On the Anomalous Behavior of the Charge Transfer Resistance of the Hydrogen Evolution Reaction at Low Overpotentials and Its Relationship with Hydrogen Absorption in Metals

A non-monotonic anomalous behavior of charge transfer resistance with applied potential during the hydrogen evolution reaction on palladium has been investigated and shown to depend on the hydrogen absorption rate by the electrode. This non-monotonic behavior, which occurs around the region where the Pd-microstructure is in thermodynamic equilibrium between the H-poor, α, and H-rich, α′, phases, cannot be captured by the usual reaction mechanism kinetic modeling. In the present paper, such non-monotonic behavior was successfully modeled thanks to the experimental estimation of the hydrogen Pd absorption rate by volumetric techniques coupled with an alternative electrochemical kinetic approach. In this approach, the actual overpotential is not referred to a fixed equilibrium potential value but to a sliding one, empirically calculated as a hidden function of the hydrogen partial pressure at the metal|electrolyte interface, which is, in turn, modulated by the applied potential itself. The results allowed the indirect estimation of the hydrogen concentration inside the Pd matrix, opening the way to an alternative electrochemical tool for investigating hydrogen absorption by metals.