The influence of cation ordering, oxygen vacancy distribution and proton siting on observed properties in ceramic electrolytes: the case of scandium substituted barium titanate.

Nico Torino,Carlotta Giacobbe,Paul F Henry,Emma Suard,Sten G Eriksson,Christopher S Knee,Tor Svendsen Bjørheim,Seikh M H Rahman

doi:10.1039/c7dt01559c

Abstract

The origin of the 2-order of magnitude difference in the proton conductivity of the hydrated forms of hexagonal and cubic oxygen deficient BaScxTi1-xO3-δ (x = 0.2 and x = 0.7) was probed using a combination of neutron diffraction and density functional theory techniques to support published X-ray diffraction, conductivity, thermogravimetric and differential scanning calorimetry studies. Cation ordering is found in the 6H structure type (space group P63/mmc) adopted by BaSc0.2Ti0.8O3-δ with scandium preferentially substituting in the vertex sharing octahedra (2a crystallographic site) and avoiding the face-sharing octahedra (4f site). This is coupled with oxygen vacancy ordering in the central plane of the face-sharing octahedra (O1 site). In BaSc0.7Ti0.3O3-δ a simple cubic perovskite (space group Pm3[combining macron]m) best represents the average structure from Rietveld analysis with no evidence of either cation ordering or oxygen vacancy ordering. Significant diffuse scattering is observed, indicative of local order. Hydration in both cases leads to complete filling of the available oxygen vacancies and permits definition of the proton sites. We suggest that the more localised nature of the proton sites in the 6H structure is responsible for the significantly lower proton conduction observed in the literature. Within the 6H structure type final model, proton diffusion requires a 3-step process via higher energy proton sites that are unoccupied at room temperature and is also likely to be anisotropic whereas the highly disordered cubic perovskite proton position allows 3-dimensional diffusion by well-described modes. Finally, we propose how this knowledge can be used to further materials design for ceramic electrolytes for proton conducting fuel cells.

Highlights

The field of Protonics, a term conceived by Iwahara[1] to describe the study of solid-state proton conductors, has increasingly attracted attention in the last few decades
X-ray powder diffraction (XPD) patterns of the BTS20 and BTS70 compositions for the VD, H and D samples were in good agreement with those previously reported by Rahman et al.[32] and appeared to be phase pure
We concluded from the Synchrotron X-ray powder diffraction (SXPD) measurements that nothing was observed to support this hypothesis for either substitution level

Summary

Introduction

The field of Protonics, a term conceived by Iwahara[1] to describe the study of solid-state proton conductors, has increasingly attracted attention in the last few decades This is due mainly to the rich potential of proton conducting materials in energy related applications, sensors and membranes.[2,3,4,5] One active research area is proton conducting – solid oxide fuel cells (PC-SOFC), which feature a proton-conducting ceramic electrolyte rather than the oxygen ion conducting ceramic electrolyte found in standard SOFC fuel cells. There remain significant drawbacks limiting the potential use of such materials including the high level of grain boundary resistance, the high temperatures required for sintering to form dense ceramics and poor thermodynamic stability under operating conditions.

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