(186c) Thermodynamic and Kinetic Characterization of Al- and Ga-Doped La-Sr-Mn Perovskites for Solar Thermochemical Water Splitting

Redox cycling of strontium doped lanthanum-manganese perovskite type metal oxides has been shown to be a promising pathway for thermochemically splitting water to produce solar H₂. In this work, we characterized the thermodynamic and kinetic properties of Al and Ga doped LSM perovskites, namely La_.60Sr_.40Mn_.60Al_.40 (LSMA6464), La_.60Sr_.40Mn_.40Al_.60 (LSMA6446), and La_.60Sr_.40Mn_.80Ga_.20 (LSMG6482), for solar thermochemical water splitting. Pelletized samples of each composition were tested in a thermogravimetric analyzer (TGA) with controllable oxygen partial pressure (pO₂), monitored using a ZrO₂ based oxygen sensor. Thermodynamic and kinetic data was gathered via relaxation experiments by stepwise changing pO₂ isothermally and observing the change in mass towards equilibrium. The isothermal experiments were run in the range between 1200°C and 1400°C. Materials were further subjected to testing in a stagnation flow reactor, to compare water splitting capabilities of each material and extend thermodynamic characterization into a wider pO₂ space. The subsequently measured nonstoichiometry, as determined from changes in mass, was used to construct an Ellingham diagram in order to determine partial molar enthalpy and entropy as a function of oxygen non-stoichiometry, for each material. Results from this study lay the groundwork for understanding the role that thermodynamic and kinetic properties of LSM based perovskites has in the solar thermochemical H₂ splitting efficiency, and the conditions and reactor configurations that are most conducive for their utilization.