A novel mixed-conducting network in all-oxide composites: overcoming traditional percolation constraints†

Abstract

Mixed-conducting composites play pivotal roles in ceramic devices for advancing efficient and environment-friendly energy consumption and conversion processes. Conventionally, these materials are synthesized via the blending of distinct conducting phases, where grain percolation of each phase is considered essential. This approach inevitably leads to intertwined networks interspersed with inactive regions, limiting the overall performance. This study challenges this conventional paradigm by proposing an alternative percolation mechanism that circumvents the need for strict grain connectivity. The mechanism is demonstrated in composites of doped ceria with iron–cobalt oxide additives, where grains of the doped ceria constitute over 80 vol% and are nearly completely percolated for efficient and rapid ionic conduction. Remarkably, even though the additive-induced electronic conducting grains occupy less than 20 vol% and are distributed as islands, the observed electronic conductivity far surpasses conventional predictions. This anomaly is attributed to the accumulation of charge carriers at ceria grain boundaries, which facilitates electronic conduction. Through extensive structural and compositional analyses at micro- and nanoscale levels, the study unveils novel insights into the intricate architecture of this advanced percolation network. Furthermore, the optimization of these composites is achieved by enriching iron and cobalt cations at ceria grain boundaries, while inhibiting grain coarsening. This delicate balance culminates in excellent and sustainable mixed conductivity for oxygen permeation, thus advancing the potential of mixed-conducting composites for applications in clean and efficient energy technologies.