INTRODUCTION. Hao Yuhn C. Hsu 1,2, Lorico DS. Lapitan Jr. 1,3, Harris M. Fulo 1,3, and Bernard John V. Tongol 1,2,3*

Transcription

1 Acta Manilana 58 (2010), pp. 1 9 Printed in the Philippines ISSN: Enhanced electrocatalytic behaviour of Pt metallic particles dispersed on poly(3,4- ethylenedioxythiophene)-modified Au electrode towards ethanol oxidation Hao Yuhn C. Hsu 1,2, Lorico DS. Lapitan Jr. 1,3, Harris M. Fulo 1,3, and Bernard John V. Tongol 1,2,3* 1 Research Center for the Natural and Applied Sciences, 2 Department of Chemistry, College of Science, and 3 The Graduate School, University of Santo Tomas, España Blvd., 1015 Manila, Philippines Abstract. Platinum particles dispersed on poly(3,4-ethylenedioxythiophene) (PEDOT) film show enhanced electrocatalytic properties compared to bulk platinum and Pt-modified Au electrode for ethanol oxidation in sulfuric acid solution. Pt particles were incorporated on a PEDOT matrix from 1.0 mm H 2 PtCl 6 in 0.1 M H 2 using potentiodynamic deposition mode. The effect of different number of Pt deposition cycles on their electrocatalytic activity has also been studied. Performance evaluation for the electrocatalytic activity towards ethanol oxidation at E = V (vs. Ag/AgCl) of the Pt metallic particles dispersed on PEDOT-modified Au electrode, bulk Pt, and Pt metallic particles-modified Au electrode were investigated for comparative purposes. Enhanced electrocatalytic properties towards ethanol oxidation were observed for Pt metallic particles dispersed on PEDOT-modified Au electrode (1810 A cm 2 ) in comparison to bulk Pt ( A cm 2 ) and Pt particles-modified Au electrode ( A cm 2 ). Cyclic voltammetric data revealed that Pt metallic particles deposited for 12 cycles exhibited the greatest electrocatalytic activity (1810 A cm 2 ) towards ethanol oxidation. The film morphology and elemental analysis of the composite electrode were analysed using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX), respectively. The enhanced electrocatalytic behaviour for the Pt metallic particles dispersed on PEDOT-modified Au electrode can be correlated with the surface properties of the composite film. Keywords: poly(3,4-ethylenedioxythiophene), Pt particles, ethanol oxidation INTRODUCTION Platinum (Pt) has been extensively investigated and was shown to be an efficient electrocatalyst for the oxidation of small organic molecules such as methanol, ethanol, * To whom correspondence should be addressed bvtongol@mnl.ust.edu.ph and formic acid [1 7]. However, the high price and scarcity of bulk Pt constitute a major hindrance for the development of direct alcohol fuel cells (DAFC) applications. In addition, platinum s active areas are susceptible to poisoning by the reaction intermediates such as carbon monoxide (CO) [1, 6]. These reasons serve as a thrust for developing low-cost Pt-based electrocatalysts with comparable or improved kinetics Acta Manilana Volume 58 (2010)

2 Hsu HYC, Lapitan Jr LDS, Fulo HM, Tongol BJV towards oxidation of small organic molecules as compared to bulk Pt electrodes. To solve this problem, the incorporation of Pt metal particles on conducting polymers as host matrix was proposed [1, 7 10]. These metal particle-conducting polymer composites are expected to exhibit enhanced electrocatalytic activities as compared to the bulk form metal electrodes. The main reason for incorporating metallic particles into porous polymeric matrices is to increase the specific area of the active surface and thereby improve the catalytic efficiency [6, 11]. As compared to bulk platinum electrodes, polymer supported platinum particles was observed to have higher tolerance to poisoning by adsorption of CO species [6 9]. In addition, conducting polymers act as a good dispersing material for metallic particles and it improves the interfacial properties between the electrode and the electrolyte [7, 12]. Among the polythiophene derivatives, poly(3,4-ethylenedioxythiophene) (PEDOT) is the best known conducting polymer because it exhibits high conductivity, reversible doping state, excellent stability, and low band-gap properties. Moreover, PEDOT is easily prepared from aqueous solutions of EDOT and forms a homogeneous and strong adherent film on Au electrode with a high surface area [13 15]. Several methods for the incorporation of metal particles on conducing polymers have been reported. These include potentiostatic deposition [17, 18], double potential step method [18, 19], potentiodynamic deposition [18, 20] and constant current [19]. Generally, the electrocatalytic behaviour of deposited Pt is slightly different by deposition mode. This observation suggests a different distribution (two dimensional, 2D or three dimensional, 3D) or state (crystalline structure and grain morphology) of the Pt metals deposited [17 22]. With the aim of developing new Pt-based electrocatalyts systems with enhanced electrocatalytic behaviour while reducing cost of preparation, this paper reports a simple fabrication of a Pt-modified PEDOT composite for the oxidation of ethanol and formic acid. Moreover, the approach of preparation of the electrocatalyst system is benign to the environment since all electrochemical syntheses were done in aqueous solution. Pt particles were imbedded by potentiodynamic deposition into a pre-synthesized PEDOT matrix. It is expected that PEDOT will serve as a three-dimensional and electronically conducting matrix and allow the formation of relatively uniform Pt particles. For comparative purposes, the electrocatalytic behaviour of bulk polycrystalline Pt electrode and Pt particlesmodified Au electrode were also investigated. Moreover, the effect of the number of scan cycles for the deposition of Pt particles on their electrocatalytic properties towards ethanol oxidation was also studied. The performance evaluation of the Pt particlesmodified PEDOT electrodes as electrocatalyst towards oxidation of ethanol and formic acid were monitored using cyclic voltammtery. Morphological and elemental analyses of Pt particles-modified electrodes were done using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis respectively. EXPERIMENTAL Materials and chemicals 3,4-ethylenedioxythiophene (EDOT) (>97%, Aldrich) was used as received. Hexachloroplatinic acid hydrate (H 2 PtCl 6 ) (>99.9%), perchloric acid (suprapur), and sulfuric acid (suprapur) were purchased from Merck. Ethanol is of analytical grade and was purchased from Scharlau, Spain. All solutions were prepared using ultrapure water (TOC < 5.00 ppb; Resistivity = 18.2 M cm). Gold wire (diameter: 0.90 mm; purity 99.95%) was purchased from Nilaco, Corp., Japan. Electrode preparation Pt particles dispersed on PEDOT-modified Au electrodes were prepared in two steps, namely: (i) aqueous electropolymerization of EDOT, and (ii) electrochemical deposition of platinum. Potentiodynamic electropolymerization of 2

3 Enhanced electrocatalytic behaviour of Pt metallic particles dispersed on poly(3,4-ethylenedioxythiophene)-modified Au electrode towards ethanol oxidation 0.01 M EDOT in 0.10 M HClO 4 on a polycrystalline Au electrode was done by cycling the potential from E = 0.0 to 1.10 V vs. Ag/AgCl for 20 cycles at a scan rate of 50 mv s 1. Then, Pt particles were pontentiodynamically deposited from a plating solution of 1 mm H 2 PtCl 6 in 0.1 M H 2 from E = V to V at a scan rate of 10 mv s 1 [6]. The number of scan cycles (4, 8, 12, 16, and 20 cycles) was varied to determine the optimum number of cycles where the Pt particles would exert the greatest catalytic activity. The amount of Pt particles loaded onto the PEDOT/Au electrode was calculated from the following equation [16], m= Q dep M FZ where the amount deposited (m) in g cm 2 was calculated by using the charge (Q dep ) (obtained through graphical integration of cyclic voltammetry curves) utilized for the deposition of Pt particles. M is the atomic weight of Pt ( gmol 1 ), F is the Faraday constant (96,500 C mol 1 ), and Z is the number of electrons transferred (taken as four for the reduction Pt 4+ Pt 0 ). The modified electrodes were carefully rinsed with ultrapure water and transferred to a CV cell containing ethanol. Evaluation of the electrocatalytic activity towards ethanol was done using 1.0 M ethanol in 0.1 M H 2 by cyclic voltammtery from E = 0.0 V to 0.90 V at a scan rate of 100 mv s 1. Instrumentation All electrochemical measurements were performed in a conventional 3-electrode electrochemical cell at room temperature. All solutions were purged by bubbling with high purity N 2 gas for 15 min and under N 2 atmosphere during electrochemical measurements. A Pt rod and a Ag/AgCl (saturated 3 M KCl) electrode were used as the counter and reference electrode, respectively. The working electrodes used were Pt particles dispersed on PEDOT-modified Au electrode, Pt particles-modified Au electrode and bulk polycrystalline Pt electrode. Before electrode modification, the Au electrodes were polished mechanically using emery paper (grade ) and cleaned by potential cycling between E = 0.2 and 1.2 V at 50 mvs 1 in 0.5 M H 2. All electrochemical measurements were carried out using EDAQ potentiostat (Australia) under control of a dedicated software (EChem, EDAQ). All potentials reported in this paper are presented in the Ag/AgCl scale. Surface morphology and elemental analyses of Pt-modified PEDOT films were recorded using a JEOL JSM 5310 SEM with an accelerating voltage of 15 kev and EDX (De La Salle University, Manila), analysis, respectively. RESULTS AND DISCUSSION Electrode preparation Synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) film The electrochemical polymerization of EDOT in aqueous solutions without the presence of surfactants has been previously described [23]. Potentiodynamic electropolymerization offers the possibility of controlling the thickness and homogeneity of the film on the electrode surface. Hence, this method was used for the modification of the polycrystalline Au electrode by PEDOT from a solution containing 0.01 M EDOT in 0.1 M HClO 4. The electrode potential was cycled from E = 0 to 1.10 V at a scan rate = 50 mv s 1 for 20 cycles in order to ensure a uniform polymer film had covered the electrode surface. Figure 1 shows the cyclic voltammograms during the electropolymerization of EDOT on polycrystalline Au electrode. The first cycle showed an increase in anodic current at E = V which corresponds to the oxidation of EDOT monomers. A constant current increment was observed from cycle to cycle attributed to the polymer oxidation and indicating film growth. A dark blue film was formed at the electrode surface after 20 cycles which is a visible indication of film growth. Examination of the surface morphology of PEDOT with SEM revealed globular 3

4 Hsu HYC, Lapitan Jr LDS, Fulo HM, Tongol BJV Fig. 1. Cyclic voltammograms during the electropolymerization of EDOT in 0.01 M EDOT in 0.1 M HClO 4 solution (u = 50 mv s 1 ; 20 cycles). microstructures. These PEDOT microstructures would provide a high surface area and can be used as support matrix for the incorporation of metal catalysts. Preparation and characterization of Pt particles electrodeposited on PEDOT film The Pt particles were electrochemically deposited on a pre-synthesized PEDOT film using potentiodynamic deposition. In this deposition mode, the deposited metal particles are activated periodically thereby leading to an improvement of their dispersion [18, 20]. Shown in Fig. 2 are the cyclic voltammograms for the potentiodynamic deposition of Pt on PEDOT film from E = V to V with different number potential cycles. It has been reported that the actual potential of [PtCl 6 ] 2 reduction (Pt 4+ Pt ) in solution is ~0.5 V [16]. However, current responses corresponding to Pt metal deposition were also found at more negative potentials [19]. This may be ascribed to the kinetic hindrance of the [PtCl 6 ] 2 reduction in the interior of the polymer film. For this reason, the potential range between E = V to V was chosen for deposition of Pt. The negative potential limit of E = V was also chosen in order to observe the formation well-defined peaks corresponding to the adsorption-desorption region of hydrogen on platinum, which confirms that a good dispersion of platinum Fig. 2. Cyclic voltammograms for the potentiodynamic deposition of Pt particles on PEDOT film at 4, 8, 12, 16, and 20 cycles from E = 0.400V to V. ( = 10 mv s 1 ). Only the last cycles were presented for clarity. in the PEDOT film is achieved. Also, the formation of hydrogen adsorption-desorption properties confirms the presence of platinum particles [18, 24]. During the Pt deposition process, an increasing trend in the cathodic current with an increasing number of potential cycles was noticed at E= V. This indicates the deposition of Pt metal particles onto the PEDOT matrix at applied potentials. Interestingly, peaks corresponding to the hydrogen adsorption-desorption were prominent from 8, 12, 16, and 20 cycles as shown in Fig. 2. This observation is indicative that Pt particles are successfully deposited and dispersed in the PEDOT matrix [16, 18]. Integration of the current density for the last cycles for each deposition of Pt revealed a difference in the intensity of hydrogen desorption (Table 1). Generally, the integrated 4

5 Enhanced electrocatalytic behaviour of Pt metallic particles dispersed on poly(3,4-ethylenedioxythiophene)-modified Au electrode towards ethanol oxidation Table 1. Platinum loading and hydrogen desorption peaks current density corresponding to the different cycles during Pt electrodeposition on PEDOT matrix No. of Deposition Cycles Peak Current Density (ma/cm 2 ) Platinum Loading ( g/cm 2 ) , , , , intensity of hydrogen desorption represents the number of available sites for hydrogen adsorption and desorption on Pt surface [16 18]. The charges for hydrogen desorption were calculated by assuming a constant double-layer charging current over the whole potential range [16]. Table 1 also shows a difference in the amount of Pt particles deposited for each number of deposition cycles. These results correlates to an increased amount of Pt loaded with an increase in number of scan cycles. Moreover, the PEDOT matrix acts as a good dispersing medium and binder to provide contact for Pt metal particles. Figure 3a c shows the surface morphology of the composite film wherein Pt metal particles were deposited at 8, 12, and 16 cycles. The SEM image shows a globular morphology of the polymer. Moreover, the actual presence of Pt on PEDOT matrix is confirmed by the EDX spectra in Fig. 3d. Close examination of the SEM images revealed that Pt particles were uniformly dispersed onto the PEDOT matrix. Also, agglomeration of Pt particles is difficult to distinguish in the present SEM images. Kuo et al. reported that Pt particles are homogeneously deposited on the surface of ITO/PEDOT-PSS substrate. The polymer matrix can act as a protective layer for the Pt nanopaticles and prevents agglomeration of particles [16]. Larger size and homogeneously dispersed Pt particles were observed on bare ITO electrode when loaded with a higher number of cycles [16]. The observed larger size of Pt particles on bare electrodes using increased number of deposition cycles may suggest the agglomeration of Pt particles. Agglomeration of electrochemically deposited Pt particles is well-documented in the literature [25 26]. Maillard et al. deposited Pt from a plating solution of 7 mm H 2 PtCl M HCl on a glassy carbon electrode by potentiodynamic deposition [25]. As revealed from TEM images, agglomerated Pt structures were observed rather than isolated nanopaticles. These aggregates range in size from 25 nm to 100 nm. Moreover, Patra et al. also observed the formation of homogenously dispersed and small clusters of Pt particles deposited by potentiostatic deposition [10]. The formation of Pt aggregates rather than nanoparticles is attributed to the 3D nucleation and growth mechanism. In this growth mechanism, primary nucleation of the Pt deposited is followed by secondary nucleation on a pre-deposited Pt surface. This is because the pre-deposited Pt surface has a higher concentration of nucleation centers compared to the bare substrate. This ultimately results to the formation of aggregated structures [10]. This explains the observed larger size of Pt particles deposited on bare Au electrode as revealed by the SEM images in Fig. 5. Electrocatalytic activity of Pt particles dispersed on modified PEDOT-modified Au electrode for ethanol oxidation The electrochemical properties of the fabricated electrodes were investigated using cyclic voltammetry in 1.0 M EtOH M H 2 aqueous solution. Figure 4 presents the voltammetric profile for ethanol oxidation on Pt particles deposited with different number of potential cycling on pre-synthesized PEDOT film. There was no observable oxidation peak 5

6 Hsu HYC, Lapitan Jr LDS, Fulo HM, Tongol BJV a b c d Fig. 3. SEM images of Pt particles dispersed on PEDOT film. Pt particles were electrodeposited on a pre-synthesized PEDOT film at different scan cycles of (a) 8, (b) 12, (c) 16 cycles, respectively, and (d) representative EDX spectra for Pt particles dispersed on PEDOT film. at E = 0.60 V for the electrode prepared for four cycles. This could be attributed to the insignificant amount of Pt particles dispersed on the PEDOT-modified Au electrode. For the succeeding cycles (i.e. 8, 12, 16 and 20 cycles), an anodic peak at E = V, was observed which suggest the presence of Pt particles. The occurrence of a peak at E = V is an indication that the ethanol dissociated producing acetaldehyde and was further oxidized to carbon dioxide (CO 2 ) [14, 17]. It was seen that the voltammetric behaviour of the catalyst towards ethanol oxidation depends on the Pt loading content. It was observed that there was a significant amount of Pt deposited on the PEDOT film as the number of scan cycles was increased further as shown on Table 1. It was also observed that the particle size increases as the number of cycles was increased further. The calculated average size of Pt metal particles deposited at 8, 12, and 16 cycles were found to be 6

7 Enhanced electrocatalytic behaviour of Pt metallic particles dispersed on poly(3,4-ethylenedioxythiophene)-modified Au electrode towards ethanol oxidation Current Density (ma/cm 2 ) cycles 12 cycles 16 cycles 20 cycles E/V vs Ag/AgCl Current Density (ma/cm 2 ) Bulk Pt Au/PEDOT/Pt Au/Pt E/V vs Ag/AgCl Fig. 4. Cyclic voltammogram profiles for the oxidation of 1.0 M EtOH for Pt-modified PEDOT/Au electrode. The Pt particles were electrodeposited on the PEDOT matrix by (a) 8, (b) 12, (c) 16, and (d) 20 cycles, respectively. Fig. 5. Cyclic voltammogram profiles for the oxidation of 1.0 M EtOH on (a) Pt-modified PEDOT/Au electrode, (b) Pt-modified Au electrode, and (c) bulk Pt electrode ( = 100 mv s 1 ). a b Fig. 6. SEM images of (a) Pt particles electrodeposited on Au electrode, and (b) Pt particles deposited on PEDOTmodified Au electrode. ~ mm, ~ mm, and ~1.119 mm, respectively. Moreover, the electrochemical activity of the systems increased with increasing number of potential cycling until reaching the maximum at 12 cycles and decreased with succeeding cycles (i.e., 16 and 20 cycles). This could be due to the accumulation of Pt particles on the PEDOT-modified electrode resulting in a saturation of the active sites of the Pt particlemodified PEDOT/Au electrode [7, 16]. When the metal loading is further increased the metal particles agglomerate during the reduction process and results in a decrease of the electrochemical active area [16]. This is because deposited Pt particles tend to agglomerate instead of dispersed as individual isolated particle on the PEDOT-modified Au. Moreover, this was mainly due to the nucleation and growth mechanism of the Pt particles, where the primary nucleation of the Pt deposit was 7

8 Hsu HYC, Lapitan Jr LDS, Fulo HM, Tongol BJV followed by secondary nucleation on a predeposited Pt surface [7, 28]. Figure 5 shows the comparison of the electrocatalytic activity of the bulk platinum substrate, Pt particles dispersed on bulk Au electrode and Pt particles dispersed on PEDOT-modified Au electrodes. The CV profile showed an enhancement of the electrocatalytic activity when platinum was dispersed as particles on the Au and PEDOT-modified Au surface. The corresponding SEM images for the Pt particles on bulk Au substrate and PEDOT support matrix are presented in Fig. 6. Apparently, the enhanced electroactivity towards ethanol oxidation can be attributed to the large surface area of the Pt particles compared to the bulk Pt. Thus, the larger surface area of the Pt particles could be correlated to an increase in the availability of the active sites and an increase in the catalytic activity. Moreover, the PEDOT-supported catalysts showed a more enhanced electrocatalytic activity compared with the Pt particles dispersed on bulk Au electrode in acidic medium. The addition of the polymer had increased the catalytic activity of the system up to 27%. Accordingly, the improved performance of the Pt particles dispersed on PEDOT-modified Au electrode could be attributed to the high surface area of PEDOT [7, 26 27], uniform distribution of the Pt particles on the polymer matrix (Fig. 6b) and because of the relatively high electric conductivity of conducting polymer [27], thus making it possible to shuttle the electrons through polymer chains between the electrodes and dispersed metal particles where the electrocatalytic reaction occurs. Moreover, the polymer matrix acts as a protective layer for the Pt particles and prevents aggregation of particles [27]. CONCLUSIONS Pt particles were successfully incorporated into a PEDOT matrix by potentiodynamic deposition. The composite electrode, Pt particles dispersed on PEDOT-modified Au electrode exhibited an increased electrocatalytic performance and promises as a good catalyst for ethanol oxidation. The electrocatalytic behaviour of the composite electrode can be related to the distribution of Pt particles embedded in the PEDOT matrix and the synergistic effects between the dispersed platinum particles and PEDOT film. Moreover, among the different number of Pt deposition cycles, the Pt particles deposited for 12 cycles exhibited the greatest catalytic activity towards ethanol oxidation. This phenomenon can be attributed to the large surface area of Pt particles surface available on the PEDOT matrix, hence, exhibiting more catalytic sites and greater catalytic activity. The enhanced catalytic activity of Pt-modified PEDOT/Au electrode opens up the possibility of a simple, environment friendly and costefficient electrode material for direct ethanol fuel cell (DEFC) applications. ACKNOWLEDGEMENT This research work was financially supported by the Philippine Council for Advanced Science and Technology Research and Development (PCASTRD) of the Department of Science and Technology (DOST). REFERENCES [1] Yano J, Shiraga T, Kitani A, Mater JN. Electrochem. Sys. 2008; 11:235. [2] Gupta SS, Datta J. J. Chem. Sci. 2005; 117:337. [3] Jiang L, Hsu A, Chu D, Chen R. Int. J. Hydrogen Energy 2009; 30:1. [4] Zhou W, Song SQ, Lia WZ, Zhou ZH, Sun G, Xin Q, Douvartzides S, Tsiakaras P, Power J. Sources 2005; 140:50. [5] Camara CA, Iwasita T, J. Electroanal. Chem. 2005; 578:315. [6] Biallozor S, Kupniewska A, Jasulaitene V. Fuel Cells 2003; 3:8. [7] Patra M, Munichandraiah N. Langmuir 2009; 25:1732. [8] Kim S, Park S. Solid State Ionics 2008; 178:1915. [9] Xue KH, Cai CX, Yang H, Zhou YM, Sun SG, Chen SP, Xu G. J. Power Sources 1998; 75:207. [10] Patra S, Barai K, Munichandraiah N. J. Syn. Met. 2008; 158:430. [11] Antolini E, Gonzalez ER. Appl. Catal. A Gen. 2008; 365:1. [12] Choi J, Park K, Lee H, Kim Y, Lee J, Sung Y. Electrochim. Acta 2003; 48:

9 Enhanced electrocatalytic behaviour of Pt metallic particles dispersed on poly(3,4-ethylenedioxythiophene)-modified Au electrode towards ethanol oxidation [13] Groenendaal LB, Jonas F, Freitag D, Pielartzik H, Reynolds JR. Adv. Mater. 2000; 12:481. [14] Zanardi C, Terzi F, Pigani L, Heras A, Colina A, Lopez-Palacios J, Seeber R. Electrochim. Acta 2007; 53:3916. [15] Terzi F, Zanardi C, Martina V, Pigani L, Seeber R. J. Electroanal. Chem. 2008; 619:75. [16] Kuo CW, Huang LM, Wen TC, Gopalan A. J. Power Sources 2006; 160:65. [17] Mikhylova AA, Molodkina EB, Khazova OA. J. Electroanal. Chem. 2001; 509:119. [18] Niu L, Li QH, Wei FH, Wu S, Liu P, Cao X. J. Electroanal. Chem. 2005; 578:331. [19] Bouzek K, Mangold K-M, Jutter K. Electrochim. Acta 2000; 46:661. [20] Niu L, Li QH, Wei FH, Chen H, Wang J. J. Electroanal. Chem. 2003; 544:121. [21] Kelaidopoulou, Abelidou E, Papoutsis A, Polychroniadsis EK, Kokkinidis G. J. App. Electrochem. 1998; 28:1101. [22] Castro Luna AM. J. App. Electrochem. 2000; 30:1137. [23] Du X, Wang Z. Electrochim. Acta 2003; 48:1713. [24] Yang H, Lu TH, Xue K, Sun SG, Lu GQ, Chen SP. J. Electrochem. Soc. 1997; 144:2302 [25] Maillard F, Schreier S, Hanzlik M, Savinova ER, Weinkaul S, Stimming U. Phys. Chem. Chem. Phys. 2005; 7:385. [26] Cherstiouk OV, Simonova PA, Savinova ER. Electrochim. Acta 2003; 48:3851. [27] Kim S, Park S. Solis State Ionics 2008; 178: [28] Liu Z, Shamsuzzoha M, Ada E, Reichert M, Nikles D. J. Power Sources 2007; 164: