On the electrochemical hydrogenation of Nb: An insight into the effect of hydrogen absorption on the kinetics of the hydrogen evolution reaction

Abstract

Abstract The electrochemical hydrogenation (galvanostatic charging) and desorption (potentiostatic discharging) of Nb in an alkaline solution was investigated, using Pd as a reference model system. The phases formed during hydrogenation and those present after desorption were investigated by X-ray diffraction (XRD). The cathodic charging induced the formation of NbH0.89 and NbH2 for Nb and PdH0.6 for Pd. Nb hydrides appeared to be irreversibly formed, unlikely to the PdH0.6 that spontaneously decomposed back into Pd. The irreversible character of absorption process in Nb is likely to be related to oxide/hydroxides formation on the electrode surface in the alkaline media, which might hinder the dehydrogenation reaction. The electrochemical behavior was evaluated through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results were analyzed according to both thermodynamic and kinetic approaches. EIS results indicated that Pd and Nb have very similar hydrogen evolution reaction (HER) kinetic parameters. A theoretical development based on the different kinetic contributions to the overall charge-transfer resistance, including the absorption step s indirect contribution, is presented.