Sulfonyl Imide Acid-Functionalized Membranes via Ni (0) Catalyzed Carbon-Carbon Coupling Polymerization for Fuel Cells

Polymer membranes, having improved conductivity with enhanced thermal and chemical stability, are desirable for proton exchange membranes fuel cell application. Hence, poly(benzophenone)s membranes (SI-PBP) containing super gas-phase acidic sulfonyl imide groups have been prepared from 2,5-dichlorobenzophenone (DCBP) monomer by C-C coupling polymerization using Ni (0) catalyst. The entirely aromatic C-C coupled polymer backbones of the SI-PBP membranes provide exceptional dimensional stability with rational ion exchange capacity (IEC) from 1.85 to 2.30 mS/cm. The as-synthesized SI-PBP membranes provide enhanced proton conductivity (107.07 mS/cm) compared to Nafion 211® (104.5 mS/cm). The notable thermal and chemical stability of the SI-PBP membranes have been assessed by the thermogravimetric analysis (TGA) and Fenton’s test, respectively. The well distinct surface morphology of the SI-PBP membranes has been confirmed by the atomic force microscopy (AFM). These results of SI-PBP membranes comply with all the requirements for fuel cell applications.


Introduction
Eco-friendly proton exchange membranes fuel cells (PEMFC) are regarded as an effective substitute for fossil fuels [1][2][3][4][5][6]. In PEMFC, the polymer electrolyte membranes (PEM) plays a vital role by converting the H 2 and oxidant into electrical energy. Therefore, PEM materials are the core research area for fuel cells. Although polyfluorosulfonic acid (PFSA) based membranes, Nafions ® , are considered as a standard commercial material for proton exchange membranes (PEM), high cost, harsh manufacturing process, and difficulties for structural modification limit its applications [7][8][9]. Aromatic hydrocarbon-based polymers with different polymer backbones [10][11][12] have already been studied as an alternative to PFSA-based membranes. Comparatively, the pre-sulfonation process (i.e., using sulfonated monomers) is found to be more convenient than post-sulfonation of the membranes but sometimes it can be very difficult to achieve the high molecular weight polymers from primarily sulfonated monomers [12][13][14][15][16]. Additionally, most of these sulfonated polymers show oxidative and hydrolytic stabilities either for their ether linkage-based polymer backbones nor for the attachment of sulfonic acid groups directly to the main polymer chain as they are susceptible to nucleophilic attacks [17,18]. Typically, the lower acidity of the sulfonic acid groups is also accountable for the poor conductivity in the fuel cells. Therefore, researchers have attempted to find out the alternative acid functional groups with nonfluorinated or partially fluorinated membranes other than perfluorosulfonic acids (PFSA) [19]. Recently, sulfonyl imide membranes have become the main interest in their super gas-phase acidity and exceptional thermal stability [20,21]. It is claimed that higher charge delocalization capability is responsible for its higher acidity [22][23][24][25][26][27][28][29][30]. As a result, researchers reported some polymer membranes using sulfonyl imide groups [31][32][33][34][35][36][37][38][39] and demonstrated as potential candidates for fuel cell applications [40][41][42][43]. Most of the sulfonyl imide membranes also show poor chemical stability due to ether linkages in the main polymer backbones [44,45]. However, poly(phenylene)s particularly poly(2,5-bezophenone) s, exhibit exceptional mechanical properties as they contain entirely aromatic backbones with pendants groups [46]. Additionally, to improve the mechanical properties of the membranes, Ni (0) catalyzed C-C coupling polymerization process can be the most suitable way to achieve high molecular weighted polymers from poly(phenylene)s [47][48][49].
Therefore, this work is an attempt at the synthesis of sulfonyl imide-based proton exchange polymer membranes (SI-PBP) from 2,5-dichlorovenzophenone (DCBP) using the Ni (0) catalyzed C-C polymerization followed by the post imidization of the poly(benzophenone)s polymers. The DCBP monomer shows good reactivity in polymerization by electron-withdrawing groups. Additionally, the absence of the ether linkages in the SI-PBP membranes provides good dimensional and chemical stability. Sulfonyl imide groups promote the proton conductivity through super acidity compared to sulfonic acid groups. The pendant sulfonyl imide groups also greatly affect the membranes' properties and attributes to make very distinct ionic channels for proton conduction throughout the polymer networks. Therefore, the SI-PBP membranes are anticipated to be a remarkable alternative to PFSA membranes in fuel cell applications.

Synthesis of Poly(Benzophenone) Polymers (PBP)
Inside in a glove box, a 3-necked catalytic flask was charged with (0.366 g, 1.19 mmol), zinc powder (4.54 g, 71.68 mmol), and triphenylphosphine (2.51 g, 9.56 mmol) and 2,5dichlorobenzophenone (DCBP) monomer (3.0 g, 11.95 mmol) were charged into another flask. With a syringe, DMAc (3-5 mL) was added into the catalytic flask, fitted with a mechanical stirrer and nitrogen inlet/outlet. It was stirred gently at 80 • C until it turned a blood-red color. Thereafter, DCBP monomer was dissolved into DMAc solvent and added into the catalytic flask through a syringe. The resultant mixture was stirred 12 h at 100 • C to get a viscous dark mixture. The viscous mixture was diluted with 8-10 mL DMAc and cooled to normal temperature. Then, the solution was poured with stirring into the distilled water containing 30% HCl. When the white solids come out, the solids were collected through filtration and washed with distilled water and methanol, respectively. Finally, the white solid polymer (PBP) was dried 12 h in a vacuum oven at 60 • C.

Sulfonation of the PBP Polymer (SPBP)
At first, PBP polymers (2.0 g, 7.96 mmol) were dissolved in chloroform (10 mL) at 25 • C. The concentrated chlorosulfuric acid (3.18 mL, 47.8 mmol) was then added slowly in an ice condition and stirred for the required time at 80 • C. The black-colored solution was then poured into the ice water and stirred vigorously to obtain yellow solids. Finally, the yellow solid polymer was rinsed with distilled water and dried at 60 • C for 24 h.

Synthesis of Sulfonyl Imide Poly(Benzophenone)s Polymers (SI-PBP)
A 100 mL flask was charged with SPBP polymer (2.0 g, 5.66 mmol), 15 mL dichloromethane (15 mL), and thionyl chloride (20 mL). The mixture was reflux with 2 mL DMF at 75 • C for 24 h. Thereafter, all the solvents were evaporated to obtain a viscous solution. Diluting the resultant viscous solution with dichloromethane (15 mL), sulfamoyl fluoride (2.24 g, 22.65 mmol) was added dropwise in an ice condition. The mixture was stirred for 12 h at 25 • C. The resultant orange color solution was then poured into methanol to obtain the sulfonyl imide poly(benzophenone) polymers (SI-PBP). After collecting the polymer by filtration it was dried at 60 • C.

Preparation of Monomer (DCBP)
The 2,5-dichlorobenzophenone (DCBP) monomer was synthesized from 2,5-dichlorotoluene followed by Friedel-Craft acylation with benzene (Scheme S1). 1 H-NMR (Nuclear Magnetic Resonance) was used to confirm the structure of the DCBP monomer ( Figure S1). The chemical structure of the DCBP monomer was confirmed by 1 H-NMR ( Figure S1). All the phenyl protons appeared at 7.35-7.84 ppm. For the mesomeric effect of the chlorine atoms, the protons in the ortho and meta position appeared at upfield near 7.39-7.42 and 7.36 ppm, respectively. Additionally, the ortho, meta, and para protons of side phenyl rings shifted to the downfield because of the electron withdrawal effect of the carbonyl group and appeared at near 7.46-7.52, 7.60-7.66 and 7.78-7.85 ppm, respectively.

Preparation of Polymers (SI-PBP)
The SI-PBP polymers were synthesized by C-C coupling polymerization of the DCBP monomer using Ni (0) catalyst (Scheme 1). The molar ratio of the catalyst plays an important role to get high molecular weighted polymers. Therefore, the molar ratio of the catalyst NiBr 2 : PPh 3 : Zn (1:10:40) was applied in the synthesis of the PBP polymers. Additionally, chlorosulfonation of PBP polymers (S-PBP) was acquired by controlling the amount and time of exposure to chlorosulfuric acid. Thereafter, sulfonimidization was carried out using SOCl 2 and sulfamoyl fluoride (FSO 2 NH 2 ), respectively. The SI-PBP polymers was characterized by FT-IR (Fourier Transform Infrared Spectrometer), 1 H-NMR, and 19 F-NMR (Figures 1 and 2) respectively. In Figure 1, the broad stretching frequencies ranging from 2800-3600 cm −1 corresponds to the -OH vibrational frequencies of the S-PBP polymers. The stretching frequencies at 1420 and 1190 cm −1 correspond to S=O groups (assy. & sym.) whereas frequency at 1680 cm −1 for C=O bonds. However, the sharp characteristic peaks for N-H stretching found at 3500 cm −1 and N-H bending was overlapped with C=O stretching at 1680 cm −1 for the SI-PBP polymers. The peaks at 1480 and 1250 cm −1 respectively represent the asymmetric and symmetric stretching vibration of S=O groups for sulfonyl imide groups. Additionally, the frequencies at 640 cm −1 for S-F bonds confirms the sulfonyl imide incorporation to the sulfuric acid groups. Figure 2 shows the 1 H-NMR and 19 F-NMR for the PBP, SPBP and SI-PBP polymers. The peaks around 6.50-7.78 ppm correspond to the protons of the aromatic rings for PBP and SPBP polymers (Figure 2a,b). Again, the -SO 3 H peaks for SPBP polymers observed at the upfield at 4.0 ppm due to the conjugation with moisture and solvent DMSO (Figure 2b). However, the aromatic protons peaks for SI-PBP polymers shifted towards the downfield at 6.50-8.60 ppm, due to the delocalization capability of the sulfonyl imide anion (Figure 2c). The characteristic lumpy N-H peaks were observed at 4.90-5.70 ppm. The characteristic 19 F peak at near −162 ppm also assured the attachment of the sulfonyl imide groups into the polymer backbones (Figure 2d).

IEC, Water Uptake and Dimensional Stability of Membranes
The details about the method of characterization and properties of the SI-PBP membranes have been discussed in the Characterizations and Measurement of Membranes' Properties section in the supporting details. Figure 3 shows a gradual increase in the water uptake (22.72 to 64.29%) with the increment of IEC from 1.85-2.30 meq./g). The charge delocalization capability of the sulfonyl imide acid groups is accountable for the higher water content of the SI-PBP membranes.    The SI-PBP membranes show comparably lower-dimensional changes (∆t and ∆l) than Nafion 211 ® . The SI-PBP membranes exhibited higher ∆t (7.5%, 12.5%, and 34.0%) and ∆l (4.81%, 8.0%, and 16.17%) values whereas Nafion 211 ® showed (∆t = 31.98 and ∆l = 14.2%) ( Table 1). Noticeably, ∆l values of SI-PBP membranes were significantly lower than the ∆t values due to the C-C bonded aromatic rigid polymer backbones.  Figure 4 represents the proton conductivity of the SI-PBP membranes at different temperatures (30-90 • C) and humidity (30-90% RH), respectively. Markedly, SI-PBP-3 membrane showed significantly higher proton conductivity (107.07 mS/cm) than Nafion 211 ® (104.5 mS/cm) under 90% RH at 90 • C (Figure 4a). Noticeably, SI-PBP 2 also exhibited comparable proton conductivity (94.80 mS/cm) to Nafion 211 ® (104.5 mS/cm) in the same condition. However, the proton conductivity of the synthesized SI-PBP membranes was also increases with the humidity (Figure 4b). The higher acidity and location of the sulfonyl imide groups in the pendant benzophenone rings are mutually liable for the higher proton conduction of the SI-PBP membranes by forming well distinct ionic channels for proton conduction.  Figure 5 depicts the thermal degradation curves for the SI-PBP membranes in an air atmosphere. The PBP polymer exhibited the thermal degradation in one step at around 380 • C whereas SI-PBP membranes showed three steps degradation. The thermal degradations at the ranges 200-330, 340-475 and 475-610 • C accounts for the conversion of the sulfonyl imide into sulfuric acid groups, removal of the -SO 3 H groups and polymer backbones degradation, respectively. Markedly, the most advantageous site of the SI-PBP polymers is the conduction of proton in two steps (i.e., with -SO 2 NHSO 2 -groups and -SO 3 H groups) before the breakdown of the main polymer backbones [43]. Additionally, the SI-PBP polymers are stable up to 550 • C. The C-C bonded aromatic backbones are mainly responsible for the extreme thermal stability of the SI-PBP membranes.  Figure 6 illustrates the chemical degradation of the SI-PBP membranes by Fenton's reagent in terms of weight losses with times at 80 • C for 9 h. All the SI-PBP membranes showed excellent chemical stability compare to sulfonated poly (arylene ether sulfone), SPAES 40. The C-C polymer backbones which are less susceptibility towards free radical attacks and presence of the fluorine atoms in the sulfonyl imide groups are responsible for the higher chemical stability of the SI-PBP membranes.     Figure 8 illustrates the polarization curves for the single-cell performance of the SI-PBP membranes. The SI-PBP 3 membranes showed higher power density with a maximum power density (0.638 W/cm 2 ) than Nafion 211 ® (0.62 W/cm 2 ). The SI-PBP 1 and SI-PBP 2 also displayed comparable cell performance as Nafion 211 ® in the whole range of current density. Dry membrane tensile stress strain properties are shown in Table 1. In dried membrane condition, the Young's modulus of SPPBP-1,2 and 3 were 1005, 1014 and 1019 Mpa compared with 208 MPa of Nafion 211. These properties are momentous in PEMFC application, and make these membranes attractive for PEM materials.

Conclusions
The polymer membranes based on sulfonamide poly(benzophenone)s (SI-PBP) were synthesized by Ni (0) catalyzed C-C coupling polymerization of 2,5-dichloro-benzophenone. The synthesized SI-PBP membranes showed IECs from 1.85 to 2.30 meq./g and water uptake from 22.72 to 64.29%. The as-synthesized SI-PBP 3 membranes exhibited higher proton conductivity (107.07 mS/cm) with maximum power density (0.638 W/cm 2 ) than Nafion 211 ® (104.5 mS/cm and 0.62 W/cm 2 ). The C-C coupled polymer backbones of the synthesized SI-PBP membranes provided outstanding thermal and chemical stability which are assured by the thermogravimetric analysis (TGA) and Fenton's test. Additionally, during the cell operation, the presence of the fluorine atoms in the pendant side-chain protects the polymer backbones from free radical attacks. Moreover, the pendant sulfonyl imide groups in the benzophenone moiety provide well-defined ion-conducting channels for proton conduction throughout the polymer network. Hence, due to the excellent thermal, chemical stability with enhanced proton conductivity, the synthesized C-C coupled SI-PBP polymer membranes can be considered as an ultimate alternative to ether linkage based per fluorinated Nations.