Electrochemical impedance study of discharge characteristics of Pd substituted MgNi-based hydrogen storage electrode alloys

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Quaternary Pd substituted MgNi-based hydrogen storage alloys Mg 0.9− x Ti 0.1Pd x Ni ( x = 0.04, 0.06, 0.08, 0.1) in amorphous phase were prepared by mechanical alloying (MA). They exhibited enhanced cycle life in our previous studies. However, their electrochemical impedances and its relationship with electrode reaction kinetics are still unclear. In this work, the impedance spectra of the above electrode alloys were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. We further observed that the rate-determining step changed from charge-transfer to hydrogen diffusion with proceeding of discharge according to the fitting analyses of spectra. The fitted results demonstrated that charge-transfer resistance R ct decreased and then increased with the depth of discharge (DOD) and it also increased with Pd content among the studied alloys. Thickness of surface passive film, which is proportional to the reciprocal of double layer capacitance near the electrode alloy surface according to Badawy et al., increased with the DOD and especially with Pd content in the alloys. Meanwhile, the thickened film on the surface of electrode alloys ensured its long cycle life. Through Randles plotting and comparing the variation of Warburg coefficient σ vs. DOD for the electrode alloys, it was found that σ increased with the DOD and substitution of Pd also improved the diffusion performance of the studied electrode alloys.