Energy shortages and environmental pollution are serious challenges that humanity is now starting to face. Proton exchange membrane fuel cells (PEMFCs) are non-polluting and efficient energy conversion devices that are expected to play a dominant role in future energy solutions. However, current PEMFC systems still face significant technological roadblocks which must be overcome before the system can become economically viable. A major impediment to the commercialization of PEMFCs is the high cost and stability of Pt-based electrocatalysts. In collaboration with Ballard Power Systems, researchers from the Nanomaterials and Energy group at the University of Western Ontario, Canada, have discovered a new metal-free catalyst, nitrogen doped graphene (N-graphene), that could make fuel cells more cost effective and stable. [D. Geng et al., Energy Environ Sci (2011) 4, 760].
Recent studies on reducing or replacing Pt-based catalysts in fuel cells have indicated that nitrogen-doped carbon materials could act as effective metal-free electrocatalysts, although the real active site remains unclear. Graphene, a new 2-dimensional carbon material, not only possesses a high surface area and excellent conductivity, it also has a unique graphitic basal plane structure. It is well known that the greater the extent of graphitization of the carbon material, the greater the durability it demonstrates. The unique properties of nitrogen-doped graphene prompted the researchers to investigate the electrochemical activities of nitrogen-doped graphene.
N-graphene has been synthesized on a large scale, using ammonia and the heat treatment of graphene. The method overcomes the shortcomings of chemical vapor deposition, which is difficult to scale up, and therefore limits the wide use of N-graphene for electrodes. The new method makes it possible for membrane electrode assemblies to be easily fabricated, and allows the extent of the electrocatalytic activity to be accurately identified. The activities of N-graphene electrodes and commercial Pt/C catalyst electrodes can be directly compared, as the exact catalyst loading is known.
Importantly, the nitrogen content can be controlled by changing the ammonia heat-treatment temperature. N-graphene has exhibited comparable, or better, activity and stability than commercial Pt/C (loading: 4.85 mgPt cm-2) towards the oxygen reduction reaction (ORR). XPS analysis indicates that quaternary type nitrogen species seem to play the most important role for ORR activity. Therefore, N-graphene as next-generation catalysts may have the potential to replace the costly Pt/C catalyst in fuel cells.
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