The impact of the morphology of the carbon support on the activity and stability of nanoparticle fuel cell catalysts

Abstract

This study explores how the morphology of nanostructured carbons impacts the morphological stability of supported Pt fuel cell nanoparticle catalysts under extended potential cycling. Using in situ small angle X-ray scattering (SAXS), we monitor the evolution of key structural parameters of four different Pt/carbon catalyst couples, involving carbons with vastly different porosity characteristics (hollow carbons, nanotubes, and carbon blacks). In line with the size of supported Pt nanoparticles, the intrinsic specific electrochemical oxygen reduction reaction (ORR) activities of all samples were comparable. However, counter to common sense, a non-monotonic trend between the carbon surface area and the ORR mass-based activity, coupled with a similar relative loss in the electrochemical surface area (ECSA), was observed. This is explained in terms of a varying effective ECSA, which is sensitively dependent on the morphology of the support. In situ SAXS monitoring revealed a mainly coalescence-based increase in mean particle size for the low surface area carbon nanotubes. In contrast, the highly microporous hollow carbons showed strongly enhanced particle stability where Ostwald ripening accounted for the observed coarsening. Altogether, our study provides new atom-scale insights into Pt/C fuel cell catalyst stability. Based on this study, supports of intermediate surface area provide the best compromise between activity and size stability, while highly graphitized or highly nanoporous supports are detrimental.