Quantifying Diffusion Across Solid-Solid Interfaces in Electrochemical Cells

Abstract

Enabling an increased rollout of hydrogen-based technologies requires decarbonisation of both hydrogen production and conversion. Solid Oxide Cells (SOCs), in the form of electrolysers and fuel cells, could play a significant role in achieving this, however lifetime and degradation of SOCs remain major obstacles to commercialisation. Understanding the mechanisms governing ionic transport in ceramics is important to improving the performance and durability of SOCs. Properties such as the oxygen tracer diffusivity, , 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 properties of single materials; however, SOCs are multilayer devices with solid-solid interfaces 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 is 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 new way of quantifying interfacial resistance. A new interfacial resistance parameter, , has been defined, which quantifies the magnitude of concentration drop across any given interface and thus offers a universal way of numerically characterising resistance to diffusion.The validity of the developed method has been experimentally tested in samples containing layers of lanthanum strontium cobalt ferrite (LSCF) and gadolinium-doped ceria (GDC), a commonly used system in SOCs. Initial data from tracer diffusion experiments as seen in Figure 1 have shown the presence of a significant concentration drop at the interface of a LSCF-GDC stack which can be fitted to the numerical model developed by the authors. This approach is 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 SOC and other diffusion systems.[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.Fig. 1. Oxygen isotopic fraction data of an LSCF layer deposited on a GDC substrate and annealed in Oxygen-18 atmosphere at 650 C for 1h obtained using cross-sectional SIMS (left) and fit of the data using the multi-layer diffusion model developed by the authors (right). Figure 1