The development of efficient bifunctional catalysts for both oxygen reduction and oxygen evolution reactions is highly desirable but challenging in energy conversion and storage systems. Here, a simple yet cost-effective strategy is developed to produce heteroatom-doped carbon nanospheres using natural cuttlefish ink as the precursor. For the oxygen reduction reaction, the catalyst exhibits more positive onset-potential and larger diffusion limiting current density compared with benchmark platinum catalyst in alkaline medium. Moreover, the as synthesized catalyst shows low onset-potential for oxygen evolution reaction, indicating its outstanding catalytic activity. The catalyst shows a potential gap of 0.75 V between the oxygen evolution reaction potential at a current density of 10 mA cm(-2) and the oxygen reduction reaction potential at the half-wave potential, outperforming most of other noble metal-free carbon catalysts in the current state of research. The remarkable catalytic performance can be assigned to heteroatoms doping, full exposure of the active sites, large surface area and enrichment of pores for sufficient contact and rapid transportation of the reactants. This study offers a new approach for the synthesis of metal-free carbon nanomaterials from natural resources, and broadens the design for the fabrication of bifunctional oxygen reduction and oxygen evolution catalysts. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.