Molecular Catalysis of Electrochemical Reactions. Overpotential and Turnover Frequency: Unidirectional and Bidirectional Systems

Molecular catalysis of electrochemical reactions is taking on an increasing importance in modern energy challenges involving the activation of small molecules. The import of the notion of overpotential from the domain of electrocatalysis is a source of confusion. The consequences are particularly deleterious for the establishment of kinetic vs thermodynamic correlations with the aim to design the "best catalyst" for a targeted reaction. Herein we propose a clarification of the notion of overpotential in the context of molecular catalysis of electrochemical reactions, examining both unidirectional and bidirectional systems. In the latter case, the turnover frequencies are defined for both monoelectronic and bielectronic systems and we show that the space-dependent departure from the equilibrium in the diffusion-reaction layer has to be considered for a correct extraction of relevant kinetic parameters from electrochemical data.