Nitrogen doped and phosphorus doped thin graphene spherical shells as efficient metal-free ORR catalysts

Abstract

Developing efficient, metal-free electrocatalysts for the oxygen reduction reaction (ORR) is essential for advancing sustainable energy technologies. In this work, we report the synthesis of nitrogen-doped (N-GS) and phosphorus-doped (P-GS) graphene spherical shells as highperformance, non-metallic ORR catalysts. N-GS was directly synthesized via chemical vapor deposition using acetonitrile as a dual carbon and nitrogen source, while P-GS was prepared 2 through post-phosphorylation of undoped graphene shells. Elemental analysis confirmed nitrogen and phosphorus doping levels of 8.59 at% and 2.80 at%, respectively. The threedimensional spherical architecture effectively prevents graphene sheet aggregation and increases accessible surface area. Heteroatom doping introduces lattice distortions and defect sites, which enhance ORR activity. Both N-GS and P-GS exhibited half-wave potentials close to that of commercial Pt/C (0.82 V), and superior kinetics with Tafel slopes of 70 and 75 mV dec -1 , respectively. Electrochemical impedance spectroscopy revealed low charge transfer resistance, and Koutecky-Levich analysis confirmed a four-electron ORR pathway for both materials. These results demonstrate that heteroatom-doped graphene shells are promising metal-free catalysts for ORR, offering a cost-effective alternative to precious metal-based catalysts in fuel cell applications.