AC-IMPEDANCE STUDIES ON METAL HYDRIDE ELECTRODES

The metal hydride (MH(2)) electrode is the negative electrode in one of the most advanced secondary batteries (i.e., nickel/metal hydride). The objective of this study is to obtain insight on the mechanism of the hydriding/dehydriding reaction in the battery by using the electrochemical impedance spectroscopy (EIS) technique. An equivalent circuit for the MH(2) electrode reaction is proposed. The rate capability of charge and discharge of the MK, electrode is determined by the kinetics of the charge-transfer reaction at the alloy surface, which is mainly represented by the EIS responses in the low frequency region. Transient and pseudo-steady-state analyses (cyclic voltammetry and potential vs. current density behavior) qualitatively and quantitatively support the EIS results. EIS studies on electrodes with (i) three types of binding additives, (ii) varying amounts of active material, and (iii) two types of alloys as active materials demonstrate the usefulness of this technique for developing electrodes with the optimum composition and structure.