Electrospun nanofiber enhanced imidazolium-functionalized polysulfone composite anion exchange membranes

Abstract

A novel method of improving interfacial compatibility in electrospun anion exchange membranes (AEMs) is developed by using imidazolium-functionalized polysulfone (IMPSF) as both electrospun fiber mats and interfiber voids filler. Scanning electron microscopy (SEM) illustrates the defect-free and fiber-retained morphology of the IMPSF electrospun AEMs. Transmission electron microscopy (TEM) shows better aggregation of ion clusters in the IMPSF electrospun AEMs. As a result, the electrospun AEMs prepared in the present work exhibit much higher hydroxide conductivity increment than most reported electrospun AEMs (around 1.7 folds in 20 °C water and 100 folds at 60 °C, relative humidity (RH) 40% as compared with the corresponding cast AEMs). Excellent interfacial compatibility and microphase separation morphology also promote swelling resistance and mechanical and alkaline stabilities of the IMPSF electrospun composite AEMs. Compared with the cast AEMs, tensile strength increment is up to 22%, alkaline stability increases more than one fold after immersing in 1 M KOH at 60 °C for 24 h. Results in the present work are helpful to componential design of the electrospun AEMs.

---

Introduction

As one of the key components of fuel cells, AEM has attracted much attention due to its potential superiority to proton exchange membranes (PEM). In alkaline working conditions, the reaction kinetics increase,^1,2 therefore non-noble metal catalysts can be adopted to reduce the cost; opposite direction of ion motion is helpful to water management; however, none of commercial AEMs behave as perfect as commercial PEMs such as Nafion, due to low hydroxide conductivity, alkaline stability and mechanical stability.

Many researches have been carried out to improve performances of AEMs. Increasing ions exchange capacity (IEC) leads to high hydroxide conductivity, while may result in excessive swelling and lower mechanical stability. AEMs have been fabricated with different functional groups such as trimethylamine,³ imidazolium⁴ and even some highly alkaline functional group like phosphonium,^5,6 guanidine⁷ and permethyl cobaltocenium.⁸ However, further improvement of hydroxide conductivity to the level of commercial Nafion is limited by pristine properties of AEMs, such as lower mobility of OH⁻ (OH⁻ = 2.69, H⁺ = 4.76 compared to K⁺ in dilute solution at 25 °C), less dissociation and solvation of OH⁻,⁹ easily neutralization of OH⁻ by CO₂ in air.¹⁰ Design of ion conductive channels has been proposed to improve efficiency of ionic functional groups. Hybrid of AEM materials with inorganic stuff, such as functionalized carbon nanotubes is reported to improve hydroxide conductivity by creating interconnected channels along carbon tube's wall, while poor compatibility may result in macro-phase separation.¹¹ Blocking, grafting and semi-interpenetrating networks (sIPNs) are useful methods for hydrophilic–hydrophobic morphology control. For instance, we have fabricated sIPN based PEMs¹² and bi-functional 1,4-diazabicyclo[2,2,2]octane (DABCO) side chain based AEMs¹³ with improved microphase separating morphologies and high performances.

Electrospinning is newly proposed as a promising method for ion conductive channels design initially for PEMs around 2006.¹⁴ Under high AC electric field, polymer solution is stretched into extra-fine nanofibers,¹⁵ which could provide large specific surface area and long connected ion conductive channels along fibers.^16,17 It has been reported that single electrospun proton conductive nanofiber exhibits more than 10 folds of conductivity as compared with its bulk membrane.¹⁶ The reasons are proposed as the orientation of ionic domains along fiber axis or towards fiber surface,¹⁸ driven by the shear force of electric field.^16,19 Although widely concerned, only a few studies focus on electrospun AEMs as compared with that on electrospun PEMs. As summarized in Table 1, materials reported as electrospun fibers in AEMs include trimethylamine functionalized polsulfone (QAPSF) and guanidinium functionalized polyether sulfone (PES-G). The main challenge is that hydroxide conductivity of the most electrospun composite AEMs could hardly improve as compared with their corresponding cast AEMs. Possible reasons are as follows: (1) electrospinning of uncharged chloromethylation polymers,^20–22 seeing no. 1 through 3 in Table 1. Shear stress of electric field has little effect on elongation of ion clusters during electrospun process. As a result, hydroxide conductivity of this kind QAPSF/PPSU electrospun AEMs declined by 27% at IEC = 2.17 mmol g⁻¹ (no. 1); further crosslinking of the system with very high IEC could elevate hydroxide conductivity due to increasing concentration of ions, however, the obtained AEMs are likely to swell too much (no. 2 and 3). (2) Using non-polyelectrolytes as interfiber voids fillers,²³ seeing no. 1 through 4 in Table 1, would reduce fraction of functional groups and also decrease hydroxide conductivity. (3) Applications of ion conductive interfiber voids fillers as well as crosslinking is helpful to improve hydroxide conductivity,²⁴ seeing no. 5 in Table 1, however, incompatibility nature between different fiber and filler materials is still likely to induce macro-phase separation and poor mechanical stability. And it is difficult to distinguish the effect of electrospun fibers from that of crosslinking.

Table 1 Hydroxide conductivity and swelling ratio of recently reported electrospun AEMs

No.	Membranes	Fiber	Filler	Cation	IEC of fiber (mmol g^−1)	Conductivity in water (20–25 °C) (mS cm^−1)		Swelling ratio^a (%)	Ref.
						Electrospun	Cast	Electrospun
a Gravimetric swelling ratio in no. 1 through 3, dimensional swelling ratio in no. 5.
1	QAPSF/PPSU	CMPSF	PPSU	TMA	2.17	11	15	61	20
2	Diamine crosslinked QAPSF/PPSU	CMPSF	PPSU	TMA	3.2	65	—	144	21
3	Diol crosslinked QAPSF/PPSU	CMPSF	PPSU	TMA	2.8	57	50	136	22
4	QAPS/PDMS	QAPSF-Cl	PDMS	TMA	1.73	22 (80 °C, RH 90%)	54	—	23
5	Crosslinked PES-G/(VBTC and MBA)	PES-G-Cl	VBTC/MBA	Guanidinim	—	30	23	10.3	24

---

In order to improve interfacial compatibility and conductivity of electrospun AEMs, a novel method is proposed in the present work by using the same alkaline fiber and interfiber voids filler materials. That is, imidazolium-functionalized polysulfone (IMPSF) is electrospun into nanofiber mats and then the interfiber voids are filled with IMPSF matrix of the same IEC by means of solubility difference of IMPSF in different solvents. Glycol/water mixture is developed to dissolve IMPSF well only at elevated temperature, so as to be employed for interfiber voids filling process. Excellent interfacial compatibility and better microphase aggregations are achieved, resulting in enhancement of hydroxide conductivity, swelling resistance and alkaline and mechanical stabilities of the IMPSF electrospun AEMs as compared with the cast AEMs of the same IEC. The present method simplifies componential design of the electrospun AEMs. It also gives an intuitive insight into the functions of IMPSF fiberization by getting rid of influences of the dissimilar fillers in most reported electrospun AEMs. The results indicate that IMPSF electrospun AEMs prepared in the present work is promising for AEMs applications.

Experimental

Materials

Udel P3500 polysulfone (PSF), 1-methylimidazol, stannic chloride (SnCl₄), dichloromethane (CH₂Cl₂), N-methyl pyrrolidone (NMP), N,N-dimethyl formamide (DMF), acetone, ethylene glycol, potassium hydroxide (KOH), hydrochloric acid (HCl, 12 wt%) and palladium chloride (PdCl₂) were obtained commercially and used as received. Chloromethyl octylether (CMOE) was laboratory-made according to our previous work.²⁵

Preparation of IMPSF electrospun and cast AEMs

Anion exchange membranes (AEMs) play a role as carrier for hydroxyl transport from cathode to the anode in alkaline fuel cells. AEMs usually consist of polymer matrix and cationic side chains. Polymer matrix determines mechanical and thermal stabilities, while cations form ionic clusters and function as carrier for hydroxide ion transport. Imidazolium is one of the most applicable hydroxide-conducting functional cationic groups^25–27 due to its low volatility, excellent thermal stability and high ion conductivity. Thus we fabricated IMPSF–Cl material according to our previous work.²⁵ IMPSF nanofiber mats were obtained by electrospinning of IMPSF–Cl directly. Electrospun conditions were as follow: 22.5 wt% in DMF solution, 10 cm from needle to collecting plate, applied voltage of 22 kV. The obtained IMPSF fiber mats were dried at 60 °C for 6 h to evaporate redundant DMF thus avoiding over-welded when compacting. Then fibers were compacted at 25 °C, 3 MPa for 20 s to increase volume fraction and interconnection between fibers.

In order to obtain dense and fibers incorporated composite membranes, interfiber voids need to be filled. IMPSF–Cl was observed to dissolve well in glycol/water mixture (volume ratio 1 : 1.5) only at elevated temperature (above 60 °C), therefore IMPSF/glycol/water solution can be used as interfiber voids filler without dissolution of the fiber mats. Fiber mats were immersed in IMPSF/glycol/water solution (5 wt%) at room temperature with swelling ratio less than 23%. It is noticed that the fiber and filler IMPSF materials possess the same IEC. Then redundant solution was removed by suction filtration, which promotes flow of solvent through the fiber mats.²⁸ The solvent-uptake membrane was dried in vacuum oven at 40 °C. The favorable mass fraction of IMPSF filler in the final electrospun AEMs was around 41.5 wt% being determined with gravimetric measurements. The schematic structure of IMPSF electrospun nanofiber enhanced composite AEMs is shown in Fig. 1. The obtained membrane was soaked in 1 M KOH solution for 48 h to exchange Cl⁻ with OH⁻ then washed with deionized water to neutral. In contrast, IMPSF cast AEM was prepared by casting from IMPSF/NMP solution. Thickness of all membranes prepared is around 100 μm.

Fig. 1 Schematic structure of IMPSF electrospun nanofiber enhanced composite AEM.

Membrane characterization

Morphologies of fiber mats and electrospun AEMs were characterized with scanning electron microscopy (SEM, TM3000, Japan). Mechanical performance of the hydrated membrane was tested with SANS CMT8102 stretching tester (Xinsansi Co, China). Membrane sample was 3 × 1 cm and tested under the same conditions. Ion clusters was observed with transmission electron microscopy (TEM, JEM-2000EX, JEOL Co.). Membrane sample was stained with PdCl₂ in 1 M HCl solution for about 3 days as ammonium could form complex with Pd²⁺ and then washed with deionized water.

IEC of the IMPSF electrospun AMEs represents moles of imidazolium per weight dry membrane, and was tested by back titration method. About 0.1 g membrane sample was immersed into 25 ml 0.01 M HCl for 24 h and then titrated with 0.01 M NaOH with phenolphthalein indicator. 25 ml 0.01 M HCl was also titrated as blank test. IEC was calculated by eqn (1), where V_b and V_a are volumes of NaOH solution used for blank test and membrane titration respectively; C_NaOH is the concentration of NaOH solution;

Membrane sample was first immersed in deionized water of different temperatures at least 12 h to keep membrane hydrated. After measurement, the membrane sample was dried in 60 °C vacuum oven for 24 h in order to measure weight and size of the dry membrane.

As calculated by eqn (2) and (3), W_wet and W_dry are weight of wet and dry membrane, respectively; l_wet = (l_w1 × l_w2)^1/2 and l_dry = (l_d1 × l_d2)^1/2 are exponential average dimension of wet and dry membrane, respectively; l_w1, l_w2 and l_d1, l_d2 are lengths and widths of wet and dry membrane, respectively.⁴

The in-plane hydroxide conductivity was measured both in water and in water vapor of different RH by four electrode AC impedance method, using Ivium Technologies A08001 equipment with scanning frequency in the range of 1 to 10⁵ Hz. All the membranes were hydrated in deionized water at least 24 h before test, then cut into about 0.6 × 4 cm slices, sandwiched between two Teflon block mounting with four platinum electrode, immersed in a glass container full of deionized water, stabilized at test temperature for 30 min using water bath. The hydroxide conductivity was calculated by eqn (4), where σ is the hydroxide conductivity of membrane in mS cm⁻¹, L is the distance between two potential electrodes in cm, R is the membrane resistance obtained from high frequency intersect of semi-circle on the complex impedance plane with the real axis impedance, A is the in-plane area in cm². Stable RH environment was obtained by placing the test cell into an environmental chamber with 60 °C and certain RH (from 98% to 40% and stable 1 h for each RH).

Alkaline stability of the IMPSF–OH membranes was tested by immersing them into 1 M KOH solutions for 24 h at different temperatures, and then washed with deionized water to neutral. Hydroxide conductivity was measured in 60 °C water before and after alkaline treatment to evaluate the stability of the electrospun AEMs.

Results and discussion

Morphology of IMPSF electrospun AEMs

A novel method, filling interfiber voids with the same IMPSF material as electrospun nanofibers, is developed in the present work to improve interfacial compatibility as compared with the electrospun AEMs reported in the literature. Morphologies of the IMPSF electrospun fiber mats and composite membrane are shown in Fig. 2. The opaque optical photograph in Fig. 2(a) indicates highly porous of the fiber mats. Since IMPSF is observed to dissolve well in glycol/water mixture (volume ratio 1 : 1.5) only at elevated temperature (above 60 °C), it is feasible to fill the interfiber voids of IMPSF/glycol/water solution and then evaporate the solvent at lower temperature (40 °C) to prepare a dense membrane. As shown in Fig. 2(b) and (c), the obtained electrospun composite AEMs are nearly as transparent as the cast AEMs, indicating defect-free and excellent interfacial compatibility between the electrospun fiber mats and interfiber voids filler components. Microstructure of the fiber mats and composite membrane are further investigated by SEM. As shown in Fig. 2(d), smooth IMPSF nanofibers have been fabricated successfully with an average diameter of around 156 nm (counted at least 100 fibers). Intersecting fibers are welded to form three-dimensional network by compacting, as shown in Fig. 2(e). After interfiber voids filling, it is obviously observed that the nanofibers remain intact on the membrane surface (Fig. 2(f) and (g)) as compared with membrane (Fig. 2(h)). Compared with the commonly used dissimilar compounds for fiber and filler that easily leads to poor interfacial compatibility and macroscopic phase separation, the method developed in the present work achieves excellent interfacial compatibility in the electrospun AEMs.

Fig. 2 Images of electrospun IMPSF fiber mats, electrospun and cast AEMs (optical photographs of IMPSF (a) fiber mats, (b) electrospun AEM, (c) cast AEM; SEM images of IMPSF (d) electrospun fiber mats with inserted histogram of fiber diameter distribution (counted at least 100 fibers), (e) surface of fiber mats after compaction, (f) surface of electrospun AEM, (g) cross-section of electrospun AEM, (h) cross-section of cast AEM). Electrospinning condition: 22 kV, syringe-collector distance of 10 cm, concentration of electrospinning IMPSF solution 22.5 wt%, mass fraction of interfiber voids filler in the composite membrane is about 41.5 wt%.

Hydroxide conductivity

Hydroxide conductivity is one of the most important properties of AEMs, directly affecting the performance of fuel cells. Fig. 3 shows hydroxide conductivity of the fully hydrated IMPSF membranes as a function of IEC, in which the fiber mats, interfiber voids filler and cast membrane have the same IEC at each data point. Hydroxide conductivity of the electrospun AEMs is about 1.7 times averagely (20 °C) as high as that of the cast AEMs and could achieve 70.2 mS cm⁻¹ for IEC 1.78 mmol g⁻¹ at 60 °C. Compared with other reported electrospun AEMs (listed in Table 1), IMPSF electrospun AEMs prepared in the present work exhibit highest conductivity increment due to excellent interfacial compatibility achieved by the design of the similar fiber and filler materials. Compared with the reported cast imidazolium-functionalized AEMs, as summarized in Table 2, with comparable IECs, hydroxide conductivity of IMPSF electrospun AEMs in the present work is in the top level.

Fig. 3 Hydroxide conductivity of the electrospun and cast AEMs as a function of IEC (IEC is the same in electrospun fiber mats, interfiber voids filler and cast membrane at each point. Hydroxide conductivity is measured in water).

Table 2 IEC, hydroxide conductivity of recently reported imidazolium functionalized AEM

Membrane material	IEC (mmol g^−1)	Conductivity in water (20 °C) (mS cm^−1)	Ref.
PSF-ImOH electrospun AEMs	1.78	38.4	This work
PSF-ImOH	1.77	31.8	4
PES-ImOH	1.45	30.0	29
IPAES-MIm	1.85	22.3 ± 0.9	30
PSF-ImOH	2.46 ± 0.25	20.7 ± 0.6	31
IM-PFEKS	1.64	17.1 (RT)	32
FPAEO-MIM	1.79	15.9	33
PES-MeIm/OH	1.65	14.9 (RT)	34
BPPO-ImOH	1.76	14.0	35
[VBMI]OH/styrene	1.45	12.4 (30 °C)	36
mSQPBI	1.49	5.11 (30 °C)	37

---

Non-fluorine AEMs usually exhibit very low hydroxide conductivity at low RH due to poor connection between ionic conductive channels, although low RH is a common and inevitable working condition in fuel cell driven cars or potable equipments.

Incorporation of electrospun nanofibers into AEMs greatly improves hydroxide conductivity at low RH. As shown in Fig. 4, when equilibrated with liquid water (data points at RH 100%), hydroxide conductivity of the electrospun membrane is around 1.6 times as that of the cast membrane, both reaching values near 100 mS cm⁻¹. However, hydroxide conductivity of the electrospun AEMs responses much faster to RH. For instance, at RH 40%, hydroxide conductivity is only around 10⁻³ mS cm⁻¹ in the cast AEMs, while reaches 0.1 mS cm⁻¹ in the electrospun AEMs (about 100 times increment). The improved hydroxide conductivity of the IMPSF electrospun AEMs at low RH suggests that they are promising for AEMs applications at reduced RH, with the potential advantages of increasing partial pressure of fuels, improving transport property of electrodes and simplifying water management of fuel cells.

Fig. 4 Relative humidity-dependent hydroxide conductivity of electrospun and cast AEMs at 60 °C (IEC = 1.78 mmol g⁻¹).

Hydroxide conductivity and mechanical stability of AEMs are greatly influenced by water sorption behavior. Fig. 5 shows water uptake and swelling ratio of the electrospun and cast AEMs as a function of IEC. It is reasonable that water uptake and swelling ratio increase with the increasing IEC. As compared with the cast AEMs, the electrospun AEMs absorb more water and water uptake of the electrospun AEMs is averagely 1.5 times as that of the corresponding cast AEMs. Higher water sorption ability contributes partially to the improvement of hydroxide conductivity in both hydrated and low RH environments. Although water uptake is higher, the electrospun AEMs exhibit lower values of swelling ratio. For instance, swelling ratio of the electrospun AEMs is just 40.6% (IEC 1.78 mmol g⁻¹, 60 °C), while the corresponding cast AEM swells so serious (about 62.7%) that the membrane could not keep its morphology. The data indicates that the electrospun AEMs have the ability of absorbing more water and swelling less as compared with the cast AEMs. It is reported that the absorbed water molecules tend to aggregate along the fiber surfaces rather than distribute uniformly in the membrane, which promotes dimensional stability under hydration.^38,39 The welded fiber networks and excellent interfacial compatibility between the IMPSF fiber and filler could also contribute to the well swelling resist behavior of the electrospun AEMs. The data presented here suggests that nonconductive interfiber voids filler as reported in most electrospun AEMs might be unnecessary in terms of swelling resistance, so as to avoid reducing mass fraction of the ion conductive components and causing interfacial incompatibility between the dissimilar components.

Fig. 5 Water uptake and swelling ratio of the electrospun and cast AEMs as a function of IEC at 60 °C.

Further studies on ionic clusters of the electrospun AEMs are conducted by TEM and shown in Fig. 6. It is well established for ionic membranes that hydrophobic polymer backbones and ionic functional groups have the tendency to aggregate respectively. Degree of hydrophilic–hydrophobic phase separation and connectivity of ionic domains usually increase with increasing IEC, ordered ionic conductive channels and longer pendent and multi-ionic side chains.^13,40,41 In this work, we emphasis on the micro-phase separation structure induced by electrospun ionic conductive channels, so that morphologies of the electrospun and cast membranes with similar IEC and imdazolium ionic side chain are investigated by TEM technology. As shown in Fig. 6, aggregation of ion clusters (black dots stained by Pd²⁺) is observed in the electrospun membrane (Fig. 6(a)), while no obvious hydrophilic and hydrophobic phase separation was observed in the cast membrane (Fig. 6(b)) that indicates the ion clusters are too small to be distinguished by TEM technology.⁴² Better aggregation of ion clusters in the electrospun membranes could contribute to interconnection of ion conductive channels and improvement of hydroxide conductivity especially at low RH. Well hydrophilic–hydrophobic microphase separation structure also helps to resist swelling of AEMs. Ionic groups in fibers are easily to be elongated and aggregate along fibers by shear force during electrospinning process.¹⁶ With good interfacial compatibility of the present IMPSF system, ionic clusters in the filling IMPSF matrix could also aggregate along the IMPSF fiber by hydrophilic attractions and promote aggregation of ion clusters.^38,43

Fig. 6 TEM images of (a) electrospun AEMs, (b) cast AEMs (IEC = 1.78 mmol g⁻¹ for both membranes).

Chemical stability

Alkaline stability of AEMs is very important due to their alkaline working condition. Imidazolium-type AEMs are easily degraded as a result of nucleophilic attack of OH⁻ on the C₂ position of planar imidazole ring.^32,44 In the literature, IEC of IMPFEK was reported to reduce to 83% of its original after alkaline treatment in 1 M NaOH solution at 60 °C for 48 h.³² Hydroxide conductivity was observed to decline from 17.3 mS cm⁻¹ to 2 mS cm⁻¹ (HCO₃⁻ from, measured at 30 °C) in 1-benzyl-3-methylimazolium (BMI)-type AEMs only immersed in 1 M KOH at 60 °C for 24 h.⁴⁵ In the present work, it is proved that electrospun fibers incorporation could weaken the attack of OH⁻ by forming better microphase separation structure. As shown in Fig. 7, ratios of hydroxide conductivity (σ_τ/σ₀) and IEC (IEC_τ/IEC₀) after and before alkaline soaking are plotted as a function of immersion temperatures. About 20% hydroxide conductivity and 73% IEC of the cast AEMs is reserved, comparable to the values reported in the literature. Although hydroxide conductivity of both membranes declines with increasing immersion temperature, it decreases much slower in electrospun AEMs (more than 40% hydroxide conductivity is reserved). And IEC of the electrospun membrane reduces much slower (83% reserved) compared with the cast membrane (73% reserved) after immersing in 1 M KOH for 24 h at 60 °C. The better alkaline stability is related to well microphase separation of the electrospun AEMs, as reported in the literature.^46–48 On one hand, better aggregation of hydrophilic domains promotes better solvation of cations and OH⁻, which could slow down the degradation speed of cations.⁴⁶ On the other hand, the attack of OH⁻ on polymer backbone could be well protected by the aggregated hydrophobic polymer main chains.^47,48

Fig. 7 σ_τ/σ₀ (IEC = 0.95 mmol g⁻¹, conductivity measured at 60 °C) and IEC_τ/IEC₀ of the IMPSF electrospun and cast AEMs after immersing in 1 M KOH for 24 h at different temperatures.

Mechanical strength

Mechanical stability influences durability as AEMs would be broken by assembling and swelling/shrinking forces during operations.⁴⁹ Fig. 8 shows the mechanical properties of the hydrated electrospun and cast AEMs. Incorporation of IMPSF electrospun fibers reinforces the composite membranes, although weight fraction of the fiber mats is less than 50 wt%. As shown in Fig. 8, tensile strength of the electrospun AEMs is 10% to 20% higher than that of the cast AEMs and the yield stress of electrospun AEMs (21.4 Mpa) is enhanced by 45% over that of the cast AEMs (14.8 Mpa) at IEC = 1.02 mmol g⁻¹ (denoted as electrospun 1.02 and cast 1.02 in Fig. 8). The result is related to the better microphase separation structure and welded fiber networks formed during compacting and filling processes, which function as skeleton to support the membranes when imposing force on them. It also suggests good interfacial compatibility of the IMPSF electrospun AEMs prepared by the present method. Although the electrospun AEMs show lower elongation at break as compared with the corresponding cast AEMs, the observed yield behavior, indicates robust of the electrospun AEMs.

Fig. 8 Stress–strain curves of hydrated electrospun and cast AEMs with IEC 1.02 and 1.46 mmol g⁻¹.

Conclusions

A novel method of fabricating electrospun composite AEMs is proposed in the present work. By adopting the same IMPSF material as both electrospun fibers and interfiber voids filler, excellent interfacial compatibility between fiber and filler is achieved and evidenced by optical and SEM images. Better aggregation of the ionic clusters is observed in the electrospun AEMs by TEM, resulting in higher hydroxide conductivity, alkaline stability and better swelling resistance. Compared with the cast AEMs, hydroxide conductivity of the IMPSF electrospun composite AEMs increases remarkably by 1.7 times in water and 100 times at RH 40%. Swelling ratio is reduced by about 35% (IEC = 1.78 mmol g⁻¹, 60 °C). Reserved hydroxide conductivity increases more than one time after immersion in 1 M KOH at different temperatures. Incorporation of IMPSF electrospun fibers reinforces the composite membranes. Tensile strength increases by about 12–22% as compared with the cast membranes.

The results in the present work are helpful to componential design of the electrospun AEMs.

Abbreviation

BPPO-ImOH	Imidazolium-functionalized poly(2,6-dimethyl-1,4-phenyleneoxide)
CMPSF	Chloromethylated polysulfone
FPAEO-MIM	Fluorinated poly(aryl ether oxadiazole)s ionomers based on imidazolium salts
IM-PFEKS	Imidazolium-functionalized poly(fluorenyl ether ketone sulfone)s
IPAES-MIm	N-Methylimidazole ionized poly(arylene ether sulfone)s
MBA	N,N-Methylene bis(acrylamide)
mSQPBI	Quaternized polybenzimidazoles (PBIs) having imidazolium moieties in the main-chain and in the side group
PDMS	Polydimethylsiloxane
PES-G	Poly(aryl ether sulfone) with hexaalkyl guanidinium groups side chain
PES-ImOH	Imidazolium-functionalized poly(arylene ether sulfone)
PES-MeIm/OH	Imidazolium-functionalized poly(arylene ether sulfone)
PPSU	Polyphenylsulfone,
PSf-ImOH	Imidazolium-functionalized polysulfones
QAPSF	Quaternized polysulfone
TMA	Trimethylamine
RT	Room temperature
VBTC	(Vinylbenzyl) trimethylammonium chloride
[VBMI]Cl/styrene	Copolymers of 1-(4-vinylbenzyl)-3-methyl-imidazolium and styrene

Acknowledgements

The authors thank the National Science Fund for Distinguished Young Scholars of China (Grant no. 21125628), the National Science Foundation of China (Grant no. 21476044 and 21406031), the Program for Liaoning Excellent Talents in University (LR2014003) for financial support of this work.

Notes and references

1. Y. J. Wang, J. Qiao, R. Baker and J. Zhang, Chem. Soc. Rev., 2013, 42, 5768–5787.
2. S. Lu, J. Pan, A. Huang, L. Zhuang and J. Lu, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 20611–20614.
3. D. W. Seo, Y. D. Lim, M. A. Hossain, S. H. Lee, H. C. Lee, H. H. Jang, S. Y. Choi and W. G. Kim, Int. J. Hydrogen Energy, 2013, 38, 579–587.
4. X. Yan, G. He, S. Gu, X. Wu, L. Du and Y. Wang, Int. J. Hydrogen Energy, 2012, 37, 5216–5224.
5. S. Gu, R. Cai, T. Luo, K. Jensen, C. Contreras and Y. Yan, ChemSusChem, 2010, 3, 555–558.
6. S. Gu, R. Cai, T. Luo, Z. Chen, M. Sun, Y. Liu, G. He and Y. Yan, Angew. Chem., Int. Ed., 2009, 48, 6499–6502.
7. L. Liu, Q. Li, J. Dai, H. Wang, B. Jin and R. Bai, J. Membr. Sci., 2014, 453, 52–60.
8. S. Gu, J. Wang, R. B. Kaspar, Q. Fang, B. Zhang, E. B. Coughlin and Y. Yan, Sci. Rep., 2015, 5, 11668.
9. J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kuernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy Environ. Sci., 2014, 7, 3135–3191.
10. H. Yanagi and K. Fukuta, ECS Trans., 2008, 16, 257–262.
11. Q. Li, L. Liu, S. Liang, Q. Dong, B. Jin and R. Bai, RSC Adv., 2013, 3, 13477–13485.
12. X. Wu, G. He, S. Gu, Z. Hu and P. Yao, J. Membr. Sci., 2007, 295, 80–87.
13. X. Wu, W. Chen, X. Yan, G. He, J. Wang, Y. Zhang and X. Zhu, J. Mater. Chem. A, 2014, 2, 12222–12231.
14. X. Li, X. Hao, D. Xu, G. Zhang, S. Zhong, H. Na and D. Wang, J. Membr. Sci., 2006, 281, 1–6.
15. S. Cavaliere, S. Subianto, I. Savych, D. J. Jones and J. Rozière, Energy Environ. Sci., 2011, 4, 4761–4785.
16. B. Dong, L. Gwee, D. S. Cruz, K. I. Winey and Y. A. Elabd, Nano Lett., 2010, 10, 3785–3790.
17. J. H. Won, H.-J. Lee, J.-M. Lim, J.-H. Kim, Y. T. Hong and S. Y. Lee, J. Membr. Sci., 2014, 450, 235–241.
18. X. Y. Sun, R. Shankar, H. G. Börner, T. K. Ghosh and R. J. Spontak, Adv. Mater., 2007, 19, 87–91.
19. M. J. Laudenslager, R. H. Scheffler and W. M. Sigmund, Pure Appl. Chem., 2010, 82, 2137–2156.
20. A. M. Park and P. N. Pintauro, Electrochem. Solid-State Lett., 2012, 15, B27–B30.
21. A. M. Park, F. E. Turley, R. J. Wycisk and P. N. Pintauro, Macromolecules, 2014, 47, 227–235.
22. A. Park, F. Turley, R. Wycisk and P. Pintauro, J. Electrochem. Soc., 2015, 162, F560–F566.
23. S. Roddecha, Z. Dong, Y. Wu and M. Anthamatten, J. Membr. Sci., 2012, 389, 478–485.
24. L. Wang, L. Dou and S. Zhang, J. Polym. Res., 2013, 20, 232.
25. X. Yan, G. He, S. Gu, X. Wu, L. Du and H. Zhang, J. Membr. Sci., 2011, 375, 204–211.
26. B. Qiu, B. Lin, L. Qiu and F. Yan, J. Mater. Chem., 2012, 22, 1040.
27. Z. Zhao, F. Gong, S. Zhang and S. Li, J. Power Sources, 2012, 218, 368–374.
28. D. S. Liu, J. N. Ashcraft, M. M. Mannarino, M. N. Silberstein, A. A. Argun, G. C. Rutledge, M. C. Boyce and P. T. Hammond, Adv. Funct. Mater., 2013, 23, 3087–3095.
29. A. H. N. Rao, R. L. Thankamony, H. J. Kim, S. Nam and T.-H. Kim, Polymer, 2013, 54, 111–119.
30. X. Li, Y. Yu, Q. Liu and Y. Meng, Int. J. Hydrogen Energy, 2013, 38, 11067–11073.
31. F. Zhang, H. Zhang and C. Qu, J. Mater. Chem., 2011, 21, 12744–12752.
32. D. Chen and M. A. Hickner, ACS Appl. Mater. Interfaces, 2012, 4, 5775–5781.
33. Q. Hu, Y. Shang, Y. Wang, M. Xu, S. Wang, X. Xie, Y. Liu, H. Zhang, J. Wang and Z. Mao, Int. J. Hydrogen Energy, 2012, 37, 12659–12665.
34. W. Lu, Z.-G. Shao, G. Zhang, Y. Zhao, J. Li and B. Yi, Int. J. Hydrogen Energy, 2013, 38, 9285–9296.
35. J. Ran, L. Wu, J. R. Varcoe, A. L. Ong, S. D. Poynton and T. Xu, J. Membr. Sci., 2012, 415–416, 242–249.
36. W. Li, J. Fang, M. Lv, C. Chen, X. Chi, Y. Yang and Y. Zhang, J. Mater. Chem., 2011, 21, 11340–11346.
37. L. c. Jheng, S. L. c. Hsu, B. y. Lin and Y. l. Hsu, J. Membr. Sci., 2014, 460, 160–170.
38. H. Y. Li and Y. L. Liu, J. Mater. Chem. A, 2014, 2, 3783–3793.
39. H. Y. Li, Y. Y. Lee, J. Y. Lai and Y. L. Liu, J. Membr. Sci., 2014, 466, 238–245.
40. J. Ding, C. Chuy and S. Holdcroft, Adv. Funct. Mater., 2002, 12, 389–394.
41. X. Yan, S. Gu, G. He, X. Wu and J. Benziger, J. Power Sources, 2014, 250, 90–97.
42. J. Han, Q. Liu, X. Li, J. Pan, L. Wei, Y. Wu, H. Peng, Y. Wang, G. Li, C. Chen, L. Xiao, J. Lu and L. Zhuang, ACS Appl. Mater. Interfaces, 2015, 7, 2809–2816.
43. Y. Yao, Z. Lin, Y. Li, M. Alcoutlabi, H. Hamouda and X. Zhang, Adv. Energy Mater., 2011, 1, 1133–1140.
44. B. Lin, H. Dong, Y. Li, Z. Si, F. Gu and F. Yan, Chem. Mater., 2013, 25, 1858–1867.
45. O. M. M. Page, S. D. Poynton, S. Murphy, A. Lien Ong, D. M. Hillman, C. A. Hancock, M. G. Hale, D. C. Apperley and J. R. Varcoe, RSC Adv., 2013, 3, 579–587.
46. S. Chempath, B. R. Einsla, L. R. Pratt, C. S. Macomber, J. M. Boncella, J. A. Rau and B. S. Pivovar, J. Phys. Chem. C, 2008, 112, 3179–3182.
47. J. Pan, C. Chen, Y. Li, L. Wang, L. Tan, G. Li, X. Tang, L. Xiao, J. Lu and L. Zhuang, Energy Environ. Sci., 2014, 7, 353–360.
48. J. Wang, G. He, X. Wu, X. Yan, Y. Zhang, Y. Wang and L. Du, J. Membr. Sci., 2014, 459, 86–95.
49. J. Wu, X. Z. Yuan, J. J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu and W. Merida, J. Power Sources, 2008, 184, 104–119.

---