From model anodes to cermet anodes

The electrochemical performance of different types of Ni-based SOFC anodes was studied. Model anodes made of a Ni gauze, of a Ni pattern, and of a Ni paste were compared with Ni-YSZ cermet anodes with three dimensionally connected Ni and YSZ phases. The Ni-YSZ anodes were deposited by screen-printing and by sputtering. Due to the different designs as well as due to the different ratios of Ni and YSZ, the anodes differ mainly in the extension of the electrochemically active reaction zone, i.e. the triple phase boundary (TPB) length. According to electrochemical impedance spectroscopy (EIS) measurements, two dispersions were found for most anode designs. The main impedance arc arises at two different relaxation frequencies depending on the anode design. In both cases, it decreases exponentially under anodic polarization and is thermally activated with an activation energy of I eV. The second impedance arc arises only under high anodic polarization. The impedance increases with increasing polarization and the activation energy is around 0.5 eV.