Sulfur Doping Induces Interfacial Water Regulation on Bi‐Based Catalysts for Efficient Electrochemical CO ₂ Reduction to Formate

Electrochemical CO 2 reduction (CO 2 RR) to formate is one of the green routes for achieving high‐value conversion of CO 2 . However, the reaction performance is limited by difficult CO 2 activation and sluggish water dissociation. Herein, sulfur‐doped bismuth nanorods (BiS x NR) are synthesized via a hydrothermal method combined with electrochemical reconstruction, which significantly enhances the catalytic activity and selectivity. In an H‐type cell, BiS x NR achieves a formate Faradaic efficiency () of 95% at −1.15 V vs. RHE. In a flow cell, a exceeding 90% is stably maintained for 60 h at 200 mA cm −2 . A series of in situ and ex situ electrochemical characterizations reveal that sulfur species serve as anchoring sites to enrich K + (H 2 O) n hydrated clusters in the electrical double layer, optimizing interfacial water activation and proton transfer while suppressing the competing hydrogen evolution reaction. Moreover, sulfur doping modulates the electronic structure of bismuth active sites, enhancing CO 2 adsorption and stabilizing the key *OCHO intermediate. This work unveils the synergistic effect of interfacial water regulation and electronic effects in boosting CO 2 RR performance, providing new insights for designing high‐performance electrocatalysts via interface engineering.