Materials and Methods Som Text Figs. S1 to S23 Tables S1 to S7 Data Sets S1 to S17 High-flux Solar-driven Thermochemical Dissociation of Co 2 and H 2 O Using Nonstoichiometric Ceria

Because solar energy is available in large excess relative to current rates of energy consumption, effective conversion of this renewable yet intermittent resource into a transportable and dispatchable chemical fuel may ensure the goal of a sustainable energy future. However, low conversion efficiencies, particularly with CO 2 reduction, as well as utilization of precious materials have limited the practical generation of solar fuels. By using a solar cavity-receiver reactor, we combined the oxygen uptake and release capacity of cerium oxide and facile catalysis at elevated temperatures to thermochemically dissociate CO 2 and H 2 O, yielding CO and H 2 , respectively. Stable and rapid generation of fuel was demonstrated over 500 cycles. Solar-to-fuel efficiencies of 0.7 to 0.8% were achieved and shown to be largely limited by the system scale and design rather than by chemistry. L ong-term storage and long-range transport of the vast, yet intermittent and unevenly distributed, solar energy resource is essential for a transition away from fossil energy (1). Chemical fuels, derived from CO 2 and/or H 2 O, offer exceptional energy density and convenience for transportation, but their production using solar energy input has remained a grand challenge (2–9). Solar-driven thermochemical approaches to CO 2 and H 2 O dissociation inherently operate at high temperatures and use the entire solar spectrum; as such, they provide an attractive path to solar fuel production at high rates and efficiencies in the absence of precious metal catalysts (10). In contrast to direct thermolysis of CO 2 and H 2 O, two-step ther-mochemical cycles using metal oxide redox reactions further bypass the COO 2 or H 2-O 2 separation problem (11). Among candidate redox materials, ferrite-based oxides exhibit relatively slow reaction rates, degradation in rates because of sintering, and losses because of uncontrolled volatilization, whereas ZnO, SnO 2 , and analogous volatile oxides that sublime during decomposition require rapid quenching of gaseous products to avoid recombination (10–18). Ceri-um oxide (ceria) has emerged as a highly attractive redox active material choice for two-step thermochemical cycling because it displays rapid fuel production kinetics and high selectiv-ity (17, 19–24), where such features result, in part, from the absence of distinct oxidized and reduced phases. However, ceria-based thermo-chemical studies to date have largely been limited to bench-top demonstrations of components or individual steps of the solar fuel production cycle ; assessment of cyclability has been limited, and the energy conversion efficiency has …