Achieving high electrode specific capacitance with materials of low mass specific capacitance: Potentiostatically grown thick micro-nanoporous PEDOT films

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Achieving high electrode specific capacitance with materials of low mass specific capacitance: Potentiostatically grown thick micro-nanoporous PEDOT films
  Achieving high electrode specific capacitance with materials of lowmass specific capacitance: Potentiostatically grown thickmicro-nanoporous PEDOT films Graeme A. Snook  a,b , Chuang Peng  a , Derek J. Fray  b , George Z. Chen  a,* a School of Chemical, Environmental and Mining Engineering, University of Nottingham Coates Building, University Park, Nottingham NG7 2RD, UK  b Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK  Received 9 August 2006; received in revised form 17 August 2006; accepted 20 August 2006Available online 22 September 2006 Abstract Electrode materials for supercapacitors are at present commonly evaluated and selected by their mass specific capacitance ( C  M ,F g  1 ). However, using only this parameter may be a misleading practice because the electrode capacitance also depends on kinetics,and may not increase simply by increasing material mass. It is therefore important to complement  C  M  by the practically accessible elec-trode specific capacitance ( C  E , F cm  2 ) in material selection. Poly[3,4-ethylene-dioxythiophene] (PEDOT) has a mass specific capacitancelower than other common conducting polymers, e.g. polyaniline. However, as demonstrated in this communication, this polymer can bepotentiostatically grown to very thick films (up to 0.5 mm) that were porous at both micro- and nanometer scales. Measured by bothcyclic voltammetry and electrochemical impedance spectrometry, these thick PEDOT films exhibited electrode specific capacitance( C  E , F cm  2 ) increasing linearly with the film deposition charge, approaching 5 F cm  2 , which is currently the highest amongst allreported materials.   2006 Elsevier B.V. All rights reserved. Keywords:  Poly[3,4-ethylene-dioxythiophene]; Electrode specific capacitance; Supercapacitor; Quartz crystal microbalance; Cyclic voltammetry;Electrochemical impedance spectrometry Poly[3,4-ethylene-dioxythiophene] (PEDOT) respondsin a fast, reversible and stable manner to relatively largepotential perturbations [1–4], and hence promises applica-tions in electricity storage devices, particularly supercapac-itors for high pulsed power [5–8]. However, it has beensuggested that the drawback of PEDOT is its relativelysmall ‘‘mass specific capacitance’’ (210 F g  1 ) because of a quite large molar mass of the monomeric unit, in compar-ison with other conducting polymers, for example, poly-pyrrole (620 F/g) and polyaniline (750 F/g) [5]. Becausethe ‘‘mass specific capacitance’’ ( C  M ) generally impressesa proportionality between mass and capacitance, activematerials with smaller values of   C  M , such as PEDOT, willlikely receive less attention in the material selection processfor commercial applications.However, it has been demonstrated that electrochemi-cally deposited polypyrrole films only show proportionalitybetween capacitance and mass, which converts to filmthickness or deposition charge on an electrode substrateof fixed surface area, when the films are relatively thin(deposition charge <5 C cm  2 ) [9,10]. An upper limit inthe electrode capacitance of less than 1.0 F cm  2 , has beenobserved with further increasing the film thickness beingineffective to the capacitance [10]. In contrast, the electrodecapacitance of electrochemically prepared polyaniline filmsreaches beyond 2.0 F cm  2 linearly with increasing the filmthickness [11]. However, another problem is encountered:only relatively thin polyaniline films (deposition charge<15 C cm  2 ) can be grown coherently, but further elec- 1388-2481/$ - see front matter    2006 Elsevier B.V. All rights reserved.doi:10.1016/j.elecom.2006.08.037 * Corresponding author. Tel.: +44 115 9514171; fax: +44 115 9514005. E-mail address: (G.Z. Chen). Electrochemistry Communications 9 (2007) 83–88  trolysis leads to non-adherent polymer/oligomers insolution [11,12]. These previous observations on polypyr-role and polyaniline challenge their use in supercapacitors,even though they are both very high in ‘‘mass specificcapacitance’’.In fact, focusing on the ‘‘mass specific capacitance’’( C  M , F g  1 ) in the evaluation of supercapacitor electrodematerials can also be misleading in many other cases. Inpractice, the capacitance of a supercapacitor does not sim-ply increase linearly with increasing the amount of the elec-trode material. This is because the charge-dischargeprocesses in all redox reaction based or pseudo-capacitiveelectrode materials involve ion intercalation and depletiondue to electric neutrality requirement [5,9]. For electrodefabrication, in addition to mass, the thickness, conductivityand porosity of the active material need to be correlativelyoptimised so that ion transfer is insignificantly or not at allrestricted by kinetics. Because the thickness of the materialon an electrode substrate of a given surface area is directlyproportional to the mass, it is impossible to gain highercapacitance by increasing the mass after the thickness hasreached the kinetic limit. Ideally, kinetic restrictions onion transfer can be avoided by using well structured porousmaterials, but this approach is practically challengingbecause pores can increase the overall resistivity, volumeand mechanical instability of the electrode. Consequently,evaluation of the electrode’s performance cannot alwaysbe appropriate by only using  C  M . A complementary evalu-ation parameter is the ‘‘electrode specific capacitance’’ ( C  E ,F cm  2 ) that measures the practically accessible capaci-tance over a unit geometric area of the electrode substrate[11]. Obviously,  C  E  is dependent on the value of   C  M  but,more importantly, reflects the utilisation effectiveness of the active materials on the electrode, and hence is of greaterpractical importance.For PEDOT, very little is known about its ‘‘electrodespecific capacitance’’ against the mass. As discussed afore,if PEDOT may be made into stable and thick films, andalso retains its excellent redox reversibility and charge-dis-charge cycling stability towards large potential perturba-tions, it will become a much preferred candidate forcommercial applications, disregarding its relatively small C  M  value. Indeed, previous work has shown linearitybetween capacitance and film thickness for galvanostati-cally grown thin PEDOT films with the deposition chargebeing up to 0.21 C cm  2 [13]. Herein, we report unprece-dented experimental findings on potentiostatically grownPEDOT films, particularly the thick ones, that have exhib-ited linearly increasing capacitance with the film thicknessup to about 0.5 mm (deposition charge: 60 C cm  2 ).Although having not yet exhausted all variation possibili-ties in experiments, the measured value of   C  E  hasapproached close to 5.0 F cm  2 which, to our knowledge,is the highest amongst all reported supercapacitor electrodematerials.In this work, electrochemical polymerisation of 3,4-eth-ylene-dioxythiophene (EDOT) was first investigated in ace-tonitrile containing 0.2 mol l  1 EDOT and 0.5 mol l  1 LiClO 4  by cyclic voltammetry on a 5 mm gold disc. Theprocess was simultaneously monitored by an electrochemi-cal quartz crystal microbalance (EQCM). Typical resultsare presented in Fig. 1. The cyclic voltammogram (CV)in Fig. 1a shows an oxidation current starting at about0.45 V vs. Ag/AgCl (3 mol l  1 KCl). However, the polymeronly began to noticeably coat at around 0.95 V as evi-denced by the fast current increase and, more importantly,by the mass increase on the mass–potential plot (MPP) inFig. 1b. The current loop seen on the CV at the positiveend of the potential scan is also indicative of the nucle-ation/growth mechanism expected for electrochemicalpolymerisation [14]. Comparing the CV and the MPP, itis seen that the current flowing between 0.5 V and 0.9 Vdid not correspond to any polymer deposition.Interestingly, in this work, the low potential oxidationwas not observed on the Pt electrode, indicative of thewell-known interaction between the gold surface and a sul-fur compound such as the EDOT molecule. Further, the S– Au interaction (likely adsorption) is spontaneous andoccurred soon after the addition of the EDOT solution     I    /      µ    A  E  / V vs Ag/AgCl (3M KCl)       ∆     M    /  n  g  E  / V vs Ag/AgCl (3M KCl) ba Fig. 1. (a) Cyclic voltammetric deposition of PEDOT from a solution of 0.2 mol l  1 EDOT and 0.5 mol l  1 LiClO 4  in acetonitrile. (b) Simulta-neously recorded mass-potential plot. Working and counter electrodes:5 mm gold disc (on quartz crystal) and 0.5 mm Pt wire. Potential scan rate:50 mV s  1 .84  G.A. Snook et al. / Electrochemistry Communications 9 (2007) 83–88  to the EQCM cell. This means that before the potentialscan was applied for recording the CV and the MMP, thegold disc was already covered by the adsorbed EDOT mol-ecules. Consequently, the corresponding part on the MPPin Fig. 1b shows almost no mass change when the pre-adsorbed monomers underwent the low potential oxidationon the Au surface. On the other hand, the MPP does sug-gest that, once the initial polymer was deposited, thegrowth was very fast and continued at potentials as lowas 0.8 V. Based on these results, potentiostatic depositionat 1.0 V was used for all subsequent depositions to avoidover oxidation. As shown later, such selected potentialhelped the growth of thick and porous PEDOT films withhigh  C  E  values.The equivalent mass,  M  eq , of the deposited polymer perelectron transferred can be calculated as  M  eq  =  F  D m / Q ,where  F   is Faraday’s constant,  D m  the change in massand  Q  the charge passed for deposition [15–18]. TheEQCM measurements at 1.0 V on thin films (and henceassumed to be rigid [17]) showed  M  eq  to be87 ± 16 g mol  1 e  1 . In theory,  M  eq  ¼ ð  M  EDOT  þ  c  M  ClO  4 Þ = ð 2  þ  c Þ  [15], where  M  EDOT  (=140.20 g mol  1 ) and  M  ClO  4 (=99.45 g mol  1 ) are respectively the molar mass of themonomeric unit and that of the dopant anion in PEDOT; c  is the molar ratio of dopant anions and monomeric unitsin the oxidised polymer; and (2 +  c ) is the number of elec-trons withdrawn from each monomeric unit during anodicdeposition of the polymer. It can then be derived that when c  is changed from 0.1 to 0.3,  M  eq  increases from 71.5 to73.9 g mol  1 e  1 . Considering possible solvation in thedeposited PEDOT films, and experimental errors, theEQCM measurements are in very satisfactory agreementwith the theoretical calculations, and are also suggestiveof high current efficiency in the electrolytic polymerisationprocess at the selected deposition potential.The anodic deposition of PEDOT was then carried outon a 1.6 mm Pt disc in the same solution as mentionedabove. The as-deposited film, which should be in the fullydoped/oxidised state, was carefully scraped off the elec-trode and analysed by infrared spectrometry (IR) and theresults confirmed the same absorption features of oxidisedPEDOT between 400 cm  1 and 1600 cm  1 as reported inliterature [19,20].Cyclic voltammograms (CVs) of the PEDOT coated Ptelectrodes were recorded in the acetonitrile with 0.5 mol l  1 LiClO 4  as shown in Fig. 2a. Previously observed CV fea-tures of thin PEDOT films in organic solutions [21,22] weresatisfactorily repeated on our CVs, particularly the oxida-tion peaks A1 and A2, and the reduction peaks C1 andC2 as shown in Fig. 2a. More importantly, both oxidationand reduction currents on the CVs increased linearly withthe potential scan rate in either wide or narrow potentialranges. These linear relationships are characteristic of con-ducting polymer coatings. The CVs were also recorded inthe aqueous 0.5 mol l  1 KCl, showing similar features tothose in Fig. 2a, except for that at the negative end of the potential scan, large reduction currents were observed,which can be attributed to the reduction of proton that canlead to hydrogen evolution. To avoid this unwanted reac-tion, CVs in the aqueous electrolyte were recorded in nar-rower and more positive potential ranges, as shown inFig. 2b and c.Clearly, these EQCM, IR and CV characterisations allconfirm that the potentiostatically grown PEDOT coatingswere of similar quality to previously reported PEDOT pre-pared by chemical or electrochemical means. Because theemphasis of this work is the electrode specific capacitanceof this polymer, the CVs shown in Fig. 2a were first inves-tigated. The CVs recorded in the wide potential range showa fast current switching at the positive end of the potentialscan, as expected from a capacitor. However, a thin tailwas seen at the negative end of the potential scan, indica-tive of the polymer being reduced to the insulating state.The conversion from the reduced state to the more con-ducting state involves oxidation, i.e. the Faradaic current,which should be avoided in the measurement of thepseudo-capacitance. Thus, a narrower and more positivepotential range was applied, leading to the more capaci-tor-like CVs, as shown in Fig. 2a.In the aqueous electrolyte, the CVs presented a morerectangular shape as from an ideal capacitor. This perfor-mance is believed to have, at least partly, benefited fromthe higher polarity and conductivity of the aqueous electro-lyte than the acetonitrile electrolyte. Fig. 2b shows anexample for a thin PEDOT film (deposition charge:0.32 C cm  2 ) in 0.5 M KCl at 50 mV s  1 . This capacitor-like behaviour was only distorted at much faster scan ratesas demonstrated in Fig. 2b in which the distorted CV wasrecorded at 2 Vs  1 . The high scan rate distortion is partlydue to the ohmic polarisations of electronic and/or ionicnature in the polymer film [4,9–11] which is significantwhen larger currents flow at very high scan rates. In addi-tion, like all conducting polymers, charge and discharge of PEDOT involve movement of ions in the film, which cancontribute partly to the high scan rate distortion. For thesame reason, when the film became thicker, the scan rate,at which the rectangular capacitive shape of the CV canbe recorded, decreased as shown by the examples given inFig. 2b–d, indicative of increased kinetic influences.Nevertheless, for those CVs with the rectangular shape,the electrode capacitance can be calculated as  C  E  =  I  c / v ,where  I  c  is half of the difference between the average pla-teau currents (positive and negative scans), and  v  the scanrate [10,13]. Considering the potential dependence of   I  c  onthe CV, the average data measured at two typical poten-tials, 0.1 V and 0.3 V, are reported here. The capacitancefrom the CVs was found to be approximately proportionalto the deposition charge. Particularly, at deposition chargesof 0.32, 2.7 and 60 C cm  2 , the average  C  E  values measuredat the two selected potentials from the CVs in Fig. 2 at thelower scan rates were found to be 0.025 F cm  2 in acetoni-trile (Fig. 2a), and 0.028, 0.27 and 4.88 F cm  2 in water(Fig. 2b–d), respectively. Note that the capacitance mea-sured in water is about 10% higher than that in acetonitrile. G.A. Snook et al. / Electrochemistry Communications 9 (2007) 83–88  85  Using the data from the aqueous electrolyte, the  C  E / Q ratios of the three films, although very different in thick-ness, agree satisfactorily with each other (0.09, 0.10 and0.08 F C  1 ), but are almost twice as large as previouslymeasured  C  E / Q  ratios (0.044–0.055 F C  1 ) for galvanostat-ically grown thin PEDOT films (deposition charge: 0.07– 0.21 C cm  2 ) in 0.1 M NaCl [13]. The increased  C  E / Q  ratioin our films is likely due to potentiostatic polymerisationoffering higher current efficiency and more uniform struc-ture and composition for growing the PEDOT films.It has been considered that the  C  E  values measured fromCVs likely include contributions from both capacitive andFaradaic currents. The latter can originate from thedynamic nature of the CV method, and from both the poly-mer itself and any redox active impurity in the electrolytesuch as dissolved oxygen [13]. As a steady-state technique,electrochemical impedance spectrometry (EIS) can effec-tively avoid the Faradaic influence [13]. Fig. 3a compares the complex plane plots of a thin (deposition charge:0.32 C cm  2 ) and a thick (60 C cm  2 ) PEDOT films mea-sured at 0.4 V. The thin film impedance followed a verticalline very well at most applied frequencies but deviationoccurred at about 35 Hz and higher. Similar behaviourwas observed for the thick film at low frequencies but thedeviation from the vertical line at high frequencies wasmore pronounced, forming an inclined section with theangle to the  Z  0 -axis being much smaller than 45  . Whilstin both cases, the deviation from the vertical line is indica-tive of the kinetic influence of ion (1) transfer across thepolymer/electrolyte interface and (2) transport inside thepolymer film [13], the frequency at which the deviationoccurs, known as the knee frequency [23], is a usefulparameter for measuring the speed of the film respondingto charge-discharge process. Fig. 3b plots the knee fre-quency against the deposition charge, showing that, aftera quick drop, the knee frequency becomes relatively stablewith increasing the film thickness. Even for the thickest filmmade in this work at a deposition charge of 60 C cm  2 , theknee frequency still remained at a practically accessiblevalue of about 0.3 Hz. It is noticed that the intercept of the impedance plot at the  Z  0 -axis is smaller for the thickfilm than that for the thin film, see the inserts in Fig. 3b,which may be attributed to the thick film having a muchlarger surface area, apart from experimental errors.Values of the imaginary impedance,  Z  00 , on the verticallines are related to the so called low frequency capacitanceas  C  E  = 1/(2 p  fZ  00 ). Therefore, plotting  Z  00 against 1/(2 p  f  )should give a straight line whose slope is the reciprocal of  C  E . The two inserts in Fig. 3a confirm such linear relations.The EIS measured values of   C  E  were also found to depend -40-30-20-100102030  E 40-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6  / V vs Ag/AgCl         I    /      µ    A -1.2-0.9-0.6-         I     /  m   A -0.7-0.5- 0.6-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6  E  /V vs Ag/AgCl         I    /  m   A -0.25-0.15- 2mVs -1 5mVs -1 -2.0-1.5-1.0- -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6  E  / V vs Ag/AgCl         I    /  m   A -0.4-0.3-0.2- 50 mV s -1 500 mV s -1 -0.25-0.15- 0.  25-1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75  E  / V vs Ag/AgCl         I     /  m   A A1A2C1C22 V s -1 50 mV s -1 a bc d Fig. 2. Cyclic voltammograms of potentiostatically (1.0 V vs. Ag/AgCl) grown PEDOT films on Pt disc (1.6 mm diameter) (a) in acetonitrile with0.5 mol l  1 LiClO 4  at different scan rates (inwards, wide range: 300, 200, 150, 100, 50 mV s  1 , narrow range: 300, 200, 100 mV s  1 ), and (b), (c) and (d) inaqueous 0.5 mol l  1 KCl. Deposition charge: (a) and (b) 0.32 C cm  2 , (c) 2.7 C cm  2 , and (d) 60 C cm  2 .86  G.A. Snook et al. / Electrochemistry Communications 9 (2007) 83–88  on the applied potential in a similar manner as the CV mea-sured  C  E . Fig. 3c plots typical  C  E  values measured by EISand CV against the deposition charge, showing that thetwo methods produced very comparable results.The inset in Fig. 4 is the side view of the Pt disc electrodewith a thick PEDOT coating. It can be measured from thisphoto that the coating was about 0.5 mm in thickness. Thepotentiostatically grown coatings were reasonably smoothas viewed by eye, but were highly porous as revealed byscanning electron microscopy (SEM) as shown in Fig. 4.Clearly, there is a large interconnecting network of poly-mer with micro- and nano-sized pores, which illustrateswhy the charge storage centres are so accessible. Themicro-sized pores help de-polarisation of the ion concen-tration and hence a high speed of charge-discharge,whereas the nano-sized pores result in a high interfacialarea and therefore a high capacitance per geometric unit(volume or area).In summary, the potential for anodic deposition of PEDOT films has been selected as 1.0 V vs. Ag/AgCl(3 mol l  1 KCl) on the basis of simultaneous CV andEQCM measurements. At this potential, coherent and por-ous PEDOT films have been grown on a 1.6 mm Pt discelectrode at high current efficiency, reaching a depositioncharge of 60 C cm  2 and a film thickness of 0.5 mm. Thedeposited PEDOT films have exhibited linearly increasingand practically accessible capacitance, approaching5 F cm  2 , as measured by both CV and EIS. This electrodespecific capacitance of PEDOT is much higher than previ-ously reported electrode specific capacitance for polypyr-role and polyaniline, even though the latter two are muchlarger in mass specific capacitance (F g  1 ). This novelexperimental finding highlights the importance of usingboth mass specific capacitance and electrode specificcapacitance to evaluate electrode materials for supercapac-itor applications. Fig. 4. SEM image of the top view of a potentiostatically grown PEDOTfilm (deposition charge: 30 C cm  1 ). Inset: photograph of the side view of a PEDOT film (60 C cm  1 ) on a Pt disc electrode (sheathed in epoxyresin). 050100150200250300050100150200250300  Z'  / ohm     Z    '    '    /  o   h  m 0.01 Hz 1.05 Hz 0.32 C cm -2 60.0 C cm -2 y = 1748x+6.829R 2  = 1 0500100015002000250030000.0 0.5 1.0 1.5 2.0 y = 10.3x+3.08R 2  = 1 03060901201501800 5 10 15 20    Z   "   /  o   h  m (2  f  ) -1  / Hz -1 0.01 Hz 0.10 Hz 0.1110100-10102030405060 Deposition charge/C cm -2    K  n  e  e   f  r  e  q  u  e  n  c  y   /   H  z 010203030405060  Z'  / ohm     Z    '    '    /  o   h  m Knee,35.6 Hz Knee,0.30 Hz y= 81.23x R 2  = 0.9996    E   l  e  c   t  r  o   d  e  c  a  p  a  c   i   t  a  n  c  e   /   F  c  m   -   2 EIS CVLinear ( EIS) 0 Deposition charge/C cm -2 cba Fig. 3. (a) Complex plane impedance plots of a thin (squares, 0.32 C cm  1 deposition charge) and a thick (triangles, 60 C cm  1 ) PEDOT filmsmeasured at 0.4 V vs. Ag/AgCl. Insets: the linear relations between  Z  00 and1/(2 p  f  ) for the two films. (b) Knee frequency as a function of film thickness(deposition charge). Inset: selection of the knee frequencies for the twofilms used for recording (a). (c) Electrode specific capacitance as a functionof film deposition charge. Values measured by EIS (filled triangles) andCV (empty squares) are both presented. G.A. Snook et al. / Electrochemistry Communications 9 (2007) 83–88  87
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