polymer batteries

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Synthetic Metals 160 (2010) 76–82 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Funct...

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Synthetic Metals 160 (2010) 76–82

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Functionalised polyterthiophenes as anode materials in polymer/polymer batteries C.Y. Wang, G. Tsekouras, P. Wagner, S. Gambhir, C.O. Too, D. Officer, G.G. Wallace ∗ ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Squires Way, Fairy Meadow, NSW 2519, Australia

a r t i c l e

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Article history: Received 10 August 2009 Received in revised form 23 September 2009 Accepted 1 October 2009 Available online 6 November 2009 Keywords: Conducting polymers Rechargeable battery Functionalised polyterthiophenes Polypyrrole All-polymer battery

a b s t r a c t A functionalised polyterthiophene, poly(3 -styryl-4,4 -didecyloxyterthiophene) (poly(OC10 DASTT)), was investigated as an anode coupled with a polypyrrole cathode in a battery with a lithium hexafluorophosphate (LiPF6 ) in 1:1 ethylene carbonate (EC):dimethylcarbonate (DMC) electrolyte. The polymer was electrodeposited on stainless steel mesh and Ni/Cu-coated nonwoven polyester fabric. A discharge capacity of 45.2 mAh/g was obtained for the battery constructed using poly(OC10 DASTT) on a Ni/Cu-coated fabric as the anode. An alternative anode material, poly(4,4 -didecyloxyterthiophene) (poly(OC10 STT)) was electropolymerised on Ni/Cu-coated fabric, and exhibited a maximum discharge capacity of 94.7 mAh/g. The capacity decreased for both polymers with repeated charge/discharge cycling. This deterioration is attributed to mechanical degradation of the polymer as evidenced by scanning electron microscopy (SEM). © 2009 Elsevier B.V. All rights reserved.

1. Introduction One of the most promising applications of inherently conducting polymers (ICPs) is in charge storage devices including supercapacitors [1] and batteries [2,3]. The fact that ICPs can be electrochemically switched between the doped (conducting) and undoped (insulating) states provides the basis for application areas. Currently there exists an active research direction in advanced batteries to make them flexible, which could lead to important applications in modern gadgets, such as roll-up displays, wearable devices, radio-frequency identification tags and integrated circuit smart cards [4–6]. ICPs are promising materials for this application due to the fact that these polymer materials may be electrochemically switched and possess useful mechanical properties in terms of tensile strength and flexibility [7,8]. Most research in rechargeable batteries has focused on the use of ICPs as cathode materials due to the good reversibility and stability of p-type ICPs [9–11]. In contrast, n-type ICPs or neutral ICPs usually exhibit low charge capacity and poor chemical stability [12,13]. Only a few studies investigating ICPs as both the cathode and anode material in rechargeable batteries [14–18] have appeared. Killian et al. [14] reported a battery sys-

∗ Corresponding author at: ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Squires Way, Fairy Meadow, NSW 2519, Australia. Tel.: +61 2 4221 3127; fax: +61 2 4221 3114. E-mail address: [email protected] (G.G. Wallace). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.10.001

tem based on polypyrrole (PPy), which exhibited a specific capacity of 22 mAh/g. The anode was PPy containing polystyrenesulfonate (PSS) dopant and the cathode was PPy containing perchlorate (ClO4 − ) as dopant. Similarly an all-polymer battery based on PPy was reported recently by Song et al. [15], where the battery was composed of a PPy containing indigo carmine (IC) anode and a PPy containing 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) cathode showing a specific capacity of 15 mAh/g at which the potential reached 300 mV. A battery based on polythiophenes was reported by Gofer et al. [16] where the cathode and anode materials were 3-(3,5-bifluorophenyl)thiophene and 3-(3,4,5-trifluorophenyl)thiophene, respectively. This battery produced a discharge capacity of 9.5–11.5 mAh/g. Another type of all-polymer battery reported by Rehan [17] with a poly(1-naphthol) cathode and a polyaniline anode gave a discharge capacity of 150 mAh/g. Polyaniline was also investigated as an anode material with a polyindole cathode by Cai et al. [18] and this cell exhibited a specific capacity of 79 mAh/g. In a previous study [19], we reported on a novel anode material that consisted of a chemically synthesised functionalised polyterthiophene poly(3 -styryl-4,4 didecyloxyterthiophene) (poly(OC10 DASTT)). A discharge capacity of 39.1 mAh/g was obtained when this material was coupled to a cathode consisting of PPy doped with hexafluorophosphate (PF6 − ). In this work we have extended our study into the use of functionalised polyterthiophenes synthesised via electropolymerisation as anode battery materials. Electropolymerisation is generally preferred to chemical preparation of ICPs and is ideal for electrochemical studies because it provides better control of film

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thickness, morphology and cleaner polymers. The chemical methods imply the use of various oxidants, even of a catalyst [20,21]. For electrochemical studies of chemically prepared ICPs, particularly for the application in batteries or supercapacitors, the polymers are either pressed [22] or pasted onto the electrode surface with the addition of binders (e.g. polyvinylidene fluoride (PVDF) or polyfluoroethylene (PTFE)) and conductive additives (e.g. carbon black, graphite). Two monomers were investigated, namely 3 -styryl-4,4 didecyloxyterthiophene (OC10 DASTT) and 4,4 -didecyloxyterthiophene (OC10 STT), the structures of which are shown below. The difference between these two monomers is the presence of an alkene-linked aromatic ring in OC10 DASTT, which would be expected to alter the electron-withdrawing nature of the resultant polymer and therefore affect the redox properties. Both monomers included strategic didecyloxy substitution in order to improve polymer regioregularity via electropolymerisation through 2- and 2 -positions. The use of electropolymerisation was expected to result in more intimate substrate/polymer interaction compared to the previous studies using this polymer prepared chemically [19]. Functionalised polyterthiophenes were electropolymerised on stainless steel mesh, Ni/Cu-coated nonwoven polyester fabric and carbon fibre mat. All of these substrates are conductive and have an inherent degree of flexibility and conformability making them possible candidates for wearable energy storage. In the battery configuration, functionalised polyterthiophenes were used as anodes and PPy was used as the cathode.

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[24]. The OC10 STT was synthesised from 2,5-dibromoithiophene and 4-decyloxythiophene-2-boronic acid following a procedure described previously by Zotti et al. [25]. The detailed synthesis, characterisation and chemical polymerisation of OC10 DASTT and OC10 STT will be reported elsewhere. 2.2. Electrodeposition of polymer and battery fabrication Electrochemical polymerisation or characterisation of the polymer was performed using an EG&G PAR 363 Potentiostat/Galvanostat, a MacLab 400, and a Chart 4 software or EChem v1.3.2 software (ADInstruments). A three-electrode cell was used that had a stainless steel mesh counter electrode and Ag/Ag+ reference electrode employing a salt bridge containing 0.1 M TBAP in CH3 CN (EAg/Ag+ = +0.7403 V). Poly(OC10 DASTT) film could not be electrodeposited potentiostatically at +0.7 V (vs. Ag/Ag+ ) on stainless steel mesh (Metal Mesh Pty. Ltd., Australia, product No. T316), Ni/Cu-coated nonwoven polyester fabric (Laird Technologies, USA, product No. 3027-217) or carbon fibre mat (Spectracorp, USA, lot 11104), but rather a few black particles formed in solution at room temperature. To decrease the polymerisation rate and thereby promote polymer deposition on the substrates, the concentration of the monomer was reduced from 5 to 1 mM and the experiment was carried out at 0 ◦ C. These conditions enabled poly(OC10 DASTT) to be successfully electrodeposited on these three substrates. Poly(OC10 DASTT) film was electrodeposited potentiostatically at +0.70 V (vs. Ag/Ag+ ) from an electrodeposition solution containing 1 mM monomer OC10 DASTT and 0.1 M tetrabutylammonium perchlorate (TBAP) in CH2 Cl2 /CH3 CN (1:1 by volume) for 20 min at 0 ◦ C in a water-ice bath. Poly(OC10 STT) was similarly electrodeposited potentiostatically at +0.70 V (vs. Ag/Ag+ ) from a solution containing 5 mM OC10 STT and 0.1 M TBAP at room temperature. The solvent was a mixture of CH2 Cl2 /CH3 CN (1:1 by volume). Prior to the electropolymerisation, solutions were de-oxygenated with N2 . Following electrodeposition, polymer electrodes were rinsed with CH3 CN. PPy films doped with PF6 − were electrodeposited galvanostatically at a current density of 1.0 mA cm−2 on stainless steel mesh as reported previously [19]. Poly(OC10 DASTT) and poly(OC10 STT) were also electrodeposited by cyclic voltammetry (CV). Batteries composed of functionalised polyterthiophene anodes (1 cm × 1 cm) and PPy cathodes (1 cm × 1 cm) were assembled into Teflon cells in an argon-filled glove box (Unilab, Mbraun, USA). The configuration of the Teflon cell has been given previously [19]. The electrolyte was 1.0 M LiPF6 in 1:1 (V/V) of ethylene carbonate (EC):dimethylcarbonate (DMC) solution, and the separator used was Celgard 2500 microporous polypropylene membrane. 2.3. Battery testing

2. Experimental 2.1. Monomer synthesis The synthesis of OC10 DASTT was carried out following the building block approach previously reported for 3-substituted bis(decyloxy)terthiophenes [23]. 4,4 -Bis(decyloxy)-3 -formyl2,2 :5 ,2 -terthiophene was prepared in 67% yield by the Suzuki coupling of 2,5-dibromo-3-formylthiophene [24] and 4-decyloxythiophene-2-boronic acid [23]. The formylterthiophene was converted to OC10 DASTT in 75% yield using Wittig chemistry in a similar manner to that used for the preparation of trans-1-((2 ,2 :5 ,2 -terthiophene)-3 -yl)-2-phenylethene

Batteries were tested using a battery testing device (Neware, Electronic Co., China). The cells were charged galvanostatically at a current density of 0.05 mA cm−2 to a cell voltage of 1.65 V, and discharged at the same current density to a cut-off voltage of 0.2 V. Three similar batteries were tested simultaneously to assess reproducibility. 2.4. Surface morphology characterisation The surface morphologies of electrodes were investigated with a scanning electron microscope (SEM, Leica Model Stereoscan 440) with a secondary electron detector. To avoid charging of polymer samples and in order to obtain clear images, all the samples were sputter-coated with a thin layer of gold in advance. SEM

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Poly(OC10 DASTT) was also electrodeposited onto these three substrates by application of a constant +0.70 V (vs. Ag/Ag+ ) potential at 0 ◦ C. Chronoamperograms obtained during polymer growth are shown in Fig. 2. The rise in current indicated the deposition of a ICP with a concomitant increase in surface area. The different current increase during polymer growth on different substrates is indicative of a greater rate of deposition on the different substrates. 3.2. Surface morphology of poly(OC10 DASTT) The surface morphology of poly(OC10 DASTT) electrodeposited at +0.70 V depended on the substrate (Fig. 3). This reflected the different polymerisation rate of poly(OC10 DASTT) on the different substrates, again demonstrating that the substrate plays an important role during polymer electrodeposition. The polymer coating on stainless steel mesh was uneven and was characterised by granular particles (Fig. 3(a)). The Ni/Cu-coated nonwoven polyester fabric showed a more even and continuous coating of polymer film (Fig. 3(b)), and better electrochemical properties were expected as a result. 3.3. Cyclic voltammetry of poly(OC10 DASTT)

Fig. 1. Potentiodynamic growth of poly(OC10 DASTT) on different substrates: (a) Ni/Cu-coated nonwoven polyester, and (b) stainless steel mesh. Electropolymerisation solution: 1 mM OC10 DASTT and 0.1 M TBAP in 1:1 CH3 CN:CH2 Cl2 . Potential range: −600 to 1200 mV, scan rate: 50 mV s−1 , 15 cycles.

examinations were carried out under an accelerating voltage of 20 kV.

CV was used to identify the intrinsic redox reaction of poly(OC10 DASTT) on different substrates (Fig. 4). The electroactivity of the polymer shown in the CVs on the two substrates was different, which verifies that the substrates cannot only affect the electropolymerisation but also the electrochemical properties of the resultant polymer. A reversible redox couple at +180 mV was observed for the polymer on stainless steel mesh (Fig. 4(a)). However, on Ni/Cu-coated nonwoven polyester fabric, a less reversible redox couple was observed together with another cathodic response at −30 mV ascribed to the coating layer of metal Ni/Cu. It is also noted that the area under the respective voltammetric waves indicated that the available charge/discharge capacity was different, suggesting that the batteries employing poly(OC10 DASTT) on these two substrates as anode materials would display different electrochemical characteristics. 3.4. Battery testing with poly(OC10 DASTT) as anode material Batteries were designed to be anode-limited, whereby the discharge capacity of the PPy cathode was much higher than the anode. The discharge capacity of the cell was calculated based on the amount of the active material (poly(OC10 DASTT)) on the

3. Results and discussion 3.1. Electrochemical synthesis of poly(OC10 DASTT) on different substrates Poly(OC10 DASTT) was successfully electrodeposited on Ni/Cucoated nonwoven polyester, carbon mat and stainless steel mesh using CV. All of the CVs obtained during growth (Fig. 1) displayed increased currents with successive cycles; indicative of ICP growth. The onset oxidation potentials for monomer oxidation were different on the three substrates investigated, namely +0.23 V for Ni/Cu-coated nonwoven polyester and +0.64 V on stainless steel mesh. This indicated that the substrates played an important role in the electrochemical growth of the polymer, and that poly(OC10 DASTT) could be more easily electropolymerised on Ni/Cu-coated nonwoven polyester. The oxidation in Fig. 1(a) between −400 and −200 mV could be attributed to copper oxidation from the Ni/Cu coating layer on this substrate, since this response was observed in the absence of monomer. The increase in current with the repeated scan may be due to increasing surface area as polymer is deposited.

Fig. 2. Chronoamperograms recorded during the electrodeposition of poly(OC10 DASTT) on (1) Ni/Cu-coated nonwoven polyester and (2) stainless steel mesh substrates. Electropolymerisation solution: 1 mM OC10 DASTT and 0.1 M TBAP in 1:1 CH3 CN:CH2 Cl2 .

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Fig. 5. The 2nd discharge curves of all-polymer poly(OC10 DASTT)/LiPF6 /PPy batteries with anode on different substrates: (a) stainless steel mesh and (b) Ni/Cu-coated nonwoven polyester fabric at the second cycle. Charge/discharge current density: 0.05 mA cm−2 .

Fig. 3. SEM images of the electrodeposited poly(OC10 DASTT) on (a) stainless steel mesh and (b) Ni/Cu-coated nonwoven polyester fabric.

anode electrode. The mass of the active material was determined by weighing the electrode with conducting polymer, and then subtracting the mass of substrate. A charge/discharge current density of 0.05 mA cm−2 was applied in this experiment, and the discharge curves are shown in Fig. 5. The curves show that the cell voltages decreased as the depth of the discharging increased, a characteristic of rechargeable batteries. It is noted that the polymer batteries possessed a sloping discharge curve, which is typical of non-stoichiometric reaction. In turn, this indicates that the struc-

Fig. 4. CVs of poly(OC10 DASTT) on different substrates: (a) stainless steel mesh and (b) Ni/Cu-coated nonwoven polyester fabric; electrolyte: 0.1 M TBAP in CH3 CN; potential range: −700 to +700 mV; scan rate: 10 mV s−1 .

ture of the polymer was changing or that no stable intermediate state was produced during the discharge process. The cells composed of these two blank substrates (i.e. without polymer) anode and PPy cathode were also investigated, and negligible capacities were observed. A maximum discharge capacity of 45.2 mAh/g was obtained at the 6th discharge cycle for the cell using the anode material poly(OC10 DASTT) on Ni/Cu-coated nonwoven polyester fabric, which was higher than the 39.1 mAh/g obtained for the same polymer chemically synthesised previously [19]. Moreover it is much higher than the 22 or 15 mAh/g that has been reported for PPy systems [14,15] or the 9.5–11.5 mAh/g for polythiophenes [15]. It is, however, still lower than the discharge capacity reported in Refs. [16,17], but it should be noted that an inorganic acid was used in these latter two systems. In comparison, the cell employing poly(OC10 DASTT) on stainless steel mesh as the anode exhibited a much lower discharge capacity of 21.1 mAh/g. This can be explained with reference to the SEM for this anode given in Fig. 2, which showed non-uniform particulate morphology and therefore poor contact with the substrate. The cell voltage at the half discharge capacity point was 0.65 V for the anode using Ni/Cu-coated nonwoven polyester fabric substrate, higher than the 0.52 V for the anode using stainless steel mesh substrate. Both these types of batteries exhibited a high reproducibility, and a very small variation of about 2% of the capacities was shown. These results indicate that the battery using Ni/Cu-coated nonwoven polyester fabric possesses better discharge properties as a rechargeable battery, in agreement with the conclusion reached previously [19]. The discharge capacities for poly(OC10 DASTT) coated stainless steel mesh or Ni/Cu-coated nonwoven polyester fabric anode as functions of the cycle number are shown in Fig. 6. It can be seen that batteries using a stainless steel mesh substrate demonstrated a promising cycle life since the discharge capacity did not fade even after 150 cycles (Fig. 6(a)), although the discharge capacity was much lower than the battery with the anode material on Ni/Cu-coated nonwoven polyester fabric substrate. Cycle life testing indicated that poly(OC10 DASTT) on stainless steel mesh substrate was electrochemically stable, and this is consistent with the reversible redox properties shown in Fig. 4(a). In contrast, batteries using poly(OC10 DASTT) on Ni/Cu-coated nonwoven polyester fabric as substrate demonstrated an activation process, and it took 6 charge/discharge cycles to reach the maximum discharge capacity of 45.2 mAh/g, after which the discharge capacity deteriorated with cycle number. The discharge capacity dropped to 37.1 mAh/g, or 82.1% of the maximum capacity, after 50 charge/discharge cycles.

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Fig. 6. Cycle life and discharge efficiency of all-polymer poly(OC10 DASTT)/LiPF6 /PPy batteries of type with anode on different substrates: (a) and (d) stainless steel mesh; (b) and (c) Ni/Cu-coated nonwoven polyester fabric.

Only 77.6% and 59.1% of the maximum discharge capacity was retained after 100 and 150 charge/discharge cycles, respectively. However, the residual discharge capacity was 26.7 mAh/g after 150 cycles, which was still higher than the battery using stainless steel mesh substrate. The discharge efficiency (Á), defined as the ratio of discharge capacity to charge capacity, was also investigated and is reported in Fig. 6. The cell with stainless steel mesh anode substrate showed

Fig. 8. Discharge curve (a) and discharge capacity (b) of poly(OC10 STT)/LiPF6 /PPy battery.

a high discharge efficiency of ∼95%. However, a discharge efficiency of <60% was shown for the cell with stainless steel mesh anode substrate in the first 20 cycles, before the discharge efficiency increased with cycling. The lower discharge efficiency can be ascribed to the discharge capacity degradation. 3.5. Surface morphological changes of poly(OC10 DASTT) with charge/discharge cycling The surface morphologies of poly(OC10 DASTT) coated anodes after 150 charge/discharge cycles were examined using SEM (Fig. 7). Little evidence of change was observed for the polymer on stainless steel mesh substrate when compared with the polymer before battery testing (Fig. 3(a)). Not surprisingly, a very steady cycle life was demonstrated for the cell employing poly(OC10 DASTT) on stainless steel mesh anode as discussed above. However, on Ni/Cu-coated nonwoven polyester fabric substrate, the polymer poly(OC10 DASTT) was severely damaged after 150 charge/discharge cycles. This explains the discharge capacity deterioration with cycle number in Fig. 6 and less reversible electrochemistry shown in the CV in Fig. 4(b). 3.6. Battery testing using poly(OC10 STT) as anode material

Fig. 7. SEM images of poly(OC10 DASTT) anode electrodes after 150 charge/discharge cycles on different substrates: (a) stainless steel mesh, and (b) Ni/Cu-coated nonwoven polyester fabric.

For comparison with poly(OC10 DASTT), another functionalised polyterthiophene, poly(OC10 STT), was tested as an anode material in batteries. The decrease in steric hindrance due to the absence of a pendant aromatic group in OC10 STT appeared to be beneficial to the electrodeposition of the polymer. Poly(OC10 STT) was successfully electrodeposited on Ni/Cu-coated nonwoven polyester fabric substrate from 5 mM monomer solution at room temperature. Chronoamperograms and CVs of

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cycles (Fig. 9(b)), which explains the deterioration of discharge capacity with cycle life in Fig. 8. 4. Conclusions

Fig. 9. Surface morphology of a poly(OC10 STT) on Ni/Cu-coated nonwoven polyester fabric (a) as-synthesised, and (b) after 50 charge/discharge cycles.

polymer electrodeposition are not shown due to the similarity with the results already shown for the electrodeposition of poly(OC10 DASTT). The properties of poly(OC10 STT) as an anode material were investigated in terms of galvanostatic charge/discharge coupled with a PPy-coated stainless steel mesh cathode. The discharge curve and cycle life are shown in Fig. 8. A sloping discharge curve was observed for the battery with poly(OC10 STT) anode on a Ni/Cu-coated nonwoven polyester fabric substrate, indicating that a non-stoichiometric reaction occurred during the discharge process similar to the analogous case of poly(OC10 DASTT). A discharge capacity of 81.3 mAh/g was obtained on the 9th cycle, which was much higher than the 45.2 mAh/g obtained for a poly(OC10 DASTT) anode on the same substrate (refer to Section 3.4). The discharge capacity increase may be partially explained by the lower molecular weight of OC10 STT compared to OC10 DASTT. The cell voltage at half discharge capacity was 0.52 V, slightly lower than the 0.65 V for poly(OC10 DASTT). These results support previous findings regarding the electrochemical properties of ICPs and the relationship to monomer structure [26]. A slow activation process in the cycle life of batteries with a poly(OC10 STT) anode was observed in Fig. 8(b). The highest discharge capacity of 94.7 mAh/g was reached on the 16th cycle. However, it decreased with cycle number, and only 49% of the maximum capacity was retained after 50 cycles. The poly(OC10 STT) anode, after charge/discharge cycling, was taken out of the battery and the surface morphology compared with the assynthesised polymer (Fig. 9). A relatively uniform, continuous film of poly(OC10 STT) was formed during electrodeposition (Fig. 9(a)). However, some cracks were formed after 50 charge/discharge

A functionalised polyterthiophene, poly(OC10 DASTT), was successfully electrodeposited onto stainless steel mesh and Ni/Cucoated nonwoven polyester fabric substrates with the formation of different surface morphologies. Reversible redox properties were observed in the CV of poly(OC10 DASTT) on stainless steel mesh substrate, and its discharge capacity during battery testing did not decrease, even after 150 charge/discharge cycles. In contrast, less reversible properties were observed for poly(OC10 DASTT) on Ni/Cucoated nonwoven polyester, yet a much higher discharge capacity of 45.2 mAh/g was obtained. However, only 59.1% of the maximum discharge capacity was retained after 150 charge/discharge cycles when using this anode, but that was still higher than the reported results from other groups. The deterioration in capacity was ascribed to the degradation of polymers as characterised by SEM. Another functionalised polyterthiophene, poly(OC10 STT), was electrodeposited onto Ni/Cu-coated nonwoven polyester and investigated as a battery anode material for comparison. A discharge capacity of 94.7 mAh/g was achieved for a poly(OC10 STT)/LiPF6 /PPy battery. Discharge capacity deterioration was also found for this polymer, with only 49% of the maximum capacity retained after 50 charge/discharge cycles. These results showed that the electrochemical properties of polymers can be affected by monomer structure and the substrate onto which polymers are electrodeposited. Future investigations will aim to solve the problem of degradation during the charge/discharge process. These polymer electrode materials provide an interesting alternative when considering the development of wearable flexible battery structures. Acknowledgement Financial support from the Australian Research Council is gratefully acknowledged. References [1] A. Rudge, I. Raistrick, S. Gottesfeld, J.P. Ferraris, Electrochim. Acta 39 (1994) 273–287. [2] A. Mohammadi, O. Inganas, I. Lundstrom, J. Electrochem. Soc. 133 (1986) 947–949. [3] P. Novak, K. Muller, K.S.V. Santhanam, O. Haas, Chem. Rev. 97 (1997) 207–282. [4] H. Nishide, K. Oyaizu, Science 319 (2008) 737–738. [5] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, Science 312 (2006) 885–888. [6] W. Sugimoto, K. Yokoshima, K. Murakami, Y. Takasu, J. Electrochem. Soc. 153 (2006) A255–A260. [7] P. Gould, Mater. Today (2003) 38. [8] S. Par, S. Jayaraman, MRS Bull. (2003) 585. [9] N. Oyama, T. Tatsuma, T. Sato, T. Sotomura, Nature 373 (1995) 598–600. [10] M. Maxfield, T.R. Jow, S. Gould, M.G. Sewchok, L.W. Shacklette, J. Electrochem. Soc. 135 (1988) 299–305. [11] J. Wang, C.O. Too, D. Zhou, G.G. Wallace, J. Power Sources 140 (2005) 162– 167. [12] R.J. Waltman, J. Bargon, Can. J. Chem. 64 (1986) 76–95. [13] G.P. Evans, in: H. Gerischer, C.W. Tobias (Eds.), Electrochemical Science and Engineering, vol. 1, VCH, Amsterdam, The Netherlands, 1990, pp. 1–74. [14] J.G. Killian, B.M. Coffey, F. Gao, T.O. Poehler, P.G. Searson, J. Electrochem. Soc. 143 (1996) 936–942. [15] H.K. Song, G.T.R. Palmore, Adv. Mater. 18 (2006) 1764. [16] Y. Gofer, H. Sarker, J.G. Killian, J. Giaccai, T.O. Poehler, P.C. Searson, Biomed. Instrum. Technol./Assoc. Adv. Med. Instrume. 32 (1) (1998) 33–37. [17] H.H. Rehan, J. Power Sources 113 (2003) 57–61. [18] Z.J. Cai, M.M. Geng, Z.M. Geng, J. Mater. Sci. 39 (2004) 4001–4003. [19] C.Y. Wang, A. Ballantyne, S.B. Hall, C.O. Too, D. Officer, G.G. Wallance, J. Power Sources 156 (2006) 610–614.

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