Electrochemical measurements of a metal hydride electrode for the Ni/MH battery

Abstract

The characteristics of the Ni/MH battery, including discharge voltage, high-rate discharge capability and charge/discharge cycle lifetime are mainly determined by the construction of the negative and positive electrodes and the composition of the hydrogen-absorbing alloy. The metal hydride (MH) alloy powder for the negative electrode of the Ni/MH battery was first pulverized and oxidized by electrochemically charging/discharging for a number of cycles. A multicomponent AB 5-based alloy Mm 0.95Ti 0.05Ni 3.85Co 0.45Mn 0.35Al 0.35 was developed as the hydrogen-absorbing alloy. The discharge characteristics of the negative electrode, including discharge capacity, cycle lifetime and exchange current density, were studied by means of the electrochemical experiments and analysis in the experimental cell. The polarization measurements show that the exchange current density of the MH electrode increases with increasing number of charge/discharge cycles and then it almost remains constant after 30 cycles. A microcracking activation, resulting from an increase in reaction surface area and an improvement in the electrode surface activation, decreases the polarization resistance and increases the hydrogen exchange current densities. Measurement of the hydrogen diffusivities for the Mm 0.95Ti 0.05Ni 3.85Co 0.45Mn 0.35Al 0.35 alloy shows that the ratio D/ a 2 ( D=hydrogen diffusivity; a=sphere radius) increases with increasing number of cycles up to 20 cycles and it remains constant after further cycling.