Effect of the stoichiometric ratio on electrochemical properties of hydrogen storage alloys for nickel-metal hydride batteries

Acto. Vol. 40. No. 7. pp. 845-848, 1995 Copyright G 1995 Ekvier Science Ltd Pruned in Great Britam. All rights reserved Ml3-4686/95 19.50 + 0.00

Electrochmico

Pergamon

EFFECT OF THE STOICHIOMETRIC RATIO ON ELECTROCHEMICAL PROPERTIES OF HYDROGEN STORAGE ALLOYS FOR NICKEL-METAL HYDRIDE BATTERIES YUKIO FUKUMOTO, MASAYOSHI MIYAMOTO, MASAO MATSUOKA and CHIAKI IWAKURA* Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 593, Japan (Receioed 4 July

1994; in reoised form

18 October 1994)

Abstract-Effect of stoichiometric ratio on the electrochemical properties of negative electrodes was investigated for alloys with composition Mm(Ni,,,Mno.4Al,,,Co~,,), (Mm = misch metal, 0.88 < x < 1.12). The discharge capacity at a current density of 0.2Ag-’ increased with an increase in unit cell volume of the alloys in the range of x = 0.96-1.12. The discharge eficiency increased with increasing x value, and was about 85.2% even at a high current density of ca 2Ag-’ in the case of x = 1.12. The improved high-rate dischargeability was explained on the basis of high diffusibility of hydrogen in the bulk of alloys as the result of the relatively low stability of the hydride, in addition to the high electrocatalytic activity for the hydrogen electrode reactions of the negative electrode. Key words: hydrogen storage alloy, stoichiometry, discharge eficiency, exchange current density, nickel metal hydride battery.

1. INTRODUCTION

With respect to the recent increasing demand for the high performance of secondary batteries and for environmental protection, nickel-metal hydride batteries using a hydrogen storage alloy as a negative electrode material have drawn much attention, because they have several advantages over the conventional secondary batteries eg higher energy density, durability against overcharge and overdischarge, compatibility with a nickel-cadmium battery, cleanliness and freedom from poisonous heavy metals[l]. Under the situation, extensive studies on the nickel-metal hydride batteries have been carried out with respect to the composition of the alloys[2-41, and surface modifications[5, 63 and so on. Nogami et aI.[2, 33 first reported that nonof stoichiometric alloys with a composition showed high& discharge Mm(NiMnAlCo),.,,_,.,, caoacitv. Notten et aI.r41 also found that hieh-rate capabiity of Lao.sNdo:2Ni,.,Coz,,Sio,~ el&trode was greatly improved by deviating the stoichiometric ratio of the alloy to the nickel rich side. Therefore, further characterization of nonstoichiometric alloys seems to be very significant for improving the performance of negative electrodes in nickel-metal hydride batteries. In this study, as a part of fundamental studies on the nickel-metal hydride batteries, misch metal based multicomponent alloys with the stoichiometric of composition and nonstoichiometric ’ Author addressed.

to

whom

all

correspondence

should

Mm(Ni,,,Mn,.,Al,,,Co~,~)~ (0.88 < x < 1.12) were selected and their crystallographic parameters and electrochemical properties were investigated.

2. EXPERIMENTAL The hydrogen storage alloys with composition Mm(Ni,.,Mn,,,A1,,,Coo.,), (0.88 < x < 1.12) were prepared via the arc melting method in an argon atmosphere. The composition of the misch metal (Mm) used was 52.56% Ce, 24.81% La, 5.51% Pr, 16.86% Nd and 0.14% Sm. As-prepared hydrogen storage alloys without any annealing treatment were pulverized mechanically into powder in an argon atmosphere. The powder of the alloys was sieved to 106-125 /1m 4 or less than 20 p m #J. The former particles were used for the electrochemical measurement, the latter particles for the X-ray diffraction measurements. Pressure-composition isotherms for hydrogen absorption and desorption were measured by the Sieverts’ method. In activation cycle, after heating the reactor filled with the alloy powder at 150°C in vacuum for 30min, hydrogen gas was introduced into the reactor, and then the reactor was cooled by water bath. This cycle was repeated three times. Crystallographic characterization of the hydrogen storage alloys was carried out by using an X-ray diffractometer(Rigaku, RINT-1100, CuKa). Lattice parameters (a, c) and unit cell volume of the hydrogen storage alloys were then calculated by using the following equations,

be

sin20 = (nZ/4)[{4(h2 + hk + 845

k2)/3a2}

+ Iz/cz]

(1)

846

Y. FIJKUMOTO et al.

and v = &l2c/2

(2)

where 1 is the wavelength of CuKcr, 0 is Bragg angle and h, k and 1are Miller indices. The working electrodes, consisting of the abovementioned hydrogen storage alloys, were prepared by the following procedureC5, 61. The alloy particles (1OOmg) were mixed with 2wt% polyvinyl alcohol (PVA) as a binder and loaded in a porous nickel substrate (1.0 x 2.5 cm). After drying at 120°C in vacuum for 1 h, the alloy-loaded substrate was covered with a porous nickel sheet, and then pressformed at 1200kgcmm2. Electrochemical measurements were carried out at 30°C using an unpressurized glass cell with three compartments separated by a sintered glass. The working electrode was positioned in a central compartment and two counter nickel electrodes having electrochemical capacity much larger than that of the working electrode were installed in the other compartments. The reference electrode was a Hg/Hg0/6M KOH electrode kept at room temperature. The electrolyte was a deaerated 6 M KOH solution. In charge-discharge cycle tests, the negative electrode was charged at 20 mA for 2.5 h and discharged at the same current to -0SV vs Hg/Hg0/6M KOH. After each charging, the circuit was kept open for 10min. Such cycle tests were carried out by use of the galvanostatic charge-discharge unit (Hokuto Denko, HJ-lOlM6, HJ-201B) and the potential of the negative electrode was recorded by using a multichannel recorder (Yokogawa, HR1300). The electrochemical capacity was calculated from discharge time and expressed in mAh per gram of hydrogen storage alloy. The electrocatalytic activity for the hydrogen electrode reaction was evaluated by a potential sweep method[4]. The negative electrode was charged at 20 mA for 15 min, and the circuit was kept open for lOmin, then the potential was swept at a rate of 1 mV s-l using a potentiostat with the aid of potential scanning unit (Hokuto Denko, HAB151). The resulting voltammograms were recorded by using an X-Y recorder (Yokogawa, 3036). An apparent exchange current densty (Jo), which might be a measure of catalytic activity of electrodes, was calculated from the slope of polarization curves by the following equation, J, = (B 7-/FXJIrl),*,

(3) where F is the Faraday constant, R and T have their usual meanings. In measuring the discharge efficiency, the activated negative electrode was charged at 20 mA for 0.5 h, and the current was interrupted for lOmin, and then the electrode was discharged at various current densities (0.05-2 A g- ‘). The discharge efftciency was determined from the ratio of the discharge capacity measured at different current densities to the total charged capacity (10 mA h).

3. RESULTS

AND DISCUSSION

Discharge capacities of the negative electrodes consisting of Mm(Ni,,,Mn,,,Al,,,Co,,,), alloys with

‘; 0

300

x=0.96 x=1.00

x=0.68

2

g

200

x=1.12

8 a w

-0

10

20

30

40

Cycle number Fig. 1. Activation profiles of Mm(Ni,,6Mn,.,A1,,,Co~,,), electrodes.

four different compositions (x = 0.88, 0.96, 1.00 and 1.12) are shown in Fig. 1 as a function of cycle number. The use of alloys with smaller x value facilitated activation of the negative electrode, and activation of the negative electrodes in the range of 0.88 6 x < 1.00 was almost completed within lstdth cycle. Although the Mm-rich nonstoichiometric alloys (x = 0.88, 0.96) showed relatively high discharge capacity (ca. 215mA hg-‘) even at the first cycle, the discharge capacity of the x = 0.88 negative electrode decreased gradually. In the case of the x = 1.12 negative electrode, however, it took cycles more than 30 to complete the activation. The discharge capacity of the negative electrode and the unit cell volume of the alloys are shown in Fig. 2 as a function of the x value in Mm(Ni,,,Mn,,,Al,,,Co~,,), . The discharge capacity increased with decreasing x value and passed through a maximum (about 285 mA h g-i) at around x = 0.96. The unit cell volume also increased with decreasing x value, because the misch metal had a larger ingredient radius, and it passed through a maximum at around x = 0.92. All alloys used in this study showed the X-ray diffraction peaks corresponding to the structure of CaCu, type and

88.5

300 ‘; 0 fE 250 $ 09. 200 8

87.5

& 2 150

87.0

5: ._ 0 100

86.5 0.85 0.90 0.95 1.00 1.05 1.10 1.15 Composition x

Fig. 2. Discharge capacity (closed circle) and unit cell volume (open circle) as a function of alloy composition x in Mm(Ni,.,Mn,.,Al,.,Co~,,),.

847

Effect of the stoichiometric ratio on nickel-metal hydride batteries

50

I

’

1.0

0

2.0

Discharge current density I A 9-l Fig. 3. Discharge efficiency as a function of the discharge current density for Mm(Ni,,,Mn,,,Al,,,Co~,,), electrodes.

x = 0.88 alloy alone showed additional peaks to be attributable to the Ce,Ni, type structure[3]. In spite of the formation of intermetallic compounds such as Ce,Ni,, the unit cell volume decreased at x = 0.88. As shown in Fig. 2, the changes in the discharge capacity at about 0.2A g- ’ were correlated to the changes in the unit cell volume[3]. Discharge efficiencies are shown in Fig. 3 as a function of discharge current density. When the discharge rate was low, the discharge efficiencies were almost the same, irrespective of alloy composition. In contrast, they increased with increasing x value when the discharge rate became high. At the discharge current density of ca. 2Ag-‘, the discharge efliciency was about 66.8% for the stoichiometric alloy electrode, but was about 85.5% for the Mm-poor nonstoichiometric alloy electrode x = 1.12. In the case of the x = 0.88 Mm-rich nonstoichiometric electrode, the discharge efflency was quite low compared with any other case. Pressure-composition isotherms measured at 30°C for hydrogen desorption in the Mm(Ni,., Mn,,,Al,,,Co,,,),H system are shown in Fig. 4. The equilibrium hydrogen pressure decreased with decreasing x value. This means that the Mm-rich nonstoichiometric alloys produce the more stable

100

1

o.8 I

I

t

hydrides than the Mm-poor nonstoichiometric alloys because the unit cell volume increases with increasing Mm content. In the case of the alloy having composition of x = 0.88, the equilibrium hydrogen pressure is relatively low in spite of the decreased unit cell volume. It is caused by the fact that the alloy contains intermetallic compounds such as Ce,Ni, which forms exceedingly stable hydride and thus contains less amounts of bulk x u 1.00 phase which works as actual hydrogen absorbing and desorbing alloy. The hydrogen storage capacity increased with decreasing x value, and passed through a maximum at around x = 0.96. The exchange current density (Jo) for the hydrogen electrode reaction was measured by the potential sweep method to evaluate the electrocatalytic activity of the negative electrodes. The measured exchange current densities are shown in Fig. 5 as a function of x value. J, increased with increasing x value. It has been reported by Notten et al.[4] the exchange current density of that alloy electrode was higher Lao.sNd,.zNi3,Co,,,Si~.~ than that of La,,Nd,,,Ni,,,Co,,,Si~.~ alloy electrode. It has also been reported by Sawa et a/.[73 that the instability of hydride in the vanadium and nickel-rich nonstoichiometric Zr-V-Ni Laves-type alloys resulted in the improved dischargeability. It was found also in the present work the Mm-poor nonstoichiometric alloy showed the improved polarization characteristics as a result of the formation of unstable hydride. The alloy with the composition of x = 1.12 exhibited the highest electrocatalytic activity for the hydrogen electrode reactions in our experiment. These results are indeed in agreement with the higher discharge efficiencies obtained with alloys with higher value for x (see Fig. 3). During the discharge process, either hydrogen diffusion in bulk of alloy or the charge transfer at the surface of the negative electrode should be the ratedetermining step in hydrogen electrode reactions[S]. Since the alloy with the composition of x = 0.88 forms more stable hydride, it is supposed that the rate of hydrogen diffusion is relatively slow, resulting in the shortage of hydrogen involved in the discharge reaction. It is the reason why this negative

0.85 0

0.2

0.4

0.6

0.8

HIM

Fig. 4. Pressure-composition desorption isotherms Mm(Ni,,,Mn,,,Al,,,Co~,,), at 30°C.

090

0.95

1.00 1.05

1.10

1.15

Composition x

1.0 for

Fig. 5. Exchange current densities for hydrogen electrode reaction as a function of alloy composition x in Mm(Nij.,MnO.,AlO,,CoO.,),.

848

Y.

FuKuMOro

electrode material showed the low discharge eniciency in high-rate discharge. On the other hand, the high electrocatalytic activity and the high discharge efficiency could be obtained because of the formation of relatively unstable hydride in the case of the alloy with the composition of x = 1.12. Acknowledgements-This work was partly supported by a Grant-in-Aid for Developmental Scientific Research no. 05555172 from the Ministry of Education, Science and Culture of Japan.

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et al.

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