Investigation of 2R-Cell Degradation under Thermal Cycling and RedOx Cycling Conditions by Electrochemical Impedance Spectroscopy

The prevalent anode supported thin electrolyte solid oxide fuel cells (ASC) used today are highly susceptible to failure under redox conditions. The expansion caused during the oxidation of Ni to NiO can destroy the support, leading to irreversible cell damage. To counter this problem, Fiaxell SOFC Technologies (Switzerland) has engineered the microstructure of their cells, to confer added redox stability to their anode supported cell (2R-cell™). This commercial cell was investigated within the Eurostars-Roxsolidcell project against thermal and redox cycling.

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.

The cell was examined after the test by scanning electron microscopy (SEM). No delamination of the cathode was observed, however, fragmentation of the anode active layer was apparent.