Monitoring the degradation of a solid oxide fuel cell stack during 10,000 h via electrochemical impedance spectroscopy

A 5-cell solid oxide fuel cell stack was tested during 10,000 h of continuous operation with simulated reformate gas as fuel (71 vol.% H-2, 20.7 vol.% CO2 and 8.3 vol.% steam) under high fuel utilization (73%) and constant current load (0.5 A cm(-2) or 25 A) at 750 degrees C. In situ electrochemical impedance spectroscopy was used to monitor the evolution of ohmic and polarisation resistances of individual cells in the stack without interrupting the current load. Impedance spectra were recorded on each cell periodically (every 1000 h) or after uncontrolled incidents happened with the test setup. It has been found that the stack degradation is mainly attributed to the increased ohmic resistance, pointing to possible causes such as interconnect corrosion and reduced effective contact areas between cells and interconnects. The degradation rate during the first 5000 h was about 1% kh(-1), but increased afterwards up to 1.5% kh(-1) due to the impact of incidents. Both types of incidents (fuel supply fluctuations and overloading failure of the electronic load) were complicated by inhomogeneous fuel distribution among cells, leading to most probably partial re-oxidation of the anode, accelerating the stack degradation. (C) 2011 Elsevier Ltd. All rights reserved.