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Electrochemical impedance study of discharge characteristics of Pd substituted MgNi-based hydrogen storage electrode alloys
Journal of Alloys and Compounds 481 (2009) 826–829
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Electrochemical impedance study of discharge characteristics of Pd substituted MgNi-based hydrogen storage electrode alloys Qifeng Tian a,b,d,∗ , Yao Zhang c,∗ , Hailiang Chu c , Yigang Ding a,b , Yuanxin Wu a,b a
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China d Department of Physics and Atmospheric Science, Dalhousie University, Halifax B3H 3J5 Canada b c
a r t i c l e
i n f o
Article history: Received 16 January 2009 Received in revised form 16 March 2009 Accepted 20 March 2009 Available online 31 March 2009 Keywords: Hydrogen storage alloys Electrochemical impedance Charge-transfer resistance Warburg coefficient
a b s t r a c t Quaternary Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) in amorphous phase were prepared by mechanical alloying (MA). They exhibited enhanced cycle life in our previous studies. However, their electrochemical impedances and its relationship with electrode reaction kinetics are still unclear. In this work, the impedance spectra of the above electrode alloys were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. We further observed that the rate-determining step changed from chargetransfer to hydrogen diffusion with proceeding of discharge according to the fitting analyses of spectra. The fitted results demonstrated that charge-transfer resistance Rct decreased and then increased with the depth of discharge (DOD) and it also increased with Pd content among the studied alloys. Thickness of surface passive film, which is proportional to the reciprocal of double layer capacitance near the electrode alloy surface according to Badawy et al., increased with the DOD and especially with Pd content in the alloys. Meanwhile, the thickened film on the surface of electrode alloys ensured its long cycle life. Through Randles plotting and comparing the variation of Warburg coefficient vs. DOD for the electrode alloys, it was found that increased with the DOD and substitution of Pd also improved the diffusion performance of the studied electrode alloys. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction MgNi-based hydrogen storage alloy is a kind of promising cathode material for Nickel-Metal Hydride (Ni-MH) batteries owing to its significant advantages, such as high discharge capacities, abundant reserves, low cost, etc. Many electrochemical studies about such alloys were reported in previous decades by Lei et al. [1,3] and Nohara et al. [2]. The major problem hindering the application of this kind of alloys might be attributed to their inferior cycle life. Recently, the partial substitution of Ti for Mg in MgNi alloys has attracted more attention, as can be seen in Lei et al. [4,6], Han et al. [5], and Ruggeri et al. [7]. It was found that Ti substitution effectively inhibited capacity degradation of the electrode alloys. Due to the excellent anti-corrosion performance of Pd, the improved cycle life of Pd substituted MgNi-based hydrogen storage alloys have been reported by us [8,9]. In recent years, several researches were also
∗ Corresponding author. Tel.: +86 27 8719 4980; fax: +86 27 8719 4465. E-mail addresses: [email protected] (Q. Tian), [email protected] (Y. Zhang).
published on the improvement of cyle life of MgNi-based electrode alloys [10–13]. Electrochemical impedance spectroscopy (EIS) is a very useful technique to investigate electrode reaction kinetics. Liu et al. studied electrochemical impedance spectra of MgNi hydrogen storage alloys at different depths of discharge (DODs) [14]. They found that charge-transfer resistance increased with the depth of discharge (DOD) and the fitted charge-transfer resistance was larger than the corresponding Warburg impedance. By means of EIS, Yuan and coworkers studied Al and Ce [15], Ti–Al [16], CoB [17,18], NiB [19], FeB [20], and MB (M = Co, Ti) [21] substituted amorphous MgNi-based alloy electrodes. But most of their studies only focused on the discussion of the variation of charge-transfer resistance of different substituted alloys at 50% DOD. Gao and co-workers investigated the Warburg impedance of Mg-rich Mg–Nd [22] and Ni-added LaMg10 Ni12−x Alx [23] electrode alloys with EIS. They concluded that Ni addition can decrease the Warburg impedance of such electrode alloys. Pd substituted MgNi-based hydrogen storage alloys were ever reported by us [8,9]. The significance of such alloys lies in their relatively long cycle life as compared with the other MgNi-based alloys.
0925-8388/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.116
Q. Tian et al. / Journal of Alloys and Compounds 481 (2009) 826–829
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However, their electrode reaction kinetics during the discharge process has not been studied in detail yet. Moreover, the relationship between the kinetics properties of the electrode alloys with its cycle life is still unclear. In this work, Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) were prepared by mechanical alloying. Their electrode reaction kinetics during the discharge process was studied by electrochemical impedance technique. With the equivalent circuit model used in the paper, the values of the equivalent circuit elements were obtained after fitting electrochemical impedance spectra and their variations along with the DOD were investigated. The rate-determining step and diffusion performance of the studied electrode alloys during discharge process were analyzed and the changes of those properties with the DOD were also discussed in the paper as well. 2. Experimental 2.1. Preparation of Pd substituted MgNi-based hydrogen storage alloys The Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) were prepared by mechanical alloying (MA). The purity of all metal powders was higher than 99.5%. Stoichiometric powder mixtures were ground by a planetary ball mill under Argon atmosphere for 120 h. In each milling pot, the ball to powder weight ratio was 30:1. The main phase structure of the alloys, which were characterized by XRD (Rigaku D/max-2500, Cu K␣, 50 kV, 200 mA) and transmission electron microscopy (JEM-2000EX), was determined to be amorphous. 2.2. Discharge tests and electrochemical impedance spectra measurements The electrodes for the electrochemical experiments were fabricated by mixing 0.1 g of alloy powder with 0.3 g of electrolytic Cu powder. The mixture was then pressed into a pellet of 1.0 cm diameter under a pressure of 30 MPa. Both sides of the pellet were coated with two foam nickel sheets, then pressed at 25 MPa and spot-welded. A nickel wire lead was attached to this pressed foam nickel sheet by spot welding. The NiOOH/Ni(OH)2 electrode and Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. The electrolyte was a 6 M KOH aqueous solution. The charge–discharge cycles of the electrodes were conducted by an automatic LAND battery test instrument. The electrodes were charged for 3 h at a current density of 300 mAg−1 , rested for 5 min and then discharged to −0.6 V vs. the Hg/HgO electrode at a current density of 100 mAg−1 . All the measurements were carried out at a temperature of 303 K. The electrochemical impedance spectra were measured at 0%, 25% 50%, 75%, and 100% DOD using an IM6e electrochemical workstation. Before the measurements were taken, the alloy electrodes were kept in a static condition for at least 10 min to stabilize their open circuit potentials. The measurements were conducted from 5 mHz to 10 kHz with an amplitude of 5 mV vs. the open circuit potential.
3. Results and discussion 3.1. Fitting of measured electrochemical impedance spectra The measured electrochemical impedance spectra were analyzed with the equivalent circuit model [14,16,24] as shown in Fig. 1. Rs is ascribed to the electrolyte resistance between the hydrogen storage alloy electrode and reference electrode. The semicircle in the high-frequency region, modeled by R1 and CPE1, results from the contact resistance and capacitance between the alloy particles and the current collector. The contact resistance and capacitance between the alloy particles generate the parameters R2 and CPE2, respectively. R3 and CPE3, representing the semicircle in the low-frequency region, contribute to the charge-transfer reaction resistance and the double layer capacitance of the electrode alloy surface, respectively. W is the Warburg impedance. Fig. 2 is the comparison of the experimental spectra with fitted curves using the above equivalent circuit model at different DODs.
Fig. 1. Equivalent circuit model used to analyze the measured electrochemical impedance spectra.
Fig. 2. Experimental and fitted impedance spectra of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at different DODs (a) DOD = 0%; (b) DOD = 25%; (c) DOD = 50%; (d) DOD = 75%; (e) DOD = 100%.
The measured spectra consisted of several semi-circles from high frequency to low frequency. The radius of circles at mid- or low frequency decreased with the DOD and Warburg diffusion impedances appeared when the DOD reached 25%. When the DOD was 100%, no apparent circles were observed in the figure except for several lines that denote Warburg diffusion impedance. That means the rate-determining step changed from charge-transfer to hydrogen diffusion gradually during the discharge process. From the figure, one can also see a good fitting between the simulated spectra and measured ones, which partially proved the correctness of the model used in the paper. 3.2. Variation of charge-transfer resistance and double layer capacitance of the alloy electrodes during the discharge process Fig. 3(a) presents the variation of fitted charge-transfer resistance Rct vs. DOD of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys. The overall trend of these electrode alloys was that the Rct decreased initially and increased afterwards with the DOD. This trend agreed well with other reports [17,24,25]. At the initial stage of discharge, the non-equilibrium of hydrogen concentration between metal hydride and its surface facilitates hydrogen oxidation and charge-transfer resistance is decreased accordingly. At the end of the discharge process, the depletion of hydrogen atoms on the alloy surface and the formation of a thin oxide layer led
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Fig. 3. Variation of the fitted charge-transfer resistance Rct , the reciprocal of fitted double layer capacitance 1/CPE3 and the fitted Warburg impedance W at various DODs of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys (a) Rct ; (b) 1/CPE3; (c) W.
to the higher resistance. That is why the Rct decreased at first of discharge and then increased subsequently. Furthermore, Rct also increased with Pd content in the studied alloys, which means that the substitution of Pd in the alloys enhanced the resistance of the surface oxide layer and made the charge-transfer more difficult. These results are also consistent with our previous work [8]. The change of the reciprocal of the fitted double layer capacitance of the alloy electrodes with the DOD is displayed in Fig. 3(b). Sustainable increase of the parameter along with the DOD was observed in the figure. Since the value of the reciprocal of capacitance is usually proportional to the thickness of the passive film according to Badawy et al. [26], one can conclude that the thickness of the passive film increased with the DOD due to continuous formation of surface oxide layers on the alloys. In addition, the thickness of the surface passive film also increased with Pd content within the alloys according to the figure. In general, the thicker film, the higher its resistance. Therefore, the variation of the reciprocal of fitted capacitance as to Pd content within the alloys is consistent with that of charge-transfer resistance as discussed above. The thicker films on the alloys, with the increase of Pd content, made charge-transfer more difficult and then increased its resistances accordingly. The increased electrode resistances partly enhanced the anti-corrosion performance of the electrode alloys and extended their cycle life accordingly.
Fig. 4. Randles plots of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at various DODs (a) x = 0.04; (b) x = 0.06; (c) x = 0.08; (d) x = 0.1.
above range. Similar results were also obtained by Liu et al. [27] when they studied the electrochemical performances of Pd-added Ti–V-based hydrogen storage alloys. They found that the hydrogen diffusion rate was also enhanced on the condition that the Pd content was within 0–0.1 in the Ti–V-based hydrogen storage alloys. According to Cui et al. [28], the effect of mass transfer is manifested through the Warburg coefficient, . With Randles plotting, that is plotting Z with ω−1/2 (ω = 2f ) for a low-frequency Warburg response, the Warburg coefficient can be obtained by measuring the slope of such plots [29–32]. Fig. 4 shows the Randles plotting of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at different DODs. It can be derived from the figure that the slopes did not change much before the DOD reached 75%. After that point it increased suddenly. The corresponding plot of slopes () vs. DOD of the alloys is demonstrated in Fig. 5. The trend of with the DOD is consistent with that of Warburg impedance as shown in Fig. 3(c), which means the rate-determining step of the electrode alloys evolved from charge-transfer to hydrogen diffusion near the end of the discharge process. Furthermore, the value of followed
3.3. Diffusion performance of the alloy electrodes during the discharge process Fig. 3(c) is the summary of the fitted Warburg impedance of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at different discharge statuses. Before the DOD reached 75%, the Warburg impedance did not change much as can be seen from the figure. When the discharge process came to its end, the hydrogen oxidation rate was lower than the requirements of the discharge current, which led to the rate-determining step switched from charge-transfer to hydrogen diffusion. The characteristic of Warburg impedance increasing at the end of discharge also corresponds to the change of Nyquist spectra as shown in Fig. 2. In addition, with the Pd content increasing from 0.04 to 0.1, the Warburg impedance decreased accordingly, which denotes that the diffusion performance of such electrode alloys increased with Pd content in the
Fig. 5. Variation of Warburg coefficient vs. DOD of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys.
Q. Tian et al. / Journal of Alloys and Compounds 481 (2009) 826–829
the opposite sequence of Pd content in the alloys. A high concentration of Pd in the alloys led to a smaller corresponding value. Larger value of means the poor hydrogen diffusion performance as concluded by Cui et al. [28]. Therefore, the enhancement of hydrogen diffusion performance with the increase of Pd by view of the Warburg coefficient agreed well with the conclusion obtained by the fitted Warburg impedance in the above discussion. 4. Conclusions The study of electrochemical impedance spectra of Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) revealed its rate-determining step changed from charge-transfer to hydrogen diffusion near the end of the discharge process. Charge transfer resistances of the electrode alloys decreased initially and then increased with the increase of the DOD. The thickness of surface oxide films increased gradually during the discharge and it also increased with Pd content in the alloys, which ensured the long cycle life of the alloys with higher Pd content. The hydrogen diffusion performance of the electrode alloys also enhanced with Pd content was proved by the fitted Warburg impedance and analyses of the Warburg coefficient , which were derived from the experimental spectra. Acknowledgements The authors are grateful to Mr. Markus Karahka from Dalhousie University of Canada for his correction on the writing. References [1] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Z. Phys. Chem. Bd. 183 (1994) 379–384. [2] S. Nohara, K. Hamasaki, S.G. Zhang, H. Inoue, C. Iwakura, J. Alloys Compd. 280 (1998) 104–106. [3] W.H. Liu, H.Q. Wu, Y.Q. Lei, Q.D. Wang, J. Wu, J. Alloys Compd. 261 (1997) 289–294. [4] H. Ye, Y.Q. Lei, L.S. Chen, H. Zhang, J. Alloys Compd. 311 (2000) 194–199.
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Electrochemical impedance study of discharge characteristics of Pd substituted MgNi-based hydrogen storage electrode alloys Qifeng Tian a,b,d,∗ , Yao Zhang c,∗ , Hailiang Chu c , Yigang Ding a,b , Yuanxin Wu a,b a
Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China d Department of Physics and Atmospheric Science, Dalhousie University, Halifax B3H 3J5 Canada b c
a r t i c l e
i n f o
Article history: Received 16 January 2009 Received in revised form 16 March 2009 Accepted 20 March 2009 Available online 31 March 2009 Keywords: Hydrogen storage alloys Electrochemical impedance Charge-transfer resistance Warburg coefficient
a b s t r a c t Quaternary Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) in amorphous phase were prepared by mechanical alloying (MA). They exhibited enhanced cycle life in our previous studies. However, their electrochemical impedances and its relationship with electrode reaction kinetics are still unclear. In this work, the impedance spectra of the above electrode alloys were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. We further observed that the rate-determining step changed from chargetransfer to hydrogen diffusion with proceeding of discharge according to the fitting analyses of spectra. The fitted results demonstrated that charge-transfer resistance Rct decreased and then increased with the depth of discharge (DOD) and it also increased with Pd content among the studied alloys. Thickness of surface passive film, which is proportional to the reciprocal of double layer capacitance near the electrode alloy surface according to Badawy et al., increased with the DOD and especially with Pd content in the alloys. Meanwhile, the thickened film on the surface of electrode alloys ensured its long cycle life. Through Randles plotting and comparing the variation of Warburg coefficient vs. DOD for the electrode alloys, it was found that increased with the DOD and substitution of Pd also improved the diffusion performance of the studied electrode alloys. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction MgNi-based hydrogen storage alloy is a kind of promising cathode material for Nickel-Metal Hydride (Ni-MH) batteries owing to its significant advantages, such as high discharge capacities, abundant reserves, low cost, etc. Many electrochemical studies about such alloys were reported in previous decades by Lei et al. [1,3] and Nohara et al. [2]. The major problem hindering the application of this kind of alloys might be attributed to their inferior cycle life. Recently, the partial substitution of Ti for Mg in MgNi alloys has attracted more attention, as can be seen in Lei et al. [4,6], Han et al. [5], and Ruggeri et al. [7]. It was found that Ti substitution effectively inhibited capacity degradation of the electrode alloys. Due to the excellent anti-corrosion performance of Pd, the improved cycle life of Pd substituted MgNi-based hydrogen storage alloys have been reported by us [8,9]. In recent years, several researches were also
∗ Corresponding author. Tel.: +86 27 8719 4980; fax: +86 27 8719 4465. E-mail addresses: [email protected] (Q. Tian), [email protected] (Y. Zhang).
published on the improvement of cyle life of MgNi-based electrode alloys [10–13]. Electrochemical impedance spectroscopy (EIS) is a very useful technique to investigate electrode reaction kinetics. Liu et al. studied electrochemical impedance spectra of MgNi hydrogen storage alloys at different depths of discharge (DODs) [14]. They found that charge-transfer resistance increased with the depth of discharge (DOD) and the fitted charge-transfer resistance was larger than the corresponding Warburg impedance. By means of EIS, Yuan and coworkers studied Al and Ce [15], Ti–Al [16], CoB [17,18], NiB [19], FeB [20], and MB (M = Co, Ti) [21] substituted amorphous MgNi-based alloy electrodes. But most of their studies only focused on the discussion of the variation of charge-transfer resistance of different substituted alloys at 50% DOD. Gao and co-workers investigated the Warburg impedance of Mg-rich Mg–Nd [22] and Ni-added LaMg10 Ni12−x Alx [23] electrode alloys with EIS. They concluded that Ni addition can decrease the Warburg impedance of such electrode alloys. Pd substituted MgNi-based hydrogen storage alloys were ever reported by us [8,9]. The significance of such alloys lies in their relatively long cycle life as compared with the other MgNi-based alloys.
0925-8388/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.116
Q. Tian et al. / Journal of Alloys and Compounds 481 (2009) 826–829
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However, their electrode reaction kinetics during the discharge process has not been studied in detail yet. Moreover, the relationship between the kinetics properties of the electrode alloys with its cycle life is still unclear. In this work, Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) were prepared by mechanical alloying. Their electrode reaction kinetics during the discharge process was studied by electrochemical impedance technique. With the equivalent circuit model used in the paper, the values of the equivalent circuit elements were obtained after fitting electrochemical impedance spectra and their variations along with the DOD were investigated. The rate-determining step and diffusion performance of the studied electrode alloys during discharge process were analyzed and the changes of those properties with the DOD were also discussed in the paper as well. 2. Experimental 2.1. Preparation of Pd substituted MgNi-based hydrogen storage alloys The Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) were prepared by mechanical alloying (MA). The purity of all metal powders was higher than 99.5%. Stoichiometric powder mixtures were ground by a planetary ball mill under Argon atmosphere for 120 h. In each milling pot, the ball to powder weight ratio was 30:1. The main phase structure of the alloys, which were characterized by XRD (Rigaku D/max-2500, Cu K␣, 50 kV, 200 mA) and transmission electron microscopy (JEM-2000EX), was determined to be amorphous. 2.2. Discharge tests and electrochemical impedance spectra measurements The electrodes for the electrochemical experiments were fabricated by mixing 0.1 g of alloy powder with 0.3 g of electrolytic Cu powder. The mixture was then pressed into a pellet of 1.0 cm diameter under a pressure of 30 MPa. Both sides of the pellet were coated with two foam nickel sheets, then pressed at 25 MPa and spot-welded. A nickel wire lead was attached to this pressed foam nickel sheet by spot welding. The NiOOH/Ni(OH)2 electrode and Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. The electrolyte was a 6 M KOH aqueous solution. The charge–discharge cycles of the electrodes were conducted by an automatic LAND battery test instrument. The electrodes were charged for 3 h at a current density of 300 mAg−1 , rested for 5 min and then discharged to −0.6 V vs. the Hg/HgO electrode at a current density of 100 mAg−1 . All the measurements were carried out at a temperature of 303 K. The electrochemical impedance spectra were measured at 0%, 25% 50%, 75%, and 100% DOD using an IM6e electrochemical workstation. Before the measurements were taken, the alloy electrodes were kept in a static condition for at least 10 min to stabilize their open circuit potentials. The measurements were conducted from 5 mHz to 10 kHz with an amplitude of 5 mV vs. the open circuit potential.
3. Results and discussion 3.1. Fitting of measured electrochemical impedance spectra The measured electrochemical impedance spectra were analyzed with the equivalent circuit model [14,16,24] as shown in Fig. 1. Rs is ascribed to the electrolyte resistance between the hydrogen storage alloy electrode and reference electrode. The semicircle in the high-frequency region, modeled by R1 and CPE1, results from the contact resistance and capacitance between the alloy particles and the current collector. The contact resistance and capacitance between the alloy particles generate the parameters R2 and CPE2, respectively. R3 and CPE3, representing the semicircle in the low-frequency region, contribute to the charge-transfer reaction resistance and the double layer capacitance of the electrode alloy surface, respectively. W is the Warburg impedance. Fig. 2 is the comparison of the experimental spectra with fitted curves using the above equivalent circuit model at different DODs.
Fig. 1. Equivalent circuit model used to analyze the measured electrochemical impedance spectra.
Fig. 2. Experimental and fitted impedance spectra of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at different DODs (a) DOD = 0%; (b) DOD = 25%; (c) DOD = 50%; (d) DOD = 75%; (e) DOD = 100%.
The measured spectra consisted of several semi-circles from high frequency to low frequency. The radius of circles at mid- or low frequency decreased with the DOD and Warburg diffusion impedances appeared when the DOD reached 25%. When the DOD was 100%, no apparent circles were observed in the figure except for several lines that denote Warburg diffusion impedance. That means the rate-determining step changed from charge-transfer to hydrogen diffusion gradually during the discharge process. From the figure, one can also see a good fitting between the simulated spectra and measured ones, which partially proved the correctness of the model used in the paper. 3.2. Variation of charge-transfer resistance and double layer capacitance of the alloy electrodes during the discharge process Fig. 3(a) presents the variation of fitted charge-transfer resistance Rct vs. DOD of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys. The overall trend of these electrode alloys was that the Rct decreased initially and increased afterwards with the DOD. This trend agreed well with other reports [17,24,25]. At the initial stage of discharge, the non-equilibrium of hydrogen concentration between metal hydride and its surface facilitates hydrogen oxidation and charge-transfer resistance is decreased accordingly. At the end of the discharge process, the depletion of hydrogen atoms on the alloy surface and the formation of a thin oxide layer led
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Fig. 3. Variation of the fitted charge-transfer resistance Rct , the reciprocal of fitted double layer capacitance 1/CPE3 and the fitted Warburg impedance W at various DODs of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys (a) Rct ; (b) 1/CPE3; (c) W.
to the higher resistance. That is why the Rct decreased at first of discharge and then increased subsequently. Furthermore, Rct also increased with Pd content in the studied alloys, which means that the substitution of Pd in the alloys enhanced the resistance of the surface oxide layer and made the charge-transfer more difficult. These results are also consistent with our previous work [8]. The change of the reciprocal of the fitted double layer capacitance of the alloy electrodes with the DOD is displayed in Fig. 3(b). Sustainable increase of the parameter along with the DOD was observed in the figure. Since the value of the reciprocal of capacitance is usually proportional to the thickness of the passive film according to Badawy et al. [26], one can conclude that the thickness of the passive film increased with the DOD due to continuous formation of surface oxide layers on the alloys. In addition, the thickness of the surface passive film also increased with Pd content within the alloys according to the figure. In general, the thicker film, the higher its resistance. Therefore, the variation of the reciprocal of fitted capacitance as to Pd content within the alloys is consistent with that of charge-transfer resistance as discussed above. The thicker films on the alloys, with the increase of Pd content, made charge-transfer more difficult and then increased its resistances accordingly. The increased electrode resistances partly enhanced the anti-corrosion performance of the electrode alloys and extended their cycle life accordingly.
Fig. 4. Randles plots of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at various DODs (a) x = 0.04; (b) x = 0.06; (c) x = 0.08; (d) x = 0.1.
above range. Similar results were also obtained by Liu et al. [27] when they studied the electrochemical performances of Pd-added Ti–V-based hydrogen storage alloys. They found that the hydrogen diffusion rate was also enhanced on the condition that the Pd content was within 0–0.1 in the Ti–V-based hydrogen storage alloys. According to Cui et al. [28], the effect of mass transfer is manifested through the Warburg coefficient, . With Randles plotting, that is plotting Z with ω−1/2 (ω = 2f ) for a low-frequency Warburg response, the Warburg coefficient can be obtained by measuring the slope of such plots [29–32]. Fig. 4 shows the Randles plotting of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at different DODs. It can be derived from the figure that the slopes did not change much before the DOD reached 75%. After that point it increased suddenly. The corresponding plot of slopes () vs. DOD of the alloys is demonstrated in Fig. 5. The trend of with the DOD is consistent with that of Warburg impedance as shown in Fig. 3(c), which means the rate-determining step of the electrode alloys evolved from charge-transfer to hydrogen diffusion near the end of the discharge process. Furthermore, the value of followed
3.3. Diffusion performance of the alloy electrodes during the discharge process Fig. 3(c) is the summary of the fitted Warburg impedance of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys at different discharge statuses. Before the DOD reached 75%, the Warburg impedance did not change much as can be seen from the figure. When the discharge process came to its end, the hydrogen oxidation rate was lower than the requirements of the discharge current, which led to the rate-determining step switched from charge-transfer to hydrogen diffusion. The characteristic of Warburg impedance increasing at the end of discharge also corresponds to the change of Nyquist spectra as shown in Fig. 2. In addition, with the Pd content increasing from 0.04 to 0.1, the Warburg impedance decreased accordingly, which denotes that the diffusion performance of such electrode alloys increased with Pd content in the
Fig. 5. Variation of Warburg coefficient vs. DOD of Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) electrode alloys.
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the opposite sequence of Pd content in the alloys. A high concentration of Pd in the alloys led to a smaller corresponding value. Larger value of means the poor hydrogen diffusion performance as concluded by Cui et al. [28]. Therefore, the enhancement of hydrogen diffusion performance with the increase of Pd by view of the Warburg coefficient agreed well with the conclusion obtained by the fitted Warburg impedance in the above discussion. 4. Conclusions The study of electrochemical impedance spectra of Pd substituted MgNi-based hydrogen storage alloys Mg0.9−x Ti0.1 Pdx Ni (x = 0.04, 0.06, 0.08, 0.1) revealed its rate-determining step changed from charge-transfer to hydrogen diffusion near the end of the discharge process. Charge transfer resistances of the electrode alloys decreased initially and then increased with the increase of the DOD. The thickness of surface oxide films increased gradually during the discharge and it also increased with Pd content in the alloys, which ensured the long cycle life of the alloys with higher Pd content. The hydrogen diffusion performance of the electrode alloys also enhanced with Pd content was proved by the fitted Warburg impedance and analyses of the Warburg coefficient , which were derived from the experimental spectra. Acknowledgements The authors are grateful to Mr. Markus Karahka from Dalhousie University of Canada for his correction on the writing. References [1] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Z. Phys. Chem. Bd. 183 (1994) 379–384. [2] S. Nohara, K. Hamasaki, S.G. Zhang, H. Inoue, C. Iwakura, J. Alloys Compd. 280 (1998) 104–106. [3] W.H. Liu, H.Q. Wu, Y.Q. Lei, Q.D. Wang, J. Wu, J. Alloys Compd. 261 (1997) 289–294. [4] H. Ye, Y.Q. Lei, L.S. Chen, H. Zhang, J. Alloys Compd. 311 (2000) 194–199.
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