Influence of highly flexible di(biphenyl)ethane units on the properties of poly(arylene piperidinium) anion exchange membranes

Abstract

Introducing angled arene units and short flexible alkyl segments into rigid aromatic heteroatom-free polymer backbones are efficient strategies to improve the performance of anion exchange membranes (AEMs). So far, only very few monomers increasing backbone flexibility in polymers prepared by polyhydroxyalkylations have been presented, often with limited reactivity and polymerizability. Here, we present the synthesis of a highly reactive monomer [m-di(biphenyl)ethane, mD] containing two angled biphenyl units bridged by an ethane link using a straightforward noble-catalyst-free reductive homocoupling reaction. A series of copolymers were produced by polyhydroxyalkylation involving p-terphenyl, N-methylpiperidone, and different concentrations of mD. Small angle X-ray scattering of the AEMs indicated enhanced ionic clustering with increasing mD content and backbone flexibility. Moreover, water and KOH (aq.) uptake, hydroxide conductivity and alkaline stability increased with the mD content. The hydroxide conductivity of an AEM containing 25% mD units reached 187 mS cm⁻¹ at 80 °C in water, and an AEM with 50% mD units exhibited a conductivity of 53 mS cm⁻¹ in 2 M KOH (aq.) solution. Using only simple nickel foam electrodes, the latter AEM reached a current density of >400 mA cm⁻² at 2.5 V without any cell optimization. In summary, this work demonstrates a convenient synthetic strategy to incorporate flexible units in rigid aromatic polymers, offering improved membrane properties and valuable insights into the design and optimization of advanced AEM materials.

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1. Introduction

Meeting current climate and sustainable energy challenges requires the innovation and development of novel clean energy devices.¹ In this context, anion exchange membrane fuel cells (AEMFCs) and anion exchange membrane water electrolyzers (AEMWEs), operating under alkaline conditions, are of significant interest in the pursuit of low-cost clean energy because of the potential for noble-metal-free catalysts and electrodes.^2,3 Anion exchange membranes (AEMs) are crucial components in these electrochemical devices, separating the electrodes and facilitating the transport of water and hydroxide ions between the electrodes.^4,5 In addition, AEMs are utilized in redox flow batteries and CO₂ electrolyzers.⁴

Initially, AEMs based on aryl ether polymers, including poly(phenylene oxide)s, poly(arylene ether sulfone)s, and poly(arylene ether ether ketone)s, were investigated.^6–8 These polymers are comparatively inexpensive, and the flexible ether bonds between the rigid phenyl rings enhance the mechanical properties of the AEMs by reducing brittleness.⁶ However, the ether bonds were later proven to cause polymer chain degradation under harsh alkaline conditions.^4,9 To meet the demands of high alkaline stability and ion conductivity, aryl ether-free poly(arylene alkylene)s synthesized via superacid-mediated polyhydroxyalkylation reactions are currently extensively studied for use as AEMs.^10–21 Polyhydroxyalkylation reactions require electron-rich arene monomers, such as terphenyls and fluorenes, and suitable ketone or aldehyde monomers and offer many possibilities to vary the polymer structure and functionality. Under optimized conditions, polyhydroxyalkylations generate high-molecular weight poly(arylene alkylene)s while tolerating various functional groups.^10,11 Hence, this type of polymerization provides a robust and versatile platform for developing high-performance AEMs.^4,10–14 However, the ether-free poly(arylene alkylene) backbones produced by polyhydroxyalkylation often lack the proper mechanical toughness, particularly when containing rigid p-terphenyl or dimethyl fluorene units. This adversely affects the film-forming and mechanical properties, as well as the stability of AEMs.^17,18

A feasible method to increase the backbone flexibility of poly(arylene alkylene)s is to copolymerize with an angled arene monomer like m-terphenyl, having a torsion angle between two adjacent benzene rings (Scheme 1a). This approach leads to much more flexible polymers compared to when the non-angled p-terphenyl monomer is used.^13,16 Bae et al. demonstrated that poly(m-terphenylene alkylene)s with pendant quaternary ammonium groups can be applied in AEMFCs to achieve a higher power density compared to the corresponding devices using poly(p-terphenylene alkylene)-based AEMs.¹³ Under similar preparation conditions, the poly(m-terphenylene alkylene)s also reached a higher molecular weight than their p-terphenyl-based isomer. This may originate from the higher solubility of poly(m-terphenylene) during the polyhydroxyalkylation process. Further research has shown that optimization of the m-terphenyl-to-p-terphenyl ratio in poly(terphenyl piperidinium) AEMs can improve stability, conductivity, and the performance of the AEMs.^13,22–24

Scheme 1 Poly(arylene piperidinium) segments containing (a) an angled arene unit (m-terphenyl²²), (b) a short flexible alkyl bridge (bibenzyl²⁵), and (c) the di(biphenyl)ethane unit studied in the present work. The latter unit combines the features of m-terphenyl and bibenzyl units, and introduces exceptional polymer chain flexibility.

Recently, Lee et al. used bibenzyl, consisting of two benzene rings bridged by an ethyl group, to introduce backbone flexibility in poly(arylene piperidinium)s (Scheme 1b), thus enabling excellent AEMFC performance, including a peak power density of 2.58 W cm⁻².²⁵ They later investigated the effect of the alkyl chain length in-between the benzene rings on the physical and electrochemical performance of the AEMs.²⁶

Currently, the range of available flexible arene monomers for polyhydroxyalkylation is limited. In addition, arene monomers with bridging alkyl segments often exhibit low reactivity in polyhydroxyalkylation polymerizations.^25–27 Consequently, there is a need for novel, reactive, and flexible arene monomers for the preparation of high-performance AEMs. Combining the characteristics of the aforementioned m-terphenyl and bibenzyl monomers presents a promising strategy for developing new monomers to increase backbone flexibility.^27–29 In this context, we have in the present work designed and synthesized a novel monomer [m-di(biphenyl)ethane (mD)] for use in polyhydroxyalkylations, incorporating both angled arene units and alkyl segments (Scheme 1c). Monomer mD was prepared via a noble-metal-free reductive coupling reaction in an almost quantitative conversion. To investigate the influence of the mD units on AEM properties, we have synthesized a series of poly(arylene piperidinium)s with different mD : p-terphenyl ratios to vary the backbone flexibility. AEMs were then prepared via solution-casting and characterized with respect to morphology, water/alkaline uptake, mechanical properties, ionic conductivity, and alkaline and thermal stability. Subsequently, the most promising AEM was selected for an initial water electrolysis evaluation, as well as tensile testing, to investigate the use of these AEMs in electrochemical energy devices.

2. Experimental section

Information on the materials used and details concerning the measurements performed are included as SI.

2.1. Monomer synthesis

The mD monomer was synthesized following a published procedure after slight modification.^30,31 Anhydrous cobalt chloride (26 mg, 0.2 mmol, 2 mol%) was added to a 50 mL Schlenk tube equipped with an oval magnetic stirring bar. The Schlenk tube was sealed with a rubber septum, evacuated, and backfilled with nitrogen thrice. Subsequently, 15 mL of anhydrous THF was injected before adding 0.8 mL 1 M trimethyl phosphine (0.8 mmol, THF solution). After the addition of 6 mL 1 M dimethyl zinc (6 mmol, heptane solution), the black solution in the Schlenk tube became clear and dark red. An amount of 2.47 g (10 mmol) 3-phenyl benzyl bromide was dissolved in 5 mL THF before being quickly added to the Schlenk tube. The tube was immediately placed in a sand bath preheated to 66 °C. The reaction was stopped after 20 min. by adding 1 mL 2 M HCl (aq.) solution. Next, the reaction mixture was dissolved in 200 mL dichloromethane (DCM) and washed with water thrice. The organic phase was collected and filtered through a pad of aluminum oxide until the solution became colorless. The resulting solution was dried with Na₂SO_4, and the organic solvent was removed using a rotary evaporator. A clear, highly viscous liquid was obtained, which spontaneously crystallized within 10 min. After drying under vacuum at room temperature for 2 h, 1.64 g (4.9 mmol) mD monomer (quantitative conversion) was ready for polymerization. ¹H NMR (400 MHz, chloroform-d) δ 7.66 (dd, J = 8.3, 1.3 Hz, 4H), 7.56–7.38 (m, 12H), 7.29 (dt, J = 7.5, 1.5 Hz, 2H), 3.13 (s, 4H). ¹³C NMR (101 MHz, chloroform-d) δ 142.24, 141.41, 128.88, 128.78, 127.58, 127.53, 127.28, 124.97, 38.15.

2.2. Polymerization and quaternization

A series of precursor copolymers were synthesized via superacid-mediated polyhydroxyalkylations of mD, p-terphenyl, and N-methyl piperidone.¹¹ The copolymers were designated as PmDPip-x, where x is the mol% of mD units. Here, the synthesis of PmDPip-100 is given as an example. To a 25 mL round-bottom flask equipped with a magnetic stirrer, mD (334 mg, 1 mmol) and N-methyl piperidone (147 mg, 1.3 mmol) were added along with 1.2 mL DCM. The mixture was cooled to 0 °C in an ice bath before adding 0.2 mL trifluoroacetic acid (TFA). Trifluoromethansulfonic acid (TFSA, 1.4 mL, 15.6 mmol) was then added dropwise. The polymerization was allowed to proceed at room temperature for 4 h, after which 10 mL of dimethyl sulfoxide (DMSO) was added to dilute the viscous solution. The solution was then poured into 100 mL of a 1 : 1 (v : v) mixture of diethyl ether and isopropanol to precipitate the product as a white powder. The product was collected and dried in a vacuum oven at 50 °C, yielding PmDPip-100 quantitatively (447 mg).

Quaternization of the precursor polymers was achieved in Menshutkin reactions with methyl iodide.¹⁶ The samples were designated as PmDPipQ-x, and here the synthesis of PmDPipQ-100 is provided as an example. To a 25 mL round-bottom flask equipped with a magnetic stirrer, PmDPip-100 (430 mg, corresponding to 1 mmol N-methylpiperidine units), K₂CO₃ (280 mg, 2 mmol), and methyl iodide (0.17 mL, 2.5 mmol, 5 equiv.) were added together with 10 mL DMSO. The flask was sealed and covered with aluminum foil. After stirring for 24 h, the solution was poured into water to precipitate the product, yielding PmDPipQ-100 as a white solid. The product was washed with a mixture of isopropanol and diethyl ether, then collected and dried under vacuum. PmDPipQ-100 was obtained quantitatively (576 mg).

2.3. Membrane casting

An amount of 200 mg of each PmDPipQ-x sample was first dissolved and diluted with dimethyl sulfoxide (DMSO) to reach a concentration of 4 wt% polymer. The solutions were passed through a PTFE filter (ϕ = 5 μm), before being poured into a Petri dish (ϕ = 6 cm). Casting was performed during 2 days at 80 °C in an air-circulating oven, after which transparent, colorless, and uniform AEMs were obtained. The AEMs were subsequently immersed in 1 M aq. NaBr for 2 days, before washing and storage in deionized water (DI water).

3. Results and discussion

3.1. Synthesis of monomers and polymers

One of the major motivations for developing devices based on AEMs is their potential to utilize non-noble metals as catalysts and electrodes. Hence, it is also important to avoid the use of noble metal-catalyzed reactions in the preparation of the AEMs. Unlike conventional methods to prepare arene-functionalized monomers that often require Pd-catalyzed Suzuki-cross-coupling reactions involving both aryl boronic acids and aryl halides,^32–34 the symmetrical mD monomer was synthesized in a cobalt-catalyzed reductive homocoupling reaction of only one benzyl bromide compound.^30,31,35 In this coupling reaction, a low-valent cobalt species abstracts the bromine atoms from the starting benzyl bromide compound, and then the combination of the generated benzylic radicals efficiently forms the C_sp³–C_sp³ bond of mD (Scheme 2a). The cobalt(II) species was then regenerated to cobalt(0) using dimethyl zinc.³⁶ The monomer was quantitatively isolated by crystallization without needing column chromatography (Fig. 1a and b). This straightforward synthetic method indicates possibilities for scaled-up production of mD. In addition, manganese has recently been identified as a potential alternative to dimethyl zinc, which may further reduce the cost of mD.³⁷

Scheme 2 (a) Synthesis of the mD monomer and (b) the preparation of the PmDPipQ-x series of AEM polymers by superacid-mediated polyhydroxyalkylations (x indicating the mol% of mD units).

Fig. 1 (a) ¹H and (b) ¹³C NMR spectra of the mD monomer recorded in chloroform-d, and (c) ¹H NMR spectra of the samples in the PmDPipQ-x series recorded in DMSO-d₆ solutions (TFA was added to shift the water signal).

The mD monomer obtained in the crystalline form was then employed in superacid-mediated polyhydroxyalkylations with N-methylpiperidone and p-terphenyl (Scheme 2b). Quaternization of the N-methyl piperidine groups in the polymer backbones was subsequently achieved by Menshutkin reactions with methyl iodide, yielding the PmDPipQ-x series of polymers (x indicating the mol% of mD units in the polymer). Compared to bibenzyl-type monomers, mD exhibited a significantly higher reactivity. This enabled a high-molecular weight polymer based on only mD and N-methyl piperidone (PmDPipQ-100) to be formed after merely 4 h. Additionally, when copolymerized with p-terphenyl, the resulting ratios between the p-terphenyl and mD units (calculated from NMR data) precisely matched the monomer feed ratios (Table S1). Similar to m-terphenyl, the presence of the mD units enhanced polymer solubility during the polymerization reaction, which may explain why PmDPipQ-25 (with the lowest mD content) displayed a relatively low intrinsic viscosity (Table 1).

Table 1 Key AEM properties

AEM	IEC (mequiv. g^−1)		WU^a (%) (80 °C)	λ^a (80 °C)	2 M aq. KOH uptake (%, 80 °C)	σ^a (mS cm^−1)		Tg (°C)	Td,95 (°C)	[η], dL g^−1
	Theoretical	Titrated				Water^a	2 M KOH
a Measured in the OH^− form.
PmDPipQ-25	2.59	2.59	197	42	112	187	33	—	313	0.73
PmDPipQ-50	2.43	2.47	502	113	151	253	53	210	420	0.81
PmDPipQ-75	2.29	2.32	769	184	171	338	42	175	423	0.82
PmDPipQ-100	2.16	2.21	—	—	174	—	43	159	424	0.86

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3.2. Membrane preparation and morphology

All the AEMs cast from DMSO solutions were transparent, mechanically robust, and foldable (180°). After casting, the membranes were ion-exchanged to the Br⁻ form and the ion exchange capacity (IEC) was determined by Mohr titrations. The results aligned well with the theoretical IEC values calculated from the corresponding polymer structures (Table 1). As the content of mD units increased from 25 to 100%, the IEC of the AEMs decreased from 2.59 to 2.16 mequiv. g⁻¹. Reduced ionic content often leads to membrane morphologies with less organized phase domains, observed by indistinct ionomer peaks in the small angle X-ray scattering (SAXS) profiles.³⁸ However, despite having the lowest IEC, PmDPipQ-100 showed the strongest ionomer peak, stronger than the ionomer peaks of all the copolymers containing p-terphenyl (Fig. 2a). Notably, the intensity of the ionomer peak gradually increased with the mD unit content of the membranes (corresponding to a decreasing IEC), which indicated that the increasing polymer chain flexibility favored the ionic clustering more than the increasing ionic content. In comparison, ionomer peaks are not normally observed for AEMs based on poly(p-terphenyl piperidinium), indicating less ordered and inefficient ionic clustering and phase separation of more stiff polymer backbones.^14,16

Fig. 2 (a) SAXS profiles of dry AEMs in the Br⁻ form, and AFM phase images of dry AEMs: (b) PmDPipQ-25, (c) PmDPipQ-50, (d) PmDPipQ-75, and (e) PmDPipQ-100 in the Br⁻ form.

While SAXS data provide information about the bulk morphology, atomic force microscopy (AFM) analysis gives details about surface morphology. As seen in Fig. 2b–e, AFM phase images also indicated distinctly phase separated morphologies for all the AEMs, with bright and dark areas indicating hydrophobic and hydrophilic phase domains, respectively. As the mD content of the membranes increased, the regularity of the phase domains increased, which aligned with the SAXS results. Concurrently, the overall area of hydrophobic phase domains increased with the mD content, which was consistent with decreasing IEC values.

3.3. Thermal properties

Previous studies have demonstrated that the chemical (alkaline) stability of the piperidinium cation is significantly improved by placement on flexible segments in the backbone to allow ring relaxation.^19,23 Moreover, the incorporation of monomer units containing flexible alkyl chains in the backbone has been reported to improve the thermal stability of piperidinium-functional AEMs.^26,27 In the present case, the thermal decomposition and stability of the AEM samples in the Br⁻ form were studied by thermogravimetric analysis (TGA) under nitrogen and air atmosphere (Fig. S7a and b, respectively), to measure the thermal decomposition temperature (T_d,95, taken at 5% weight loss). Membrane PTPipQ-1 (with only p-terphenyl units) has been reported to reach T_d,95 = 264 °C under nitrogen.¹⁴ The incorporation of 25% flexible mD units raised the T_d,95 value of PmDPipQ-25 to 313 °C (Table 1). Increasing the content of flexible mD units to 50% resulted in an additional increase of T_d,95 to 420 °C for PmDPipQ-50. Further increases in the mD content, beyond 50%, did not lead to any significant increases in T_d,95, and PmDPipQ-75 and PmDPipQ-100 reached T_d,95 values of 423 and 424 °C, respectively. Based on this finding, we speculate that enhancing backbone flexibility raises the thermal stability of the piperidinium cation. However, this positive effect levels out after reaching a certain critical backbone flexibility.

As expected, the TGA thermograms recorded under air showed lower T_d,95 values compared to those under nitrogen (Fig. S7b). The thermo-oxidative decomposition revealed a multi-step process, with the first two steps most probably related to the degradation of the piperidinium cations. Also in this case, the T_d,95 values increased with the mD content, and PmDPipQ-25, -50, -75, and -100 showed values of 276, 302, 387, and 411 °C, respectively.

The AEMs were characterized by differential scanning calorimetry (DSC, Fig. S7c), and glass transitions were observed at T_g = 210, 175, 159 °C for PmDPipQ-50, PmDPipQ-75, and PmDPipQ-100, respectively. Hence, as expected, the T_gs of the AEMs decreased, and the specific heat capacity (ΔC_p) increased, with the mD content. These features indicated increasing backbone flexibility with the mD content. Moreover, PmDPipQ-50, PmDPipQ-75, and PmDPipQ-100 had T_gs below their respective T_d,95 values, suggesting that these samples were in the rubbery-melt state when thermally decomposing in the TGA experiments. Hence, the high backbone flexibility under these conditions may have efficiently prevented the distortion of piperidinium rings, which may in turn explain the observed increase in the thermal stability. In contrast, no T_g was detected for PmDPipQ-25 below its degradation temperature, which may explain why the T_d,95 for this AEM was the lowest among all four samples.

3.4. Aqueous uptake and swelling

For application in AEMFCs, AEMs with a suitable level of water uptake are a prerequisite for efficient ion conduction and water management.³⁹ The water uptake typically depends on, e.g., the IEC value, polymer backbone flexibility, degree of crystallinity, and the molecular weight. In the present case, the water uptake and swelling ratio of the AEMs in the PmDPipQ-x series (OH⁻ form) were measured between 20 and 80 °C, and the results are shown in Fig. 3a, c, and e, respectively. On the one hand, AEMs with a high mD content have lower IEC values compared to those with a high p-terphenyl content, which should result in a reduced water uptake with increasing mD content. On the other hand, an increased mD content can be expected to significantly increase the polymer backbone flexibility of the polymers, which promotes water uptake. Although these two parameters linked to the mD content have opposing effects, the overall impact of the increased backbone flexibility provided by the mD units dominated. Hence, the water uptake of the PmDPipQ-x membranes increased with the mD content. Membrane PmDPipQ-25, with the lowest mD content, exhibited the lowest water uptake (197% at 80 °C) in this work. Correspondingly, PmDPipQ-50 and PmDPipQ-75 showed a higher water uptake, and the water uptake started to increase progressively with temperature above 60 °C. PmDPipQ-75 softened considerably when the water uptake reached 769%, corresponding to a hydration number (λ) of 184, at 80 °C. Consequently, the OH⁻ concentration of the swollen PmDPipQ-75 decreased to approximately 1.2 mol L⁻¹. For PmDPipQ-100, the progressive increase in water uptake started already at 40 °C, in agreement with the lower T_g observed by DSC. This membrane became a hydrogel and lost all mechanical strength after 8 h immersion in water at 80 °C. The swelling ratio of the AEMs followed the same trend as the water uptake (Fig. 3c and e), with PmDPipQ-25 and PmDPipQ-50 showing controlled swelling ratios at 80 °C.

Fig. 3 Water uptake, swelling and ionic conductivity data of the AEMs in the PmDPipQ-x series between 20 and 80 °C (note different scales on vertical axes): water uptake (a) and uptake of 2 M aq. KOH (b), through-plane swelling ratio in water (c) and in 2 M aq. KOH (d), in-plane swelling ratio in water (e) and in 2 M aq. KOH (f), OH⁻ conductivity in the fully hydrated state [immersed in water] (g) and ionic conductivity in 2 M aq. KOH solution (h).

In practical applications, AEMWEs generally operate with electrolyte solutions rather than pure water.⁴⁰ Since the uptake of alkaline solution of AEMs is often significantly lower than the uptake of water, it is relevant to evaluate the alkaline solution uptake of the PmDPipQ-x AEMs. Hence, the electrolyte uptake and swelling ratio were measured in 2 M aq. KOH between 20 and 80 °C (Fig. 3b, d and e). As expected, the electrolyte uptake and swelling ratio of the AEMs increased with both temperature and mD content. In contrast to the water uptake data, the difference in electrolyte uptake between the AEMs was modest. PmDPipQ-100 exhibited the highest alkaline solution uptake, reaching 174% at 80 °C. All AEMs remained mechanically robust after the alkaline solution uptake, also at 80 °C.

3.5. Ionic conductivity

The in-plane OH⁻ conductivity of the AEMs fully immersed in water was measured between 20 and 80 °C using electrochemical impedance spectroscopy (EIS). In addition, the through-plane ionic conductivity of the AEMs immersed in 2 M aq. KOH solution was measured between 20 and 80 °C, using a previously reported method.⁴¹ The OH⁻ conductivity is an essential property of AEMs and depends on various factors, such as water/electrolyte uptake, temperature, and membrane morphology. As described above, increasing amounts of mD in the PmDPipQ-x series reduced the IEC values, but increased the uptake of water and 2 M aq. KOH solution at 80 °C. As seen in Fig. 3g, the OH⁻ conductivity of the PmDPipQ-x membranes in the fully hydrated state generally increased with decreasing IEC and increasing water uptake. Compared to poly(p-terphenyl piperidinium) [PTPipQ100, with only p-terphenyl in the backbone, 132 mS cm⁻¹, 302% water uptake at 80 °C],^14,16 PmDPipQ-25 exhibited higher conductivity (184 mS cm⁻¹) at a lower water uptake. PmDPipQ-50 and PmDPipQ-75 reached very high OH⁻ conductivities, i.e., 253 mS cm⁻¹ and 338 mS cm⁻¹ at 80 °C, respectively. These levels of conductivities have only rarely been reported previously. However, a gel-state ion-solvating polybenzimidazole membrane doped with 25 wt% KOH has recently been reported by Henkensmeier et al. to reach 313 mS cm⁻¹ at 80 °C.⁴⁷ Furthermore, a highly swollen ion-solvating membrane based on a sulfonated polybenzimidazole has been reported by the same authors to reach an even higher conductivity, 358 mS cm⁻¹ when doped with 1 M KOH at 80 °C.⁴⁸ This value further increased to 682 mS cm⁻¹ with 3 M KOH. As mentioned above, the water uptake of PmDPipQ-75 increased to 769% at 80 °C, which transformed the membrane to a self-supporting gel-like material with an OH⁻ concentration close to 1.2 M. Furthermore, the dimensional increase during the conductivity measurements between 20 and 80 °C may have led to a certain overestimation of the OH⁻ conductivity of this membrane, since the standard OH⁻ conductivity calculation was based on the dimensions of the membrane equilibrated at room temperature. It should be mentioned that the PmDPipQ-75 sample swelled significantly in the EIS cell during the temperature increase from 20 to 80 °C. This dimensional change may have led to an overestimation of the OH⁻ conductivity of this membrane, since the standard calculation of the OH⁻ conductivity was based on the initial thickness of the membrane equilibrated at room temperature. The OH⁻ conductivity of PmDPipQ-100 was not reported because of the very high water uptake at 80 °C. The uptake decreased significantly when the AEMs were immersed in aq. KOH solution. Moreover, the backbone flexibility appeared to have a significantly lower effect on the ionic conductivity in 2 M aq. KOH solution, and the ionic conductivity did not increase with the mD content. PmDPipQ-50 had a moderate alkaline solution uptake and reached the highest conductivity among the AEMs, just above 50 mS cm⁻¹ at 80 °C, while PmDPipQ-25 with the lowest alkaline solution uptake among the AEMs reached the lowest OH⁻ conductivity at 80 °C, i.e., 33 mS cm⁻¹.

3.6. Alkaline stability

Reaching a sufficient alkaline stability of the AEMs is a major challenge for application in AEMFCs and AEMWEs. Previous research has demonstrated that the alkaline stability of the N,N-dimethyl piperidinium cation can be significantly improved by reducing the constraints of the piperidine ring in the polymer structure, either by increasing backbone flexibility or by tethering the cations to the polymer backbone via flexible side chains.³

Consequently, the alkaline stability of the PmDPipQ-x was expected to increase with the mD content. However, after storage in 5 M aq. KOH at 80 °C for 480 h, all the AEMs exhibited approximately 6% total ionic loss despite different contents of the flexible mD units (Fig. 4). Hofmann elimination was identified as the dominating degradation reaction leading to ionic loss, as indicated by the small signals at 4.9, 5.3, 6.5, (b, c, d in Fig. 4) arising from the vinyl group in the degradation product, and at 9.7 ppm (a in Fig. 4) originating from the protonated tertiary amine group in the degradation product. As expected, the ratios of these signals were 1 : 1 : 1 : 1.

Fig. 4 ¹H NMR spectra of AEM samples after storage in 5 M aq. KOH solutions at 80 °C for 480 h, displaying small signals (a–d) emerging from the degradation products by Hofmann β-elimination.

The present AEMs showed slightly higher alkaline stability compared to the corresponding poly(arylene piperidinium) AEMs based on terphenyl and quaterphenyl,^14,34 which may result from the higher backbone flexibility induced by the mD units. However, the content of flexible mD units appeared to have little influence on the alkaline stability of the PmDPipQ-x AEMs. The reason may be that the flexible units had a limited effect on the local chain rigidity around the piperidinium rings along the polymer backbone. Despite different mD contents, the piperidinium rings are still surrounded by rigid biphenyl and p-terphenyl units that may distort the rings (Scheme 2b). Moreover, all the PmDPipQ-x AEMs had almost the same uptake of KOH solution (Fig. 3b), which may also have contributed to a similar alkaline stability. The alkaline stability of quaternary ammonium cations generally increases with the λ (hydration number, [Η₂Ο]/[OH⁻]).⁴² Since the uptake of pure water increased with the mD content, the alkaline stability of the AEMs in pure water can be expected to increase with the mD content.

3.7. Tensile testing

Good mechanical properties of AEMs are required to prevent rupture and provide support for the electrochemical cell structure. Some of the PmDPipQ-x membranes showed high alkaline stability and OH⁻ conductivity in 2 M KOH (aq.) solution, and the mechanical properties of the AEMs were characterized to further assess the suitability for water electrolyzer applications. The stress–strain data of the AEMs recorded in the dry state and after immersion in 2 M KOH (aq.) solution are presented in Fig. 5a and b, respectively. As the mD content increased from 25 to 75%, both the tensile strength and the elongation at break increased. In the dry state, the PmDPipQ-50 and PmDPipQ-75 demonstrated a tensile strength over 75 MPa with an elongation at break exceeding 30%. These values surpass the corresponding data of membranes where polymer backbone flexibility was introduced solely through twisted arene moieties³⁴ or short alkyl chains.²⁶ PmDPipQ-100, without any p-terphenyl units, exhibited a lower elongation at break compared to PmDPipQ-50 and PmDPipQ-75, and showed the lowest tensile strength among the AEMs. Seemingly, the mechanical properties of the former sample deteriorated because of the very high mD content, which made the AEM very soft and weak. The findings agree with the previous findings that an appropriate polymer backbone flexibility may enhance the mechanical properties of AEMs by reducing brittle behavior.²² Since AEMs often operate immersed in KOH solutions, the stress–strain curves of the AEMs in 2 M KOH (aq.) were also measured (Fig. 5b). As expected, under this condition, the AEMs became considerably softer than in the dry state, resulting in decreased tensile strength and increased elongation at break. PmDPipQ-50 and PmDPipQ-75 exhibited a tensile strength over 20 MPa and an elongation at break exceeding 70%, making them promising candidates for use in alkaline electrochemical devices.

Fig. 5 Stress–strain data of AEMs in the dry state (a) and after immersion in 2 M KOH (aq.) solution (b).

3.8. Initial water electrolysis testing

Displaying high alkaline stability and OH⁻ conductivity, as well as good mechanical properties, the PmDPipQ-50 membrane was selected for a preliminary evaluation in a water electrolyzer to investigate the applicability of AEMs based on poly(arylene piperidinium)s containing mD units in electrochemical devices. The single cell assembly followed a previously reported method.^43,44 A 60 μm thick PmDPipQ-50 membrane was sandwiched between a pair of pure nickel foam sheets as electrodes in the cell hardware. The test was conducted with 2 M KOH (aq.) at 40, 60 and 80 °C, respectively. The polarization curves are displayed in Fig. 6a, with the corresponding galvanostatic electrochemical impedance spectroscopy (GEIS) results presented in Fig. 6b. An inset image in Fig. 6b compares the potentiostatic electrochemical impedance spectroscopy (PEIS) and GEIS data at 80 °C. The in situ conductivities estimated from PEIS data recorded at 1.3 V (without any gas evolution), and from GEIS data recorded at 10 mA cm⁻² and 100 mA cm⁻² (with increasing gas evolution), were 12, 8, and 6 mS cm⁻¹, respectively. The difference between the conductivities estimated from the PEIS and GEIS data may be attributed to the entrapment of H₂ gas in the nickel foam, causing additional resistance contributions from the porous electrodes. These in situ conductivities were significantly lower than the ex situ conductivity results, which may indicate additional resistance contributions from the electrolyzer cell fixture, including contact resistance between the electrode and the flow fields. For this reason, the current density only reached 400 mA cm⁻² at 2.5 V and 80 °C (Fig. 6a). Consequently, more advanced electrodes and catalysts that better match the flexible membrane are required to increase the AEMWE performance.

Fig. 6 (a) Water electrolysis polarization curves for cells assembled with PmDPipQ-50 membranes and nickel foam electrodes, and (b) the corresponding galvanostatic EIS data with an inset plot of the detailed potentiostatic and galvanostatic EIS data at 80 °C. (c) H₂ in O₂ concentrations reached at different current densities in AEMWEs assembled with PmDPipQ-50 at 40, 60, and 80 °C, respectively.

The lower explosion limit of H₂/O₂ mixtures is merely 3.8 mol% at 80 °C,⁴⁵ and the H₂ crossover is therefore a critical factor when evaluating the safety of AEMWEs. Transport mechanisms, including diffusion and convection of H₂-supersaturated electrolyte from the cathode into the anode compartment, contribute to the overall permeation of H₂ through the membrane.⁴⁶ The H₂/O₂ level in the cell assembled with PmDPipQ-50 is shown in Fig. 6c. At 80 °C, the H₂/O₂ level remained below 0.7% at 50 mA cm⁻² and decreased to 0.4% at 100 mA cm⁻², thus ensuring safe operation of AEMWEs assembled with this AEM. In addition, the cell voltage (Figure S8) required to achieve 50 mA at 40 °C before and after the H₂ crossover measurement, which was performed during 100 h between 40 and 80 °C, remained essentially constant. This further indicated the durability of PmDPipQ-50 in AEMWE applications.

4. Conclusions

A novel monomer mD, with two angled biphenyl units separated by an ethylene bridge, was synthesized to introduce polymer backbone flexibility in AEMs prepared by polyhydroxyalkylations. Copolymerization of p-terphenyl and N-methylpiperidone with controlled concentrations of mD, produced a series of AEMs with different ratios of flexible mD units and stiff p-terphenyl units. The mD monomer demonstrated high reactivity in the polyhydroxyalkylations and produced high-molecular weight polymers with varying backbone flexibility and good film-forming properties. Increasing contents of mD units promoted ionic clustering, as well as the uptake of water and electrolyte solution, which resulted in high ionic conductivities. Moreover, the presence of the mD units was found to improve both the thermal and alkaline stability of the AEMs. However, the mD content had seemingly limited impact on the alkaline stability, possibly because of small differences in the uptake of the alkaline solution and a limited effect of the mD units on the relaxation of the piperidinium rings along the polymer backbone. An AEMWE cell assembled with a selected AEM and plain nickel foam electrodes produced a current density of 402 mA cm⁻² at 2.49 V. The results also indicated that further optimization of cell configuration and catalyst may significantly improve the performance of electrolyzers. The study demonstrated the high potential of introducing flexible chain segments to improve and tune AEMs for use in different electrochemical energy devices.

Author contributions

Si Chen: conceptualization, methodology, investigation, validation, writing – original draft. Yifan Xia: investigation, methodology, validation, writing – review & editing. David Aili: funding acquisition, supervision, writing – review & editing. Patric Jannasch: methodology, funding acquisition, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The materials and measurement methods used in this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5ta04171f.

Acknowledgements

We thank the Swedish Energy Agency (grants 50519-1, 45057-1, 37806-3, and 45515-3), the Swedish Research Council (grant 2023-04525), the Swedish Foundation for Strategic Research, SSF (grant ARC19-0026), and the European Union's Horizon 2020 Research and Innovation Action program (862509, NEXTAEC) for financial support. We also thank Dr Dmytro Serhiichuk for his assistance with the tensile testing.

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