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

Metal oxides are reviewed as catalysts to convert H 2 O and CO 2 to fuels using solar energy. For photochemical conversion, TiO 2 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 CO 2 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 Ag 2 O 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 CO 2 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.