Effective transport properties in porous composites applied to MIEC SOC electrodes

Abstract

Semi-analytical models describing transport phenomena governed by the Laplace equation (like conduction of charge carriers or heat) are presented for the case of a porous composite with two solid phases and one pore-phase (i.e., two conducting and one insulating phase), closing the existing gap in literature for fast and accurate predictions for this particular case. The models allow for an efficient screening of promising concepts and material combinations, as they are computationally much more efficient compared to numerical simulations on a 3D geometry. Furthermore, the semi-analytical models are also applicable, if not the full 3D geometry of the microstructure is available. Three different semi-analytical models (Maxwell, Xu and MST models) are compared and validated using a microstructure dataset of perovskite-CGO solid oxide cell electrodes obtained by stochastic modeling. Based on results from both, numerical and semi-analytical models, the effects of the resulting composite transport properties are discussed for the application example of these fully ceramic electrodes. CGO and the used LSTN perovskite are both mixed ionic and electronic conductors (MIECs), which leads to different reaction mechanisms and associated requirements for the microstructure design compared to, e.g., Ni-YSZ. Due to the MIEC-property of both solid phases, the transports of neither the electrons nor the oxygen ions are limited to a single phase. Consequently, the composite conductivity, which is inherent to MIEC electrodes, opens a much larger design space for microstructure optimization compared to conventional electrodes with single-phase conductivity, that are prone to percolation failure.