Understanding Pt Nanoparticle Anchoring on Graphene Supports through Surface Functionalization

Proton exchange membrane fuel cells (PEMFCs) that use hydrogen as fuel to produce electricity have attracted extensive interests over the past decades.[1] The high efficiency and zero carbon- emission make PEMFCs promising candidates for viable propulsion systems in electric vehicels.[2] One of major challenges that hinder the commercialization of PEMFCs vehicles is the poor stability of Pt nanoparticles on carbon supports, which is mainly due to (1) the dissolution and re-deposition of Pt and (2) migration of Pt nanoparticles over carbon supports.[3],[4] Both of them will cause the agglomeration of Pt nanoparticles, resulting in loss of Pt electrochemical surface area (ECSA) and the increasing of activation over-potential. In this regard, the enhancement of Pt nanoparticle anchoring strength and dispersion on carbon supports is highly desirable in PEMFCs as well as in other catalysis processes. Presented here is a comprehensive study of the interaction between catalyst nanoparticles and carbon supports in terms of the electronic structure change and its effects on the electrocatalytic performance of supported catalysts. Graphene was chosen as an ideal model system for catalyst support because the unique 2-D structure allows the direct investigation of the interaction with supported metal nanoparticles at their interface. We developed a facile strategy to covalently graft p-phenyl SO3H or p-phenyl NH2 groups onto the graphene surface. The functional groups were found to not only facilitate the homogeneous distribution of Pt nanoparticles over the surface of graphene supports and reduce the Pt average particle size but also strengthen the interaction of the Pt atoms with the functional groups and, consequently, minimize the migration/coalescence of the Pt nanoparticles in the course of accelerated durability tests. The experimental results from both X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) demonstrate the electron density shift from Pt to graphene supports with the strength of the Pt−graphene interaction following the trend of Pt/p-phenyl NH2-graphene > Pt/p-phenyl SO3H-graphene > Pt/graphene. This study will shed light on strategies to improve not only the durability but also the activity of the metal nanoparticles via the functionalization of the catalyst supports in the catalysis field (1) Gasteiger, H. A.; Markovic, N. M. Science 2009, 324, 48. (2) Debe, M. K. Nature 2012, 486, 43. (3) Xie, J.; Wood, D. L.; More, K. L.; Atanassov, P.; Borup, R. L. Journal of The Electrochemical Society 2005, 152, A1011.