3D Microstructure Effects in Ni-YSZ Anodes: Prediction of Effective Transport Properties and Optimization of Redox Stability

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 °C. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φ(eff)), constriction factor (β), and tortuosity (τ). The effective conductivity (σ(eff)) is described as the product of intrinsic conductivity (σ(0)) and the so-called microstructure-factor (M): σ(eff) = σ(0) × M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, M(pred) = εβ(0.36)/τ(5.17), and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (M(sim)). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers.