Comprehensive theoretical study on ionic transport numbers of triple ionic-electronic conducting oxides determined by the electromotive force method

Abstract

Triple ionic-electronic conducting (TIEC) oxides, as an emerging class of materials with complex conduction properties, are used in diverse electrochemical energy devices. Accurately determining the transport numbers (tx) of TIEC oxides is significant. In this study, we theoretically evaluate the reliability of the electromotive force (EMF) method in determining tx of TIEC oxides and address the issue that tx obtained by the EMF method is an apparent value (txapp). Initially, based on a precise defect distribution model, it reveals the non-uniform distributions of transport numbers within the TIEC membrane. Subsequently, through a comprehensive multiple-factor analysis, it discloses that compared with temperature and thermodynamic parameters of defect reactions, the gradient of gas partial pressure in concentration cells is the primary influencing factor affecting txapp. Notably, under conditions of relatively small gradients of gas partial pressure, txapp can be approximated as the average value of tx at both sides of the membrane. Motivated by these findings, we propose a new EMF method, which can determine the accurate tx of materials at a specific gas composition, rather than an apparent one. Overall, the finding of this study furnishes valuable insights into determining transport numbers of TIEC oxides.