Stack degradation in dependence of operation parameters; the Real-SOFC sensitivity analysis

The EU Integrated Project Real-SOFC aims at improving the understanding of degradation in SOFC stacks and extending the durability of planar SOFC stacks to degradation rates suitable for stationary application. As part of the Real-SOFC project three series of SOFC stacks each with 2 or 4 planar anode-supported cells were operated for durations of up to 3000 h (and longer) under varying fuel and electrical load conditions. The durability tests on these short stacks were conducted galvanostatically at 800 and 700 °C in dependence of current-density (0.3, 0.5 or 0.7 A/cm²), of fuel composition (hydrogen: H2 + 3.10% H2O or methane: CH4/H2O (S/C=2)) and of fuel utilization (8, 40, 60 or 75%). A pronounced difference in degradation behaviour was observed between the stacks operated at different current densities. The degradation behaviour was, however, not influenced by the choice of fuel (hydrogen or methane) and was hardly influenced by the fuel utilization. Lowest degradation rates of about 20 m?ocm²/kh were determined for the tests of a short stack with cells with LSM cathodes operated at 800 °C and a current-density of 0.3 A/cm² and of a short stack with cells with LSCF cathodes operated at 700 °C and a current-density of 0.5 A/cm². Post-test characterization of the cathode with respect to chromium poisoning was performed on cells from several stacks. No clear relationship between the degradation rate of the stacks and amount of Cr incorporated in the cathode could be established. The major difference was a change in microstructure of the cathode in the region near the electrolyte interface; in the stacks operated at lower current densities the structurally changed zone was clearly thinner than in those stacks operated at higher currents.