Behavior and mechanisms of Ni/ScSZ cermet anode deterioration by trace tar in wood gas in a solid oxide fuel cell

In order to determine both the criterion for diagnosing the deterioration of Ni/ScSZ cermet anodes in solid oxide fuel cells (SOFCs) by tar contaminants in wood gas and the tolerance limit of tar in wood gas for such anodes, the influence of tar concentration in wood gas on anode deterioration behavior was examined by means of scanning electron microscopy. We found that the anode degradation mechanism consisted of three phenomena: the disappearance of Ni particles, the destruction of sintered ScSZ, and carbon deposition. Furthermore, the Ni particle disappearance occurred at lower tar concentrations than did sintered ScSZ destruction and apparent carbon deposition. Therefore, we propose that the disappearance of Ni particles be set as the criterion for confirming deterioration of Ni/ScSZ cermet anodes in SOFCs by tar. On the basis of this criterion, the tolerance limit of toluene in fuel gas was determined to be 3 g/Nm3 when the operating temperature, steam to carbon molar ratio, and current density were 1073 K, 1, and 0.5 A/cm2, respectively. The tolerance limit for tar for the fuel cell constructed herein was one to two orders of magnitude higher than that for internal combustion engines.