Degradation of Catalysts at High Temperatures

Research on the degradation of catalysts has mainly focused on low-temperature PEMFCs ( 90 °C) [28-42]. For high-temperature operation, studies on eatalyst degradation have been in the areas of phosphoric acid fuel cells (PAFCs) and PBI-based MEAs [41-43]. Since the catalysts used in PAFCs are the same as those in PEMFCs, the degradation mechanisms should be applicable to high-temperature PEMFCs. Normally, catalyst degradation includes two parts Pt catalyst degradation and carbon support oxidation. 

Pt degradation is manifested by Pt particle agglomeration, Pt particle isolation, and Pt dissolution. The observable phenomena are an increase in Pt particle size and a loss of electroactive surface area. For example, Pt particle size is 3 nm at the beginning of lifetime (BOL) testing, while after the durability testing or accelerated aging testing the particle size could increase to 10 or even 20 mn [26-39]. 

In a PEMFC, the potential is usually high at the eathode side under operating conditions, especially under open circuit conditions, where the following reaetion occurs [32]  

It is also observed that the Pt degradation rate in PEMFCs is dependent on operating conditions, cell voltage, and temperature. Unfortunately, the literature results are not comparable because different conditions and methods were used. A surface area loss rate of 7.14 x 10 m /g-Pt/hr over a 1000-hour period of operation was reported at a current density of 1.07 A/cm, a backpressure of 30 psig, and 100% RH [34], A 63% surface area loss was reported for an ME A after 10,000 potential cycles in the voltage range of 0.6-1.0 V at 80 °C [31]. 

An increase in temperature ean result in an increase in Pt degradation. Bi et al. studied catalyst degradation by cycling the MEA potential between 0.87-1.2 V at 40, 60, and 80 °C, respeetively [26]. They found that the reduction in surface area with the potential cyeling eould be deseribed by Equation 18.10  

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