UMD Theses and Dissertations
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Item ELECTROSPUN CERIA-BASED FIBERS FOR ENERGY CONVERSION APPLICATIONS(2014) Gibbons, William Tilden; Jackson, Gregory S; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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)36H2O, 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.Item Quantifying the Role of Cerium Oxide as a Catalyst in Solid Oxide Fuel Cell Anodes(2009) DeCaluwe, Steven Craig; Jackson, Gregory S; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solid Oxide Fuel Cells (SOFCs) are an important electrochemical power conversion device, due largely to their high efficiencies and ability to directly oxidize a variety of fuels, including hydrogen, carbon monoxide, and light hydrocarbons. Conventional Ni-based SOFC anodes are prone to failure due to carbon deposition or unwanted metal oxidation. For this reason, ceria (CeO2) is being explored to replace or supplement Ni in SOFC anodes. CeO2, a mixed ionic-electronic conductor (MIEC), has been shown to improve SOFC anodes' resistance to carbon deposition and sulfur poisoning. Optimization of ceria-based anodes has proven difficult due to the unknown role of ceria during SOFC operation. The electrochemical mechanisms and reaction rates needed to describe fuel oxidation on ceria anodes are not well understood, and thus it is not clear how to model the coupling between electrochemistry and mass transport in complex SOFC geometries containing ceria. Both Ce4+ and Ce3+ are present during fuel cell operation, and the ionic and electronic conductivities are determined by the abundance of Ce3+. The in situ spatial distribution of valence states, then, is expected to have a major impact on ceria's role in SOFC anodes. This work aims to describe the fundamental role of ceria in SOFC anodes by building a numerical SOFC model for the electrochemical oxidation of small molecules. Porous-media SOFC models are developed and validated against experimental data, correcting previous errors in transport equations. The thermodynamic and kinetic parameters for such a model are obtained from experimental measurements on thin-film ceria anodes, including electrochemical measurements and novel in situ X-ray photoelectron spectroscopy measurements. Fitting thin-film MIEC model results against experimental data leads to the identification and estimation of several key parameters in the proposed H2 oxidation mechanism, with results demonstrating the importance of charge transfer, bulk oxide diffusion and adsorption reactions at the electrode surface. This provides a basis for modeling porous media composite SOFC electrodes with distributed electrochemistry as demonstrated in this work.