Improving solar fuel production performance from H2O and CO2 thermochemical dissociation using custom-made reticulated porous ceria

Abstract

Thermochemical CO2- and H2O-splitting cycles for sustainable fuel generation were investigated by using custom-made reticulated ceria foams integrated in a solar-heated cavity reactor. A parametric study revealed the suitable conditions to produce H2/CO with maximum fuel rates and yields per unit mass of redox material. Various operating parameters such as the total pressure during the reduction step, the gas inlet flowrates, the temperature, or the reactive gas content during the oxidation step were studied in detail. A series of on-sun experiments including more than 20 cycles under different cycling conditions were carried out with relevant performance repeatability and stability. With a ceria foam reduction temperature in the range 1400–1470 °C and a reduction pressure of 103 mbar, the ceria foams produced 281 μmol of H2 and 332 μmol of CO per gram of material during oxidation below 1000 °C under cooling with a maximum production rate of 3.0 mL/min.g and 10.2 mL/min.g, respectively. Direct syngas production was also evidenced during on-sun experiments with simultaneous H2O- and CO2-splitting, which yielded a H2:CO ratio ranging from 0.7 to 1.14. The dual-scale porous ceria structures were characterized using X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis to confirm material thermal stability during cycling. Finally, optimization of fuel production capacity was achieved by maximizing the total amount of ceria foam loaded into the reactor, yielding 667 mL of CO (409 μmol/g) and 513 mL of H2 (314 μmol/g) per cycle. A maximum solar-to-fuel efficiency of 10.1% was calculated for CO2-splitting vs. 4.9% for H2O-splitting cycle. This study thus demonstrated noteworthy fuel production performance in an efficient monolithic reactor under real concentrated solar radiation, from highly reactive customized ceria foams.