Co2.8Mg0.2O4 as a promising thermochemical energy storage material with lower reduction onset temperature and higher energy density

Co-based oxides have been considered as one of the most promising materials for thermochemical energy storage (TCES) systems, however, the high operation temperature limits their applications. Specially, when Co 3 O 4 -based materials are used in concentrated solar power (CSP) system, a large mirror field area is required to meet the high endothermic temperature, which significantly increases the cost of a CSP system. In addition, the high cost of Co-based materials also limits its application. Improving the energy density can effectively reduce the use of the materials. Thus, in this work, to reduce the reduction onset temperature and increase the energy density of Co-based oxides, the TCES performance of Co 3-x Mg x O 4 oxides is investigated. Co 2.8 Mg 0.2 O 4 shows the best TCES performance. The reduction onset temperature of Co 2.8 Mg 0.2 O 4 is ∼775.6 °C, which is 133.7 °C lower than that of the pure Co 3 O 4 (∼909.3 °C). Mg-decoration can also improve the energy density of Co 3 O 4 by 8.02 %. Furthermore, the in situ XRD spectra of the oxides indicate that precipitation of MgO from the Co 3 O 4 lattice during the reduction reaction result in a metastable structure of the oxide, which is conducive to its reduction reaction. Density functional theory calculations indicate that Co 2.8 Mg 0.2 O 4 possessed a higher defect formation energy and shorter chemical bonds than the pure cobalt oxide , which result in its lower reduction onset temperature and higher energy density. The cost-benefit analysis results showed that Mg-decoration could significantly reduce the area of mirror field and the use of energy storage material for a CSP system.