Journal of Electroanalytical Chemistry

Journal of Electroanalytical Chemistry 688 (2013) 262 268 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem IrO 2 coated TiO 2 nanopore arrays electrode for SPE HBr electrolysis Linsong Zhang a,b, Zhi-Gang Shao a,, Hongmei Yu a, Xunying Wang a,b, Baolian Yi a a Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China b Graduate School, Chinese Academy of Sciences, Beijing 100049, PR China article info abstract Article history: Available online 8 September 2012 Keywords: SPE HBr electrolysis Anodization TiO 2 nanopore arrays IrO 2 catalyst IrO 2 nanoparticles coated TiO 2 nanopore arrays (IrO 2 /TNPs) electrode has been successfully synthesized by depositing IrO 2 on the surface of size-controllable TiO 2 nanopore arrays (TNPs), which were fabricated by using anodic oxidation of pure titanium meshes in electrolyte solutions. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), liner sweep voltammetry (LSV), SPE cell polarization curves and electrochemistry impedance spectroscopy (EIS) were adopted to characterize their structures, properties and performances. The IrO 2 /TNPs electrode has ordered microstructure and more porous surface morphology than that of IrO 2 /Ti electrode. The electrochemical tests showed that IrO 2 /TNPs electrode exhibited higher catalytic activity than IrO 2 /Ti electrode. And the cell voltage can be as low as 1.16 V at 1000 ma cm 2 and 70 C, which is 90 mv lower than that of the cell with IrO 2 /Ti electrode (1.25 V). The increase in performance is attributed to the ordered microstructure and porous surface which decrease the interface contact resistance and charge transfer resistance. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction The development of hydrogen economy and the expectation that hydrogen will be one of the candidates for fueling future cars have led to extensive work on technologies for hydrogen production [1]. The solid polymer electrolyte (SPE) water electrolysis is considered to be a very promising hydrogen production technology from renewable energy sources [2,3]. However, an obstacle to produce hydrogen from water electrolysis is the cost of the large electrical power input, which is imposed by both the thermodynamics of the hydrogen oxygen couple and the irreversibility of the oxygen electrode [4]. An alternative method of hydrogen production may be a hybrid process, in which HBr electrolysis may be a step [1,5,6]. Moreover, hydrogen bromine fuel cells and electrolyzers are promising candidates for the storage of electrical energy in load-leveling applications and renewable sources applications, especially solar and wind power, to cope with their intermittent nature [7,8]. These cells consist of a bromine electrode and a hydrogen electrode with a proton-conducting membrane between them. The electrochemical reactions for the hydrogen bromine system are nearly reversible. Consequently, good energy storage efficiencies can be obtained even at high current density operation [9,10]. The half-cell reactions in electrolyzer are: Anode : 2Br ¼ Br 2 þ 2e u h ¼ 1:098 V ð1þ Corresponding author. Tel.: +86 411 84379153. E-mail address: zhgshao@dicp.ac.cn (Z.-G. Shao). Cathode : 2H þ þ 2e ¼ H 2 u h ¼ 0:000 V ð2þ Electrolyzing 35% HBr at 90 C, the electrolyzer cell voltage is 0.78 V at 200ASF (215 ma cm 2 ) [5], which is much less than that of SPE water electrolysis (about 1.49 V) [11]. With increased HBr concentration the cell voltage will be further decreased. In SPE water electrolysis cells, porous titanium materials without modification or just coating a layer of gold or Pt are used as current collectors and flow fields [3,11 19]. In direct methanol fuel cells (DMFCs), Ti meshes with coating of Pt Ru or Pt RuO 2 catalysts are used as anodes and show better performance than that of conventional anode [20,21]. However, the literatures about the modification method of porous titanium materials to improve cell performance are very limited. Although high performance was reported by using porous titanium materials as current collectors and flow fields or electrodes, the obvious defect is the low catalytic activity of the coating layer. In recent years, highly ordered TiO 2 nanotube arrays (TNTs) prepared by anodization have attracted significant interest due to their features related to large surface area, non-toxicity, good stability and low production cost, which makes them valuable functional materials in many areas such as water photoelectrolysis, dyesensitized solar cells, gas sensors, and solid-state heterojunction solar cells [22 26]. Recently, Liu et al. [27,28] reported TiO 2 nanopore arrays (TNPs), which possess strong mechanical stability and outstanding photochemical performance. In this work, we report on the preparation and characterization of IrO 2 /TNPs electrode, and compare the use of IrO 2 /TNPs, IrO 2 /Ti and IrO 2 /TNTs electrodes in SPE HBr electrolysis. 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.08.008

L. Zhang et al. / Journal of Electroanalytical Chemistry 688 (2013) 262 268 263 2. Experimental 2.1. Preparation of IrO 2 /TNPs electrodes and physical characterizations TNPs were prepared by anodization of Ti meshes (4Ti6-031FA, Dxmet) in 100 ml of electrolytic solution. Before anodization the Ti mesh was sonicated in acetone and chemically polished. Water (10 wt.%), ammonium fluoride (NH 4 F, 0.5 wt.%, Tianjin Damao Chemical Reagent Factory), and ethylene glycol (Tianjin Kermel Chemical Reagent Co., Ltd.) were mixed together thoroughly and used as the electrolyte [29]. A two electrode configuration was used for anodization. Ti mesh was used as the anode electrode and a graphite plate as the cathode electrode [30]. The distance between the anode and the cathode is approximately 2 cm. The Ti mesh was subjected to potentiostatic anodization at room temperature, rinsed with DI water to remove the occluded ions and then blow dried in air. The IrO 2 /TNPs electrodes were prepared by thermal decomposition. The catalyst precursor solution, H 2 IrCl 6 dissolved in isopropanol, was coated on the TNPs and dried in air. And this procedure was repeated several times until the desired catalyst loading was achieved. Afterward, calcination was performed at 450 C for 2 h in a ceramic tube furnace. Finally, it was cooled to room temperature and weighted. For comparison, IrO 2 /Ti and IrO 2 /TNTs electrodes with the same loading were prepared by thermal decomposition method. XRD analyses of all samples were performed with a Shimadzu diffractometer (XRD-6000) with a Cu Ka radiation source (k = 1.54056 Å) to characterize the crystalline structure. Field emission scanning electron microscopy (FESEM) imaging was performed with a Hitachi S-4800 microscope. Fig. 1. XRD patterns of IrO 2 /Ti (a) and IrO 2 /TNPs electrodes anodized at various potentials: (b) 20 V 30 s, (c) 30 V 30 s, and (d) 40 V 30 s. 2.2. Electrochemical analysis A1cm 1 cm IrO 2 /TNPs and a flag-shaped Pt electrode were used as working electrode and counter electrode, respectively. The reference electrode was a saturated calomel electrode (SCE) which was connected to the cell by a glass capillary. The linear sweep voltammetry (LSV) was measured in 2 M HBr aqueous solution at room temperature between 0.6 and 1.1 V (vs. SCE) at a scan rate of 5 mv s 1. 2.3. Preparation of membrane electrode assembly (MEA) Iridium black (Johnson Matthey) and Pt/C catalyst (70 wt.%, Johnson Matthey) were used as the anode and the cathode catalyst respectively. A homogeneous ink consisted of catalyst, Nafion solution (5 wt.%, DuPont) and isopropanol was sprayed onto each side of a Nafion 115 membrane. The catalyst loading were about 2.7 mg cm 2 iridium black and 1 mg cm 2 Pt/C catalyst. MEA was fabricated by hot pressing the catalyst coated membrane and carbon paper (TGP-H-060, Toray) at 140 C and 1 MPa. Carbon paper was used as the cathode diffusion layer. The active area of the MEA was about 5 cm 2. 2.4. SPE HBr electrolysis cell assembly and evaluation Fig. 2. XRD patterns of IrO 2 /Ti (a) and IrO 2 /TNPs electrodes anodized at 20 V for different anodization time: (b) 30 s, (c) 60 s, and (d) 5 min. The performance for SPE HBr electrolysis was tested in a single cell. IrO 2 /TNPs electrode was used as the anode. The MEA and the IrO 2 /TNPs anode were positioned between two graphite plates. In HBr electrolysis, the cell was run at 70 C and ambient pressure. 500 ml of 2 M HBr solution was fed to the anode. The electrochemistry impedance spectroscopy (EIS) was carried out at 1.00 V with PARSTAT 2273 (EG and G instruments). And the frequency varied from 10 khz to 50 mhz and 10 mv amplitude of sinusoidal potential perturbation was employed. The impedance data were obtained by calculation and simulation with ZSimpWin software. 3. Results and discussion 3.1. XRD and SEM characterization Figs. 1 and 2 show the XRD patterns of IrO 2 /Ti and IrO 2 /TNPs electrodes synthesized by the thermal decomposition of H 2 IrCl 6 isopropanol solution. The rutile-structure IrO 2 and metallic titanium crystalline phases are observed in all samples. The strongest peak is identified as the titanium substrate because the coatings are too thin so that the X-ray can penetrate to the titanium substrate. Besides IrO 2 and titanium, the diffraction peaks of anatase TiO 2 (25.3, JCPDS No. 21-1272) are identified in all IrO 2 /TNPs electrodes, which is consistent with several reports that show the anatase phase of TiO 2 obtained through electrochemical oxidation and sintering [25,27,28,31]. The XRD patterns show a slight decrease in the line broadening of IrO 2 diffraction peaks after the anodization. This result demonstrated that both the crystallinity and the crystal size increased with anodization time and anodization potential. To investigate the morphologies of TNPs and IrO 2 /TNPs electrodes, field emission scanning electron microscopy (FESEM) was conducted. Fig. 3A shows that the surface of Ti mesh without anodization is smooth. However, ordered nanoporous structure is observed on the anodized Ti meshes with average pore diameter from about 50 nm (Fig. 3B) to 20 nm (Fig. 3F), indicating that the

264 L. Zhang et al. / Journal of Electroanalytical Chemistry 688 (2013) 262 268 Fig. 3. FESEM images of as-anodized Ti meshes (A F) and IrO 2 /TNPs electrodes anodized at different potentials (a f). (A, a): without anodization, (B, b): 60 V 30 s, (C, c): 50 V 30 s, (D, d): 40 V 30 s, (E, e): 30 V 30 s, and (F, f): 20 V 30 s. Fig. 4. FESEM images of as-anodized Ti meshes (A F) and IrO 2 /Ti, IrO 2 /TNPs and IrO 2 /TNTs electrodes anodized with different anodization time (a f). (A, a): without anodization, (B, b): 30 V 30 s, (C, c): 30 V 60 s, (D, d): 30 V 5 min, (E, e): 30 V 30 min, and (F, f): 30 V 1 h.

L. Zhang et al. / Journal of Electroanalytical Chemistry 688 (2013) 262 268 265 average pore diameter decreases with decreasing anodization potentials. The ordered nanopore structure possesses large specific surface area and fast charge transport property, which are advantageous for the loading of catalysts and catalytic reaction [31]. Fig. 3a f shows the surface of IrO 2 /Ti and IrO 2 /TNPs electrodes. The smooth Ti surface is uniformly sheathed by a compact layer of IrO 2 granular film (Fig. 3a). From Fig. 3b f, it can be seen that the TNPs are extensively covered by aggregated IrO 2 nanoparticles in diameter ranging from 10 nm to 20 nm. It is certain that the IrO 2 film of IrO 2 /TNPs electrodes have a regular porous structure with many cavities, which indicated that IrO 2 /TNPs electrodes possess bigger specific surface area than IrO 2 /Ti electrodes. Fig. 4 shows the influence of anodization time on the microstructure. The average pore diameter of the nanopore is increased with anodization time (Fig. 4B F). High density and well ordered TiO 2 nanotube arrays are grown vertically from the Ti meshes when the anodization time is longer than 5 min (Fig. 4E and F). Compared with IrO 2 /TNPs electrodes, the IrO 2 /TNTs electrodes have less porous structure. The TiO 2 nanotube arrays with bigger diameters, more IrO 2 nanoparticles could be filled into the inner portion of the nanotubes, forming a close contact structure with TiO 2 nanotube arrays [32]. 3.2. Electrochemical analysis The electrochemical performances of IrO 2 /TNPs electrodes were evaluated by LSV measurements in 2 M HBr solution, and the results are shown in Figs. 5 7. To get higher electrochemical performances, the anodization potential and anodization time were optimized. It can be found that the performances of the IrO 2 /TNPs Fig. 6. LSV of IrO 2 /Ti and IrO 2 /TNPs electrodes anodized with different anodization time in 2 M HBr solution (5 mv s 1 ). Fig. 7. LSV of IrO 2 /Ti, IrO 2 /TNPs and IrO 2 /TNTs electrodes anodized with different anodization time in 2 M HBr solution (5 mv s 1 ). Fig. 5. LSV of IrO 2 /Ti and IrO 2 /TNPs electrodes anodized at different potentials in 2 M HBr solution (5 mv s 1 ). electrodes were clearly better than that of the IrO 2 /Ti electrode, particularly at high potential. The optimized anodization potential is 20 V (Fig. 5) and anodization time is 30 s (Fig. 6). When the anodization potential is below 20 V or anodization time less than 30 s, the IrO 2 catalyst coating is less porous and electrochemical performances decrease. However, if the anodization potential is too high or the anodization time is too long, the increase in TiO 2 crystallinity will augment the ohmic resistance and decrease its electrochemical performance (see Fig. 7).

266 L. Zhang et al. / Journal of Electroanalytical Chemistry 688 (2013) 262 268 Fig. 8. Polarization curves of the SPE HBr electrolysis cells at 70 C using different anode electrodes with IrO 2 loading of 0.5 mg cm 2. Cell-1: IrO 2 /Ti electrode; and Cell-2: IrO 2 /TNPs electrode. Fig. 11. Effect of anodization time of IrO 2 /TNPs electrodes on cell performance with IrO 2 loading of 0.4 mg cm 2. Fig. 9. The Nyquist diagrams of the SPE HBr electrolysis cells at 70 C using different anode electrodes with IrO 2 loading of 0.5 mg cm 2. Cell-1: IrO 2 /Ti electrode, and Cell-2: IrO 2 /TNPs electrode. Fig. 12. Effect of IrO 2 loading of IrO 2 /TNPs electrode on cell performance. 0.43 A cm 2 and 0.24 A cm 2, for IrO 2 /TNPs electrodes and IrO 2 / TNTs electrodes anodized at 30 V for 30 s and 1 h, respectively. The IrO 2 /TNTs electrode leads to a decrease of 0.19 A cm 2 in the peak current density at the highest experimental potential for the low conductivity of TiO 2. The performance of IrO 2 /TNTs electrode may be improved if the TNTs are decorated with metals to lower its resistance. 3.3. SPE HBr electrolysis cell performance Fig. 10. Effect of anodization potentials of IrO 2 /TNPs electrodes on cell performance with IrO 2 loading of 0.4 mg cm 2. In order to examine the electrochemical performances of the IrO 2 /TNTs electrodes, the samples anodized with different anodization time were tested and the results are shown in Fig. 7. It can be found that the peak current densities at 1.1 V (vs. SCE) are Fig. 8 shows the polarization curves comparison of the two cells with IrO 2 /Ti electrode (Cell-1) and IrO 2 /TNPs electrode (Cell-2), in which it is clear that Cell-2 yields much better performance than Cell-1 in all regions of the polarization curve. In the SPE HBr electrolysis cell, Ir IrO 2 /Ti or Ir IrO 2 /TNPs is the anode electrode and the Ir black on the MEA also catalyzes the anode reaction. However, the MEA in the electrolysis cells are the same and the only difference between the two cells is the IrO 2 /TNPs electrode. Accordingly, the increase in performance is attributed to the ordered microstructure and porous surface of IrO 2 /TNPs electrode. It can be seen that Cell-2 shows lower voltage rise than does Cell-1 at very low current densities. The lower voltage rise means that the IrO 2 / TNPs electrode possesses a faster rate of bromide oxidation than IrO 2 /Ti electrode [33]. In the high current density regions, the performance of Cell-2 surpasses that of Cell-1 significantly and the

L. Zhang et al. / Journal of Electroanalytical Chemistry 688 (2013) 262 268 267 magnitude of performance enhancement is increased with increasing current density. At the current of 1000 ma cm 2, the cell voltage of Cell-2 can be as low as 1.16 V, which is 90 mv lower than that of the Cell-1 (1.25 V). It indicates that the ohmic resistance of Cell-2 is smaller than that of Cell-1. This can be attributed to the fact that the IrO 2 /TNPs electrode which possesses ordered porous surface well contacted with the MEA and the bipolar plate thus decreases the interface contact resistance. To get more insights on the different characteristics of the two cells, an analysis of the electrochemical impedance was carried out at 1.0 V, 70 C. A comparison of Nyquist plots is reported in Fig. 9. Only one semicircular loop can be observed in the Nyquist plot. The high frequency intercept on the real axis R X of the Cell-2, which represents the total ohmic resistance of the single cell [34], is smaller than that of the Cell-1. Moreover, the arc diameter of the Cell-2, which is a measure of charge transfer resistance R ct [34], is also smaller than that of the Cell-1. R X and R ct of the two cells can be calculated through simulation with LR X (R ct Q) equivalent circuit. The calculation results show that the ohmic resistance R X of Cell-1 and Cell-2 are 0.167 X cm 2 and 0.147 X cm 2, respectively. The difference in R X indicates that the IrO 2 /TNPs electrode has better interfacial contact with MEA and bipolar plate than that of IrO 2 /Ti electrode, because the cell components were kept under the same conditions for the two cells. Additionally, the charge transfer resistance R ct (0.309 X cm 2 ) of Cell-2 is also smaller than that of Cell-1 (0.412 X cm 2 ), which further suggests that the IrO 2 /TNPs electrode yields a more efficient electrochemical active layer and a faster rate of bromide oxidation than the IrO 2 /Ti electrode. These results are certainly consistent with the polarization curves in Fig. 8. The IrO 2 /TNPs electrode gets advantages over the conventional IrO 2 /Ti electrode, which can be associated to the large specific surface and fast charge transport property of the porous microstructures [31]. 3.4. The effect of anodization potentials on HBr electrolysis performance The effect of the anodization potentials on the performances of a SPE HBr electrolyzer single cell are shown in Fig. 10. Five electrodes are tested, of which some other parameters (catalyst loading, anodization time, MEA) are the same except the anodization potentials. As can be seen from Fig. 10, all IrO 2 /TNPs electrodes show better performance than IrO 2 /Ti electrode. The performances first increased and then decreased with anodization potentials. There is only minor difference at the low current densities for the five electrodes. However, at high current densities, the differences increased obviously. The IrO 2 /TNPs electrode anodized at 20 V for 30 s gives the best performance, which is in accordance with the LSV investigation. Since other parameters are the same, the sample which anodized at 20 V for 30 s must have the smallest interfacial resistance [35]. 3.5. Influences of anodization time on HBr electrolysis performance To evaluate the influences of the anodization time on the performances of a SPE HBr electrolyzer single cell, IrO 2 /TNPs, IrO 2 /Ti and IrO 2 /TNTs electrodes are compared in SPE HBr electrolysis. Performances of the four cells are shown in Fig. 11. The cell with IrO 2 /TNPs electrode anodized for 30 s gives better performance than the one with IrO 2 /Ti electrode in all regions of the polarization curve, and anodized for 5 min shows comparable performance with the one with IrO 2 /Ti electrode. However, the IrO 2 /TNTs electrode anodized for 30 min shows worse performance than IrO 2 /Ti electrode even in low current density regions, which is in accordance with results of LSV measurement (Fig. 7). Therefore, it can be inferred that the increase in cell electrolysis voltage with the anodization time is probably related to the increased ohmic resistance of TiO 2 nanotube arrays. 3.6. The effect of catalyst loading on HBr electrolysis performance The effect of IrO 2 catalyst loadings on the cell performance was investigated, as shown in Fig. 12. Four IrO 2 /TNPs electrodes are tested, of which some other parameters (anodization potential, anodization time, MEA) are the same except the IrO 2 catalyst loading. With increased IrO 2 catalyst loading, the ohmic resistance decreases, as seen from the slops of high current density regions of polarization curves. And the IrO 2 /TNPs electrode with 0.5 mg cm 2 IrO 2 loading gives the best performance. However, the performance decreases with the increase of IrO 2 loading from 0.5 mg cm 2 to 0.8 mg cm 2, which may be due to the increased ohmic resistance and mass transfer resistance because of the thicker coating [33]. Based on these results, an optimum IrO 2 loading on the IrO 2 /TNPs electrode is 0.5 mg cm 2. Moreover, the loading of IrO 2 on IrO 2 / TNPs electrode and MEA need to be further optimized to get balance between good performance and low catalyst loading. 4. Conclusions IrO 2 nanoparticles coated on TiO 2 nanopore arrays (IrO 2 /TNPs) electrode has been successfully synthesized by depositing IrO 2 on the surface of size-controllable TiO 2 nanopore arrays (TNPs), which could be achieved by altering anodization potential and time. The IrO 2 /TNPs electrodes have a porous structure and large surface area due to the ordered nanopore arrays substrate. Comparing with the cell of IrO 2 /Ti electrode, the cell with IrO 2 /TNPs electrode showed much better performance especially at high current density. The cell voltage can be as low as 1.16 V at 1000 ma cm 2 and 70 C, which is 90 mv lower than that of the cell with IrO 2 /Ti electrode (1.25 V). The optimized anodization condition is 20 V for 30 s and the optimum IrO 2 loading on anodized mesh is 0.5 mg cm 2. EIS further revealed that the higher performance resulted from its smaller ohmic resistance and charge transfer resistance. The increase in ordered microstructure, porosity and surface area of IrO 2 /TNPs electrode, which is advantageous for catalytic reaction, is perceived as the reason for the corresponding increase in the performance of SPE HBr electrolysis. Acknowledgements We thank the National High Technology Research and Development Program of China (863 Program, 2011AA050701 and 2012AA052002) and the National Natural Science Foundations of China (Nos. 20936008 and 21076208) for financial support. References [1] E. Peled, A. Blum, M. Goor, Fuel cells exploratory fuel cells hydrogen bromine fuel cells, in: G. Editor-in-Chief: Jürgen (Ed.), Encyclopedia of Electrochemical Power Sources, Elsevier, Amsterdam, 2009, pp. 182 191. [2] F. Barbir, Sol. Energy 78 (2005) 661 669. [3] S.A. Grigoriev, P. Millet, S.V. Korobtsev, V.I. Porembskiy, M. Pepic, C. Etievant, C. Puyenchet, V.N. Fateev, Int. J. Hydrogen Energy 34 (2009) 5986 5991. [4] E.N. Balko, J.F. McElroy, A.B. LaConti, Int. J. Hydrogen Energy 6 (1981) 577 587. [5] G.H. Schuetz, P.J. Fiebelmann, Int. J. Hydrogen Energy 5 (1980) 305 316. [6] P. Sivasubramanian, R.P. Ramasamy, F.J. Freire, C.E. Holland, J.W. Weidner, Int. J. Hydrogen Energy 32 (2007) 463 468. [7] M. Goor-Dar, N. Travitsky, E. Peled, J. Power Sources 197 (2012) 111 115. [8] R.F. Savinell, S.D. Fritts, J. Power Sources 22 (1988) 423 440. [9] S.D. Fritts, R.F. Savinell, J. Power Sources 28 (1989) 301 315. [10] J.A. Kosek, A.B. Laconti, J. Power Sources 22 (1988) 293 300. [11] J. Xu, R. Miao, T. Zhao, J. Wu, X. Wang, Electrochem. Commun. 13 (2011) 437 439. [12] S.D. Greenway, E.B. Fox, A.A. Ekechukwu, Int. J. Hydrogen Energy 34 (2009) 6603 6608.

268 L. Zhang et al. / Journal of Electroanalytical Chemistry 688 (2013) 262 268 [13] S.A. Grigoriev, P. Millet, K.A. Dzhus, H. Middleton, T.O. Saetre, V.N. Fateev, Int. J. Hydrogen Energy 35 (2010) 5070 5076. [14] H. Ito, T. Maeda, A. Nakano, Y. Hasegawa, N. Yokoi, C.M. Hwang, M. Ishida, A. Kato, T. Yoshida, Int. J. Hydrogen Energy 35 (2010) 9550 9560. [15] A. Marshall, B. Børresen, G. Hagen, M. Tsypkin, R. Tunold, Electrochim. Acta 51 (2006) 3161 3167. [16] A.T. Marshall, S. Sunde, M. Tsypkin, R. Tunold, Int. J. Hydrogen Energy 32 (2007) 2320 2324. [17] P. Millet, D. Dragoe, S. Grigoriev, V. Fateev, C. Etievant, Int. J. Hydrogen Energy 34 (2009) 4974 4982. [18] S. Siracusano, V. Baglio, A. Di Blasi, N. Briguglio, A. Stassi, R. Ornelas, E. Trifoni, V. Antonucci, A.S. Aricò, Int. J. Hydrogen Energy 35 (2010) 5558 5568. [19] G. Wei, Y. Wang, C. Huang, Q. Gao, Z. Wang, L. Xu, Int. J. Hydrogen Energy 35 (2010) 3951 3957. [20] Z.-G. Shao, F. Zhu, W.-F. Lin, P.A. Christensen, H. Zhang, J. Power Sources 161 (2006) 813 819. [21] Z.-G. Shao, W.-F. Lin, F. Zhu, P.A. Christensen, M. Li, H. Zhang, Electrochem. Commun. 8 (2006) 5 8. [22] L. Xing, J. Jia, Y. Wang, B. Zhang, S. Dong, Int. J. Hydrogen Energy 35 (2010) 12169 12173. [23] H. Chen, S. Chen, X. Quan, H. Yu, H. Zhao, Y. Zhang, J. Phys. Chem. C 112 (2008) 9285 9290. [24] X.-F. Gao, W.-T. Sun, Z.-D. Hu, G. Ai, Y.-L. Zhang, S. Feng, F. Li, L.-M. Peng, J. Phys. Chem. C 113 (2009) 20481 20485. [25] X. Li, Y. Hou, Q. Zhao, G. Chen, Langmuir 27 (2011) 3113 3120. [26] G. Liu, K. Wang, N. Hoivik, H. Jakobsen, Sol. Energy Mater. Sol. Cells 98 (2012) 24 38. [27] Y. Liu, B. Zhou, J. Bai, J. Li, J. Zhang, Q. Zheng, X. Zhu, W. Cai, Appl. Catal. B 89 (2009) 142 148. [28] Y. Liu, X. Gan, B. Zhou, B. Xiong, J. Li, C. Dong, J. Bai, W. Cai, J. Hazard. Mater. 171 (2009) 678 683. [29] S.K. Mohapatra, M. Misra, V.K. Mahajan, K.S. Raja, J. Phys. Chem. C 111 (2007) 8677 8685. [30] S. Li, G. Zhang, D. Guo, L. Yu, W. Zhang, J. Phys. Chem. C 113 (2009) 12759 12765. [31] H. He, P. Xiao, M. Zhou, Y. Zhang, Q. Lou, X. Dong, Int. J. Hydrogen Energy 37 (2012) 4967 4973. [32] M. Ouyang, R. Bai, L. Yang, Q. Chen, Y. Han, M. Wang, H. Yang, H. Chen, J. Phys. Chem. C 112 (2008) 2343 2348. [33] H. Su, B.J. Bladergroen, V. Linkov, S. Pasupathi, S. Ji, Int. J. Hydrogen Energy 36 (2011) 15081 15088. [34] X. Yuan, H. Wang, J. Colin Sun, J. Zhang, Int. J. Hydrogen Energy 32 (2007) 4365 4380. [35] W. Xu, K. Scott, Int. J. Hydrogen Energy 35 (2010) 12029 12037.