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J. Sheng et al. / Materials Today Nano 7 (2019) 100044 3 Fig. 1. Schematic diagram of redox flow batteries and important indicators of the membrane. The role of additives in the membrane morphology and thus on the performance of the VRFB is explored by the aforementioned work. They showed that with the increasing poly(vinylpyrrolidone) (PVP) content and the increased molecular weight of PVP, it leads to enlarged and more interconnected pores in a series of asymmetric PES/PVP membranes. PVP causes thermodynamic enrichment and kinetic hindrance during the phase separation process by reducing the miscibility of casting solutions and simultaneously weakens the interaction between the solvent and non-solvent by increasing the viscosity of the casting solution [54,55]. As a result, the dramatic upsurge of vanadium permeability and the membrane with the highest PVP content displayed a poor CE of 66.8% and a VE of 81.6%, and the EE first reaches a peak at 76.1% and declines to 54.6%. However, additional optimization in the PVP content was able to achieve a decent CE of 92.4% and an EE of 76.1%, which is similar to the previously reported NF membranes. Furthermore, Yuan et al. [56] proposed a highly stable composite PES-based membrane doped with the corresponding salts of acidic phosphotungstic acid (TPA) and alkaline PVP, created through acid-base interaction by partially substituting the proton of TPA with alkaline nitrogen of PVP (Fig. 3d), which provides well-connected cross-linking net- works that enhance the selectivity of the stable PES matrix. The rapid proton conduction occurs owing to the hopping of the pro- tons from the TPA cluster to either the adjacent TPA or to the alkaline nitrogen in the PVP, resulting a high VE and EE of 88% and 87% at a current density of 80 mA cm2, respectively. 3. Hydrophilic modification Among the different methods, hydrophilic modification is another effective way to improve the conductivity of PES mem- branes. To improve the ion conductivity of PES membranes, photoinitiated adsorption was proposed by Li et al. [57]. They grafted a vinyl monomer (sodium p-styrenesulfonate) on phase- separated PSF membranes by UV-assisted polymerization that enhances the hydrophilicity of PSF membranes. Initially, a photo initiator called benzophenone was coated on the PES surface and excited to a short lifetime single state by the irradiated photons. As a result, benzophenone forms a radical site at PES to relax its triplet state by extracting the primary hydrogen from the side methyl groups of PES and initiates polymerization of vinyl monomers (Fig. 4a). The grafted PES consists of a typical asym- metric structure made of a thin skin layer and a spongy sublayer, but the pore size after grafting decreases to 1.75 nm from 3.65 nm. Although the reduced pore size provides a steric hin- drance to vanadium ions, the increasing hydrophilicity resulting from the polymer grafting facilitates the diffusion of the vanadium ions along with protons. In addition, the UV irradiation during the polymerization process could possibly escalate the structural degradation and reduce the chemical stability of the membrane. Thus, to retain the structural integrity, polymer blends were introduced to enhance the hydrophilicity, thereby increasing the proton conductivity [47,58].PDF Image | membrane for aqueous redox flow batteries
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