This article presents a study on the high-flux solar-driven thermochemical dissociation of CO₂ and H₂O using nonstoichiometric ceria. The research team, including William C. Chueh, Christoph Falter, Mandy Abbott, Danien Scipio, Philipp Furler, Sossina M. Haile, and Aldo Steinfeld, developed a solar reactor that efficiently captures concentrated solar radiation. The reactor uses nonstoichiometric ceria, which is a porous material that can undergo thermal reduction and oxygen evolution. The ceria was prepared by ball-milling with starch and graphite, followed by heat treatment to produce a mechanically robust porous monolith. The solar reactor is designed as a cavity-receiver with a cylindrical volume and a circular aperture. It includes a compound parabolic concentrator to enhance solar flux concentration and reduce re-radiation losses. The cavity is lined with thermally-insulating porous alumina tiles and closed with a quartz window. Reacting gases are injected into the annular gap between the porous ceria cylinder and the insulation tiles, and product gases exit through an axial outlet port. The experiments were conducted at the Paul Scherrer Institute's High-Flux Solar Simulator, which uses ten high-pressure xenon arcs to provide intense thermal radiation. Gaseous products were analyzed using various detectors, including gas chromatography, infrared-based, thermal conductivity-based, and paramagnetic alternating pressure-based detectors. The study calculates the solar-to-fuel energy conversion efficiency, considering the energy required for oxygen evolution and the energy input necessary to maintain reaction temperature. The efficiency is defined as the ratio of the fuel energy output to the solar energy input plus the energy required for inert gas separation. The results show that the efficiency is significantly influenced by the rate of fuel evolution, which is much higher than the rate of oxygen evolution. The reactor energy balance is analyzed, assuming the chemical reaction proceeds to completion and temperature gradients are negligible. The re-radiation power through the aperture is calculated based on the solar power, Stefan-Boltzmann constant, and solar concentration ratio. The heat loss due to inert gas heating is also considered. The remaining power loss is attributed to conductive, radiative, and convective losses at the reactor wall, some of which is due to active water-cooling. The study also includes X-ray diffraction patterns and morphology evolution of the ceria after heat-treatment.This article presents a study on the high-flux solar-driven thermochemical dissociation of CO₂ and H₂O using nonstoichiometric ceria. The research team, including William C. Chueh, Christoph Falter, Mandy Abbott, Danien Scipio, Philipp Furler, Sossina M. Haile, and Aldo Steinfeld, developed a solar reactor that efficiently captures concentrated solar radiation. The reactor uses nonstoichiometric ceria, which is a porous material that can undergo thermal reduction and oxygen evolution. The ceria was prepared by ball-milling with starch and graphite, followed by heat treatment to produce a mechanically robust porous monolith. The solar reactor is designed as a cavity-receiver with a cylindrical volume and a circular aperture. It includes a compound parabolic concentrator to enhance solar flux concentration and reduce re-radiation losses. The cavity is lined with thermally-insulating porous alumina tiles and closed with a quartz window. Reacting gases are injected into the annular gap between the porous ceria cylinder and the insulation tiles, and product gases exit through an axial outlet port. The experiments were conducted at the Paul Scherrer Institute's High-Flux Solar Simulator, which uses ten high-pressure xenon arcs to provide intense thermal radiation. Gaseous products were analyzed using various detectors, including gas chromatography, infrared-based, thermal conductivity-based, and paramagnetic alternating pressure-based detectors. The study calculates the solar-to-fuel energy conversion efficiency, considering the energy required for oxygen evolution and the energy input necessary to maintain reaction temperature. The efficiency is defined as the ratio of the fuel energy output to the solar energy input plus the energy required for inert gas separation. The results show that the efficiency is significantly influenced by the rate of fuel evolution, which is much higher than the rate of oxygen evolution. The reactor energy balance is analyzed, assuming the chemical reaction proceeds to completion and temperature gradients are negligible. The re-radiation power through the aperture is calculated based on the solar power, Stefan-Boltzmann constant, and solar concentration ratio. The heat loss due to inert gas heating is also considered. The remaining power loss is attributed to conductive, radiative, and convective losses at the reactor wall, some of which is due to active water-cooling. The study also includes X-ray diffraction patterns and morphology evolution of the ceria after heat-treatment.