Descriptors for\nElectrochemical CO<sub>2</sub> Reduction\nin Imidazolium-Based Electrolytes

Electrochemical CO<sub>2</sub> reduction (CO<sub>2</sub>R) allows\nus to close the carbon cycle and store intermittent renewable energy\ninto chemical products. Among these, syngas, a mixture of hydrogen\nand carbon monoxide, is particularly valuable due to its high market\nshare and the low energy required for its electrocatalytic production.\nIn addition to catalyst optimization, lately, electrolyte modifications\nto achieve a suitable CO/H<sub>2</sub> ratio have also been considered.\nIonic liquid (IL)-based electrolytes have enabled high faradaic efficiency\ntoward CO, depending on the chemical properties of the IL. In this\nwork, we rationalized through density functional theory (DFT) descriptors\nthe competition between hydrogen evolution (HER) and CO<sub>2</sub>R on silver in imidazolium-based electrolytes, developing a DFT-based\nanalytical model. The electrolyte anion regulates the concentration\nratio between cationic and carbene species of ILs cation, respectively,\nbetween the 1-ethyl-3-methylimidazolium cation (EMIM<sup>+</sup>)\nand carbene (EMIM:) species and between the 1-butyl-3-methylimidazolium\ncation (BMIM<sup>+</sup>) and carbene (BMIM:). The latter species,\nif formed, hinders the CO<sub>2</sub>R by blocking the active sites\nor trapping CO<sub>2</sub> in solution. In the case of weak Lewis\nbase anions as fluorinated ones, EMIM<sup>+</sup> (BMIM<sup>+</sup>) cations, which serve as cocatalysts in CO<sub>2</sub>R, are more\nabundant, allowing high CO partial current densities and high electrochemically\nactive surface area. Applying the here-defined descriptors to ILs\nnot yet tested makes it possible to predict the HER and CO<sub>2</sub>R selectivity on silver, thus enabling guidelines for designing better\nILs for CO<sub>2</sub>R.