Science 351, 361–365 (2016)

Nitrogen-doped carbon nanostructures have emerged as promising electrocatalysts for a range of important chemical reactions including the oxygen reduction reaction (ORR), which lies at the heart of proton exchange membrane fuel cells. The exact nature of the active sites in these ORR catalysts is, however, unclear, which limits the design of high-performance materials. In particular, there is debate over whether the sites involve pyridinic nitrogen (in which the nitrogen is bonded to two carbon atoms) or graphitic nitrogen (in which the nitrogen is bonded to three carbon atoms). Takahiro Kondo, Junji Nakamura and colleagues at the University of Tsukuba have now identified pyridinic nitrogen as the key species with the help of a series of graphite-based model catalysts.

The researchers tested the ORR performance of graphite catalysts that had different concentrations of pyridinic and graphitic nitrogen, using X-ray photoelectron spectroscopy (XPS) to characterize the chemical composition of the materials. The ORR activity of the catalysts was found to be dependent on the concentration of the pyridinic nitrogen. Post-reaction XPS experiments, combined with carbon dioxide adsorption measurements, ultimately suggested that the active sites are carbon atoms next to pyridinic nitrogen — sites that have Lewis basicity and that can adsorb oxygen, the initial step of the ORR. The relationship between catalytic activity and pyridinic nitrogen concentration was also found to hold for nitrogen-doped graphene nanosheet catalysts.