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2 J. Sheng et al. / Materials Today Nano 7 (2019) 100044 efficiency (CE) and energy efficiency (EE) of RFBs [30,31]. The function of the membrane in RFBs and the important indicators are illustrated in Fig. 1. As a kind of perfluorosulfonic cationic mem- brane, a series of Nafion membranes produced by the Dupont company provides high proton conductivity, and high chemical and electrochemical stability, which is a common membrane for RFBs [32e34]. However, the exorbitant price of Nafion membranes hindered their extensive utilization in RFBs [35]. Thus, developing more advanced membranes with both superior performances and low cost always attracts the attention of researchers. This review aims to summarize recent research advances on categories of the improvement method on membranes to enhance the performance of RFBs, mainly including the controlling of the pore size, hydro- philicity, and some other aspects. By providing an overview of the developments of membrane materials in RFBs, we hope to inspire various ways to explore more advanced membranes with both high performances and low cost. 2. Pore size modification The porous membranes differ from traditional ion-exchange membranes in a sense that the ion selectivity in porous mem- branes is obtained either by size sieving or by Donnan's exclusion because the stoke radii of vanadium ions are much larger than those of the hydronium ions. The elimination of the ion-exchange groups from the porous membrane overcomes the limitation caused by the ion-exchange groups and significantly enhances the stability of the membrane under harsh operating conditions of VRFBs. The first ever porous membrane for VRFB application was reported by Zhang et al. [36] using a hydrolyzed polyacrylonitrile nanofiltration (NF) membrane with finger-like pores with an ul- trathin top layer prepared by the phase inversion method (Fig. 2a). The main concept lies in tuning the porosity of the membrane to obtain higher selectivity between the protons and the vanadium ions. The membrane demonstrated an efficiency of 95% at 80 mA cm2, which remains stable for around 200 cycles, demonstrating excellent chemical stability of the membrane. To further improve the selectivity of the NF membrane, an idea of introducing a thin layer of filler onto the membrane to reduce the pore size was introduced by the same group [37]. The silica was introduced onto the NF membrane to decrease the pore size and thereby to enhance the selectivity by almost four times via in situ hydrolysis of tetraethyl orthosilicate. The silica-modified NF membranes exhibited a much higher CE of 98% at 80 mA cm2, which is even higher than the commercial Nafion 115. Besides, the versatility of the concept was also verified by using the polysulfone (PSF)/sulfonated poly(ether ether ketone) (SPEEK) blend that ob- tainedaCEof97%at80mAcm2 andanEEof80%at40mAcm2 (Fig. 2b) [37]. Overall, the silica-modified NF membranes demon- strated better performance, but the stability of the membrane remained unknown. The commercial Daramic polyethylene/silica microporous membranes with an average pore size of 0.15 mm and 57% porosity were also used in an iron-vanadium RFB that yielded an EE of ~70% [38,39]. Later on, a hydrophilic separator composed of agglomerated silica particles enmeshed in a fibril polyvinyl chloride matrix that possesses a unique porous structure with an average pore size of 45 nm (Fig. 2c) and a porosity of 65% (Fig. 2d) was incorporated in the VRFB by Wei et al. [40]. These pores serve as ion transport channels resulting in lower ohmic overpotential. Nevertheless, the large pore size allows protons and hydrated vanadium ions to diffuse through with greater freedom, causing poor selectivity and high self-discharge. Consequently, the cell can only acquire a CE of 89.2%, a voltage efficiency (VE) of 87.0%, and an EE of 78.1%, which are significantly lower than the commercial Nafion 115 at the same current density of 50 mA cm2. Therefore, to enhance the selec- tivity of ion permeation and realize improved performance in the flow cell than the traditional ion-exchange membranes, funda- mental factors affecting the performance of the porous membranes such as the morphology, preparation technique, additives, and so on were researched extensively over the years. After the successful implementation of the NF membrane in the VRFB, many re- searchers optimized the fundamental aspects of the porous mem- branes to achieve even better performance than the ion-exchange membranes. Polyethersulfone (PES) has also been widely studied as a membrane material for the flow battery owing to its excellent mechanical and chemical stability and tunable morphology. PES membranes created by phase inversion methods usually hold an asymmetric structure composed of a skin layer that defines the selectivity and a porous support layer, which governs the ion transport resistance. Although the large pore size of the skin layer is preferred to reduce the ion transport resistance, it also leads to a higher amount of active ion crossover. The asymmetric morphology arising from the kinetic properties is usually driven by the con- centration distribution, viscosity of the casting solution, activity gradients of non-solvents, evaporation time, and humidity [41e45]. In general, increasing the porosity increases the proton conduc- tivity, but the selectivity of the membranes is still a major chal- lenge. Two different mechanisms of phase separation by immersion precipitation and instantaneous and delayed demixing are usually used to tune the morphology of the PES matrix. In a ternary system, instantaneous phase separation occurs when the precipitation path crosses the bimodal phase and forms two distinct phases imme- diately after immersion, leading to the porous top layer, whereas in delayed demixing, the phase separation does not occur for a considerable amount of time after immersion because the precip- itation path does not cross the binodal immediately, resulting a dense skin layer [46]. Xu et al. [47] showed that the finger-like pores become larger and the appearance of macrovoids decreases, as shown in the scan electron microscope (SEM) images in Fig. 3a, owing to the delayed demixing caused by the higher viscosity of the concentrated polymer solution with increasing concentration of SPEEK in the cast solution. SPEEK is highly hydrophilic and possesses excellent pro- ton conductivity and miscibility with a large range of polymers [48e50]. The porous skin layer obtained by the addition of SPEEK to PES facilitates the diffusion of solvent molecules into the cast so- lution transforming the finger-like pores to sponge-like pores by changing the thermodynamic properties of the cast solution and affecting the phase inversion method [51]. However, the CE and EE obtained by the membrane in the full cell is only 92.8% and 78.4%, respectively, which are lower than the traditional Nafion 115 membrane. Similarly, Chen et al. [52] reported a porous hierarchical PES/SPEEK membrane obtained through the hard template method using phenolphthalein as the template. They removed phenol- phthalein from the pore walls of the PES/SPEEK/phenolphthalein composite membrane, leading to well-defined nanoscale hierar- chical pores on the selective layer. In addition, the increase in vis- cosity of the casting solution owing to the incorporation of phenolphthalein promotes the formation of a spongy structure, as shown in Fig. 3b. However, with the increasing phenolphthalein concentration, a decrease in the CE was observed owing to the high vanadium permeability resulting from the larger and inter- connected pores. The VRFB cell performance of the SPEEK/PES composite was further improved by Li et al. [53]. They increased the selectivity of the porous PES/SPEEK composite membrane by add- ing double ion-selective layers (skin layer and top layer) by coating an ultrathin layer of Nafion on top of the PES/SPEEK membrane (Fig. 3c) created by the phase inversion method.PDF Image | membrane for aqueous redox flow batteries
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