Meaning
Point defect in a crystal lattice where an oxygen atom is missing from its designated site. An oxygen vacancy acts as a local site of positive charge that attracts electrons to maintain electrical neutrality in the crystal. These defects are fundamental to the operation of mixed ionic electronic conductors and certain cathode materials.
Charge Neutrality
Balance within the structure requires the reduction of nearby metal cations when an atom is removed. The presence of an oxygen vacancy often correlates with the appearance of transition metals in lower oxidation states. This shift changes the electronic properties of the material and can increase its total conductivity.
Ionic Conductivity
Empty sites provide a path for the movement of oxygen ions through the lattice. By increasing the concentration of the oxygen vacancy, a developer improves the rate at which ions diffuse through a solid oxide fuel cell membrane. This mechanism allows the device to operate at lower temperatures while maintaining high efficiency.
Surface Reactivity
Termination of the crystal at the gas interface creates active sites for chemical adsorption. A high density of the oxygen vacancy on the surface of a catalyst particle promotes the breakdown of gas molecules. This property is exploited in gas sensors and exhaust treatment systems to improve the speed of detection and conversion.
In battery cathodes, surface vacancies can also facilitate the exchange of lithium between the particle and the liquid electrolyte. This enhancement reduces the charge transfer resistance at the interface and allows for faster charging times in electric vehicle batteries.
