By Will Soutter
An international team of researchers has developed a more affordable way to make fuel cells for vehicles - using chemically modified graphene nanoplatelets instead of costly metals like platinum as the electrocatalyst.
The graphene nanoplatelets (GnPs) are have halogens (chlorine, bromine and iodine) chemically bonded to the edge of each graphene layer. This forces the edges of the layers apart, increasing the surface area of the platelets.
In this study, published and free to access in Nature's Scientific Reports, the halogen-functionalized edges of the GnPs were shown to be good binding sites for oxygen, and very effective catalysts for the oxygen reduction reaction (ORR), a chemical process which is central to the operation of a fuel cell in which oxygen and hydrogen molecules break up and rearrange to form water.
Schematic showing the effect of halogens on GnPs. From left to right: untreated GnPs, chlorine (Cl), bromine (Br) and iodine (I).
The edge-halogenated graphene nanoplatelets, or XGnPs, as they have been dubbed, could be a great step forward for fuel cell technology, as the method used to produce them is relatively simple and cheap to perform. This is in contrast to more traditional fuel cell catalysts, which are based on expensive metals like platinum.
Simple, scalable process
The XGnPs are produced by ball-milling of graphite powder fast enough to break the bonds between the carbon atoms. The nanoscale flakes of graphite this creates, called graphene nanoplatelets, are treated with the halogen gases, which react with the exposed carbon atoms along the edge of the platelets.
Previous attempts to create a non-metallic electrocatalyst material have not taken off - they have tended to suffer from poor cycle stability over longer periods of operation, and issues with efficiency during production. The XGnPs, however, have shown performance which is better than these previous attempts, and even better in some ways than the catalysts which are commonly used today.
Best of all worlds
This new graphene-based material seems well placed to fix all of the issues with previous electrocatalyst materials which have been hampering the commercialization of fuel cells. Prof. Liming Dai from Case Western Reserve University comments:
"Our metal-free catalysts are made using an affordable and scalable process. They are also more stable than platinum catalysts and tolerate carbon monoxide poisoning and methanol crossover."
Once again, the entry of graphene into a sector of the materials market is shown to be potentially revolutionary. GnPs also have fewer technical hurdles to clear than CVD graphene to reach commercial production. This discovery could have huge implications for fuel cell technology, potentially bringing large-scale commercial applications in vehicles and local power generation forward by several years.
"Facile, scalable synthesis of edge-halogenated graphene nanoplatelets as efficient metal-free eletrocatalysts for oxygen reduction reaction" - Nature Scientific Reports, 2013. DOI: 10.1038/srep01810
"Metal-free catalyst outperforms platinum in fuel cell" - Case Western Reserve University
"Noble way to low-cost fuel cells, halogenated graphene may replace expensive platinum" - UNIST