Hydrogen Absorption into and Subsequent Diffusion Through Hydride-Forming Metals

In most theoretical and experimental investigations, it has been assumed that the rate-determining step (RDS) of hydrogen insertion (intercalation, ingress, cathodic charging/injection/introduction) into and desertion (deintercalation, egress, anodic extraction) from hydride-forming electrodes is hydrogen diffusion through the electrode. In practice, however, the rate of hydrogen insertion into and desertion from the electrode is simultaneously determined by the rates of two or more reaction steps, such as hydrogen ion transport through the electrolyte by diffusion and migration (ohmic potential drop), interfacial charge (electron) transfer (cathodic discharge of hydrogen ions), interfacial hydrogen transfer, and subsequent hydrogen diffusion through the electrode [1]. The RDS of the series-connected overall hydrogen insertion reaction is defined as the most strongly impeded/disturbed “slowest” step deviating far from its thermodynamic equilibrium state that represents the highest hydrogen overpotential and/or impedance pertaining to the step. In this respect, the mechanism of hydrogen insertion into and from a hydride-forming electrode has been extensively studied.