Thermodynamic Guiding Principles for Designing Efficient High-Temperature Solar Thermochemical Redox Materials for Fuel Production

The advancement of solar thermochemical fuel production technology critically relies on improvements in active redox materials and solar reactor designs. Previous studies typically predicted the solar-to-fuel efficiency of candidate active materials by adopting a simplified equilibrium approach, which could overpredict the efficiency. In this study, we describe a general thermodynamic framework to predict upper efficiency limits for both state-of-the-art and hypothetical materials in order to guide material design. The framework is based on conservation of mass and energy as well as Gibbs’ criterion and allows for efficiency optimization by simultaneous consideration of material properties and reactor system conditions. A global map of solar-to-fuel efficiencies versus change in reaction entropy and enthalpy is presented for both existing and hypothetical materials revealing performance trade-offs due to competing factors: thermodynamic favorability, thermal losses, sweep gas demand, and oxidizer preheating. A hypothetical material design resulting in peak efficiency can be identified for a set of reactor operating conditions.