Modification of Nafion „ Membrane via a Sol-Gel Route for Vanadium Redox Flow Energy Storage Battery Applications

Nafion 117(N-117)/SiO2-SO3Hmodifiedmembraneswere prepared using the 3-Mercaptopropyltrimethoxysilane (MPTMS) to react with H2O2 via in situ sol-gel route. Basic properties including water uptake, contact angle, ion exchange capacity (IEC), vanadium ion permeability, impedance, and conductivity were measured to investigate how they affect the charge-discharge characteristics of a cell. Furthermore, we also set a vanadium redox flow energy battery (VRFB) single cell by the unmodified/modified N-117 membranes as a separated membrane to test its charge/discharge performance and compare the relations among the impedance and efficiency. The results show that the appropriate amount of SiO2-SO3H led into the N-117 membrane contributive to the improvement of proton conductivity and vanadium ion selectivity. The permeability was effectively decreased from original 3.13 × 10 cm/min for unmodified N-117 to 0.13 × 10 cm/min for modified membrane.The IEC was raised from original 0.99mmol/g to 1.24mmol/g. The modified membrane showed a good cell performance in the VRFB charge/discharge experiment, and the maximum coulombic efficiency was up to 94%, and energy efficiency was 82%. In comparison with unmodified N-117, the energy efficiency of modified membrane had increased more than around 10%.


Introduction
Search of a higher efficiency, lower pollution, and greener alternative energy has become an important trend for rapid growth of nowadays global economy.Recently, scientists are actively involved in the exploitation of renewable, sustainable, and clean energy, such as wind turbine and photovoltaic, to produce clean and sustainable energy [1,2].However, power produced from those devices is fluctuating, and it is easily affected by the climate change.Consequently, electrical energy storage is needed to buffer the peak power on electrical grid.There are several available storage technologies, namely, hydropump, compressed air energy storage, and secondary batteries [3,4].Great accomplishment has been made to develop new types of redox flow storage battery (RFB) [5][6][7].RFB is a promising energy storage technology due to its low cost and long cycle life, which could be up to 13,000 cycles.
Vanadium redox flow battery (VRFB) is one of the most promising technologies for mid-to-large scale (KW-MW) energy storage, which was first put forward by Sum and Skyllas-Kazacos in 1985 [8]. High cycle life, low cost, reasonable efficiency, and safe operation of VRFB make it very attractive in many energy related applications, such as load leveling, peak shaving, and voltage stabilizing.VRFB is the most mature energy storage technology among others [6,9,10].A constant supply of V 2+ /V 3+ ions and VO 2+ /VO 2 + ions, dissolved in sulfuric acid, is provided to the negative and positive electrodes, respectively, through two pumps connected to external storage tanks, as illustrated in Figure 1.
VRFB is a flow battery where electrolytes are circulating between electrolytic cells and storage tank.During chargedischarge cycle, reaction of (4) took place on the electrodes.Positive half-cell reaction: Negative half-cell reaction: Overall reaction: The direction of above reactions is reversed, during charge-discharge cycle.Two electrodes are separated by a separation membrane.This membrane is electrical insulated and is highly ionic conductive.The H + ion is the major charge carrier inside the membrane.However, due to the electrical field and concentration gradient across the membrane, vanadium ions (VO 2+ , VO 2 + , V +2 , V +3 ) are also migrated/diffused through the membrane.The crossover of vanadium ions not only reduces the discharge cell voltage but also reduces its Faraday efficiency.During battery operation, the water is transported from one half-cell to the other half-cell by osmosis dragging and diffusion.The phenomena of water transfer cause dilution of one half-cell electrolytes and electrolyte concentration of the other half-cell.The water transport properties of Selemion CMV, AMV, and DMV (Asahi Glass, Japan) had been studied by Mohammadi et al. [11].Many other composite membranes were also fabricated and tested, including cationic exchange membranes [12][13][14], anionic exchange membranes [15][16][17], and amphoteric ion exchange membranes [18,19].Nowadays commercial available cationic separate membranes of Nafion based membranes have been developed by DuPont, USA.Although its film has high ionic conductivity, chemical stability, and thermal stability, it cannot overcome the penetration of vanadium ions, which caused the decrease of energy density of battery.
Separation membrane plays an important role in VRFB system.An ideal separated membrane should exhibit low vanadium ions diffusion (crossover) and high proton conductivity.Many modifications were made on Nafion membranes, either to decrease their vanadium permeability or to develop a new membrane with low cost and low vanadium permeability [20][21][22].Some investigations [23,24] have reported that the inorganic-organic hybrid route, sol-gel method, enables inorganic silica or TiO 2 particles to be led into the channel structure of Nafion membrane and then improve the cycle performance of the VRFB.In fact, it has been verified that both Nafion/SiO 2 and Nafion/TiO 2 hybrid membranes show nearly the same IEC and proton conductivity as that of pristine Nafion 117 (N-117) membrane.This could be a promising strategy to overcome the vanadium ions crossmixed.
Nevertheless, Vijayakumar et al. [25] had proposed explanation with many spectroscopic analyses, showing that SiO 2 is still present in the structure of Nafion-SiO 2 composite membrane even after 30 cycles of charge/discharge operation, but its ion diffusivity was similar as that of pristine N-117 membrane.Note that under the highly acidic condition porous SiO 2 material condenses, shrinks, and forms agglomeration, which in turn decreases the amount of interactions area between SiO 2 and Nafion, causing unbinding V 4+ ion transport via its usual pathway of reversible binding to the sulfonic acid groups of a side of Nafion channel walls.
Based on above deduction, we used the silica bonding sulfonic acid groups (SiO 2 -SO 3 H) with the sulfonated 3mercaptopropyl trimethoxysilane (MPTMS) to react with an oxidized reagent H 2 O 2 via in situ sol-gel route to modify Nafion membrane, expecting that the unbinding V 4+ ion can be bound with either SiO 2 or -SO 3 H group and avoid binding with the side -SO 3 H of Nafion membrane.It could improve the ion crossover and raise the proton conductivity to obtain better cell performances for the VRFB charge/discharge experiment.[23,24].The pretreated N-117 film was dried for 3 hours at 110 ∘ C and then soaked in a sulfonated MPTMS solution (MPTMS/EtOH/H 2 O 2 = 1 mL : 5 mL : 10 mL) in a two-neck reaction vessel with a mechanical stirrer.The N-117/SiO 2 -SO 3 H membrane was fabricated, and the hydrolysis/polycondensation reaction was allowed to proceed for 0.5 hours (NM-0.5H), 1 hour (NM-1H), and 24 hours (NM-24H), respectively.The formation of SiO 2 -SO 3 H via the solgel reaction was embedded to inside channel network of the Nafion membrane.

Experimental
The structural characteristics of membranes were analyzed using a Fourier transform infrared spectrometer FT-IR (U-2001, HITACHI, Japan) in transmission mode, wavelength ranging from 400 to 4000 cm −1 , with a 4 cm −1 resolution.Thermophysical properties of N-117 and N-117/SiO 2 -SO 3 H hybrid membranes were performed by a differential scanning calorimeter, DSC-Q10 (TA Instruments, USA), in a nitrogen atmosphere, at a heating rate of 10 ∘ C/min.Figure 2 is the spectra of VOSO 4 solution at four different concentrations, and the maximum absorption peak for VO 2+ ions is located at 766 nm.A calibration curve based on the results of Figure 2 was carried out in the concentration range from 0.005 to 0.15 M VOSO 4 solution, at wavelength  max = 766 nm, as shown in Figure 3.

Water Uptake and Contact Angle.
The water uptake is defined as the ratio of the weight of absorbed water to that of the dry membrane.The water uptake was calculated by (5).

Water uptake
where   is the weight of the wetted membrane after the membrane is immersed in H 2 O for 24 hours and   is the weight of the dry membrane.The contact angle between the water and the membranes was directly measured by a contact angle measuring instrument (FTA-125, APPR, Germany) for evaluation of their hydrophilic/hydrophobic properties.

Ion Exchange Capacity.
Ion exchange capacity was measured by the typical acid-base titration (inverse-titrated method).Membranes in acidic form were first immersed in excessive 0.1 M NaOH solution for 24 hours to exchange the fixed H + ions by Na + ions.The unreacted NaOH solution with membrane was inverse-titrated by 0.1 M HCl solution, and the IEC could be calculated by ( 6): where  NaOH × NaOH is the total mmoles of NaOH and  HCl ×  HCl is consumed moles by HCl solution inverse-titrated.7) and (8).
where A is area, L is thickness, R is electric resistance,  is conductivity, and R 1 and R 2 are the electric resistance of the cell with and without a membrane, respectively.as negative and positive electrolytes.The effective area of electrode was 5 × 5 cm 2 and the volume of the electrolytes solution in each half-cell was 30 mL and cyclically pumped into the corresponding half-cell.The charge and discharge test was carried out using a Battery Cycler Test System (WBCS3000S, WonATech, Korea) and CT2001C-10 V/2 A (Wuhan Land Co., China) between 0.8 and 1.8 V at a current density of 20 mA cm −2 .

Preparation of N-117/SiO 2 -SO 3 H Hybrid Membranes
3.1.1.The Syntheses of Sulfonated MPTMS and N-117/SiO 2 -SO 3 H Hybrid Membranes.MPTMS was oxidized by an oxidizing agent, H 2 O 2 in EtOH solution to form a strong sulfuric acid (-SO 3 H) group.The pH value of the sulfonated solution was reduced with increase of reaction time.Figure 4 shows that the pH of the solution decreases strongly for all H 2 O 2 contents in the first 400 s.The more complete the sulfonated reaction of MPTMS, the lower the pH value it reaches.This is due to the formation of sulfonic acid groups.The solution has the lowest pH value at a MPTMS/EtOH/H 2 O 2 volume ratio of 0.5 : 2.5 : 5.
The N-117/SiO 2 -SO 3 H hybrid membranes were prepared using the sulfonated MPTMS to react with an oxidized reagent H 2 O 2 via sol-gel method.The content of SiO 2 -SO 3 H increases with the increasing of sol-gel reaction time, as shown in Table 1.The SiO 2 -SO 3 H content of samples NM-0.5H,NM-1H, and NM-24H was 1.51 wt%, 1.91 wt%, and 1.99 wt%, respectively.The reaction times for NM-0.5H,NM-1H, and NM-24H were 0.5 h, 1 h, and 24 h, respectively.was confirmed by FT-IR spectra, as shown in Figure 5.The spectrum of the N-117 membrane has been reported in previous research [27].In comparing spectrum of N-117 with NM-1H, a new peak at 1110 cm −1 can be observed, which is attributed to the vibration of Si-O-Si groups [24,28,29].However, there are not any new peaks of N-117 membrane found in that region.In addition, Figure 5 (red line) shows there is one specific peak appearing at 3238 cm −1 .This peak is corresponding to the O-H stretching vibration of SO 3 H groups [28,29].

FT-IR
Furthermore, for both spectrums of N-117 and NM-1H, the band at 1309 cm −1 is from the antisymmetric CF 3 stretch, which appears overlapped with the antisymmetric and symmetric CF 2 stretching modes, around 1230 and 1150 cm −1 , and S=O antisymmetric and symmetric stretching bands from SO 3 H groups would appear at 1435 and 1320 cm −1 , respectively [24].There are no peaks found in 2500∼2600 cm −1 (stretching vibration of -SH group) for the NM-1H, which indicates the -SH groups of MPTS have been oxidized to sulfonic acid groups [27].Moreover, there is a stronger absorption peak at 1638 cm −1 for NM-1H membrane, showing that some physical-absorbed water may be present in the NM-1H membrane.That is associated with SiO 2 particles and either bulk water, (H 2 O)  , or highly hydrated oxonium ions, H 3 O + (H 2 O)  [25,27].Exo.groups which come from the condensation of Si-OH [30] and formation of crystalline SiO 2 particles and release of H 2 O for sample NM-1H.The unmodified N-117 has not shown any peaks in these regions.

Vanadium Ion (V 4+
) Permeability.The membrane was placed between two compartments, compartment A and compartment B, and vanadium concentration in compartment B ( B ) was measured after experimental time t from the calibration curve in Figure 3.
According to the previous literature [31], V 4+ permeability can be obtained by (9).t 0 is the initial time of experiment, and t 0 is equal to zero in this case. A0 is the initial vanadium concentration and P is permeability.A and L are the membrane area and thickness, respectively. B is the solution volume in compartment B.

Water Uptake, Contact Angle, IEC, and Other Properties of Membranes
. Table 1 lists basic properties of four membranes, N-117, NM-0.5H,NM-1H, and NM-24H, including water uptake, contact angle, IEC, and permeability.Water uptake is one of basic factors of ion exchange membrane.
An optimal amount of water uptake enables the membrane to achieve good ion conductivity.The membrane contains excessive water uptake which results in the vanadium ion cross-over mixing and even reduces its mechanical properties.The water uptakes of the modified N-117 membranes (NM-0.5H,NM-1H, and NM-24H) are in the range of 17 to 18 wt%, which are lower than that of N-117 which is 21 wt%.Contact angles were measured to evaluate the changes in the hydrophilic/hydrophobic properties.In general case, Nafion series membranes are water-swellable.When water droplets come into contact with membrane, the membrane was swollen by water, and a projection phenomenon could be observed on the surface of membrane as exhibited in Figure 8.The contact angles of the N-117/SiO 2 -SO 3 H membranes were in the range of 82 ± 2 ∘ to 86 ± 2 ∘ , which are slightly lower than that of N-117(88 ∘ ± 2).These hybrid membranes were slightly hydrophilic.
IEC is defined as the ability of H + proton to exchange between positive and negative electrolytes.The separate membrane with a high IEC will exhibit great proton conductivity for VRFB system.As listed in Table 1, the IEC for an unmodified N-117 and three modified membranes, NM-0.5H,NM-1H, and NM-24H, are 0.99 mmol/g, 1.23 mmol/g, 1.24 mmol/g, and 1.24 mmol/g, respectively.The ion exchange capacity increases with the reaction time and SiO 2 -SO 3 H from NM-0.5H to NM-1H.There is no further increase when the reaction time is extended to 24 hours.It can be found that the modified membranes have improved its IEC by using the sulfonated MPTMS.

Electrochemical Impedance Spectroscopy.
The membrane conductivity was measured with a single cell.A cationic exchange membrane (N-117) was used as both separator and reference for comparison with the modified membranes.The resistance, area resistance, and conductivity of membranes can be estimated from the electrochemical impedance spectroscopy data.The Nyquist plots and Equivalent circuit are Z  (ohm) often used to model the electrochemical system.Figure 9 is the Nyquist plots of the impedance data with the simulating Equivalent circuit model for N-117 and NM-1H sample.
The semicircle in high frequency region is concerned with the charge transfer process.The membrane resistance R s representing the resistance of electrolyte solution can be obtained from the intercept of the impedance curve on the -axis at frequency close to infinity, that is, at Z  = 0.The charge transfer resistance R s +  ct is related to across electrode/solution interface charge transfer process.The semicircle  ct is the charge transfer resistance of the electrode, as similar to R s , and can be obtained from the semicircle diameter at frequency close to zero and at Z  = 0 [32,33].
The semicircle in low frequency region represents the mass diffusion control process.This resistance is called Warburg resistance when the electrochemical system achieves in mass transfer control process.The Warburg resistance equals The R s value of membranes is in the order as NM-24H (36.40 mΩ), N-117 (23.70 m Ω), NM-1H (22.32 mΩ).The NM-1H modified membrane has a slightly lower area resistance (R A = 0.56 Ωcm 2 ) than that of N-117 (R A = 0.59 Ωcm 2 ).In general, a membrane with high area resistance exhibits low conductivity.

Single Cell Performance for VRFB System.
A single cell for VRFB was charged to 1.8 V and discharged to 0.8 V at a constant current density of 20 mA/cm 2 .The columbic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) are defined according to (10): where  charge and  discharge are charge/discharge capacity and  charge and  discharge are the middle point voltage of charge/discharge, respectively.The higher CE, meaning lower capacity loss, is mainly due to the lower crossover diffusion rate for vanadium ions and the side reactions.Higher VE  indicates lower battery resistances, which are referred to the electrodes, separator, flow path, and so on.The charge-discharge experiments were carried out by a single cell.Figure 10 displays the charge-discharge curves of the VRFB with a 2.0 M vanadium electrolyte and using N-117 and NM-1H modified membrane as a separator, respectively.Comparing the charge-discharge curves of VRFB between N-117 and NM-1H modified membranes, we can find that the discharge capacity of the VRFB with NM-1H modified membrane is higher than N-117 membrane.The relationships among the CE, VE, and EE are listed in Table 3.The CE and EE of the VRFB with NM-1H membrane are 94% and 82%, and the CE and EE of the VRFB for N-117 membrane are 82% and 71%, respectively.The VE of the VRFB is close to around 87% for both N-117 and NM-1H membrane.The VRFB with NM-1H modified membrane displays higher CE and EE than that of N-117 membrane.

Discussion
The investigation of the pH change for sulfonated MPTMS solution and the FTIR spectra in Figures 4 and 5 shows that the present condition of synthesis, at a MPTMS/EtOH/H 2 O 2 volume ratio of 1 : 5: 10, enables the MPTMS to be oxidized completely.
By full hydrolysis/polycondensation and in situ sol-gel route, the inorganic SiO 2 -SO 3 H groups are introduced into the channel network structure of N-117 membrane.The deposition of SiO 2 particle on the N-117 membrane can be confirmed by the presence of a new absorption band and a specific band, at 1110 cm −1 and 1638 cm −1 of the FTIR spectra, which are referred to Si-O-Si groups and physiabsorbed water associated with SiO 2 particles.
Furthermore, the stretching vibration absorption peak of -SH group in the MPTST disappeared in the range of 2500∼2600 cm −1 , and the specific surfonic acid bands in the range 900∼1000 cm −1 and 1300∼1400 cm −1 can be observed for the NM-1H modified membrane.It can be corroborated that the -SH groups of MPTS have been oxidized to the sulfonic acid groups.Summarizing these FT-IR spectra and DSC data, it can be established that the N-117/SiO 2 -SO 3 H modified membranes could be successfully synthesized via the sol-gel reaction.
By combining the results from the basic properties of membranes in Table 1, the contact angle of NM-1H membrane, 86 ∘ ± 2, is similar to that of N-117, 88 ∘ ± 2. This can be explained as follows: the strong hydrophilicity of sulfonated group can reduce the contact angle, but the hydrophobicity of silica group will cause the contact angles to increase, under the cross effect between the hydrophilic and hydrophobic groups; the contact angles of hybrid membranes are similar to that of N-117.The water uptake of the hybrid membranes is based on above reasons, too.
In addition, Table 1 shows that the permeability of modified membrane reduces obviously with the reaction time, and the SiO 2 -SO 3 H amounts are increased with reaction time.However, the formation of SiO 2 -SO 3 H via the sol-gel route grafting on the channel network of the Nafion membrane reaches equilibrium after reacting around one hour; hence the permeability of NM-1H sample, 0.13 × 10 −6 cm 2 /min, is similar to that of NM-24H sample, 0.12 × 10 −6 cm 2 /min.It is clear that the permeability was effectively decreased for modified membrane, because SiO 2 -SO 3 H particles result in the polar clusters inside the Nafion membrane and hinder the vanadium ion diffusion.In fact, these results verified that both Nafion/SiO 2 and Nafion/TiO 2 hybrid membrane show nearly the same IEC and proton conductivity as that of pristine N-117 membrane.The results of this research suggest that this approach is a promising strategy to overcome the vanadium ions cross-mixed problem.
From Table 1, it can be proved that the IEC of N-117/SiO 2 -SO 3 H modified membrane was raised more, and the permeability was reduced stronger than those of other references [23][24][25].The outcomes can be attributed to the introduction of silica and surfonic acid groups into the pore cluster of N-117.
In the same case, resistance and conductivity data of N-117/SiO 2 -SO 3 H modified membrane for the EIS test of VRFB shown in Table 2 indicate the proton conductivity of NM-1H modified membrane is higher than that of N-117, signifying that the increasing proton conductivity is referred to the SiO 2 -SO 3 H immobilized in the network channel of N-117.At the same time, the NH-24H modified membrane has the lowest proton conductivity.As shown in Table 1, the IEC of NH-24H membrane is the same as that of NM-1H membrane, implying that the equivalent of sulfonic acid for both NH-1H and NH-24H is equal.However, the SiO 2 -SO 3 H amount of NH-24H membrane, 1.99 wt%, is larger than that of NM-1H, 1.91 wt%, inferring that the SiO 2 content of NH-24H is more than that of NM-1H, but its -SO 3 H groups are similar to that of NM-1H.It can be deduced that the higher resistance for NH-24H sample was ascribed to the SiO 2 particles.
Furthermore, as shown in Figure 10 and Table 3, test data of a vanadium redox single cell performance, it can be seen that the columbic efficiency of the single cell using NM-1H as a separator was increased from 82% to 94%; fortunately, the voltage efficiency was not reduced, holding at 87%.Related references [23][24][25] revealed that the voltage efficiency of modified membrane was reduced because of the higher area resistance of SiO 2 or TiO 2 inorganic particles.
The EE of the NM-1H modified membrane is higher than that of unmodified N-117.It could be attributed to the -SO 3 H groups of NM-1H membrane having improved the proton conductivity and the SiO 2 -SO 3 H groups having restrained vanadium ion cross-mixed, especially shown in the increase of CE.

Conclusions
The present condition of synthesis, at a MPTMS/EtOH/H 2 O 2 volume ratio of 1 : 5 : 10, enables the MPTMS to be oxidized completely to form a sulfonated SiO 2 -SO 3 H functional group.The N-117/SiO 2 -SO 3 H modified membranes could be synthesized using an oxidation reaction and a simple sol-gel route.It can be confirmed by FT-IR spectra and DSC that the SiO 2 -SO 3 H groups can be successfully grafted on the N-117 network structure to form modified membranes.The permeability was effectively decreased and the ion exchange capacity was raised obviously for modified membrane.It is mainly attributed to its SiO 2 -SO 3 H particles which hinder the diffusion of vanadium ion and then improve the transfer of H + proton.
In the EIS test of VRFB, the improvement of proton conductivity can be attributed to the introduction of SiO 2 -SO 3 H into the Nafion channel network; however, excessive SiO 2 in the modified membrane would cause the resistance to increase.The NM-1H modified membrane has a lower R s , lower area resistance, and higher proton conductivity than those of unmodified N-117 membrane.Hence, the NM-1H sample exhibits a superior cell performance than the pristine N-117 in the VRFB charge/discharge experiment.The maximum coulombic efficiency is up to 94%, and energy efficiency is 82% for the NM-1H modified membrane.

Figure 1 :
Figure 1: Schematic of a vanadium redox flow battery.

9 Figure 4 :
Figure 4: The reaction time dependence of pH value for the sulfonated MPTMS solution.
Spectra of Membranes.The structural comparison of N-117/SiO 2 -SO 3 H and pretreated N-117 membranes

Figure 5 :
Figure 5: FT-IR spectra and for the pretreated N-117 and NM-1H modified membranes.

Figure 6 :Figure 7 :
Figure 6: DSC thermogram of N-117and NM-1H modified membranes, in a nitrogen atmosphere at a heating rate of 10 ∘ C/min.

Figure 8 :
Figure 8: Water drops into the NM-1H membrane; after 30 secs, the membrane was swollen and caused a projection phenomenon.

Figure 9 :
Figure 9: Nyquist plots of the impedance data for N-117 and NM-1H membranes.

Figure 10 :
Figure 10: Charge-discharge curves of single cell performance for VRFB by N-117 and NM-1H membrane, respectively, as a separator; a current density is 20 mA cm −2 .

2 O 2 solution at 80 ∘ C for one hour, followed by washing with deionized H 2 O for 30 minutes at 80 ∘ C, then immersed in 1 M H 2 SO 4 solution for 1 hour at 80 ∘
[26]rate change of the vanadium ion in VOSO 4 solution absorbance is used to calculate the diffusion coefficient, that is, permeability, according to Fick's First Law and Beer's Law[26].The diffusion cell has two compartments, compartment A and compartment B. The former was filled with 50 mL of 2.0 M VOSO 4 /2.0 M H 2 SO 4 solution, and the latter was filled with 50 mL of 2.0 M MgSO 4 /2.0 M H 2 SO 4 solution.

Table 1 :
Comparison of basic properties of N-117 and N-117/SiO 2 -SO 3 H membranes.

Table 2 :
Comparison of resistance and conductivity for N-117 as well as modified membranes.
dl  2 −   −  ct , where  dl is electric double layer capacity and  is Warburg constant.The resistance (R s and R A ) and conductivity of N-117 and modified membranes were summarized in Table2.

Table 3 :
The single cell efficiency for VRFB system based on the pristine N-117 membrane and modified NM-1H membrane.