Impact of La doping on the thermochemical heat storage properties of CaMnO_3-δ

Recently, CaMnO₃ has been proposed as a promising candidate for high temperature thermochemical heat storage. The material reversibly releases oxygen in response to changes in oxygen partial pressure (pO₂) in the temperature range (800-1000°C) suitable for Concentrated Solar Power (CSP) plants. However, it undergoes decomposition at pO₂<0.008 atm and at temperature ≥ 1100°C, limiting its value. In this study, doping with La on the A-site (10 and 30 cat%) was explored as an approach for tuning thermochemical behavior (i.e. reaction enthalpy, reaction entropy, reaction extent, reduction onset temperature, and thermal stability). The thermodynamic properties were determined from mass loss measurements at selected pO₂. Both 10 and 30 cat% La stabilized the base oxide against decomposition up to 1200°C at pO₂=0.008 atm, providing access to higher operating temperatures. At 10 cat% La, both the reaction enthalpy and the reaction entropy increased, whereas both decreased at the 30 cat% doping level. The chemical heat storage capacity of Ca_0.9La_0.1MnO₃ (~265 kJ/kg_ABO3) was found to be comparable to that of undoped CaMnO₃, which is in turn much higher than that of Ca_0.7La_0.3MnO₃ (~97 kJ/kg_ABO3) and slightly lower than was previously reported for CaMn_0.9Fe_0.1O₃ under similar conditions. The differences between the heat storage capacities of these materials can be fully understood in terms of the differences in the enthalpies and entropies of reduction. Entropy is not a metric usually considered when estimating the thermochemical storage capacity of a material, but it is an essential property because, as discussed here, the reaction extent depends monotonically on this term. Accordingly, materials with large entropy of reduction and intermediate enthalpy of reduction, where the latter metric represents a compromise between the heat per mole of reacted oxide and reaction extent, are desirable for chemical heat storage applications.