Abnormal Behaviors in Hydrogen Transport: Importance of Interfacial Reactions

Abstract

As described in Sect. 3.1, the redox reactions of hydrogen absorption into and desorption from hydride-forming metals and oxides proceed via one of the following two mechanisms: (1) the one-step (direct) and (2) two-step mechanisms [1–3]. In both mechanisms, the (faradaic) charge-transfer reaction on the electrode surface is the essential step for hydrogen absorption and desorption, leading to reduced and oxidized species, respectively. Many of the ac-impedance results obtained on hydride-forming metals and oxides indicated slow charge-transfer kinetics, i.e., a large resistance for the charge-transfer reaction at the electrode/electrolyte interface [4–9]. In addition, physicochemical and electrochemical studies on Pt, Pd, and Ni single crystals demonstrated that an absorbed state for hydrogen exists at the electrode subsurface, which represents hydrogen atom residing just beneath the topmost surface layer [10–13]. This means that hydrogen transport in hydride-forming metals and oxides may involve the hydrogen transfer reaction between the adsorbed state (MHads) on the electrode surface and the absorbed state (MHabs) at the electrode subsurface. As a result, one cannot rule out the possibility that in addition to hydrogen diffusion, interfacial charge transfer, hydrogen transfer, or both may determine the overall rate of hydrogen transport. In fact, abnormal behaviors in hydrogen transport have been revealed by current transient analysis, which showed a strong deviation from the diffusion-control model. The key feature is that the log (current I) versus log (time t) curve exhibits a simple two-stage shape, but the absolute value of its slope is lower than 0.5 in the early stage, and no plateau region appears in the plot of It 1/2 versus log t [14–18].