Effect of high and low temperature on the electrochemical performance of LaNi4.4−xCo0.3Mn0.3Alx hydrogen storage alloys

Different electrochemical measurements such as activation and maximum capacity, cyclic stability, self-discharge property, high-rate dischargeability and electrochemical impedance spectroscopy have been carried out to investigate the electrochemical performances of LaNi4.4−xCo0.3Mn0.3Alx hydrogen storage alloys at low and high temperatures. We have selected 273 K and 323 K as low and high temperatures respectively. Samples W, X, Y and Z were used to represent alloys LaNi4.4Co0.3Mn0.3, LaNi4.3Co0.3Mn0.3Al0.1, LaNi4.2Co0.3Mn0.3Al0.2 and LaNi4.1Co0.3Mn0.3Al0.3. XRD patterns show the alloys are all single-phase alloys with hexagonal CaCu5 type structure. The highest maximum discharge capacity is 340.3 mA h/g for alloy X, which is higher than other alloys at same temperature and high temperature. After 50 charge/discharge cycles, capacity retention of 93.6% has been observed at low temperature for alloy Z. The least capacity loss was 13.3% at 273 K during the self-discharge test. At both studied temperatures, alloy Z has the highest high-rate dischargeability (HRD) of 56.6% and 44.9% respectively at a discharge current density of 1800 mA/g. The electrochemical impedance spectroscopy (EIS) analysis was carried out to determine the charge-transfer resistance (Rct) which was further used in the analysis of the HRD. Alloy Z with the highest HRD at 273 K has a lower Rct of 0.9272 mΩ due to the dissolution or presence of Al into the electrolyte which can effectively reduce the total resistance of alloys. Apparently, at the two different testing temperatures, all the alloys possess better electrochemical properties at 273 K than 323 K.