Approach to high proton/vanadium selectivity in an ionic exchange membrane via a hyperbranching design

Abstract

A series of non-perfluorinated sulfonated polyamide hyperbranched polymers (HBP) were synthesized via a one-step polycondensation reaction for their application as ion exchange membranes for vanadium flow batteries. The homogeneous HBP membranes were manufactured via a solution casting method to exhibit high proton/vanadium (H⁺/V) selectivity up to 14.32 × 10⁴, four times that of commercial Nafion 117 membrane, resulting from their high ion exchange capacity and hyperbranched design. At the same time, their great mechanical and thermal properties indicated that the HBP membranes are highly stable, and such HBP membranes with a hyperbranched structure are promising candidates for application in vanadium redox flow batteries.

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1. Introduction

The vanadium redox flow battery (VRB) has received considerable attention due to its easy design, high energy efficiency, long life cycle and environmentally friendly operation.¹ The ion exchange membrane (IEM) is one of the key components of a VRB system because it provides proton conductive channels and separates the positive and negative electrodes. Therefore, IEMs for VRB application must possess high proton conductivity, low vanadium ion permeability and satisfactory mechanical and chemical stability.^2–5 Nafion membranes currently dominate the VRB market due to their great overall performance;^2,6,7 however, they suffer from poor proton/vanadium (H⁺/V) selectivity, defined as the ratio of proton conductivity to vanadium ion permeability,^8,9 which results in decaying voltage efficiency, coulombic efficiency and energy efficiency of the Nafion equipped VRB.^3,10–14 Great efforts have been made towards the modification of Nafion membranes in order to improve H⁺/V selectivity.^15–17

Various low cost, novel polymeric IEMs have also been developed to replace Nafion membrane for VRB application.^18–21 Polymeric IEMs with sulfonic acid groups pending on the side chains improved the H⁺/V selectivity by increasing the proton conduction efficiency.^22–26 However, the trade-off between proton conductivity and mechanical strength of the linear sulfonated polymeric IEMs restricted their practical application in VRBs.²⁷ There are several reported studies of block copolymers as membranes for use in VRBs.^28–31 Wang et al.²⁸ prepared an amphiphilic block copolymer membrane with improved H⁺/V selectivity, but lower proton conductivity as compared to Nafion 117. Grafting techniques, in which side chains are bonded to the main polymer backbone, are widely used as alternative methods to prepared IEMs for VRB application.^32–37 Qiu et al. prepared a grafted IEM that exhibited significantly lower VO²⁺ permeability, but the conductivity was much lower than that of Nafion 117.³⁶ The trade-off among proton conductivity, mechanical strength and VO²⁺ permeability can be more or less avoided in the cross-linked polymeric IEMs^3,38–43 and branched polymeric IEMs.⁴⁴ At the same time, tightening the molecular structure of these two types of IEMs can effectively block vanadium ion permeation. Unfortunately, it was difficult to achieve high ion exchange capacity (IEC) in these cross-linked and branched polymeric IEMs and the H⁺/V selectivity was limited.

In this study, a series of the hyper-branched polyamides (HBP) were designed to achieve high H⁺/V selectivity via a unique molecular structure design. As illustrated in Scheme 1a, vanadium ions were too large to cross the tight framework of the HBP macromolecules (Scheme 1b). As shown in Scheme 1b, the HBP were directly polymerized from two small monomers to form an A2B3 hyperbranched structure. The space in the HBP macromolecules is too small for the vanadium ions to transfer through. At the same time, ultra-high IEC values (higher than 2.3 mmol g⁻¹) were obtained due to the unique molecular structure of HBP. Compared to the Nafion 117 membrane, the hyperbranched membranes exhibited almost doubled proton conductivity (0.28 S cm⁻¹ at 80 °C) and tripled vanadium permeation resistivity (less than 8 × 10⁻⁷ cm² min⁻¹). Along with their high H⁺/V selectivity, the HBP membranes also displayed satisfactory mechanical and chemical stabilities, indicating a promising practical application in VRBs.

Scheme 1 (a) Schematic of proton conduction and vanadium permeation resist through the HBP membrane; (b) Synthesis route of the HBP macromolecules.

2. Experimental

2.1 Materials

2,4-Diaminobenzenesulfonic acid (DSA, Adamas Reagent, Ltd.), trimesic acid (TA, Shanghai Dibo Chemical Reagent Co., Ltd.) and anhydrous lithium chloride (LiCl, Shanghai Qiangshun Chemical Reagent Co., Ltd.) were dried under vacuum at 80 °C for 24 h before use. N-Methyl-2-pyrrolidone (NMP) and triphenylphosphite (TPP) were purchased from Sinopharm Chemical Reagent Co., Ltd., and were used as received. Pyridine (Py Shanghai Lingfeng Chemical Reagent Co., Ltd.) was purified before use by distillation from KOH.

2.2 HBP synthesis

In accordance with Scheme 1b, TA (m mmol), p-DSA (n mmol, where m : n = 1.05 for HBP1, 1.02 for HBP2 and 1.11 for HBP3), LiCl (0.32 g), TPP (1 mL), Py (3 mL) and NMP (4 mL) were combined in a 50 mL two-necked round-bottom flask. The reaction mixture was kept at 100 °C for 2 h with stirring under argon atmosphere. The resulting polymers were washed repeatedly with methanol and deionized water. The final products were dried under vacuum at 80 °C for 24 h.

2.3 Membrane preparation

Membranes of ca. 100 μm thickness were prepared using a traditional solution-cast technique by casting uniform solutions of HBPs in dimethyl sulfoxide (DMSO) onto clean glass plates and drying at 80 °C. The dried membranes were removed and immersed into 1 M HCl solution for 12 h at room temperature, and then washed repeatedly with deionized water to neutral pH (7).

2.4 Characterizations

¹H nuclear magnetic resonance (NMR) spectra for the polymers were recorded using a Bruker AVANCE III HD spectrometer (400 MHz) in deuterated dimethyl sulfoxide (DMSO-d₆) solution. FTIR spectra of the synthesized polymers were collected on a Shimadzu IR Prestige-21 instrument. Thermogravimetric analyses (TGA) of the polymers were carried out using a NetzschSTA 409 PC TG-DTA instrument, under nitrogen atmosphere with a heating rate of 10.00 °C min⁻¹ at a temperature range from 30 °C to 800 °C. Cross-sectional images of the membranes were obtained via field emission scanning electron microscopy (SEM, SU8010, Japan). Tapping mode atomic force microscopy (AFM) analysis of the HBP membranes was performed with a Digital Instruments Multimode Atomic Force Microscope (AFM, Model AJ-III). The viscosity of 1 wt% HBP solutions in DMSO was measured using a rheometer (HAAKE RheoStress RS600) at 25 °C. Shear viscosity and shear stress were recorded over a range from 0 to 1000 s⁻¹. Measurement of the tensile stress of the wet membranes was performed on Labthink XLW (PC) at the strain rate of 25 mm min⁻¹ at room temperature.

The chemical stability of the HBP membranes was also examined. A sheet of each membrane was immersed in a 3 mol L⁻¹ H₂SO₄ solution with concentration of 0.1 mol L⁻¹ VO₂⁺ for 7 days at room temperature. The membrane was then washed repeatedly with deionized water until pH 7 and dried at 60 °C for 12 h. The chemical stability was evaluated by recording the weight of the membrane before and after soaking in solution; and the weight loss was calculated according to eqn (1).

where W₀ and W represent the weight of the membrane before and after immersion in VO₂⁺ H₂SO₄ solution.

Proton conductivities of the hydrated membranes were measured by the four-probe method using an electrochemical workstation CHI 604B (CH Instruments Inc.) with AC impedance method over a frequency range of 1–10⁵ Hz and 5 mV oscillating voltage. Proton conductivity was measured at a temperature range of 25–80 °C with a relative humidity of 100%. Proton conductivity of the hybrid membranes was calculated according to the following equation:

where σ is the proton conductivity (S cm⁻¹), d is the distance between the two electrodes, t and w are the thickness (cm) and width (cm) of the membranes, respectively, and R is the resistance (Ω) from the measured impedance data.

IEC values of the membranes were measured using the classical acid–base titration method. The membrane amounting to its dry weight was immersed in 1 M NaCl solutions overnight at room temperature to liberate the H⁺ ions. The replaced protons in the solutions were titrated with a 0.01 M NaOH solution using phenolphthalein as the indicator. The IEC values were calculated using the following equation from the titration data:

where C (M) is the concentration of the 0.01 M NaOH solution, V (mL) is the volume of the NaOH solution and W is the weight (g) of the dry membrane.

Vanadium ion permeation measurements were carried out by an “H” type diffusion test cell method using two glass cells separated by the membranes at room temperature. The diffusion cell was filled with 0.5 M VOSO₄ in 2 M H₂SO₄ aqueous solution and the sampling cell was filled with the same volume of 2 M H₂SO₄ aqueous solution. The two cells were magnetically stirred throughout the experiments at room temperature. Samples were taken at regular time intervals, and the concentration of vanadium ions in the sample side was measured using a UV-vis spectrometer (Specord 200, Germany).

Water uptake and volume swelling ratio of the hydrated membranes were determined by immersing the membranes in deionized water at room temperature overnight. Water on the membrane surface was wiped off and the wet weight and volume of the membranes were quickly measured and recorded. The membranes were then dried at 80 °C under vacuum for 12 h and the dried weight and volume were immediately measured and recorded. Water uptake and volume swelling ratios of the membranes were calculated using the following equations:

where W_wet and W_dry are the weight of wet and dry membranes, respectively. V_wet and V_dry are the corresponding volumes of the wet and dry membranes.

3. Results and discussion

The structures of the three synthesized HBP macromolecules were confirmed via ¹H NMR spectra, as shown in Fig. 1a, and FT-IR spectra, as shown in Fig. 1b. The –COOH signal appeared at 11.9 ppm and a –CONH signal was observed at 10.8 ppm in the ¹H NMR spectra of the HBPs. The bands at 1740 cm⁻¹ and the absorption peaks at 945 cm⁻¹ in the FT-IR spectra of the HBPs were attributed to the stretching vibrations of C=O and the bending vibrations of O–H in –COOH, respectively.

Fig. 1 (a) ¹H NMR spectra and (b) FTIR spectra of HBP macromolecules.

Proton conductivity and VO²⁺ permeability of the HBP membranes were compared with those of the Nafion 117 membrane in Fig. 2a and b. All of the three prepared HBP membranes performed better in terms of proton conductivity compared with the Nafion 117 membrane over the entire temperature range (Fig. 2a). As the best among the three membranes, the HBP1 membrane exhibited proton conductivity as high as 0.28 S cm⁻¹ at 80 °C, which was much higher than that of the Nafion 117 membrane (0.19 S cm⁻¹ at 80 °C). The proton conductivity of the HBP membranes was also higher than most of the reported alternative IEMs for VRBs.^8,20,45–48 The superior proton conductive performance of the HBP membranes was easily be attributed to their higher IEC values compared to Nafion 117 (Table 1), as well as their specially designed microstructure; this can be attributed to the following reasons: first, very high ion exchange capacities greater than 2.3 mmol g⁻¹ (Table 1) were achieved for the HBP membranes' first order proton conductive channels (PCCs) due to the high –SO₃H density in the monomers and their hyperbranched molecular structure. Second, the HBP molecules were tightly linked to each other via hydrogen bonds among the end-capped –COOH groups; therefore, narrow channels were formed (second order PCC) for the transportation of the hydrated protons. The tapping mode phase image (Fig. 2c) showed clear nano-scale phase separation in the HBP membranes, particularly for the HBP1 membrane, which was consistent with the superior proton conductivity. In addition, the activation energies of proton conductivity for the HBP membranes were calculated to be similar to that of the Nafion 117 membrane, as shown in Table 1.

Fig. 2 (a) Arrhenius plots of proton conductivity for the HBP membranes and the Nafion 117 membrane; (b) VO²⁺ concentration–time curves of the HBP membranes and the Nafion 117 membrane; (c) AFM tapping phase image (1500 nm × 1500 nm) and (d) cross-sectional SEM images of HBP1, HBP2 and HBP3 membranes.

Table 1 IEC, activation energy, proton conductivity, VO²⁺ permeability and H⁺/V selectivity of HBP membranes and Nafion 117 membrane

Membranes	HBP1	HBP2	HBP3	Nafion 117
IEC (mmol g^−1)	2.33	2.46	2.34	0.91
Activation energy (kJ mol^−1)	15.27	16.34	14.12	17.12
Proton conductivity at 30 °C (S cm^−1)	0.113	0.112	0.090	0.087
VO^2+ permeability (10^−7 cm^2 min^−1)	7.89	7.90	7.86	23.95
H^+/V selectivity	14.32 × 10^4	14.18 × 10^4	11.45 × 10^4	3.63 × 10^4

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At the same time, the hyperbranched structure of the HBP molecules, as well as the narrow second order PCC among the HBP molecules made it difficult for the large-sized vanadium ions to cross over the membranes. As a result, VO²⁺ concentrations in the sampling cells for all three HBP membranes appeared to be remarkably lower than that of the Nafion 117 membrane during the entire diffusion time (Fig. 2b). The excellent combination between the first and second order PCCs resulted in the compact net structure that was observed in the SEM micrographs of the membranes (Fig. 2d). The VO²⁺ permeability values of the three HBP membranes were calculated to be 7.89 × 10⁻⁷, 7.90 × 10⁻⁷ and 7.86 × 10⁻⁷ cm² min⁻¹, respectively, normalized for membrane thickness. These values were substantially lower than those of the Nafion 117 membrane (23.95 × 10⁻⁷ cm² min⁻¹), as shown in Table 1. This vanadium permeability is also lower or similar to the reported alternative IEMs with similar thickness.^20,46

With both superior proton conductive property and vanadium permeation resistivity versus the Nafion 117 membrane, H⁺/V selectivity of the HBP membranes at room temperature was calculated as shown in Table 1. Generally speaking, H⁺/V selectivity can be used to assess the performance of proton conductivity and VO²⁺ permeability, with higher H⁺/V selectivity demonstrating better performance for VRB applications.⁴⁹ Not surprisingly, all three HBP membranes exhibited much higher H⁺/V selectivity compared to that of the Nafion 117 membrane. The H⁺/V selectivity of the HBP membranes was ca. 4 times higher than that of the Nafion 117 membrane, with the HBP1 membrane showing the highest H⁺/V selectivity; this may indicate a better VRB performance based on HBP membranes than on Nafion 117 membrane.

In addition to conductivity, water uptake of the IEM also strongly depends on the IEC of the membrane.^10,50 Although appropriate water in the IEM is beneficial for the proton conduction because water is considered as proton carrier according to the Grotthuss mechanism,^21,51,52 excess water in the membrane will cause poor dimensional stability.^53,54 Table 2 depicts the water uptake and volume swelling of the HBP membranes. The HBP membranes showed water uptake ratios of 31.71%, 34.22% and 34.42%, respectively, all of which were significantly higher than the water uptake of the Nafion 117 membrane. Conversely, the volume swelling ratios of the three HBP membranes were lower than those of the Nafion 117 membrane, indicating better dimensional stability. The improved dimensional stability was attributed to hydrogen bonding between –COOH groups and the hyper-branched structure of the HBP macromolecules.

Table 2 Water uptake and volume swelling of the membranes

Membranes	HBP1	HBP2	HBP3	Nafion 117
Water uptake (%)	31.71	34.22	34.42	21.03
Volume swelling (%)	27.48	28.86	22.86	36.29

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To evaluate the thermal stability of the HBP membranes, thermogravimetric analyses (TGA) was conducted (Fig. 3a). The initial weight loss at about 150 °C was ascribed to the evaporation of adsorbed solvent and water, while the weight loss at 270–290 °C was attributed to cleavage of the sulfonic acid groups from the HBP polymer. The final weight loss at around 600 °C was the result of decomposition of the HBP polymer main chain.⁵⁵ The TGA study provided evidence of sufficient thermal stability for the intended VRB application. In addition to thermal stability, mechanical stability is also very important for practical VRB operation. Stress–strain curves of the HBP membranes in the wet state are shown in Fig. 3b. Tensile strength of the HBP1 membrane was high, at 18 MPa, which was approximately equal to the tensile strength of the Nafion 117 membrane. The great mechanical stability of the HBP1 membrane is contributed by the hyperbranched structure and hydrogen bond between the –COOH groups among the HBP molecules within the membrane. As the designing molecular weight of the HBP macromolecules increased, reflected by the shear viscosity of the HBP solution in Fig. 4, elongation at the break of the HBP membrane strongly decreased; this was particularly true for the HBP3 membrane, due to its relatively low content of end-capping –COOH groups, which reduced the degree of hydrogen bonding. Ultimately, the mechanical stability of all three HBP membranes, particularly the HBP1 and HBP2 membranes, satisfied the application requirements for VRBs.

Fig. 3 (a) TGA curves and (b) stress–strain curves of the HBP membranes.

Fig. 4 Shear viscosity (blue circle) and shear stress (black cube) of (a) DMSO solvent, (b) 1 wt% HBP1 DMSO solution, (c) 1 wt% HBP2 DMSO solution and (d) 1 wt% HBP3 DMSO solution measured at 25 °C.

The mechanical strength of the HBP membranes can also be related to the shear viscosity of the HBP polymers (Fig. 4). With only 1 wt% HBP polymer dissolved in DMSO solvent, shear viscosity of the solution was enhanced by two orders of magnitude compared to the original DMSO (Fig. 4a), indicating the high molecular weight of the HBP molecules. The HBP3 DMSO solution exhibited the largest shear viscosity among the three polymers, indicating a substantially greater molecular weight of HBP3. The proportion of –COOH, among which hydrogen bonds were built to form the membrane, was therefore concluded to be too small in HBP3 for the membrane to achieve great mechanical strength.

The chemical stability of HBP and Nafion 117 membranes was measured via weight loss after 7 days of immersion in VO₂⁺ H₂SO₄ solution. The results are shown in Table 3. The Nafion 117 membrane exhibited outstanding chemical stability, with an extremely low weight loss of 0.01%. The chemical stability of the three HBP membranes was about 6%, similar to the traditional non-fluorine IEMs,⁵ with the HBP1 membrane exhibiting the best chemical stability among the three.

Table 3 Chemical stability of the HBP membranes and Nafion 117 membrane

Membranes	HBP1	HBP2	HBP3	Nafion 117
Weight loss (%)	5.67	5.91	6.24	0.01

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4. Conclusion

In summary, we successfully synthesized a new type of non-perfluorinated polyamide with a hyperbranched structure. The prepared HBP membranes performed much better than the commercial Nafion 117 membrane in terms of both proton conduction and vanadium-permeation resist. In other words, high H⁺/V selectivity was achieved via this facile design. By considering the superior thermal and mechanical stability and satisfactory chemical stability, the HBP membranes, particularly the HBP1 membrane, were promising alternatives to the Nafion IEM for VRB application.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (No. 21503197, 21233006 and 21473164) and Fundamental Research Funds for the Central University, China University of Geosciences (Wuhan) (No. CUG150627 and CUG150615).

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