Proton Conducting Ceramics; Synthesis, Characterization and Sample Cell Environments

Abstract: Acceptor doped Perovskite ceramics have been shown to exhibit high levels of protonic conduction in humid atmospheres. However significant grain boundary resistance and long sintering times required to achieve sufficient grain size growth has hindered their widespread application as electrolyzers, fuel cells and gas separation membranes. Furthermore there is insufficient detailed knowledge on the local environment of the H+ and its behavior in relation to the crystal structure under real working conditions. The works of the thesis describe the synthesis, structural characterization and conductivity of BaZr0.5In0. 5O3—δ and BaSn1-xScxO3—δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5). Traditional solid state sintering was used to prepare the samples which were then characterized by a variety of methods such as electrochemical impedance spectroscopy, thermogravimetric analysis, X-ray and neutron diffraction. Spark plasma sintering processing technique has been explored. BaZr0.5In0. 5O3—δ processed using the spark plasma sintering method were found to exhibit higher grain boundary conductivity compared to solid state sintered samples that had fewer grain boundaries under wet H2 (5% H2 + 95% Ar). The highest proton conductivity was found in the SPS samples under wet H2 atmosphere. In BaSn1-xScxO3—δ the deuteron was found to occupy the 24k crystallographic position by Rietveld analysis of room temperature neutron powder diffraction data. A detailed understanding on the environment of the deuteron in Scandium doped BaSnO3 has been developed. Conductivity was found to peak at x ≈ 0.3-0.4. Experimental facilities at the ISIS neutron facility at Oxford, UK, for in-situ studies of fuel cell materials were demonstrated and their capability to bring about new insights on the workings of protonic conduction shown. The diffraction experiments as a function of temperature using in-situ impedance and in-situ gas flow cells gave detailed insight into the conductivity and hydration/dehydration behavior using deuterated BaZr0.5In0. 5O3—δ samples with the crystal structure remaining intact over the whole temperature interval despite expanding on deuteration and contacting on de-deuteration. The oxygen anisotropic temperature factor u11 was found to replicate the cell unit expansion-contraction behavior as the deuteron was inserted or removed from the crystal structure. Keywords: proton conduction, perovskites, neutron diffraction, in-situ cell, Rietveld refinements, deuteron position, BaSnO3, BaZrO3