Predicting the solar thermochemical water splitting ability and reaction mechanism of metal oxides: A case study of the hercynite family of water splitting cycles

A screening method is developed to determine the viability of candidate redox materials to drive solar thermal water splitting (STWS) and the mechanism by which they operate using only the reduction enthalpy of the material. This method is applied to the doped-hercynite water splitting cycle, as well as FeAl₂O₄ and CoAl₂O₄, materials which have not been previously experimentally demonstrated for STWS. Density functional theory (DFT) calculations of reduction energies coupled with our screening method predict H₂ production capacities for iron and cobalt aluminate spinels to be in the order FeAl₂O₄ > Co_0.5Fe_0.5Al₂O₄ > CoAl₂O₄ with relative H₂ production capacity ratios of approximately 1.0 to 0.7 to 2 × 10^-4, respectively. Experimental measurements for 1500/1350 °C redox temperatures validate the H₂ production capacity predicted by the screening method by demonstrating H₂ production ratios of 1.0 to 0.6 to 0. Un-doped hercynite (FeAl₂O₄) is shown to be a viable STWS material for the first time with a higher H₂ production capacity than traditional doped-hercynite materials. Theory and experiments show that redox of the aluminate family of spinel materials operates via an O-vacancy mechanism rather than a stoichiometric one, which is more typical for ferrites. The screening approach is generally useful for predicting the ability of new complex materials to drive STWS and the mechanism by which they operate, thus, providing a method to identify promising new candidate STWS materials.