Abstract
(Englisch)
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Oxygen ion conductors, typically yttria-zirconia or ceria-gadolinia solid solutions, are already well established in industry, particularly as sensors for catalyser control in automobile applications, but also for metallurgy and process control. Rapid developments are proceeding with a view to their applications in high temperature fuel cells. There is reason to believe that 'proton conducting' electroceramic materials can be equally significant, with their potential relevance to hydrogen processing in energy technology, initiation and catalysis of hydrogen-exchange reactions in chemical engineering, and even as hydrogen isotope separators. Charge transfer within these materials is by mobile hydrogen ions, hence the name. In the present project electroceramics of this type are synthesised, processed, sintered and characterised for their charge exchange and transport properties. Typical operating temperatures for hydrogen ion ('proton') - conductive solid electrolyte devices lie in the range 400 to 600°C (1), much lower than those for oxygen mobility in electroceramics, and more compatible with conventional engineering materials in chemicalengineering systems.
Ongoing research within the present programme (2) addresses the utility of such materials for fuel cells, sensors, hydrogen separation membranes, and catalysis. The EPFL participation specifically targets the electrode - electrolyte contact problem for these applications. In identifying candidate protonic conductors, it is noted that the perovskite cerate-based solid solution SrCe1-xDxO3-x/2 (D=dopant) (3) has one of the highest proton conductivities but it is not very stable, while the material BaZr1-xDxO3-x/2 (4) is particularly stable. Therefore the system Ba0.5Sr0.5Ce0.475Zr0.475Yb0.05O2.975, with intermediate composition, has been chosen and investigated for the first time as an electrolyte. The conductivity appears slighly better than the conductivity of SrCe1-xYbxO3-x/2, in contrast to the literature results (5,6). At room temperature, we have observed a cubic structure of the system Ba0.5Sr0.5Ce0.475Zr0.475Yb0.05O2.975 while an orthorhombic structure is given for the system Ba0.5Sr0.5Ce0.5Zr0.5O3 (7). Nickel oxide is a standard component in SOFC anode composites on zirconia electrolytes. Its compatibility with the electrolyte, in particular interfacial reactivity and interdiffusion, is under study. Pretreatment of the electrolyte prior to electrode deposition has been shown to influence strongly the resultant interface kinetics for charge transfer reactions.
In the experimental programme, the phase purity and structure of the proton conducting material was verified by X-ray diffraction, with imaging by scanning electron microscopy and analysis by EDAX. Stoichiometry variation as a function of gas environment at elevated temperature was monitored by thermogravimetric analysis, these techniques being applied either by the contracting laboratory or in coordination with project partners. Electrochemical testing including impedance spectrometry was also accomplished. Results have been presented at two international conferences (8,9), and the programme report to the European Commission is attached as an annex.
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