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 high-performance, 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 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 three-dimensional 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.