Sulfonated covalent organic nanosheets (SCONs) with a functional group (-SO3H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm(-2)), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm(-2)) in 2 M methanol at 60 degrees C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules. (C) 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.