Characterising Ionic Transport Resistance at Solid-Solid Interfaces in SOFC Using Isotopic Exchange and Numerical Modelling

Abstract

Understanding the mechanisms governing ionic transport in ceramics is important for improving the performance and durability of Solid Oxide Fuel Cells (SOFCs). Properties such as the tracer diffusivity, D* , can be measured by Isotopic Exchange Depth Profiling (IEDP) as developed by Kilner et al. [1]. This method has, so far, primarily been used to characterise the material properties of single materials; however, SOFCs are multilayer devices with solid-solid interface that may also affect transport. Some investigations into the diffusion behaviour across multiple layers using the IEDP technique have been done in thin film samples [2][3]. Profiles obtained from these studies appear to show an abrupt concentration drop at the interface between certain materials, indicating an interface which significantly impacts the diffusion behaviour (and by extension the overall cell performance). However, no attempt was made to quantify this interface effect with a theoretical modelling approach.A finite-difference model for diffusion in a system containing multiple layers with interfaces has been developed. It numerically solves Fick’s second law of diffusion with various boundary conditions. This model can be used to fit experimental data obtained from tracer diffusion SIMS data, yielding a new way of quantifying interfacial resistance. A new interfacial resistance parameter, r int , has been defined, which quantifies the resistance to diffusion across an interface.The validity of the developed method has been experimentally demonstrated in a sample consisting of a layer of lanthanum strontium cobalt ferrite (LSCF) on a gadolinium-doped ceria (GDC) substrate, both common materials in SOFCs. Initial data from tracer diffusion experiments have shown the presence of a significant concentration drop at the interface of the LSCF-GDC stack, which can be fitted to the numerical model developed by the authors. This approach could be used to measure the changing interface properties under various ageing conditions, as well as the influence of interlayers and material selection on the interfacial resistances both in SOFC and other diffusion systems with interfaces, such as solid state batteries.[1] J. A. Kilner, B. C. H. Steele, and L. Ilkov. Oxygen self-dffusion studies using negative-ion secondary ion mass-spectrometry (sims). Solid State Ionics, 12(MAR):89-97, 1984.[2] K. Develos-Bagarinao, H. Yokokawa, H. Kishimoto, T. Ishiyama, K. Yamaji, and T. Horita. Elucidating the origin of oxide ion blocking effects at gdc/srzr(y)o-3/ysz interfaces. Journal of Materials Chemistry A, 5(18):8733-8743, 2017.[3] Katherine Develos-Bagarinao, Haruo Kishimoto, Jeffrey De Vero, Katsuhiko Yamaji, and Teruhisa Horita. Effect of la0.6sr0.4co0.2fe0.8o3-delta microstructure on oxygen surface exchange kinetics. Solid State Ionics, 288:6-9, 2016.Figure: Isotope Exchange Depth Profile Data taken by linescan from an LSCF-GDC multi-layer sample. Original image data and isotopic fraction profile, as well as the corresponding fit obtained using the numerical model is shown. Figure 1