(288d) Solar Thermochemical Water Splitting Using Iron Aluminates

Technologies like solar thermochemical hydrogen (STCH) production, which leverage the oxygen-exchange properties of metal oxides to split water, are attractive because they can harness the entire solar spectrum, whereas photovoltaic-driven alternatives (e.g., water electrolysis) rely only on specific wavelength ranges. Generally, this approach involves alternating between two steps namely, (1) the endothermic removal of oxygen from an oxide and (2) the exothermic oxidation of the reduced oxide with water to produce hydrogen. The endo/exothermic nature of each reaction implies that opposite changes in temperature are required to drive the hydrogen yield per cycle. However, in practice, implementing a temperature swing between redox regimes imposes significant irreversibilities that arise from sensible heating of the solid, thus resulting in inefficient operation unless heat is recovered via ultra-high temperature thermal energy storage (a technology still in its infancy). Therefore, in this work, a scalable prototype reactor – situated within a well-insulated cavity receiver – was designed and operated such that the water-splitting (oxidation) step was intentionally initiated at temperatures where reduction is favorable, thereby avoiding the complications associated with solid-solid heat recuperation. To mitigate the undesired thermodynamic consequence of performing the exothermic oxidation reaction at higher temperatures, iron aluminates, a low-cost class of materials with an exceptional isothermal capacity for hydrogen production, were considered for this prototype demonstration. The stable experimental results, obtained under concentrated radiation from a 45 kW_e high-flux solar simulator, provide compelling motivation for the commercial viability of solar thermochemical routes for hydrogen production.