Quantification of Ni coarsening in infiltrated SOFC anodes by combining 3D tomography, impedance spectroscopy and mechanistic modelling

Nickel coarsening is one the main degradation mechanisms that affect the lifetime of solid oxide fuel cell (SOFC) anodes. In this study we present an integrated experimental/modelling approach to quantify the effect of the Ni microstructural evolution on the electrochemical performance of infiltrated anodes. Symmetric cells made of a scaffold of scandia-stabilised zirconia (ScSZ) impregnated with different volume fractions of Ni were produced and annealed at constant temperature while recording the impedance every 30 mins. The microstructure of the fresh and degraded samples was reconstructed with focused ion beam SEM tomography and analysed to quantify the change in three-phase boundary (TPB) density, Ni percolation and Ni particle size upon annealing. A physically-based electrochemical model, based on conservation equations, was used to link the microstructural degradation to the evolution of impedance spectra. The model was able to decouple the microstructural contribution of Ni coarsening in impedance spectra and to interpolate the TPB density change with time, thus linking the microstructural evolution due to Ni coarsening to the increase in polarisation resistance. The analysis reveals a high coarsening rate in the first few hours of annealing. The degradation gradually slows down and a stable polarization resistance is approached, provided that a sufficient volume fraction of Ni is present to guarantee electronic percolation. On the contrary, an insufficient Ni volume fraction causes a gradual decrease in Ni percolation, resulting in a dramatic reduction in current collection with corresponding increase in polarization and ohmic resistance. Thus, the combination of mechanistic modelling and 3D tomographic analysis allow for an insight into the mechanisms that control Ni coarsening in infiltrated SOFC electrodes.