Oxygen-Reduction Active Carbon Supports for Pt Cathode Catalysts in Polymer Electrolyte Fuel Cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely considered one of environmentally friendly and highly efficient electric power sources for a wide variety of power applications ranging from portable and stationary power supplies to transportation. However, some of the most critical barriers to their commercialization are the intrinsic high cost and insufficient long-term performance durability. Among others, the spherical carbon black particle support used for Pt catalyst in MEAs have a number of intrinsic structural disadvantage. It is partially responsible for current insufficient performance and durability of MEAs. They include (1) inefficient O2 transport due to significant tortuosity in carbon black-based cathode, (2) non-uniform distribution of ionomer due to significant microporosity of carbon blacks, (3) catastrophic corrosion during start-up/shut-down events due to low corrosion resistance, (4) low electrical conductivity, and (5) weak binding sites with Pt nanoparticles. University at Buffalo, SUNY has developed a new type of high-surface areas (up to 850 m2/g) and large size nitrogen-diped graphene tubes (NGT) in a highly controlled manner and at high yields. The diameters of these graphene tubes can be tuned from 50 to 500 nm, which is much larger than conventiona carbon nanotubes (20 nm). It should be noted that the graphene tube is low cost using inexpensive dicyandiamide as precursors ($1-2/kg) and the synthesis method is easily scaled up. Apart from the obvious advantage of high electronic conductivity and enhanced corrosion resistance, the large tube size provided better Pt nanoparticle dispersion and more facil mass transfer including O2 and H2O. Importantly, the NGT exhibited oxygen reduction reaction (ORR) activity in acidic media comparable with state of the art PGM-free catalsyts. Thus, the novel ORR active nitrogen-doped graphene tubes are able to not only offer remarkable support effects by geometrically and electronically modifying the deposited Pt nanoparticles, but also provide a large amount of complementary nonprecious ORR active sites, thereby significantly improving the overall activity and durability of the Pt catalysts. Thus, relative to conventional carbon blacks, the highly graphitized and active N-GT is an ideal support for low-PGM catalysts for high-performance MEAs with enhanced mass transport and durability along with ideal interfaces with ionomer.