Ceria-based electrospun fibers for renewable fuel production via two-step thermal redox cycles for carbon dioxide splitting

Abstract

Zirconium-doped ceria (Ce(1-x)Zr(x)O2) was synthesized through a controlled electrospinning process as a promising approach to cost-effective, sinter-resistant material structures for high-temperature, solar-driven thermochemical redox cycles. To approximate a two-step redox cycle for solar fuel production, fibrous Ce(1-x)Zr(x)O2 with relatively low levels of Zr-doping (0 < x < 0.1) were cycled in an infrared-imaging furnace with high-temperature (up to 1500 °C) partial reduction and lower-temperature (∼800 °C) reoxidation via CO2 splitting to produce CO. Increases in Zr content improve reducibility and sintering resistance, and, for x≤ 0.05, do not significantly slow reoxidation kinetics for CO production. Cycle stability of the fibrous Ce(1-x)Zr(x)O2 (with x = 0.025) was assessed for a range of conditions by measuring rates of O2 release during reduction and CO production during reoxidation and by assessing post-cycling fiber crystallite sizes and surface areas. Sintering increases with reduction temperature but occurs primarily along the fiber axes. Even after 108 redox cycles with reduction at 1400 °C and oxidation with CO2 at 800 °C, the fibers maintain their structure with surface areas of ∼0.3 m(2) g(-1), higher than those observed in the literature for other ceria-based structures operating at similarly high temperature conditions. Total CO production and peak production rate stabilize above 3.0 mL g(-1) and 13.0 mL min(-1) g(-1), respectively. The results show the potential for electrospun oxides as sinter-resistant material structures with adequate surface area to support rapid CO2 splitting in solar thermochemical redox cycles.

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

“…The difference δ red − δ ox determines the maximum molar amount of fuel capable of being produced per cycle and per mole of ceria. Various porous ceria structures have been investigated such as 3D ordered macroporous structures,11 electrospun fibers,12 monoliths,13 felts,14 and reticulated porous ceramics (RPC) 14. Although structures with submicron‐sized pores exhibited relatively fast oxidation rates, they lacked morphological stability as sintering occurred at elevated temperatures 11.…”

Morphological Characterization and Effective Thermal Conductivity of Dual-Scale Reticulated Porous Structures

“…Ceria-based oxides have emerged as highly attractive redox materials because of the rapid oxygen transport in the bulk [14,15,16,17,18]. Various porous structures made of ceria have been investigated for enhanced reaction rates [18,19,20], including structures with submicron-sized interconnected pores, but these are problematic to retain because of partial sintering at elevated temperatures [19]. Furthermore, their high optical thickness inhibits penetration of concentrated solar radiation, resulting in non-uniform heating and temperature distributions [14].…”

Modeling of a concentrated-solar, falling-particle receiver for ceria reduction

“…A material with near blackbody absorption characteristics over the bulk of the solar spectrum has the potential to improve g sol by as much as 50% increase over baseline, but if the desired performance is particle reduction for fuel production, such a material must provide good thermochemistry to achieve high g chem . This has been the focus of many materials thermochemical investigations including with doped cerias (Kaneko et al, 2011, Petkovich et al, 2011Scheffe et al, 2013;Gibbons et al, 2014) and more recently with perovskites (McDaniel et al, 2013). However, none of these studies have done a thorough analysis on the radiative properties of these alternative materials to assess their effectiveness at radiation absorption in a directly irradiated receiver as presented here.…”

“…On the other hand, the high temperatures (>1400 K) associated with ceria reduction have encouraged researchers to explore alternative Ce 1Àx M x O 2Ày (where M is a cation dopant such as Zr) to lower temperatures for the reduction reaction R1 and thereby reduce both re-radiation losses and costs associated with such hightemperature receiver operation (Le Gal and Abanades, 2012;Scheffe and Steinfeld, 2012). However, ceria dopants explored to date -including Zr (Kaneko et al, 2011;Petkovich et al, 2011;Scheffe et al, 2013;Gibbons et al, 2014), Sc (Lee et al, 2013;Scheffe et al, 2013), Pr (Meng et al, 2012), and Hf (Scheffe et al, 2013) -still require temperatures above 1400 K and as such present real challenges on particle receiver designs.…”

“…While researchers have explored a variety of material structures for ceria reduction in solar receivers including ordered and disordered porous structures Furler et al, 2012;Rudisill et al, 2013;Gibbons et al, 2014), falling particles provide a unique approach to mitigate structural sintering that plagues fixed structures. However, the authors know of no studies to date that have explored particle receiver designs for efficient ceria particle reduction.…”