Accelerated Lifetime Tests for SOFCs
Considering the expected lifetimes for Solid Oxide Fuel Cells of 5 to 10 years, durability is still a major issue. Even though quite a number of research institutions and companies have proven an acceptable degradation rate of cells, stacks and systems for periods ranging from a few hundred hours to several years, neither reliable degradation models nor methodologies to evaluate the durability in a short timeframe are available. Thus any kind of modification on the cell or stack level, which might affect the durability, requires an expensive and time consuming repetition of the durability test. Accelerated lifetime tests, which enable a rapid degradation analysis, and degradation models, that enable an extrapolation of the results, would be highly desirable.
In most durability investigations a cumulative degradation rate is evaluated from the cell voltage decrease in a galvanostatic operating mode. In addition, extensive post test analysis is performed to detect the failure causes. This approach does not reveal any quantitative information on the different underlying degradation mechanisms. If aggravated stress is applied by increasing operating temperature, current density or gas utilization, the degradation of cathode, electrolyte, anode, contact layers and interconnects is accelerated in different ways. Therefore the cell voltage alone is insufficient to understand the performance degradation and its acceleration due to aggravated stress.
To deconvolute the degradation of cathode, electrolyte and anode (i) electrochemical impedance spectroscopy, (ii) impedance data analysis by the distribution of relaxation times and, (iii) a subsequent CNLS-Fit to a physically meaningful equivalent circuit model is suggested. Cell tests were performed at different stress levels by varying temperature and current density as well as fuel and oxidant composition. Based on this extensive data set, the interplay between stress level and impact of the different stresses on degradation mechanisms in cathode, electrolyte and anode will be presented and guidelines for the design of accelerated tests will be discussed.