Correlating Cathode/Electrolyte Interface Characteristics to SOFC Performance

Abstract

Correlating chemical composition and microstructure characteristics of both electrode/electrolyte interfaces to the performance of a solid oxide fuel cell (SOFC) is of fundamental importance for further improvement [1]. Focused ion beam / scanning electron microscopy (FIB/SEM) reconstruction is one of the most advanced techniques for the quantitative assessment of microstructure characteristics of primary material phases. Progress was recently made in approaching also the local distribution of secondary phases and impurities by combining FIB/SEM with TEM/EDXS elemental analysis (transmission electron microscopy / energy dispersive x-ray spectroscopy) [2]. Applying this method to the cathode/electrolyte interface; LSCF cathode (La1-xSrxCo1-yFeyO3- δ) / GDC barrier layer (Gd-doped Ceria) / YSZ electrolyte (Y2O3 stabilized ZrO2) reveals important findings:Formation of the secondary phase SrZrO3 (SZO) dominates interface resistance and cell performance, depending on volume and local distribution (Fig. 1 and 2) [2,3]. This is confirmed by in-situ impedance and performance measurements of symmetrical and full cells (Fig. 1). A model is introduced, which simulates the locally resolved contribution of a secondary phase like SZO to the charge transfer reaction and GDC/YSZ interdiffusion (ID) to the ion diffusion at a heterogeneous cathode/electrolyte interface. It will be demonstrated, that the barrier layer functionality is determined by the applied sintering temperature. Functionality is achieved by a custom-tailored thick ID layer at high sintering temperatures. In contrast are low GDC sintering temperatures; then functionality fails, and a continuous SZO layer forms (Fig. 2 (a)) which increases the cathode overpotential over two orders of magnitude.The results of this contribution are fundamental for understanding the microstructure characteristics of secondary phases and their correlation to SOFC performance. They convey intuitive knowledge to fabricate high performance cathode/electrolyte interfaces for individual cell concepts.