RESUMO
The water electrolysis reaction involves a large kinetic overvoltage, and considerable research efforts are currently devoted to the search for better electrocatalysts. It is commonly expected that, at least, in principle, an ideal electrocatalyst would enable significant reaction rates close to the equilibrium voltage. In the present work, we question this expectation. For reactions, such as water electrolysis, which involve a significant change in the concentration between the reactant and product states, the position of the equilibrium voltage generally becomes decoupled from the onset of macroscopic kinetic currents. The reason is the dependence of the equilibrium voltage on the concentrations of both reactant and product species, whereas the forward rate of the reaction does not, in general, depend on the latter. Based on a new ideal gas reference for association/dissociation reactions, we develop a formalism to decompose the equilibrium voltage of electrolysis reactions into two distinct contributions: first, a contribution due to unbalanced relative concentrations between reactants and products second, a contribution due to the (mis)alignment of reactant and product states within the potential energy surface. The latter defines an intrinsic "kinetic reference voltage" that agrees remarkably well with the experimentally observed onset of water electrolysis, providing a new perspective on the origin of a significant fraction of the respective overvoltage. We expect the concept of kinetic reference voltages/potentials to be also useful in the context of other reactions involving significant concentration changes from the reactant to the product.
