ELECTROSPUN CERIA-BASED FIBERS FOR ENERGY CONVERSION APPLICATIONS
Gibbons, William Tilden
Jackson, Gregory S
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Electrospun ceria-based fibers are explored for two energy conversion applications. An electrospinning facility was developed and the electrospinning process and subsequent fibrous material processing was optimized to enable rapid, scalable, and inexpensive production of ceramic fibers with diameters ranging from 50 nm to 5 μm. In this work, electrospinning of ceria-based fibers with various dopants were produced by spinning a sol with polyvinylypyrolidone (PVP), polar solvents, Ce(NO3)36H2O, and additional metal salts as desired. PVP removal by oxidation, followed by calcination in air, produced CeO2-based fibers. Non-woven ceramic textile mats were electrospun for integration into a compact water-gas-shift membrane reactor with a metallic Pd-based membrane for pure H2 production. The fibrous mats (CeO2 doped with 2 wt% Pd and/or 10 wt% Cu), were characterized for water-gas-shift (WGS) catalysis. High initial activity was followed by slow deactivation over 60 hours during time-on-stream testing at 400°C. Ex-situ characterization of the catalyst indicated that reduction and surface segregation of the Cu caused the deactivation which could be reversed by a brief oxidation treatment above 400°C. To test the Pd/Cu-doped ceria mats in the membrane reactor application, a H2-selective membrane system was constructed from a 5 μm-thick Pd/Cu (60/40 wt%) alloy foil supported on porous stainless steel. The electrospun fibers, mechanically pressed against the membrane foil, provided stable pure H2 production for over 300 hours at 400°C. The integration of the catalyst and H2 membrane achieved super-equilibrium conversion to H2 for some feed conditions. Though the membrane system showed stable performance, the oxidative treatments induced rapid membrane degradation, and are not a viable route for catalyst re-activation in such systems. A second application was investigated for the electrospun ceria-based fibers involving their use as a structured working material for solar-driven thermochemical redox cycles. These cycles use concentrated sunlight to drive endothermic oxide (CeO2) reduction at high temperatures (up to 1700 K) and lower temperature re-oxidation with CO2 and/or H2O to produce CO and/or H2 for subsequent fuel production. The electrospun fibers offer a cost-effective, flexible, and scalable path to the production of such working materials because the nature of the synthesis offers extensive composition control and the fiber structure reduces surface area loss at high temperatures. Un-doped CeO2 as well as Zr and Pr doped CeO2 fibers were studied to understand the affect of high temperature exposure on the overall structure of powder and fiber materials, and the affect of dopant concentration and structure on reduction and fuel production kinetics. Under the conditions studied, Pr doping (5/10 mol %) promoted grain growth, and did not improve reduction yields over un-doped CeO2. Doping with Zr (2.5, 5, 10, 20 mol %) inhibited sintering, increased reduction yields, and slowed oxidation kinetics. The fiber structures showed faster oxidation kinetics than the parallel powders likely due to shorter diffusion lengths, higher surface areas, and improved mass transfer. Long term thermal cycling of Zr doped fibers between 1673 K and 1023 K indicated rapid fuel production and a gradual loss of surface area, but a highly porous structure remained after 100 cycles over 30 hours. A final surface area of 0.3 m2 g-1 was measured via Kr adsorption.