The favourable thermodynamic properties of Fe-doped CaMnO3 for thermochemical heat storage

Abstract

The CaMnO₃ oxide can reversibly release oxygen over a relatively wide range of temperatures and oxygen partial pressures (pO₂) and is thus a promising candidate for thermochemical heat storage in Concentrated Solar Power (CSP) plants. Moreover, it is composed of earth-abundant, inexpensive and non-toxic elements and exhibits a high-energy storage density, which are desirable characteristics for decreasing the deployment costs of the system. However, it undergoes decomposition at pO₂ ≤ 0.008 atm and temperature ≥ 1100 °C. Here the possibility of overcoming this limitation and extending the operating temperature range by B-site doping with Fe (CaFe_xMn_1−xO_3−δ0) is explored. Two doping levels are investigated, x = 0.1 and 0.3. The enthalpy of reduction was determined from a measurement of continuous equilibrium non-stoichiometry curves (δ, T) at several pO₂, enabling an evaluation of the heat storage capacity with high accuracy over widely ranging oxygen non-stoichiometry. Introduction of 0.1 Fe (CaFe_0.1Mn_0.9O_3−δ0) prevented CaMnO₃ decomposition up to 1200 °C at pO₂ = 0.008 atm, thus widening the operating temperature range and the oxygen reduction extent. The increase in the accessible nonstoichiometry translates into an increase in the heat storage capacity (Q_M (kJ mol_ABO3⁻¹)) from ∼272 kJ kg_ABO3⁻¹ in CaMnO₃ to ∼344 kJ kg_ABO3⁻¹ in CaFe_0.1Mn_0.9O_3−δ0. While even larger changes in oxygen content were accessible in CaFe_0.3Mn_0.7O_3−δ0, the oxidation state changes are accompanied by a lower enthalpy of reduction, resulting in a diminished heat storage capacity of ∼221 kJ kg_ABO3⁻¹.