Surface coverage-limited hydrogen uptake into nickel under cathodic charging

We investigate hydrogen (H) uptake into 50 mu m-thick pure nickel (Ni) foils under cathodic charging in 1 M H2SO4 solution. We measure H concentration using melt extraction as a function of charging time and overpotential. For every overpotential, H concentration reaches a time-independent, equilibrium value after ~8 h of charging. The measured H concentrations are systematically lower than expected for the applied overpotentials and approach an upper bound asymptotically as a function of charging current density. To rationalize these findings, we propose that H uptake is surface H coverage-dependent and reaches an upper limit when the sample surface becomes saturated with H. Saturation of surface coverage, in turn, occurs when the dominant H evolution re-action (HER) pathway switches from Volmer-Tafel to Volmer-Heyrovsky.