Reverse electrodialysis (RED), based on ion-selective membranes, is one of the most promising technologies for capturing osmotic energy. As key elements of the RED system, ion-selective membranes must meet the crucial demands of mechanical stability, anti-fouling characteristics, easy fabrication, and high power density; however, this still remains a challenge. In this study, we demonstrated a large-scale, mechanically stable, and high-porosity membrane obtained by combining carbon nanomaterials and hyperbranched polyethyleneimine (h-PEI), thereby achieving a high power density of 5.0 W·m⁻² with seawater and river water. Carbon nanofibers (CNFs) were subsequently bridged with graphene and h-PEI to strengthen the interaction between the CNFs, reduce the channel size and increase the space charge density, mechanical strength, and toughness. The large-scale and mechanically stable membrane fabricated using the modified CNFs exhibited anion selectivity and high ionic conductivity, thereby achieving a high-performance osmotic energy conversion. Furthermore, the anti-fouling property of the membrane was confirmed by the stability of the osmotic energy conversion in a solution with algae, which can be attributed to the high porosity of carbon nanomaterials. This economic and convenient method for the ion-selective membrane preparation is believed to be promising for large-scale osmotic energy harvesting.