Low permeable composite membrane based on sulfonated poly(phenylene oxide) (sPPO) and silica for vanadium redox flow battery

The proton conductivity and vanadium permeability of organic-inorganic (sulfonated poly (phenylene oxide) (sPPO)-nano sized sulfonated silica (sSiO2)) hybrid membrane were investigated for application in a vanadium redox flow battery (VRFB) system. Significant attention is being paid to PPO polymers as a replacement for Nafion® membranes due to their relatively low cost and ease of sulfonation. The attachment of a sulfonic acid (SO3H) functional group to PPO and SiO2 was confirmed using Fourier Transform Infrared Spectroscopy (FTIR). The hybrid membrane (sPPO-2% sSiO2) exhibited increased thermal stability, water uptake (WU), ion exchange capacity (IEC) and proton conductivity (IC) compared with a conventional organic sPPO membrane. The proton conductivity of the hybrid membrane increased considerably compared to sPPO alone, resulting from the 2% sSiO2 nanoparticles added homogeneously to the polymer matrix. The proton conductivities of the sPPO and hybrid membranes were 0.050 and 0.077 S/cm, respectively. The increased proton conductivity of the hybrid membrane was attributed to the enhanced hydrophilic properties of SO3H in the membranes. In addition, inorganic particles in the polymer matrix acted as a barrier for vanadium ion crossover. During VRFB unit cell operation, vanadium ion (VO2+) crossovers were measured as 14.66, 1.955 and 0.173 mmol L−1 through Nafion®212, sPPO and hybrid membranes, respectively, and VO2+ permeability were 2.22 × 10−7, 2.50 × 10−8 and 4.76 × 10−9 cm2 min−1 for Nafion®212, sPPO and hybrid membranes, respectively. Based on our experimental results, low cost organic-inorganic hybrid membranes as prepared provide an efficient alternative membrane material for advanced VRFB systems.