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.