Hydrogen Transport Under Impermeable Boundary Conditions
The hydrogen injection reaction into metals and oxides involves hydrogen absorption, followed by hydrogen diffusion through the bulk electrode. There are two models that describe hydrogen absorption in an alkaline solution: (1) the one-step (direct) mecha
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Hydrogen Transport Under Impermeable Boundary Conditions
4.1
Redox Reactions of Hydrogen Injection and Extraction
The hydrogen injection reaction into metals and oxides involves hydrogen absorption, followed by hydrogen diffusion through the bulk electrode. There are two models that describe hydrogen absorption in an alkaline solution: (1) the one-step (direct) mechanism and (2) the two-step mechanisms [1–3]. 1. One-step absorption mechanism: hydrogen is directly absorbed into the electrode as follows M þ H2 O þ e $ MHabs þ OH
(4.1)
where MHabs denotes a hydrogen atom absorbed at the electrode subsurface just beneath the topmost surface layer of the metal M. 2. Two-step mechanism: hydrogen is first adsorbed on the electrode surface (Volmer adsorption) and then transferred from the adsorbed state (MHads) on the electrode surface to the absorbed state (MHabs) at the electrode subsurface, M þ H2 O þ e $ MHads þ OH
(4.2)
MHads $ MHabs
(4.3)
Then, the absorbed hydrogen atom diffuses toward the interior of the electrode. The hydrogen evolution reaction occurs simultaneously with the cathodic hydrogen injection. Hydrogen evolution proceeds by the chemical desorption (Tafel reaction) or electrochemical desorption (Heyrovsky reaction) of the adsorbed hydrogen atoms [4–6]. Therefore, the hydrogen evolution reaction provides an alternative reaction path parallel to hydrogen injection. The adsorbed hydrogen
S.-I. Pyun et al., Electrochemistry of Insertion Materials for Hydrogen and Lithium, Monographs in Electrochemistry, DOI 10.1007/978-3-642-29464-8_4, # Springer-Verlag Berlin Heidelberg 2012
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4 Hydrogen Transport Under Impermeable Boundary Conditions
Fig. 4.1 Schematic diagrams of the (a) hydrogen injection and (b) extraction reactions
state for hydrogen evolution is identical to that for hydrogen injection, and, thus hydrogen injection is closely associated with the hydrogen evolution reaction. It is difficult to uniquely determine the contribution of hydrogen injection to the measured cathodic current because part of the electric charge is consumed by hydrogen evolution. In order to study the mechanism of hydrogen transport, therefore, it is more useful to analyze the anodic currents measured during hydrogen extraction from the electrode than the cathodic currents. The hydrogen injection and extraction reactions are schematically shown in Fig. 4.1a, b, respectively, along with the corresponding profiles of the hydrogen concentration cH across the electrode under the impermeable boundary condition. As shown in Fig. 4.1b, the hydrogen extraction reaction proceeds through the following reactions: 1. Hydrogen diffusion within the bulk electrode 2. Hydrogen transfer of MHabs at the electrode subsurface to MHads on the electrode surface 3. Electrochemical oxidation of the adsorbed hydrogen atom on the electrode surface involving the charge transfer reaction The hydrogen extraction reaction has been studied using a wide range of electrochemical techniques, for example, cyclic voltammetry [7–10], ac-imped
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