The effect of the hydrogen diffusion on the hydrogen electrode reaction has been studied and rigorous kinetic expressions for the Tafel–Heyrovsky–Volmer mechanism has been derived. The analysis of the dependences of the current density (j) on overpotential (η), particularly oriented to the hydrogen oxidation reaction (hor), leads to the following main results: (i) in the Tafel–Volmer (TV) route, the current density reaches a maximum value (j_max) less or equal to the limiting diffusion current density (j_L); (ii) j_max is always equal to j_L for the Heyrovsky–Volmer (HV) route; (iii) in the simultaneous occurrence of both routes (THV), the current density always reaches the j_L value, although in the range of overpotentials of applied interest (0 ≤ η/V ≤ 0.6) the j_max value, characteristic of the TV route, can also be obtained.

The Levich–Koutecky plots have also been analysed and it has been demonstrated that the j(η) dependence for the hor under activated control cannot always be obtained from these plots.