A study of the hydrogen absorption reaction into α- and β-LaNi5Hx porous electrodes by using electrochemical impedance spectroscopy

The hydrogen absorption reaction into α- and β-phase LaNi5Hx porous electrodes in 6 M KOH solution has been investigated as a function of applied potential by analysing the a.c. impedance spectra complemented with cyclic polarization curves. The onset of the hydrogen evolution reaction (HER) can be accounted for in terms of the transition of the Warburg impedance to a capacitive loop at −0.980 V for the α-phase porous electrode and at −0.920 V for the β-phase porous electrode. The charge-transfer resistance, Rct, values from the β-LaNi5Hx electrode are much smaller than those from the α-LaNi5Hx electrode over the whole applied potential range. The α- to β-phase transition at −0.980 V and the β- to α-phase transition at −0.920 V are discussed in terms of the applied potential dependence of Rct for the hydrogen absorption reaction.