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

Abstract Balancing the activation of H 2 O is crucial for highly selective CO 2 electroreduction (CO 2 RR), as the protonation steps of CO 2 RR require fast H 2 O dissociation kinetics, while suppressing hydrogen evolution (HER) demands slow H 2 O reduction. We herein proposed one molecular engineering strategy to regulate the H 2 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 2 O density at the interface and the relative content of proton suppliers (free and coordinated H 2 O), suppressing the HER. The adsorbed H 2 O was stabilized via the second sphere effect and its dissociation was promoted by weakening the O−H bond, which accelerates the subsequent *CO 2 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 CO 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.