Spirocyclic quaternary ammonium cations for alkaline anion exchange membrane applications: an experimental and theoretical study

Abstract

Spirocyclic quaternary ammonium (QA) cation based anion exchange membranes (AEMs) were prepared and studied by both experimental measurements and theoretical calculations. Spirocyclic QA cations, including 5-azoniaspiro[4.4]nonane ([ASN]⁺), 5-azoniaspiro[4.5]decane ([ASD]⁺), and 5-azoniaspiro[4.6]undecane ([ASU]⁺) were synthesized as small molecule model compounds and investigated in terms of their chemical stability in alkaline media at elevated temperatures. Compared with nonspirocyclic QA cations, spirocyclic QAs possessed higher alkaline stability, which correlates with their higher energy barrier value associated with a transition state. Furthermore, analogous spirocyclic QA cation based AEMs with promising alkaline stability in alkaline medium were synthesized via photo-cross-linking. The results indicated that alkaline stability studies on small molecule cations may provide a basis for evaluating that in corresponding polymer membranes, and also inspired a feasible way for the preparation of alkaline stable AEMs.

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Introduction

Alkaline anion exchange membrane fuel cells (AEMFCs), which are greatly boosted by the promise of utilizing non-noble metal catalysts, are attracting increasing international interest.^1–3 Compared with comprehensively studied proton exchange membrane fuel cells (PEMFCs), AEMFCs have the following advantages including independence of non-platinum metal (such as Co, Ag, and Ni) catalysts, carbon free supports, excellent oxygen reduction kinetics, and enhanced tolerance to CO₂ impurities in gaseous feeds.^4–6

As one of the key components of AEMFCs, anion exchange membranes (AEMs) play a vital role in influencing the performance of AEMFCs.^2,7–12 In general, an ideal AEM candidate should possess high hydroxide ion conductivity, excellent chemical stability, high mechanical strength, limited swelling, durability and long-lifetime device performances.^13–16 Among the requirements of AEMs, the alkaline stability has been considered as one of the major challenges that limit the practical applications of AEMFCs.^2,11 To achieve the long-lifetime fuel cell performance, both polymer backbone and attached cations should maintain high stability in strongly basic solution at elevated temperatures (60–80 °C).^17–31 Recently, a variety of AEMs based on polyphenylenes,^32–34 polysulfones,^35–37 polystyrenes,³⁸ polyethylenes,^39,40 polynorbornenes,⁴¹ poly(arylene ether ketone)s,⁴² and poly(phenylene oxide)s^43–45 have been studied. These AEMs show relatively high hydroxide ion conductivity and potential applications in AEMFCs.

Recently, it has been reported that cationic group chemistry has more essential influence on the alkaline stability of AEMs.^9,26,27 Once a proper cationic-group is identified, high alkaline stable AEMs can be rationally designed. Among the cations investigated, quaternary alkylammonium (QA) cation based AEMs have been extensively studied due to their relatively high hydroxide ion conductivity, ease of functionalization, and adequate alkaline stability over a short-time scales.^9,46 However, a major disadvantage of QA cations (in OH⁻ form) is their poor stability in high pH aqueous solutions over time because of the base-induced Hofmann elimination (E₂),⁴⁷ nucleophilic substitution (S_N2),^48–50 and (or) ylide formation⁵¹ side reactions. Therefore, alternative N-based cations, such as pyrrolidinium⁵² and imidazolium^53,54 based AEMs have been synthesized and investigated by experimental studies and theoretical calculations. Some of these alternative AEMs show improved alkaline stability.

More recently, Marino and Kreuer reported that the aliphatic N-spirocyclic QAs are quite stable in high pH aqueous solutions (10 M NaOH solution).⁵⁵ Afterwards, Jannasch and co-workers prepared AEMs functionalized with bis-N-spirocyclic QAs. Spirocyclic QA groups were incorporated into aromatic poly(arylene ether sulfone)s (adjoined through fused phenyl rings along polymer backbones) by using a synthetic route comprising polycondensation, bromination, and cycloquaternization.⁵⁶ The resultant AEMs showed reasonable alkaline stability, however, degraded at elevated temperatures, most probably due to the ring-opening substitution at the benzylic positions.

Inspired by these results, here, we report the synthesis and characterization of spirocyclic QA cations, without benzylic positions, for alkaline stable AEMs. A series of small molecule spirocyclic QA cations, including 5-azoniaspiro[4.4]nonane bromide ([ASN][Br]), 5-azoniaspiro[4.5]decane bromide([ASD][Br]), and 5-azoniaspiro[4.6]undecane bromide ([ASU][Br]) were synthesized. The alkaline stability of these small molecule model compounds was systematically investigated by both experimental (via ¹H nuclear magnetic resonance analysis (NMR)) and theoretical approaches (density functional theory (DFT) calculations). Spirocyclic QA based AEMs with enhanced alkaline stability were further synthesized by photo-cross-linking and investigated with respect to their ionic conductivity, thermal properties, and alkaline stability.

Experimental

Materials

1-Methylpyrrolidine, pyrolidine, piperidine, hexamethyleneimine, benzyl chloride, 1,4-diazabicyclo[2.2.2]octane (DABCO), iodomethane, 1-bromobutane, acetonitrile, 1,4-dibromobutane, trimethylamine, isopropyl bromide, K₂CO₃, 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AAPH), styrene, acrylonitrile, allyl bromide, benzoin ethyl ether, dimethyl sulfoxide (DMSO), ethyl ether, ethyl acetate, potassium hydroxide, and hydrochloric acid were used as purchased without further purification. All of the vinyl monomers were removed inhibitor by passing the liquid through a column filled with basic alumina and then stored at 5 °C before use. Deionized water was used throughout the experiments.

Synthesis of quaternary ammonium salts

Synthesis of 1-butyl-1-methylpyrrolidinium bromide ([BMPy][Br]). A mixture of 1-methylpyrrolidine and an equivalent molar amount of 1-bromobutane was stirred at room temperature for 48 h. The product was washed three times with ethyl ether and dried under vacuum at room temperature. White solid (yield: 83%). ¹H NMR (400 MHz, D₂O): 0.874–0.976 (t, 3H), 1.299–1.427 (m, 2H), 1.693–1.809 (m, 2H), 2.118–2.248 (s, 4H), 2.973–3.039 (s, 3H), 3.255–3.338 (t, 2H), 3.413–3.531 (m, 4H).

Synthesis of 1-methyl-4-aza-1-azoniabicyclo[2.2.2]octane iodide ([MAABCO][I]). [MAABCO][I] was synthesized by stirring a mixture containing DABCO and an equivalent molar amount of iodomethane at room temperature under nitrogen atmosphere. The product was washed three times with ethyl ether and dried under vacuum at room temperature. White solid (yield: 84%). ¹H NMR (400 MHz, D₂O): 2.941–3.074 (s, 3H), 3.253–3.440 (t, 6H), 3.085–3.244 (t, 6H).

Synthesis of benzyltrimethylammonium chloride ([BTMA][Cl]). [BTMA][Cl] was synthesized by stirring a mixture containing benzyl chloride and an equivalent molar amount of trimethylamine at room temperature under nitrogen atmosphere. After the removal of solvent, the product was washed with ethyl ether three times and dried under vacuum at room temperature. White solid (yield: 88%). ¹H NMR (400 MHz, D₂O): 3.057–3.125 (s, 9H), 4.457–4.511 (s, 2H), 7.519–7.615 (m, 5H).

Synthesis of 5-azoniaspiro[4.4]nonane bromide ([ASN][Br]). A mixture of 1,4-dibromobutane (4.32 g, 0.02 mol) and K₂CO₃ (5.5 g, 0.04 mol) in dimethylacetamide (15 mL) was stirred at room temperature under a nitrogen atmosphere for 48 h, and then pyrrolidine (1.4 g, 0.02 mol) was added dropwise over 30 min. The product was washed three times with ethyl ether and dried under vacuum at room temperature. Colorless viscous oil (yield: 64%). ¹H NMR (400 MHz, D₂O): 2.119–2.267 (s, 8H), 3.450–3.602 (t, 8H).

Synthesis of 5-azoniaspiro[4.5]decane bromide ([ASD][Br]). A mixture of 1,5-dibromopentane (4.6 g, 0.02 mol) and K₂CO₃ (5.5 g, 0.04 mol) in acetonitrile (15 mL) was stirred at room temperature under a nitrogen atmosphere for 48 h. While pyrrolidine (1.4 g, 0.02 mol) was added dropwise over 30 min. The product was washed with ethyl ether three times and dried under vacuum at room temperature. Colorless viscous oil (yield: 64%). ¹H NMR (400 MHz, D₂O): 1.630–1.713 (m, 2H), 1.819–1.910 (m, 4H), 2.100–2.207 (m, 4H), 3.301–3.381 (t, 4H), 3.464–3.584 (t, 4H).

Synthesis of 5-azoniaspiro[4.6]undecane bromide ([ASU][Br]). A mixture of 1,6-dibromohexane (4.88 g, 0.02 mol) and K₂CO₃ (5.5 g, 0.04 mol) in acetonitrile (15 mL) was stirred at room temperature under a nitrogen atmosphere for 48 h. While pyrrolidine (1.4 g, 0.02 mol) was added dropwise over 30 min. The product was washed with ethyl ether three times and dried under vacuum at room temperature. Colorless viscous oil (yield: 82%). ¹H NMR (400 MHz, D₂O): 1.657–1.734 (m, 4H), 1.815–1.940 (m, 4H), 2.076–2.220 (m, 4H), 3.382–3.453 (t, 4H), 3.457–3.535 (t, 4H).

Synthesis of N,N-diallylpyrrolidinium bromide ([DAPy][Br]). [DAPy][Br] was synthesized by stirring a mixture containing pyrrolidine (2.84 g, 0.04 mol) and allyl bromide (9.67 g, 0.08 mol) at room temperature under a nitrogen atmosphere for 48 h. The product was washed with ethyl ether three times and dried under vacuum at room temperature. White solid (yield: 92%). ¹H NMR (400 MHz, D₂O): 2.094–2.258 (s, 4H), 3.397–3.596 (t, 4H), 3.799–3.990 (d, 4H), 5.616–5.723 (m, 4H), 5.935–6.113 (m, 2H).

Synthesis of N,N-diallylpiperidinium bromide ([DAPi][Br]). [DAPi][Br] was synthesized by stirring a mixture containing piperidine (3.4 g, 0.04 mol) and allyl bromide (9.67 g, 0.08 mol) at room temperature under a nitrogen atmosphere for 48 h. The product was washed with ethyl ether three times and dried under vacuum at room temperature. White solid (yield: 83%). ¹H NMR (400 MHz, D₂O): 1.572–1.709 (m, 2H), 1.813–1.950 (m, 4H), 3.252–3.375 (t, 4H), 5.621–5.744 (m, 4H), 5.933–6.071 (m, 2H).

Synthesis of N,N-diallylhexamethyleneiminium bromide ([DAHM][Br]). [DAHM][Br] was synthesized by stirring a mixture containing hexamethyleneimine, (3.96 g, 0.04 mol) and allyl bromide (9.67 g, 0.08 mol) at room temperature under a nitrogen atmosphere for 48 h. The product was washed with ethyl ether three times and dried under vacuum at room temperature. White solid (yield: 66%). ¹H NMR (400 MHz, D₂O): 1.624–1.700 (m, 4H), 1.845–1.931 (s, 4H), 3.369–3.441 (t, 4H), 3.381–3.892 (d, 4H), 5.611–5.716 (t, 4H), 5.953–6.082 (m, 2H).

Synthesis of spirocyclic QA based polymer

The spirocyclic QA based homopolymers were synthesized by free radical polymerization of cationic monomer (with initiator) in aqueous solution. For example, a solution of [DAPy][Br] (0.928 g, 4.0 mmol) was dissolved in water (6 mL) and purged with nitrogen for 10 min before the initiator AAPH (18.6 mg, 0.07 mmol) was added. The mixture was stirred at 60 °C for 24 h under a nitrogen atmosphere. The obtained product was washed with cold diethyl ether three times and then dried under reduced pressure. [DAPi]⁺ and [DAHM]⁺ cation based homopolymers were synthesized under the same way.

Preparation of alkaline anion exchange membranes

AEMs were prepared via photo-cross-linking of a mixture containing styrene/acrylonitrile (1 : 3 weight ratio, 75 wt%), [DAPy][Br] (or [DAPi][Br], or [DAHM][Br]) (0.116 g, 25 wt%), divinylbenzene (cross-linker, 3 wt% based on the weight of monomer), and benzoin ethyl ether (photoinitiator, 1 wt% based on the weight of monomer) was stirred and ultrasonicated to obtain a homogeneous solution. The obtained mixture was cast into a home-made glass mold and photo-cross-linked by irradiation with UV light of 250 nm wavelength at room temperature for 40 min. The prepared polymeric membranes were abbreviated as [PAPy][Br], [PAPi][Br] and [PAHM][Br], for [DAPy][Br], [DAPi][Br] and [DAHM][Br] based AEMs, respectively.

The resultant polymeric membranes were immersed in 1 M NaOH solution at 60 °C under a nitrogen atmosphere to convert the membranes from Br⁻ to OH⁻ form. Such a procedure was repeated at least three times to make the complete conversion of anions. The polymeric membranes were thoroughly washed with deionized water until the KOH residue was removed from the membranes. The obtained polymeric membranes were abbreviated as [PAPy][OH], [PAPi][OH], and [PAHM][OH], respectively, and then stored in deionized water under nitrogen atmosphere.

Characterization

The alkaline stability of organic cations was tested using a Varian spectrometer at 400 MHz spectrometer in CD₃OD/D₂O/NaOH (2, 4, and 8 M NaOH aqueous solutions, respectively) solutions under a nitrogen atmosphere. Fourier transform infrared (FTIR) spectra of polymeric membranes were recorded on a Varian CP-3800 spectrometer in the range of 4000–400 cm⁻¹. Thermogravimetric analysis (TGA) was performed under nitrogen flow by Universal Analysis 2000. All the samples were heated from 30 to 800 °C at a heating rate of 20 °C min⁻¹. Scanning electron microscopy (SEM) images were taken with a Philips Model XL 30 FEG microscope with an accelerating voltage of 10 kV.

Hydroxide ion conductivity

The resistance value of the polymeric membranes was measured by four-point probe alternating current (ac) impedance spectroscopy, using an electrode system connected with an electrochemical workstation (Zahner IM6EX) over the frequency range from 1 Hz to 1 MHz. Hydroxide conductivity (σ, S cm⁻¹) of the AEMs was calculated as:
where R is the resistance value (Ω) of the membrane, l is the distance (cm) between two electrodes, and W and T are the width (cm) and thickness (cm) of the membrane, respectively. All the membrane were measured under fully hydrated conditions, and be equilibrated for at least 30 min at each given temperature.

Water uptake and swelling ratio

The membrane samples (in OH⁻ form) were fully hydrated in water and weighed immediately (W_wet) and dried at 80 °C under a vacuum to obtain a constant weight (W_dry). The water uptake value of the samples was calculated as follows:
where W_wet is the weight of water-swollen membranes, and W_dry is the weight of dried membranes.

The swelling ratio of the membrane samples were characterized by a linear expansion ratio, evaluated by the difference between wet and dry dimensions of a membrane sample (4 cm in length and 1 cm in width). The swelling degree was calculated as:

where L_wet and L_dry are the lengths of wet and dry samples, respectively.

Ion exchange capacity (IEC) measurements

Ion exchange capacity (IEC) of membranes was measured by a conventional back-titration method. The membrane samples were immersed in 0.01 M HCl standard solution for 24 h. The resulting solution was back-titrated with a NaOH standard solution with phenolphthalein as an indicator. The IEC value was calculated as:
where W_dry is the mass of the dried membrane, C_NaOH and C_HCl are the concentration of NaOH and HCl solution, respectively; V_NaOH and V_HCl are the volume of NaOH solution and HCl solution consumed in the titration, respectively.

Mechanical property measurements

The mechanical properties of all the AEMs was measured in terms of tensile strength and elongation at break. All the evaluations were carried out at room temperature, in air, using a universal tensile testing machine (KJ-1065B, Kejian-tech, China) with a 50 N loading cell. All the polymeric membranes were dried before the evaluation.

Computational details and analysis

Theoretical calculations on the single molecules of QAs were carried out by the DMol³ density functional code as implemented in Materials Studio (Version 7.0).^57,58 Generalized gradient approximation functional by Beck exchange and Lee, Yang, and Parr correlation (GGA-BLYP) as well as the double numerical plus polarization (DNP) basis set were applied in all calculations.⁵⁹

The self-consistent field (SCF) converged with a threshold value of 10⁻⁶ ha was applied to fully optimize the single molecules. During the geometry optimization, the convergence criteria were set as 5 × 10⁻⁶ ha for energy, 0.001 ha Å⁻¹ for gradient, and 0.005 Å for displacement. Considering of the solvent effect on the cations, the calculations used a dielectric constant ε = 78.54 for water.

All the optimized single molecules were evaluated by a frequency analysis to ensure the frequencies are all normal. The complete linear synchronous transit/quadratic synchronous transit (LST/QST) method was applied to search transition state (TS) structures.⁶⁰ The obtained TS structures exhibit only one imaginary frequency. To ensure the TS geometries are the direct connection between the corresponding reactants and products, the transition state confirmation embedded with the nudged-elastic band (NEB) method was applied in this study.

Results and discussion

Alkaline stability of QA cations

In order to accurately evaluate the alkaline stability of the synthesized AEMs, the chemical structure changes of polymeric membranes were characterized by NMR spectroscopy. However, it has already been demonstrated that the NMR signals will be highly weakened by the polymer backbones.⁶¹ Therefore, the analogous small molecule cations, including 5-azoniaspiro[4.4]nonane ([ASN]⁺), 5-azoniaspiro[4.5]decane ([ASD]⁺), 5-azoniaspiro[4.6]undecane ([ASU]⁺), 1-butyl-1-methylpyrrolidinium ([BMPy]⁺), 1-methyl-4-aza-1-azoniabicyclo-[2.2.2]octane ([MAABCO]⁺), and benzyltrimethylammonium ([BTMA]⁺) were first synthesized (see Table 1). The chemical structure and purity of these organic cations were confirmed by quantitative ¹H NMR spectra.

Table 1 Molecular structures and abbreviations of QAs studied in this study

Molecular Structure	Name	Abbreviations
	5-Azoniaspiro[4.4]nonane	[ASN]^+
	5-Azoniaspiro[4.5]decane	[ASD]^+
	5-Azoniaspiro[4.6]undecane	[ASU]^+
	1-Butyl-1-methylpyrrolidinium	[BMPy]^+
	1-Methyl-4-aza-1-azoniabicyclo[2.2.2]octane	[MAABCO]^+
	Benzyltrimethylammonium	[BTMA]^+

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The influences of alkaline concentration, and reaction time on the chemical stability of the QA cations were evaluated. Fig. 1 shows the ¹H NMR spectra of synthesized organic cations ([ASN]⁺, [ASD]⁺, [ASU]⁺, [BMPy]⁺, [BTMA]⁺, and [MAABCO]⁺) before and after the exposure to 2 M NaOH mixed solution (V_CD3OD/V_D2O = 3 : 1) at 80 °C for 168 h. The reason that methanol was added as a co-solvent is because the methanol could accelerate the organic cation degradation and dissolve the decomposed cations much better than in pure aqueous solution.⁶² It can be seen from Fig. 1 that all the organic cations exhibited new peaks after the exposure to 2 M mixed NaOH solution for 168 h, indicating the occurrence of cation degradation. The degradation degree of organic cations can be calculated via the relative integrated intensities of corresponding NMR peaks.⁶³

Fig. 1 ¹H NMR spectra of (A) [ASN]⁺, (B) [ASD]⁺, (C) [ASU]⁺, (D) [BMPy]⁺, (E) [BTMA]⁺, and (F) [MAABCO]⁺ before and after exposure to 2 M NaOH/CD₃OD/D₂O solution (V_CD3OD : V_D2O = 3 : 1) at 80 °C for 168 h.

Among the organic cations studied, new peaks at around 1.5, 1.6, and 2.4 ppm probably due to the ring-opening reaction of [ASN]⁺ were observed (Fig. 1A). The emerging aliphatic signals at 1.5 and 1.6 ppm are attributed to the proton of 7′ and 8′, suggesting the substitution at 2 (5 or 6 or 9) to be the primary degradation mechanism.⁵⁵ The degradation degree of [ASN]⁺ in 2 M NaOH solution for 168 h was calculated to be 4.5%, while 2.8%, 6.4%, are observed for [ASD]⁺ and [ASU]⁺, respectively (see Fig. 1A–C).⁵⁵ However, the degradation degree observed for non-spirocyclic QA cations, [BMPy]⁺ (9.1%), [BTMA]⁺ (15.2%) and [MAABCO]⁺ (14.6%), was significantly higher than that for the spirocyclic QA cations, indicating relatively poorer alkaline stability (see Fig. 1D–F).

To further evaluate the chemical stability of QA cations synthesized in this work, the alkaline stability measurements were conducted by ¹H NMR spectra in 4 M (see Fig. S1†) and 8 M (see Fig. S2†) NaOH mixed solutions (V_CD3OD/V_D2O = 3 : 1) at 80 °C, respectively. It can be clearly seen that all the spirocyclic QA cations exhibit relatively higher alkaline stability if compared with non-spirocyclic QA cations (Fig. S1†). Among the organic cations studied, [ASD]⁺ shows the highest alkaline stability (about 3.9% degraded) (see Fig. S1B†). The alkaline stability measurements in 8 M NaOH solution, again, further confirm the highest alkaline stability of [ASD]⁺ (see Fig. S2B†).

Therefore, the alkaline stability order of the QA cations studied in this work can be concluded to be [ASD]⁺ > [ASN]⁺ > [ASU]⁺ > [BMPy]⁺ > [MAABCO]⁺ > [BTMA]⁺ (see Fig. 2). Obviously, spirocyclic QAs are more stable than non-spirocyclic QAs in alkaline medium. These results motivate us to further synthesize spirocyclic QA cation based AEMs for AEMFCs.

Fig. 2 Degradation of spirocyclic QAs under different test conditions: (A) in 2 M NaOH mixed solution (V_CD3OD/V_D2O = 3 : 1) at 80 °C for 168 h; (B) in 4 M NaOH mixed solution (V_CD3OD/V_D2O = 3 : 1) at 80 °C for 96 h; and (C) in 8 M NaOH mixed solution (V_CD3OD/V_D2O = 3 : 1) at 80 °C for 72 h.

Computational analysis

To further evaluate the alkaline stability of the QAs synthesized in this work, a theoretical study was applied. All the computational calculations are based on an ideal condition, that is, 0 K and 1 atm. Fig. 3 shows the Mulliken charge population analysis on the spirocyclic QAs. As it can be seen that the ring-opening reaction could occur on α-C or β-C atoms. However, due to the strong electron-withdrawing effect from nitrogen atom, the neighboring α-C atoms show positive charge and the faraway β-C atoms show negative charge, thus the OH⁻ tends to attack the α-C instead of β-C atoms. To make parallel comparison and confirm the reaction pathway from a computational view, therefore, we supposed that the ring-opening substitution reaction mainly happens on α-C of the five-membered ring (see Scheme 1).

Fig. 3 The Mulliken charge population analysis of QAs: (a) [ASN]⁺, (b) [ASD]⁺, (c) [ASU]⁺ and (d) [MAABCO]⁺. The Mulliken atomic charge on QA cations shows that α-C is electropositive, while β-C is electronegative. The α-C atoms (C1, C2) on the five-membered ring of spirocyclic QAs with more positive charge are vulnerable to be attacked by OH⁻ than β-C atoms. Therefore, the ring-opening reaction of spirocyclic QAs will occur on α-Cs on the five-membered ring. However, compared with other α-Cs of spirocyclic QAs (>0.025|e|), α-Cs of [MAABCO]⁺ (0.009|e| and 0.011|e|) show relatively weak positive charge, which are relatively difficult to be attacked by OH⁻, leading to a high E_barrier value.

Scheme 1 Degradation mechanism of spirocyclic QAs in alkaline solution.

Fig. 4A schematically illustrates an energy barrier associated with a transition state during hydroxide nucleophilic attack on a spirocyclic QA cation. The relative energies of all pairs of spirocyclic QAs and hydroxide anion in initial state were set as zero. Here, E_barrier represents the energy that OH⁻ needed to overcome the energy difference between transition state (TS) and initial state. The higher the E_barrier value is, the harder for QAs to be attacked by OH⁻. Fig. 4B shows the E_barrier values of all the QAs studied in this work. The [ASD]⁺ shows the highest E_barrier value of 39.5 kcal mol⁻¹, indicating the most stable cation in basic solution. The order of the E_barrier based on the computational analysis for spirocyclic QA cations is [ASD]⁺ > [ASU]⁺ > [ASN]⁺, which is inconsistent with the experimental results. It should be noted that [ASN]⁺ shows a relative lower E_barrier value (36.6 kcal mol⁻¹), indicating a poorer alkaline stability. Due to the Jahn–Teller distortion, the [ASN]⁺ is not really symmetric, which leads to different charge values on the α-Cs. There is a higher positive charge on α-C of [ASN]⁺ (0.28 and 0.29|e|) that makes it easier to be attacked than [ASD]⁺ and [ASU]⁺. In addition, it is found that the [ASD]⁺, [ASN]⁺, [ASU]⁺, and [BMPy]⁺ exhibit similar steric hindrance for the α-Cs on the five-membered ring. Therefore, the charge population could be the decisive factor for these species. Meanwhile, [MAABCO]⁺ shows a relatively high E_barrier value (38.8 kcal mol⁻¹), which is even higher than that of [ASN]⁺ (36.6 kcal mol⁻¹), [ASU]⁺ (37.4 kcal mol⁻¹) and [BMPy]⁺ (37.2 kcal mol⁻¹). This result is in contrast with the alkaline stability of the QA cations tested in alkaline solution. From Fig. 3d and compared with other α-Cs of spirocyclic QA cations (>0.025|e|), it can be seen that there are six weakly positive-charged α-Cs on [MAABCO]⁺ (0.009|e| and 0.011|e|), which makes them relatively unattractive for OH⁻ and not so easy to be attacked by OH⁻ on a single site, leading to a high E_barrier value when react with one OH⁻ (see Fig. 3d). The lowest E_barrier value of [BTMA]⁺ (33.2 kcal mol⁻¹) suggest the worst alkaline stability. This result is consistent with the cation degradation degree observed in experimental study. However, in practical situation, the [MAABCO]⁺ should be easier to be attacked because of the rich α-C sites with less steric hindrance provide more chance to nucleophilic attack. This is why [MAABCO]⁺ shows a high E_barrier value in theoretical computation but is still easy to degrade in alkaline solution.

Fig. 4 (A) Energy diagram of spirocyclic QAs along the reaction coordinates; (B) energy barriers of [ASD]⁺, [ASN]⁺, [ASU]⁺, [BMPy]⁺, [MAABCO]⁺, [BTMA]⁺.

Based on the results of alkaline stability of spirocyclic QA cations, [ASN]⁺, [ASD]⁺, and [ASU]⁺ based homopolymers were synthesized via free radical polymerization (Scheme S1†). The chemical structure of obtained homopolymers was characterizated by ¹H NMR spectra, as shown in Fig. 5A–C. The signal at 3.6 ppm assigned to the α-protons of the spiro nitrogens and in Fig. 5A, signals appeared at 2.4 and 1.5 ppm depicted the cycloaliphatic methylene protons (2, 3, 9, 10 and 1, 4). These results indicated the successful synthesis of the spirocyclic QA cation based polymers.

Fig. 5 ¹H NMR spectra of (A) [ASN]⁺, (B) [ASD]⁺, and (C) [ASU]⁺ cation based polymers.

From the view point of practical application, however, AEMs with high hydroxide conductivity, high mechanical properties, and low swelling degree are requisite. On the basis of the alkaline stability of the organic cations studied above, analogous spirocyclic QA based AEMs were prepared by photo-cross-linking of [DAPy][Br] ([DAPi][Br] or [DAHM][Br]) with styrene and acrylonitrile, using benzoin ethyl ether as photoinitiator and divinylbenzene (DVB) as a cross-linking agent. The prepared polymeric membranes were then immersed in alkaline solution to convert the membranes from Br⁻ to OH⁻ form. The spirocyclic QA cation based AEMs are free-standing, transparent, and could be very easily cut into appropriate sizes (see Schemes 2 and S2†).

Scheme 2 Synthetic route of spirocyclic QA based anion exchange membrane.

Fig. S3A† depicts the FTIR spectra of [PAPy][OH], show an absorption band at about 2250 cm⁻¹ due to the absorption of cyano groups (C≡N). While the absorption peak at 2928 cm⁻¹ belongs to –CH₃ and –CH₂–. The characteristic absorption peak of spirocyclic cations appeared at 1183 cm⁻¹ and the absorption peaks at 1659 cm⁻¹ belong to benzene rings. These results clearly confirm the successful synthesis of [PAPi][OH], [PAPi][OH], and [PAHM][OH] membranes. The morphology of the [PAPi][OH] membrane was investigated by scanning electron microscopy (SEM) (see Fig. S4A†), which showed that the prepared membranes are uniform, compact, without any visible pores.

Fig. 6A shows the typical TGA curves of the resulted spirocyclic QA cation based polymeric membranes, which were recorded under a nitrogen flow from 30 to 800 °C (heating rate: 20 °C min⁻¹). The weight loss for [PAHM][OH] at 200 °C was 3.3%, and less than 2% weight loss was observed below 200 °C for [PAPy][OH] and [PAPi][OH], indicating a high thermal stability, far beyond the requirements for application in AEMFCs. The obvious degradative weight loss starts at about 350 °C was attributed to the degradation of the polymer backbones. Fig. 6B shows the long-term thermal stability of the synthesized AEMs, which was recorded under a nitrogen flow at 150 °C. It clearly depicted that all the polymeric membranes showed less than 4% weight loss for 12 h at 150 °C.

Fig. 6 (A) TGA curves of the produced membranes in nitrogen at a heating rate of 20 °C min⁻¹. (B) Isothermal TGA thermograms of spirocyclic QA based membranes at 150 °C under the nitrogen flow.

The [PAPi][OH] was especially thermal stable with the weight loss less than 2%. These results further indicate that the spirocyclic QA cation based membranes possess good thermal stability and are qualified for the AEMs application.

Table 2 shows the values of IEC, water uptake, and swelling degree for the synthesized spirocyclic QA cation based membranes. All the resultant polymeric membranes exhibited relatively higher conductivity (>1.0 × 10⁻² S cm⁻¹) at 40 °C. No obvious changes for the water uptake and swelling degree were observed at 40 °C and 80 °C, respectively, probably due to the relatively high crosslinking degree of the membranes. In addition, the relatively low IEC value also limited the swelling degree and water uptake, which also limited the enhancement of conductivities. Compared with AEMs reported, the prepared membranes showed comparable conductivity while with relatively lower swelling degree and water uptake, which are desirable for the AEMFCs applications when they are hydrated.^49,52,56

Table 2 Ion Exchange Capacity (IEC), water uptake, swelling degree, and conductivity of spirocyclic QA based membranes

Membrane	IEC value^a (mequiv. g^−1)		Water uptake (%)		Swelling degree (%)		Conductivity (×10^−2 S cm^−1)
	theor^a	exptl^b	40 °C	80 °C	40 °C	80 °C	40 °C	80 °C
a Calculated from monomer ratio.b Data were obtained based on the average of three trials.
[PAPy][OH]	1.20	0.97	21.9	32.5	21.3	22.4	1.03	1.89
[PAPi][OH]	1.20	1.15	22.8	33.2	17.5	25.3	1.24	1.92
[PAHM][OH]	1.20	1.09	25.3	35.6	33.7	37.4	1.12	2.03
[PEMPy][OH]^52	1.49	1.41	87.07 (30 °C)	—	24.23 (30 °C)	—	1.27 (30 °C)	1.77 (60 °C)
[PVMIIm][OH]^49	1.02	0.93	63.51 (30 °C)	—	20.73 (30 °C)	—	1.03 (30 °C)	1.53 (60 °C)

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The alkaline stability of the spirocyclic QA cation based AEMs in alkaline medium was evaluated by monitoring the conductivity in N₂-saturated NaOH solution at various temperature over time. Fig. 7 shows the ionic conductivity of the membranes as a function of the temperature. The conductivities of the membranes increase with increasing temperature because both the mobility and ion transport of anions increase with the temperature increases. Fig. 7A–C show that all the spirocyclic QA cation based polymeric membranes, [PAPy][OH], [PAPi][OH] and [PAHM][OH], slightly decreased the conductivity in 1 M NaOH solution at 80 °C over time. Among these AEMs, [PAPi][OH] showed a loss of conductivity by 1.6% for 168 h at 80 °C, while [PAPy][OH] and [PAHM][OH] showed a loss of conductivity by 4.8% and 7.9%, respectively, under the same experimental condition. The alkaline stability of the [PAPi][OH] was further studied in 10 M NaOH solution at 30 °C. It can be seen that about 2.1% of hydroxide conductivity was decreased after 168 h test (see Fig. 7D). The results also indicated that the spirocyclic QA cation based polymeric membranes exhibited comparable chemical stability if compared with the substituted pyrrolidinium cation based membranes.⁵²

Fig. 7 Conductivity Arrhenius plots of AEMs (A) [PAPy][OH]; (B) [PAPi][OH]; (C) [PAHM][OH] after immersion in N₂-saturated 1 M NaOH solution at 80 °C for various times; and (D) [PAPi][OH] after immersion in N₂-saturated 10 M NaOH solution at 30 °C for various times.

The conductivity of AEMs is highly affected by the IEC values. The changes of IEC values and mechanical properties of AEMs before and after the alkaline stability test were also evaluated (see Table 3). It should be noted that the IEC values of three AEMs in 1 M NaOH solution at 80 °C were almost unchanged after 168 h test, which is consistent with the results of conductivity test. However, the mechanical properties (tensile strength and elongation at break) of all the AEMs slightly decreased after immersion in NaOH solution for 168 h. In this study, the prepared polymeric membranes are partially cross-linked. The membranes swelled in aqueous solution during the alkaline stability test. Therefore, it is supposed that the decrease of tensile strength and elongation at break is not because of the degradation of polymeric membranes in alkaline solution, but maybe due to the disentanglement of uncross-linked polymer chains in the membranes. Furthermore, no new absorption peaks and obvious shift were observed by FTIR spectra of [PAPi][OH] membrane before and after the alkaline stability test (see Fig. S3B†), and no visible pores were observed after the alkaline stability test by SEM as well (see Fig. S4B†). All these results confirm the alkaline stability of the spirocyclic QA cations and indicate that spirocyclic QA cation based membranes are highly alkaline stable.

Table 3 The change of Ion-Exchange Capacity (IEC) values and mechanical properties of AEMs after immersion in 1 M NaOH solution at 80 °C^a

Membrane	Time (h)	IEC value (mequiv. g^−1)	Tensile strength (MPa)	Elongation at break (%)
a Based on the average of three trials.
[PAPy][OH]	0	0.97 ± 0.044	10.12 ± 1.02	9.12 ± 1.02
	24	0.96 ± 0.037	7.33 ± 0.83	6.21 ± 0.78
	168	0.93 ± 0.051	5.52 ± 9.12	4.52 ± 0.62
[PAPi][OH]	0	1.15 ± 0.038	9.25 ± 0.93	8.33 ± 0.83
	24	1.15 ± 0.046	7.15 ± 0.67	7.12 ± 0.72
	168	1.12 ± 0.033	4.92 ± 0.93	4.16 ± 0.59
[PAHM][OH]	0	1.09 ± 0.035	8.43 ± 0.88	7.75 ± 0.97
	24	1.11 ± 0.031	6.21 ± 0.77	6.11 ± 0.78
	168	1.03 ± 0.042	5.13 ± 1.12	4.62 ± 0.86

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Conclusions

In conclusion, a series of spirocyclic QA cations and their corresponding polymeric membranes were designed and synthesized as the AEMs for fuel cells. The alkaline stability of the spirocyclic QA cations was studied by both quantitative ¹H NMR spectroscopy and computational analysis. Compared with nonspirocyclic QA cations, spirocyclic QA cations exhibited higher alkaline stability probably due to the high transition-state energy of the degradation reactions.

The spirocyclic [ASD]⁺ cation and corresponding polymeric membranes exhibited the highest chemical stability in alkaline solution at elevated temperatures. The results suggested that alkaline stability studies on small molecule model cations may provide a basis for evaluating the properties of corresponding polymer membranes, and also inspired a feasible approach for the preparation of spirocyclic QA based AEMs with improved long-term stability and performance.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant No. 21425417 and 21274101, the National Basic 2012CB825800) and by project funding from the Priority Academic Program Development of Jiangsu Higher Education Institutions.

References

1. G. Merle, M. Wessling and K. Nijmeijer, J. Membr. Sci., 2011, 377, 1–35.
2. J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer and K. Scott, Energy Environ. Sci., 2014, 7, 3135–3191.
3. C. Yang, S. Wang, W. Ma, S. Zhao, Z. Xu and G. Sun, J. Mater. Chem. A, 2016, 4(10), 3886–3892.
4. X. Ge, Y. He, M. D. Guiver, L. Wu, J. Ran, Z. Yang and T. Xu, Adv. Mater., 2016, 28, 3467–3472.
5. C. Yang, S. Wang, W. Ma, L. Jiang and G. Sun, J. Membr. Sci., 2015, 487, 12–18.
6. J. Yan and M. A. Hickner, Macromolecules, 2010, 43, 2349–2356.
7. H. W. Zhang, D. Z. Chen, Y. Xianze and S. B. Yin, Fuel Cells, 2015, 15, 761–780.
8. N. Li, Y. Leng, M. A. Hickner and C.-Y. Wang, J. Am. Chem. Soc., 2013, 135, 10124–10133.
9. Y. Ye and Y. A. Elabd, Macromolecules, 2011, 44, 8494–8503.
10. O. D. Thomas, K. J. Soo, T. J. Peckham, M. P. Kulkarni and S. Holdcroft, J. Am. Chem. Soc., 2012, 134, 10753–10756.
11. N. Li, M. D. Guiver and W. H. Binder, ChemSusChem, 2013, 6, 1376–1383.
12. J. Ran, L. Wu, Y. Ru, M. Hu, L. Din and T. Xu, Polym. Chem., 2015, 6, 5809–5826.
13. J. Cheng, G. He and F. Zhang, Int. J. Hydrogen Energy, 2015, 40, 7348–7360.
14. J. Pan, L. Zhu, J. Han and M. A. Hickner, Chem. Mater., 2015, 27, 6689–6698.
15. T.-H. Tsai, S. P. Ertem, A. M. Maes, S. Seifert, A. M. Herring and E. B. Coughlin, Macromolecules, 2015, 48, 655–662.
16. M. Tanaka, M. Koike, K. Miyatake and M. Watanabe, Polym. Chem., 2011, 2, 99–106.
17. A. D. Mohanty, S. E. Tignor, J. A. Krause, Y.-K. Choe and C. Bae, Macromolecules, 2016, 49(9), 3361–3372.
18. K. M. Meek, J. R. Nykaza and Y. A. Elabd, Macromolecules, 2016, 49, 3382–3394.
19. L. Zhu, J. Pan, C. M. Christensen, B. Lin and M. A. Hickner, Macromolecules, 2016, 49, 3300–3309.
20. L. Wu, Q. Pan, J. R. Varcoe, D. Zhou, J. Ran, Z. Yang and T. Xu, J. Membr. Sci., 2015, 490, 1–8.
21. H.-S. Dang and P. Jannasch, Macromolecules, 2015, 48, 5742–5751.
22. J. Wang, S. Gu, R. B. Kaspar, B. Zhang and Y. Yan, ChemSusChem, 2013, 6, 2079–2082.
23. J. Miyake, K. Fukasawa, M. Watanabe and K. Miyatake, J. Polym. Sci., Part A: Polym. Chem., 2014, 52, 383–389.
24. W.-H. Lee, Y. S. Kim and C. Bae, ACS Macro Lett., 2015, 4, 814–818.
25. K. M. Meek, J. R. Nykaza and Y. A. Elabd, Macromolecules, 2016, 49, 3382–3394.
26. L. Zhu, J. Pan, Y. Wang, J. Han, L. Zhuang and M. A. Hickner, Macromolecules, 2016, 49, 815–824.
27. K. M. Meek and Y. A. Elabd, Macromolecules, 2015, 48, 7071–7084.
28. A. D. Mohanty, C. Y. Ryu, Y. S. Kim and C. Bae, Macromolecules, 2015, 48, 7085–7095.
29. S. Miyanishi, T. Fukushima and T. Yamaguchi, Macromolecules, 2015, 48, 2576–2584.
30. M. Zhang, J. Liu, Y. Wang, L. An, M. D. Guiver and N. Li, J. Mater. Chem. A, 2015, 3, 12284–12296.
31. A. M. A. Mahmoud, A. M. M. Elsaghier and K. Miyatake, RSC Adv., 2016, 6, 27862–27870.
32. K. H. Gopi, S. G. Peera, S. Bhat, P. Sridhar and S. Pitchumani, Int. J. Hydrogen Energy, 2014, 39, 2659–2668.
33. R. Janarthanan, J. L. Horan, B. R. Caire, Z. C. Ziegler, Y. Yang, X. Zuo, M. W. Liberatore, M. R. Hibbs and A. M. Herring, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 1743–1750.
34. R. Janarthanan, S. K. Pilli, J. L. Horan, D. A. Gamarra, M. R. Hibbs and A. M. Herring, J. Electrochem. Soc., 2014, 161, F944–F950.
35. M. T. Pérez-Prior, A. Várez and B. Levenfeld, J. Polym. Sci., Part A: Polym. Chem., 2015, 53, 2363–2373.
36. A. D. Mohanty, Y.-B. Lee, L. Zhu, M. A. Hickner and C. Bae, Macromolecules, 2014, 47, 1973–1980.
37. B. Hu, L. Liu, Y. Zhao and C. Lü, RSC Adv., 2016, 6, 51057–51067.
38. J. Wang, R. He and Q. Che, J. Colloid Interface Sci., 2011, 361, 219–225.
39. T. A. Sherazi, J. Y. Sohn, Y. M. Lee and M. D. Guiver, J. Membr. Sci., 2013, 441, 148–157.
40. Y. Li, Y. Liu, A. M. Savage, F. L. Beyer, S. Seifert, A. M. Herring and D. M. Knauss, Macromolecules, 2015, 48, 6523–6533.
41. Y. Zha, M. L. Disabb-Miller, Z. D. Johnson, M. A. Hickner and G. N. Tew, J. Am. Chem. Soc., 2012, 134, 4493–4496.
42. C. Wang, B. Shen, C. Xu, X. Zhao and J. Li, J. Membr. Sci., 2015, 492, 281–288.
43. Y. Yang and D. M. Knauss, Macromolecules, 2015, 48, 4471–4480.
44. Y. Li and T. Xu, J. Appl. Polym. Sci., 2009, 114, 3016–3025.
45. H.-S. Dang, E. A. Weiber and P. Jannasch, J. Mater. Chem. A, 2015, 3, 5280–5284.
46. A. D. Mohanty and C. Bae, J. Mater. Chem. A, 2014, 2, 17314–17320.
47. H. Long, K. Kim and B. S. Pivovar, J. Phys. Chem. C, 2012, 116, 9419–9426.
48. G. Wang, Y. Weng, D. Chu, D. Xie and R. Chen, J. Membr. Sci., 2009, 326, 4–8.
49. F. Gu, H. Dong, Y. Li, Z. Si and F. Yan, Macromolecules, 2013, 47, 208–216.
50. A. G. Wright and S. Holdcroft, ACS Macro Lett., 2014, 3, 444–447.
51. J. Ni, C. Zhao, G. Zhang, Y. Zhang, J. Wang, W. Ma, Z. Liu and H. Na, Chem. Commun., 2011, 47, 8943–8945.
52. F. Gu, H. Dong, Y. Li, Z. Sun and F. Yan, Macromolecules, 2014, 47, 6740–6747.
53. H. Dong, F. Gu, M. Li, B. Lin, Z. Si, T. Hou, F. Yan, S. T. Lee and Y. Li, ChemPhysChem, 2014, 15, 3006–3014.
54. J. Li, X. Yan, Y. Zhang, B. Zhao and G. He, RSC Adv., 2016, 6, 58380–58386.
55. M. Marino and K. Kreuer, ChemSusChem, 2015, 8, 513–523.
56. T. H. Pham and P. Jannasch, ACS Macro Lett., 2015, 4, 1370–1375.
57. B. Delley, J. Chem. Phys., 1990, 92, 508–517.
58. B. Delley, J. Chem. Phys., 2000, 113, 7756–7764.
59. A. D. Becke, J. Chem. Phys., 1988, 88, 2547–2553.
60. N. Govind, M. Petersen, G. Fitzgerald, D. King-Smith and J. Andzelm, Comput. Mater. Sci., 2003, 28, 250–258.
61. S. C. Price, K. S. Williams and F. L. Beyer, ACS Macro Lett., 2014, 3, 160–165.
62. K. J. Noonan, K. M. Hugar, H. A. Kostalik IV, E. B. Lobkovsky, H. D. Abruña and G. W. Coates, J. Am. Chem. Soc., 2012, 134, 18161–18164.
63. G. Cospito, G. Illuminati, C. Lillocci and H. Petride, J. Org. Chem., 1981, 46, 2944–2947.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR characterization of the synthesized compounds and TGA analysis of prepared AEMs. See DOI: 10.1039/c6ra22313c

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