Photosynthesis of formate from CO₂ and water at 1% energy efficiency via copper iron oxide catalysis

Solar conversion of carbon dioxide and water to value-added chemicals remains a challenge. A number of solar-active catalysts have been reported, but they still suffer from low selectivity, poor energy efficiency, and instability, and fail to drive simultaneous water oxidation. Herein, we report CuFeO₂ and CuO mixed p-type catalysts fabricated via widely employed electroplating of earth-abundant cupric and ferric ions followed by annealing under atmospheric air. The composite electrodes exhibited onset potentials at +0.9 V vs. RHE in CO₂-purged bicarbonate solution and converted CO₂ to formate with over 90% selectivity under simulated solar light (air mass 1.5, 100 mW cm^-2). Wired CuFeO₂/CuO photocathode and Pt anode couples produced formate for over 1 week at a solar-to-formate energy conversion efficiency of ∼1% (selectivity >90%) without any external bias while O₂ was evolved from water. Isotopes and nuclear magnetic resonance analyses confirmed the simultaneous production of formate and O₂ at the stand-alone couples.