Fe3O4/CaO Mixture Enables Hydrogen Storage Comparable to Metal Hydrides

Abstract

A new category of hydrogen storage material is reported. The material is functionally similar to metal hydrides but is chemically composed of two metal oxides: iron oxide, which reversibly reacts with hydrogen to produce iron and water, and calcium oxide, which reversibly absorbs the water in situ. Hydrogen is stored simply upon contact with the material, with the reduction of iron oxide and absorption of water taking place in sequence. The direction of the reaction is easily controlled by changing the system temperature, pressure, or both. On the applicational side, the material is tested in a fixed bed for almost 100 cycles over the course of five months with a stable storage capacity of 10 g H2/kg. To reach a high degree of charge, a high pressure (10 bar) and relatively low temperature (350 °C) are needed. To investigate these effects in detail, thermogravimetric studies on the performance of the two components are conducted separately, revealing the inhibiting effect of water vapor on the reduction of iron oxide at lower pressures. By adjusting temperature and production rates during discharging, the process allows the generation of high-purity, pressurized H2 (up to 5.4 bar at 500 °C), making it directly usable by power-generating devices. On the theoretical side, a detailed thermodynamic analysis is conducted to support the experimental results and explain the optimal reaction conditions. This material behaves similarly to a metal hydride in terms of operational simplicity while offering additional advantages of material cost (7 $/kg H2 capacity) and stability in the presence of oxygen or water vapor. In comparison, magnesium hydride can reach a higher practical storage density of 6 wt %, but its sluggish kinetics often require more than 30 bar for hydrogenation, and it has a significantly higher material cost at 25 $/kg H2 capacity. The simplicity, low cost, and high oxygen resistance of the FeOx/CaO mixture make it highly interesting in the field of hydrogen storage.