NdBaInO4 based triple (electronic, ionic and protonic) conductor for solid oxide fuel cell applications
Creators: name: Zhou, Yu
Publisher: name: Imperial College London
Year: 2021

description: Oxide-ion conducting materials have never failed to attract intensive attention due to their potential to be used for the applications of solid oxide fuel cell (SOFC) devices. With the aim of reducing the operating temperatures of SOFC devices to the intermediate high temperature range (500oC-800 oC), the design and synthesis of a new structure family to be used as the electrolyte material could be crucial. In this thesis, the potential of calcium-doped layered perovskite compounds, BaNd1-xCaxInO4-x/2 (where x is the Ca content), as protonic conductors was experimentally investigated. The single phase of monoclinic crystal structure with the P21/c symmetry was confirmed in the as-synthesized BaNd1-xCaxInO4-x/2 solid solutions by XRD characterisations. The acceptor-doped ceramics exhibited improved total conductivities that were 1-2 orders of magnitude higher than those of the parent material, BaNdInO4. The highest total conductivity of 2.6 x 10-3 Scm-1 was obtained for the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 oC in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250 oC-750 oC). At 500 oC, the total conductivity of the 20% substituted sample in humid air (~3% H2O) was 1.3 x 10-4 Scm-1. The incorporation of water vapour decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) eV to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol% protonic defects, which caused an expansion in the lattice according to the in-situ X-ray diffraction data. Combining studies of the impedance behaviour with 4-probe DC conductivity measurements obtained in humid air which showed a decrease in the resistance of the x=0.2 sample, it could be concluded that experimental evidence indicates that BaNd1-xCaxInO4-x/2 exhibits triple (oxygen-ion, proton and hole) conduction in wet atmospheres. The oxygen and deuterium isotope exchange depth profiling measurements on the BaNd0.8Ca0.2InO3.90 sample were successfully performed where valid diffusion kinetics for oxygen diffusion in this unique layered perovskite were firstly obtained. The obtained surface exchange coefficients for oxygen of the BaNd0.8Ca0.2InO3.90 sample measured under wet atmospheres were significantly higher than those measured in dry 18O2, while the diffusion coefficients in bulk material were decreased in wet atmosphere and a higher the activation energy for oxygen diffusion was achieved according to the Arrhenius plot. The activation energy for oxygen diffusion was increased from 1.08(8) eV to 1.86(11) eV after water incorporation, which implied a hindering effect of protonic defects on the oxygen diffusion process in the bulk material. Besides, fast grain boundary diffusion ‘tail’ was confirmed in the 500 ℃ and 550 ℃ (H218O+O2) wet exchange depth profiles and the grain boundary diffusion products, D_gb∙δ, of 6.64 x 10-14 and 5.31 x 10-13 cm3/s at 500 ℃ and 550 ℃ respectively were calculated. The chemical stability of the obtained phase in the BaNd0.8Ca0.2InO3.90 sample was examined after long duration wet annealing by XRD and STEM-EDS materials characterization methods, where A-site cation exsolution of calcium and barium to the sample surface was confirmed forming corresponding CaCO3 and BaCO3 secondary phases on the surface. In this work, processing high density Calcium substituted BaNdInO4 as an electrolyte was investigated. By means of electrochemical performance characterization methods together with the isotope exchange depth profile measurement, the contribution of different charge carriers in this layered-perovskite structural material was probed. It was shown that BaNd1-xCaxInO4-x/2 exhibits triple (oxygen-ion, proton and hole) conduction in wet atmospheres and the migration of protons are much faster than that of oxide-ions. All these results implied a promising potential of this ceramic, with a unique layered perovskite structure, to be used as the electrolyte material for the intermediate high temperature SOFC devices.