Assessing nonstoichiometric oxides for solar thermochemical fuel production

The high temperatures at which two-step solar thermochemical fuel production proceeds (e.g. 1000 to 1500 °C) canrender both surface and bulk kinetics of porous nonstoichiometric infinitely fast relative to gas sweep rates. In suchcase, the material operates under quasi-equilibrium conditions, and the macroscopically observed oxygen evolutionand hydrogen production profiles are limited by the thermodynamic characteristics of the oxide. Recognition of thisbehavior enables the development of material-specific cycling strategies that maximize the process efficiency takinginto account factors such as the energy for sweep gas and solid state heating and for mechanical pumping. Buildingon 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 ofknown 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 ofheating and cooling the oxide, for most materials considered, the overall efficiency is highest when the temperature forthe water splitting half-cycle is relatively low. In such case, the thermodynamic driving force for the hydrogen evolutionreaction is large, hastening the pace of the reaction. Achieving the predicted efficiencies, however, may requiresurface 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 thermochemicalmaterials on an optimized rather than ad hoc basis. That is, for each candidate, the maximum efficiency and optimalconditions, within some range of constraints, and can be determined, rather than comparing materials at arbitrarycycling conditions which may inherently favor one material over another.