Characterization of metal hydride electrodes via microperturbation and in situ intrinsic resistance measurement

Microperturbation methods, including galvanostatic intermittent titration using applied microcurrent pulses (GIT), small amplitude cyclic voltammetry, and electrochemical impedance spectroscopy (EIS), together with in situ intrinsic (i.e., physical) resistance measurements, have been applied to investigate charge/discharge kinetics of a LaNi sub 4.4 Sn sub 0.25 metal hydride (MH) electrode as a function of cycling. The results show that electrode capacity loss was caused by reduced ability to absorb hydrogen. High frequency semicircles in the Nyquist plots may be attributed to hydrogen transition between the adsorbed and the absorbed states, and are unrelated to contact resistance between the current collector and the hydride particles, although this showed contact resistance between alloy particles and current collector. This contradicts a generally accepted interpretation of EIS data. It was difficult to obtain hydrogen diffusion resistance from EIS results when the potential change in the electrode charge-discharge plateau is smaller than the EIS voltage perturbation, because long charge/discharge times in the low frequency range change the state of discharge. Alternating current EIS is more suitable than alternating voltage studies to investigate hydrogen diffusion kinetics in MH alloys with a flat potential plateau. Similarly, it is also difficult to obtain hydrogen diffusion resistance from small amplitude cyclic voltammetry because of the size of the charge/discharge capacitance. However, the total reaction resistance may be obtained from GIT.