Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

We develop an approach for extracting gas−solid kinetic information from convoluted experimental data and demonstrate it on isothermal carbon dioxide splitting at high-temperature using CoFe 2 O 4 /Al 2 O 3 (i.e., a "hercynite" cycle based on Co-doped FeAl 2 O 4 ) active material. The reaction kinetics equations we derive account for competing side reactions, namely catalytic CO 2 splitting on and O 2 oxidation of doped hercynite, in addition to CO 2 splitting driven by the oxidation of oxygendeficient doped hercynite. The model also accounts for experimental effects, such as detector dead time and gas mixing downstream of the reaction chamber, which obscure the intrinsic chemical processes in the raw signal. A second-order surface reaction model in relation to the extent of unreacted material and a 2.4th-order model in relation to CO 2 concentration were found to best describe the CO generation of the doped hercynite. Overall, the CO production capacity was found to increase with increasing reduction temperature and CO 2 partial pressure, in accordance with previously predicted behavior. The method outlined in this paper is generally applicable to the analysis of other convoluted gas−solid kinetics experiments.