Synthesis of Aliphatic Acids from CO₂ and Water at Efficiencies Close to the Photosynthesis Limit Using Mixed Copper and Iron Oxide Films

The photocatalytic conversion of CO₂ and water into value-added chemicals remains a great challenge. This study shows that uniformly mixed copper and iron oxide (CuO/CuFeO₂; CFO) bulky heterojunction films are capable of converting CO₂ and water into C1–C6 aliphatic acid anions and O₂ at a solar-to-chemical energy conversion (STC) efficiency close to 3% under simulated sunlight in the absence of any sacrificial chemicals or electrical biases. When the CFO film is simply wired to a Pt foil, C1 (formate, with selectivity of 100%) and O₂ are produced at a near-stoichiometric ratio at an STC efficiency of ∼5% via the Z-scheme charge transfer mechanism. The CFO films are durable over 1 week and recyclable over 5 weeks under continuous irradiation. The addition of chloride enhances formate production, with an STC efficiency of 10%, while inhibiting the deformation of CFOs. Density functional theory computations support the observed selectivity and durability.