Fuel cells have attracted much attention due to excellent properties, such as high energy conversion rate, low operating temperature and environmental friendliness. However, the bottleneck of fuel cells lies in sluggish oxygen reduction reaction (ORR) on the cathode. Platinum-based materials are regarded as ideal ORR catalysts, but they have the inherent drawbacks of high cost and scarcity.

Nitrogen doped carbon-based materials have been proven to be promising alternatives, and the ORR catalytic activity of carbon materials depends closely on the form of doped N atoms. It is generally believed that pyridine nitrogen creates the ORR active site, while other nitrogen doping forms are rarely reported in high-performance ORR catalysts. 

Recently, a research team led by Prof. WANG Dan from the Institute of Process Engineering (IPE) of the Chinese Academy of Sciences introduced a new doping form of nitrogen, i.e., sp-hybridized nitrogen (sp-N), into chemically defined sites of ultrathin graphdiyne.

Researchers found that the as-prepared sp-N-doped graphdiyne catalyst exhibited excellent comprehensive ORR performance. In addition, the doping site and proportion of sp-N atoms could be well controlled. As shown in Figure 1, a new type of sp-hybridized nitrogen atom has been successfully introduced into specific sites of graphdiyne through the pericyclic reaction.

Theoretical and experimental results indicated that, compared with other nitrogen configurations, sp-N atoms made the neighbouring carbon atoms more positively charged, which created a beneficial chemical environment for O₂ adsorption in the ORR.

The sp-N-doped 2D carbon catalyst presented an excellent comprehensive ORR performance, and an excellent catalytic activity in acidic solution.

The optimal sp-N-doped graphdiyne presented catalytic activity superior to that of Pt/C, as well as better stability and methanol resistance than Pt/C in both alkaline and acidic solutions (Figure 2).

The doping strategy to incorporate sp-N atoms into carbon nanomaterials in a controllable way, and the understanding of the doping mechanism, may open new opportunities for site-specific doping of sp-N atoms into other catalysts and thereby broaden the scope of their applications. 

Figure 1. Synthesis of sp-N-doped few-layer graphdiyne. (Image by ZHAO Yasong)   

Figure 2. Electrocatalytic ORR activity of N-doped graphidyne and commercial Pt/C in O₂-saturated 0.1 M KOH. (Image by ZHAO Yasong)