Mechanical and microstructural evolution of Ni-YSZ electrodes in solid oxide cells after long-term operation: insights from high-speed nanoindentation mapping and Gaussian mixture model clustering

• Validating high-speed nanoindentation mapping to assess the Ni-3YSZ hardness and elastic modulus. • Good agreement achieved between conventional nanoindentation and HSNM results. • H and E maps replicated the microstructure of Ni-3YSZ electrode. • H and E of Ni-3YSZ are sensitive to the microstructural degradation. • Ni agglomeration and reoxidation increased the H and E in aged SOFCs. Nickel-3 mol.% yttria-stabilized zirconia (Ni-3YSZ) fuel-electrodes of Solid Oxide Cells (SOCs) are susceptible to electrochemical, mechanical and microstructural degradation during operation. Mitigating this degradation constitutes one of the major challenges for the SOC industry. This work investigates the evolution of the hardness ( H ) and elastic modulus ( E ) associated with microstructural changes, using high-speed nanoindentation mapping (HSNM) technique combined with statistical analysis as a novel methodology to assess the Ni-3YSZ degradation under realistic operational conditions. Two industrially manufactured Ni-3YSZ cermets from electrode-supported cells are examined: one aged 20,500 h under fuel cell (SOFC) mode and another aged 2,500 h in electrolysis (SOEC) mode. A non-operated reference cell (Fresh) is also analyzed to establish direct comparisons. Conventional nanoindentation and HSNM are used complementarily to quantify local mechanical properties, while the datasets are statistically treated using Gaussian Mixture Model (GMM) clustering to distinguish the mechanical response of the constitutive phases. Complementary microstructural characterization, including FIB-FESEM tomography, enables quantification of phase evolution and porosity changes. Results show that the fuel electrode of the aged SOEC cell exhibits no significant variations in H or E compared to the fresh sample, with only slight Ni agglomeration, indicating limited microstructural degradation. In contrast, the aged SOFC electrode shows significant increases in both properties, attributed to agglomeration and reoxidation of Ni, and porosity reduction. In all cases, H and E maps replicate the Ni-3YSZ microstructure, and their good agreement with the microstructural analyses confirms the effectiveness of combining HSNM with GMM for assessing Ni-3YSZ degradation. This outcome could guide future strategies to quantify and predict degradation phenomena for improving the durability of SOCs.