Preparation and characterization of sulfonated poly(aryl ether ketone)s containing 3,5-diphenyl phthalazinone moieties for proton exchange membrane

Abstract

A series of sulfonated poly(phthalazinone ether ketone)s containing 3,5-diphenyl phthalazinone moieties (SPPEK-dPs) were prepared by the sulfonation of poly(aryl ether ketone)s containing 3,5-diphenyl phthalazinone moieties (PPEK-dPs) which were synthesized via direct nucleophilic polycondensation from 4-(4-hydroxyphenyl)-2,3-phthalazin-1-ketone (DHPZ), 4-(3,5-diphenyl-4-hydroxyphenyl)-2,3-phthalazin-1-ketone (DHPZ-dP) and 4,4-difluorobenzophenone (DFB). The molecular structures were assessed by FTIR and ¹H-NMR spectroscopy. The ion exchange capacity (IEC) of these sulfonated polymers were in the range of 0.99–1.81 mmol g⁻¹. SPPEK-dP proton exchange membranes demonstrated good mechanical properties as well as dimensional, thermal, and oxidative stability. The proton conductivities of SPPEK-dP membranes increased with DHPZ-dP content and temperature. The proton conductivity of SPPEK-dP-55 was 13.18 × 10⁻² S cm⁻¹ at 95 °C. Furthermore, the methanol diffusion coefficients of SPPEK-dP membranes were 0.12 × 10⁻⁷ cm² s⁻¹ to 1.09 × 10⁻⁷ cm² s⁻¹ depending on the molar ratio of DHPZ-dP. Remarkably, the selectivity of SPPEK-dP membranes was 5–7 times higher than that of Nafion 117 membranes under the same conditions. All of the above properties indicate that SPPEK-dPs have potential applications in proton exchange membranes for direct methanol fuel cells.

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1. Introduction

Sulfonated polymers have received significant interest for their applications in proton exchange membranes (PEMs).^1–4 These materials, including the highly successful Nafion®, exhibit excellent oxidative stability, chemical stability and high proton conductivity. However, there remain significant limitations to current commercial materials, such as high methanol crossover rate and low operational temperature (methanol fuel cell, <80 °C).^5–7 To overcome these problems, more novel sulfonated aromatic polymers have been synthesized and evaluated as alternatives to Nafion.^8–12

Based on previous reports, these substituted aromatic polymers can be roughly classified into two major categories. One is the main-chain-type sulfonated aromatic polymers, such as sulfonated poly(aryl ether ketone)s,¹² sulfonated poly(ether ether ketone)s,^13–15 poly(arylene ether sulfone ketone)s¹⁶ and poly(phthalazinone ether ketone)s.^17,18 Compared to Nafion® membranes, these polymers show much lower methanol permeability and better thermal stability which can be attributed to the different microstructures between Nafion and main-chain-type sulfonated aromatic polymers.¹⁴ The backbones of main-chain-type sulfonated aromatic polymers are less hydrophobic and the sulfonic acid functional group is less acidic (i.e. less polar), which result in less hydrophilic/hydrophobic separation and thus prevent methanol from permeating the transport channels.^14,19 However, the desire for improving proton conductivity of these sulfonated aromatic polymers remained. To address this need, side-chain-type sulfonated aromatic polymers, such as sulfonated poly(ether sulfone)s,¹ sulfonated naphthalene-based poly(arylene ether ketone) copolymers,^20,21 and sulfonated polysulfone,²² have been studied. By introducing sulfonic acid functional groups to the side chains of aromatic polymers, beneficial aggregation of ionic clusters and more distinct hydrophilic/hydrophobic separation were achieved.²³ The microphase separation of these polymers serves to compartmentalize water into the hydrophilic polymer side chain domains, resulting in effective membrane water management and excellent proton conductivities.^1,24,25

In the present work, a series of novel side-chain sulfonated aromatic polymers, sulfonated poly(aryl ether ketone)s containing 3,5-diphenyl phthalazinone moieties (SPPEK-dPs), were synthesized. The molecular structures were characterized by ¹H-NMR and FTIR. To assess their potential in proton exchange membranes, solubility, ion exchange capability, thermal stability, oxidative stability, water uptake and swelling rate, proton conductivity and methanol permeability were systematically studied.

2. Experimental

2.1. Materials

4-(4-Hydroxyphenyl)-2,3-phthalazin-1-ketone (DHPZ) and 4-(3,5-diphenyl-4-hydroxyphenyl)-2,3-phthalazin-1-ketone (DHPZ-dP) were supplied by Dalian Polymer New Material Company (China); 4,4-difluorobenzophenone (DFB) was purchased from Yanji Chemical Company (China); xylene, anhydrous potassium carbonate and concentrated sulfuric acid were purchased from Bodi Chemical Company (China); sulfolane was purchased from Liaoyang Guanghua Chemical Co., LTD, and purified by distillation. All other solvents and chemicals were obtained from commercial sources and used as received.

2.2. Synthesis of PPEK-dPs and SPPEK-dPs

A series of PPEK-dP polymers were synthesized via direct nucleophilic polycondensation from DHPZ, DHPZ-dP and DFB by varying the feed ratios of DHPZ-dP and DHPZ. These polymers are referred to as PPEK-dP-x, where x is the molar fraction of DHPZ-dP in the total feed of DHPZ-dP and DHPZ; the molar fraction of DFB is always equal to the total feed of DHPZ-dP and DHPZ. For example PPEK-dP-35 was synthesized as follows: DHPZ-dP (0.35 mmol), DHPZ (0.65 mmol), DFB (1 mmol) and K₂CO₃ (1.2 mmol) were fed into a three-neck round-bottom flask equipped with an overhead mechanical stirrer, a water cooled condenser and nitrogen purge inlet and outlet, followed by the addition of 30 mL sulfolane as the solvent and 25 mL xylene as the water-carrying agent. The reaction mixture was stirred under N₂ at 160 °C for 2 h to remove the water and xylene, and then polymerized at 200 °C for at least 20 h. After polymerization, the reaction product was poured into hot water and boiled to remove impurities such as inorganic salts. Finally, the polymer PPEK-dP-35 was obtained after drying the product under vacuum at 100 °C for 12 h.

SPPEK-dPs were prepared by sulfonating PPEK-dPs. Concentrated sulfuric acid was the solvent and sulfonating agent in the reaction. First, PPEK-dP-x (5 w/w%) was dissolved in concentrated sulfuric acid. The mixture then reacted at 30 °C for 7 h. The sulfonated product was subsequently precipitated into cold water and washed repeatedly to neutral with deionized water. SPPEK-dP-x was obtained after drying the product under vacuum at 80 °C.

2.3. Preparation of SPPEK-dP proton exchange membranes

SPPEK-dP membranes were prepared by the solution casting method. A typical procedure for the membrane fabrication is as follows. The SPPEK-dP-x sample was dissolved in NMP to form a 10% homogeneous solution which was then cast onto a clean, smooth glass plate held at 60 °C for 10 h to evaporate NMP. Finally, a SPPEK-dP-x proton exchange membrane with a thickness of about 40 μm was obtained by drying at 80 °C for 12 h under vacuum.

2.4. Characterization of polymers and membranes

The chemical structures of PPEK-dPs and SPPEK-dPs were confirmed using ¹H-NMR (Bruker Avance II 400M) and FTIR spectroscopy (Thermo Nicolet Nexus 330 fourier transform infrared spectrometer). In ¹H-NMR measurements, deuterated chloroform was used as the solvent for PPEK-dPs, and deuterated dimethyl sulfoxide (DMSO-d₆) was used as the solvent for SPPEK-dPs. In the FTIR measurement, the membranes were measured directly using in transmission geometry.

The solubility of the polymers was carried out by dissolving polymer in varies solvents at a concentration of 10 g L⁻¹.

The inherent viscosities of the polymers were obtained with a Ubbelohde capillary viscometer at 25 °C using NMP as solvent, where the concentration of the solution was 5 g L⁻¹.

The thermogravimetric analysis (TGA) of SPPEK-dP-x was performed on a NETZSCH TG-209 with a heating rate of 10 °C min⁻¹ under N₂. The heating range was from room temperature to 800 °C.

The ion exchange capability (IEC) was determined by titration. First, 0.1 g of SPPEK-dPs powder was dissolved in 10 mL NMP. Deionized water (10 mL) was then added to the solution. Methyl red was added to the titration chamber as an indicator. The solution was subsequently titrated with 0.02 mol L⁻¹ NaOH solution until the color changed to yellow. The titrated IEC was obtained from the following equation:

where V_NaOH is the volume of the consumed NaOH solution, C_NaOH is the molarity of NaOH solution, and M_dry is the weight of dry membrane sample.

The oxidative stability was evaluated by immersing the membrane into Fenton's reagent at 25 °C (30 ppm FeSO₄ in 30% H₂O₂) and 80 °C (2 ppm FeSO₄ in 3% H₂O₂), respectively. During the test, membranes were probed gently with a glass stick every hour to check for membrane degradation. The oxidative stability of the membranes was characterized by the time required for breakdown. For each membrane, at least four samples were tested and their average value was calculated.

Water uptake of SPPEK-dP membranes was measured by equilibrating a membrane sample with deionized water at 25 °C and 80 °C for 24 h, respectively. The membranes were then removed from the water, wiped using absorbent paper, and weighed immediately. The membranes were dried under vacuum at 80 °C for at least 48 h and weighed again. Water uptake was calculated according to the following equation:

where W_wet and W_dry are the weight of the membrane in the wet and dry states, respectively.

The dimensional stability of the SPPEK-dP membranes was evaluated according to eqn (3). The membranes were dried under vacuum at 80 °C for 48 h, and then the lengths of the membranes were measured. After that, the membranes were soaked in deionized water at 25 °C and 80 °C for 24 h, respectively and the dimensions were measured again. The swelling ratios were calculated according to the following equation:

where L₁ and L₀ are the lengths of the soaked membrane and dry membrane, respectively.

The mechanical properties of membranes were measured at 25 °C using an Instron 5567 instrument with a stretching rate of 2 mm min⁻¹. For each sample, at least four samples were used and their average value was calculated.

The proton conductivity of the sulfonated membranes was measured by a four-probe electrochemical impedance spectroscopy technique as described previously in the literature.^2,26 The resistance of the tested membrane was obtained using a CHI-604D electrochemical workstation (Shanghai Huachen). Membranes with a size of 1 cm × 4 cm were initially hydrated by immersion in deionized water at room temperature for 48 h. The membranes were then sandwiched between two teflon slices in a thermostatic bath. The measurements were performed at 30 °C, 50 °C, 75 °C, 85 °C, 95 °C, respectively. For each sample, at least three membranes were tested and their average values were calculated. The proton conductivities of the membranes were calculated using the following equation:²⁷

where R is the resistance of the membrane, L represents the distance between two electrodes (i.e. the thickness of the membranes) and S is the area of the interface between the membranes and the electrodes.

Methanol permeability was assessed by a method described in the literature.¹⁵ The methanol permeability coefficient was calculated according to the methanol concentration, which was measured from the penetrants by gas chromatography (Agilent 6890N).

3. Results and discussion

3.1. Synthesis of PPEK-dPs and SPPEK-dPs

As shown in Scheme 1, a series of heterocyclic poly(aryl ether ketone)s containing 3,5-diphenyl phthalazinone moieties (PPEK-dPs) were synthesized via the aromatic nucleophilic substitution reaction of DFB with DHPZ-dP and DHPZ in sulfolane in the presence of anhydrous potassium carbonate. Both DHPZ-dP and DHPZ contain two labile protons, in O–H and N–H, which can react with potassium carbonate to form the corresponding phenolate anions and aza-nitrogen anions. Subsequently, these anions underwent a nucleophilic displacement with DFB to produce PPEK-dP. SPPEK-dPs were prepared by sulfonating PPEK-dPs as described in Scheme 1.

Scheme 1 Synthesis of PPEK-dPs and SPPEK-dPs.

3.2. Characterization of PPEK-dPs and SPPEK-dPs

The structures of the PPEK-dPs copolymers were confirmed by ¹H-NMR spectra as shown in Fig. 1. The single peak at 8.63 ppm was ascribed to the typical signals of the protons (H6 and H18) of phthalazinone, which has been used as the reference signal to assign the other protons of phthalazinone based polymers.²⁸

Fig. 1 ¹H-NMR spectra of PPEK-dPs.

The shift at δ = 7.78 ppm was assigned to the proton H14 of phenyl groups and δ = 7.70 ppm was assigned to H2. The integration value of H14 increased with increasing molar feed ratios of DHPZ-dP. The DHPZ-dP segments content was calculated from the integration ratios of H14 to H6 and H18. The results are presented in Table 1 and are in good agreement with the molar feed ratios of DHPZ-dP. Furthermore, viscosity measurements showed that PPEK-dPs have high molecular weight, indicating that the polymers were synthesized.

Table 1 Inherent viscosity of PPEK-dPs

Polymer	DHPZ-dP/DHPZ	η1^a (dL g^−1)	DHPZ-dP content^b
a Measured at 25 °C in NMP.b Average number of DHPZ-dP segments per repeat unit.
PPEK-dP-25	25/75	1.07	0.251
PPEK-dP-35	35/65	1.05	0.356
PPEK-dP-45	45/55	1.10	0.445
PPEK-dP-55	55/45	1.10	0.546

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Fig. 2 shows the ¹H-NMR spectra of SPPEK-dPs. The signals at δ = 8.49 ppm corresponded to the protons H6 and H18 of phthalazinone. The signals at δ = 8.49–6.60 ppm corresponded to the ortho-protons H1, H10 and H22 of –O–Ar. Compared to the PPEK-dPs, the H12 signals were shifted downfield and their intensity also increased with increasing the molar ratio of DHPZ-dP. These results indicate that an electrophilic substitution reaction, i.e. sulfonation, occurred, confirming the synthesis of SPPEK-dPs.

Fig. 2 ¹H-NMR spectra of SPPEK-dPs.

The structures of PPEK-dPs and SPPEK-dPs were further analyzed via FTIR spectroscopy, as seen in Fig. 3. The absorption peak at 1661 cm⁻¹ was attributed to the C=O stretching vibrations in the aromatic rings. The absorption peaks at 1598 cm⁻¹ and 1499 cm⁻¹ were ascribed to vibrational modes of the phenyl rings. The peaks at 1235 cm⁻¹ were attributed to aromatic C–O–C absorption. All of the above peaks appearing in the FTIR spectra suggested that PPEK-dPs were successfully synthesized. In the FTIR spectra of SPPEK-dP-25 and SPPEK-dP-55, two sharp absorption peaks appeared at 1035 cm⁻¹ and 1010 cm⁻¹ which were attributed to the asymmetric and symmetric stretching vibrations of O=S=O in the sulfonic acid groups, respectively. Additionally, the intensity of the two peaks increasing as the molar ratio of DHPZ-dP increased, demonstrating that the sulfonation reaction occurred at the pendant phenyl groups. In summary, all experimental data from ¹H-NMR and FTIR spectroscopy confirmed that SPPEK-dPs were synthesized successfully.

Fig. 3 FTIR spectra of PPEK-dPs and SPPEK-dPs.

3.3. Properties of SPPEK-dP polymers

The viscosities η₂ and IEC values of SPPEK-dPs are presented in Table 2. As seen, the viscosities of the SPPEK-dPs were in the range of 6.32 dL g⁻¹ to 9.31 dL g⁻¹, much higher than that of the PPEK-dPs. The hydrophilic nature of the sulfonic acid groups enhanced the viscosity during the measurements in NMP. The results indicated that the sulfonation reaction was carried out successfully and no obvious degradation of polymer chains occurred under the sulfonation condition. The theoretical IEC (IEC_T) values of the SPPEK-dP membranes were obtained assuming the complete conversion of H11 (Fig. 1) to sulfonic acid groups. The measured IEC (IEC_M) values are virtually identical to the IEC_T values, suggesting complete sulfonation.

Table 2 Viscosity and IEC value of SPPEK-dPs

Polymer	η2^a (dL g^−1)	IECT (mmol g^−1)	IECM (mmol g^−1)
a Measured at 25 °C in NMP.
SPPEK-dP-25	7.08	1.01	0.99
SPPEK-dP-35	6.32	1.33	1.30
SPPEK-dP-45	9.31	1.62	1.55
SPPEK-dP-55	7.83	1.87	1.81

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Both the IEC_T and IEC_M values of SPPEK-dPs increased as the molar feed ratio of DHPZ-dP was increased. Thus, facile control of the IEC values of SPPEK-dPs was achieved by adjusting the molar feed ratios of DHPZ-dP and DHPZ.

Table 3 summarizes the results of the solubility measurements of PPEK-dPs and SPPEK-dPs. Both PPEK-dPs and SPPEK-dPs exhibited good solubility in NMP and DMAc at room temperature. Moreover, SPPEK-dPs were soluble in DMSO and DMF while PPEK-dPs were partially insoluble and/or swollen. The two type polymers were both insoluble in acetone and water no matter whether at room temperature or under heating condition. These results indicated that the introduction of sulfonic acid group to the pendant phenyl groups enhanced solvation. The better solubility of SPPEK-dPs afforded good processing performance; their solution-processable nature is beneficial for lowering the cost of membrane fabrication.

Table 3 Solubility of PPEK-dPs and SPPEK-dPs^a

Polymer	DMAc^b	NMP^c	DMSO	CHCl3	DMF^d	EGME^e	Acetone	H2O
a + soluble; ± partially soluble; ⊥ swelling; − insoluble.b N,N-Dimethylacetamide.c N-Methyl pyrrolidone.d Dimethylformamide.e Ethylene glycol monomethyl ether.
PPEK-dP-25	+	+	±	+	⊥	−	−	−
PPEK-dP-35	+	+	±	+	⊥	−	−	−
PPEK-dP-45	+	+	±	+	⊥	−	−	−
PPEK-dP-55	+	+	±	+	⊥	−	−	−
SPPEK-dP-25	+	+	+	−	+	⊥	−	−
SPPEK-dP-35	+	+	+	−	+	⊥	−	−
SPPEK-dP-45	+	+	+	−	+	⊥	−	−
SPPEK-dP-55	+	+	+	−	+	⊥	−	−

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The TGA curves for SPPEK-dPs are shown in Fig. 4. Two decomposition stages were observed in the TGA curves. The first degradation stage began at 330 °C (defined as T_d1) and was attributed to the loss of sulfonic acid groups. The second decomposition step began at about 485 °C (defined as T_d2) and corresponded to the degradation of the polymer backbone. T_d1 of SPPEK-dP-55 is 331 °C, lower than SPPEK-dP-45 (343 °C), SPPEK-dP-35 (347 °C) and SPPEK-dP-25 (357 °C). This suggests that the bond between the sulfonic acid groups and the pendant phenyl group is the weakest bond in SPPEK-dP-x.²⁹ Thus, increased weight loss was observed for polymers containing more sulfonic acid groups.

Fig. 4 TGA curves of SPPEK-dPs in N₂.

According to a previous report,³⁰ the desulfonation process of Nafion 117 occurred at about 290 °C, where the gradual mass loss was about 6.4%. This suggests that the thermal stability of SPPEK-dPs is higher than Nafion 117 and sufficient for the requirements of PEM applications.

3.4. Properties of SPPEK-dP proton exchange membranes

The mechanical properties of SPPEK-dP membranes were measured as displayed in Table 4. Tensile moduli of 1.2–1.5 GPa and tensile stresses at maximum loads of 70–80 MPa are both higher than those of Nafion 117 membranes (0.26 GPa and 26.65 MPa, respectively).³¹ The elongation at break (20–58%) of SPPEK-dP membranes was lower than that of Nafion 117 membranes (>200%) because of the rigid phenyl and heterocyclic structures in the SPPEK-dPs polymer backbones. This suggests that Nafion 117 membranes are more flexible, however, SPPEK-dPs remain potential proton exchange membrane materials.

Table 4 Mechanical properties of SPPEK-dP membranes

Membrane	Tensile modulus (GPa)	Tensile strength (MPa)	Elongation at break (%)
SPPEK-dP-25	1.4	78.9	57.8
SPPEK-dP-35	1.4	79.6	24.5
SPPEK-dP-45	1.2	71.6	21.4
SPPEK-dP-55	1.5	70.1	23.0
Nafion 117	0.26	26.65	200

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The oxidative stability of SPPEK-dPs was measured by using Fenton's reagent (Table 5). Generally, all SPPEK-dP membranes exhibited good oxidative stability, owing to their wholly aromatic structure. The SPPEK-dP-25 membranes showed the highest oxidative stability whereas SPPEK-dP-55 showed the lowest oxidative stability. The oxidative stability of the membranes decreased with increasing number of sulfonic acid groups (or the molar ratio of DHPZ-dP, or the IEC_M value). This can be attributed to the hydrophilicity of sulfonic acid groups, intensifying the rate of oxidation.

Table 5 The oxidative stability of SPPEK-dPs

Membrane	25 °C		80 °C
	Breaking time (h)	Dissolution time (h)	Breaking time (h)	Dissolution time (h)
SPPEK-dP-25	41.0	100.5	6.7	13.5
SPPEK-dP-35	20.3	60.3	4.9	12.5
SPPEK-dP-45	16.9	50.0	4.0	5.8
SPPEK-dP-55	14.3	28.0	3.0	4.1

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Swelling ratio and water uptake are also important parameters for evaluating PEMs, and these properties are listed in Table 6. The hydrophilic regions of the polymer chains can absorb water and increase the cluster sizes, producing interconnected channels for proton transfer. However, large swelling ratios may affect the dimensional stability of the membranes. In this work, water uptake of SPPEK-dPs at 25 °C is in the range of 10.1–27.6%, while that at 80 °C is in the range of 13.4–31.2%. As expected, the water uptake of SPPEK-dP membranes increased with increasing IEC values or temperature. The maximum swelling ratio of SPPEK-dPs was 8.1 × 7.9% at 80 °C which was slightly larger than that at 25 °C (1.8 × 1.0%), but still less than 10%. The results indicate that SPPEK-dP membranes with higher water uptake and lower swelling ratio are dimensionally stable.

Table 6 Swelling ratio and water uptake of SPPEK-dP membranes

Membrane	IECM (mmol g^−1)	Swelling ratio (length × width%)		Water uptake (%)
		25 °C	80 °C	25 °C	80 °C
SPPEK-dP-25	0.99	1.8 × 1.0	2.3 × 1.5	10.1	13.4
SPPEK-dP-35	1.30	3.4 × 2.6	4.2 × 3.4	12.7	17.7
SPPEK-dP-45	1.55	4.3 × 4.5	6.0 × 5.3	20.1	21.2
SPPEK-dP-55	1.81	5.4 × 5.1	8.1 × 7.9	27.6	31.2

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The proton conductivities at various temperatures for SPPEK-dP membranes are illustrated in Fig. 5. For comparison, the proton conductivity of Nafion 117 membranes was measured under the same conditions. Fig. 5 shows that the proton conductivity of SPPEK-dP membranes increased with increasing temperature. The conductivity of SPPEK-dP membranes was in the range of 0.36 × 10⁻² to 3.99 × 10⁻² S cm⁻¹ at 30 °C, and 2.25 × 10⁻² to 13.18 × 10⁻² S cm⁻¹ at 95 °C, which is considered to be promising for PEMs in fuel cell application. For instance, the conductivity values of SPPEK- dP-55 membranes are 3.99 × 10⁻² S cm⁻¹ at 30 °C and 13.18 × 10⁻² S cm⁻¹ at 90 °C, close to those of Nafion 117.

Fig. 5 The proton conductivity at various temperatures for SPPEK-dPs membranes.

Table 7 shows the methanol diffusion coefficients of SPPEK-dP membranes. Methanol diffusion in Nafion 117 membranes was measured under the same conditions for comparison. It is apparent that the methanol diffusion in SPPEK-dP membranes increased as the molar ratio of DHPZ-dP increased. More sulfonic acid groups provided more hydrophilic domains and increased the hydrodynamic solvent transport of water and methanol.³² Notably, the methanol diffusion in all SPPEK-dP membranes in the present work was much lower than that of Nafion 117 membranes by almost one order of magnitude. For example, the methanol diffusion coefficient of SPPEK-dP-55 is 1.09 × 10⁻⁷ cm² s⁻¹ while that of Nafion 117 is 1.4 × 10⁻⁶ cm² s⁻¹. It is attributed to the different microstructure between SPPEK-dPs and Nafion 117.³¹ SPPEK-dPs with many pendant phenyls and heterocyclic structures are rigid and have fewer hydrophilic domains than Nafion 117.

Table 7 Methanol diffusion of SPPEK-dPs membranes

Membrane	Thickness (μm)	Methanol diffusion (10^−7 cm^2 s^−1)
SPPEK-dP-25	24	0.12
SPPEK-dP-35	34	0.35
SPPEK-dP-45	30	0.51
SPPEK-dP-55	32	1.09
Nafion 117	198	10.40

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The selectivity, the ratio of proton conductivity to methanol permeability, is another important parameter for evaluating membrane performance in direct methanol fuel cells. As shown in Fig. 6, SPPEK-dP-35 exhibits the highest selectivity of 42.4 S s cm⁻³ and SPPEK-dP-25 shows the lowest selectivity of 30.0 S s cm⁻³. The selectivity of SPPEK-dPs is about 5–7 times higher than that of Nafion 117, suggesting that SPPEK-dP membranes have great potential for direct methanol fuel cell applications.

Fig. 6 Selectivity of SPPEK-dPs membranes at room temperature.

4. Conclusions

SPPEK-dPs were produced by sulfonation of PPEK-dPs which were synthesized by direct nucleophilic polycondensation from DHPZ, DHPZ-dP and DFB. The sulfonation level was controlled by varying the molar ratio of DHPZ-dP. SPPEK-dP polymers show many favorable properties for PEM applications. Specifically, SPPEK-dPs exhibit good processing performance, with good solubility in NMP and DMAc at room temperature and good oxidative stability in Fenton's reagent (breaking time > 14 h at room temperature and dissolution time > 4 h at 80 °C). Additionally, SPPEK-dP membranes exhibit good mechanical properties and dimensional stability. In this work, the tensile strength of SPPEK-dP membranes was about 70 MPa, and elongation at break was in the range of 20% to 57%. Furthermore, SPPEK-dP membranes have appropriate water uptake and low swelling ratios at 25 °C or 80 °C. Finally, SPPEK-dP membranes are particularly well-suited for applications as PEMs as indicated by their combination of good proton conductivity and low methanol diffusion. The proton conductivities of SPPEK-dP membranes approached that of Nafion 117 membranes, especially at higher temperature (>80 °C). Moreover, methanol diffusion in all SPPEK-dP membranes was significantly lower than that of Nafion 117 membranes by almost one order of magnitude (1.09 × 10⁻⁷ cm² s⁻¹ versus 1.4 × 10⁻⁶ cm² s⁻¹). Above all, the selectivity of SPPEK-dPs is at least 5 times higher than Nafion 117. It is an encouraging result for the use of SPPEK-dP materials in proton exchange membranes.

Acknowledgements

The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (No. 21276037 and No. 21476038) and the project of Dalian municipal science and technology plan (No. 2014J11JH127 and No. 2015J12JH208).

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