On the development of thermochemical hydrogen storage: An experimental study of the kinetics of the redox reactions under different operating conditions

This work aims at investigating the reduction/oxidation (redox) reaction kinetics on iron oxide pellets under different operating conditions of thermochemical hydrogen storage. In order to reduce the iron oxide pellets (90% Fe₂O₃, 10% stabilizing cement), hydrogen (H₂ ) is applied in different concentrations with nitrogen (N₂ ), as a carrier gas, at temperatures between between 700 °C and 900 °C, thus simulating the charging phase. The discharge phase is triggered by the flow of a mixture out of steam (H₂O) and N₂ at different concentrations in the same temperature range, resulting in the oxidizing of the previously reduced pellets. All investigations were carried out in a thermo-gravimetric analyzer (TGA) with a flow rate of 250 mL/min. To describe the obtained kinetic results, a simplified analytical model, based on the linear driving force model, was developed. The investigated iron oxide pellets showed a stable redox performance of 23.8% weight reduction/gain, which corresponds to a volumetric storage density of 2.8 kWh/(L bulk), also after the 29 performed redox cycles. Recalling that there is no H₂ stored during the storage phase but iron, the introduced hydrogen storage technology is deemed very promising for applications in urban areas as day-night or seasonal storage for green hydrogen.