Investigation of Local Electrochemical Performance and Local Degradation in an Operating Solid Oxide Fuel Cell

In order to characterize a new SOFC stack design, to investigate degradation processes and to validate models, a diagnostic test station was designed and realized. It allows to characterize repeat-elements locally by measuring the local potential, current density and temperature over the active area. The active area is segmented in 18 small electrically insulated measurement points (segments), in addition to a main segment. The profile of current density was investigated up to an average of 0.6A/cm2 and 67% of fuel utilization, showing as expected a different response depending on the position along the flowpath. In addition, the local Nernst potentials were measured by temporarily disabling the polarization of the concerned segments. The temperature profile was also investigated as a function of the output current, showing a large heat transfer with the test furnace in this single-element configuration. In addition, local degradation behavior was studied over 1900 hours. In particular, it was found that the repeat-element showed a large sensitivity to fuel composition, with larger degradation rates under pure hydrogen than under diluted fuel mixture. Impedance spectroscopy results showed large differences in degradation behavior, depending on the location in the repeat-element and on the polarization history. In particular, segments that had not been polarized during the test showed lower ohmic resistances than the polarized ones, and zones located near the gas inlets degraded more than those near the outlet. With this experiment, it was clearly demonstrated that degradation processes depend on local operating conditions.