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Title: Ceria-Based Anodes for Next Generation Solid Oxide Fuel Cells
Author: Mirfakhraei, Behzad
Advisor: Birss, Viola
Thangadurai, Venkataraman
Keywords: Chemistry--Physical;Energy;Materials Science
Issue Date: 1-May-2015
Abstract: Mixed ionic and electronic conducting materials (MIECs) have been suggested to represent the next generation of solid oxide fuel cell (SOFC) anodes, primarily due to their significantly enhanced active surface area and their tolerance to fuel components. In this thesis, the main focus has been on determining and tuning the physicochemical and electrochemical properties of ceria-based MIECs in the versatile perovskite or fluorite crystal structures. In one direction, BaZr0.1Ce0.7Y0.1M0.1O3-δ (M = Fe, Ni, Co and Yb) (BZCY-M) perovskites were synthesized using solid-state or wet citric acid combustion methods and the effect of various transition metal dopants on the sintering behavior, crystal structure, chemical stability under CO2 and H2S, and electrical conductivity, was investigated. BZCY-Ni, synthesized using the wet combustion method, was the best performing anode, giving a polarization resistance (RP) of 0.4 Ω.cm2 at 800 oC. Scanning electron microscopy and X-ray diffraction analysis showed that this was due to the exsolution of catalytic Ni nanoparticles onto the oxide surface. Evolving from this promising result, the effect of Mo-doped CeO2 (nCMO) or Ni nanoparticle infiltration into a porous Gd-doped CeO2 (GDC) anode (in the fluorite structure) was studied. While 3 wt. % Ni infiltration lowered RP by up to 90 %, giving 0.09 Ω.cm2 at 800 oC and exhibiting a ca. 5 times higher tolerance towards 10 ppm H2S, nCMO infiltration enhanced the H2S stability by ca. 3 times, but had no influence on RP. In parallel work, a first-time study of the Ce3+ and Ce4+ redox process (pseudocapacitance) within GDC anode materials was carried out using cyclic voltammetry (CV) in wet H2 at high temperatures. It was concluded that, at 500-600 oC, the Ce3+/Ce4+ reaction is diffusion controlled, probably due to O2- transport limitations in the outer 5-10 layers of the GDC particles, giving a very high capacitance of ca. 70 F/g. Increasing the temperature ultimately diminished the observed capacitance, likely as the chemical reduction of GDC at high temperatures is irreversible.
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