Kinetics of ethanol electrooxidation at Pd electrodeposited on Ti

Electrochemistry Communications 9 (2007) 2334 2339 www.elsevier.com/locate/elecom Kinetics of ethanol electrooxidation at Pd electrodeposited on Ti Jianping Liu a, Jianqing Ye a, Changwei Xu a,b, *, San Ping Jiang c, Yexiang Tong a, * a School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China b Department of Chemistry and Institute of Nanochemistry, Jinan University, Guangzhou 510632, China c School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore Received 27 May 2007; received in revised form 18 June 2007; accepted 19 June 2007 Available online 3 July 2007 Abstract Pd nanoparticles dispersed well on Ti were successfully prepared by the electrodeposition method used in this study. The results show that Pd has no activity for ethanol oxidation in acid media and is a good electrocatalyst for ethanol oxidation in alkaline media when the OH concentration is greater than 0.001 M. The ph and ethanol concentration affects the ethanol oxidation. The reaction orders for OH and ethanol are 0.2 and 1. The anodic transfer coefficient (a) is 0.1. The diffusion coefficient (D) of ethanol is calculated as 0.42 10 5 cm 2 s 1 (298 K) when the concentration of KOH and ethanol is both 1.0 M. The overall rate equation for ethanol oxidation on Pd/Ti electrode in alkaline media is given as j ¼ 1:4 10 4 C 0:2 KOH C ethanol exp 0:28F g RT. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Fuel cells; Ethanol electrooxidation; Kinetics; Palladium; Ti 1. Introduction * Corresponding authors. Address: Department of Chemistry and Institute of Nanochemistry, Jinan University, Guangzhou 510632, China. Tel.: +86 20 84110071; fax: +86 20 84112245. E-mail addresses: cwxuneuzsu@126.com (C. Xu), cestyx@mail.sysu. edu.cn (Y. Tong). Direct alcohol fuel cells (DAFCs) generate electric power by feeding liquid fuels directly to anode, which makes them easier to design small and lightweight power sources. DAFCs are promising power sources for portable electronic devices and electric vehicles [1,2]. Direct ethanol fuel cells (DEFCs) have attracted more and more attention as ethanol has no toxicity compared to methanol and can be easily produced in great quantity by fermentation of sugar-containing raw materials [3,4]. Pt and Pt-based catalysts have been extensively investigated as the elctrocatalysts for the oxidation of liquid fuels such as methanol and ethanol [5 8]. However, the high price and limited supply of Pt constitute a major barrier to the development of DAFCs. Our previous work on the development of Pt-free electrocatalysts for alcohol oxidation has focused on Pd electrocatalysts [9 12]. The results revealed that Pd is a good electrocatalyst for ethanol oxidation in alkaline media and shows higher activity and better steady-state behaviour than Pt. A unique tolerance of Pd surface against CO poisoning was observed in several studies [13 16]. The usage of Pd is interesting as it is at least fifty times abundant on the earth than Pt. Although Pd exhibits a high activity for ethanol oxidation in alkaline media, a little information is devoted to the kinetics for ethanol oxidation on Pd. Here, titaniumsupported Pd nanoparticles were fabricated by electrodeposition method and used as the catalysts for ethanol oxidation with varying the concentration of KOH and ethanol. It is well known that the electrodeposition is one of the most efficient methods for the growth of metal nanoparticles [17 20]. This is a powerful technique for the deposition of many metals since it is rapid and facile, allowing easy control of the nucleation and growth of metal nanoparticles with differing sizes, shapes and distributions [21]. The Pd nanowire arrays have been successfully prepared by electrodeposition method [11,22]. Ti was used as the substrate due to its excellent chemical stability. It is well know that titanium-supported catalysts present significant 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.06.036

J. Liu et al. / Electrochemistry Communications 9 (2007) 2334 2339 2335 electrocatalytic activity for methanol oxidation [23,24]. Ti mesh anodes prepared by deposition of the platinum directly on the surface have been used in direct methanol fuel cells (DMFCs) and show a higher performance than carbon black supported platinum anodes [25,26]. 2. Experimental Electrodeposition was carried out in an aqueous solution containing 0.5 g L 1 Pd(NH 3 ) 4 Cl 2 and 0.8 g L 1 Na 2 EDTA to prepare Pd nanoparticles on Ti foil. The ph value of the solution was adjusted to 8.5 with NH 3 Æ H 2 O. The Pd nanoparticles on Ti were prepared using the potential of 0.6 V for 5 min, resulting in a palladium of 0.08 mg cm 2. All of the solutions were prepared by using distilled water and were purged with Ar before experiments. Electrochemical experiments were performed on IM6e electrochemical workstation (Zahner Electrik, Germany). A standard three-electrode cell was used and was controlled at 25 C using a water bath during the experiments. A platinum foil (3.0 cm 2 ) and saturated calomel electrode (SCE, 0.241V versus SHE) were used as counter and reference electrodes, respectively. A salt bridge was used between the cell and the reference electrode. Scanning electron microscopic (SEM) analysis was performed with a Hitachi S-5200. indicates that the Pd nanoparticles on Ti were successfully prepared by the electrodeposition method used in this study. Fig. 2 shows the cyclic voltammograms for the Pd/Ti electrode in 0.5 M H 2 SO 4 solution (solid line) and 0.5 M H 2 SO 4 containing 1.0 M ethanol (dotted line) with a palladium loading of 0.08 mg cm 2. The sweep rate is 5 mv s 1 in the potential range from 0.07 V to 0.93 V. By comparing with the cyclic voltammogram in the absence of ethanol, no any ethanol oxidation peak can be clearly observed in 0.5 M H 2 SO 4 solution containing 1.0 M ethanol. The results show that Pd has no activity for ethanol oxidation in acidic media. Fig. 3 shows the cyclic voltammograms of ethanol oxidation in 1.0 M KOH solution containing 1.0 M ethanol on Pd/Ti electrode with a palladium loading of 0.08 mg cm 2. The sweep rate is 5 mv s 1 in the potential range of 0.9 to 0.1 V. By comparing with the cyclic voltammogram in the absence of ethanol, an ethanol oxidation peak can be clearly observed. The ethanol electrooxidation on Pd/Ti electrode was characterized by two well-defined 3. Results and discussion Fig. 1 shows the TEM image of Pd nanoparticles deposited on Ti by electrodeposition method. The Pd particles are well dispersed on Ti and average particle size is about 8 nm. The distribution of Pd nanoparticles appears to be uniform. The Pd deposit surface consists of fine clusters of approximate spheres with a porous structure. The result Fig. 2. Cyclic voltammograms of Pd/Ti electrode with a Pd loading of 0.08 mg cm 2 in 0.5 M H 2 SO 4 solution containing 1.0 M ethanol with a sweep rate of 5 mv s 1. Fig. 1. SEM image of Pd nanoparticles supported on Ti foil with a palladium loading of 0.08 mg cm 2. Fig. 3. Cyclic voltammograms for ethanol oxidation on Pd/Ti electrode with a palladium loading of 0.08 mg cm 2 in 1.0 M KOH solution containing 1.0 M ethanol with a sweep rate of 5 mv s 1.

2336 J. Liu et al. / Electrochemistry Communications 9 (2007) 2334 2339 current peaks on the forward and reverse scans. In the forward scan, the oxidation peak is corresponding to the oxidation freshly chemisorbed species coming from ethanol adsorption. The reverse scan peak is primarily associated with removal of carbonaceous species not completely oxidized in the forward scan than the oxidation of freshly chemisorbed species [27,28]. The magnitude of the peak current on the forward scan indicates the electrocatalytic activity of the Pd/Ti for the oxidation reaction of ethanol. The onset potential for ethanol oxidation is 0.65 V. The peak potential is 0.20 V and the peak current density is 6.4 ma cm 2. The results show that Pd/Ti is a good electrocatalyst for ethanol oxidation in alkaline media. Since the value of ph affects the ethanol oxidation on Pd/Ti electrode, it is important to study the effect of KOH concentration on ethanol oxidation. The effect of KOH concentration on ethanol oxidation on Pd/Ti electrode was studied in 1.0 M ethanol with different KOH concentration and the results are depicted in Fig. 4. At concentration lower than 0.001 M KOH, no obvious oxidation current was recorded, indicating an important dependence of the ethanol oxidation on the ph. An oxidation reaction Fig. 4. (a) Linear sweep curves for ethanol oxidation on Pd/Ti electrode with a palladium loading of 0.08 mg cm 2 in 1.0 M ethanol containing different concentration of KOH solution with a sweep rate of 5 mv s 1. (b) The relation between ethanol oxidation current density and oxidation potential with different KOH concentration in a fixed ethanol concentration of 1.0 M. enhancements with increasing the KOH concentration at concentration higher than 0.001 M KOH. The results suggest that the kinetics of the ethanol oxidation reaction were improved by the greater availability of OH ions in solution and/or a higher OH coverage of the electrode surface [29]. The ethanol oxidation peak potential shifts negatively with increasing the KOH concentration. The value for the shifted potential is determined by the following equation: DE ¼ 0:0591 ph ð1þ It can be seen that the peak current increases with the increasing in the KOH concentration from Fig. 4. The best performance was found when the KOH concentration is around 2.0 M at a fixed ethanol concentration of 1.0 M. The peak current declines with the further increasing the KOH concentration. The ethanol oxidation on Pd/Ti in alkaline media corresponds to the following reactions CH 3 CH 2 OH þ 3OH $ CH 3 CO ads þ 3H 2 O þ 3e ð2þ OH $ OH ads þ e ð3þ CH 3 CO ads þ OH ads! CH 3 COOH ð4þ CH 3 COOH þ OH! CH 3 COO þ H 2 O ð5þ It has been suggested that the rate-determining step is Eq. (4). Ethanol oxidation is determined by the degree of CH 3 CO ads and OH ads coverage. OH ads adsorbed on the electrode is important for ethanol oxidation. OH ads adsorbed on the electrode will take the place of CH 3 CO ads if the concentration of OH is too high, so OH ads is available in excess and CH 3 CO ads is insufficient at the electrode surface. When the KOH concentration is around 2.0 M at a fixed ethanol concentration of 1.0 M, ethanol oxidation gives a best performance. The Tafel plots for ethanol oxidation are given in Fig. 5a. At KOH concentrations greater than 0.1 M, the Tafel slopes were identical and suggest the same reaction mechanism occurs in this ph range. The Tafel slope in this range was 210 mv dec 1, which is in accordance with that for ethanol oxidation on Pd/C in alkaline media [12]. The Tafel slope increases to 315 mv dec 1 in 0.01 M KOH which may have resulted from the lower reaction activity in this medium because of the lower ph. The relation between oxidation current and oxidation potential is according to the Butler Volmer equation [30] j ¼ j 0 exp anf log j ¼ log j 0 þ anf 2:303; ð7þ where j 0 is the exchange current density; a is the anodic transfer coefficient; n is the number of transferred electrons and g is overpotential. The an is 0.28 from the Eq. (7) and the Tafel slope at KOH concentrations greater than 0.1 M. The n is 2.8 for ethanol oxidation [31]. Soa is 0.1, showing the ethanol ð6þ

J. Liu et al. / Electrochemistry Communications 9 (2007) 2334 2339 2337 h OH ¼ K 1 C 0:2 KOH exp bf ; ð9þ where h OH is the fractional coverage of OH ; K 1 is rate constant; b is transfer coefficient. Fig. 6 shows the effect ethanol concentration on ethanol oxidation at a fixed KOH concentration of 1.0 M. The oxidation current increases with the ethanol concentration at low ethanol concentration, indicating that the ethanol oxidation reaction was controlled by ethanol concentration at the electrode surface. The current decreases at higher ethanol concentration. The best performance was found at the ethanol concentration of around 4.0 M. It may be the effect to be due to depletion of OH ads by CH 3 CO ads at the electrode surface. Therefore, it can be concluded that at low ethanol concentration, the peak currents were controlled by the diffusion transport of ethanol due to the excess availability of OH ion. At higher ethanol concentrations, peak currents were controlled by the diffusion transport of OH ions because of the excess availability of ethanol and insufficient OH ions [36]. The reaction order for ethanol was determined as 1by the dependence of logj versus logc ethanol at low fixed potentials in Fig. 7. The relation between the coverage of Fig. 5. (a) Tafel plots for ethanol oxidation in 1.0 M ethanol with different KOH concentration; (b) The plots between the dependence of logj versus logc KOH at several low fixed potentials in a fixed ethanol concentration of 1.0 M. oxidation reaction on Pd/Ti in alkaline media is irreversible electrode process. For an irreversible electrode reaction, the diffusion coefficient of ethanol can be calculated according to the following equation [32]: 1=2 j p ¼ 0:4958 10 3 anf nfc ethanol D 1=2 v 1=2 ð8þ RT D is diffusion coefficient of ethanol; j p is the peak current density; v is sweep rate. It is well known that the diffusion coefficient of ethanol determined by the temperature, the concentration of KOH and ethanol. The D is calculated as 0.42 10 5 cm 2 s 1 (298 K) when the concentration of KOH and ethanol is both 1.0 M. The exchange current density for 1.0 M ethanol oxidation obtained from Eq. (7) in different concentration of KOH is 0.008 (0.01 M), 0.011 (0.1 M), 0.058 (1.0 M), 0.060 (1.5 M), 0.065 (2.0 M), 0.098 (3.0 M), and 0.087 ma cm 2 (4.0 M). Fig. 5b shows the dependence of logj versus logc KOH at several low fixed potentials. The reaction order for OH was 0.22, close to 0.2. The adsorption of OH in alcohol solution follows Temkin-type isotherms [33,34]. The fractional coverage of OH can be given as [35] Fig. 6. (a) Linear sweep curves for ethanol oxidation on Pd/Ti electrode with a palladium loading of 0.08 mg cm 2 in 1.0 M KOH containing different ethanol concentration with a sweep rate of 5 mv s 1. (b) The relation between ethanol oxidation peak current density with different ethanol concentration in a fixed KOH concentration of 1.0 M.

2338 J. Liu et al. / Electrochemistry Communications 9 (2007) 2334 2339 Fig. 7. The plots between the dependence of logj versus logc ethanol at several low fixed potentials in a fixed KOH concentration of 1.0 M. intermediate CH 3 CO ads and ethanol concentration can be expressed as [31] h CH3 CO ads ¼ K 2 C ethanol ; ð10þ where h CH3 CO ads is the coverage of intermediate CH 3 CO ads ; K 2 is rate constant. Assuming that the chemical reaction given by Eq. (4) is rate-determining step in ethanol oxidation on Pd/Ti in alkaline media, a kinetic expression can be given in the following equation: j ¼ Kh OH h CH3 CO ads ð11þ Then, by combining Eqs. (9) and (10), the overall rate equation for ethanol oxidation is following equation: j ¼ KC 0:2 KOH C ethanol exp (12): bf ð12þ Here, K is the overall rate constant, K = K 1 K 2. The following equation is obtained from Eqs. (6) and j 0 exp anf ¼ KC 0:2 KOH C ethanol exp bf ð13þ b = an = 0.28; the average value of K is 1.4 10 4 ms 1 calculated from the Eq. (13). So, the Eq. (12) can be given as j ¼ 1:4 10 4 C 0:2 KOH C ethanol exp 0:28F RT g ð14þ 4. Conclusions The kinetics of ethanol electrooxidation was studied at Pd nanoparticles dispersed on Ti by the electrodeposition method. The results show that Pd has no activity for ethanol oxidation in acidic media and is a good electrocatalyst for ethanol oxidation in alkaline media at KOH concentration greater than 0.001 M. The value of ph affects the ethanol oxidation and the best performance was found when the KOH concentration is around 2.0 M in 1.0 M ethanol. The best performance was also found when the ethanol concentration is around 4.0 M at KOH concentration fixed at 1.0 M. The reaction orders for OH and ethanol are 0.2 and 1. The anodic transfer coefficient (a) of ethanol oxidation is 0.1, showing the ethanol oxidation reaction on Pd/ Ti in alkaline media is irreversible electrode process. The diffusion coefficient (D) is calculated as 0.42 10 5 cm 2 s 1 (298 K) when the concentration of KOH and ethanol is both 1.0 M. The overall rate equation for ethanol oxidation on Pd/Ti electrode in alkaline media is given as j ¼ 1:4 10 4 C 0:2 KOH C ethanol exp 0:28F g RT. The Pd nanoparticles dispersed well on Ti in this study possess good electrocatalytic properties for ethanol electrooxidation in alkaline media and may be a great potential in ethanol sensor and direct ethanol fuel cells. Acknowledgements This work was financially supported by the Natural Science Foundations of China (20573136), the Natural Science Foundations of Guangdong Province (06023099 and 04205405) and Chinese Postdoctoral Prizing Funding (Xu Changwei, 20060390734). References [1] V.M. Barragán, A. Heinzel, J. Power Sources 104 (2002) 66. [2] H.L. Tang, S.L. Wang, M. Pan, S.P. Jiang, Y.Z. Ruan, Electrochim. Acta 52 (2007) 3714. [3] S.Q. Song, P. Tsiakaras, Appl. Catal. B 63 (2006) 187. [4] E. Antolini, J. Power Sources 170 (2007) 1. [5] C.W. Xu, P.K. Shen, X.H. Ji, R. Zeng, Y.L. Liu, Electrochem. Commun. 7 (2005) 1305. [6] Z.D. Wei, L.L. Li, Y.H. Luo, C. Yan, C.X. Sun, G.Z. Yin, P.K. Shen, J. Phys. Chem. B 110 (2006) 26055. [7] S.L. Chen, M. Schell, Electrochim. Acta 44 (1999) 4773. [8] S.L. Chen, M. Schell, J. Electroanal. Chem. 478 (1999) 108. [9] C.W. Xu, L.Q. Cheng, P.K. Shen, Y.L. Liu, Electrochem. Commun. 9 (2007) 997. [10] C.W. Xu, P.K. Shen, Y.L. Liu, J. Power Sources 164 (2007) 527. [11] H. Wang, C.W. Xu, F.L. Cheng, S.P. Jiang, Electrochem. Commun. 9 (2007) 1212. [12] P.K. Shen, C.W. Xu, Electrochem. Commun. 8 (2006) 184. [13] A. Capon, R. Parsons, J. Electroanal. Chem. 44 (1973) 285. [14] M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11375. [15] M. Hara, U. Linke, T. Wandlowski, Electrochim. Acta 52 (2007) 5733. [16] F.P. Hu, C.L. Chen, Z.Y. Wang, G.Y. Wei, P.K. Shen, Electrochim. Acta 52 (2006) 1087. [17] Z.D. Wei, S.H. Chan, J. Electroanal. Chem. 569 (2004) 23. [18] O. Ordeig, C.E. Banks, F.J. Campo, F.X. Muñoz, J. Davis, R.G. Compton, Electroanal. 18 (2006) 247. [19] X.B. Ji, C.E. Banks, A.F. Holloway, K. Jurkschat, C.A. Thorogood, G.G. Wildgoose, R.G. Compton, Electroanal. 18 (2006) 2481. [20] P. Liu, X.A. Guo, H. Huang, Q.Q. Yang, Y.X. Tong, G.A. Hope, Adv. Mater. 18 (2006) 1873. [21] C.M. Welch, R.G. Compton, Anal. Biochem. 384 (2006) 601. [22] X.B. Ji, C.E. Banks, W. Xi, S.J. Wilkins, R.G. Compton, J. Phys. Chem. B 110 (2006) 22307. [23] R.G. Freitas, M.C. Santos, R.T.S. Oliveira, L.O.S. Bulhões, E.C. Pereira, J. Power Sources 158 (2006) 164. [24] M.B. Oliveira, L.P.R. Profeti, P. Olivi, Electrochem. Commun. 7 (2005) 703.

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