Ionomigration of Neutral Phases in Ionic Conductors

Abstract

Without sensing any physical force, a neutral object in an ion conducting solid can move in a uniform electrochemical field by coupling a global ion wind with localized counterion diffusion at the interface. This happens to pores and gas bubbles at 840 degrees C in a fast O-2(-) conductor, yttria-stabilized zirconia (YSZ), despite having cations that are essentially frozen with lattice diffusivities 10(12) times slower than the O-2(-) diffusivity. Through-thickness migration and massive electro-sintering in thin YSZ ceramics are observed at voltages similar to those in YSZ fuel cells and electrolysis cells. This effect should apply to any electrochemically-loaded multiphase ionic conducting solid, with or without an electric field, and can lead to electrolyte sintering, phase accumulation and electrode debonding, resulting in unexpected benefit or damage in electrochemical devices. As the velocity obeys a pseudo Stokes-Einstein equation, inversely proportional to the object size, an especially enhanced size effect is expected in nanocomposites.