Apatite electrolytes : design, synthesis and modeling

Abstract

The crystal chemistries of a range of germanate, silicate and rhenate apatites were studied as potential electrolytes for solid oxide fuel cells (SOFCs). While these compounds are often described as complying with the composition A10(BO4)6O2 (A = rare earth, alkali earth, alkali and B = germanium, silicon, transition metals), the character of apatite electrolytes is far more interesting. Indeed, it is the underlying complexity and flexibility of these substances that endows them with such promise as ion conductors. In this thesis, the nature of “hybrid” apatites is examined, and the critical role of extrastoichiometric oxygen associated in BO5 units for facilitating high conductivity with low activation energy considered. Following this approach, a taxonomy of polymorphic and polysomatic apatites is devised that provides a basis for the rationale design of new electrolytes and the optimization of performance. Apatites can be synthesized in various ways, including solid state sintering, sol-gel methods and hydrothermal reactions; but the former is favored since large quantities are readily fabricated for division and multi-technique characterization. X-ray, neutron and electron diffraction were employed to refine crystal structures and locate “interstitial” oxygen, supported by spectroscopic and microscopic methods to identify defects and recognize superlattices. In this manner, correlations between ionic conductivity and crystal chemistry were established.