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1 CONJUGATED POLYMER ELECTROCHEMICAL DEVICES FOR ENERGY STORAGE AND ELECTROCHROMIC WINDOWS By DAVID Y. LIU A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011
2 2011 David Y. Liu
3 To Mom, Dad, and soon to be wife, Lizzy
4 ACKNOWLEDGMENTS I would like to start by thank ing my family and friends for all their love, support, and encouragement Most importantly I want to thank my parents Raymond and Julie for their dedication and sacrifice over the years, and moving the family to the U nited States so that my brother and I would have better e ducatio n and more opportunities. I want to thank my older brother Patrick for being a great role model and friend. I have always looked up to you, and thank you for always being there for me. I thank my best friend and soon to be wife, Lizzy for her unconditiona l love, support, patience, and understanding over the five years we have been apart. Thank you! I also thank her parents, Mr. and Mrs. Fitzgerald, for welcoming me into their family I am v ery grateful for all the wonderful people I have met over the years especially my roommates: Ken Graham, Jared Lynch, Michael Hyman, and Jeff Carter. I have very enjoyed living with you all and you will all be missed. I thank my research advisor Professor John Reynolds, for his guidance, support mentorship, and optimi sm over the years. By working and learning from Dr. Reynolds, I hav e become a more optimistic and balanced scientist. I am very grateful to have been able to conduct research in the Reynolds group and have had the freedom to pursue research projects that i nterested me. I am grateful for all the research colleagues I have met over the years. You all have made my graduate studies much enjoyable. I th ank Ken Graham, Romain Stalder, Frank Arroyave, Eric Shen Jianguo Mei, Paula Delgado and Pam Cohn. I am especially grateful for the research scientists and post doc: Aubrey Dyer, Chad Amb, Ryan Walzack, James Leonard, Mike Craig, Leandro Estrada, and Svetlana Vasilyeva. I also
5 have to thank all the undergraduat es from the Faraday Cage, especially Andy Chilton for our collaboration and friendship. During my PhD studies, I was fortunate to collaborate on several projects. I want to acknowledge Pat Kinlen and Eve Fabrizo from Crosslink, and Vince B allarotto from t he University of Maryland. I also thank Steve Miles from the electronic shop for designing and constructing the triggering circuit. I thank my committee members Professors Ken Wagener, Ron Castellano, David Wei, and Heather Ray for our discussions on care er goals I would also like to thank everyone from the Polymer Floor who has helped with all of the various parts of graduate school, including Sara Klossner, Gena Borrero, Cheryl Googins, and Annyetta Douglas. I also thank Lori Clark and Dr. Ben Smith fro m the graduate office
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF ABBREVIATIONS ................................ ................................ ........................... 13 ABSTRACT ................................ ................................ ................................ ................... 15 CHAPTER 1 ELECTROCHEMISTRY OF CONJUGATED POLYMERS: INTRODUCTION, PRINCIPLES, AND APPLICATIONS ................................ ................................ ...... 17 Introductory Remarks ................................ ................................ .............................. 17 Electronic Properties of Conjugated Polymers ................................ .................... 18 Charge Injection ................................ ................................ ................................ ...... 22 From Monomers to Polymer Films ................................ ................................ .......... 24 Electrochemical versus Chemical Polymerization ................................ ............ 2 4 Electrochemical Polymerization ................................ ................................ ........ 25 Chemical Polymerization ................................ ................................ .................. 28 Processing Methods ................................ ................................ ......................... 29 Electrochemical Processes in Conjugated Polymer Electrodes .............................. 31 Application of Electrochemical Devices ................................ ................................ .. 35 Energy Storage ................................ ................................ ................................ 36 Electrochromism ................................ ................................ ............................... 39 Overview of Dissertation ................................ ................................ ......................... 41 2 EXPERIMENTAL METHODS AND CHARACTERIZATION TECHNIQUES ........... 44 Introduction ................................ ................................ ................................ ............. 44 Materials ................................ ................................ ................................ ................. 44 General Chemicals ................................ ................................ ........................... 44 Electrochemical Supercapacitor (ESC) Materials ................................ ............. 45 Electrochromic Device (ECD) Materials ................................ ........................... 45 Electrochemistry ................................ ................................ ................................ ..... 46 General Setup ................................ ................................ ................................ .. 46 Investigation of Contact Resistance ................................ ................................ 46 Cell Setup and Film Handling ................................ ................................ ........... 48 Device Fabrication ................................ ................................ ................................ .. 49 Electrochemical Supercapacitors ................................ ................................ ..... 49 Supercapacitor Module ................................ ................................ .................... 50
7 Absorptive/Transmissive Window type Electrochromic Devices ...................... 52 Optical Spectroscopy ................................ ................................ .............................. 53 Synchronization of Electrochemical and Optic al Data ................................ ...... 53 Trigger Circuit ................................ ................................ ................................ ... 55 Colorimetry ................................ ................................ ................................ ............. 56 Photography ................................ ................................ ................................ ........... 57 Solution Conductivity ................................ ................................ .............................. 58 3 DEVELOPING CONJUGATED POLYMERS FOR ELECTROCHEMICAL SUPERCAPACITORS ................................ ................................ ............................ 59 Introduction ................................ ................................ ................................ ............. 59 Electrochemical Polymerization and Characterization of CP Electrodes ................ 68 Poly(ProDOT Me 2 ) ................................ ................................ ........................... 68 Poly(BEDOT Isoindigo butyl 2 ) ................................ ................................ .......... 70 Electrochemical Supercapacitors ................................ ................................ ............ 75 PProDOT Me 2 based Type I ESCs ................................ ................................ .. 75 Poly(BEDOT Isoindigo) based Type I ESCs ................................ .................... 77 Poly(BEDOT Isoindigo) based Type III ESC ................................ .................... 79 Supercapacitor Modules ................................ ................................ ................... 82 Chapter Summary ................................ ................................ ................................ ... 89 4 DEVELOPING ABSORPTIVE/TRANSMISSIVE BLACK WINDOW TYPE ELECTROCHROMIC DEVICES ................................ ................................ ............. 91 Introduction ................................ ................................ ................................ ............. 91 Black to Transmissive Electrochromes ................................ ................................ ... 95 Opt imization of Transmittance and Switching S peed ................................ ............ 105 Transparent Counter Material ................................ ................................ ............... 108 Charge Balance ................................ ................................ ................................ .... 111 Black to Transmissive Electrochromic Devices ................................ .................... 112 Chapter Summary ................................ ................................ ................................ 116 5 IN SITU SP ECTROSCOPIC ANALYSIS OF SUB SECOND SWITCHING POLYMER ELECTROCHROMES ................................ ................................ ........ 118 Introduction ................................ ................................ ................................ ........... 118 Electrochromic Polymers ................................ ................................ ...................... 120 ECP Magenta ................................ ................................ ................................ 121 ECP Green ................................ ................................ ................................ ..... 125 ECP Black ................................ ................................ ................................ ...... 128 Chapter Summary ................................ ................................ ................................ 132 6 CONCLUSIONS AND FUTURE WORK ................................ ............................... 134 Electrochemical Supercapacitors ................................ ................................ .......... 134 Electrochrom ic Devices ................................ ................................ ........................ 136
8 LIST OF REFERENCES ................................ ................................ ............................. 139 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 150
9 LIST OF TABLES Table page 2 1 Calculated resistance for silver paste and copper tape contacts ........................ 48 4 1 Relative luminance and CIE L*a*b* Color Coordinates for BASF Black films ..... 99 4 2 Transmittance and switching speed for BASF Black films ................................ 101 4 3 Relative lumin ance and CIE L*a*b* Color Coordinates for Random Black ....... 102 4 4 Transmittance and switching speed for Random Black ................................ .... 104 4 5 Conductivity of supporting electrolyte solutions ................................ ................ 105 4 6 Switching speed of Random Black ................................ ................................ ... 106 4 7 Relative luminance and CIE L *a*b* Color Coordinates for ECDs. .................... 114 4 8 Transmittance and Switching Speed Summary of ECD ................................ ... 116 5 1 Summary of ECP transmittance contrast for EC P Magenta, Green, and Black ................................ ................................ ................................ ................ 131 5 2 Summary of ECP s witching time for EC P Magenta, Green, and Black .......... 131
10 LIST OF FIGURES Figure page 1 1 Electronic properties of conjugated p olymers. ................................ .................... 18 1 2 Doping scheme of conducting polymers ................................ ............................. 19 1 3 Representative repeat unit structures of conjugated polymers in the neutral state ................................ ................................ ................................ .................... 20 1 4 Allowed electronic transitions for conducting polymers. ................................ ..... 21 1 5 Dopi ng methods for conjugated polymers ................................ .......................... 22 1 6 Common solution processing techniques for soluble conjugated polymers. ....... 30 1 7 Electrochemical characte rization of conjugated polymers ................................ .. 32 1 8 Represen tative chronoabsorptometry data ................................ ........................ 34 1 9 Overview schematic of electrochemical devices ................................ ................. 36 1 10 Ragone plot representing specific power and specific energy outputs ............... 37 1 11 Common redox states of methyl viologen ................................ ........................... 41 2 1 Schematic of electrochemical model ................................ ................................ .. 47 2 2 Electrochemical cell designs for the preparation of ESC electrodes ................... 49 2 3 E ncapsulation of Supercapacitors ................................ ................................ ...... 50 2 4 Supercapacitor Module ................................ ................................ ....................... 51 2 5 Supercapacitor Module with bipolar electrodes. ................................ ................. 52 2 6 Schematic of in situ electrochemical spectroscopy instrumentation ................... 54 2 7 Schematic of the external circuit ................................ ................................ ......... 55 2 8 CIE 1976 L*a*b* ................................ ................................ ................................ 57 3 1 p do pable and n dopable CPs utilized in electrochemical supercapacitors ........ 60 3 2 Operating principle of electrochemical supercapacitor ................................ ....... 64 3 3 Supercapacitor modular design ................................ ................................ .......... 66
11 3 4 p dopable and n dopable CPs utilized in supercapacitors. ................................ 67 3 5 Electrochemistry of PPro DOT Me 2 ................................ ................................ ..... 69 3 6 Electrochemistry of PBEDOT iI butyl 2 ................................ ................................ 71 3 7 Electrochemical polymerization of PEDOT iI butyl 2 ................................ ............ 72 3 8 Electrochemical polymerization of PBEDOT iI butyl 2 from solutions of 2 mM monomer in 0.5 M LiBTI/BZN ................................ ................................ ............. 73 3 9 p doping and n doping responses of PBEDOT iI butyl 2 ................................ ..... 74 3 10 Oxidative and reductive switching of PBEDOT iI butyl 2 ................................ ...... 75 3 11 Type I PProDOT Me 2 Supercapacitor ................................ ................................ 76 3 12 Type I PBEDOT iI butyl 2 Supercapacitor ................................ ............................ 78 3 13 Operative voltage range of PBEDOT iI butyl 2 based Type I supercapacitor ...... 79 3 14 Voltage limits of Type III PEDOT iI butyl 2 based ESC ................................ ........ 80 3 15 Type III PBEDOT iI butyl 2 Supercapacitor ................................ .......................... 81 3 16 Supercapacitor charging/discharging CV ................................ ........................... 83 3 17 Supercapacitor galvanic charging/discharging ................................ ................... 85 3 18 Supercapacitor a ssem bly ................................ ................................ ................... 86 3 19 Tandem supercapacitor. ................................ ................................ ..................... 87 3 20 Supercapacitor CV ................................ ................................ ............................. 88 4 1 S truct ures of ECP Black p olymers ................................ ................................ ..... 93 4 2 Schematic design of absorptive/transmissive window type elec trochromic device ................................ ................................ ................................ ................. 95 4 3 C yclic voltammogram of ECP Black ................................ ................................ ... 95 4 4 Spectroelectro chemical series for ECP Black ................................ .................... 97 4 5 Absorbance and relative l uminance of BASF Black ................................ ........... 98 4 6 Potential square wave step ch ronoabsorpometry for BASF Black ................... 1 00 4 7 Absorbance and relative l uminance of Random Black ................................ ..... 101
12 4 8 Potential square wave step chronoabsorpometry for Random Black.. ............. 103 4 9 Comparative absorption spectra for BASF Black and Random Black .............. 104 4 10 Chronoabsorptometry of Random Black films with potential square wave steps ................................ ................................ ................................ ................. 107 4 11 Chronoabsorptometry of Random Black ................................ .......................... 108 4 12 Electrochemistry of MCCP ................................ ................................ ................ 109 4 13 Trans mittance and relative l uminance of MCCP ................................ ............... 110 4 14 Calibration plots of absorbance and charge density ................................ ......... 112 4 15 Spectroelectrochemical series of window ECD ................................ ................ 113 4 16 Photographs of window ECDs ................................ ................................ .......... 115 4 17 Chronoabsorptometry of ECDs ................................ ................................ ......... 116 5 1 Structures of ECP Magenta, ECP Green, and ECP Black ............................... 120 5 2 Spectral e volution of ECP Magenta ................................ ................................ .. 122 5 3 Chronoabsorptogram of ECP Magenta ................................ ............................ 124 5 4 Spectral e volution of ECP Green ................................ ................................ ...... 126 5 5 Chronoabsorpt ogram of ECP Green ................................ ................................ 127 5 6 Spect ral Evolution of ECP Black ................................ ................................ ...... 129 5 7 Chronoabsorptogram of ECP Black ................................ ................................ 130 6 1 Proposed Poly(ProDOT) ................................ ................................ ................... 135 6 2 Visual representat ion of voltage profile waveform ................................ ............ 137
13 LIST OF ABBREVIATION S ACN acetonitrile BZN benzonitrile CB conduction band CP conjugated polymer CV cyclic voltammetry D A D donor acceptor donor DCM dichloromethane DPV differential purlse voltammetry ECD electrochromic device ECP electrochromic polymer E 1/2 half wave potential E g band gap FET field effect transistor ESC electrochemical supercapacitor HOMO highest orbital molecular orbital ITO indium tin oxide LED light emitting diode LCAO linear combination of atomic orbitals LUMO lowest unoccupied molecular orbital MCCP minimally color changing polymer MO molecular orbital OLED organic light emitting diode OPV organic photovoltaic
14 PEDOT:PSS poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) PET poly(ethylene terephthalate) PC propylene carbonate PMMA poly(methyl methacrylate) SWNT single walled carbon nanotubes TDAT total data acquisition time VB valence band
15 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CONJUGATED POLYMER ELECTROCHEMICAL DEVICES FOR EN ERGY STORAGE AND ELECTROCHROMIC WINDOWS By David Y. Liu December 2011 Chair: John R. Reynolds Major: Chemistry This dissertation details the development of conjugated polymer (CP) based electrochemical devices for energy storage and electrochromic windo ws. Since operating principles such as redox processes, electron transfer at the electrode/electrolyte interface, and the interactions amo ng current, charge, and voltage, are common in CP based electrochemical devices, the interplay of these functions as they relate to structure and device performance is crucial. In this work, a variety of electrochemical and optical techniques are utilized to characterize electrochemical dev ices and CPs contained therein. The first portion of this dissertation focuses on CP based electrochemical supercapacitors (ESCs) for energy storage, wit h an emphasis on improving the voltage window through structure modification and modular device assembly. Specifically, CP based electrodes are developed through electrochemical polymer izations of electron rich 3,4 propylenedioxythiophene (ProDOT) and donor acceptor donor (D A D) units. These e lectron rich poly(ProDOT) based ESCs are observed to operate with h igh electrochemical stability across a 1.0 volt range. Extension of the voltage window to 1.8 V is realized in D A D based ESCs due to the ability of this class of CPs to undergo
16 both oxidation and reduction Further, a novel methodology to increase the voltage window is presented utilizing a modular device assembly method The second portion of this dissertation focuses on the optimization of rapidly switching window type electrochromic devices (ECDs). In this work, the c omponents of black ECDs are developed and investigated utilizing solution processable black electrochrom es as active layers and minimally color changing polymer (MCCP) as the counter material. The optimized system was achieved by incorporation of these polymers into a dual electrochromic polymer device configuration with the appropriate charge balance, along with exploration of high ly conductive electrolytes. A novel electronic spectroscopy method was developed by using an external trigger to communicate between a potentiostat and a fiber optic spectrophotometer with a CCD array detector The detector is cap able of rapid data acquisition to track the electrochromic change, with one spectrum collected every 2 ms, in order to elucidate properties of solution processed electrochromic films A set of ECPs demonstrated sub second response times on the order of 400 700 ms with high transmittance contrasts over 40% T.
17 CHAPTER 1 ELECTROCHEMISTRY OF CONJUGATED POLYMERS: INTRODUCTION, PRINCIPLES, AND APP LICATIONS Introduct ory Remarks Organic p olymers have traditionally been considered to be thermally and electrically insulating materials. This picture began to change with reports of conducting behavior in polymers first reported in iodine doped poly ( pyrrole ) (PPy) in 1963 by Weiss, 1 and perhaps most famously in iodine doped poly(acetylene) (PA) by Heege r, MacDiarmid, and Shirakawa in 1977. 2,3 This Nobel Prize winning work initiated the field of conducting (and conjugated) polymers Since their discovery, the inherent combination of mechanical properties of plastics (flexibi lity, pro cessability, and robustness ) with electrical properties has been a major focus. Conjugated polymers (CPs) can exhibit high ele ctrical conductivity when doped, as in the case of trans poly ( acetylene ) where Heege r et al demonstrated a change of seven orders in magnitude upon iodine doping. 2,3 Moreover, the organic nature of CPs allows for fine tuning of intrinsic properties through synthetic modificatio ns. These features, in addition to the ability to solution process, have contributed conducting polymers towards the interest on development of potential low cost, large area, flexible, and lightweight electronic devices such as field effect transistors, 4 6 light emitting diodes, 7 9 chemical sensors, 10,11 memories, 12 photovoltaic cells, 13 17 energy storage and electrochromic devices. 18 22 A common element shared among organic electronic devices is a change in their properties upon application of an electric field. As such, electrochemistry becomes an important tool for the characterization of fundamental materials or device properties. This dissertation foc uses on the characterization of ne w conjugated polymers
18 specifically for energy storage and electrochromic devices, along with the application of novel electrochem ical device concepts. Before delving into electrochemical device concepts and applications, an introductory background on the fundamental properties of conjugated polymers is provided. Electronic Properties of Conjugated Polymers The inherent electronic properties of conjugated polymers arise from a continuous array of overlapping orbitals supported by a bond backbone. Figure 1 1a depicts the simple linear combinations of atomic orbitals (LCAO) to model the molecular orbital (MO), whereas Figure 1 1 b illustrates the expansion of this concept from monomer to conjugated polymer, such that the number of overlapping orbitals are so closely spaced that they may be considered as a continuum. As this simple schematic demonstrates, the highest occupied mol ecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are respectively analogous to the top of the valence band (VB), and to the bottom of the conduction band (CB), as described by band theory. Figure 1 1 Electronic properties of Con jugated Polymers. a ) Ener gy level splitting of orbitals with respec t to molecular orbital theory. b ) Conceptual model of the buildup of electronic energy bands using thiophene (Th) to represent conjugated polymers c ) Band diagrams showing the differences in energy for metals, semiconductors, and insulators. (Adapted from Thompson) 183
19 The band theory classifies materials into three categories (metal, semiconductor, and insulator) depending on the energy gap (band gap, E g ) between the VB and CB as depicted in Figure 1 1 c Case (i) depicts a zero band gap and therefore electrons can access a multitude of energy states within the conduction band. This access allows electrons to move freely throughout the material and ther efore the material is conducting and metallic. Case (ii) depicts a low band gap (< 3.0 eV) such that thermal excitation of electrons from the filled VB to the empty CB is sufficient to promote conductivity. Case (iii) depicts a large band gap (> 3 eV) such that thermal excitation of electrons cannot overcome the large energy gap, and t hese electrons remain localized in the VB and the material is therefore insulating (non conducting). Semiconducting polymers can be rendered conductive via charge injection e ither by the addition of electrons (reduction, n doping) or removal of electrons (oxidation, p doping). The removal of one electron from the VB creates a radical cation (or polaron) and further removal of another electron creates a dication, which is refer red to as a bipolaron (Figure 1 2a). The polaron and bipolaron states exist at mid gap as illustrated by the band model shown in Figure 1 2b. Figure 1 2 Do ping scheme of conducting polymers represented by a ) poly ( thiophene ) chain and b ) band models as a neutral chain, polaron, and bipolaron.
20 At high doping levels, the polyheterocycle becomes a bipolaron eventually leads to the formation of bipolaron bands. 23 The p resence of these charge carriers (polarons and bipolarons) ultimately gives rise to electrical cond uctivity in conjugated polymers. In a real world example, the conductivity of iodine doped PA approaches that of metals such as copper and iron. As Figure 1 3 demonstrates, the field of conducting polymers has expanded to other relatively high conducting polymers such as poly(thiophene) poly(pyrrole) an d commercial available poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate) ( PEDOT:PSS ) with conductivi ty as high as 10 3 S/cm. 184 Figure 1 3 Representative repeat unit structures of conjugated polyme rs in the neutral state a ) polyacetylene (PA 1 1 ) polypyrrole (PPy 1 2 ) polythiophene (PTh 1 3 ) and poly(3,4 ethylenedioxythiophene) (PEDOT 1 4 ) b ) Conductivity scale comparing metals, sem iconductors, and insulators to tunable CPs (Adapted from Skotheim ) 24
21 Since the process of doping results in introduction of new electronic transitio n s and is accompanied by a structural reorganization, a change in the absorption of electromagnetic radiation is induced. Electrochromic materials are those that can undergo a reversible change in absorption (or transmittance) under an electrical bias Gen erally, the visible region of spectrum is the most highly investigated, as electrochromic properties in this area have potential utility in electronic displays and smart windows Optical absorptions in CPs arise from the allowed electronic transitions from the VB to polaron and bipolaron states created upon doping. In the neutral, undoped state, the minimum energy transition, as illustrated in Figure 1 4 is the energy difference between the VB and CB (band gap) and is referred to as the transition. The creation of polaron and bipolaron states at mid gap expands the number of possible electronic transitions and therefore depletes the transition Figure 1 4 b depicts the smaller energy separation of these states, leading to broad absorptions at lo nger wavelengths. Figure 1 4 Allowed electronic transition s for conducting polymers in the neutral, polaron, and bipolaron states.
22 Charge Injection Charge injection, in general, can lead to significant changes in the conductivity and optical propert ies of conjugated polymers. Four different types of charge injection processes are illustrated in Figure 1 5. Figure 1 5 Doping methods for conjugated polymers, and the redox reactions associated with each type. (Adapted from Heeger ) 25 Depending on t he applied potential, oxidation or reduction of CPs leads to the removal or addition of electrons such that the polymer is then positively or negatively charged. This oxidation or reduction change in turn has an immediate consequence, in that electro neutr ality within the polymer film must be maintained. Redox process is thus accompanied by charge balancing counter ions being inserted into, or extracted from, the supporting electrolyte. Conversely, ions moving in and out of conjugated polymers induce concom itant changes in the electrical and optical properties. In the redox
23 representation shown below, the anion (tetrafluoroborate, BF 4 ) and cation (tetrabutylammonium, TBA + ) stabilize the oxidized and reduced species respectively. (1 1) (1 2) Conjug ated polymers can also be doped by chemically induced charge transfer. This process requires an oxidizing or reducing agent to react with CPs. Common oxidizing agents include iodine and nitrosium hexafluorophosphate, while common reducing agents include sodium naphthalide and hydrazine. Positively and negatively charged CPs retain charge neutralization by the presence of counter ions derived from the oxidant or reductant. Through che mical doping of CPs, high conductivity can be achieved, leading to applications such as anti static coatings 26 and transparent electrode materials. 27 Metal polymer interfacial doping is similar to electrochemical doping, but in the absence of counter ions. 28 This type of charge in jection is often found in fie ld effect transistors (FETs), 29 and light emitting diodes (LEDs) 30 Lastly, photo induced doping can convert light into el ectricity. As light is absorbed by CPs, an electron is promoted from the valence band to the conduction band. F luorescence and /or phosphorescence are observed if the excited states recombine and return to the ground state via a radiative decay mechanism However, if there is electron transfer from the CP excited state to an acceptor, such as fullerene C 60 then charge separation of the two charge carriers (the electron and resulting hole) will occur. This concept is the operating principle of photovolta ic devices used to convert sunlight into electricity 31
24 With a high degree of control, electrochemistry is one of the most useful charge injection techniques. Moreover, the transfer of cha rge is easily monitored by controlled variation in the potential. From Monomers to Polymer Films For practical electrochemical and optical studies, conducting polymers must be intimately adhere d onto conductive substrates allowing the redox states of th e polymer modified electrodes to be controlled by a potentiostat connected to the sample. To pr epare polymer coated electrodes, there are two approaches to consider: (1) electr ochemical polymerization of monomer to form conjugated polymers directly onto th e conductive substrate and (2) solution process ing of chemically prepared CPs by casting techniques such as spray cast ing 32 34 spin cast ing 13,35,36 drop cast ing 37 39 inkjet 4 0 42 and flexo printing. 43 46 The following sections detail and compare the principles, advantages, and drawbacks of electrochemical and chemical polymerization methods as well as the ability to form thin polymer films. Electrochemical versus Chemical Polymerization Electrochemical polymerization offers several adv antages from the viewpoint of materials preparation and structural control From the perspective of a synthetic chemist monomers are typically more straightforward to prepare and isolate than their polymeric analogue s and only small amounts (10 50 mg) ar e needed for electrochemical polymerizations. In addition, electrochemical polymerization methods and various electrochemical conditions have been shown to allow control over polymer film thickness and morphology In particular, electrochemical polymerizat ions afford porous films of high capacitance and are especially suitable for charge storage applications. For example, electrochemically prepared poly(pyrrole) (PPy), poly( 3,4
25 ethylenedioxythiophene) (PEDOT), and poly( 3,4 propylenedioxythiophene) ( PProDOT) have all demonstrated high capacitive properties ideal for electrochemical supercapacitors. 21,47 51 A discussion on these materials and other conjugated polymers suitable for electrochemical supercapacitors are elaborat ed in C hapter 3. Additi onally electrochemical polymerization s can be carried out directly on conducting substrate s to ensure that intimate contact is made at the interface. In scenarios where high surface area electrodes are used, for example single walled carbon nanotubes (SWN Ts) for charge storage applications, 52 55 electrochemical polymerizat ion is the preferred approach to deposit a film into these interpenetrating porous electrode s whereas solution processing may only coat the exposed surface. Although chemically prepared polymers may be more laborious to synthesize, there are significant a dvantages presented by this method. Chemically prepared polymers allow for more complete characterization, including elucidating the molecular weight distribution, exploring the thermal characteristics (such as T g crystallization, and thermal stability), and analysis of the polymer end groups. Furthermore, solution processability of chemically prepared polymers facilitates practical large scale printing. Electrochemical Polymerization Various possible mechanism s have been reported for electrochemical polymerization of polyheter o cycles. 56 59 The most wi dely accepted electropolymerization mechanism proceeds via oxidation of the monomer unit to generate radical cation species in solution, which are able to couple with ano ther surrounding radical cation or monomer unit to form a dimer. This reaction involv e s an electrochemical oxidation and chemical coupling followed by the elimination of protons. Further oxidation of the dimer and radical cation couplings produces oligomers that are soluble. Elongation of the
26 oligomer either through a linear chain or a cro ss linked structure deposits the insoluble electroactive polymer onto the working electrode surface. 60 Electrochemical polymerization begins with the formation of oligomers in solution f ollowed by the deposition of conjugated polymers, which consists of nucleation and growth steps. The nucleation and growth model for conjugated polymer s is borrowed from the model used to describe the electro deposition of metals. 184 The nucleation mechanis m can be described as (1) instantaneous in which there is a set number of nucleation sites are formed without additional nucleation sites, and (2) progressive where the nucleation sites are generated continuously throughout the process In addition, conjug ated polymer s can grow in two different orientations ; that is, growth can occur perpendicular to the electrode surface (1D), and parallel to the surface (2D). When the deposition occurs in both directions s imultaneously, the growth is 3D. The polymer d epo sition can be carried out and monitored by a number of electrochemical processes, most commonly by (1) cyclic voltammetry ( also referred to as potentiodynamic cycles), (2) potentiostatic (constant potential), and (3) galvanostatic (constant current) method s. Potentiodynamic polymerization inv olves a concurrent potential and current change during the deposition. As the potential continuously changes during the potentiodynamic scan, the growing polymer is switching between the insulating and conducting states As such, there is a constant exchange of electrolyte and solvent through the deposited polymer matrix that leads to a disordered yet advantageously porous film. 61 The extent of electrochemical polymerization by potentiodynamic technique is heavily dependent on the potential range, scan rate and number of cycles.
27 The electrosynthesis of CPs, by potentiostatic and galvanostatic polymerizations makes possible a corre lation between the charge transferred during the electrochemical this relationship: Q = nFN = it (1 3 ) where Q (C) is the charge passed, F is Faraday stant ( 96,485.3 C/mol ) n is the number of electrons in the redox reaction, N (mol) is the amount of reactant i is current (A) and t is time (s). The extent of polymerization is related to the total amount of electrolysis time and the charge passed, and the thickness of the resulting film is therefore c ontrolled by the deposition time. It has been shown that potentiostatic and galvanostatic polymerizations of poly(pyrrole) lead to non adhesive dendritic type polymer film s of low uniformity. Conversely, p otentio dynamic polymerizations of PPy result in highly adhesive, homogeneous, and smooth films. 62 This experimental observation was accounted for by the large number of nucleation sites dur ing the potentiodynamic polymerization. 63 The e lectrochemical conditions can also influence the electropolymerization process and polymer film morphology. For example, i t has been demonstra ted that relatively high oxidation potentials can lead to cross linked defects. 64 On the other hand, low potentials can lead to weakly charged intermediates and only form oligomers. 65,66 Furthermore, the selection of solvent and electrolyte can significantly influence the electropolymerization of conjugated polymers. 67 70 Recent, Bendikov systematically studied the concurrent influence of both solvent and electrolyte during the
28 electropolymerization of PEDOT and found the solvent, not the electrolyte, more heavily impacted the morphological structure and electrochromic pro perties of PEDOT films. 71 Chemical Polymerization Oxidative polycondensation and metal mediated cross coupling s are generally used for chemical polymerizations of polyheterocycles Examples of metal mediated cross coupling reactions include Grignard metathesis, Stille, and Suzuki Miyaura reactions Oxidative polymerization of heterocycles is analogous to electrochemical polymerization in that the methodology involves the oxidation of the mon omer, for example with FeCl 3 as the oxidant. Oxidative polymerizations are utilized for the synthesis of soluble homopolymers, and this route is less cost intensive than metal mediated polymerizations. However, b ecause the oxidized polymer is less soluble than the neutral counterpart, high molecular weight polymers are often difficult to achieve. Another drawback of oxidative polymerization is the possibility of side reactions occurring at available positions. Furthermore, ferric ions from the oxidant can be trapped in the polymer backbone, affecting the overall device performance. 72 Grignard metathesis polymerization 73 as developed by the McCullough group has yielded poly ( thiophenes ) with high molecular weights and high degree of regioregularity. 73 This synth etic route utilizes 2,5 dibromo thiophene derivati ves, Grignard reagents such as methyl magnesium bromide (MeMgBr), and catalytic amounts of Ni catalyst (Ni(dppp)Cl 2 ). With the said components, the reaction is highly oxygen and moisture sensitive and requires careful chemical handling. The polymerization proceeds through a quasi living chain growth mechanism such that the molecular weight is controllable with narrow molecular weight distributions. 74,7 5
29 Stille polycondensation has been demonstrated as a versat ile polymerization methodology yielding functional materials. 76 The Stille coupling reaction takes place between aryl or vinyl stannanes and aryl halides with catalytic amounts of palladium to form new carbon carbon bonds. In addition to high molecular weight polymers and high yields, the Stille polymerization also afford s stereospecific and regioselective conjugated polymers. Fu rthermore, the coupling reactions are less oxygen and moisture sensitive than other organometallic components, such as Grignard reagents, although the organotin compounds are toxic and expensive. Another viable polymerization route is Suzuki Miyaura reaction employing organoboronic acid s aryl halides, and catalytic amounts of palladium. Through this polymerization route, CPs with high molecular weight and narrow molecular weight distributions have been prepared. This coupling reaction traditionall y requires basic reagents to activate the boronic acid/ester to form the new carbon carbon bond s 77 As such, there are some limitations with this polymerization route. However, base free Su zuki polymerization conditions have recently been developed 78 For instance, Brookin s et al de monstrated the optimization of a base free Suzuki polymerization for poly (fluorine) functional ized with carboxylic acid group s. 79 Processing Methods A number of solution processing techniques for conducting polymers have been developed for both fundamental studies and large scale printing. While the spray casting technique was predominantly used in this work, other solution processing techniques are introduced to illustrate other viable processing opportunities in this field. Material processing t echniques that are commonly impleme nted for small scale research include spray casting and spin coating shown in Figure 1 6, in which
30 fundamental structure property relationship studies and small scale device fabrications are carried out. In these techniques, conjugated polymers are dissol ved in organic solvents such as toluene, dichloromethane, or chloroform at a concentration ranging from 1 5 mg/mL Aqueous solutions can even be used 80 Spray casting utilizes a pressurized airbrush sprayer to process the polymer on to an electrode surface. Spray cast films are relatively smooth to the naked eye and the polymer thickness ranges from 0.2 1.0 m 33 Spin coating is another commonly used processing technique, especially for organic semiconducting materials for use in photovoltaic devices Typically, polymer films are relatively thin, ranging from (50 150 nm) with a 1 2 nm R MS surface roughness. 81 Figure 1 6 Common solution processing techniques for soluble conjugated polymers.
31 Large scale device prototypes require advanced processing techniques such as i nkjet and flexo printing. Inkjet printing has previously been utilized for processing active layers in organic photovoltaics ( OPVs ) and organic light emitting diodes ( OLEDs ) This technique is especially useful for printing materials with precise control. For example, Foulger have demonstrated printing resolution on the micrometer scale. 84 In addition, flexo printing offers a different method of processing soluble electroactive materials. T his technique uses flexible printing and inked plates in a roll to roll method as shown in Figure 1 6. The ink roller transfers the active material from the ink reservoir to an adjacent roller that controls the uniformity and thickness. The plate cylinder controls the pattern, while the impression cylinder applies pressure to the substrate. Electrochemical Processes in Conjugated Polymer Electrodes The second major area of conjugated polymer electrochemical research concerns the electrochemical processes occurring at the electrodes made from, or coated with, CPs. Fundamental and applied electrochemical processed as related to CPs are discussed in this section. Cyclic voltammetry (CV) remains one of the most useful electrochemical processes to study conjugated polymers and CP based electrochemical devices by providing both quantitative and qualitative information. CV (also referred to as potentiodynamic cycling ) measures the cell current while sweeping the applied potential between the working and reference electrodes. The obtained voltammogram reveals several important properties, such as the polymer's redox switching range, the relative response rate of redox processes, the electrochemical stability, and the degree of reversibility.
32 The cyclic voltammogram of PProDOT Me 2 shown in Figure 1 7 a elucidates several electrochemical properties such a s half wave potential (E 1/2 ), scan rate dependence, and film capacit ance. The half wave potential, measured by taking the average of the peak anodic and cathodic current potentials, is the potential at which the concentration s of the oxidized and reduced species are equal. Figure 1 7 Electrochemical characterization of conjugated polymers. a) Cyclic voltammogram for PProDOT Me 2 switching in 0.1 M LiBTI/ACN at 50 mV/s b) Scan ra te dependence of polymer film. c) Peak currents as a function of scan rate In an ideal case the cyclic voltammogram of a reversible system would show symmetrical and mirror image anodic and cathodic peaks, identical potentials and current levels, and a peak to peak difference ( E peak ) of 59 mV. By varying the scan rate of cyclic voltammetry experiments, the polymer's scan rate dependence can be revealed. This is accomplished b y monitoring the peak anodic (i p,a ) and cathodic (i p,c ) current responses as a function of scan rate, as illustrated in Figure 1 7b. In the case of electrode bound species such as electroactive conjugated polymers, the c urrent response is linearly proportional to the scan rate and can be described by Eq uation 1 4 : i p = n 2 F 2 A / 4 RT (1 4)
33 where n is the number of electrons, F A is the surface area of the working electrode (cm 2 is the concentration of surface bound electroactive centers (mol/cm 3 ) and is the scan rate (V/s). A linear relationship between current and scan rate indicates that the polymer is well adhered to the el ectrode surface. Generally, E quation 1 4 ho lds true for thin polymer films. W ith thick films however the anodic and cathodic peak currents vary with the square root of scan rate, indicating that the redox process is limited by the diffusion of counter ions transporting in and out of the film. In a nother word the polymer is characteristic of an insulator (resistor), and the shape of the cyclic voltammogram is usually oblong. In addition to the peak anodic and cathodic currents that are characteristics of redox (faradaic) reactions, the cyclic volta mmogram shown in Figure 1 7 also exhibits a broad current plateau that arise s from capacitive currents through double layer charging at the polymer electrolyte interface. 89,90 H igh levels of capacitive currents are desirable for charge storage applications. Chapter 3 will elaborate the use of cyclic voltammograms and scan rate dependence experiments to evaluate the capacitance of polymer films and electrochemical supercapacitors as well as the charging and discharging rates of ESCs Another electrochemical process applied to conjugated polymers is square wave potential cycles as represented in Figure 1 8a. The working electrode is switched instanta neously between two potentials while the current time (chronoamperometry) or the charge time (chronocoulometry) profile is monitored.
34 Figure 1 8 Representative chronoabsorptometry data. a) Representation of potential square wave cycles between 0.4 to +0.8 V vs. Ag/Ag + at 0.5 second periods for an electroc hromic polymer (Random Black). b) Chronoabsorptometry experiment o f a film of Random Black (monitored at = 550 nm) on an ITO coated sub strate in 0.1 M LiBTI / ACN where the potential is stepped between the neutral state ( 0.4 V) and the oxidized state (+0.8 V vs. Ag/Ag + ). The quantitative analysis of chronoamperometry is described by the Cottrell equation: i = nFACD t (1 5) where, n is the number of electrons transferred per molecule, F is Faraday's constant (96,485 C/mol), A is the surface area of the working electrode (cm 2 ), D is the diffusion coefficient of the redox active sites (cm 2 /s), C is the bulk concent ration (mol/cm 3 ), and t is the time in seconds. The analysis of charge in chronocoulometry is carried out by integrating the Cottrell equation to obtain the following equation for linear diffusion controlled systems: Q = 2nFACD t 1/2 (1 6)
35 wh ere n is the number of electrons, A is the electrode area (cm 2 ), D is the diffusion constant (cm 2 /s), C is the concentration (mol/c m 3 ) of the analyte, and t is the time in seconds. As it will be elaborated in C hapters 4 and 5, the switching of electrochromic polymers between their colored and bleached states can be accomplished by potential square wave cycles between their neutral and oxidized states respectively. This electrochemical process mimics the instant aneous electrochromic devices. By coupling potential square wave cycles with in situ UV vis NIR experiments (chronoabsorptometry), the optical co ntrast and switching speed of e lectrochromic polymers and devices are evaluate d. Fi gure 1 8b exhi bits a n example of a chronoabsorptogram for a black to transmissive polymer switching between the neutral ( 0.4 V) and oxidized states (+0.8 V) A t ransmittance contrast of 50 % T was demonstrated by applying a 0.5 second potential square wave switching period. Application of Electrochemical Devices The o verall architecture of electrochemical devices consists of two electrodes, where oxidation of electroactive materials occurs at the anode and reduction (or re neutralization) takes p lace at the cathode. A layer of electrolyte is placed between electrodes to electrically balance the oxidized and reduced species during electrochemical switches. This general assembly is applied to all electrochemical devices presented in this dissertatio n though slight modifications are necessary, as illustrated by the schematic for energy storage (supercapacitor) and electrochromic devices in Figure 1 9
36 Figure 1 9 Overview schematic of electrochemical devices and specific components for energy sto rage and electrochromic devices. Fundamental differences in energy storage and electrochromic device architecture are indicated. Foremost, energy stora ge devices utilize a separator, a porous membrane placed between electrodes which not only prevent s an e lectrical short, but also allow s rapid transport of ionic charge carriers needed to complete the circuit or charge neutrality during the passage of current in an electrochemical cell. On the other hand, separators are not desirable in window type electroc hromic devices since the membrane affects the overall device color purity. As such, spacer membranes are utilized around the outer edges of the device electrode to prevent an electrical short. At the same time in order to retain color purity in electrochro mic window devices, the conducting substrates and electrolyte must both be highly transparent and near colorless. Indium tin oxide ( ITO ) coated glass substrates and transparent SWNTs 91 as w ell as polyethylene terephthalate (PET) 92 substrates have been incorporated in smart window electrochromic devices. Energy Storage Electrochemically dopable materials for charge storage materials include carbon, inorganic metal oxides, and organic conjugated polymers. Depending on materials and
37 cell configurations, energy storage devices are classified into four categories (capacitor, supercapacitor, battery, and fuel cell), with each type yielding different energy and power outputs as illustrated in the Ragone Plot in Figure 1 1 0 Figure 1 1 0 Ragone plot representing specific power and specific energy outputs for various energy s torage devices. (Adapted from Winter and Brodd ) 48 In batteries and fuel cells, electrical energy is generated by the conversion of chemical energy at the cathode and anode through redox rea ctions. These energy storage devices are generally high energy and low power. The major difference between a battery and fuel cell is the location of energy storage and conversion. Batteries operate under a closed system in which the anode and cathode part icipate in redox reactions in the same compartment. In fuel cells, redox reactions involve oxygen or hydrogen loading from the environment to the anode or cathode. On the other side of the spectrum in the Ragone plot are capacitors. These are electrochemic al devices capable of delivering high power and low energy. Capacitors operate solely via electrostatic interactions with high reversibility and high lifetime (over 100,000 cycles).
38 Supercapacitor s taking advantage of both the electrochemical double laye r and redox active charging abilities bridge the gap between capacitors and batteries on the Ragone Plot with intermediate energy and power densities. 50 Electrochemical double layer supercapacitors store charge electrostatically using reversible adsorption of electrolyte ions. The charge separation occurs upon polarization at the electrode electrolyte interface, and the subsequent double layer capacitance is directly proportional to the electrolyte dielectric constant and surface area, and inversely proportional to the effective thickness. 51 Furthermore, reversible surface adsorption allows fast energy uptake and output. The absence of faradic process can improve the overall cycling lifetime by avoiding swelling and expansion of charge storing materials. Active materials with fast, reversible redox reactions utilize faradic processes to stor capacitive 2 and MnO 2 as well as organic conducting polymers have been employed as pseudo capacitive materials in supercapacitors. Ruthenium oxide has been widely investigated for its conductivity and multiple oxidation states, and specific capacitances of more than 600 F/g have been reported. 93 However, a low voltage window (1 .2V) and the high cost of ruthenium limits the practical utilization of ruthenium oxide in electrochemical energy storage devices. Electrically conducting polymers offer alternative redox active materials in supercapacitors. Poly ( aniline ) poly ( pyrrole ) poly ( thiophene ) and their derivative s have shown promise as p dopable materials in supercapacitor applications. Furthermore, donor acc eptor donor polymers as n dopable materials have the potential to improve
39 the overall power and energy densities. 94 The performance of these materials in electrochemical supercapacitors is detailed in C hapter 3. Electrochromism Electrochromic materials are those that exhibit a reversible change in absorption (or transmittance) upon oxidation or reduction As optical changes may occur in the ultra violet (UV), visible (Vis), near infrared (NIR) mid infrared (MIR) as well as in the microwave region of the spectrum, these various electrochromic changes are exploited in different application driven electrochemical devices. 95 97 Electroc hromic materials with large absorption contrast in the NIR have shown promise in electrochromic variable optical attenuator s (EC VOA) and for modulating fiber optic signals for optical telecommunications. 98 Optical attenuators are key components in fiber optic communication networks, enabling the control of optical signal intensity and the regulation of signal powers in amplified wavelength division multiplexed networks. 99,100 A more active area of electrochromic research has been directed towards optical changes in the visible spectrum, giving rise to applications such smart windows, electrochromic displays, and reflectance mirrors. Electrochromic materials with high degrees of optical modulation in the visible spectrum include both inorganic (Prussian blue, transition metal oxides), and organic (viologens, conjugated polymers) system s. Prussian blue (ferric ferrocyanide) is one of the earliest studied electrochromic material s and was first prepared on a solid electrode by Neff in the electrochemical reduction of solutions containing iron (III) and hexacyanoferrate (II) ions. 101 Thin films of Prussian blue exhibit multichromic properties such that upon reduction in aqueous medium containing potassium ions become colorless (Everitt salt, ES) O xidation with a chloride
40 m edium affords a yellow compound (Prussian yellow, PY). The described electrochromic properties were studied by Itaya 102 using the following redox reactions: (1 7) (1 8) Tr ans ition metal oxides h ave been o ne of the most widely studied classes of electrochromic materials. Depending on the metal oxide, either anodic or cathodic coloration is observed ; Cr, Mn, Fe, C o, Ni, Rh, and Ir are anodically coloring, whereas Ti, Nb, Mo, Ta, and W are cathodically coloring. Of these transition metal oxides, tungsten trioxide has been the most widely studied and has been used in commercial electrochromic applications. 103 Tungste n trioxide and other transition metal oxide electrochromic materials, are deposited via thermal evaporation, sputtering, and chemical vapor deposition. 104 While as deposited tungsten trioxide films are highly transmissive, electrochemical reduction represented in the equation below, induces a color change from colorless to dark blue: (1 9) Viologens represent a class of small molecule organic electrochromes formed by the disubs tituted bipyridinium salts. The most common class of viologens is dimethy l 4,4 bipyridi nium (methyl viologen, MV) which is colorless in the dication form illustrated in Figure 1 1 1 Reduction of the dication forms the highly delocalized and stable radical cation which exhibits a blue color.
41 Figure 1 1 1 Common redox states of methyl viologen Conjugated polymers have recently emerged as one of the most promising electrochromic materials, offering color tunability, high transmittance in the bleached states, large optical contrast, fast switching, and solution processability. 105 107 Electrochromic materials are most useful if they are revers ibly switchable between a colored and highly transmissive state, as they can find application in absorptive/ transmissive window type electrochromic devices. Recently, the color palette of solution processable, vibrantly colored to highly transmissive elec trochromes was completed by discovery of the long sought after yellow to transmissive electrochromic polymer facilitating the pursuit of a full color electrochromic display 108 Electroche mical and optical studies for selected color to transmissive electrochromic polymers are discussed in C hapters 4 and 5 particularly ECP Black as the active material, which has potential in electrochromic smart windows and ele ctronic displays. Overview of Dissertation T his dissertation combines fundamental studies on the redox and optical properties of organic molecules and conjugated polymer s with deep application driven studies for energy storage and electrochromic devices. This body of work has been c hallenging, yet especially rewarding as it fo stered the design and synthesis of organic molecules, the characterization s of conjugated polymer s and the utilization of CPs for electrochemical devices The thesis is an accumulation of multiple projects and research collaborations, focused on a unique niche that integrates aspects of
42 conjugated polymers and electrochemistry. Experimental methods and characterization techniques employed to acquire data presented in this dissertation are summarized in Chapter 2 One of the emerging fields of conjugated polymer research is in the development of electrode materials in charge storage applications. The possibility of manufacturing low cost, lightweight, and flexible power storage devices for applications such as por table consumer electronics, military applications, and electronic vehicles has motivated the advancement of this field. Chapter 3 details the design and synthesis of electron rich dioxythiophene molecules for electrochemically polymerized electrodes in sym metric ( Type I ) electrochemical supercapacitors. A novel acceptor isoindigo is introduced as a donor acceptor donor electropolymerizable monomer for asymmetric ( Type III ) supercapacitors. Cyclic voltammetry and galvanic charging discharging cycles are utilized to gain insight into their capacitive behaviors. A novel lamination technique for device encapsulation is described, as well as the ability to tune capacitance and v oltage by the construction of device modules. Electronic devices specifically electrochromic windows that can switch from an absorptive colored state to a highly transmissive state, are gaining interest for applications in military camouflage and privacy window tint ing Chapter 4 focuses on the optical and colorimetric properties of a novel solution processable black to transmissive electrochromic polymer in the development of absorptive/transmissive window type devices with an emphasis on switching speed The correlation between current and voltage in t he context of polymer thickness and electrolyte composition is investigated.
43 In addition, a new spectroscopic characterization method to rapidly acquire full spectral transitions of sub second electrochrom ic films is described for the first time in Chapter 5. This analytical technique introduces a novel method to measure switching speeds of electrochromic materials: ECP Magenta, ECP Green, and ECP Black. Finally, experimental outlooks of an automated spray cast setup and an inkjet printer suitable for solution processable polymers are described. Conclusions, perspectives, and path forward for the presented work are described in Chapter 6.
44 CHAPTER 2 EXPERIMENTAL METHODS AND CHARACTERIZATION TEC HNIQUES Introduction In C hapter 1, the fundamental properties of conjugated polymers were described and related to energy storage and electrochromic applications In order to gain a deeper understanding of our materials for these applic ations, several electrochemical and spectroscopic techniques were used This c hapter presents an overview of the experimental techniques and methodologies used in the preparation of this dissertation. As previous dissertations from the Reynolds group have given extensive background overviews on electrochemical and optical spectroscopy techniques, only key experimental methods and characterization techniques are described. Materials General Chemicals Electrochemical grade electrolytes were purchased when available, and were purified as needed according to the literature 109 1 Ethyl 3 methyl 1 H imidazolium bis(trifluoromethylsulfonyl) imide (EMI BTI) was purchased from Covalent Associates Inc. Lithium bis (trifluoromethylsulfonyl)imide (Li BTI), tetrabutylammonium tetrafluoroborate (TBABF 4 99%) and tetrabutylammonium hexaflurophosphate (TBAPF 6 99%) were purchased from Sigma Aldrich and recrystallized prior to use. Lithium tetrafluorobor ate (LiBF 4 99%) was purchased from Acros. Poly(methyl methacrylate) (PMMA, M w = 996,000 g/mol) was purchased from Aldrich and used as received. Acetonitrile (ACN, HPLC grade), dichloromethane (DCM, HPLC grade), and toluene (certified ACS grade) were purchased from Fischer Scientific. Acetonitrile was
45 distilled over calcium hydride prior to use. Propylene carbonate (PC, anhydrous 99.7%) was purchas ed from Sigma Aldrich with a Sure Seal Benzonitrile (BZN, 99% extra pure) was purchased from Acros. Acetonitrile, propylene carbonate, and benzonitrile were degassed via four freeze pump thaw cycles prior to use inside the glove box. Gel electrolytes us ed in electrochemical devices were prepared by dissolving 1.0 M electrolyte of choice with 13% m/v PMMA in PC. Electrochemical Supercapacitor (ESC) Materials 2,2 Dimethyl 3,4 propylenedioxythiophene (ProDOT Me 2 ) was synthesized via the transetherificatio n between 3,4 dimethoxythiophene and 2,2 dimethyl 1,3 propanediol according to the methodologies previously reported. 110 Bis(3,4 ethylenedioxythien 2 yl) dibutylisoindigo (Bis ED OT isoindigo butyl 2 ) was prepared by Dr. Leandro Estrada via dibromo dibutylisoindigo and 2 trimethylstannyl 3,4 ethylenedioxythiophene (EDOT SnMe 3 ) Gold coated Kapton substrates (Gold 1000 ; 1 mil Kapton/ 3M 966 adhe sive) were purchased from Astral Technology Unlimited. Contacts for the electrodes were made from conductive adhesive copper tape (1131, 3M). Separator materials consist of coffee filter M thickness). Thermal encapsulation of electrochemical devices (Wilson Jones laminator, LP35HS) was carried out with heat seal laminating pouch (GBC, 3 mil). Electrochromic Device (ECD) Materials Indium tin oxide (ITO) coated glass substrates in cuvette size (CG 51IN CUV, 7 x 50 x 0.7 mm, R s = 5 15 ) and device size (CG 40IN S115, 25 x 75 x 1.1 mm, R s = 4 8 ) were purchased from Delta Technologies, Ltd. Contacts for the electrodes were
46 made from conductive adhesive copper tape (3M). The spacer mate rials applied between the ECD electrodes were double Two different epoxies were used to encapsulate our ECDs: hardman epoxy and Loctite E OONS Hysol epoxy adhesive. Electrochemistry General Setup Electrochemistry was carried out using a three electrode cell with a platinum (Pt) wire or a Pt flag as the counter electrode, a Ag/Ag + ref erence electrode prepared with 10 mM silver nitrate (Bioanalytical Systems, Inc.) solution in 0.1M TBABF 4 /ACN supporting electrolyte, and a platinum (or gold) button (0.02 cm 2 ) or ITO coated glass slides described previously as the working electrode. Electrochemical polymerizations were carried out in solutions composed of either 10 mM or 2 mM monomer in 0.1 M electrolyte solution unless otherwise noted. All supporting electrolyte solutions were purged with argon prior to electrochem ical and optical measurements, and a constant flow of argon was suppli ed over the electrochemical cell to maintain an inert atmosphere. An EG&G Princeton Applied Research Model 273 potentiostat was used to generate all electrochemical processes under the control of the Coreware software from Scribner and Associates. Investig ation of Contact Resistance One scenario that may impede our redox switching is the presence of high contact resistance between the copper tape and electrodes (in this case, ITO substrates ) In order to confirm that the contact between the copper tape and the ITO/glass substrate does not have any appreciable resistance that would otherwise hinder our electrochemical properties, a model study was conducted. Relative contact resistance
47 between the copper tape adhesive and copper wire/silver paste was measured according the diagram shown in Figure 2 1 A via the working electrode. The counter and reference electrodes were then shorted and c oupled to the ITO/glass substrate. Figure 2 1 Schematic of electrochemical model used to investigate the relat ive contact resistance between a) silver paste and b) copper tape. The measured current with an applied voltage of 1.0V was relatively cons tant across the copper tape adhesive and the copper wire/silver paste. As such, the relative resistance across the two leads is also similar on the basis of significant difference in the measure d current and calculated resistance was ob served by changing the distance between the two contact leads. Based on the results shown in Table 2 1 we can conclude that the resistance of the adhesive from the copper tape and the silver paste is negligible, although h igh er resistance was obse rved along the ITO substrate by varying the distance between the two contacts on the ITO/glass substrate surface.
48 Table 2 1 Calculated resistance for silver paste and copper tape contacts as a function of distance. Distance between Contacts on ITO (cm) Applied Voltage (V) Measured Current (mA) Cu Tape Ag Paste Cu Tape Ag Paste 2.0 1.0 25 27 40 37 1.0 34 40 29 25 0.0 75 84 13 12 Cell Setup and Film Handling Electrochemical polymerizations were accomplished by repeated potentiodynamic scanning of the working electrode between low potential where no redox reactions rinsed with monomer free supporting electrolyte solution before electrochemical characterizations were carried out. Two electrochemical cell designs were used in the preparation of ESC electrodes. As illustrated in Figure 2 2 a, the gold/Kapton is stabilized vertically between two Teflon substrates. Electrochemical polymerization in this cell design generates CP electrodes of reproducible surface areas (0.79 cm 2 ). In the second cell design displayed in Figure 2 2 b, the gold/Kapton substrate is placed horizontally between two glass cells. Here, gold/Ka pton is adhered to both sides of the substrate to construct a bipolar electrode. The use of two o rings and a clamp (not shown in the diagram) to stabilize the components together ensures that both the top and bottom halves could be used as an electrochemi cal cell. After electrochemical polymerization on one half, the procedure is carried out on the reverse side to yield a bipolar CP electrode. The surface area of the electrochemical cell is 2.82 cm 2
49 Figure 2 2 Electrochemical cell designs for the prep aration of ESC electrodes. a) Gold/Kapton electrodes are placed vertically between two Teflon substrates, with shadow mask area = 0.79 cm 2 b) Gold/Kapton bipolar electrode is placed horizontally between two glass electrochemical cells with areal surface a rea of 2.83 cm 2 Device Fabrication Electrochemical Supercapacitors A schematic diagram for electrochemical supercapacitors utilizing CP e lectrodes is shown in Figure 2 3 a. The assembly method for these devices employ s a sandwich type configuration of two conductive electrodes coated with electroactive polymers which face each other and are separated by a porous membrane containing ionically conductive gel electrolyte. The use of another CP as the counter electrode ensures charge balance of the redox reac tions which take place on the cathode and the anode during charging and discharging.
50 Figure 2 3 Encapsulation of Supercapacitors. a) General electrochemical supercapaci tor assembly and encapsulation b) thermal encapsulat ion method with laminator, and c) photograph of typical ESC Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. Prior to ESC encapsulation, CP films are neutralized and oxidized potentiostaticall y to prepare the anode and cathode respectively. In the case of PProDOT Me 2 the films were held at 0.6V and +0.9 V vs. Ag/Ag + for 30 seconds. In the case of PEDOT iI butyl 2 films were neutralized and oxidized potentiostatically at 0.0 and +0.7 V vs. Ag/Ag + for Type I ECSs and at 0.35 V for electrodes in Type III ESCs. The pre packaged supercapacitor assembly is placed between thermally sealable laminate sheets and passed through a laminator as illustrated in Figure 2 3 b. Auxiliary laminate materials are trimmed around the edges to yield the final device as shown in the photograph (Figure 2 3 c). Supercapacitor Module The super capacitor modules described in C hapter 3 are assembled by two different methods, via external or internal connections. In the e xternal configuration, individual ESCs are connected using alligator cl ips, from the cathode of one device to the anode of another device, and so on for modules in series. Conversely, ESCs in parallel are coupled cathode to cathode, and anode to anode resp ectively Figure 2 4
51 depicts the assembly of six total ESCs, with two individual devices coupled in series with three sets connected in parallel. Figure 2 4 Supercapacitor Module. a) Schematic of ESC module with two devices in series and three sets in parallel and b) Photograph of externally connected ESC module. A schematic diagram for an internal ESC module is shown in Figure 2 5 a. The assembly of these modules employs a sandwich type configuration in which a CP coated bipolar elect rode is positioned internally between two monopolar electrodes to ensure charge balance of the redox species. The electrodes are coupled in series to the exterior electrodes to complete a charged ESC module. T hermal encapsulation method of the thin flexibl e layers nicely compliments the fabrication of these modules as a single stack as shown in Figure 2 5 b.
52 Figure 2 5 Supercapacitor Module with bipolar electrodes. a) Schematic of internal ESC module employing bipolar electrodes. b) Photograph of an in ternal ESC module. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. Absorptive/Transmissive Window type Electrochromic Device s Two transparent conductive electrode substrates were spray coated with electrochromic polymer s of controlled absorbance. The working electrode ( with cathodically coloring ECP ) wa s oxidatively doped while counter electrode with MCCP was neutralized prior to device assembly to ensure the balan ce o f charge. Highly transparent gel electrolyte for electrochromic devices was composed of 1.0 M LiBTI, 13 wt % PMMA, PC Devices were encapsulated using commercially fast (3 5 minutes) curable epoxy. Blank devices consisting of two facing ITO/glass slid es with the gel electrolyte between them with no electroactive polymer layers were prepared for all background optical measurements.
53 Optical Spectroscopy Optical experiments were carried out on a Varian Cary 500 UV vis NIR spectrophotometer. Spectroelectrochemical measurements were taken to study the electrochromic behaviors of CPs. The polymer films were prepared by spray casting from 2 mg/mL DCM or toluen e solutions. An airbrush (Iwata eclipse HP BS) was utilized at 25 30 psi argon pressure. All polymer solutions were filtered through 0.45 m Whatman Teflon (PTFE) syringe filters prior to spraying. The thickness of the electrochromic films was controlled b y monitoring the absorbance at max during spraying. The electrochromic behavior was observed by recording the spectral changes upon oxidation of the polymer film. T he optical band gap was determined from the low energy absorption edge (onset of the t ransition) of the neutral absorption spectrum of the polymer film. Synchronization of Electrochemical and Optical Data The Ocean Optics spectrophotometer comes with a deuterium light source that radiates in the 215 400 nm and a tungsten halogen light so urce that radiates between 400 2000 nm ranges. Both light sources require at least thirty minutes to warm up. Spectrometer components consisting of the fiber optics, sample holders, and detectors (190 900 nm) were placed inside a home built enclosure t o eliminate extraneous light. The overall in situ electrochemical optical spectroscopy instrument is illustrated in Figure 2 6 T he trigger circuit, described in the next section, is connected to the working electrode of the cell for input via BNC (Bayonet N eill Concelman) cable, to the reference electrode via an identical BNC cable, and also attached to the spectrophotometer via BNC/D Sub connector.
54 Figure 2 6 Schematic of in situ electrochemical spectroscopy instrumentation. PC1 controls the potentiostat to signal in a square wave to the triggering circuit that is also coupled to the electrochemical cell. The triggering circuit sends an output to the spectrometer detector to start collecting spectra data, which are displayed on PC2. Rep resentative potential square wave and transmittance results are shown for ECP Black (Abs = 0.7 a.u. at = 550 nm). To prepare for measurements, the reference cell is placed in the enclosure, and the reference and background intensity spectra (both by pho ton count) are collected. The time between spectra can be set between 2 6 ms by an adjustable knob located on the trigger circuit. A potential equal to the lower edge of the square wave is then held for 10 s to prime the triggering circuit and the samp le of interest prior to the experiment, which gave a potential for the trigger to contrast with the first rising edge of the square wave. The potentiostat and the spectrophotometer are activated in parallel, and spectra are collected upon a triggering even t The collected measurements are combined with the background and reference photon counts to calculate percent transmittance by E quation 2 1 where f is the film spectrum, b is the background spectrum and r is the reference spectrum.
55 (2 1) Trigger Circuit The design of the circuit board as illustrated in Figure 2 7 and subsequent construction the trigger system was completed by Steve Miles from the UF electronic s shop. Figure 2 7 Schematic of the external circuit used to electrically tr igger the spectrometer detector during electrochemical switchings of electrochromic polymers. The purpose of this trigger circuit is to instruct the spectrophotometer to collect spectral data as soon as a rising or falling edge of a potential square wave ( greater than 0.4V) is output from the potentiostat. Technically, this is accomplished by means of a series of monostable multivibrators, three of which generate the actual output signal
56 (designated A, B, and C) and two of which (designated D and E) retrigg er the output generators, as illustrated in Figure 2 6 The monostables control the overall time during which the spectrophotometer will be collecting data or total data acquisition time (TDAT). The parameters involved in the calculation of this TDAT are adjustable by means of several internal and external potentiometers. Using these potentiometers, the RC time constant associated with each monostable can be adjusted, and thus the TDAT can be varied from 2 ms to 10 s. Furthermore, the TDAT corresponds to the total number of spectra the spectrophotometer will collect after a triggering event. By calcula ting the upper limit, pre cise data acquisition in terms of both amount and synchronization is ensured. The number to be entered is tabulated w ith E quation 2 2. The number of periods over which data is to be collected (N), the frequency of the waveform (f 0 ), and the time in between spectral acquisitions (t s ), are all incorporated into the following formula: Number of Spectra (2 2) where t s is adjustable by a potentiometer on the triggering circuit, from 2 ms to 6 ms. Since the triggering circuit activates as soon as an input in the rising/falling edge format described above is given, it is possible to synchronize electrochemical and spectra l data acquisition right at the point where oxidation or neutralization (reduction) begins, and thus correlate both data types without the element of human error. Colorimetry In 2000, the Reynolds group introduced colorimetry as a method for quantitativel y measuring the color of electrochromic film s. 111 C olorimetric measurements are performed with a Minolta CS 100 Chroma Meter. The samples are illuminated from
57 behind with a Graphiclite D5 000 GLX Transparency Viewer (GTI Graphic Technology, Inc.) in a light booth designed to exclude ext ernal light for both colorimetric and photographic measurements. The Minolta CS 100 records color coordinates in the CIE 1931 Yxy format, in which Y stands f or photometric luminance in units of cd/m 2 x and y are chromaticity coordinates designed to represent the hue and saturation. The relative luminance of the sample, normalized to the standard, is determined by (Y / Y n x 100) in which Y is the luminance of the sample and Y n is the luminance of the reference background. The CIE 1931 Yxy color coordinates are converted into the CIE 1976 L*a*b* format as an improved representation of the color space established by the Commission Internationale de l'Eclairage 112 Illustrated in Figure 2 8 the L*a*b* color coordinates represent lightness on a scale of 0 to 100 (L*) and different levels of hue and saturation (a* and b*). Figure 2 8 CIE 1976 L*a*b* representation of lightness (L*) color coordinates (a* b*) 113 Photography The use of photography to represent the color of electrochromic films has recently become an important as pect of our research. Photographs are vital supplementary data used to compliment optical spectroscopy and colorimetry results. Because photographs
58 can provide an immediate visual representation of our electrochromic films or devices, it is important to be as accurate as possible. A Nikon D90 DSLR, which is a digital single lens reflex camera, can save photographs in JPEG and RAW formats. While JPEG files, which can be resized and cropped, are used in this dissertation, the RAW files remained unedited as ou r actual data. The following manual camera settings were used: ISO Sensitivity: L 0.3 White Balance: 5000K, 0,0 Aperature: f/5.6 Shutter Speed: 1/80 s Since the Nikon D90 was recently implemented, photographs in Chapters 3 and 4 were taken with an automatic point and shoot camera (FinePix S7000, Fuji Photo Film). All photographs are presented as received without altering the color brightness or saturation parameters. Solution Conductivity Conductivity of electrolyte solutions was measured using Fish er Accumet four probe conductivity meter (cell constant = 1.0) calibrated in traceable solutions with exact conductivity values at room temperature.
59 CHAPTER 3 DEVELOPING CONJUGATE D POLYMERS FOR ELECTROCHEMICAL SUPERCAPACITORS Introduction Elec trochemical supercapacitors (ESC s ) utilizing conjugated polymers (CPs) are envisioned to bridge the gap between capacitors and batteries with intermed iate power and energy densities Conjugated polymers are pseudocapacitive materials, where the bulk of the active material undergoes fast redox reaction s to provide capacitive responses. Pseudocapacitance delivers charge storage capabilities predominantly via faradic processes in the bulk with a small percentage coming from non f aradic double layer capacitance at the polymer and electrolyte interface 51,114 Conjugated polymer ESC s can be designed to ensure that the entire volume of the material is in volved in the charge storage process. T he energy stored in CP based supercapacitors can be superior to carbon based supercapacitors since charge st orage occurs in the bulk volume, whereas carbon based material s only store charge at the surface through doub le layer ion adsorption/desorption. However, carbon based materials have fast kinetics since only the surface of the material is being accessed. Additionally there is great potential in utilizing CPs for charge storage devices as these redox materials exh ibit reduced cost, weight, and environmental impact relative to other redox materials such as inorganic metal oxides. 51,94 While highly regarded a s the first conducting polymer discovered by Heege r et al poly ( acetylene ) also functioned as the prototype material in polymeric charge storage applications. 115 117 Poly ( acetylene ) electrodes have the ability to be used as both the cathode and the anode with p doping and n doping capabilities; however, the advancement of poly ( acetylene ) based batteries was hindered because of inherent
60 instability and processing difficulties. Although poly ( acetylene ) set the platform for CP based batteries, the ability to utilize the capacitive currents of conjugated polymers was later suggested by Feldbe rg in his electrochemical analyses o f poly ( pyrrole ) and poly ( thiophene ) 89 Initial reports of conjugated polymers in redox active ESCs 118 120 have since stimulated further research efforts in developing CPs for charge storing applications. I n particular these include poly ( aniline ) poly ( pyrrol e ) poly ( thiophene ) 121 126 as shown in Figure 3 1, alon g with other well studied p dopable and n dopable CPs for energy storage. Figure 3 1 p dopable and n dopable CPs utilized in electrochemical supercapacitors. P dopable materials include: (3 1) trans polyacetylene (PA), (3 2) poly(p phenylene) (PPP), (3 3) polyaniline (PANi), (3 4) poly( N methyl aniline), (3 5) polypyrrole (PPy), (3 6) polythiophene (PTh), (3 7) 3 substituted polythiophene, (3 8) poly(3,4 ethylenedioxythiophene) (PEDOT). n dopable materials include: (3 1) trans polyacetylene, (3 2) poly(p phenylene),(3 9) poly( 3 4 fluorophenyl)thiophene), (3 10 ) poly(1 cyano 2(2 [3,4 ethylenedioxythiophenyl] 1 (2 thienyl)vinylene (PThCNVEDT), (3 11) poly(cyclopental[2,1 b;3,4 4 one]) (PcDT), (3 12) poly(5 amino 1,4 naphthoquinone)
61 Poly ( aniline ) has demonstrated charge storing capabilities in both non aqueous and aqueous electrolytes with specific capacitances over 200 F/g. 127 The cycle life of polyaniline is limited for Li doped PANi electrode s where the specific capacitance dropped from 100 F/g to 70 F/g over 5000 cycles 128 The stability of P ANi can be improved by modifyi ng the parent compound to poly( N methyl aniline). By blocking the site proton exchange the polymer is stabilized against chemical degradation. 129 Poly ( pyrrole ) has also shown promise in both battery and supercapacitor research with a specific capacitance of 40 85 F/g 119,130,131 Similar to PA N i, PPy switches well in both aqueous and n on aqueous solutions. Poly ( thiophene ) albeit with reports of high capacitance values of 250 F/g, has shown limited progress in energy storage because of its electrochemical instability. 132 However, electron rich derivatives of polythiophene, such as poly(3,4 ethylenedioxythiophene) (PEDOT), have been extensively studied as a supercapacitor electrode because of their excellent electrochemical stability, fast redox switching speeds and high specific capacitances. 133 135 Subsequently, derivatives of poly(3,4 alkylenedioxythiophene)s that are structurally similar to PEDOT have been developed for electrochemical devices. Chemically synthesized poly(3,4 propylenedioxythiophene) (PProDOT) and similar derivatives have be en well characterized as electrochromic polymers due to their high optical contrast, rapid switching times, electrochemical lifetimes, and high coloration efficiency. 34,136,137 Although PProDOTs exhibit excellent electrochromic properties, their capacitive behavior has not received as much attention. Previously, Stenger Smith prepared Type II supercapacitor s based on two different p dopable electron rich conjugated polymers, PEDOT and PProDOT, in which capacitive properties were
62 investigated as a function of supporting electrolytes 138 Their results showed improved electrochemical stability in ionic liquid electrolytes, specifically, 1 ethyl 3 methyl 1H imidazolium bis(trifluoromethanesulfonyl)imide (EMI BTI). Most recently, Irvin investigated PProDOT based supercapacitors utilizing ionic l iquid electrolyte blen ds at low temperatures from 30 C up to 60C, with 10% capacity loss after 10,000 cycles at 70% depth of disch arge. 139 In this c hapter, the electrochemical properties PProDOT Me 2 films were investigated and incorporated into ESCs after excellent film properties were observed. In addition to electron rich CPs, donor acceptor donor (D A D) systems have the potential to increase the voltage range of ESCs since both oxidation and reduction processes are utilized To date, only a few selected acceptors have been studied as ESC electrodes including poly 3 (phenylthiophene) derivatives, 119,120,140 poly(diheteroaryl cyanovinylene), 141,142 poly(cyclopentadithioph eneone), 123 and poly(amino naphthoquinone). 143 R ecently Mei introduced a novel acceptor isoindigo in D A D oligome rs for molecular bulk heterojunction solar cells where power conversion efficiencies of ca. 1.8% were a chieved. 81 Due to the ease of synthesis and derivatization, isoindigo was further inc orporated into D A D conjugated polymers with varying electron donating strengths w ith LUMO on the range of 3.8 to 3.9 eV and band gaps between 1.6 and 2.0 eV 144 Inspired by the photovoltaic results, we naturally extended isoindigo into our supercapacitor work. In order to probe isoindigo as an acceptor unit for electrochemical supercapacitors, a donor acceptor donor unit that is prone to electrochemical
63 polymerization was designe d. 3,4 ethylenedioxythiophene (EDOT) is an excellent candidate as the electron rich unit owing to the low oxidation potential, lack of undesirable electropolymerization sites, and polymer stability. 145,146 As such, bis(3,4 ethylenedioxythien 2 yl) isoindigo (BEDOT iI butyl 2 ) was synthesized by Dr. Leandro Estrada and investigated as electrodes for electrochemical supercapacitors Due to the ability of electron rich polymers to oxidize and D A D polymer to both oxidize and reduce they may find use in a variety of ESC configurations termed Type I, II, III and IV by Rudge. 119 Type I a nd Type III ESCs consist of symmetric electrode materials where o nly p dopable CPs are used in Type I and n dopable CPs are used in Type III configurations Type II ESCs use two different p dopable CPs, and Type IV consists of two different n dopable mater ials. The operating principle of Type I and Type III supercapacitors is illustrated in Figure 3 2. In the ch arged state for Type I ESCs one of the CP electrodes is fully oxidized (denoted by PProDOT + ) while the other electrode is fully neutral (PProDOT 0 ). In the discharged state both the cathode (the fully oxidized CP electrode) and the anode (t he neutral CP electrode) become 50% oxidized. As such, at most 50% of the p doping charging capacity is available for Type I supercapacitors. The utilization of p doping and n doping redox processes can improve the voltage window in supercapacitors as depicted in Figure 3 2b In the charged state for Type III ESCs the CP on the cathode is fully oxidized and the CP on the anode is fully reduced. In the discharged doping capacity are accessible.
64 Figure 3 2 Operating principle of a) Type I and b) Type III electrochemical supercapacitor Since the voltage of an electrochemical device is defined as the potential difference across the cathode and the anode, Type I ESCs attain a voltage window between 0.5 to 1.2 V. Type III ESCs are more attractive as higher voltage windows are attainable, generally between 1.5 t o 3.0 V. However, there is a trade off between electrochemical stability and lower voltages with Type III devices. The energy of supercapacitors charged to a voltage difference of V can be calculated with the following equation: (3 1) (3 2) where E is energy in joules, C is capacitance in F/g, V is the operating voltage difference in V, ED is energy density in Whr/kg, and m is mass in kg. Energy density is evaluated by accounting for the mass of either the active materials (po lymer and electrolyte) or the entire ESC device. Power density is the
65 average amount of energy delivered per unit time, which is essentially the discharge time of ESCs. Power can be determined from the following equations: (3 3) (3 4) where P is power in W, i is the average current in A, V is the operation voltage difference in V, m is the mass in kg, J is the average current density in A/cm 2 and A is the area in cm 2 Both the average current and voltage are determined from the cyclic voltammogram. Because the cyclic v oltammograms presented in this c hapter are highly rectangular, the current was selected at the average voltage range Additionally, cyclic voltammograms were used to evaluate the capacitance of our films and dev ices using the following equations: (3 5) (3 6) w here C a is the aerial capacitance in mF/cm 2 Q is charge in C, V is voltage in V, i is current density in mA/cm 2 and is the scan rate in V/s. Specific capacitance, C s is dete rmine by measuring the polymer mass o f both electrodes. Through this c hapter, cyclic voltammograms are often graphed with current density (mA/cm 2 ) and capacitance (mF/cm 2 ) as a function of voltage.
66 Based on the power and energy calculations shown above, t he operating voltage window range is a n important metric to improve. One strategy to increase the voltage is realized by connecting multiple supercapacitors in series. The coupling of individual supercapacitors in series has a cumulative effect in the over all voltage; that is, the total voltage is the summation of the voltage across each individual device. Alternatively, increased current density is achievable by way of coupling supercapacitors in parallel since the total current in a parallel orientation i s the summation of the current from each supercapacitor. S chematic s of three individual devices, connected in series (a), and in parallel (b) are illustrated i n Figure 3 3 with representative calculations. Figure 3 3 Supe rcapacitor modular design and representative calculations. This c hapter describes the electrochemical polymerization and capacitive properties for a set of p dopable and n dopable CPs as show n in Figure 3 4. The fi rst part describes the electrochemical polymerization and capacitive p roperties of PProDOT Me 2 as p doping CP electrodes for Type I electrochemical supercapacitors.
67 Figure 3 4 p dopable and n dopable CPs utilized in supercapacitors. Exhibiting excellent capacitive behavior, rapid redox sw itching, and stable electrochemical responses, films of PProDOT Me 2 are attractive charge storing materials for ECP based supercapacitors. As such, Type I electrochemical supercapacitors consisting of PProDOT Me 2 as the active layer displayed robust capac itive currents within a 1.0 V window, rapid redox switching up to 500 mV/s, and a stable electrochemical switching lifetime of over 32,000 cycles at 100 mV/s. These results are comparable to the Type II supercapacitors based on the parent PEDOT and PProDOT mentioned above 138 and position us for further development of these ESCs. A novel electroactive polymer poly(Bis EDOT Isoindigo) was also applied to electrochemical suercapacitors. Due to the ability of the D A polymers to be fully
68 oxidized (p doped) and reduced (n doped), they may find applicability in Type III and IV ESCs where n dopable materials are necessary. Poly(Bis EDOT Iso indigo) was first used as an electroactive material for Type I supercapacitors, and also probed for Type III capabilities. PBEDOT iI butyl 2 performed well in Type I supercapacitor s with 0.49 Wh/kg energy, while ret aining about 90% of electroactivity over 10,000 cycles. As high as 1.8 V was achieved in Type III supercapacitors, although the undesirable electrochemical instability limits ultimate use. Additionally we describe the extension of voltage and capacitive currents by coupling multiple supercapacit ors in tandem. To date, there has been little reported effort in d eveloping electroactive polymer based supercapacitor modules and, to the best of our knowledge, no previous report of compact laminated devices. 147 In this c hapter, a stable 4.0 V tandem device was assembled with four Type I supercapacitors in series. Additionally, a four fold increase in capacitive currents was obtained with parallel constructs. The potential of a tandem sup ercapacitor is realized by combining b oth serial and parallel assemblies to achieve high cell voltage and capacity simultaneously. These studies demonstrate the ability to couple supercapacitors, which with the use of bipolar electrodes, are ultimately en capsulated as a single supercapacitor module. Electrochemical Polymerization and Characterization of CP Electrodes Poly(ProDOT Me 2 ) The e lectrochemical polymerization of PProDOT Me 2 by repeated potentiodynamic scans on gold/Kapton substrates is shown in Figure 3 5a. T he gold/ Kapton electrodes provide high conductivity and excellent mechanical flexibility suitable for electrochemical supercapacitors.
69 Figure 3 5 Electrochemistry of PProDOT Me 2 a) Electrodeposition of PProDOT Me 2 on Au/Kapton substrates via 5 repetitive potentiodynanmic cycles from 1.0 to +1.31 V vs. Ag/Ag + at 50 mV / s using 10 mM ProDOT Me 2 in 0.1 M Li BTI/ACN supporting electrolyte b) PProDOT Me 2 redox switching in 0.1 M LiBTI/ACN supporting electrolyte at 50 mV/ s from 0.7 V to 0.8 V vs Ag/Ag + for 20 cycles. c) Electrochemical redox switching of PProDOT Me 2 as a function of increasing sc an rates from 10 to 1000 mV/s. d) Anodic and cathodic peak current as a function of scan rate showing linear dependence. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. The initial anodic sweep display ed a monomer oxidation peak potential (E p,m ) of 1.21 V vs. Ag/Ag + Subsequent scans exhib i t increases in current density between 0.5V to 1.0V vs. Ag/Ag + which is indicative of CP electrodepostion on the working electrode. The resulting polymer was then electrochemically cycled in monomer free supporting electrolyte, demonstrating a highly re producible redox (doping/dedoping) process centered at 0.31 V (E 1/2 ) vs. Ag/Ag + which is calculated from the average of the pea k anodic potential ( 0.23V vs. Ag/Ag + ) and the peak cathodic potential ( 0.39V vs. Ag/Ag + ) as illustrated in Figure 3.5b. Redox switching of PProDOT Me 2 films demonstrate d broad capacitive current responses that are ideal for supercapacitors. Since it is
70 necessary to have the same amount of polymer on each electrode to maintain charge balance upon charging and discharging in super capacitor devices, the amount of electrodeposited polymer on the electrode was controlled by the number of potentiodynamic cycles during electropolymerization. The polymer electrochemistry was monitored as a function of increasing scan rates (10 to 1000 mV/s) as show n in Figure 3 5c. T he peak currents were also probed as a function of scan rate, as illustrated in Figure 3 5d. The li near dependence indicate d that the redox process is electrode confined and well behaved to relatively rapid switching rates. Poly(BEDOT Isoindigo butyl 2 ) BE DOT iI butyl 2 was electrochemically polymerized via potentiodynamic cycling, as illustrated in Figure 3 6a. BE DOT iI butyl 2 monomer presented a n anodic peak at 0.68 V vs. Ag/Ag + relatively close to that reported for B i EDOT against the same reference (E p,a = 0.56V). 148 Electropolymerization by potentiodynamic cycles is an effective method as evidenced by the evolution of current upon sweeps between 0.2 to 0. 6V vs. Ag/Ag + after 10 repeated scans. Subsequent redox switching between 0.0 and 0.6 V in 0.1M LiBTI/ACN illustrates that PEDOT iI butyl 2 films maintain a reproducible current response after the first ten cy cles. In another set of experiments, s imilar cur rent responses were observed when redox switched in 0.1 M LiBTI / PC supporting electrolyte; however red ox switching in benzonitrile r esult ed in over 50% loss of electroactivity after 10 cycles. Nevertheless, the capacitive nature of PEDOT iI butyl 2 films in spite of a small voltage window (< 1.0), enabled well behaved and reversible CV responses in propylene carbonate solutions that are comparable in shape to those of PEDOT 148 and PProDO T. 149
71 Figure 3 6 Electrochemistry of PBEDOT iI butyl 2 a) Electrodeposition of PBEDOT iI butyl 2 on Au/button electrode via 10 repetitive potentiodynanmic cycles from 0.4 to +0.95 V vs. Ag/Ag + at 50 mV / s using 2 mM PBEDOT iI butyl 2 in 0.1 M Li BTI/BZN supporting electrolyte b) PBEDOT iI butyl 2 redox switching in 0.1 M LiBTI/ACN supporting electrolyte at 50 mV / s from 0.1V to 0.6 V vs Ag/Ag + for 20 cycles c) Electrochemical redox switching of PBEDOT iI butyl 2 as a function of increasing scan rates, from 10 to 3000 mV/ s d) Anodic and cathodic peak current as a function of scan rate showing linear depende nce. The films were cycled using scan rate ranging from 10 3000 mV/s, as illustrated in Figure 3 6c, and demonstrated that the film retains capacitive behavior even at large scan rates The rise of an anodic peak centered at 0.52 V and a corresponding cath odic peak at 0.49 V vs. Ag/Ag + along with slight ly resistive behavior with increasing scan rates, suggests the redox behavior is not ideal yet the capacitive behavior is still pronounced at fast scan rates. The linear dependence of peak currents as a function of scan rates shown in Figure 3 6 d indicates that th e polymer is well behaved up to relatively rapid switching rates and is well adhered to the electrode.
72 Because the bulk of the CP participates in the redox process, the capacitance response can b e improved by increasing the amount of electrodeposited material. To do so, two approaches were considered: (1) increasing the monomer concentration, and (2) increasing the number of potentiodynamic scans during electropolymerization. The influence of inc r eased monomer concentration is illustrated in Figure 3 7, in which BEDOT iI butyl 2 was electropolymerized from ( a ) 1.0 mM BEDOT iI butyl 2 and (b ) 2.0 mM BEDOT iI butyl 2 in 0.5 M LiBTI/BZN supporting electroly te solutions. Figure 3 7 Electrochemical po lymerization of PEDOT iI butyl 2 from a) 1.0 mM BEDOT iI butyl 2 and b) 2.0 mM BEDOT iI butyl 2 in 0.5 M LiBTI/BZN supporting electrolyte. Redox switching of subsequent PEDOT iI butyl 2 films in 1.0M LiBTI/PC for c) film deposited fr om 1.0 mM monomer solution and d) 2.0 mM monomer solution. ( courtesy of Dr. Leandro Estrada) After 10 repeated cycles in identical electrochemical conditions (excluding the monomer concentration) it is evident from the difference in current response s as highlighted by the dotted lines that increasing the concentration increases the current
73 density and leads to more material being electrodeposited. Subsequent examination of the polymer current responses shown in Figure 3 7 d indicated an improvement of 33%. This set of electrochemi cal conditions demonstrates a simple and effective route to improve the film capacitance, and the effects of monomer concentration as a function to electrolyte concentration and solvent should be considered for future study. Another approach to increasing film capacitance was tested by way of varying the number of potentiodynamic scans. Solutions consisting of 2 mM BEDOT iI butyl 2 in 0.5 M LiBTI/BZN supporting electrolyte were cycled between 0.45V to +0.9V vs. Ag/Ag + for 15, 20, 50, and 75 cycles as shown by the CVs in Figure 3 8a d Figure 3 8 Electrochemical polymerization of PBEDOT iI butyl 2 from solutions of 2 mM monomer in 0.5 M LiBTI/BZN for a) 15 cycles, b) 20 cycles, c) 50 cycles) and (d) 75 cycles. (e) Redox switching of formed films in 1.0 M LiBTI/PC. ( courtesy of Dr. Leandro Estrada) Continuous cycling yielded more polymer deposition on the electrode, as revealed by the increasing anodic and cathodic peak current densities of the polymer oxidation/reduction shown in Figure 3 8e. In the ideal case, a highly capacitive film would result in a rectangular shaped CV; however resistance is unavoidable in real systems, especially as the polymer films become thicker.
74 Oxidative and reductive electrochemistry of PEDOT iI butyl 2 is elucidated by cyclic voltammetry (CV) and differential pulse voltamm etry (DPV) i n Figure 3 9. T he polymer exhibits an oxidation peak near 0.4 V vs. Ag/Ag + and a reduction peak around 1.3 V vs. Ag/Ag + The d ifference between the oxidation and reduction pea ks can be used to predict the attainable voltage window in Type III ESCs. In this case, we expect an operative voltage window of 1.5 to 2.0 V since the potential difference between the oxidation and reduction peaks is ca. 1.7 V. Figure 3 9 p doping an d n doping responses of PBEDOT iI butyl 2 carried out by a) cyclic voltammetry and b) differential pulse voltammetry. The oxidative and reductive switching of PEDOT iI butyl 2 can also provide insight into the device cycle life. By probing the p doping and n doping current responses independently over 40 CV cycles as shown in Figure 3 10, it can be seen that the electrochemical stability of the se processes differ. After initial oxidative break in cycles, the films retained over 90% of their ele ctroactivity. Unfortunately reductive switching was met with electrochemical instability, as a majority of the electroactivity was lost after 40 cycles. Proper choice of electrolyte and solvent may improve reductive electrochemistry as demonstrated by St alde r in the reductive electrochemistry of n type conjugated polyisoindigo. 150
75 Figure 3 10 Oxidative and reductive switching of PBEDOT iI butyl 2 over 40 cycles between 0.4V to 0.9V vs Ag/Ag + for p doping and 0.4V to 1.3V vs. Ag/Ag + for n doping in a glove box Electrochemical Supercapacitors PProDOT Me 2 based Type I ESC s The charging/discharging CV of a Type I PProDOT Me 2 based supercapacitor as a two electrode cell is highlighted in Figure 3 1 1 a. The supercapacitor demonstrates a near ly ideal and highly capacitive response as gleaned from the rectangular shaped CV. As seen from theory in the case of a pure capacitive pro cess, a rapid increase and plateau of current density upon charging followed by its mirror image upon discharging are distinctive signs of capacitive behavior in these electrochemical capacitor processes. The device maintains stable current (0.3 mA/cm 2 ) th roughout the 1.0 V operating window. The charging/discharging cyclic voltammogram monitored at 50 mV/s shows minimal current density loss after 50 redox cycles, as the device was broken in. It is important to note that the sharp increase and decrease in cu rrent density at 0.0 V and 1.0 V respectively demonstrate the ability of the supercapcitor to charge and discharge rapidly.
76 Figure 3 1 1 Type I PProDOT Me 2 Supercapacitor. a) Two electrode device with symmetric PProDOT Me 2 electrodes. Type I ESC chargin g and discharging at 50 mV/s between 0.0 and 1.0 V. b) Device current density and capacitance for a separate device at 0.5 V, with 85% capacity retained over 32,000 charge/discharge cycles at 100 mV/s. The inset displays selected CV to represent 32,000 cyc les c) Device charging and discharging at as a function of scan rate from 5 to 500 mV/s. d) Current and capacitance as a function of scan rate for symmetric PProDOT Me 2 Type I supercapacitor. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. The lifetime of the supercapacitor was monitored by means of repeated cyclic voltammetric charging and discharging cycles (Figure 3 1 1 b). After m ore than 32,000 redox cycles at 100 mV/s cycling rate, the supercapacitor retained 85% of its charge storing capacity This demonstrate s its ability to perform with a high degree of stability. The inset in Figure 3 1 1 b shows selected CV s to represent the 32,000 cycles. Despite a steady decline in current density over tens of thousands of cycles, the overall capacitive
77 behavior of PProDOT Me 2 supercapacitor is remarkable thr oughout the switching duration. The ability of PProDOT Me 2 based ESCs to charge and discharge at high scan rates is illustrated in Figure 3 1 1 c. Scan rate dependence studies between 5 to 500 mV/s over a 1.0 V window demonstrate the ability of the supercapacitor to charge and discharge effe ctively at low and h igh scan rates. Even at fast scan rates, the device is capable of maintaining the near ideal rectangular CV shape as excellent capacitive behavior is observed throughout the voltage window at all scan rates with rapid charging and discharging responses. I n Figure 3 1 1 d, a linear relationship is seen between the average current density (monitored at 0.5 V) and the scan rate. This trend indicates that the polymeric materials in the device are not only highly electroactive, but the electrolyte in the device e ffectively compensates both the neutral and oxidized polymer electrodes, especially throughout the entire matrix of the film. More importantly, the supercapacitor areal capacitance is nearly consistent throughout the scan rate regime. This observation is c onsistent with ideal electrochemical supercapacitors as the capacitance should be independent of charging/discharging rates. Poly(BEDOT Isoindigo) based Type I ESC s The charge/discharge CV for a PBEDOT iI butyl 2 based Type I supercapacitor is shown in Figure 3 1 2 a. The CV response demonstrates a nearly ideal behavior operating at a rate of 50 mV/s for 10 cycles. The sharp increase/decrease in current density at the edges of the voltage window highlights the inherent ability of the device to rapidly char ge/discharge. The operative voltage window is from 0 to 0.5 V, which is within range of that previously reported for PEDOT/PEDOT (0.8 V, Type I) 121 and PEDOT/PProDOT (0.5 V, Type II) 138 ESCs.
78 Figure 3 1 2 Type I PBEDOT iI butyl 2 Supercapacitor. a) CV switching of Type I ESC based on PBEDOT iI butyl 2 at 50 mV/s between 0.0 and 0.5 V. b) Long term CV switching at 200 mV/ s between 0.0 to 0.5V for 30,000 cycles. c) ESC charging and discharging as a function of scan rate from 10 to 500 mV/s. d) Current and capacitance as a function of scan rate for symmetric PBEDOT iI butyl 2 Type I ESC monitored at V = 0.25V. The lifetime of the supercapacitor is monitored using repeated charge/discharge CV as illustrated in Figure 3 1 3 b. After 30,000 cycles at 200 mV/s, the device lost 40% of its electroactivity Interestingly, there is an asymmetric redox process which takes place upon repeated cycles. The discharge process is more susceptible to electrochemical instability as evident by the rapid current lost. Furthermore, the asymmetric current response was accentuated in the scan rate dependence experiments shown in Figure 3 1 2 c A possible explanation for this observation is the ohmic polarization that causes a deviation from the ideal rectangular capacitive behavior. The ohmic barrier was previously observed and explained for Type I ESC based on PA N i electrodes 151 and PEDOT electrodes 152 It is due to a drop in
79 conductivity at the cathode caused by its nearly undoped state, and is a limitation f or Type I supercapacitors because it limits the power that can be obtained from the device when releasing its energy. Nevertheless, by plotting the anodic peak current and the normalized capacitance (at V = 0.25 V) as a function of scan rate illustrated in Figure 3 1 2 d, it is clear that the normalized capacitance gradually decreases with increasing scan rates. Since these results reflect the current and capacitive response at V=0.25 V, the scan rate dependence may become more non ideal if the currents are m onitored at V = 0 or 0.5 V. P oly(BEDOT Isoindigo) based Type III ESC The voltage range for a PBEDOT iI butyl 2 based Type I supercapacitor was found to be optimized at 0.5 V as shown in Figure 3 1 3 ; although, it is possible to operate at a maximum voltage o f ca 0.8 V without losing much of the capacitive profile for a Type I ESC. As such, a desire to increase the voltage window has prompted the use of PEDOT iI butyl 2 electrodes in Type III ESCs. Figure 3 1 3 Operative voltage range of PBEDOT iI butyl 2 based Typ e I supercapacitor between 0.5 to 0.8V. As illustrated by the reductive electrochemistry of PEDOT iI butyl 2 in Figures 3 9 and 3 1 0 the nature of the n doping process is h ighly faradic with strong redox
80 responses centered vs. Ag/Ag + This results in an extended voltage window for potential Type III supercapacitors, but with possible electrochemical instability at extreme voltage ranges. While n doping redox switching of PEDOT iI butyl 2 films demonstrates electrochemical in stability, incorporation of the isoindigo acceptor into Type III supercapacitors is certainly worthwhile. The reasons for this are two fold: potential enhancement of the operating voltage window of the supercapacitors, and access to a fundamental understan ding of the structure property relationships of isoindigo as part of D A D electroactive polymers. Prior to Type III E SC assembly, PEDOT iI butyl 2 films were primed to their neutral V vs. Ag/Ag + V w as determined from the midpoint potential between the oxidation and reduction peaks. Once encapsulated, the supercapacitor is set to its discharged state. To ensure that the supercapacitors were indeed in their fully discharged state, the devices were pote ntiostatically held at 0.0 V for 2 minutes prior to device testing. The operating voltage range of a Type III ESC was determined by incrementally increasing the voltage from 0.0V to 3.25 V as illustrated in Figure 3 1 4 Figure 3 1 4 Voltage limits of Type III PEDOT iI butyl 2 based ESC cycling at 50 mVs.
81 The cyclic voltammogram indicates that the bulk of the faradaic and non faradaic charging capacity is between 1.0 to 2.75 V, corresponding well with earlier predictions based on p doping and n doping f ilm electrochemistry. This voltage window is comparable to other Type III E SCs reported in literature, such as poly(diheteroaryl cyanovinylene), 141 ,142 and poly(cyclopentadithiopheneone) 123 operating at a 2 V window, and poly(3 methylthiophene) 126 and poly(3 (phen ylthiophene)) derivatives at 2.5 3 V. 140 It is important to note both the capacitive and faradic current responses of Type III E SCs in Figure 3 1 5 a. Figure 3 1 5 Type III PBEDOT iI butyl 2 Supercapacitor. a) CV of PEDOT iI butyl 2 based Type III ESC switching from 0.5 to 3.0 V at 50 mV/s. b) Type III device switching with 2.5 V window at 200 mV/s for 400 cycles c) Device charging and discharging at as a function of scan rate from 10 to 1000 mV/s. d) Peak current density as a function of scan rate at 1.5V and 2.25V for PBEDOT iI butyl 2 Type III supercapacitor.
82 That is, the current density is broad across the operating voltage range with strong faradic currents centered at 1.68 V and 2.24 V for anodic peaks, and 1.68 V and 2.20 V for cathodic peaks Long term redox switching of a Type III ESC is illustrated in Figure 3 1 5 b. The instability of PEDOT iI butyl 2 films on gold button under repeated switching, as shown in Figure 3 10, indeed translate s to Type III ESC s as close to 50% of the electroactivity was lost after the first 100 cycles, and the electroactive response is nearly depleted after 400 scans. While further adjustments are needed in order to improve the stability of PBED OT iI butyl 2 to n doping, this material may hold promise in modular E SCs as discussed in the next section 153 Scan rate dependence studies between 10 to 1000 mV/s over a 2.5 V operating w indow is shown in Figure 3 1 5 c. Non ideal behavior is observed as the redox peaks begin to shift at high scan rates. This observation is accentuated in the scan rate versus the current density plot illustrated in Figure 3 1 5 d. It can be seen that the highe r redox process (V = 2.25) causes a non ideal behavior as the peak current and scan rate are not perfectly linear. Supercapacitor Modules It is well known that efforts to increase energy density in CP based supercapacitors demand both high and stable operating voltage s In this context, n dopable polymers capable of spanning the volta ge were introduced in previous section s and have been developed by the community 119,154 While Type III and IV supercapacitors are able to operate in the 2.0 3.0 voltage range, electrochemical instability is often an issue. 94 As a result, there is a trade off between high energy density and stability at lower volta ges. To improve the voltage window without
83 employing n dopable polymers, we have developed a strategy in which solely p dopable polymers are utilized and incorporated into tandem supercapacitors. The assembly of multiple supercapacitors has a cumulative ef fect in which a widening of device voltage is expected when coupled in series and an increase in capacitive currents when coupled in parallel. As shown via cyclic voltammetry experiments presented in Figure 3 1 6 a single PProDOT Me 2 based ESC (i), servin g as the model device, shows robust capacitive properties up to 1.0 V while passing 0.3 mA/cm 2 of current. With approximately 60 PProDOT Me 2 and residual electrolyte per electrode, as determined post electrochemical switching the specific capacitanc e relative to total polymer mass is 55 F/g with an average energy density of 6 Wh/kg. Figure 3 1 6 Supercapacitor charging/di scharging CV at 50 mV/s for single symmetric PProDOT Me 2 supercapacitor, two tandem se rial supercapacitors, and two tandem parallel supercapacitors. Polymer electrodes prepared via five potentiodynamic electropolymerization cycles. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Soc iety.
84 As two supercapacitors were coupled in series (ii), the CV exhibits capacitive behavior and reversibility throughout the 2.0 V window with 0.18 mA/cm 2 of current, or 15 F/g relative to the total polymer mass. The ability to maintain a steady current during the charging/discharging process highlights the capacitive properties of the PProDOT Me 2 along with the ability to successfully couple supercapacitors. While the tandem serial supercapacitor improve d the voltage window, it is important to note the c oncurrent diminution of current with additional devices. As two supercapacitors are coupled in series, the current decreased due to added resistance. To address this issue, tandem supercapacitors are assembled in parallel to increase the current. Relative to a single supercapacitor (i), the charging/discharging CV of tandem parallel supercapacitors, as illustrated in (iii), exhibits a two fold increase in current while maintaining steady capacitive behavior and reversibility throughout the 1.0 V experiment. Galvanic charging/discharging steps were used on these supercapacitors to further characterize the behavior of PProDOT Me 2 for charge storing applications and the ability to couple supercapacitor devices effectively. By applying a constant current of 0.5 mA/cm 2 to the tandem supercapacitors as a function of time shown in Figure 3 1 7 the galvanic charging discharging curves highlight the relative charging/discharging times and the operating voltage window. T he single supercapacito r used as the model in (Fi gure 3 1 7 i) requires 12 s to charge/discharge to 1.0 V. The charging discharging curves for the tandem serial supercapacitor (Figure 3 1 7 ii) exhibit a 2.0 V window, which is in good agreement with the cyclic voltammogram. In contrast, the tandem parallel supercapacitor (Figure 3 17 iii) requires 24 s for charging/ discharging to 1.0 V.
85 Figure 3 1 7 Supercapacitor galvanic charging/discharging at 0. 5 mA/cm 2 for single symmetric PProDOT Me 2 supercapac itor, two tandem s erial supercapacitors, and two tandem parallel supercapacitors. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 America n Chemical Society. Here, the times are approximately doubled from that of the single supercapacitor since the applied current passing through the tandem device is shared between two individual devices. While longer charging/discharging times are expected for the tandem supercapacitor in parallel, it allows for opportunities in ride through power applications where extended charging/discharging times are needed. The repetitive near triangular curves emphasize that the PProDOT Me 2 ESCs are reversibly cycling though the voltage dro p suggests internal resistance as shown in Figure 3 1 7 A close examination of the galvanic charging/discharging curves indicates an ohmic drop, while the cyclic voltammogram (Figure 3 1 6 ) shows a n asymmetry in the current response. These observations have been well attribute d these behavior s to a drop of conductivity in the negative electrode as a result of its nearly undoped state. 151 In the case of the Type I supercapacitors discussed here,
86 and perhaps more noticeable in the tandem supercapacitors, our observations are in agreement and consistent with those reported in literature. 151,152 While this is a limitation in electroactive polymer based supercapacitors, these issues can be circumvented both by lower ing the single cell voltage, 151 and by the addition of proper interfacial contacts within the cell 155 For the purpose of comparing the effects of multiple tan dem supercapacitors, four identical Type I supercapacitors were assembled in series. As illustrated by the cyclic voltamm etry profiles in Figure 3 1 8 a, the tandem serial supercapacitor assembly has an overall cell voltage of 4.0 V. Figure 3 18 Supercap acitor Assembly. a) Four symmetric PProDOT Me 2 supercapacitors in series to enhance vol tage window from 1.0 to 4.0 V. b) Four symmetric PProDOT Me 2 supercapacitors in parallel to increase current by four fold. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. The charging/discharging CVs exhibit high capacitive behavior and reversibility throughout the extended voltage re gime. Similarly, the ability of tandem parallel supercapacitors to effectively charge/discharge is demonstrated by the cyclic voltammetry profiles i n Figure 3 1 8 b. The tandem parallel supercapacitor assembly has
87 an overall four fold increase in current den sity, which is in good agreement with previous supercapacitor devices. To capitalize on the tandem supercapacitor concept, we connected individual devices both in series and in parallel to simultaneously extend the cell voltage and increase the capacitance Six identical supercapacitors were integrated with two individual devices connected in series and three sets of these two ESC configurations connected in parallel. With this device organization, we expect the assembly to operate with a 2.0 cell voltage and exhibit a three fold increase of current den sity. Figure 3 19 sh ows the CV of the six supercapacitor assembly (designated 2S 3P) superimposed on the response of two supercapacitors in series. Figure 3 19 T andem supercapacitor with two devices in series and three sets in parallel (2S 3P). Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. The advantage of tandem supercapacitor development allows for design fl exibility in voltage and capacitance values. Although not fully optimized, we do not envision a limit to the number of supercapacitors coupled in tandem.
88 In our efforts to develop tandem supercapacitors as an individual module that is stackable and lightwe ight, bipolar electrodes were utilized as the central component. Weight considerations can benefit from the module architecture as inactive materials, such as inner membranes and packaging materials, are minimized and active materials are maximized. The performance of the supercapacitor module is illustrated via the cyclic voltammograms in Figure 3 2 0 Figure 3 2 0 Supercapacitor CV for single supercapacitor and supercapacitor module at 50 mV/s. Polymer electrodes prepared via three potentiodynamic electropolymerization cycles. Adapted with permission from Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010 2 3586. Copyright 2010 American Chemical Society. In these experiments, the extent of electrochemical polymeriz ation for PProDOT Me 2 was maintained to three potentiodynamic scans. As such, the CV of the single supercapacitor device (Figure 3 20 i) displays an average current of 0.2 mA/cm 2 to 1.0 V. In comparison to the supercapacitor module (Figure 3 2 0 ii), the volt age window was successfully extended to 2.0 V and the module exhibits 0.1 mA/cm 2 of capacitive current. The development of supercapacitor modules enables us to stack multiple
89 supercapacitors as a single component, although this precludes analyses of indivi dual devices for the possibility of defects. Chapter Summary In this c hapter, an electron rich derivative of dioxythiophene, PProDOT Me 2 was electrochemically polymerized over gold/Kapton substrates to investigate its capacitive properties for charge st orage applications in supercapacitors. Redox switching of the PProDOT Me 2 films indicates stable electrochemical response while scan rate dependence studies from up to 1000 mV/s suggest the electroactive polymer to be electrode confined and well behaved. P ProDOT Me 2 was also used to construct polymeric electrodes on gold/Kapton substrates in Type I electrochemical supercapacitors. The supercapacitor devices display robust capacitive behavior over a 1.0 V operating window with scan rates between 5 and 500 mV /s. The specific capacitance of 55 F/g was calculated based on the active material with average energy density of 6 Wh/kg. Also, the device retains over 85% of its charging/discharging capacity over 32,000 redox cycles. A novel acceptor isoindigo was emp loyed as an electropolymerizable D A D monomer with EDOT units f lanking the acceptor core for Type I and Type III electrochemical supercapa citors. A PEDOT iI butyl 2 based Type I ECS demonstrated an operative voltage window of 0.5 V with 0.49 Wh/kg energy (based on approximately 0.4 mg of polymer) while retaining ab out 90% of electroactivity over 10,000 cycles. A voltage window of 1.8 V was achieved in Type III supercapacitors, although the undesirable electrochemical instability limits ultimate use. As a strategy to improve the voltage window and capacitance of electrochemical supercapacitors, tandem supercapacitors coupled both in series and in parallel, and
90 ultimately as a single encapsulated module, were successfully devised and tested. Using ProDOT Me 2 based Type I supercapacitors, an assembly of four tandem supercapacitors in series and in parallel resulted in an extension of the voltage window to 4.0 V and capacitance to 25 mF/cm 2 The flexibility of supercapacitor design allows for voltage improveme nts, capacitance improvements or both via serial and parallel constructs to meet power and energy requirements. Furthermore, development of tandem supercapacitors encapsulated within a single stackable energy storage device was successfully fabricated wit h PProDOT Me 2 on gold/Kapton bipolar electrodes, extending the supercapacitor voltage to 2.0 V.
91 CHAPTER 4 DEVELOPING ABSORPTIV E/TRANSMISSIVE BLACK WINDOW TYPE ELECTROCHROMIC DEVIC E S Introduction Organic electrochromic polymers (ECPs) have been developed over the past decades to the point of near commercial feasibility While strong research efforts have also been focused on inorganic metal oxide electrochromic materials, organic electrochromic polymers have several advantages 103,104,156 Organic ECPs are available in a variety of colors that switch to highly transmissive states. 157 The ability to access the range of organic ECPs has been accomplished by tuning the electron rich nature and the extent of conjugation of CPs 105,158 The recent yellow to transmissive ECP based on alternating propylenedioxythiophene phenylene copolymer completed the full color palette of solution processable organic electrochromes, facilitating the pursuit of a full color electrochromic display 107,108 B lack to transmissive ECPs are one of the most promising electrochromes giving rise to possible utility for ECPs in electronic papers, electrochromic smart windows, privacy goggles, and camouflages. In particular, privacy applications are finding great interest among government related programs for military and camouflages p urposes. For these applications, rapidly switching black to transmissive electrochromic materials are necessary, ideally on the order of video rate (30 50 ms) with large optical contrast and high transmittance and luminance (> 80 % T and 80% Y ) in the ble ached state. Our effort s to address these requirements take a multidisciplinary approach. Foremost, black to transmissive electrochromic polymers that are both highly electroactive and electrochemically stable are required. Solubility and ease of processi ng over large transparent electrodes is also desirable. Finally incorporation of
92 ECPs into window type devices requires the use of a dual electrochromic polymer device configuration that is neutral ly colored and highly transmissive to visible light in the bleached state. T he first black to transmissive electrochromic device was demonstrated by Unur using a pseudo 3 electrode electrochromic device (P3 ECD) 159 The P3 ECD, which operates un der the control of a bipotentiostat, consists of a transparent electroactive polymer (PProDO P N E tCN) as the counter electrode material, and two active electrochromic materials, specifically cathodically coloring green to transmissive ECPs and magenta to transmissive ECPs as the working electrodes. By color mixing of green and magenta ECPs in the neutral state, a black color was exhibited. Conversely, the oxidation of both working electrodes caused the device to bleach. While the pseudo 3 electrode electro chromic device exhibit ed a black to transmissive device via the color mixing of two polymers, it would be advantageous to only use one electrochromic polymer. T he first solution processable black to transmissive electrochromic polymer was developed by Beaujuge from the Reynolds group 160 Using a donor acceptor approach, easily oxidized ECPs with broad absorption bandwidths extending over the entire visible spectrum tha t can be fully bleached were synthesized. A series of donor acceptor oligomers differing in the ratio of the electron rich (3,4 bis(2 ethylhexyloxy)thiophene) to electron deficient (2,1,3 benzothiadiazole) substituents were first synthesized, followed by t he polymerizations of the oligomers. The variation of the relative contribution of electron rich and electron deficient moieties controlled the two band absorption in the visible spectrum. In the optimized feed ratio of two different monomers, the discrete
93 absorption bands merged into a broad absorption acr oss the entire visible spectrum, however, reproducible large scale production was difficult. This was realized in collaborations with BASF in which the polymer was synthesized and provided to the Reynolds group in bulk for further electrochemistry and electrochromic studies. Samples The structure of BASF Black is shown in Figures 4 1a. Figure 4 1 Structures of ECP Black Polymers. a ) BASF Black and b) Random Black More recently, S hi demonstrated a novel and reproducible synthetic approach to access black to transmissive electrochromic polymers via a random Stille coupling polymerization. 161 The random polymerization of electron rich (2,5 dibromo and 2,5 tributylstannyl 2 ethylhexyloxy substituted 3,4 propylenedioxythiophene) and electron deficient (4,7 dibromo 2,1,3 benzothiadiaz ole) moieties in different feed ratios lead to black ECPs that demonstrated high molecular weights (M n between 10 to 15 kDa) and low polydispersities (1.3 to 1.6). The structure of this polymer is shown in Figure 4 1b, and Bl A comparative
94 electrochromic and colorimetric analysis of these two ECP Black polymers are detailed in the following section in the optimization of black to transmissive window type ECDs. In addition, ECPs that are electroactive and transparent in both redox states are needed as counter electrodes in window ECDs. Nishide pr eviously introduced an organic nitroxide radical polymer, poly(2,2,6,6 tetramethylpiperidinyloxy 4 yl methacrylate) (PTMA), that is highly transparent and color less in the visible region, and exhibits rapid electron transport 162 This non color changing polymer (NCCP) has previous ly found potential uses in ECDs. 163,164 Furthermore the Reynolds group in collaboration wit h BASF recently introduced N C 18 substituted poly(3,4 propylenedioxypyrrole) (PProDOP N C 18 ) as the minimally color ch anging poly mer (MCCP) suitable for counter electrode materials in ECD s Detailed analyses of MCCP, including electrochemical electrochromic and colorimetric properties, are presented in this c hapter. The architecture of window ECDs utilizes a dual electrochromic polymer device configuration as shown in Figure 4 2, in which ECP Black functioning as the working electrode is charge balanced by the counter material Two ITO glass substrates are coated with the spray processable ECPs, and a transparent conducting gel electrolyte is applied between two electrodes. When the device is in the dark neutral state, ECP Black is neutral while counter material is oxidized. The application of a voltage across the device causes ECP Black to oxidiz e and the counter material to neutralize. The concurrent and reversible process of these electrochromic films gives rise to the switching of ECDs between the dark and bleached states.
95 Figure 4 2 Schematic design of absorptive/transmissive window typ e electrochromic device in the a) neutral and b) bleached states. Black to Transmissive Electrochromes Thin films of BASF Black and Random Black were spray cast onto ITO coated glass slates from 2 mg / mL toluene solution and electrochemically cycled from 0.3 to +0. 8 V vs. Ag/Ag + in 0.1 M LiBTI /ACN Redox switching by potentiodynamic cycling is illustrated in Figure 4 3 demonstrating a highly reproducible and electrochemically stable current response upon 50 repeated scans. The relatively low onset of oxid at ion, 0.19 V vs. Ag/Ag+ for BASF Black and Random Black, is ideal for long term redox switching as electrochromic films and devices. Figure 4 3 Cyclic voltammogram of ECP Black. a) BASF Black and b) Random Black at 50 mV/s in 0.1 M LiBTI/ACN for a total of 50 cycles. Only every 10 th CV scan is displayed in the figure.
96 Figure 4 4 shows the full spectroelectrochemical behavior of both ECP Black polymers. Thin films of polymer were spray cast onto indium tin oxide (ITO) coated glass slides and then ele ctrochemically cycled until a stable and reproducible current response was observed prior to analysis. As seen in the neutral spectrum of BASF Black in Figure 4 4a, a sharp absorption band with max at 575 nm is present, whereas, the neutral spectrum for Random Black shown in Figure 4 4b lacks a distinct absorption peak and is broadly absorbing across the visible spectrum. It is important to note the differences in these absorption bandwidths. Since a portion of visible light is not absorbed in 390 550 and 650 750 nm ranges, thin films of BASF Black films are purple/inky black unless an adequate amount of polymer is deposited onto ITO coated substrates to obtain a dark neutral film. The spectroelectrochemical series of BASF Black and Random Black were car ried out from 0.3 V to +0.0V vs. Ag/Ag + in 100 mV increments and from 0.05 V to +0.7 V in 50 mV increments. Upon oxidation of the polymer films, the absorption in the visible region is reduced, while polaronic transitions in the near IR around 950 nm aris e. The intensity of the polaronic transitions deplete s at higher oxidation levels, giving rise to bipolaronic absorptions beyond 1600 nm. Effective bleaching in the visible region occurs in both ECP Black films and high levels of transmissivity are observe d. It is important to note the rise of dual bands in the visible range as the neutral polymer films oxidize to the bleached state. The dual band creates an intermediate green color which is present for a brief period of time during the electrochromic switc h. The duration of this dual band is analyzed using a novel electronic spectroscopy setup described in C hapter 5.
97 Figure 4 4 Sp ectroelectrochemical series for ECP Black. a ) BASF Black and b) Random Black between 0.3 V to +0.7 V vs. Ag/Ag + in 0.1 M LiBTI/ACN supporting electrolyte. Applied potentials between 0.3 to 0.0V are in 100 mV increments and +0.05 to +0.7 V in 50 mV increments. To construct black window ECDs with high transmittance in the bleached state, large optical contrast, and fast switching speeds, several device parameters were optimized. Since these characteristics depend strongly on the thickness of ECP Black films, a set of four BASF Black and four Random Black films of differing optical density were spray cast onto ITO co ated glass substrates. The set of neutral and corresponding bleached spectra for BASF Black films (A = 0.14, 0.30, 0.70, 0.97 a.u. measured at max = 550nm) are shown in Figure 4 5a, indicating that more optically dense films are less capable of attaining h igh ly transmissive states. Film 4 with the maximum absorbance of 0.94 a.u. has a corresponding absorbance of 0.18 a.u. in the bleached state. On the other hand, the thinnest film of 0.14 a.u. in the neutral state is associated with an absorbance of 0.03 a.u. in the bleached state. Upon further analysis of the spectra in Figure 4 5 a, films that exhibit high contrast correspond to lower transmittance levels in the bleached state. Because of this interplay, a balance between the optical contrast and bleached state transmittance is needed.
98 Figure 4 5 Absorbance and Relative Luminance of BASF Black. a) Absorption spectra in the n eutral and bleached states and b) relative luminance from neutral to bleached states for BASF Black films at four different thickness (A = 0.14, 0.3, 0.7, 0.94 a.u.). To evaluate the color changes of BASF Black films with respect to the human eye, the L*a*b* color coordinates (CIE 1976 L*a*b* Color Space) for these polymer films of differing optical density were measured as a function of applied potential. The relative luminance change in Figure 4 5 b, as determined by colorimetry, varied from 52 % Y for the thickest to 19 % Y for the thinnest prepared film, demonstrating the ability to modulate relative luminance based on film thickness. In their neutral state, BASF Black films exhibited L* from 43 for the thickest to 89 for the thinnest film. The neutral absorption of the thickest film displayed a relatively dark purple/blue color with L* = 43, a* = 7 and b* = 24. This observa tion corresponds well with the visibl e absorption shown in Figure 4 5 a as small amount of red and blue light is transmitted by the polymer. In comparison to the fully oxidized polymer films, L* varied from 84 for the thickest and 97 for the thinnest film. Correspondingly, a* and b* color coordinates approach zero, which demonstrate the capacity of films to reach a highly transmissive state. The relative
99 luminance values and CIE L*a*b* color coordinates for films 2 and 3 are summarized in below in Table 4 1 Table 4 1 Relative luminance and CIE L*a*b* Color Coordinates for BASF Black films in the neutral ( 0.2 V vs. Ag/Ag + ) and oxidized (+0.7 V vs. Ag/Ag + ) states. Film Absorbance(a.u.) Luminance Color Coordinates Neutral Color Coordinates Oxidized %Y b a Y L a b L a b 1 0.14 94 19 89 1.5 6.8 97 1 0.5 2 0.3 86 34 77 3 10 94 1 1 3 0.7 73 50 55 5.7 20 88 3 3 4 0.97 65 52 43 7 24 84 4 3.6 a Relative luminance in the bleached state. In order to study the switching speed of BASF Black films which will govern their utility in ECDs, in situ electrochemical optical spectroscopy measurements (chronoabsorptometry) were carried out. The film switching rate was examined by monitoring the tr ansmittance change at a single wavelength as a function of time elapsed upon the application of potential square waves between the fully neutral and oxidized states. Polymer films were potentially stepped between 0.2 V and +0.65 V vs. Ag/Ag + at periods of 60 s 10 s 1 s and 0.5 s as shown in Figure 4 6. In the case of a BASF Black film at A = 0.14 optical density, a transmittance change (monitored at 550 nm) of 22 T was recorded at long switch ing time (60 s), and decreasing the switch ing period to 0.5 s reduced the contrast to only 21 T. This observation indicates that longer switch ing times are not necessary to realize the full optical contrast for this thin film. However, a longer switching period was necessary to obtain the full contrast f or the thicker films represented in Figure 4 6b. An optically dense film of A = 0.7 reached the full transmittance contrast of 53 T over a 60 s switching period, and was subsequently reduced to 35 T at a 1 s switch rate.
100 Figure 4 6 Potential squ are w ave step chronoabsorpometry for BASF Black. a) BASF Black film 1 (A = 0.14 a.u .) and b) BASF Black film 3 (A = 0.7 a.u.) monitored at 550 nm between 0.2 to +0.65 V vs. Ag/Ag + in 0.1 M LiBTI/ACN electrolyte solution, at 60 s, 10 s, 1s, and 0.5 s swit ching periods. An alternative method of representation for ECP switching time is to determine the time necessary for the full (or a specific percent of full) transmittance contrast to be reached. A range of transmittance contrast values (95%, 50%, and 25%) were analyzed to understand the relationship between switching time and transmittance contrast. The thickest film of A = 0.97 a.u. required over 3 s to reach 95% of the full transmittance
101 contrast (T 95% ), whereas the 50% and 25% transm ittance contrasts (T 50% andT 25% ) were achieved in approximately 1 s and 600 ms respectively. The thinnest film demonstrated sub second switching times, varying from 240 ms (t 95% ) to 50 ms (t 25% ). The range of switching speeds indicates how the response tim es can be modulated and optimized as a function of thickness. The full summary of transmittance contrast and switching speeds i s compiled in Table 4 2. Table 4 2 Transmittance and switching speed for BASF Black films Film Absorbance (a.u.) Transmittance Switching Speed (sec) %T b a T Color to Bleached Bleached to Color t 95% t 50% t 25% t 95% t 50% t 25% 1 0.14 92 22 0.12 0.09 0.05 0.24 0.15 0.12 2 0.3 86 35 1.18 0.3 0.18 1.12 0.57 0.35 3 0.7 73 53 2.57 0.66 0.38 2.6 1.0 0.56 4 0.97 66 54 3.5 0.86 0.5 3.23 1.43 0.71 a Transmittance in the bleached state Analogously, thin films of Random Black were spray cast to differing optical densit ies (A = 0.12, 0.34, 0.5, 0.7 measured at max = 550nm), with the spectra of neutral and bleached films shown i n Figure 4 7a. Figure 4 7 Absorbance and Relative Luminance of Random Black. a) Absorption spectra in the n eutral and bleached states and b) relative luminance from neutral to bleached states for Random Black films at four different thickness (A = 0.12, 0.34, 0.5, 0.7 a.u.)
102 Th is set of Random Black film s further confirms the interplay between optical contrast and the transmittance of bleached films, and empha sizes the need to optimize film thickness for specific electrochromic applications. The pe rcent relative luminance values of the four Random Black films were measured colorimetrically as a function of applied potential using the CIE L*a*b* color space, and the results are summarized in Table 4 3. As discussed previously, relative luminance is a mathematical representation of the color of visible light as calibrated to the human eye. The greatest luminance contrast was observed in film 3 (47 % Y), while film 1 had the highest relative luminance in the bleached state (94% Y). The analysis of Table 4 3 indicates the ability to modulate and optimize relative luminance and luminance contrast as a function of film thickness. Table 4 3 Relative luminance and CIE L*a*b* Color Coordinates for Random Black films in the neutral ( 0.2 V vs. Ag/Ag + ) and oxidized (+0.7 V vs. Ag/Ag + ) states. Film Absorbance (a.u.) Luminance Color Coordinates Neutral Color Coordinates Oxidized %Y b a Y L a b L a b 1 0.12 94 18 90 2 4 98 0.9 0.18 2 0.34 86 39 75 7.7 13 95 1.4 0.7 3 0.5 80 47 64 5.4 12 92 2.7 1.9 4 0.7 72 46 58 5 11 88 3 2.6 a Relative luminance in the bleached state. Furthermore, a significant fact was revealed by examining the CIE L*a*b* color coordinates, particularly in comparison to BASF Black films. With similar optical densities, the L* and a* color coordinates of Random Black (film 3) are comparable to those of BASF Black (film 4); however, the b* coordinate was different, with b* = 11 for Random Black and b* = 20 for BASF Black. Negative b* value signifies blue on the CIE color coordinates, which in the case of BASF Black is more prominent and corresponds
103 well with the absorption spectra. The transition of a* and b* color coordinates toward zero demonstrates that the films reach a highly transmissive state. Given that active electrochromic components will strongly influence the properties window ECDs, the switching speed of Random Black was also investigated by chronoabsorptometry. Polymer films were potentially stepped between 0.2 V and +0.65 V vs. Ag/Ag + at periods of 60 s, 10 s, 1 s, and 0.5 s, as shown in Figure 4 8a for film 1. A transmittance change (monitored at 550 nm) of 22 T was recorded for long switch period s (60 s), and decreasing the switch time to 0.5 s reduced the contrast to only 20 T. I n the case of film 4, the full transmittance contrast of 50 T was reached during the 60 s period and decreased by 40% when the switching period 1 s was applied as shown by the chronoabsorptogram in Figure 4 8b. Figure 4 8 Potential square w ave step chronoabsorpometry for Random Black. a) Random Black film 1 (A = 0.17 a.u.) and b) Random Black film 4 (A = 0.7 a.u.) monitored at 550 nm between 0.2 to +0.65 V vs. Ag/Ag + in 0.1 M LiBTI/ACN electrolyte solution, at 60 s, 10 s, 1s, and 0.5 s swit ching periods. The times necessary to reach a specific percent of the full transmittance contrast for Random Black are summarized in Table 4 4. In particular, the thickest film (film 4) required 5.5 s to reach 95% of the full contrast (T 95% ), whereas 1 s and 500 ms were
104 needed to achieve T 50% and T 25% respectively. The thinnest film (film 1) demonstrated sub second switching times, ranging from 400 ms (t 95% ) to 50 ms (t 25% ). Table 4 4 Transmittance and switching speed for Random Black Film Absorbance (a. u.) Transmittance Switching Speed (sec) %T b a T Color to Bleached Bleached to Color t 95% t 50% t 25% t 95% t 50% t 25% 1 0.17 93 22 0.4 0.1 0.05 0.17 0.08 0.05 2 0.34 83 39 1.11 0.35 0.18 0.8 0.42 0.29 3 0.5 78 47 1.57 0.48 0.24 1.13 0.5 0.33 4 0.7 70 50 5.5 1.0 0.47 2.6 0.9 0.53 a Transmittance in the bleached state In light of the promising electrochromic and colorimetric properties of spray processed ECPs both BASF Black and Random Black position us well for developing window ECDs Because the color purity of Random Black was more neutra l l y black, as confirmed by CIE L*a*b* color coordinates and by the comparative neutral absorption spectra shown in 4 9, it is evident that Random Black is the superior candidate of the two polymers for windo w type applications. Figure 4 9 Comparative absorption spectra for BASF Black and Random Black with normalized absorbance at = 550 nm for films in the dark and bleached states.
105 Optimization of Transmittance and S witching S peed In order to optimize the switching speed of Random Black films, a series of supporting electrolyte compositions were investigated since highly conductive electrolyte solutions are expected to improve the ion transport and thus the switching speed. 178,179 The conductivity resu lts compiled in Table 4 5 suggest two general trends, that is, the increase in conductivity with both high electrolyte concentration and with acetonitrile as the solvent. Table 4 5 Conductivity of supporting electrolyte solutions in various combinations electrolyte concentration, electrolyte, and solvent. Concentration (M) Electrolyte Solvent Conductivity (mS/cm) 0.1 LiBTI PC 1.6 0.1 TBABF 4 PC 2.2 0.1 TBAPF 6 PC 2.6 0.1 LiBTI ACN 9.1 0.1 LiBF4 ACN 7.1 0.1 TBABF 4 ACN 10.7 0.1 TBAPF 6 ACN 9.9 0.1 EMI BTI ACN 11.4 1.0 TBABF 4 ACN 34.9 1.0 LiBF 4 ACN 20.8 1.0 EMI BTI ACN 50.4 1.0 Na 2 SO 4 H 2 O 88.7 The conductivity of tetrabutylammonium tetrafluoroborate (TBABF 4 ) and lithium tetrafluoroborate (LiBF 4 ) improved over two fold with increasing electrolyte concentration. A remarkable five fold increase was observed in the case of 1 ethyl 3 methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI BTI) Furthermore, improved conductivities were observed in A CN, owing to higher ion mobility compared to PC. A queous supporting electrolyte demonstrated the highest ionic conductivity, although our organic ECPs did not respond electrochemically. In light of high conductivity measurements, electrochromic measurement s were focused on TBABF 4 and EMI B TI
106 electrolytes in acetonitrile. E lectrolyte concentrations were also varied to elucidate the differences in switching times. Subsequently, films of Random Black were analyzed in the selected conditions outlined in Table 4 6. The transmittance of these films was compared first ranging between 39 to 41% T in the neutral state and 78 to 80 % T in the oxidized states. With reproducible transmittance levels, we were then able to compare the switching speed in different electrolyte solutions. Unfortunately our results did not demonstrate a strong relationship between the switching speed and electrolyte conductivity. Only slight improvement in switchin g speed was observed with 1.0 M EMI BTI electrolyte concentrat ion. One important factor not considered at the time of these experiments is the size of counter ions. It has been reported that the cation and anion heavily influence the reduction and oxidatio n process respectively. 60 As such, while EMI BTI demonstrate d high conductivity, the large anion may impede the ion transport during electrochemical cycling Table 4 6 Swi tching speed of Random Black in various electrolyte compositions Supporting Elect rolyte Transmittance %T Switching Speed (s) Colored to Bleached Bleached to Colored Neutral Oxidized Contrast t 95% t 50% t 95% t 50% 0.1 M TBABF 4 / ACN 41 80 39 0.6 0.19 0.26 0.18 0.1 M EMI BTI / ACN 40 78 38 0.5 0.17 0.23 0.16 1.0 M TBABF 4 / ACN 39 78 39 0.4 0.15 0.19 0.12 1.0 M EMI BTI / ACN 39 78 39 0.38 0.14 0.22 0.15 In addition to electrolyte composition, the investigation of appl ied potentials was carried out in particular to probe the effects of potential limits. Because the potential
107 square wave steps were selected from the CV in Figure 4 3, that is, from 0.3 V to +0.8 V vs. Ag/Ag + higher applied potential square waves may facilitate ion transport and possibly improve both the switching speed and the optical contrast. Beginning with a simple proof of concept shown in Figure 4 10, the applied potential range varied from (a) 0.2 to +0.8 V, and (b) 0.8 to +0.8 V. With switching periods of 5 s, 3 s, 1 s, and 0.5 s, it can be seen in Figure 4 10b that the polymer film retained higher co ntrast at fast switching rates ( 1 s and 0.5 s ), although with lower transmittance in the neutral state This observation is attributed to the additional 0.6 V in the reduction process. Figure 4 10 Chronoabsorptometry of Random Black films with potential square wave steps in 5 s, 3 s, 1 s, and 0.5 s periods from a ) 0.2V to +0.8V and b) 0.8V to +0.8V vs. Ag/Ag+.
108 Our observations of increasing contrast with increasing potential ranges le d to the next set of experiments; that is, systematically in crease the potential to determine t he limits of Random Black films as depicted in Figure 4 11 Figure 4 11 Chronoabsorptometry of Random Black (A=0.48) switching at 0.5 s periods between different applied potentials Taking the left block (a) in Fi gure 4 11 as the reference point, an increase in the oxidation potential to +1.5V (b) significantly improved the transmittance to 67% T (from 56 % T) in the bleached state, albeit with a concurrent increase in the transmittance of the neutral state. However with the application of a potential window from 1.5 V to + 1.5 V (c) we were able to improve the contrast while maintaining a deeply absorbing neutral film that is also highly transmissive in the oxidized state Slight improvement was seen with an applied potential of 2 to + 2 V (d) and beyond this range (e) exhibited the potential limits of Random Black films Transparent Counter Material With the ability to tune the electron rich nature and the extent of conjugation of conjugated polymers the position the neutral and charge carrier transitions can be
109 tuned across the UV vis NIR spectrum. Conducting polymers of reduced conjugation have high energy band gaps with the majority of the light absorption in the UV region as in the case for t he family of N alkylated PProDOP, which have band gaps over 3.0 eV. In addition, as these polymers are highly electroactive at low oxidation potentials and highly transparent in both the neutral and oxidized states, they may find use as counter electrodes in window electrochromic devices. Because of its electrochromic properties, N alkylated PProDOP will be referred to as minimally color changing polymer (MCCP) in this dissertation. MCCP was synthesized by M. Craig of the Reynolds group in collaboration wit h BASF. Redox properties of MCCP are exhibited by CV and DPV in Figure 4 1 2 with potentiodynamic cycling between 0.5 V to +0.4 V vs. Ag/Ag + in 0.1 M LiBTI/ACN for 30 repeated cycles. Both the CV and DPV demonstrated onsets of oxidation ca. 0.05 V vs. Ag /Ag + and anodic peaks (E p,a ) centered ca. 0. 1 V vs. A g/Ag + which are relatively low oxidation potentials suitable for long term redox switching. Figure 4 1 2 Electrochemistry of MCCP. a) CV switching of MCCP between 0.5 V to +0.4 V vs. Ag/Ag + in 0.1 M LiBTI/ACN for 30 cycles b) DPV of MCCP in the oxidation and neutralization process.
110 The transmittance of MCCP in the neutral and oxidized states is displayed in Figure 4 1 3 a Thin f ilms were spray cast from a 2 m g/mL solution of MCCP in toluene t o A = 0.15 a.u at = 375 nm. Highly transparent films were observed with transmittance s over 80% T in both the neutral and oxidized states although an intermediate pink hue was detected at V = 0.0 V vs. Ag/Ag + Given the fact that the faint pink hue was observed at this potential, it is reasonable to associate the pink color with the polaron transition. Despite this observation the pink intermediate is not expected to adversely affect window ECDs since the colors of interest are at the extreme volt ages. Figure 4 1 3 Transmittance and Relative Luminance of MCCP. a) Transmittance spectra of MCCP at 0.5 V, 0.0 V, and 0.5V with corresponding photographs and b) Colorimetry studies of MCCP with relative luminance and CIE L*a*b color coordinates be tween the neutral and oxidized states. The high levels of transmittance correlate well with the relative luminance results s hown in Figure 4 1 2b. As the applied potential incrementally increase d by 100 mV to the oxidized state (+0.5 V) the film of MCCP de monstrated only slight variations in the relative luminance and CIE color coordinates, meaning that MCCP films are excellent counter electrode candidates in window ECDs.
111 Charge Balance From a practical perspective, the most useful combinations of Random Black and MCCP in a dual electrochromic polymer device are those in which the redox charge for both electroactive materials is the same. Observed by Padilla, a balanced system prevents, or at least minimizes, undesirable reactions that would otherwise decrease the device switching lifetime. This was reported in dual electrochromic devices using complimentary ECPs (poly(3,4 ethylenedioxythiophene and poly[bis(3,4 ethylenedioxy)thienyl) N m ethylcarbazole]). 165 In the case of unbalanced systems, individual electrochromic polymer s did not reach their full switching capacity. This reasoning was employed on our window ECDs. C al ibration plots relating the charge density as a function of film absorbance were first generated with four films of Random Black spray casted to A (5 50 nm) = 0.27, 0.48, 0.69, 1.15 and six films of MCCP, A (3 5 0 nm) = 0.02, 0.07, 0.13, 0.2, 0.35, 0.49. Sub sequently, the charge density of the individual films was determined from potential square wave cycle s between the 0.2 to + 0.8 V vs. Ag/Ag + for ECP Black and 0.5 to + 0. 4 V vs. Ag/Ag + for MCCP). Taking into con sideration the ECD design, that is Random Black as the working electrode (anode) and MCCP as the counter electrode ( cathode ) charge density of Random Black was determined from the oxidation process whereas MCCP charge density was measured from the neutra lization (reduction) process Figure 4 1 4 shows t he calibration curves for Random Black (1a) and MCCP (1b) with respective linear fit lines. The coefficients of determination (R 2 ) for both plots are above 0.98. This statistical parameter demonstrates a linear relationship between the charge density and absorbance, and is utilized to correlate the absorbance and charge density during film preparation.
112 Figure 4 1 4 Calibration plots of absorbance as a function of charge density for a) Random Black and b) MCCP films. Black to Transmissive Electrochromic Devices With the fundamental electrochromic and colorimetric properties of the active components characterized, window electrochromic devices were assembled on 1.0 by 1.5 inch ITO substrates consisting of equal amounts of Random Black and MCCP based on charge density. Prior to device encapsulation, the films were switched between their neutral and oxidized states until reproducible current density responses were observed. In addition, Random Black films ( as the working electrode ) were
113 potentiostatically held for 30 s at +0.8 V vs. Ag/Ag + and MCCP films at 0.5 V vs. Ag/Ag + for 30 s ( as the counter electrode ) This procedure ensured polymer charge balance during operation of the device. Both polymers were covered with a thin layer of transparent gel electrolyte comprised of 1.0 M LiBTI/PC/PMMA and assembled into a sandwich type device configuration. The spectroelectrochemical behavior of the window ECD was measured to examine the spectral changes during r edox switching. Using a Cary 500 UV Vis NIR spectrophot ometer equipped with a tungsten halogen visible light source, a voltage sufficient to fully neutralize the active layers was applied across the device ( 0.5 V), followed by the incremental increase of applied voltages until the fully oxidized state was achieved (1.8 V). At 0.5 V, the device exhibited an absorption bandwidth (bold line) in the visible region characteristic of Random Black and has a dark neutral color as evident in the photograph shown i n Figure 4 1 5 Figure 4 1 5 Spectroelectrochemical series of window ECD (D3) based on Random Black and MCCP films, with applied voltages from 0.3 to 0.0V in 100 mV increments and +0.05 to +0.7 V in 50 mV increments.
114 With an incremental increase of applied voltage from 0.5 to +1.8 V, the cathodically coloring polymer becomes oxidized and the absorption in the visible region is reduced. The rise of a polaronic (ca. 950 nm) peak and bipolaronic (ca. 1500) peaks at high doping l evels led to effective bleaching of the visible absorption (dotted line) and a high level of ECD transmissivity was subsequently realized. The relative luminance in the extreme states demonstrates a contrast of 47 % Y, with a luminance value of 20% Y in t he neutral state ( 0.5V) and 67% Y in the oxidized state (+1.8 V). The values obtained for the ECD were near those of the individual Random Black films at A = 0.7 a.u. As window ECDs were originally designed to achieve high levels of transmissivity and lum inance in the bleached state, the use of thinner films would allow for this, albeit with a subsequent lowering of the optical contrast. Two thinner ECDs, D1 a nd D2 (as summarized in Table 4 7 ) exhibit relative luminance ranging from 88 to 77 % Y in the ble ached state with a luminance contrast of 37 and 42 % Y respectively Table 4 7 Relative luminance and CIE L*a*b* Color Coordinates for ECDs in the neutral ( 0. 5 V) and oxidized (+ 1. 8 V) states ECD Random Black Absorbance Luminance Neutral ( 0.5 V) Bleached (1. 8 V) %Y b Y L a b L a b D1 0.3 88 37 7 9 9 95 1 2 D2 0.48 77 42 66 9 12 90 1 4 D3 0.7 67 47 52 11 12 85 1 6 The color purity of window ECDs was evaluated with the CIE L*a*b* system. Most of the color change was found to vary along the lightness scale (L*), since the a* and b* color coordinates did not significantly differ between the three ECDs. This observation is
115 indicative of an achromatic transition between the neutral and bleached states and can be seen by the photographs in Figure 4 1 6 Figure 4 1 6 Photographs of window ECDs in the neutral ( 0.5 V) and oxidized (+1. 8 V) states with corresponding CIE L*a*b* color coordinates. The switching speed of two window ECDs, based on Random Black films of A = 0.48 and A = 0.3 optical density, was determined from the chronoabsorptometry shown in Figure 4 1 7 The transmittance change of the device was measured between 0.5 V and +1. 8 in 15 s intervals with a corresponding transmittance contrast of 38% T and 31% T for D2 and D1 respectively. While the t 95% in both devices were on the order of seconds, the t 50% and t 25% were under 1 s, suggesting that video rate for display applications may be achievable with furth er switching speed optimization, as summarized in Table 4 8.
116 Figure 4 1 7 Chronoabsorptometry of ECDs (Random Black for A = 0.3 and A = 0.48) from 0.5 to 1.5V at 15 s switch period Table 4 8 T r ansmittance and Switching Speed Summary of ECD ECD Absorbance (a.u.) Transmittance Colored to Bleached (s) Bleached to Colored (s) %T b T t 95% t 50% t 25% t 95% t 50% t 25% D1 0.3 80 31 4.79 0.14 0.1 5.73 0.63 0.26 D2 0.48 72 38 7.8 0.18 0.1 9.14 0.61 0.21 Chapter Summary I n summary, electrochromic and colorimetric properties of black to transmissive electrochromes and minimially color changing polymer were investigated and optimized for high optical contrast in the bleached state and fast response rate. Interplay between film thickne ss, switching speed, optical contrast, and transmittance in the ble ached was elucidated. A linear relationship between film thickness and optical contrast was observed; whereas, an inverse correlation between film thickness and both switching speed and tra nsmittance level in the bleached state was realized. Black to transmissive electrochromic windows based on a black electrochrome as the active layer and minimally color changing polymer as the counter material were demonstrated with transmittance contrast between 31 and 38 % T and luminance
117 contrast between 37 and 47 % Y. In the bleached state, the transmittance and luminance values were 88% T and 80% Y respectively. The switching speed between the dark and bleached states ranged from 5 to 9 s at the 95% transmittance contrast, and sub second at the 50% transmittance contrast.
118 CHAPTER 5 IN SITU SPE CTROSCOPIC ANALYSIS OF SUB SECOND SWITCHING POL YMER E LECTROCHROMES Introduction Electrochromic materials for applications such as smart windows, visual displays, and electronic papers have shown great progress with recent improvements in structural modifications and metal mediated polymerizations of electro active conjugated polymers as well as in advanced nanostructure templating and processing techniques of metal oxide materials. As electrochromic materials are finding their niche in various display applications, the accurate measurement and determination o f the response time is required as the switching speed ultimately governs their usefulness for specific functions. For instance residential privacy windows target a response time on the order of minutes, whereas information displays and electronic reader s require a 100 ms to 500 ms benchmark. For use in video displays, electrochromic materials must attain switching speeds on the range of 30 ms to 50 ms. While the full switch response times of electrochromic materials are approaching sub second, there i s continued effort to increase the switching speed, specifically by structure property optimization, ionic conductivity of supporting electrolyte, and 3D nanostructure templating 166 169 In parallel, as electrochromic materials are accessing sub second switching speeds, new analytical tools to probe rapid electrochromic t ransitions are necessary. Spectroscopic characterization remains one of the most valuable techniques in the analysis of electrochromic materials. When considering the rate at which an electrochromic material switches, there are two approaches: the magnit ude of contrast when the film is switched at a specific rate ( %
119 contrast to be reached (time). In both scenarios, analysis of the response time requires in situ electrochemical experiments coupled to electronic s pectroscopy. In the former case, the spectrometer measures the electrochrome's optical transmittance at controlled switching rates. In the latter case, the electrochromic material is switched between the two optical states and saturation is allowed to be reached, while the optical transition is measured with respect to time. In this situation, synchronization between the potentiostat that induces the redox reaction, and the spectrometer, that measures the resulting optical changes is necessary to determi ne the precise moment of an electrochromic change, which can be difficult, particularly for rapidly switching electrochromic materials. This c hapter describes a novel technique developed to synchronize the electrochemical and optical responses of sub secon d switching electrochromic polymers. The method involves the use of an external circuit directly coupled to the electrode on which the electrochromic film is supported, which upon the application of an electrochemical bias from the potentiostat, an output signal is transmitted to the spectrometer detector to trigger the measurement of optical signals. While the commercial spectrophotometers, rapidly switching films requir e this trigger setup since the synchronized electrochemical and optical responses give an accurate analysis of the switching speed. T he use of a novel spectroscopic setup to probe the switching times of three electrochromic polymers (ECP Magent a, 170 ECP Green 171,172 and ECP Black 160,161,173 ) is described, all with response times near the 500 ms benchmark for applications in electronic inf ormation and e reader displays.
120 Electrochromic Polymers The synthet ic protocols of ECP Magenta 34 ECP Green, 172 and ECP Black 161 have been previously reported. The repeat unit of each polymer is shown in Figure 5 1, with corresponding photographs and L a b values ( based on the Commission Internati onale L a b Color Space) in their fully colored and bleached states. The number average molecular weights of the polymers were measured by GPC in THF vs. polystyrene standards and found to be 10.5 kDa for ECP Magenta, 24 kDa for ECP G reen, and 12.7 kDa for ECP Black. The applied voltages for ECP Magenta were 0.6V and 0.75V vs. Ag/Ag + for the colored and bleached states respectively, and 0.6V and 0.95V vs. Ag/Ag + for ECP Green, and 0.7 and 0.85V vs. Ag/Ag + for ECP Black. Figure 5 1. Structures of ECP Magenta, ECP Green, and ECP Black with photographs of the films in the colored and bleached states and their corresponding L a b L a b Color Space). The app lied voltages for ECP Magenta were 0.6V and 0.75V vs. Ag/Ag + for the colored and bleached states respectively, and 0.6V and 0.95V vs. Ag/Ag + for ECP Green, and 0.7 and 0.85V vs. Ag/Ag + for ECP Black.
121 Each film was subjected to optical absorbance measure ments to monitor their relative film thickness, which was optimized at A ( max ) = 0.7. Absolute film thickness measurements were obtained with a Veeco Dektek 150 profilometer by Danielle Salazar The average thickness of ECP Magenta was found to be 375 + 60 nm with a film sprayed to A ( 545 ) = 0.7. For ECP Green, the average thickness is 350 + 130 nm, and 380 + 80 nm for ECP Black at A ( max ) = 0.7. ECP Magenta As the first solution processable electrochromic polymer exhibiting high color purity in electrochromic devices (ECDs), the electron rich ECP Magenta is an ideal candidate for analysis using the novel triggered spectrometer setup. A film of ECP Magenta, sp rayed to an absorbance of 0.7 a.u. at max is stepped between full neutralized and fully oxidized states ( 0.6 V and +0.75 vs. Ag/Ag + ), and the optical spectra captured every 2 ms during the electrochromic transition. As seen in the spectral profile in F igure 5 2 a, the film in the colored neutral state has a transmittance of 20% at max (545 nm) that increases to near 70% transmissivity when the bleached state is reached. In Figure 5 2b, only selected spectra (a total of 20 spectra) are given to clearly show the electrochromic transition. Further, by tracing the peaks shown by the circular markers in Figure 5 2b, it is evident that ECP Magenta does not switch monochromatically as the max shifts during the electrochromic switch. The full transition from the neutral to oxidized states is further confirmed by the near stationary isobestic point centered at 625 nm, which indicates, spectroscopically, a smooth transition from one species to another with no intermediates.
122 Figure 5 2 Spectral Evol ution of ECP Magenta. a) ECP Magenta film (abs at max = 0.7) switching from 0.6 V to +0.75 vs. Ag/Ag + with the full potential step and hold occurring within 0.5 seconds. Spectrum recorded every 2 ms for a total of 25 0 spectra in 500 ms. b) Same ECP Mag enta film and switching experiment with spectrum shown every 20 ms for clarity, for a total of 25 spectra in 500 ms. Circular markers are placed to emphasize the shifting of max during the electrochromic switch.
123 It is important to note that this transmitt ance profile is not a typical spectroelectrochemical experiment where the electrochromic film is monitored at steady state ( i.e. constant applied potential). Instead the optical responses are continuously captured by the spectrometer detector at 1 spectru m for every 2 ms during a switch between the two extreme states. Shown in Figure 5 2 represents only one full transition (colored to bleached) to avoid spectral overlapping. At the 0.5 second switch, ECP Magenta exhibits a 48% transmittance contrast at max and slightly higher transmittance contrasts observed at 1 and 5 second potential square waves (48.8 and 50 % Subsequently, the contrast levels and switching times of ECP Magenta were analyzed to determine the response time while retaining a high level of transmittance contrast. Experiments were carried out at the 1 second switch which allowed a near full contras t as exhibited in Figure 5 3 The 545 nm wavelength ( max in the neutral state) was selected to probe the transmittance contrast as it is expected that this is where the maximum contrast occurs. ECP Magenta achieved the full optical contrast (50 % dur ing a 5 second switch, and retained 97.6% of the contrast during the 1 second switch. These results allow us to determine the time necessary for a specific level of optical contrast to be reached. For example, it is common to determine the times to reach 98% and 95% of the full transmittance contrast (t 98% and t 95% ) and are typically reported for display characterizations as the remaining small changes in transmittance (3% and 5%, respectively) are nearly imperceptible to the human eye. The first striki ng observation in Figure 5 3 is the asymmetric response of the ECP Magenta in comparing the bleaching (0 to 1000 ms) and the coloring (1000 to 2000 ms)
124 processes. The asymmetric time dependence of the transmittance indicates that the oxidation and reducti on rates are different, with the oxidation process occurring faster than the reduction process, as indicated by the increased rate of reaching asymptotic limits. Figure 5 3 Chronoabsorptogram of ECP Magenta. Time dependence of ECP Magenta film at 545 nm with an applied 1 second potential square wave from 0.6V to +0.75V vs. Ag/Ag + in 1.0M TBABF 4 /ACN Analyses of switching times reveal the t 98% for the bleaching process (a transmittance of 67% T as indicated by the dotted line on the first switch) is a chieved in 400 ms, whereas the neutralization process ( a tr ansmittance of 21 % T ) is reached in 680 ms. The asymmetry is not unexpected as there are a variety of electronic and electrochemical factors ( e.g., differences in polymer conductivity, electric field strength, and ion diffusion rates ) for each redox state contributing to the speed at which the respective oxidation or reduction processes occur 174 176 However, the response times can be corrected through the balancing of oxidation and neutralization potential ranges
125 as suggested by the work of Pa dilla et al. 177 The authors compared the electrochromic potential window of PProDOT derivatives relative to current density, charge density, and percent transmittance, and revealed a specific potential window in which a high level of electroch romic contrast occurred and regions of inefficient electrochromic change. In addition, MacFarlan e et a l inves tigated the effects of electrolytes on the redox cycling of electrochromic materials 178 Electrolyte selection with respect to cation and anion size can optimize the mobility of the ions within the bulk of the electrochromic polymer, and the authors determined tosylate anion to be ideal for PEDOT, for example 179 ECP Green A second polymer, ECP Green, was chosen for analysis due to its structural similarities and differences to ECP Magenta. By introducing an electron deficient heterocycle 2,1,3 benzothiadiazole (B TD) into the ECP Magenta backbone, the absorption spectrum is consequently affected by the donor acceptor interaction, yet the bis(alkyloxy) substituted propylenedioxythiophene (PProDOT (CH 2 OR) 2 ) donors allow the low redox potential and high level of organ ic solvent processability. In the case of ECP Green, the donor acceptor effect contributes a dual band absorption centered at 443 nm and 682 nm as illustrated in Figure 5 4 a. The transmittance response exhibited in Figure 5 4 a consists of 25 total spectr a for a 0.5 second switch A de tailed analysis of the transmittance change at both absorption peaks illustrated in Figure 5 4 b reveals the bleaching process for both absorption peaks are nearly linear, and over 90% the electrochromic change to occur within the first 250 ms. This is followed by a plateau of the time dependence of the transmittance in the bleaching stage. By tracing the electrochromic onset at both wavelengths, as shown by the dotted lines in Figure 5 4 b, it can be seen that both band s bleach nearly simultaneously.
126 Figure 5 4 Spectral Evolution of ECP Green. a) ECP Green film (Abs at max = 0.7) switching from 0.6 V to 0.95 vs. Ag/Ag + with the full potential step and hold occurring within 0.5 seconds Spectrum shown every 20 ms for a total of 25 spectra in 500 ms. b) Time dependence of ECP Green transmittance at 443 nm and 682 nm, switching from the colored state to the bleached state. In the colored, neutral state ( A at max = 0.7), ECP Green has a tr ansmittance of 30% at the high energy band centered at 443 nm and 18% at the low energy band centered at 682 nm. The film was electrochemically switched between 0.6V and 0.95V vs. Ag/Ag + yielding transmittances of 70% and 58% at 443 nm and 682 nm respec tively
127 in the bleached state. In the case of dual band absorption donor acceptor ECPs, analysis of transmittance contrast and switching speeds need to be carried out at these two different absorption regions ideally simultaneously to ensure film quality and reproducibility. By extracting the transmittance responses at 443 nm and 682 nm as a function of time from Figure 5 4 a, the optical contrast and response time of ECP Green at both absorption bands is then compared in Figure 5 5 Figure 5 5 Chronoabsorptogram of ECP Green. Time dependence of ECP Green transmittance at 443 nm and 682 nm at 1 second potential square wave from 0.6V to 0.95V vs. Ag/Ag + in 1.0M TBABF 4 / ACN A significant observation seen in Figure 5 5 is the more symmetric respo nse of ECP Green (at both wavelengths) to that of ECP Magenta in comparing the bleaching (0 to 1000 ms) and the coloring (1000 to 2000 ms) processes. Rather than the asymmetric coloring/bleaching processes exhibited by ECP Magenta, the oxidation and neutr alization processes of ECP Green are both balanced and also exhibit a shorter response time as suggested by the earlier reached plateaus in the results of Figure 5 5 Further analyses of Figure 5 5 indicate the t 98% for oxidation and neutralization of ECP
128 Green at 682 nm are 420 ms and 330 ms, respectively. The switching speeds of ECP Green at both wavelengths are listed and compared with films of ECP Magenta, of the same optical density (at max ) and summarized in Table s 5 1 and 5 2 ECP Black As one of the most promising electrochromic polymers for a variety of window and display applications, the need to understand the optical properties and transitions during switching is important. We have recently reported a black to transmissive polymeric electroc hrome prepared by the random incorporation of monomeric units while controlling the polymerization feed ratios of donors to acceptors. This results in the polymeric backbone of ECP Black being randomly comprised of electron rich 3,4 propylenedioxythiophene (ProDOT) groups, as found in ECP Magenta, and the electron deficient 2,1,3 benzothiadiazole (BTD), as found in ECP Green. This results in the polymer having a broad absorption of the visible light from 400 to 700 nm in the neutral state, switching to a h igh level of transmission in the oxidized state, as shown in Figure 5 6 a. Without a distinct peak wavelength to probe the switching speed, 550 nm was selected as this is the wavelength at which the human eye is most sensitive to changes in light intensity 180 As shown by the transmittance profile in Figure 5 6 a, ECP Black is absorptive (21% T) in the colored, neutral state, spanning a large portion of the visible spectr um, and highly transmissive (71 % T) upon oxidation. It is important to note that Figure 5 6 shows only selected spectra (1 spectrum for every 24 ms for a total of 27 spectra) in order to clearly illustrate the dual band absorption at intermediate oxidation states. The complete transmittance profile is shown in Figure 5 6 b.
129 Figure 5 6 Spectral Evolution of ECP Black. a) Spectra of ECP Black film switching at a 0.5 second potential step (spectrum shown every 24 ms for a total of 27 spectra), along with the spectrum w hen the film is held longer at 0.85V vs. Ag/Ag + for a total of 5 s (dark solid line). b) ECP Black film (Abs at 550 = 0.7 a.u.) switching from 0.7 V to 0.85 vs. Ag/Ag + for 1 cycle at 0.5 second potential steps. Spectrum shown every 6 ms for a total of 83 spectra in 500 ms. An important aspect of the rapid spectroscopic technique is highlighted with ECP Black. As the film is electrochemically switched between the colored neutral and highly transmissive bleached states ( 0.7 V to +0.85V vs. Ag/Ag + ), ECP Black transitions from state. The temporal evolution of the dual band absorption can be tracked with
130 representative dotted lines shown in Figure 5 6 Starting in t he colored state with the broadly absorbing spectrum, this single broad absorption band evolves into a dual band after 12 spectra were collected (represented by 4 spectra). The total time to reach the start of the dual band transition is approximately 70 ms, and this green intermediate tone is present for approximately 100 ms before bleaching out into the transmissive state. It is worth mentioning that the green intermediate tone, with a lifetime of approximately 100 ms, is in essence imperceptible by the human eye, and is only now detectable with this spectroscopic technique. In addition, spectra captured in the bleached states at the 500 ms and 5000 ms mark are plotted in Figure 5 6 to illustrate the relative transmittance levels. As ECP Black is capab le of rapid switching in the sub second time frame, longer switching times (0.5 and 5 s) do not significantly improve the transmittance contrast as the polymer film is completely oxidized. The switching time of ECP Black is determined from the time depend ence of the t ransmittance illustrated in Figure 5 7 The oxidation and neutralization processes for ECP Black are near identical as suggested by the symmetric results, and the t 98% response times were determined to be 480 ms and 570 ms, respectively as su mmarized in Table s 5 1 and 5 2 Figure 5 7 Chronoabsorptogram of ECP Black monitored at 550 nm at 1 second potential square wave from 0.7V to 0.85V vs. Ag/Ag + in 1.0M TBABF 4 / ACN
131 Table 5 1 Summary of ECP transmittance contrast for ECP Magenta, Gr een and Black. Transmittance contrasts monitored when the potential square waves between the extreme states were held for 5s, 1s, 0.5s, 0.2s, and 0.1s. Transmittance Contrast ( % Polymer [a] 5s 1s 0.5s 0.2s ECP Magenta (545 nm) 50 48.8 48.4 39 ECP Green (443 nm) 44 41 40 30 ECP Green (682 nm) 44.4 43 41.6 33 ECP Black (550 nm) 50 49.5 48.5 40 [a] All polymer films were sprayed max Table 5 2 Summary of ECP switching time for ECP Magenta, Green and Black. Switching times tabulated at t 98% t 95% t 90% t 75% t 50% and t 25 % for colored to bleached and bleached to colored transitions. Polymer [a] Time (ms) Colored to Bleached Bleached to Colored t 98% t 95% t 90% t 75% t 50% t 25% t 98% t 95% t 90% t 75% t 50% t 25% ECP Magenta 545 nm 400 330 270 190 110 60 680 500 350 210 150 100 ECP Green 443 nm 400 330 275 200 125 60 340 320 300 260 200 150 ECP Green 682 nm 420 330 260 180 110 60 330 320 300 260 210 160 ECP Black 550 nm 480 400 330 230 160 100 570 500 440 370 290 190 [a] All polymer films were sprayed max Table 5 1 features the level of transmittance contrast attained at a specific switching rate. While the full and complete switch was achieved within 5 s for all ECPs, a significant trend observed is the high level of transmittance contrast retained when switching every 0.5 s. Using 5 seconds as the point of reference for a full switch the polymers were able to retain over 93% of the optical contrast whe n switched every second. At a switching time of 0.5 s (10x faster than the full switch ), the polymers retained over 90% of the transmittance contrast. Again, it is important to remember that these small percent transmittance differences are rather imperc eptible to the human
132 eye. Although, at times shorter than the 0.5 s mark, the optical contrast starts to steadily decline, while the polymers remaining responsive and demonstrating % the range of 14 40 %T. Even at the 0.1 s switch time ECP Magen ta remarkably displayed an optical contrast of 24 % Furthermore, Table 5 2 highlight s the time s in which a specific percentage of the transmittance is reached. The times were calculated at several different contrast levels to illustrate the interplay of switching speed and transmittance specifically at 98%, 95%, and 90% of the full optical contra st, to probe a region of perceived full optical change; 75% and 50% of the full optical contrast, where the bulk of electrochromism occurs; and finally the lower extreme of 25% of the full optical contrast to realize how fast the ECPs can respond. The maj ority of the transmittance contrast was reached within the 250 ms time frame ( i.e. at 90% of the full contrast) for ECP Magenta, ECP Green, and ECP Black, and the 98% contrast levels were achieved in less than 680 ms for both the coloring and bleaching pro cesses of all polymers. As the bulk of the electrochromic change (75% and 50% contrast) occurred between 150 ms 370 ms, this is the time frame in which most of the color change is noticeable. Finally, video rate was nearly accomplished at the 25% contr ast level where ECP Magenta exhibited a 60 ms coloring response time. Certainly parameters such as film morphology and doping/dedoping processes at the film electrolyte interface will need optimization to improve the optical contrast without sacrificing t he switching speed. Chapter Summary The need of an electronic spectroscopy technique to study electrochromic polymers in light of their rapid switching speed has prompted the enhanced in situ electrochemical and spectroscopy coupled instrumentation reporte d in C hapter 5 This
133 introduces a new methodology to investigate electrochromic materials by associating a time parameter with a specific spectrum during the electrochromic transition. As such, the technique has enabled (i) accurate measurements of fast polymer electrochromes, (ii) simultaneous analyses of transmittance at multiple wavelengths as a function of time, and (iii) the ability to detect intermediate color tones during an electrochromic switch and the times associated with them. Using this meth od to probe a series of solution processable electrochromic polymers, ECP Magenta, ECP Green, and ECP Black, the response times were determined on the range of 400 to 700 ms with high transmittance contrast. ECP Green was analyzed at both peak wavelengths simultaneously to demonstrate similar bleaching properties for both optical transitions. In addition, spectra of ECP Black indicated the black polymer film to switch through to an intermediate green tone before bleaching out in the transmissive state. W hile the capabilities of the electronic spectroscopy were demonstrated with electrochromic polymers, analysis of other rapidly switching electrochromic materials such as organic small molecules and inorganic metal oxides can be carried out with this instru mentation. With advancements in structure property relationship of polymer electrochromes, nanostructure templating of metal oxides, as well as optimization of doping dedoping processes, we envision the 50 ms milestone response time for electrochromic mat erials will be realized for video display applications.
134 CHAPTER 6 CONCLUSIONS AND FUTU RE WORK Research and development of conjugated polymer electronic devices represent many exciting new frontiers. Conjugated polymers offer a broad range of attractive properties that have stimulated fundamental and applied research for chemists, physicists, and en gineers. The application of conjugated polymers in low cost, flexible, and lightweight organic electronic device prototypes is a reality, and continued discoveries and optimizations will bring them closer to commercialization Electrochemical Supercapacito rs In this dissertation Type I supercapacitors were demonstrated with electron rich PProDOT Me 2 and PBEDOT isoindigo electrodes. Both electroactive materials were prepared electrochemically on gold/Kapton substrates after which their voltage window s and capacitance s were analyzed via cyclic voltammetry. Low operating voltage window s were observed in Type I PBEDOT isoindigo systems relative to Type I PProDOT Me 2 and other Type I CP based ESCs. To improve the voltage window, PBEDOT isoindigo electrodes were employed in Type III cells. In comparison to Type I ESCs, the voltage range increased over two fold; however electrochemical instability limited long term cycling. The need to further improve the voltage window with the use of Type III cells led to the modular assembly of ESCs. The fabrication of module ESCs was accomplished by connecting multiple Type I ESCs in series and in parallel, which respectively enhanced the voltage range and device capacitance. Ultimately, we envision electrochemical supercap acitors, while maintaining their flexible and lightweight architecture, as rollable and stackable devices that can meet the
135 power and energy needs of space limited applications. The ability to process highly capacitive, conjugated polymers will allow roll to roll production of modular ESCs. As such, while numerous conducting polymers can be utilized in supercapacitor modul es, highly conductive, electrochemically and mechanically stable conjugated polymers are ideal. Future research in conjugated polymer bas ed electrochemical supercapacitors should also focus on improving the film capacitance and pow er and energy densities. I n my opinion, the following ideas are promising: (1) development of conjugated polymer with side chain electroactive groups and (2) inco rporation of high surface area electrodes into CP based ESCs. Abruna recently introduced a hybrid PEDOT with pendant thioether functionalities as redox and charge storing sites. 181,182 The proposed concept is to improve the energy densities while maintaining the high switching rate. based on ProDOT for enhanced charge storage. The concept was to electropo lymerize the monomer and the dimer analogues to afford the polymers shown in Figure 6 1, and to extend the capacitance with various pendant phenyl groups. Figure 6 1 Proposed Poly(ProDOT) with various redox active pendan t groups for enhanced charge storage.
136 Although our electrochemical results were irreproducible and inconclusive, I believe the proper selection of pendant groups, such as naphthalene or oligothiophenes, will demonstrate enhanced charge storage. High surfac e electrodes, such as carbon foam, nickel foam, and single walled carbon nanotubes (SWNT), when combined with the redox capacitance of conjugated polymers, can improve device performance. Further, if the electrodes are thin, flexible and mechanically robus t, they may have potential for use in modular electrochemical supercapacitors. At the very least, the supercapacitor volume will increase with h igh surface area electrodes. We may eventually move away from our lamination encapsulation method and use a heat sealing and vacuum sealing pouch for device construction. Electrochromic Devices In this work, black to transmissive electrochromic windows based on black electrochrome s as the working electrode and minimally color changing polymer as the counter material were demonstrated with transmittance contrast s between 31 and 38 % T and luminance contrast s between 37 and 47 % Y. In the bleached state, the transmittan ce and luminance values were 88 % T and 80% Y respectively. The switching speed between the dark and bleached states ranged from 5 to 9 s at the 95% transmittance contrast, and sub second at the 50% transmittance contrast. Since one of the factors that control s switching speed corresponds to the rate at which counter ions penetrate through the film, op en film morphologies are desirable. Future work should be focused on creating a porous film to allow fast ion penetration with the intent of increasing the switching speed. This can be accomplished by
137 incorporating sacrificial additive s i n the spray castin g solution. By dissolution of sacrificial additives in the proper solvent, high film porosity may be achieved. Furthermore, by modifying the potential square wave technique with a multi step voltage profile, we may be able to facilitate rate of ion penetra tion through the polymer films. This can be carried out with a short initial voltage step, followed by an instantaneous drop to the final potential, as illustrated by the voltage profile model in Figure 6 2. Figure 6 2 Visual representation of voltage profile waveform. The black line represents the baseline applied potentials while the red lines represent the voltage spikes. By employing a large over potential for a short period we may be able to rapidly switch the fil m and/or device to a high contrast state, and maintain stability by immediately reducing the potential to one in which the device is fully oxidized or neutral, yet electrochemically stable. Chapter 5 introduced a novel electronic spectroscopy method to st udy rapidly switching electrochromic polymers. While this technique was applied to conjugated polymers, other rapidly switching electrochromic materials can also be studied. An extension of this technique would be to replace the current CCD array detector with an
138 advanced detector that extends to the NIR so that the kinetics of polaron and bipolaron formation can be elucidated.
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150 BIOGRAPHICAL SKETCH David Y. Liu was born in Taipei, Taiwan in September of 1983. He spent most of his early childhood in Taiwan, before moving to Orlando, Florida in 1990. After spending six years in Florida, David and his family moved to Raleigh, North Carolina. Once in NC, David attended Leesville Middle and High School. In the fall of 2002, David began his undergraduate education at the University of North Carolina at Chapel Hill (UNC C H ) as a chemistry major. In the summer of 2004, he began working as a technician in a biochemistry lab with Professor Andrew Lee. In 2005, he began conducting undergraduate research with Professor Muhammad Yousaf on self assembled monolayers. David graduat ed with a bachelor of science in chemistry from UNC in the spring of 2006. In the fall of 2006, David returned back to Florida and joined Professor John Reynolds' research group. He received his Ph.D. in chemistry from the University of Florida in December of 2011.