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Preparation and electrochemical properties of MgNi–MB (M = Co, Ti) composite alloys
Journal of Alloys and Compounds 450 (2008) 375–379
Preparation and electrochemical properties of MgNi–MB (M = Co, Ti) composite alloys Guang He, Li-Fang Jiao, Hua-Tang Yuan ∗ , Yun-Yun Zhang, Yi-Jing Wang Institute of New Energy Material Chemistry, Nankai University, Tianjin 30071, China Received 16 July 2006; received in revised form 13 October 2006; accepted 28 October 2006 Available online 1 December 2006
Abstract Alloys MgNi–MB (M = Co, Ti) were successfully synthesized by means of mechanical alloying (MA). The XRD spectroscopy suggested that the alloys were amorphous. The discharge capacity and cycle life of these alloys were tested, showing that as borides were introduced, the cycling life of the alloys became much better than MgNi alloy. For instance, 10 h composite MgNi–CoB and MgNi–TiB retained 53.2% and 54.1% of the initial capacity after 30 cycles, while the MgNi alloy kept only 23.3%. The exchange impedance spectroscopy and the potentiodynamic polarization curves proved that the electrochemical properties of the composite alloys were improved significantly. MgNi–CoB was used as an example to study the mechanism of electrochemical hydrogen storage. © 2006 Elsevier B.V. All rights reserved. Keywords: Mg-based hydrogen alloys; Borides; Electrochemical properties
1. Introduction Mg-based hydrogen storage alloys are considered as one of the most promising candidates of the third generation alloys because of lower density, cost, and rich natural resources [1], especially for their discharge capacity which is as high as 999 mAh/g and much higher than that of other alloys such as AB5 type, AB2 type, and AB3 type, so they have drawn much attention in this field [2,3]. These alloys, however, reveal relatively high H-desorption temperatures, slow kinetics of H-sorption and charge–discharge cycling stability, which make them difficult for practical applications as negative materials of Ni/MH batteries [4]. A large amount of work, such as the optimization of alloy composition [5–10] and surface modifications [11–15] has been done to solve these problems. It is known that borides are used as the ceramic materials because of the strong hardness and excellent anti-corrosion property in alkaline or acid solution by forming a thin film on the surface of the as-protected composites [16–18]. As Co and Ti are common elements doped in Mg-based hydrogen storage alloys [19–24], we prepared MB (M = Co, Ti) and MgNi–MB alloys
∗
Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (H.-T. Yuan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.127
in order to improve the electrochemical properties of MgNi alloy. The phase structure and electrochemical properties of the alloys were tested, and the effects of the borides were studied primarily. 2. Experimental details 2.1. Preparation of the alloys MgNi alloy was prepared from mixed powders of pure Mg and Ni (<75 m) at an atomic ratio of 1:1 by MA. Original reactants were carried out for 100 h using a planetary ball mill (ZKX-2B, Nanjing) in a stainless steel vessel at a speed of 450 rpm under argon atmosphere. The weight ratio of milling balls to reagent powders was selected to be 20:1. MB (M = Co, Ti (<75 m)) alloys were prepared in the same way except for the milled time changed to 80 h. Composite alloys were also synthesized in a similar method by balling milling MgNi and 10 wt% MB (M = Co, Ti) for 5, 10 and 15 h. The crystal structure and surface configuration of the alloys were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu ␣ radiation) and scanning electron microscopy (SEM, Hitachi X-650).
2.2. Electrochemical measurements Negative electrodes were constructed through mixing as-prepared composites with nickel powders at 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
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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/g and discharged at 25 mA/g up to the cut-off voltage set at −0.5 V (versus Hg/HgO). The electrolyte solution is a 6 M KOH aqueous solution. The testing time between charge and discharge was 10 min. Solartron 1250 + 1287 electrochemical apparatus was used for polarization and EIS measurements. The AC amplitude was 5 mV and the frequency range employed between 104 and 10−1 Hz. The potential range of potentiodynamic polarization experiment employed was between −1.2 and −0.2 V (versus Hg/HgO). The sweep rate was 1 mv/s.
3. Results and discussion 3.1. Phase structure In order to investigate the microstructures of the alloys, X-ray diffractometry was carried out. As can be seen in Fig. 1, before composite, though weak traces of Ni and Co can be observed from the XRD patterns of MgNi and CoB, dominant broad peaks appear in the region of 40–45◦ for MgNi, CoB and TiB, suggesting that the main phase of the alloy has formed an amorphous structure. After composite for 5 h, the weak peaks of Ni and Co disappear and the broad peaks become wider, which indicates that the alloys MB (M = Co, Ti) and Ni element have dissolved
Fig. 1. The XRD patterns of the alloys.
in the main phase MgNi. As composite alloys for 5 h are completely amorphous in the XRD patterns, it can be concluded that composite alloys for 10 and 15 h have a similar structure. The particles of MgNi–MB (M = Co, Ti) alloys milled 10 h are cubical grains with large interfaces and defects, as shown in Fig. 2. The grain size varies between 1 and 4 m in diameter. These observations do not all agree with the results of MgNi alloys as-prepared by other authors [25–27]. In addition, composite MgNi–TiB particles seem to be smaller and less conglomerated than composite MgNi–CoB, that is, MgNi–TiB particles have larger specific area which may contribute to higher capacity. 3.2. Electrochemical properties The alloys were constructed experimental Ni–MH cells to test their capacity and cycle stability as negative electrodes. Figs. 3 and 4 are discharging capacity and cycling life curves of the composite alloys with cycle number. In Fig. 3, electrodes need no activation process and show the maximum discharge capacity at the first cycle. MgNi has the highest initial capacity and the worst cycling stability among these alloys. In the first 30 cycles, the discharge capacity decreases from 403.7 to 94 mAh/g and the capacity maintaining rate (Cn/C1 ) is only 23.3% in Fig. 4. After composite by MB (M = Co, Ti), the initial discharge capacities of the alloys decrease to some extent, but the cycling stability improves distinctly, revealing that borides added in MgNi alloy protect the electrodes from corrosion in the alkaline solution effectively. Take 10 h composite MgNi–TiB as an example which is the best among the electrodes, its initial discharge capacity is as high as 370.7 mAh/g and keeps more than 200 mAh/g after 30 cycles. With different composite hours, the cycling stability of the electrode alloys performs differently. For MgNi–CoB, the capacity maintaining rates are 43.4%, 53.2% and 45.4%. For MgNi–TiB, the values are 39.1%, 54.1% and 50.9%. It is noticed that the stability of 10 h composite alloys is the best for both of the two kinds of alloys. On conventional view, more milling
Fig. 2. SEM micrographs of MgNi–MB (M = Co, Ti) alloys: (A) MgNi–CoB and (B) MgNi–TiB.
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Fig. 4. The cycle life of the alloys electrodes. R1, solution resistance; R2, chargetransfer reaction resistance; CPE1, component of phase in electric circuit; W1, Warburg resistance.
Fig. 3. (a) The discharge capacity of the MgNi–CoB alloys electrodes. (b) The discharge capacity of the MgNi–TiB alloys electrodes.
hours have two advantages as making alloys equally dissolved in the phase and creating more crystal lacunas, both of which are helpful for absorbing and desorbing hydrogen atoms. However, lacunas make larger specific area contact with the alkaline solution in the same time which may accelerate the speed of corrosion, so it is important to find a proper milling time. According to the specific information on the discharging capacity and cycling stability of the MgNi and MgNi–MB (M = Co, Ti) alloy electrodes given in Table 1, 10 h the best composite time because the capacity maintaining rates are higher than others. As electrodes of 10 h-composite-alloys are better than others, EIS Nyquist diagrams and potentiodynamic polarization were
Fig. 5. The EIS patterns of the alloys electrodes.
used to test these two alloys and MgNi alloy. Figs. 5 and 6 show the EIS Nyquist diagrams for the three electrode alloys and their proposed equivalent circuit. It can be seen clearly that the spectra consist of a semicircle followed by a sloped straight line. Kuriyama et al. [28] ascribed the lower-frequency semicircle to the charge-transfer reaction resistance and the line to the Warburg impedance suggesting the diffusion of H from surface of the alloy to the inner phase. The semicircles for 10 h composite MgNi–MB (M = Co, Ti) are unobvious and the radii are
Table 1 Cycle stability of the electrodes
MgNi MgNi–CoB (5 h) MgNi–CoB (10 h) MgNi–CoB (15 h) MgNi–TiB (5 h) MgNi–TiB (10 h) MgNi–TiB (15 h)
C1st (mAh/g)
C15th (mAh/g)
C30th (mAh/g)
C15th /C1st (%)
C30th /C1st (%)
403.7 338.7 300.9 317.9 364.2 370.7 325.2
134.3 195.0 195.2 200.7 184.1 247.9 207.9
94.0 147.0 160.0 144.4 142.4 200.7 165.6
33.3 57.6 64.0 63.1 50.5 66.9 63.9
23.3 43.4 53.2 45.4 39.1 54.1 50.9
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Fig. 6. The EIS equivalent circuit of the alloys electrodes. R1: solution resistance; R2: charge-transfer reaction resistance; CPE1: component of phase in electric circuit; W1: Warburg resistance. Table 2 The EIS parameters of the alloys electrodes
( cm2 )
R2 W1 ( cm2 )
MgNi
MgNi–CoB
MgNi–TiB
0.4202 0.6264
0.1174 0.3242
0.0953 0.2521
smaller, implying that the values of the electrochemical resistance is smaller than that of MgNi alloy and partly concealed in the reaction resistance [29]. Moreover, it is reported in Ref. [29] that the spectra of Mg-based hydrogen storage consists of two semicircles and a linear, that is, a semicircle appears in the higher frequency region. The main reason for that are different depths of discharge (DOD). Alloy electrodes tested in this experiment are not activated by charge–discharge process, so only one semicircle can be seen in the spectra. On the basis of the equivalent circuit by means of Zview, the values of charge-transfer reaction resistance and the Warburg resistance were obtained. It was reported [30] that surface MgNi alloy in alkaline solution was inclined to be eroded to Mg(OH)2 which increased the reaction resistance and restrained atom H from diffusing into the inner phase. In Table 2, the result reveals that for composite electrodes, the values of the two listed resistance R2 and W1 are much smaller than in MgNi, so MB (M = Co, Ti) in the alloy prevent the formation of Mg(OH)2 and improve the cycle life of the electrodes. To confirm the effect of MB (M = Co, Ti) on anticorrosion, potentiodynamic polarization was employed to investigate the corrosive behavior of the alloy electrodes in our experiment. The potentiodynamic polarization of MgNi milled 100 h, MgNi–CoB and MgNi–TiB milled 10 h are shown in Fig. 7 with
Fig. 7. Potentiodynamic polarization curves of the alloy electrodes. Table 3 Tafel fitting data of the alloy electrodes
Ec (V) log i ic (mA/cm2 )
MgNi
MgNi–CoB
MgNi–TiB
−0.898 −3.78 1.66 × 10−4
−0.89 −3.87 1.35 × 10−4
−0.892 −4.32 4.79 × 10−5
the results obtained by Tafel fitting [31] and are summarized in Table 3. For the three alloys, their values of Ec are almost equivalent, but the corrosion rate (ic ) of the composite alloys are much lower than for MgNi, especially for the MgNi–TiB alloy. The result is accordant with that of EIS, suggesting that the composite MB (M = Co, Ti) reduce the rate of corrosion of the electrodes in the alkaline electrolyte. 3.3. Mechanism of hydrogen storage Ten hours composite MgNi–CoB alloy was taken as an example to study the charge–discharge process. SEM micrographs of MgNi–CoB alloy freshly prepared and the alloy after 30 charge–discharge cycles are shown in Fig. 8. It can be seen that
Fig. 8. SEM micrographs of MgNi–CoB alloys: (A) image of the as milled alloy, (B) image of the electrode after 30 cycles.
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it easy for hydrogen absorption–desorption and the corrosive rate decreased distinctly. In order to investigate the effect of the MB (M = Co, Ti), SEM and XRD were used for 10 h composite MgNi–CoB alloy. According to the XRD comparison between alloy as-prepared and after 30 cycles we supposed that CoB was changed to Co(OH)2 film which played an important role for protecting the alloy electrode from corrosion in KOH solution. Acknowledgement The work was supported by NSFC (50571046, 20573058). References Fig. 9. XRD pattern of the electrode after 30 charge–discharge cycles. Ni was introduced when pellets were prepared.
a great number of very fine grains are observed in Fig. 8(B) including some with filamentous shape indicating the cracking of the alloy during charge–discharge process. In addition, surface of the electrode after 30 cycles becomes smoother on cycling, which is a trace of oxidation [30]. In Fig. 9, for the XRD pattern of the electrode after 30 charge–discharge cycles, besides the 3 strongest Ni peaks, the several main weak peaks can be indexed to -Co(OH)2 (PCPDFWIN 30-0443). This result is quite different with that of pure MgNi alloy which suggests that Mg(OH)2 and Ni(OH)2 should be the main products after more than four charge–discharge cycles [30]. In a recent work [32], amorphous CoB was prepared by chemical method and investigated as an anode material. The electrode of CoB showed excellent electrochemical hydrogenation property and the XRD pattern of the electrode showed the same result, in which only Co(OH)2 appeared. It seems that Co(OH)2 does not restrain atom H diffusing in the alloy like Mg(OH)2 , on the contrary, it behaves like a protecting film. The same result was acquired in our previous work [33], in which CoSi alloys were coated by Co(OH)2 after discharging and XPS was used to test the state of the alloys. It was found that the alloys were still in metallic state inside the electrodes, so the Co(OH)2 only existed on the surface of the electrodes. The composite CoB in this experiment may have a similar effect: CoB coated the MgNi alloy in the composite process and was oxidized to Co(OH)2 on the surface of MgNi alloy in alkaline solution. In this case, Co(OH)2 film partly prevents further corrosion and improves the cycle stability for the electrodes. 4. Conclusion In this work, MgNi–MB (M = Co, Ti) alloys were prepared and their electrochemical characteristics were measured as anode electrodes. The results showed that though initial capacity decreased slightly, the composite alloys possessed much better cycle stability than pure MgNi alloy. EIS and potentiodynamic polarization suggested that composite process made
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Preparation and electrochemical properties of MgNi–MB (M = Co, Ti) composite alloys Guang He, Li-Fang Jiao, Hua-Tang Yuan ∗ , Yun-Yun Zhang, Yi-Jing Wang Institute of New Energy Material Chemistry, Nankai University, Tianjin 30071, China Received 16 July 2006; received in revised form 13 October 2006; accepted 28 October 2006 Available online 1 December 2006
Abstract Alloys MgNi–MB (M = Co, Ti) were successfully synthesized by means of mechanical alloying (MA). The XRD spectroscopy suggested that the alloys were amorphous. The discharge capacity and cycle life of these alloys were tested, showing that as borides were introduced, the cycling life of the alloys became much better than MgNi alloy. For instance, 10 h composite MgNi–CoB and MgNi–TiB retained 53.2% and 54.1% of the initial capacity after 30 cycles, while the MgNi alloy kept only 23.3%. The exchange impedance spectroscopy and the potentiodynamic polarization curves proved that the electrochemical properties of the composite alloys were improved significantly. MgNi–CoB was used as an example to study the mechanism of electrochemical hydrogen storage. © 2006 Elsevier B.V. All rights reserved. Keywords: Mg-based hydrogen alloys; Borides; Electrochemical properties
1. Introduction Mg-based hydrogen storage alloys are considered as one of the most promising candidates of the third generation alloys because of lower density, cost, and rich natural resources [1], especially for their discharge capacity which is as high as 999 mAh/g and much higher than that of other alloys such as AB5 type, AB2 type, and AB3 type, so they have drawn much attention in this field [2,3]. These alloys, however, reveal relatively high H-desorption temperatures, slow kinetics of H-sorption and charge–discharge cycling stability, which make them difficult for practical applications as negative materials of Ni/MH batteries [4]. A large amount of work, such as the optimization of alloy composition [5–10] and surface modifications [11–15] has been done to solve these problems. It is known that borides are used as the ceramic materials because of the strong hardness and excellent anti-corrosion property in alkaline or acid solution by forming a thin film on the surface of the as-protected composites [16–18]. As Co and Ti are common elements doped in Mg-based hydrogen storage alloys [19–24], we prepared MB (M = Co, Ti) and MgNi–MB alloys
∗
Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (H.-T. Yuan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.127
in order to improve the electrochemical properties of MgNi alloy. The phase structure and electrochemical properties of the alloys were tested, and the effects of the borides were studied primarily. 2. Experimental details 2.1. Preparation of the alloys MgNi alloy was prepared from mixed powders of pure Mg and Ni (<75 m) at an atomic ratio of 1:1 by MA. Original reactants were carried out for 100 h using a planetary ball mill (ZKX-2B, Nanjing) in a stainless steel vessel at a speed of 450 rpm under argon atmosphere. The weight ratio of milling balls to reagent powders was selected to be 20:1. MB (M = Co, Ti (<75 m)) alloys were prepared in the same way except for the milled time changed to 80 h. Composite alloys were also synthesized in a similar method by balling milling MgNi and 10 wt% MB (M = Co, Ti) for 5, 10 and 15 h. The crystal structure and surface configuration of the alloys were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu ␣ radiation) and scanning electron microscopy (SEM, Hitachi X-650).
2.2. Electrochemical measurements Negative electrodes were constructed through mixing as-prepared composites with nickel powders at 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
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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/g and discharged at 25 mA/g up to the cut-off voltage set at −0.5 V (versus Hg/HgO). The electrolyte solution is a 6 M KOH aqueous solution. The testing time between charge and discharge was 10 min. Solartron 1250 + 1287 electrochemical apparatus was used for polarization and EIS measurements. The AC amplitude was 5 mV and the frequency range employed between 104 and 10−1 Hz. The potential range of potentiodynamic polarization experiment employed was between −1.2 and −0.2 V (versus Hg/HgO). The sweep rate was 1 mv/s.
3. Results and discussion 3.1. Phase structure In order to investigate the microstructures of the alloys, X-ray diffractometry was carried out. As can be seen in Fig. 1, before composite, though weak traces of Ni and Co can be observed from the XRD patterns of MgNi and CoB, dominant broad peaks appear in the region of 40–45◦ for MgNi, CoB and TiB, suggesting that the main phase of the alloy has formed an amorphous structure. After composite for 5 h, the weak peaks of Ni and Co disappear and the broad peaks become wider, which indicates that the alloys MB (M = Co, Ti) and Ni element have dissolved
Fig. 1. The XRD patterns of the alloys.
in the main phase MgNi. As composite alloys for 5 h are completely amorphous in the XRD patterns, it can be concluded that composite alloys for 10 and 15 h have a similar structure. The particles of MgNi–MB (M = Co, Ti) alloys milled 10 h are cubical grains with large interfaces and defects, as shown in Fig. 2. The grain size varies between 1 and 4 m in diameter. These observations do not all agree with the results of MgNi alloys as-prepared by other authors [25–27]. In addition, composite MgNi–TiB particles seem to be smaller and less conglomerated than composite MgNi–CoB, that is, MgNi–TiB particles have larger specific area which may contribute to higher capacity. 3.2. Electrochemical properties The alloys were constructed experimental Ni–MH cells to test their capacity and cycle stability as negative electrodes. Figs. 3 and 4 are discharging capacity and cycling life curves of the composite alloys with cycle number. In Fig. 3, electrodes need no activation process and show the maximum discharge capacity at the first cycle. MgNi has the highest initial capacity and the worst cycling stability among these alloys. In the first 30 cycles, the discharge capacity decreases from 403.7 to 94 mAh/g and the capacity maintaining rate (Cn/C1 ) is only 23.3% in Fig. 4. After composite by MB (M = Co, Ti), the initial discharge capacities of the alloys decrease to some extent, but the cycling stability improves distinctly, revealing that borides added in MgNi alloy protect the electrodes from corrosion in the alkaline solution effectively. Take 10 h composite MgNi–TiB as an example which is the best among the electrodes, its initial discharge capacity is as high as 370.7 mAh/g and keeps more than 200 mAh/g after 30 cycles. With different composite hours, the cycling stability of the electrode alloys performs differently. For MgNi–CoB, the capacity maintaining rates are 43.4%, 53.2% and 45.4%. For MgNi–TiB, the values are 39.1%, 54.1% and 50.9%. It is noticed that the stability of 10 h composite alloys is the best for both of the two kinds of alloys. On conventional view, more milling
Fig. 2. SEM micrographs of MgNi–MB (M = Co, Ti) alloys: (A) MgNi–CoB and (B) MgNi–TiB.
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Fig. 4. The cycle life of the alloys electrodes. R1, solution resistance; R2, chargetransfer reaction resistance; CPE1, component of phase in electric circuit; W1, Warburg resistance.
Fig. 3. (a) The discharge capacity of the MgNi–CoB alloys electrodes. (b) The discharge capacity of the MgNi–TiB alloys electrodes.
hours have two advantages as making alloys equally dissolved in the phase and creating more crystal lacunas, both of which are helpful for absorbing and desorbing hydrogen atoms. However, lacunas make larger specific area contact with the alkaline solution in the same time which may accelerate the speed of corrosion, so it is important to find a proper milling time. According to the specific information on the discharging capacity and cycling stability of the MgNi and MgNi–MB (M = Co, Ti) alloy electrodes given in Table 1, 10 h the best composite time because the capacity maintaining rates are higher than others. As electrodes of 10 h-composite-alloys are better than others, EIS Nyquist diagrams and potentiodynamic polarization were
Fig. 5. The EIS patterns of the alloys electrodes.
used to test these two alloys and MgNi alloy. Figs. 5 and 6 show the EIS Nyquist diagrams for the three electrode alloys and their proposed equivalent circuit. It can be seen clearly that the spectra consist of a semicircle followed by a sloped straight line. Kuriyama et al. [28] ascribed the lower-frequency semicircle to the charge-transfer reaction resistance and the line to the Warburg impedance suggesting the diffusion of H from surface of the alloy to the inner phase. The semicircles for 10 h composite MgNi–MB (M = Co, Ti) are unobvious and the radii are
Table 1 Cycle stability of the electrodes
MgNi MgNi–CoB (5 h) MgNi–CoB (10 h) MgNi–CoB (15 h) MgNi–TiB (5 h) MgNi–TiB (10 h) MgNi–TiB (15 h)
C1st (mAh/g)
C15th (mAh/g)
C30th (mAh/g)
C15th /C1st (%)
C30th /C1st (%)
403.7 338.7 300.9 317.9 364.2 370.7 325.2
134.3 195.0 195.2 200.7 184.1 247.9 207.9
94.0 147.0 160.0 144.4 142.4 200.7 165.6
33.3 57.6 64.0 63.1 50.5 66.9 63.9
23.3 43.4 53.2 45.4 39.1 54.1 50.9
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Fig. 6. The EIS equivalent circuit of the alloys electrodes. R1: solution resistance; R2: charge-transfer reaction resistance; CPE1: component of phase in electric circuit; W1: Warburg resistance. Table 2 The EIS parameters of the alloys electrodes
( cm2 )
R2 W1 ( cm2 )
MgNi
MgNi–CoB
MgNi–TiB
0.4202 0.6264
0.1174 0.3242
0.0953 0.2521
smaller, implying that the values of the electrochemical resistance is smaller than that of MgNi alloy and partly concealed in the reaction resistance [29]. Moreover, it is reported in Ref. [29] that the spectra of Mg-based hydrogen storage consists of two semicircles and a linear, that is, a semicircle appears in the higher frequency region. The main reason for that are different depths of discharge (DOD). Alloy electrodes tested in this experiment are not activated by charge–discharge process, so only one semicircle can be seen in the spectra. On the basis of the equivalent circuit by means of Zview, the values of charge-transfer reaction resistance and the Warburg resistance were obtained. It was reported [30] that surface MgNi alloy in alkaline solution was inclined to be eroded to Mg(OH)2 which increased the reaction resistance and restrained atom H from diffusing into the inner phase. In Table 2, the result reveals that for composite electrodes, the values of the two listed resistance R2 and W1 are much smaller than in MgNi, so MB (M = Co, Ti) in the alloy prevent the formation of Mg(OH)2 and improve the cycle life of the electrodes. To confirm the effect of MB (M = Co, Ti) on anticorrosion, potentiodynamic polarization was employed to investigate the corrosive behavior of the alloy electrodes in our experiment. The potentiodynamic polarization of MgNi milled 100 h, MgNi–CoB and MgNi–TiB milled 10 h are shown in Fig. 7 with
Fig. 7. Potentiodynamic polarization curves of the alloy electrodes. Table 3 Tafel fitting data of the alloy electrodes
Ec (V) log i ic (mA/cm2 )
MgNi
MgNi–CoB
MgNi–TiB
−0.898 −3.78 1.66 × 10−4
−0.89 −3.87 1.35 × 10−4
−0.892 −4.32 4.79 × 10−5
the results obtained by Tafel fitting [31] and are summarized in Table 3. For the three alloys, their values of Ec are almost equivalent, but the corrosion rate (ic ) of the composite alloys are much lower than for MgNi, especially for the MgNi–TiB alloy. The result is accordant with that of EIS, suggesting that the composite MB (M = Co, Ti) reduce the rate of corrosion of the electrodes in the alkaline electrolyte. 3.3. Mechanism of hydrogen storage Ten hours composite MgNi–CoB alloy was taken as an example to study the charge–discharge process. SEM micrographs of MgNi–CoB alloy freshly prepared and the alloy after 30 charge–discharge cycles are shown in Fig. 8. It can be seen that
Fig. 8. SEM micrographs of MgNi–CoB alloys: (A) image of the as milled alloy, (B) image of the electrode after 30 cycles.
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it easy for hydrogen absorption–desorption and the corrosive rate decreased distinctly. In order to investigate the effect of the MB (M = Co, Ti), SEM and XRD were used for 10 h composite MgNi–CoB alloy. According to the XRD comparison between alloy as-prepared and after 30 cycles we supposed that CoB was changed to Co(OH)2 film which played an important role for protecting the alloy electrode from corrosion in KOH solution. Acknowledgement The work was supported by NSFC (50571046, 20573058). References Fig. 9. XRD pattern of the electrode after 30 charge–discharge cycles. Ni was introduced when pellets were prepared.
a great number of very fine grains are observed in Fig. 8(B) including some with filamentous shape indicating the cracking of the alloy during charge–discharge process. In addition, surface of the electrode after 30 cycles becomes smoother on cycling, which is a trace of oxidation [30]. In Fig. 9, for the XRD pattern of the electrode after 30 charge–discharge cycles, besides the 3 strongest Ni peaks, the several main weak peaks can be indexed to -Co(OH)2 (PCPDFWIN 30-0443). This result is quite different with that of pure MgNi alloy which suggests that Mg(OH)2 and Ni(OH)2 should be the main products after more than four charge–discharge cycles [30]. In a recent work [32], amorphous CoB was prepared by chemical method and investigated as an anode material. The electrode of CoB showed excellent electrochemical hydrogenation property and the XRD pattern of the electrode showed the same result, in which only Co(OH)2 appeared. It seems that Co(OH)2 does not restrain atom H diffusing in the alloy like Mg(OH)2 , on the contrary, it behaves like a protecting film. The same result was acquired in our previous work [33], in which CoSi alloys were coated by Co(OH)2 after discharging and XPS was used to test the state of the alloys. It was found that the alloys were still in metallic state inside the electrodes, so the Co(OH)2 only existed on the surface of the electrodes. The composite CoB in this experiment may have a similar effect: CoB coated the MgNi alloy in the composite process and was oxidized to Co(OH)2 on the surface of MgNi alloy in alkaline solution. In this case, Co(OH)2 film partly prevents further corrosion and improves the cycle stability for the electrodes. 4. Conclusion In this work, MgNi–MB (M = Co, Ti) alloys were prepared and their electrochemical characteristics were measured as anode electrodes. The results showed that though initial capacity decreased slightly, the composite alloys possessed much better cycle stability than pure MgNi alloy. EIS and potentiodynamic polarization suggested that composite process made
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