▎ 摘　　要

The inhomogeneous surface of state-of-the-art carbon supports leads to a weak Pt-C interaction and, thereby, the non-uniform dispersion of Pt nanoparticles and is responsible for the poor activity and stability of proton exchange membrane fuel cells (PEMFCs). In order to improve the surface uniformity, polybenzimidazole (PBI) has been covalently grafted onto both carbon black (Vulcan XC-72) and graphene by surface-initiated polymerization. Pt nanoparticles decorated on PBI-grafted carbon black and graphene show much narrower particle size distribution and smaller average particle size than on their pristine counterparts. Oxygen reduction reactions (ORRs) carried out on a rotating disk electrode (RDE) show the higher activity of Pt/PBI-XC72 and Pt/PBI-graphene than Pt/XC72 (ETEK-BASF) and Pt/graphene, respectively. With the insertion of negatively charged SO3H-carbon black (FCB) between positively charged PBI-graphene sheets, the lamellar structure of the graphene becomes more expanded, further enhancing the ORR activity and giving rise to a higher mass activity of 183 mA/mg(pt) at 0.9 V vs. RHE on Pt/PBI-graphene+FCB than the 101 mA/mg(pt) of Pt/PBI-graphene and the 149 mA/mgpt of commercial Pt/XC72. The accelerated degradation testing (ADT) with transmission electron microscopy (TEM) characterizations demonstrated that the PBI functionalization helps to strongly anchor Pt nanoparticles on the surface of both the carbon black and the graphene, which slows down the dissolution and migration/coalescence of the Pt nanoparticles during the durability tests. Furthermore, Pt/PBI-XC72 and Pt/PBI-graphene+FCB cathode catalysts have been applied in PEMFCs and they show encouraging mass activities in membrane electrode assembly (MEA) configuration. The beginning of life (BOL) MEA performance of the Pt/PBI-graphene+FCB shows a dramatic increase of the limiting current from ca.500 mA/cm(2) to 2250 mA/cm(2), which further confirms the effective prevention of graphene restacking because of FCB insertion. In addition, the presence of the highly stable graphitic structure of graphene leads to the significant enhancement of durability during accelerated stress tests (AST) in PEMFCs: where the cell voltage at 1500 mA/cm(2) after 1000 cycles (1.0-1.5 V) can be retained >60% for Pt/PBI-graphene+FCB while for Pt/XC72, it decays more rapidly to 0 V. This study suggests the promise of using Pt/PBI-graphene+FCB hybrid cathode catalysts in fuel cells to achieve the DOE targets (i.e. <30 mV loss at 1500 mA/cm(2) after 30 K potential cycling from 1.0 V to 1.5 V). (C) 2015 Elsevier Ltd. All rights reserved.