CO2 electroreduction on single atom catalysts: Is water just a solvent?

The electrochemical reduction of CO2 on single atom catalysts (SAC) has emerged as a highly promising yet intricate process, requiring an in-depth understanding of each elementary step of the reaction. Most of the theoretical studies pertaining the screening of new efficient catalysts neglect important effects such as the capability of the catalytic site to bind and activate CO2, the occurrence of competing reaction paths via formation of different isomers in the intermediate steps, and the role of water. In this work we will show that these are key aspects of the CO2 reduction reaction (CO2RR). Employing density functional theory, we investigated a series of Transition Metal atoms (Sc-Cu, Mo-Ag, W-Au) embedded in nitrogen doped graphene (TM@4N-Gr) and their activity in the CO2 reduction. Our results show that water is not only solvating the reaction intermediates, but it acts as a ligand itself, competing with CO2 when binding to the catalytic site. This coordination chemistry effect largely affects the stability of the chemical intermediates. Furthermore, only a small fraction of the investigated SACs can bind and activate CO2, and that in most cases the reaction proceeds via formation of the OCHO intermediate and not of the COOH one, as often assumed.