Mixed Co, Cu and Mn-based Metal Oxides for Thermochemical Energy Storage Application

The potential of metal oxides for thermochemical heat storage in solar power plants at high temperature via reversible redox reactions has been largely demonstrated, and cobalt oxide and manganese oxide commonly appear as the most attractive simple oxides. However, drawbacks of pure oxides such as slow reaction kinetics, low reversibility, loss-in-capacity over cycles or sintering, could be tackled by the addition of a secondary oxide This work presents the experimental evaluation of mixed oxides from the Co-Cu-O, Mn-Cu-O, and Co-Mn-O systems. Within the studied series of mixed oxides, the Co-Cu-O system with low amounts of Cu (<= 10 mol%) shows very good cycling stability and high reaction enthalpy (similar to 570 kJ.kg(-1)). Among the mixed oxides studied in the Mn-Cu-O system, the compositions with Cu amounts in the range 40-80 mol% feature promising redox properties with complete reaction reversibility, even though sintering remains an issue. In contrast, compositions with Cu amounts below 30 mol% cannot be cycled because of the formation of the hausmannite phase during reduction, which inhibits further reoxidation. The compositions with less than 40% Mn in the Co-Mn-O system retain an interesting enthalpy for thermochemical energy storage in CSP plants for a cheaper material than pure cobalt oxide, but the reaction enthalpy decreases with the Mn content. In this system, the cycling ability is lost over 70 mol% Mn due to hausmanite phase formation, similarly to the case of Mn-Cu-O system.