Isothermal Redox Kinetics of Co3O4-Fe2O3 Nano-Composite as a Thermochemical Heat Storage Material

Isothermal redox kinetics of as-received Co3O4 (AC), 1 h ball milled Co3O4 (BC), and 1 h ball milled Co3O4-15wt.% Fe2O3 (BCF) was investigated at various temperatures (1130, 1100, 1070, and 1040 °C for reduction and 830, 860, and 890°C for re-oxidation) by thermogravimetric method. It was found that mechanical activation with and without Fe2O3 addition decreases the rate of reduction in all isothermal reduction temperatures, while it improves the rate of re-oxidation in lower re-oxidation temperatures. Mechanical activation with and without Fe2O3 addition preserves and decreases the rate of re-oxidation at higher re-oxidation temperatures, respectively. In addition, according to the results, the re-oxidation kinetics was slower than reduction kinetics. A model-free method was used to calculate the redox activation energies. It was found that mechanical activation individually increases the reduction activation energy, while mechanical activation along with Fe2O3 addition decreases the reduction activation energy in comparison with as-received Co3O4. The results showed that reduction activation energies of AC, BC, and BCF samples varies depending on the reacted fraction (α) and are in the range of  140.6 – 166.6 kJ/mol, 175.5 – 213.7 kJ/mol and 111.5 – 121.3 kJ/mol, respectively. The results also showed that although both mechanical activations with and without Fe2O3 addition decreases the re-oxidation activation energy in comparison with as-received cobalt oxide, but the impact of mechanical activation without Fe2O3 addition on activation energy decline, is higher. Moreover, it was found that activation energies for the re-oxidation of AC, BC, and BCF samples are negative, variations depending on the reacted fraction (α), and are in the range of  -76.8 to -133.5 kJ/mol, -440.2  to -471.9 kJ/mol, and -190.8 to -196.05 kJ/mol, respectively.