Degradation analysis of CFY-stacks MK35x - a guide for exact measurement

Degradation rates of DP/P0<0.5 %/1000 h are the target for SOC stacks in systems operating more than 20.000 h, which is defined by the end of life (EoL) criteria for power degradation of 10 %. Planar stack based on CFY interconnect (MK352) is a good candidate for stationary systems. The degradation mechanisms of MK352 stack components are known from pre-tests, samples tests and described in the literature. Especially the oxide scales growth on CFY interconnects and chromium poisoning of the air electrode are well investigated. Based on this knowledge Fraunhofer IKTS evaluated the different degradation mechanisms at stack level by variation of operating conditions. Different degradation parameters like increased temperature, current density and air humidification were varied and tested, to determine degradation and to separate the acceleration factors. It is shown, that the major degradation originates from oxidation of interconnect on air side and interaction with cathode contacting layer. Further investigations regarding the interactions with cathode and other components are ongoing. Exact measurement of degradation at stack level is the basis for accelerated testing, coupled with a separate investigation of sample tests. Over the last years it was shown that the used hardware for tests is affected by degradation. The constant current operation is the best option for long term testing and analysis of degradation. It is analysed by simulation and compared to constant voltage and constant power operation. Testing conditions and their impact on the test results will be explained. For test duration <3000 h the minimal deviation of measured values leads to degradation rate uncertainty up to +/-1.0 %/1000 h. It is shown, that failures in temperature measurement has a huge impact on power output and calculated degradation rates. The calibration of measuring equipment before and after durability test is essential for exact measurement of degradation rates.