One of the major hurdles for commercialization of the fuel cell technology is the sluggish oxygen reduction reaction (ORR) at cathode. So far, high cost and scarce precious platinum (Pt) and its alloys have been considered to be the most reliable cathodic ORR electrocatalysts in fuel cells. In addition to the high cost, however, Pt and its alloys are also suffered from methanol crossover/carbon monoxide (CO) poisoning effects and poor operation stability. Therefore, it is essential to search for non-precious metal [earlier post] or metal-free electrocatalysts with a high catalytic activity and long-term operation stability to reduce or replace Pt-based ORR electrocatalysts in fuel cells. Although extensive efforts have been devoted to the development of non-precious metal-based electrocatalysts, their practical application is still out of sight due largely to their limited electro-catalytic activity, poor cycle stability and sometimes environmental hazard.

Recently, carbon-based materials doped with heteroatoms, such as boron (B), halogen (Cl, Br, I), nitrogen (N), phosphorus (P), sulfur (S), and their mixtures, have attracted tremendous attentions as metal-free ORR electrocatalysts...Although the basic catalytic mechanism has been established, the full potential of these carbon-based, metal-free catalysts is hard to achieve without the synthetic capability for large-scale production of the heteroatom-doped, carbon-based materials at low cost. However, commonly affordable chemical vapor deposition (CVD) for the preparation of carbon nanotubes and graphene sheets and/or Hummers’ methods for graphene oxide production are too expensive and in-volve environmentally hazardous reagents, and thus inappropriate for large-scale production.

In this study, we have, for the first time, synthesized a series of edge-selectively halogenated (Cl, Br and I) graphene nanoplatelets (ClGnP, BrGnP and IGnP; collectively designated as XGnPs) by simply ball-milling graphite flake in the presence of chlorine (Cl₂), bromine (Br₂) or iodine (I₂), respectively.

—Jeon et al.

The technology to make the metal-free ORR catalysts builds on a simple and cheap industrial process several of the researchers developed to make graphene sheets from graphite. Inside a ball miller—a canister filled with steel balls—the researchers broke graphite down into single-layer graphene nanoparticles. While the canister turned, they injected chlorine, bromine or iodine gas to produce different catalysts. In each case, gas molecules replaced carbon atoms along the zigzag edges of nanoplatelets created by milling.

Not only were the edges then favorable to binding with oxygen molecules, but the bond strength between the two oxygen atoms weakened. The weaker the oxygen bonds became, the more efficiently the oxygen was reduced and converted to water at the cathode.

In testing, a cathode coated with iodine-edged nanoplatelets performed best. A cathode coated with bromine-edged nanoparticles generated 7% less current than the commercial cathode coated with platinum, the chlorine-edged nanoplatelets 40% less. In a test of durability, electrodes coated with the nanoplatelets maintained 85.6% to 87.4% of their initial current after 10,000 cycles while the platinum electrodes maintained only 62.5%.

Carbon monoxide was added to replicate the poisoning that many scientists blame for the poor performance of platinum at the cathode. The performance of the graphene-based catalysts was unaffected. When methanol was added to replicate methanol crossover from the anode to cathode in direct methanol fuel cells, the current density of the platinum catalyst dropped sharply. Again, the graphene-based catalysts were unaffected.

The research was led by Jong-Beom Baek, director of the Interdisciplinary School of Green Energy/Low-Dimensional Carbon Materials Center at South Korea's Ulsan National Institute of Science and Technology. Fellow authors include: In-Yup Jeon, Hyun-Jung Choi, Min Choi, Jeong-Min Seo, Sun-Min Jung, Min-Jung Kim and Neojung Park, from Ulsan; Sheng Zhang from Case Western Reserve; and Lipeng Zhang and Zhenhai Xia from North Texas.

This initial research proves such catalysts work better than platinum. We are working now to optimize the materials.

—Jong-Beom Baek

This research is partially supported by UNIST-WCU, US-Korea NBIT, and DOD-AFOSR-MURI projects.

Resources

• In-Yup Jeon, Hyun-Jung Choi, Min Choi, Jeong-Min Seo, Sun-Min Jung, Min-Jung Kim, Sheng Zhang, Lipeng Zhang, Zhenhai Xia, Liming Dai, Noejung Park and Jong-Beom Baek (2013) Facile, scalable synthesis of edge-halogenated graphene nanoplatelets as efficient metal-free electrocatalysts for oxygen reduction reaction. Scientific Reports 3, Article number: 1810 doi: 10.1038/srep01810