Investigation of the influence of nickel content on the correlation between the hydrogen equilibrium pressure for hydrogen absorbing alloys and the capacity of MH electrodes in open and closed cells

Abstract.

Two series of overstoichiometric AB_2.4 alloys [(Zr_0.35Ti_0.65)(V_1.33Cr_0.4Fe_0.27)_{2– x} Ni_{0.4+ x} and (Zr_0.5Ti_0.5)(V_0.8Mn_0.8Cr_0.4)_{2– x} Ni_{0.4+ x} ], differing in the Zr to Ti ratios in group A and the presence of Mn or Fe in group B, were examined to consider the influence of various amounts of nickel on the structural parameters following the sorption of hydrogen. To predict the electrochemical behaviour of the prepared alloys as negative electrodes for Ni-MH cells, the pressure-composition isotherms (PCT) determined for the gas/solid phase system were correlated with the electrochemical pressure-composition (EPC) isotherms estimated from the rest potential of the alloy electrode. For preliminary assessment of the practical usefulness of the prepared samples, the electrodes made of all the alloys were subjected to charge/discharge measurements in a half-cell in 6 M KOH solution and the discharge capacities were estimated. Of the alloys with Fe and Mn components, the samples (Zr_0.35Ti_0.65)(V_0.93Cr_0.28Fe_0.19Ni_1.0) and (Zr_0.5Ti_0.5)(V_0.68Mn_0.68Cr_0.34Ni_0.7) provided the highest capacities. These alloys were chosen for testing the charge/discharge cycleability in closed Ni-MH cells. The reversibility of the cell with the former sample decreased significantly around the 25th cycle, whereas the discharge capacity of the cell with the latter sample remained almost unchanged during 100 cycles of testing.