Quantitative Study of LSCF and LSM-YSZ Cathode Microstructure by FIB/SEM Tomography

Abstract

The mixed ionic-electronic conducting cathode (La, Sr)(Co, Fe)O3-δ (LSCF) is utilized worldwide for solid oxide fuel cells (SOFCs) operated at intermediate temperatures [1] because of its excellent oxygen reduction reaction rate. The challenge of using LSCF as cathode material is the complex nature of the interface to Zirconia-based electrolytes, which may counteract this advantage [2, 3]. The composite cathode (La, Sr)MnO3 (LSM) – Y2O3 doped ZrO2(YSZ) is an adequate alternative for higher temperatures only. LSM-YSZ offers fewer regions of oxygen reduction (triple phase boundaries) due to spatially separated ionically (YSZ) and electronically (LSM) conductive phases. If electrochemical and microstructural parameters of both types of electrodes are quantified by appropriate techniques, the performance can be modeled and compared for different operating conditions. Advanced imaging techniques such as focused ion beam/scanning electron microscopy (FIB/SEM) or X-ray tomography are promising techniques for microstructure quantification enabling 3D reconstructions of µm- and sub-µm-scaled multiphase electrodes [4, 5, 6]. Reconstructing the composite cathode, in particular, comprises the challenge of overcoming the weak material contrast between LSM and YSZ using scanning electron microscopy. This contribution will show how FIB/SEM tomography can help to identify different microstructural parameters such as porosity, particle size, surface area and tortuosity. We will compare LSCF and LSM-YSZ cathodes and enlighten new challenges in image reconstruction, parameter acquisition and parameter interpretation for the different material sets. [1] H. Yokokawa, et al., J. Power Sources, 182, p. 400 (2008) [2] J. Szasz, et al., ECS Trans., 66(2), p. 79 (2015) [3] Yokokawa, et al., Solid State Ionics, 262, p. 454 (2014) [4] J.R. Wilson, et al., Nature Materials 5(17), p. 541 (2006) [5] J. Joos, et al., Electrochim. Acta 82, p. 268 (2012) [6] Y. C. K. Chen-Wiegart, et al., J. Power Sources, 218, p. 348 (2012) Figure 1