Nitrogen and fluorine dual-doped porous graphene nanosheets (NFPGNS) have been successfully synthesized as efficient bifunctional metal-free electrocatalysts for overall water splitting via a simple chemical-etching method. Pyridinic N doping rich configurations have been proven beneficial for the electrochemical process. The onset voltage of water splitting on the NFPGNS is lower than 1.60 V, only slightly higher than that found for Pt/C electrocatalysts. Particularly, an onset potential of 1.45 V vs. RHE on the NFPGNS for the OER is lower than some metal based electrocatalysts, involving Pt/C. DFT calculations reveal the origin of the electrocatalytic activity on the NFPGNS for the HER and OER; heteroatom-doped graphene materials modify the electron acceptor-donor properties of carbon via a synergistic coupling effect between heteroatoms. This leads to favorable electronic structures tuning the C sites around the heteroatoms, introducing a stronger adsorption strength and consequently, a lower value for the Gibbs free energy.