Microstructure and hydrogen absorption behavior of Zr0.9Ti0.1Mn0.66V0.46Ni1.1 under electrochemical and gaseous media conditions

H.A. Peretti^†, A. Visintin^‡, H.L. Corso^†, A. Bonesi^‡ and W.E. Triaca^‡

^† Centro Atómico Bariloche, Comisión Nacional de Energía Atómica, C.C. 439, (8400) San Carlos de Bariloche, Argentina. peretti@cab.cnea.gov.ar
^‡ Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Suc. 4, C.C. 16, (1900) La Plata, Argentina. avisintin@inifta.unlp.edu.ar

Abstract&— Rechargeable alkaline batteries of nickel-metal hydride have been widely studied due to the interest in replacing the cadmium electrode by metal hydride electrodes of low or null environmental impact. Recent developments include Laves phases based on ZrCr2 with multiple substitutions to improve electrode performance. In this work, results are presented on the electrochemical behavior of the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1, its structural and morphologic characterization and its behavior under gaseous hydrogen absorption- desorption cycles. The pressure-composition-temperature (PCT) curves show a high hydrogen storage capacity (H/M ~ 3.4, where H/M = x is the number of H atoms per alloy formula unit in the metal hydride MH_x). It is also found a steep slope in the PCT isotherms, instead of a horizontal plateau corresponding to the two-phase equilibrium. Electrode activation was studied by voltammetric cycles, while the electrochemical capacity of hydrogen absorption was determined by galvanostatic measurements. For comparison, the Zr_0.9Ti_0.1CrNi alloy was also studied. The discharge capacities found are about 330 mAh/g for both alloys, but the activation is achieved faster for Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 than for Zr_0.9Ti_0.1CrNi.

Keywords&— Batteries; metal hydrides; hydrogen storage; electrochemistry; over-stoichiometry.

I. INTRODUCTION

In the last years there has been a large increase in the use of rechargeable alkaline batteries for domestic uses. The nickel-cadmium battery is among the most utilized batteries. Due to the toxicity of Cd, the necessity arises of replacing the Cd electrode by another one leading to no environmental pollution.

Bearing this in mind and due to the demand of high performance energy sources, rechargeable batteries of Ni-metal hydride (Ni-MHx) have been developed and commercialized. With these batteries, the following characteristics are looked for: high energy density, high power density without loss of rate capability, long charge-discharge cycle life and better environmental compatibility.

This type of batteries uses as negative electrode a hydrogen storage alloy, in which the hydrogen atoms are incorporated to the solid by the reversible formation of a hydrided phase. In current commercial batteries the utilized alloys are of the AB5 type, based on the compound LaNi5 with variations in the composition.

Recently, the interest in the study of Zr based Laves phase metal hydrides, i.e. AB2 type intermetallic compounds, where A = Zr and B = Ni, V, Mn, Cr, etc., has increased due to their higher hydrogen storage capacity and, hence, higher electric charge of the battery. However, in the initially studied alloys, the power density was not comparable to the AB5 type. This situation encouraged research to optimize their composition (Kim et al., 1998a, b; Klein et al., 1998).

The activation of the alloy plays a fundamental role in the absorption electrodic process, since it defines the reaction rate of the hydrogen with the metal and the incorporation to its structure. During activation, several different processes occur (Sastri et al., 1998; Anani et al., 1994; Visintin et al., 1996), such as: i) reduction of surface oxides that interfere with hydrogen, ii) reduction of particle size due to cracks produced by the volume increase, iii) changes in the chemical composition and/ or surface structure of the metal.

In this work, a technique is presented allowing for the acceleration of activation rate by the utilization of cyclic voltammetry in combination with charge-discharge galvanostatic cycles, which makes possible the determination of the real hydrogen storage capacity of the electrodes. This technique was applied to the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy of over-stoichiometric composition, equivalent to the one optimized by Kim et al. (1998a). Furthermore, a characterization of this alloy is carried out from the metallurgical point of view and from the behavior of hydrogen absorption in gaseous phase.

The same electrochemical technique was also applied to the Zr_0.9Ti_0.1CrNi alloy for comparative and complementary purposes regarding the results presented by Visintin et al. (1998)

II. EXPERIMENTAL

The alloy samples were prepared by arc melting adequate proportions of the constituent elements of purity better than 3N, under high purity inert atmosphere in a cooled copper hearth. The small button-shaped ingots (» 20 g) were turned over and remelted at least once in order to get good homogeneity. The obtained alloys of the AB₂ type, of compositions Zr_0.9Ti_0.1CrNi and Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1, were used without any subsequent thermal treatment.

The electrodes were prepared by cold pressure of the powders using equal parts by weight of alloy (75 mg, particle size < 105 m m) and of Teflon treated carbon (Vulcan XC-72). A Ni wire was used as current collector. The electrochemical measurements were carried out by using a 7 M KOH solutions at 30 °C and a Hg/HgO reference electrode.

To perform the experiments the conventional galvanostatic charge-discharge technique was used, in which the alloy electrodes are placed into an electrochemical cell, subjecting them to successive charge-discharge cycles by means of a cycling apparatus conceived and constructed in the laboratory. The charge is achieved by a cathodic current of C/2 during a period of time long enough to overpass 20% of the total capacity, and the discharge by an anodic current between 2 and 6 mA down to a potential of -0.6 V.

In order to accelerate the activation processes the electrodes were subjected to successive voltammetric cycles by means of a Princeton Applied Research Potentiostat/ Galvanostat, model 273, keeping the anodic and cathodic limit potentials constant and the cycling velocity at 0.001 V/s. To determine the capacity of the electrode every 50 voltammetric cycles, a galvanostatic charge-discharge cycle was carried out in the same conditions as indicated above, with a discharge current of 2 mA.

The crystal structure of the alloys was investigated by X-ray diffractometry (XRD), while the microstructure morphology was characterized by using scanning electron microscopy (SEM) and microanalysis from energy dispersive spectroscopy (EDS).

The behavior of the alloy regarding gaseous hydrogen storage was investigated by measuring pressure-composition-temperature (PCT) curves by means of the classical volumetric technique, using a Sievert type apparatus made up at the laboratory (Peretti et al., 1996).

III. RESULTS

In order to get the alloy activated by using the galvanostatic conventional charge-discharge technique, a large number of cycles is needed (Bonesi et al., 1999). The periodic potential treatments increase the capacity and improve the kinetic properties of the electrodes in comparison with the galvanostatic charge-discharge conventional methods (Visintin et al., 1998). By using the cyclic voltammetric technique, the activation processes of the AB₂ type alloys were evaluated for the hydrogen absorption within a KOH electrolytic solution at 30 °C. The storage capacity was determined by the galvanostatic technique under the previously described conditions.

By comparing the alloys of different compositions, as shown in Fig. 1, it is observed that the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy reaches its maximum capacity at 25 voltammetric cycles, while the Zr_0.9Ti_0.1CrNi alloy requires 175 cycles.

Analyzing a complete charge- discharge cycle (Fig. 2), by following the potential we can observe the higher capacity of the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy with respect to that of the Zr_0.9Ti_0.1CrNi alloy, which is attained after a smaller number of cycles and lower overpotentials, indicating that the processes are more reversible.

Figure 3 shows the electric potential of Zr_0.9Ti_0.1Mn_0.66 V_0.46Ni_1.1 as a function of time, presenting its maximum capacity at 25 voltammetric cycles. At the end of 200 cycles, processes of electrode degradation have taken place.

Figure 1. Capacity percent against voltammetric cycles. v: 1mV/s, E(lower): -1.4 V, E(upper): -0.4 V, 7M KOH, 30 °C.

Figure 2. Charge- discharge cycle at constant current after voltammetry. I(charge): 6 mA, I(discharge): 2 mA. Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1: 25 voltammetric cycles. Zr_0.9Ti_0.1CrNi: 50 voltammetric cycles. 7M KOH, 30 °C.

From the XRD results of the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy (Fig. 4), only a single hexagonal phase can be identified corresponding to a C14 type structure. The lattice parameters determined from the diffraction angles are: a = 4.992 Å and c = 8.153 Å. These values differ in less than 0.7 % from the ones presented by Kim et al. (1998a) for the same nominal composition and are consistent with the fact that no peaks are detected corresponding to other phases.

Figure 3. Discharge at constant current after voltammetry. I(discharge) 2 mA, 7M KOH.

Figure 4. XRD of Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 obtained with Ka (Cu) radiation.

In the SEM image of Zr_0.9Ti_0.1Mn_0.66 V_0.46Ni_1.1 (Fig. 5), regions of different contrast can be observed. Results of EDS microanalysis on clear zones reveal a composition increase of 22 at. % for Zr, 94 at. % for Ti and 48 at. % for Ni with respect to the nominal values, at the expense of a depletion of V and Mn. The resulting atomic proportions of Zr, Ti and Ni within these regions indicate the presence of a compound of approximate composition Zr_0.8Ti_0.2Ni, which would be a variant of the intermetallic compound ZrNi with partial substitution of Zr by Ti, in very small quantities as to be clearly detected by X-rays. In the rest of the microstructure (grey zone), the detected composition is consistent with the nominal values. These results indicate the existence of microsegregation of alloying elements.

Figure 5. SEM image of Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 microstructure.

Figure 6. Pressure-composition-temperature (PCT) curves of Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1.

The (PCT) curves obtained at 100, 140 and 180 °C are shown in Fig. 6, together with the isotherm at 30 °C determined for the same alloy by Kim et al. (1998a).

We can see that the measured isotherms present a steep slope with a slight plateau tendency similar to the one reported (Kim et al., 1998a). Saturation for the isotherm at 100 °C occurs at hydrogen pressures above 2000 kPa for a value of H/M ~ 3.4, while at 500 kPa we have H/M ~ 3.2 .

IV. DISCUSSION

From the electrochemical measurements, we obtained a storage capacity of 324 mAh/g for Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 and of 330 mAh/g for Zr_0.9Ti_0.1CrNi, practically of the same order for both alloys. However, the activation of the former is achieved faster, indicating that the substitution of Cr by Mn and V leads to an acceleration of the activation processes, possibly due to the formation of oxides that can be reduced more easily.

The fact that the measured charge capacity C for the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy as well as the maximum H/M values given by the (PCT) curves resulted smaller than the ones reported by Kim et al., (1998a) (C = 392 mAh/g and H/M = 3.6), can be ascribed to possible differences in the alloy composition. This assumption is supported by the small differences observed in the lattice parameters. As known, the behavior of this type of alloys is very sensitive to small variations of contents of the alloying elements.

Regarding the microsegregation of elements observed by SEM in Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1, this is an effect occurring during solidification, usually observed in alloys without any further homogenization thermal treatment. This effect is more likely to be expected for alloys with a higher number of alloying elements and undergoing rapid solidification. These are just the conditions given in our case as well as in the case of Kim et al. (1998a), where an electric arc furnace was employed to melt the alloys. A similar microsegregation effect was observed in the Zr_0.9Ti_0.1CrNi alloy without thermal treatment, as reported in Visintin et al. (1998).

V. CONCLUSIONS

The cyclic voltammetric technique combined with galvanostatic charge- discharge cycles accelerates activation processes and makes it possible to know the real hydrogen storage capacity (i.e. electric charge) of electrodes made from Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 and Zr_0.9Ti_0.1CrNi Laves phase AB₂ type alloys. The corresponding charge capacities determined are 330 and 324 mAh/g, respectively.

The substitution of Cr by Mn and V produces an acceleration of the activation processes, possibly due to the formation of oxides that can be reduced more easily.

The pressure-composition isotherms of the Zr_0.9Ti_0.1 Mn_0.66V_0.46Ni_1.1 alloy determined at temperatures of 100, 140 and 180 °C, present the same characteristics of the isotherm at 30 °C reported in the bibliography regarding the absence of plateau. This feature can be attributed to the microsegregation of alloying elements occurring during solidification, since in both cases the alloys have not been subjected to homogenizing thermal treatments after melting.

ACKNOWLEDGMENTS

This work has been partially funded by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) of Argentina, the Comisión de Investigaciones Científicas (CIC) of the Province of Buenos Aires, the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), and the Cooperativa de Electricidad Bariloche (CEB). The authors acknowledge the collaboration rendered by Julio Andrade Gamboa, Ernesto Scerbo, Carlos Cottaro, Osvaldo Cartelli and Ernesto Aranda.

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Received: May 11, 2001.
Accepted for publications: June 7, 2002.

Recommended by Subject Editor A. L. Cukierman and Guest Editors E. L. Tavani and J. E. Perez Ipiña.