Redox Oxides-Based Solar Thermochemistry and Its Materialization to Reactor/Heat Exchanger Concepts for Efficient Solar Energy Harvesting, Transformation and Storage

Ca-Mn-based perovskites doped in their A- and B-site were synthesized and comparatively tested versus the Co₃O₄/CoO and (Mn,Fe)₂O₃/(Mn,Fe)₃O₄ redox pairs with respect to thermochemical storage and oxygen pumping capability, as a function of the kind and extent of dopant. The perovskites' induced heat effects measured via differential scanning calorimetry are substantially lower: the highest reaction enthalpy recorded by the CaMnO_3–δ composition was only 14.84 kJ/kg compared to 461.1 kJ/kg for Co₃O₄/CoO and 161.0 kJ/kg for (Mn,Fe)₂O₃/(Mn,Fe)₃O₄. Doping of Ca with increasing content of Sr decreased these heat effects; more than 20 at % Sr eventually eliminated them. Perovskites with Sr instead of Ca in the A-site exhibited also negligible heat effects, irrespective of the kind of B site cation. On the contrary, perovskite compositions characterized by high oxygen release/uptake can operate as thermochemical oxygen pumps enhancing the performance of water/carbon dioxide splitting materials. Oxygen pumping via Ca_0.9Sr_0.1MnO_3–δ and SrFeO_3–δ doubled and tripled, respectively, the total oxygen absorbed by ceria during its re-oxidation versus that absorbed without their presence. Such effective pumping compositions exhibited practically no shrinkage during one heat-up/cool-down cycle. However, they demonstrated an increase of the coefficient of linear expansion due to the superposition of “chemical expansion” to thermal-only one, the effect of which on the long-term dimensional stability has to be further quantified through extended cyclic operation.