High oxide ion conductivity below 500∘C in the garnets LaxY3−xFe5O12+δ

Abstract

We address the issue of achieving high oxide ion conductivity at low temperatures $(\ensuremath{\le}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ in stable electrolytes for solid oxide fuel cell (SOFC) techonology. We report here a high oxide ion conductivity $(\ensuremath{\sim}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$ in the La substituted yttrium iron garnets $\mathrm{L}{\mathrm{a}}_{x}{\mathrm{Y}}_{3\text{\ensuremath{-}}x}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12+\ensuremath{\delta}}$ at low temperatures $(\ensuremath{\sim}450{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$. The judicious substitution of high electropositive $\mathrm{L}{\mathrm{a}}^{3+}$ ions in the yttrium iron garnet $(x\ensuremath{\le}0.45)$ affords continuous and easy oxide ion conduction pathways through polyhedral networks even at a low temperature of $\ensuremath{\sim}127{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The conduction pathways have been visualized from the maximum entropy method analyzed soft bond valence sum distribution, obtained from neutron diffraction data. Our study also shows that these iron garnets have high structural stability over a broad range of temperature (at least over $27\text{--}800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$, and compatible thermal expansion coefficient $(\ensuremath{\sim}10.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}{\mathrm{C}}^{\ensuremath{-}1})$ with that of the commonly used interconnect and electrode SOFC materials. We reveal that the oxide ion conduction in the present garnet is based on an excess oxide ion concentration mechanism. The observed high oxide-ion conductivity at low temperatures in the present trivalent substituted garnets is a remarkable finding for the development of an efficient SOFC technology.