Sulfonated Poly(ether ether ketone)–Zirconia Organic–Inorganic Hybrid Membranes with Enhanced Ion Selectivity and Hydrophilicity for Vanadium Redox Flow Batteries

Proton-exchange membranes (PEMs) are the pivotal components of vanadium redox flow batteries (VRFBs) and play a critical role in the comprehensive output performance of VRFB systems. Currently, the most widely commercialized membranes are Nafion series membranes produced by DuPont, Wilmington, DE, USA, but the high vanadium permeability and cost hinder their large-scale promotion. Hence, there is an active demand for developing a low-cost, high-performance, and energy-efficient PEM to promote the commercialization of VRFB systems. In this paper, sulfonated poly(ether ether ketone) (SPEEK) as matrix and zirconia nanoparticles as inorganic filler were used for composite modification to prepare a series of SPEEK-ZrO₂ organic-inorganic hybrid membranes for VRFBs. The thickness of these membranes was 50-100 μm. Compared with Nafion 115 (thickness 128 μm), composite membranes demonstrated obvious cost advantages. The results showed that the SP-Z-X series membranes had higher water uptake (53.26-71.1%) and proton conductivity (0.11-0.24 S cm^-1). SP-Z-5 displayed the best comprehensive output performance at 200 mA cm^-2 (CE: 99.01%, VE: 81.95%, EE: 81.11%). These hybrid membranes are very cost-effective and exhibit high potential for application in VRFB applications, and are expected to lead to the industrial application of VRFBs on a large scale.