Synergistic Behavior of Phosphonated and Sulfonated Groups on

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Synergistic behavior of phosphonated and sulfonated groups on proton conductivity and their performance for hightemperature proton exchange membrane fuel cells (PEMFCs) Koorosh Firouz Tadavani, Amir Abdolmaleki, Mohammad Reza Reza Molavian, Sedigheh Borandeh, Elahe Sorvand, and Mohammad Zhiani Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01065 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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Synergistic behavior of phosphonated and sulfonated groups on proton conductivity and their performance for high-temperature proton exchange membrane fuel cells (PEMFCs) Koorosh Firouz Tadavania, Amir Abdolmalekia,b,∗, Mohammad Reza Molaviana, Sedigheh Borandeha, Elahe Sorvanda, Mohammad Zhiania

a

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,

Iran. b

Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71467-13565,

Iran.

∗

Corresponding author: Tel.: +98 3133913249; fax: +98 3133912350 E-mail address: [email protected], [email protected] (A. Abdolmaleki)

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Abstract Some new polybenzimidazoles (PBIs) containing different acidic groups (–SO3H and – PO3H2) as proton donors, and amphoteric groups (imidazole and –PO3H2) as proton transmitters were synthesized. Proton conductivity was 0.015 S.cm-1 for PBI1 and 0.075 S.cm-1 for PBI2 at ambient temperatures in water. At 140 ºC and RH = 30%, proton conductivity equaled 0.126 S.cm-1 for PBI2 and 0.003 S.cm-1 for Nafion. Moreover, PBI2 showed high proton conductivity under anhydrous conditions at 140 ºC. It exhibited a porous morphology due to its sulfonated hydrophilic side chains. Theoretical studies showed that hydrogen bonding (HB) interactions of side chains and main chain groups leads to the formation of pores in the polymer structure. The presence of pores and hydrophilic groups in polymer structure increases water uptake and the durability of water molecules at high temperatures. The formation of a great HB network between acidic and amphoteric groups helps proton transport under anhydrous conditions. Keywords: Fuel Cell; Proton Exchange Membrane; Hopping Mechanism; Quantum Theory of Atoms in Molecules; Density Functional Theory.


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1. Introduction Developing highly proton-conductive polymer electrolyte membranes (PEMs) at anhydrous conditions is one of the most important issues in high-temperature proton exchange membrane fuel cells (PEMFCs)

1, 2

. As a perfluorosulfonic acid (PFSA)

membrane, Nafion is the most well-known commercial membrane used in PEMFCs. However, the proton conductivity of Nafion dramatically decreased at temperatures higher than 100 ºC

3-6

. The proton conductivity of PEMs is affected by different

parameters such as polymer structure, side chains, and distribution; position, number, and acidity of ionic groups; and polymer morphology

7-9

. Recently, many

researchers have investigated these parameters. Dishari et al. examined the effects of some of these parameters, including film thickness, hydration number, and polymer−substrate interaction, on proton dissociation in Nafion

10

. Moreover, Li et

al. explored the effect of densely sulfonated hydrophilic polymer block of poly(ether sulfone)s on proton conductivity 7. At high temperatures, hopping mechanism is the dominant mechanism for proton transfer, occurring through hydrogen bonding (HB) networks (between sulfonated groups and water molecules), and the network does not form completely in anhydrous conditions

11-13

. Molavian et al. studied proton

transfer in various polymers 14. Poly(benzimidazole)s (PBIs) potentially have shown the ability of proton transfer through hopping mechanism 14. The imidazole group in PBIs acts as a carrier and helps increase proton transfer. In addition, replacing water molecules with other high-boiling-point protogenic groups such as phosphoric acid is a promising approach for developing anhydrous membranes and increasing their efficiency at high temperatures. PBIs are usually doped with sulfuric acid and phosphoric acid to form an acid-base complex 15 and, therefore, proton conductivity is enhanced by increasing the acidic content. Besides, the preparation of porous 3 ACS Paragon Plus Environment

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membranes is developed to increase acid absorption capacity of membranes

16

.

Numerous studies have been conducted on the synthesis of porous PBIs. Ma et al. reported the synthesis of a phosphoric acid doped cross-linked poly(benzimidazole) with high stability for high temperature PEMFC

17

. Also, Li et al. synthesized

porous poly(benzimidazole) with high ion-exchange capacity (IEC=15.7 mol. phosphoric acid per PBI unit)

18

. However, the efficiency of the phosphoric acid-

doped membranes reduced during cell operation due to the acids’ washing out process19-22. Insertion of acidic groups by covalent bonding on the polymer chain prevents conductivity decrease due to acid washing out during cell operation

22

.

Although numerous sulfonated membranes have been developed during the past decade, most of them suffer from water-dependent proton transfer 23. Our previous studies and some other reports show that at low humidities, the phosphonate group has a more amphoteric character than the sulfonate group for acting as a proton carrier

14, 24-26

. Furthermore, we demonstrated that the presence of a second

protogenic group enhances the sulfonate group’s proton conductivity at low humidity, while the proton conductivity of the diprotogenic system increases at lowhumidity conditions 24. Herein, for the first time, we reported the synthesis of a new PBI containing different vicinal strong protogenic groups (–SO3H and –PO3H2 groups), covalently bonded onto the polymer backbone. The amphoteric nature and moisture-absorption affinity of –PO3H2 group had a synergetic effect on the sulfonated groups’ conductivity 27. Designing membranes containing bulky highly sulfonated side chains enhanced free volume and created hydrophilic channels to increase the H3O+ transmittance, and processability of the polymer 28.


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2. Experimental 2.1. Materials

The materials included 5-Hydroxyisophthalic acid (Merck Chemicals Co.), sodium hydroxide (Merck Chemicals Co.), 4,4'-oxybis(benzene-1,2-diamine) (Alfa Aesar), fuming sulfuric acid (Merck Chemicals Co.), 4,4'-sulfonylbis(chlorobenzene) (Merck Chemicals Co.), poly(phosphoric acid) (PPA) (Merck Chemicals Co.), ammonia (Sigma Aldrich), sodium hydride (Sigma Aldrich), dimethyl sulfoxide (DMSO) (Sigma Aldrich). DMSO was dried and distillated over barium oxide.

2.2. Techniques Bruker Avance 400MHz spectrometer was employed to record 1H-NMR spectra, and solid-state

31

P-NMR spectrum was recorded on Bruker Avance II 400WB

spectrometer. Jasco-680 FT-IR spectrophotometer (Japan) was utilized to record FT-IR spectra (KBr pellet). Vibration bands are reported as wavenumber (cm-1). Band intensities were described as strong (s), medium (m), weak (w), and broad (br). Elemental analysis was performed with a CHNS-932, Leco. Thermal gravimetric analysis (TGA) was done at the heating rate of 10 °C/min from 25 to 800 °C under nitrogen atmosphere by an STA503 win TA (Bahr-Thermoanalyse GmbH, Hüllhorst, Germany). The polymer’s inherent viscosity was measured in DMSO at 25 ºC by a standard procedure employing a Cannon Fenske routine viscometer. Mechanical properties were measured with a Testometric Universal Testing Machine M350/500 (UK) according to ASTM D882 standards at room temperature. Proton conductivity was measured by a Scribner 850e fuel cell test station over the frequency range of 1-105 5 ACS Paragon Plus Environment

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Hz. Specific surface area and pore size distributions of the PBIs were evaluated using Brunauer–Emmett–Teller (BET) equation (BELSORP mini, Japan) after preheating the samples to 150◦C for 2h to eliminate the adsorbed water. A differential scanning calorimetry (DSC) device (NETZSCH, Germany) was used to study the glass-transition temperatures (Tg) of the blend samples. Samples were kept at 50 °C for 30 min under isothermal conditions. Samples then were scanned from 50 °C to 500 °C at a heating rate of 10 °C/min. 2.3. Computational details Optimized structures were obtained by DFT calculations with the CAM-B3LYP method and 6-311G basis set. Frequency calculations were performed with the same method and basis set without symmetry restriction. The Gaussian 09 rev. D01 was used for optimization and frequency calculation

29

. Related wave functions (WFX file) were

gained from Gaussian and used AIMAll to calculate hydrogen bond energy and determine the nature of interactions between different functional groups (–SO3H, – PO3H2 and imidazole) in the polymer chain 30. 2.4. Polymer synthesis 2.4.1. Synthesis of PBI1 For the synthesis of PBI1, 0.230 g (1.0 mmol) of 4,4'-oxybis(benzene-1,2-diamine) and 0.182 g (1.0 mmol) of 5-hydroxyisophthalic acid were mixed with 20 g of poly(phosphoric acid) in a 50 mL two-necked round-bottom flask equipped with a magnetic stirrer. Polymerization was carried out under N2 atmosphere at 130 ºC for 4 h and, then, at 200 ºC for 20 h. The gel-like solution was poured into 100 mL of distilled water and neutralized to pH=9 with ammonia solution. The resultant precipitates were


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collected by filtration, repeatedly washed with deionized water, and dried at 80 °C for 8 h under vacuum conditions (0.386 g, 92%). Electrophilic phosphonation occurred on active 5-hydroxyisophthalic acid simultaneously in poly(phosphoric acid) 30, 31. FT-IR (KBr): ν = 3330-3500 (NH, s, br), 1613 (C=N, m), 1596 (C=C, w), 1565 (m), 1411 (C-N, s, sh), 1221-1250 (s, br), 1155 (s), 808 (m) cm-1; 1H-NMR (400 MHz, DMSO-d6): δ = 13.01 (s, 2H, NH, aromatic), 10.14 (1H, OH), 8.44 (1H, CH, aromatic), 8.01 (1H, CH, aromatic), 7.64 (4H, CH, aromatic), 7.13 (1H, CH, aromatic), 7.01 (2H, CH, aromatic), 5.60 (2H, OH) ppm; 31P-NMR (400 MHz): δ = -4.86 (br); Elem. Anal. Calc for C20H13N4O5P: C, 57.15%; H, 3.12%; N, 13.33%. Found: C, 57.02%; H, 3.08%; N, 13.19%. Scheme 1. 2.4.2. Synthesis of PBI2 2.4.2.1. Synthesis of 3,3'-disulfonated-4,4'-dichlorodiphenyl sulfone First, 0.574 g (2 mmol) of 4,4`-dichlorodiphenyl sulfone was dissolved in 2 mL of fuming H2SO4 at room temperature and the mixture was heated at 110 ºC for 6 h. Afterwards, the resultant solution was cooled to room temperature. The brown solution was poured into 4 mL of ice-cold water and a white precipitate was obtained after adding 1.8 g of NaCl. The resulting precipitate was filtered and dissolved in deionized water and neutralized to pH=7 with NaOH. After adding 1.8 g of NaCl, a white precipitate was attained which recrystallized in a mixture of water-ethanol (3:7 v/v). FT-IR (KBr, cm-1): ν=1140, 1329 (s) (SO2), 1213 (s), 1123 (m) (SO3-), 1633, 1579 (s) (aromatic), 1673 (w) (aromatic); Elem. Anal. Calc for C12H6O8S3Cl2Na2: C, 29.34%; H, 1.23%; S, 19.58%. Found: C, 29.14%; H, 1.29%; S, 19.25%. 7 ACS Paragon Plus Environment

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2.4.2.2. Synthesis of PBI2 First, 0.420 g (1 mmol) of PBI1 was dissolved in 10 mL of dry DMSO. The excess amount of NaH was slowly and carefully added to the solution at room temperature. The mixture was stirred at 80 ºC under nitrogen atmosphere for 4 h, and then the excess amount of 3,3'-disulfonated-4,4'-dichlorodiphenyl sulfone (4.913 g = 10 mmol) was added to the reaction mixture to perform complete modification and prevent crosslinking. The solution was stirred for 20 h at 130 ºC. The mixture was poured into water and, after adding H2SO4 (pH = 4), the brown precipitates was collected and subsequently washed with water and dried at 80 ºC for 24 h under vacuum conditions (yield: 1.175 g, 71%). FT-IR (KBr): ν=3330- 3500 (OH, s, br), 1619 (C=N, m, br), 1417 (C-N, s, sh), 1320 (w) (SO2), 1250 (s), 1164 (s) (SO3-), 1053 (s), 1014 (s), 964 (m), 809 (s), 571 (s) cm-1 ; 1HNMR (400 MHz, DMSO-d6): δ = 8.39 (12H), 7.84 (10H, aromatic), 7.21 (5H, CH, aromatic), 6.26 (2H, OH) ppm; Elem. Anal. Calc for C56H34N4O29PS9Cl3: C, 40.70%; H, 2.07%; N, 3.39%; S, 17.46%. Found: C, 41.55%; H, 2.22%; N, 3.48%; S, 16.97%. Scheme 2 2.5. Membrane preparation PBI1 and PBI2 were separately dissolved in the minimum value of DMSO and poured into glass dishes. The membranes were dried after the evaporation of the solvent for 12 h and, then, at 60 °C for 4 h under vacuum conditions. After cooling, membrane was immersed in distilled water so that it was removed from the glass dish. Both PBI1 and


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PBI2 resulted in flexible membranes (Fig. 1 shows digital-camera images of prepared membranes). Fig. 1. 2.6. Membrane characterization 2.6.1. Ion exchange capacity (IEC) The membranes in the acidic form were immersed in 100 mL of 2M NaCl for 24 h. The resultant solutions were titrated with 0.01N NaOH. The IEC of membranes was calculated as follows (Equation 1):

Equation 1. IEC =

∆ ×

( . )

where ∆ is the volume of NaOH, C is the concentration of NaOH, and Wd is the weight of the dry membrane. 2.6.2. Water uptake The membranes were placed at 100 ºC for 15 h until their weight stopped changing. Then, the membranes were immersed in distilled water for 4 h. After the membranes were taken out, their surfaces were cleaned with tissue paper and they were quickly weighted on a microbalance. The water uptake of the membranes was calculated as follows (Equation 2): ( )

Equation 2. Water uptake (%) =

× 100

in which Wd represents membrane weight in the dry form (before water uptake), and Ww denotes membrane weight after water uptake in the wet form. 2.6.3. Proton conductivity and PEMFC performance


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The PBIs’ resistance was obtained using impedance spectroscopy technique over the frequency range of 1-105 Hz. The proton conductivity of the membrane was measured in different conditions. The proton conductivity of PBI2 membrane (with a thickness about 130 µm) was measured at the constant humidity of 60%, 30% and 0% at temperature of 40-140 oC. To adjust humidity to 60% and 30% at different temperatures, cell temperature and humidifier temperature were fitted at respective values based on literature

32

. Proton conductivity σ (S.cm-1) was calculated based on the following

equation (Equation 3):

Equation 3. σ = where R (Ω) shows the resistance of the membrane, A (cm2) is the area of the membrane, and l (cm) denotes the distance between reference electrodes. To prepare the electrodes, Pt/C 30% wt (fuel cell store) was dispersed in a solution containing 10% wt of PBI for 3 min. The prepared ink was painted on carbon cloth (LT 1200, fuel cell store) and the electrodes were dried at 80 oC for 80 min. Membrane electrode assembly (MEA) was fabricated by hot-pressing the electrodes and membrane (PBI2 with 130 µm of thickness) at 135 ºC and 4 MPa for 4 min. The Pt loading of electrodes equaled 0.5 mg cm-2. MEA was conducted using homemade 5cm2 single-cell hardware with a serpentine flow pattern. The cell was first activated using a constantvoltage (0.6V) protocol which was reported in details in literature 33. Polarization curves were achieved by scanning the cell potential from OCV to 0.25 V with the scan rate of 5 mV S-1 at 120 ºC with dry H2/O2 at the flux rate of 200 mL min-1 and 15 psi pressure of gases for 100 h. 3. Results and discussion


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3.1. Polymer synthesis and characterizations PBI1 was synthesized by the condensation polymerization of 4,4'-oxybis(benzene-1,2diamine) and corresponding the diacid in poly(phosphoric acid) (Scheme 1). PBI2 was synthesized by grafting the sulfonated side chains to PBI1 backbone (Scheme 2) to improve proton conductivity by increasing the density of sulfonate groups through hydrophilic side chains and creating a highly porous morphology. This design led to the formation of a hydrogen-bonded network increased the free volume

28

27

that held water molecules very tightly and

. Moreover, the solubility of the polymer with a large side

group improved in the organic solvent by reducing the chain packing and structure rigidity 34, 35. The solubility of both PBI1 and PBI2 was measured in different common organic solvents (Table 1). PBI1 shows good solubility in DMSO and is partially soluble in other polar aprotic solvents. PBI2 is soluble in NMP and DMAc by the addition of 5% V/V NH3 due to the increased number of acidic groups in side chains. The inherent viscosity and yield of polymers are depicted in Fig. 2.

Table 1.

Fig. 2.

The FT-IR spectra of PBI1 and PBI2 are presented in S1. For PBI1, the characteristic absorption bands around 3200-3500 cm-1 were ascribed to the stretching vibration of self-associated hydrogen-bonded N-H…N 36 and OH stretching vibration. Furthermore, the sharp bands at 1615 cm-1 were attributed to C=N stretching vibration. The peak at 1414 cm-1 was ascribed to the C-N stretching vibration. The absorption bands at 1155 and 1255 cm-1 were related to the symmetric and asymmetric P=O stretching of the


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phosphonated group 37, 38. Broad absorption bands at 2500-3000 cm-1 for N+-H vibration were observed due to imidazole ring protonation in the presence of –PO3H2 group 39. For PBI2, the broad absorption band at 3200-3500 cm-1 was attributed to the O-H stretching of –SO3H and –PO3H2 groups. The peaks at 1619 cm-1 and 1417 cm-1 were recognized as the absorption bands for the C=N and C-N stretching of the imidazole ring, respectively. The broad and strong peaks at 1150-1250 cm-1 were attributed to S=O and P=O stretching vibrations which were overlapping

37-40

. The signal stretching

vibration of the C-Cl group appeared at 571 cm-1. The 1H-NMR spectrum of PBI1 in which the peak at 13.11 ppm was identified as the NH proton of benzimidazole group is illustrated in S2. The peak at 10.17 ppm corresponded to the phenolic O-H group 41, 42 and the peak at 5.4 ppm was assigned to the O-H proton of the –PO3H2 group 43, 44. The signal of PBI1 aromatic proton appeared between 6.9 and 8.5 ppm. To confirm that the phosphonate group was covalently attached to the benzene ring, PBI1 was characterized by 31P-NMR (S3) and the peak at 4.86 ppm was assigned to the phosphonate group on the benzene ring. Based on previous reports, phosphonate groups covalently bonded to the benzene ring exhibit a peak in the range of -1.00 to -10.00 ppm 45, 46. The 1H-NMR spectrum of PBI2 is demonstrated in S4. The peak at 6.37 ppm was assigned to the hydrogen of –PO3H2 groups. Acidic protons of –PO3H2 shifting from 5.55 to 6.37 ppm confirmed the strong H-bonding between –PO3H2 and –SO3H groups in a grafted structure. The aromatic protons of PBI2 had characteristic peaks at 7.038.84 ppm. The peaks at 8.24-8.84 ppm were assigned to the aromatic protons vicinal to –SO3H groups in pendant groups 42, 47. The peaks at 13.02 and 10.32 ppm corresponded with the N-H proton of the imidazole ring and phenolic O-H group 12 ACS Paragon Plus Environment

26, 41

, respectively,

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which disappeared in PBI2. Broadening of the solvent's water and absorbed water of the polymer chain confirmed the water uptake of PBI2 due to the addition of highly sulfonated hydrophilic side chains and porous morphology of the membrane. The porous structure of the membrane is completely confirmed by scanning electron microcopy (SEM) image and computational studies.

3.2. Thermal analysis Membrane thermal stability is a widely used criterion for determining the temperature limit of membrane operation. The thermal stability of PBI1 and PBI2 was studied by thermal gravimetric analysis (TGA) in nitrogen atmosphere at the heating rate of 10 oC min-1. Typical TGAs of PBI1 and PBI2 are shown in S5. Figure S5 shows three steps thermal degradation of PBI1. The first weight loss region at 210–250 ºC was due to the removal of trapped solvent (DMSO); the weight loss at 260320 ºC was attributed to the removal of –PO3H2 groups; and the weight loss starting from 540 ºC was related to the degradation of the main chain of the polymer. For PBI2, the first weight loss occurring at 80-120 ºC corresponded with the evaporation of humidity absorbed by polymer chains and represents the high moisture uptake of the membrane compared with PBI1. This result is also confirmed by the broadening of water molecule peak at the 1H-NMR spectrum of PBI2. The weight loss around 200-240 º

C occurs due to the evaporation of trapped DMSO. The third weight loss step (290-360

°

C) for PBI2 was due to the destruction of –PO3H2 and –SO3H groups. The final step in

TGA started at 450 ºC and was attributed to the degradation of polymer chains. This step covered a wide temperature range due to the degradation of main and side chains.


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Also, PBI2 showed a lower char yield compared with PBI1 which is related to the higher density of sulfonate group in PBI2. Differential scanning calorimetry (DSC) analysis was performed to investigate different thermal behaviors of PBI1 and PBI2 (S6). As shown in S6, by adding side chains to the polymer backbone (PBI2), Tg decreased (154 °C for PBI2 compared to 273 °C for PBI1) due to the increased free volume between polymer chain and weakening of intermolecular interactions. 3.3.Mechanical properties The mechanical properties of PBI1 and PBI2 were measured in dry and hydrated states at room temperature and the results are reported in Table 2 and S7. Tensile strength reduced by adding side groups into the PBI2 backbone. In contrast, PBI2 showed a higher elongation than PBI1. Based on previous reports, tensile strength is reduced by increasing the amount of acid

4, 48

. Moreover, the presence of the side chain increased

the free volume and reduced the intermolecular interaction between the polymer chains, thereby decreasing strength. Based on previous literature reports on PBIs, higher free volume causes to decrease in tensile strength and increase in tensile strain

49

. The

mechanical properties of the membrane were measured at the fully hydrated state. The tensile strength of both membranes decreased in the wet form compared to the dry form. Nevertheless, elongation increased due to the increased hydrogen bonds in the wet form and the plasticizing effect of water molecules. Table 2. 3.4. X-Ray diffraction analysis (XRD)


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The XRD patterns for PBI1 and PBI2 are depicted in Fig. 3. PBI1 showed three sharp peaks at 2Ɵ = 7º, 25º, and 33º, and exhibited a semi-crystalline structure. On the contrary, PBI2 demonstrated a broad peak at 2Ɵ = 19º that indicates an amorphous morphology. The observed crystallinity in PBI1 can be attributed to strong intermolecular interactions (imidazole-imidazole and phosphoric acid groups). In PBI2, grafting of long side chains to the polymer backbone distorted the structural order in PBI1, and the membrane manifested an amorphous morphology. The results were also confirmed by SEM images. Fig. 3 3.5. Morphology The morphology of the prepared membrane was investigated by SEM, and the images are illustrated in Fig. 4. The PBI1 membrane exhibited a uniform morphology and, on the other side, the PBI2 membrane showed a highly porous morphology. The achieved morphology for PBI2 was also confirmed by theoretical studies. This microporous structure provided hydrophilic channels and facilitated proton transfer through the membrane. Also Yang et. al. reported addition of side chains increase free volume and acid absorption of membrane

49

. In addition, BET analysis was employed to prove the

porosity of prepared membranes. PBI1 showed a very low surface area (about 1.7 m2.g1

) and pore diameter (6 nm), while PBI2 exhibited a higher surface area and pore

diameter (about 47.9 m2.g-1 and 36 nm, respectively), indicating the higher porosity of PBI2. Fig. 4

3.6.Water uptake 15 ACS Paragon Plus Environment

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The water uptake for PBI1 and PBI2 is depicted in Fig. 5a. PBI1 showed an ordinary water uptake (about 27%), comparable with Nafion (Fig. 5a). Introducing densely sulfonated hydrophilic side chains increased the water uptake of PBI2 twofold more than PBI1. PBI2 attracted water molecules in hydrophilic holes through the intramolecular interaction with polar side groups. Also the hydrophilicity of PBI2 was enhanced in comparison with PBI1, the higher water-absorption capacity of PBI2 results from the porous morphology which is the most important factor in water-uptake enhancement. Fig 5. 3.7. Ion exchange capacity (IEC) and proton conductivity Proton conductivity depends on many factors, including strength, number, and position of ionic groups. IEC (determined by titration) specifies the density of ionizable protogenic functional groups in the membrane. IEC is strongly affected by vehicle and Grotthuss mechanisms 39. Fig. 5b describes the IEC values for PBI1 and PBI2 compared with Nafion. IEC values for PBI1 and PBI2 were found to be higher than those of Nafion, which can be attributed to the synergetic effect of phosphonate and sulfonate groups in the membranes. PBI2 also showed higher IEC values than PBI1, which is due to the addition of sulfonated side chains to polymer backbone. The proton conductivity of PBI1 membrane was measured in deionized water using AC impedance spectroscopy at 25 °C and equaled 0.0152 S.cm-1. The relatively good proton conductivity of PBI1 can be ascribed to the presence of –PO3H2 groups in the polymer structure. To improve the proton conductivity of PBI1, highly sulfonated side chains were introduced into the polymer backbone by covalent bonding. The modified membrane comprising covalently bonded –SO3H and – 16 ACS Paragon Plus Environment

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PO3H2 groups (PBI2) depicted a higher conductivity (0.0751 S.cm-1) in deionized water using AC impedance spectroscopy at 25 °C that was comparable with Nafion 117 (0.070 S.cm-1). Due to the high proton conductivity of PBI2 in deionized water, its proton conductivity was measured at different temperatures and humidity levels. The proton conductivity of PBI2 at various relative humidity levels (RH=30%, 60%) is demonstrated in Fig. 6a. The membrane demonstrated reasonable proton conductivity and a sharp increase at temperatures higher than 100 °C. As already mentioned, the formation of hydrophilic pores trapped water molecules and improved the proton conductivity of the membrane in low-humidity conditions

34

.

When the proton conductivity of PBI2 at RH=30% was compared with that of Nafion (Fig. 6b), a large gap was observed between their performances at low humidity, confirming the excellent efficiency of PBI2. In addition, the Arrhenius plot was depicted for PBI2 at RH=60% and RH=30% (Fig. 7). Activation energies were approximately 10.64 to 12.38 kJ.mol-1, representing the suitable performance of the membrane. The obtained activation energies for PBI2 were lower than those of Nafion 117 and many other previously reported PBIs

17, 48, 50-52

, suggesting that

the membrane can be used in anhydrous conditions. Fig.6 Fig. 7 When the active humidity was 0 (at temperatures lower than 100 °C) (Fig. 8), PBI2 proton conductivity decreased due to the reduction in internal humidity. At temperatures higher than 100 ºC, proton conductivity was enhanced due to the better proton dissociation and increased chain mobility which assisted proton transfer through the hopping mechanism. In fact, the proton conductivity of PBI2 switched 17 ACS Paragon Plus Environment

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from the vehicular mechanism to hopping mechanism at temperatures higher than 100 ºC. PBI2 was also doped with phosphoric acid, and the proton conductivity of phosphoric acid-doped membrane was measured at 80-180 °C and anhydrous conditions (Fig. 9). Based on Fig. 9, the proton conductivity of the membrane increased by temperature enhancement. This behavior could be attributed to the presence of phosphoric acid as a high boiling-point carrier. Regarding the porous morphology of PBI2, this membrane was able to trap phosphoric acid and showed a higher proton conductivity than the non-porous PBIs reported in the literature 53. Fig. 8. Fig. 9. As previously reported, the proton conductivity of Nafion was dramatically decreased at RH

Synergistic Behavior of Phosphonated and Sulfonated Groups on

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