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The effects of surface treatment on metal hydride electrodes using a weak acid solution containing Ni(II)
Journal of Alloys and Compounds 316 (2001) 131–136
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The effects of surface treatment on metal hydride electrodes using a weak acid solution containing Ni(II) a,b a, a Junmin Nan , Yong Yang *, Zugeng Lin a
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, Xiamen University, Xiamen 361005, China b School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Received 4 May 2000; received in revised form 12 October 2000; accepted 1 November 2000
Abstract A novel surface treatment method using a weak acid solution containing Ni 21 ions (WANi) for AB 5 -type hydrogen storage alloys is introduced. The properties of treated and untreated metal-hydride (MH) electrodes were investigated using several electrochemical and spectroscopic methods. Ex-situ scanning tunnelling microscopy (STM) and electrochemical results showed that the WANi process modified the alloy surface layer under modest reaction conditions and enhanced the initial electrochemical performances of the electrodes. In-situ confocal microprobe Raman spectroscopic results demonstrated that the formation, growth and properties of alloy surface oxide layers exhibited some differences on the microscopic level. In addition, manganese and cobalt segregated and progressively enriched the alloy surface and were subsequently oxidized / reduced during charge–discharge processes. The changes in the composition and structure of the surface layer of the electrodes are believed to be an important factor causing the deterioration of MH electrodes. 2001 Elsevier Science B.V. All rights reserved. Keywords: Metal hydride electrodes; Surface treatment methods; Surface properties; Charge–discharge performance; In-situ and ex-situ spectroscopic characterizations
1. Introduction The metal hydride / nickel (MH / Ni) battery, which replaced the toxic cadmium electrode in the Cd / Ni battery by a metal hydride electrode, has recently found wide commercialization in portable electric applications and electric vehicles due to its environmental friendly characteristics [1–6]. Improving the performance of MH electrodes by means of surface treatment has received considerable attention in recent years. Up to now, metal coatings such as copper and nickel plating, and alkaline, acid and reducing agents such as NaBH 4 treatment methods have been developed in laboratories and factories [7–13]. However, some aspects, such as the production cost, must be reduced for these methods to be widely used in the industrial production of MH / Ni batteries. In addition, the absence of in-situ and microscopic surface experimental techniques limits our understanding of the degradation processes of MH electrodes. Because a stable Ni-rich surface layer on hydrogen *Corresponding author. E-mail address: [email protected] (Y. Yang).
storage alloys is essential to the performance of metal hydride electrodes [14], a novel surface treatment process called the ‘weak acid solution containing Ni 21 ions’ (WANi) method, has been developed in our laboratory. Moreover, electrochemical, ex-situ and in-situ spectroscopic techniques such as scanning tunnelling microscopy (STM), Raman spectroscopy and photoelectrochemical microscopy (PEM) have been combined to investigate the surface properties of MH electrodes [15–17]. In this paper, we report some results concerning the applications of electrochemical, ex-situ STM and in-situ Raman spectroscopic techniques for the study of the morphology and reactivity of alloys and the degradation process of MH electrodes. In addition, we also discuss the activation mechanism of the WANi process.
2. Experimental La 0.54 Ce 0.32 Pr 0.03 Nd 0.11 Ni 3.5 Co 0.8 Mn 0.4 Al 0.3 hydrogenstorage alloy ingots were supplied by the Beijing General Research Institute for Non-ferrous Metals. The treatment solution consists of 1 M NH 4 Cl, 0.1 M NiSO 4 and 0.3 M
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01412-2
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citric acid [18]. The pH of the solution was adjusted to pH 4 using HCl and NH 4 OH. The treatment temperature was 908C. The MH electrodes with and without treatment were called the WANi electrode or the bare electrode, respectively. All electrochemical experiments were performed at room temperature (|208C). Solid MH electrodes were prepared to study the morphology and Raman spectra of the surface layers of the alloys. Alloy ingots were cut into a cylindrical rod of diameter 1 mm, and the rods were sealed in a Teflon sheath. After one side was abraded with successively fine grades of silicon carbide paper and thoroughly rinsed with distilled water in an ultrasonic washing machine, a nickel slice was welded to the other side as a conductive lead. STM experiments were performed by means of a Nanoscope IIIa (Digital Instruments, USA). The electrochemical experiments were performed by an IM6 (ZAHNER-elektrik, Germany). The electrolyte solution was 30 wt% KOH11 wt% LiOH; a slice of Pt and Hg / HgO were used as the counter and reference electrode respectively in a three-compartment electrolytic cell. Powder MH electrodes were constructed by packing the paste of alloy particles (grinding the alloy ingot to 300 mesh powders) and binders [made from a mixture of PTFE (polytetrafluoroethylene) and CMC (cellulose methylcarboxylate) in the ratio 3:7 by 60 wt% PTFE and 3 wt% CMC] into a foamed nickel substrate (1031032 mm). The electrodes were subsequently cold-compressed to 0.7 mm thickness, and were used to study the charge–discharge performance. Two sintered nickel hydroxide electrodes were tightly placed on both sides as counter electrodes. Cycle tests were carried out with an automatic battery instrument (Arbin Inc., USA). The charge and discharge current density was 100 mA / g, and the overcharging charge and cut potential was 125% and 20.6 V (vs. Hg / HgO), respectively. Raman spectra of the powder MH electrodes were obtained using a confocal microprobe Raman system (LabRam I, Dilor, France). The excitation light was obtained from an air-cooled He–Ne laser (623.8 nm) at a power of 13 mW and a spot of ca. 3 mm.
3. Results and discussion
3.1. Electrochemical reactivity of MH electrodes before and after WANi treatment Polarization curves for bare and WANi electrodes (Fig. 1) were obtained in an increasing potential direction at a scan rate of 0.5 mV/ s. For the bare electrode, the initial equilibrium potential was only 20.708 V, which differed from the standard equilibrium potential (20.9324 V) of a MH electrode [19,20]. This may be due to a compact surface oxide layer that formed during the mechanical crushing, storage and transportation processes in which the surface oxide layer restricts adsorption–desorption of
Fig. 1. Polarization curves of solid bare (A) and WANi (B) electrodes.
hydrogen atoms and their diffusion into the electrode. Compared with the bare electrode, the exchange current density of the WANi electrode increased i 0b /i 0a 5 367.6 / 3.241 5 113.4-fold, and decreased from its equilibrium potential to 20.8 V. Based on our XPS results [14] and the proposed formation mechanism of the WANi treatment process (see Section 3.4 for more details), it is believed that the enhanced electrochemical activity was caused by the Ni-rich surface that formed after treatment. Since the hydrogen adsorption–desorption process on pristine MH electrodes is a reversible reaction, the equilibrium potential observed in the polarization curve will be closely related to the equilibrium hydrogen pressure in alloy surface layers [21]. Therefore, the hindering influence of a surface layer on the diffusion of adsorbed hydrogen atoms can be neglected and the electrocatalytic activity of the surface layer is (mainly due to the Ni-rich layer) enhanced effectively. It is well known that MH electrodes can be activated by cyclic charge–discharge or cyclic potential-scanning experiments. Fig. 2 depicts the equilibrium potential curves of the bare (curve A) and WANi (curve B) electrodes as a function of activation cycle. In our experiments, the electrodes were progressively activated with cyclic potential-scanning in the potential range 20.8 to 21.15 V at a scan rate of 3 mV/ s. As a comparison, the equilibrium potential curve of the nickel-plated electrode is also shown as curve C in Fig. 2 for further comparison with the function of a pure nickel coating which was obtained without using acid solution. As expected, diffusion of adsorbed hydrogen atoms from the alloy surface into the bulk alloy might be enhanced by an increase of hydrogen adsorbed on the surface and the reaction activity of the alloy surface layer for decomposition of hydrogen molecules into hydrogen atoms. From Fig. 2 it can be seen that the equilibrium potential of WANi electrodes attained the activation potential more quickly than that of bare electrodes. In
J. Nan et al. / Journal of Alloys and Compounds 316 (2001) 131 – 136
Fig. 2. Equilibrium potentials as a function of activation time for solid MH electrodes. Cyclic potential sweeping in 20.8 to 21.15 V at a rate of 3 mV/ s was used to activate the electrodes. (A) Bare electrode, (B) WANi electrode, (C) nickel-plated electrode.
addition, the activation performance of the nickel-plated electrode was even worse than the former two methods, indicating that the nickel layer did not play the role of catalytic site. From the STM image of the nickel-plated electrode (not shown), it was observed that a compact nickel layer was formed. Thus, the poorer performance of the electrodeless nickel-plating system can be understood. Because the activation step using acidic solution was omitted in the treatment process, the joint hindering effects of the compact nickel coating and surface oxide layers on the diffusion of adsorbed hydrogen atoms would be responsible for the poorer activation performance of nickel-plating electrodes without acid treatment.
3.2. The charge–discharge performance of powder MH electrodes Surface treatment effects can be seen from changes in the performance of powder MH electrodes after treatment. Fig. 3 shows the discharge capacity as a function of cycle number for bare and WANi electrodes with different treating times. In addition, Fig. 4 shows the charge and discharge energy versus cycle number for the electrodes. Because the WANi electrode treated for 16 min had similar characteristics to the other two WANi electrodes, only the results for this electrode are present here. The discharge capacity of WANi electrodes increased from 0.5 to 78, 101 and 138 mAh / g with increasing treatment period in the first cycle. The discharge capacity increased more quickly, and reached a higher maximum discharge capacity (|290 mAh / g), on about the tenth cycle. By contrast, the bare electrode needed more than 30 cycles to attain its maximum discharge capacity (|278 mAh / g). Combining the results given in Fig. 4, the WANi process increased the initial charge–discharge performance of the electrodes. In particular, it is worth emphasizing
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Fig. 3. Discharge capacity as a function of cycle number for powder bare electrodes and WANi electrodes with different treatment times. (A) Bare electrode, (B–D) WANi electrodes treated for 5, 16 and 25 min, respectively.
that, after two to three cycles, a discharge capacity of 200 mAh / g, which corresponds to the actual discharge capacity of the MH electrode in a MH / Ni battery, was obtained after WANi treatment. This would be valuable for the production of sealed MH / Ni batteries, which needs activated alloy particles to produce the MH electrode. As a comparison, the repeated hydrogen absorption–desorption treatment method [22] was used to activate the alloy particles, but it has a higher cost of production equipment and this complex process even breaks the bulk structure of the alloys, while these aspects are avoided with the WANi process. Fig. 5 shows two typical degradation curves for the discharge capacity of the bare powder electrodes and WANi electrodes, which were treated for 16 min. For both electrodes, after reaching their maximum discharge capacities, the discharge capacity decreased progressively and approached similar values on about the 100th cycle, and decreased about 20–24% on the 500th cycle compared
Fig. 4. Discharge energy as a function of cycle number for the powder bare electrode and WANi electrode treated for 16 min. (A) Bare electrode, (B) WANi electrode.
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Fig. 5. Degradation curves for the discharge capacity of the powder bare electrode and WANi electrode treated for 16 min. (A) Bare electrode, (B) WANi electrode.
with their maximum discharge capacity. This result supports our previous conclusion [23] from nickel- and copper-coated MH electrode studies. Namely, the initial electrochemical properties of the MH electrode can be greatly increased by appropriate treatment methods while the cycle lifetime mainly depends on the bulk properties of the alloys.
3.3. Raman spectra of the surface oxide layer on powder MH electrodes To understand the structure and composition of surface oxide layers on powder MH electrodes, the surface layers of alloy particles in an MH electrode were studied using Raman spectroscopy. This is also useful for gaining an understanding of the degradation processes of MH electrodes during charge–discharge cycles. Although Raman bands were not found for bare and WANi treated alloy particles before the cycle tests, some Raman bands for surface oxides appeared and were enhanced with the activation processes of the electrodes. Fig. 6 presents the Raman spectra of powder WANi electrodes treated for 16 min and cycled 500 times. Analysis of the Raman spectra of MH electrodes at different charge–discharge stages and in different potential regions will be given in detail in a future publication. Here, we are only interested in the possible oxide species formed on the alloys after the cycle tests. In our experiment, with the help of a confocal microprobe Raman spectrometer, we can see that the size distribution and pulverization of the alloy particles were irregular after 500 charge–discharge cycles. Therefore, taking advantage of the Raman sampling spot of ca. 3 mm, the multiple-spot sampling method was used. In addition, depending on the difference in color of the sampling spots recorded by a CCD camera and in the spectra, the sampling spots can be freely chosen on the sites of alloy particles or binder.
Fig. 6. Raman spectra of a WANi electrode treated for 16 min after 500 cycles at different sampling spots.
Raman bands, located at 289.0, 387.1 and 731.8 cm 21 in Fig. 6b,d,e, arise from the mixed binders (PTFE1CMC): they correspond very well with the spectra of a solid mixture of PTFE1CMC. Besides the Raman peaks for mixed binders, the spectral peaks located in the spectral region between 480 and 700 cm 21 were attributed to the oxides formed on the alloys in the charge–discharge cycles. A series of Raman experiments for different oxides formed on pure alloy compositions such as La, Ce, Ni, Co and Mn metals were performed in this study [24]. No distinctive Raman bands for La, Ce and Al electrodes and their oxides were found. In addition, the Raman spectra of pure Ni, Co and Mn electrodes and their oxides were also quite complex [24]. Based on the spectra shown in Fig. 6 and a comparison of the results, it appears that a composite oxide layer was formed on the alloys. However, their Raman bands were different from those of the oxides which usually form on the surface layer of individual Ni, Co and Mn electrodes. Some useful information can be obtained by combining the results of Fig. 6 with the Raman bands of individual alloy components [24–29] and our photoelectrochemistry analysis results [14–17]: (i) We tentatively assign the Raman bands located around 600.1 cm 21 , with a broad shoulder peak (see Fig. 6e) appearing at every sampling spot (see Fig. 6a–e), to the composite oxide layer including the oxides of Ni, Co, La and Mn. (ii) For the Raman bands with a sharp peak appearing at some sampling spots, we assign the peaks at 324.3, 628.4 and 668.9 cm 21 (Fig. 6b) to the manganese oxides, and the peaks at 485.5, 498.1 and 688.2 cm 21 (Fig. 6e) to the cobalt oxides. The origin of these Raman bands is attributed to the progressive segrega-
J. Nan et al. / Journal of Alloys and Compounds 316 (2001) 131 – 136
tion and enrichment of manganese, cobalt and other alloy components to the alloy surface due to their affinity for water [30]. From the microscopic viewpoint, the oxides formed and their segregation behavior have also been proposed in other studies [1,31]. It was proposed that the segregation of lanthanum and manganese occurs during the cycle tests and gives rise to the performance degradation of the MH electrode. Furthermore, it is believed that these processes accelerate the pulverization of the alloy particles and increase the contact resistance between particles and particles and substrate, and are an important aspect causing the deterioration of MH electrodes. (iii) Because the Raman patterns of the WANi electrode are similar to that of a bare electrode (this will be reported in a future paper), the similarity of the degradation process for both electrodes can be concluded.
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hydrogen atoms [33], otherwise a hydrogen fire will be observed in the alloys. In the WANi process, the above points have been fully considered. The reaction mechanism is proposed as follows. Because of the acidic characteristics of the WANi treatment solution, the surface oxide layer and some components of the alloy are dissolved in solution and active hydrogen atoms are produced first. At the same time, an active Ni-rich layer is formed during the treatment process, either by replacing Ni 21 with other active components such as Al, and / or the reduction of Ni 21 with the resultant active hydrogen atoms. The proposed reaction processes that take place on alloy surface layers in the WANi process are MO 1 2H 1 5 M 21 1 H 2 O
(1)
3M 1 2H 1 5 M 21 1 2 MH
(2)
MM9 1 Ni 21 5 M 9 21 1 MNi
(3)
3.4. Discussion of the activation mechanism of the WANi treatment process
Ni 21 1 2MH 5 MMNi 1 2H 1
(4)
Generally, hydrogen-storage alloys can be activated by treatment with strong acidic solutions [32]. In this process, alloy components react with the acidic solution, some components dissolving in the treating solution and other products, such as hydrogen atoms, are adsorbed on the surface and even diffuse into the bulk alloy. As a result, the following composition and structural changes of the alloy surface layer, the hydrogen absorption–desorption capability and the reactivity of the alloys will be increased. However, it should be noted that this kind of active surface layer may also be oxidized and results in degradation of the alloy particles in humid air and electrolyte solution. Therefore, a coating is usually required, deferring the oxidation process until after the acid activation step [32]. Moreover, this process should be carefully controlled to avoid its negative effects on the diffusion of adsorbed
Here, M and MO denote the surface components and their oxides, respectively, and M9 denotes the active components which react with Ni 21 . The surface STM images of solid WANi electrodes, and the electrodes treated with the same mole concentration of weak acid (WA) solution as in the WANi process, are shown in Fig. 7. In our previous publication [17], the STM image of the bare electrode consisted of crystalline particles with diameters of 4–6 nm. From Fig. 7, we can see that some agglomerates and large holes appear for the WA electrode. In contrast, although the surface of the WANi electrode was also rough, the roughness was less and the large holes almost disappeared. It is believed that Ni 21 produce compensation effects, which are helpful for producing an even surface layer with a large amount of Ni active
Fig. 7. STM images of MH electrodes. (a) WANi electrode, (b) WA electrode.
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reaction sites in the treatment process. It can also be seen that the STM results are in agreement with the explanation of the activation mechanism of the treatment process. In addition, our XPS results [14] indicated that a Ni-rich surface layer was produced on the alloy surface after the WANi process, thus it seems that the proposed mechanism is reasonable.
4. Conclusion The proposed WANi process modifies the properties of the alloy surface layer under modest conditions and enhances the initial electrochemical performance of MH electrodes. It is shown that the method is valuable for the production of sealed MH / Ni batteries. Through a study of the capacity degradation of bare and treated MH electrodes, a better understanding of the relationship between the surface treatment and cyclic performance of MH electrodes has been obtained. Furthermore, it is demonstrated that STM and Raman techniques are powerful and suitable experimental tools for the study of MH electrodes.
Acknowledgements The authors are grateful for financial support from the National Science Foundation (Nos. 29925310 and 29673033) and the New Material Division of the National High-Technology Research and Development Committee. We would like to thank Prof. B.W. Mao, Prof. Z.Q. Tian, and Mr. C.H. Shi for assistance with the STM and Raman experiments. Y. Yang would also like to thank the Ministry of Education, PRC, for additional financial support for young researchers.
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The effects of surface treatment on metal hydride electrodes using a weak acid solution containing Ni(II) a,b a, a Junmin Nan , Yong Yang *, Zugeng Lin a
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, Xiamen University, Xiamen 361005, China b School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Received 4 May 2000; received in revised form 12 October 2000; accepted 1 November 2000
Abstract A novel surface treatment method using a weak acid solution containing Ni 21 ions (WANi) for AB 5 -type hydrogen storage alloys is introduced. The properties of treated and untreated metal-hydride (MH) electrodes were investigated using several electrochemical and spectroscopic methods. Ex-situ scanning tunnelling microscopy (STM) and electrochemical results showed that the WANi process modified the alloy surface layer under modest reaction conditions and enhanced the initial electrochemical performances of the electrodes. In-situ confocal microprobe Raman spectroscopic results demonstrated that the formation, growth and properties of alloy surface oxide layers exhibited some differences on the microscopic level. In addition, manganese and cobalt segregated and progressively enriched the alloy surface and were subsequently oxidized / reduced during charge–discharge processes. The changes in the composition and structure of the surface layer of the electrodes are believed to be an important factor causing the deterioration of MH electrodes. 2001 Elsevier Science B.V. All rights reserved. Keywords: Metal hydride electrodes; Surface treatment methods; Surface properties; Charge–discharge performance; In-situ and ex-situ spectroscopic characterizations
1. Introduction The metal hydride / nickel (MH / Ni) battery, which replaced the toxic cadmium electrode in the Cd / Ni battery by a metal hydride electrode, has recently found wide commercialization in portable electric applications and electric vehicles due to its environmental friendly characteristics [1–6]. Improving the performance of MH electrodes by means of surface treatment has received considerable attention in recent years. Up to now, metal coatings such as copper and nickel plating, and alkaline, acid and reducing agents such as NaBH 4 treatment methods have been developed in laboratories and factories [7–13]. However, some aspects, such as the production cost, must be reduced for these methods to be widely used in the industrial production of MH / Ni batteries. In addition, the absence of in-situ and microscopic surface experimental techniques limits our understanding of the degradation processes of MH electrodes. Because a stable Ni-rich surface layer on hydrogen *Corresponding author. E-mail address: [email protected] (Y. Yang).
storage alloys is essential to the performance of metal hydride electrodes [14], a novel surface treatment process called the ‘weak acid solution containing Ni 21 ions’ (WANi) method, has been developed in our laboratory. Moreover, electrochemical, ex-situ and in-situ spectroscopic techniques such as scanning tunnelling microscopy (STM), Raman spectroscopy and photoelectrochemical microscopy (PEM) have been combined to investigate the surface properties of MH electrodes [15–17]. In this paper, we report some results concerning the applications of electrochemical, ex-situ STM and in-situ Raman spectroscopic techniques for the study of the morphology and reactivity of alloys and the degradation process of MH electrodes. In addition, we also discuss the activation mechanism of the WANi process.
2. Experimental La 0.54 Ce 0.32 Pr 0.03 Nd 0.11 Ni 3.5 Co 0.8 Mn 0.4 Al 0.3 hydrogenstorage alloy ingots were supplied by the Beijing General Research Institute for Non-ferrous Metals. The treatment solution consists of 1 M NH 4 Cl, 0.1 M NiSO 4 and 0.3 M
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01412-2
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citric acid [18]. The pH of the solution was adjusted to pH 4 using HCl and NH 4 OH. The treatment temperature was 908C. The MH electrodes with and without treatment were called the WANi electrode or the bare electrode, respectively. All electrochemical experiments were performed at room temperature (|208C). Solid MH electrodes were prepared to study the morphology and Raman spectra of the surface layers of the alloys. Alloy ingots were cut into a cylindrical rod of diameter 1 mm, and the rods were sealed in a Teflon sheath. After one side was abraded with successively fine grades of silicon carbide paper and thoroughly rinsed with distilled water in an ultrasonic washing machine, a nickel slice was welded to the other side as a conductive lead. STM experiments were performed by means of a Nanoscope IIIa (Digital Instruments, USA). The electrochemical experiments were performed by an IM6 (ZAHNER-elektrik, Germany). The electrolyte solution was 30 wt% KOH11 wt% LiOH; a slice of Pt and Hg / HgO were used as the counter and reference electrode respectively in a three-compartment electrolytic cell. Powder MH electrodes were constructed by packing the paste of alloy particles (grinding the alloy ingot to 300 mesh powders) and binders [made from a mixture of PTFE (polytetrafluoroethylene) and CMC (cellulose methylcarboxylate) in the ratio 3:7 by 60 wt% PTFE and 3 wt% CMC] into a foamed nickel substrate (1031032 mm). The electrodes were subsequently cold-compressed to 0.7 mm thickness, and were used to study the charge–discharge performance. Two sintered nickel hydroxide electrodes were tightly placed on both sides as counter electrodes. Cycle tests were carried out with an automatic battery instrument (Arbin Inc., USA). The charge and discharge current density was 100 mA / g, and the overcharging charge and cut potential was 125% and 20.6 V (vs. Hg / HgO), respectively. Raman spectra of the powder MH electrodes were obtained using a confocal microprobe Raman system (LabRam I, Dilor, France). The excitation light was obtained from an air-cooled He–Ne laser (623.8 nm) at a power of 13 mW and a spot of ca. 3 mm.
3. Results and discussion
3.1. Electrochemical reactivity of MH electrodes before and after WANi treatment Polarization curves for bare and WANi electrodes (Fig. 1) were obtained in an increasing potential direction at a scan rate of 0.5 mV/ s. For the bare electrode, the initial equilibrium potential was only 20.708 V, which differed from the standard equilibrium potential (20.9324 V) of a MH electrode [19,20]. This may be due to a compact surface oxide layer that formed during the mechanical crushing, storage and transportation processes in which the surface oxide layer restricts adsorption–desorption of
Fig. 1. Polarization curves of solid bare (A) and WANi (B) electrodes.
hydrogen atoms and their diffusion into the electrode. Compared with the bare electrode, the exchange current density of the WANi electrode increased i 0b /i 0a 5 367.6 / 3.241 5 113.4-fold, and decreased from its equilibrium potential to 20.8 V. Based on our XPS results [14] and the proposed formation mechanism of the WANi treatment process (see Section 3.4 for more details), it is believed that the enhanced electrochemical activity was caused by the Ni-rich surface that formed after treatment. Since the hydrogen adsorption–desorption process on pristine MH electrodes is a reversible reaction, the equilibrium potential observed in the polarization curve will be closely related to the equilibrium hydrogen pressure in alloy surface layers [21]. Therefore, the hindering influence of a surface layer on the diffusion of adsorbed hydrogen atoms can be neglected and the electrocatalytic activity of the surface layer is (mainly due to the Ni-rich layer) enhanced effectively. It is well known that MH electrodes can be activated by cyclic charge–discharge or cyclic potential-scanning experiments. Fig. 2 depicts the equilibrium potential curves of the bare (curve A) and WANi (curve B) electrodes as a function of activation cycle. In our experiments, the electrodes were progressively activated with cyclic potential-scanning in the potential range 20.8 to 21.15 V at a scan rate of 3 mV/ s. As a comparison, the equilibrium potential curve of the nickel-plated electrode is also shown as curve C in Fig. 2 for further comparison with the function of a pure nickel coating which was obtained without using acid solution. As expected, diffusion of adsorbed hydrogen atoms from the alloy surface into the bulk alloy might be enhanced by an increase of hydrogen adsorbed on the surface and the reaction activity of the alloy surface layer for decomposition of hydrogen molecules into hydrogen atoms. From Fig. 2 it can be seen that the equilibrium potential of WANi electrodes attained the activation potential more quickly than that of bare electrodes. In
J. Nan et al. / Journal of Alloys and Compounds 316 (2001) 131 – 136
Fig. 2. Equilibrium potentials as a function of activation time for solid MH electrodes. Cyclic potential sweeping in 20.8 to 21.15 V at a rate of 3 mV/ s was used to activate the electrodes. (A) Bare electrode, (B) WANi electrode, (C) nickel-plated electrode.
addition, the activation performance of the nickel-plated electrode was even worse than the former two methods, indicating that the nickel layer did not play the role of catalytic site. From the STM image of the nickel-plated electrode (not shown), it was observed that a compact nickel layer was formed. Thus, the poorer performance of the electrodeless nickel-plating system can be understood. Because the activation step using acidic solution was omitted in the treatment process, the joint hindering effects of the compact nickel coating and surface oxide layers on the diffusion of adsorbed hydrogen atoms would be responsible for the poorer activation performance of nickel-plating electrodes without acid treatment.
3.2. The charge–discharge performance of powder MH electrodes Surface treatment effects can be seen from changes in the performance of powder MH electrodes after treatment. Fig. 3 shows the discharge capacity as a function of cycle number for bare and WANi electrodes with different treating times. In addition, Fig. 4 shows the charge and discharge energy versus cycle number for the electrodes. Because the WANi electrode treated for 16 min had similar characteristics to the other two WANi electrodes, only the results for this electrode are present here. The discharge capacity of WANi electrodes increased from 0.5 to 78, 101 and 138 mAh / g with increasing treatment period in the first cycle. The discharge capacity increased more quickly, and reached a higher maximum discharge capacity (|290 mAh / g), on about the tenth cycle. By contrast, the bare electrode needed more than 30 cycles to attain its maximum discharge capacity (|278 mAh / g). Combining the results given in Fig. 4, the WANi process increased the initial charge–discharge performance of the electrodes. In particular, it is worth emphasizing
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Fig. 3. Discharge capacity as a function of cycle number for powder bare electrodes and WANi electrodes with different treatment times. (A) Bare electrode, (B–D) WANi electrodes treated for 5, 16 and 25 min, respectively.
that, after two to three cycles, a discharge capacity of 200 mAh / g, which corresponds to the actual discharge capacity of the MH electrode in a MH / Ni battery, was obtained after WANi treatment. This would be valuable for the production of sealed MH / Ni batteries, which needs activated alloy particles to produce the MH electrode. As a comparison, the repeated hydrogen absorption–desorption treatment method [22] was used to activate the alloy particles, but it has a higher cost of production equipment and this complex process even breaks the bulk structure of the alloys, while these aspects are avoided with the WANi process. Fig. 5 shows two typical degradation curves for the discharge capacity of the bare powder electrodes and WANi electrodes, which were treated for 16 min. For both electrodes, after reaching their maximum discharge capacities, the discharge capacity decreased progressively and approached similar values on about the 100th cycle, and decreased about 20–24% on the 500th cycle compared
Fig. 4. Discharge energy as a function of cycle number for the powder bare electrode and WANi electrode treated for 16 min. (A) Bare electrode, (B) WANi electrode.
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Fig. 5. Degradation curves for the discharge capacity of the powder bare electrode and WANi electrode treated for 16 min. (A) Bare electrode, (B) WANi electrode.
with their maximum discharge capacity. This result supports our previous conclusion [23] from nickel- and copper-coated MH electrode studies. Namely, the initial electrochemical properties of the MH electrode can be greatly increased by appropriate treatment methods while the cycle lifetime mainly depends on the bulk properties of the alloys.
3.3. Raman spectra of the surface oxide layer on powder MH electrodes To understand the structure and composition of surface oxide layers on powder MH electrodes, the surface layers of alloy particles in an MH electrode were studied using Raman spectroscopy. This is also useful for gaining an understanding of the degradation processes of MH electrodes during charge–discharge cycles. Although Raman bands were not found for bare and WANi treated alloy particles before the cycle tests, some Raman bands for surface oxides appeared and were enhanced with the activation processes of the electrodes. Fig. 6 presents the Raman spectra of powder WANi electrodes treated for 16 min and cycled 500 times. Analysis of the Raman spectra of MH electrodes at different charge–discharge stages and in different potential regions will be given in detail in a future publication. Here, we are only interested in the possible oxide species formed on the alloys after the cycle tests. In our experiment, with the help of a confocal microprobe Raman spectrometer, we can see that the size distribution and pulverization of the alloy particles were irregular after 500 charge–discharge cycles. Therefore, taking advantage of the Raman sampling spot of ca. 3 mm, the multiple-spot sampling method was used. In addition, depending on the difference in color of the sampling spots recorded by a CCD camera and in the spectra, the sampling spots can be freely chosen on the sites of alloy particles or binder.
Fig. 6. Raman spectra of a WANi electrode treated for 16 min after 500 cycles at different sampling spots.
Raman bands, located at 289.0, 387.1 and 731.8 cm 21 in Fig. 6b,d,e, arise from the mixed binders (PTFE1CMC): they correspond very well with the spectra of a solid mixture of PTFE1CMC. Besides the Raman peaks for mixed binders, the spectral peaks located in the spectral region between 480 and 700 cm 21 were attributed to the oxides formed on the alloys in the charge–discharge cycles. A series of Raman experiments for different oxides formed on pure alloy compositions such as La, Ce, Ni, Co and Mn metals were performed in this study [24]. No distinctive Raman bands for La, Ce and Al electrodes and their oxides were found. In addition, the Raman spectra of pure Ni, Co and Mn electrodes and their oxides were also quite complex [24]. Based on the spectra shown in Fig. 6 and a comparison of the results, it appears that a composite oxide layer was formed on the alloys. However, their Raman bands were different from those of the oxides which usually form on the surface layer of individual Ni, Co and Mn electrodes. Some useful information can be obtained by combining the results of Fig. 6 with the Raman bands of individual alloy components [24–29] and our photoelectrochemistry analysis results [14–17]: (i) We tentatively assign the Raman bands located around 600.1 cm 21 , with a broad shoulder peak (see Fig. 6e) appearing at every sampling spot (see Fig. 6a–e), to the composite oxide layer including the oxides of Ni, Co, La and Mn. (ii) For the Raman bands with a sharp peak appearing at some sampling spots, we assign the peaks at 324.3, 628.4 and 668.9 cm 21 (Fig. 6b) to the manganese oxides, and the peaks at 485.5, 498.1 and 688.2 cm 21 (Fig. 6e) to the cobalt oxides. The origin of these Raman bands is attributed to the progressive segrega-
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tion and enrichment of manganese, cobalt and other alloy components to the alloy surface due to their affinity for water [30]. From the microscopic viewpoint, the oxides formed and their segregation behavior have also been proposed in other studies [1,31]. It was proposed that the segregation of lanthanum and manganese occurs during the cycle tests and gives rise to the performance degradation of the MH electrode. Furthermore, it is believed that these processes accelerate the pulverization of the alloy particles and increase the contact resistance between particles and particles and substrate, and are an important aspect causing the deterioration of MH electrodes. (iii) Because the Raman patterns of the WANi electrode are similar to that of a bare electrode (this will be reported in a future paper), the similarity of the degradation process for both electrodes can be concluded.
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hydrogen atoms [33], otherwise a hydrogen fire will be observed in the alloys. In the WANi process, the above points have been fully considered. The reaction mechanism is proposed as follows. Because of the acidic characteristics of the WANi treatment solution, the surface oxide layer and some components of the alloy are dissolved in solution and active hydrogen atoms are produced first. At the same time, an active Ni-rich layer is formed during the treatment process, either by replacing Ni 21 with other active components such as Al, and / or the reduction of Ni 21 with the resultant active hydrogen atoms. The proposed reaction processes that take place on alloy surface layers in the WANi process are MO 1 2H 1 5 M 21 1 H 2 O
(1)
3M 1 2H 1 5 M 21 1 2 MH
(2)
MM9 1 Ni 21 5 M 9 21 1 MNi
(3)
3.4. Discussion of the activation mechanism of the WANi treatment process
Ni 21 1 2MH 5 MMNi 1 2H 1
(4)
Generally, hydrogen-storage alloys can be activated by treatment with strong acidic solutions [32]. In this process, alloy components react with the acidic solution, some components dissolving in the treating solution and other products, such as hydrogen atoms, are adsorbed on the surface and even diffuse into the bulk alloy. As a result, the following composition and structural changes of the alloy surface layer, the hydrogen absorption–desorption capability and the reactivity of the alloys will be increased. However, it should be noted that this kind of active surface layer may also be oxidized and results in degradation of the alloy particles in humid air and electrolyte solution. Therefore, a coating is usually required, deferring the oxidation process until after the acid activation step [32]. Moreover, this process should be carefully controlled to avoid its negative effects on the diffusion of adsorbed
Here, M and MO denote the surface components and their oxides, respectively, and M9 denotes the active components which react with Ni 21 . The surface STM images of solid WANi electrodes, and the electrodes treated with the same mole concentration of weak acid (WA) solution as in the WANi process, are shown in Fig. 7. In our previous publication [17], the STM image of the bare electrode consisted of crystalline particles with diameters of 4–6 nm. From Fig. 7, we can see that some agglomerates and large holes appear for the WA electrode. In contrast, although the surface of the WANi electrode was also rough, the roughness was less and the large holes almost disappeared. It is believed that Ni 21 produce compensation effects, which are helpful for producing an even surface layer with a large amount of Ni active
Fig. 7. STM images of MH electrodes. (a) WANi electrode, (b) WA electrode.
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reaction sites in the treatment process. It can also be seen that the STM results are in agreement with the explanation of the activation mechanism of the treatment process. In addition, our XPS results [14] indicated that a Ni-rich surface layer was produced on the alloy surface after the WANi process, thus it seems that the proposed mechanism is reasonable.
4. Conclusion The proposed WANi process modifies the properties of the alloy surface layer under modest conditions and enhances the initial electrochemical performance of MH electrodes. It is shown that the method is valuable for the production of sealed MH / Ni batteries. Through a study of the capacity degradation of bare and treated MH electrodes, a better understanding of the relationship between the surface treatment and cyclic performance of MH electrodes has been obtained. Furthermore, it is demonstrated that STM and Raman techniques are powerful and suitable experimental tools for the study of MH electrodes.
Acknowledgements The authors are grateful for financial support from the National Science Foundation (Nos. 29925310 and 29673033) and the New Material Division of the National High-Technology Research and Development Committee. We would like to thank Prof. B.W. Mao, Prof. Z.Q. Tian, and Mr. C.H. Shi for assistance with the STM and Raman experiments. Y. Yang would also like to thank the Ministry of Education, PRC, for additional financial support for young researchers.
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