The effect of dissolution, migration and precipitation of platinum in Nafion®-based membrane electrode assemblies during fuel cell operation at high potential

MEAs comprising Nafion® perfluorosulfonic acid-type membranes have been submitted to accelerated ageing operation conditions at high cell voltage, and characterised during the course of time in terms of gas consumption, gas crossover, and water production at the anode and cathode. Aged and end-of-life MEAs have been characterised using atomic force and scanning electron microscopy with energy dispersive analysis by X-ray spectroscopy, as well as transmission electron microscopy. A representative sample has been examined using X-ray photoelectron spectroscopy to identify the nature of the platinum species present in the membrane. Under high cell potential, platinum dissolves from the cathode, and migrates through ion-exchange sites in the membrane through a concentration effect. On encountering crossover hydrogen, platinum precipitates within the membrane as dispersed particles that comprise a metallic core and a shell of oxidised platinum. Platinum charged species act as centres for the generation of free radicals that chemically degrade the membrane, leading to increased porosity, gas crossover and ultimately membrane and MEA failure. Mass spectroscopic analysis of water generated even at open circuit has enabled identification of degradation fragments, the nature of which points to degradation both at polymer end groups, and functionalised pendant groups.