Te electrochemical performance of fuel cells is greatly affected by the oxygen reduction reaction (ORR) at the cathode because of its sluggish reaction kinetics. To efficiently catalyze the ORR, platinum-loaded carbon is the most commonly used electrocatalyst. However, its large-scale production for commercial applications has been hindered by the high cost of Pt as well as by the time-dependent drift and CO deactivation problems of Pt-based electrodes. Consequently, intensive research is underway to develop new ORR electrocatalyst alternatives to minimize or replace Pt.

… In spite of tremendous efforts in developing precious-metal-free and entirely metal-free ORR electrocatalysts, it still remains a challenge to develop efficient catalysts that are comparable or even superior to commercial Pt/C. Here, we first synthesized graphene quantum dot/graphene (GQD/G) hybrid nanoplatelets by hydrothermal self-assembly and then codoped the GQD/G with boron and nitrogen to obtain BN-doped GQD/G(BN-GQD/G) hybrid nanoplatelets by annealing at high temperature for different time periods. The optimized samples show excellent ORR electrocatalytic activity with more positive onset potential than commercial Pt/C, and they also have large current densities.

—Fei et al.

Electron microscope images show flake-like nanoplatelets made of graphene quantum dots drawn from coal and graphene oxide sheets, modified with boron and nitrogen. The nanoplatelets feature enough edge to make them suitable as catalysts for applications such as fuel cells. Courtesy of the Tour Group. Click to enlarge.

The lab group discovered that boiling down a solution of GQDs and graphene oxide sheets (exfoliated from common graphite) combined them into self-assembling nanoscale platelets that could then be treated with nitrogen and boron.

The hybrid material combined the advantages of each component: an abundance of edges where chemical reactions take place and excellent conductivity between GQDs provided by the graphene base. The boron and nitrogen collectively add more catalytically active sites to the material than either element would add alone.

The GQDs add to the system an enormous amount of edge, which permits the chemistry of oxygen reduction, one of the two needed reactions for operation in a fuel cell. The graphene provides the conductive matrix required. So it’s a superb hybridization. The efficiency is better than platinum in terms of oxygen reduction, permitting one to sidestep the most prohibitive hurdle in fuel-cell generation—the cost of the precious metal.

—James Tour

(a) RDE linear sweep voltammograms of ORR on different compositions of the hybrid material—DF-GQD/G-30, N-GQD/G-30, BN-GQD/G-10, BN-GQD/G-30, BN- GQD/G-60, BN-G-30, and the baseline Pt/C—at a rotating speed of 900 rpm and scan rate of 5 mV s-1. (b) Electrocatalytic activity given as the kinetic current density at 0.5 V for all samples in (a). Credit: ACS, Fei et al. Click to enlarge.

Rice graduate student Huilong Fei is the paper’s lead author. Co-authors are graduate students Ruquan Ye, Gonglan Ye, Yongji Gong, Zhiwei Peng and Errol Samuel; research technician Xiujun Fan; and Pulickel Ajayan, the Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry and chair of the Department of Materials Science and NanoEngineering, all of Rice.

Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of materials science and nanoengineering and of computer science.

The Office of Naval Research Multidisciplinary University Research Initiative (MURI) program, the Air Force Office of Scientific Research and its MURI program supported the research.

Resources

• Huilong Fei, Ruquan Ye, Gonglan Ye, Yongji Gong, Zhiwei Peng, Xiujun Fan, Errol L. G. Samuel, Pulickel M. Ajayan, and James M. Tour (2014) “Boron- and Nitrogen-Doped Graphene Quantum Dots/Graphene Hybrid Nanoplatelets as Efficient Electrocatalysts for Oxygen Reduction” ACS Nano doi: 10.1021/nn504637y