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 extr
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Hydrogen Absorption into and Subsequent Diffusion Through Hydride-Forming Metals
3.1
Introduction
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. A detailed knowledge of the hydrogen insertion and desertion reactions has been acquired using various electrochemical techniques such as cyclic voltammetry [2–5], ac-impedance spectroscopy [6–17], the galvanostatic potential transient technique (chronopotentiometry) [18, 19], and potentiostatic current transient technique (chronoamperometry) [20–26]. Among these, ac-impedance spectroscopy has been widely used to identify the various reaction steps and to determine the rate-determining step, since it is an exceptionally powerful tool for separating the dynamics of several electrode processes with different relaxation times [27, 28]. In parallel with hydrogen insertion into hydride-forming electrodes, hydrogen absorption into and diffusion through metals has been widely studied in electrochemical permeation double cells with a metallic planar electrode [29–32] which
S.-I. Pyun et al., Electrochemistry of Insertion Materials for Hydrogen and Lithium, Monographs in Electrochemistry, DOI 10.1007/978-3-642-29464-8_3, # Springer-Verlag Berlin Heidelberg 2012
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3 Hydrogen Absorption into and Subsequent Diffusion Through Hydride-Forming Metals
were first introduced by Devanathan and Starchurski [33]. The theoretical formulations for the potentiostatic or galvanostatic transient method [32–36], the steady-state stepwise method [37], and the steady-state galvanostatic pulse method [38] usually consider a constant concentration of absorbed hydrogen on (potentiostatic boundary condition) or constant flux (galvanostatic boundary condition) into the metal surface. Furthermore, theoretical studies and experimental evidence of the hydrogen absorption reaction (HAR) using electrochemical impedance spectroscopy (EIS) have been presented for metal electrodes with
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