Semi-Interpenetrating Network-Type Cross-Linked Amphoteric Ion-Exchange Membrane Based on Styrene Sulfonate and Vinyl Benzyl Chloride for Vanadium Redox Flow Battery.

Clean energy is the main requirement for human life. Redox flow battery may be an alternative to fossil fuels. An ion-exchange membrane is the heart of the redox flow battery. In the present study, we synthesize semi-interpenetrating cross-linked copolymer amphoteric ion-exchange membranes (AIEMs) with a partially rigid backbone. The styrene sulfonate and vinyl benzyl chloride monomers are used as the cationic and anionic moieties into the AIEMs. Three different types of quaternizing agents are used to convert a primary amine into a quaternary amine group. Here, we avoid the use of the carcinogenic chemical CMME, commonly used for the synthesis of anion-exchange membranes. The prepared membranes exhibit good electrochemical and physicochemical properties with a high acidic stability. The membranes also show moderate water uptake and dimensional change. The ZWMO membrane shows better properties among the AIEMs, with an ionic conductivity of 3.12 × 10-2 S cm-1 and 5.49 water molecules per functional group. The anion and cation-exchange capacities of the ZWMO membranes are calculated to be 1.11 and 0.62 mequiv/g. All AIEMs show good thermal and mechanical stabilities, calculated by differential scanning calorimetry, dynamic mechanical analysis, and universal testing machine analysis. The membranes show low vanadium ion permeability than the commercial membrane Nafion for their use in vanadium redox flow batteries. Further, the AIEMs are applied in redox flow batteries as separators and deliver good results with the charging and discharging phenomena, with 87% voltage efficiency and 91% current efficiency.

Introduction

Vanadium redox flow batteries (VRFBs) are considered to be a promising system for energy storage because of their excellent features like low cost, long life, easy operation, and deep discharge capability, as well as high current efficiency.[1−5] This pioneer work was done by Skyllas-Kazacos in 1985.[6−8] However, increasing sustainable development of the world requires huge amounts of energy.[9,10] However, because of the instability in the supply of electricity and intermittent nature, there is a demand for a large-scale energy storage system. VRFBs are the prominent candidates that convert chemical energy into electrical energy on a large scale for a long time. In a redox flow battery, the four oxidation states formed by the redox couples are separated by an ion-exchange membrane. So, an ion-exchange membrane plays a key role in the operation of a redox flow battery. This design mitigates the crossover contamination of redox couples.[11]

Nafion is used as a separator in VRFBs because of its good proton conductivity and high chemical stability. However, the expensiveness and VO2+ ion crossover limit its use.[12] The cross-contamination of VO2+ ions between the positive and electrolyte tanks would result in severe capacity decay and voltage declination in an open circuit.[9] Various other membranes are also available, but their uses are limited because of the poor stability in the V+5 oxidation state.[13] Because of the high demand, researches have been carried out to develop such kinds of ion-exchange membranes which show good ionic conductivity (IC), bear good chemical stability, as well as provide low VO2+ crossover across the membranes. Different types of cation-exchange membranes have been developed containing sulfonic acid groups such as sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(ether sulfone), and sulfonated polyimide for VRFBs because of their extremely good proton conductivity and chemical and mechanical stability.[14−17] These sulfonated aromatic polymer-based IEMs are also being widely used in direct methanol fuel cells because of their low methanol permeability. However, as to attain high conductivity, a high degree of sulfonation is desired, which may often result in their high swelling ratio and reduced mechanical stability.

Radiation grafting is also a method for the synthesis of cost-effective membranes by maintaining proper grafting conditions.[18,19] Various kinds of anion-exchange membranes (AEM) have also been employed as they show low vanadium ion permeability because of the Donnan exclusion/charge repulsion phenomenon.[20,21] Although AEMs show good resistance toward vanadium ion permeation, they face the problem of lower mobility of the SO42+/HSO4– ions through the membrane matrix than the H+ ions.[21,22] Higher functional group-containing polymers also have inferior properties like less mechanical stability because of the high degree of swelling and more water uptake (WU). To synthesize a polymer electrolyte membrane bearing improved characteristics, blending of acidic and basic polymers is also a facile and easier way.[23,24] By blending both the groups, one can improve the mechanical, chemical, and electrochemical stabilities by maintaining both the components in desired amounts. Polyetherimide (PEI), which is a commercially available polymer, exhibits good thermal, mechanical, and IC values.[25] Blending of PEI with SPEEK would result in the synthesis of an acid–base membrane in which the sulfonic group of SPEEK acts as an acidic site, whereas the nitrogen-containing group acts as a basic site. One more important thing observed is the hydrogen-bond formation between the hydroxyl groups of SPEEK and the amine group of PEI, which results in a cross-linking phenomenon, because of which swelling could be controlled. Likewise, amphoteric ion-exchange membranes (AIEMs) containing both cationic and anionic sites are generating huge attention toward VRBFs.[25] The controllability in the composition of both anionic and cationic functional groups is possible in AIEMs, which can be extended for wide applications.[26,27]

Herein, we have developed semi-interpenetrated network (SPIN) type of AIEMs based on the cationic site of sodium-p-styrene sulfonate and anionic site in the form of vinyl benzyl chloride (VBC). Quaternization of the synthesized membranes was achieved through the reaction with three different kinds of aminating agents. The membranes were characterized by Fourier transform infrared (FTIR) spectroscopy to confirm their molecular structure and functional groups. Important physicochemical characterizations like WU, ion-exchange capacity (IEC), linear expansion ratio (LER), and IC were measured to evaluate their performance toward electrochemical processes. Charging–discharging cycles were performed to estimate their suitability for VRFBs, and their performances were examined with respect to current, voltage, and energy efficiency (EE).

Experimental Section

Synthesis of Semi-Interpenetrating AIEMs

Semi-interpenetrating (SPIN)-type AIEMs were synthesized by dissolving poly vinyl chloride (PVC) into cyclohexenone. After obtaining the transparent mixture of PVC, predetermined amounts of styrene, sodium-p-styrene sulfonate, and VBC were added into the solution. Free-radical copolymerization was carried out with the help of azobisisobutyronitrile initiator at 80 °C for 7 h, and the color of the solution turned into pale yellow from being transparent. This solution was then casted on a clean glass plate with a doctor’s blade. After complete drying, the membranes were peeled off and then subjected to quaternization in three different quaternizing agents, viz iodomethane, trimethyl amine, and n-methyl morpholine, and denoted as ZWMI, ZWA, and ZWMO, respectively, as shown in Scheme .

Scheme 1

Schematic Representation for the Synthesis of AIEM by Different Quartenizing Agents

Physicochemical Characterization of AIEMs

To evaluate the performance of the synthesized AIEMs, the membranes were physicochemically characterized by means their IEC, both cation- and anion-exchange groups, IC, WU, number of water molecules per ionic site (λ), and dimensional changes. The details are given in the Supporting Information.

Chemical, Thermal, and Mechanical Characterization of AIEMs

The synthesized CEMs and AEMs were characterized functionally, thermally, and mechanically using FTIR spectroscopy, thermogravimetric analysis, and universal testing machine (UTM). The FTIR spectra of the membranes are recorded using the KBr pellet method with a spectrum GX series 49387 spectrometer in the frequency range 4000–400 cm–1. The stress–strain property of the membrane samples (2.5 cm long, 0.5 cm width, and 0.15 mm thick) is determined using a Zwick Roell Z 2.5 tester. The test Xpert II-V3.5 software was used for the data analysis. The details are given in the Supporting Information.

VO2+ Permeation Measurements of VRFB Single-Cell Performance

To conduct VO2+ permeability measurements, the membrane pieces of an effective area (7.06 cm2) were sandwiched between two half-cells, one cell filled with 1.5 mol L–1 VOSO4 in 3.0 mol L–1 H2SO4 (40 mL) and the other cell filled with a solution of 1.5 mol L–1 MgSO4 in 3.0 mol L–1 H2SO4 (40 mL). MgSO4 is used to balance the ionic strength and minimize the osmotic pressure between the two solutions. The VO2+ ion permeability in the second compartment was checked periodically by UV–vis analysis and calculated by the following equation of Fick’s law[28]where VB is the volume of VO2+, CB is the VO2+ concentration in MgSO4 solution, t is time, A is the effective area of the membrane, L is the thickness of the membrane, P is VO2+ permeability, and CA is the initial concentration of VO2+ in the first compartment.

The charging and discharging characteristics of the VRFB cells were determined with the prepared AIEMs membranes. The charging of the cell was done at a current density of 116 mA/cm2. During the whole process, a piece of membrane is sandwiched between two graphite electrodes clamped with a bare copper wire and connected to a power source. The testing membrane acts as a solid electrolyte separator between the anodic and cathodic compartments. A two-channel peristaltic pump was used to maintain the turbulent flow at a fixed flow rate. Both the chambers were filled with 40 mL of 1.5 M VOSO4 in 3 M H2SO4 solution. The VRB single-cell performance was evaluated in terms of current efficiency (CE), calculated by using the following equation[29]where Cdis (discharging capacity) and Cch (charging capacity) can be calculated aswhere Idis is the current recorded during the discharging process and tdis is the time taken for the discharging process. Similarly, ICh is the current recorded during the charging process and tCh is the time taken for the charging process.

For the performance evaluation of a redox flow battery, energy efficiency (EE) was evaluated from the current efficiency (CE) and voltage efficiency (VE), and was calculated by the following eq ..

Results and Discussion

The functional characterization of the synthesized AIEMs was carried out by FTIR spectroscopy as shown in Figure . The peaks at 1237, 1465, and 1667 cm–1 arise because of the −C=C– stretching found in the benzene ring of styrene, VBC, and sodium-p-styrene sulfonate. The occurrence of these peaks confirms the interpenetration of the monomers into the PVC matrix. Another peak that arises at a frequency of 2958 cm–1 is due to the stretching vibration of the −C–H– bond of the aromatic ring. An absorption peak at 1108 cm–1 confirms the presence of the sulfonate group, whereas an absorption peak at 1667 cm–1 indicates the presence of −N–H stretching. Another absorption peak at 2239 cm–1 is associated to the stretching vibration of the −C–N– groups of amines. All the characteristic peaks are present in the membrane, revealing the presence of both the functional groups within the membranes. The scanning electron microscope (SEM) images at different magnifications were also taken to demonstrate the dense nature of the membranes, as shown in Figure A–C. The cross-sectional image of ZWMO was recorded showing the nonporous nature of the formed membrane. The proper integrity of the monomers used for the preparation of AIEM can be easily demonstrated here by observing the compactness, which in result provides the dense nature. Figure D shows the cross-sectional view of the ZWMI membrane which confirms the dense nature of the membrane. Figure E shows the mapping of different elements present in the membrane matrix by different colors. The presence of nitrogen and sulfur in the membrane is shown in Figure F by different colors. Later on, the occurrence of both the acidic and basic groups was confirmed by energy-dispersive X-ray analysis.

Figure 1

FTIR spectra of different AIEMs.

Figure 2

SEM images of the ZWMO membrane (A–C) and ZWMI membrane (D). Elemental mapping (E) and mapping of sulfur and nitrogen in the ZWMI membrane (F).

The thermal and mechanical characterizations of AIEMs were performed by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and UTM analysis. Figure shows the thermomechanical stability of different AIEMs by DMA. The modulus values for all the membranes were found to be comparable to the reported membranes, and the modulus value for ZWMO was found to be maximum among the membranes, which is 14% higher than that of the ZWA membrane and 21% higher than that of the ZWMI membrane. The temperature of tan δ also increases from the ZWA membrane to the ZWMO membrane, which confirms the higher thermomechanical stability of the composite membranes (Figure ).[30] The DSC analysis measured the glass transition temperature of the AIEMs, as shown in Figure S-1. The Tg value calculated for the ZWA membrane was found to be 81.92 °C, whereas it was 81.24 °C in the case of ZWMI membrane, which is higher than that of PVC because of the interpenetrating network with VBC.[31] It is clear from the data that the membranes maintain their thermal stability after functionalization. The mechanical stability of all the prepared membranes was tested at room temperature in dry state by UTM, and the results are shown in Figure and the corresponding values are presented in Table . The ZWMO membrane showed the maximum value of Young’s modulus, 14.66 MPa, whereas the others showed 11.33 and 5.5 MPa, respectively. This result can also be justified by the WU value. ZWMO shows higher amounts of WU when compared with the rest of the two membranes, and we know that the water absorbed by the membrane is responsible for membrane swelling, thus providing support to the membrane when placed under an external force, that is, stress. ZWMO also showed the maximum value for strain, that is, 3.60%, whereas the lowest value is shown by ZWA, which is 2.26. This may be due to the internal cross-linking between the functional groups and the polymer matrix, which acts as a barrier for WU. Less WU and high degree of cross-linking do not allow the membrane to elongate when force is applied on it.[32]

Figure 3

Modulus and temperature of tan δ for different AIEMs.

Figure 4

UTM spectra of different AIEMs.

Table 1

Mechanical Properties of Different AIEMs

membrane type	modulus (MPa)	stress (MPa)	strain %
ZWA	5.5	16.62	2.26
ZWMI	11.33	17.71	1.67
ZWMO	14.66	46.07	3.60

The kinetics of WU play an important role in ion-exchange membranes. The WU of AIEM was measured gravimetrically by equilibrating the membranes in water. The WU in AIEMs varied from 15 to 21%. The water molecule per functional group (water content/hydration number) λ was calculated using the following equation[33]

The λ value for the ZWMI membrane is calculated to be 4.54, which is enhanced by 7.4 and 20% for the ZWA and ZWMO membranes, respectively (Table ). The enhancement of the hydration number in the ZWA and ZWMO membranes may be due to the presence of a higher number of functional groups present in the membrane. Transportation of ions in the membrane is carried out by absorbed water.[34,35] WU by a membrane up to an extent is good for ion transport and stability, but high amounts will cause diminishment in mechanical stability.[36] The anion-exchange capacity (AEC) and cation-exchange capacity (CEC) for the prepared AIEMs that were found to be maximum for the ZWMO membrane are 1.11 and 0.62 mequiv/g, respectively, which slightly differ from the theoretical IEC values, and can be explained by the formation of ionic knots between both the acidic and basic groups. The occurrence of intrinsic electrostatic force of attraction between these two functional groups will provide no free exchangeable ion, which results in slightly lower values of IEC. The difference must be related to the effective degree of quaternization of the aminating agent used. ZWMI and ZWO AIEMs also show good values for AEC and CEC, as shown in Table . WU, LER, and IEC of the membrane are related with each other. IEC is dependent upon the presence of the functional group in the membrane matrix; as the functional group increases, IEC also increases with WU. Table shows the WU and LER % in AIEMs, and it is clear from the data that as WU increases LER also increases. The existence of both acidic and basic groups within a common polymer chain shows a self-cross-linking behavior because of the establishment of hydrogen bonding between two atoms having different electronegativities. Because of this, one of the functional groups, either acidic or basic, gets inhibited by the other. In summary, we can say the protection of one by the other results in a lower value of the ion-exchange capacity.

Table 2

IEC, IC, WU, LER, Hydration Number (λ), and Activation Energy of Membranes

membrane type	AEC (mequiv/g)	PEC (mequiv/g)	WU (%)	LER (%)	IC × 10–2 (S cm–1)	hydration no. (λ)	Ea (kJ/mol)
ZWMI	0.73	0.45	18	14	2.77	4.54	26.50
ZWA	0.84	0.57	15	11	1.84	4.88	35.56
ZWMO	1.11	0.62	21	19	3.12	5.49	22.94

IC of AIEMs was measured for each membrane from 30 to 90 °C by Nyquist plots to check their performance for high-temperature applications, and the corresponding values are presented in Figure and Table . The IC values for AIEMs were calculated to be from 1.84 × 10–2 to 3.12 × 10–2 S cm–1. The enhancement in IC was also recorded for all the three membranes by enhancing the temperature from 30 to 80 °C. The enhancement in IC with temperature may be due to the higher mobility of ions and proton and higher proton diffusion coefficient at higher temperatures.[37,38] The activation energy for the conduction of proton/ions reveals the minimum energy required for the proton/ion transport from a functional group to another and is, therefore, an important parameter of IEM. Temperature-dependent IC is used to determine the activation energy for proton/ion conduction. The Arrhenius-type plots shown in Figure are used to calculate the activation energies for AIEMs.[39,40]Table shows that among the activation energies for AIEMs, the ZWMO membrane has the lowest value than the other membranes.

Figure 5

Arrhenius plots for different AIEMs.

The crossover of vanadium ions has become a very crucial problem for VRFB, as it causes several electrochemical energy losses and increases the rate of self-discharge. Therefore, the vanadium ion crossover resistance across the membrane has been measured. The diffusion of vanadium ions is calculated by taking a sample from the model solution of MgSO4 after a certain interval of time. It is clear from Table that the vanadium crossover through the prepared AIEMs is very low as compared to that through the Nafion 117 membrane. For ZWMO, the value of diffusion coefficient is 4.97 × 10–8 cm2 s–1, which is about 15% of that of Nafion 117 (3.57 × 10–7 cm2 s–1). The reason behind this much lower value for the diffusion coefficient for AIEMs could be the Donnan exclusion phenomenon.[41,42] Because of this effect, the cationic functional groups which are present within the polymer matrix prevent the VO2+ ion permeation by static charge repulsion. However, for ZWA and ZWMI, the VO2+ ion crossover value is slightly greater than that of ZWMO; this may be due to the lower amount of WU, which in turn makes AIEMs more favorable toward acid uptake, providing a medium for VO2+ ion movement.

Table 3

VO2+ Permeability (after 4 h) for Different Membranes

membrane type	VO2+ × 10–8 cm2 s–1
ZWA	4.59
ZWMI	4.93
ZWMO	5.27

All the prepared AIEMs are chosen for VRFB application, and the performances are compared with the benchmark Nafion 117 membrane. The charging of all the prepared AIEMs is performed at a constant current density of 116 mA/cm2 for 4 h. The discharging voltage and current values were recorded manually by applying a constant resistance of 1 kΩ for all AIEMs. The charging and discharging characteristics of all the membranes are shown in Figures and 7, respectively. However the charging voltage is slightly higher for the Nafion membrane because of its excellent proton conduction and lower area resistance, but the discharge capacity for AIEMs is greater than that for Nafion, as shown in Figure . This could be due to the lower diffusion coefficient of VO2+ ions across AIEMs compared to that of the Nafion, as shown in Table . The VE values for all the membranes calculated during the charging and discharging processes of VRFB are found to be 87.05, 86.62, 84.51, and 87.78% for the ZWA, ZWMI, ZWMO, and Nafion membranes, respectively. The values of CE % for all the membranes calculated during the charging and discharging processes of VRFB were found to be 91.30, 90.50, 89.83, and 89.43% for the ZWA, ZWMI, ZWMO, and Nafion 117 membranes, respectively as shown in Table . Nafion 117 possesses good proton conductivity and hence bears high VE (Figure ). However, here, the reason for the low CE in the charging phenomenon at high current density is the high polarization of the cell at the high current density (Figure ). The higher values of CE and VE in case of AIEMs are due to the reduction in the cross-mixing of the redox pairs in the anolyte and catholyte, respectively, because of the Donnan exclusion phenomenon and lower WU.

Figure 6

Charging of VRFB with different AIEMs at a constant current density.

Figure 7

Discharging of VRFB with different AIEMs at a resistance of 1 kΩ.

Table 4

VE % and Current Efficiency (CE) % of Different AIEMs during VRFB Operation

membrane type	VE (%)	CE (%)
ZWA	87.05	91.30
ZWMI	86.62	90.50
ZWMO	84.51	89.83
Nafion	87.78	89.43

Figure 8

EE vs number of cycles with different AIEMs. Inset shows the CE vs number of cycles and VE vs number of cycles with different AIEMs.

Further, the discharge capacity retention of AIEMs was also tested after 10 repeated cycles, and the performances were evaluated in terms of EE, CE, and VE. It was observed that the decay rate for AIEMs is slower than that for Nafion. This is also again attributed to the resistance for the permeability of vanadium ions. ZWMO displays better consistency, which is 91.51%, whereas it is 86.54% for Nafion 117 after 10 cycles (Figure S-3). This kind of behavior of the prepared AIEMs delivers decisive capability toward VRB to convert chemical energy into electrical energy. This low-cost and ecofriendly method brings out a novel synthesis of AIEMs bearing stable EE, VE, and CE, which, when applied in VRFBs, offers an alternate of Nafion 117 substitution.[43]

The acid and VO2+ ion stabilities of the AIEMs were also tested for the application of membranes in VRFBs . To check the acid stability of the synthesized AIEMs, the pieces of 2 × 2 cm2 area were soaked in 5 M H2SO4 solution for 10 days, and the physicochemical properties were compared with that of the untreated membranes (Table ). It is clear from Table that a slight reduction in IC (2%), IEC (2%), and weight of membranes is subjected to the degradation in the functional groups (quaternary amine and sodium salt of sulfonic acid), but no degradation in the main chain was observed. A loss in weight of 1% was noted, which is smaller than that of the respective IC and IEC values, which also reveals the sustainability of the polymer backbone. An electrolytic solution of vanadium ions generally tries to degrade the polymers which bear hydrocarbon chains because of its strong oxidizing nature. It is better to check the stability of membranes by the conventional cycling method. However, for this, a whole setup is required, which is time-consuming. The second method to check the stability and durability is the soaking test in which membranes are immersed for a long interval of time into a predetermined concentration of the vanadium electrolytic solution, that is, 1.5 M VOSO4 in 3 M H2SO4. The stability of the prepared membrane was tested for 10 days, in which no remarkable change either in the morphology or stability was noticed. The physiochemical parameters show a minimal change with no color change in the membranes (Table ).

Table 5

Acid Stability and Vanadium Ion Stability of Different AIEMs

	% loss in different parameters, with the membranes exposed to 5 M H2SO4 and 1.5 M VO2+
	5 M H2SO4 10 days			1.5 M VO2+ in 3 M H2SO4 10 days
membrane type	IEC	weight	conductivity	weight	change in physical appearance
ZWA	2.47	1.54	2.27	0	slight change in color
ZWMI	2.34	2.09	1.75	0	no change
ZWMO	2.68	2.14	2.49	0	no change

Conclusions

Cross-linked semi-interpenetrating (SPIN)-type AIEMs were synthesized successfully by inserting hydrophilic groups in the form of sodium-p-styrene sulfonate and VBC into a rigid PVC backbone. The prepared AIEMs show relatively low IEC values than the theoretical IEC values because of the formation of ionic knots between the acidic and basic groups. The membranes exhibit good IC as well as mechanical and chemical stabilities. The prepared membranes were applied in VRFBs and compared with the state-of-the-art commercially available Nafion 117 membrane, whereas their performance was recorded in terms of VE % and CE %. The ZWMO membrane shows 87.05% VE and 91.30% CE. It shows the CR % value of 91.33%, which is higher than that of the Nafion 117 membrane. Methanol crossover resistance across the membrane was also performed. The ZWMO membrane shows the lowest methanol permeability value of 2.97 × 10–7 cm2 s–1 and selectivity of 2.43 × 105. The results suggest that the SPIN-type AIEMs can be a good candidate for applications in redox flow batteries and other energy devices.