Thermochemical Hydrogen Storage via the Reversible Reduction and Oxidation of Metal Oxides

In this work, we perform a technical analysis of a cyclic reduction and oxidation process of metal oxides applied as a hydrogen storage route. Hydrogen is used to reduce a metal oxide and form steam, and regenerated by oxidizing the reduced metal oxide with steam. The reduced metal acts as the solid storage medium, which could be transported or used for stationary applications. Suitable metal oxides were screened according to the following performance indicators: specific energy demand (enthalpy change of reaction), thermodynamic conversion extents, specific storage density, resistance to sintering (melting point), and material cost. Iron oxide was then selected as the most promising candidate. A thermodynamic model was developed to determine favorable process temperatures of >400 °C for the hydrogen storage step and 100-500 °C for the hydrogen release step. The energy demand of the storage was calculated to be 27% of the lower heating value of hydrogen. Vis-a-vis other hydrogen storage methods, the iron-based storage exhibits drawbacks for on-board mobile applications but could be attractive for large-scale and long-term storage applications.