(174d) Computationally Accelerated Discovery and Experimental Demonstration of Materials for Solar Thermochemical Hydrogen Production

While solar energy is the most abundant renewable energy resource, the capture, storage, and distribution of it remains a challenge. Solar thermochemical hydrogen production (STCH) provides a promising route for efficient utilization of this disperse resource since it allows for use of the entire solar spectrum to convert water to an energy dense fuel, H₂. However, despite a significant number of materials having been examined, an optimal redox material to drive this process has yet to be developed. In order to be viable for economic hydrogen production, materials must for have high hydrogen productivity, fast reduction and oxidation kinetics, low thermal reduction temperatures, and long term stability and reactivity. In this work we utilize a computationally accelerated, accurate, and experimentally validated materials-by-design approach involving ab initio and machine-learned models to design and demonstrate durable materials with improved thermodynamic and kinetic properties for STCH.

The thermodynamics of new materials are screened for stability, oxygen vacancy formation energy, and extent of reduction using machine learned models and DFT calculations. The effect of charged defects and their associated large electronic entropy on the predicted STCH behavior is assessed and compared to traditional charged-neutral screening methods. The effect of temperature on the parameters including crystal structure and cation disorder and their relationship to predicted STCH ability will be discussed. Compositional (doping) control is utilized to further optimize materials’ thermodynamic properties.

Thermogravimetric analysis (TGA) experiments are used to quantify the thermodynamic properties of new materials which provide a direct metric for comparison to computational results and help identify optimal operating conditions. Hydrogen production and kinetic parameters of new materials are measured during redox cycling in a stagnation flow reactor (SFR) and are provide feedback and validation to computational results. The long-term stability of new materials is critical to producing hydrogen economically. The stability of spinel particles is demonstrated over 200 redox cycles in the SFR and during on-sun operation at the high flux solar furnace at the National Renewable Energy Laboratory.