Air-cathodes are a critical component for microbial fuel cells (MFCs) and need to have high catalytic performance for the oxygen reduction reaction (ORR). As an important two-dimensional material, graphene has been explored in various applications including ORR catalysts for MFCs. However, the reported graphene for MFC cathodes was usually small flakes/powders, which cannot be directly coated onto metal meshes without binders. Here, we report a binder-free nitrogen-doped graphene (NG) sheet in situ grown on nickel mesh as an efficient catalyst layer for MFC air-cathodes. By optimizing the growth parameters of NG, the maximum power density of MFCs based on NG can be boosted up to 1470 ± 80 mW m−2, which is 32% higher than that of the conventional Pt/C air-cathode. The optimized NG air-cathode has a low internal resistance (21 ± 3 Ω), only 20% of that of the Pt/C air-cathode. These results provide a proof-of-concept for the binder-free NG air-cathode as an alternative to the costly Pt cathode for MFCs.