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 behaviorof charge transfer resistancewith applied potential during the hydrogen evolution reaction on palladiumhas been investigated and shown to depend on the hydrogen absorptionrate by the electrode. This non-monotonic behavior, which occurs aroundthe region where the Pd-microstructure is in thermodynamic equilibriumbetween the H-poor, & alpha;, and H-rich, & alpha;& PRIME;, phases,cannot be captured by the usual reaction mechanism kinetic modeling.In the present paper, such non-monotonic behavior was successfullymodeled thanks to the experimental estimation of the hydrogen Pd absorptionrate by volumetric techniques coupled with an alternative electrochemicalkinetic approach. In this approach, the actual overpotential is notreferred to a fixed equilibrium potential value but to a sliding one,empirically calculated as a hidden function of the hydrogen partialpressure at the metal|electrolyte interface, which is, in turn, modulatedby the applied potential itself. The results allowed the indirectestimation of the hydrogen concentration inside the Pd matrix, openingthe way to an alternative electrochemical tool for investigating hydrogenabsorption by metals.