Kinetics of the Electrochemical Loading of Hydrogen into Thin Yttrium Films Covered by Palladium

Abstract

The behavior of a thin-film yttrium electrode (150-200 nm) capped by a palladium layer (20 nm) toward electrochemical hydrogen uptake was studied in 1 M NaOH electrolyte under galvanostatic conditions. The system is of interest for smart optical windows/mirrors in devices with changeable transparency. The electrochemical behavior of the Y/Pd electrode depended on many factors, e.g., whether the loading is performed on a fresh or used electrode, current density, presence of reducible components in the electrolyte, time of spontaneous open-circuit potential relaxation and/or anodic discharge. Residual hydrogen kept by the system and its interaction with some electrolyte components is a reason for this behavior. A decisive role of the palladium layer in electrochemical kinetics was elucidated for the first time. We assume that the kinetics of the hydrogen uptake by underlying yttrium as well as the corresponding hydrogen release process is controlled by diffusion from/through hydrogenated palladium. The constant potential hydrogenation process realized at 0.045 V vs. the reversible hydrogen electrode (RHE) was proposed to be the α-β Pd hydride transition. Formation of the β-Y dihydride phase is completed on the way to the −0.095 V potential level (vs. RHE) and is irreversible. In contrast, the β-γ Y hydride transition realized at this potential level is reversible and could be repeated many times. The last potential drop to −0.345 V (vs. RHE) is a hydrogen evolution process and, simultaneously, a metal-semiconductor (and optical) transition in the phase. In a discharge process, a β-Y dihydride of different stoichiometry and α-Pd hydride are probably the only species that could be obtained. The kinetics depends extremely on the discharge conditions (open-circuit relaxation or anodic polarization), and the process shows two potential levels. Under anodic hydrogen release, surface Pd oxidation could also take place. We have proposed and discussed a scheme for hydrogenation/release in the Y/Pd electrode under different experimental conditions. © 2002 The Electrochemical Society. All rights reserved.