Nitrogen-doped graphitic carbons as non-Pt catalysts for oxygen reduction reaction (ORR) have been paid much attention in recent years because commercialization of fuel cells requires less expensive and abundant catalytic materials. Identification of the active site of nitrogen-doped carbon materials for ORR is thus urgently required, but still under debate. Currently, the debate focuses on whether the active site is created by pyridinic N (N bonded to two carbon atoms) or graphitic N (N bonded to three carbon atoms, also called substituted N or quaternary N). To determine the active site conclusively, we prepared model catalysts of highly oriented pyrolytic graphite (HOPG) with pyridinic N (pyri-HOPG) or graphitic N (grap-HOPG). The active sites and adsorption properties were examined by ORR and post-ORR X-ray photoelectron spectroscopy (XPS). We have thus determined the active nitrogen species in carbon [1].The graphitic-N doping was performed by mild bombardment with a nitrogen ion beam. To prepare the pyri-HOPG samples, edge patterning was first performed by bombarding the sample with an Ar⁺ ion beam through a slit-patterned Ni mask as shown in Fig.1 A-D. The edged HOPG samples were then exposed to NH₃ at 973 K. The catalytic performances of the model catalysts were measured by cyclic voltammetry (CV) in acidic electrolyte (0.1 M H₂SO₄). It was found that the pyri-HOPG model catalyst shows high activity at high voltages, compared to the very low ORR activities of the N-free model catalysts (Fig.1 E, F). The pyri-HOPG sample with lower N concentration (N: 0.60 at%) is much more active than the grap-HOPG sample with higher N concentration (N: 0.73 at%). Since the pyri-HOPG sample is nearly free of graphitic N, the ORR results indicate that it is pyridinic N rather than the graphitic N that reduces the ORR overpotential and creates the active site. It is thus concluded that the ORR active sites in nitrogen-doped carbon materials are created by pyridinic N.

Based on the model catalyst study, we then try to prepare catalytically active carbon surfaces covered with pyridinic nitrogen-containing aromatic molecules. That is, bottom-up preparation of catalysts using HOPG electrode covered with pyridinic nitrogen-containing aromatic molecules (dibenz[a,c] acridine (DA) molecule). The DA molecules were deposited on HOPG with different coverage by simply dropping solutions of the DA molecules at room temperature. Scanning tunneling microscopy (STM) measurements revealed that a well-ordered two-dimensional structure of DA monolayer is formed on HOPG surfaces with high densities via π-π interaction, rather than aggregates to form three-dimensional clusters. The nitrogen concentration of the DA-covered HOPG surfaces was estimated to be 0.5~1.5 at.% by XPS. The DA-covered HOPG model catalysts reveled activities of ORR. The specific activity per pyridinic nitrogen atom was estimated to be 0.08 (e sec-1 pyriN-1) at 0.3 eV, which is comparable to that for pyridinic nitrogen incorporated graphene sheets (0.07 ~ 0.14 (e sec-1 pyriN-1))[1]. The current densities at 0.1, 0.2, and 0.3 V vs RHE were in proportional to the surface coverage of DA molecules, indicating that the ORR active site was created by DA molecule adsorbed on HOPG. The present studies clearly show that fixing nitrogen-containing aromatic molecules on graphitic carbon materials is one of the promising approaches to prepare active ORR carbon catalysts.

Acknowledgement: This study is supported by JST PRESTO and NEDO project.

REFERENCES

[1] D. Guo, R. Shibuyaa, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science, 351 (2016) 361.