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Effects of chemical coating with Ni on electrochemical properties of Mg2Ni hydrogen storage alloys
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RARE METALS Vol. 26, No. 6, Dec 2007, p . 611 E-mail: [email protected]
ScienceDirect
Effects of chemical coating with Ni on electrochemical properties of Mg2Ni hydrogen storage alloys TANG Yougen,SHEN Jianbin, XU Ejun, and JIANG Jinzhi Chemistry and Chemical Engineering School, Central South University, Changsha 410083, China (Received 2006-08-17)
Abstract: The effects of nickel coating on the electrochemical properties of Mg2Nihydrogen storage alloys are presented in this paper. X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques were employed to examine the crystal structure and surface morphologies of the bare and Ni-coated Mg2Ni alloys. The electrochemical properties of alloys were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that Ni coating not only decreased the charge transfer resistance, but also decreased the H atom diffusion resistance for Mg2Ni alloys. It was also found that Ni coating effectively improved the discharge capacity, but decreased the cycling performance of the as-synthesized Ni-coated MgzNi alloys. The discharge current has a great impact on the cycling performance of the as-synthesized Ni-coated MgzNi alloys. Key words: hydrogen storage alloys; chemical coating; cyclic voltammetry; electrochemicalimpedance spectroscopy
[This work was$mncially supported by the National High Technology Program of China,(No. 2001AA501433)]
1. Introduction In recent years, MgzNi based hydrogen storage alloys have been recognized as most promising new negative active materials used in MH-Ni battery because of their high theoretical capacity, low production cost and abundant resources. However, they are inadequate for practical use for MH-Ni battery due to their poor kinetic property and cycling stability. To overcome these shortcomings, intensive efforts have been carried out such as partial substitution of Mg by transition metals [l-31, surface modifications [4-81 and mechanical alloying [9-121. However, there was little breakthrough in improving the kinetic property and cycling stability of Mg2Ni based hydrogen storage alloys. It is well known that properties of hydrogen storage alloys, such as activation, lunetics, anti-oxidation and anti-corruption in the electrolyte, were closely related to their surface characteristics. In view of its favorable electroCorrespondingauthor: SHEN Jianbin
chemical properties, metal Ni was chemically coated on the surfaces of Mg2Ni alloys and electrochemical properties of the as-synthesized materials have been readily investigated.
2. Experimental The Mg2Nialloys were prepared according to the powder metallurgical sintering techniques described below. In a typical procedure, appropriate amounts of Mg and Ni powders (100-150 pm, purity of higher than 99.5 wt.%) were thoroughly mixed and pressed into pellets. The pellets were first sintered at 600°C for 5 h under an argon atmosphere and then ball-milled into powders with a size of less than 100 Clm. The powders were chemically coated with 10 wt.% Ni at 30°C using a solution listed in Table 1. The pH value of solution was controlled in the range of 8-8.5 by NH3.H20.The rotation speed was kept at
E-mail: shenjianbin@ gmail.com
RARE METALS, Vol. 26, No. 6,Dec 2007
612
150 r/min and the coating time was 1 h. Table 1. Basic composition of the solution for nickel coating of M&Ni alloy
Component Concentration/ (mo1.L-I
NiS046H 2O
Na3C&07.2H~O
0.30
0.50
The surface morphology was examined using a KYKY2800 scanning electron microscope. Structure and phase identifications of the alloy powders were confiied by X-ray diffraction (XRD) with Cu K, radiation. The test electrodes for the charge/discharge cycling test and electrochemical measurements were fabricated according to the following procedures: 0.4 g MgzNi alloys were mixed thoroughly with 1.6 g fine nickel powders (diameter < 3 pn).The powders were pressed into pellets with 10-mm diameter and 1-mm thickness at 2 kN/cm2. Electrode properties were tested in a half-cell using an Ni(OH)2/NiOOH counter electrode, Hg/HgO reference electrode and KOH electrolyte of 6 m o a . Each electrode was charged for 5 h at 200 mA/g and discharged to -500 mV versus Hg/HgO electrode at different currents. When the discharge capacity was calculated, only the weight of the hydrogen storage alloy was considered.
3. Results and discussion
WCl 0.25
NaH2P0yH20
Thiourea
0.50
0.004 (EL-')
Ni-coated all01
Y . , ,
---\
\\ .(
60
80
1 10
Discharge capacity / (mAh.g-') Fig. 1. Discharge curves (the first cycle) of the bare and nickel-coated M g a i alloy electrodes.
Discharge capacities of the bare and Ni-coated alloy electrodes as a function of cycle life are shown in Fig. 2. It is clear that the capacity decay of the two
-.-
E .
Rare alloy Ni-coated alloy
-0-
v
3.1. Chargddischargecycling performance Discharge curves of the bare and nickel-coated Mg2Ni alloy electrodes at a discharge current density of 50 mA/g in the fmt cycle are given in Fig. 1. It shows that discharging performance of the MgzNi alloy electrode was greatly improved by nickel coating. No discharge plateau was found for the bare alloy electrode, but a clear discharge plateau was observed at about 0.73 V (vs. Hg/HgO electrode) for the materials after metal nickel coating. Discharge capacities for the bare and nickel-coated alloy electrodes were calculated to be about 16.5 and 85.5 mAh/g, respectively. These results show that the nickel coating of MgzNi alloy is a very effective method of increasing the discharge capacity of the alloy electrode.
2 1 0
6
0
,
---.-.\ ,
4
I
,
8
8-m .-.-.-
I
I
12
I
I
16
0
Cycle number Fig. 2. Discharge capacities of the bare and nickelcoated M&Ni alloy electrodes as a function of cycle life.
kinds of electrodes proceeds very fast. For instance, after 15 cycles, the remaining capacity for the bare and the Ni-coated alloy electrode were found to be about only 9.5 mAh/g and 33.5 mAh/g, respectively. The poor cycling stability of the electrodes may suggest the formation of magnesium hydroxide on the surface during the chargeldscharge procedure in
Tang XG et aZ., Effects of chemical coating with Ni on electrochemical properties of ...
613
an electrolyte with hgh concentration (6 mol/L KOH). The oxidation of the Ni-coated alloy cannot be prevented to a great extent because: (1) the chemically coated nickel on the alloy surface is not homogeneous, and (2) the repeated charging and discharging processes lead to gradual pulverization of the alloys, whch again results in the increase of surface area, and then make degradation of the alloys continue.
to surface of Mg2Ni alloy and for electron transfer, but cannot provide significant corrosion protection because of its open structure. When decreasing discharge current, the discharge process lasts longer and the oxide film becomes thlcker. As a consequence, the measured discharge capacity is lower and the cycle life is poorer for the electrode with a lower discharge current than that with a higher discharge current.
3.2. Effects of discharge current on discharge capacity and cycle lie
3.3. Electrochemical characteristics
In general, the hlgher is the discharge current, the shorter is the cycle life, and the smaller is the initial capacity of the electrode. The same phenomenon occurs to A E 3 5 and AI32 systems. However, for the Ni-coated Mg2Ni alloy (Fig. 3), the initial discharge capacity increased and the cycling stability was improved firstly with increasing discharge current from
Fig. 4 shows cyclic voltammetry curves for the bare and Ni-coated MgzNi alloy electrodes. prior to CV examination, the two electrodes were charged for 5 h at 200 mNg in a 6 mol/L KOH solution at room temperature. Results show that metal nickel coating greatly increases the reduction current, as well as the oxidation current. The anodic peak current was ascribed to the oxidation of hydrogen absorbed in the alloy according to the equation [131: MH, + nOH- M + nHzO + ne(1) 0.16 0.12 0.08 d 0.04 6 0.00 -0.04 -0.08 -
. .w,
a
5
d
2
4 6 Cycle number
8
1 0 '
Fig. 3. Effect of discharge current on the cycle life of Ni-coatedM&Ni alloy.
50 to 300 mNg, and then deteriorated with further increasing the discharge current from 300 to 400 M g . It should be concluded that there exits an optimal discharge current for the highest discharge capacity and the best cycling performances. The observations mentioned before may be due to a fast degradation of Ni-coated MgzNi alloys in the chargddischarge duration. This fast degradation of Ni-coated MgzNi alloy can be caused by serious oxidation of magnesium. The discharge process depends strongly on the oxide film thickness. The film can act as a barrier for H atom diffusion from phase
"
-0.12 -0.16 -
-0.20
It is reported that the anodic peak current and peak area can be used to evaluate the electrochemical reactive activity and discharge capacity of the working electrode [ll-121. So, it can be concluded that Ni coating can greatly improve the electrochemical reactive activity and discharge capacity of Mg2Ni alloy electrode. In order to further clanfy the effects of Ni coating on the electrode behavior of MgzNi alloys, the elec-
RARE METALS, Vol. 26, No. 6,Dec 2007
614
trochemical impedance spectra (EIS) (Fig. 5) of the bare and Ni-coated Mg2Ni alloy electrodes were examined. Prior to CV examination, the two electrodes were charged for 5 h at 200 mAfg. As shown in Fig. 5, the EIS of the two electrodes are composed of three parts which are a smaller semicircle in the high-frequency region related to the contact resistance and capacitance between the current collector and the pellets of alloy powders, a larger distorted semicircle in the low-frequency region being related to the electrochemical reaction resistance on the surface between the electrode and electrolyte, and the liner region being related to hydrogen difFusion in the alloys. So the equivalent circuit of the EIS for the MgZNi alloy electrode can be summarized as shown in Fig. 6 [14-151.
MgzNi alloy electrode.
Fig. 6. Equivalent circuit of the EIS for MgzNi alloy electrode. R1: charge transfer resistance at the interface of electroddelectrolyte; Rz: contact resistance between alloy particles and collector; R3: resistance of electrolyte; el:capacitance between electrode and electrolyte; Cz: capacitance between the current collector and the pellet; 2,: Warburg impedance of hydrogen diffusion.
3.4. Phase structure and surface morphology Fig. 7 shows XRD patterns of the as-synthesized bare and Ni-coated Mg2Ni alloys. It is clear that hexagonal phase was present on the bare Mg2Ni alloys.
--- 1.434Hz
.
14.34 mHz
3500 4000 3000 2500
d (b)
Mg
00 . .-
A Ni * - -
.
.
. Ni
-
b 2000
8
1.0
2.0
3.0
4.0
5.0
RIR Fig. 5. EIS of the bare and Ni-coated M g a i alloy electrodes (charged for 5 h at 200 mA/g )
It can be concluded from Fig. 6 that the two semicircles in the high-frequency region are similar, indicating that there are no obvious effects of Ni coating on the contact resistance of the Mg2Ni alloy electrode. However, in the low-frequency region, the semicircle and the slope of the liner of Ni-coated alloy electrode are smaller than that of the bare alloy electrcde, which indicates that Ni coating can effectively reduce charge transfer resistance on the electrode/electrolyte surface and hydrogen diffusion resistance. For the Ni-coated electrode, the smaller charge transfer resistance and hydrogen diffusion resistance bring forth a higher charge efficiency and a lower anodic polarization potential [ 161. Therefore, Ni coating can increase the discharge capacity of the
* 1500 5 1000 500
q0
20
30
40
50
60
70
80
2e/ (01 Fig. 7. X-ray diffraction patterns of Mgfli alloys: (a) bare; (b) Ni-coated.
However, for the Ni-coated alloy, the characteristic peaks of the MgzNi alloy phase are broadened, which might accord to the presence of Ni ultrafine particles on the surface produced in the chemical coating process. SEM images of the bare and nickel-coated alloys are given in Fig. 8. It shows that the surfaces of the bare alloy were smooth. However, surfaces of the nickel-coated sample were rather rough and protruding because the surfaces were covered with ultrafine Ni particles. Therefore, metal nickel coating was believed to increase the specific surface area, leading to a reduced electrochemical charge transfer resistance and hydrogen diffusion resistance.
Tang KG et aL, Effects of chemical coating with Ni on electrochemical properties of
...
615
References
Fig. 8. SEM images of the bare and Ni-coated Mg2Ni alloys: (a) bare; (b) Ni-coated.
4. Conclusion The effects of Ni coating on the electrochemical properties of Mg2Ni alloys have been investigated. It was found that Ni coating improved the initial discharge capacity of the MgzNi alloy electrode from 16.5 to 85.5 mAh/g, but it had a negative effect on its cycle life. The cycle life and initial discharge capacity of the as-synthesized Ni-coated MgzNi alloys were greatly dependent on the discharge current. XRD and SEM results showed that Ni coating led to a dense and ultrafiie Ni particles layer producing on the surfaces and an increase of the specific surface area. CV and EIS analyses showed that Ni coating reduced charge transfer resistance as well as hydrogen diffusion resistance, which resulted in the discharge capacity improvement of the as-synthesized MgzNi alloy electrodes.
Ye H., Lei Y.Q., Chen L.S., et al., Electrochemical characteristics of amorphous Mgo.9&,,Ni (M = Ni, Ti, Zr,Co, and Si) ternary alloys prepared by mechanical alloying, J. Alloys Compd., 2000, 311 (2): 194. Iwakura C., Nohara S., and Inoue H., Preparation and characterization of Mgo.gT&,VO.&i-carbon material composites for electrochemicaluse, Solid State Ionics, 2002,148 (3-4): 499. Wang Z.M., Zhou H.Y., Gu Z.F., et al., Preparation of Mg2&EXNi(RE = La, Ce, FY, Nd, Y) alloys and their electrochemical characteristics, J. Alloys Compd., 2004,381: 234. Luo J.L. and Cui N., Effects of microencapsulation on the electrode behavior of MgzNi-based hydrogen storage alloy in alkaline solution, J. Alloys Cornpd., 1998,264 (1-2): 299. Chen J., Bradhurst D.H., Dou S.X., et al., Effect of chemical coating with Ni on the electrode properties of MgzNi alloy, J. Alloys Compd., 1998, 280 (1-2): 290. Wang C.Y., Yao P., Bradhurst D.H., et al., Surface modification of Mg2Ni alloy in an acid solution of copper sulfate and sulfuric acid, J. Alloys Compd., 1999,285: 267 Hamptona M.D., Lomnessa J.K., and Giannuzzib L.A., Surface study of liquid water treated and water vapor treated Mg2.35Ni alloy, In?. J. Hydrogen Energy, 2002,27: 79. Yang H.B., Yuan H.T., Ji J.T., et al., Characteristics of Mg2Ni,-.7&z(M = Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn)alloys after surface treatment, J. Alloys Compd., 2002,330-332:640 Inoue H., Ueda T., Nohara S., et al., Effect of ball-milling on electrochemical and physicochemical characteristics of crystalline Mg2Ni, Electmchim. Acta, 1998,43 (14-15): 2215. Cui N., P He., and Luo J.L.. Magnesium-based hydrogen storage materials modified by mechanical alloying, Acta MateK, 1999,47: 3737. Jiang J.J. and Gasik M., An electrochemicalinvestigation of mechanical alloying of MgNi-based hydrogen storage alloys, J. Power Sources, 2000, 89: 117. Shang C.X., Bououdinal M., Song Y., et al., Mechanical alloying and electronic simulations of (MgH2+ M) systems (M = Al, Ti, Fe, Ni, Cu,and Nb)
6Z6 for hydrogen storage, Int. J. Hydrogen Energy, 2004, 29: 73. [13] Matsupka M., Kohono T., and Iwakura C., Electrochemical properties of hydrogen storage alloys modified with foreign metals, Electrochim Acta, 1993,38 (6): 787. [14] Chen W.X., Study on cyclic voltammetry and electrochemical impedance of hydrogen storage alloy electrode, Chin. J. Power Sources (in Chinese), 2000,
RARE METALS, Vol. 26, No. 6,Dec 2007
24 (4): 200. [15] Luo J.L. and Cui N., Effects of microencapsulation on the electrode behavior of Mg2Ni-based hydrogen storage alloy in alkaline solution, J. Alloys Compd., 1998,264:299. [16] Yang X.G, Lei Y.Q., Shu K.Y., et al., Contribution of rare-earths to activation property of Zr-based hydride electrodes, J. Alloys Compd., 1999, 293-295: 632.
RARE METALS Vol. 26, No. 6, Dec 2007, p . 611 E-mail: [email protected]
ScienceDirect
Effects of chemical coating with Ni on electrochemical properties of Mg2Ni hydrogen storage alloys TANG Yougen,SHEN Jianbin, XU Ejun, and JIANG Jinzhi Chemistry and Chemical Engineering School, Central South University, Changsha 410083, China (Received 2006-08-17)
Abstract: The effects of nickel coating on the electrochemical properties of Mg2Nihydrogen storage alloys are presented in this paper. X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques were employed to examine the crystal structure and surface morphologies of the bare and Ni-coated Mg2Ni alloys. The electrochemical properties of alloys were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that Ni coating not only decreased the charge transfer resistance, but also decreased the H atom diffusion resistance for Mg2Ni alloys. It was also found that Ni coating effectively improved the discharge capacity, but decreased the cycling performance of the as-synthesized Ni-coated MgzNi alloys. The discharge current has a great impact on the cycling performance of the as-synthesized Ni-coated MgzNi alloys. Key words: hydrogen storage alloys; chemical coating; cyclic voltammetry; electrochemicalimpedance spectroscopy
[This work was$mncially supported by the National High Technology Program of China,(No. 2001AA501433)]
1. Introduction In recent years, MgzNi based hydrogen storage alloys have been recognized as most promising new negative active materials used in MH-Ni battery because of their high theoretical capacity, low production cost and abundant resources. However, they are inadequate for practical use for MH-Ni battery due to their poor kinetic property and cycling stability. To overcome these shortcomings, intensive efforts have been carried out such as partial substitution of Mg by transition metals [l-31, surface modifications [4-81 and mechanical alloying [9-121. However, there was little breakthrough in improving the kinetic property and cycling stability of Mg2Ni based hydrogen storage alloys. It is well known that properties of hydrogen storage alloys, such as activation, lunetics, anti-oxidation and anti-corruption in the electrolyte, were closely related to their surface characteristics. In view of its favorable electroCorrespondingauthor: SHEN Jianbin
chemical properties, metal Ni was chemically coated on the surfaces of Mg2Ni alloys and electrochemical properties of the as-synthesized materials have been readily investigated.
2. Experimental The Mg2Nialloys were prepared according to the powder metallurgical sintering techniques described below. In a typical procedure, appropriate amounts of Mg and Ni powders (100-150 pm, purity of higher than 99.5 wt.%) were thoroughly mixed and pressed into pellets. The pellets were first sintered at 600°C for 5 h under an argon atmosphere and then ball-milled into powders with a size of less than 100 Clm. The powders were chemically coated with 10 wt.% Ni at 30°C using a solution listed in Table 1. The pH value of solution was controlled in the range of 8-8.5 by NH3.H20.The rotation speed was kept at
E-mail: shenjianbin@ gmail.com
RARE METALS, Vol. 26, No. 6,Dec 2007
612
150 r/min and the coating time was 1 h. Table 1. Basic composition of the solution for nickel coating of M&Ni alloy
Component Concentration/ (mo1.L-I
NiS046H 2O
Na3C&07.2H~O
0.30
0.50
The surface morphology was examined using a KYKY2800 scanning electron microscope. Structure and phase identifications of the alloy powders were confiied by X-ray diffraction (XRD) with Cu K, radiation. The test electrodes for the charge/discharge cycling test and electrochemical measurements were fabricated according to the following procedures: 0.4 g MgzNi alloys were mixed thoroughly with 1.6 g fine nickel powders (diameter < 3 pn).The powders were pressed into pellets with 10-mm diameter and 1-mm thickness at 2 kN/cm2. Electrode properties were tested in a half-cell using an Ni(OH)2/NiOOH counter electrode, Hg/HgO reference electrode and KOH electrolyte of 6 m o a . Each electrode was charged for 5 h at 200 mA/g and discharged to -500 mV versus Hg/HgO electrode at different currents. When the discharge capacity was calculated, only the weight of the hydrogen storage alloy was considered.
3. Results and discussion
WCl 0.25
NaH2P0yH20
Thiourea
0.50
0.004 (EL-')
Ni-coated all01
Y . , ,
---\
\\ .(
60
80
1 10
Discharge capacity / (mAh.g-') Fig. 1. Discharge curves (the first cycle) of the bare and nickel-coated M g a i alloy electrodes.
Discharge capacities of the bare and Ni-coated alloy electrodes as a function of cycle life are shown in Fig. 2. It is clear that the capacity decay of the two
-.-
E .
Rare alloy Ni-coated alloy
-0-
v
3.1. Chargddischargecycling performance Discharge curves of the bare and nickel-coated Mg2Ni alloy electrodes at a discharge current density of 50 mA/g in the fmt cycle are given in Fig. 1. It shows that discharging performance of the MgzNi alloy electrode was greatly improved by nickel coating. No discharge plateau was found for the bare alloy electrode, but a clear discharge plateau was observed at about 0.73 V (vs. Hg/HgO electrode) for the materials after metal nickel coating. Discharge capacities for the bare and nickel-coated alloy electrodes were calculated to be about 16.5 and 85.5 mAh/g, respectively. These results show that the nickel coating of MgzNi alloy is a very effective method of increasing the discharge capacity of the alloy electrode.
2 1 0
6
0
,
---.-.\ ,
4
I
,
8
8-m .-.-.-
I
I
12
I
I
16
0
Cycle number Fig. 2. Discharge capacities of the bare and nickelcoated M&Ni alloy electrodes as a function of cycle life.
kinds of electrodes proceeds very fast. For instance, after 15 cycles, the remaining capacity for the bare and the Ni-coated alloy electrode were found to be about only 9.5 mAh/g and 33.5 mAh/g, respectively. The poor cycling stability of the electrodes may suggest the formation of magnesium hydroxide on the surface during the chargeldscharge procedure in
Tang XG et aZ., Effects of chemical coating with Ni on electrochemical properties of ...
613
an electrolyte with hgh concentration (6 mol/L KOH). The oxidation of the Ni-coated alloy cannot be prevented to a great extent because: (1) the chemically coated nickel on the alloy surface is not homogeneous, and (2) the repeated charging and discharging processes lead to gradual pulverization of the alloys, whch again results in the increase of surface area, and then make degradation of the alloys continue.
to surface of Mg2Ni alloy and for electron transfer, but cannot provide significant corrosion protection because of its open structure. When decreasing discharge current, the discharge process lasts longer and the oxide film becomes thlcker. As a consequence, the measured discharge capacity is lower and the cycle life is poorer for the electrode with a lower discharge current than that with a higher discharge current.
3.2. Effects of discharge current on discharge capacity and cycle lie
3.3. Electrochemical characteristics
In general, the hlgher is the discharge current, the shorter is the cycle life, and the smaller is the initial capacity of the electrode. The same phenomenon occurs to A E 3 5 and AI32 systems. However, for the Ni-coated Mg2Ni alloy (Fig. 3), the initial discharge capacity increased and the cycling stability was improved firstly with increasing discharge current from
Fig. 4 shows cyclic voltammetry curves for the bare and Ni-coated MgzNi alloy electrodes. prior to CV examination, the two electrodes were charged for 5 h at 200 mNg in a 6 mol/L KOH solution at room temperature. Results show that metal nickel coating greatly increases the reduction current, as well as the oxidation current. The anodic peak current was ascribed to the oxidation of hydrogen absorbed in the alloy according to the equation [131: MH, + nOH- M + nHzO + ne(1) 0.16 0.12 0.08 d 0.04 6 0.00 -0.04 -0.08 -
. .w,
a
5
d
2
4 6 Cycle number
8
1 0 '
Fig. 3. Effect of discharge current on the cycle life of Ni-coatedM&Ni alloy.
50 to 300 mNg, and then deteriorated with further increasing the discharge current from 300 to 400 M g . It should be concluded that there exits an optimal discharge current for the highest discharge capacity and the best cycling performances. The observations mentioned before may be due to a fast degradation of Ni-coated MgzNi alloys in the chargddischarge duration. This fast degradation of Ni-coated MgzNi alloy can be caused by serious oxidation of magnesium. The discharge process depends strongly on the oxide film thickness. The film can act as a barrier for H atom diffusion from phase
"
-0.12 -0.16 -
-0.20
It is reported that the anodic peak current and peak area can be used to evaluate the electrochemical reactive activity and discharge capacity of the working electrode [ll-121. So, it can be concluded that Ni coating can greatly improve the electrochemical reactive activity and discharge capacity of Mg2Ni alloy electrode. In order to further clanfy the effects of Ni coating on the electrode behavior of MgzNi alloys, the elec-
RARE METALS, Vol. 26, No. 6,Dec 2007
614
trochemical impedance spectra (EIS) (Fig. 5) of the bare and Ni-coated Mg2Ni alloy electrodes were examined. Prior to CV examination, the two electrodes were charged for 5 h at 200 mAfg. As shown in Fig. 5, the EIS of the two electrodes are composed of three parts which are a smaller semicircle in the high-frequency region related to the contact resistance and capacitance between the current collector and the pellets of alloy powders, a larger distorted semicircle in the low-frequency region being related to the electrochemical reaction resistance on the surface between the electrode and electrolyte, and the liner region being related to hydrogen difFusion in the alloys. So the equivalent circuit of the EIS for the MgZNi alloy electrode can be summarized as shown in Fig. 6 [14-151.
MgzNi alloy electrode.
Fig. 6. Equivalent circuit of the EIS for MgzNi alloy electrode. R1: charge transfer resistance at the interface of electroddelectrolyte; Rz: contact resistance between alloy particles and collector; R3: resistance of electrolyte; el:capacitance between electrode and electrolyte; Cz: capacitance between the current collector and the pellet; 2,: Warburg impedance of hydrogen diffusion.
3.4. Phase structure and surface morphology Fig. 7 shows XRD patterns of the as-synthesized bare and Ni-coated Mg2Ni alloys. It is clear that hexagonal phase was present on the bare Mg2Ni alloys.
--- 1.434Hz
.
14.34 mHz
3500 4000 3000 2500
d (b)
Mg
00 . .-
A Ni * - -
.
.
. Ni
-
b 2000
8
1.0
2.0
3.0
4.0
5.0
RIR Fig. 5. EIS of the bare and Ni-coated M g a i alloy electrodes (charged for 5 h at 200 mA/g )
It can be concluded from Fig. 6 that the two semicircles in the high-frequency region are similar, indicating that there are no obvious effects of Ni coating on the contact resistance of the Mg2Ni alloy electrode. However, in the low-frequency region, the semicircle and the slope of the liner of Ni-coated alloy electrode are smaller than that of the bare alloy electrcde, which indicates that Ni coating can effectively reduce charge transfer resistance on the electrode/electrolyte surface and hydrogen diffusion resistance. For the Ni-coated electrode, the smaller charge transfer resistance and hydrogen diffusion resistance bring forth a higher charge efficiency and a lower anodic polarization potential [ 161. Therefore, Ni coating can increase the discharge capacity of the
* 1500 5 1000 500
q0
20
30
40
50
60
70
80
2e/ (01 Fig. 7. X-ray diffraction patterns of Mgfli alloys: (a) bare; (b) Ni-coated.
However, for the Ni-coated alloy, the characteristic peaks of the MgzNi alloy phase are broadened, which might accord to the presence of Ni ultrafine particles on the surface produced in the chemical coating process. SEM images of the bare and nickel-coated alloys are given in Fig. 8. It shows that the surfaces of the bare alloy were smooth. However, surfaces of the nickel-coated sample were rather rough and protruding because the surfaces were covered with ultrafine Ni particles. Therefore, metal nickel coating was believed to increase the specific surface area, leading to a reduced electrochemical charge transfer resistance and hydrogen diffusion resistance.
Tang KG et aL, Effects of chemical coating with Ni on electrochemical properties of
...
615
References
Fig. 8. SEM images of the bare and Ni-coated Mg2Ni alloys: (a) bare; (b) Ni-coated.
4. Conclusion The effects of Ni coating on the electrochemical properties of Mg2Ni alloys have been investigated. It was found that Ni coating improved the initial discharge capacity of the MgzNi alloy electrode from 16.5 to 85.5 mAh/g, but it had a negative effect on its cycle life. The cycle life and initial discharge capacity of the as-synthesized Ni-coated MgzNi alloys were greatly dependent on the discharge current. XRD and SEM results showed that Ni coating led to a dense and ultrafiie Ni particles layer producing on the surfaces and an increase of the specific surface area. CV and EIS analyses showed that Ni coating reduced charge transfer resistance as well as hydrogen diffusion resistance, which resulted in the discharge capacity improvement of the as-synthesized MgzNi alloy electrodes.
Ye H., Lei Y.Q., Chen L.S., et al., Electrochemical characteristics of amorphous Mgo.9&,,Ni (M = Ni, Ti, Zr,Co, and Si) ternary alloys prepared by mechanical alloying, J. Alloys Compd., 2000, 311 (2): 194. Iwakura C., Nohara S., and Inoue H., Preparation and characterization of Mgo.gT&,VO.&i-carbon material composites for electrochemicaluse, Solid State Ionics, 2002,148 (3-4): 499. Wang Z.M., Zhou H.Y., Gu Z.F., et al., Preparation of Mg2&EXNi(RE = La, Ce, FY, Nd, Y) alloys and their electrochemical characteristics, J. Alloys Compd., 2004,381: 234. Luo J.L. and Cui N., Effects of microencapsulation on the electrode behavior of MgzNi-based hydrogen storage alloy in alkaline solution, J. Alloys Cornpd., 1998,264 (1-2): 299. Chen J., Bradhurst D.H., Dou S.X., et al., Effect of chemical coating with Ni on the electrode properties of MgzNi alloy, J. Alloys Compd., 1998, 280 (1-2): 290. Wang C.Y., Yao P., Bradhurst D.H., et al., Surface modification of Mg2Ni alloy in an acid solution of copper sulfate and sulfuric acid, J. Alloys Compd., 1999,285: 267 Hamptona M.D., Lomnessa J.K., and Giannuzzib L.A., Surface study of liquid water treated and water vapor treated Mg2.35Ni alloy, In?. J. Hydrogen Energy, 2002,27: 79. Yang H.B., Yuan H.T., Ji J.T., et al., Characteristics of Mg2Ni,-.7&z(M = Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn)alloys after surface treatment, J. Alloys Compd., 2002,330-332:640 Inoue H., Ueda T., Nohara S., et al., Effect of ball-milling on electrochemical and physicochemical characteristics of crystalline Mg2Ni, Electmchim. Acta, 1998,43 (14-15): 2215. Cui N., P He., and Luo J.L.. Magnesium-based hydrogen storage materials modified by mechanical alloying, Acta MateK, 1999,47: 3737. Jiang J.J. and Gasik M., An electrochemicalinvestigation of mechanical alloying of MgNi-based hydrogen storage alloys, J. Power Sources, 2000, 89: 117. Shang C.X., Bououdinal M., Song Y., et al., Mechanical alloying and electronic simulations of (MgH2+ M) systems (M = Al, Ti, Fe, Ni, Cu,and Nb)
6Z6 for hydrogen storage, Int. J. Hydrogen Energy, 2004, 29: 73. [13] Matsupka M., Kohono T., and Iwakura C., Electrochemical properties of hydrogen storage alloys modified with foreign metals, Electrochim Acta, 1993,38 (6): 787. [14] Chen W.X., Study on cyclic voltammetry and electrochemical impedance of hydrogen storage alloy electrode, Chin. J. Power Sources (in Chinese), 2000,
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