(345h) Enhanced Radiative Heat Transfer of Concentrated Solar Energy in Hierarchically Ordered Porous Structures

We report on the testing of novel porous ceria structures for CO2 splitting via a redox thermochemical cycle driven by concentrated solar energy. These structures were 3D-printed using a Direct Ink Writing (DIW) technique and were designed with hierarchically ordered porosity gradients to circumvent Beer-Lambert’s exponential attenuation law and thus enable longer propagation of the incoming radiation. Experimental testing was performed in a solar-driven thermogravimetric analyzer, in which the ceria samples were directly exposed to high-flux irradiation, mimicking the realistic operation of solar reactors. At the same time, their mass change, temperature, and product gas composition were continuously monitored during the redox reactions. Temperatures and reaction extents were compared with those obtained with a reticulated porous ceramic (RPC) structure with comparable mass per unit volume, serving as the state-of-the-art reference. Remarkably, the 3D-printed structures achieved a higher and more uniform temperature profile than the RPC: for a radiative flux equivalent to 1280 suns, equilibrium temperatures in the range 1320-1480°C were measured for the 3D-printed structures vis-a-vis 870-1340°C for the RPC. This is attributed to enhanced volumetric absorption, attaining the so-called volumetric effect where front temperatures are lower than the inner temperatures, reducing radiative heat losses. The higher and more uniform temperatures led to a higher oxygen exchange during the redox cycle, doubling the CO yield.