Superior thermochemical energy storage performance of the Co3O4/CoO redox couple with a cubic micro-nanostructure

Co3O4 has been regarded as one of the most promising materials for redox energy storage due to its high theoretical conversion and complete redox reversibility. However, when undergoing charge-discharge cycles at high temperature, the material becomes sintered. The reversibility of the cobaltous oxides highly deteriorates, and the reoxidation reaction rate notably decreases. In addition, the obvious thermal hysteresis in the redox reaction process also limits the energy storage performance of the material. In this work, Co3O4 with different micro-nanostructured morphologies is synthesized to evaluate its thermochemical energy storage performance. The capacity of the synthesized and commercial Co3O4 is tested by thermogravimetric analysis (TGA). The fresh and recycled oxides are well characterized. Results suggest that all samples show complete reversibility, and the conversion decreases slightly after 30 cycles. The synthesized samples show better performance, especially cubic Co3O4. The reoxidation rate of the commercial sample is approximately 150 mu mol/min/g, but that of the cubic sample is approximately 300-200 mu mol/min/g in the first five cycles. The oxidation rate of the cubic sample decreases and finally remains at about 180 mu mol/min/g from the 5th cycle to the 30th cycle. After 5 charge discharge cycles, the commercial Co3O4 is severely sintered, while the cubic oxides are only slightly sintered. After 30 redox cycles, the morphology of commercial oxides completely changed, while that of the cubic oxide retains the approximate shape of a cube. Moreover, the thermal hysteresis values of the commercial and cubic Co3O4 are 24.7 degrees C and 10.1 degrees C, respectively, suggesting that the energy loss of cubic oxides is much smaller. The higher exothermic temperature of the cubic Co3O4 redox process also indicates that it can supply high-grade thermal energy.