Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions II: Kinetic analyses

•Coupled Air Brayton cycle and two-step solar thermochemical cycle.•Heat storage based on CoO/Co3O4 redox reactions.•Kinetic analyses conducted using isothermal and non-isothermal thermogravimetry.•Thermolysis follows pressure-dependent Avrami–Erofeyev nucleation model.•Oxidation is pressure-independent, follows an ionic diffusion model. A two-step solar thermochemical cycle based on Co3O4/CoO redox reactions integrated into an Air Brayton cycle is considered for thermochemical heat storage. The two-step cycle encompasses (1) the thermolysis of Co3O4 to CoO and O2 driven by concentrated solar irradiation and (2) the re-oxidation of CoO with O2 to Co3O4, releasing heat and completing the cycle. The cycle steps can be decoupled, allowing for thermochemical heat storage and integration into an Air Brayton cycle for continuous electricity production. Kinetic analyses to identify the rate limiting mechanisms and determine kinetic parameters for both the thermolysis of Co3O4 and the re-oxidation of CoO with O2 were performed using a combination of isothermal and non-isothermal thermogravimetry. The Co3O4 thermolysis between 1113 and 1213K followed an Avrami–Erofeyev nucleation model with an Avrami constant of 1.968 and apparent activation energy of 247.21kJmol−1. The O2 partial pressure dependence between 0% and 20% O2–Ar was determined with a power rate law, resulting in a reaction order of 1.506. Ionic diffusion was the rate limiting step for CoO oxidation between 450 and 750K with an apparent activation energy of 58.07kJmol−1 and no evident dependence on O2 concentration between 5% and 100% O2–Ar. Solid characterization was performed using scanning electron microscopy and X-ray powder diffraction.