Microstructural and electrochemical degradation of infiltrated SOFC anodes

The polarization resistance of solid oxide fuel cell (SOFC) anodes can be significantly reduced by infiltrating nickel nanoparticles into an ion-conducting ceramic scaffold, thus allowing operation even at intermediate temperatures. However, the stability of infiltrated anodes is still too poor to be used in real applications as the performance undergoes a rapid degradation [1], whose causes are still unclear. In this study we present an experimental/modelling approach to quantify the different contributions that lead to the rapid increase in anode polarization resistance. YSZ symmetric cells with scandia-stabilised zirconia (ScSZ) scaffolds were impregnated with different volume fractions of Ni and annealed at constant temperature while recording the impedance every 30 mins in reducing atmosphere. The 3D 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 [2] was used to decouple and quantify the different microstructural and electrochemical contributions in the evolution of impedance spectra. The analysis reveals evidence of Ni coarsening, as the Ni specific surface area decreases from 6.4 to 3.3 um-1 and the TPB density decreases from 19.8 to 11.7 um-2 after 200 h of annealing. The Ni tortuosity factor increases from 4.2 to 7.7, suggesting a reduction in Ni percolation. The polarization resistance shows a rapid increase within the first 3 h, followed by a slower degradation rate of ca. 0.09 Ohm cm2 h-1 and stabilization after about 100 h, resulting in a final resistance being an order of magnitude larger than the impedance of the fresh electrode. The model suggests that there is an underlying microstructural evolution of Ni, which is responsible for the long-term electrochemical degradation. The rapid increase in resistance within the first 24 h is attributed to additional phenomena occurring at the nanoscale at the TPB. The combination of mechanistic modelling and 3D tomographic analysis allows for a quantitative understanding of the mechanisms that control the degradation in infiltrated anodes.