Reversible and Selective CO2 to HCO2 - Electrocatalysis near the Thermodynamic Potential.

Reversible catalysis is a hallmark of energy-efficient chemical transformations, but can only be achieved if the changes in free energy of intermediate steps are minimized and the catalytic cycle is devoid of high transition-state barriers. Using these criteria, we demonstrate reversible CO2 /HCO2 - conversion catalyzed by [Pt(depe)2 ]2+ (depe=1,2-bis(diethylphosphino)ethane). Direct measurement of the free energies associated with each catalytic step correctly predicts a slight bias towards CO2 reduction. We demonstrate how the experimentally measured free energy of each step directly contributes to the <50 mV overpotential. We also find that for CO2 reduction, H2 evolution is negligible and the Faradaic efficiency for HCO2 - production is nearly quantitative. A free-energy analysis reveals H2 evolution is endergonic, providing a thermodynamic basis for highly selective CO2 reduction.