(381a) The Chemistry of the “Herycnite Cycle” Solarthermal Water Splitting Reactions

Muhich, C. L., University of Colorado at Boulder
Weston, K., University of Colorado at Boulder
Arifin, D., University of Colorado Boulder
McDaniel, A. H., Sandia National Laboratories
Coker, E. N., Sandia National Laboratories
Musgrave, C. B., University of Colorado
Weimer, A. W., University of Colorado at Boulder

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.