Why do RuO2 electrodes catalyze electrochemical CO2 reduction to methanol rather than methane or perhaps neither of those?

Abstract

The electrochemical CO₂ reduction reaction (CO₂RR) on RuO₂ and RuO₂-based electrodes has been shown experimentally to produce high yields of methanol, formic acid and/or hydrogen while methane formation is not detected. This CO₂RR selectivity on RuO₂ is in stark contrast to copper metal electrodes that produce methane and hydrogen in the highest yields whereas methanol is only formed in trace amounts. Density functional theory calculations on RuO₂(110) where only adsorption free energies of intermediate species are considered, i.e. solvent effects and energy barriers are not included, predict however, that the overpotential and the potential limiting step for both methanol and methane are the same. In this work, we use both ab initio molecular dynamics simulations at room temperature and total energy calculations to improve the model system and methodology by including both explicit solvation effects and calculations of proton-electron transfer energy barriers to elucidate the reaction mechanism towards several CO₂RR products: methanol, methane, formic acid, CO and methanediol, as well as for the competing H₂ evolution. We observe a significant difference in energy barriers towards methane and methanol, where a substantially larger energy barrier is calculated towards methane formation than towards methanol formation, explaining why methanol has been detected experimentally but not methane. Furthermore, the calculations show why RuO₂ also catalyzes the CO₂RR towards formic acid and not CO(g) and methanediol, in agreement with experimental results. However, our calculations predict RuO₂ to be much more selective towards H₂ formation than for the CO₂RR at any applied potential. Only when a large overpotential of around -1 V is applied, can both formic acid and methanol be evolved, but low faradaic efficiency is predicted because of the more facile H₂ formation.