Abstract
Thermochemical processes are considered promising pathways to utilize solar energy for fuel production. Several physico-chemical, kinetic and thermodynamic properties of candidate oxides have been studied, yet their morphological stability during redox cycling under radiative heating is not widely reported. Typically when it is reported, it is for large-scale directly irradiated reactors (~1–10 kWth) aimed at demonstrating high efficiency, or in indirectly irradiated receivers where the sample surface is not exposed directly to extreme radiative fluxes. In this work, we aimed to emulate heat flux conditions expected in larger scale solar simulators, but at a smaller scale where experimentation can be performed relatively rapidly and with ease compared to larger prototype reactors. To do so, we utilized a unique infrared (IR) laser-based heating system with a peak heat flux of 2300 kW/m2 to drive redox cycles of two candidate materials, namely nonstoichiometric CeO2-δ and La0.6Sr0.4MnO3-δ. In total, 200 temperature-swing cycles using a porous ceria pellet were performed at constant pO2, and 5 cycles were performed for both samples by introducing H2O vapor into the system during reduction. Porous ceria pellets with porosity (0.55) and pore size (4–7 μm) were utilized because of their similarity to other porous structures utilized in larger-scale reactors. Overall, we observed that reaction extents initially decreased along with the decrease in reaction rates up to cycle 120 because of the change in structure and sintering. In the case of H2O splitting, ceria outperformed LSM40 in total H2 production because of the low pO2 during oxidation, where the oxidation of LSM40 is less favorable than that of ceria.
Highlights
Thermochemical processes are one of several promising pathways capable of utilizing solar energy to produce hydrocarbon fuel precursors or pure H2 via the splitting of CO2 and/or H2 O [1,2,3,4].These operate via two-step reduction and oxidation cycles using metal oxides as a reactive intermediate
The X-ray powder diffraction (XRD) pattern of fresh LSM40 powder is shown in Figure 2 and alongside comparisons to the pattern fresh LSM40 powder is shown in Figure and alongside comparisons to literature areofshown in Figure confirmed the 2perovskite structure of LSM40 the literature are shown in Figure confirmed theICP-MS
We demonstrated thermochemical redox cycles driven by direct irradiation of porous samples at a much smaller scale than is typical of solar reactors
Summary
Thermochemical processes are one of several promising pathways capable of utilizing solar energy to produce hydrocarbon fuel precursors or pure H2 via the splitting of CO2 and/or H2 O [1,2,3,4]. These operate via two-step reduction and oxidation (redox) cycles using metal oxides as a reactive intermediate. Marxer et al recently achieved a solar-to-fuel efficiency of 5.25% using an engineered reticulated porous ceramic (RPC) of ceria contained within a cavity-based receiver [8] It contains large pores (i.e., 2.5 mm mean diameter) for enhanced radiative heat transfer and small pores (i.e., 10 μm mean diameter) for enhanced reaction rates. Perovskites with ABO3 structure have gained interest because of the potential
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