Effect of alumina additions on the anode|electrolyte interface in solid oxide fuel cells

The longevity of a solid oxide fuel cell (SOFC) stack is curtailed by the fragility of its ceramic components. At Ceramic Fuel Cells Limited (CFCL), 15 wt.% alumina is added to the commonly used 10 mol% Y2O3-ZrO2 (YSZ) electrolyte to improve both the fracture toughness and grain-boundary conductivity of the electrolyte. This study investigates the effect of such addition of alumina on the anode|electrolyte interface; more specifically, which reactions occur with the Al2O3 at the interface and how these reactions influence fuel cell performance. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the formation of NiAl2O4 in the alumina regions in the electrolyte. The NiAl2O4 is observed to grow into the adjacent grain boundaries to form an interconnected NiAl2O4 network up to 4 μm deep into the electrolyte. Impedance spectroscopy shows that the formation of NiAl2O4 does not affect the grain bulk ionic conductivity. The grain-boundary conductivity is markedly reduced at low temperatures. However, at the high SOFC operating temperature at CFCL (850 °C) the contribution of the grain-boundary conductivity to the total conductivity is diminished, and the NiAl2O4 is found not to have an effect on the total electrolyte conductivity and is deemed not to be a detrimental reaction.