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6 J. Sheng et al. / Materials Today Nano 7 (2019) 100044 Fig. 4. (a) Schematic of modified polysulfone membranes used in the VRFB. Reproduced from Li et al. [57] with permission from the Elsevier. (b) Design principles of a VRFB with a porous membrane bearing a zeolite flake layer. (c) High-resolution transmission electron microscopy (HRTEM) image of ZSM-35 zeolite viewed in the [010] direction. The inset shows that a ZSM-35 framework drawn by diamond 4.0 (section in color: viewed in the [010] direction) fits perfectly with an HRTEM image viewed in the [010] direction. Reproduced from Yuan et al [59] with permission from the John Wiley & Sons, Inc. (d) The influence of the solvent treatment on the morphology and performance of the porous PES membrane. Reproduced from Lu et al. [60] with permission from the Royal Society of Chemistry. VRFB, all-vanadium redox flow battery; PES, polyethersulfone. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) titanate (BT) and grafting the blend with grafted polystyrene sul- fonated acid. As the BT content in the membrane increased, more oxygen vacancies were created that cause more F to enter into the oxygen octahedron of BT, leading to the reduction in the degree of hydrophilic sulfonic acid grafting on the branched chain. Although this phenomenon decreases the ion conductivity, occurrence of microphase separation provides favorable channels for proton conduction between the highly hydrophobic CeF skeleton and sulfonic acid group. Similarly, to obtain connected ion trans- portation channels, Cao et al. [68] grafted hydrophilic PVP in the pores and on the surface of PVDF-based porous membranes first by immobilization of PVP on the PVDF substrate via cross-linking re- action using potassium persulfate as a cross-linking agent, followed by a solvent preswelling treatment using ethanol as the solvent, as depicted in Fig. 5c. Owing to the decrease in porosity and the pore size owing to PVP immobilization, the membrane can obtain a CE of 94.3% at 80 mA cm2. However, the firm immobilization of PVP on the pores tends to block the pores and increases the area specific resistance of the flow cell, and a solvent preswelling treatment is needed to enlarge the pores. In addition to PVDF, poly(vinylidene fluoride-co-hexafluoropropylene) was also used owing to its high hydrophobicity [69]. The polybenzimidazole (PBI)-based membrane, as another type of well-investigated membranes, exhibited chemical stability and good mechanical property. The drawback of this kind of ion- exchange membranes is their moderate proton conductivity, which usually manifested in unsatisfied VE. Recently, Ding et al. [70] used a polycondensation method to obtain the sulfonated polybenzimidazole (SPBI) membranes, enhancing the water uptake capability. The SPBI membrane shows higher average CE, VE, and EE values than that of the Nafion 115 membrane. Peng et al. [71] tried to graft non-ionic N-substitution hydrophilic side chains into the PBI membranes, forming hydrophilic clusters, which make the proton transfer more efficient. 4. Other modifications Apart from introducing hydrophobic polymers, another inter- esting strategy to enhance the selectivity is to exploit Donnan's exclusion mechanism by introducing positively charged groups on the pore walls to restrict the permeability of vanadium ions through the pores and thereby increase the membrane's selectivity. In 2013, Zhang et al. [72] created a porous membrane with chlor- omethylated polysulfone resin and pyridine by the vapor-induced phase inversion method that looks similar to a bulk sponge with thousands of highly symmetrical micron-sized pores separated by ultrathin walls. All pores are grafted with weak alkaline groups and possess a uniform positive charge. As a result, protons can travel freely, but the large vanadium ions are rejected owing to Donnan's exclusion and the size sieving effect. By loosening the pore walls, the membrane can achieve proton conductivity similar to an aqueous sulfuric acid solution, and at the same time, it acts as a multilayered barrier to the vanadium ions, leading to an EE higher than 81% at 120 mA cm2. This specially designed membrane morphology provides an excellent solution for increasing the ion permeation selectivity while achieving a high VE and thus a higher EE. Except for the size sieving and Donnan's exclusion, another strategy to enhance the selectivity is layer-by-layer (LBL) self- assembly [73e75]. A composite membrane possessing high selectivity for the vanadium ions was demonstrated by leaching out low-molecular-weight imidazole from a composite membrane of imidazole and SPEEK, followed by assembling two oppositely charged polyelectrolytes, poly(diallyldimethylammonium chloride) (polycation) and poly(sodium styrene sulfonate) (polyanion), on the obtained porous substrate. Although the membrane exhibited higher proton conductivity and high selectivity compared with the commercial Nafion membrane, the coulombic efficiencies obtained by the membrane at various current densities are much lowerPDF Image | membrane for aqueous redox flow batteries
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