Solar Redox Cycling of Ceria Structures Based on Fiber Boards, Foams, and Biomimetic Cork-Derived Ecoceramics for Two-Step Thermochemical H<sub>2</sub>O and CO<sub>2</sub> Splitting

Journal of CO2 Utilization

Julbe

et al. 2020

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Solar thermochemical cycles offer a viable option for the production of green synthetic fuels from CO 2 and H 2 O. Two-step cycles using redox materials consist of a high-temperature reduction creating oxygen vacancies, followed by a re-oxidation step with an oxidant gas (CO 2 and/or H 2 O), resulting in CO and/or H 2 production. This study focuses on the thermochemical performance in a solar reactor of a new kind of composite reactive material: reticulated ceria foam with uniform perovskite coating, forming a dual-phase layered heterostructure. La 0.5 Sr 0.5 Mn 0.9 Mg 0.1 O 3 (LSMMg) was selected as perovskite coating due to its high fuel productivity and thermochemical stability upon cycling. The perovskite coating improves the reduction step by increasing the reduction extent reached by the reactive material. The results revealed significant enhancement of oxygen exchange in the dual-phase composite material during reduction compared to individual components. The enhanced reduction extent had a beneficial effect on the oxygen release rate and the total amount of fuel produced by CO 2 and H 2 O splitting, whereas an adverse impact on the peak rate during H 2 /CO evolution was noticed. Decreasing the reduction pressure allowed enhancing the non-stoichiometric oxygen extent, while the re-oxidation extent increased with the inlet oxidant molar fraction. With suitable operating conditions (reduction at 0.100 bar and oxidation in 100% CO 2 ), the LSMMg-coated ceria foam produced a higher amount of fuel but with lower fuel production rate in comparison with pure ceria foam. This study points out the beneficial effects of composite redox materials consisting of LSMMg-coated ceria foam in enhancing the oxygen exchange capacity of ceria for solar fuel production.

Section: Introductionmentioning

confidence: 99%

Two-step CO2 and H2O splitting using perovskite-coated ceria foam for enhanced green fuel production in a porous volumetric solar reactor

Haeussler

Journal of CO2 Utilization

Julbe

et al. 2020

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“…For instance, a ceria felt made of compressed ceria fibers yielded slightly higher fuel yields because it was reduced at a higher temperature of ~1650°C and oxidized at a lower temperature below 900°C [45]. However, extended material degradation was observed after exposure to high-flux radiation, and the low thermal conductivity of the ceria felt resulted in undesired temperature gradients across the felt, as also noticed for directly-irradiated ceria fiber boards [27]. Macroporous structures for volumetric solar radiation absorption and homogeneous temperature distribution were also considered.…”

Section: Resultsmentioning

confidence: 94%

“…The directly-irradiated solar reactor (SUNFUEL) was previously developed for two-step redox cycles using various forms of reacting materials including reticulated ceria foams, perovskite-coated foams, or porous biomimetic cork-templated ceria granules [27,29,37,38]. It was placed at the focus of a medium-size vertical axis solar furnace (same characteristics as those mentioned in the previous section).…”

Section: Directly-heated Solar Reactormentioning

confidence: 99%

“…Such reactor concepts basically consist of monolithic or particle-based configurations with temperature-swing redox cycling. The main evaluated ceria morphologies for this application were three-dimensionally ordered macroporous (3DOM) structures [18,19], bioinspired materials [20,21], porous monoliths [22][23][24][25], fibers [26,27], and reticulated porous ceramics (RPC) [28,29]. Alternatively, the membrane configuration was also proposed involving mixed ionicelectronic conducting membranes for isothermal and continuous operation [30][31][32].…”

Section: Introductionmentioning

confidence: 99%

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Two-step thermochemical cycles using fibrous ceria pellets for H2 production and CO2 reduction in packed-bed solar reactors

Sustainable Materials and Technologies

Haeussler

2021

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The thermochemical redox activity and performance of commercial-grade fibrous ceria pellets were determined including fuel production rates and yields from H 2 O and CO 2 dissociation. Two solar reactors integrating the ceria pellets undergoing two-step thermochemical cycling (with temperatureswing between alternating redox steps) were experimentally tested. They consisted of packed-bed tubular and cavity-type solar reactors with direct or indirect heating of the reacting materials. The obtained fuel production rates compared favorably with the performance of other previously considered microstructured ceria materials such as templated foams, sintered felts, bioinspired or ordered macroporous morphologies. H 2 production rates reached 2.3 mL.g -1 .min -1 (with H 2 /O 2 ratios approaching 2) in the indirectly-heated tubular reactor after reduction at 1400°C and oxidation upon free cooling below 950°C in steam atmosphere (57% molar content). The highest fuel production rate (~9.5 mL.g -1 .min -1 ) and peak solar-to-fuel energy efficiency (~9.4%) were reached in the directlyirradiated cavity-type reactor (with a reduction step at 1400°C under reduced pressure, and an oxidation step in pure CO 2 below 950°C). The reduction extent was favored at low p O2 ( up to 0.056) and the fuel yields were thus improved notably (up to 315 µmol/g) by decreasing the total pressure below 0.1 bar during the reduction step. The fuel production rate increased when the oxidation temperature decreased (because the reaction was thermodynamically more favorable). An increase in CO 2 partial pressure further promoted the oxidation kinetics (and decreased the p CO /p CO2 ratio, thus shifting the thermodynamic equilibrium toward CO product). The fibrous ceria structures exhibited stable morphologies with high fuel production performance similar to less scalable porous structures, thus offering the potential for versatile applications in packed-bed solar reactors.

“…n ZnO n MgO (5) where n CO and n CO2 indicate the mole amounts of produced CO and CO 2 obtained by time integration of their production rates over the reaction duration. Conventionally, Zn is extracted from ores via smelting and refining processes [19], while Mg is produced by Pidgeon, Magnetherm, and electrolytic methods [20].…”

Section: Introductionmentioning

confidence: 99%

Solar Metallurgy for Sustainable Zn and Mg Production in a Vacuum Reactor Using Concentrated Sunlight

Chuayboon

2020

Sustainability

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Solar carbothermal reduction of volatile metal oxides represents a promising pyro-metallurgical pathway for the sustainable conversion of both metal oxides and sunlight into metal commodities and fuels in a single process. Nevertheless, there are several scientific challenges in discovering suitable metal oxides candidates for the ease of oxygen extraction from metal oxides to enhance the reaction extent and in designing reactors for the efficient absorption of incident solar radiation to minimize losses. In this study, ZnO and MgO were considered as volatile metal oxides candidates, and their reaction behaviors were studied and compared through gas species production rate, metal oxides conversion, and yield. A solar reactor prototype was developed to facilitate solar carbothermal reduction of ZnO and MgO with different reducing agents comprising activated charcoal and carbon black. The process was operated in a batch operation mode under vacuum and atmospheric pressures to demonstrate the flexibility and reliability of this system for co-production of metals (Zn/Mg) and CO. As a result, decreasing total pressure enhanced conversion of ZnO and MgO, leading to increased Zn and Mg. However, in the case of ZnO, CO yield decreased with decreasing total pressure at the expense of favored CO2 as a result of the decrease of residence time. In contrast, CO2 formation was negligible in the case of MgO, and CO yield thus increased with decreasing pressure. Using activated charcoal as the reducing agent exhibited better conversion of both ZnO and MgO than carbon black thanks to the higher available specific surface area for chemical reactions. MgO and ZnO conversion above 97% and 78%, respectively, and high-purity Mg and Zn content were accomplished, as evidenced by the recovered products at the reactor outlet and filter containing pure metal. In addition, Mg product exhibited strong oxidation reactivity with air, thus requiring inert atmosphere for the handling of Mg-rich powders to avoid direct exposure to air.