Synergistic Atomic‐Vacancy Engineering in Bi ₂ S _3‐δ /Co‐N‐C: Boosting Photoelectrocatalytic Hydrogen Production via Formaldehyde Oxidation

Abstract Efficient photoelectrochemical fuel generation requires catalysts overcoming sluggish charge kinetics. Bi 2 S 3‐δ /Co‐N‐C dual‐function catalysts are engineered by anchoring Co single atoms (SAs) onto sulfur‐vacancy‐rich Bi 2 S 3‐δ supported by Co‐N‐C. This system simultaneously achieves photoelectrocatalytic hydrogen evolution at 1534 µmol·cm −2 ·h −1 (92.94% FE) and formaldehyde oxidation (0.163 h −1 , 99.16% FE). Sulfur vacancies upshift the S‐p‐band center and shorten Co─S bonds, while Co SAs and sulfur vacancies reduce the *HCOOO formation barrier by 49%. The introduction of sulfur vacancies also significantly lowers the d‐band center of Co and reduces the orbital energy of sulfur, thereby enhancing the hybridization capability between S and Co atoms. The strengthened orbital hybridization between Co and S establishes an efficient interfacial charge transfer pathway. Sulfur vacancies trap electrons and Co SAs mediate electron transport, enabling dual‐channel charge transfer through Co─S bonds and Co SAs, which extend carrier lifetimes and enhance internal electric fields by 2.6–7.5 times versus Bi 2 S 3‐δ , Co‐N‐C, Bi 2 S 3‐δ /ZIF‐67, and Bi 2 S 3 /Co‐N‐C. Enriched holes on Bi 2 S 3‐δ within Bi 2 S 3‐δ /Co‐N‐C oxidize adsorbed formaldehyde molecule‌s. By replacing oxygen evolution with thermodynamically favorable formaldehyde oxidation (hydrogen production rate increased to 3.1 times), our design lowers the overall energy barrier, enabling coupled solar fuel production and pollutant remediation.