A Quaternized Polysulfone Membrane for Zinc-Bromine Redox Flow Battery

A quaternized polysulfone (QNPSU) composite membrane is fabricated for zinc-bromine redox flow battery. The structure of the membrane is examined by FT-IR spectra and SEM.The conductivity of the membrane is tested by electrochemical analyzer. After a zinc-bromine battery with this composite membrane is operated at different voltage while charging and at different current while discharging to examine the performance of themembrane, it is found that the discharge voltage was 0.9672V and the power density was 6mW/cm at a current of 0.1 A, which indicated that the novel compositemembrane is a promisingmaterial for the flow battery.


Introduction
Recently, many people have focused on the zinc-bromine redox flow battery because it is considered highly fascinating for energy storage.Because Zn-Br flow battery has an advantage in cost [1], the cost of electrolyte, to a great extent, determines the overall cost of the battery.Zinc, as a very common metal, is of low cost and abundant.And the bromine can be extracted in sewage and swamp.Moreover, zinc-bromine battery is of high energy density and power density, as well as good charge and discharge properties [2][3][4][5].Although, up to now, too many experiments have been carried out, there are still a few technical problems unresolved [6,7].The problems are the fact that bromine diffusion towards the zinc electrode could not be completely worked out so far.In the zinc-bromine redox flow battery, the membrane with good properties in ionic conductivity, mechanical strength, and chemical stability is required [8][9][10][11][12].However, up to now, the fabrication of this kind of membrane is very challenging.
In this paper, we prepared a thin quaternized polysulfone (QNPSU) composite membrane for zinc-bromine redox flow battery, which, to our knowledge, has not been reported previously.QNPSU composite membrane provides good mechanical strengths with low cost.The initial results indicate that the new membrane is a promising material for zincbromine redox flow battery.

The Composition of Chloromethyl Ethyl
Ether.The procedure for chloromethyl ethyl ether preparation was as follows.Formaldehyde solution (50 g) was dissolved in the alcohol with a volume of 30 mL in a reaction kettle equipped with mixer, dropping funnel, and then thermometer.52.5 g phosphorus trichloride was added to the solution with magnetic stirring in ice bath.Temperature was controlled ranging from 30 ∘ C to 35 ∘ C and stirring continued for one hour.Keeping still for 10 minutes, the solution was separated into two layers.Upper solution was chloromethyl ethyl ether which is what we need.
The equation is given as follows: 2.2.Chloromethylated Polysulfone Preparation.The synthesis of Chloromethylated polysulfone (CMPFS) was prepared according to a modification of a published procedure [13].First, 7 g polysulfone was dissolved in 100 cm 3 (mL) 1,2dichloroethane with magnetic stirring at room temperature.0.5 g zinc chloride anhydrous was added to the abovementioned 10 mL chloromethyl ethyl ether.This solution was added dropwise into the reaction kettle and the reaction proceeded for 8 h at 70 ∘ C. The final reaction solution was light red.The white product was precipitated in hot water.The resulted step was dried in high vacuum to remove any residual solvent and kept in a vacuum before further use.The preparation of the chloromethylation of polysulfone is as shown in Scheme 1.

Preparation of CMPFS Composite
Membrane.5 g CMPFS was dissolved in 30 cm 3 (mL) N-methyl-2-pyrrolidone (NMP) and then the mixture was stirred at 60 ∘ C. Composite membranes of CMPFS were prepared by CMPFS-NMP solution evenly spreading on the glass for 30 min at 60 ∘ C and then dried at 100 ∘ C.

Preparation of the Quaternized Polysulfone Membrane.
The procedure for the preparation of the quaternized polysulfone membrane was as follows.The composite membranes of CMPFS were bathed in 100 cm 3 (mL) trimethylamine (TMA) till the membranes softened.Then the membrane was rinsed with copious amounts of deionised (DI) water, immersed in aqueous NaOH (1.0 mol/dm 3 ) solution for 12 h at 25 ∘ C, and then dried.
The preparation of the quaternized polysulfone membrane is as shown in Scheme 2.

Conductivity Measurement. Membrane conductivity was
measured with a four-point probe and frequency response electrochemical analyzer CHI604D (Shanghai Chenhua Instrument Co., Ltd.).The polymer membranes were held at the desired temperature and humidity for 0.5 h, to ensure that a steady state was achieved, and measurements were taken at 1 min intervals.
2.6.Battery Tests.The membranes were cut into 4 cm × 4 cm.High-density graphite plates are used directly as framework components and electrodes.There is a 5 mm depth slot on one side of graphite plates to make solution circulate smoothly.The molarities of zinc-bromine solution are about 4 mol/L so that the amount is nearly half of the optimal value not only for the conductivity but also for the bromine solubility [14][15][16][17].
The anode and cathode electrodes were filled with electrolyte through constant pumping.This research carried out the battery charging and discharging tests through battery testing system 7.5.X (Neware Co., Ltd.).The charge was conducted by a designated current, while the discharge was at a constant current.

FT-IR Analysis.
Figure 1 shows the infrared spectra of quaternized polysulfone membrane.We can find that S=O vibration peaks are at 1220 and 1325 cm −1 [18].The broad peak at around 3380 cm −1 was attributed by O-H and residual water.The quaternary ammonium group stretching vibration peaks are at 2970 cm −1 [19].The data suggests that there was a successful synthesis of quaternized polysulfone composite membrane.

Morphology of QNPSU Composite Membrane.
Figure 2 shows an image of the microporous structure of the composite membrane.From the dense structure of composite membrane cross-section in different magnification, we can see that the membrane cross-section was comparatively dense and no holes were produced.

Proton Conductivity of the Quaternized Polysulfone.
AC impedance diagram includes frequency, real part, and imaginary part.The real parts of impedance serve as the horizontal axis and the imaginary parts serve as the vertical axis; namely,  =   +   .The AC impedance curve is as shown in Figure 3. Internal resistance of the battery is about 0.5 Ω and the conductivity of the membrane is about 0.01 S/cm in 25 ∘ C with 100 RH%.

Ions Permeability Experiments.
Two different kinds of membranes are prepared for the permeability experiments, namely, commercial Nafion membrane and quaternized polysulfone composite membrane.Experimental procedure is as follows.20 mL bromine water with a certain molar concentration is contained in a sealed container and the bottom of the container is quaternized polysulfone composite membrane which is pasted onto the container to let ions cross through it.Then we put the container into a bottle and seal the bottle.After some time of nine days, we find some red liquid in the bottle and collect it; subsequently, we measure the volume of the collected liquid and the measurement result is 15.8 mL.In the same way, we change the commercial Nafion membrane instead of quaternized polysulfone composite membrane and repeat the above steps and another measurement data of the collected liquid is obtained, namely, 9 mL.Compared with the two different measured volumes, we find that quaternized polysulfone membrane is more useful to let the solution pass through it than the commercial Nafion membrane.So quaternized polysulfone membrane is more suitable for zinc-bromine redox flow battery.different charge voltage.Because the theoretical open circuit potentials for the battery were 1.82 V, the present charge test was carried out with a voltage slightly higher than 1.8 V. From the data, we see that the battery performance increased with the voltage increasing at the same temperature and pressure.This indicates that there was still a small degree of porosity in the membrane.This factor would need to be investigated in more detail in subsequent development of the membrane for redox flow battery applications.

Battery
Figure 5 shows polarization curves of the battery with quaternized polysulfone at different discharge voltage.As expected, battery voltage declined with current density increasing while discharging.For example, at a cell current of 0.1 A, the discharge voltage was 0.9672 V and the power density was 6 mW/cm 2 , while, at 0.15 A current, the voltage was 0.4724 V and the power density was 4.4 mW/cm 2 .The relatively good performance was mainly attributed to the high proton conductivity of the composite membrane.The major cause for voltage loss of the battery was associated with the performance of the electrode layers and solution.This indicates that the improvement of the zinc-bromine battery performance can be made through further development of the electrodes and solution.

Conclusions
A quaternized polysulfone (QNPSU) composite membrane has been fabricated for zinc-bromine redox flow battery.The structure of the membrane is examined by FT-IR spectra and SEM.The successful synthesis of membrane was comparatively dense and no holes were produced.Tested by electrochemical analyzer, the conductivity of the membrane is about 0.01 S/cm in 25 ∘ C with 100 RH%.A zinc-bromine battery with this composite membrane is operated at different voltage while charging and at different current while discharging.The battery performance of the membrane shows that the discharge voltage was 0.9672 V and the power density was

Figure 4 :Figure 5 :
Figure 4: Polarization curves of the battery with quaternized polysulfone membranes at different charge voltage.
Performance.Figure4shows polarization curves of the battery with quaternized polysulfone membranes at