The hydrogen electrode reaction (HER) has been subjected to numerous studies, theoretical and experimental, which agree in that on metallic electrodes the reaction takes place through the elementary steps of Volmer, Heyrovsky and/or Tafel. Diverse procedures with different levels of approximations were employed (rate determining steps, neglect of the backward reaction, etc.) for the evaluation of the kinetic parameters from the current-potential experimental dependences under steady state on different type of substrates (smooth, porous, microelectrodes, etc.). Nevertheless, the kinetic studies were limited basically to the hydrogen evolution reaction (her), which implies the characterization of the HER only in the cathodic overpotentials region. Then, the hydrogen oxidation reaction (hor) was much less studied and moreover, it continues being considered by many authors under pure diffusion control. This means that the rate of the activated oxidation reaction is infinite, which is opposed with the observed reaction rates for the her, violating the principle of microscopic reversibility. Therefore, the usual methodologies are analysed. Then, a more rigorous kinetic treatment is described, which takes into account that the reaction mechanism is unique and independent of its evaluation in the cathodic (her) or anodic (hor) overpotentials region or else if it is analysed around the equilibrium potential through the polarization resistance. Finally, the requirements that should be satisfied for the design of the experiments for the evaluation of the kinetic parameters are also considered.
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