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Electrochemical hydrogen storage behaviors of ultrafine Co–P particles prepared by direct ball-milling method
Electrochimica Acta 51 (2006) 4285–4290
Electrochemical hydrogen storage behaviors of ultrafine Co–P particles prepared by direct ball-milling method Yuliang Cao a , Wenchao Zhou a , Xiaoyan Li a , Xinping Ai a,1 , Xueping Gao b , Hanxi Yang a,∗,1 a
b
Department of Chemistry, Wuhan University, Wuhan 430072, China Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China
Received 6 August 2005; received in revised form 23 September 2005; accepted 10 December 2005 Available online 26 January 2006
Abstract Ultrafine particles of Co–P were synthesized by direct ball milling of Co and P powders and also investigated as a reversible hydrogen storage electrode material. The electrochemical results demonstrated that the reversible charge–discharge capacity of the Co–P electrode can reach more than 300 mA h/g. In addition, the cycling ability and high rate capability of the Co–P electrode are excellent with only 5% capacity decay after 100 cycles at a high rate of 300 mA/g. The temperature-programmed desorption measurements (TPD) of the Co–P electrode revealed that the charge and discharge reactions of the Co–P electrode proceeds predominantly through electrochemical hydrogen storage mechanism and the electrooxidation of cobalt contributes only a negligible part to the reversible electrochemical capacity. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ultrafine particles; Co–P alloy; Hydrogen storage; Anodic material; Electrochemical reversibility
1. Introduction Materials with high electrochemical hydrogen storage capacity and rate capability are greatly needed for use as negative electrode materials in various applications from portable electronics to electric vehicles. In past decades, many metal and alloy hydrides have been explored and successfully used for commercial batteries [1–3]. In order to improve the energy densities of the electrochemical battery systems, the search for new hydrogen storage materials has been continuously carried out and particularly, nano-sized materials have been paid considerable research attention as a promising new entry to the hydrogen storage electrode materials [4–6]. In recent years, a number of nanotubes [7–11] and alloy nanoparticles [12,13] have been shown to have larger hydrogen storage capacities than conventional hydrogen storage alloys; however, the room temperature kinetics and cycling stability of these materials are not satisfactory. Some transition metal–metalloid alloys, such as cobalt boride and nickel phosphide, have been known to have strong
ability for hydrogen adsorption [14] and excellent catalytic activity for electrochemical oxidation of hydrogen [15–18], which may serve as a new type of hydrogen storage medium. Recently, Mitov et al. [16] reported the electrochemical absorption–desorption properties of Co–B and Wang et al. [15] observed a reversible electrochemical hydrogen storage capacity of about 300 mA h/g from Co–B nanoparticles. These results seem to indicate that it is possible to use ultrafine transition metal–metalloid alloy to construct high capacity hydrogen storage electrode. In this paper, we prepared the ultrafine Co–P particles by direct ball-milling method and investigated the electrochemical hydrogen storage behaviors of the alloy particles. Based on the structural and electrochemical characterization of this material, we discussed the possible mechanisms of electrochemical hydrogen storage reactions on the ultrafine Co–P particles. 2. Experimental 2.1. Preparation of Co–P particles
∗ 1
Corresponding author. Tel.: +86 27 68754526; fax: +86 27 87884476. E-mail address: [email protected] (H. Yang). ISE member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.12.007
The Co–P powders were prepared by direct ball milling of cobalt and phosphor powders using a planet-type miller (ND4L, China). The starting material was a mixture of cobalt powder
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(purity, 99.0%) and red phosphor powder (purity, 99.3%) with the molar ratio of Co:P = 1:0.75, and the weight ratio of balls to powder was set to be 10:1. The starting materials were sealed in a steel milling pot and then filled with Ar. The ball-milling treatment (MA) was conducted at a speed of 240 rpm for 12 h. After milling, the resulting alloy powders were taken out in an Ar-filled glove box. 2.2. Structural characterization The size and morphology of the Co–P particles were examined by transmission electron microscopy (TEM) on a FEI Tacnai 20 microscope. The crystalline structure of the Co–P particles was characterized by powder X-ray diffractometry (XRD) on a Shimadzu XRD-6000 diffractometer with Cu K␣ source. The electrode samples for characterization of the changes in the crystalline phases at different charge and discharge states were taken from the cells, rinsed with distilled water and then dried for direct ex situ XRD analysis. 2.3. Electrochemical measurements The Co–P electrode was prepared by mixing the Co–P powder, acetylene black and poly-tetrafluoroethylene (PTFE) emulsion with isopropanol to form an electrode paste, then rolling the paste into a ca. 0.1 mm thick film and finally pressing the electrode film onto a nickel net. The Co–P electrode so prepared consisted of 80% Co–P powders, 12% acetylene black and 8% PTFE by weight. The cyclic voltammetry were performed using a threeelectrode test cell. The experimental cells were constructed with a Co–P electrode as anode, a sintered nickel electrode as cathode, and an Hg/HgO electrode in 6 M KOH as reference electrode. The charge–discharge measurements were carried out on the test cells of a sandwiched design with a 2 cm2 Co–P anode inbetween two nickel cathodes. 2.4. Chemical desorption measurements
Fig. 1. A TEM microgram of the Co–P powder. The selected area electron diffraction is inserted in top right corner.
Co–P powers are consisted of small grains of 100–200 nm size and each grain is composed of very fine particles. The selected area electron diffraction (SAED) of the sample as inserted in Fig. 1 shows a broad and featureless diffraction ring, seemingly suggesting a polycrystalline structure of the powders. However, the XRD pattern of the material as shown in Fig. 2 gives a few of very weak diffraction lines, which can be indexed from the hexagonal Co structure (P63/mmc, No. 194), indicating the existence of crystalline Co structure. This discrepancy between the SAED and XRD data may arise from the fact that the Co–P powders are consisted of a large number of nano-polycrystals, which leads to an indistinct diffracted ring in the SAED measurements and only exhibit a weak and incomplete XRD pattern. In addition, the EDAX analysis of the sample demonstrated that the Co and P elements are distributed uniformly at nano-sized scale without apparent separation of Co and P phases. These results
The temperature-programmed desorption experiments (TPD) were carried out on a chemisorption analyzer (Autochem II, Micromeritics). Before the TPD measurements, the Co–P powders were firstly purged with purified argon for 15 min to move out the adsorbed gases on the sample, and then changed to a flow of 10 vol.% H2 in Ar gas for hydrogen absorption. For the TPD measurements of the Co–P electrode charged and uncharged, the electrode samples were firstly purged with a flow of purified argon to obtain a stabilized baseline and then scanned from room temperature up to 550 ◦ C at a heating rate of 10 ◦ C/min, using argon as a carrier gas. 3. Results and discussion 3.1. Structural features of ball-milled Co–P particles Fig. 1 shows the TEM image of the ball-milled Co–P particles prepared in this work. It can be clearly visualized that the
Fig. 2. The XRD pattern of the Co–P powder.
Y. Cao et al. / Electrochimica Acta 51 (2006) 4285–4290
seem to indicate that the Co element in the powders exists in the form of nano-sized crystallites, which are combined with the surrounded P atoms. Since we failed to detect any single phases of phosphides, it is reasonable to attribute the Co–P powders to a Co–P alloy rather than a compound of Co phosphide from the chemical point of view. It should be mentioned that very recently, Zhang et al. [19] reported a CoP3 synthesis by a similar ball milling of the stoichiometric mixture of Co and P powders. Possibly because the atomic ratio of Co:P is non-stoichiometric and the rotating speed for ball milling is relatively low (250 rpm) in our work, we could not find the formation of any cobalt phosphides as reported in [19], and in addition, we could not find the existence of separate phase of elemental phosphor either. Therefore, we can only attribute the ball-milled Co–P powder to a non-stoichiometric alloy compound. 3.2. Electrochemical behaviors of the Co–P powder Fig. 3 shows a typical voltammogram of the Co–P powder in a 6 M KOH solution. The main CV feature of the sample is a pair of remarkable cathodic and anodic current peaks centered at −1.0 and −0.7 V, respectively, suggesting a reversible electrochemical oxidation–reduction process occurring on the Co–P powder. Since the cathodic peak is very similar to the CV features for electrochemical reduction of H2 O in the potential position and peak shape, and no other reduction reactions can take place in this potential region at the subsequent cathodic scan, it is most likely that the cathodic peak in the CV curve arises from the hydrogen adsorption on the Co–P electrode. At the reversed anodic scan, a strong oxidation peak appears at the onset potential of −0.85 V and reaches its maximum value at −0.7 V. Since the equilibrium potentials of Co and P elements are at −0.83 and −2.05 V (versus Hg/HgO) [20], it is difficult to assign the anodic current peak to be given rise by the oxidation of elemental Co or P. Nevertheless, the close resemblance of this anodic peak to the CV features of electrochemical oxidation of hydrogen, as observed previously on various hydrogen stor-
Fig. 3. A typical CV curve of the Co–P electrode in a 6 M KOH solution. Sweep rate, 10 mV/s.
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age electrodes [21,22] strongly suggests that the anodic peak in Fig. 3 arises mainly from the electrochemical oxidation of adsorbed hydrogen. Of course, the anodic oxidation of Co could not be completely excluded, particularly when the potential scan goes to a more positive potential region. In previous works, the electrochemical adsorption and oxidation of hydrogen has also been observed as a dominate reaction for a thin-film cobalt electrode [23] and Co–B alloy electrodes [15,16] and the partial oxidation of Co surface is suggested as a subsidiary process occurring only at more positive potentials. Thus, we can use the electrochemical hydrogen adsorption–desorption reaction of the Co–P materials for construction of reversible hydrogen storage electrodes. To examine the reversible electrochemical capacity of the Co–P material, we constructed the experimental Ni–MH cells using the Co–P negative electrodes and Ni(OH2 ) positive electrodes, and cycled the cells at conventional charge–discharge conditions. Fig. 4 shows the charge–discharge curves of the cells at a constant current of 300 mA/g. As it can be seen, the charging voltage plateaus appeared at 1.5–1.6 V, and the average discharge voltage of the cells were at 1.2 V, which are consistent with those observed for the conventional Ni–MH cells [24]. Very similar to the hydrogen storage behaviors of conventional alloy electrodes [25], the Co–P electrode also shows an activation process from the initial reversible capacity of ∼200 mA h/g to its highest capacity of 310 mA h/g at the 30th cycle. This capacity corresponds to the charge–discharge of 0.95 mol hydrogen for one molar Co–P electrode material, very close to an atomic ratio of H/M = 1, as expected from the hydrogen storage reaction of most alloy hydrides [26]. In order to examine the kinetic properties of the Co–P electrode, we measured the rate capability of the cells at various discharging currents of 100–1000 mA/g. As shown in Fig. 5, the delivered discharge capacities of the Co–P electrode are 300, 274 and 240 mA h/g at the high rates of 300, 500 and 1000 mA/g, corresponding to a capacity utilization of 99%, 90% and 80%, respectively, in comparison with the discharge capacity (303 mA h/g) of the Co–P electrode at 100 mA/g. These data demonstrated that the Co–P electrode has an excellent rate capa-
Fig. 4. Charge and discharge curves of the Co–P electrode at a constant current of 300 mA/g.
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Fig. 5. Discharge curves of the Co–P electrode at different current rates.
bility, and is capable of serving as a high power capability anode for the rechargeable alkaline batteries. Fig. 6 shows the specific discharge capacity of the Co–P electrode as a function of cycle number at a high rate of 300 mA/g. The discharge capacity of Co–P electrode increased with cycling number at first 30 cycles and then remained a quite stable value of ca. 310 mA h/g at successive cycles, showing excellent capacity retention for high rate cycling. 3.3. The reaction mechanism of the Co–P electrode From thermodynamic analysis, the standard oxidation potential of elemental Co is −0.83 V (versus Hg/HgO) in alkaline solution, slightly more positive than the standard oxidation potential (−0.93 V, versus Hg/HgO) of hydrogen, therefore, it needs to clarify whether the reversible capacity of the Co–P electrode is given rise from the electrochemical hydrogen storage of the Co–P electrode, or from the electrochemical oxidation–reduction of the elemental Co in the Co–P electrode. In earlier studies of the electrochemical oxidation of Co in alkaline solution, Meier et al. [27] and Novose-
Fig. 6. Cycling performance of the Co–P electrode at a high rate of 300 mA/g.
leskii et al. [28] observed a pair of weak and reversible CV peaks from thin-film Co surfaces and assigned the CV peaks to the electrochemical dissolution–precipitation reaction: Co(OH)2 + 2e− ↔ Co + 2OH− . White et al. [23] studied the electrochemical performance of the Co-coated hydrogen storage alloys and found that the Co-coating film can not only participate in electrochemical oxidation, but also can adsorb hydrogen, resulting in additional contribution to the discharge capacity. Later, Antonov et al. [29] investigated the dissolution of hydrogen in Co and found that the loading amount of hydrogen in Co in terms of H/Co atomic ratio is close to 0.95. Recently, a number of works reported the reversible charge–discharge behaviors of amorphous Co–B alloy particles and attributed the reversible electrochemical capacities to result from the electrochemical adsorption–desorption of hydrogen [15–17]. To get a clearer understanding of the reaction mechanism of the Co–P electrode, we used XRD and TPD to characterize the changes in the crystalline structure and in the adsorption properties of the Co–P electrode after charge and discharge. Fig. 7 gives the XRD patterns of the Co–P electrode at different charge and discharge states. Before charge, the XRD lines of the bare Co–P electrode can be all attributed to metallic Co, similar to that shown in Fig. 1, except for a new peak at 2θ = 18◦ due to PTFE binder [JCPDS 47–2217]. When the Co–P electrode was charged firstly to fully charged state at a current of 300 mA/g, the XRD pattern remained almost unchanged, expect for a few of new diffraction peaks of KOH hydrate appearing at 21.6◦ and 36.7◦ (Fig. 7b), showing no structural change occurring during charging process. However, if the Co–P electrode was fully discharged to −0.65 V, the XRD pattern (Fig. 7c) showed some characteristic peaks of -Co(OH)2 [JCPDS 30–0443], indicating the formation of Co(OH)2 at the late stage of deep discharge. At following charge, these XRD peaks of -Co(OH)2 were still present and remained their intensities almost unchanged (Fig. 7d), implying that the Co(OH)2 was not reduced at the charging potential interval. Since the Co(OH)2 was produced only at the more positive potential in late stage of discharge, it
Fig. 7. XRD patterns of the Co–P electrode: (a) freshly prepared, (b) fully charged, (c) completely discharged to −0.65 V and (d) recharged to −0.93 V.
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Fig. 8. Changes of the OCP with charging times for: (a) pure Co power electrode and (b) the Co–P electrode.
can contribute only a small part to the discharge capacity of the Co–P electrode. In fact, the open circuit potential (OCP) of the Co–P electrode can also provide a hint for distinguishing the possible working mechanisms of the electrode. Fig. 8 compares the OCPs of the Co–P electrode and the pure Co electrode measured after a given time of cathodic polarization. It can be seen from the plots in Fig. 8 that once the Co–P electrode is cathodically polarized, its OCP shifts rapidly towards negative potential region of hydrogen generation and maintains steadily at the standard electrode potential of hydrogen (−0.93 V) even the Co–P electrode is kept at the charging potential (−1.0 V) only for 10 min. However, the OCP of the pure Co electrode does not change very much and can only reach the equilibrium potential (−0.83 V) of a Co electrode. These phenomena suggest that hydrogen absorption can only occur effectively on the Co–P electrode, but not on pure Co powders. In other words, the existence of elemental phosphor plays an important role in the electrochemical hydrogen storage of the Co–P electrode. Though there have been a number of works showing the differences in the electronic structure of the Co–P compounds from their parent elements of Co and P [30,31], it is not sufficient for us to explain the detailed interactions of hydrogen atoms with the Co–P particles. The hydrogen absorption of the Co–P powder can be further convinced from the temperature-programmed desorption (TPD) experiments. Fig. 9 shows the TPD profiles of hydrogen on the Co–P powders, which were pretreated with or without of a flow of 10 vol.% H2 loaded Ar gas under ambient pressure and temperature. The sample without hydrogen pretreatment shows only a weak, broad and featureless band at 160–200 ◦ C, possibly due to desorption of the residual gas, such as oxygen and nitrogen, absorbed on the Co–P particles in experimental treatments. When the sample is purged with H2 , three distinct hydrogen desorption peaks appear at 300, 400 and 460 ◦ C, implying that the Co–P powder has a certain ability to absorb hydrogen molecules and the absorbed hydrogen may bind to the sample in different adsorption sites. From the integrated area of three desorption peaks in Fig. 9, it can be calculated that the hydrogen storage capacity of the sample is about 5.63 mL/g (∼0.05 wt.%) at this experimental condition. If the sample was exposed in a
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Fig. 9. TPD spectra of the Co–P powder with purge of 0% (dashed line) and 10% H2 (real line) in Ar.
flow of pure hydrogen, the hydrogen storage capacity should be increased to ca. 0.5 wt.%, corresponding to a discharge capacity of 134 mA h/g for the hydrogen storage electrode. Since all the hydrogen desorption steps occur at quite high temperatures and the amount of desorbed hydrogen is much higher than that expected from a physisorption reaction, it is most likely that the hydrogen absorption on the Co–P powder takes place by the dissolution of hydrogen in the bulk phase of Co metal, or strongly chemical adsorption of hydrogen on the Co–P particles. This hydrogen storage mechanism is very different from the lithium storage mechanism in the CoPx compounds as reported by Zhang et al. [19], in which lithium intercalation into the CoPx lattice proceeds through the formation of Co and Lix P, leading to a structural breakdown of the host lattice. In contrary, the hydrogen storage in the Co–P particles does not cause any structural changes during the charge–discharge processes. In order to ensure the electrochemical hydrogen storage of the Co–P particles, we also measured the hydrogen desorption properties of the Co–P electrode by TPD measurements. Fig. 10 compares the TPD profiles of the Co–P electrodes taken from the fully charged cells and uncharged cells. As it can be seen in Fig. 10, the TPD profile of the uncharged electrode appears
Fig. 10. TPD spectra of the Co–P electrode completely charged (real line) and uncharged (dashed line) before TPD measurements.
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as a flat baseline, showing no sign of hydrogen desorption from the electrode. However, for the electrode fully charged, a huge desorption band appears in the TPD curve with four distinct desorption peaks at ∼300, 360, 400 and 450 ◦ C, respectively. Except for the TPD peak at 360 ◦ C, the other three peaks at 300, 400 and 450 ◦ C can be seen in the TPD curve of the Co–P particles with hydrogen pretreatment (Fig. 9), reflecting the same absorption types of gaseous hydrogen occurring on the Co–P electrode. The appearance of a new desorption peak at 360 ◦ C for the Co–P electrode is clearly related to an electrochemical adsorption or absorption of hydrogen on the electrode, although we are not able to explain the mechanism for the electrochemical hydrogen storage. By calculating the total area of the TPD peaks in Fig. 10, the amount of hydrogen released from the Co–P electrode is 84.65 mL/g, corresponding to a discharge capacity of 202 mA h/g, which is considerably lower than the realized discharge capacity (282 mA h/g) of this electrode. This capacity loss is probably arisen from the release or oxidation of hydrogen atoms when the electrode was taken from the cell and exposed in air during experiment. Nevertheless, the large amount of desorbed hydrogen (>200 mA h/g) cannot be accounted for simply by a chemical adsorption reaction of hydrogen on the surface of the Co–P particles, but can only be attributed by the electrochemical hydrogen storage of the material, since a surface adsorption of hydrogen on a gram of Co powders (φ = 20 nm) can give only a electrochemical capacity of ∼50 mA h/g theoretically, even if every atoms of Co surface is occupied by a hydrogen atom. Based on the above experimental results, it can be concluded that the reversible charge–discharge processes of the Co–P material proceed mainly through the electrochemical storage and oxidation of hydrogen and the electrode reaction can be expressed as: [Co–P] + H2 O + e ↔ [Co–P]H + OH−
(1)
Finally, we have to point out that at present stage, we are not clear how the hydrogen atoms is bonded in the Co–P particles and how the P element plays a crucial role for the reversible electrochemical hydrogen storage reactions of the material. Further work to reveal the interactions between the hydrogen atoms and Co, P elements in the alloy particles may help to solve these problems. 4. Conclusions In summary, we prepared the ultrafine Co–P particles simply by a direct ball-milling method and investigated the electrochemical properties of this material. It is found that the reversible charge–discharge capacity of the Co–P electrode can reach more than 300 mA h/g at a high rate of 300 mA/g and remains steadily at continuous cycling. Experimental results from electrochemical and TPD measurements revealed that the charge and discharge reactions of the Co–P electrode proceeds predominantly through electrochemical storage and oxidation of
hydrogen produced by electrochemical reduction of water, and the electrooxidation of cobalt contributes only a negligible part to the reversible electrochemical capacity. Acknowledgement The authors thank financial support by the 973 Program, China (Grant No. 2002CB211800) and the National Science Foundation of China (No. 20573080). References [1] U. K¨ohler, J. K¨umper, M.J. Ullrich, J. Power Sources 105 (2) (2002) 139. [2] B.-Y. Liaw, X.-G. Yang, Solid State Ionics 152/153 (2002) 217. [3] X.-P. Gao, Y. Wang, Z.-W. Lu, W.-K. Hu, F. Wu, D.-Y. Song, P.-W. Shen, Chem. Mater. 16 (13) (2004) 2515. [4] X.-P. Gao, Z.-W. Lu, Y. Wang, F. Wu, D.-Y. Song, P.-W. Shen, Electrochem. Solid State Lett. 7 (5) (2004) A102. [5] T.W. Hong, Y.J. Kim, J. Alloys Compd. 333 (2002) L1. [6] Z. Dehouche, J. Goyette, J.K. Bose, R. Schulz, Int. J. Hydrogen Energy 28 (2003) 983. [7] L. Schlapbach, A. Z¨uttel, Nature 414 (6861) (2001) 353. [8] M.S. Dresselhaus, I.L. Thomas, Nature 414 (6861) (2001) 332. [9] J. Chen, S.-L. Li, Z.-L. Tao, Y.-T. Shen, C.-X. Cui, J. Am. Chem. Soc. 125 (2003) 5284. [10] C. Liu, Y.-Y. Fan, M. Liu, H.-T. Cong, H.-M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [11] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [12] K. Tanaka, M. Sowa, Y. Kita, T. Kubota, N. Tanaka, J. Alloys Compd. 330–332 (2002) 732. [13] M.J. Jurczyk, J. Alloys Compd. 307 (2000) 279. [14] K. Kaghavachari, Q. Fu, G. Chen, L. Li, C.-H. Li, D.C. Law, R.F. Hicks, J. Am. Chem. Soc. 124 (50) (2002) 15119. [15] Y.-D. Wang, X.-P. Ai, H.-X. Yang, Chem. Mater. 16 (24) (2004) 5194. [16] M. Mitov, A. Popov, I. Dragieva, Colloids Surf. A: Physicochem. Eng. Aspects 149 (1999) 413. [17] M. Mitov, A. Popov, I. Dragieva, J. Appl. Electrochem. 29 (1999) 59. [18] J.-X.Yu.L. Wang, Y.-D. Wang, H. Dong, H.-X. Yang, J. Electrochem. Soc. 151 (8) (2004) A1124. [19] Z.S. Zhang, J. Yang, Y.N. Nuli, B.F. Wang, J.J. Xu, Solid State Ionics 176 (2005) 693. [20] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker Inc., New York, 1985. [21] M. Kopczyk, G. Wojcik, G. Mlynarek, A. Sierczynska, M. BeltowskaBrzezinska, J. Appl. Electrochem. 26 (1996) 639. [22] A. Lasia, D. Gregoire, J. Electrochem. Soc. 142 (1995) 3393. [23] B.S. Haran, B.N. Popov, R.E. White, J. Electrochem. Soc. 145 (1998) 3000. [24] K.C. Hong, J. Alloys Compd. 321 (2) (2001) 307. [25] H. Lee, S.M. Lee, J.Y. Lee, J. Electrochem. Soc. 146 (10) (1999) 3666. [26] J.-M. Wu, J. Li, W.-P. Zhang, F. Muo, L. Tai, R. Xu, J. Alloys Compd. 248 (1997) 180. [27] H.G. Meier, J.R. Vilche, A.J. Arvia, J. Electroanal. Chem. 134 (1982) 251. [28] I.M. Novoseleskii, N.R. Menglisheva, Electrochim. Acta 29 (1984) 21. [29] V.E. Antonov, T.E. Antonova, M. Baier, G. Grosse, F.E. Wagner, J. Alloys Compd. 239 (1996) 198. [30] R.C. Ambrosio, E.A. Ticianelli, J. Electrochem. Soc. 150 (9) (2003) E438. [31] P. Lagarde, J. Rivory, G. Vlaic, J. Noncryst. Solids 57 (1983) 275.
Electrochemical hydrogen storage behaviors of ultrafine Co–P particles prepared by direct ball-milling method Yuliang Cao a , Wenchao Zhou a , Xiaoyan Li a , Xinping Ai a,1 , Xueping Gao b , Hanxi Yang a,∗,1 a
b
Department of Chemistry, Wuhan University, Wuhan 430072, China Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China
Received 6 August 2005; received in revised form 23 September 2005; accepted 10 December 2005 Available online 26 January 2006
Abstract Ultrafine particles of Co–P were synthesized by direct ball milling of Co and P powders and also investigated as a reversible hydrogen storage electrode material. The electrochemical results demonstrated that the reversible charge–discharge capacity of the Co–P electrode can reach more than 300 mA h/g. In addition, the cycling ability and high rate capability of the Co–P electrode are excellent with only 5% capacity decay after 100 cycles at a high rate of 300 mA/g. The temperature-programmed desorption measurements (TPD) of the Co–P electrode revealed that the charge and discharge reactions of the Co–P electrode proceeds predominantly through electrochemical hydrogen storage mechanism and the electrooxidation of cobalt contributes only a negligible part to the reversible electrochemical capacity. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ultrafine particles; Co–P alloy; Hydrogen storage; Anodic material; Electrochemical reversibility
1. Introduction Materials with high electrochemical hydrogen storage capacity and rate capability are greatly needed for use as negative electrode materials in various applications from portable electronics to electric vehicles. In past decades, many metal and alloy hydrides have been explored and successfully used for commercial batteries [1–3]. In order to improve the energy densities of the electrochemical battery systems, the search for new hydrogen storage materials has been continuously carried out and particularly, nano-sized materials have been paid considerable research attention as a promising new entry to the hydrogen storage electrode materials [4–6]. In recent years, a number of nanotubes [7–11] and alloy nanoparticles [12,13] have been shown to have larger hydrogen storage capacities than conventional hydrogen storage alloys; however, the room temperature kinetics and cycling stability of these materials are not satisfactory. Some transition metal–metalloid alloys, such as cobalt boride and nickel phosphide, have been known to have strong
ability for hydrogen adsorption [14] and excellent catalytic activity for electrochemical oxidation of hydrogen [15–18], which may serve as a new type of hydrogen storage medium. Recently, Mitov et al. [16] reported the electrochemical absorption–desorption properties of Co–B and Wang et al. [15] observed a reversible electrochemical hydrogen storage capacity of about 300 mA h/g from Co–B nanoparticles. These results seem to indicate that it is possible to use ultrafine transition metal–metalloid alloy to construct high capacity hydrogen storage electrode. In this paper, we prepared the ultrafine Co–P particles by direct ball-milling method and investigated the electrochemical hydrogen storage behaviors of the alloy particles. Based on the structural and electrochemical characterization of this material, we discussed the possible mechanisms of electrochemical hydrogen storage reactions on the ultrafine Co–P particles. 2. Experimental 2.1. Preparation of Co–P particles
∗ 1
Corresponding author. Tel.: +86 27 68754526; fax: +86 27 87884476. E-mail address: [email protected] (H. Yang). ISE member.
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.12.007
The Co–P powders were prepared by direct ball milling of cobalt and phosphor powders using a planet-type miller (ND4L, China). The starting material was a mixture of cobalt powder
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(purity, 99.0%) and red phosphor powder (purity, 99.3%) with the molar ratio of Co:P = 1:0.75, and the weight ratio of balls to powder was set to be 10:1. The starting materials were sealed in a steel milling pot and then filled with Ar. The ball-milling treatment (MA) was conducted at a speed of 240 rpm for 12 h. After milling, the resulting alloy powders were taken out in an Ar-filled glove box. 2.2. Structural characterization The size and morphology of the Co–P particles were examined by transmission electron microscopy (TEM) on a FEI Tacnai 20 microscope. The crystalline structure of the Co–P particles was characterized by powder X-ray diffractometry (XRD) on a Shimadzu XRD-6000 diffractometer with Cu K␣ source. The electrode samples for characterization of the changes in the crystalline phases at different charge and discharge states were taken from the cells, rinsed with distilled water and then dried for direct ex situ XRD analysis. 2.3. Electrochemical measurements The Co–P electrode was prepared by mixing the Co–P powder, acetylene black and poly-tetrafluoroethylene (PTFE) emulsion with isopropanol to form an electrode paste, then rolling the paste into a ca. 0.1 mm thick film and finally pressing the electrode film onto a nickel net. The Co–P electrode so prepared consisted of 80% Co–P powders, 12% acetylene black and 8% PTFE by weight. The cyclic voltammetry were performed using a threeelectrode test cell. The experimental cells were constructed with a Co–P electrode as anode, a sintered nickel electrode as cathode, and an Hg/HgO electrode in 6 M KOH as reference electrode. The charge–discharge measurements were carried out on the test cells of a sandwiched design with a 2 cm2 Co–P anode inbetween two nickel cathodes. 2.4. Chemical desorption measurements
Fig. 1. A TEM microgram of the Co–P powder. The selected area electron diffraction is inserted in top right corner.
Co–P powers are consisted of small grains of 100–200 nm size and each grain is composed of very fine particles. The selected area electron diffraction (SAED) of the sample as inserted in Fig. 1 shows a broad and featureless diffraction ring, seemingly suggesting a polycrystalline structure of the powders. However, the XRD pattern of the material as shown in Fig. 2 gives a few of very weak diffraction lines, which can be indexed from the hexagonal Co structure (P63/mmc, No. 194), indicating the existence of crystalline Co structure. This discrepancy between the SAED and XRD data may arise from the fact that the Co–P powders are consisted of a large number of nano-polycrystals, which leads to an indistinct diffracted ring in the SAED measurements and only exhibit a weak and incomplete XRD pattern. In addition, the EDAX analysis of the sample demonstrated that the Co and P elements are distributed uniformly at nano-sized scale without apparent separation of Co and P phases. These results
The temperature-programmed desorption experiments (TPD) were carried out on a chemisorption analyzer (Autochem II, Micromeritics). Before the TPD measurements, the Co–P powders were firstly purged with purified argon for 15 min to move out the adsorbed gases on the sample, and then changed to a flow of 10 vol.% H2 in Ar gas for hydrogen absorption. For the TPD measurements of the Co–P electrode charged and uncharged, the electrode samples were firstly purged with a flow of purified argon to obtain a stabilized baseline and then scanned from room temperature up to 550 ◦ C at a heating rate of 10 ◦ C/min, using argon as a carrier gas. 3. Results and discussion 3.1. Structural features of ball-milled Co–P particles Fig. 1 shows the TEM image of the ball-milled Co–P particles prepared in this work. It can be clearly visualized that the
Fig. 2. The XRD pattern of the Co–P powder.
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seem to indicate that the Co element in the powders exists in the form of nano-sized crystallites, which are combined with the surrounded P atoms. Since we failed to detect any single phases of phosphides, it is reasonable to attribute the Co–P powders to a Co–P alloy rather than a compound of Co phosphide from the chemical point of view. It should be mentioned that very recently, Zhang et al. [19] reported a CoP3 synthesis by a similar ball milling of the stoichiometric mixture of Co and P powders. Possibly because the atomic ratio of Co:P is non-stoichiometric and the rotating speed for ball milling is relatively low (250 rpm) in our work, we could not find the formation of any cobalt phosphides as reported in [19], and in addition, we could not find the existence of separate phase of elemental phosphor either. Therefore, we can only attribute the ball-milled Co–P powder to a non-stoichiometric alloy compound. 3.2. Electrochemical behaviors of the Co–P powder Fig. 3 shows a typical voltammogram of the Co–P powder in a 6 M KOH solution. The main CV feature of the sample is a pair of remarkable cathodic and anodic current peaks centered at −1.0 and −0.7 V, respectively, suggesting a reversible electrochemical oxidation–reduction process occurring on the Co–P powder. Since the cathodic peak is very similar to the CV features for electrochemical reduction of H2 O in the potential position and peak shape, and no other reduction reactions can take place in this potential region at the subsequent cathodic scan, it is most likely that the cathodic peak in the CV curve arises from the hydrogen adsorption on the Co–P electrode. At the reversed anodic scan, a strong oxidation peak appears at the onset potential of −0.85 V and reaches its maximum value at −0.7 V. Since the equilibrium potentials of Co and P elements are at −0.83 and −2.05 V (versus Hg/HgO) [20], it is difficult to assign the anodic current peak to be given rise by the oxidation of elemental Co or P. Nevertheless, the close resemblance of this anodic peak to the CV features of electrochemical oxidation of hydrogen, as observed previously on various hydrogen stor-
Fig. 3. A typical CV curve of the Co–P electrode in a 6 M KOH solution. Sweep rate, 10 mV/s.
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age electrodes [21,22] strongly suggests that the anodic peak in Fig. 3 arises mainly from the electrochemical oxidation of adsorbed hydrogen. Of course, the anodic oxidation of Co could not be completely excluded, particularly when the potential scan goes to a more positive potential region. In previous works, the electrochemical adsorption and oxidation of hydrogen has also been observed as a dominate reaction for a thin-film cobalt electrode [23] and Co–B alloy electrodes [15,16] and the partial oxidation of Co surface is suggested as a subsidiary process occurring only at more positive potentials. Thus, we can use the electrochemical hydrogen adsorption–desorption reaction of the Co–P materials for construction of reversible hydrogen storage electrodes. To examine the reversible electrochemical capacity of the Co–P material, we constructed the experimental Ni–MH cells using the Co–P negative electrodes and Ni(OH2 ) positive electrodes, and cycled the cells at conventional charge–discharge conditions. Fig. 4 shows the charge–discharge curves of the cells at a constant current of 300 mA/g. As it can be seen, the charging voltage plateaus appeared at 1.5–1.6 V, and the average discharge voltage of the cells were at 1.2 V, which are consistent with those observed for the conventional Ni–MH cells [24]. Very similar to the hydrogen storage behaviors of conventional alloy electrodes [25], the Co–P electrode also shows an activation process from the initial reversible capacity of ∼200 mA h/g to its highest capacity of 310 mA h/g at the 30th cycle. This capacity corresponds to the charge–discharge of 0.95 mol hydrogen for one molar Co–P electrode material, very close to an atomic ratio of H/M = 1, as expected from the hydrogen storage reaction of most alloy hydrides [26]. In order to examine the kinetic properties of the Co–P electrode, we measured the rate capability of the cells at various discharging currents of 100–1000 mA/g. As shown in Fig. 5, the delivered discharge capacities of the Co–P electrode are 300, 274 and 240 mA h/g at the high rates of 300, 500 and 1000 mA/g, corresponding to a capacity utilization of 99%, 90% and 80%, respectively, in comparison with the discharge capacity (303 mA h/g) of the Co–P electrode at 100 mA/g. These data demonstrated that the Co–P electrode has an excellent rate capa-
Fig. 4. Charge and discharge curves of the Co–P electrode at a constant current of 300 mA/g.
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Fig. 5. Discharge curves of the Co–P electrode at different current rates.
bility, and is capable of serving as a high power capability anode for the rechargeable alkaline batteries. Fig. 6 shows the specific discharge capacity of the Co–P electrode as a function of cycle number at a high rate of 300 mA/g. The discharge capacity of Co–P electrode increased with cycling number at first 30 cycles and then remained a quite stable value of ca. 310 mA h/g at successive cycles, showing excellent capacity retention for high rate cycling. 3.3. The reaction mechanism of the Co–P electrode From thermodynamic analysis, the standard oxidation potential of elemental Co is −0.83 V (versus Hg/HgO) in alkaline solution, slightly more positive than the standard oxidation potential (−0.93 V, versus Hg/HgO) of hydrogen, therefore, it needs to clarify whether the reversible capacity of the Co–P electrode is given rise from the electrochemical hydrogen storage of the Co–P electrode, or from the electrochemical oxidation–reduction of the elemental Co in the Co–P electrode. In earlier studies of the electrochemical oxidation of Co in alkaline solution, Meier et al. [27] and Novose-
Fig. 6. Cycling performance of the Co–P electrode at a high rate of 300 mA/g.
leskii et al. [28] observed a pair of weak and reversible CV peaks from thin-film Co surfaces and assigned the CV peaks to the electrochemical dissolution–precipitation reaction: Co(OH)2 + 2e− ↔ Co + 2OH− . White et al. [23] studied the electrochemical performance of the Co-coated hydrogen storage alloys and found that the Co-coating film can not only participate in electrochemical oxidation, but also can adsorb hydrogen, resulting in additional contribution to the discharge capacity. Later, Antonov et al. [29] investigated the dissolution of hydrogen in Co and found that the loading amount of hydrogen in Co in terms of H/Co atomic ratio is close to 0.95. Recently, a number of works reported the reversible charge–discharge behaviors of amorphous Co–B alloy particles and attributed the reversible electrochemical capacities to result from the electrochemical adsorption–desorption of hydrogen [15–17]. To get a clearer understanding of the reaction mechanism of the Co–P electrode, we used XRD and TPD to characterize the changes in the crystalline structure and in the adsorption properties of the Co–P electrode after charge and discharge. Fig. 7 gives the XRD patterns of the Co–P electrode at different charge and discharge states. Before charge, the XRD lines of the bare Co–P electrode can be all attributed to metallic Co, similar to that shown in Fig. 1, except for a new peak at 2θ = 18◦ due to PTFE binder [JCPDS 47–2217]. When the Co–P electrode was charged firstly to fully charged state at a current of 300 mA/g, the XRD pattern remained almost unchanged, expect for a few of new diffraction peaks of KOH hydrate appearing at 21.6◦ and 36.7◦ (Fig. 7b), showing no structural change occurring during charging process. However, if the Co–P electrode was fully discharged to −0.65 V, the XRD pattern (Fig. 7c) showed some characteristic peaks of -Co(OH)2 [JCPDS 30–0443], indicating the formation of Co(OH)2 at the late stage of deep discharge. At following charge, these XRD peaks of -Co(OH)2 were still present and remained their intensities almost unchanged (Fig. 7d), implying that the Co(OH)2 was not reduced at the charging potential interval. Since the Co(OH)2 was produced only at the more positive potential in late stage of discharge, it
Fig. 7. XRD patterns of the Co–P electrode: (a) freshly prepared, (b) fully charged, (c) completely discharged to −0.65 V and (d) recharged to −0.93 V.
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Fig. 8. Changes of the OCP with charging times for: (a) pure Co power electrode and (b) the Co–P electrode.
can contribute only a small part to the discharge capacity of the Co–P electrode. In fact, the open circuit potential (OCP) of the Co–P electrode can also provide a hint for distinguishing the possible working mechanisms of the electrode. Fig. 8 compares the OCPs of the Co–P electrode and the pure Co electrode measured after a given time of cathodic polarization. It can be seen from the plots in Fig. 8 that once the Co–P electrode is cathodically polarized, its OCP shifts rapidly towards negative potential region of hydrogen generation and maintains steadily at the standard electrode potential of hydrogen (−0.93 V) even the Co–P electrode is kept at the charging potential (−1.0 V) only for 10 min. However, the OCP of the pure Co electrode does not change very much and can only reach the equilibrium potential (−0.83 V) of a Co electrode. These phenomena suggest that hydrogen absorption can only occur effectively on the Co–P electrode, but not on pure Co powders. In other words, the existence of elemental phosphor plays an important role in the electrochemical hydrogen storage of the Co–P electrode. Though there have been a number of works showing the differences in the electronic structure of the Co–P compounds from their parent elements of Co and P [30,31], it is not sufficient for us to explain the detailed interactions of hydrogen atoms with the Co–P particles. The hydrogen absorption of the Co–P powder can be further convinced from the temperature-programmed desorption (TPD) experiments. Fig. 9 shows the TPD profiles of hydrogen on the Co–P powders, which were pretreated with or without of a flow of 10 vol.% H2 loaded Ar gas under ambient pressure and temperature. The sample without hydrogen pretreatment shows only a weak, broad and featureless band at 160–200 ◦ C, possibly due to desorption of the residual gas, such as oxygen and nitrogen, absorbed on the Co–P particles in experimental treatments. When the sample is purged with H2 , three distinct hydrogen desorption peaks appear at 300, 400 and 460 ◦ C, implying that the Co–P powder has a certain ability to absorb hydrogen molecules and the absorbed hydrogen may bind to the sample in different adsorption sites. From the integrated area of three desorption peaks in Fig. 9, it can be calculated that the hydrogen storage capacity of the sample is about 5.63 mL/g (∼0.05 wt.%) at this experimental condition. If the sample was exposed in a
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Fig. 9. TPD spectra of the Co–P powder with purge of 0% (dashed line) and 10% H2 (real line) in Ar.
flow of pure hydrogen, the hydrogen storage capacity should be increased to ca. 0.5 wt.%, corresponding to a discharge capacity of 134 mA h/g for the hydrogen storage electrode. Since all the hydrogen desorption steps occur at quite high temperatures and the amount of desorbed hydrogen is much higher than that expected from a physisorption reaction, it is most likely that the hydrogen absorption on the Co–P powder takes place by the dissolution of hydrogen in the bulk phase of Co metal, or strongly chemical adsorption of hydrogen on the Co–P particles. This hydrogen storage mechanism is very different from the lithium storage mechanism in the CoPx compounds as reported by Zhang et al. [19], in which lithium intercalation into the CoPx lattice proceeds through the formation of Co and Lix P, leading to a structural breakdown of the host lattice. In contrary, the hydrogen storage in the Co–P particles does not cause any structural changes during the charge–discharge processes. In order to ensure the electrochemical hydrogen storage of the Co–P particles, we also measured the hydrogen desorption properties of the Co–P electrode by TPD measurements. Fig. 10 compares the TPD profiles of the Co–P electrodes taken from the fully charged cells and uncharged cells. As it can be seen in Fig. 10, the TPD profile of the uncharged electrode appears
Fig. 10. TPD spectra of the Co–P electrode completely charged (real line) and uncharged (dashed line) before TPD measurements.
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as a flat baseline, showing no sign of hydrogen desorption from the electrode. However, for the electrode fully charged, a huge desorption band appears in the TPD curve with four distinct desorption peaks at ∼300, 360, 400 and 450 ◦ C, respectively. Except for the TPD peak at 360 ◦ C, the other three peaks at 300, 400 and 450 ◦ C can be seen in the TPD curve of the Co–P particles with hydrogen pretreatment (Fig. 9), reflecting the same absorption types of gaseous hydrogen occurring on the Co–P electrode. The appearance of a new desorption peak at 360 ◦ C for the Co–P electrode is clearly related to an electrochemical adsorption or absorption of hydrogen on the electrode, although we are not able to explain the mechanism for the electrochemical hydrogen storage. By calculating the total area of the TPD peaks in Fig. 10, the amount of hydrogen released from the Co–P electrode is 84.65 mL/g, corresponding to a discharge capacity of 202 mA h/g, which is considerably lower than the realized discharge capacity (282 mA h/g) of this electrode. This capacity loss is probably arisen from the release or oxidation of hydrogen atoms when the electrode was taken from the cell and exposed in air during experiment. Nevertheless, the large amount of desorbed hydrogen (>200 mA h/g) cannot be accounted for simply by a chemical adsorption reaction of hydrogen on the surface of the Co–P particles, but can only be attributed by the electrochemical hydrogen storage of the material, since a surface adsorption of hydrogen on a gram of Co powders (φ = 20 nm) can give only a electrochemical capacity of ∼50 mA h/g theoretically, even if every atoms of Co surface is occupied by a hydrogen atom. Based on the above experimental results, it can be concluded that the reversible charge–discharge processes of the Co–P material proceed mainly through the electrochemical storage and oxidation of hydrogen and the electrode reaction can be expressed as: [Co–P] + H2 O + e ↔ [Co–P]H + OH−
(1)
Finally, we have to point out that at present stage, we are not clear how the hydrogen atoms is bonded in the Co–P particles and how the P element plays a crucial role for the reversible electrochemical hydrogen storage reactions of the material. Further work to reveal the interactions between the hydrogen atoms and Co, P elements in the alloy particles may help to solve these problems. 4. Conclusions In summary, we prepared the ultrafine Co–P particles simply by a direct ball-milling method and investigated the electrochemical properties of this material. It is found that the reversible charge–discharge capacity of the Co–P electrode can reach more than 300 mA h/g at a high rate of 300 mA/g and remains steadily at continuous cycling. Experimental results from electrochemical and TPD measurements revealed that the charge and discharge reactions of the Co–P electrode proceeds predominantly through electrochemical storage and oxidation of
hydrogen produced by electrochemical reduction of water, and the electrooxidation of cobalt contributes only a negligible part to the reversible electrochemical capacity. Acknowledgement The authors thank financial support by the 973 Program, China (Grant No. 2002CB211800) and the National Science Foundation of China (No. 20573080). References [1] U. K¨ohler, J. K¨umper, M.J. Ullrich, J. Power Sources 105 (2) (2002) 139. [2] B.-Y. Liaw, X.-G. Yang, Solid State Ionics 152/153 (2002) 217. [3] X.-P. Gao, Y. Wang, Z.-W. Lu, W.-K. Hu, F. Wu, D.-Y. Song, P.-W. Shen, Chem. Mater. 16 (13) (2004) 2515. [4] X.-P. Gao, Z.-W. Lu, Y. Wang, F. Wu, D.-Y. Song, P.-W. Shen, Electrochem. Solid State Lett. 7 (5) (2004) A102. [5] T.W. Hong, Y.J. Kim, J. Alloys Compd. 333 (2002) L1. [6] Z. Dehouche, J. Goyette, J.K. Bose, R. Schulz, Int. J. Hydrogen Energy 28 (2003) 983. [7] L. Schlapbach, A. Z¨uttel, Nature 414 (6861) (2001) 353. [8] M.S. Dresselhaus, I.L. Thomas, Nature 414 (6861) (2001) 332. [9] J. Chen, S.-L. Li, Z.-L. Tao, Y.-T. Shen, C.-X. Cui, J. Am. Chem. Soc. 125 (2003) 5284. [10] C. Liu, Y.-Y. Fan, M. Liu, H.-T. Cong, H.-M. Cheng, M.S. Dresselhaus, Science 286 (1999) 1127. [11] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [12] K. Tanaka, M. Sowa, Y. Kita, T. Kubota, N. Tanaka, J. Alloys Compd. 330–332 (2002) 732. [13] M.J. Jurczyk, J. Alloys Compd. 307 (2000) 279. [14] K. Kaghavachari, Q. Fu, G. Chen, L. Li, C.-H. Li, D.C. Law, R.F. Hicks, J. Am. Chem. Soc. 124 (50) (2002) 15119. [15] Y.-D. Wang, X.-P. Ai, H.-X. Yang, Chem. Mater. 16 (24) (2004) 5194. [16] M. Mitov, A. Popov, I. Dragieva, Colloids Surf. A: Physicochem. Eng. Aspects 149 (1999) 413. [17] M. Mitov, A. Popov, I. Dragieva, J. Appl. Electrochem. 29 (1999) 59. [18] J.-X.Yu.L. Wang, Y.-D. Wang, H. Dong, H.-X. Yang, J. Electrochem. Soc. 151 (8) (2004) A1124. [19] Z.S. Zhang, J. Yang, Y.N. Nuli, B.F. Wang, J.J. Xu, Solid State Ionics 176 (2005) 693. [20] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker Inc., New York, 1985. [21] M. Kopczyk, G. Wojcik, G. Mlynarek, A. Sierczynska, M. BeltowskaBrzezinska, J. Appl. Electrochem. 26 (1996) 639. [22] A. Lasia, D. Gregoire, J. Electrochem. Soc. 142 (1995) 3393. [23] B.S. Haran, B.N. Popov, R.E. White, J. Electrochem. Soc. 145 (1998) 3000. [24] K.C. Hong, J. Alloys Compd. 321 (2) (2001) 307. [25] H. Lee, S.M. Lee, J.Y. Lee, J. Electrochem. Soc. 146 (10) (1999) 3666. [26] J.-M. Wu, J. Li, W.-P. Zhang, F. Muo, L. Tai, R. Xu, J. Alloys Compd. 248 (1997) 180. [27] H.G. Meier, J.R. Vilche, A.J. Arvia, J. Electroanal. Chem. 134 (1982) 251. [28] I.M. Novoseleskii, N.R. Menglisheva, Electrochim. Acta 29 (1984) 21. [29] V.E. Antonov, T.E. Antonova, M. Baier, G. Grosse, F.E. Wagner, J. Alloys Compd. 239 (1996) 198. [30] R.C. Ambrosio, E.A. Ticianelli, J. Electrochem. Soc. 150 (9) (2003) E438. [31] P. Lagarde, J. Rivory, G. Vlaic, J. Noncryst. Solids 57 (1983) 275.