Effects of TiO₂ doping on the performance of thermochemical energy storage based on Mn₂O₃/Mn₃O₄ redox materials

A thermochemical energy storage (TCES) system can adjust problems of unstable energy supply for solar concentrating power plants. Mn₂O₃/Mn₃O₄ system is a promising TCES system, but it has the problem of a difficult reoxidation process. In this paper, TiO₂ was doped into the manganese oxide TCES system to solve this problem and the factors which influence the performance of this method were analyzed. The different performances between commercial Mn₂O₃ (Mn) and Mn₂O₃ synthesized by the Pechini method (PCMn), and different scales of doping agents (25Ti, 100Ti) were compared. Because of the formation of the Mn₂TiO₄, adding TiO₂ into the manganese oxide TCES system could improve its reoxidation process obviously. During single complete redox process, PCMn had better performance than Mn whether doped with TiO₂ or not, but Mn had a higher optimum oxidation temperature and a narrow temperature range of the redox reactions after adding TiO₂. Adding 25Ti could bring higher energy storage density than adding 100Ti, and the optimal doping ratio was 0.05. As the doping ratio of 25Ti was increased, the activation energy (E _a) was increased and then decreased. The E _a of the samples doped with 25Ti was higher than that doped with 100Ti. Moreover, the E _a of the 25Mn0.05 was decreased firstly and then was increased in the later stage of the reaction. The doped Mn samples exhibited better performance and lower attenuation than the doped PCMn samples after 30 cycles. During cyclic tests, the Mn₂TiO₄ was initially formed at the boundary between Mn₂O₃ and TiO₂, and it was generated continuously with the extension of operating time. Therefore, the operating temperature, morphology of the Mn₂O₃, the doping agents, the doping ratio, and the phase change with the operating time should be all considered when doping TiO₂ into the Mn₂O₃/Mn₃O₄ TCES system to improve its performance. Moreover, the results obtained from Mn-Ti systems would make a lot sense when other similar systems are considered, such as Mn-Fe, Mn-Si, Mn-Cr, etc.