The Chemistry of the “Herycnite Cycle” Solarthermal Water Splitting Reactions

Solar thermal water splitting (STWS) is a promising technology for producing hydrogen gas 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, is attracting increased attention due to its ability to undergo isothermal water splitting. However, the hercynite STWS mechanism was not well understood. 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. The O-vacancy mechanism operates by the formation and filling of O-vacancies in the spinel structure during the reduction and oxidation steps respectively, rather than via the phase changes which characterize the displacement reaction mechanism.