Hydrogen atom diffusion into electrode

Consider a corroding metal, and let hydrogen evolution be the electronation reaction. The formation of hydrogen atoms adsorbed on the metal surface is an essential intermediate step in the electrodic evolution of hydrogen. What happens to these adsorbed hydrogen atoms They can get desorbed in either a chemical or electrodic reaction as hydrogen molecules that diffuse out into the solution or collect in bubbles of hydrogen gas. This is the visible way out from the metal surface. 

A mathematical model describing the processes of AC charging-discharge with account for EDL charging, intercalation of hydrogen into carbon, and its nonsteady-state solid-phase diffusion, electrode kinetics, ion transport over the electrode thickness, and characteristics of its porous structure was developed and confirmed experimentally. It is shown that the maximum path of diffusion of hydrogen atoms all other conditions being equal is inversely proportional to the hydrophilic specific surface area of the electrode. 

Figure 11.29 schematically illustrates the described processes that lead to formation of a hydrogen bubble. It also indicates that the adsorbed hydrogen atoms formed electrochemically can react in two ways, which are in competition they may form molecules that dissolve into the electrolyte, eventually forming a bubble, or they may diffuse into the metal as atoms. The presence of adsorbed surfactants and the surface roughness of the electrode can influence the nucleation of bubbles and, as a result, the effective hydrogen pressure. 

Figure 6.49. Schematic illustration of a cathode process with hydrogen adsorption in an electrode. On the cathode, a layer of adsorbed atomic hydrogen is built up the hydrogen penetrates into the cathode metal by diffusion. In internal pores, the atomic hydrogen combines to hydrogen molecules. By thermodynamic equilibrium, a given concentration of atomic hydrogen on the surface will have a corresponding equilibrium gas pressure in the internal pores.

Estimates given in section 2.7 show that such a reaction is quite probable in principle, and could compete with desorption involving HaO". Pleskov et al.[121] have demonstrated experimentally the existence of two parallel paths for electrochemical desorption, i.e. reactions involving H2O and HgO". In these experiments, the photoelectrons emitted by the cathode into the solution were captured by an acceptor, i.e. by an H30 ion. The hydrogen atoms produced as a result of this reaction diffused towards the electrode where the... 

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