Thermochemical CO₂ splitting via redox cycling of ceria reticulated foam structures with dual-scale porosities

Abstract

Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO2. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with dmean = 2.5 mm and porosity = 0.76-0.82, enables volumetric absorption of concentrated solar radiation for efficient heat transfer to the reaction site during endothermic reduction, while the smaller void size range within the struts, with dmean = 10 μm and strut porosity = 0-0.44, increases the specific surface area for enhanced reaction kinetics during exothermic oxidation with CO2. Characterization is performed via mercury intrusion porosimetry, scanning electron microscopy, and thermogravimetric analysis (TGA). Samples are thermally reduced at 1773 K and subsequently oxidized with CO2 at temperatures in the range 873-1273 K. On average, CO production rates are ten times higher for samples with 0.44 strut porosity than for samples with non-porous struts. The oxidation rate scales with specific surface area and the apparent activation energy ranges from 90 to 135.7 kJ mol(-1). Twenty consecutive redox cycles exhibited stable CO production yield per cycle. Testing of the dual-scale RPC in a solar cavity-receiver exposed to high-flux thermal radiation (3.8 kW radiative power at 3015 suns) corroborated the superior performance observed in the TGA, yielding a shorter cycle time and a mean solar-to-fuel energy conversion efficiency of 1.72%.

Splitting CO2 with a ceria‐based redox cycle in a solar‐driven thermogravimetric analyzer

“…CeO 2 powder (Sigma Aldrich, 99.9% purity, particle size < 5 μm), stoichiometric amounts of ZrO 2 (Tosoh, TZ‐0, 99.95% purity, surface area 14.5 m 2 g −1 ), and 30 vol % spherical carbon pore‐forming agent particles (HTW Hochtemperatur‐Werkstoffe GmbH, Sigradur K, particle size 0.4–12 μm, for DS‐RPC samples) were mixed with water in a 5:1 mass ratio. 0.84 wt % organic deflocculating agent (Zschimmer & Schwarz, Dolapix CE64), 20 wt % polyvinyl alcohol binder (PVA, Zschimmer & Schwarz, Optapix RA 4G) and antifoaming agent (Zschimmer & Schwarz, Contraspum KWE) were added and processed according to a previously reported recipe 17, 31. Organic polyurethane sponges (Foam Partner, Fritz Nauer AG) with 8, 10, and 35 pores per inch (ppi) were then immersed into the slurry, dried in air, and finally sintered for 5 h at 1873 K. The solid‐to‐water ratio was decreased linearly for the ZrO 2 containing RPCs from 5:1 for x  = 0 to 4.8:1 for x  = 0.2 due to the increasing viscosity with ZrO 2 content.…”

“…In contrast, the foam‐type RPC structures offered enhanced heat and mass transport properties for high‐temperature processing 16. They have been developed with interconnected single‐scale porosity (SS‐RPC)15 as well as with interconnected dual‐scale porosity (DS‐RPC) 17, 18. In the latter, the larger mm‐sized pores enabled efficient volumetric radiative absorption during reduction (Eq.…”

“…(1)) due to appropriate optical thickness, while the smaller µm‐sized pores within the struts enabled fast reaction rates during oxidation with CO 2 and H 2 O (Eq. 2) due to an increased specific surface area 17. RPCs with 76% overall porosity and 18% strut porosity, obtained by the addition of 30 vol % carbon pore forming agent into the ceramic slurry,17, 19 offered a trade‐off between large specific surface area (0.066 m 2 g −1 ) for fast oxidation kinetics and large mass loading per unit volume for high fuel yield, while being morphologically stable over more than 200 redox cycles at T red  = 1773 K and T ox  = 1023 K 18.…”

“…2) due to an increased specific surface area 17. RPCs with 76% overall porosity and 18% strut porosity, obtained by the addition of 30 vol % carbon pore forming agent into the ceramic slurry,17, 19 offered a trade‐off between large specific surface area (0.066 m 2 g −1 ) for fast oxidation kinetics and large mass loading per unit volume for high fuel yield, while being morphologically stable over more than 200 redox cycles at T red  = 1773 K and T ox  = 1023 K 18. The specific fuel yield per cycle, given by δ red − δ ox , can be increased by alternative redox materials with a higher oxygen exchange capacity compared to ceria,20 for example by the introduction of 4+ valence dopants such as Zr 4+ 2, 21, 22, 23, 24, 25, 26 or Hf 4+ 21, 27.…”

“…Measurements using a solar reactor are unpractical because of the large volume of the sample required to fill the cavity and because the weight change of the sample cannot be monitored. Supplementing previous studies with RPC structures performed in solar reactors15, 18 and standard thermogravimetric analyzers,17 this work further investigates the influence of the porous structure and Zr‐dopant concentration on the overall rates of both reduction/oxidation reactions, as well as on the reduction extent and specific fuel yield, with samples subjected to high‐flux conditions relevant to the operation of solar reactors. We analyze the O 2 and CO evolutions during reduction and oxidation, respectively, of SS‐RPC, DS‐RPC, and fibers made of undoped ceria.…”