Interfacial role of Ionic liquids in CO2 electrocatalytic Reduction: A mechanistic investigation

Ionic liquids (ILs) can significantly reduce the overpotential of CO2 electrocatalytic reduction reaction (CO2RR) and thus show a huge application potential in the CO2 conversion. However, it has not been clear what role ILs play in the reaction occurring at the solid–liquid interface of the electrodes. In this work, we performed comprehensive DFT calculations to investigate the mechanism of CO2RR to CO on Ag electrode surfaces with ILs. Our results showed that the Ag(1 1 0) surface exhibits better catalytic performance than both Ag(1 0 0) and Ag(1 1 1) surfaces since the energy barrier of the transition state (Ts) is lower and the intermediate (*COOH) is more stable on the former surface. When ILs exist near the surface, the energy barrier of the Ts decreases and varies when the CO2 molecule is localized at different positions of the [Emim]+ cation. The optimized structures showed that the CO2 molecule prefers to stay near the C4/5 position rather than the C2 position. It was also found that proton transferring on the Ag surface by the hydrogen bond mode has a lower energy barrier than the shuttling mode, which indicates that the IL can act as an assist-catalyst by forming hydrogen-bonding complex in the reaction. Furthermore, the role of water was explored by using the implicit-solvent model. It was found that the solvation effect of the water always decreases the energy barrier, and the decline is more pronounced when there are ILs. Molecular dynamics (MD) simulations also showed that near the electrode surface, each CO2 molecule is enclosed by 3–4 [Emim]+ cations with their C4/5 more likely approaching the CO2. Such a distribution embodies the mesoscale multi-ions synergistic catalytic mechanism that will be elucidated in our future work.