Near-isothermal ferrite/alumina ("hercynite cycle") twostep Red/Ox cycle for solar-thermal water splitting

Hydrogen productivity exceeding 350 micromoles H₂/g total redox material has been demonstrated for near-isothermal processing using the "hercynite cycle" for oxidation with steam carried out at 1350°C following 1500°C reduction. This temperature difference driving the redox is quite narrow compared to standard 500oC temperature swing (T-swing) redox processing. Such processing substantially reduces the difficult solid/solid heat recuperation required for standard Tswing systems and the thermal stresses associated with heating/cooling active materials during redox cycling. Focused ion beam (FIB) milling followed by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) after 200 redox cycles shows that the ferrite/alumina is well-dispersed, indicating a robust active redox material. Efficiency analysis identifies isothermal processing with perfect steam/steam heat exchange as the highest theoretically possible efficiency. Since isothermal processing at the highest reduction temperatures is unlikely due to simultaneous redox (producing both H₂ and O₂ together), near-isothermal processing provides for the best scenario to achieve the highest solar-thermal process efficiency possible.