A Promising Proton Exchange Membrane: High Efficiency in Low

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A Promising Proton Exchange Membrane: High Efficiency in Low Humidity Koorosh Firouz Tadavani, Amir Abdolmaleki, Mohammad Reza Molavian, and Mohammad Zhiani ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Energy Materials

Abstract

In this research, two new proton conductive membranes consisting of –SO3H groups are synthesized and their proton transfer properties are studied in different conditions. By indirect

insertion

of

the

sulfonate

group

onto

the

imidazolic

nitrogen

of

poly(benzimidazole-imide) (PBII) by sultone, water uptake increases to 160 % and maximum proton conductivity (0.067 S.cm-1) is observed at 80 ºC and RH=60% (PBII2). However, at temperatures higher than 80 ºC, the proton conductivity of PBII2 becomes similar to that of Nafion (lower proton conductivity at high temperatures). Nevertheless, when the sulfonate group is directly attached to the imidazolic nitrogen by ClSO3H (PBII3), water uptake drops to approximately 0% and shows very poor conductivity at ambient temperature. By increasing the temperature, proton conductivity is amplified and at 160 ºC and RH=0 %, the proton conductivity of the membrane reaches 0.0251 S.cm-1. At low temperatures, due to highly strong electrostatic interactions, the proton cannot transfer easily. Nevertheless, at high temperatures, sufficient energy is provided for proton transfer through the hopping mechanism. Finally, some theoretical calculations were conducted in order to support both the experimental findings and the nature of interactions. Keywords: fuel cell; hopping mechanism; proton exchange membrane; density functional theory (DFT); quantum theory of atoms in molecules (QTAIM)

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1. Introduction As a clean and safe energy resource, Fuel cells (FCs) are a good candidate for replacing fossil fuels. Among various types of FCs, proton exchange membrane fuel cells (PEMFCs) have received considerable attention due to their advantages including favorable power-toweight ratio and fast start-up time. As a key component in FCs, proton exchange membranes (PEM) face some limitations. Nafion is the most common commercial PEM and shows high conductivity at temperatures lower than 80 °C. However, since proton exchange in Nafion is highly dependent on the presence of water molecules, any reduction in relative humidity will reduce the proton conductivity of such membranes

1-11

. On the

other hands, higher operation temperatures (> 80 °C) are highly favorable because they not only prevent the Pt electrodes from being poisoned by CO adsorption but also enhance the rate of reaction at the Pt surface

3, 12-13

. At temperatures higher than 100 °C, the humidity

and the proton conductivity of Nafion decrease. Development of membranes with appropriate proton conductivity at low humidity is the most important challenge in high-temperature PEMFCs and can lead to a revolution in the commercialization of fuel cells. Therefore, anhydrous proton exchange membranes which are able to operate at high temperatures are needed in fuel cells technology. For this purpose, imidazole groups have attracted special attention as they exist in two tautomeric forms. In fact, a proton transfer pathway can be recognized when a proton moves from the imidazolium cation to other imidazole groups. The reported pKa for benzimidazole and benzimidazolium cation are 12.9 and 5.3 respectively; therefore, benzimidazole group facilitates proton transfer through the hopping mechanism. Forming unprotonated and


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protonated benzimizoles provide protonic defect in membrane and may lead to local disorder. Therefore, the proton could easily transfer between the benzimidazole and benzimidazolium ion through the hydrogen-bonding. At high temperatures and low humidity (in absence of water as carrier), benzimidazole groups are proton transfer sponsorship. However, the proton exchanges of poly(benzimidazole)s (PBI) are generally lower than those of Nafion due to the absence of hydrophilic channels that are developed in Nafion 14-21. According to previous reports, the magnitude of ionic conductivity is given as 22

: σ(T)=∑〖ni qi µi 〗

where ni and qi are respectively the number and charge of carriers, while µi is their mobility. The carriers in poly(benzimidazole) membranes are adequately high, but their conductivity is very low (10-8 S.cm-1). This is due to the strong hydrogen bonding between imidazole and imidazolium cation which leads to an increase in the dense packing as well as a decrease in the carrier mobility of PBI

22

. Introducing additional ionic side-groups

(strong acidic groups) enhances the q values and varied mobility of ionic groups. In this regard, Chen et. al. synthesized a series of poly(benzimidazole)s with bulky side chains, and the polymer with bulkier side chains showed higher proton conductivity 23. Also, Tang et.al. reported the synthesis of benzyl-methyl phosphoric acid grafted-PBI to improve the proton conductivity of membrane 24. In the present study, the -SO3H group is directly attached to imidazolic nitrogen in polymer main chain for the first time. The direct insertion of -SO3H to the imidazole ring formed an ionic structure with new characteristics. The sulfonate group was also inserted onto the imidazolic nitrogen by a four-carbon sidechain to explore conductivity, water uptake and



etc. in these two membranes. By changing the length of side chains containing the –SO3H group (n=0 and n=4) where n is the number of CH2, the synthesized membranes indicated very different characteristics. When the 1,4-butane sultone is used for the sulfonation of poly(benzimidazole-imide) (PBII), water uptake becomes very high (PBII2) due to the long side chain and the creation of hydrophilic holes

25-27

. For this membrane, high proton

conductivity is observed at low temperatures while membrane humidity and conductivity are drastically reduced (similar to Nafion) at high temperatures (>80 °C). When the –SO3H group is directly attached to the imidazolic nitrogen, although this group is a hydrophilic one, a hydrophobic behavior (fast movements on a wet surface) is observed in the PBII3 polymer membrane. To the best of our knowledge, such an observation is not reported elsewhere. Unlike PBII2, PBII3 shows poor conductivity at low temperatures and high conductivity at temperatures higher than 120 °C. To prove the experimental results, the intermolecular interactions in the membranes were explored by computational studies. 2. Experimental 2.1.

Materials


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Poly(phosphoric

acid)

(PPA),

benzene-1,2-diamine,

isophthalic

acid,

N,N-

dimethylacetamide (DMAc), ammonia, sodium hydride, sodium bicarbonate, 1,4-butane sultone, chlorosulfonic acid, N-methyl pyrrolidone (NMP), sulfuric acid, nitric acid, 85% hydrazine hydrate, Pd/C, sodium hydride, and dimethyl sulfoxide (DMSO) were purchased from Sigma-Aldrich and Merck Chemical Co. All the solvents were dried and distilled before use.

2.2. Techniques 1

H-NMR spectra of monomers and polymers were recorded by the Bruker Avance 400

MHz spectrometer the in deuterated DMSO. The FT-IR spectra with KBr pellet were recorded by a Jasco-680 FT-IR spectrophotometer (Japan). Vibration bands were reported as wavenumber (cm-1). The inherent viscosities of synthesized polymers (0.5% w/v) were measured by a Cannon-Fenske routine viscometer in DMSO at 25 °C. Elemental analysis was carried out with a CHNS-932, Leco. The polymer thermograms were recorded with an STA 503 win TA (Bahr-Thermoanalyse GmbH, Hüllhorst, Germany) under nitrogen atmosphere from 25 °C to 800 °C by the heating rate of 10 °C.min-1. Tensile measurements were determined with Testometric Universal Testing Machine M350/500 (UK) according to the ASTM D882 standard at room temperature. The dimensions of samples were 35 mm × 20 mm × 0.06 mm at the cross-head speed of 10.0 mm min-1. The Philips Xpert MPD diffractometer equipped with a Cu Kα anode (λ = 1.51418 Å) was utilized to record X-ray diffraction (XRD) patterns in 2θ range of 10–80 at the speed of 0.05 °/min. Philips XL30 scanning electron microscopy (SEM) was used to study the morphology of membranes at an accelerating voltage of 10 kV after sputter coating with gold. Proton conductivity 6 ACS Paragon Plus Environment


measurements were performed by fuel cell test station (Scribner 850 e) over a frequency range of 1 to 105 Hz. Brunauer–Emmett–Teller (BET) equation was employed to measure specific surface area of the PBIIs using BELSORP mini, (Japan) after preheating the samples to 150◦C for 2h to eliminate the adsorbed water. 2.3. Computational details Gaussian 09 was employed for all DFT calculations

28

. The geometry of structures were

optimized of structures by the M06-2x/6-311g(2d,p) level of theory without any symmetry restriction. Moreover, frequency calculations were conducted at the same level of calculation. The AIMAll program package

29

was employed to explore the strength and

nature of bonds through the quantum theory of atoms in molecules (QTAIM). 2.4. Monomer synthesis 2.4.1. Synthesis of 1,3-bis(1H-benzo[d]imidazol-2-yl)benzene (A) For the synthesis of monomer A, 1.66 mmol (0.280 g) of isophthalic acid and 3.6 mmol (0.390 g) of 1,2-phenylenediamine were conducted into a 50 mL two-necked round-bottom flask containing 20 g of poly(phosphoric acid) and reaction was performed for 6 h at 210 °C under N2 atmosphere. The viscos solution was added into 150 mL of distilled water and then the pH of the mixture was adjusted to 9 using ammonia solution. The obtained precipitates were filtrated and collected, repeatedly washed with distilled water, and dried at 80 °C for 6 h (0.49 g, 96%) (Scheme S1) 30-31. FT-IR (KBr): ν = 3389 (NH, s, br), 1625 (C=N, m), 1593 (w), 1553 (w), 1461 (m), 1438 (C-N, s), 1278 (s), 730 (s), 686 (s) cm-1; Elem. Anal. Calc for C20H14N4: C, 77.40 %; H, 4.55 %; N, 18.05 %. Found: C, 77.21 %; H, 4.67 %; N, 18.13 %. 7 ACS Paragon Plus Environment

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2.4.2. Nitration of 1,3-bis(1H-benzo[d]imidazol-2-yl)benzene (B1) 1,3-bis(5-nitro-1H-benzo[d]imidazol-2-yl)benzene (B1) was synthesized by the nitration of A (Scheme S2). Moreover, 0.40 g (1.30 mmol) of monomer A was dissolved in 6 mL of concentrated sulfuric acid at room temperature. After cooling to 0 °C in the ice bath, 3 mL of HNO3/H2SO4 solution (1:1) was added dropwise to the mixture and stirred at 0 °C for 4 h. The mixture was poured into 100 mL of cold distilled water, the yellow precipitate was filtered, and then sodium bicarbonate solution was added. The precipitate was washed several times with distilled water until the wash water had nearly neutral pH. The orange precipitate was dried at 80 °C for 6 h (0.501 g, 97%)

30-31

. FT-IR (KBr): ν = 3200-3400

(NH, s, br), 1625 (C=N, m), 1594 (w), 1514 (NO2, s), 1491 (m), 1451 (s), 1337 (NO2, s), 1267 (s), 1122 (s), 760 (s), 700 (s) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 13.52 (2H, NH, Aromatic), 8.81 (1H, CH, Aromatic), 8.23 (2H, CH, Aromatic), 8.08 (2H, CH, Aromatic), 7.94 (2H, CH, Aromatic), 7.57 (3H, CH, Aromatic);

13

C-NMR (400 MHz,

DMSO-d6): δ = 154.3 (C), 144.0 (C), 142.0 (C), 138.5 (C), 130.0 (CH), 129.2 (C), 128.3 (CH), 125.2 (CH), 117.5 (CH), 114.2 (CH), 111.5 (CH); Elem. Anal. Calc for C20H12N6O4: C, 60.00 %; H, 3.02 %; N, 20.99 %. Found: C, 59.87 %; H, 3.07 %; N, 20.81 %.

2.4.3.

Synthesis

of

4,4'-(1,3-phenylenebis(5-nitro-1H-benzo[d]imidazole-2,1-diyl))bis

(butane-1-sulfonic acid) (B2)

1.5 mmol (0.6 g) of B1 was added to 10 mL of dry NMP. After obtaining a uniform solution, NaH (excess) was added to the solution at room temperature during 10 min. The reaction was carried out for 5 h at 80 °C under N2 atmosphere. Then, 5 mmol (excess) of 1,4-butane sultone was added to the mixture and reaction continued for 24 h at 120 °C. 8 ACS Paragon Plus Environment


After cooling to ambient temperature, the mixture was precipitated in acetone and solids was filtered and washed with acetone several times (0.93 g, 87%). The reaction scheme is depicted in (Scheme S3). FT-IR (KBr): ν = 3450 (OH, s, br), 2943 (C-H, Aliphatic, s), 1655 (C=N, s), 1510 (NO2, m), 1491 (m), 1451 (s), 1340 (NO2, s), 1205 (s, –SO3H), 1046 (s), 760 (s), 700 (s) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 8.34 (1H, CH, Aromatic), 7.92 (2H, CH, Aromatic), 7.86 (1H, CH, Aromatic), 7.69 (2H, CH, Aromatic), 7.34 (2H, CH, Aromatic), 7.11 (2H, CH, Aromatic) 4.37 (4H, CH2, Aliphatic), 3.35 (CH2, Aliphatic, H2O), 2.37 (DMSO), 1.89 (4H, CH2, Aliphatic), 1.54 (4H, CH2, Aliphatic); Elem. Anal. Calc for C28H26N6Na2O10S2: C, 46.93 %; H, 3.66 %; N, 11.73 %; S, 8.95 %. Found: C, 46.74 %; H, 3.67 %; N, 11.90 %; S, 8.85 %.

2.4.4. Synthesis of 2,2'-(1,3-phenylene)bis(5-nitro-1H-benzo[d]imidazole-1-sulfonic acid) (B3)

Compound 2,2'-(1,3-phenylene)bis(5-nitro-1H-benzo[d]imidazole-1-sulfonic acid) (B3) was prepared from monomer B1 and ClSO3Na, following the same procedure for B2 (yield = 94%) (Scheme S4). FT-IR (KBr): ν = 3450 (OH, s, br), 1658 (C=N, s), 1517 (NO2, m), 1494 (m), 1450 (s), 1346 (NO2, s), 1201 (s, –SO3H), 1040 (s), 766 (s), 680 (s) cm-1; 1HNMR (400 MHz, DMSO-d6): δ = 9.04 (1H, CH, Aromatic), 8.42 (2H, CH, Aromatic), 8.36 (1H, CH, Aromatic), 8.29 (2H, CH, Aromatic), 8.12 (2H, CH, Aromatic), 7.76 (2H, CH, Aromatic);

13

C-NMR (400 MHz, DMSO-d6): δ = 154.3 (C), 142.0 (C), 138.5 (C), 137.5

(C), 129.2 (C), 129.0 (CH), 128.3 (CH), 125.2 (CH), 117.5 (CH), 114.2 (CH), 111.5 (CH); Elem. Anal. Calc for C20H12N6O10S2: C, 42.86 %; H, 2.16 %; N, 14.99 %; S, 11.44 %. Found: C, 42.84 %; H, 1.97 %; N, 14.78 %; S, 11.56 %. 9 ACS Paragon Plus Environment

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2.4.5. Synthesis of 2,2'-(1,3-phenylene)bis(1H-benzo[d]imidazol-5-amine) (C1)

0.4 g of B1 dissolved in 20 mL of absolute ethanol and 0.05 g of 5% Pd/C was added to the solution. Afterwards, at 50 °C, 1 mL of 85% hydrazine hydrate was added dropwise to the solution over 30 min and refluxed at 85 °C for 24 h. The reaction mixture was filtered to remove Pd/C and the obtained solution was poured into distilled water to form a precipitate which was separated by filtration (0.245 g, 72%) (Scheme S5). FT-IR (KBr): ν = 34323325 (NH2, s, br), 1634 (C=N, s), 1585 (s), 1528 (w), 1438 (C-N, s), 1356 (m), 1311 (w), 1174 (m), 803 (m), 691 (s), 616 (s) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 12.48 (2H, NH, Aromatic), 8.77 (1H, CH, Aromatic), 8.07 (2H, CH, Aromatic), 7.62 (2H, CH, Aromatic), 7.33 (2H, CH, Aromatic), 6.67 (1H, CH, Aromatic), 6.57 (2H, CH, Aromatic), 5.03 (4H, NH2); Elem. Anal. Calc for C20H16N6: C, 70.57 %; H, 4.74 %; N, 24.69 %. Found: C, 70.50 %; H, 4.72 %; N, 24.32 %.

2.4.6. Synthesis

of

4,4'-(1,3-phenylenebis(5-amino-1H-benzo[d]imidazole-2,1-diyl))bis

(butane-1-sulfonic acid) (C2)

Diamine C2 was prepared from dinitro B2 as described for C1, except that in the final stage, acetone was used instead of distilled water to precipitate it (yield = 62%) (Scheme S6). FT-IR (KBr): ν = 3432-3325 (NH2, OH, s, br), 2937 (C-H, Aliphatic, m), 1623 (C=N, s), 1445 (C-N, s), 1211 (–SO3H, s), 1037 (s), 608 (s) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 8.27 (1H, CH, Aromatic), 7.81 (2H, CH, Aromatic), 7.70 (1H, CH, Aromatic), 7.48 (2H, CH, Aromatic), 6.94 (2H, CH, Aromatic), 6.71 (2H, CH, Aromatic) 5.19 (4H, NH2) 4.42 (4H, CH2, Aliphatic), 3.31 (CH2, Aliphatic, H2O), 2.41 (DMSO), 1.93 (4H, CH2,



Aliphatic), 1.69 (4H, CH2, Aliphatic); Elem. Anal. Calc for C28H30N6Na2O6S2: C, 51.21 %; H, 4.60 %; N, 12.80 %; S, 9.76 %. Found: C, 51.37 %; H, 4.67 %; N, 13.23 %; S, 10.15 %.

2.4.7. Synthesis

of

2,2'-(1,3-phenylene)bis(5-amino-1H-benzo[d]imidazole-1-sulfonic

acid) (C3)

Diamine C3 was prepared from dinitro B3 as described for C2 (yield = 87%) (Scheme S7). FT-IR (KBr): ν = 3432-2700 (NH2 stretching, OH stretching related –SO3H, s, br), 1620 (C=N, s), 1449 (C-N, s), 1211 (–SO3H, s), 1044 (s), 603 (s) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 8.96 (1H, CH, Aromatic), 8.20 (2H, CH, Aromatic), 7.83 (1H, CH, Aromatic), 7.31 (2H, CH, Aromatic), 6.89 (2H, CH, Aromatic), 6.66 (2H, CH, Aromatic) , 4.83 (4H, NH2); Elem. Anal. Calc for C20H14N6Na2O6S2: C, 44.12 %; H, 2.59 %; N, 15.44 %; S, 11.78 %. Found: C, 44.36 %; H, 2.61 %; N, 15.29 %; S, 11.52 %. 2.5. Polymer synthesis 2.5.1. Synthesis of PBII1 Poly(benzimidazole-imid) (PBII1) was obtained from diamine C1 and commercially available aromatic dianhydride 5,5'-carbonyl bis(2-benzofuran-1,3-dione) through a twostep polycondensation mechanism (Scheme 1). In the first stage, poly(amic acid) (PAA) was synthesized by adding 0.151 g of dianhydride to 0.160 g of diamine C1 in 6 mL of DMAc that was stirred for 2 h at 0 °C and then 4 h at room temperature. The resulted viscous solution was subsequently cast into a glass plate to achieve PAA film. Then, the film was converted to polyimide by exposure at 80 °C for 6 h and then at 150 °C for 6 h (yield = 89%)

32

. FT-IR (KBr): ν = 3440 (NH, s), 1776 (C=O, w), 1713 (C=O, s), 1624


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(C=N, s), 1438 (m), 1369 (m), 1300 (m), 1260 (m), 1110 (m), 805 (w) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 13.44 (2H, NH, Aromatic), 9.14 (2H, CH, Aromatic), 8.33 (4H, CH, Aromatic), 8.24 (4H, CH, Aromatic), 7.91 (4H, CH, Aromatic), 7.34 (2H, CH, Aromatic) ppm; Elem. Anal. Calc for C37H18N6O5: C, 70.92 %; H, 2.90 %; N, 13.41 %. Found: C, 70.21 %; H, 2.68 %; N, 13.83 %.

Scheme 1. Synthesis of PBII1

2.5.2. Synthesis of PBII2 and PBII3 12 ACS Paragon Plus Environment


The PBII2 (yield = 93%) and PBII3 (yield = 91%) were obtained from diamine C2 and C3 respectively, using the same method for PBII1 (Scheme 2, Scheme S8 and Scheme S9). PBII2: FT-IR (KBr): ν = 3437 (OH, s), 2933 (C-H, Aliphatic, m), 1778 (C=O, w), 1708 (C=O, s), 1643 (C=N, s), 1438 (m), 1369 (m), 1300 (m), 1238 (–SO3H, s), 1150 (–SO3H, s), 1010 (–SO3H, s), 805 (w) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 7.13 - 8.81 (16H, CH, Aromatic), 4.47 (4H, CH2, Aliphatic), 3.27 (CH2, Aliphatic, H2O), 2.49 (DMSO), 1.90 (4H, CH2, Aliphatic), 1.48 (4H, CH2, Aliphatic); Elem. Anal. Calc for C45H34N6O11S2: C, 60.13 %; H, 3.81 %; N, 9.35 %; S, 7.13 %. Found: C, 59.84 %; H, 3.88 %; N, 9.43 %; S, 7.02 %. PBII3: FT-IR (KBr): ν = 3437 (OH, s), 1779 (C=O, w), 1714 (C=O, s), 1637 (C=N, s), 1430 (m), 1358 (m), 1303 (m), 1241 (–SO3H, s), 1139 (–SO3H, s), 1019 (–SO3H, s), 801 (w) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 7.17 - 8.92 (16H, CH, Aromatic); Elem. Anal. Calc for C37H18N6O11S2: C, 56.49 %; H, 2.31 %; N, 10.68 %; S, 8.15 %. Found: C, 56.61 %; H, 2.39 %; N, 10.35 %; S, 8.28 %. Scheme 2. Schematic structure of PBII 1-3


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2.6. Membrane preparation PBII1-3 membranes were prepared by the following procedure: The polymers in the amic acid form were dissolved in a minimum amount of DMAc and conducted into glass petridishes. The membranes were prepared by solvent evaporation for 12 h at 60 °C and for 2 h at 120 °C under vacuum condition. To form imide groups, the membrane was heated for 6 h at 150 °C. After cooling, it was immersed in distilled water to remove membrane from the glass dish (Figure S1). 2.7.

Membrane characterization



Page 14 of 42

2.7.1. Ion exchange capacity (IEC) The IEC values were measured by following equation according to the method reported in pervious literature11 also more details are presented in SI (section 1).

Equation 1. ICE =

∆ ×

( . )

2.7.2. Water uptake and swelling ratio Water uptake and swelling ratio was measured according to equations 2, 3 and 4 based on procedures in previous reports 33 and more details are presented in SI (section 1).

Equation 2. Water uptake (%) =

( )

× 100

Equation 3. Swelling ratio (%) (In-Plan) =

( × )( × ) ( × )

Equation 4. Swelling ratio (%) (Thickness) =

( )( ) ( )

×

×

2.7.3. Proton conductivity The proton conductivity was measured according to equation 5 and procedures of proton conductivity measurement and MEA preparation33 reported in detail in SI (section 1).

Equation 5. proton conductivity (σ) =

3.

Results and discussion 3.1. Synthesis and characterizations of monomers and polymers The prepared polymers were classified into three groups, non-modified and modified through the direct and indirect insertion of –SO3H groups. The FT-IR spectra of monomers 15 ACS Paragon Plus Environment

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A, B1, and C1 are presented in (Figure S2). For monomer A, the characteristic absorption bands around 3200-3500 cm-1 were attributed to the stretching vibration of self-associated hydrogen-bonded N-H…N, and the absorption band at 3057 cm-1 was related to the stretching vibration of C-H sp2. The sharp band at 1624 cm-1 was attributed to C=N stretching vibration. The absorption band at 1438 cm-1 was ascribed to a C-N stretching vibration. For monomer B1, bands at 1515 and 1337 cm-1 were assigned to the –NO2 group stretching vibration. A comparison between elemental analysis of monomer A and B1 reveal that the nitrogen and oxygen content increase from 18.13% and 0% (for monomer A) to 20.81% and 16.25% (for monomer B1) which confirms nitration of compound A. The broad absorption bands at 3271 and 3353 cm-1 are attributed to the N-H stretching of –NH2 groups in monomer C1. In comparing C1 and B1, the diminishing of nitro group bands (1515 and 1337 cm-1) indicates the formation of the amine group. The FT-IR spectra related to dinitro and diamine compounds are depicted in Figure S3 and Figure S4. Absorption bands located at the range of 1100-1250 cm-1 show the –SO3H group for all sulfonated compounds (B2, B3, C2, C3, PBII2, and PBII3). Also, increase sulfur content of B2 (8.85%) and B3 (11.56%) compare B1 (0%) and C2 (10.15%) and C3 (11.52%) compare C1 (0%) confirms sulfonation of all compounds. Moreover, the C-H sp3 starching bond related to butane fragment (1,4-butane sultone) confirms the presence of the sulfonated group. The FT-IR spectra of PBII1-3 (Figure S5) demonstrate that broad absorption bands at around 1710 and 1775 cm-1 are attributed to symmetric and asymmetric stretching vibrations of the imide bond. All the monomers were characterized by 1H-NMR (Figures S6-S11). The diminishing of imidazolic NH peaks at sulfonated dinitro compounds (Figure S7 and Figure S8) by 16 ACS Paragon Plus Environment


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ClSO3H and 1,4-butane sultone indicates the sulfonation of related compounds. Furthermore, the presence of aliphatic CH2 peaks confirms the existence of sultone (Figure S7). To prove only N-substitution (not C-substitution) occurred in compound B3, the

13

C-

NMR was recorded for B1 and B3 (Figures S12 and S13). After substitution of Imidazolic nitrogen by SO3H group, it expects that peaks 2 and 5 in B1 shifted to high filed in B3. Comparison between the

13

C-NMR spectrum of B1 and B3 exhibit that obtained patterns

completely matched with expected results (Figure S14). Both compounds show almost the same pattern, expect in some position (peaks 2 and 5). The peaks 1, 3, 9, 10 and 11 are related to benzene ring containing the nitro group. Also, peaks 4, 6, 7 and 8 ascribed to benzene ring containing two benzimidazole group. The reduction of nitro compounds can be proved by the appearance of amine peak at 4.5-5.5 ppm (Figures S9-S11). Figure 1 illustrates the 1H-NMR related to PBII1-3. The peaks at 13-14 ppm indicate imidazolic hydrogen, and the diminishing of this peak at PBII2 and 3 confirms the sulfonation of the membrane.


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Figure 1. 1H-NMR spectra of PBII1-3 3.2. Thermal gravimetric analysis (TGA) One of the criteria for membrane stability, which determines the temperature limit of the membrane operating, is thermal stability. The thermal stability of poly(benzimidazole-



imide)s was studied by TGA at the heating rate of 10 °C min-1 under nitrogen atmosphere. Typical TGAs of PBII1-3 are presented in Figure S15. PBII1 shows a one-step weight loss (higher than 520 °C) which is attributed to the degradation of polymer backbone, while PBII2 indicates two steps of thermal degradation. The first weight loss at the 300–450°C region is due to the degradation of polymer side chains and –SO3H groups. The second weight loss started from 620 °C and is related to the main chain degradation. In PBII3 diagram, the first weight loss stage (200- 300 °C) is due to the destruction of–SO3H groups. The weight loss starting from 520 °C is related to the degradation of polymer backbone 3.3. X-Ray diffraction analysis Figure 2 depicts the XRD patterns related to PBII1-3 and confirms that sulfonated polymers can be less or more amorphous than non-modified polymer. Malik et al. reported the synthesis of some poly(ether ether ketone)s with different degrees of sulfonation and illustrated that the amount of sulfonation can increase or decrease the crystallinity of polymers

34

. Herein, sulfonated polymers show different behaviors in crystallinity. When

sulfonation occurs by the direct insertion of the –SO3H group onto imidazolic nitrogen, crystallinity increases compared to PBII1. This behavior can be attributed to the higher order of polymer main chain in PBII3 compared with PBII1 because of the electrostatic character of interactions in PBII3. For PBII2, sulfonation through butane sultone increases irregularity and causes a decrease in crystallinity and PBII2 is thus more amorphous than PBII1.


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Figure 2. XRD patterns related to PBIIs 1-3 3.4. Morphology characterization The morphology of synthesized membranes was characterized by scanning electron microscopy (SEM). Figure 3. illustrates the SEM image related to synthesized membranes in two different conditions. In normal casting, all three membranes exhibit a uniform morphology (images a-c). High water uptake of PBII2 could be related to producing of hydrophilic channels in the membrane. Regarding hydrophilic channels in membrane works in presence of water, to explore the effect of water uptake on the morphology of prepared membranes and confirm the presence of hydrophilic channels, the SEM images were recorded at highly swollen after freeze-drying (images d-f). The PBIIs 1 and 3 still shows a uniform morphology (images d and f) that can be attributed to the strong interactions between polymer chains but PBII2 exhibit a porous structure. The longer side chains in PBII2 could alter the morphology of this membrane due to change in many parameters such



as solubility and separation of hydrophilic (-SO3H group) and hydrophobic (main chain) domains and etc. In fact, this longer side chain in PBII2 help to the construction of hydrophilic channels which confirmed by observing porous structure in highly swollen form. Also, the BET analysis was performed to prove SEM results and porosity of PBII2. Specific area for PBII1 and PBII3 was lower than 5.0 m2.g-1 while it obtained around 240.1 m2.g-1 for PBII2.

Figure 3. SEM images (surface view) related to a) PBII1, b) PBII2, and c) PBII3 for normal casting after drying at 100 °C under vacuum and d) PBII1, e) PBII2, and f) PBII3 in highly swollen after freeze drying 3.5. Mechanical properties The mechanical properties of polymers in dry form were measured at room temperature, and the obtained stress and strain data are reported in Table 1. According to SEM images, the presence of the side chain in PBII2 enhances the free volume 35-37, while intermolecular interaction and rigidity decrease compared to PBII1 membrane which reduced its tensile 21 ACS Paragon Plus Environment

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strength. Moreover, PBII3 indicates the highest tensile strength (compared to PBII1 and PBII2) due to strong electrostatic intermolecular interactions. Also, for PBII3, the strain value decreased compared to PBII1 and 2 which can be ascribed to the stronger intermolecular interactions between polymer chains. Table 1. Water uptake, IEC and mechanical properties Water uptake (%)

IEC (meq.g-1)

Strain (%)

Stress (MPa)

PBII1

13

0.85

2.54

14.71

PBII2

160

2.20

2.84

6.23

PBII3

1

2.47

1.01

17.38

3.6.Water uptake (WU) and Swelling Ratio (SR)

The water uptake level is a critical parameter for membrane conductivity. Higher water uptake usually results in high conductivity for membranes. Water uptake usually depends on the degree of sulfonation or IEC. The water uptakes of membranes are presented in Table 1 and Figure 4. As expected, the presence of the –SO3H group in PBII2 led to a sharp increase in water uptake. This is due to the loss of packing and the formation of hydrophilic holes through the butane side chain. Nevertheless, the presence of hydrophilic – SO3H groups in polymer structure causes a severe hydrophobic PBII3 membrane. This polymer shows a fast stimulus response to moisture while being in contact with a wet surface such as hand palm or wet paper (the video presented in supporting information 22 ACS Paragon Plus Environment


(Video S1)). This unique behavior is ascribed to a very strong hydrogen bond creation (with energy around the covalent bond) and ionic structure of PBII3. The strength and nature of these interactions were studied and described in the Computational Study section. The strong bond with the electrostatic character links the chains together at a level which might be cross-linking. The strong interaction between the ionic groups in PBII3 strongly closed pack chains prevented the penetration of water to the membrane and led to the hydrophobic characteristics of this polymer. The small –SO3H groups connector causes to more packing of hydrophobic chains to each other, prevents water penetration, and gives a strong hydrophobic property to the PBII3 membrane. Also, the swelling ratio was measured for all three membranes and showed a trend same as WU. According to the SR results the PBII3 shows lowest swelling ratio value among three membranes which lead to better dimensional stability for fuel cell application.

Figure 4: Water uptake and Swelling Ratio of PBII1-3 (%) 3.7. IEC and Proton conductivity


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The IEC values for all membranes was measured and reported in table 1. For PBII1, regarding imidazole group in natural form is not able to act as proton donner, the PBII1 was treated with 1M sulfuric acid for 24h. therefore, IEC value for PBII1 was measured in the protonated form of a membrane which shows in Scheme S10. For PBII2 and PBII3 due to covalently connected -SO3H groups on the main chain, IEC was measured without any treatment. Lower IEC value of PBII1 compare with PBII2 and PBII3 maybe is due to lower pKa imidazolium cations than -SO3H group. Difference between IEC values of PBII2 and PBII3 is related to higher molecular weight PBII2 than PBII3. One of the most important parameters in fuel cells’ efficiency is the proton conductivity of PEMs. Proton transfer in the polymeric matrix can be done through two mechanisms: 1. “proton hopping” or “Grotthus mechanism”, 2. “vehicular mechanism” or “diffusion mechanism” in which water acts as a transmitter in the presence of humidity. At a high temperature and low humidity, a proton can only be transported on a short distance (

A Promising Proton Exchange Membrane: High Efficiency in Low

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