Effect of Magnetic Field on Properties of AuPt Particles Magnetoelectrodeposited

Transcription

1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 27, NUMBER 6 DECEMBER 27, 2014 ARTICLE Effect of Magnetic Field on Properties of AuPt Particles Magnetoelectrodeposited on Carbon Paper Jin-qiu Zhang a,b, Da Li a, Miao-miao Chen a, Mao-zhong An a,b, Pei-xia Yang a, Peng Wang b a. School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin , China b. School of Municipal and Environmental Engineering, State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin , China (Dated: Received on July 18, 2014; Accepted on September 9, 2014) AuPt nano particles are bi-functional catalysts for oxygen reduction reaction and oxygen evolution reaction that takes place on air electrodes in lithium air batteries. In this work, magnetic field was applied during electrodeposition for the preparation of AuPt particles. With the increase of the magnetic flux density under constant current density, the grain size decreases from 1 µm to 200 nm and the activity of the AuPt catalyst increases. The magnetic field orienting vertically to the electric field has a promotion effect on increasing the catalytic ability of AuPt/carbon electrode. By pulse plating, the grain size decreases to about 100 nm. By adjusting parameters of the electric field and the magnetic field, controllable in situ preparation of AuPt catalyst with various morphology and catalytic activity can be achieved. Key words: Magneto-electrodeposition, Catalyst, Lithium-air battery, Air electrode, Pulse plating I. INTRODUCTION Lithium air battery has attracted a great deal of attention because of its higher theoretical energy density than all other battery types [1]. In a lithium air battery, oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) take place on the side of the oxygen electrode [2]. In the process of OER, the prereaction product Li 2 O 2 has a strong ionic bond that is difficult to be reduced and decomposed, which causes the actual charging voltage up to 4.2 V in the absence of the catalyst. If byproducts, such as Li 2 CO 3, are generated during the discharge, the charging voltage can be increased to 4.5 V. Therefore, catalysts are needed to accelerate the reactions [3]. Usually, catalysts were loaded on porous carbon cathode to prepare air electrode by hydrothermal synthesis method. Transition metal oxides [4 8] and noble metals, such as MnO 2, Pt, Au, Au@Pt, and Ru@Pt, were studied extensively [9 13]. Since most catalysts only have good catalytic actions on ORR, not on OER, bifunctional catalysts were developed, such as Pt 0.5 Au 0.5 nano particles and Pd-MnO 2 [14, 15]. Comparatively, Pt 0.5 Au 0.5 catalyst has a better discharging ability, via whole mass of the electrode, that is about 720 mah/g, Author to whom correspondence should be addressed. zhangjinqiu@hit.edu.cn, Tel.: , FAX: while that of Pd-MnO 2 is about 545 mah/g. Although PtAu catalyst has a prospective application in the lithium air battery, the present chemical synthesis preparation technology has some shortcomings: (i) The load amount of PtAu particles is 40wt% of the electrode, which is a high proportion and causes an increase of production cost. (ii) Many complex conditions, such as Argon environment, long-time supersonic dispersion, and high-temperature vacuum dry etc., are used during preparation, which are uneasily controllable. (iii) Some chemical components are left on the catalyst and the electrode during the synthesis process, which would decrease the catalytic activity. Electrodeposition is an in situ technology for catalyst preparation without chemical residue and has a lower loading mass on carbon compared to other methods. However, few researches have been reported on electrodeposition of AuPt catalyst and the catalytic properties of AuPt alloy obtained by electrodeposition are not clear. We pulsed electrodeposited AuPt particles in diameter of nm on carbon in previous work [16], which demonstrated that AuPt catalysts could be loaded on carbon paper directly on a larger scale than that obtained by hydrothermal synthesis. Generally, in the case of stable alloy contents, catalyst with a higher specific area has a bigger electrochemical activity area and a better catalytic activity. Consequently, the target of electrodeposition of AuPt particles is to decrease their diameters and increase their roughness, which could improve their activity area. 704 c 2014 Chinese Physical Society

2 Chin. J. Chem. Phys., Vol. 27, No. 6 AuPt Particles Magneto-electrodeposited on Carbon Paper 705 It was reported that magneto-electrodeposition was effectively in decreasing grain size [17]. Also, under the influence of magnetic field, the morphologies of Co deposits were different. In an aqueous electrolyte, H is a diamagnetic element, while O is a paramagnetic element. Additional magnetic moment directions that are generated by H and O in the effect of magnetic field are opposite, so hydrated ions in an aqueous solution and their electric crystallization process are influenced by the magnetic field. Besides, the crystal itself has the magnetic anisotropy, which cause it take under different magnetizing forces in different directions. The convective magnetohydrodynamic effects (MHD) are generated by the magnetic field and have action on the mass transport and their consequence for the morphology of the deposits. It has been observed that the limiting current density i lim of the electrode surface increases with increasing magnetic flux density. Magnetoelectrodeposition is becoming a new branch in electrochemical preparation technology of metals whether they are magnetic metals or not. It can be deduced that magnetic field will affect the electrodepositon of AuPt catalyst. In this work, magneto-electrodeposition was carried out to in situ load AuPt particles on carbon paper. The effects of magnetic field and pulse deposition parameters on properties of catalyst were studied. Especially, some special micromorphologies of AuPt particles were presented. II. EXPERIMENTS Electrodeposition cell was set in a static magnetic field generated by two pieces of rubidium-iron-boron magnets (78 mm 57 mm 17 mm, Shanghai Yan-Tai metal materials Co., Ltd) or two electric magnets (Mianyang Li-Tian Co. Ltd). Magnetic flux density of the magnetic field ranged from 40 mt to 1300 mt by adjusting the distance of two pieces of magnets. B//J is defined as magnetic field oriented parallelly to electric field (Fig.1(a)), while B J is defined as magnetic field oriented vertically to electric field (Fig.1(b)). Electrodeposition baths of 25 C contained 0.5 mol/l H 2 SO 4, 0 20 mmol/l H 2 PtCl 6, and 0 20 mmol/l HAuCl 4. In different electrodeposition process, the total concentrations of HAuCl 4 and H 2 PtCl 6 were kept as 20 mmol/l with the ratios varying. A gold plate or a piece of graphite plate was used as the anode. Constant current density of 45 ma/cm 2 was carried out for 17 s to electrodeposit AuPt catalysts on carbon paper (15 mm 15 mm 0.3 mm, Toray Industries, Inc.) in the magnetic field of T. AuPt catalysts were also pulse electrodeposited on the carbon paper using a pulse power or a PARSTAT 2273 (AMETEK Co.) in the magnetic field of mt. For X-ray diffraction (XRD, PANalytical B.V.) measurements, constant current density of 4 ma/cm 2 was carried out for 30 min FIG. 1 Sketch of the magnetic field (a) B//J orienting parallelly to electric field and (b) B J orienting vertically to electric field. to electrodeposit Au, Pt or AuPt coatings on graphite sheets of 0.1 mm thickness (T-GRAF-2000, Polygore, Shenzhen Ying-Wei-Bo Co. Ltd.). The micromorphologies of samples were observed by scanning electron microscope (SEM, Quanta 200FEG, FEI Co.) and transmission electron microscope (TEM, TECNAI G2, FEI Co.), while the compositions of the AuPt particles were also measured at the same time by use of energy disperse spectroscopy (EDS, Quanta 200FEG, FEI Co.). The activity areas of AuPt catalysts were approximatively calculated as Eq.(1) shows by integral area of hydrogen adsorption peaks in the 25th cyclic voltammetry (CV) that were measured at the scan rate of 50 mv/s using PARSTAT 2273 in an argon-saturated 0.1 mol/l HClO 4 aqueous solutions where saturated calomel electrode (SCE) was used as the reference electrode and a platinum plate of 1 cm 2 was used as the counter electrode [18]. EAA = Q 0.21m (1) whereas, EAA is the electrochemical activity area (cm 2 /g), Q is the electric quantity of reduction reaction calculated from the CV curve (mc), m is the mass of the catalyst (g), and 0.21 is the specific adsorption capacitance of Pt to hydrogen (mc/cm 2 ). The activities of AuPt catalysts on ORR and OER were measured by CV at the scan rate of 10 mv/s in an oxygen-saturated ionic liquid electrolyte composed by EMI-TFSI (1-ethyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide) and Li-TFSI (lithium-bis(trifluoromethanesulphonyl)imide) at mass ratio of 1:0.4. The 15th cycle scanning curves are shown in figures without special description. The carbon paper loaded with AuPt catalyst was the work electrode. A platinum wire was used as the counter electrode. The reference electrode was Ag wire with potential of V vs. Li/Li +, which was evaluated experimentally by submerging the Ag electrode in the ionic liquid electrolyte containing a clean lithium electrode and monitoring the potential between the Ag wire and the lithium foil. c 2014 Chinese Physical Society

3 706 Chin. J. Chem. Phys., Vol. 27, No. 6 Jin-qiu Zhang et al. FIG. 2 SEM images in different magnification scanning of (a) and (b) of AuPt particles on carbon paper that were electrodeposited under constant current density of 10 ma/cm2. III. RESULTS AND DISCUSSION A. Effect of magnetic parameters on micromorphologies of AuPt particles By use of constant current electrodeposition, the effect of the magnetic orientations and the magnetic flux densities were studied. As shown in Fig.2, the AuPt particles that were constant current electrodeposited on the carbon paper without magnetic field are cauliflowerlike in size of about 1 µm (Fig.2(a)) attaching on the carbon fiber (Fig.2(b)). While the magnetic field was superimposed on the electric field during the electrodeposition, the micromorphologies of AuPt particles are different from that obtained without the magnetic field. When B J and magnetic flux density was 400 mt, as seen in Fig.3(a), AuPt particles electrodeposited on the carbon paper are hemispherical. The diameters of the particles are around 1 µm. While the magnetic flux density was adjusted to 700 mt, as shown in Fig.3(b), the growth of the particles shows an obvious trend of parallel to the direction of the magnetic field. At the same time, the rough mushroom-like particles also have a growing trend in the direction of the electric field. Because of B J, for the AuPt particles, there are two different growth directions, which are duplex and cause the particles fusing to a few islands whose length are about 1 2 µm. On the surface of these islands, there are terrace-like markings that are actually the border of fused particles whose original diameter is about 0.5 µm. The magnetic flux density was maintained at 700 mt, the magnetic direction was adjusted to B//J. It can be seen from Fig.3(c) that the diameters of AuPt particles are about 0.5 µm in general, and the AuPt particles also look like small mushrooms with smooth surfaces and clear borders, which were caused by the two parallel fields where the particles have only one preferential growth direction. If a particle of charge moves with velocity in the presence of an electric field and a magnetic field, it will experience Lorentz force, the combination of electric and magnetic force on a point charge due to electromagnetic fields. Under vertical magnetic field (B J) metal cations move in helix in the area perpendicular to the FIG. 3 SEM images in different magnification scanning of (a), (b), (c) and (d) and (e), (f), (g) and (h) of AuPt particles on carbon paper that were magneto- electrodeposited at various magnetic flux densities and directions. (a) and (e) 400 mt and B J, (b) and (f) 700 mt and B J, (c) and (g) 700 mt and B//J, and (d) and (h) 1.3 T and B//J, respectively. cathode surface, which weakens the concentration polarization. While under parallel magnetic field (B//J) metal cations move in gyration in the area parallel to the negative surface, which promotes the concentration polarization. So, the differences of the concentration polarization cause the different micromorphologies of the deposits in Fig.3 (b) and (c). In the condition of B//J, the magnetic flux density was increased to 1.3 T. The AuPt particles in Fig.3(d) are spherical with diameter of 0.2 µm and closely arranged, which indicates that the increase of the magnetic flux density has a decreasing effect on the diameters of the particles during magneto-electrodeposition. However, as seen in Fig.3(h), there are some larger particles with diameter of around 1 µm on the surface of smaller particles layer, which is different from other three samples shown in Fig.3 (e), (f) and (g). These larger particles perhaps are the preferential growth rec 2014 Chinese Physical Society

4 Chin. J. Chem. Phys., Vol. 27, No. 6 AuPt Particles Magneto-electrodeposited on Carbon Paper 707 TABLE I Contents of Au and Pt in the AuPt particles measured by EDS, deposite with different magnetic field and direction. Au/at% Pt/at% Without magnetic field B//J, 700 mt B J, 700 mt FIG. 4 XRD patterns of (a) graphite (G), (b) Pt coating electrodeposited without magnetic field, (c) Au coating electrodeposited without magnetic field, (d) AuPt coating electrodeposited without magnetic field, (e) AuPt coating electrodeposited at 700 mt magnetic field parallel to electric field (B//J) and (f) AuPt coating electrodeposited at 700 mt magnetic field vertical to electric field (B J). sult of some smaller particles in the parallel direction of magnetic field caused by stronger magnetic flux density. B. Effect of magnetic parameters on structures of AuPt particles In order to determine the effect of magnetic field on coating structures, AuPt coatings were electrodeposited with or without the effect of magnetic field and then XRD measurements (Fig.4) were carried out. For identification of the peaks, Au and Pt coatings were electrodeposited without magnetic field as shown in Fig.4 (b) and (c). Because the AuPt coatings are porous and not thick enough, the peaks of the substrate also appear in the XRD patterns. Despite the peaks of carbon substrate, the peaks at about 40, 46, 67, and 81 are AuPt alloy peaks [19], which are similar to that of Pt, not Au. Since the preferred orientation is (111), interplanar distances of (111) are compared in the follow discussion. The interplanar distance of Pt(111) is Å, while that of AuPt(111) are Å (Fig.4(d)), Å (Fig.4(e)), and Å (Fig.4(f)), respectively. The diameter of a gold atom is 1.44 Å that is bigger than 1.39 Å of a Pt atom. The increase of the interplanar distances indicates that Au atoms enter the platinum s crystal lattices. The full width at half maximum (FWHM) of AuPt(111) deposited in different magnetic field are (Fig.4(d)), (Fig.4(e)), and (Fig.4(f)). The wider FWHM is, the smaller grain size is. The AuPt alloy deposited in B//J has the biggest interplanar distance, while it also has the smallest grain size. It is worth noting that there are two weak peaks at 29.2 and 31.5 in Fig.4(d). However, the peak at 29.2 disappears while AuPt coating was electrodeposited in the magnetic field of B J (Fig.4(f)), and the both peaks disappear while AuPt coating was electrodeposited in the magnetic field of B//J (Fig.4(e)), which implys that the magnetic field paralleled to the electric field maybe cause the preferred orientation structure of AuPt alloy more obvious and without other impurity peaks. The compositions of AuPt alloys are shown in Table I by measurement of EDS. The Au content of AuPt alloy deposited in B//J is less than that obtained in B J or without magnetic field, which also demonstrates that the magnetic field paralleled to the electric field in 700 mt has an obvious influence on the structure of AuPt alloy. C. The catalytic abilities of AuPt particles The catalytic abilities on ORR and OER of the AuPt catalysts deposited in various conditions can be compared by the CV curves in Fig.5. Peak C1 in Fig.5 is the peak of ORR during negative scanning process, while peak A1 is the corresponding peak of OER during positive scanning process, which corresponds to a reaction in Eq.(2) as below [20, 21]. O 2 + e + Li + LiO 2 (2) The stronger of peak C1 is, the higher catalytic ability of the AuPt catalyst is. So, as seen in Fig.5, the magnetic field oriented vertically to the electric field has a promotion effect on increasing the catalytic ability of the AuPt catalyst. However, the peak C1 height of the samples deposited under B//J is as much as that deposited without the magnetic field, which demonstrates that their catalytic abilities are closer. Peak A2 and peak A3 are peaks of OER during positive scanning process, while their corresponding ORR peaks are not significant in the curves. These peaks correspond to Eq.(3) and Eq.(4) as below [20, 21]. LiO 2 + e + Li + Li 2 O 2 (3) Li 2 O 2 + 2e + 2Li + 2Li 2 O (4) The potentials of the peaks in Fig.5 have a good agreement with that reported by Abraham group [21], which c 2014 Chinese Physical Society

5 708 Jin-qiu Zhang et al. Chin. J. Chem. Phys., Vol. 27, No. 6 FIG. 5 CV curves of AuPt/carbon electrodes in an oxygensaturated EMI-TFSI/Li-TFSI electrolyte at a scan rate of 10 mv/s. AuPt catalysts were constant current electrodeposited under the 700 mt magnetic field (a) vertical and (b) parallel to the electric field, or (c) without magnetic field superimposed. FIG. 6 Negative scanning parts of CV curves of samples in Fig.3 in an argon-saturated 0.1mol/L HClO4 aqueous solutions. TABLE II The catalytic activity areas of the AuPt particles measured by EDS, deposite with different magnetic field and direction (S: activity area). B J, 400 mt B J, 700 mt B//J, 700 mt B//J, 1.3 T m/mg S/(m2 /g) indicates that the AuPt catalyst in situ deposited on the carbon paper has the bi-functional catalytic ability. The catalytic activity area of the AuPt catalysts listed in Table II were derived from CV shown in Fig.6 and according to Eq.(1). As seen from Table II, the magnetic field orienting vertically to the electric field also has a promotion effect on increasing the catalytic activity area of the AuPt catalyst. Under the magnetic field of B J, with the increasing of the magnetic flux density, the catalytic activity area also increases, which has a relationship with the decrease of the particle size. However, under the magnetic field of B//J, with the increasing of the magnetic flux density, the catalytic activity area decreases, which may be caused by the huge particles grown on the surface of the small particles (Fig.3(h)). These huge particles occupy more mass ratio of the whole catalyst and cause a lower value when being calculated. D. Effect of pulse plating parameters on properties of AuPt particles Compared with constant current electrodeposition, pulse plating has an effect on decreasing the particle size. The micromorphologies of AuPt particles prepared FIG. 7 SEM images (a) and (c) and TEM images (b) and (d) in different magnification scanning of AuPt particles on carbon paper at the pulse width of 10%, the peak current density of 100 ma/cm2, the pulse frequency of 0.5 khz, [HAuCl4 ]: [H2 PtCl6 ]=2:1, the magnetic flux density of 220 mt, and the magnetic field direction of B J. by pulse plating are shown in Fig.7. The AuPt particles are rough and spherical in range of nm under a relative lower magnetic flux density. Our previous orthogonal experimental results indicate that pulse width and current density have the biggest and the second biggest influence on the activity area of AuPt particles, respectively. That is to say, the pulse plating parameters have more obvious effect than the magnetic field parameters on the activity area of the AuPt catalyst. The catalytic activities of the sample in Fig.7 scanning for various cycles are shown in Fig.8. With the increase of the scanning cycles from the 1st to the 3rd, the potential of the reductive peaks move to a negative direction, while the current of the reductive peaks decreases, which indicates the catalytic activities decrease obviously. Some AuPt catalyst particles falling off from the carbon fiber during the scanning may be the reac 2014 Chinese Physical Society

6 Chin. J. Chem. Phys., Vol. 27, No. 6 AuPt Particles Magneto-electrodeposited on Carbon Paper FIG. 8 CV curves of AuPt particles on carbon paper at the pulse width of 10%, the peak current density of 100 ma/cm2, the pulse frequency of 0.5 khz, [HAuCl4 ]: [H2 PtCl6 ] of 2:1, the magnetic flux density of 220 mt, and the magnetic field direction of B J in an oxygen-saturated EMI-TFSI/Li-TFSI electrolyte at the scan rate of 10 mv/s for various cycles of 1st, 2nd, 3rd, 5th, and 15th. son of the decrease of the catalytic activities. After 3rd cycle, the potential and the current of the reductive peaks keep stable till to the 15th cycle, which demonstrate that the stability of the AuPt catalyst is good [22]. By adjusting the pulse width or the current density, different micromorphologies of the AuPt particles were obtained, which are shown in Fig.9. When the pulse width increased to 40%, the grains are flat and rough without sharp boundary while grain sizes change obviously (Fig.9(a)). However, when the current density decreased to 10 ma/cm2, the strip grains were deposited with diameter of less than 50 nm and length is about 300 nm (Fig.9(b)). The grains show an obvious growing direction, which may be because the smaller current density causes a slower growth speed of new grains. The grain growth is the main crystal process, so the growing direction is so obvious. CV curves in Fig.10 show that the above samples have different catalytic abilities. By adjusting pulse plating parameters, AuPt catalysts with various morphology and catalytic activity could be achieved in control. The effect of various pulse plating parameters need more and detailed research to get smaller grain, more appropriate compositions and better catalytic ability of AuPt catalyst. IV. CONCLUSION AuPt nano-particles were magneto-electrodeposited on the carbon papers. The morphologies of AuPt particles were influenced by the orientation of the magnetic field significantly, and the grain sizes decrease from about 1 µm to 200 nm with the increase of the magnetic flux density under constant current density. The AuPt alloy deposited in B//J has a bigger interplanar distance, a smaller grain size and a lower Au content 709 FIG. 9 SEM images in different magnification scanning of (a) and (b) and (c) and (d) of AuPt particles on carbon paper electrodeposited at the pulse frequency of 0.5 khz, [HAuCl4 ]:[H2 PtCl6 ]=2:1, the magnetic flux density of 220 mt, the magnetic field direction of B J. (a) and (c) The pulse width of 40% and the peak current density of 100 ma/cm2, (b) and (d) the pulse width of 10% and the peak current density of 10 ma/cm2. FIG. 10 CV curves of samples with different pulse width and peak current density in an oxygen-saturated EMI-TFSI/LiTFSI electrolyte at the scan rate of 10 mv/s. (a) 40%, 100 ma/cm2, (b) 10%, 10 ma/cm2. than that deposited in B J or without magnetic field. Compared with the magnetic field parameters, the pulse electrodeposition parameters have more significant effect on decreasing the size of AuPt particles. By pulse plating, the grain sizes decrease to about 100 nm. The magnetic field oriented vertically to the electric field has a promotion effect on increasing the catalytic ability of AuPt/carbon electrode. Magneto-electrodeposition is a convenient and prospective in situ technology for preparation of metal catalysts. The future work will focus on studying the controllable in situ preparation and the effect of each magneto-electrodeposition parameter on properties of AuPt particles for lithium air batteries. c 2014 Chinese Physical Society

7 710 Chin. J. Chem. Phys., Vol. 27, No. 6 Jin-qiu Zhang et al. V. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No ), the China Postdoctoral Science Foundation (No.2013M531049), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the Fundamental Research Funds for the Central Universities (No.HIT.NSRIF ). [1] Z. Huang, B. Chi, J. Pu, and J. Li, Prog. Chem. 25, 260 (2013). [2] C. O. Laoire, S. M. Mukerjee, K. M. Abraham, E. J. Plichta, and M. A. Hendrickson, J. Phys. Chem. C 113, (2009). [3] S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé, and P. G. Bruce, Angew. Chem. Int. Ed. 50, 1 (2001). [4] A. Débart, A. J. Paterson, J. Bao, and P. G. Bruce, Angew. Chem. Int. Ed. 47, 4521 (2008). [5] D. Zhang, Z. H. Fu, Z. Wei, T. Huang, and A. S. Yu, J. Electrochem. Soc. 157, A362 (2010). [6] J. X. Li, N. Wang, Y. Zhao, Y. H. Ding, and L. H. Guan, Electrochem. Commun. 13, 698 (2011). [7] Y. Cao, Z. K. Wei, J. He, J, Zang, Q. Zhang, M. S. Zheng, and Q. F. Dong, Energy Environ. Sci. 5, 9765 (2012). [8] L. Zhang, Z. L. Wang, D. Xu, J. J. Xu, X. B. Zhang, and L. M. Wang, Chin. Sci. Bull. 57, 4210 (2012). [9] Y. Yang, M. Shi, Q. F. Zhou, Y. S. Li, and Z. W. Fu, Electrochem. Commun. 20, 11 (2012). [10] Y. Lu, Z. Y. Wen, J. Jin, Y. M. Cui, M. F. Wu, and S. J. Sun, J. Solid State Electrochem. 16, 1863 (2012). [11] Z. Peng, S. A. Freunberger, Y. Chen, and P. G. Bruce, Science 337, 563 (2012). [12] S. Wang, N. Kristian, S. Jiang, and X. Wang, Nanotechnology 20, (2009). [13] D. O. Atienza, T. C. Allison, and Y. J. Tong, J. Phys. Chem. C 116, (2012). [14] Y. C. Lu, Z. C. Xu, H. A. Gasteiger, S. Chen, K. H. Schifferli, and S. H. Yang, J. Am. Chem. Soc. 132, (2010). [15] A. K. Thapa, Y. Hidaka, H. Hagiwara, S. Ida, and T. Ishihara, J. Electrochem. Soc. 158, A1483 (2011). [16] J. Q. Zhang, G. L. Chen, M. Z. An, and P. Wang, Int. J. Electrochem. Sci. 7, (2012). [17] M. Uhlemann, A. Krause, J. P. Chopart, and A. Gebert, J. Electrochem. Soc. 152, C817 (2005). [18] C. H. Cui, H. H. Li, and S. H. Yu, Chem. Sci. 2, 1611 ( 2011). [19] Z. H. Yu, M. Tian, Q. Y. Jiao, and D. G. Xia, Acta Phys. Chim. Sin. 22, 1015 (2006). [20] J. Hassoun, F. Croce, M. Armand, and B. Scrosati, Angew. Chem. Int. Ed. 50, 2999 (2011). [21] M. J. Trahan, S. Mukerjee, E. J. Plichta, M. A. Hendrickson, and K. M. Abraham, J. Electrochem. Soc. 160, A259 (2013). [22] D. S. He, Y. Han, J. Fennell, S. L. Horswell, and Z. Y. Li, Appl. Phys. Lett. 101, (2012). c 2014 Chinese Physical Society