Anomalous behaviour of hydrogen extraction from hydride-forming metals and alloys under impermeable boundary conditions

Hydrogen injection into and extraction from hydride-forming electrodes has been routinely investigated under the assumption that the electrode is homogeneous in structure and hydrogen transport through such electrodes is purely controlled by hydrogen diffusion. However, various kinds of abnormal behaviours in hydrogen transport, which could hardly be explained in terms of the diffusion-control model, have been quite frequently reported by many researchers. This review provides a comprehensive survey of the anomalous behaviours of hydrogen transport observed in such hydride-forming electrodes as Pd and metal-hydrides, with particular emphasis on hydrogen extraction under the impermeable boundary conditions during the potentiostatic current transient measurement involving the potential stepping. After a brief discourse on the conventional diffusion-control model, the topics related to the boundary conditions at the electrode surface during hydrogen extraction are extensively reviewed. In particular, it is shown that the diffusion-controlled constraint should be no longer valid at the electrode surface for hydrogen extraction in case hydrogen diffusion is influenced by either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. Subsequently, the atypical behaviours of current transient due to hydrogen diffusion in the presence of traps and in the coexistence of two hydride phases are treated in detail. Each of the hydrogen extraction models suggested is discussed with the aid of the anodic current transients numerically calculated based upon the theoretical electrode potential curve, and then it is exemplified by hydrogen extraction from Pd and metal-hydrides in aqueous solutions.     

Further to the “Reply to remarks on Discussion of three models used for the investigation of insertion/extraction processes by the potential step chronoamperometry technique [C. Montella, Electrochim. Acta 50 (2005) 3746] by H.-C. Shin, S.-I. Pyun, K.-N. Jung” [Electrochim. Acta 51 (2006) 2775]

Some issues have been critically discussed regarding the cell-impedance-controlled model inspired by the recent publication entitled “Reply to remarks on Discussion of three models used for the investigation of insertion/extraction processes by the potential step chronoamperometry technique [C. Montella, Electrochim. Acta 50 (2005) 3746] by H.-C. Shin, S.-I. Pyun, K.-N. Jung” [Electrochim. Acta 51 (2006) 2775]. In particular, two points on the definition of the internal cell resistance and the intersection phenomenon of chronoamperometric curves (CAs) have been revisited on the basis of the combination of experimental and theoretical approaches.     

Remarks on “Discussion of three models used for the investigation of insertion/extraction processes by the potential step chronoamperometry technique” [C. Montella, Electrochim. Acta 50 (2005) 3746-3763]

The cell-impedance-controlled concept to explain the chronoamperometric curves obtained from intercalation compounds is revisited and inspired by the recent publication entitled “Discussion of three models used for the investigation of insertion/extraction processes by the potential step chronoamperometry technique” [C. Montella, Electrochim. Acta 50 (2005) 3746-3763]. The main issues the author pointed out have been critically discussed and then, the validity of the cell-impedance-controlled model has been verified from the combination of experimental and theoretical approaches.     

Comments of the paper ‘Capacity intermittent titration technique (CITT). A novel technique for determination of Li solid diffusion coefficient of LiMn2O4’ [X.-C. Tang, X.-W. Song, P.-Z. Shen, D.-Z. Jia, Electrochim. Acta 50 (2005) 5581-5587]

The article by Tang et al., which was published recently [Electrochim. Acta, Electrochim. Acta 50 (2005) 5581-5587], proposed a novel technique for the determination of diffusion coefficients of intercalated species in host materials. The capacity intermittent titration technique (CITT) is an extension of the ratio of potentiostatic-to-galvanostatic charge (RPG) technique developed initially by the same authors. By the CITT, the Li⁺ solid-state diffusion coefficient within LiMn₂O₄ was determined by these authors at different voltages and different galvanostatic-charge currents. In these comments, the ratio of potentiostatic-to-galvanostatic charge is reexamined by taking finite charge-transfer kinetics at the interface and Ohmic drop into consideration. The measurement error of diffusion coefficients evaluated by CITT while using the approximation of infinitely fast charge-transfer kinetics is predicted quantitatively.