Understanding the Fundamentals of the “Hercynite Cycle” and It Operational Behavior Under Pseudo-Isothermal Water Splitting Conditions

economically, efficiently and cleanly. Two-step, metal oxide based STWS cycles generate H₂ by a metal oxide undergoing sequential high temperature reduction and water re-oxidation.  The “hercynite cycle”, which is based on iron reduction in a spinel structure, relies on intimate contact between CoFe₂O₄ and Al₂O₃ phases though the exact mechanism was not well understood. In this work we study both the mechanism of the “hercynite cycle” and its operation under pseudo-isothermal conditions. Through a combination of computational and experimental studies using periodic boundary condition density functional theory, high temperature XRD and EDS we have determined that the hercynite cycle operates via an O-vacancy mechanism rather than the displacement reaction mechanism as previously suggested.  When operating under PITWS conditions we were able to produce H₂ in excess of 200 μmol/g, which enables the Hercynite cycle to maintain the high production rates of isothermal water splitting but with higher production rates.