Cobalt oxide is a candidate material for thermochemical heat storage via reversible reduction and re-oxidation reactions. In this research, the relationship between Gibb’ s free energy (Δ G) with reaction temperature (T) and oxygen partial pressure (PO2) and the relationship between equilibrium temperature (Te) and PO2 were investigated theoretically. It was found that an increase in reduction temperature decreases the reduction Δ G. Also, increasing PO2 increases the Te and reduces Δ G. In addition, isothermal reduction kinetics of Co3O4-5wt% Al2O3 (CA) and Co3O4-5wt% Y2O3 (CY) were investigated at various temperatures (1040-1130° C) by thermogravimetric analysis. Results showed that the CA sample desorbs more oxygen than the CY sample in similar conditions. A model-free method was used to calculate the reduction activation energies. It was found that activation energy required for reduction of CA and CY samples, depending on conversion fraction (α ), is in the range of 40-65 kcal/ mol and 25-50 kcal/mol, respectively. Furthermore, results showed that the reduction activation energy of CA and CY samples decreased and increased as the conversion fraction (α ) increased, respectively. The difference in the performance of alumina and yttria additions on the reduction of cobalt oxide was attributed to their ionic radii difference, the ability to create new compounds with different decomposition temperatures, and their different effect on the sintering of cobalt oxide particles.

The effect of Al2O3 (1-10 wt %) and Y2O3 (1-10 wt %) additions on thermochemical heat storage properties of Co3O4/CoO system was investigated by thermogravimetry, XRD, and SEM analyses. Results showed that the addition of Al2O3 to Co3O4 at constant 8 h mechanical activation improved the redox cycle stability and increased oxygen sorption value and rate. It was found that oxygen sorption and their rate decreased with increasing the alumina content to more than 8 wt %. The formation of the spinel phase and an increase in its amount by increasing the alumina content led to a decrease in the oxygen sorption capacity. SEM studies showed that Al2O3 prevented the sintering and particle growth of cobalt oxide particles during reduction and re-oxidation processes. In addition, results showed that the addition of Y2O3 in all ranges to Co3O4 improved the redox cycle stability of cobalt oxide; however, it significantly decreased the oxygen sorption in the Co3O4/CoO system. XRD patterns of a sample containing 10 wt % yttria before the redox process indicated the presence of only Co3O4 phase; however, after three redox cycles, other phases including CoO and Y2O3 appeared.