Oxygen nonstoichiometry, defect equilibria, and thermodynamic characterization of LaMnO3 perovskites with Ca/Sr A-site and Al B-site doping

Abstract

This work encompasses the thermodynamic characterization of four doped lanthanum manganites, namely La0.6 A0.4Mn1−yAlyO3 (A = Ca, Sr and y = 0, 0.4), all showed to be promising redox materials for the solar thermochemical splitting of H2O and CO2 to H2 and CO. We present oxygen nonstoichiometry measurements in the temperature range T = 1573 K–1773 K and oxygen partial pressure range pO2 = 4.5066 × 10−2 bar–9.9 × 10−5 bar. For a given T and pO2, oxygen nonstoichiometry is found to be higher when replacing the divalent dopant Sr in La0.6Sr0.4MnO3 by the divalent Ca but also increases significantly when additionally doping 40 mol-% Al to the Mn-site. La0.6Ca0.4Mn0.6Al0.4O3 revealed the highest mass specific oxygen release, 0.290 mol O2 per kg metal oxide at T = 1773 K and pO2 = 2.360 × 10−3 bar and 0.039 mol kg−1 at T = 1573 K and pO2 = 4.5066 × 10−2 bar. It is shown that the chemical defect equilibrium of all four perovskites can be accurately described by the two simultaneous redox couples Mn4+/Mn3+ and Mn3+/Mn2+. Thermodynamic properties, namely partial molar enthalpy, entropy and Gibbs free energy are consequently extracted from the defect models. Partial molar enthalpy decreases with increasing oxygen nonstoichiometry for the Al-doped perovskites whereas the opposite trend is observed for the others. The enthalpy falls within the range 260–300 kJ mol−1 for all the materials. Equilibrium hydrogen yields upon oxidation with H2O are determined as a function of redox conditions. Although reduction extents of the perovskites are greater compared to CeO2, oxidation with H2O and CO2 is thermodynamically less favorable. This leads to lower mass specific fuel productivity compared to CeO2 under most conditions relevant for solar thermochemical cycles.