Function of cobalt in the new type rare-earth Mg-based hydrogen storage electrode alloys

The role of cobalt on the structural and electrochemical properties of the La0.7Mg0.3Ni3.4−xMn0.1Cox (x=0-1.15) hydrogen storage alloys were investigated systematically. XRD and Rietveld analyses show that all alloys consist mainly of the (La,Mg)Ni3 phase and the LaNi5 phase. P-C isotherms illustrate that the equilibrium pressure for hydrogen decreases monotonically with increasing x. Electrochemical studies indicate that the maximum discharge capacity first increases from 397.5 (x=0) to 403.1 mA h/g (x=0.75) and then decreases to 388.2 mA h/g (x=1.15). Moreover, with increasing Co substitution, the cycling durability of the alloy electrodes was markedly improved due to the decrease in the pulverization of the alloy particles and the corrosion of La in KOH solution. The high rate dischargeability, electrochemical impedance spectrum, linear polarization, Tafel polarization and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases and then decreases with increasing Co content.