Investigation of 2R-Cell Degradation under Thermal Cycling and RedOx Cycling Conditions by Electrochemical Impedance Spectroscopy
A 2R-cell with a 10cm2 active area LSCF cathode on a CGO buffer layer, was placed between alumina felts, mounted inside a seal-less open flange setup kept in an oven, and operated at a temperature of 780°C. A nickel foam and a gold grid were used for the current collection at the anode and cathode respectively. Dry hydrogen (180 Nml/min) was fed to the anode and air (500 Nml/min) to the cathode. 20 thermal cycles were performed between 780 and 100°C, using 200°C/h heating ramps and natural cooling, followed by 20 redox cycles, during which air was fed to the anode for 1 hour at 780°C, causing the cell potential to drop to zero.
V-i polarisation curves and electrochemical impedance spectroscopy (EIS) measurements were performed before and after each consecutive cycle in order to monitor the performance losses. The OCV remained stable during the whole test whereas the area specific resistance (ASR) measured at 0.5 A/cm2 increased by +1.3 mΩcm2 per thermal cycle and by +1.8 mΩcm2per redox cycle. The EIS measurements showed that the degradation during the thermal cycling was mainly due to an increase of the ohmic losses. In the case of the redox-cycling, the cell degradation after stabilisation principally came from high frequency polarisation losses (cf. Figure), which were attributed to changes in the anode functional layer microstructure.