Molecular Engineering of Cation Solvation Structure for Highly Selective Carbon Dioxide Electroreduction

Abstract

Balancing the activation of H2O is crucial for highly selective CO2 electroreduction (CO2RR), as the protonation steps of CO2RR require fast H2O dissociation kinetics, while suppressing hydrogen evolution (HER) demands slow H2O reduction. We herein proposed one molecular engineering strategy to regulate the H2O activation using aprotic organic small molecules with high Gutmann donor number as a solvation shell regulator. These organic molecules occupy the first solvation shell of K+ and accumulate in the electrical double layer, decreasing the H2O density at the interface and the relative content of proton suppliers (free and coordinated H2O), suppressing the HER. The adsorbed H2O was stabilized via the second sphere effect and its dissociation was promoted by weakening the O−H bond, which accelerates the subsequent *CO2 protonation kinetics and reduces the energy barrier. In the model electrolyte containing 5 M dimethyl sulfoxide (DMSO) as an additive (KCl-DMSO-5), the highest CO selectivity over Ag foil increased to 99.2 %, with FECO higher than 90.0 % within −0.75 to −1.15 V (vs. RHE). This molecular engineering strategy for cation solvation shell can be extended to other metal electrodes, such as Zn and Sn, and organic molecules like N,N-dimethylformamide.