Secondary Phases at Cathode/Electrolyte Interfaces

Abstract

Understanding the nature of a cathode/electrolyte interface in solid oxide fuel cells (SOFC) is still a challenge, especially if this interface is composed of a mixed conducting cathode, i.e., La1-xSrxCo1-yFeyO3- δ (LSCF), and a bilayer electrolyte, i.e., Gd-doped Ceria (GDC) interlayer and Y2O3 stabilized ZrO2 (YSZ) electrolyte. The interface resistance can vary over more than two orders of magnitude [1], resulting from variable volume and local distribution of the poorly ionic conducting secondary phase SrZrO3(SZO) [2] and GDC-YSZ interdiffusion (ID) [3]. This contribution will introduce a COMSOL Multiphysicsmodel, which simulates the performance characteristics of such heterogeneous cathode/electrolyte interfaces. The model is based on 3D-reconstruction data of an interface region, which was characterized (i) electrically by in-situ electrochemical impedance spectroscopy and (ii) by ex-situ elemental analysis using TEM/EDXS. Our model calculations quantify the contributions of both secondary phases, SZO and ID to the cathodic polarization resistance, but also a contribution of ID to the ohmic resistance, corresponding well to the performance measurements. Our results further reveal that interface morphology, i.e. volume and local distribution of secondary phases, plays a significant role. When GDC is sintered at high temperatures (see Fig.1 at 1300°C) to a continuous and dense ID layer (green color), SZO formation is still significant (turquoise color), but a comparatively small area of “free” pathways for oxygen ions from LSCF→GDC→ID→YSZ can reduce the polarization resistance up to two orders of magnitude. On the contrary; when GDC is sintered at lower temperatures, a much thicker and continuous SZO layer blocks the diffusion of oxygen ions, corresponding to the highest polarization resistance. From our investigation we conclude that because secondary phases are inherently present and their local contribution overshadows every high-performing material property, it is of particular interest to deepen the understanding of functional cathode/electrolyte interfaces by using high-resolution analysis techniques.