Publications

Air separation via a two-step solar thermochemical cycle based on (Ba,La)xSr1-xFeO3-δ: Thermodynamic analysis

Bush, Hagan E.; Nguyen, Nhu P.; Farr, Tyler; Loutzenhiser, Peter G.; Ambrosini, Andrea A.

A two-step solar thermochemical cycle was considered for air separation to produce N2 based on (Ba,La)xSr1-xFeO3-δ perovskite reduction/oxidation (redox) reactions for A-site fractions of 0 ≤ x ≤ 0.2. The cycle steps encompassed (1) thermal reduction and O2 release via concentrated solar input and (2) re-oxidation with air to uptake O2 and produce high-purity N2. Thermogravimetry at temperatures between 400 and 1100 °C in atmospheres of 0.005 to 90% O2/Ar at 1 bar was performed to measure equilibrium nonstoichiometries. The compound energy formalism was applied to model redox thermodynamics for both Ba2+ and La3+ substitution. Non-linear regression was used to determine the empirical parameters based on the thermogravimetric measurements. The model was used to define partial molar reaction enthalpies and entropies and predicted equilibrium oxygen nonstoichiometry as functions of oxide stoichiometry, site fraction, temperature, and O2 partial pressure. The thermodynamic analysis showed the materials are appealing for air separation at temperatures below 800 °C.