Electrochemical characteristics of AB5-type hydrogen storage alloys

The kinetic behaviour of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 metal hydride, used as a negative electrode in the nickel/metal-hydride (Ni/MH) batteries, was investigated using electrochemical impedance spectroscopy (EIS) at different state of charge (SOC). Impedance measurements were performed in the frequency range from 50 kHz to 1 mHz. Electrochemical impedance spectrum of the metal hydride electrode was interpreted by an equivalent circuit including the different electrochemical processes taking place on the interface between the MH electrode and the electrolyte. Electrochemical kinetic parameters such as the charge-transfer resistance Rtc, the exchange current density I0 and the hydrogen diffusion coefficient DH were determined at different state of charge. The results of EIS measurements indicate that the electrochemical reaction activity of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 metal hydride electrode was markedly improved with increasing state of charge (SOC). The transformation α–β is probably a limiting step in the mechanisms of hydrogenation of metal hydride electrode.

Research highlightsIn AB5-type hydrogen storage alloys, Mm(Ni,Mn,Al,Co)5 alloys have now been widely used as negative electrodes of Ni–MH rechargeable batteries. However, cobalt is also the most expensive metal among the alloying compounds. It almost takes up about 40–50% of the total cost of the commercial Mm(Ni,Mn,Al,Co)5 alloy raw materials. Therefore, it seemed interesting to investigate low-Co or Co-free AB5-type alloys. For this reason, we have studied the kinetic behaviour of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 metal hydride, used as a negative electrode in the Ni–MH batteries, using electrochemical impedance spectroscopy (EIS) at different state of charge (SOC). Results of EIS measurements revealed that the electrochemical reaction activity of the LaNi3.55Mn0.4Al0.3Co0.4Fe0.35 metal hydride electrode was markedly improved with increasing state of charge (SOC). The real surface area of the electrode is determinated by EIS.