Construction of Ion-Accessible Regions within Hydrophobic Membranes for Methylene Blue-Based Flow Batteries

Polyvinylidene fluoride (PVDF) membranes are widely used in energy-related devices, such as flow batteries and osmotic energy capture. However, most PVDF-based membranes are intrinsically hydrophobic, which leads to a low conductivity. Here, we use a moderate strategy to graft hydrophilic sodium p-styrenesulfonate (NaSS) in PVDF membranes from the surface into the interior, enabling ion-accessible regions to form within the membranes. By creating and optimizing nanoscale pores in PVDF membranes to allow NaSS and redox mediation systems to diffuse into the membrane, the grafting reaction between NaSS and PVDF from outside to inside can be realized at temperate temperatures (35 °C–45 °C), which favors maintaining membrane stability. Through adjustment to the grafting degree, we can tailor the ion transport channels to obtain membranes with different conductivities. Using the methylene blue (MB)–vanadium flow battery for exemplification, our membranes enable the battery to afford a Coulombic efficiency of 99.94% and a voltage efficiency of 87.55% at a current density of 40 mA cm–2, which is significantly higher than that of the battery using a Nafion 212 membrane (CE ∼ 99.74%, VE ∼ 75.08%). Our strategy is simple and moderate and provides an efficient way to create hydrophilic ion transport channels within hydrophobic membranes.