Tunable thermodynamic activity of La_xSr_1−xMn_yAl_1−yO_3−δ(0 ≤ x ≤ 1, 0 ≤ y ≤ 1) perovskites for solar thermochemical fuel synthesis

Abstract

Nonstoichiometric metal oxides with variable valence are attractive redox materials for thermochemical and electrochemical fuel processing. To guide the design of advanced redox materials for solar-driven splitting of CO₂ and/or H₂O to produce CO and/or H₂ (syngas), we investigate the equilibrium thermodynamics of the La _<i>x</i> Sr_1-<i>x</i> Mn _<i>y</i> Al_1-<i>y</i> O_3-<i>δ</i> perovskite family (0 ≤ <i>x</i> ≤ 1, 0 ≤ <i>y</i> ≤ 1) and La_0.6Ca_0.4Mn_0.8Al_0.2O_3-<i>δ</i> , and compare them to those of CeO₂ as the baseline. Oxygen nonstoichiometry measurements from 1573 to 1773 K and from 0.206 to 180 mbar O₂ show a tunable reduction extent, increasing with increasing Sr content. Maximal nonstoichiometry of 0.32 is established with La_0.2Sr_0.8Mn_0.8Al_0.2O_3-<i>δ</i> at 1773 K and 2.37 mbar O₂. As a trend, we find that oxygen capacities are most sensitive to the A-cation composition. Partial molar enthalpy, entropy and Gibbs free energy changes for oxide reduction are extracted from the experimental data using defect models for Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ redox couples. We find that perovskites exhibit typically decreasing enthalpy changes with increasing nonstoichiometries. This desirable characteristic is most pronounced by La_0.6Sr_0.4Mn_0.4Al_0.6O_3-<i>δ</i> , rendering it attractive for CO₂ and H₂O splitting. Generally, perovskites show lower enthalpy and entropy changes than ceria, resulting in more favorable reduction but less favorable oxidation equilibria. The energy penalties due to larger temperature swings and excess oxidants are discussed in particular. Using electronic structure theory, we conclude with a practical methodology estimating thermodynamic activity to rationally design perovskites with variable stoichiometry and valence.