Architecting a Bias-Free Photoelectrochemical CO₂ Reduction System for Sustainable Formic Acid

Solar-driven photoelectrochemical CO₂ reduction represents a promising approach for the production of renewable liquid fuel but is limited by low photocurrent, the need for an external bias, and low carbon efficiency. This work employs a TiO₂-CdS/Se-ZnSe/S photoanode to drive the sulfur oxidation reaction, achieving a photocurrent density of 12.7 mAcm^-2 under AM 1.5G illumination and with an 87% retention after 100 h of continuous operation. Furthermore, through tailoring the adsorption capability for the ^*OCHO intermediate, the Cu₆Sn₅ catalyst exhibits a Faradaic efficiency of 92.8% for formic acid at -1.15 V in acidic media and maintains stability above 90% during a 120-h test. Finally, the constructed system achieves bias-free photoelectrochemical CO₂ reduction to HCOOH and delivers a yield of up to 172.9 µmolh^-1cm^-2 over an 85-h long-term test, outperforming conventional solar-driven systems. These findings highlight a cost-effective strategy for solar-driven liquid fuel production and provide valuable design concepts and insights into the development of photoelectrochemical systems.