Electrochemical Methods for Assessing Kinetic Factors in the Reduction of CO₂ to Formate: Implications for Improving Electrocatalyst Design

Abstract

Thermochemical insights are often employed in the rationalization of reactivity and in the design of electrocatalysts for CO2 reduction reactions targeting C–H bond-containing products. This work identifies experimental methods for assessing kinetic aspects of reactivity. These methods are illustrated using [Fe4N(CO)12]−, which produces formate from CO2 at −1.2 V versus SCE in either a MeCN/H2O solvent (95:5) or pH 6.5 buffered water. Elementary rates for each reaction step are identified along with the rate-determining step (RDS) as C–H bond formation. Transition state kinetics were determined from an Eyring analysis for the rate-determining C–H bond formation step using temperature-dependent electrochemical measurements. A lower measured ΔG⧧ (298 K, 12.3 ± 0.1 kcal mol–1) in a pH 6.5 aqueous solution, compared with a ΔG⧧(298 K) of 15.0 ± 0.1 kcal mol–1 in a MeCN/H2O solvent (95:5), correlates with faster observed reaction rates and provides a kinetic rationalization for the solvent-dependent chemistry. Taken together, the experimentally determined kinetic insights highlight that enhancement of the local concentration of CO2 at catalyst–hydride sites should be a primary focus of ongoing catalyst design. This will both enhance reaction rates and increase selectivity for C–H bond formation over competing H–H bond formation, because that step is fast in H2 evolution reactions.