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

By M. Takacs, M. Hoes, M. Caduff, T. Cooper, J. Scheffe, A. Steinfeld. 2016

146 citations recorded by the source corpus.

Abstract This work encompasses the thermodynamic characterization of four doped lanthanum manganites, namely La 0.6 A 0.4 Mn 1−y Al y O 3 ( A  = Ca, Sr and y  = 0, 0.4), all showed to be promising redox materials for the solar thermochemical splitting of H 2 O and CO 2 to H 2 and CO. We present oxygen nonstoichiometry measurements in the temperature range T  = 1573 K–1773 K and oxygen partial pressure range p O2  = 4.5066 × 10 −2  bar–9.9 × 10 −5  bar. For a given T and p O2 , oxygen nonstoichiometry is found to be higher when replacing the divalent dopant Sr in La 0.6 Sr 0.4 MnO 3 by the divalent Ca but also increases significantly when additionally doping 40 mol-% Al to the Mn-site. La 0.6 Ca 0.4 Mn 0.6 Al 0.4 O 3 revealed the highest mass specific oxygen release, 0.290 mol O 2 per kg metal oxide at T  = 1773 K and p O2  = 2.360 × 10 −3  bar and 0.039 mol kg −1 at T  = 1573 K and p O2  = 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 Mn 4+ /Mn 3+ and Mn 3+ /Mn 2+ . 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 H 2 O are determined as a function of redox conditions. Although reduction extents of the perovskites are greater compared to CeO 2 , oxidation with H 2 O and CO 2 is thermodynamically less favorable. This leads to lower mass specific fuel productivity compared to CeO 2 under most conditions relevant for solar thermochemical cycles.

Subfields: Biomedical Engineering, Electronic, Optical and Magnetic Materials, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/c36bff778f53f3baf1b539850a65a9c99b0742ab