<p>Ion exchange membranes are essential components of vanadium redox flow batteries (VRFBs), significantly influencing their efficiency and lifespan. Sulfonated poly(ether ether ketone) (SPEEK)-based membranes are being explored as alternatives to Nafion membranes; however, their lower proton conductivity remains a significant challenge. While highly sulfonated SPEEK exhibits enhanced proton conductivity, its excessive swelling and poor mechanical properties limit its practical application. This study investigates the development of highly sulfonated SPEEK-based nanocomposite membranes incorporating graphene oxide (GO), kaolinite clay, epoxy resin (bisphenol-A-epichlorohydrin), and isophorone diamine (IPDA) as a cross-linker in varying compositions. SPEEK with a degree of sulfonation of 81% was used. GO and the amphiphilic nature of kaolinite help restrict vanadium ion crossover while facilitating proton transport through water channels. The membranes demonstrated a proton conductivity of 6.12 × 10⁻⁵ S cm⁻¹ at 50% relative humidity (RH) and a low vanadium ion permeability of 9.067 × 10⁻⁸ cm² min⁻¹. Epoxy resin and hardener were included to form a semi-interpenetrating polymer network to enhance mechanical stability. However, the poor mechanical strength suggests that the degree of sulfonation is the primary factor governing membrane durability. Interfacial modification due to the presence of IPDA also helps retard vanadium ions. This study highlights the potential of highly sulfonated SPEEK-based nanocomposite membranes for VRFB applications. By optimizing nanocomposite composition, a balance between high proton conductivity and low vanadium ion permeability can be achieved, making these membranes promising candidates for next-generation VRFB systems.</p>