Electroreduction of zirconia – a multi-step process

Abstract

The phenomenon of electroreduction, or electrocoloration, in yttria-stabilised zirconia (YSZ) has garnered significant attention due to its dual role as a possible degradation mechanism in solid oxide electrolysers and as a beneficial effect during the flash sintering of ceramics with tailored properties. Despite extensive investigation over several decades, the precise mechanisms underlying the transformation of the transparent, purely ionic conductor YSZ into a black, mixed ionic-electronic conductor, and eventually into a metallic state, remain inadequately understood. In this study, we present a comprehensive analysis that integrates electrical characterisation during electroreduction with in situ microscopy and ex situ spectroscopy techniques. Our findings enable us to delineate three primary stages: a reversible electrocoloration associated with the development of blackening fingers, an accelerated electroreduction facilitated by the formation of mixed ionic-electronic conducting pathways between the anode and cathode, and a runaway-type process that induces morphological changes and filamentary phase transformations in the surface region.