Hydrogen pure diffusion control

The analytical model given by Bockris and co-workers for the potentiostatic pulse technique involved only pure diffusion control and did not take into account hydrogen-entry kinetics or trapping. However, a more general model has since been... 

Equation (96) can be solved analytically for two cases, as characterized by the kinetics of hydrogen transfer across the metal interface (i) pure diffusion control and (ii) diffusion control with a limited rate of entry (see Section III.1). As with the permeation methods, in both cases the layer of adsorbed hydrogen is assumed to adjust very rapidly to a potential step. 

An additional test for diffusion control is provided by the dependence of the steady-state anodic current on membrane thickness. In potentiostatic charging, the hydrogen concentration is fixed in the case of pure diffusion control, and therefore should be proportional to 1/L (see Section III). This test is not necessarily applicable for a constant flux boundary condition because C /L,... 

Hence, the steady-state flux of hydrogen through an iron membrane under pure diffusion control should be proportional to the square root of the cathodic current density, as found experimentally by Bodenstein. Deviations from the square-root dependence are observed, particularly at extreme current densities, but this relationship seems to hold generally. Hence, hydrogen absorption by iron is considered to proceed through the adsorbed intermediate. If hydrogen evolution were independent of hydrogen transfer into the metal (see Section 11.1), should be proportional to /, but this relationship has not been observed experimentally. 

The negative surface potential of the anionic micelles prevents the approach of hydrated electrons and sensitizer cations, e.g. holes, (Fig. 4.4). The preferred pathway of reaction of e q is here hydrogen formation (2ejq — H2 + 2 OH-). Hence, with such a system, the prevention of electron-hole recombination is achieved. While in pure aqueous solution the lifetime of the sensitizer cation is only several microseconds (due to the diffusion controlled back reaction with erq) the lifetime in anionic micellar systems can be as long as days or weeks. 

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