Nitrogen and Phosphorus Co-doped Hollow Carbon Spheres as Efficient Metal-Free Electrocatalysts for the Oxygen Reduction Reaction

Replacement of rare- and noble-metal catalysts with earth-abundant, low-cost, stable, and efficient metal-free catalyst substitutes is highly desirable, but full of challenges for the oxygen reduction reaction (ORR). Herein, N, P co-doped hollow carbon spheres (N,P-HCS) are prepared through a simple pyrolysis method, using hollow carbon spheres (HCS), melamine, and phytic acid (PA) as the C, N, and P sources, respectively. Compared to the N-doped HCS (N-HCS) and P-doped HCS (P-HCS), the N, P co-doped sample exhibited much higher ORR catalytic activity. Moreover, in comparison with commercial Pt/C, the as-prepared N,P-HCS showed comparable ORR activity, but stronger endurance to methanol and higher durability in alkaline solution. The high performance of N,P-HCS can be primarily ascribed to the large surface area, high conductivity, porous carbon structure, and high content of heteroatoms (N and P). Considering the excellent catalytic activity and durability, after further structural optimization, N,P-HCS could be a promising metal-free catalyst candidate for the ORR in fuel cells.