Tailoring the Porous Structure of Mono-dispersed Hierarchically Nitrogen-doped Carbon Spheres for Highly Efficient Oxygen Reduction Reaction

The search for a low-cost metal-free cathode material with excellent mass transfer structure and catalytic activity in oxygen reduction reaction (ORR) is one of the most challenging issues in fuel cells. In this work, nitrogen-rich m-phenylenediamine is introduced into the synthesis of porous carbon spheres to tune the pore structure and nitrogen-doped active sites. As a result, more pyridinic N and pyrrolic N functional species were observed at the interior and surface of the carbon spheres. The introduction of m-phenylenediamine also regulated the nucleating of precursors, an urchin-like mesoporous surface structure ensures point contact and less agglomeration between each particle was obtained. With optimized proportion of micropores/mesopores and improved nitrogen-contained functional species, the ORR activity can be remarkably improved. The half-wave potential of this catalyst could achieve to 0.81 V (versus RHE) which is only 42 mV lower than commercial Pt/C catalyst. Furthermore, the optimized cathode catalyst achieved a 69 mW cm⁻² maximum power density when operated in direct methanol fuel cells at room temperature.