Investigation of reactive perovskite materials for solar fuel production via two-step redox cycles: Thermochemical activity, thermodynamic properties and reduction kinetics

The investigation and optimization of solar fuels production by H2O and CO2 splitting reactions using non-stoichiometric redox materials as oxygen carriers relies on materials-related studies. The thermochemical cycles performance strongly rely on the thermodynamics and kinetics of redox reactions, as well as the chemical composition and morphology of the reactive redox materials commonly based on ceria and perovskites. This study focusses on the evaluation and selection of suitable non-stoichiometric metal oxides for two-step thermochemical cycles with high fuel production yields, rapid reaction rates, and performance stability. The redox activities of different A- and B-site substituted perovskite materials (ABO(3)) were experimentally investigated (with A = La, Sr, Y, Ca, Ce, Pr, Sm and B = Mn, Co, Fe, Mg, Al, Ga, Cr). The reactive powders were synthesized via modified Pechini methods providing a porous microstructure especially suitable for thermochemical cycles, while their redox activity was evaluated by thermogravimetric analysis. This experimental screening highlighted the difficulty to combine high reduction extent (delta) achievable by the reactive material with complete reoxidation extent and fast reaction rates. From the redox activity study of manganite perovskites, La0.5Sr0.5Mn0.9Mg0.1O3 (LSMMg) was pointed out as a good compromise between CO2 splitting activity and thermal stability, possibly competing with ceria as a promising material for two-step thermochemical cycles. Both thermodynamic and kinetic studies were also performed to provide a better understanding of the mechanisms involved in thermochemical cycles. Thermodynamic properties derived from experimental delta(T,p(O2)) diagrams were used to predict the upper bounds for both reduction extent and fuel production performance at equilibrium. Regarding kinetics, the activation energy during LSMMg reduction was shown to increase with the increase of the non-stoichiometry extent.