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Carbon nanotube and polypyrrole supercapacitors Izadi-Najafabadi, Ali 2006

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Carbon Nanotube and Polypyrrole Supercapacitors by A l i Izadi-Najafabadi B . A . S c , T h e Univers i ty of B r i t i s h C o l u m b i a , 2003 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F A P P L I E D S C I E N C E i n T H E F A C U L T Y O F G R A D U A T E S T U D I E S (Elec t r ica l and C o m p u t e r Engineering) T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A A p r i l 2006 © A l i Izadi-Najafabadi , 2006 11 Abstract Supercapacitors are electrochemical devices intended to combine the h igh power density of convent ional capacitors w i t h the h igh energy density of batteries. T h e y can store energy electrostat ically at the interface between a meta l electrode and an electrolyte (double layer capacitance) a n d / o r Faradaic energy storage through reversible successive redox processes (pseudocapaci-tance). T h e amount of accessible surface area of the electrode to the elec-t rolyte and the amount of t ime required for electronic charges and ionic charges to reach the electrode/electrolyte interface are the performance de-te rmin ing factors. 15 electrolytes are evaluated for their ionic conduc t iv i ty and potent ia l op-erat ing range. A m o n g organic electrolytes N a P F 6 dissolved i n acetonitr i le is determined to have the highest conduct iv i ty at 4.65 S / m . T h e operat ing voltage range of a l l electrolytes are l imi t ed due to impur i t ies . Different types of polypyrro le , a conduct ing polymer , exh ib i t ing double layer capacitance and pseudocapacitance are examined under different op-erat ing condi t ions. A m o n g the ten different types of po lypyrro le films eval-uated, the best film achieves a m a x i m u m discharge energy densi ty of 16.6 ( J /g ) and a m a x i m u m discharge power density of 1 .15(W/g) according to single electrode measurements. To improve the performance of the po lypyr -role films, composi te electrodes based on electrodeposi t ing polypyrro le on carbon fiber paper are produced. A cell based on these electrodes achieves a m a x i m u m discharge energy density of 12.5 ( J /g ) and a m a x i m u m discharge power density of 0.0134 ( W / g ) based on the mass of b o t h electrodes. A novel ca rbon nanotube film fabricat ion method is employed to assemble supercapacitor cells w i t h carbon nanotube electrodes. T h e best perform-ing cell achieves a m a x i m u m discharge energy densi ty of 4.79 ( J /g ) and a m a x i m u m discharge power density of 1.6 ( W / g ) . i i i Contents Abstract i i Contents i i i List of Tables v i i i List of Figures x i List of Abbreviations x x i i i Preface x x v Acknowledgements x x v i i 1 Introduction 1 1.1 Supercapacitors 1 1.2 A i m 2 1.3 Scope of the W o r k 4 2 Background 5 2.1 His to ry 5 2.2 Capac i to r Basics 9 3 Supercapacitor Electrochemistry 14 3.1 E lec t rochemica l C e l l 14 3.2 Elec t rochemica l Po ten t i a l 17 3.2.1 Ion Transpor t 18 3.2.2 Reac t ion K ine t i c s 19 3.3 Non-Farada ic and Faradaic Processes 21 3.4 C o m p a r i s o n of Capac i to r and Ba t t e ry 25 Contents i v 4 S t a t e o f t h e A r t 30 4.1 Industry 30 4.1.1 Patent Survey Resul ts 32 4.2 A c a d e m i a 35 4.3 Observations 41 5 E l e c t r o c h e m i c a l T e c h n i q u e s 43 5.1 Con t ro l l ed Curren t M e t h o d s 43 5.2 Con t ro l l ed Po ten t ia l Me thods 45 5.3 C y c l i c a l V o l t a m m e t r y 47 5.4 Impedance Spectroscopy 49 6 E l e c t r o l y t e s 53 6.1 In t roduct ion 53 6.2 Background 54 6.2.1 C o n d u c t i v i t y 54 6.2.2 Opera t ing Vol tage Range 58 6.2.3 Capaci tance 60 6.3 E x p e r i m e n t a l Setup 60 6.4 Expe r imen ta l Resul ts 66 6.4.1 Elect rolytes 66 6.4.2 Separaters 74 6.5 S u m m a r y 79 7 P o l y p y r r o l e 80 7.1 In t roduct ion 80 7.2 Synthesis Me thods 82 7.2.1 C h e m i c a l Depos i t ion 83 7.2.2 Elec t rochemica l Depos i t ion 85 7.3 E x p e r i m e n t a l Setup 89 7.4 E x p e r i m e n t a l Resul ts 93 7.4.1 C h e m 0 . 0 5 M 93 7.4.2 C h e m 0 . 1 M 112 7.4.3 C h e m 0 .148M 123 7.4.4 C h e m 0 . 1 7 5 M 135 7.4.5 Kane to 5min 146 7.4.6 Kane to 15min 156 7.4.7 Kane to 30min 167 Contents v 7.4.8 Y a m a u r a 5min 178 7.4.9 Y a m a u r a 15 m i n 188 7.4.10 Y a m a u r a 30 m i n 198 7.5 A n a l y s i s 207 7.5.1 O C P 207 7.5.2 I S F S L O W R E 209 7.5.3 C V 1 0 212 7.5.4 I S F S R E 213 7.5.5 G C 216 7.5.6 I S F S B 225 7.5.7 Potent ios ta t ic 227 7.6 S u m m a r y 233 8 Polypyrrole-Carbon Fiber Paper Electrode 235 8.1 In t roduct ion 235 8.2 Performance of C a r b o n F i b e r Paper 236 8.3 E lec t rodepos i t ion on C F P 236 8.4 G C Measurements on C F P - P P y Cel l s 238 8.5 S u m m a r y 242 9 Carbon Nanotube Film Electrode 243 9.1 In t roduct ion 243 9.2 M W N T Elect rodes 244 9.3 E x p e r i m e n t a l Setup 249 9.4 E x p e r i m e n t a l Resul ts 251 9.4.1 O C P Measurements 252 9.4.2 Impedance Spectroscopy 253 9.4.3 Ga lvan ic Cycles 259 9.4.4 I S F S Resul ts 274 9.5 S u m m a r y 276 10 Conclusion 278 •10.1 S u m m a r y of Resul ts 278 10.2 Recommendat ions 280 Bibliography 282 Contents v i A Supercapacitor Patent Survey 289 A . I U S patents on electrodes for electrochemical capacitors . . . . 290 A . 2 U S patents on electrolytes and separaters for electrochemical capacitors 327 A . 3 U S patents on overall design of an electrochemical capacitor . 359 B MATLAB Code 399 B . l Ga lvan i c Techniques 399 B . l . l G C m a i n 399 B . l . 2 vol tage.pc 400 B . 2 Potent ios ta t ic 401 B.2 .1 P S m a i n 401 B . 2.2 current.ps 402 C Electrolyte and Separator Results 403 C . l Elec t ro ly tes 403 C . l . l I S F S N O R E 403 C . l . 2 I S F S L O W R E 419 C .1 .3 I P S 435 C . 1.4 C V + I S F S R E . 465 C . 2 Separators 481 D Polypyrrole Results 496 D . l Transmiss ion E lec t ron Microscopy 496 D . 2 C h e m i c a l F i l m s 500 D . 2.1 0 . 0 5 M 500 D.2.2 0 . 1 M 507 D.2.3 0 .148M 514 D.2 .4 0 .175M 521 D . 3 K a n e t o F i l m s 528 D.3.1 5min 528 D.3 .2 15min 535 D.3 .3 30min 542 D.4 Y a m a u r a F i l m s 549 D.4.1 5min 549 D.4.2 15min 556 D.4.3 30min 563 Contents v i i E M W N T R e s u l t s 570 E . l C e l l 1 570 E . l . l I S F S Resul ts 570 E.1 .2 O C P Resul ts 571 E .2 C e l l 2 572 E.2.1 I S F S Resul ts 572 E.2 .2 O C P Resul ts 573 E . 3 C e l l 3 574 E.3.1 I S F S Resul ts 574 E.3 .2 O C P Resul ts 575 E .4 C e l l 4 576 E.4 .1 I S F S Resul t s 576 E.4 .2 O C P Resul ts 577 E . 5 C e l l 5 578 E.5.1 I S F S Resul ts 578 E.5 .2 O C P Resul ts 579 E .6 C e l l 6 580 E.6.1 I S F S Resul ts 580 E.6 .2 O C P Resul ts 581 V l l l List of Tables 2.1 Die lec t r ic constants of materials 11 2.2 B r e a k d o w n voltage of insulators 12 3.1 Types of capacitors 27 3.2 Types of batteries 28 3.3 C o m p a r i s o n of electrochemical capacitors and batteries . . . . 29 4.1 L i s t of supercapaci tor companies '. 31 4.2 Top 5 Supercapaci tor Companies 33 4.3 Top 10 electrochemical capacitor research groups 36 6.1 Factors de termining conductance of an electrolyte 57 6.2 P h y s i c a l properties of solvents 61 6.3 L i s t of electrolytes 62 6.4 E lec t ro ly te test ing protocol 63 6.5 Elec t ro ly te I S F S N O R E fi t t ing results 66 6.6 E lec t ro ly te conduc t iv i ty from I S F S N O R E 67 6.7 Molecu la r weights of salts 68 6.8 E lec t ro ly te I S F S L O W R E fi t t ing results . 69 6.9 E lec t ro ly te capacitance from I S F S N O R E and I S F S L O W R E . . 70 6.10 E lec t ro ly te non-ideal i ty from I S F S N O R E and I S F S L O W R E . 71 6.11 Measured potent ia l range of electrolytes 73 6.12 Separater in format ion 74 6.13 Separator impedance spectroscopy results . . . 75 6.14 Separator conduc t iv i ty results 76 6.15 Capac i tance associated w i t h separater /electrolyte combina-t ion from C V measurement 78 7.1 C h e m i c a l deposi t ion of polypyrrole : monomer concentrat ion vs. deposi ted po lymer mass 83 7.2 Po lypyr ro le est imated thickness 89 List of Tables ix 7.3 Polypyr ro le test ing protocol 92 7.4 C h e m 0 .05M G C 1 fit parameters 99 7.5 ' C h e m 0 .05M I S F S B fit results 103 7.6 C h e m 0 . 0 5 M I S F S B fit results 104 7.7 C h e m 0 . 0 5 M P S fit, parameters 106 7.8 C h e m 0 . 0 5 M G C 2 fit parameters 110 7.9 C h e m 0 . 1 M G C l fit parameters 116 7.10 C h e m 0 . 1 M I S F S B fit results 118 7.11 C h e m 0 . 1 M I S F S B fit results 118 7.12 C h e m 0 . 1 M P S fit parameters 119 7.13 C h e m 0 . 1 M G C 2 fit parameters 122 7.14 C h e m 0 .148M G C l fit, parameters 127 7.15 C h e m 0 .148M I S F S B fit results 130 7.16 C h e m 0 .148M I S F S B fit results 130 7.17 C h e m 0 .148M P S fit parameters 131 7.18 C h e m 0 .148M G C 2 fit parameters 134 7.19 C h e m 0 .175M G C l fit parameters 138 7.20 C h e m 0 .175M I S F S B fit results 140 7.21 C h e m 0 .175M I S F S B fit results 141 7.22 C h e m 0 .175M P S fit parameters 141 7.23 C h e m 0 .175M G C 2 fit parameters 145 7.24 K a n e t o 5 m i n G C l fit parameters 149 7.25 K a n e t o 5 m i n I S F S B fit results • 151 7.26 K a n e t o 5 m i n I S F S B fit results 151 7.27 K a n e t o 5 m i n P S fit parameters 153 7.28 K a n e t o 5 m i n G C 2 fit parameters 155 7.29 K a n e t o 15 m i n G C l fit parameters 159 7.30 K a n e t o 15 min I S F S B fit results 162 7.31 K a n e t o 15 m i n I S F S B fit results 162 7.32 Kane to 15 m i n P S fit parameters 162 7.33 K a n e t o 15 m i n G C 2 fit parameters 166 7.34 Kane to 30 m i n G C l fit parameters 170 7.35 Kane to 30 m i n I S F S B fit results 173 7.36 K a n e t o 30 m i n I S F S B fit results ( 173 7.37 Kane to 30 m i n P S fit parameters 174 7.38 Kane to 30 m i n G C 2 fit parameters 177 7.39 Y a m a u r a 5 m i n G C l fit parameters 181 7.40 Y a m a u r a 5 m i n I S F S B fit results 182 List of Tables x 7.41 Y a m a u r a 5 m i n I S F S B fit results 184 7.42 Y a m a u r a 5 m i n P S fit parameters 184 7.43 Y a m a u r a 5 m i n G C 2 fit parameters 188 7.44 Y a m a u r a 15 m i n G C 1 fit parameters 191 7.45 Y a m a u r a 15 m i n I S F S B fit, results 193 7.46 Y a m a u r a 15 m i n I S F S B fit results 193 7.47 Y a m a u r a 15 m i n P S fit parameters 194 7.48 Y a m a u r a 15 m i n G C 2 fit parameters 196 7.49 Y a m a u r a 30 m i n G C 1 fit parameters 201 7.50 Y a m a u r a 30 m i n I S F S B fit results 202 7.51 Y a m a u r a 30 m i n I S F S B fit results 203 7.52 Y a m a u r a 30 m i n P S fit, parameters 203 7.53 Y a m a u r a 30 m i n G C 2 fit parameters 207 7.54 Potent ios ta t ic results 232 7.55 S u m m a r y of capacitance values for polypyrrole films 233 8.1 F i t parameters for C F P + P P y cel l 240 9.1 Electrodes for the M W N T cells 247 9.2 M W N T cell testing p ro toco l 251 9.3 M W N T specific capaci tance based on impedance spectroscopy 258 9.4 F i t parameters for G C tests on M W N T cells 261 9.5 Discharge power, energy and efficiency of G C tests on M W N T cells 262 9.6 M W N T specific capacitance values 276 A . l N u m b e r of U S patents by countries 289 A . 2 U S patents on electrodes for electrochemical capacitors . . . . 290 A . 3 U S patents on electrolytes and separaters for electrochemical capacitors 327 A . 4 U S patents on overall design of an electrochemical capaci tor . 359 x i List of Figures 1.1 Ragone plot for electr ical energy storage devices 2 1.2 C o m p a r i s o n of commerc ia l supercapacitors and batteries . . . 3 2.1 Leyden jar 6 2.2 F i r s t e lectrolyt ic capaci tor 7 2.3 Bas ic capaci tor 9 2.4 Capac i to r discharge c i rcui t 13 2.5 C i r c u i t mode l for basic capacitors 13 3.1 Bas ic electrochemical cel l configurat ion 16 3.2 Free energy changes du r ing a react ion 20 3.3 Electrode-electrolyte interface 22 3.4 Bas ic mode l for an electro chemical capaci tor 23 3.5 Improved mode l for an electrochemical capacitor 23 3.6 P r a c t i c a l mode l for an electrochemical capacitor 24 3.7 St ructure of an electrochemical capacitor 26 4.1 N u m b e r of supercapacitor U S patents granted for the pe r iod of 1980-2001 32 5.1 Schematic of a cyc l ica l vo l t ammet ry profile 48 5.2 Nyqu i s t plot representation 51 6.1 I l lus t ra t ion of decomposi t ion l imi t s of an electrolyte as de-duced from a cycl ic vo l t ammet ry experiment 58 6.2 A p p r o x i m a t e potent ia l ranges of some aqueous and non-aqueous electrolytes 59 6.3 Elec t ro ly te test setup 65 7.1 Pyr ro l e monomer 82 7.2 T E M micrograph of polypyrro le 82 List of Figures x i i 7.3 S E M micrograph of a chemical ly deposited P P y 84 7.4 S E M micrograph of a Y a m a u r a P P y fi lm 86 7.5 S E M micrograph of a K a n e t o P P y film 87 7.6 Po lymer dry mass vs. deposi t ion t ime 88 7.7 C h e m 0 .05M C V 1 0 96 7.8 C h e m 0 . 0 5 M G C 1 98 7.9 C h e m 0 . 0 5 M G C 1 Ragone plot 100 7.10 C h e m 0 . 0 5 M I S F S B B o d e plot 102 7.11 C h e m 0 . 0 5 M Potent ios ta t ic ho ld at 0 V vs. R E 106 7.12 C h e m 0 . 0 5 M Paras i t ic current profile 108 7.13 C h e m 0 . 0 5 M G C 2 I l l 7.14 C h e m 0 . 0 5 M G C 2 Ragone plot 112 7.15 C h e m 0 . 1 M C V 1 0 113 7.16 C h e m 0 . 1 M G C 1 115 7.17 C h e m 0 . 1 M G C 1 Ragone plot 117 7.18 C h e m 0 . 1 M I S F S B Bode plot 117 7.19 C h e m 0 . 1 M Potent ios ta t ic ho ld at 0 V vs. R E 119 7.20 C h e m 0 . 1 M Paras i t ic current profile 120 7.21 C h e m 0 . 1 M G C 2 121 7.22 C h e m 0 . 1 M G C 2 Ragone plot 123 7.23 C h e m 0 .148M C V 1 0 124 7.24 C h e m 0 .148M G C 1 126 7.25 C h e m 0 .148M G C 1 Ragone plot 127 7.26 C h e m 0 .148M I S F S B Bode plot 129 7.27 C h e m 0 .148M Potent ios ta t ic ho ld at 0 V vs. R E 131 7.28 C h e m 0 .148M Paras i t ic current profile 132 7.29 C h e m 0 .148M G C 2 fit results 133 7.30 C h e m 0 .148M G C 2 Ragone plot 134 7.31 C h e m 0 .175M C V 1 0 136 7.32 C h e m 0 .175M G C l 137 7.33 C h e m 0 .175M G C l Ragone plot 138 7.34 C h e m 0 .175M I S F S B Bode plot 139 7.35 C h e m 0 .175M Potent ios ta t ic ho ld at 0 V vs. R E 142 7.36 C h e m 0 .175M Paras i t ic current profile 143 7.37 C h e m 0 .175M G C 2 144 7.38 C h e m 0 .175M G C 2 Ragone plot 145 7.39 K a n e t o 5 m i n C V 1 0 147 7.40 K a n e t o 5 m i n G C l 148 List of Figures xiii 7.41 K a n e t o 5 m i n G C l Ragone plot 149 7.42 K a n e t o 5 m i n I S F S B B o d e plot 150 7.43 K a n e t o 5 m i n Potent ios ta t ic ho ld at 0 V vs. R E 152 7.44 K a n e t o 5 m i n Paras i t ic current profile 153 7.45 K a n e t o 5 m i n G C 2 154 7.46 K a n e t o 5 m i n G C 2 Ragone plot 155 7.47 K a n e t o 15 m i n C V 1 0 157 7.48 K a n e t o 15 m i n G C l 158 7.49 K a n e t o 15 m i n G C l Ragone plot 159 7.50 K a n e t o 15 m i n I S F S B B o d e plot 161 7.51 K a n e t o 15 m i n Potent ios ta t ic ho ld at 0 V vs. R E 163 7.52 K a n e t o 15 m i n Paras i t ic current, profile 164 7.53 K a n e t o 15 m i n G C 2 165 7.54 K a n e t o 15 m i n G C 2 Ragone plot 166 7.55 K a n e t o 30 m i n C V 1 0 168 7.56 K a n e t o 30 m i n G C l 169 7.57 K a n e t o 30 m i n G C l Ragone plot 170 7.58 K a n e t o 30 m i n I S F S B B o d e plot 172 7.59 K a n e t o 30 m i n Potent ios ta t ic ho ld at 0 V vs. R E 174 7.60 K a n e t o 30 m i n Paras i t ic current profile 175 7.61 K a n e t o 30 m i n G C 2 176 7.62 K a n e t o 30 m i n G C 2 Ragone plot 177 7.63 Y a m a u r a 5 m i n C V 1 0 179 7.64 Y a m a u r a 5 m i n G C l 180 7.65 Y a m a u r a 5 m i n G C l Ragone plot 182 7.66 Y a m a u r a 5 m i n I S F S B B o d e plot 183 7.67 Y a m a u r a 5 m i n Potent ios ta t ic ho ld at 0 V vs. R E 185 7.68 Y a m a u r a 5 m i n Paras i t ic current profile 185 7.69 Y a m a u r a 5 m i n G C 2 187 7.70 Y a m a u r a 5 m i n G C 2 Ragone plot 187 7.71 Y a m a u r a 15 m i n C V 1 0 189 7.72 Y a m a u r a 15 m i n G C l 190 7.73 Y a m a u r a 15 m i n G C l Ragone plot 191 7.74 Y a m a u r a 15 m i n I S F S B B o d e plot 192 7.75 Y a m a u r a 15 m i n Potent ios ta t ic ho ld at 0 V vs. R E 194 7.76 Y a m a u r a 15 m i n Paras i t ic current profile 195 7.77 Y a m a u r a 15 m i n G C 2 197 7.78 Y a m a u r a 15 m i n G C 2 Ragone plot 197 List of Figures x i v 7.79 Y a m a u r a 30 m i n C V 1 0 198 7.80 Y a m a u r a 30 m i n G C l 200 7.81 Y a m a u r a 30 G C l Ragone plot 200 7.82 Y a m a u r a 30 m i n I S F S B Bode plot 202 7.83 Y a m a u r a 30 m i n Potent ios ta t ic ho ld at 0 V vs. R E 204 7.84 Y a m a u r a 30 m i n Paras i t ic current profile 205 7.85 Y a m a u r a 30 m i n G C 2 206 7.86 Y a m a u r a 30 m i n G C 2 Ragone plot 206 7.87 O p e n C i r c u i t Po ten t ia l fit parameters 208 7.88 I S F S L O W R E fit parameters 211 7.89 C V 1 0 Resul ts for C h e m i c a l F i l m s 212 7.90 C V 1 0 Resul ts for K a n e t o F i l m s . 213 7.91 C V 1 0 Resul ts for Y a m a u r a F i l m s 214 7.92 I S F S R E fit parameters 215 7.93 G C M a x discharge power and energy 217 7.94 G C M a x discharge power and energy densities 218 7.95 G C Average discharge power densities 219 7.96 Po lypyr ro le on G o l d 221 7.97 Po lypyr ro le par t ia l ly lifted off gold 223 7.98 Var i ab le R s and R F F i t Resul t 224 7.99 I S F S B fit, parameters 226 7.100 Potent ios ta t ic fit parameters 228 7.101 Potent ios ta t ic to ta l charge 229 7.102 Paras i t ic current 231 8.1 S E M M i c r o g r a p h of bare C F P 237 8.2 S E M M i c r o g r a p h of C F P coated w i t h P P y 238 8.3 G C F i t for C F P + P P y cel l 239 8.4 G C C F P + p p y Ragone plot 241 9.1 T E M M i c r o g r a p h of M W N T 245 9.2 S E M Micrographs of a M W N T f i lm formed on a separater . . 246 9.3 Assembly process of a M W N T cell 248 9.4 M W N T C e l l under compression 250 9.5 O C P F i t parameters 252 9.6 Ce l l s 1&4 I S F S N O C O M P R E S S I O N , I S F S C O M P R E S S I O N and I S F S L O W F R E Q Bode plots 254 List of Figures x v 9.7 C e l l 2 & 5 I S F S N O C O M P R E S S I O N , I S F S C O M P R E S S I O N and I S F S L O W F R E Q B o d e plots . . 255 9.8 C e l l 3&6 I S F S N O C O M P R E S S I O N , I S F S C O M P R E S S I O N and I S F S L O W F R E Q B o d e plots 256 9.9 F i t parameters for IS low freq tests 257 9.10 C e l l 1 G C 1 V - 0 V 263 9.11 C e l l 1 G C 1 . 5 V - 0 V 263 9.12 C e l l 1 G C 1 . 5 V - 1 . 5 V 263 9.13 C e l l 1 G C 1 V - 0 V Ragone plot 264 9.14 C e l l 1 G C 1 . 5 V - 0 V Ragone plot 264 9.15 C e l l 4 G C 1 V 0 V 265 9.16 C e l l 4 G C 1 . 5 V 0 V 265 9.17 C e l l 4 G C 1 . 5 V 1 . 5 V 265 9.18 C e l l 4 G C 1 V 0 V Ragone plot 266 9.19 C e l l 4 G C 1 V 0 V Ragone plot 266 9.20 C e l l 2 G C 1 V 0 V 267 9.21 C e l l 2 G C 1 . 5 V 0 V 267 9.22 C e l l 2 G C 1 . 5 V 1 . 5 V 267 9.23 C e l l 2 G C 1 V 0 V Ragone plot 268 9.24 C e l l 2 G C 1 . 5 V 0 V Ragone plot 268 9.25 C e l l 5 G C 1 V 0 V 269 9.26 C e l l 5 G C 1 . 5 V 0 V 269 9.27 C e l l 5 G C 1 . 5 V 1 . 5 V 269 9.28 C e l l 5 G C 1 V 0 V Ragone plot 270 9.29 C e l l 5 G C 1 . 5 V 0 V Ragone plot 270 9.30 C e l l 3 G C 1 V 0 V 271 9.31 C e l l 3 G C 1 . 5 V 0 V 271 9.32 C e l l 3 G C 1 . 5 V 1 . 5 V 271 9.33 C e l l 3 G C 1 V 0 V Ragone plot 272 9.34 C e l l 3 G C 1 . 5 V 0 V Ragone plot 272 9.35 C e l l 6 G C 1 V 0 V 273 9.36 C e l l 6 G C 1 . 5 V 0 V 273 9.37 C e l l 6 G C 1 . 5 V 1 . 5 V 273 9.38 C e l l 6 G C 1 V 0 V Ragone plot 274 9.39 C e l l 6 G C 1 . 5 V 0 V Ragone plot 274 9.40 F i t parameters for I S F S 275 9.41 Discharge power and energy densities for M W N T cells . . . . 277 List of Figures x v i C . l T B A P + A N I S F S N O R E F i t 1 404 C .2 T B A P + A N I S F S N O R E F i t 2 404 C .3 T B A P + P C I S F S N O R E F i t 1 405 C .4 T B A P + P C I S F S N O R E F i t 2 405 C .5 T E A P + A N I S F S N O R E F i t 1 406 C .6 T E A P + A N I S F S N O R E F i t 2 406 C . 7 T E A P + P C I S F S N O R E F i t 1 407 C .8 T E A P + P C I S F S N O R E F i t 2 407 C . 9 T B A C + A N I S F S N O R E F i t 1 408 C I O T B A C + A N I S F S N O R E F i t 2 408 C . l l T B A C + P C I S F S N O R E F i t 1 409 C.12 T B A C + P C I S F S N O R E F i t 2 409 C.13 T E A C + A N I S F S N O R E F i t 1 410 C.14 T E A C + A N I S F S N O R E F i t 2 410 C.15 T E A C + P C I S F S N O R E F i t 1 411 C.16 T E A C + P C I S F S N O R E F i t 2 411 C.17 T B A B + A N I S F S N O R E F i t 1 412 C.18 T B A B + A N I S F S N O R E F i t 2 412 C.19 T B A B + P C I S F S N O R E F i t 1 413 C.20 T B A B + P C I S F S N O R E F i t 2 413 C.21 N a P F 6 + A N I S F S N O R E F i t 1 414 C.22 N a P F 6 + A N I S F S N O R E F i t 2 414 C.23 N a P F 6 A Q I S F S N O R E F i t 1 415 C.24 N a P F 6 A Q I S F S N O R E F i t 2 415 C.25 B M I B I S F S N O R E F i t 1 416 C.26 B M I B I S F S N O R E F i t 2 416 C.27 B M I P I S F S N O R E F i t 1 417 C.28 B M I P I S F S N O R E F i t 2 417 C.29 E M I T F S I I S F S N O R E F i t 1 418 C.30 E M I T F S I I S F S N O R E F i t 2 418 C.31 T B A P + A N I S F S L O W R E F i t 1 420 C.32 T B A P + A N I S F S L O W R E F i t 2 420 C.33 T B A P + P C I S F S L O W R E F i t 1 421 C.34 T B A P + P C I S F S L O W R E F i t 2 421 C.35 T E A P + A N I S F S L O W R E F i t 1 422 C.36 T E A P + A N I S F S L O W R E F i t 2 422 C.37 T E A P + P C I S F S L O W R E F i t 1 423 C.38 T E A P + P C I S F S L O W R E F i t 2 423 List of Figures x v i i C.39 T B A C + A N I S F S L O W R E F i t 1 424 C.40 T B A C + A N I S F S L O W R E F i t 2 424 C.41 T B A C + P C I S F S L O W R E F i t 1 425 C.42 T B A C + P C I S F S L O W R E F i t 2 425 C.43 T E A C + A N I S F S L O W R E F i t 1 426 C.44 T E A C + A N I S F S L O W R E F i t 2 426 C.45 T E A C + P C I S F S L O W R E F i t 1 427 C.46 T E A C + P C I S F S L O W R E F i t 2 427 C.47 T B A B + A N I S F S L O W R E F i t 1 428 C.48 T B A B + A N I S F S L O W R E F i t 2 428 C.49 T B A B + P C I S F S L O W R E F i t 1 429 C.50 T B A B + P C I S F S L O W R E F i t 2 429 C.51 N a P F 6 + A N I S F S L O W R E F i t 1 430 C.52 N a P F 6 + A N I S F S L O W R E F i t 2 430 C.53 N a P F 6 A Q I S F S L O W R E F i t 1 431 C .54 N a P F 6 A Q I S F S L O W R E F i t 2 431 C.55 B M I B I S F S L O W R E F i t 1 432 C.56 B M I B I S F S L O W R E F i t 2 432 C.57 B M I P I S F S L O W R E F i t 1 433 C.58 B M I P I S F S L O W R E F i t 2 433 C.59 E M I T F S I I S F S L O W R E F i t 1 434 C.60 E M I T F S I I S F S L O W R E F i t 2 434 C.61 T B A P + A N I P S l O m H z B o d e plot 436 C.62 T B A P + A N I P S l O O m H z B o d e plot 436 C.63 T B A P + A N I P S l O m H z Impedance plot 437 C.64 T B A P + A N I P S l O O m H z Impedance plot 437 C.65 T B A P + P C I P S l O m H z B o d e plot 438 C.66 T B A P + P C I P S l O O m H z Bode plot 438 C.67 T B A P + P C I P S l O m H z Impedance plot 439 C .68 T B A P + P C I P S l O O m H z Impedance plot 439 C.69 T E A P + A N I P S l O m H z B o d e plot 440 C.70 T E A P + A N I P S l O O m H z B o d e plot 440 C.71 T E A P + A N I P S l O m H z Impedance plot 441 C.72 T E A P + A N I P S l O O m H z Impedance plot 441 C.73 T E A P + P C I P S l O m H z B o d e plot 442 C.74 T E A P + P C I P S l O O m H z B o d e plot 442 C.75 T E A P + P C I P S l O m H z Impedance plot 443 C.76 T E A P + P C I P S l O O m H z Impedance plot 443 List of Figures x v i i i C .77 T B A C + A N I P S l O m H z B o d e plot 444 C.78 T B A C + A N I P S l O O m H z B o d e plot 444 C.79 T B A C + A N I P S l O m H z Impedance plot 445 C.80 T B A C + A N I P S l O O m H z Impedance plot 445 C.81 T B A C + P C I P S l O m H z B o d e plot 446 C.82 T B A C + P C I P S l O O m H z B o d e plot 446 C.83 T B A C + P C I P S l O m H z Impedance plot 447 C.84 T B A C + P C I P S l O O m H z Impedance plot 447 C.85 T E A C + A N I P S l O O m H z B o d e plot 448 C.86 T E A C + A N I P S l O O m H z Impedance plot 448 C.87 T E A C + P C I P S l O m H z B o d e plot 449 C.88 T E A C + P C I P S l O O m H z B o d e plot 449 C.89 T E A C + P C I P S l O m H z Impedance plot 450 C.90 T E A C + P C I P S l O O m H z Impedance plot 450 C.91 T B A B + A N I P S l O m H z B o d e plot 451 C.92 T B A B + A N I P S l O O m H z B o d e plot 451 C.93 T B A B + A N I P S l O m H z Impedance plot 452 C.94 T B A B + A N I P S l O O m H z Impedance plot 452 C.95 T B A B + P C I P S l O m H z B o d e plot 453 C.96 T B A B + P C I P S l O O m H z B o d e plot 453 C .97 T B A B + P C I P S l O m H z Impedance plot 454 C.98 T B A B + P C I P S l O O m H z Impedance plot 454 C.99 N a P F e + A N I P S l O m H z B o d e plot 455 C . 1 0 0 N a P F 6 + A N I P S l O O m H z B o d e plot 455 C . l O l N a P F g + A N I P S l O m H z Impedance plot 456 C 1 0 2 N a P F 6 + A N I P S l O O m H z Impedance plot 456 C . 1 0 3 N a P F 6 A Q I P S l O m H z B o d e plot 457 C 1 0 4 N a P F 6 A Q I P S l O O m H z B o d e plot 457 C . 1 0 5 N a P F 6 A Q I P S l O m H z Impedance plot 458 C . 1 0 6 N a P F 6 A Q I P S l O O m H z Impedance plot 458 C 1 0 7 B M I B I P S l O m H z B o d e plot 459 C . 1 0 8 B M I B I P S l O O m H z B o d e plot 459 C . 1 0 9 B M I B I P S l O m H z Impedance plot 460 C . 1 1 0 B M I B I P S l O O m H z Impedance plot 460 C . 1 1 1 B M I P I P S l O m H z B o d e plot 461 C . 1 1 2 B M I P I P S l O O m H z B o d e plot 461 C . 1 1 3 B M I P I P S l O m H z Impedance plot 462 C 1 1 4 B M I P I P S l O O m H z Impedance plot 462 List of Figures x i x C.115 E M I T F S I I P S l O m H z B o d e plot 463 C . 1 1 6 E M I T F S I I P S l O O m H z B o d e plot 463 C . 1 1 7 E M I T F S I I P S l O m H z Impedance plot 464 C . 1 1 8 E M I T F S I I P S l O O m H z Impedance plot 464 C . 1 1 9 T B A P + A N C V + I S F S R E 466 C . 120 T B A P + P C C V + I S F S R E 467 C.121 T E A P + A N C V + I S F S R E 468 C . 1 2 2 T E A P + P C C V + I S F S R E 469 C.123 T B A C + A N C V + I S F S R E 470 C . 1 2 4 T B A C + P C C V + I S F S R E 471 C . 1 2 5 T E A C + A N C V 472 C.126 T E A C + P C C V + I S F S R E 473 C . 1 2 7 T B A B + A N C V + I S F S R E 474 C . 1 2 8 T B A B + P C C V + I S F S R E 475 C . 1 2 9 N a P F 6 + A N C V + I S F S R E 476 C . 1 3 0 N a P F 6 A Q C V + I S F S R E 477 C.131 B M I B C V + I S F S R E 478 C . 132 B M I P C V + I S F S R E 479 C . 133 E M I T F S I C V + I S F S R E 480 C . 134 Resul ts for 2 5 0 0 M M + T E A P + P C 482 C . 135 Resul ts for E D + N A P F 6 + A N 483 C . 136 Resul ts for E D + T E A P + P C 484 C . 137 Resul ts for E C + N a P F 6 + A N 485 C . 138 Resul ts for E C + T E A P + P C 486 C . 139 Resul ts for E D G 3 + B M I B 487 C.140 Resul ts for E D G 3 + B M I P 488 C.141 Resul ts for E D G 3 + E M I T F S I 489 C.142 Resul ts for E D G 3 + A q N a P F 6 490 C . 143 Resul ts for E D G 3 + N a P F 6 + A N 491 C . 144Resul ts for E D G 3 + T E A P + P C 492 C .145Resu l t s for N Z A 1 1 0 + T E A P + P C 493 C . 146 Resul ts for T F 4 4 - 2 5 + N a P F 6 + A N 494 C . 147 Resul ts for T F 4 4 - 2 5 + T E A P + P C " . . 495 D . l E lec t rodepos i t ion setup for T E M sample 497 D . 2 Depos i t ion profile for T E M sample 498 D . 3 Impedance spectroscopy o n T E M gr id 498 D . 4 T E M gr id after deposi t ion 499 List of Figures xx D . 5 C h e m 0 . 0 5 M I S F S N O R E F i t 1 501 D .6 C h e m 0 . 0 5 M I S F S N O R E F i t 2 501 D . 7 C h e m 0 . 0 5 M I S F S L O W R E F i t 1 502 D .8 C h e m 0 . 0 5 M I S F S L O W R E F i t 2 502 D . 9 C h e m 0 . 0 5 M 1 S F S M I D R E F i t 1 503 D.10 C h e m 0 . 0 5 M I S F S M I D R E F i t 2 503 D . l l C h e m 0 . 0 5 M I S F S R E 504 D.12 C h e m 0 . 0 5 M O p e n C i r c u i t Po ten t ia l Profi le 505 D.13 C h e m 0 . 0 5 M Galvanos ta t ic and Potent ios ta t ic Resul ts 506 D.14 C h e m 0 . 1 M I S F S N O R E F i t 1 508 D.15 C h e m 0 . 1 M I S F S N O R E F i t 2 508 D.16 C h e m 0 . 1 M I S F S L O W R E F i t 1 509 D.17 C h e m 0 . 1 M I S F S L O W R E F i t 2 509 D.18 C h e m 0 . 1 M I S F S M I D R E F i t 1 510 D.19 C h e m 0 . 1 M I S F S M I D R E F i t 2 510 D.20 C h e m 0 . 1 M I S F S R E 511 D.21 C h e m 0 . 1 M O p e n C i r c u i t Po ten t i a l Profi le 512 D.22 C h e m 0 . 1 M Galvanos ta t ic and Potent ios ta t ic Resul ts 513 D.23 C h e m 0 .148M I S F S N O R E F i t 1 515 D.24 C h e m 0 .148M I S F S N O R E F i t 2 515 D.25 C h e m 0 .148M I S F S L O W R E F i t 1 516 D.26 C h e m 0 .148M I S F S L O W R E F i t 2 516 D.27 C h e m 0 .148M I S F S M I D R E F i t 1 517 D.28 C h e m 0 .148M I S F S M I D R E F i t 2 517 D.29 C h e m 0 .148M I S F S R E 518 D.30 C h e m 0 .148M Open C i r c u i t Po ten t ia l Profi le 519 D.31 C h e m 0 .148M Galvanos ta t ic and Potent ios ta t ic Resul ts . . . . 520 D.32 C h e m 0 .175M I S F S N O R E F i t 1 522 D.33 C h e m 0 .175M I S F S N O R E F i t 2 522 D.34 C h e m 0 .175M I S F S L O W R E F i t 1 523 D.35 C h e m 0 .175M I S F S L O W R E F i t 2 523 D.36 C h e m 0 .175M I S F S M I D R E F i t 1 524 D.37 C h e m 0 .175M I S F S M I D R E F i t 2 524 D.38 C h e m 0 .175M I S F S R E 525 D.39 C h e m 0 .175M O p e n C i r c u i t Po ten t i a l Profi le 526 D.40 C h e m 0 .175M Galvanos ta t ic and Potent ios ta t ic Resul ts . . . . 527 D.41 K a n e t o 5 m i n I S F S N O R E F i t 1 529 D.42 K a n e t o 5 m i n I S F S N O R E F i t 2 529 List of Figures x x i D.43 K a n e t o 5 m i n I S F S L O W R E F i t 1 530 D.44 K a n e t o 5 m i n I S F S L O W R E F i t 2 530 D.45 K a n e t o 5 m i n I S F S M I D R E F i t 1 531 D.46 K a n e t o 5 m i n I S F S M I D R E F i t 2 531 D.47 K a n e t o 5 m i n I S F S R E 532 D.48 K a n e t o 5 m i n O p e n C i r c u i t Po ten t i a l Profi le 533 D.49 K a n e t o 5 m i n Galvanos ta t ic and Potent ios ta t ic Resul ts . . . . 534 D.50 K a n e t o 15 m i n I S F S N O R E F i t 1 536 D.51 K a n e t o 15 m i n I S F S N O R E F i t 2 536 D.52 K a n e t o 15 m i n I S F S L O W R E F i t 1 . 5 3 7 D.53 K a n e t o 15 m i n I S F S L O W R E F i t 2 537 D.54 K a n e t o 15 m i n I S F S M I D R E F i t 1 538 D.55 K a n e t o 15 m i n I S F S M I D R E F i t 2 538 D.56 K a n e t o 15 m i n I S F S R E 539 D.57 K a n e t o 15 m i n O p e n C i r c u i t Po ten t i a l Profi le 540 D.58 K a n e t o 15 m i n Galvanos ta t ic and Potent ios ta t ic Resul ts . . . 541 D.59 K a n e t o 30 m i n I S F S N O R E F i t 1 543 D.60 K a n e t o 30 m i n I S F S N O R E F i t 2 543 D.61 K a n e t o 30 m i n I S F S L O W R E F i t 1 544 D.62 K a n e t o 30 m i n I S F S L O W R E F i t 2 544 D.63 K a n e t o 30 m i n I S F S M I D R E F i t 1 545 D.64 K a n e t o 30 m i n I S F S M I D R E F i t 2 545 D.65 K a n e t o 30 m i n I S F S R E . . . 546 D.66 K a n e t o 30 m i n O p e n C i r c u i t Po ten t i a l Profi le 547 D.67 K a n e t o 30 m i n Galvanos ta t ic and Potent ios ta t ic Resul ts . . . 548 D.68 Y a m a u r a 5 m i n I S F S N O R E F i t 1 550 D.69 Y a m a u r a 5 m i n I S F S N O R E F i t 2 550 D.70 Y a m a u r a 5 m i n I S F S L O W R E F i t 1 551 D.71 Y a m a u r a 5 m i n I S F S L O W R E F i t 2 551 D.72 Y a m a u r a 5 m i n I S F S M I D R E F i t 1 552 D.73 Y a m a u r a 5 m i n I S F S M I D R E F i t 2 552 D.74 Y a m a u r a 5 m i n I S F S R E 553 D.75 Y a m a u r a 5 m i n O p e n C i r c u i t Po ten t i a l Profi le 554 D.76 Y a m a u r a 5 m i n Galvanos ta t ic and Potent ios ta t ic Resul ts . . . 555 D.77 Y a m a u r a 15 m i n I S F S N O R E F i t 1 557 D.78 Y a m a u r a 15 m i n I S F S N O R E F i t 2 557 D.79 Y a m a u r a 15 m i n I S F S L O W R E F i t 1 558 D.80 Y a m a u r a 15 m i n I S F S L O W R E F i t 2 558 List of Figures x x i i D.81 Y a m a u r a 15 m i n I S F S M I D R E F i t 1 559 D.82 Y a m a u r a 15 m i n I S F S M I D R E F i t 2 559 D .83 Y a m a u r a 15 m i n I S F S R E 560 D.84 Y a m a u r a 15 m i n O p e n C i r c u i t Po ten t i a l Profi le 561 D.85 Y a m a u r a 15 m i n Galvanos ta t ic and Potent ios ta t ic Resul ts . . 562 D.86 Y a m a u r a 30 m i n I S F S N O R E F i t 1 564 D.87 Y a m a u r a 30 m i n I S F S N O R E F i t 2 564 D.88 Y a m a u r a 30 m i n I S F S L O W R E F i t 1 565 D.89 Y a m a u r a 30 m i n I S F S L O W R E F i t 2 565 D.90 Y a m a u r a 30 m i n I S F S M I D R E F i t 1 566 D.91 Y a m a u r a 30 m i n I S F S M I D R E F i t 2 566 D.92 Y a m a u r a 30 m i n I S F S R E 567 D.93 Y a m a u r a 30 m i n O p e n C i r c u i t Po ten t ia l Profi le 568 D . 94 Y a m a u r a 30 m i n Galvanosta t ic and Potent ios ta t ic Resul ts . . 5 6 9 E . l C e l l 1 I S F S B o d e plots 570 E .2 C e l l 1 O C P Resul ts 571 E . 3 C e l l 2 I S F S B o d e plots 572 E .4 C e l l 2 O C P Resul ts 573 E .5 C e l l 3 I S F S B o d e plots 574 E .6 C e l l 3 O C P Resul ts 575 E . 7 C e l l 4 I S F S Bode plots 576 E .8 C e l l 4 O C P Resul ts 577 E . 9 C e l l 5 I S F S B o d e plots 578 E .10 C e l l 5 O C P Resul ts 579 E . l l C e l l 6 I S F S B o d e plots 580 E.12 C e l l 6 O C P Resul ts 581 XX111 List of Abbreviations A N Acetonitr i le or acceptor number B M I B i-butyl-3-niethyl- imidazol ium tetrafluoroborate B M I P l-butyl-3-methyl- imidazol iurn hexafhiorophosphate C B M Carbon based materials C E Counter electrode C F P Carbon fiber paper C N T Carbon nanotube C P Conduct ing polymer C P E Constant phase element C V Cyc l ica l vol tammetry D N Donor number E M I T F S I l -ethyl-3-methyl- imidazol ium bis(trif luoromethane sulfonyl)imide G C Galvanostat ic cycl ing IS Impedance spectroscopy ISFS Impedance spectroscopy frequency scan I S F S B Impedance spectroscopy frequency scan wi th applied D C bias I S F S L O W R E Impedance spectroscopy frequency scan down to l m H z using reference electrode I S F S M I D R E Impedance spectroscopy frequency scan down to lOmHz using reference electrode I S F S N O R E Impedance spectroscopy frequency scan wi th no reference electrode IPS Impedance potential spectroscopy M W N T Mul t i -wa l l carbon nanotube N C Negative current (discharging cycle) O C P Open circuit potential P C Propylene carbonate or positive current (charging cycle) P S Potentiostat ic P P y Polypyrrole R E Reference electrode S C E Saturated calomel electrode S E M Scanning electron microscope / microscopy S H E Standard hydrogen electrode S W N T Single-wall carbon nanotube T B A B Tetrabuty lammonium tetrafluoroborate T B A C Tetrabuty lammonium perchlorate T B A P Tetrabuty lammonium hexafluorophosphate T E A C Tetraethylammonium perchlorate T E A P Tetraethylainmoniurn hexafluorophosphate T E M Transmission Electron Microscope / Microscopy List of Abbreviations xxiv T M N Transi t ion metal ni tr ide T M O Transi t ion metal oxide W E Work ing electrode X X V Preface T h i s thesis has been prepared according to the standards set by the fac-u l ty of graduate studies for the degree of master of appl ied science. T h e style deviates from a t rad i t iona l thesis and is closer to a manuscr ip t thesis. T h e intended objectives or audience for the thesis can be broken into three cate-gories. F i r s t and foremost, i t is hoped that this document outlines the efforts undertaken to achieve the target degree. Chapters 6-10 where the experimen-ta l work is discussed could be used to evaluate the work performed. G i v e n the size of the text, a brief summary is presented at each of the exper imental chapters (CH6-9) , to make the results more accessible. T h e second objective is to provide as much relevant in format ion as possible to the current project sponsor, E P O D I N C . (and po ten t ia l future sponsors) i n terms of what has been achieved here and elsewhere i n the field. Chapte r 4 and the appendix A t ry to present what others have accomplished especially from an indus t r i a l perspective; while chapters 6-10 present what has been achieved here. T h e final objective has been to provide sufficient informat ion for new graduate students p lann ing to continue this work. Those interested i n the field are s trongly recommended to also read the book by professor C o n w a y 1 of Univers i ty of O t t a w a [2]. W h e n I started graduate studies, m y intent ion was to focus on synthesis of carbon nanotubes and nanoelectronic devices based on them. However to-ward the end of the first year of the masters program, an oppor tun i ty arose to work on the supercapacitor project. A s t ime went on, more funding oppor-tuni t ies were presented for the supercapacitor project, whi le the synthesis project hi t many obstacles. A s such I t r ied (somewhat i n vain) to pursue b o t h projects. Due to personal interest i n the synthesis project more t ime was spent on the synthesis project, especially i n keeping up to date w i t h the Sadly professor Conway passed away on Ju ly 9, 2005 Preface x x v i scientific l i terature. However i n the end the supercapacitor project proved fruitful and as such the thesis is based on that work. B e y o n d lack of any presentable results (excluding the molecular d y n a m i c s imulat ions); a negative result of the emphasis on the synthesis work was a lower degree of awareness of scientific l i terature related to supercapacitors. T h i s p roblem was somewhat addressed dur ing the latter part of the experi -menta l work. D u r i n g the w r i t i n g of this thesis, i t was hoped to rectify the problem completely by presenting a l l relevant l i terature. A no the r result of this approach is the t ime line for the experiments: T h e polypyrrole-carbon fiber paper experiments discussed i n chapter 7 were started i n A p r i l of 2004 and they were performed on and off t i l l January of 2005. T h e experiments regarding electrolytes and separators as discussed i n chap-ter 6 were predominant ly performed dur ing the summer of 2005, whi le a few experiments were carr ied out i n 2004 as wel l . T h e polypyrro le experiment discussed i n chapter 6 were performed i n J u l y - A u g u s t 2005. T h e m u l t i w a l l nanotube experiments discussed i n chapter 9 were performed i n A u g u s t of 2005; while some experiments not presented here were performed earlier. W h i l e the exper imenta l results presented here are not spectacular; there is every ind ica t ion to believe that the field of supercapacitors is ever ex-panding and requires greater at tention. T h e probabi l i ty of any more great breakthroughs i n the future has d iminished; however there is a great deal of improvements left to be achieved i n order to fully u t i l ize the po ten t ia l of supercapacitors. W h a t por t ion of this work w i l l be carr ied out by indus t ry vs. academia remains to be seen. XXV11 Acknowledgements T h e work presented here has benefited from substant ia l guidance and as-sistance from various sources. F i r s t and foremost I would like to express m y grat i tude toward those involved i n m y undergraduate thesis project, wh ich formed the basis for m y entry to graduate school and the work presented here. I wou ld l ike to thank M r . H o k T i n E d d y Yeung , my undergraduate thesis project partner, and professors D a n B i z z o t t o , George Sawatzky and M i c h a e l Wolf , the sponsors of that project. I w o u l d l ike to express m y grat i tude toward professor D a v i d Pulfrey, who played a c ruc ia l role i n my admission to the graduate program at the electr ical and computer engineering department and mentored me dur ing the program. W h i l e this thesis does not cover the work performed on molecular dynamic s imulat ions of carbon nanotubes, I wou ld l ike to acknowledge the tireless efforts of professor Wal te r Scott for teaching me this topic and helping me i n cod ing the s imulat ions. I intend to base m y future P h D work on this topic. D u r i n g the undergraduate thesis project, I had the oppor tun i ty to meet and learn from some of the graduate students of the project sponsors; and these students continued to greatly assist me dur ing this work as well . D r . A n d r a s P a t t a n t y u s - A b r a h a m , has always been a reliable source for any ques-tions i n the rea lm of physics and chemistry. A substant ia l por t ion of the ex-per imenta l work presented here would have not been possible wi thou t some sort of assistance from professor B izzo t t o ' s group. M r . R o b i n Stoodley has been m y electrochemistry handbook whi le I have also been greatly helped by professor B izzo t t o ' s other students: M s . A m a n d a Musgrove and M s . A y a Sode. I a m also very grateful toward D r . M a r i o Beaudo in , the manager of the clean r o o m facility, for teaching me many mic ro fabricat ion techniques and always responding to m y 11th hour equipment problems. I wou ld l ike to Acknowledgements x x v i i i express my apprecia t ion toward D r . L i Y a n g (Nano Imaging faci l i ty of the S imon Fraser Univers i ty) for teaching me electron microscopy and p rov id ing guidance dur ing the imaging sessions. D u r i n g this work, I had the pleasure of work ing w i t h some except ional undergraduate students. M s . D a w n Tse H u i T a n worked on the polypyrro le-carbon fiber composites por t ion of this work (chapter 7) as a summer student i n 2004. Her work led to a pub l i ca t ion [1]. Subsequently she performed her undergraduate thesis project evaluat ing separators, wh ich led to procurement of c rucia l separators from Gore . M s . J o a n n a W i n g Y u L a m worked tirelessly dur ing the summer of 2005, sacrificing many weekends and evenings, exam-in ing new electrolytes and separators (chapter 5) and evaluat ing fabr icat ion methods of carbon nanotube films (chapter 8). She continues to be work ing on the supercapacitor project. D u r i n g summer of 2005, M r . D a v i d Evans , an N S E R C summer student worked on different growth methods of po lypyr ro le for actuators and supercapacitors. H i s exper imenta l insight and enthusiasm greatly benefited the experiments discussed i n chapter 6. I wou ld l ike to thank m y supervisor, professor J o h n M a d d e n , for this great opportuni ty . Toward the rest of the lab members, I owe m y grat i tude for their constant support: T o M r . A r a s h Taksh i , I a m very grateful for your car ing and attentive nature; thank y o u very much for a l l your guidance. T o M s . M y a War ren , I greatly benefited from your theoret ical insights. T h a n k you also for your recommendat ions on reference electrodes. R e a d i n g your thesis greatly helped me i n w r i t i n g this thesis. To M s . M i r a n d a Jamieson, I really appreciated your encouragement and cheerful spir i t . To M r . C h i W a h E d d i e Fok, thank you very much for the filtration set up and the u l t ra -sonicator (Chapter 9). I a m also very grateful for a l l the discussions we have had regarding supercapacitors. To M r . T issaphern M i r f a k h r a i , i t was great fun t ak ing the chemistry courses together and discussing molecular d y n a m i c simulat ions of carbon nanotubes. T o M r . M a t t h e w Cole , since your a r r iva l the lab has been much more fun. T a k i n g m u tua l pleasure i n each others exper imental mishaps was a great pass t ime that I shal l dearly miss. I would like to dedicate this work to m y parents and sister, for their un-condi t ional support . I hope one day, I can make up for their selfless efforts i n m a k i n g my life so pleasant. 1 Chapter 1 Introduction 1.1 S u p e r c a p a c i t o r s If you are reading this thesis, on a screen or as a pr int , y o u are consuming electrical energy (or energy has been consumed to pr int the document) . T h e electrical energy be ing consumed is at some point stored and condi t ioned through capacitors regardless of the source of the electr ical energy and its me thod of consumpt ion . Capaci tors are used wherever electr ici ty is used, from random access memory units to smal l devices such as cellular phones to large scale power stations. T h e capabi l i ty of a capaci tor to store electrical energy is referred to as capacitance[2]. Capaci tors that have high specific capacitance, i e . , capacitance per uni t mass, are referred to as supercapaci tors 1 . W h i l e conventional capacitors, have high power density, i.e., they can dissipate energy i n a very short t ime; they have a low energy density, i.e., they can only store a smal l amount of energy. Bat ter ies and fuel cells on the other hand have a h igh energy density and a low power density. Supercapacitors also referred to as electrochemical capaci tors 2 or Ul t ra -capaci tors are electrical energy storage devices f i l l ing the performance gap between batteries and conventional capacitors as shown i n figure 1.1. x T h e term supercapacitor has been registered as a trademark by N ippon Electr ic Cor-porat ion; however it continues to be used in a generic fashion by the scientific community. 2 Elect rochemical capacitors are not the same as electrolytic capacitors. Electrolyt ic capacitors are a subgroup of conventional capacitors; they have lower capacitance than supercapacitors. Electro ly t ic capacitors are based on thin-f i lm oxides formed electrolyt i-cally wi th a gel electrolyte on transit ion metals[2]. Chapter 1. Introduction 2 CD I Ik a> 5 o a. 0.5 1 5 10 Specific Energy (Wh/kg) SO 100 Figure 1.1: Compar i son of power density vs. energy density for various elec-t r ica l energy storage devices. F igure from [3]. Reproduced w i t h permission from Elsevier Science L t d . 1.2 A i m T h e demand for electr ical energy storage devices capable of combined h igh power and energy densities has been created by evolut ion of exis t ing products and development of new products . Mul t imedia -enab led cel l phones are an example of an evolved technology whereby the power density of batteries is reaching l imi t s of del iver ing the energy required by the cell phone whi le being used as a T V or a video camera. E lec t r i c vehicles are an emerging field where batteries and fuel cells alone are not able to provide the necessary energy i n a short burst of t ime required for achieving acceleration rates available i n conventional cars. Wherever portable electr ical energy has been needed, batteries have dom-inated the market . However i n add i t ion to low power density, batteries have l imi ted shelf life and the cycle life for rechargeable batteries is rather short. For most applicat ions the shortcomings of batteries have been tolerated and when possible the bat tery performance has been extended through engineer-ing solutions on how the bat tery is operated as part of the whole system. Chapter 1. Introduction 3 However the performance requirements of new appl icat ions combined w i t h emergence of new materials have led to development of electrochemical ca-pacitors w i t h the a i m of exceeding the power density, shelf life t ime and cycle life t ime of batteries by at least an order of magni tude whi le ma in ta in ing rea-sonable energy densities. fl) 4 5>10 CD 0 c 10 o X Bolder Pb Acid n Hawker Pb Acid V Horizon Pb Acid 0 Optima Pb Acid O Panasonic NiHD Sanyo Li Ion + Varta NiHD X Maxwell 1000F o Maxwell 2700F * Panasonic 2000F + Panasonic 800F 0 SaftGen2 144F Saft Gen3 132F • Superfarad 250F §10 2 1 D- 10" 10 10 Energy Density (Wh/kg) 10° Figure 1.2: C o m p a r i s o n of power density vs. energy densi ty for a few com-merc ia l electrochemical capacitors and batteries. Bat ter ies are marked i n blue and capacitors are marked i n red. D a t a points from [4]. In add i t ion to power density and energy density, another impor tan t figure of meri t is cost. E lec t rochemica l capacitors have to achieve their performance targets at s imi lar or lower cost compared to batteries. Chapter 1. Introduction 4 1.3 Scope of the Work A s the t i t le suggests the focus of this work is on electrochemical capac-itors based on polypyrrole , a conduct ive polymer , and carbon nanotubes. Po lypyr ro le has been selected due to previous experience w i t h this mater ia l [5]. A s i t w i l l be discussed later, the jus t i f icat ion for usage of conduct ing polymers and carbon materials as electrodes for electrochemical capacitors is the potent ia l ab i l i ty of these types of materials to achieve the performance targets while main ta in ing low fabricat ion cost. T h e layout of the text is as follows. T h e next chapter w i l l provide some background informat ion regarding the t ime evolut ion of capacitors and the pr inc ip le of operat ion of basic capacitors. T h e fol lowing chapter provides the under ly ing pr inciple of e lectrochemical capacitors and compares them to batteries. T h e current state of electrochemical capacitors is discussed i n chapter 4. Chapte r 5 presents the electrochemical methods used i n the experiments performed. There are two m a i n components to an electrochemical capacitor: the elec-t ro ly te and the electrodes. Chap te r 6 discusses various potent ia l electrolytes available for electrochemical capacitors. Measurements carr ied out on differ-ent electrolytes along w i t h compat ib le separator materials are also presented. T h e following three chapters (7, 8, 9) discuss the electrodes that have been developed and the results obta ined based on these electrodes. Chap te r 10 summarizes the results obtained and provides a list of recommendations on how to improve upon what has been achieved. A p p e n d i x A accompanies chapter 4 by p rov id ing a U S patent survey on electrochemical capacitors. A p p e n d i x B covers the M A T L A B code used for da ta f i t t ing. A p p e n d i x C presents the exper imenta l da t a associated w i t h chapter 6: electrolytes and separators. A p p e n d i x D supplements the po lypyr -role test results i n chapter 7. A p p e n d i x E contains test results on M W N T cells. 5 Chapter 2 Background T h e his tor ica l account provided here is based on Chapte r one of Conway ' s monogram [2] and a review article by K o t z and Ca r l en [3]. 2.1 H i s t o r y T h e discovery of electr ical charge storage on surfaces could be a t t r ibu ted to the rubb ing of amber i n ancient t imes 1 . T h e first instrument to make usage of such charge storage was the L e y d e n jar , invented almost s imultaneously by K l i e s t at K a m i n (former Pruss ia , current ly i n Poland) and Musschenbroek at L e y d e n (Netherlands) i n the mid-eighteenth century 2 . T h e Leyden ja r consists of a glass v i a l , conta ining an ac id and an immersed meta l electrode. T h e outside of the ja r is covered w i t h a me ta l foi l , thus the glass m e d i u m is used as a dielectr ic between the meta l foil and the immersed electrode whi le the acid acts as an electrolyte. W h i l e i n the Leyden jar , dou-ble layer charging occurs due to presence of the acid, the capacitance of the device is p r imar i l y due to the glass ac t ing as a dielectric mater ia l between two conductors 3 . T h e Leyden jars were usual ly charged using electrosta-t ic generators such as the Hawkesbee machine; a l though B e n F r a n k l i n used l ightening i n his famous ki te experiments to charge a Leyden ja r as wel l . 1 T h e word electron is derived from the Greek word for amber. 2 T h e real discovery may have happened accidentally by Kl iest 's student, Cunues, while working wi th an electrostatic generator in his attempt to electrify water. Once he con-nected the generator across the water, he received a shock that nearly ki l led h im. Th is accident prompted Kl iest to investigate the cause. 3 A s it wi l l be explained later, the Leyden jar in effect contains two series capacitors, one is the capacitance of the double layer between the acid and the immersed electrode, the other is the capacitance due to the glass acting as a dielectric material between two conductors, i.e., the foil on the outer surface of the glass and the electrolyte inside. The ca-pacitance of the double-layer is much higher than the capacitance due to the glass; however in a series configuration, the total capacitance is dominated by the smaller capacitor. Chapter 2. Background 6 F igure 2.1: A col lect ion of L e y d e n jars at M u s e u m Boerhave [6]. Reproduced under G N U Free Documenta t ion License [7]. W h i l e the engineering beh ind Leyden jars was well unders tood and i m -proved devices were made, i t took over two centuries to uncover the sci-entific principles beh ind the ab i l i ty of the Leyden jar to function. Proper unders tanding of electr ici ty was in i t ia ted by Ga lvan i ' s discovery of an imal electr ici ty and Vol ta ' s discovery of vol taic electr ici ty toward the end of the eighteenth century. Faraday moved the field forward through his discovery of the relat ionship between charge and current, and the chemical equivalence of charge. T h e proper unders tanding of charge i n terms of electrons came about th rough the works of T h o m s o n (charge to mass ra t io of negative charge carriers produced i n the ion iza t ion of low pressure gases) and M i l l i k a n (charge of an electron). Based on Faraday 's law, He lmho l t z was able to deduce a fundamental uni t for electr ical charge. H i s discovery enabled h i m to conduct quant i ta t ive experiments wh ich l a id the foundations for the science of electrochemistry. E lec t rochemis t ry was then moved forward by advances i n spectroscopy and theoret ical unders tanding of energy states of atoms and ions i n solutions, th rough the works of B o h r , Sommerfeld, Schroedinger and Heisenberg. Chapter 2. Background 7 O n the appl ica t ion side, capacitors based on the Leyden ja r were con-structed and used throughout the eighteenth, nineteenth and early twentieth century; these devices were referred to as condensers 4 . V o l t a improved upon the Leyden ja r w i t h his invention: the electrophorous. T h i s device relied on ebonite (a ha rd plastic) as the dielectric mate r ia l instead of glass, placed between two meta l electrodes, as such i t can be considered as the first sol id-state capaci tor [9]. B y W o r l d W a r I, capacitors employing m i c a as the dielectr ic mater ia l be-came common. Capaci tors based on rol led papers and ceramic materials were also developed. T h e first patent for an electrochemical capaci tor is at-t r ibu ted to Becker and his employer Genera l E l ec t r i c for their 1957 patent [10]. Becker developed the first electrochemical capaci tor based on electrical energy storage at the interface between a porous carbon mater ia l and an aqueous electrolyte, as shown i n figure 2.2. F igure 2.2: D i a g r a m of Becker 's capacitor as d rawn i n his patent appl ica t ion. 10 is an insu la t ing container. 11 is an acidic electrolyte. 12 marks the porous carbon electrodes [10]. 4The word condenser was coined by Volta as a reference to the ability of the device to condense energy [8]. This terminology was further reinforced with development of devices relying on air as the dielectric material. Many languages still continue to use the equivalent word for a condenser to refer to capacitors. //-•10 Chapter 2. Background 8 In 1969, the S O H I O corpora t ion began to market capacitors based on porous carbon electrodes s imi lar to Becker 's [3, 11], however, w i t h improved device geometry. T h e carbon capacitors developed by Becker and Boos (SO-H I O ) are predecessors to the modern double layer charging carbon superca-pacitors. A different type of capaci tor based on pseudocapacitance (see 3.3) was developed by Conway and coworkers s tar t ing in 1975 th rough efforts at the Cont inen ta l G r o u p and the Hooker Corpora t ion . These capacitors were s im-i lar to batteries as they rel ied on an electrochemical react ion for charge stor-age, however they were able to m i m i c the response behaviour of t rad i t iona l capacitors. T h e foundations l a id by Conway formed the basis for modern electrochemical capacitors based on t rans i t ion meta l oxides such as RuOi-W h i l e the capacitor manufactur ing business grew, a migra t ion away from N o r t h A m e r i c a to Japan , fol lowing the same t rend as i n the microelec-t ronic sector occurred. B y 1994, capaci tor manufactur ing was domina ted by Japanese companies, who accounted for 75% of wor ldwide p roduc t ion [12]. T h e type of capacitors being developed gradual ly started to change as wel l . In 1987, 99% of capacitors commercia l ly produced could be labeled as conventional capacitors, whereby s imi lar to the L e y d e n jar , a dielectric m e d i u m is used to store charges at me ta l electrodes. B y 1992, 96% of ca-pacitors produced belong to the conventional class, the remain ing 4% can be considered as double-layer electrochemical capacitors. In recent t imes the push toward commerc ia l iza t ion of electric vehicles as a result of concern regarding rap id deplet ion and envi ronmenta l impacts of fossil fuels, coupled w i t h the emergence of new materials , has generated a substant ial effort from b o t h indus t ry and academia to develop viable super-capacitors. D u r i n g the 90s large-scale u l t racapaci tor development programs were in i t ia ted by the U S , E U and Japanese governments p rov id ing funding for their respective home markets. T h i s push has lead to emergence of new companies such as Skeleton w i t h the sole purpose of supercapaci tor produc-t ion, whi le t rad i t iona l capaci tor manufacturers such as N E C have entered this market as wel l . Chapter 2. Background 9 2.2 C a p a c i t o r B a s i c s T h e basic capacitor comprises two paral le l conduct ive plates separated by an insula t ing med ium (the dielectric) as shown in figure 2.3. Lead Wire Dielectric Medium Lead Wire Figure 2.3: Bas ic capaci tor W h e n a voltage is appl ied across the two terminals of the capacitor, equal numbers of opposing charges b u i l d up on the two plates un t i l the voltage across the dielectric mate r ia l matches the appl ied voltage, at this point the flow of charge, i.e., current stops as there is no longer a potent ia l difference between the capaci tor and the voltage source. If the source is removed and the two terminals of the capaci tor are kept open, the charges w i l l remain on the plates as there is no place for them to go to. T h e amount of charges remain ing on the plates, is related to the applied voltage across the plate as governed by Gauss 's law, i.e. , the linear relation-ship between flux of electric field and charge: V »D = p (E *dA = Q t = KCQ (2.1) (2-2) (2.3) Chapter 2. Background 10 Whereby e is the p e r m i t t i v i t y of the dielectric m e d i u m between the two plates wh ich is related to en, p e r m i t t i v i t y of free space and K, the dielectric constant (sometimes referred to as relative pe rmi t t iv i ty ) of the insulator . Q is the amount of charge on each plate. $ is the flux of electric field. For the paral lel plate geometry shown the result would be: eEA = Q (2.4) E = (2.5) A = lw (2.6) e^V = Q (2.7) Cparallel plate ^~d (^ 'S) CV = Q (2.9) Whereby C is the capacitance, a parameter that only depends on the geometry and dielectric constant of the insulator . Vc is the voltage across the dielectric med ium. Fo l lowing the same process, the capacitance associated w i t h a cy l ind r i ca l capacitor and an spherical capacitor can be calculated: Ccylindrical = 27Te-— (2-10) ln[b/a) Cspherical = 47T6- (2.11) b — a L is the height of the cyl inder , b is the diameter for the outer plate and a is the diameter for the inner plate i n b o t h cases. U s i n g the equat ion for the spherical case, the capacitance of an isolated meta l l ic sphere can be calculated by a l lowing the outer diameter to go to infini ty: Cisolated sphere — l i m C'spherical — 47T6Q (2.12) fc—>oo N o w if a capacitor is charged from 0 V to V c appearing across the dielectric, then the energy stored i n the capaci tor is: W = J Vdq = JJ VCdv = ^CVg (2.13) Chapter 2. Background 11 A s i t is apparent from equat ion 2.13, i n order to increase the energy stored i n the capacitor , the capacitance and the voltage need to be increased. T h e capacitance can be increased through geometrical modif icat ions and usage of mate r ia l w i t h h igh dielectr ic constant. T h e fol lowing table lists the dielectric constant of some materials: Table 2.1: Die lec t r ic constants of materials . Values are given for 1 a tm, 20°C unless noted otherwise. D a t a from [12, 13] M a t e r i a l Die lec t r ic Cons tant M a t e r i a l Die lec t r ic Cons tant Ace ton i t r i l e 37.5 A i r (dry) 1.00054 A l u m i n u m Ox ide 9.3-11.5 A r g o n 1.00052 Benzene 2.28 D i a m o n d 5.7 E b o n i t e 2-3.5 H e l i u m 1.000065 Hydrogen 1.00025 Ice ( -30°C) 99 M e t h a n o l 33.0 M i c a 5-7 Neon 1.00013 Ni t rogen 1.00055 Paper 1.2-2.6 Paraffin 1.9-2.4 Polyethylene 2.2-2.4 Polystyrene 2.5-2.7 Propylene Carbona te 69 Salt 5.9 S i l i con 11.8 Sulfur 2-4.2 T a n t a l u m O x i d e 27.6 T i t a n i u m dioxide 114 V a c u u m 1 Water 80 A s table 2.1 indicates, bo th the chemical and phys ica l compos i t ion (e.g. water vs. ice) of the insulator affect its dielectric constant. W h i l e i t is tempt-ing to choose the mater ia l w i t h the highest dielectric constant and app ly a very large potent ia l to store a large amount of energy, these two factors are not independent of each other. B e y o n d a certain potent ia l difference, referred to as the breakdown voltage, the insu la t ing mater ia l conducts electr ici ty as a result of chemical changes caused by the h igh electric field. T h i s process is usual ly non-reversible, i.e., once an insulator is broken down it no longer behaves as an insulator at any voltage. T h e electric field corresponding to the breakdown voltage is referred to as dielectric strength; values for a few insulators are given i n the fol lowing table. Chapter 2. Background 12 Table 2.2: Breakdown voltage of insulators. D a t a from [14] Mater ia l Dielectric Strength ( M V / m ) Mater ial Dielectric Strength ( M V / m ) A i r (dry) 3 D i a m o n d 10 M i c a 160 Paper 14-16 Polyethylene 500-700 Polystyrene 400-600 So far the capacitor being discussed does not have any resistance asso-ciated w i t h i t . Such a capaci tor wou ld have instantaneous response t ime corresponding to an infinite power. Based on equat ion 2.9, the impedance of such a device i n frequency doma in wou ld be: ZM = -L, (2.14) However a real capacitor has a finite response t ime due to two factors: the dielectric constant is frequency dependent, i.e., the dielectric requires a finite amount of t ime to r e spond 5 and the capacitor 's plates and leads have a finite resistance. A s such a real capaci tor is usual ly represented by an ideal capacitor i n series w i t h a resistor. T h e resistor is usual ly referred to as Equiva len t Series Resistance ( E S R ) . T h e impedance of a real capaci tor is usual ly represented by: Z(u>) = Rs + - ± - (2.15) R e a l voltage sources such as batteries also have a finite resistance associ-ated w i t h them. To calculate the power of the capaci tor dur ing a charging cycle and a discharge cycle, one wou ld need to know the impedance of the charging source and the discharge load. T o have a comparable benchmark, i t is assumed that the load has an in ternal resistance equal to the capacitor 's E S R . T h i s condi t ion is referred to as matched impedance; under such an arrangement the capacitor achieves m a x i m u m discharge power. 5 T h e dielectric constant is a macroscopic quanti ty which is related to a microscopic quanti ty called polarizabil i ty. Tota l polar izabi l i ty comprises three terms: electronic, ionic and dipolar. In heterogeneous materials there is also a fourth term associated wi th in-terfacial polar izat ion. Each of these terms has a frequency dependence and as such the dielectric constant is frequency dependent[15]. Chapter 2. Background 13 A s s u m i n g the capaci tor has an i n i t i a l voltage of Vc-(O) at t ime zero, and app ly ing a load equal to E S R : w v Figure 2.4: Capac i t o r discharge c i rcui t «w = & ) } =*vcit) = v' o ( 0 ) e^ ( 2 1 6 ) = ^ = V c ( 0 ^ ( 2 . 1 7 ) Whereby Pji(t) designates the power transferred to the load. A s it is apparent the peak power of the capaci tor occurs right at the moment the load is applied. A n o t h e r impor tan t measure of the capaci tor is i ts t ime constant. For a resistive load i n series w i t h the capacitor: T = (RS + Rl)C (2.18) T h e t ime constant designates the t ime required for the capaci tor to dis-charge from its i n i t i a l voltage to a voltage value lowered by a factor of e _ 1 . T h e exponent ia l decay of the capaci tor voltage suggests that i t wou ld take an infinite amount of t ime for the capaci tor to discharge to OV. However i n real-i ty this is not the case, since insu la t ing materials s t i l l conduct a finite amount of current as opposed to zero current. T o account for this leakage current, the mode l used for convent ional capacitors contains a para l le l resistor, Rp w i t h the capacitor , as shown i n figure 2.5. c Figure 2.5: C i r c u i t mode l for basic capacitors 14 Chapter 3 Supercapacitor Electrochemistry Elec t rochemis t ry is a fascinating complex in terdisc ipl inary field w i t h i m -portant indus t r i a l impl ica t ions . T h e intent of this chapter and the chapter after i t , is to provide a very basic unders tanding of the field as required for the in terpre ta t ion of the experiments discussed i n later chapters. T h e mater-ia l p rovided here heavi ly draws from the electrochemistry book by B a r d and Faulkner [16], Conway ' s monogram [2] and class notes provided by Professor D a n B i z z o t t o for the graduate chemistry course on interfacial electrochem-istry. Chap te r 3 of M s . M y a Warren ' s thesis [17] has greatly helped i n clar i fying some of the ideas discussed here. T h e p r i m a r y impor tance of electrochemistry for supercapacitors is the un-der ly ing electrochemical process wh ich allows these devices to function. In some cases the electrode mater ia l , e.g., conduct ing polymers and the elec-t rolyte , e.g., so l id electrolytes are fabricated through electrochemical meth-ods. 3.1 E l e c t r o c h e m i c a l C e l l Elec t rochemis t ry is concerned w i t h charge transfer across interfaces be-tween chemical phases[16]. There are two types of phases wh ich give rise to an interface: an electrolyte and an electrode. A n electrolyte is a phase where charge is carr ied by the movement of ions. A n electrode is a phase where charge is carr ied by electronic movement 1 . E l e c t r o n i c movement covers charge transport by electrons, holes, solitons, polarons and bipolarons. Chapter 3. Supercapacitor Electrochemistry 15 T h e word electrolyte usual ly conjures up the image of a l i qu id , however there are sol id electrolytes such as sod ium /^-alumina and N a t i o n ® . M o s t common electrolytes are made by dissolving a salt i n a solvent; however there are solvent free electrolytes cal led ionic l iquids . These are essentially salts that due to the rmodynamic ins tab i l i ty are l i qu id except at very low temperatures. A n interface can arise between two immisc ib le electrolytes or an electrolyte and an electrode. W h i l e i t may be desirable to investigate a single interface i n isolat ion, electrochemical techniques rely on the electr ical pa th to be closed, as such at least two electrodes are needed. T h e col lect ion of interfaces is cal led an electrochemical cell . T h e basic electrochemical cell has two electrodes and an electrolyte; as such it consists of two interfaces. T h e electrode wh ich comprises the interface of interest w i t h the electrolyte is referred to as the work ing electrode ( W E ) . T h e other electrode which completes the electr ical pa th is cal led the counter electrode ( C E ) . If the electrodes do not have the same chemical and phys ica l composi t ion , there w i l l be a measurable potent ia l difference between the electrodes. T h i s potent ia l exist regardless of charge flow. T h e cell potent ia l arises due to potent ia l differences between the phases. In the basic electrochemical cel l , the cell potent ia l depends on the work ing electrode-electrolyte interface and the counter electrode-electrolyte interface; however i n most cases the interface of interest is only the work ing electrode-electrolyte interface. To s tudy just the work ing electrode-electrolyte interface, a t h i rd electrode, called the reference electrode ( R E ) is added to the system. T h e reference electrode is made up of phases hav ing constant composi t ion , as such its potent ia l is fixed, thus by measuring the potent ia l difference between W E and R E , i t is possible to s tudy just the work ing electrode-electrolyte interface. A glass tube w i t h a meta l wire conta in ing a saturated solut ion i n ionic contact w i t h the electrolyte through a porous p lug is usual ly used as the reference electrode. T h e redox couple used i n the reference electrode, determines the reference potent ia l . Chapter 3. Supercapacitor Electrochemistry 16 S tandard hydrogen electrode ( S H E ) is the s tandard reference electrode, as it corresponds to a potent ia l of zero. S H E consists of a P l a t i n u m wire i n contact w i t h a stream of hydrogen gas i n an aqueous solut ion. However such a reference electrode is difficult to use, so other reference electrodes have been made. T h e most common reference electrode used is the saturated calomel electrode ( S C E ) which is made of mercury i n contact w i t h a saturated aqueous K C 1 solut ion. T h e potent ia l of S C E vs. S H E is 0.242 V . D u r i n g experiments, the potent ia l of W E vs. R E is measured and the current flowing through the cell from C E to W E is adjusted accordingly. T h e current passing through the R E is negligible and as such its impedance i n most experiments is not i m p o r t a n t 2 . T h e potent ia l of W E vs. R E is p r imar i l y determined by the work ing electrode-electrolyte interface, however there is also a potent ia l drop associated w i t h the electrolyte pa th between W E and R E . T o min imize this resistive drop, R E is p laced as close as possible to W E . Function Generator Volt Meter Power Supply WE RE CE Figure 3.1: Bas ic electrochemical cell configuration 2 A n exception is when impedance spectroscopy is performed. See 5.4 Chapter 3. Supercapacitor Electrochemistry 17 3.2 Electrochemical Potential Before the cell potent ia l can be defined a zero energy level needs to be established. B y convention, the energy of a charged par t ic le at an infinite distance from a charged interface is considered to be zero. T h e work associated w i t h m o v i n g a charged part icle from infini ty to the interior of an uncharged phase is referred to as the chemical potent ia l : //,. T h e electrical potent ia l associated w i t h moving a charged par t ic le from inf ini ty across a charged interface to an empty phase is referred to as the inner potent ia l or G a l v a n i potent ia l : <f> (Th i s is an electrical potent ia l measured i n Vol t s as opposed to energy measured i n Joules). T h e electrochemical potent ia l , fl is the energy required for m o v i n g of a charged par t ic le from zero energy to an uncharged phase across a charged interface: Where z is the charge of the charged part icle, and F is the Faraday con-stant (the chemical equivalent of charge). T h e chemical potent ia l associated w i t h an electron i n a meta l is s imp ly i ts Fe rmi energy 3 . Ions follow M a x w e l l - B o l t z m a n n statistics, and their chemical potent ia l is governed by the law of mass act ion: Where u° is the s tandard chemical potent ia l and a is the act ivi ty . Fo r low concentrations a is s imply the ionic concent ra t ion 4 . 3 I n a metal a continuum of electronic levels exist, and electrons wi l l fill these levels start ing from the ground state. The highest occupied level at absolute zero is the Fermi energy. A t higher temperatures, there is a difference between the chemical potential and the Fermi energy, however up to room temperature this difference is negligible.[18]. 4 A t high concentrations, for example for ionic l iquids, due to ion-ion interactions, cor-rections are needed. // = fl + zF(f> (3.1) H = fi° + RTln{a) (3.2) Chapter 3. Supercapacitor Electrochemistry 18 N o w for an electrochemical cell based on the following reversible redox react ion couple: RA ^ ne~A + OA Oc + nec =± Rc where the subscript A refers to anode and C refers to cathode. Under equi-l i b r i u m , i.e., no net current, the electrochemical potent ia l of the reactant species and the product species have to be balanced so that no net charge transfer takes place: PRA + POc + Kec = P-nel + A + PRC (3-3) N o w the cel l voltage, i.e., the voltage difference between W E (cathode) and C E (anode) can be derived: EceU = <t>c~4>A = \{p°oA - fi0Oc - RT.lr£±) (3.4) nb ° Uc E q u a t i o n 3.4, is the Nerst equat ion wh ich i n general form can be wr i t t en as: Ecell = E0 + S n ^ - (3.5) Zr &react T h e Nerst equation is very useful for example for der iv ing the open cel l voltage ( O C V ) of some batteries by t ak ing into account the half reactions t ak ing place at each electrode, however there are impor tan t l imi ta t ions as this re lat ionship has been derived under equ i l ib r ium condit ions and w i t h the assumpt ion that the law of mass ac t ion holds. Under non-equi l ib r ium condit ions, i.e., when current is flowing, how fast ions move, i.e., mass t ransport , and how fast reactions take place, i.e., reac-t ion kinet ics , need to be considered. 3.2.1 Ion Transport There are three ways to t ransport ions: convection, diffusion and migra-tions. For batteries and supercapacitors, convection is not impor tan t as there is no mechanica l ly induced t ransport of ions, i.e., the N e w t o n i a n forces ac t ing on the electrolyte are negl ig ib le 5 . 5 A n exception is forces associated wi th surface tension arising when ions are moving through a porous separator layer or a porous electrode. Chapter 3. Supercapacitor Electrochemistry 19 Ignoring convection, ion t ransport is governed by: Ji = -DiVCi-^DiCiV<l> (3.6) til Where Ji is the ion flux (m,ol s _ 1 c m - 2 ) , Di is the diffusion coefficient ( c m 2 s - 1 ) , C{ is the ionic concentrat ion (mol cm"3), and V C ; is the concen-t ra t ion gradient (mol c m - 4 ) , zi is the charge of species i (C), R is the gas constant ( J m o / - 1 I ^ - 1 ) , T is the temperature (AT) and V</> is the electric field present (V cm*1). T h i s equat ion represents the flux for ion type i ; the flux for each type of ion has to be considered to calculate the net current. ; T h e first part of the equation is the diffusion dr iven t ransport governed by F i c k ' s first law. Diffusion is movement of species due to a concentrat ion gradient. T h e second part refers to ion t ransport v i a migra t ion governed by the Eins te in-Smoluchowski equation. M i g r a t i o n refers to movement of ions due to an electric field, i.e., an electr ical potent ia l gradient. Diffusion and migra t ion are a result of a gradient i n the electrochemical potent ia l th rough the system. For batteries and supercapacitors, a saturated electrolyte is used and as such diffusion does not play an impor tan t role for bu lk of the electrolyte as there is m i n i m a l concentrat ion gradient. Ion movement w i t h i n the electrolyte is more affected by the migra t ion te rm due to the potent ia l difference between the two electrodes. T h e region where the electrolyte approaches the electrode, there is a con-centrat ion gradient due to the physical configuration of the electrode, i.e., its permeabi l i ty to the electrolyte a n d / o r electrochemical reactions consuming ions. In this region diffusion and migra t ion bo th play an impor tan t role i n movement of ions. 3.2.2 Reaction Kinetics For electrochemical reactions, s imi lar to most chemical reactions, the rate of react ion or react ion kinet ics is governed by Ar rhen ius ' s law: k = Ae n.T (3.7) Chapter 3. Supercapacitor Electrochemistry 20 Where k is the rate constant w i t h uni ts of inverse of t ime, EA is the act ivat ion energy corresponding to the energy barrier associated w i t h the reaction, and the coefficient A is cal led the frequency factor corresponding to the p robab i l i ty of surmount ing the energy barrier. T h e rate constant equation can be further refined according to the act ivated complex theory to relate the react ion rate to fundamental constants: Where K, is the t ransmission coefficient ranging from a value of 0 to 1, k is the B o l t z m a n n constant, h is the P l a n c k constant and A G is the change i n free energy. N o w the overal l conversion rate of the reactant to the product is given by: k T _ A G k = K—e h (3.8) (3.9) (3.10) (3.11) (3.12) Product Reaction coordinate Figure 3.2: Free energy changes dur ing a react ion. D i a g r a m from [16]. R e -produced w i t h permission from J o h n W i l e y & Sons L t d . Chapter 3. Supercapacitor Electrochemistry 21 Where kj is the forward react ion rate, CR is the concentrat ion of reactants, kb is the backward react ion rate and Cp is the concentrat ion of products . Under equ i l ib r ium, no net current flows and as such vnet would be zero. Set t ing vnet equal to zero, turns equation 3.9 to the Nerst equation. N o w the voltage and current relat ionship associated w i t h the cell can be derived by so lv ing the mass t ransport equat ion 3.6 under boundary condi-tions set by the react ion rates. 3.3 N o n - F a r a d a i c a n d F a r a d a i c P r o c e s s e s Cons ider ing the basic electrochemical cell under equ i l ib r ium, the Nerst equation implies that there would be a measurable potent ia l difference be-tween W E and C E . T h i s cell voltage arises as the electrochemical potentials for a l l species involved equate. A s the chemical potent ia l of each species is fixed and different, from one another, the al ignment of the electrochemical potentials induces an electr ical potent ia l gradient at each interface. T h e change i n the electr ical potent ia l at the interface induces opposing charges to b u i l d up at the interface, this b u i l d up of charge is referred to as double layer charging. For an ideal ly polar izable electrode, i.e., an electrode that does not transfer charge, app ly ing an external potent ia l s imply bui lds up more charge at the interface. W h i l e no mater ia l behaves as an ideal polar izable electrode, for cer tain po-tent ia l ranges depending on the electrode and electrolyte mater ia l , no charge transfer occurs. In this manner the electrode-electrolyte interface behaves as an electrostatic capacitor . A s no charge transfer takes place, this process is called non-Faradaic . W h e n charge transfer across the interface occurs, i.e., an electrochemical react ion takes place, the process is cal led Faradaic . A s the potent ia l gradient across the interface has a sharp profile, the double layer thickness is very t h in as such the corresponding capacitance is large. Based on equat ion 2.9 the capacitance per uni t area of the double layer can be est imated: ^ = ^ (3-13) Chapter 3. Supercapacitor Electrochemistry 22 - Metal- - Solution -,0> (+) Solvated cation © © ,o, © v—' "Ghost" of anion repelled IHP OHP ' r o m electrode surface Figure 3.3: Electrode-electrolyte (metal-solution) interface. <f> is the inner potent ia l as discussed i n section 3.2. Image from [16]. Repro-duced w i t h permiss ion from J o h n W i l e y & Sons L t d . Where t is the double layer thickness. T h e electrolyte side of the double layer is thought to be composed of several layers 6 . T h e layer nearest to the electrode surface is called the inner layer or the inner He lmho l t z plane as marked by I H P i n figure 3.3. T h i s layer only consists of solvent molecules and i n some cases ions and molecules that are specifically adsorbed onto the electrode surface. T h e solvated ions i n the electrolyte can approach the electrode only as far as the outer He lmhol t z layer ( O H P in figure 3.3). T h e interaction be-tween these ions and the charges on the electrode side are thought to be only long range electrostatic forces. These ions are considered as non-specifically adsorbed. Due to the rmal mot ion , these ions are d is t r ibu ted through a re-gion cal led the diffuse layer which extends from the O H P to the bulk of the solut ion. 6 T h e model discussed here is the basic Helmholtz model. There is an advanced model in better agreement wi th experiments, called the Gouy-Chapman-Stern model. For more information please refer to section 13.3 of [16]. Chapter 3. Supercapacitor Electrochemistry 23 T h e length scale corresponding to the double layer thickness, i.e., t, is less t han l O n m ; the exact thickness depends on the electrolyte concentrat ion, the size of ions, the solvent used and the potent ia l present across the double layer. A t such a length scale, the use of the dielectric constant to estimate the double layer specific capacitance becomes questionable, as the dielectric constant is a bu lk property. Fur thermore whi le the bu lk dielectric constant of a mater ia l would not change by app ly ing a moderate D C potent ia l , on a molecular scale there would be changes due to po la r iza t ion v i a the external potent ial . T h e G o u y - C h a p m a n - S t e r n mode l takes into account the applied potent ia l and the charge concentrat ion. Based on this mode l and exper imental observation, the double layer capacitance has a range of 10 to 50 [2, 16]. For a purely non-Faradaic basic electrochemical cell , the behaviour can then be modeled using a series resistor w i t h two capacitors, whereby the series resistor accounts for the res is t ivi ty of the electrolyte between W E and C E , and each capacitor represents the double layer capacitance at each interface: / v v Figure 3.4: Bas ic mode l for an electrochemical capacitor For actual electrochemical cells, even when non-Faradaic processes domi -nate, a smal l amount of electrochemical reactions take place ma in ly i n the form of reactions between the ions i n the double layer and oxygen molecule and impur i t ies present i n the electrode and electrolyte materials. S imi l a r to the mode l for leakage current i n conventional capacitors, these parasi t ic reactions can be modeled by p lac ing a resistor i n paral le l w i t h the capacitor: F igure 3.5: Improved mode l for an electrochemical capacitor Chapter 3. Supercapacitor Electrochemistry 24 T h e overall impedance of the above cell would then be given by: Z(u)=RS+1+„2g:: A + i T ^ r + R2PCECCE 7T PCE^CE (3.14) W h e n electrochemical capacitors are used i n appl icat ions, a reference elec-trode is not used, therefore d is t inguishing between the double layer at each interface is not possible, as such a prac t ica l mode l wou ld represent the net effect of the two double layer capacitances. A s s u m i n g CWE = CCE a n d RPWE — RPCE then equat ion 3.14 would simplify to: RP juRpC Whereby Rp as shown: Z(u) = Rs.+ 2RPWE and C 1 + LU2R2PC2 1 + u>2R2C2 (3.15) T h e system could be then represented i / Figure 3.6: P r a c t i c a l mode l for an electrochemical capaci tor In this model Rs is analogous to the E S R discussed before; i t represents a l l the series resistance i n the device wh ich is determined by the electrolyte and the conduc t iv i ty of the electrodes and the connect ion to the current collector. Reca l l i ng the ion t ransport equat ion 3.6: J% = -DiVCi - | ^ A W T h e series resistance does represent migra t ion wel l , since mig ra t ion is s im-i lar to O h m ' s law as the flux of ions is l inear ly p ropor t iona l to the potent ia l v i a a constant: ZjF Ji Migration ryrrn, Kl t Z-F R oc —£=-DiCi RTt Chapter 3. Supercapacitor Electrochemistry 25 However diffusion is not very wel l represented by a resistor. For a more realist ic mode l a diffusion element needs to be inc luded. T h e impedance associated w i t h diffusion element wou ld depend on the phys ica l properties of the electrode and the type of electrolyte used. For most of the discussion here, the prac t ica l mode l shown i n figure 3.6 w i l l be used; where diffusion is c r i t i ca l to the exper imental results, its impact w i l l be discussed. 3.4 C o m p a r i s o n o f C a p a c i t o r a n d B a t t e r y T h e basic electrochemical capaci tor and electrochemical bat tery b o t h con-sist of two electrodes, an electrolyte and a separator layer keeping the two electrodes apart . W h i l e the geometric s tructure of these devices is s imi lar their pr inciples of operat ion is different: electrochemical capacitors r un on non-Faradaic processes while batteries run on Faradaic processes. For an electrochemical capacitor , the energy is stored d i rec t ly electrostat-ica l ly at each double-layer; whi le for a bat tery the energy is stored indi rec t ly as chemical energy to be released though charge transfer across the double layer [2]. T h i s difference i n the under ly ing principles of operat ion results i n major differences between properties of electrochemical capacitors and bat-teries. T h e first major difference between properties of batteries and electrochem-ica l capacitors is energy dens i ty W h i l e as stated at the beginning the goal of supercapacitors or electrochemical capacitors is to achieve higher energy densities; the most advanced supercapacitors s t i l l have lower energy density t han the latest batteries. A s deduced i n the previous section, the double layer has a specific capac-i tance range of 10-50 /iF/cm,2; now for an aqueous electrolyte that would correspond to a charge density of 10-50 fiC/crn2 (assuming a I V operat ing voltage range). N o w using an a tomic density of 1 0 1 5 c m - 2 , the charge per a tom would be 10-50 x 1 0 ~ 1 5 / / C , i.e., 0.06-0.31 electrons per a tom whereby these electrons are from the conduct ion b a n d 7 . For a Faradaic react ion on 7 T h e discussion on charge density for non-Faradaic and Faradaic processes is based on the treatment provided in Conway's monogram. See section 2.4.1 of [2] for more details Chapter 3. Supercapacitor Electrochemistry 26 Electrolyte, Separator 2-10 A Figure 3.7: S t ruc ture of an electrochemical capacitor . A bat tery has a s im-i lar geometric configuration. T h e potent ia l profile expresses the energy stored i n the double layer at each electrode-electrolyte interface. Image from [3]. Reproduced w i t h permiss ion from Elsevier Science L t d . the other hand, one or two valence electrons per a tom are involved i n the process. Such difference i n charge density is the m a i n reason for difference i n energy density. Ano the r l i m i t i n g factor is the relat ionship between charge stored and the resul t ing energy stored i n the system. A s shown i n chapter 2, for a capaci tor the following relat ionship holds: Esuper-capacitor — ~^QV (3.16) W h i l e for a battery, the energy stored is equal to the resul t ing cell voltage and the net charge transfered: Ebattery — QV (3.17) Chapter 3. Supercapacitor Electrochemistry 27 T h i s discussion inadvertent ly may suggest that theoret ical ly for a superca-paci tor i t is not possible to exceed the energy density of a battery, however that conclusion is not necessarily true. W h i l e the voltage associated w i t h a bat tery is inherently set by the half reactions t ak ing place at each electrode-electrolyte interface; the voltage associated w i t h a supercapacitor can be increased and i t is only l im i t ed by the decomposi t ion voltage associated w i t h the electrolyte (and electrode mater ia l ) . T h i s is the m a i n d r iv ing point be-h i n d adopt ing organic or ionic l i qu id as electrolytes. W h i l e the Faradaic nature of batteries provides a higher energy density i n most cases, as these devices rely on charge transfer; their response t ime is l imi t ed by chemical kinet ics and as such their power density is lower t han that of supercapacitors. For a supercapacitor the response t ime is m a i n l y governed by the conduc t iv i ty of the electrolyte (especially conduc t iv i ty of the electrolyte through the electrode mater ia l ) . So far batteries and supercapacitors have been described i n black-whi te terminology as Faradaic and non-Faradaic; however an intermediate s i tua t ion is possible. Under cer tain the rmodynamic condi t ions, the potent ia l V of the electrode is a continuous function of the charge Q transfered such that a derivative dQ/dV exists. Such a process wou ld have ' a capaci t ive behaviour and i t is referred to as pseudocapacitance. Systems w i t h pseudocapacitance have been ta i lored toward act ing as a supercapaci tor and also as a battery. T h e fol lowing tables, based on Conway ' s book, summar ize available capac-i tor and bat tery technologies: Table 3.1: Types of capacitors and their mode of energy storage [2]. Type Storage Mode Examples Dielectr ic Electrostat ic M ica ,My la r , paper Electrolyt ic Electrostat ic T a 2 0 5 , A1 20 3 Double-layer Electrostat ic Carbon fibers, felts, nanotubes and powders Redox oxide film Pseudocapacitance R u O x , I rGv, C03O4 Redox nitr ide film Pseudocapacitance V N , N b N , T a 3 N 5 Redox polymer film Pseudocapacitance polyani l ine, polypyrrole, polythiophene Chapter 3. Supercapacitor Electrochemistry 28 Table 3.2: Types of batteries and their mode of energy storage [2]. T y p e S t o r a g e M o d e E x a m p l e s Lechlanche, z i n c - M n 0 2 A l k a l i n e , z i n c - M n 0 2 P r i m a r y Faradaic M g - A g C l M g - P b C l 2 A l - a i r L e a d acid , P b - P b 0 2 Nicke l -cadmium, N i - O - O H - C d Secondary Faradaic Nickel-hydrogen, N i - O - O H - m e t a l hydr ide Nickel -z inc , N i - O - O H - Z n Zinc-a i r L i - T i S 2 L i - M o S 2 Secondary Pseudocapaci tance L i - M n 0 2 L i - C - C o 0 2 Na-S T h e mode of operat ion ci ted i n the tables is the most dominant factor while other factors may play a role albeit a minor role. In a l l electrochemical systems at the interface between an electrode and an electrolyte the double layer exists; however the con t r ibu t ion of the double-layer is l imi t ed to 2-5% of to ta l charge storage capaci ty for bat tery systems. Ano the r major difference between batteries and electrochemical capacitors is cyclabi l i ty . For electrochemical capacitors i t is possible to cycle the device for thousands of cycles wi thou t much degradation, however for batteries the electrochemical reactions are not completely reversible. For p r ima ry batter-ies, i t is not easily possible to recharge the bat tery upon discharge. Secondary batteries can be recharged upon discharge, however their performance drops rapid ly over t ime and they can not achieve the same number of cycles of operat ion as for e lectrochemical capacitors. T h e differences between batteries and electrochemical capacitors suggest that these devices would perform complementary roles. T h i s section con-cludes w i t h an overal l compar ison between these devices. Chapter 3. Supercapacitor Electrochemistry 29 Table 3.3: C o m p a r i s o n of electrochemical capacitors and batteries. Based on [2]-Battery Capacitor 1. Ideally has constant discharge and Has in t r ins ica l ly s loping charge and recharge potent ia l , except for L i i n - recharge curve tercala t ion systems 2. Because of (1) does not have a good int r ins ic state-of-charge ind ica-t ion except for L i intercala t ion sys-tems It has a good state-of-charge indica-t ion 3. Has moderate or h igh energy den- Usua l ly has low energy density sity, depending on the electrode ma-ter ia l used 4. Has relat ively low power density as l i m i t e d by kinet ics 5. Has short cycle life, usual ly far be-low 1000 cycles, due to i r revers ibi l i ty of redox and phase-change processes i n three dimensions 6. Has significant temperature de-pendence (Faradaic resistance) 7. Has shorter l ifetime (active life and shelf life) due to degradat ion or reconstruct ion of active materials Has moderate or h igh power density, depending on the electrolyte Has long cycle life, usual ly many thousands of cycles, due to s im-ple add i t ion or wi thd rawa l of charge i n purely double-layer type. For pseudocapacitance, the cycle life may be shorter a l though s t i l l much longer than pure ly Faradaic . Has minor temperature dependence. Has long l ifetime except for corro-sion of current collectors, etc. 8. E lec t ro ly te conduc t iv i ty changes dur ing charging depending on chem-is t ry of cell reactions Elec t ro ly te conduc t iv i ty can de-crease dur ing charging due to ion ad-sorpt ion. Chapter 4 State of the Art 30 Elec t rochemica l capacitors are be ing r igorously pursued bo th by indus t ry and academia. A s a l luded i n the previous section, there are four compet ing and sometimes complementary electrode technologies for supercapacitors: • Trans i t ion meta l oxides ( T M O ) exh ib i t ing pseudocapacitance • Trans i t ion meta l ni t r ides ( T M N ) exh ib i t ing pseudocapacitance • C o n d u c t i n g polymers ( C P ) exh ib i t ing p r imar i l y pseudocapacitance • C a r b o n based materials ( C B M ) runn ing on non-Faradaic a n d / o r pseudo-capacitance A l o n g w i t h electrodes, compat ib le electrolytes/separators and overal l cel l design are also major factors i n i m p r o v i n g electrochemical capacitors. Re -cent developments on these fronts as reported through scientific journals and patent disclosures are presented here. 4.1 I n d u s t r y Elec t rochemica l capacitors are an example of a technology uncovered by basic science and fostered by indus t r i a l needs. A l t h o u g h the first electro-chemical capacitor was realized i n 1957 (Becker 's patent); i t took more than 30 years for a successful commerc ia l product . T h e first mass commerc ia l iza t ion of an electrochemical capaci tor was car-r ied out by Panasonic (a M a t s u s h i t a group company) i n the form of the Gold Capacitor i n 1978. T h e Gold Capacitor wh i ch is s t i l l available today is a double-layer type capaci tor based on act ivated carbon fiber electrodes w i t h an organic electrolyte [2, 19]. Since then a number of companies have developed and successfully marketed electrochemical capacitors. Chapter 4. State of the Art 31 Table 4.1: L i s t of companies w i t h commercia l ly available supercapaci tor products . In a lphabet ica l order. C o m p a n y Name L o c a t i o n Technology c a p - X X [20] A u s t r a l i a C B M Cooper Industries [21] U S C B M E L N A [22, 23] J apan C B M E P C O S [24] Germany, Japan C B M E P P S C O R E ( M A X F A R A D ) [25] K o r e a C B M E V A N S Capac i to r C o [26] U S E l e c t r o l y t i c + T M O M a x w e l l Technologies [27] U S C B M N E C / T O K I N [28] J apan C B M Ness Corpora t ion [29] K o r e a C B M , T M O Panasonic ( G o l d Capaci tors ) [30] J apan C B M T a v r i m a [31] C a n a d a C B M Skeleton [32, 33] U S , Sweden, E s t o n i a C B M T h e companies l is ted i n table 4.1 are those w i t h readi ly available p rod-ucts. T h i s list of companies indicates carbon based electrodes have had more commerc ia l success thus far. These electrodes have been wide ly used as their current p roduc t ion cost is lower compared to the other electrode materials due to the ma tu r i ty of the indus t r i a l p roduc t ion of ca rbon based mater ia ls and inherently cheaper and easier methods of product ion . T h e carbon mater ia l used i n commerc ia l devices are p r imar i ly , ca rbon cloths, carbon fiber paper, carbon felts and carbon powder. T h e carbon mater ia l is treated i n a process called: A c t i v a t i o n which enables the mate r ia l to develop pseudocapacitance i n add i t ion to double layer charging. W h i l e meta l oxides offer better performance i n terms of energy and power density; these systems are very expensive when R u t h e n i u m is used. Cheaper metals can be used, at a lower performance w i t h the overal l cost of the sys tem s t i l l higher than the ca rbon based systems. T h e superb performance of the RuC>2 based supercapacitors has created a niche market for these devices i n m i l i t a r y and space appl icat ions . There are no known commerc ia l products re ly ing on meta l nitr ides, a l though some companies are exp lor ing them. Chapter 4. State of the Art 32 C o n d u c t i n g polymers are relat ively new engineering materials and their i n -dus t r ia l adopt ion w i l l take t ime. Al ready , commerc ia l ly available electrolyt ic capaci tors 1 employ conduct ing polymers as a dielectric and sometimes as electrodes to form flexible capacitors. 4.1.1 Patent Survey Results A n impor tan t factor i n successful p roduc t ion and marke t ing of any engi-neered product is creation and protect ion of intel lectual property associated w i t h the product i n terms of patents and indus t r ia l designs. T o evaluate the status of electrochemical capacitors a comprehensive search of U S patents have been carried out; the relevant patents have been l isted i n A p p e n d i x A . T h e patent survey provides useful informat ion regarding the pace and the nature of recent developments i n the field as shown here. 1980 1985 1990 1995 2000 Filing Year Figure 4.1: Number of supercapaci tor U S patents granted for the per iod of 1980-2001 (Tota l : 343 Patents) . T h e year is according to the filing date. : A s mentioned before electrolytic capacitors are a sub-class of conventional capacitors, i.e., they have two metall ic electrodes wi th a dielectric in between. The term electrolytic is used because the dielectric material is often formed electrochemically by oxidizing a metal using an electrolyte. Sometimes the dielectric is a porous material such as paper wetted wi th an electrolyte to act as a dielectric; most organic solvents have a high dielectric constant and as such they are well suited for this appl icat ion. Chapter 4. State of the Art 33 One of the interesting results of the patent survey is the ident i ty of com-panies act ively involved i n acqui r ing supercapacitor technology: three of the top five companies do not even d i rec t ly market supercapacitors: Table 4.2: Top 5 Supercapaci tor companies according to # of U S Patents . C o m p a n y # of U S Patents A r e a of Exper t i se M o t o r o l a 47 C B M , C P , T M N and T M O Electrodes Po lymer Elec t ro ly tes A s a h i Glass [41] 24 C B M Electrodes Organic Elec t ro ly tes M a t s u s h i t a G r o u p 18 C B M and C P Electrodes Organic , Po lymer and So l id Elec t ro ly tes M a x w e l l 15 C B M Electrodes Cur ren t Col lec tor Technology W i l s o n Grea tba tch [42] 13 C B M , T M O Electrodes Aqueous and Organic Elec t ro ly tes D u r i n g the 90s, M o t o r o l a has been the most active company i n this area. A s cel l phones have become ever more complex, their demand for energy and power has risen. M o t o r o l a has taken an active role i n developing solutions to meet the energy needs of i ts own cell phones. It has vigorously pursued a l l possible electrode technologies (27 patents di rect ly on electrodes). For elec-trolytes i t has focused on po lymer electrolytes possibly due to size, weight and lifetime considerations for i ts appl ica t ion . Po lymer electrolytes provide a wide operat ing voltage range and as such devices based on them could achieve h igh power density and energy density while being sma l l and l ight weight. Po lymer electrolytes have relat ively long lifetime; they should be able to meet the two year life cycle of cell phones. However these electrolytes are potent ia l ly more expensive t h a n t yp i ca l aqueous or organic electrolytes; but i t is expected that the overal l cost w i l l be comparable to bat tery technol-ogy used for cell phones. G i v e n Moto ro l a ' s indus t r i a l experience i n batteries, i t is wel l posi t ioned to develop commerc ia l supercapacitors complement ing batteries (and possibly micro fuel cells) for portable electronic devices; how-ever its technology may not be appl icable to larger scale appl icat ions such as electric vehicles due to cost. There are no indicat ions that M o t o r o l a plans to d i rec t ly market an electrochemical capacitor . Chapter 4. State of the Art 34 A s a h i Glass is a chemicals company s imi lar to D u P o n t . It has focused on developing carbon-based electrodes and compat ib le organic electrolytes for these electrodes. T h e technology that it is developing is wel l sui ted for a wide range of applicat ions i n terms of size and the potent ia l cost cou ld be compet-i t ive . It does not appear that A s a h i Glass intends to market supercapacitors directly, instead it is act ively cooperat ing w i t h E l n a . M a t s u s h i t a group has been act ively market ing electrochemical capacitors for sometime through its Panasonic brand. It holds c r i t i ca l patents for ac-t iva ted carbon based and conduct ing po lymer based electrodes. It has also developed many types of electrolytes ta i lored to specific electrodes. G i v e n the size of its t r ad i t iona l capaci tor market; M a t s u s h i t a w i l l r emain to be a dominant player in the field. Ul t racapac i tors consti tute one of the three product lines tha t M a x w e l l Technologies produces. T h e company has been one of the early players i n the field, and over t ime has established a dominant pos i t ion i n developing ul t racapaci tors w i t h very low E S R . It already markets a wide range of elec-t rochemica l capacitors; its latest p roduct is a 4 8 V , 144 module at a relat ively l ight weight of 15kg for automotive appl ica t ion [43]. W i l s o n Grea tba tch is a manufacturer of components for implan tab le med-ica l devices. It has had a great deal of experience w i t h batteries for i m -plantable medica l devices. Its patents are a imed at addressing energy needs for i ts devices and they are best sui ted for batteries a l though they could be used for electrochemical capacitors as wel l (as they c la im) . It is un l ike ly that the company would d i rec t ly compete i n the electrochemical capaci tor market . W h i l e the number of patents can indicate the major players i n a field; i t does not necessarily identify the c r i t i ca l players involved. H y p e r i o n Ca ta lys i s In ternat ional is one of those companies. W h i l e i t only holds 4 relevant U S patents; its patents cover the app l ica t ion of carbon nanotubes for electro-chemical capacitors. A s i t w i l l be discussed later carbon nanotubes ho ld the potent ia l to extend the life t ime of electrochemical capacitors (and L i Ion Bat ter ies) by enhancing the mechanical s tabi l i ty of electrodes. Chapter 4. State of the Art 35 In terms of patents covering key technologies for conduct ing polymers , the first U S patent can be traced back to researchers at M I T for electrochemical devices based on poly(3-methyl thiophene) i n 1985 (patent# 4,717,673). A broader patent covering p-doped conduct ive po lymer such as polypyrro le , polythiophene, polyani l ine and their derivatives was granted to the French group A l c a t e l (patent# 5,442,197). A s mentioned before P innac le Research Inst i tute has been the pioneer i n the area of meta l oxide and meta l n i t r ide electrodes for electrochemical ca-pacitors. M o t o r o l a , E P C O S , Evans Capac i to r and T / J C o r p o r a t i o n have been very active i n this area as wel l . M o t o r o l a also pioneered h y b r i d elec-t rochemical capacitors whereby the compos i t ion of the two electrodes are significantly different from one an other. A s for electrolytes, i t is rather difficult to identify holders of key technolo-gies as almost composi t ion of a l l possible electrolytes have been patented by many players w i t h minor differences. Showa Denko K . K . appears to have solely focused on electrolytes for electrochemical capacitors as i ts 7 rel-evant U S patents are a l l related to electrolytes, specifically sol id po lymer electrolytes. It is wor th also ment ioning the patents held by the U S government through the R & D insti tutes at the department of energy (3), U S A i r Force (5), U S A r m y (9) and U S N a v y (2); at 19 patents the U S government is ahead of many companies active i n this area. T h e U S A r m y holds significant patents related to t rans i t ion meta l oxide electrochemical capacitors. 4.2 A c a d e m i a W h i l e off the shelf metal-oxide and conduc t ing po lymer based electrochem-ica l capacitors may not be wide ly available yet; these electrode materials poses properties that enable t h em to achieve higher performance compared to purely carbon based electrodes. T h e potent ia l of these electrode materials is h ighl ighted by publ icat ions from several universit ies and R & D insti tutes. Chapter 4. State of the Art 36 Table 4.3: Top 10 supercapacitor research groups i n a lphabet ica l order. Team Leader Ins t i tu t ion Name Technology Col labora tors D . Belanger Univers i te du Quebec a M o n t r e a l (Canada) C P , C B M , T M O N R C , T . Brousse (Ecole Poly technique de l 'Univers i te de Nantes) B . Conway Univers i ty of O t t a w a (Canada) T M O , C P , T M N P innac le Research Insti-tute M . E n d o Shinshu Univers i ty (Japan) C B M A s a h i C h e m i c a l , N i s -shinbo, M . Dressel-h a u s ( M I T ) E . Frackowiak P o z n a n Univers i ty of Technol-ogy (Poland) C P , C B M , T M O F . B e g u i n ( C N R S ) C . C . H u N a t i o n a l C h u n g C h e n g Univers i ty (Taiwan) T M O , C B M Y . H . Lee S u n g k y u n k w a n Unive r s i ty (Korea) C B M ( C N T T M O M . Mast ragos t ino Univers i ty of B o l o g n a (Italy) C P , C B M P. S i m o n M . M o r i t a Y a m a g u c h i Univers i ty (Japan) Electrolytes P. S i m o n C I R I M A T , U M R C N R S (France) C P , C B M M . Mas t ragos t ino J . P . Zheng F l o r i d a A & M (US) T M O E V A N S Capac i to r Table 4.3 l ists the most active researchers ( in academia) work ing on super-capacitors based on searches through ISI Web of Knowledge and Engineer ing V i l l a g e 2 onl ine databases. O n l y E n g l i s h language publ ica t ions have been considered. T h e con t r ibu t ion of each group is briefly presented here. Professor Belanger 's group [44] has focused on low cost t rans i t ion oxide materials such as M n 0 2 and F e 2 0 3 . T h e y have also worked on act ivated carbon based aqueous capacitors. T h e i r goal is to fabricate hyb r id capacitors w i t h a posi t ive electrode based on low cost t rans i t ion me ta l oxides and a carbon based negative electrode. Such a device would take advantage of the high energy density of t rans i t ion meta l oxides; and the longer life t ime of Chapter 4. State of the Art 37 carbon based electrodes. In one of their recent publ icat ions [45], they present a hyb r id supercapac-i tor w i t h Mn02 posi t ive electrode and act ivated carbon negative electrode capable of an energy density of 10 W h / k g (36J/g) and a power density of 3600 W / k g (only the mass of the active electrodes is used for the calcula-t ion, i.e., mass of electrolyte,separator,. . . is not accounted for) while the cell is galvanostat ical ly cycled between 0 and 2 .2V. 0 . 6 5 M K2SO4 is used as electrolyte. Hydrogen evolut ion on the act ivated carbon electrode and oxy-gen evolu t ion at the M11O2 electrode is detected; however by reducing the window of operat ion to 1.5V the electrolyte breakdown is prevented. A s ment ioned before Professor Conway pioneered electrochemical energy storage devices based on pseudocapacitance. He has had numerous publ ica -tions covering b o t h the theoret ical and exper imenta l aspects of electrochemi-cal capacitors. H i s last pub l ica t ion [46] addresses h y b r i d supercapacitor (also referred to as asymmetr ic capacitors) . For systems w i t h a non-Faradaic and a Faradaic electrode to perform wel l , two operat ing condit ions have to be considered. F i r s t and foremost the charge stored and discharged by the system (also referred to as cell charge-capacity (Ah) ) has to be l imi t ed by the non-Faradaic electrode, so that the Faradaic-type bat tery electrode is discharged down to an appropriate state of charge; this ensures an opt imized energy density and long cycle life. If the Faradaic electrode is discharged a l l the way its life t ime wou ld d imin ish ; i t is best to design the system such that the capaci ty of the non-Faradaic electrode is about 1/3 of the capaci ty of the Faradaic electrode. Secondly T h e dis-charge/charge rate have to be chosen according to the Faradaic electrode; charging and discharging the device at faster rates wou ld undermine the life-t ime of the Faradaic electrode. One more art icle [47] from Professor Conway wor th ment ioning compares Ru02 w i t h two nitr ides of molybdenum: MoN and MoN2. W h i l e the ca-pacitance of the nitr ides are comparable to the ru then ium oxide electrodes, their operat ing voltage range is about hal f of the ru then ium oxide system, as the nitr ides decompose at potentials above 0.7 V vs. S H E . and as such these ni tr ides would have l imi t ed applicat ions. Chapter 4. State of the Art 38 Professor E n d o [48] has been researching synthet ic carbon materials for over three decades now. H e is one of the prominent researchers i n the field of carbon nanotubes. H i s group act ively collaborates w i t h bo th theorists such as M . S. Dresseihaus ( M I T ) and companies such as Showa Denko. H i s group has pioneered the work on extending the life t ime of lead-acid and L i - i o n batteries by adding a minute amount of ca rbon nanotubes to the anode elec-trode to restrict the degree of ac tua t ion of electrode dur ing charge/discharge cycles [49]; this may become the first mass commerc ia l appl ica t ion of carbon nanotubes. T h e E n d o group's act ivi t ies on electrochemical capacitors have been on development of carbon based electrodes and non-aqueous electrolytes specif-ica l ly ionic l iquids. The i r latest art icle [50] discusses two novel ionic l iquids: N , N - d i e t h y l - N -methy l (2-methoxye thy l )ammonium tetrafluoroborate (referred to as D E M E -B F 4 ) and N , N-d ie thy l -N-methyl (2-methoxye thy l ) a m m o n i u m bis (trifluo-romethylsulfonyl) imide (referred to as D E M E - T F S I ) . D E M E - B F 4 has an operat ing potent ia l of - 3 V to + 3 V vs. A g / A g C l and D E M E - T F S I has an operat ing potent ia l of - 3 V to 2.7 V vs. A g / A g C l . Such an operat ing window exceeds even that of i m m i d a z o l i u m based ionic l iquids. B o t h of these elec-trolytes are tested w i t h act ivated carbon electrodes and compared to a cel l runn ing on a propylene carbonate electrolyte. Due to the wider potent ia l range capacitors using these electrolytes achieve much higher energy den-sity. T h e gain i n voltage range is expected to off-set the increase i n in ternal resistance and as such l i m i t the reduct ion i n power. Professor Frackowiak i n co l labora t ion w i t h Professor Begu in ( C N R S U n i -versity, France) has been act ively developing electrode materials based on act ivated carbon, carbon nanotubes, ca rbon nanotubes coated w i t h conduct-ing polymers , carbon nanotubes coated w i t h amorphous manganese dioxide while invest igat ing phosphon ium based ionic l iquids and polyethylene oxide-KOH -H2O po lymer electrolytes. T h e latest paper from their col laborat ive effort, [51] investigates capaci-tance of m u l t i w a l l carbon nanotubes coated w i t h polypyrro le and po lyan i -line. T h e composite electrodes typ ica l ly conta in 20 w t % nanotubes and 80 Chapter 4. State of the Art 39 w t % polymer . T h e po lymer is synthesized chemical ly rather than electro-chemical ly as the electrochemical method can not mass produce po lymer or produce a composite w i t h large polymer to nanotube rat io. T h e m u l t i w a l l nanotube polypyrro le composite exhibi ts a specific capacitance of 1 9 0 F / g while the other composi te achieves 3 6 0 F / g (the specific capacitance is ca l -culated based on capacitance corresponding to one electrode d iv ided by the mass of that electrode). B y using the polypyrro le composi te as the negative electrode and the polyani l ine composite as the posit ive electrode a specific capacitance of 320 F / g is achieved. Professor H u has ma in ly focused on amorphous ru then ium oxide based electrochemical capacitors. H i s latest pub l i ca t ion [52] discusses improve-ments on the electr ical connect ion between the stainless steel current collec-tors and the active electrode mater ia l by usage of t h in films of gold. A cell compromis ing of an act ivated carbon loaded w i t h R u O x electrode and stain-less steel loaded w i t h t h in films of gold and R u O ^ runn ing i n 0 . 1 M H2SO4 exhibi ted a specific capacitance of 1 5 8 0 F / g (per mass of only R u O x ) dur ing cyc l ica l vo l t ammet ry from - 0.2 V to 0.9 V vs. A g / A g C l at a rate of 1 m V / s . Professor Lee's group has focused on carbon nanotubes special ly single-wal l carbon nanotubes as electrode materials while explor ing incorpora t ion of polypyrrole or ru then ium oxide i n the single wa l l carbon nanotube electrodes to achieve better performance. The i r 2001 paper [53] on single-wall carbon nanotube supercapacitors is one of the most c i ted nanotube electrochemical capacitor articles. In that paper they presented a device w i t h single-wall carbon nanotube electrodes on nickel current collectors runn ing i n 7.5 M K O H solut ion exh ib i t ing a m a x i m u m specific capacitance of 1 8 0 F / g energy densities i n the range of 6-7 W h / k g (21.6-25.2 J / g ) and power densities i n the range of 0.2-5 k W / k g . T h e connection between the current collectors and the electrodes was enhanced by pressing the single-wall carbon nanotube electrodes onto the nickel current collectors under 1000 ps i . T h e i r latest paper [54] presents composite electrodes based on nickel oxide and mul t iwa l l carbon nanotubes. Different loadings of ca rbon nanotubes have been at tempted. T h e best result corresponds to 15 w t % which leads to a specific capacitance of 1 6 0 F / g . Chapter 4. State of the Art 40 Professor Mast ragos t ino ' s group has focused p r imar i l y on conduct ing poly-mer electrodes; they could be considered as the number one group global ly i n this area. A l o n g w i t h Professor Simon 's group they have studied various conduct ing polymers as electrode materials for supercapacitors. In 2003 they publ ished results of an E U project [55] that developed a supercapacitor w i t h a cell voltage of 3 V and 1.5 k F capacitance. A s the project was a imed at elec-t r ic vehicles a long cell life t ime was needed, as such they used a hyb r id con-s t ruc t ion whereby the posit ive electrode was poly(3-methyl thiophene) and the negative electrode was act ivated carbon. Propylene carbonate w i t h 1 M te t rae thy lammonium tetrafluoroborate was used as electrolyte w i t h 2 sheets of 25pm th ick Teflon® based separators. T h e cell exhib i ted an energy density of 31 W h / k g (112J /g) , average power density of 512 W / k g and m a x i m u m power density of 9 k W / k g when discharged from 3 V 'to 1 V at a current density of 5 m A / c m 2 (mass of bo th electrodes is taken into account, every th ing else is not included) . T h e final cells tha t they made were packaged such that they could be readi ly used. T h e mater ia l used i n their design is low cost and as such commerc ia l iza t ion of their device is very realistic. In a more recent pub l i ca t ion [56], the electrolyte has been replaced w i t h N -b u t y l - N - m e t h y l p y r r o l i d i n i u m bis( tr i f luoromethanesulfonyl) imide ionic l i qu id . T h e operat ing cell voltage has been extended to 3 .6V. R u n n i n g the hyb r id cells w i t h the ionic l iquids electrolyte galvanostat ical ly from 3 .6V to 1.5V at 1 0 m A / c m 2 exhibi ts an energy density of 2 4 W h / k g (87J/g) and a m a x i m u m power density of 1 4 k W / k g . T h e separation distance between the electrodes i n this cell is bigger t han the other cell , as no separator has been used. If a separator is used, i t is expected that the in ternal resistance of the cell can be further reduced and as such the power density wou ld be further increased. T h e group at Y a m a g u c h i universi ty has focused on development of elec-trolytes for supercapacitors. T h e i r most c i ted work is on ethylene carbonate based organic electrolytes [57]. M o r e recently they have publ ished work on pro ton conduct ing electrolytes for electrochemical capacitors [58]. A non-aqueous polymer ic gel composed of poly(ethylene oxide) modif ied w i t h poly-methacrylate d issolving anhydrous H 3 P O 4 has been used as a sol id electrolyte w i t h act ivated carbon fiber c lo th electrodes. A t 50% doping w i t h H 3 P O 4 a conduc t iv i ty of 0.22 m S / c m is observed. A specific capacitance of 220 F / g runn ing at a current densi ty of 0 . 5 m A / c m 2 is achieved. Chapter 4. State of the Art 41 Professor Zheng has been work ing on electrochemical capacitors p r i m a r i l y ru then ium oxide based systems for over a decade whi le co l labora t ing w i t h E V A N S Capac i to r and D A R P A . H i s latest publication[59] presents theo-re t ica l predict ions on electrochemical capacitors w i t h in tercala t ion carbon electrodes. It is assumed propylene carbonate w i t h t e t r ae thy lammonium tetrafmoroborate is used as electrolyte. H i s calculat ions predict energy den-sities of 70-114 W h / k g (252-410 J / g ) based on electrode mater ia l only, 14-30 W h / k g (50-108 J / g ) based on electrode mater ia l and electrolyte, 8-20 W h / k g (29-72 J / g ) based on electrode mater ia l , electrolyte, and separator paper, and 7-15 W h / k g (15-54 J /g ) for a packed capacitor. 4 .3 O b s e r v a t i o n s Based on the ci ted l i terature, the current state of development of electro-chemical capacitors can be l is ted as the fol lowing. 1. C a r b o n based electrodes whi le costly efficient are not able to achieve h igh energy density on their own. 2. To achieve h igh energy density, h igh power density electrochemical ca-pacitors a h y b r i d design is essential. 3. Compos i te electrodes are necessary to min imize diffusion t ime for elec-trodes. 4. Th inne r electrodes w i t h a pore size d i s t r ibu t ion ta i lored to the elec-t ro ly te used are necessary to achieve better energy and power densities. 5. A m o n g conduct ive pol