Diffusion of Molecular Hydrogen

The inhomogeneous structure of the a-Si H films most likely leads to H2 motion that cannot be described as uniformly diffusive throughout the film. It has been suggested that H2 motion occurs along microvoids, present even in device quality films, that are aligned preferentially perpendicular to the films surface (Vanderheiden et al., 1987). These oriented microvoids can be considered as remnant micro-columnar structure in the films. The fact that the amount of H2 that can be reintroduced can never 

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Sometimes very low Tafel slopes are claimed (<15mV) [99,253]. It seems difficult to interpret such an observation in terms of a specific mechanism. It is more probable that anomalously low Tafel slopes are the result of a combined thermal activation of the reaction and of the electrode surface state, resulting in a reaction rate limited by the diffusion of molecular hydrogen away from the electrode. Supersaturation of the electrode ad-layer by the evolved gas can also play a decisive role [254,255]. This phenomenon has been amply discussed in the case of Cl2 evolution on oxide electrodes [256], but the same idea can be applied to the case of H2 evolution [257,258],... 

On the other hand, if one considers the diffusion of molecular hydrogen coupled to the reaction and charge transfer, the anodic hydrogen overpotential is given by... 

It was concluded first by Knorr and coworkers [1] that diffusion of molecular hydrogen is the rate-determining step of hydrogen evolution on smooth platinum and palladium electrodes pretreated anodically. The same conclusion holds for the anodic processes given by Eq. 1 and 2. The interpretation based on steady-state measurements [1] is valid at cathodic current densities — i < 1 mA/cm in strong acid electrolytes (0d.H+ 1)- The correctness of this interpretation has been substantiated by a number of independent techniques which allow the concentration Ch2 of H2 molecules adjacent to the surface to be determined ... 

Small, charged solutes, however, do not possess the same mobility. Voltammetric studies by Buzzeo et al. [258] indicate that the rate of diffusion of molecular oxygen in an IL is 30 times faster than that of the superoxide radical O2. Similarly, pulse radiolysis experiments [259] indicate that the diffusion of a neutral hydrogen atom in an IL is roughly an order of magnitude faster than that of an unbound electron. These charged species are clearly slowed by their interaction with the charge distribution of the medium. 

FTIR diffuse reflectance spectroscopy was recently applied by Kazanski et al. (61) to monitor the interactions of molecular hydrogen with silica,... 

Both Pd and Pt are capable of absorbing large volumes of molecular hydrogen, and Pd is used for the purification of H2 by diffusion. 

This diffusion of nascent hydrogen is not inhibited even by a pressure of 14 atmospheres. Molecular hydrogen only passes through iron at a measurable velocity when the temperature is raised. Below 325° C. the velocity is inappreciable, at 350° C. it becomes perceptible, and at 850° C. the velocity is some forty times as great.3... 

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