Fabrication of a proton exchange membrane via blended sulfonimide functionalized polyamide
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A novel linear non-fluorinated sulfonimide functionalized polyamide (SPA) polymer electrolyte was successfully synthesized via an aromatic sulfonimide monomer with superior thermal stability and superacidity. The aromatic sulfonimide remains stable below 220 °C. To fabricate membranes with strong mechanical strength and dimensional stability, the polymer was blended with various quantities of PVdF. The PVdF/SPA blend membranes exhibit an excellent capacity of water uptake and high dimensional stability. However, their proton conductivity was found to be substantially lower than that of Nafion 211. Analysis on the SEM images of the PVdF/SPA blend membranes reveals that the low proton conductivity is primarily caused by the large pore structures (>1 μm), which lead to breakdown of the continuous proton transport channels.
KeywordsPolymer Electrolyte Proton Exchange Membrane Fuel Cell Thermo Gravimetric Analysis Sulfonic Acid Group High Proton Conductivity
Proton exchange membrane (PEM) fuel cell technology is considered as one of the key technologies for the incipient hydrogen economy [1, 2]. With an energy conversion efficiency of 40–60 %, PEM fuel cells convert hydrogen into electricity directly with water as the only byproduct [3, 4]. In general, a PEM plays two important roles in a fuel cell by transporting hydrated protons and blocking the mass crossover between the anode and the cathode. While commercial PEMs based on perfluorosulphonic acid polymers, of which Nafion® is viewed as the “gold standard”, often display high proton conductivity, excellent chemical stability, strong mechanical strength and flexibility, and potentially, long-term durability , they suffer from fuel permeation, electro-osmotic drag of water, reduced proton conductivity under relatively low humidity, environmentally unfriendliness, and high cost . Furthermore, degradation of Nafion® membranes during fuel cell operation was also found to be a serious issue for stable fuel cell performance . It has been confirmed that the membrane degradation is attributed to the chemical attack by hydroxyl free radicals produced from hydrogen peroxide, which is generated catalytically by metal impurities [8, 9]. The “weak” sites of Nafion®, such as the polymer end groups (–CF2COOH) produced during membrane processing, are most accessible for the attack.
Chen et al. . investigated Nafion® membrane degradation using X-ray photoelectron spectroscopic technique. A decomposed product of cross-linking S–O–S was detected, indicating there is an additional “weak” site, –SO3H, during the operation of PEMFC. In general, the sulfonic acid groups in sulfonated PEMs may also undergo degradation, which further affects the performances of fuel cells. To date, the majority of PEMs that have been reported depends on sulfonic acid to promote conductance of protons [10, 11, 12, 13]. To avoid membrane degradation, new functional groups, such as phosphoric acid or sulfonimide group, to be utilized in the membrane materials as proton carriers, appear to be highly attractive alternatives.
One of the functional groups, CF3SO2–, is known to be notoriously electron withdrawing, which enables the negative charge on the anionic species, N−(SO2CF3)2 (TFSI), to be significantly delocalized. This unique property allows the TFSI salt to be widely utilized in lithium ion batteries [14, 15, 16] and ionic liquids [14, 15, 16, 17, 18, 19, 20, 21]. Most recently, this functional group was also used in single ion conductive polymer electrolytes because the compounds yield high Li+ ion transfer numbers arising from the high degree of charge delocalization and the anion immobilization with the anionic species being covalently bonded to the polymer backbones [22, 23, 24, 25, 26, 27, 28]. In addition to the superior electrochemical performance, these materials also display excellent thermal and chemical stabilities. Moreover, extensive efforts have been devoted to the development of sulfonamide-based polymer membranes for potential replacement of Nafion as PEMs because of their strong gas-phase superacidity, excellent thermal stability, capability to promote oxygen reduction kinetics at cathode, and weak adsorption on platinum catalyst [29, 30, 31, 32, 33, 34, 35, 36, 37, 38]. Summer and co-workers  developed a class of polymers with a similar structure of Nafion but the sulfonic acid group is replaced by a sulfonimide group. This sulfonimide substituted membrane exhibits proton conductivity two-fold higher than that of Nafion 117. It also displays higher water uptake (WU) than that of Nafion 117 facilitated by the internal pores and/or the channel structures enabled by the sulfonimide acid groups. Nevertheless, utilization of the trifluoromethane group in the sulfonimide functionalized PEMs may result in membrane degradation and higher material costs.
To overcome the shortcoming but at the same time retain the good proton conductivity of Nafion, we designed and synthesized a non-fluorinated sulfonimide based PEM with tunable structure and properties. A non-fluorinated sulfonimide precursor, 4,4′-dicarboxyldiphenylsulfonimide, was first synthesized. With high thermal stability up to 320 °C and strong acidity, the compound can be used to fabricate linear, comb-like or network structured SPA polymer electrolytes by taking advantage of the two carboxyl acid linkers. In this paper, we report synthesis and characterizations of a linear aromatic SPA based polymer electrolyte with a rigid structure. In order to fabricate mechanically strong membranes, a polyvinylidene fluoride (PVdF) was employed as a binder. The main properties of the blend membranes, specifically, the mechanical strength, WU and swelling, ion exchange capacity (IEC) and proton conductivity, were investigated and compared with the corresponding values of Nafion 211. The blend membranes display a higher water retention rate since sulfonimides interact with water more strongly than sulfonic acid . Moreover, they also show good thermal stability up to 220 °C and better swelling property. However, these membranes exhibit lower proton conductivity than that of Nafion 211 arising from the interrupted internal pores and channel structures due to the poor compatibility between the flexible structure of the aliphatic PVdF binder and the rigid structure of the aromatic SPA polymer, based on the SEM analysis. The result provides useful information on how to design performance-enhanced blend membranes with tunable macro/micro porous structures through selecting appropriate structurally compatible polymers.
4,4′-Diaminodiphenylsulfone (DADS), p-toluenesulfonamide and p-toluenesulfonylchloride were purchased from Sigma Aldrich. Pyridines, triphenylphosphite (TPP), KMnO4 and dimethyl sulfoxide (DMSO) were used as received without further purification. DADS was dried at 100 °C for 24 h, Pyridine was dried with KOH and distilled. N-methyl-2-pyrrolidone (NMP) was distilled from P2O5 and calcium chloride (CaCl2) was dried under vacuum at 180 °C for 24 h before use.
Preparation design and development of organic and ionic liquids for gas storage
Synthesis of 4,4′-dimethyldiphenylsulfonimide (MBSI)
A homogeneous solution was prepared by dissolving p-toluenesulfonamide (17.122 g, 100 mmol) and NaOH 4.225 g, 105 mmol) into 150 ml of deionized water at 90 °C. Subsequently, p-toluenesulfonylchloride (19.074 g, 100 mmol) was slowly added into the solution within 2 h, and the reaction was then kept at 90 °C for another 12 h. After adjusting the pH of the mixture to 7 by using a diluted HCl, the reaction was left overnight. A sufficient amount of HCl was added to adjust the pH of the solution to 1, yielding a white precipitate. The product was collected via filtration and then dried under vacuum overnight at 120 °C. The final product (20.665 g) was obtained upon recrystallization from the deionized water. The yield was 63.5 %.
Synthesis of 4,4′-dicarboxyldiphenylsulfonimide monomer (CBSI)
A 4,4′-dicarboxyldiphenylsulfonimide monomer was obtained by oxidizing the methyl group of MBSI into carboxyl group. 4,4′-Dimethyldiphenylsulfonimide (20.665 g, 63.51 mmol) and LiOH·H2O (2.690 g, 63.51 mmol) were then dissolved completely into deionized water (300 mL) at 95 °C. KMnO4 (20.068 g, 127.0 mmol) was slowly added into the solution within 2 h. Subsequently, the reaction mixture was stirred at 95 °C overnight. Upon removal of MnO2 and the unreacted KMnO4 by filtration, the filtrate was acidified with concentrated HCl. The final desired product was collected via acidification at last 5 times with a yield of 76.1 %.
Synthesis of sulfonimides functionalized polyamide based polymer electrolyte (SPA)
SPA was synthesized and purified based on the procedure described by Yamazaki et al. . A mixture of CBSI (3.855 g, 10 mmol), DADS (2.480 g, 10 mmol), CaCl2 (2.80 g, 25.5 mmol), TPP (5.2 ml) was added into a mixed solvent of NMP (20 ml) and pyridine (15 ml) and the reaction was kept at 100 °C for 12 h with stirring under argon atmosphere. After the reaction was completed, the mixture was cooled to 70 °C and then poured into 200 ml cold methanol. A white precipitate was obtained by filtration and the pure product was collected upon continuous washing with methanol and deionized water. The product was finally dried at 140 °C under a reduced pressure for 6 h. The molecular weight of the compound was evaluated using gel permeation chromatography with DMF as the eluent. The weight-average molecular weight (Mw) of the SPA was 34,800 g/mol and the polydispersity index was 3.67, suggesting that the SPA polymer was successfully synthesized.
Preparation of PVdF/SPA blend membranes
A series of PVdF/SPA blend membranes were prepared via a solution cast technique using DMSO as a solvent. The 8 wt% of the SPA solution and 8 wt% of the PVdF solution in DMSO were first prepared. The well mixed solution was then casted onto glass plates and dried at 80 °C. The dried membranes were removed from the glass plates and further dried at 140 °C under vacuum for 24 h. The prepared blend membranes were activated by soaking in 0.5 M H2SO4 at room temperature for 12 h, and subsequently washed with ultrapure water repeatedly until the pH values of the wash solutions became 7. The average thicknesses of the membranes measured were 50.96 ± 4.75 μm in a dry state.
Thermal analysis of the PVdF/SPA blend membranes was done via thermo gravimetric analysis (TGA), using SDT TA Instruments 2960 Simultaneous DTA-TGA, at 10.00 °C/min ramp up to 800 °C, under nitrogen atmosphere. Samples were dried in a vacuum oven at 140 °C for 6 h before test.
Scanning electron microscopy
Surface morphologies of the PVdF/SPA blend membranes were investigated using JEOL JSM-6701F field emission scanning electron microscopy. Test specimens were sputter-coated with platinum before imaging.
The tensile strength and elongation at break of the PVdF/SPA blend membranes were measured according to ASTM D882, REF ASTM using an Instron Universal Materials Testing System (model 5544) with a 10 N load cell at 25 °C with constant relative humidity of 50 %. Rectangular-shaped samples were cut from the films (10 mm wide with a gauge length of 20 mm). The thickness of the samples was measured with a digital micrometer with a precision of 1 μm and sensitivity set at 40 %.
Water uptake and swelling
Ion exchange capacity
Results and discussion
Thermal and mechanical strength
Water uptake and water swelling
The importance of membrane quality on proton conductivity is further underscored from the comparison between the SEM images and the measured proton conductivity. It is evident from Fig. 7 that the morphologies of the blend membranes are largely affected by the PVdF content. The smoothness of the membranes, which indicating formation of continuous conducting channels essential for enhancement of proton conductivity, increases with the decrement of PVdF content in the blends. PVdF35 displays the smoothest surface and the highest proton conductivity, even with the lowest WU among all the blend membranes. The increase of the formation of pores with the increment of PVdF content in the blend membranes may be attributed to the poor compatibility between PVdF and SPA. Despite the presence of polar groups in the blend, which enhances the membrane strength through dipole–dipole interactions, the structural rigidness of SPA may be the root cause for the poor compatibility, which becomes increasingly pronounced as the PVdF proportion increases in the blend.
Ion exchange capacity
Tabulated data of properties of PVdF/SPA blend membranes and Nafion 211
Water uptake (%)
Water swelling (%)
Proton conductivity (×10−3 S cm−1)a
Tensile stress (MPa)
Elongation at break (%)
A sulfonimide based SPA polymer with an adjustable linear, comb-like or network structure, was designed and successfully synthesized. To enhance the mechanical strength and to improve dimensional stability of the polymer, PVdF was utilized as a binder for fabrication of blend membranes with various binders to polymer ratios. Subsequently, the performance of these membranes as potential PEM materials for applications in fuel cells was examined. All PVdF/SPA blend membranes exhibit higher WU and better dimensional stability than Nafion 211. Unfortunately, they also display much lower proton conductivity than the Nafion membrane despite the higher WU. The SEM images suggest that the free water stored in the pores is primarily responsible for the high WU. The large porosity in the membrane breaks down the continuous proton conduction channels. To boost the conductivity of the SPA membranes to be competitive to that of Nafion, the quality of the polymer films must be significantly improved with higher density and better uniformity, which requires selecting more miscible binders with the SPA polymer and optimizing the preparation conditions of the polymer blends. In addition, doping with metal oxides into the polymer blends may also enhance the membrane performance, which is the subject of our future studies.
The authors gratefully acknowledge support of a start-up grant from NUS, a POC grant from National Research Foundation of Singapore and the National Natural Science Foundation of China (No. 21233006).
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