TiO2 Containing Hybrid Composite Polymer Membranes for Vanadium Redox Flow Batteries

In recent years, vanadium redox flow batteries (VRFB) have captured immense attraction in electrochemical energy storage systems due to their long cycle life, flexibility, high-energy efficiency, time, and reliability. In VRFB, polymer membranes play a significant role in transporting protons for current transmission and act as barriers between positive and negative electrodes/electrolytes. Commercial polymer membranes (such as Nafion) are the widely used IEM in VRFBs due to their outstanding chemical stability and proton conductivity. However, the membrane cost and increased vanadium ions permeability limit its commercial application. Therefore, various modified perfluorinated and non-perfluorinated membranes have been developed. This comprehensive review primarily focuses on recent developments of hybrid polymer composite membranes with inorganic TiO2 nanofillers for VRFB applications. Hence, various fabrications are performed in the membrane with TiO2 to alter their physicochemical properties for attaining perfect IEM. Additionally, embedding the -SO3H groups by sulfonation on the nanofiller surface enhances membrane proton conductivity and mechanical strength. Incorporating TiO2 and modified TiO2 (sTiO2, and organic silica modified TiO2) into Nafion and other non-perfluorinated membranes (sPEEK and sPI) has effectively influenced the polymer membrane properties for better VRFB performances. This review provides an overall spotlight on the impact of TiO2-based nanofillers in polymer matrix for VRFB applications.


Introduction
Fossil fuel energies have been commonly used as an energy source for stationary, mobile, and transport applications for the past several decades [1][2][3][4]. However, fossil fuel energy is associated with numerous disadvantages such as air pollution and greenhouse gas emissions [5][6][7]. Apart from the environmental concern, fossil fuel depletion has a significant impact on industrialization and globalization [8]. Fossil fuels are highly extracted from the earth because of their higher necessity for the present energy needs. The worldwide population growth is rapidly increasing, which is also a significant reason for the higher extraction and utilization of fossil fuels. The significant population growth created energy shortage-related issues [9,10]. Moreover, the emission of greenhouse gases such as CO 2 , CH 4 , SO 2, and N 2 O are the major byproducts of fossil fuels during the energy conversion process [5,11,12]. The byproducts of fossil fuels create serious environmental pollution and increase the Earth's atmospheric temperature [13][14][15]. To overcome fossil fuel-related issues such as fossil fuel depletion, energy shortage, and greenhouse gas emissions, worldwide researchers are searching for various kinds of renewable and alternative energy resources. The development of alternative energy needs to be green and clean for the environment. The excellent energy sources of solar, wind, hydro, geo, and bio energy have been effectively considered renewable and alternative energies for the present and future [16][17][18][19][20]. The energy derived from the primary renewable energy resources can be stored and utilized using
Compared to different RFB systems, the VRFB is considered an efficient battery system because of the different oxidation states of vanadium electroactive species (vanadium ions: V 2+ , V 3+ , VO 2+ , and VO 2 + ) used in the electrolyte solution in anolyte and catholyte tanks as shown in Figure 1a [48,[68][69][70]. Moreover, it possesses numerous benefits such as an efficient option for large-scale energy storage, reasonable discharge capacity, cycle life, high safety, room temperature operation, and environmental friendliness [70][71][72]. Thus, the VRFB has already proved its performance in the large-scale energy system (0.3 MW/1.3 MWh, 0.45 MW/1.44 MWh, and 2 MW/8 MWh) [69,73]. To improve the VRFB's overall performance, intense research focuses on the major components, namely electrodes, bipolar plates, membranes, and electrolytes. The performance of VRFB can be altered by employing the co-solvents and different kinds of additives in the electrolyte solution for boosting the steadiness, electroactive species solubility, and overall electrochemical performances. Different kinds of additives have been utilized as stabilizing agents such as zwitterion-type molecule (pyridinium propyl sulphobetaine with sulfonic and pyridine groups), inositol or phytic acid, Trishydroxymethyl aminomethane, Polyacrylic acid, l-glutamic acid, antimony ions, sodium formate, methanesulfonic acid, K 2 SO 4 , Li 2 SO 4 , KHSO 4 , Na 2 SO 4 , CH 3 SO 3 H, MgCl 2 for different oxidation states of vanadium ions [74][75][76][77][78][79][80][81]. In VRFB, the bipolar plate serves as a multifunctional component for constructing and operating the unit cell system [82][83][84]. The major role of the bipolar plate is collecting the current (electron pathway), distributing electrolytes to the electrodes via flow field, internally connecting every single cell in the stacked VRFB unit cell, acting as structural support to the stack, and chemically discrete every single cell. The choice of bipolar plate materials is most important because of their working condition in high acidic conditions (~3M H 2 SO 4 ) and various applied potentials [82,83]. The developments of bipolar plates for VRFB are mainly classified into three categories, namely graphitic, metallic, and carbon polymer composites bipolar plates [82][83][84][85][86][87]. The following essential component in VRFB is electrode materials, which is the component of direct contact with electrolyte that induces the vanadium redox reaction that occurs during the charge and discharge process [88][89][90][91]. The electrode materials possess the properties of excellent chemical stability, higher electrochemical active sites, and greater electrical conductivity behavior, which can effectively serve as efficient electrode materials for the VRFB system. Carbon felt, carbon nanofiber, graphite felt, and carbon paper have been commonly studied as efficient electrode materials for VRFB. To further improve the stability and electrochemical reactions, the electrodes were modified with different concepts such as heteroatom doped, metal oxide nanoparticle decoration, graphene deposition, functionalization with SO 3 H, and halogen atom doped [88,89,92,93]. The center part of the VRFB is a polymer membrane, which is one of the most significant components of the device. The primary function of polymer membrane in VRFB is to separate the anode and cathode part, transport the proton from anode to cathode or cathode to anode side during the unit cell operation, and control the crossover of vanadium ion.

Nafion-TiO 2 Hybrid Polymer Membrane for Vanadium Redox Flow Batteries
The perfluorosulfonic acid ionomer with a long-side chain (Nafion membrane) is commonly used as one of the efficient membrane candidates for the VRFB application because of its higher proton conductivity and excellent chemical stability [144,145]. However, the production cost and vanadium ion crossover due to larger-sized ion cluster formation limits its advantages [146][147][148]. To overcome the higher vanadium ion crossover-related issues, different kinds of inorganic fillers were introduced in the Nafion polymer through various concepts for developing the hybrid membrane. In this connection, efficient attempts have been made with TiO 2 introduction in Nafion polymer to control the vanadium permeability in Nafion membranes. Teng et al. developed Nafion/Organic silica modified TiO 2 composite membrane by sol-gel method and used it for VRFB applications to overcome the solubility issue of TiO 2 in the Nafion/TiO 2 hybrid membrane [138]. Figure 2a illustrates the hydrolysis and polymerization reactions formed during the Nafion/organic silica modified TiO 2 composite membrane preparation. Due to the filling of modifiers (Organic silica modified TiO 2 nanoparticles) into Nafion membrane polar clusters, the modified Nafion membrane exhibited increased thickness with the decrease in water uptake and IEC than the Nafion membrane. Organically modified TiO 2 into the Nafion clusters in the composite membrane exhibited lower vanadium permeability than the Nafion membrane as shown in Figure 2b. It has been observed that approximately 1.7 mL of water was transferred in the modified Nafion membrane while 2.7 mL of water was transferred across the unmodified Nafion membrane. As a result, the modified Nafion membrane exhibited greater water transport resistance than the unmodified Nafion membrane, which was reasoned by the presence of inorganic additives in the membrane. The Si and Ti distribution and titania-silica phase formation has been observed inside the modified Nafion membrane. Interestingly, a new Si-O-Ti vibration peak (in Figure 2c) was observed at 919.89 cm −1 in the modified Nafion membrane, which confirms the composite membrane. As shown in Figure 2b, VRFB with the modified Nafion membrane exhibited lower vanadium crossover, which can exhibit higher discharge capacity than the Nafion membrane. Concurrently, the larger area resistance of the modified Nafion membrane increased IR drop resulting in a little higher average charge voltage. The coulombic efficiency of 94.8% and 90.8% was seen in VRFB with a modified Nafion membrane and Nafion membrane, respectively. As a result of increased area resistance in the modified composite membrane, it exhibited little lower voltage efficiency of 82.2% than the Nafion membrane, 84.8%. Therefore, in the VRFB application, an appropriate amount of Si/Ti content can be used for modified Nafion membrane preparation. The self-discharge of the VRFB single cell through opencircuit voltage (OCV) after being charged to SOC at 75% of the modified Nafion membrane showed better performance than the Nafion membrane. The modified Nafion membrane maintained OCV above 65 h beyond 0.8V, higher than the unmodified Nafion membrane (30 h). This resulted from the modified Nafion membrane exhibiting lower vanadium ions crossover between the compartments in the VRFB system. Furthermore, the VRFB single cell with modified Nafion membrane up to 100 cycles exhibited no CE decay but a slight EE reduction. This shows the high stability of the modified Nafion membrane in highly acidic vanadium electrolyte solution [138].
Nafion/TiO 2 hybrid membrane gained attention for VRFB applications in order to reduce vanadium ions permeation. Thus, Wang et al. prepared Nafion/TiO 2 hybrid membranes by a practical and simple hydrothermal method and investigated their performances in VRFB cells [123]. The introduction of TiO 2 by hydrothermal synthesis into polar clusters does not intrude the ion exchange groups (-SO 3 H) of the Nafion membrane. Thus, the Nafion/TiO 2 hybrid membrane exhibited nearly the same IEC value of 0.85 mmol g −1 as the Nafion membrane. Due to partial filling of Nafion pores by TiO 2 particles, Nafion/TiO 2 hybrid membrane showed lower water uptake of 19.13% than the Nafion membrane. In the presence of an 8.3% TiO 2 modifier, there has been a slightly increased area resistance in Nafion/TiO 2 hybrid membrane. After membrane modification, the hybrid membrane exhibited lower vanadium permeability of 6.72 × 10 −6 cm 2 min −1 than the unmodified Nafion membrane (2.26 × 10 −5 cm 2 min −1 ) due to the partial filling of TiO 2 particles in the Nafion membrane polar clusters. The Nafion/TiO 2 hybrid membrane exhibited excellent chemical stability confirmed by XRD patterns. As per the degree of self-discharge of the VRFB unit cell, OCV for both the membranes gradually decreases at first and then sharply drops. The time for OCV of VRFB with hybrid membrane was nearly 37 h beyond 1.2 V, which was higher than Nafion membrane (14 h). This was mainly attributed to the lower vanadium crossover of the Nafion/TiO 2 hybrid membrane. As a result, VRFB unit cells with the hybrid membrane exhibited higher CE (88.8%) and EE (71.5%) than the Nafion membrane [123]. In another approach, Ye et al. (2020) developed a hybrid membrane by solution casting method with superhydrophilic TiO 2 nanotubes dispersed in the matrix of Nafion for VRFB applications [68]. The surface and cross-sectional images of the recast Nafion (rN212) and hybrid membrane (rN212/T1.5) were examined through SEM (Figure 3a-h) for the confirmation of nanotube distribution in the hybrid membrane. Here, many pinholes with a heavy and compact structure are observed in rN212, while the size and amount were lowered in the rN212/T1.5 membrane due to TiO 2 blending. Meanwhile, TiO 2 blending creates straight rods with regular round holes in the rN212/T1.5 membrane. Predominantly, the shorter TiO 2 nanotubes from long tubes during stirring and ultrasonication are responsible for rN212/T1.5 membrane mechanical strength. The chemical binding and dispersion of Ti 4+ ions in the Nafion matrix improve the mechanical and chemical stability of the rN212/T1.5 membrane. The water uptake and swelling properties of the rN212/T1.5 membrane are lower than Nafion 212 membrane due to TiO 2 nanotubes filling the Nafion pinhole matrix, which lowered the water absorption and polymer swelling. The reduced vanadium ion behavior of the rN212/T1.5 membrane was attributed to the impassable and stretchable ion pathway during TiO 2 nanotube blending. Meanwhile, the high loading of nanotubes increases vanadium permeation by nanoparticle aggregation. Additionally, the rN212/T1.5 membrane observed decreased conductivity due to the parallel alignment of the nanotube on the membrane surface, which possibly blocked the proton transport. Concurrently, the rN212/T1.5 membrane has higher ion selectivity of 10.68 × 10 4 S min cm −3 than the Nafion 212 membrane 8.32 × 10 4 S min cm −3 as shown in Figure 3j. Hence, it has been suitable for VRFB applications. The mechanical strength of the rN212/T1.5 membrane (26.54 MPa) is comparable to the Nafion 212 membrane (27.31 MPa). It has been observed that the OCV of VRFB with rN212/T1.5 membrane sustained a longer time than the Nafion membrane. This is because of lower vanadium crossover by nanotube incorporation. It has been identified that the rN212/T1.5 membrane shows CE of 98.3% and EE of 84.4% higher than Nafion 212 membrane, which has stable performances after 1400 cycles as shown in Figure 3k. Thus, the rN212/T1.5 membrane exhibited long-term stability during VRFB operation [68].

Polyimide Polymer with TiO 2 for Vanadium Redox Flow Batteries
Among different polymer-based non-fluorinated membranes, sPI membranes gained promising attraction in VRFB applications owing to their outstanding chemical stability, high selectivity, low vanadium permeation, and low cost with mechanical and thermal stability [139,[149][150][151]. Despite this, sPI exhibited lower antioxidant and proton conductivities in VRFB application. To overcome this issue, Li et al. incorporated mesoporous TiO 2 filler material into the sPI membrane for enhancing antioxidant ability and membrane proton transport by the blending method [139]. The successful blending of TiO 2 into the sPI was indicated by bright points (size < 1.0 µm) on the surface of the sPI/TiO 2 membrane and the amorphous nature of the composite membrane was obtained. Although the presence of an inorganic TiO 2 component in the composite membrane enhances its chemical stability, the composite membrane exhibited lower water uptake (32.94%) and swelling ratio than the pure sPI membrane. The reason behind this involves the presence of minimal hydrophilic nature of TiO 2 reduces water absorption and swelling. In proton conductivity, the composite sPI/TiO 2 membrane (3.12 × 10 −2 S cm −1 ) showed higher than the pure sPI membrane (2.47 × 10 −2 S cm −1 ). This was due to the collegial effect of the mesoporous hydrated inorganic TiO 2 particle and hydrated sulfonic group promoting the transport of protons in the composite membrane through adsorbing water by mesoporous TiO 2 . An increase in the degree of sulfonation of sPI enhances the proton conductivity value, but the swelling affects the membrane stability. Due to the improved size strength of the composite membrane, the vanadium ions permeability of 2.02 × 10 −7 cm 2 min −1 was observed, which was lesser than the Nafion 117 membrane (17.10 × 10 −7 cm 2 min −1 ). Additionally, the sPI/ TiO 2 composite membrane (1.54 × 10 −5 S min cm −3 ) exhibited more ion selectivity than the pure sPI (1.30 × 10 −5 S min cm −3 ) and the Nafion 117 (0.34 × 105 S min cm −3 ). The sPI/TiO 2 composite membrane performance in VRFB was performed with different current densities from 20 to 80 mA cm −2 . As a result, the sPI/TiO 2 composite membrane exhibited higher charge and discharge capacities than the Nafion 117 membrane due to the reduced vanadium ion permeability of the composite membrane. VRFB with an sPI/TiO 2 membrane at the same current density showed higher CE than the Nafion 117 membrane. Furthermore, the sPI/TiO 2 membrane and the Nafion 117 membrane exhibited adjacent VE at various current densities. At low current density, the sPI/TiO 2 membrane displayed higher EE than the Nafion 117. For the sPI/TiO 2 membrane, about 160 h, the OCV decreases up to 1.3 V and thus 65 h only for the Nafion 117 membrane. Meanwhile, CE, VE, and EE remain unchanged for up to 50 cycles of VRFB with the sPI/TiO 2 membrane. This indicated the high durability, and chemical and cyclic stability of the sPI/TiO 2 membrane, achieved by the incorporation of TiO 2 [139].

Sulfonated Poly(Ether Ether Ketone)-TiO 2 Based Hybrid Composites for Vanadium Redox Flow Batteries
As an alternative to a commercial membrane for VRFB, the sPEEK membrane obtained attention due to its chemical, mechanical and thermal stability with low cost, low vanadium ion permeability, and easy preparation. Increasing the degree of sulfonation in sPEEK creates a broad water channel in sPEEK that increases the membrane swelling, resulting in increased vanadium permeability and decreased membrane mechanical stability. Here, inorganic particles are introduced into the sPEEK membrane to reduce water channel size and increase the mechanical stability of the membrane.
In this connection, Ji et al. used TiO 2 inorganic filler for preparing a uniform sPEEK/ TiO 2 composite membrane by optimizing DS and TiO 2 with a solvent casting method using DMSO solvent [141]. Increasing the sulfonation time increases the IEC, which increases the membrane's water uptake and swelling ratio. It has been shown in the order of 7 h sPEEK > 5-h sPEEK > 3 h sPEEK. Increased proton conductivity is also enhanced by a high water swelling ratio, but this decreases the membrane's mechanical strength. Here, 5 h sPEEK exhibited a similar water uptake and swelling ratio as Nafion 117 membrane. The high loading of TiO 2 nanoparticles incorporated into the membrane decreases water uptake, swelling ratio, IEC, and IC (Figure 4a-d). Excessive TiO 2 acts as a blocking agent, decreasing the swelling ratio and free volume in the membrane. Thus, it prevents proton transport across the membrane. Although 5 h sPEEK/5% TiO 2 loading exhibited an identical proton conductivity value of 17.6 mS cm −1 as Nafion 117. Additionally, the composite membrane showed better oxidative and chemical stability in VRFB. The TiO 2 nanoparticle incorporation also reduces the vanadium ions permeation due to the barrier effect that creates a smaller water channel. It has been observed the 5 h sPEEK/5% TiO 2 composite membrane showed 2.45 × 10 −7 cm 2 min −1 of vanadium permeation lower than Nafion 117 as represented in Figure 4e,f. Additionally, the good dispersion of TiO 2 with sulfonic acid backbone attributed become the increased selectivity in the 5 h sPEEK/5% TiO 2 composite membrane. The self-discharge of VRFB with the 5 h sPEEK/5% TiO 2 composite membrane by charging to SOC of 50% has been evaluated. Initially, the OCV decreases slowly to about 1.2V and then drops sharply to 0.8V. Hence, the 5 h sPEEK/5% TiO 2 composite membrane (107 h) exhibited three times higher self-discharge due to vanadium crossover than Nafion 117 (34 h). It has been observed that there was no decline in charge and discharge capacity with the increased cycles during VRFB single-cell operation with the composite membrane, as shown in Figure 4g-j. Furthermore, VRFB with the Nafion 117 with 180 µm thickness (0.27% per cycle) exhibited two times higher decay rates than the composite membrane with 118 µm thickness (0.11% per cycle). The composite membrane exhibited higher CE and EE than the Nafion membrane. In addition, the 5 h sPEEK/5% TiO 2 composite membrane is intact after 100 cycles, which is evidence of higher stability [141].
To improve the mechanical strength and vanadium permeation resistivity, an idea of different kinds of inorganic nanoparticles (Al 2 O 3 , SiO 2, and TiO 2 ) incorporated into the sPEEK membrane for composite membrane preparation (S/A, S/S, and S/T) by solution casting method at different concentrations (2.5%, 5%, 7.5% and 10%) has been developed [124]. Further, the introduction between the -SO 3 H group of sPEEK and the OH group of inorganic nanoparticles strengthens the compatibility of the composite membrane. Their dense and smooth surface identifies the uniform distribution of inorganic nanoparticles in the composite membrane without a pinhole. The composite membrane exhibited a more decreased WU value compared to sPEEK, but more than Nafion 117. In the same way, the composite membrane exhibited SR of S/A-10% (17.7%), S/S-10% (17.5%), and S/T-10% (19.3) lower than the sPEEK (26.5%) membrane due to the low content of sPEEK and inorganic nano-oxides interaction. These decreasing results are attributed to high mechanical strength for composite membranes with increased dimensional stability. Additionally, the composite membrane also experiences lower proton conductivity and IEC value due to the reduction of free sulfonic acid groups by higher loading of nano-oxides. The OH groups in nano-oxides occupied the free -SO 3 H groups of sPEEK by intermolecular H 2 bonding. Consequently, the composite membranes exhibited low vanadium ion permeability due to lower WU and IEC. The nano-oxides in the composite membrane act as a barrier for proton transport and suppress the vanadium ion permeability. Due to the incorporation of Al 2 O 3 , SiO 2, and TiO 2 , the composite membranes showed enhanced breaking strength with decreasing elongation percentage that increases the membrane mechanical strength. Furthermore, the 5% loading of nano-oxides in sPEEK exhibited a proper balance between physicochemical and mechanical properties for VRFB application. The VRFB single cell performance with composite membranes has been evaluated at various current densities (40 to 200 mA cm −2 ). The composite membranes S/A-2.5%, S/A-5%, S/A-7.5% and S/A-10% exhibited an increasing CE than Nafion 117 due to their low vanadium ion crossover. As a result of decreased vanadium ion crossover and increased ionic selectivity, the S/A-5% (85.0%), S/S-5% (84.8%), S/T-5% (83.5%) exhibited higher EE than Nafion 117 (79.5%) at 80 mA cm −2 . The composite membrane exhibited good self-discharging time due to low VO 2+ permeability, SR, and high mechanical strength. Because of Al 2 O 3 (13 nm size), proper dispersion in sPEEK membrane, S/A-5% membrane possesses a longer self-discharge time (170 h) than the Nafion 117 (46 h) and sPEEK (80 h). The composite membrane S/A-5% and S/T-5% remain at 50% capacity after 200 cycles, whereas the Nafion 117 membrane loses more than 75% capacity indicating the composite membrane has higher stability [124]. In another study, a novel sPEEK/TiO 2 double-deck (S/T) membrane was prepared for VRFB using the solvent casting method [140]. In this study, a TiO 2 inorganic particle was used to improve the antioxidant ability and overall membrane stability of the nonfluorinated sPEEK membrane. From Figure 5a-c, SEM analysis indicated the membrane double-deck structure without cracks that indicated the successful deposition of TiO 2 layer on sPEEK membrane holes. In the S/T membrane, the TiO 2 layer deposition on the sPEEK membrane reduces the H 2 O diffusion channel and thus decreases H 2 O absorption. Therefore, the S/T membrane exhibited a lower H 2 O uptake and swelling ratio. Furthermore, the S/T membrane displayed decreased proton conductivity due to the proton-nonconductive TiO 2 layer. The vanadium permeability of the S/T membrane (6.66 × 10 −7 cm 2 min −1 ) is lower than Nafion 117 (36.5 × 10 −7 cm 2 min −1 ) and sPEEK (19.1 × 10 −7 cm 2 min −1 ). This was due to the distribution of the TiO 2 layer on the sPEEK membrane that reduces the ion transport channel with increased dead-end pockets that prevent vanadium ion crossover. Comparatively, the S/T membrane exhibited higher selectivity of 9.46 × 10 4 S min cm −1 than the Nafion 117 membrane due to lower permeability (Figure 5d). The VRFB cell with an S/T membrane exhibited higher CE (97.0%) and EE (85.8%) higher than the Nafion 117 membrane (CE, 93.3% and EE, 83.7%) as shown in Figure 5e. In addition to this, the composite membrane showed a lower decay rate than Nafion 117, with outstanding VRFB cycle performances [140]. Lou et al. prepared an sPEEK/TiO 2 composite membrane to overcome the stabilityrelated issues and efficient VRFB performances by solvent casting method [142]. TiO 2 has been selected in this direction due to its cost-effectiveness, feasibility, and super hydrophilicity. Thus it improved the proton conductivity, ionic selectivity, and chemical stability. The uniform and successful dispersion of TiO 2 nanoparticles in the sPEEK matrix has been effectively achieved. The size and uniformity of TiO 2 filler into sPEEK altered the water channel size and suppressed the vanadium transport by repulsing the larger stoke radius vanadium ion. Hence, the sPEEK/TiO 2 -5% (0.18 × 10 −7 cm 2 min −1 ) membrane exhibited lower vanadium ion permeability than the Nafion 212 (1.87 × 10 −7 cm 2 min −1 ) and sPEEK (0.35 × 10 −7 cm 2 min −1 ). On the other hand, sPEEK/TiO 2 -5% showed decreased proton conductivity (18.3 mS cm −1 ) compared to the sPEEK membrane (25.28 mS cm −1 ) due to TiO 2 filling into the sPEEK composite membrane. The electrochemical performance of the VRFB unit cell with composite membrane showed a considerable charge-discharge performance at 50 mA cm −2 current density. As a result, the composite membrane exhibited a higher charge voltage than Nafion 212 and oppositely exhibited a discharge process. This was due to the higher thickness of the sPEEK/TiO 2 -5% membrane, in which the proton transport pathway was elongated. Furthermore, the composite membrane showed a high discharge time of 87.5 h higher than the Nafion 212 membrane, which showed only 15.7 h. In addition to this, the composite membrane showed a higher CE of 99.3% due to decreased vanadium permeability. An increase in current density means the VE and EE of both composite membrane and Nafion 212 start decreasing. The reason behind this implied the increased ohmic resistance and potential due to increased current density. During long cycle operation at 120 mA cm −2 , the composite membrane exhibited outstanding CE (over 99.3%), EE (84.8%) with a discharge capacity of 95.4% at 100th cycle and 86.8% at the 200th cycle. Therefore, the composite membrane exhibited the highest stability with enhanced mechanical strength and ionic selectivity due to inorganic TiO 2 nanoparticles [142].
Further altering the sPEEK/TiO 2 composite has enhanced performances in VRFB membranes. The concepts of incorporation of TiO 2 with graphene oxide (GO) and TiO 2 sulfonation have shown considerable attention in preparing the hybrid membranes. The nanosheet GO and titanium dioxide nanoparticle (TiO 2 ) inorganic nanofillers are introduced into the sPEEK matrix to develop the sPEEK/GO/TiO 2 hybrid membrane by the solvent casting method [143]. The blocking effect caused by GO is responsible for vanadium permeation, whereas the GO nanosheets modification performed by TiO 2 aids proton transportation. Additionally, the GO inoxidizability influences the membrane dimensional stability by reinforcing the membrane. In order to increase the hybrid membrane thickness, the various amounts of nanoparticles also increased (0.5 GO, 0.5T, 1.0T, 2.0T). The blocking effect of S/0.5GO/0.5T exhibited decreased vanadium permeability (~0.23 × 10 −7 cm −2 min −1 ) which is 1/7th of Nafion (~1.6 × 10 −6 cm −2 min −1 ) that is due to the GO nanosheets and TiO 2 , which acts as a blocker for vanadium ions transport in the membrane. Further, increased TiO 2 particle loading created increased vanadium ions cross over because a large number of TiO 2 nanoparticles aggregated and developed big pores in the hybrid membrane. The prolonged iron transport channel developed by membrane thickness directly influences membrane resistance. The increase in additive loading decreases the swelling ratio of the hybrid membrane, which increases membrane stability. The ultrasonic and magnetic stirring method introduced TiO 2 into the GO sheet, developing a small ions transport channel that transports only ions with a small stroke radius and hence decreases vanadium ion transport. The hybrid membrane S/0.5GO/0.5T exhibited the highest CE of~99.2% than Nafion (96.2%) due to the blocking effect of GO and TiO 2 nanoparticles. Additionally, the S/0.5GO/0.5T hybrid membrane showed~97.20% of discharge capacity even after 100 cycles with outstanding cycle performances [143].
The development of wide molecular and ion channels by the high degree of sulfonation in sPEEK membrane resulted in high vanadium permeability and low mechanical strength. To overcome this fact, the inorganic fillers are introduced into the sPEEK membrane. In most cases, the inorganic fillers in the sPEEK membrane reduce the membrane proton conductivity. Therefore, functional groups, namely -SO 3 H, -COOH, and -NH 2 are added as inorganic fillers by oxidation, sulfonation, and acidification reaction. In this way, the -SO 3 H group has been introduced into TiO 2 nanoparticles by the sulfonation process for the preparation of the sPEEK/sulfonated TiO 2 composite membrane for VRFB application [126]. As shown in Figure 6a, the sulfonation functional group in TiO 2 nanopar-ticles has been successfully confirmed by IR analysis. The -SO 3 H group not only optimizes the ionic channels but also increases the composite membrane proton conductivity. The enhancement in composite membrane thermal stability was due to the intermolecular hydrogen bond formation by -SO 3 H addition to TiO 2 in sPEEK/s-TiO 2 membrane. Furthermore, the -SO 3 H group in TiO 2 opens the more ion transport channels, resulting in higher proton conductivity. This results in a low vanadium permeability, but not as low as the Nafion 117, sPEEK, and sPEEK/TiO 2 membrane. The sPEEK/sTiO 2 composite membrane (7.13 × 10 4 S min cm −3 ) exhibited two times higher selectivity than the sPEEK membrane (3.40 × 10 4 S min cm −3 ) and three times than Nafion 117 (2.66 × 10 4 S min cm −3 ) as shown in Figure 6b. The VRFB with the sPEEK/sTiO 2 composite membrane exhibited the best charge-discharge performance (Figure 6c-f). Moreover, the sPEEK/sTiO 2 provided a high EE of 82.3% and high discharge capacity because of the high selectivity of the membrane. The efficient unit cell performance is mainly attained because of the presence of sulfonated TiO 2 in the membrane matrix, where sulfonated TiO 2 acts as a barrier for vanadium ion permeability and further improves the proton transportation with the existence of -SO 3 H in TiO 2 [126].
Adding acidic groups to nanoparticle surfaces reduces the swelling properties and vanadium iron crossover with improved ionic conductivity in the membrane as shown in Figure 7a. The sulfonation on the particle surface -OH groups resulted in the -SO 3 H functionalization. In the same way, TiO 2 was sulfated to sTiO 2 with enhanced acidic properties and has been incorporated at a low loading ratio into the sPEEK matrix by the solution casting method, used for obtaining a hybrid membrane with enhanced selectivity ionic conductivity and mechanical stability [125]. The sulfonation of TiO 2 was effectively introduced in the sPEEK. The increased hydrophilic properties of sPEEK in the hybrid membrane due to the sTiO 2 incorporation enhance the WU due to a higher concentration of H + . Moreover, the SR of the hybrid membrane due to sTiO 2 fillers on the sPEEK matrix acts as a barrier. The enhanced amount of SO 3 H groups in the hybrid membrane due to sTiO 2 in the hybrid membrane possesses an increased conductivity (20.9-25.1 ± 0.2 mS cm −1 ) compared to sPEEK (16.4 ± 0.3 mS cm −1 ), sPEEK/TiO 2 (15.9 ± 0.2 mS cm −1 ) and IEC. The sPEEK/sTiO 2 −3(3% sTiO 2 ) exhibits a lower vanadium permeation (7.0 × 10 −7 cm −2 min −1 ) than sPEEK/TiO 2 (17.4 × 10 −7 cm −2 m −1 ) as shown in Figure 7b-e. This was due to the restraining vanadium ion movement and also the stabilization of proton transport channels by -SO 3 H groups of sTiO 2 . The improvement in IEC and proton conductivity has been attributed to the hydrophilicity developed by −OH and −SO 3 H groups (scheme). The decreased vanadium ion permeability of the sPEEK/sTiO 2 -3 hybrid membrane exhibited an increased selfdischarge time of 55 h, which was higher than Nafion 117 (33 h), sPEEK (37 h), sPEEK/TiO 2 (41 h). The VRFB unit cell with sPEEK/sTiO 2 -3 hybrid membrane showed increased CE and EE as shown in Figure 7f,g. The membrane mechanical stability and chemical stability were enhanced by an intramolecular interaction between OH and -SO 3 H groups in the hybrid membrane [125]. The TiO 2 -based inorganic nanofillers have effectively altered the physicochemical properties of membranes and VRFB unit cell performances, as represented in Tables 1 and 2. The introduction of TiO 2 in the polymer matrix effectively controls the higher amount of water uptake by the membrane and the swelling ratio of the membrane. More interestingly, the vanadium ion permeability is significantly lowered to the Nafion, SPI and sPEEK membranes after introducing of TiO 2 in the membrane. Thus, the TiO 2 nanoparticles in the membrane acted as a barrier for the easier movement of vanadium ions through ionic clusters, which is the primary reason for obtaining the lower permeability. With the further modification of TiO 2 as a sulfonation and other organic compounds, the proton conductivity and stability have been further tuned. Based on the reasonable proton conductivity and lower vanadium ion crossover, most of the TiO 2 -containing membranes achieved higher selectivity than the bare polymer membrane. Thus, the TiO 2 -based hybrid membrane revealed excellent VRFB performances during the unit cell operation. Apart from the membrane applications, TiO 2 has been widely considered an efficient electrode material for various applications such as photoelectrochemical devices, batteries, and fuel cells [152][153][154]. The different concepts and structural properties of TiO 2 nanostructures developed for photoelectrochemical devices [152,[155][156][157] can be further altered and considered as a new kind of TiO 2 -based additive for VRFB membranes. This comprehensive review will be more resourceful for the polymer and membranes scientific community to choose the further development in the TiO 2 -containing polymer membranes for VRFB and other energy conversion and storage systems.

Conclusions
VRFB has gained great attraction in extensive scale energy storage systems due to its long cycle life, easy maintenance, and high efficiency. In VRFB, ion exchange membrane (IEM) plays a key role in battery performance. An excellent IEM should possess high proton conductivity, good chemical stability, low vanadium permeability, low cost, high mechanical strength, etc. DuPont Nafion perfluorinated sulfonic acid polymer membrane has been commonly used in VRFB due to its increased proton conductivity and chemical stability. Nafion membranes have limitations in VRFB due to their high cost and high vanadium permeability. To overcome these issues, incorporating inorganic nanoparticles into Nafion has been performed to decrease vanadium permeability. This comprehensive study reviewed TiO 2 as an inorganic filler for VRFB membranes. It has been confirmed that TiO 2 is one of the best inorganic fillers due to its low price, high stability, and easy preparation method. The TiO 2 occupies the Nafion matrix, thus decreasing the swelling ratio by reducing the size of the water transport channel. Thus, it decreases the vanadium ion cross-over. Modification such as organic silica modified TiO 2 has also been introduced into Nafion to increase the chemical stability in a high acidic electrolyte environment with decreased vanadium ion permeability. Apart from this, different non-perfluorinated polymer membranes (sPEEK, sPI, etc.), have also been developed as IEM as an alternative to the Nafion membrane. Low chemical stability and high vanadium permeation were identified as the major drawback of this type of hydrocarbon membrane (such as sPEEK). The DS in the sulfonated membranes increased the proton conductivity but also increased vanadium ion permeability. Therefore, such membranes are modified by incorporating inorganic nanofillers and modified nanofillers to enhance membrane performances. However, nanofiller introduction decreases the proton conductivity by occupying the SO 3 H groups. Therefore, functional group modification (SO 3 H, COOH, NH 2 ) on nanofiller has been accomplished to enhance membrane proton conductivity. The sulfonation of TiO 2 alters the ion transport channels and increases proton conductivity. In addition to this, the nanoparticle incorporation increases mechanical stability, suppresses the vanadium ion permeability, enhances selectivity, and improves the overall VRFB unit cell performances. Hence, this review encompasses the overall developments in TiO 2 -based hybrid composite membranes for VRFB applications.