152 Journal of Alloys and Compounds, 192 (1993) 152-154 JALCOM 2050

Electrochemical characterization of hydrogen storage alloys modified with metal oxides

Chiaki Iwakura, Yukio Fukumoto, Masao Matsuoka, Tatsuoki Kohno and Katsuhide Shinmou Department of Applied Chemistry, University of Osaka Prefecture, 1-1 Gakuen-cho, Sakai, Osaka 593 (Japan)

Abstract

Negative electrodes were made from hydrogen storage alloys (MmNi3.6Mno4Alo.3Coo.7) modified by mixing them with metal oxide powders. The discharge capacity of cathodes was greatly increased by the modification with metal oxides with high electric conductivity, for example RuO2 and C0304. An exchange current density (Jo) for the hydrogen electrode reaction was also increased remarkably by modification with metal oxides. High-rate discharge- ability was found to increase asymptotically with an increment of Jo. An impedance analysis indicated that the modification of the alloy powder with the metal oxides decreased the charge-transfer resistance. The mode of action of metal oxides was discussed on the basis of the electrocatalytic activity of the modified surface.

1. Introduction

Nickel hydride batteries using hydrogen storage alloys as a negative electrode material have recently attracted great attention. Batteries of this kind are expected to have several advantages over the con- ventional lead-acid and nickel-cadmium batteries; e.g. high energy density, high-rate capability, toler- ance to overcharge and overdischarge, lack of poi- sonous heavy metals and no electrolyte consumption during the charge-discharge cycles [1, 2]. The charac- teristics of the batteries could be changed arbitrarily by designing the composition of the hydrogen storage alloys. The purpose of this study is to improve the characteristics of the negative electrodes by mixing MmNi3.6Mno.4Alo.3Coo. 7 alloy particles with various metal oxides. The mode of action of the metal oxides was investigated by electrochemical techniques.

2. Experimental details

An ingot of MmNi3.6Mno.4A10.3Coo.7 alloy, prepared by an arc-melting technique was pulverized mechani- cally to a particle size of 20-63 ~m. The powder of this alloy (112 mg) was mixed with various metal oxides. The mixture was filled in a porous nickel substrate together with a small amount of polyvinyl alcohol solution as a binder, followed by vacuum drying at 120 °C and by pressing at a pressure of 160 kg cm -e. The negative electrode thus prepared was set in the central part of the three-compartment cell made of Pyrex glass. The two counter-electrodes used were the

same nickel positive electrode (Ni(OH)~/NiOOH) as that employed in commercial nickel-cadmium batter- ies. The capacity of the counter-electrodes was designed to be sufficiently large so that the cell capacity was determined by that of the negative electrode. The mer- cury oxide electrode (Hg/HgO/6M KOH) was used as a reference electrode.

The electrochemical characteristics of the negative electrode were examined by using a charge/discharge unit (Hokuto Denko, HJ-201B type) combined with a high impedance recorder (Nichia, ER-6 type). These measurements were made at 30 °C and 1 atm unless otherwise noted. The negative electrode was charged for 2.5 h at 20 mA and after resting for 10min was dis- charged at 20mA to - 0 . 5 V vs. Hg/HgO. Prior to each electrochemical measurement, the negative elec- trode was activated by repeating the charge and dis- charge cycles 30 times. High-rate dischargeability was determined from the ratio of high-rate capacity at 200 mA to total capacity, which was assumed to be the sum of the high-rate capacity and the additional nor- mal-rate capacity at 20 mA. The electrocatalytic activity for the hydrogen electrode reaction (HER) was evalu- ated by the exchange current density (Jo) of the modified negative electrodes. Jo was determined from the polarization resistance measured by the potential sweep method (1 mV s -1) after charging for 15 rain at 20 mA [3].

J/~1 = J o V / R T (1)

AC impedance measurement was carried out using a frequency analyzer (NF Electronic Instruments 5010A type) in conjunction with a potentiostat (Hokuto

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C. lwakura et al. / Metal oxide-modified hycb'ogen storage alloys 153

Denko, HA-501). The frequency was scanned step-wise in the range between 0.01 and 10000Hz under the potentiostatic conditions.

Pressure composition isotherms of the MmNi3~,- Mno.4Alo.~Co. 7 H z, system were measured by the Siev- ert method.

3. Results and discussion

The discharge capacity of negative electrodes modified with 20 wt.% metal oxides is shown in Fig. 1 as a function of the cycle number. The discharge capac- ity based on the pressure-composition isotherms of the MmNi~6Mn0.4Al03Co0.7-H2 system was calculated to be about 270 mAh g ~ [4]. After repeating charge and discharge 30 times, the saturated value of the discharge capacity experimentally determined for the unmodified negative electrode was found to be 200 mAh g ~. This saturation capacity is far less than the calculated value mainly because of insufficient current collection. Modi- fication of the negative electrodes with La20~, fl- MnO~, NiO and A1203 powders caused an unfavorable influence as indicated by the capacity reduction (100- 170 mAh g ~). This fact indicates that utilization efficiency of the hydrogen storage alloy decreased by mixing with metal oxides having low electric conductiv- ities. In contrast, the discharge capacity increased sig- nificantly up to 240-260 mAh g ~ with modification by RuO~. Co304 and CoO powders, indicating that the surface condition of the hydrogen storage alloys is very important for improving the characteristics of the nega- tive electrodes.

The steady-state charging potentials of the modified negative electrodes measured as a function of the cycle number revealed that modification with RuO2 de- creased the overpotential even at the first cycle. The electrocatalytic activity for the hydrogen electrode reac- tion (HER) would be strongly affected by modification of the alloys with metal oxides. To evaluate the electro-

300

2oo

~ iO0 ~ RuOa -~ A12%

~" ~ CO304 ~ NIO --m-- COO --0- L0203 -o-- Unmodified ~ B-Mn02

0 l I

i0 20 30

Cycle number

Fig. 1. Activation profiles of MmNi3.~,Mno4A10 ~Coo v electrodes modified with metal oxides (20 wt.%).

catalytic nature of the modified negative electrode, the exchange current density (Jo) was measured: the value of Jo was increased by modification in the order of CoO < Co~O4 < RuO~.

The relationship between high-rate dischargeability and the exchange current density for HER is shown in Fig. 2. The high-rate dischargeability is a very important characteristic from a practical viewpoint. The modifica- tion with RuOe significantly increased the value of Jo. The asymptotical dependence of high-rate dischargeabil- ity on Jo indicates that the rate capability is controlled by the electrochemical surface reaction under the condi- tion of quite rapid hydrogen diffusion in the alloy. However, the diffusion of hydrogen becomes rate deter- mining when the value of Jo increases to a sufficiently high level. The best result of high-rate dischargeability and Jo was obtained by modification with RuO e powder.

The AC impedance technique was applied to get detailed information on the kinetics of the hydrogen electrode reaction. Impedance diagrams plotted in the complex plane generally consist of two overlapped semi- circles. Judging from the available literature [5 7], a semicircle in the high-frequency region is ascribed to the contact resistance between the alloy particles and porous nickel substrate. Another semicircle m the low-frequency region is ascribed to the charge-transfer resistance for the hydrogen electrode reaction. The modification with metal oxides having low electric conductivities decreased the double-layer capacitance, and increased both the contact resistance and the charge-transfer resistance remarkably. In contrast, it was disclosed that modifica- tion with RuO 2 powder having high clectric con- ductivity decreased both the contact resistance and the charge-transfer resistance to about half the values for unmodified negative electrode accompanying an incre- ment of the double-layer capacitance. It might be suggested that modification of the hydrogen storage

85

RuOz

8c "-" C0304

7s u ~

70 / A1203

I 65 zl~<- N10 I I

200 300 400

Jo (mA/g)

Fig. 2. Relationship between the high-rate dischargeability and the exchange current density of modilied electrodes.

154 C. lwakura et al. / Metal oxide-modified hydrogen storage alloys

alloy with R u O 2 powder not only increased the effective surface area but also improved the electrocatalytic activity.

4. Conclusions

Acknowledgment

The authors wish to express their thanks to The Asahi Glass Foundation for financial support for a part of this work.

Changes in the catalytic nature of negative electrodes by modification with powder of various metal oxides were examined. Modification with a few metal oxides (RuO2, Co3 04 and CoO) increased the discharge capacity and the high-rate dischargeability of the negative electrodes. It was elucidated that such metal oxides improved the exchange current density (Jo) for the hydrogen electrode reaction. A well-defined relationship between the high- rate dischargeability and Jo could be recognized. AC impedance analysis indicated that catalytic activity of the negative electrode might be improved by mixing the metal oxides. The high performance of the modified negative electrode was explained on the basis of improved electro- catalytic activity and reduced contact resistance as well as an increment of effective surface area.

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