Microstructural changes in nickel-ceria fuel electrodes at elevated temperature

Durability testing of low temperature solid oxide cells is challenging as degradation phenomena related to microstructural changes like nickel-agglomeration are slow. In the present study, a nickel/gadolinia doped ceria (GDC) fuel electrode with a porous GDC-interlayer towards the zirconia electrolyte was investigated. The electrode, designed for operating temperatures of 600(degrees)C, was tested at an elevated temperature of 900(degrees)C for up to 1100 h to accelerate aging. Contrary to every expectation, the electrodes showed continuous improvement in electrochemical performance. Impedance spectroscopy, the distribution of relaxation times analysis, scanning electron microscope and transmission electron microscope were applied to correlate electrochemical and microstructural changes. Structural analysis showed a significant Ni agglomeration accompanied by a decrease in triple phase boundary density. Furthermore, a minor particle growth in the GDC-phase decreased the volume-specific double phase boundary GDC/pore. Considering these microstructural changes, the decrease in active reaction sites should have increased the polarization resistance, but a decrease of about 32% was observed. The discrepancy between polarization resistance improvement and microstructural degradation might be attributed to an activation of the GDC-surfaces in the electrode and the porous GDC-interlayer.