Studies on the Hydrogen Evolution Reaction

Two different reaction mechanisms are considered as being experimentally established  

According to the Volmer-Tafel mechanism, the discharge of a proton forms an adsorbed hydrogen atom (Volmer reaction). Two such adsorbed hydrogen atoms then combine to form molecular hydrogen (Tafel reaction) 

Alternatively, according to the Volme-Heyrovsky mechanism, the Volmer discharge reaction is followed by the Heyrovsky electrochemical desorption reaction 

In either case a fraction of the adsorbed atomic hydrogen may enter the metal 

The electrochemical hydrogen permeation technique has proved to be a valuable tool in the study of these reaction mechanisms. This is mainly due to the ability to estimate the amount of an intermediate (Hads) in the reaction scheme. Such studies have been presented, for example, by Devanathan and Stachurski, by Bockris et and by Iyer et The applicability of the Volmer-Tafel reaction scheme can be evaluated by considering the kinetic expressions for reactions (22) and (23), together with equilibrium in the absorption process (25) 

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