Solar CO recycling has received wide attention primarily to address global CO emission and to convert CO and water to value-added chemicals. Despite a long research history over the past four decades, the technology remains in an early stage, with low CO conversion efficiency and selectivity. CO is highly stable and has limited solubility in water, and its reduction requires multiple proton-coupled electron transfers, resulting in a range of carbon intermediates (C1–C3) as well as a larger amount of H over CO conversion products. For the realization of solar CO recycling, the system of interest should be operated sustainably, which requires the development of not only energy-efficient and cost-effective materials but also stand-alone, complete reaction processes (CO reduction and water oxidation) operating for long periods without any external bias. A range of semiconductors (mostly p-types) have been studied for CO conversion, including GaP, InP, GaAs, Si, CuO, and CuFeO, all of which have narrow bandgaps (Eg) and sufficient Fermi levels (EF) capable of reducing CO. Although promising, these aterials inherently require potential biases to drive the CO reduction reaction and compete with other metallic electrodes, whereas complete reactions (CO reduction and water oxidation) have been rarely demonstrated due to large overpotentials. Photocathode-photoanode couples have been demonstrated to operate, yet the syntheses of materials are complicated and the energy conversion efficiency is low. We have searched for high-efficiency, low-cost, and scalable p-type materials and found that CuFeO and CuO mixed materials meet all requirements. To our surprise, this material converted CO to formate with selectivity greater than 90% over 1 week and simultaneously produced molecular oxygen via water oxidation when simply wired to an inert anode (Pt foil) without any external bias under circum-neutral pH. The solar-to-formate (STF) energy efficiency was in the range of 0.7–1.2%, which is the highest reported value and comparable to the efficiency of photosynthesis occurring in most plants. For comparison, CuFeO, CuO, and even CuO were fabricated; however, their efficiencies for formate production were much lower, and no oxygen was evolved.


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