Thermodynamic properties, defect equilibria, and water splitting behavior of Ga- doped LSM perovskite

Abstract

Solar thermochemical hydrogen (STCH) production via thermochemical redox cycling of metal oxides is a promising pathway for production of green hydrogen. Understanding the intrinsic thermodynamic properties of metal oxide candidates employed in these cycles is vital for better understanding process technoeconomics and reactor design. Herein, for the candidate Ga- doped La–Sr–Mn perovskite water splitting oxide (La_0.6Sr_0.4)_0.95Mn_0.8Ga_0.2O_3−δ (LSMG6482), we present a detailed characterization of the partial molar thermodynamic properties, defect equilibria, and water splitting behavior. Measurements of equilibrium oxygen non-stoichiometry (δ) were obtained from thermogravimetric relaxation experiments to describe the equilibrium behavior of LSMG6482 (and CeO_2−δ as a reference) in a pO₂ range of 10^−6.84 atm ≤ pO₂ ≤ 10^−2.94 atm and a temperature range of 1200 °C ≤ T ≤ 1400 °C. From this data, the partial molar changes of enthalpy and entropy (Δ_o and Δ_o), were extracted by fitting the experimental data to an oxygen defect model, and through Van't Hoff analysis were compared to prior published Al- doped La–Sr–Mn perovskites. It was found that the magnitude of Δ_o and Δ_o was greater than those of Al- doped LSM perovskites at δ ≥ 0.1, with Δ_o(δ = 0.1) = 286 kJ mol⁻¹ and Δ_o(δ = 0.1) = 132 J mol⁻¹ K⁻¹. An Ellingham diagram and water splitting model was developed using Δ_o and Δ_o to observe trends in H₂ yield and steam conversion to identify suitable operating conditions for LSMG6482 and contrasted with the state-of-the-art CeO_2−δ. Overall, it was found that LSMG6482 is most suitable for cycling under smaller temperature swings than ceria and is suitable for lower reduction temperature operation (∼1300 °C vs. ∼1500 °C), but at the expense of low steam conversion to H₂.