• • The pre-doped Bi catalyst achieves >95% Faradaic efficiency (FE) for formate across a current density range of 100–500 mA cm−2 in a flow cell, demonstrating industrial relevance for high-rate production.
• • Long-term stability is proven with >95% FE maintained for over 70 hours at 100 mA cm−2 in a membrane electrode assembly (MEA), addressing the sulfur leaching failure of conventional Bi2S3.
• • A solar-driven system achieves a solar-to-formate conversion efficiency of 4.4%, validating the potential for renewable-powered carbon valorization.
• • Mechanistic studies confirm that trace sulfur doping increases electron density, stabilizes the *OCHO intermediate, and suppresses hydrogen evolution, providing a design principle for selective CO2RR catalysts.