Catalyst Degradation Under Potential Cycling as an Accelerated Stress Test for PBI-Based High-Temperature PEM Fuel Cells-Effect of Humidification

摘要

In the present work, high-temperature polymer electrolyte membrane fuel cells were subjected to accelerated stress tests of 30,000 potential cycles between 0.6 and 1.0 V at 160 degrees C (133 h cycling time). The effect that humidity has on the catalyst durability was studied by testing either with or without humidification of the nitrogen that was used as cathode gas during cycling segments. Pronounced degradation was seen from the polarization curves in both cases, though permanent only in the humidified case. In the unhumidified case, the performance loss was more or less recoverable following 24 h of operation at 200 mA cm(-2). A difference in degradation behavior was verified with electron microscopy, X-ray diffraction, and electrochemical impedance spectroscopy. The strong effect of humidification is explained by drying of the phosphoric acid that is in the catalyst layer(s) versus maintaining humidification of this region. Catalyst degradation due to platinum dissolution, transport of its ions, and eventual recrystallization is reduced when this portion of the acid dries out. Consequently, catalyst particles are only mildly affected by the potential cycling in the unhumidified case.