Electrochemical Reduction of CO₂ to Methane on Platinum Catalysts without Overpotentials: Strategies for Improving Conversion Efficiency

The electrochemical CO2 reduction to hydrocarbons and alcohols with sustainable energies is a promising technology for reducing atmospheric CO2 and storing electricity as chemical energy. However, the development of catalysts with high activities, high product selectivities, and low overpotentials to drive the reaction remains a major challenge for practical applications. On Pt, CO2 reduction to CO occurs easily, but further reduction to hydrocarbons and alcohols is difficult because of the strong adsorption of CO on the Pt surface. Consequently, Pt is considered an inactive catalyst and has been studied less than other transition metals such as Cu and Au. Herein, we show that adsorbed CO is selectively reduced to methane on carbon-supported Pt catalysts at low CO2 partial pressures and, crucially, at potentials close to the thermodynamic equilibrium potential of the reaction (0.16 V vs reversible hydrogen electrode), i.e., without overpotentials. Although the estimated apparent faradaic efficiency of 6.8% is not sufficiently high for commercial applications, this is the first demonstration of electrochemical methane generation without an overpotential. We suggest that the overpotential deposition (OPD) of hydrogen atoms at the atop sites of the Pt surface facilitates the reduction of adsorbed CO to methane and that the onset of hydrogen OPD coincidentally matches the equilibrium potential of methane generation. Mechanistic considerations also answer some questions raised by the present experiments, such as why low CO2 pressures are favorable for selective methane generation and why the faradaic efficiency is low. Overall, the results provide useful information for rationally designing effective CO2RR catalysts.