Structural and Chemical Analysis By Transmission Electron Microscopy of Pt-Ru Membrane Precipitates in Proton Exchange Membrane Fuel Cell Aged Under Reformate

Carbon supported platinum-ruthenium (Pt-Ru/C) nanoparticles are used as anode catalysts in PEMFCs operated under reformate owing to their good carbon monoxide tolerance. The stability of these catalysts during fuel cell operation is still not well known. In this work, we have studied by transmission electron microscopy (TEM) the microstructural evolution of a membrane/electrode assembly after a 1,000 h ageing test under reformate (26 ppm of CO) representative of a stationary application. The analyses clearly show evidence of Ru dissolution from the anode with the observation of Pt-Ru precipitates in the membrane and of small Ru precipitates in some areas of the microporous layer near the anode. This Ru dissolution happens despite a rather low estimated anode overpotential (around 0.2V). The distribution of the Pt-Ru membrane precipitates was found to be similar to that observed in aged MEAs with pure Pt catalysts, i.e. a precipitation band several micrometres away from the cathode catalyst layer made of the largest precipitates along with smaller precipitates between the band and the anode catalyst layer. The structure and the chemistry of the membrane precipitates were accurately analysed ¹. The high resolution TEM images and EDS (Energy Dispersive X-Ray Spectroscopy) Pt, Ru elemental maps show that the largest precipitates display a singular flower shape consisting of a Pt-rich face-centred cubic ( fcc) crystallographic structure core surrounded by a Ru-rich hcp (hexagonal close-packed) crystallographic structure shell (Fig. 1 a and b). These results suggest a precipitate formation mechanism (Fig. 1c) where the Ru reduction is catalysed by Pt within the membrane. Moreover, the localization of the precipitation band near the cathode seems to indicate that the Pt in the precipitates comes from the dissolution of cathodic Pt/C and that both Pt and Ru ions are reduced by the hydrogen crossover.

In addition, the degradation of a MEA aged in a stack under 10 ppm of CO was studied. The MEA degradation was analysed in different zones of the MEA surface area; near the gas inlet, outlet and in the middle. As the durability tests were applied with continuous registration of current density maps thanks to a S++ ® current scan line device, the observed degradation heterogeneities has been correlated to the local conditions.

¹  P. A. Henry, L. Guétaz, N. Pelissier, P.-A. André, S. Escribano, J. Power Sources 275 (2015) 312-321

Aknowledgement : This work was supported by the European Union’s Seventh Framework Program (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°256776 (FCH-JU project Premium Act).