| One of the primary causes of degradation of solid oxide fuel cells is the >60% volume increase that results when Ni-YSZ (yttria-stabilized zirconia) anodes are exposed cyclically to air and fuel at high temperatures, resulting in the oxidation/reduction of the Ni phase ('redox cycling'). By studying Ni foils, and both electrolyte- and anode-supported Ni-YSZ anodes, using thermogravimetric analysis, 3-electrode electrochemistry, and microscopy, it is shown that multiple redox cycles (at both 750 and 800°C) cause significant morphological changes which can lead to electrolyte cracking and interfacial delamination. However, redox cycling also leads to performance activation in humidified H2. In order to prevent redox cycling damage during air exposure, it is shown that cooling the Ni-YSZ samples to <600°C at between 3 and 7.5°C/min significantly slows down the oxidation rate of Ni, thus minimizing the volume expansion associated with NiO formation. Another promising solution involves the application of a cathodic current during air exposure, which serves to maintain the anode compartment in a reducing environment, thus preventing NiO formation. |
