The Dual Role of Graphene As a Catalyst Support and Protection Cap for Monolayer Pt Catalysts

Abstract

Production of cost-effective polymer electrolyte membrane (PEM) fuel cell materials has been of a great interest over the past decade. At the cathode side, the oxygen reductionreaction (ORR) is the key reaction, for which Pt remains the most widely used catalyst. In order to leapfrog the current limits to Pt use, three main targets have to be achieved. First, is an enhancement of the catalytic activity per Pt atom; second, is a reduction of Pt loading; and third is an increase the catalyst longevity. Here, we demonstrate the dual role of single graphene layer, both as a growth platform and as a protective cap for 2D Pt monolayers catalysts, where all three targets can be achieved. Using iterative under potential deposition (UPD), monolayers layers of Pt catalyst are grown on top of a monolayer graphene sheet, supported on 50 nm (111) Au substrate. X-ray absorption spectroscopic (XAS) and transmission electron microscopy (TEM) analysis show that Pt monolayers growth is dictated by the graphene-teplated epitaxy. Intimate contact between Pt/graphene induces a localized compressive strain on Pt monolayers ranges 3-10% with an overall compressive strain of 3.5% according to extended x-ray absorption fine structure (EXAFS) analysis. In addition, cyclic voltammetry (CV) analysis shows Pt monolayers fully wetting ~100 mm2area of graphene under-layer with only a ~1 nm ultra-thin layer of Pt, while avoiding ripening as shown through TEM images. Atomic Force Microscopy (AFM) analysis demonstrates that Pt monolayers prefer to follow Frank-van der Merwe growth (i.e. layer-by-layer growth mode) rather than Volmer-Weber growth (i.e. island growth mode), where root mean square (rms) of surface roughness remains quite similar while increasing Pt loading is increased. Pt/graphene hybrid catalysts show superior catalytic activity for ORR relative to the graphene-free counterparts. A combination of the graphene-imposed compressive strain and electron transfer, push the Pt d-band center up, lowering the overpotential needed for ORR to occur. The graphene/Pt hybrid also show superior activity towards ORR for all shallow Pt mass loadings relative to graphene-free cases, which can be attributed to the interface strain. Due to their intimate epitaxial contact, graphene and Pt, therefore, form a new hybrid catalyst that outperforms Pt. Furthermore, the graphene/Pt cap hybrid shows the graphene protecting Pt MLs from both dissolution and from ripening, with almost no Pt loss after 1000 fuel cell operating cycles. Our demonstration of a graphene-Pt hybrid opens the door for graphene/metal or metal/graphene architectures with potential applications in, and not limited to, energy, thermo-electric and electronics field. Figure 1