Review of the Two-Step H(2)O/CO(2)-Splitting Solar Thermochemical Cycle Based on Zn/ZnO Redox Reactions

This article provides a comprehensive overview of the work to date on the two‑step solar H(2)O and/or CO(2) splitting thermochemical cycles with Zn/ZnO redox reactions to produce H(2) and/or CO, i.e., synthesis gas—the precursor to renewable liquid hydrocarbon fuels. The two-step cycle encompasses: (1) The endothermic dissociation of ZnO to Zn and O(2) using concentrated solar energy as the source for high-temperature process heat; and (2) the non-solar exothermic oxidation of Zn with H(2)O/CO(2) to generate H(2)/CO, respectively; the resulting ZnO is then recycled to the first step. An outline of the underlying science and the technological advances in solar reactor engineering is provided along with life cycle and economic analyses.