Development and characterization of novel reduction-oxidation active materials for two-step solar thermochemical cycles

Abstract

Solar thermochemistry enables concentrating solar technologies to store or produce energy and materials in new, more versatile ways. In this work, binary and perovskite metal oxide candidates for high-temperature reduction-oxidation (redox) thermochemical cycles were synthesized and characterized to determine their potential for solar applications. First, the experimental infrastructure required to study rapidly reacting, high temperature metal oxides was developed. A high flux solar simulator (HFSS) capable of rapid heating was coupled to an upward flow reactor (UFR) to thermally reduce oxide samples, and O2 product gas flows were measured to calculate thermal reduction rates. The radiative input from the HFSS was characterized and coupled to computational models of the UFR to predict gas dynamics and redox sample heating. Dispersion modeling was used to correct temporal O2 measurements downstream of reducing samples. Thermal reduction experiments with the well-studied binary oxide pair Co3O4/CoO were performed to validate the computational models. Next, the UFR and a thermogravimetric analyzer (TGA) were used to evaluate candidate materials. Fe2O3/Fe3O4 were kinetically characterized via TGA and evaluated in thermodynamic cycle models. The results suggested the oxides were promising candidates for solar thermochemical electricity production. Al-doped SrFeO3-δ was synthesized and reaction models were developed with TGA to predict equilibrium nonstoichiometry and redox thermodynamics. The results were incorporated into a thermodynamic cycle model, and redox cycling experiments were performed in the UFR. The analyses determined that the oxides were well-suited to air separation for NH3 production.