While the subscripts n = 1, 2 and 4 in equation (6 and 7) correspond respectively to the hydronium (H3O + ), Zundel (H5O2 + ) and Eigen (H9O4 + ) cations frequently used as models of aqueous hydronic species, the relevant transition state through which hydrogen adsorption proceeds has been recently shown to involve an intermediate n = 3 complex (H7O3 + ) in which the transferring proton is strongly oriented toward the electrode surface. The Volmer step is followed by the Tafel reaction where two adsorbed hydrogen intermediates H* react to form molecular hydrogen H2, 𝐾𝑡 2 𝐻 * ↔ 𝐻2 (7) 𝐾-𝑇 where kT and k-T are the rate constants of the forward and reverse reactions, respectively. Notably, the Tafel reaction is a purely chemical surface reaction involving only a minor amount of charge transfer required to compensate for desorption-induced changes in the surface dipole affecting the electrode potential.