Redox study of anode supported solid oxide fuel cell

The common technology for solid oxide fuel cells (SOFC) is based on a cermet anode of nickel with yttrium stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this anode technology is the redox tolerance. In this study, two techniques are used to quantify the anode expansion after one redox cycle at different temperatures. First, the increase of porosity is measured using scanning electron microscopy (SEM) image quantification. The second method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. An influence of redox cycling temperature on anode volume expansion was already observed by Ettler et al. but not fully understood. Here a model based on nickel oxidation mechanisms is proposed to explain the relation between redox temperature and volume expansion. The same methods are used to quantify anode expansion after consecutive redox cycles at constant temperature. In particular, small cracks caused at low oxidation temperature (600°C) were closed again after a subsequent reduction. The quantifying technique is then applied to cells tested in real stack conditions. The cell corners can undergo redox cycles depending on stack design and fuel utilization. The study of this location can give information on the number of cycles experienced by the anode support. Moreover, cracks in the electrolyte show infiltration of NiO that close the openings. Important infiltration can produce a short circuit between anode and cathode. The presence of strontium chromate on the infiltrated NiO was observed.