Redox Mediated Molecular Chemical Catalysis of Electrochemical Reactions: A Co-Catalytic Boosting Effect in CO₂ Electroreduction Catalyzed by an Iron Porphyrin

Redox mediators play a crucial role in electrochemical systems by facilitating electron transfer between electrodes and catalytic centers. This study investigates the enhancement of molecular electrocatalysis through the cocatalytic use of redox mediators, focusing on the electrochemical reduction of CO2 to CO catalyzed by iron tetraphenylporphyrin (TPPFe). A range of redox mediators with redox potentials near that of the TPPFe(I)/TPPFe(0) couple were examined. Cyclic voltammetry revealed significant boosting effects in catalytic current and reduction of overpotential, depending on the mediator’s redox potential. Mediators that were either similar or harder to reduce than TPPFe enabled electron shuttling over an extended diffusion-reaction layer, increasing the effective number of catalytically active species. Mediators being easier to reduce than the chemical catalysts offer opportunity to achieve a smaller overpotential but at a reduced effective turnover frequency. A kinetic model was developed to quantitatively describe this cocatalytic mechanism, emphasizing that the observed current enhancement arises from physical redistribution of catalytic activity. This work establishes a general framework to distinguish purely outersphere redox-mediated effects from specific mediator–catalyst interactions and provides design principles for integrating redox mediators in molecular electrocatalysis. These findings open directions for optimizing catalyst performance in energy-relevant or organic electrochemical transformations and developing more complex systems involving, in particular, proton-coupled electron transfer or hydride mediators.