| Summary: | 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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