Structural modifications to nickel cermet anodes in fuel cell environments

Restructuring of Ni in cermet anodes of solid oxide fuel cells (SOFCs) has been studied using both bulk fuel cells and thin foil anodes. The bulk cells were button cells (23 mm in diameter) with cermet anodes (30–70 μm thick) made up of nickel and gadolinium-doped ceria (Ni/CGO). The cells were operated (under current load) at 700 °C in moist H2 or moist H2 with low levels of H2S. Scanning electron microscopy (SEM) was used to characterize the microstructure before and after testing. The thin foil samples (100–150 nm thick) were cermets of nickel and yttria doped zirconia (Ni/YSZ) and these were exposed (without current load) at 700 °C to dry H2, moist H2 or moist H2 with H2S (1 ppm). Transmission electron microscopy (TEM) and SEM were used to analyze the microstructural changes in these samples. The anodes from the bulk cells exhibited terracing of Ni grains in all instances, with the extent of terracing increasing with exposure to H2S, and with increasing H2S levels and exposure time. The thin foil anodes showed much more extensive Ni restructuring leading to agglomeration and faceting of Ni grains. This was accompanied by debonding from YSZ, commencing at triple points, where some combination of three Ni/YSZ grains meet. The amount of restructuring increased with increasing H2 concentration in the gas, and was accelerated by the presence of H2S and/or H2O. Evidence is presented that indicates that terracing may represent the early stages of Ni agglomeration.