Some theories of hydrogen overvoltage

The essential stages in the overall process of hydrogen discharge and gas evolution at a cathode may be assumed to be as follows  

Stages (2), (3) and (4) constitute the discharge reaction which we may express as 

The problem is to decide which stages are rate-determining. If stage (1) were to be rate-determining, then the process overall would be diffusion-limited due to concentration polarization. Usually, one of the stages (2) to (5) is the slowest. Which one, depends upon many factors such as the operating conditions and the nature of the cathode. 

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