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

Balancing the activation of H O is crucial for highly selective CO electroreduction (CO RR), as the protonation steps of CO RR require fast H O dissociation kinetics, while suppressing hydrogen evolution (HER) demands slow H O reduction. We herein proposed one molecular engineering strategy to regulate the H O 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 H O density at the interface and the relative content of proton suppliers (free and coordinated H O), suppressing the HER. The adsorbed H O was stabilized via the second sphere effect and its dissociation was promoted by weakening the O-H bond, which accelerates the subsequent *CO 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 FE 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.