Likely near-term solar-thermal water splitting technologies

Thermodynamic and materials considerations were made for some two- and three-step thermochemical cycles to split water using solar-thermal processing. The direct thermolysis of water to produce H2 using solar-thermal processing is unlikely in the near term due to ultra-high-temperature requirements exceeding 3000 K and the need to separate H2 from O2 at these temperatures. However, several lower temperature (<2500 K) thermochemical cycles including ZnO/Zn, Mn2O3/MnO, substituted iron oxide, and the sulfur–iodine route (S–I) provide an opportunity for high-temperature solar-thermal development. Although zirconia-based materials are well suited for metal oxide routes in terms of chemical compatibility at these temperatures, thermal shock issues are a major concern for solar-thermal applications. Hence, efforts need to be directed towards methods for designing reactors to eliminate thermal shock (ZrO2 based) or that use graphite (very compatible in terms of temperature and thermal shock) with designs that prevent contact of chemical species with graphite materials at high temperatures. Fluid-wall reactor configurations where inert gases provide a blanket to protect the graphite wall appear promising in this regard, but their use will impact process efficiency. For the case of S–I up to 1800 K, silicon carbide appears to be a suitable material for the high-temperature H2SO4 dissociation. There is a need for a significant amount of work to be done in the area of high-temperature solar-thermal reactor engineering to develop thermochemical water splitting processes.