The performance and stability of Solid Oxide Cells (SOCs) is partially determined by the structure and evolution of the interfaces within them. New approaches to the modelling of multi-layer structures are outlined, enabling analysis of experimental data obtained from multi-layer SOC samples.
A new framework for modelling self-diffusion across multiple layers with an interface between them was derived. No existing model that is able to account for the changing diffusivities across a multi-layer structure has been developed previously. This is achieved by deriving a finite-difference-based numerical solution to the new system of equations that emerges. This enabled the quantification of interface blocking effects that were identified in previous experiments. Two different approaches are tested; one assumes the interface to exist in infinitesimal layer in space, while the other assumes an interface of finite thickness at the interface with diffusivities that are lower than other materials in the system. These models were tested against a series of experimental data, where La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) was deposited on yttria-stabilised zirconia (YSZ) substrates using pulsed laser deposition (PLD) followed by heat treatment before isotope exchange depth profiling (IEDP). The heat treatment resulted in progressively larger drops in isotopic fraction at the interface. This profile was fitted quantitatively with both multi-layer diffusion models, showing an increase in interface resistance with treatment temperature.
A different modelling approach looked into the rate-limiting reaction steps of the oxygen reduction reaction (ORR) in mixed ionic and electronic conductors (MIECs). COMSOL simulations of impedance spectra were systematically performed to investigate the effect of different ORR steps acting as rate-limiting and how these would be reflected in experimental data. The study recommends the testing of thin, dense films under different oxygen partial pressures as a method of determining the rate-limiting step in the ORR exhibited by different MIEC materials.