Influence factors of capacity loss after short-time standing of metal-hydride electrode and its EIS model

Considerable irreversible capacity loss was observed after the electrode standing at small state-of-charge (SOC) for only a short term. Influence of SOC, standing time and the replacement elements of LaNi5-type hydrogen storage alloys to standing was discussed. Charge-transfer resistance, exchange current density (I0), and hydrogen diffusion coefficient were determined based on the study of electrochemical impedance spectrum (EIS), linear polarization (LP) and constant potential step (CPS), respectively. The oxidation of metal Ni on the alloy surface after standing was responsible for the rapid deterioration of capacity, charge-transfer resistance and I0. Galvanostatic, LP, EIS and CPS measurements suggested the presence of an oxide and/or corrosion layer on the alloy surface of raw material and electrode after standing at 0% SOC for 2 d. It was proved that the first small semicircle in high-frequency region of EIS was related to this layer. Novel EIS model in metal hydride electrode was proposed accordingly.