A sustainable bias-free photoelectrochemical system for CO2 reduction into formate coupled with aldehyde synthesis

Abstract Coupling CO₂ reduction with selective oxidation of organic molecules enables simultaneous CO₂ valorisation and value-added chemical synthesis, made more sustainable when driven by light. Here, we present a bias-free tandem photoelectrochemical system that converts CO₂ to formate and 4-methoxybenzyl alcohol (4-MeOBA) to 4-methoxybenzaldehyde (4-MeOBD) under solar irradiation. The photoanode consists of a graphitic carbon nitride (g-C₃N₄)/tungsten oxide (WO₃) heterojunction thin film with a cobalt oxide (Co₃O₄) co-catalyst on a carbon paper (CP), driving alcohol oxidation with a faradaic efficiency (FE) of 72% and an aldehyde formation rate of 0.55 μmol h⁻¹. Subsequently, photogenerated electrons are transferred to a tin nanoparticles decorated cathode (SnNPs/CP), reducing CO₂ to formate with a FE of 97%. The heterojunction at the WO₃/g-C₃N₄ interface facilitates Z-scheme charge transfer, enabling spatially separated redox reactions without external bias. Importantly, all components are earth-abundant, offering a sustainable and scalable platform for solar-driven CO₂ conversion with valuable fine chemical synthesis.