Metal oxide composites and structures for ultra-high temperature solar thermochemical cycles

Conceptually, thermochemical cycles are heat engines that drive endothermic chemical reactions, e.g., splitting water into hydrogen and oxygen. The two-step metal oxide cycles (typically ferrite-based) are particularly attractive since they are relatively simple, use non-corrosive materials, and involve gas–solid reactions requiring no difficult separations. Additionally, they are potentially the most efficient renewable-energy driven processes for hydrogen production. We are developing a novel concentrating solar power (CSP) driven metal-oxide-based heat engine, the CR5, at the heart of which are rings of a reactive solid that are thermally and chemically cycled to produce oxygen and hydrogen from water in separate and isolated steps. The monolithic ring structures must have high geometric surface area for gas–solid contact and for adsorption of incident solar radiation, and must maintain structural integrity and high reactivity after extensive thermal cycling to temperatures of at least 1,400 C. We have demonstrated through laboratory and on-sun testing that cobalt ferrite/zirconia mixtures fabricated into monolithic structures suitable for the CR5 are mechanically robust and maintain productivity over tens of cycles. We have also demonstrated that carbon dioxide splitting (CDS) to carbon monoxide and oxygen is a thermodynamically favorable alternative to water splitting that can be conducted with both iron- and cerium-based materials.