- Email: [email protected]
Structure and electrochemical hydrogen storage behaviors of alloy Co2B
Electrochemistry Communications 9 (2007) 925–929 www.elsevier.com/locate/elecom
Structure and electrochemical hydrogen storage behaviors of alloy Co2B Yi Liu, Yijing Wang *, Lingling Xiao, Dawei Song, Lifang Jiao, Huatang Yuan Institute of New Energy Materials Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China Received 20 October 2006; accepted 28 November 2006 Available online 5 January 2007
Abstract The alloys Co2B were prepared by two ways of high temperature solid phase process and arc melting, the structure of the alloys was characterized by XRD and SEM. It showed that it was structure of tetragonal Co2B.The electrochemical experimental results demonstrated that the Co2B prepared by two means both showed excellent cycling stability. The initial discharge capacity of Co2B prepared by the high temperature solid phase process was 480.3 mA h g1, there was no distinct declination after 70 charge–discharge cycles and the capacity kept about 195 mA h g1. Co2B prepared by the high temperature solid phase process showed very good electrochemical reversibility in CV curves. The hydrogen storage mechanism was also discussed, it confirmed that the high initial capacity of Co2B prepared by the high temperature solid phase process was due to the oxidation of Co and B2O3, and it was irreversible. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Co2B alloy; Hydrogen storage; High temperature solid phase process; Arc melting; Negative electrode
1. Introduction In recent years, hydrogen storage alloys have been widely investigated as a high capacity negative material of nickel-metal hydride secondary batteries [1–3]. Traditional hydrogen storage alloy materials include AB5 type [4,5], AB2 type [6], AB type [7], Mg-based alloys [8– 10] and so on. Among the hydrogen storage alloys, the discharge capacity of AB5 type is comparatively low, while the activation of the AB2 and AB type alloys is difficult, and the cycle stability of the Mg-based alloys is rather poor. Many new excellent hydrogen storage alloys have been synthesized. Kadir et al. [11–13] reported the discovery of a new type of ternary alloys with the general formula of R–Mg–Ni (R = rare earth, Ca, Y), which had been considered as a new group alloys of the negative electrode materials, but they had poor cycle stability in KOH electrolyte due to their serious corrosion [14]. Recently, it has been reported that amorphous Co–B prepared by reduction with NaBH4 can absorb a large amount of hydrogen [15–18]. *
Corresponding author. E-mail address: [email protected] (Y. Wang).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.11.034
Mitov et al. [15,16] reported the cyclic voltammetry test of the Co–B amorphous alloy particles, and proposed that the cathodic and anodic peaks were due to electrochemical absorption and desorption of hydrogen, respectively. Wang et al. successfully synthesized ultrafine amorphous alloy particles CoB [17] and use it as new type negative electrodes. It reported the atom ratio of Co and B was 1.9:1, the hydrogen storage capacity of the alloy was about 300 mA h g1 and showed excellent reversible ability. These results seem to indicate that it is possible to use Co–B alloy to construct high capacity hydrogen storage electrode. In this work, Co2B alloys are prepared by two methods, high temperature solid phase process and arc melting, as negative electrode. The structure and electrochemical properties are also investigated. 2. Experiment 2.1. Preparation and structural characterization Co2B alloys were prepared by two means: high temperature solid phase process and arc melting. Firstly mixed
3. Results and discussion 3.1. Material characterization The XRD patterns of Co2B alloys prepared by two means are shown in Fig. 1. Main phases of alloy A and B are tetragonal Co2B (PCPDFWIN 89-1994). There is a small wide peak of B2O3 and a small peak of Co in the curve of alloy A, indicates that a small part of boron was oxidized to B2O3, and also a little Co exist. In the pattern of alloy B, there is a few peaks of Co3B (PCPDFWIN 89-3675). Fig. 2 shows the SEM images of the Co2B alloys. It can be seen that the powders of alloy A consist of cubical grains, most of the grain size varies between 1 and 2 lm in diameter, from Fig. 2c, the surface of Co2B alloy powder is covered with different small size particles. The powders of alloy B consist of bigger cubical grains and conglomerate together, most of the grain size is about 5 lm in diameter. 3.2. Electrochemical performance Fig. 3 shows the discharge capacity varying with cycle number for Co2B alloys as negative electrodes at the cur-
20
30
312 400
202 310
002 40
Alloy A
Alloy B
•• •
•
10
112 220
110
200
Relative intensity
∗
2.2. Electrochemical measurements Negative electrodes were constructed through mixing asprepared alloys with carbonyl nickel powders in a weight ratio of 1:3. The powder mixture was pressed under 30 MPa pressure into a small pellet of 10 mm in diameter and 1.5 mm thickness. Electrochemical measurements were conducted in a three compartment cell using a Land battery test instrument. NiOOH/Ni(OH)2 and Hg/HgO were used as the counter electrode and the reference electrode. In each charge–discharge cycle test, the negative electrode was charged for 5 h at 100 mA g1 and discharged at 25 mA g1 up to the cut-off voltage set at 0.6 V (vs. Hg/HgO). The electrolyte solution is a 6 M KOH aqueous solution. The testing time between charge and discharge was 10 min. Cyclic voltammetry studies were conducted by a Solartron SI1287 Electrochemical Interface with 1255B Frequency Response Analyzer at the rate of 0.0001, 0.0002, 0.0005 V/s from 1.2 V to 0.4 V (vs. Hg/HgO).
Co
∗ B2O3 • Co3 B
∇
powders of pure Co and B at an atomic ratio of 2:1, and the powder mixture was pressed under 15 MPa pressure into a small pellet of 10 mm in diameter. Method A was put the pellet in a pipe furnace, under the temperature of 1073 K, preserved under argon atmosphere for 10 h. Method B was arc melting under Ar atmosphere, using the prepared pellet. Both alloys were mechanically crushed and grounded to powder of 200 mesh size for test. The crystal structure and surface configuration of the alloys were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu Ka radiation), scanning electron microscopy (SEM, Hitachi X-650).
211
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
∇
926
50
60
70
80
2Theta(°) Fig. 1. The XRD patterns of Co2B alloys by solid phase process (alloy A) and arc melting (alloy B).
rent density 25 mA g1. In the case of the electrodes, alloy A and alloy B show the similar electrochemical properties. The alloy A has the higher initial discharge capacity of 480.3 mA h g1, and in the first three cycles, the discharge capacity decreases distinctly, up to 198.6 mA h g1, then, from the 3rd cycle to the 70th cycle, the discharge capacity keep steady about 195 mA h g1. The discharge curve is very similar to the reported Co–B alloy [17]. The alloy B need several cycles to activate, it has the lower initial discharge capacity of 119.2 mA h g1, and in the first 20 cycles, the discharge capacity increases gradually to 171.8 mA h g1, and then the discharge capacity keep steady about 170 mA h g1 in the later 50 cycles, and the maximum discharge capacity is 177.5 mA h g1 in the 36th cycle. From Fig. 3, it can be seen that both alloys electrodes show very well cycle stability, the capacity retention are 96.9% and 95.6% in the 70th cycle respectively. Fig. 4 shows the first circle potential–charge/discharge capacity curves for Co2B alloys as negative electrodes at the current density 25 mA g1. As the Fig. 4 shows, there is a very long sufficient electric discharge plateau, while the charging equilibrium potentials are 1.02 V, 1.03 V respectively, and the discharging equilibrium potentials are 0.72 V, 0.75 V respectively. The second discharge plateau can be seen at about 0.70 V in the charging curve of alloy A, which may result from the B2O3 phase and Co phase. Fig. 5 shows the cyclic voltammetry (CV) tests of the Co2B alloy A electrode in a 6 M KOH solution. The curves are very similar to the reported CV test of the Co–B amorphous alloy particles [16], which was proposed that the cathodic and anodic peaks were due to electrochemical absorption and desorption of hydrogen, respectively. From the Fig. 5, in the anodic scanning process, there are strong oxidation peaks in the region of 0.62 V to 0.70 V (vs. Hg/HgO). The equilibrium potentials of Co and B elements are at 0.83 V and 1.81 V (vs. Hg/HgO) in the alkaline solution respectively [19], so it is impossible to assign the
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
927
Fig. 2. SEM micrographs of alloy A: (a) and (c), alloy B: (b) and (d).
ried out at different charging and discharging states. Fig. 6 shows the XRD patterns of alloy A full charged and completely discharged of the 1st and 5th cycle, 500
Alloy A Alloy B
450
Discharge capacity(mAh g-1)
anodic current peaks to be the oxidation of Co or B. In the cathodic scanning process, there are strong reduction peaks in the region of 0.98 V to 1.10 V (vs. Hg/HgO). The potential value of the oxidation peak and the reduction peak are very near in each CV test, indicating that the Co2B alloy electrode has very good electrochemical reversibility. As the scanning rate decreases, the potential value of the oxidation peak and the reduction peak become more consistent, this phenomenon suggests the possibility that the hydrogen adsorption-desorption reaction occurring. Considering that the potential positions and shapes of these current peaks very well resemble those frequently observed for the electrochemical hydrogen storage reactions on MH electrodes [20], it is reasonable to attribute the pair of the reduction–oxidation peaks to a reversible electrochemical hydrogen adsorption–desorption reaction occurring on the Co2B electrode.
400 350 300 250 200 150 100 50 0 0
3.3. Electrode reaction mechanism In order to confirm the hydrogen storage mechanism of the Co2B electrode, XRD patterns of the alloy A are car-
10
20
30
40
50
60
70
Cycle Number(n) Fig. 3. The cycling discharge-ability of the Co2B alloys electrodes by solid phase process (alloy A) and arc melting (alloy B) (discharging current 25 mA g1).
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
Relative intensity
∇
0.70
∇
0.75
Ο
d
Ni Ο
∇
Potential(V)
∇ ∇
0.80
∇
0.85
Ο
e
∇
0.90
Ο
∇
0.95
∇
Alloy A Alloy B
Ο
∇
∇
1.00
∇
Co(OH)2
Ο Co2B
∇
1.05
∇
928
Ο
Ο
c
ΟΟ
b
Ο Ο
Ο
0.65
a
0.60
10
0.55 0
100
200
300
400
500
20
30
40
50
60
70
80
2Theta(°)
-1
Capacity(mAh g ) Fig. 4. The discharge and charge curves of the first cycle.
0.25
0.0001 V/S 0.0002 V/S 0.0005 V/S
0.20 0.15 0.10
Current(A)
0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -1.2
-1.0
-0.8
-0.6
-0.4
Potential(V) Fig. 5. CV curves of the alloy A electrode in 6 M KOH solution.
compared with the freshly prepared alloy A. There are only diffraction peaks of Co2B except for the three strongest Ni peaks introduced in the process of the preparation of the negative electrode when charged at the 1st cycle, but after discharged to 0.6 V, the diffraction peaks of Co2B become weaker, and the diffraction peaks of Co(OH)2 appear. At the 5th cycle, there are also diffraction peaks of Co(OH)2 and Co2B, and the diffraction peaks of Co2B become very weaker, and the diffraction peaks of Co(OH)2 become stronger compared to the 1st charge–discharge cycle. This result may caused by the surface oxide of cobalt, Co(OH)2 was produced mainly by chemical corrosion of the electrode in alkaline solution which is consistent with the previous report [17]. To explain this result, we can take such an assumption as follows: the main phase of alloy Co2B was surrounded by Co and B2O3 on the surface as seen in the XRD pattern at first, in the process of the first charge–discharge cycle, the Co and B2O3 were oxidized in the KOH solution, for it is the reason why the first discharge capacity is so high, and the Co was oxidized to form
Fig. 6. XRD patterns of the alloy A electrode: (a) initial, (b) charged at the 1st cycle, (c) discharged to 0.6 V at the 1st cycle, (d) charged at the 5th cycle and (e) discharged to 0.6 V at the 5th cycle.
Co(OH)2 on the surface of the Co2B alloy. In this case, only a little Co2B phase can be observed from the XRD patterns. It also explained that the very high initial discharge capacity of the alloy A was due to the oxidation of Co and B2O3, and was irreversible, and also it should be stressed that the very high initial capacity is not the intrinsic property of Co2B. As shown in Fig. 6, the diffraction intensity of Co(OH)2 and Co2B keep almost the same in the state of full charged and completely discharged at the 5th cycle, indicating there is no significant phase transformation or bulk oxidation occurs in the Co2B alloy particles during charge and discharge cycles. And also, there is no diffraction peaks of any Co can be found in Fig. 6, so it couldn’t be the formation of CoHx and the phase transition of Co as been reported [21] in this case. Nevertheless, the large amount of desorbed hydrogen (200 mA h g1) cannot be simply accounted for by the reaction of Co/Co(OH)2 on the surface of the Co2B particles for the discharge capacity of pure Co due to the reaction of Co/Co(OH)2 was only 50 mA h g1 [22], but can only be attributed by the electrochemical hydrogen storage of the material Co2B alloy. So the assumption is reasonable and the alloy Co2B involved in the hydrogen storage is credible. If the Co2B alloy is hydrogenated to form Co2BH, the theoretical capacity of this alloy should be 207 mA h g1, which is extraordinarily agree with the experiment result, for there was a little Co and B2O3 phases existing in the alloy A, which would be oxide in the first and second charge–discharge cycle. As the result, the reversible capacity of 195 mA h g1 for the Co2B alloy is reasonable. From the analysis above, we can assume the hydrogen storage mechanism as follows: Co2 B þ H2 O þ e $ Co2 BH þ OH The excellent cycle life of the Co2B alloy is most likely due to the particle size. For the alloy B, the grain size is a little bigger compared to the alloy A from the SEM image, so the discharge capacity is relatively lower.
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
4. Conclusion In summary, hydrogen storage alloys Co2B were synthesized successfully by high temperature solid phase process and arc melting. The main phase of each alloy is tetragonal Co2B. Both alloys possess high cycling capacity and stability, the discharge capacity keep about 195 mA h g1 and 170 mA h g1 after 20 charge–discharge cycles, and there is nearly no declination while the cycle numbers increases, the capacity retention are 96.9% and 95.6% in the 70th cycle respectively. A proper mechanism is constructed and explains the hydrogen storage process, the further work is on the way. Acknowledgement The work was supported by NSFC(20573058,50571046) and SRF for ROCS, SEM. References [1] L. Schlapbach, A. Zu¨ttel, Nature 414 (2001) 353. [2] S.C. Han, P.S. Lee, A. Zu¨ttel, J. Alloys Compd. 306 (2000) 219. [3] J.J. Jiang, M. Gasik, J. Laine, M. Lampinen, J. Alloys Compd. 322 (2001) 281. [4] B. Liao, Y.Q. Lei, L.X. Chen, J. Power Source 129 (2004) 358.
929
[5] T. Kohno, H. Yoshida, F. Kawashima, J. Alloys Compd. 311 (2000) 5. [6] M.Y. Song, D. Ahn, I.H. Kwon, S.H. Chough, J. Electrochem. Soc. 148 (9) (2001) A1041. [7] H. Yukawa, Y. Takahashi, M. Morinaga, Comput. Mater. Sci. 14 (1999) 291. [8] N.H. Goo, J.H. Woo, K.S. Lee, J. Alloys Compd. 288 (1999) 286. [9] T. Abe, T. Tachikawa, Y. Hatano, K. Watanabe, J. Alloys Compd. 332 (2002) 792. [10] J.W. Liu, H.T. Yuan, J.S. Cao, Y.J. Wang, J. Alloys Compd. 392 (2005) 300. [11] K. Kadir, T. Sakai, I. Uehara, J. Alloys Comp. 257 (1997) 115. [12] K. Kadir, N. Nuriyama, T. Sakai, I. Uehara, L. Eriksson, J. Alloys Comp. 284 (1999) 145. [13] K. Kadir, T. Sakai, I. Uehara, J. Alloys Comp. 287 (1999) 264. [14] H.G. Pan, Y.F. Liu, M.X. Gao, Y.Q. Lei, Q.D. Wang, J. Electrochem. Soc. 150 (2003) A565. [15] M. Mitov, A. Popov, I. Dragieva, Colloids Surf. A: Physicochem. Eng. Aspects 149 (1999) 413. [16] M. Mitov, A. Popov, I. Dragieva, J. Appl. Electrochem. 29 (1999) 59. [17] Y.D. Wang, X.P. Ai, H.X. Yang, Chem. Mater. 16 (2004) 5194. [18] J.X. Yu, L. Wang, Y.D. Wang, H. Dong, H.X. Yang, J. Electrochem. Soc. 151 (2004) A1124. [19] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker Inc., New York, 1985. [20] K. Hong, J. Alloys Compd. 321 (2001) 307. [21] S.R. Chung, K.W. Wang, T.P. Perng, J. Electrochem. Soc. 153 (2006) A1128. [22] A. Durairajan, B.S. Haran, B.N. Popov, R.E. White, J. Power Sources 83 (1999) 114.
Structure and electrochemical hydrogen storage behaviors of alloy Co2B Yi Liu, Yijing Wang *, Lingling Xiao, Dawei Song, Lifang Jiao, Huatang Yuan Institute of New Energy Materials Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China Received 20 October 2006; accepted 28 November 2006 Available online 5 January 2007
Abstract The alloys Co2B were prepared by two ways of high temperature solid phase process and arc melting, the structure of the alloys was characterized by XRD and SEM. It showed that it was structure of tetragonal Co2B.The electrochemical experimental results demonstrated that the Co2B prepared by two means both showed excellent cycling stability. The initial discharge capacity of Co2B prepared by the high temperature solid phase process was 480.3 mA h g1, there was no distinct declination after 70 charge–discharge cycles and the capacity kept about 195 mA h g1. Co2B prepared by the high temperature solid phase process showed very good electrochemical reversibility in CV curves. The hydrogen storage mechanism was also discussed, it confirmed that the high initial capacity of Co2B prepared by the high temperature solid phase process was due to the oxidation of Co and B2O3, and it was irreversible. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Co2B alloy; Hydrogen storage; High temperature solid phase process; Arc melting; Negative electrode
1. Introduction In recent years, hydrogen storage alloys have been widely investigated as a high capacity negative material of nickel-metal hydride secondary batteries [1–3]. Traditional hydrogen storage alloy materials include AB5 type [4,5], AB2 type [6], AB type [7], Mg-based alloys [8– 10] and so on. Among the hydrogen storage alloys, the discharge capacity of AB5 type is comparatively low, while the activation of the AB2 and AB type alloys is difficult, and the cycle stability of the Mg-based alloys is rather poor. Many new excellent hydrogen storage alloys have been synthesized. Kadir et al. [11–13] reported the discovery of a new type of ternary alloys with the general formula of R–Mg–Ni (R = rare earth, Ca, Y), which had been considered as a new group alloys of the negative electrode materials, but they had poor cycle stability in KOH electrolyte due to their serious corrosion [14]. Recently, it has been reported that amorphous Co–B prepared by reduction with NaBH4 can absorb a large amount of hydrogen [15–18]. *
Corresponding author. E-mail address: [email protected] (Y. Wang).
1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.11.034
Mitov et al. [15,16] reported the cyclic voltammetry test of the Co–B amorphous alloy particles, and proposed that the cathodic and anodic peaks were due to electrochemical absorption and desorption of hydrogen, respectively. Wang et al. successfully synthesized ultrafine amorphous alloy particles CoB [17] and use it as new type negative electrodes. It reported the atom ratio of Co and B was 1.9:1, the hydrogen storage capacity of the alloy was about 300 mA h g1 and showed excellent reversible ability. These results seem to indicate that it is possible to use Co–B alloy to construct high capacity hydrogen storage electrode. In this work, Co2B alloys are prepared by two methods, high temperature solid phase process and arc melting, as negative electrode. The structure and electrochemical properties are also investigated. 2. Experiment 2.1. Preparation and structural characterization Co2B alloys were prepared by two means: high temperature solid phase process and arc melting. Firstly mixed
3. Results and discussion 3.1. Material characterization The XRD patterns of Co2B alloys prepared by two means are shown in Fig. 1. Main phases of alloy A and B are tetragonal Co2B (PCPDFWIN 89-1994). There is a small wide peak of B2O3 and a small peak of Co in the curve of alloy A, indicates that a small part of boron was oxidized to B2O3, and also a little Co exist. In the pattern of alloy B, there is a few peaks of Co3B (PCPDFWIN 89-3675). Fig. 2 shows the SEM images of the Co2B alloys. It can be seen that the powders of alloy A consist of cubical grains, most of the grain size varies between 1 and 2 lm in diameter, from Fig. 2c, the surface of Co2B alloy powder is covered with different small size particles. The powders of alloy B consist of bigger cubical grains and conglomerate together, most of the grain size is about 5 lm in diameter. 3.2. Electrochemical performance Fig. 3 shows the discharge capacity varying with cycle number for Co2B alloys as negative electrodes at the cur-
20
30
312 400
202 310
002 40
Alloy A
Alloy B
•• •
•
10
112 220
110
200
Relative intensity
∗
2.2. Electrochemical measurements Negative electrodes were constructed through mixing asprepared alloys with carbonyl nickel powders in a weight ratio of 1:3. The powder mixture was pressed under 30 MPa pressure into a small pellet of 10 mm in diameter and 1.5 mm thickness. Electrochemical measurements were conducted in a three compartment cell using a Land battery test instrument. NiOOH/Ni(OH)2 and Hg/HgO were used as the counter electrode and the reference electrode. In each charge–discharge cycle test, the negative electrode was charged for 5 h at 100 mA g1 and discharged at 25 mA g1 up to the cut-off voltage set at 0.6 V (vs. Hg/HgO). The electrolyte solution is a 6 M KOH aqueous solution. The testing time between charge and discharge was 10 min. Cyclic voltammetry studies were conducted by a Solartron SI1287 Electrochemical Interface with 1255B Frequency Response Analyzer at the rate of 0.0001, 0.0002, 0.0005 V/s from 1.2 V to 0.4 V (vs. Hg/HgO).
Co
∗ B2O3 • Co3 B
∇
powders of pure Co and B at an atomic ratio of 2:1, and the powder mixture was pressed under 15 MPa pressure into a small pellet of 10 mm in diameter. Method A was put the pellet in a pipe furnace, under the temperature of 1073 K, preserved under argon atmosphere for 10 h. Method B was arc melting under Ar atmosphere, using the prepared pellet. Both alloys were mechanically crushed and grounded to powder of 200 mesh size for test. The crystal structure and surface configuration of the alloys were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu Ka radiation), scanning electron microscopy (SEM, Hitachi X-650).
211
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
∇
926
50
60
70
80
2Theta(°) Fig. 1. The XRD patterns of Co2B alloys by solid phase process (alloy A) and arc melting (alloy B).
rent density 25 mA g1. In the case of the electrodes, alloy A and alloy B show the similar electrochemical properties. The alloy A has the higher initial discharge capacity of 480.3 mA h g1, and in the first three cycles, the discharge capacity decreases distinctly, up to 198.6 mA h g1, then, from the 3rd cycle to the 70th cycle, the discharge capacity keep steady about 195 mA h g1. The discharge curve is very similar to the reported Co–B alloy [17]. The alloy B need several cycles to activate, it has the lower initial discharge capacity of 119.2 mA h g1, and in the first 20 cycles, the discharge capacity increases gradually to 171.8 mA h g1, and then the discharge capacity keep steady about 170 mA h g1 in the later 50 cycles, and the maximum discharge capacity is 177.5 mA h g1 in the 36th cycle. From Fig. 3, it can be seen that both alloys electrodes show very well cycle stability, the capacity retention are 96.9% and 95.6% in the 70th cycle respectively. Fig. 4 shows the first circle potential–charge/discharge capacity curves for Co2B alloys as negative electrodes at the current density 25 mA g1. As the Fig. 4 shows, there is a very long sufficient electric discharge plateau, while the charging equilibrium potentials are 1.02 V, 1.03 V respectively, and the discharging equilibrium potentials are 0.72 V, 0.75 V respectively. The second discharge plateau can be seen at about 0.70 V in the charging curve of alloy A, which may result from the B2O3 phase and Co phase. Fig. 5 shows the cyclic voltammetry (CV) tests of the Co2B alloy A electrode in a 6 M KOH solution. The curves are very similar to the reported CV test of the Co–B amorphous alloy particles [16], which was proposed that the cathodic and anodic peaks were due to electrochemical absorption and desorption of hydrogen, respectively. From the Fig. 5, in the anodic scanning process, there are strong oxidation peaks in the region of 0.62 V to 0.70 V (vs. Hg/HgO). The equilibrium potentials of Co and B elements are at 0.83 V and 1.81 V (vs. Hg/HgO) in the alkaline solution respectively [19], so it is impossible to assign the
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
927
Fig. 2. SEM micrographs of alloy A: (a) and (c), alloy B: (b) and (d).
ried out at different charging and discharging states. Fig. 6 shows the XRD patterns of alloy A full charged and completely discharged of the 1st and 5th cycle, 500
Alloy A Alloy B
450
Discharge capacity(mAh g-1)
anodic current peaks to be the oxidation of Co or B. In the cathodic scanning process, there are strong reduction peaks in the region of 0.98 V to 1.10 V (vs. Hg/HgO). The potential value of the oxidation peak and the reduction peak are very near in each CV test, indicating that the Co2B alloy electrode has very good electrochemical reversibility. As the scanning rate decreases, the potential value of the oxidation peak and the reduction peak become more consistent, this phenomenon suggests the possibility that the hydrogen adsorption-desorption reaction occurring. Considering that the potential positions and shapes of these current peaks very well resemble those frequently observed for the electrochemical hydrogen storage reactions on MH electrodes [20], it is reasonable to attribute the pair of the reduction–oxidation peaks to a reversible electrochemical hydrogen adsorption–desorption reaction occurring on the Co2B electrode.
400 350 300 250 200 150 100 50 0 0
3.3. Electrode reaction mechanism In order to confirm the hydrogen storage mechanism of the Co2B electrode, XRD patterns of the alloy A are car-
10
20
30
40
50
60
70
Cycle Number(n) Fig. 3. The cycling discharge-ability of the Co2B alloys electrodes by solid phase process (alloy A) and arc melting (alloy B) (discharging current 25 mA g1).
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
Relative intensity
∇
0.70
∇
0.75
Ο
d
Ni Ο
∇
Potential(V)
∇ ∇
0.80
∇
0.85
Ο
e
∇
0.90
Ο
∇
0.95
∇
Alloy A Alloy B
Ο
∇
∇
1.00
∇
Co(OH)2
Ο Co2B
∇
1.05
∇
928
Ο
Ο
c
ΟΟ
b
Ο Ο
Ο
0.65
a
0.60
10
0.55 0
100
200
300
400
500
20
30
40
50
60
70
80
2Theta(°)
-1
Capacity(mAh g ) Fig. 4. The discharge and charge curves of the first cycle.
0.25
0.0001 V/S 0.0002 V/S 0.0005 V/S
0.20 0.15 0.10
Current(A)
0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -1.2
-1.0
-0.8
-0.6
-0.4
Potential(V) Fig. 5. CV curves of the alloy A electrode in 6 M KOH solution.
compared with the freshly prepared alloy A. There are only diffraction peaks of Co2B except for the three strongest Ni peaks introduced in the process of the preparation of the negative electrode when charged at the 1st cycle, but after discharged to 0.6 V, the diffraction peaks of Co2B become weaker, and the diffraction peaks of Co(OH)2 appear. At the 5th cycle, there are also diffraction peaks of Co(OH)2 and Co2B, and the diffraction peaks of Co2B become very weaker, and the diffraction peaks of Co(OH)2 become stronger compared to the 1st charge–discharge cycle. This result may caused by the surface oxide of cobalt, Co(OH)2 was produced mainly by chemical corrosion of the electrode in alkaline solution which is consistent with the previous report [17]. To explain this result, we can take such an assumption as follows: the main phase of alloy Co2B was surrounded by Co and B2O3 on the surface as seen in the XRD pattern at first, in the process of the first charge–discharge cycle, the Co and B2O3 were oxidized in the KOH solution, for it is the reason why the first discharge capacity is so high, and the Co was oxidized to form
Fig. 6. XRD patterns of the alloy A electrode: (a) initial, (b) charged at the 1st cycle, (c) discharged to 0.6 V at the 1st cycle, (d) charged at the 5th cycle and (e) discharged to 0.6 V at the 5th cycle.
Co(OH)2 on the surface of the Co2B alloy. In this case, only a little Co2B phase can be observed from the XRD patterns. It also explained that the very high initial discharge capacity of the alloy A was due to the oxidation of Co and B2O3, and was irreversible, and also it should be stressed that the very high initial capacity is not the intrinsic property of Co2B. As shown in Fig. 6, the diffraction intensity of Co(OH)2 and Co2B keep almost the same in the state of full charged and completely discharged at the 5th cycle, indicating there is no significant phase transformation or bulk oxidation occurs in the Co2B alloy particles during charge and discharge cycles. And also, there is no diffraction peaks of any Co can be found in Fig. 6, so it couldn’t be the formation of CoHx and the phase transition of Co as been reported [21] in this case. Nevertheless, the large amount of desorbed hydrogen (200 mA h g1) cannot be simply accounted for by the reaction of Co/Co(OH)2 on the surface of the Co2B particles for the discharge capacity of pure Co due to the reaction of Co/Co(OH)2 was only 50 mA h g1 [22], but can only be attributed by the electrochemical hydrogen storage of the material Co2B alloy. So the assumption is reasonable and the alloy Co2B involved in the hydrogen storage is credible. If the Co2B alloy is hydrogenated to form Co2BH, the theoretical capacity of this alloy should be 207 mA h g1, which is extraordinarily agree with the experiment result, for there was a little Co and B2O3 phases existing in the alloy A, which would be oxide in the first and second charge–discharge cycle. As the result, the reversible capacity of 195 mA h g1 for the Co2B alloy is reasonable. From the analysis above, we can assume the hydrogen storage mechanism as follows: Co2 B þ H2 O þ e $ Co2 BH þ OH The excellent cycle life of the Co2B alloy is most likely due to the particle size. For the alloy B, the grain size is a little bigger compared to the alloy A from the SEM image, so the discharge capacity is relatively lower.
Y. Liu et al. / Electrochemistry Communications 9 (2007) 925–929
4. Conclusion In summary, hydrogen storage alloys Co2B were synthesized successfully by high temperature solid phase process and arc melting. The main phase of each alloy is tetragonal Co2B. Both alloys possess high cycling capacity and stability, the discharge capacity keep about 195 mA h g1 and 170 mA h g1 after 20 charge–discharge cycles, and there is nearly no declination while the cycle numbers increases, the capacity retention are 96.9% and 95.6% in the 70th cycle respectively. A proper mechanism is constructed and explains the hydrogen storage process, the further work is on the way. Acknowledgement The work was supported by NSFC(20573058,50571046) and SRF for ROCS, SEM. References [1] L. Schlapbach, A. Zu¨ttel, Nature 414 (2001) 353. [2] S.C. Han, P.S. Lee, A. Zu¨ttel, J. Alloys Compd. 306 (2000) 219. [3] J.J. Jiang, M. Gasik, J. Laine, M. Lampinen, J. Alloys Compd. 322 (2001) 281. [4] B. Liao, Y.Q. Lei, L.X. Chen, J. Power Source 129 (2004) 358.
929
[5] T. Kohno, H. Yoshida, F. Kawashima, J. Alloys Compd. 311 (2000) 5. [6] M.Y. Song, D. Ahn, I.H. Kwon, S.H. Chough, J. Electrochem. Soc. 148 (9) (2001) A1041. [7] H. Yukawa, Y. Takahashi, M. Morinaga, Comput. Mater. Sci. 14 (1999) 291. [8] N.H. Goo, J.H. Woo, K.S. Lee, J. Alloys Compd. 288 (1999) 286. [9] T. Abe, T. Tachikawa, Y. Hatano, K. Watanabe, J. Alloys Compd. 332 (2002) 792. [10] J.W. Liu, H.T. Yuan, J.S. Cao, Y.J. Wang, J. Alloys Compd. 392 (2005) 300. [11] K. Kadir, T. Sakai, I. Uehara, J. Alloys Comp. 257 (1997) 115. [12] K. Kadir, N. Nuriyama, T. Sakai, I. Uehara, L. Eriksson, J. Alloys Comp. 284 (1999) 145. [13] K. Kadir, T. Sakai, I. Uehara, J. Alloys Comp. 287 (1999) 264. [14] H.G. Pan, Y.F. Liu, M.X. Gao, Y.Q. Lei, Q.D. Wang, J. Electrochem. Soc. 150 (2003) A565. [15] M. Mitov, A. Popov, I. Dragieva, Colloids Surf. A: Physicochem. Eng. Aspects 149 (1999) 413. [16] M. Mitov, A. Popov, I. Dragieva, J. Appl. Electrochem. 29 (1999) 59. [17] Y.D. Wang, X.P. Ai, H.X. Yang, Chem. Mater. 16 (2004) 5194. [18] J.X. Yu, L. Wang, Y.D. Wang, H. Dong, H.X. Yang, J. Electrochem. Soc. 151 (2004) A1124. [19] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution, Marcel Dekker Inc., New York, 1985. [20] K. Hong, J. Alloys Compd. 321 (2001) 307. [21] S.R. Chung, K.W. Wang, T.P. Perng, J. Electrochem. Soc. 153 (2006) A1128. [22] A. Durairajan, B.S. Haran, B.N. Popov, R.E. White, J. Power Sources 83 (1999) 114.