Assessing Nonstoichiometric Oxides in Solar Thermochemical Fuel Production

The high temperatures at which two-step solar thermochemical fuel production proceeds (e.g. 1000 to 1500 °C) can render both surface and bulk kinetics of porous nonstoichiometric infinitely fast relative to gas sweep rates. In such case, the material operates under quasi-equilibrium conditions, and the macroscopically observed oxygen evolution and hydrogen production profiles are limited by the thermodynamic characteristics of the oxide. Recognition of this behavior enables the development of material-specific cycling strategies that maximize the process efficiency taking into account factors such as the energy for sweep gas and solid state heating and for mechanical pumping. Building on a previously established and experimentally validated model for predicting gas evolution profiles in the quasi-equilibrium regime [T. C. Davenport, M. Kemei, M. J. Ignatowich, and S. M. Haile, Int. J. Hydr. Energy 42, 16932-16945 (2017)], we develop here a computational approach for predicting cycles that maximize solar-to-fuel efficiency. The optimization is carried out using as inputs the experimentally measured enthalpy and entropy of reduction of known and fully characterized nonstoichiometric oxides. The optimized cycles are defined in terms of the temperature, the duration time, and the sweep gas flow rate of each half cycle. Significantly, despite a large energy penalty of heating and cooling the oxide, for most materials considered, the overall efficiency is highest when the temperature for the water splitting half-cycle is relatively low. In such case, the thermodynamic driving force for the hydrogen evolution reaction is large, hastening the pace of the reaction. Achieving the predicted efficiencies, however, may require surface engineering to avoid limitations due to slow surface reaction kinetics at reaction temperatures below ~1000 °C. Most importantly, this approach serves as a framework for assessing the efficacy of candidate thermochemical materials on an optimized rather than ad hoc basis. That is, for each candidate, the maximum efficiency and optimal conditions, within some range of constraints, and can be determined, rather than comparing materials at arbitrary cycling conditions which may inherently favor one material over another.