Issue 6, 2012

Synthesis and characterisation of vanadium doped alkaline earth lanthanum germanate oxyapatite electrolyte

Abstract

The crystal chemistry and oxygen conductivity of vanadium-doped apatites of nominal composition [La8AE2][Ge6−xVx]O26+x/2 (AE = Ca, Sr, Ba; 0 ≤ x ≤ 1.5) were studied to establish their potential as intermediate temperature solid oxide fuel cell (SOFC) electrolytes. Single-phase products obtained for x ≤ 0.5 were found, using a combination of powder synchrotron X-ray and neutron diffraction, to be P63/mapatites. The ionic conductivities extracted by complex impedance spectroscopy showed that small vanadium amendments enhanced oxygen mobility at intermediate temperatures (500–700 °C) by more than one order of magnitude, as the incorporation of V5+ through displacement of Ge4+ is charge balanced with interstitial O2− that improves ionic conduction. The most promising composition was La7.88Ca2Ge5.35V0.65O26.15 that delivered σ = 3.44 × 10−4 S cm−1 at 500 °C. The superstoichiometric oxygen was delocalised, without fixed X-ray or neutron scattering centres. Crystal chemistry systematics demonstrate that the Ca-apatite was superior because the relatively small framework expanded the tunnel through reduced (La/AE)O6 metaprism twisting (φ), ensuring the P63/m symmetry was adopted, which favours the passage of O2− with lower activation energy.

Graphical abstract: Synthesis and characterisation of vanadium doped alkaline earth lanthanum germanate oxyapatite electrolyte

Supplementary files

Article information

Article type
Paper
Submitted
04 Aug 2011
Accepted
19 Nov 2011
First published
19 Dec 2011

J. Mater. Chem., 2012,22, 2658-2669

Synthesis and characterisation of vanadium doped alkaline earth lanthanum germanate oxyapatite electrolyte

H. Li, T. Baikie, S. S. Pramana, J. F. Shin, P. R. Slater, F. Brink, J. Hester, K. Wallwork and T. J. White, J. Mater. Chem., 2012, 22, 2658 DOI: 10.1039/C1JM13752B

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