Optimal Porosity Distribution of Non-Uniform Macroporous Ceria in Solar Thermochemical Water Splitting

Solar-driven thermochemical water and carbon dioxide splitting is a promising technology which produces synthesis gas in a sustainable way. Previous reactor models showed that ceria-based oxides with macro-porous structure could offer higher surface area and facilitate the endothermal solid-gas reaction by increasing the solar heat absorption. However, non-uniform boundary heat flux and variant fluid concentration along the reactor would lead to large temperature and partial pressure gradient profiles, and . Therefore, a carefully designed non-uniform porosity distribution of the macroporous ceria, matching the heat and flow patterns, could enhance the thermal performance, reduce the concentration difference, increase the reactant use, and essentially improve the overall fuel yield behavior in the solar-driven reactor.

We developed a multi-scale and multi-physics modeling framework for the complete solar-driven two-step redox cycle process, including reduction and oxidation reactions. Based on the volume-averaged conservation equations of mass, momentum, and energy, this model incorporated morphology related effective properties of the porous ceria, coupled the heat and mass transfer interactions, and non-equilibrium local heterogeneous chemistry between solid and fluid phase. The effective transport properties of the porous media were determined by direct pore-level simulations and Monte Carlo ray tracing utilizing the digital 3D micro-structures obtained by X-ray computed tomography.

The porosity distribution was optimized for enhanced reaction rate and fuel yield, considering different operational conditions (temperature, mass flow rate and species concentration). The optimization objectives were to maximize the conversion efficiency and rate of solar energy into chemical energy, considering their constraints given by the conservation equations.

The results indicated that, under certain operation conditions, volumetric radiation absorption could be increased and species concentration could be smoothened and made more uniform within the non-uniform porous reactor. The advancements in the reaction rate and fuel yield for the optimized non-uniform porosity distribution are quantified.

The optimized non-uniform porosity distribution function of the ceria provides a practical opportunity in additive manufacturing, material engineering, and scalable manufacturing.