Fluorenyl phenolphthalein groups containing a multi-block copolymer membrane for alkaline fuel cells

Abstract

An anion-conducting aromatic multi-block copolymer (PEs-A_xB_y) was synthesized by a block co-polycondensation reaction between fluorene and phenolphthalein containing oligomers. The quaternized multi-block copolymers (QPEs) resulted in ductile transparent membranes with well-ordered multi-block structures. In chloroform, a thin polymer film (PE-A₂₂B₂₄) with hexagonal well-ordered geometry was obtained. Meanwhile, in NMP/DMAc a dense thin film for QPE-A₂₂B₂₄ was obtained. As an additional attractive feature, the pore structure and pore-geometry of the membrane thin film may be controlled by the membrane casting medium and conditions. The presence of hydrophilic (biphenyl fluorene)–hydrophobic (connector)–hydrophilic (phenolphthalein) groups was responsible for phase separation, and interconnected ion conducting pathways. Among many synthesized anion exchange membranes (AEMs), QPE-A₂₂B₂₄ exhibited excellent stabilities, 0.95–2.24 meq g⁻¹ ion exchange capacity and 95 mS cm⁻¹ conductivity, at 80 °C. The reported AEM retained the conductivity after 1000 h of boiling treatment. QPE-A₂₂B₂₄ membranes were assessed as suitable candidates for alkaline fuel cell applications.

---

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) have attracted extensive attention as clean and renewable energy sources.¹ The commercialized perfluorosulfonic acid based polymer electrolyte membrane (PEM) (Nafion; DuPont), is state-of-the-art for PEMFCs because of high proton conductivity, thermal, chemical and mechanical stabilities.² Requirement of precious metal (platinum) electro-catalysts, insufficient stability of membranes under fuel cell operating conditions, environmental incompatibility and high cost of perfluorinated membrane impede commercialization of PEMFCs.³

Anion exchange membrane fuel cell (AEMFC) was proposed as an alternative,⁴ because of good oxygen reduction kinetics, transition metal (Co, Ni, etc.) electro-catalysts and relatively high fuel cell performance.⁵ In addition, AEMFCs offer fuel flexibility (e.g. methanol, ethanol, ethylene glycol, etc.), low over-potential for fuel oxidation and fuel crossover.⁶ Variety of anion exchange membranes (AEMs) based on organic–inorganic hybrid materials,^2,7 aliphatic ethers,⁸ acrylate derivative,⁹ aromatic compounds,¹⁰ polyamides,¹¹ polyacrylamide,¹² poly aromatic ether,¹³ polyether amide,¹⁴ fluoro backbone,¹⁵ were reported.

Intensive research efforts were devoted to develop the AEMs with well defined hydrophilic–hydrophobic segments for improved IEC, conductivity, thermal stability, transparency, reflecting index, and low dielectric constant.¹⁶ We are reporting a multi-block copolymers containing hydrophilic (biphenyl fluorene)–hydrophobic (connector)–hydrophilic (phenolphthalein) blocks to achieve significant phase separation. Due to bulky aromatic structure (phenolphthalein and fluorenyl), breath figures were formed by water condensation employed as a physical micropattern,¹⁷ with good stabilities. Incorporation of phenolphthalein and fluorenyl blocks,¹⁸ and rigid bisphenol with pendant lactone are expected to provide reactive sites for grafting the functional groups.¹⁹

We designed well-ordered multi-block copolymers (PE-A_xB_y) containing phenolphthalein–fluorene oligomers. Three types of multi-block copolymers (PE-A₁₂B₁₈, PE-A₁₈B₂₀, and PE-A₂₂B₂₄) with varied block lengths were synthesized via coupling reactions. Controlled chloromethylation and ammination at specific position of fluorenyl and lactone group of the poly (aryl ethers) was achieved via Friedal–Crafts alkylation reaction.

2. Experimental section

2.1 Materials

4,4′-Diflurophenyl sulfone (FPS), 4,4′-difluorobenzophenone, 9,9′-bis(4-hydroxyphenyl)fluorene (BHF), phenolphthalein, chloromethyl methyl ether (CMME), trimethylamine (35 wt%) were purchased from Aldrich Chemicals and used as received. 1,1,2,2-Tetrachloroethane (TCE), N,N-dimethylacetamide (DMAc), methanol, acetone, chloroform, toluene, tetrahydrofuran, ethanol, K₂CO₃, NaOH, and ZnCl₂ (anhydrous) were obtained from S.D. Fine chemicals, India, and used without further purification. Deionized (DI) water obtained from Milli-Q system was used for all experiments.

2.2 Preparation of multi-block copolymer

A typical procedure for preparation of precursor multi-block copolymers (PEs-A_xB_y) (x and y represents the number of repeat units in the hydrophilic segments) has been described below.

2.2.1 Synthesis of oligomer A. Fluorene-containing oligomer (A₂₂) was synthesized in a round-bottom flask (500 mL) fitted with Dean–Stark trap to collect the water produced during the progress of polymerization. FPS (12.71 g, 50 mmol), BHF (18.24 g, 52.08 mmol), and K₂CO₃ (18.50 g, 133.86 mmol) were charged under nitrogen. The solvent mixture of DMAc (160 mL) and toluene (40 mL) was added and the round-bottomed flask was immersed into an oil bath preheated at 145 °C to dissolve the monomers completely. The polymerization reaction was continued for 2 h. Then the Dean–Stark trap was replaced by condenser and temperature was increased to 170 °C followed by addition of FPS (0.50 g, 1.42 mmol) at to ensure end capping of the oligomer and reaction was further continued for another 3 h. The hot reaction mixture was poured into DI water and stirred for 1 h for purification. By similar procedure, other oligomer (A₁₂ & A₁₈) was also prepared. The degree of polymerization (x) of the oligomer A was estimated from integral ratio of the NMR spectrum. This was further supported by gel permeation chromatography (GPC, Waters 2695) equipped with Styragel (hr 0.5, hr 4E, and hr 5) columns with reflective index detector. Molecular weight was calibrated with standard polystyrene samples.

2.2.2 Synthesis of oligomer B. Oligomer B₂₄ was synthesized in a three-neck round-bottomed flask (500 mL) fitted with Dean–Stark trap. 4,4,-Diflurobenzophenone (10 g, 52.08 mmol), phenolphthalein (17.2 g, 54.00 mmol), K₂CO₃ (13.8 g, 100 mmol), N,N-dimethylacetamide (160 mL) and of toluene (40 mL), were charged under nitrogen. The reaction temperature was maintained at 145 °C for 3 h then the Dean–Stark trap was replaced by water cooled condenser and the reaction temperature was increased to 170 °C. After 4 h reaction light brown coloured viscous solution was continued. A small amount of phenolphthalein (0.50 g, 1.57 mmol) was added to the mixture for end-capping of the oligomer with hydroxide containing terminal groups. The mixture was poured drop wise in large excess of hot DI water. The precipitated crude oligomer was washed with DI water and methanol several times, filtered with Whatman filter paper (0.45 μm, pore size) and dried under vacuum at 80 °C. Other oligomers (B₁₈ & B₂₀) was also prepared by similar procedure. Degree of polymerization was measured with NMR spectrum and by GPC.

2.2.3 Preparation of multi-block copolymer (A₂₂B₂₄). A three necked 500 mL round-bottomed flask equipped with Dean–Stark trap was charged with oligomer A₂₂ (13.5 g, 10 mmol), B₂₄ (12.1 g, 10 mmol), FPS (0.254 g, 10 mmol), K₂CO₃ (3.455 g, 25 mmol), CaCO₃ (25.048 g, 250.00 mmol), DMAc (100 mL), and toluene (25 mL) under nitrogen. The polymerization reaction was carried out at 145 °C for 3 h. Then the Dean–Stark trap was replaced by condenser and temperature was increased to 170 °C and maintained for 7 h. The reaction mixture was further diluted with 100 mL DMAc and poured in boiling water. Obtained crude product was washed with hot water and methanol for several times, filtered and dried at 80 °C under vacuum. The obtained multi-block copolymer was again dissolved in chloroform (400 mL) and reprecipitated in acetone (600 mL). Purification process was repeated for several times for complete removal of calcium carbonate trapped in polymer matrix. Complete removal of impurity was confirmed by EDX spectra.

2.3 Chloromethylation of multi-block copolymer

For chloromethylation reaction, a 50 mL pressure glass bottle was charged with PE-A₂₂B₂₄ (1.0 g) and TCE (20 mL) with continuous stirring condition. After dissolution of copolymer (ambient temperature), ZnCl₂ (0.1414 g) solution in THF (1.0 mL) and CMME (3.5 mL) were added. The reaction was carried out at 45 °C for 180 h to obtain yellow mixture. Obtained mixture was diluted with ca. 20 mL of TCE and poured drop wise into excess of methanol. The crude product was washed with deionized water and methanol several times. White powdered chloromethylated copolymer was obtained after filtration and drying at 80 °C under vacuum.

2.4 Membrane preparation and quaternization

The chloromethylated copolymer CPE-A₂₂B₂₄ (1.000 g) was dissolved in TCE (25 mL) and solution was filtered with Whatman filter paper (0.45 μm). The filtrate was cast as thin film onto a flat glass plate and dried at 60 °C overnight to obtain transparent, tough membrane (60 ± 5 μm). Quaternization was achieved by aqueous trimethylamine solution (30 wt%) at room temperature (48 h). Obtained quaternized membrane was washed with water several times and immersed in NaOH solution (1 M) for 48 h to convert into alkaline form. The membrane was characterized using different instruments; water uptake, hydrolytic stability, ion exchange capacity, counter ion transport number, and membrane conductivity, described in ESI (Section S1–S6).†

3. Results and discussion

3.1 Synthesis of precursor and multi-block copolymers

Series of precursors (poly(arylene ether sulphone ketone)) containing fluorenyl and phthalide groups were synthesized by nucleophilic substitution reaction (Scheme 1). The polymers were designated as PE-A₁₂B₁₈, -A₁₈B₂₀, -A₂₂B₂₄ (where A and B represent hydrophilic block, and numerical value represents the repeating unit). Their chemical structures were confirmed by ¹H, ¹³C NMR, FT-IR, elemental analysis and molecular weight by GPC measurements (Fig. S1–S6, ESI† and Tables 1 & 2). Length of oligomers was determined by integral ratio between terminal shorten peak and their perspective polymerized proton peak. Length of fluorenyl group containing oligomer (A) was shorter than calculated feed amount. While phthalide group containing oligomer (B) showed less variation. Heating time (3–5 h) was kept less for both cases to avoid the unfavourable side chain reaction. Synthesized hydrophilic block and multiblock copolymer were soluble in organic solvents (chloroform, dichloroethane, 1,1,2,2-tetrachloroethane, DMAc, DMF, THF, and DMSO). Molecular weight of synthesized precursor and polymer was measured by GPC (Tables 1 & 2). Average molecular weight (M_n) of polymers was several times higher than the corresponding copolymer. Structure of multi-block copolymer was proposed by ¹H & ¹³C NMR and FT-IR studies (Scheme 1). Good agreement between copolymer composition and oligomer feed ratio, confirmed controlled polymerizations.

Scheme 1 Synthesis of multi-block copoly(arylene ether)s containing fluorene and phenolphthalein groups.

Table 1 Molecular weight (g mol⁻¹) of phenolphthalein–fluorene containing oligomer

Oligomer	Expected oligomer length	Obtained oligomer length^a	Obtained oligomer length^b	Mn	Mw	Mp	Mz	Mz+1	Mn/Mw
a Estimated from the ^1H NMR spectra.b Estimated from the GPC data.
A12	18	12	12	6585	7265	7868	7929	8543	1.103
A18	20	18	17	9607	9987	10 021	10 375	10 761	1.040
A22	24	22	25	14 403	21 283	20 938	28 396	35 266	1.477
B18	18	18	24	12 029	18 065	18 065	16 796	32 291	1.501
B20	20	20	25	12 817	17 840	16 364	23 670	29 681	1.392
B24	24	24	28	14 569	20 379	19 227	27 224	34 398	1.399

---

Table 2 Molecular weight of the multi-block polymer

Polymer	Expected A/B length	Obtained A/B length	Mn	Mw	Mw/Mn
PE-A12B18	18/18	12/18	61 400	77 978	1.270
PE-A18B20	20/20	18/20	62 600	76 872	1.228
PE-A22B24	24/24	22/24	77 500	10 4000	1.342

---

3.2 Synthesis and characterization of chloromethylated well-ordered multi-block copolymers (CPEs-A_xB_y)

Chloromethylation of multi-block copolymer was achieved by the method reported earlier^16b (Scheme 1). Chemical structure and degree of chloromethylation (DC) for multi-block copolymer were estimated by elemental analysis, ¹H & ¹³C NMR, and FT-IR spectra. ¹H NMR spectra for chloromethylated polyether (CPE) (Fig. 1) showed two new peaks at 4.59 and 4.62 ppm assigned to methylene protons in –CH₂Cl group (absent in precursor PE). Peaks between 6.8 and 7.8 ppm were altered (Fig. 1b) due to chloromethylation at 2- & 7-position of fluorenyl group and 2′-position of phthalimide group.

Fig. 1 ¹H NMR spectra for: (a) PE-A₂₂B₂₄; (b) CPE-A₂₂B₂₄; and (c) QPE-A₂₂B₂₄ (IEC = 1.8 meq g⁻¹) membranes.

DC values were calculated by integral ratio between peaks aroused at 4.59 and 4.63 ppm, for multi-block copolymer and increased with time (¹H NMR spectra) (Fig. 2, and S7, ESI†). In ¹³C NMR, two new peaks at 45.1 and 46.9 ppm were generated due to methylene carbon (Fig. S8, ESI†), which indicate chloromethylation of phthalide groups. DC values are depicted in Table 3. At 45 °C, DC value was increased with reaction time (Fig. 2). Beyond 2.35 chloromethyl groups per repeated unit, polymer was gelated (insoluble in any organic solvent) due to high degree of cross-linking. Variation of peak was further confirmed by the FT-IR (Fig. S9, ESI†).

Fig. 2 ¹H NMR spectra for CPE-A₂₂B₂₄ membrane with varied reaction conditions: (a) without chloromethylation (PE-A₂₂B₂₄); (b) DC = 0.80 (80 equivalent CME at 45 °C for 24 h); (c) DC = 1.50 (80 equivalent CME at 45 °C for 48 h); and (d) DC = 2.04 (80 equivalent CME at 45 °C for 180 h).

Table 3 DC for CPEs, IEC, water uptake, λ, and κ^m values for QPE membrane^e

Membrane	Reaction time^a (h)/DC	IEC^b (meq g^−1)	IEC^c (meq g^−1)	Water uptake (%)	λ^X	tm̄	κ^m^d (mS cm^−1)	Φw	Φm	P/10^−7 cm^2 S^−1
										30% (MeOH–H2O)	50% (MeOH–H2O)
a Reaction temperature: 45 °C; degree of chloromethylation calculated by ^1H NMR.b IEC calculated by ^1H NMR.c 45 °C: IEC calculated from titration methods.d κ^m values were measured at 60 °C in water.e λ^X represents number of absorbed water molecules per ammonium group.^16b
QPE-A12B18	24/0.62	0.58	0.55	22.26	22.46	0.73	3.7	—	—	—	—
	48/1.52	1.47	1.33	37.76	15.75	0.85	40	—	—	—	—
	180/2.12	2.05	1.95	59.92	17.05	0.90	73	—	—	—	—
QPE-A18B20	24/0.95	0.86	0.83	28.57	19.10	0.77	8.2	—	—	—	—
	48/1.80	1.75	1.71	40.36	13.09	0.88	53	—	—	—	—
	180/2.25	2.16	2.10	66.03	17.45	0.93	79	—	—	—	—
QPE-A22B24	24/1.05	0.98	0.95	24.30	14.19	0.81	12	0.27	0.34	5.3	12.1
	48/1.95	1.84	1.80	50.90	15.69	0.90	59	0.30	0.38	1.1	8.4
	180/2.35	2.29	2.24	82.46	20.42	0.94	91	0.35	0.40	0.7	3.2

---

3.3 Membrane preparation and quaternization

The chloromethylated multi-block copolymers (CPEs) were dissolved in TCE for membrane casting (100 μm). Transparent membranes with ductile nature were dipped in trimethylamine solution (35 wt%) for 48 h at room temperature for quaternization (Fig. S10a, ESI†). Pale yellow coloured transparent quaternized membranes (QPEs) (Fig. S10b & c, ESI†) were equilibrated in NaOH (1 M) for 24 h at room temperature to convert into hydroxide form. QPEs membranes were less soluble in organic solvent (DMSO, CHCl₃, NMP, DMAc, TCE etc.) and insoluble in acetone, THF, and methanol. The chemical structure of the QPEs were analysed by ¹H, ¹³C NMR, and FT-ATR spectra (Fig. S11–S13, ESI†). ¹H NMR spectrum for QPE-A₂₂B₂₄ (IEC: 1.80 meq g⁻¹) showed two strong peaks at 2.93 and 3.01 ppm, assigned to methyl proton of quaternary ammonium groups (Fig. 1c). ¹H NMR spectrum for QPE-A₂₂B₂₄ membrane (IEC: 2.24 meq g⁻¹) was not recorded due to its low solubility in common organic solvents. Substitution of chloro- with ammonium groups was confirmed by magnetic field shifting from 4.62 ppm to 4.82 ppm.

Integral ratios between two peaks (4.85 and 2.93–3.01 ppm) were quantitatively used to determine the IEC values (Table 3). IEC values obtained by classical titration were compared with those obtained from NMR spectra, and good agreement was found. Different carbon peaks (¹³C NMR spectra) are systematized for QPE membranes in Fig. S12 (ESI).† Disappearance of carbon peaks at 45.1 and 46.9 (CPEs), and appearance of additional peaks at 52.60, 53.00, 64.44, and 69.33 ppm (QPEs) confirmed quaternization. FT-ATR spectra also showed new absorption bands at 1685 and 3295 cm⁻¹, due to quaternary ammonium groups (Fig. S13, ESI†).²⁰

3.4 Surface morphology and dynamic radius

Surface morphology of polymer and membrane were studied by SEM and AFM analysis (Fig. 3). Polymer PE-A₂₂B₂₄ was dissolved in chloroform and an aliquot of 100 μL of polymer solution was cast onto the copper stub at 25 °C (90% humidity of casting chamber) and dried. SEM image for resultant membrane showed hexagonal well-ordered geometry (Fig. 3a), which was further confirmed with vertically aligned pore (1.62 μm) by AFM analysis (Fig. 3b). Pore size and arrangement were altered with decrease in humidity of casting chamber (Fig. 3a, c & d). This experiment was repeated six times and similar observations were recorded. Same morphology was also obtained in low boiling solvent (THF, CS₂, DCM, Acetone etc.) due to formation of breath figures by water condensation.^17a Highly concentrated (>10 mg mL⁻¹) polymer solution was casted into thin film and dried under low humidity (<10%) at high temperature (>40 °C). No well-defined morphology was observed in the prepared membrane, which supported formation of breath figures. Under dissolution of polymer in NMP, DMAc, or TCE (high boiling solvent), dense membrane structure without hexagonal morphology or cracks and holes was obtained (Fig. 3e & f and S10d, (ESI)†). Thus, as attractive feature of prepared polymer, pore structure and pore-geometry may be controlled by membrane casting conditions and medium.

Fig. 3 Surface morphology of synthesized membrane: (a) high resolution SEM image of PE-A₂₂B₂₄; (b) 3D AFM images of PE-A₂₂B₂₄ for pore shape in the membrane matrix (casting solvent chloroform; humidity of the casting chamber 90%, pore size 1.62 μm); (c) SEM image of PE-A₂₂B₂₄ (casting solvent chloroform; humidity of the casting chamber 60%, pore size 1.02 μm); (d) SEM image of PE-A₂₂B₂₄ multi-block copolymer PE-A₂₂B₂₄ in chloroform ((casting solvent chloroform; humidity of the casting chamber 50%, pore size 0.9 μm)); (e & f) CPE-A₂₂B₂₄ membrane; and QPE-A₂₂B₂₄ membrane (membrane were cast in 90% humidity in DMAc solvent); scale bar 2 μm in each case.

EDX results of QPE-A₂₂B₂₄ membrane (Fig. S10e, (ESI)†) showed that membranes are completely metal free (impurity). Average dynamic radius of synthesized oligomer, multi-block copolymer, and chloromethylated copolymer was estimated with DLS analysis at constant solute mole fraction (2 × 10⁻⁴) in chloroform solvent (Fig. S14, ESI†). Average radius of polymer increased during copolymerization, but after chloromethylation size was reduced may be due to partial cross-linking of block copolymer.

3.5 IEC, transport number, water uptake and hydroxide ion conductivity of quaternized membranes

The ion-exchange capacity (IEC) provides information for density of ionisable quaternary ammonium groups in the membrane matrix, which are responsible for hydroxide conductivity. IEC value depends on degree of chloromethylation of multi-block copolymer (Table 3). IEC value depends on degree of chloromethylation of multi-block copolymer (Table 3). IEC value for QPE-A₂₂B₂₄ (2.24 meq g⁻¹) is even higher than Nafion 117 membrane (0.92 meq g⁻¹).⁷ Excess of water uptake led to the membrane swelling and thus deterioration in membrane conductivity and stability. Transport number for hydroxyl ions in the membrane matrix (t_m̄) was estimated by membrane potential measurements using Teorell–Meyer–Sievers's (TMS) theory (Table 3).²¹ t_m̄ values varied between 0.73 and 0.94, and increased with degree of chloromethylation.

Presence of water is necessary in the membrane matrix for hydroxide conduction. But, water uptake for QPEs membrane increased with IEC value (Table 3). QPE-A₂₂B₂₄ (IEC: 2.24 meq g⁻¹) membrane showed 82.46 wt% water uptake at 30 °C, which is favourable for fuel cell application. In general, membrane contains free (absorbed by ionic groups and hydrophilic domains) and bound water in the matrix. Number of water molecules absorbed per ammonium group (λ) was estimated by IEC values (Table 3). Relatively high λ values for QPE-A₁₂B₁₈ (IEC: 0.55 meq g⁻¹) membrane may be attributed to relatively large hydrophilic polymer chain responsible for more water absorption and therefore phase separation. Membrane conductivity for QPE membrane under 100% humidity conditions increased with temperature and IEC (Fig. 4). Membrane conductivity increased with IEC value. As a reference, QPE-A₂₂B₂₄ membrane (IEC: 2.24 meq g⁻¹) showed 95 mS cm⁻¹ conductivity at 80 °C. Interestingly, membrane conductivity also increased with molecular weight of polymer, due to polymer entanglement and high water uptake. Activation energy (E_a) was estimated from Arrhenius plot by following equation: (Fig. S15, ESI†)

E_a = −b × R (1)

where b is the slope of the regression lines (ln k^m vs. 1000/T (K⁻¹) plots) and R is the gas constant (8.314 J⁻¹ K⁻¹ mol⁻¹). The activation energy values (8–16 kJ mol⁻¹) were independent on IEC and increased with molecular weight. The apparent activation energy of multi-block QPE membranes was comparable or somewhat lower than those of reported AEMs (9.92–23.03 kJ mol⁻¹).^16a

Fig. 4 Variation of hydroxide ion conductivity for different QPE membranes with temperature for: (1) QPE-A₁₂B₁₈ (IEC = 0.55 meq g⁻¹); (2) QPE-A₁₂B₁₈ (IEC = 1.33 meq g⁻¹); (3) QPE-A₁₂B₁₈ (IEC = 1.95 meq g⁻¹); (4) QPE-A₁₈B₂₀ (IEC = 0.83 meq g⁻¹); (5) QPE-A₁₈B₂₀ (IEC = 1.71 meq g⁻¹); (6) QPE-A₁₈B₂₀ (IEC = 2.10 meq g⁻¹); (7) QPE-A₂₂B₂₄ (IEC = 0.95 meq g⁻¹); (8) QPE-A₂₂B₂₄ (IEC = 1.80 meq g⁻¹); (9) QPE-A₂₂B₂₄ (IEC = 2.24 meq g⁻¹).

For QPE-A₂₂B₂₄ membranes, methanol permeability value was measured by method reported in Section S7 (ESI)† and included in (Table 3). Across prepared membranes, methanol permeability values increased with methanol concentration. Methanol permeability of QPE-A₂₂B₂₄ membrane (IEC: 2.24 meq g⁻¹) is 0.7 × 10⁻⁷ cm² s⁻¹ at 30% MeOH, and 3.2 × 10⁻⁷ cm² s⁻¹ at room temperature. Compared to Nafion® membrane (13.2 × 10⁻⁷ cm² s⁻¹ at room temperature),² QPE-A₂₂B₂₄ membrane (IEC: 2.24 meq g⁻¹) has lowered methanol permeability. Lower methanol permeability values indicates that the methanol crossover rate for QPE-A₂₂B₂₄ membrane (IEC: 2.24 meq g⁻¹) membrane will be potentially lower than that of Nafion® membrane, which may improve fuel cell performance.

Some salient features (IEC, and membrane conductivity) of reported multi-block copolymer AEMs in the literature are compared with prepared QPE-A₂₂B₂₄ membrane, in Table 4, under similar experimental conditions.^16a,22–24 These properties of optimized QPE-A₂₂B₂₄ membrane are superior than other reported multi-block copolymer AEMs.

Table 4 Comparison of QPE-A₂₂B₂₄ membrane conductivity and IEC values

Membrane	Conductivity (mS cm^−1)	IEC (meq g^−1)	Reference
QPE-A22B24	91	2.29	This study
PSGOH-1.2	74	1.89	22
QPE-b	51	2.24	16a
QPE-e	45	1.56	16a
QPES-c	5.24	1.25	23
AAEM	7.33	—	24

---

3.6 Stability, solvent uptake, and solubility studies of QPE membranes

Thermal stabilities of synthesized oligomer, polymer, CPEs and QPEs membranes were analysed by thermogravimetrical analysis (TGA) (Fig. S16, ESI†). All thermo-grams showed two-steps weight loss. First step weight loss (∼1%) was observed at 50–200 °C due to evaporation of absorbed/bound water. Second step weight loss (∼30%) between 465 and 700 °C was attributed to the polymer degradation. For CPE-A₂₂B₂₄ and QPE-A₂₂B₂₄ membranes, degradation peaks between 200 and 400 °C were assigned to the degradation of side chain (chloromethylated or amminated). In DSC studies under N₂ environment between 30 and 300 °C, first endothermic transition temperature for all monomers and polymers (dry oligomer, polymer, CPE and QPE membranes) was lower than 100 °C, and area of endothermic regions increased with the step-wise progress of polymerization, and thus reduction in membrane crystallinity (Fig. S17, ESI†). The storage modulus (E′), loss modulus (E′′) and tan δ for QPE-A₂₂B₂₄ (IEC: 2.24 meq g⁻¹) membrane were determined by DMA at 10 Hz frequency between 30 and 410 °C (Fig. S18 (ESI)†). QPE-A₂₂B₂₄ membrane (IEC: 2.24 meq g⁻¹) showed 0.83 GPa initial storage modulus, while 0.063 GPa loss modulus and 351 °C glass transition temperature was observed.

The presence of water molecules in the membrane matrix facilitates the dissociation of functional groups and hydroxyl group transport. However, excessive water uptake deteriorated membrane mechanical properties and dimensional stability. For QPE-A22B24 membranes, change in volume fraction in water (Φ_w), and change in volume fraction in methanol (Φ_m), (Table 3) were estimated by method reported earlier.²⁵ Water absorption occurred due to presence of hydrophilic phenolphthalein and quaternary ammonium groups. Methanol–water uptake was little high as compared to water uptake. Volume change by swelling in equilibration with water or methanol confirmed dimensional stability of prepared QPE-A22B24 membrane for fuel cell application.

Durability of QPE-A₂₂B₂₄ (IEC = 2.24 meq g⁻¹) membrane was also tested in hot degassed deionised water (80 °C) for 1000 h (Fig. 5). Membrane conductivity was reduced up to 10 h, and afterwards it attained constant value. In similar fashion, after 1000 h, about 15% reduction in membrane IEC was observed. Thus reported anion conducting membranes are quite good durable and suitable for alkaline fuel cell application. Synthesized membrane QPE-A₂₂B₂₄, conductivity and IEC properties was compared with previous report showed that synthesized membrane had excellent AEM properties.

Fig. 5 Time course of hydroxide ion conductivity for QPE-A₂₂B₂₄ (IEC = 2.24 meq g⁻¹) membrane in water at 80 °C.

4. Conclusions

A series of multi-block AEMs with high molecular weights containing phenolphthalein and fluorenyl groups were synthesized. Structure of the designed AEM was confirmed by different spectroscopic techniques. By intelligent optimization of reaction conditions and multi-block copolymers precursor, chloromethylation at specific positions for both blocks, was successfully achieved. Herein, we are reporting multi-block AEMs, in which both hydrophilic blocks were joined by a hydrophobic moiety, to achieve good hydrophobic–hydrophilic balance and hydroxide conduction. Membrane properties were highly dependent on DC/QC, and QPE-A₂₂B₂₄ (IEC = 2.24 meq g⁻¹) membrane showed 2.35 DC value. It seems, nearly complete chloromethylation was achieved on fluorenyl group, due to high electron density. Partial chloromethylation on phenolphthalein group was assigned due to relatively low electron density. Conductivity, IEC, stabilities and durability assessment of QPE-A₂₂B₂₄ (IEC = 2.24 meq g⁻¹) membrane revealed it a potential candidate for alkaline fuel cell application.

These membranes were architected by introducing fluorenyl and phthalide groups and showed high phase separation, water content, stabilities and other desired properties. Concept of utilizing multi-block AEM based on high molecular weight aromatic polymers, containing hydrophilic blocks joined together by hydrophobic moiety, is verified for improving hydroxide conductivity without sacrificing stabilities and other desired properties for alkaline fuel cell application.

Acknowledgements

One of the authors (Ravi P. Pandey) is thankful to University Grand commission (UGC), New Delhi, for providing Senior Research Fellowship (SRF). Financial assistance of Ministry of New and Renewable Energy, New Delhi, Gov. of India, (project no. 102/79/2010-NT) is acknowledged. Instrumental support received from Analytical Science Division, CSIR-CSMCRI, is also gratefully acknowledged.

References

1. (a) K. Matsumoto, T. Fujigaya, H. Yanagi and N. Nakashima, Adv. Funct. Mater., 2011, 21, 1089; (b) R. Akiyama, D. Hirayama, M. Saito, J. Miyake, M. Watanabe and K. Miyatake, RSC Adv., 2013, 3, 20202.
2. T. Chakrabarty, A. K. Singh and V. K. Shahi, RSC Adv., 2012, 2, 1949.
3. (a) E. Antolini and E. R. Gonzalez, J. Power Sources, 2010, 195, 3431; (b) Z. Zhao, J. Wang, S. Li and S. Zhang, J. Power Sources, 2011, 196, 4445.
4. (a) H. Zhang and P. K. Shen, Chem. Rev., 2012, 112, 2780; (b) X. Li, Y. Yu, Q. Liu and Y. Meng, ACS Appl. Mater. Interfaces, 2012, 4, 3627.
5. (a) Y. Zha, M. L. Disabb-Miller, Z. D. Johnson, M. A. Hickner and G. N. Tew, J. Am. Chem. Soc., 2012, 134, 4493; (b) N. Li, Q. Zhang, C. Wang, Y. M. Lee and M. D. Guiver, Macromolecules, 2012, 45, 2411–2419.
6. (a) G. Couture, A. Alaaeddine, F. Boschet and B. Ameduri, Prog. Polym. Sci., 2011, 36, 1521; (b) N. J. Robertson, H. A. Kostalik, T. J. Clark, P. F. Mutolo, H. D. Abruña and G. W. Coates, J. Am. Chem. Soc., 2010, 132, 3400.
7. B. P. Tripathi, M. Kumar and V. K. Shahi, J. Membr. Sci., 2010, 360, 90.
8. D. Stoica, F. Alloin, S. Marais, D. Langevin, C. Chappey and P. Judeinstein, J. Phys. Chem. B, 2008, 112, 12338.
9. Y. Luo, J. Guo, C. Wang and D. Chu, J. Power Sources, 2010, 195, 3765.
10. Q. H. Zeng, Q. L. Liu, I. Broadwell, A. M. Zhu, Y. Xiong and X. P. Tu, J. Membr. Sci., 2010, 349, 237.
11. Y. Pérez-Padilla, M. A. Smit and M. J. Aguilar-Vega, Ind. Eng. Chem. Res., 2011, 50, 9617.
12. T. Nonaka and K. Fujita, J. Membr. Sci., 1998, 144, 187.
13. R. Bai, Q. Li, L. Liu, Q. Miao and B. Jin, Chem. Commun., 2014, 50, 2791–2793.
14. G. Wang, Y. Weng, D. Chu, D. Xie and R. Chen, J. Membr. Sci., 2009, 326, 4.
15. K. Matsui, E. Tobita, K. Sugimoto, K. Kondo, T. Seita and A. Akimoto, J. Appl. Polym. Sci., 1986, 32, 4137.
16. (a) K. Miyatake, B. Bae and M. Watanabe, Polym. Chem., 2011, 2, 1919; (b) M. Tanaka, K. Fukasawa, E. Nishino, S. Yamaguchi, K. Yamada, H. Tanaka, B. Bae, K. Miyatake and M. Watanabe, J. Am. Chem. Soc., 2011, 133, 10646.
17. (a) A. Muñoz-Bonilla, E. Ibarboure, E. Papon and J. Rodriguez-Hernandez, Langmuir, 2009, 25, 6493; (b) P. Escalé, L. Rubatat, L. Billon and M. Save, Eur. Polym. J., 2012, 48, 1001.
18. (a) R. Guo, O. Lane, D. VanHouten and J. E. McGrath, Ind. Eng. Chem. Res., 2010, 49, 12125; (b) A. Gugliuzza, M. C. Aceto, F. Macedonio and E. Drioli, J. Phys. Chem. B, 2008, 112, 10483.
19. Y. Xiong, Q. L. Liu and Q. H. Zeng, J. Power Sources, 2009, 193, 541.
20. A. K. Singh, S. Prakash, V. Kulshrestha and V. K. Shahi, ACS Appl. Mater. Interfaces, 2012, 4, 1683.
21. E.-Y. Choi, H. Strathmann, J.-M. Park and S.-H. Moon, J. Membr. Sci., 2006, 268, 165.
22. J. Wang, S. Li and S. Zhang, Macromolecules, 2010, 43, 3890.
23. L. Li and Y. Wang, J. Membr. Sci., 2005, 262, 1.
24. G. Wang, Y. Weng, D. Chu, R. Chen and D. Xie, J. Membr. Sci., 2009, 332, 63.
25. R. P. Pandey and V. K. Shahi, J. Membr. Sci., 2013, 444, 116.

---

Footnote

† Electronic supplementary information (ESI) available: NMR data, FTIR, ATR spectra, DLS, TGA, DSC, DMA profile, photographs of the membrane, SEM image, detailed about used instrument, water uptake, hydrolytic stability, ion exchange capacity, counter ion transport number, and membrane conductivity are available in the online version of this article. See DOI: 10.1039/c4ra01999g

---