Solid Oxide Fuel Cell Degradation, Recovery and Control via the Electrical Terminals

Abstract

This paper presents results from an investigation concerning load-induced degradation, recovery, and control of solid oxide fuel cells (SOFCs). In this study, commercially available SOFCs were subject to extended over-current conditions, followed by periods of open-circuit operation. During times of current loading, degradation was observed in the cells’ electrical performance through polarization and electrochemical impedance spectroscopy (EIS) measurements. These measurements showed an increase in the polarization curve’s ohmic region slope, i.e. large-signal resistance, as well as an increase in the cell’s small-signal low-frequency impedance. The degradation was temporary however, as the electrical performance recovered during times of open-circuit operation. These results, attributed to electrochemically-induced oxidation and reduction of nickel in the anode, suggest the degradation phenomenon is controllable via the electrical terminals. As such, an additional test was performed on an SOFC powering a pulse-width modulated load, with the load’s duty-cycle negatively proportional to the cell’s large-signal resistance. Polarization and EIS measurements taken during this test showed that despite the controlled load, degradation occurred throughout the test. However, post-test scanning electron microscope images revealed cracks in the cell’s cathode along the boundary between the active and bulk layers. This type of cracking was not observed in the original degradation and recovery tests, suggesting that the degradation observed in the controlled load test was irreversible and caused by a separate phenomenon.