Hierarchical polybenzimidazole-grafted graphene hybrids as supports for Pt nanoparticle catalysts with excellent PEMFC performance

Abstract

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 183mA/mgPt at 0.9V vs. RHE on Pt/PBI-graphene+FCB than the 101mA/mgPt of Pt/PBI-graphene and the 149mA/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.500mA/cm2 to 2250mA/cm2, 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 1500mA/cm2 after 1000 cycles (1.0–1.5V) can be retained >60% for Pt/PBI-graphene+FCB while for Pt/XC72, it decays more rapidly to 0V. This study suggests the promise of using Pt/PBI-graphene+FCB hybrid cathode catalysts in fuel cells to achieve the DOE targets (i.e.<30mV loss at 1500mA/cm2 after 30K potential cycling from 1.0V to 1.5V).