PROCOMAL · Study of local environment and microscopic motion of protons in yttrium doped barium cerate by neutron diffraction, neutron scattering and microscopic simulation

Final Activity Report Summary - PROCOMAL (Study of local environment and microscopic motion of protons in yttrium doped barium cerate by neutron diffraction, neutron scattering ...)

One of the major challenges in the development of hydrogen fuel cells remains the choice of electrolyte. Severe demands exist on a number of its properties including very high proton and very low electronic and oxide ion conductivity, high thermal and chemical stability and durability. Yttrium doped barium cerate (BCY) meets many of the above requirements. It has been studied intensively since the first observation of its high proton conductivity at elevated temperatures (> 600 degrees Celsius). Barium cerate has a perovskite-type structure, in which substitutions of Ce4+ by Y3+ cause the formation of oxygen vacancies. The doped material is hygroscopic; it absorbs water dissociatively resulting in the formation of hydroxyl groups. At temperatures above 400 - 500 degrees Celsius, hydrogen atoms in the structure become mobile, leading to proton conductivity. Using a unique set-up, neutron scattering data has been measured, which allows correlating the structural phase transitions in the BCY lattice, the hydrogen content and the mean-squared displacement (MSD, a measure of a quasi-vibrational proton motion) of H atoms. This type of information is crucial for determining under which conditions the proton motion, and thus conductivity, dominates over the oxide ion and electronic conductivity, which are detrimental to the operation of a fuel cell. A steady uptake of hydrogen (water) was observed by the BCY system in the temperature range 150 - 600 degrees Celsius and it is accompanied by an increase MSD. However, dehydration of the system together with a decrease in MSD, even under moist air, began around 600 degrees Celsius. Viewing the temperature dependence of the total conductivity measurements on the same system, it is noted that the greatest difference in conductivity between the dry and wet BCY sample occurs at the temperature corresponding to the maximum in MSD as observed by neutron scattering. According to the neutron data, conductivity beyond this temperature has to be mainly oxide ion or electronic conductivity. Neutron scattering gives more detailed information about the proton motion from the analysis of so-called quasi-elastic broadening. From these measurements it is estimated the diffusion coefficients of H atoms in the BCY sample as approximately 7*10-10 m2s-1 (at 400 degrees Celsius), 2*10-9 m2s-1 (at 650 - 750 degrees Celsius). Going beyond the determination of the diffusion coefficient was difficult, other techniques are necessary to suggest plausible models of hydrogen motion and more effort at this stage is devoted to developing a microscopic model of the barium cerate network. An existing classical potential for barium cerate is being modified by introducing a dynamical shell model to account for ion polarisability. The next step is to see whether this model can reproduce the experimental density of states as measured by inelastic neutron scattering (INS).

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