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Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction.

The cathodic oxygen-reduction reaction (ORR) is one of the most crucial factors in the performance of a fuel cell. The development of efficient ORR electrocatalysts is thus of great significance for the commercialization of fuel cells. Platinum-based materials have long been investigated as active catalysts for ORR; however, the large-scale application of fuel cells has been hampered by the high cost and inadequacy of this metal. Recently, nonprecious-metal and metal-free catalysts for ORR have attracted enormous interest as an alternative to platinum-based catalysts. In particular, nitrogen-doped carbon materials, which are typical metal-free catalysts, exhibit excellent electrocatalytic activity for ORR as a result of their unique electronic properties derived from the conjugation between the nitrogen lone-pair electrons and the graphene p system. Generally, nitrogen-doped carbon materials can be prepared by the pyrolysis of transition-metal macrocyclic compounds or mixtures of metal salts and nitrogen-containing precursors. In these processes, the transition metals play an important role not only in the formation of graphitic frameworks, but also in the introduction of nitrogen active sites. Drawbacks are the use of expensive precursors and the need for extra steps to remove metal species. Furthermore, metal nanoparticles encapsulated in the graphite framework still remain even after a tedious removal process. The nature of the nitrogen atoms in nitrogen-doped carbon materials and whether they are really the active catalytic sites is still controversial. Thus, the development of nitrogen-doped carbon materials with excellent electrochemical performance but without any metal components is an urgent issue. Such materials would not only be promising candidates for ORR catalysts but would aid attempts to elucidate the correlation between the structure, composition, and electrochemical activity of nitrogen-doped carbon materials.