Fabrication of a polymer electrolyte membrane with uneven side chains for enhancing proton conductivity

Abstract

Construction of effective proton transport channels in proton exchange membranes is the key to the design of high performance proton conductive materials. Enhancement of proton conductivity of polymer electrolyte membranes was achieved by broadening the proton transfer channels via attaching acid groups to both long and short side chains of polymer electrolytes simultaneously. To demonstrate the effectiveness of the uneven side chains on the conductive properties of polymer membranes, three types of polyamide based electrolyte membranes with long side chains, short side chains and long/short side chains were prepared. It was found that among the three types of membranes with the same ion exchange capacity (IEC) value, the one with uneven side chains exhibits the highest proton conductivity. An increase of the IEC value in the uneven side chain membrane leads to a significant increase of proton conductivity. The study provides useful insight into the structural design of polymer electrolyte materials with high conductivity for fuel cell applications.

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1. Introduction

The design of low cost, durable and highly conductive electrolyte materials presents a significant challenge for the development of polymer electrolyte membrane fuel cells (PEMFCs).^1–7 High ionic conductivity, sufficient water uptake and moderate dimensional swelling are among the most important attributes of a PEMFC device.^8–10 Although perfluorosulfonic acid ionomer membranes, represented by Nafion®, possess these attributes, they suffer from reduced proton conductivity under relatively low humidity, electro-osmotic drag of water, fuel permeation, high cost and environmental unfriendliness caused by the use of a polymeric fluorobackbone.^11–13 As an alternative to Nafion®, non-fluorinated acid ionomer membranes have been developed.^14–17 However, these materials usually suffer from inferior ionic conductivity at the same level of the ion exchange capacity (IEC) as the value of Nafion®. Indeed, much of the recent research efforts to search for novel polymer electrolyte membranes (PEMs) with improved proton conductivity have been focused on increasing the IEC value of the membranes.^18–20 Significantly high proton conductivity can be achieved as the IEC value increases. Unfortunately, a high IEC value often leads to excessive water uptake and dimensional swelling. The PEM may even become dissolvable in water at an elevated temperature. The adverse effect of high IEC on the PEM is a significant technical challenge for development of robust and versatile PEM materials for fuel cell applications.

A mechanistic understanding on proton transport in a proton exchange membrane may shed light on the design of novel PEMs from molecular level. Several principle scattering models on the morphology of Nafion®, such as cluster-network model,²¹ local-order model²² and sandwich-like model,²³ have been proposed to investigate the structure–property relationship with a particular emphasis on transport properties.¹² It was found that the microstructural features, such as connectivity and hydrophilic-site distribution, dictate water uptake and ion transport in Nafion®.²⁴ Therefore, construction of highly effective proton transfer channels in a PEM by controlling the connectivity and hydrophilic-site distribution is an important factor that should be taken into account for PEM material design. Pan et al.²⁵ built an OH⁻ conducting highway by properly constructing ion-aggregating structures of an alkaline polymer electrolyte (APE). The OH⁻ conductivity in the APE reached 0.1 S cm⁻¹ at 80 °C with a moderate IEC value of 1.0 mmol g⁻¹. In our previous work,^26–28 we designed and synthesized a series of PEMs with rigid-flexible hybrid structures that are responsible for the well-formed proton pathways. The flexible segments of the backbone are to facilitate proton conducting highways via an increase of the “bulk water” in the PEM materials to enlarge the ionic cluster size.^29,30 Here, the bulk water is referred to the water molecules in the interstitial space of ionic clusters and deemed to mediate ion transport more efficiently than the “surface water” on surfaces of the clusters.

In this paper, we report a new approach to construct high-efficiency ionic conduction channels via controlling the distribution of hydrophilic sites in the polymer backbone. The approach is based on the sandwich-like proton conduction model of Nafion®²³ proposed by Haubold, et al. As schematically illustrated in Fig. 1a, protons move along the well-constructed conduction channels (core) between two –SO₃H layers (shell) with the size of the channels in the range of 10–30 Å. The ionic conductivity of the PEM is largely determined by the amount of “bulk water”, which depends on the size of proton conduction channels. The “bulk water” content as well as the proton conduction efficiency can be improved by staggering acidic functional groups with uneven lengths attached to the polymer backbone as schemed in Fig. 1b. Compared with proton conductors that share similar polymer backbones but contain only either short (Fig. 1c) or long (Fig. 1d) acidic groups, the proton conduction channels of the polymer with uneven side chains capped by acidic groups are therefore broadened.

Fig. 1 The schematic diagrams of (a) a sandwich-like model of Nafion® morphology, (b) the broadened proton channels (BPCs), (c) the small proton channels (SPCs) with short side chains, (d) the small proton channels with long side chains, and (e) the designed polymer electrolyte with the broadened proton channels.

With the design strategy outlined above, we successfully synthesized a series of polyamide based polymer electrolytes with alternating long and short side chains of polyamide (LSPA). Two functional groups were introduced to the polymer backbone, with the sulfonic acid to form a short side chain and the sulfonimide group to form a long side chain (Fig. 1e). The polyamides have been widely used in various fields due to its excellent thermal, chemical and mechanical properties.^31–33 The negative charges of the sulfonamide are delocalized in the proximity of the sulphonyl groups due to their strong electron-withdrawing property. It leads to proton atoms are weakly attached to through an electrostatic interaction with high mobility. Therefore, the sulfonamide group was also determined as a superacid, which has been widely utilized for development of alternative PEMs.^34–36 To confirm the feasibility of the design strategy, another two PEMs were also synthesized, one with only short side chains of polyamide (SPA) capped by sulfonic acid groups and another possessing only long side chains of polyamide (LPA) with sulfonimide as the proton sources. We demonstrate that the LSPA membrane has the highest water retention capacity with the proton conductivity two times higher than the values of the SPA and LPA membranes at the same IEC value.

2. Methods

2.1 Materials

Benzene sulfonamide (Sigma-Aldrich), p-toluenesulfonylchloride (Sigma-Aldrich), dimethyl-5-aminoisophthalate (Sigma-Aldrich), potassium permanganate (GCE), lithium hydroxide monohydrate (Alfa-Aesar), triphenyl phosphite (TPP) (Alfa-Aesar) and sodium chloride (Alfa-Aesar) were used as purchased. 4,4′-(9-Fluorenylidene)dianiline (FIDA) (Sigma-Aldrich), decanedioic acid (DDA) (Sigma-Aldrich), 2,4-diaminobenzenesulfonic acid (DBSA) (Sigma-Aldrich), calcium chloride (GCE) and lithium chloride (Sigma-Aldrich) were dried under vacuum at 100 °C for 24 h before use. Pyridine was dried with KOH and distilled. N-Methyl-2-pyrrolidone (NMP) was distilled from P₂O₅ and calcium chloride (CaCl₂) was dried under vacuum at 180 °C for 24 h. Deuterated dimethyl sulfoxide (DMSO) (Cambridge Isotope Laboratories) was used for NMR characterization.

2.2 Synthesis

The precursor to be used for construction of long side chains, bis(phenylsulfonyl imide isophthalate amide) (BPSIIA), was synthesized by following the procedure described in our previous work.³⁷ The synthetic procedure of the polymer electrolytes (LSPA-0.49, LSPA-0.67, SPA-0.49 and LPA-0.49) with the postfixes of the theoretical IEC values was shown in Scheme 1.^38,39 A mixture of the calculated amount of FIDA, DDA, DBSA, BPSIIA, TPP and LiCl was added to a mixed solvent of NMP and pyridine and the reaction was kept at 100 °C for 12 h under nitrogen atmosphere. The mixture was then cooled to 70 °C and transferred into cold methanol with stirring to precipitate the white solid polymers. The white precipitates were filtered and washed with methanol and water repeatedly and dried under vacuum at 140 °C for 24 h. The weight-average molecular weights (M_w) are in the range of 235 000–309 000 g mol⁻¹, with the polydispersity indexes close to 1 (Table 1). The elemental analysis results of the polymers are in good agreement with the theoretical values (Table 2), indicating that the polymers were successfully synthesized.

Scheme 1 The synthetic procedure.

Table 1 The molecular weight of the PEMs

Polymers	Mw (×10^4 g mol^−1)	Mn^a (×10^4 g mol^−1)	PDI
a Measured at room temperature using DMF as the solvent and polystyrene as the standard.
LPA-0.49	29.3	27.4	1.07
LSPA-0.49	30.9	29.4	1.05
LSPA-0.67	25.9	24.0	1.08
SPA-0.49	23.5	21.6	1.09

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Table 2 The elemental analysis of the PEMs

	C (wt%) (theoretical)	H (wt%) (theoretical)	N (wt%) (theoretical)	S (wt%) (theoretical)
LPA-0.49	72.70 (71.72)	6.09 (5.88)	6.14 (5.68)	2.39 (3.00)
LSPA-0.49	74.15 (71.60)	6.65 (6.40)	5.69 (5.57)	1.72 (2.14)
LSPA-0.67	72.63 (69.95)	6.35 (6.21)	5.96 (5.74)	2.90 (2.88)
SPA-0.49	73.49 (71.46)	6.98 (6.91)	6.04 (5.46)	0.93 (1.29)

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2.3 PEM preparation

PEMs were prepared via a solution cast technique using dimethyl sulfoxide (DMSO) as a solvent. 0.25 g of the polymer electrolyte was dissolved in 8 mL of DMSO and the solution was then cast onto a glass plate followed by drying at 90 °C overnight. The samples were further dried at 80 °C under vacuum for one day to obtain membranes. The prepared membranes were activated by soaking in 0.5 M H₂SO₄ at room temperature for 24 h, and subsequently washed with ultrapure water repeatedly until the pH values of the wash solutions became 7. The average thicknesses of the membranes were around 40 μm in a dry state.

2.4 Thermal stability

Thermal stability of the polymer electrolytes was determined by thermo gravimetric analysis (TGA) using SDT TA instrument 2960 Simultaneous DTA-TGA. The measurements were performed at a rate of 10.00 °C min⁻¹ up to 600 °C under nitrogen atmosphere. Prior to the measurement, the samples were dried at 140 °C in a vacuum oven for 6 h.

2.5 Mechanical strength

The tensile strength and the elongation of the PEMs were performed on ASTM D882, REF ASTM, using an Instron Universal Materials Testing System (model 5544) with a 10 N load cell at 25 °C with the constant relative humidity of 50%. Rectangular-shaped samples were cut from the films (10 mm wide with a gauge length of 40 mm). The thickness of the samples was measured with a digital micrometer with a precision of 1 μm and sensitivity of 40%.

2.6 SEM images

The morphologies of the PEMs were determined by using the Scanning Electron Microscopy (SEM) with QUANTA 200 FEG. Samples were prepared by platinum sputtering under 5 × 10⁻² mbar at room temperature (120 s, 30 mA) with a Baltec SCD050 apparatus.

2.7 Water uptake and swelling

Prior to the water uptake measurement, the polymer electrolytes were first immersed in ultrapure water at room temperature for 24 h and then weighed immediately upon removal of the surface-attached water using a clean tissue paper (W_wet). The membranes were then dried under vacuum at 120 °C for 24 h and weighed immediately (W_dry). The water uptake (WU) for the polymer membranes were calculated using eqn (1):

Polymer electrolyte membranes with fixed dimensions (4 cm × 1 cm) were first immersed in ultrapure water for 24 h and the length was measured (L_wet). The membranes were then dried under vacuum at 120 °C for 24 h and the length was measured (L_dry). The water swelling (WS) for the PEMs was calculated from eqn (2):

2.8 Ion exchange capacity (IEC)

Ion exchange capacity (IEC) of the PEMs was measured according to the following steps. 0.1–0.2 g of the polymer electrolyte membranes was immersed first in 100 mL of 1 M HCl for 24 h and then in 100 mL of 1 M NaCl for 48 h to allow complete conversion of H⁺ ions in sulfonamide to Na⁺ ions. The amounts of H⁺ were measured by titration with 0.01 M NaOH using phenolphthalein as the indicator. The IEC value of the PEMs was calculated using eqn (3):where V_NaOH and [NaOH] refer to the volume and concentration of the NaOH solution, respectively, and W is the weight of the membrane.

2.9 Proton conductivity

The proton conductivity of the PEMs was measured by a four-electrode method with electrochemical impedance spectroscopy (EIS) using a potentiostat-galvanostat AUTOLAB model PGSTAT12/30/302 over the frequency range of 1 Hz–4 MHz with the oscillating voltage of 5 mV. Prior to the test, the membranes were immersed in 1 M of HCl for 48 h, and then washed with deionized water until the pH becomes 7. The proton conductivity measurement for each membrane was carried out from 30 to 80 °C at an interval of 10 °C under 100% relative humidity. The proton conductivity (σ, S cm⁻¹) was calculated from the impedance data from eqn (4):where d is the distance between the electrodes (cm), t and w are the thickness (cm) and the width (cm) of the membranes, respectively, and R refers to the resistance (Ω) obtained from the impedance data.

3. Results and discussion

3.1 FTIR spectroscopies

Fig. 2 shows the FTIR spectra of the polymer electrolytes. The characteristic band around 3400 cm⁻¹ (a) represents the N–H stretch of the amide groups, reflecting the formation of polyamides, while the band around 3300 cm⁻¹ (b) corresponds to the N–H stretch of the sulfonimide groups.²⁹ The absorption bands in the range of 3000–3100 cm⁻¹ (c) arise from the C–H stretch of aromatic rings and the bands at 2800–3000 cm⁻¹ (d) are assigned to the C–H stretch in the aliphatic alkyl chains. The typical absorption bands of the S=O and S–O groups in sulfonimide groups were observed in the range of 1355–1050 cm⁻¹ (e).^40,41

Fig. 2 The FTIR spectra of the polymer electrolytes.

3.2 ¹H NMR spectroscopies

The chemical structures of the polymer electrolytes were confirmed by the ¹H NMR spectra (Fig. 3). As the overlap of the signals and the different amount of the components of the polymer electrolytes, the assignation of peaks to the signals of the proton was mainly adjusted by the chemical shift and the coarse integration and whether the signal of the proton was appeared in ¹H NMR spectra. Obviously, different hydrogens (H_a to H_g) of the hydrophobic contents on all polymer electrolytes are clearly identified. The signals of the hydrogen (H_n to H_t) were observed in LPA-0.49 but not in SPA-0.49, clearly suggesting that they belong to the BPSIIA side group. At the same time, the proton signals of the sulfonic acid groups (H_h to H_k) of SPA-0.49 can be easily distinguished. All of the proton signals appear on LSPA-0.49 and LSPA-0.67, which confirms the presence of both the sulfonic acid group and the BPSIIA group in these two membranes.

Fig. 3 The ¹H NMR spectra of the polymer electrolyte membranes.

3.3. Thermal stability

The TGA curves of the polymer electrolytes are depicted in Fig. 4. The weight loss around 100 °C is attributed to the desorption of moisture, while the weight loss before 200 °C is due to the high melting point organic solvent (NMP) left in the polymer matrix.²⁹ A two-step degradation in the range of 200–280 °C and 350–500 °C, respectively, is observed, consistent with the results reported by Meziane et al. for PEMs containing sulfonimide groups.⁴⁰ The first degradation is attributed to the cleavage of the carbon–sulfur bonds, which produces styryl and sulfonyl or sulfonate radicals. A reaction between the styryl radical and the hydrogen of the polymer chains occurs at about 450–500 °C, resulting in degradation of the polymer backbone.⁴² The thermal stability up to 200 °C exceeds the requirement of PEMs for applications in PEM fuel cells (∼80 °C).

Fig. 4 The TGA curves of the polymer electrolytes.

3.4 Mechanical stability

Fig. 5 depicts the tensile strengths and the elongations at break of the PEMs. For purpose of comparison, the corresponding properties of a Nafion® 117 membrane are also displayed here. The synthesized membranes display tensile stresses in the range of 23–53 MPa, higher than that of Nafion® 117. The results suggest that these membranes exhibit suitable mechanical stability required for PEM fuel cell applications. Furthermore, the incorporation of the sulfonimide groups into the PEMs results in stronger mechanical properties than the incorporation of the sulfonic acid groups in the same PEMs. Bouchet et al. also found that the incorporation of sulfonimide groups can significantly increase the tensile stress of the PS–PEO–PS membrane by one order of magnitude,⁴³ in which strong ionic crosslinking between the sulfonimide and the polyamide domains induces self-healing in some ionomer-type polymers.⁴⁴ Finally, the elongation at break of the PEMs is substantially smaller than that of Nafion® 117, reflecting the short deformation rate in the fabrication process of membrane electrolyte assembly (MEA).

Fig. 5 The tensile stresses and elongation breaks of the polymer electrolyte membranes and Nafion® 117.

3.5 Water uptake and swelling

According to the sandwich-like model, water uptake and water swelling directly reflect the size of proton conduction channels in a membrane.^30,45 Fig. 6 indicates that at the same IEC level, similar water uptakes of 7.38 wt% and 7.52 wt% were obtained for SPA-0.49 and LPA-0.49 membranes, respectively. However, the LSPA-0.49 membrane exhibits a much higher water uptake of 9.10 wt%, clearly indicating that the uneven side chains in the polymer facilitate accommodation of water. The relationship between the hydration, defined as the number of water molecules per proton (λ_H2O), and the structural change (water swelling) was also calculated to gain insight into the influence of water states on proton conductivity in the PEMs. For the LPA-0.49 and SPA-0.49 membranes, the same hydration value (λ_H2O = 11) and similar water swelling (1.35% vs. 1.27%) were obtained. However, the LSPA-0.49 membrane exhibits a higher hydration value of 13 and modestly higher water swelling of 4.77% due to the uneven side chain structural configuration. The significantly enhanced λ_H2O value and water swelling are again ascribed to the side-chain tailored structure of the LSPA membrane, in which the acid groups attached to both the long and the short side chains give rise to broadened proton conduction channels. The broadened proton conduction channel of the LSPA-0.49 was directly confirmed by the SEM images of surfaces morphology with large size and continuous proton conduction channels (Fig. 7a). In contrast, the LPA-0.49 exhibit small size proton conduction channels (Fig. 7b). In addition, the water uptake and the water swelling of the LSPA membrane are further increased to 13.51 wt% and 6.58%, respectively, as the IEC value increases to 0.67. Given the same hydration value of Nafion® 117 (λ_H2O = 13), the LSPA-0.67 membrane displays minor dimensional swelling (6.58% vs. 20.10%, Table 3). The excellent dimensional stability of the PEMs in water is attributed to the polyamide based polymer backbone,^46,47 which enables the polyamide based PEMs to sustain better methanol resistance and longer device operation.

Fig. 6 Water uptake and dimensional stability of the polymer electrolyte membranes and Nafion® 117.

Fig. 7 The SEM images of the surface morphology of the PEMs. (a) LSPA-0.49, (b) LPA-0.49.

Table 3 Physical properties of the PEM membranes and Nafion® 117

	IEC (mmol g^−1)		Water uptake (%)	Dimensional swelling (%)	Proton conductivity^a (S cm^−1)	Tensile stress (MPa)	Elongation at break (%)
	Calculated	Measured
a Measured at 80 °C with 100% relative humidity.
LPA-0.49	0.49	0.39	7.38	1.35	0.029	52.67	9.90
SPA-0.49	0.49	0.38	7.52	1.27	0.023	23.19	6.60
LSPA-0.49	0.49	0.38	9.10	4.77	0.050	29.14	9.80
LSPA-0.67	0.67	0.59	13.51	6.58	0.067	26.40	10.90
Nafion® 117	—	0.91	14.38	25.10	0.155	16.9	225

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3.6 Proton conductivity

The proton conductivities of the polymer electrolytes as a function of temperature are depicted in Fig. 7 and, for clarity, also tabulated in Table 3. Proton conductivity of a PEM is known to be strongly dependent on the content of the hydrophilic component, i.e. the IEC value. As a result, the same polymer backbone based PEMs with the same content of hydrophilic and hydrophobic components, e.g. SPA-0.49, LPA-0.49 and LSPA-0.49, should in principle display a similar degree of proton conductivity.⁴⁸ However, as shown in Fig. 8, the proton conductivity of LSPA-0.49 is two times higher than the values of LPA-0.49 and SPA-0.49. This is obviously attributed to the broadened proton transport channels in the LSPA-0.49 membrane (Fig. 7a). With an increase of the IEC value of LSPA to 0.67, a higher proton conductivity of 0.067 S cm⁻¹ was obtained. In other words, an increase of the IEC value by 37% gives rise to an increase of proton conductivity of the LSPA membrane by 60% at room temperature and 33% at 80 °C. We note here that the conductivity of LSPA-0.67 is about two times lower than that of Nafion® 117 at 80 °C due to the significantly lower IEC value (0.67 vs. 0.91). Nevertheless, the conductivity is still much higher than the values recently reported for sulfonimide functionalized PEMs (0.04 S cm⁻¹ vs. 0.01 S cm⁻¹ at 60 °C), even with the IEC value roughly 3 times lower.⁴⁹ Unfortunately, the membranes become increasingly brittle as the IEC value increases. To further enhance the proton conductivity, better structural design of side chains but still with the uneven side chain topology built in the polymer architecture is necessary.

Fig. 8 Proton conductivities of the polymer electrolyte membranes.

4. Conclusion

We have successfully developed a facile approach to improve proton conductivity of polymer electrolyte membranes with a relatively low IEC value. To demonstrate the effectiveness of an uneven side chain topology on enhancement of proton conductivity in polymer membranes, three types of polyamide based polymer electrolytes with acid groups attached to the long, the short and the long/short side chains were designed and successfully synthesized. The chemical structure and the thermal stability of the polymers were characterized with FTIR, ¹H NMR and TGA. The physical properties, i.e. mechanical stress, water uptake, volume swelling, chemical structure and proton conductivity, were measured and systematically compared. The results indicate that the uneven side chain membrane LSPA exhibits the largest size of proton transport channels at the same IEC value as evidenced by its higher water uptake capacity (“bulk water”) and higher volume swelling (broadened proton transport channels). Therefore, the LSPA-0.49 membrane displays a 100% of higher proton conductivity than that of the LPA-0.49 and SPA-0.49 membranes as a result of the well-formed proton conduction channels. It was found that the polyamide based polymer electrolyte membranes display significantly better dimensional and mechanical stability than a commercial Nafion® 117 membrane due to their lower water-swelling. With an increase of the IEC value to 0.67, the proton conductivity of the LSPA-0.67 membrane becomes close to 10⁻¹ S cm⁻¹, which is two times lower than the value of the Nafion® 117 membrane but significantly higher than the values of the LPA and SPA membranes. The present study provides an important insight into the design of polymer structures for enhancement of proton conductivity of PEM materials.

Acknowledgements

The authors gratefully acknowledge support of the National Natural Science Foundation of China (No. 21233006, 21473164, and 21503197), and Fundamental Research Funds for the Central University, China University of Geosciences, Wuhan (No. CUG150620, CUG150615).

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