A computational study of electrochemical CO₂ reduction to formic acid on metal-doped SnO₂

Electrochemical reduction of CO2 to formic acid (HCOOH) can contribute to the renewable energy transition as a liquid carrier of renewably hydrogen. Here, we investigated the catalytic requirements of SnO2 electrodes for efficient CO2 reduction to HCOOH using density functional theory and microkinetics simulations. Hydroxylation of the surface is a prerequisite to achieve a high activity with predicted current densities in agreement with experiment. The resulting surface is selective to HCOOH production with a negligible contribution of the hydrogen evolution reaction. Mechanistically, it is found that the reaction proceeds via hydrogenation of adsorbed CO2 to carboxylate (COOH), which is then further hydrogenated to the desired product. Doping of the surface by commonly used elements (Bi, Pd, Ni and Cu) identifies Bi as the preferred promoter to substantially improve the current density. Brønsted-Evans-Polanyi relations are established for the two key steps in the mechanism. Overall, carboxylate formation is the rate-controlling step. The CO2 reduction activity is analyzed in terms of two descriptors, namely the free energies for the two protonation steps, showing that Bi presents the highest activity.