Understanding the Role of Entropy in Promoting Electrocatalytic CO2 Reduction

Abstract

A global push towards increased sustainability requires technologies that enable the long-term storage of renewable electricity and the sustainable synthesis of chemicals. The electrochemical reduction of CO2 to fuels and chemicals represents a promising avenue to meet these goals. Yet, despite extensive research, many fundamental aspects of the interfacial interactions that enable this reaction remain unclear. One simple question that has not been addressed to date is the mechanism through which the applied potential controls the rate of CO2 reduction. To provide insight, we measured CO2 reduction activation energies on Ag electrodes in the presence of different cations. Building on our earlier work (J. Am. Chem. Soc. 2016, 138, 25, 7820–7823), where some of us demonstrated the important role of structural modifications to imidazolium cations in controlling the rate of CO2 reduction reactions, we expected that these structural modifications impact the reaction rate through changes to the activation energy. However, our findings revealed a much more fascinating picture of the impact of imidazolium cations on CO2 reduction rates.By measuring the activation energy in the presence of imidazolium cations with different structural features, we find that introducing 1-ethly-3-methylimidazolium (EMIM) results in an apparent activation energy of zero. Under these conditions, the applied potential modifies the reaction rate solely through changes to the pre-exponential factor of the Arrhenius equation.Based on literature on homogeneous catalysis, we suggest that our finding indicates that the rate of CO2 reduction in the presence of EMIM is entirely controlled by the formation entropy of the CO2 reduction transition state, with the potential likely influencing the rate by modifying the degree of ordering of imidazolium cations at the electrode surface. We further show that proton abstraction from EMIM forms a neutral compound, resulting in the elimination of the rate enhancement. Our results provide important new insight into the mechanism through which the applied potential controls the rate of electrocatalytic reactions and we expect them to be of broad importance to a better understanding of the connection between interface structure and electrocatalytic rates.