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Proton conductive ceramic materials for an intermediate temperature fuel cell Jankovic, Jasna

Abstract

Development of intermediate temperature (200-500°C) fuel cells could possibly overcome many disadvantages of both the high temperature (600-1000°C) solid oxide fuel cells (SOFC) and the low temperature (70-100°C) proton exchange membrane fuel cells (PEMFC) in terms of materials durability, cost, application, and overall system structure. A change in materials, especially the proton conductive electrolyte, is required to achieve this. However, to date, no solid proton conductors have been developed that work satisfactorily in this temperature range. The goal of this thesis was to develop a ceramic proton-conducting material to be used as a dense electrolyte, as well as within the anode structure of an intermediate temperature fuel cell. Investigated ceramic materials were based on oxygen deficient ceramic oxides – undoped and Ce- and La-doped Ba₂In₂O₅, which were expected to show proton conductivity within the intermediate temperature range due to water and/or proton incorporation into their defect structure. Five different compositions of brownmillerite materials, Ba₂In₂-xyCexLayO₅+ x/₂ (x=0.25 and 0.5; y=0.25 and 0.5) were synthesized via the solid-state reaction and the glycine-nitrate process, characterized and electrochemically investigated in order to find a suitable proton-conductive electrolyte. The materials were characterized using X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), particle size analysis (PSA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. The electrical conductivities of the ceramics were determined using ac impedance spectroscopy. Among the tested materials, undoped Ba₂In₂O₅ produced by the glycine-nitrate process was selected as the material with the highest total conductivity (between 0.02 S/cm and 0.7 S/cm) and stability in hydrogeniii containing atmospheres and at temperatures between 300°C and 480°C. High proton transport numbers (e.g., 0.84 at 300°C) and relatively high open circuit voltage values of the air, Pt Ba₂In₂O₅ Pt, 50%H₂/50%N₂ cell (e.g., 0.81 V at 300°C) confirmed the predominant proton conductivity of this material. Although highly proton conductive in a hydrogencontaining atmosphere, Ba₂In₂O₅ showed poor performance as an electrolyte in an intermediate temperature fuel cell due to the incorporation of oxygen on the cathode side with associated blocking of the proton conduction. Application of sintered porous Ba₂In₂O₅ in a cermet with a metal catalyst in the anode structure was shown to be beneficial.

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