Renewable energy can be generated from the mixing of seawater with river water by reverse electrodialysis (RED). As part of the RED system, ion exchange membranes (IEMs) are key factors to the success of future RED energy generation. This research presents the synthesis and characterization of a new kind of nanocomposite cation exchange membrane (CEM) by using oxidized multi-walled carbon nanotubes (O-MWCNTs) blended with sulfonated poly (2,6-dimethyl-1,4-phenylene oxide) (SPPO). The nanocomposite CEM showed simultaneous improvement of membrane anti-fouling performance and energy generation performance in RED systems. The physicochemical and electrochemical properties of nanocomposite CEMs were enhanced compared to pristine SPPO CEMs. The results indicated that the optimal inorganic loadings were 0.3–0.5 wt%, which showed the best anti-fouling performance and highest power density in RED. The results show that O-MWCNTs are promising materials to improve properties of IEMs, and nanocomposite IEMs are competitive candidates for application in electrochemical systems like RED.