Review: Photochemical and Thermochemical Production of Solar Fuels from H2O and CO2 Using Metal Oxide Catalysts

Metal oxides are reviewed as catalysts to convert H2O and CO2 to fuels using solar energy. For photochemical conversion, TiO2 has been found to be the most stable and useful oxide material, but it is currently limited by its large bandgap and a mismatch between its conduction band and the redox couples for water splitting and CO2 reduction. A theoretical framework has been utilized to understand the basic thermodynamics and energetics in photochemical energy conversion systems. This is applied to model systems comprised of Ag2O and AgCl to examine why the former reacts thermochemically in air, while the latter reacts photochemically. For thermochemical conversion, zinc-, ceria-, and ferrite-based redox cycles are examined and examples of high-temperature solar reactors driven by concentrated solar radiation are presented. For CO2 splitting, theoretical solar-to-fuel energy conversion efficiencies can be up to 26.8% for photochemical systems, and can exceed 30% for thermochemical systems, provided that sensible heat is recovered between the redox steps.