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Novel Sulfonated Co-poly(ether imide)s Containing Trifluoromethyl, Fluorenyl and Hydroxyl Groups for Enhanced Proton Exchange Membrane Properties: Application in Microbial Fuel Cell Anaparthi Ganesh Kumar, Asheesh Singh, Hartmut Komber, Brigitte Voit, Bikash Ranjan Tiwari, MD Tabish Noori, Makarand M. Ghangrekar, and Susanta Banerjee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03452 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

Novel Sulfonated Co-poly(ether imide)s Containing Trifluoromethyl, Fluorenyl and Hydroxyl Groups for Enhanced Proton Exchange Membrane Properties: Application in Microbial Fuel Cell Anaparthi Ganesh Kumar1, Asheesh Singh1, Hartmut Komber2, Brigitte Voit2, Bikash Ranjan Tiwari3, Md. Tabish Noori4, Makarand M. Ghangrekar3 and Susanta Banerjee1* 1

Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur -721302, India.

2

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Strasse 6, 01069 Dresden, Germany.

3

Department of Civil Engineering, Indian Institute of Technology Kharagpur,

Kharagpur -721302, India. 4

Department of Agriculture and Food Engineering, Indian Institute of Technology Kharagpur,

Kharagpur -721302, India. Keywords: Sulfonated polyimide; hydrolytic stability; proton conductivity; peroxide radical resistance; microbial fuel cell.

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Abstract

A hydroxyl group-containing new cardo diamine monomer was synthesized, namely 9,9-bis (hydroxy-

(4’-amino(3-trifluoromethyl)biphenyl-4-oxy)-phenyl)-9H-fluorene

(mixture

of

isomers, HAPHPF). HAPHPF along with a sulfonated diamine monomer, 4,4’-diaminostilbene2,2’-disulfonic acid (DSDSA), were used to prepare a series of new sulfonated copolyimides by polycondensation with 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA). The degree of sulfonation (DS < 1) was adjusted by the feed ratio of DSDSA/HAPHPF and the copolymers were named as DHN-XX, where XX denotes the mole percentage of DSDSA (XX = 50, 60 and 70). The copolymers showed high molecular weights. The copolymer structure and composition were confirmed by FTIR and NMR techniques. Copolymer membranes were prepared through solution cast route by using dimethylsulfoxide as a solvent. The membranes showed high thermal, mechanical, hydrolytic and peroxide radical stability, and low water uptake and low swelling ratios. Well-separated hydrophilic and hydrophobic phase morphology was observed in TEM and AFM images of the copolymer membranes and was further supported by the SAXS studies. The proton conductivity of the DHN-70 was as high as 97 mS cm-1 at 80 oC and the value is significantly higher than that of the non-hydroxylated analogue. The membranes also showed superior microbial fuel cell (MFC) performance, similar like Nafion® 117 under similar test conditions. The chemical oxygen demand removal values provide substantial evidence that the fabricated membranes can be utilized in bioelectrochemical systems.


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1. Introduction. Fuel cells are important power source devices for future applications in automotive and portable applications.

1

There are different types of fuel cells depending on the application

temperature and the electrolyte that determines the kind of chemical reaction.

2

Commonly

known low temperature (< 100 oC) fuel cells are proton exchange membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC) and microbial fuel cells (MFC). The role of the polymeric membrane in PEMFC, alkaline fuel cells (AFC) and MFC is that of separator (placed in between anode and cathode), barrier (to both fuels at anode or cathode) and ion transporter (typically H+ or OH− from anode to cathode). 3-5 Perfluorinated sulfonic acid (PFSA) or Nafion® copolymer membranes are the mostly used PEM materials in a fuel cell because of their excellent thermal, mechanical and proton conduction properties. methanol permeability,

6

7

However, Nafion® membranes have also some drawbacks, e.g., high high cost, and restricted operating temperatures (≤ 80 °C).

8,9

These

issues provide great scope to researchers to develop alternative PEMs that have high proton conductivity and peroxide radical resistance, low methanol permeability, and are relatively cheap in production. Accordingly, over the years a large number of hydrocarbon-based sulfonated copolymers were developed by several researchers, such as sulfonated copolyimides (S-coPIs), 10–12 sulfonated copoly(arylene ethers) (S-coPAEs), 13–15 sulfonated co-poly(ether ether ketones) (S-coPEEKs), 16,17 sulfonated copoly(benzimidazoles) (S-coPBIs) 18,19 and sulfonated copoly(triazoles) (S-coPTAs). 20,21

Among these, S-coPIs exhibited high thermal, chemical and mechanical stability. However,

these polymers have limited solubility, low proton conductivity at a low degree of sulfonation (DS), and poor chemical and peroxide stability at high DS. In addition, due to excessive swelling 3 ACS Paragon Plus Environment


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in their hydrated state, these polymers have inferior dimensional stability. It is well known, that incorporation of fluorine in polymers helps in enhancing the polymer solubility, improve peroxide stability, and many other properties.

22

Accordingly, a number of -

CF3 (3F) or >C(CF3)2 (6F) groups containing polymers have been prepared to improve the electron density of the carbonyl carbon of the imide groups and to make the polymers more resistant to the attack from peroxide radicals.

22–24

Moreover, due to the hydrophobic nature of

these groups, the fluorinated polyimides show a low dimensional swelling in different humidity conditions and better hydrophilic and hydrophobic phase-segregated morphology that is responsible for high proton conductivity. 24 Polymers with kink-shaped moieties have also shown improved properties, not only for chemical, thermal, mechanical and peroxide stability, but also with regard to proton conductivities and fuel cell performance.

14

Therefore, over the years a

number of kink-shaped fluorene-based sulfonated poly(ether imide)s

25

has also been prepared

and their PEM properties have been investigated. Recently, a sulfonated poly(arylene ether sulfone) with hydroxyl groups has been prepared by Kwon et al.

13

It is reported there that

incorporation of phenolic –OH groups provides additional hydrogen bonding without increasing solubility of the polymers in water. In addition, the membranes showed high proton conductivities due to their enhanced phase-separated morphology and improved fuel cell performance compared to the non-hydroxylated analogous. Taking into account the beneficial effects of 3F, fluorenyl, and phenolic–OH groups, a new diamine monomer was designed and synthesized, namely 9,9-bis(hydroxy- (4’-amino(3trifluoromethyl)biphenyl-4-oxy)-phenyl)-9H-fluorene (mixture of isomers, HAPHPF). S-coPIs with different degree of sulfonation (DS) were prepared by the polycondensation reaction of an equimolar

amount

of

the

6-membered

dianhydride

1,4,5,8-naphthalenetetracarboxylic 4

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dianhydride (NTDA) and a mixture of the fluorenyl moiety containing diamine HAPHPF and the sulfonated diamine 4,4’-diaminostilbene-2,2’-disulfonic acid (DSDSA) of different molar percentage. Accordingly, the present study reports the synthesis and characterization of a series of new S-coPIs having all three beneficial groups. The synthesized polymers were structurally well characterized, their physical properties, morphology and proton conductivities were investigated, and those were compared with their analogous without OH groups. The fuel cell performances of the S-coPIs membranes were also tested using a single-chamber microbial fuel cell (sMFC).

2. Experimental. 2.1 Materials. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTDA), platinum (10 wt% loading on activated carbon), palladium (1 wt% on activated carbon), and Nafion® 117 solution (∼5%) were purchased from Sigma-Aldrich. 4,4’-Diaminostilbene-2,2’-disulfonic acid (DSDSA) was purchased from Alfa Aesar Ltd. Both NTDA and DSDSA were dried at 120 °C for 12 h prior to use. 9,9-Bis(3-methoxy-4-(4’-nitro(3-trifluoromethyl)biphenyl-4-oxy)-phenyl)-9H-fluorene was prepared according to the reported procedure.

25

Nafion®117 was purchased from Alfa Aesar

Ltd. and was heat treated before use according to the reported procedure.

26

Carbon felt was

purchased from Panex® 35, Zoltek Corporation. Pyridine, fuming hydrochloric acid (37%), triethylamine (TEA), hydrazine hydrate (80%), benzoic acid, sulfuric acid (98%), hydrogen peroxide (30%) and dimethylsulfoxide (DMSO) were purchased from E. Merck. The synthesis of monomer (mixture of isomers, HAPHPF) and copolymers (S-coPIs), and characterization details are provided in supporting information (SI). 5 ACS Paragon Plus Environment


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2.2. Synthesis 2.2.1 2.2.2

Synthesis of monomer (Scheme 1). Synthesis

of

9,9-bis(hydroxy-

(4’-nitro(3-trifluoromethyl)biphenyl-4-oxy)-

phenyl)-9H-fluorene (mixture of isomers, 2) Pyridine (143.82 mL, 141.24 g, 17.85 mol) was taken in 1 L round bottom flask equipped with a reflux condenser, nitrogen inlet and a stir bar. In it, concentrated HCl (160 mL) was added slowly under constant stirring and nitrogen blanket. The temperature of the reaction mixture raised to 110 °C due to this addition. The temperature of the reaction mixture was further increased to 210 oC and water was distilled off over a period of 5-6 h. To this hot melt, 9,9bis(methoxy- (4’-nitro(3-trifluoromethyl) biphenyl-4-oxy)-phenyl)-9H-fluorene (1) (Scheme 1) (20 g, 25.5090 mmol) was added slowly over a period of 1 h. The compound (1) gets dissolved in hot melt and the solution was allowed to stir at 160 °C for 18 h. The hot solution poured slowly in ice cold water (500 mL) and a white compound was precipitated out. The compound was dried under vacuum at 120 °C for 12 h. Yield: 90%. Structure of 9,9-bis(hydroxy- (4’nitro(3-trifluoromethyl)biphenyl-4-oxy)-phenyl)-9H-fluorene (meta/para isomer) with atom numbering was shown in Figure 1. FTIR (KBr, cm-1): 3460 (-OH stretching), 3071 (aromatic C-H stretching); 1633 (aromatic C=C stretching), 1418, 1461 (asymmetric C-O-C stretching); 1069 (C-F stretching), 1049 (symmetric C-O-C stretching), 845 (C-N stretching of –NO2), 815 (aromatic C-H bending).


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Figure 1: Structure of 9,9-bis(hydroxy- (4’-nitro(3-trifluoromethyl)biphenyl-4-oxy)-phenyl)9H-fluorene (meta/para isomer) with atom numbering.

1

H NMR (DMSO-d6, 30°C): 9.78 (s, 2H; OH), 8.28 (d, 4H; 21), 8.04 (d, 2H; 15), 8.0-7.9 (8H;

4,17,20), 7.51 (d, 2H; 1), 7.46 (t, 2H; 3), 7.41 (t, 2H; 2), 7.03 (d, 2H; 18), 6.87 (d, 2H; 13), 6.83 (d, 2H;10), 6.72 ppm (dd, 2H; 9).

2.2.3

Synthesis of 9,9-bis(hydroxy- (4’-amino(3-trifluoromethyl)biphenyl-4-oxy)-phenyl)-9H-

fluorene (mixture of isomers, HAPHPF).

9,9-Bis(hydroxy-

(4’-nitro(3-trifluoromethyl)biphenyl-4-oxy)-phenyl)-9H

fluorene

(2)

(Scheme 1) (10.0 g, 10.9540 mmol) was taken in a round bottom three-necked flask equipped with a reflux condenser and nitrogen inlet. The compound was dissolved in 1:1 mixture of 300 mL ethanol / THF, and 0.3943 g of palladium (1 wt %) on activated carbon was added. The temperature of the reaction mixture was raised to 80 oC and 50 mL of hydrazine hydrate was added dropwise over a period of 1 h using a dropping funnel and the reaction was continued for 7 ACS Paragon Plus Environment


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another 4 days. After this period, the hot reaction mixture was filtered and the maximum amount of ethanol, THF and excess hydrazine hydrate were distilled off from the filtrate using a rotary evaporator. The concentrated solution was poured into 500 mL distilled water and the precipitate was filtered, washed thoroughly with distilled water, and dried under vacuum at 60 oC. Finally, the crude product was purified by column chromatography (Silica bedded column, dichloromethane as eluent). A white compound was collected and dried under vacuum at 60 °C for 12 h. Yield: 60%. FTIR (KBr, cm-1): 3460 (O-H stretching), 3235, 3382 (N-H stretching), 3045 (aromatic C-H stretching); 1625 (aromatic C=C stretching), 1408, 1451 (asymmetric C-O-C stretching); 1061 (C-F stretching), 1044 (symmetric C-O-C stretching), 810 (aromatic C-H bending). Atom numbering (Figure S1) and NMR signal assignments of HAPHPF monomer (three isomers) is provided in the SI part (Section 1.1). 1H, 13C and 19F NMR data are summarized in section 3. [NMR studies on HAPHPF monomer].

2.2.4 Synthesis of copolymers (S-coPIs). The S-coPIs were synthesized by single step high-temperature polycondensation reaction of an equimolar amount of a dianhydride (NTDA) with a combination of two different diamines (DSDSA and HAPHPF) (Scheme 2). A typical synthetic procedure for the preparation of one of the S-coPI, DHN-50, is given here. A 50 mL three-necked round-bottom flask equipped with guard tube, condenser, stir bar and nitrogen inlet was charged with DSDSA (0.2105 g, 0.5680 mmol), triethylamine (0.2 mL) and 10 mL of m-cresol. The temperature of the reaction mixture was raised to 80 °C using an oil bath and stirred for 4 h till complete dissolution of DSDSA. After this, NTDA (0.3048 g, 1.1370 mmol), HAPHPF (0.4846 g, 0.5680 mmol), benzoic acid 8 ACS Paragon Plus Environment

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(0.4164 g) and additional 10 mL of m-cresol were added to the reaction flask. The oil bath temperature was slowly raised to 200 °C and the reaction was continued for 12 h. During this period the viscosity of the reaction medium increased dramatically. The polymer solution was cooled down to 60-80 °C, diluted with additional 10 mL of m-cresol and precipitated in large excess (300 mL) isopropanol. The fibrous precipitate was collected by filtration, washed two times with 200 mL acetone to remove any residual solvent. The obtained product was dried under vacuum 120 °C for 12 h. The other copolymers (DHN-60 and DHN-70) were prepared using the same procedure by varying the DSDSA/HAPHPF mole ratio. Structure of DHN-XX copolymers with atom numbering (only one isomeric structure of the HAPHPF unit is shown) was shown in Figure 2.

Figure 2: Structure of DHN-XX copolymers with atom numbering (only one isomeric structure of the HAPHPF unit is shown).

1

H NMR (DMSO-d6, 90°C): 8.9-8.6 (27), 8.40 (35), 8.1-7.7 (4,16,18,21,30,33), 7.7-7.25

(1,2,3,22,34), 7.1-6.65 ppm (9.9’,10,10’,13,13’,19). 13

C NMR (DMSO-d6, 60°C): 162.8 (25), 155.3 (14), 150.5 (6), 148.6 (12), 147.9 (11’), 146.2

(31), 143.7 and 143.5 (8), 141.0 (12’), 140.3 (11), 139.4 (5), 138.5 (20), 136.8 (8’), 135.2 (32),



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134.9 (23), 133.6 (29), 133.2 (17), 132.1 (18), 130.4 (27), 129.5 (22), 129.3 (34), 128.1 (35), 128-127.2 (2,3,30), 127.0 (21), 126.9-126.4 (26,28), 126.4-125.4 (1,9’,33), 125.0 (16), 123.4 (q, 1

JCF = 273 Hz, 24), 121.9-121.3 (9,13’), 120.4 (4), 119.4 (10), 118.0 123.4 (q, 1JCF = 32 Hz, 15),

117.5-116.2 ppm (10’,13,19). 19

F NMR (DMSO-d6, 60°C): -60.5 ppm (24).

2.3

Measurements and characterizations.

Fourier transform infrared (FTIR) spectra of the copolymer membranes were recorded using NEXUS 870 FTIR Thermo Nicolet spectrophotometer using Attenuated Total Reflectance (ATR) mode and powder samples were recorded using KBr pellets. 1

H (500.13 MHz),

13

C (125.76 MHz) and

19

F (470.59 MHz) nuclear magnetic resonance

(NMR) spectra were recorded on an Avance III 500 NMR spectrometer (Bruker) at 60 °C (13C, 19

F) and 90 °C (1H), respectively. To ensure full 1H relaxation a pulse delay of 60 s was applied

in the 1H NMR measurements. The monomer was measured at 30 °C. The spectra were referenced to the solvent signal (DMSO-d6: δ(1H) = 2.50 ppm, δ(13C) = 39.6 ppm). The 19F NMR spectra were referenced on external C6F6. The signal assignments were confirmed by 1H-1H and 1

H-13C correlated spectra. The molecular weight of the copolymers was determined by size-exclusion chromatography

(SEC) using DMAc with 3 g L-1 LiCl as an eluent at a rate of 1 mL min-1. Sample concentration was 1 mg mL-1. The apparatus consists of HPLC pump 1200 (Agilent Technologies), a Polar Gel-M column (300 x 7.5 mm), a refractive-index (RI) detector K-2301 (Knauer) and a MiniDAWN light scattering (LS) detector (Wyatt Technology). For calibration, linear PMMA with a molecular weight between 500 and 1,000,000 Da was used. 10 ACS Paragon Plus Environment

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The inherent viscosity was calculated by measuring the viscosity of copolymers (concentration 0.5 g dL-1 in NMP solvent at 30 °C) using Ubbelohde Viscometer. Thermogravimetric analysis (TGA) was carried out using TGA-Q50 instrument (TA Instruments) at a heating rate of 10 K min-1 under air atmosphere. The differential scanning calorimetry (DSC) was carried out using DSC-Q20 instrument (TA Instruments) at a heating rate of 20 K min-1 under a nitrogen atmosphere. The dynamic mechanical analysis was performed using NETZSCH DMA-242 E Artemis instrument (Netzsch) at a heating rate of 5 K min-1 at 10 Hz. 1 N load was applied to the membranes to maintain the amplitude up to 30 µm and samples dimensions were 15 mm × (0.07 to 0.08) mm × 5 mm. Mechanical properties were studied using a universal testing machine (TINIUS-OLSEN-H5KS). Three replicas of rectangular specimens for each sample of 25 mm length, 10 mm width and thickness of (0.07 to 0.08 mm) were used for the measurements. The values reported from the best experiments and correspond the reported Stress-Strain plot. The standard deviation of the measurements was below 4% of the mean value. The measurements were done at room temperature (35 oC) at a crosshead speed of 5% per minute of the gauge length (25 mm). Membrane morphologies were investigated using transmission electron microscope (TEM) instrument (FEI-TECNAI G2 20S-TWIN) at 100 kV. Samples were stained with 0.5 M AgNO3 aqueous solution for 12 h, rinsed with plenty of water and dried at ambient temperature. Stained samples were embedded with epoxy resin and sectioned to yield 300 nm thick slices at -60 oC using a Leica Ultracut UCT (Leica EM FCS) with a cutting speed of 0.6 mm s-1 and were dispersed on copper grids. The phase-separated morphology of copolymer membranes was studied by small-angle X-ray 11 ACS Paragon Plus Environment


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scattering (SAXS) instrument (XEUSS SAXS) in transmission mode. Prior to the test samples were stained in 0.5 M AgNO3 aqueous solution for 12 h, rinsed with plenty of water and dried at ambient temperature. The experiments were performed using Genix microsource (Xenocs), operated at 50 kV voltage and 0.6 mA current. The X-rays were generated from Cu Kα (λ = 1.54 Ao) source collimated with Xenocs FOX2D mirror (two pairs of scatter less slits). The 2D images are converted to 1D images using angular integration. The scattering vectors (q) were calculated by using the equation (1). q = 4π sinθ/λ

(1)

where, θ is the scattering angles and the inter-domain spacings (d) were calculated from corrected intensities scattering maxima using d = 2π/q. Surface roughness and topography were investigated by atomic force microscopy (AFM) using Agilent 5500 AFM instrument. Phase images were obtained with tapping mode under ambient conditions (26 oC) using etched silicon tip (square pyramid shape; force constant 42 N m-1; spring constant 1 N m-1; the radius of curvature of the tip 99

n. c.

n. c.

Measured in this study under same conditions; bdissolved or broken into pieces; cno change.

3.6 Microstructure of the DHN-XX copolymer membranes. The microstructure of the electrolyte membranes has a significant effect on the proton transport properties. Cross-sectional transmission electron microscopy (TEM) micrographs of DHN-XX copolymer membranes are presented in Figure 10. The ionic clusters morphology in random copolymers varies depending on the size and shape of the hydrophilic and hydrophobic repeat unit structures.

11,17,42,43

The cross-sectional TEM images of DHN-XX copolymer

membranes indicate the formation of microphase-separated morphology where black spherical region represents the hydrophilic ionic clusters (sulfonated groups) and the brighter region corresponds to the hydrophobic domains (copolymer backbone). 44 The size of hydrophilic ionic clusters increased as the degree of sulfonation increased.

44

Due to hydrogen bonding of the

phenolic groups, the distance between the ionic clusters decreases and this is also responsible to obtain connected ionic clusters morphology. 45,46



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Figure 10. Ag+ stained TEM micrographs depict the morphology of DHN-XX copolymers. The surface morphology of the membranes was investigated by Atomic Force Microscopy (AFM). The surface topography and phase contrast images of the DHN-XX copolymers are presented in Figure 11. In typical PEMs, two types of structured domains are observed, i.e., hard and soft structured domains.

25,45,46

The hard and soft structures are formed from hydrophobic

and hydrophilic (i.e., aggregation of sulfonic acid groups) segments respectively. Figure 11 clearly shows the phase separated morphology for all DHN-XX copolymer membranes. The images showed surface roughness and phase angles from 12 to 22.5 nm and 17 to 95°, respectively. The color contrasts in the topographic images [Figure 11. (a), (c), (e)] were due to the penetration of the tip in soft structures. In all images [Figure 11. (b), (d), (f)] phase contrast arises from the aggregation of sulfonic acid groups; in addition hydrogen bonding of phenolic – OH groups generates closely connected ionic channels that become more prominent in DHN-70 13,45,47,48

and that are also responsible for its highest WU. 12 Both TEM and AFM images confirm

the formation of a strongly phase-separated morphology for all the copolymers and for closely connected ionic channels in DHN-70. In contrast, previously reported analogous DAN-XX copolymers without –OH groups showed less developed and separated ionic clusters. 25 36 ACS Paragon Plus Environment

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Figure 11. AFM images of DHN-XX copolymer membranes (tapping mode) of scan area 2.5 × 2.5 µm. (a), (c) and (e) represent topographical height images; (b), (d) and (f) represent phase images. Small-angle X-ray scattering (SAXS) studies of the copolymer membranes and Nafion® 117 were performed for better understanding the membrane morphology. The SAXS patterns (Figure

12) show that the ionic scattering maximum of DHN-70 and DHN-60 shifted to a larger q value in comparison to the DAN-70 and DAN-60 membranes, which is in accordance with their ionic cluster sizes observed in TEM and AFM. As the mole ratio of hydrophilic repeat unit structure increases, the mode of arrangement and packing of ionic cluster morphology also changes in the copolymer membranes, and consequently, the scattering vector (q) was shifted to larger values. As DS increases, the size or local density of the ionic clusters increases or the phase separation between hydrophilic and hydrophobic domains decreases, which leads to a shifting of the scattering maximum towards larger q.

49

For Nafion®117, the ionic scattering maximum was far

higher than for DHN-XX and DAN-XX copolymer membranes due to the high local 37 ACS Paragon Plus Environment


concentration of ionic clusters of sulfonic acid groups separated from polymer main chain which leads to a high degree of phase segregation. 50 The broad SAXS profile of DHN-XX and DANXX copolymer membranes demonstrates the long-range order of ionic domains present in these materials.

Nafion 117

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DHN-70 DHN-60

DAN-70 DAN-60 0.1

1

-1

10

Scattering vector q (nm )

Figure 12. The SAXS patterns of DHN-XX and DAN-XX copolymer membranes and of Nafion® 117.

3.7. Proton conductivity and single-chamber MFC performance of DHN-XX membranes. The proton conductivity of the membranes was calculated using equation (6) and the values are listed in Table 5. The DHN-XX membranes with IEC ranging from 1.18 to 1.87 meq


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-1

-1

-1

g showed proton conductivity between 15 to 43 mS cm at 30 ºC and 35 to 97 mS cm at 80 ºC in their hydrated state. At room temperature, an increase in proton conductivity was observed in DHN-XX copolymers compared to analogous DAN-XX copolymers with the same DS values, e.g. the proton conductivity of DHN-70 was increased by 30 % compared to the analogous DAN-70 with similar IEC values. This is due to the higher WU of DHN-XX copolymers and the presence of phenolic –OH groups that form hydrogen bonds with the water molecules through electrostatic forces of attraction.

7,13

By that, water channels are formed that are responsible for

attaining large and connected ionic clusters that led to a higher proton conductivity of the DHNXX copolymers compared to the DAN-XX copolymers, despite having similar IEC values. 12,45,48 The DHN-XX copolymers also showed higher proton conductivity at relative humidity (RH) of 60% and 80% than the DAN-XX copolymers. This is because the phenolic –OH groups are assisting to the sorption of more water in the closely spaced ionic clusters, which results in the enlarged ionic clusters through hydrogen bonding. The temperature dependent proton conductivities of the S-coPI membranes were plotted in

Figure 13. The activation energy values for the proton conduction were calculated from the slope of linearly fitted Arrhenius plots. The activation energies of all the copolymers were in the range of 14.14 to 14.67 kJ mol-1 and the values were marginally higher than that of Nafion® 117 (13.56 kJ mol-1). This result supports that at higher temperature proton conductivity increases due increase in mobility of the protons (Table 5). 47,51,52



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Figure 13. Temperature-dependent proton conductivity plots of DHN-XX copolymer membranes. For a comprehensive understanding of the improvement in proton conductivity of S-coPI membranes, the effective proton concentration (CH+) and effective proton mobility (µH+) were calculated at different temperatures by using the equation (7) and (8), respectively.

27

Size and

effective connectivity of ionic clusters in DHN-XX copolymer membranes showed higher proton mobility in comparison to the DAN-XX copolymer membranes (Table 5).

47,51,52

This further

indicates the superiority of DHN-XX copolymer membranes over DAN-XX copolymer membranes in PEM application.


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Table 5. Proton conductivity, activation energy and proton mobility of the S-coPIs. Polymer

dM (g cm-3)

IECw (meq -1 g )

-1

σ (mS cm )

14.6

30 ºC 1.12

80 ºC 1.06

30 ºC 0.17

80 ºC 0.37

DHN-50

1.07

1.18

15

21

DHN-60

1.09

1.51

20

34

24

54

14.2

1.39

1.30

0.34

0.73

DHN-70

1.17

1.87

36

63

44

97

14.1

1.52

1.41

0.69

1.42

DAN-50a

1.05

1.17

10

15

11

20

11.8

1.11

1.04

0.13

0.21

DAN-60a

1.08

1.49

16

25

16

39

14.7

1.37

1.31

0.23

0.53

DAN-70a

1.15

1.85

31

43

32

75

15.4

1.75

1.65

0.58

1.28

1.98

0.91

45

87

98

165

13.56

1.31

1.27

1.03

2.18

®

a

80% RH 80 ºC

µH+ (10 cm-2 s-1 V-1) -3

Hydrated state 30 80 ºC ºC 15 34

Nafion 117a

60% RH 80 ºC

CH+ (meq cm-3)

Ea (kJ mol-1)

Measured or calculated in this study under similar conditions.

Microbial fuel cells (MFC) are bioelectrochemical systems that utilize electrogenic microorganisms as a biocatalyst to carry out oxidation of organic matter present in wastewater, a clean and green fuel, to generate electricity.

53

Among various factors which influence the

performance of MFC, the proton exchange membrane has a significant contribution to the power generated. Nafion® 117 is the commonly used membrane in MFCs.

53,54

However, it would be

imperative to bring out technical advancement in the fabrication of PEMs using different materials for diversification and to draw different scientific benefits. The microbial fuel cell performance tests of the S-coPI membranes and Nafion® 117 were performed at 30 ± 2 oC using a single-chambered microbial fuel cell (Figure 3) for a period of 30 days, during which no drop in performance of MFC (Figure S10) and no visible sign of bio-fouling in the membranes was



Page 42 of 58

noted. Typically, polarization studies were conducted to determine the optimum power density and the corresponding current density, which can be achieved for the MFCs operated with different membrane configurations (Figure 14). The maximum power density exhibited by the MFCs is given in Table 6. These values are in accordance with the proton conductivity of the DHN-XX membranes.

Figure 14. Power density vs. current density curves for MFCs (during polarization). The voltage vs. current density curve exhibits three different resistances corresponding to three different regions of overpotential prevailing in a bioelectrochemical system, viz. charge transfer resistance (active resistance) owing to activation overpotential (at low current density range), 42 ACS Paragon Plus Environment

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ohmic resistance owing to ohmic overpotential (at intermediate current density range), and diffusion resistance owing to concentration overpotential (at high current density range).

31

Charge transfer resistance arises due to the inherent activation energy barrier associated with the initiation of an oxidation or reduction reaction which can be responsible for decelerating the reaction kinetics of the corresponding reaction. In order to decrease the effect of activation overpotential, platinum catalyzed electrodes are used in all the MFCs. The ohmic resistance, also known as the internal resistance of the system, is the resistance offered by the electrolyte, proton exchange membrane, and electrical connections to the flow of charge particles, i.e. electrons and protons. On account of the similarity in system architecture, electrode configuration and electrolyte for all the MFCs under consideration, the difference in internal resistance values can be attributed to the difference in the proton exchange membranes used. The internal resistance is calculated from the slope of the linear portion of the voltage vs. current density curve (Figure 15). Among the fabricated membranes, DHN 70 with highest values for proton conductivity (44 mS cm-1 at 30 o

C) as well as IEC (1.87 meq g-1) leads to lower internal resistance values (206 Ω) of the

corresponding MFC-2 as compared to MFCs incorporating other S-coPI membranes. The internal resistance values of the MFCs are listed in Table 6. The MFC incorporating Nafion® 117 membranes was found to have an even lower internal resistance of 199 Ω as compared to all other fabricated membranes. However, the difference in internal resistance and power density values of MFC-1 in comparison to MFC-2 was marginal. All these results suggest that DHN-70 can be used as alternate membrane material to Nafion® 117 in an MFC application. In addition, the membranes prepared in this investigation showed better results in terms of power density as compared to many other hydrocarbon based membranes when incorporated in MFC.

55-58



Page 44 of 58

However, it is always not possible to compare the membrane performance owing to different fuel cell configurations and their operating conditions. 4,5,59

Figure 15. Voltage vs. current density curves for MFCs (during polarization).


Page 45 of 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6. Chemical oxygen demand (COD), coulombic efficiency (CE), and power density values of DHN-XX, DAN-XX and Nafion®117 membranes. Polymer

Code

IECw -1 (meq g )

Nafion®117a

MFC1

0.91

83.0±2.3

52.2±2.4

199

Maximum Power density (mW m-2) 536

DHN-70

MFC-2

1.87

80.8±2.5

44.8±3.1

206

500

DAN-70a

MFC-3

1.85

78.0±1.5

43.9±2.4

216

440

DHN-60

MFC-4

1.51

79.5±2.3

42.6±3.1

219

401

DAN-60a

MFC-5

1.49

76.8±2.8

36.2±2.2

228

382

DHN-50

MFC-6

1.18

77.8±2.3

35.5±1.7

232

355

DAN-50a

MFC-7

1.17

77.5±4.2

35.1±3.1

240

346

a

COD (%)

CE (%)

Internal resistance (Ω)

Measured or calculated in this study under similar conditions.

MFCs have been also projected to facilitate wastewater treatment in addition to power generation. In the present study, synthetic wastewater having acetate as the sole electron donor for all the MFCs was used. The electrogenic microorganisms present in the inoculum inhabit the anode electrode and utilize acetate to carry out various life functions. Highest COD removal of 83.0 ± 2.3% was found in the MFC-1 incorporating Nafion® 117 membranes (Table 6). Although, the COD removal values for MFCs incorporating S-coPI membranes were lower than for MFC-1, the difference was only in the range of 2 - 7%. Average COD removal values of the MFCs are listed in Table 6 and the values are comparable for all the S-coPI membranes. These values provide substantial evidence that the DHN-XX copolymer membranes can be utilized in bioelectrochemical systems.



Page 46 of 58

Coulombic efficiency (CE) is described as the ratio of the number of coulombs generated in an MFC to the theoretical number of coulombs originally present in the substrate utilized (equation 9).

29

Average coulombic efficiency values are shown in Table 6 and the values followed the

same trend as that of power generation by the MFCs.

4. Conclusions. A new cardo diamine monomer (HAPHPF) containing phenolic –OH groups was synthesized. Detailed characterization of the monomer by different NMR techniques indicated the formation of three different isomers. The monomer was used along with DSDSA to prepare several copolyimides (DHN-XX) with a controlled degree of sulfonation on reaction with NDTA. The degrees of sulfonation of the copolymers determined by titrimetric analysis, SEM-EDX sulfur mapping, and 1H NMR spectroscopy, respectively, were in good agreement with the feed composition of the monomers indicating that different isomers of HAPHPF do not influence the imidization reaction, and high-molecular-weight polymers with the expected composition are formed. Also, the copolymers were readily soluble in several aprotic polar organic solvents at room temperature with a concentration of 10 % (w/v). The solution cast membranes showed higher thermal and mechanical stability than analogous sulfonated copolymers without any –OH groups. Due to the formation of additional intermolecular hydrogen bonds between the adjacent polymer chains through –OH groups in DHN-XX, the membranes showed higher storage modulus than corresponding DAN-XX copolymer membranes not containing phenolic groups. The DHN-XX copolymers showed a good balance between WU and swelling ratio values and the obtained WU values are higher than that of DAN-XX copolymers. By increasing the DS of the copolymers, formation of connected and well developed ionic clusters was observed in TEM 46 ACS Paragon Plus Environment

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and AFM images which was further supported by SAXS patterns. The DHN-XX copolymers showed superior peroxide radical resistance in comparison to DAN-XX copolymer membranes. Additionally, the phenolic –OH groups in DHN-XX copolymers form hydrogen bonds with the water molecules which are responsible for the higher hydrolytic stability compared to that of DAN-XX copolymers. The DHN-XX copolymer membranes showed also higher proton conductivity values than their non-hydroxylated analogues, the DAN-XX copolymer membranes. The proton conductivity of DHN-70 copolymer was increased by 30 % compared to the analogous DAN-70 copolymer of similar IEC values. This was attributed to the presence of phenolic –OH groups that form additional hydrogen bonds with the water molecules through electrostatic forces. In addition, application of DHN-XX copolymer membranes in microbiotic fuel cells (MFCs) demonstrated excellent power density and coulombic efficiency, which were found to be comparable with that of MFC using Nafion® 117. Thus, it is imperative to conclude that the fabricated membrane qualifies to be used as an alternative membrane to Nafion® 117 for application in bioelectrochemical systems.

Associated content Supporting information The supporting information is available free of charge on ACS publications website at DOI: XXX. Monomer and polymer syntheses, details of the measurements and characterization techniques, microbial fuel cell experimental section and additional discussion on NMR analysis of monomer and copolymers.



Page 48 of 58

Author information *

Corresponding author: Tel. +91-03222-283972. S. Banerjee. (Email: [email protected])

Notes The authors declare no competing financial interests.

Acknowledgements A.G. Kumar acknowledges IIT Kharagpur, India for the financial support in the form of a fellowship to carry out this work. Ch. Harnisch (Leibniz-Institut für Polymerforschung Dresden e. V.) is acknowledged for SEC measurements. Thanks to, Dr. Soumendu Bisoi and Arun Kumar Mandal from IIT Kharagpur for their time to time support carrying this work.

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