Effect of Different Quaternary Ammonium Groups on the Hydroxide Conductivity and Stability of Anion Exchange Membranes.

Anion exchange membrane fuel cells (AEMFCs) are encouraging electrochemical structures for the competent and complaisant conversion of energy. Herein, the development of brominated poly(2,6-dimethyl phenylene oxide) (BPPO)-based anion exchange membranes (AEMs) with different quaternary ammonium groups for AEMFCs was reported. The successful preparation of AEMs was proved by utilizing proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. They were explored in terms of water uptake (W R), ion exchange capacity (IEC), hydration number (λ), linear swelling ratio (LSR), morphology, tensile strength (TS), and elongation at break (E b). The alkaline stability of the prepared AEMs was assessed and compared with each other. The experimental outcomes demonstrated that the N-methylpyrrolidinium-based membrane (MPyPPO) exhibited higher alkaline stability, whereas the N-methylimidazolium-based membrane (MImPPO) showed the lowest alkaline stability among the prepared AEMs. Similarly, the hydroxide conductivity of the prepared AEMs was measured and compared with each other. The pyrrolidinium-based membrane (MPyPPO) exhibited higher hydroxide conductivity among the prepared AEMs.

Introduction

For both automotive and stationary uses, fuel cells are regarded as efficient eco-friendly energy conversion systems.[1] Much research has been emphasized on the fabrication of proton exchange membrane fuel cells (PEMFCs), which now stand on the brink of broad commercialization for automotive uses.[2] The perfuorosulfonic acid (PFSA) membrane is a vital component of today’s PEMFCs, which acts as a separator between the cathode and the anode and migrates protons from the former to the latter electrode. PFSA membranes such as Nafion and Aquivion have high proton conductivity and excellent mechanical, thermal, and chemical stability, facilitating the PEMFC systems with good power output and long lifetimes.[2] In spite of the significant progress in the development of PEMFCs and the beneficial properties of the PFSA membranes, large-scale deployment of PEMFCs is still lacking and highly needed. The lack of fuel flexibility and the use of expensive and rare electrocatalysts are the two serious and genuine deficiencies of the current PEMFC technology. These deficiencies have recently directed researchers to deviate their concern to AEMFCs.[3−7]

AEMFCs appear as a possible option for PEMFCs. Over the last decade, they were sufficiently developed. Working under alkaline circumstances along with OH– as the charge bearer rewarded AEMFCs with high prestige including the fortuity to utilize non-platinum metal catalysts like nickel, cobalt, and silver in the cathode rapid oxygen reduction reaction, etc.[3,8−11] Nevertheless, the advancement of AEMFCs presently gets affected because of lack of convenient AEMs, which typically require excellent alkaline stability and sufficient OH– conductivity to offer the persisting high pursuance of the system.[3,12] Therefore, the construction of highly conductive and alkaline stable AEMs is extremely necessary for the practical uses of AEMFCs.

Due to the nucleophilicity of OH–, it is expected to react with quaternary ammonium (QA) cations, which are restrained to the polymer of the anion exchange membrane. QA cations are acknowledged to degenerate via Hofmann β-elimination, nucleophilic substitution, and different rearrangement reactions, including Stevens rearrangement in the absence of β-hydrogens under strongly alkaline conditions.[13−15] Consequently, the degeneration rate of QA cations not only depends on their type but also on the format of the polymer architecture and the explicit locations of cations in the polymeric architecture. For instance, electron-withdrawing groups in the polymer architecture, namely, polysulfone activate invasion by OH– lead to chain scission.[16] Poly(phenylene oxide) (PPO) contains only phenylene rings bonded by ether bridges and is notably highly stable under an alkaline environment.[3] Moreover, PPO is free from the chloromethylation reaction. It utilizes chloromethyl ether (CME), which is hazardous for human health and regarded as a banned chemical. Bromination of PPO is a useful, eco-friendly method for producing BPPO, which desperately controls the utilization of CME to develop AEMs. Due to these advantages, we have selected BPPO as the polymer backbone for this work. By propagating hydrophilic materials, the hydrophobic surface of BPPO-based membranes can be efficiently changed to the hydrophilic one. The AEMs developed from the BPPO backbone exhibited excellent thermal, chemical, and mechanical stabilities.[17,18] BPPO holds sufficient −CH2Br functional groups compared with other polymer architectures, including poly(ether sulfone) (PES), polysulfone (PS), and poly(vinylidene fluoride) (PVDF), which can easily react with amines without entering the base membrane and utilization of cross-linkers.[18,19] Moreover, it is a highly stable polymer architecture and much vulnerable to the attack of a nucleophile.[18,19]

In the recent past, AEMs based on several cations, such as ammonium,[20−24] phosphonium,[25−27] guanidinium,[28−30] imidazolium,[31,32] metal cation,[33] and benzimidazolium cations,[34−36] were synthesized very rapidly and considered broadly. These AEMs exhibited excellent potential in AEMFCs; however, the chemical stability, particularly the alkaline stability of these AEMs showed major challenges that limit the application of AEMs in practical uses.[37] Primary deterioration generally involves hydroxide ion attack upon the polymer architecture and the cationic groups of AEMs in alkaline solution. It has been established that cationic group chemistry plays a considerable role in giving alkaline stability to AEMs.[37] Hence, it is highly desirable to study AEMs with many cationic groups to find optimum quaternary ammonium groups for AEMs in terms of hydroxide conductivity and chemical stability.

In this research, we prepared AEMs from BPPO, followed by quaternization with different ammonium groups (imidazolium, ammonium, piperidinium, and pyrrolidinium). The physicochemical properties of the prepared AEMs such as water uptake (WR), ion exchange capacity (IEC), linear swelling ratio (LSR), morphology, and thermal, alkaline, and mechanical stabilities were investigated in detail with the aim to find out the optimum quaternary ammonium group for AEMs. The impact of quaternary ammonium groups on the hydroxide conductivity and alkaline stability of the prepared AEMs was studied and compared with each other.

Experimental Section

Materials

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Sigma-Aldrich Co., Ltd., P. R. China. 2,2′-Azobisisobutyronitrile (AIBN) and N-bromosuccinimide (NBS) were purchased from Adamas Reagent Co., Ltd., China. Chlorobenzene was bought from Shanghai Titas Co., Ltd., P. R. China. Potassium chromate (K2CrO4), trimethylamine, N-methylimidazole, sodium sulfate (Na2SO4), N-methypyrrolidine, sodium hydroxide (NaOH), N-methylpiperidine, chloroform (CHCl3), sodium chloride (NaCl), silver nitrate (AgNO3), ethanol (EtOH), and 1-methyl-2-pyrrolidone (NMP) were obtained from Sinopharm Chemical Reagent Co., Ltd., P. R. China and were utilized as attained. Deionized (DI) water was utilized throughout the work.

Bromination of Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)

It was done as described in Figure (38) (see Section S1 in the Supporting Information for more details).

Figure 1

Fabrication of AEMs with different quaternary ammonium groups.

Fabrication of N-Methylimidazole, N-Methylpipridine, and N-Methypyrrolidine Quaternized Anion Exchange Membranes

We utilized a solution casting procedure to fabricate N-methylimidazole, N-methylpyridine and N-methypyrrolidine quaternized AEMs as described in our previous research.[18,19,39−43] Initially, to attain a homogeneous solution of BPPO, we dissolved 0.8 g of it into 1-methyl-2-pyrrolidone at room temperature. Measured amounts of N-methylimidazole, N-methypyrrolidine, N-methylpyridine were added into the above-attained homogeneous solution of BPPO. Then, the mixture of BPPO and amines was stirred at 40 °C overnight to complete the reaction. Lastly, they were casted onto a glass plate overnight at 60 °C. Then, they were peeled off from the glass plate, cleaned with distilled water, and stored in it. They were named MImPPO, MPyPPO, and MPiPPO, respectively. The chemical structure of the fabricated AEMs is represented in Figure .

Fabrication of the Quaternized Poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) Membrane

It was carried out as described in Figure (18,44) (see Section S2 in the Supporting Information for more details).

Characterizations

Instrumentations

Detailed instrumental analyses, e.g., Fourier transform infrared spectroscopy, 1H NMR spectroscopy (Q800 dynamic mechanical analyzer, DMA, TA Instruments), atomic force microscopy, scanning electron microscopy, and thermogravimetric analysis, were used (see Section S3 in the Supporting Information for more details).

Ion Exchange Capacity

Classical Mohr’s method was utilized to determine IEC as reported in the literature.[19,45,46] Initially, the AEM samples were soaked into a NaCl (1 mol L–1) solution for 2 days to convert all charge sites into the Cl– form. They were washed with DI water to withdraw excessive NaCl. They were then soaked into a Na2SO4 (0.5 mol L–1) solution for 2 days. The quantity of Cl– ions discharged was deliberated by titration with 0.05 (M) AgNO3 by utilizing K2CrO4 as an indicator. It was determined by utilizing the following relationshipwhere C, V, and m show the concentration of the AgNO3 solution, the titer value during titration, and the dry mass of the membrane, respectively.

Water Uptake, Linear Swelling Ratio, and Hydration Number

Water uptake was determined to explore the hydrophilicity of ion exchange membranes (IEMs). To attain dry weight, they were dried in a vacuum oven for 1 day at 60 °C and veraciously weighed. The wet weight was attained by soaking membranes into distilled water for 2 days and weighing the membrane samples after eradication of surface water with tissue paper. It was determined from the difference in mass before and after completely drying the membranes as relative weight gain per gram of the dry sample by utilizing the following relationship.[19,41,45,46]where Wdry and Wwet show the weights of dry and wet membranes, respectively.

The linear swelling ratio (LSR) was determined by soaking the membranes (4 cm × 2 cm) into distilled water for 1 day. It was calculated by utilizing the following relationship.where Ldry and Lwet denote the lengths of dry and wet membranes, respectively.

Hydration number (λ) is the number of water molecules per ion. It was determined by utilizing the following equation.

Alkaline Stability Test

It was investigated by determining the variations in hydroxide conductivity before and after soaking into 1 M NaOH at 60 °C for 30 days.

Hydroxide Conductivity

It was determined by utilizing a four-point probe method using an Autolab PGSTAT 30 (Eco Chemie, the Netherlands) with an AC current amplitude of 0.1 mA in the galvanostatic mode and a frequency range of 1 MHz–100 Hz. Bode plots were utilized to measure the frequency region over which the magnitude of the impedance was constant; the ionic resistance was then obtained from the associated Nyquist plot. For this, the AEM was set into a Teflon cell. Herein, it was in contact with two current collecting electrodes and two potential sensing electrodes. It was fully soaked in distilled water and the impedance spectrum was determined. It was performed rapidly to curtail the potential error resulted from the reaction of OH– with dissolved carbon dioxide in the AEM, which leads to the synthesis of carbonate/bicarbonate anions and decreased AAEM conductivity. It was determined by utilizing the following equation.where R represents the membrane resistance, L shows the distance between potential sensing electrodes, and w and d are the width and thickness of the membrane, respectively.

Results and Discussion

Synthesis of BPPO

BPPO was attained by bromination of the methyl group of PPO. It was successfully done at 135 °C by utilizing AIBN as the initiator and N-bromosuccinimide (NBS) as the bromination agent. The degree of bromination (DB) depends on the concentration of NBS used during the bromination reaction. The 1H NMR spectrum of the attained BPPO is represented in Figure S1. The characteristic benzyl bromide peak was obtained at 4.3 ppm. DB was determined from the integral area ratio between the benzyl bromide group and the unreacted benzyl signal, which is present at 2.1 ppm. The DB of the prepared BPPO was 47%.

Synthesis of AEMs

AEMs including MImPPO, QPPO, MPiPPO, and MPyPPO were successfully prepared by treating BPPO with N-methylimidazole, trimethylamine, N-methylpiperidine, and N-methyl pyrrolidine, respectively. Figure depicts the 1H NMR spectra of the prepared AEMs. In the case of MImPPO, the new peak at 5.2–5.5 ppm displayed the successful synthesis of AEM. For QPPO, the peak at 4.5 ppm represents the −CH2–N bond, which exhibits an effective quaternization reaction. Similarly, the fruitful synthesis of MPiPPO and MPyPPO AEMs was established by the peak at 4.5 ppm.

Figure 2

1H NMR spectra of the prepared AEMs with different quaternary ammonium groups.

FTIR spectroscopy was also employed to prove the successful preparation of AEMs from BPPO and different amines. Figure represents the FTIR spectra of pristine BPPO and prepared AEMs. The band at 750 cm–1 is accompanied by the C–Br stretching vibration in the pristine BPPO membrane.[18] The C–Br band disappeared from the spectrum of the resultant AEMs[18,47] and a peak at 960 cm–1 appeared in the spectrum of all of the prepared membranes after the reaction with amines. It is associated with the C–N stretching vibration in AEMs. Therefore, the synthesis of AEMs was confirmed by FTIR and 1H NMR tests.

Figure 3

FTIR spectra of the pristine BPPO and prepared AEMs with different quaternary ammonium groups.

Thermal and Mechanical Stability

Under an alkaline environment, the AEMFC system works at a medium temperature, which in turn requires the AEMs to show suitable thermal stability. The thermal degradation of the prepared AEMs was investigated via thermogravimetric analysis (TGA). Figure represents the TGA thermograms of pristine BPPO and the prepared AEMs, which were measured under a nitrogen atmosphere at a temperature range of 25–800 °C. The weight loss occurred in three consecutive steps. The first weight loss was detected below 130 °C probably due to evaporation of the residual solvent and adsorbed water from the prepared membranes. The deterioration of quaternary ammonium groups into the polymer architecture at around 230 °C leads to the second weight loss stage. The final weight loss due to polymer architecture degradation at around 450 °C corresponds to the final stage.

Figure 4

TGA thermograms of pristine BPPO and the prepared AEMs with different quaternary ammonium groups.

The mechanical properties of the prepared anion exchange membranes were investigated in the wet state through dynamic mechanical analysis (DMA) at ambient temperature and the obtained results are given in Table . They exhibited excellent mechanical stability with a TS of 15.52–25.13 MPa and an Eb of 20.90–25.45%. In the case of the membrane MImPPO, the structure of the membrane becomes rigid after the incorporation of N-methylimidazole into the polymer matrix, which can be easily noticed from the reported value of Eb (Table ). The N-methylimidazole-based anion exchange membrane (MImPPO) exhibited a lower water uptake of 29.7% compared to the other studied anion exchange membranes. The anion exchange membrane with lower water uptake showed lower flexibility. Therefore, the polymer chain of MImPPO becomes highly stiff and its flexibility was lower than the other prepared membranes. On the other hand, the N-methylpyrrolidinium-based anion exchange membrane (MPyPPO) showed a higher water uptake of 75% compared to the other studied membranes. Due to this higher water uptake, it showed higher flexibility. Hence, the polymer chain becomes much flexible for MPyPPO, which corresponds to its higher value of Eb compared to the other prepared anion exchange membranes, as shown in Table .

Table 1

Composition and Mechanical Stability of the Prepared AEMs with Different Quaternary Ammonium Groups

samples	BPPO (g)	amine (g)	IECT (mmol g–1)	TS (MPa)	Eb (%)
MImPPO	0.8	0.11	2.0	25.13	20.90
QPPO	0.8	0.16	2.0	22.62	21.0
MPiPPO	0.8	0.17	2.0	18.21	24.81
MPyPPO	0.8	0.20	2.0	15.52	25.45

Morphology

The morphological features of surfaces and cross sections of the prepared AEMs with different quaternary ammonium groups were investigated in detail and the attained results are represented in Figure . The prepared AEMs were free from pores, holes, or cracks. Besides, the prepared anion exchange membranes exhibited a uniform, compact, and homogeneous morphology. Therefore, they exhibited a homogeneous morphology, which is extremely required for their applications into AEMFCs.

Figure 5

SEM micrograph of the surface (right) and cross section (left) of the prepared AEMs with different quaternary ammonium groups.

Ion Exchange Capacity

Table represents the IEC of the prepared AEMs. In this work, the IEC of QPPO is found to be the highest compared to other prepared AEMs. Among the other considered amines, trimethylamine has the smallest size. Consequently, the moiety has the largest nucleophilicity, which in turn leads to the higher IEC of QPPO than other prepared membranes. On the one hand, the membrane MPiPPO exhibited lower IEC because of the larger size of N-methylpiperidine among the series of amines investigated. The IEC of the prepared AEMs follows the order QPPO > MImPPO > MPyPPO > MPiPPO.

Table 2

Determined Ion Exchange Capacity, Water Uptake, Linear Swelling Ratio, and Hydration Number of the Prepared AEMs with Different Quaternary Ammonium Groups

		WR (%)		LSR (%)
samples	IEC (mmol g–1)	25 °C	60 °C	25 °C	60 °C	λ at 25 °C
MImPPO	1.77	29.7	32.4	7.8	9.2	9.32
QPPO	1.81	32.1	39.3	9.3	11.6	9.86
MPiPPO	1.68	54.3	57.2	12.3	15.1	18.0
MPyPPO	1.73	75.0	92.9	20.8	23.9	24.1

Water Uptake and Linear Swelling Ratio

The water uptake of the prepared AEMs was determined in the hydroxide ion (OH–) form. Table represents the water uptake of the prepared AEMs at 25 and 60 °C. The prepared pyrrolidinium-based anion exchange membrane (MPyPPO) showed the highest water uptake (75% at 25 °C), whereas the prepared imidazolium-based anion exchange membrane (MImPPO) exhibited the lowest water uptake (29.7% at 25 °C) among the prepared anion exchange membranes. It can be elucidated by the variance in the basicity of the attached group. Among the attached groups explored in this work, the highest basicity corresponds to N-methyl pyrrolidine (pKa = 10.32) and the lowest basicity corresponds to N-methylimidazole (pKa = 6.95), which is parallel to the obtained results where the highest water uptake was found for MPyPPO, whereas the lowest water uptake for MImPPO. Therefore, the water uptake of the prepared anion exchange membranes followed the order MPyPPO > MPiPPO > QPPO > MImPPO. Water uptake was enhanced with the increase in temperature (Table ). At 60 °C, the prepared AEMs MImPPO, QPPO, MPiPPO, and MPyPPO exhibited water uptakes of 32.4, 39.3, 57.2, and 92.9%, respectively. This enhancement in water uptake was associated with the increase of free volume and the rapid mobility of OH–.[48]

The linear swelling ratio (LSR) was also investigated in the hydroxide ion (OH–) form. Table denotes the LSR of the prepared AEMs at 25 and 60 °C. The LSR agreed with the water uptake at both temperatures revealed (Table ). The MPyPPO exhibits a higher LSR, whereas MImPPO exhibits a lower LSR. Therefore, the prepared AEMs showed excellent dimensional stability, which is essential for their applications in AEMFCs.

Hydroxide Conductivity

It was determined in distilled water with a four-point probe method with a temperature range of 20–80 °C. Figure shows the attained results. For the prepared AEMs, ionic conductivity and temperature were correlated linearly. It was higher than 10 mS cm–1 (Figure ) at 20 °C of the prepared AEMs, which increased with temperature because of the increase in the free volume and accelerated mobility of OH– with temperature.[28,49] With the increase in temperature from 20 to 80 °C, the ionic conductivities of the prepared membranes MImPPO, QPPO, MPiPPO, and MPyPPO also increased from 14 to 34, 23 to 47, 27 to 67, and 34 to 84 mS cm–1, respectively. It has been observed that MPyPPO exhibited higher OH– conductivity (34 mS cm–1 at 20 °C), whereas MImPPO showed lower OH– conductivity (14 mS cm–1 at 20 °C) among the prepared membranes, which can be explained on the basis of the difference in the water uptake and basicity of the prepared AEMs. N-Methyl pyrrolidine has a pKa value of 10.32,[50] which suggests that the basicity of the pyrrolidinium-based membrane (MPyPPO) is the highest among the prepared AEMs. Higher basicity resulted in higher ionic conductivity. Immense water uptake makes every OH– hydrated by more water molecules and supported ion migration, which is identical to the case of the proton exchange membrane (PEM).[48] The binding between the pyrrolidinium cation and the hydroxide ion (OH–) was weaker, which results in the easy dissociation of free OH–. Therefore, the ionic conductivity of MPyPPO is the highest compared to the other prepared membranes. On the other hand, the OH– conductivity of MImPPO is the lowest among the explored membranes because of the lowest basicity of N-methylimidazole (pKa = 6.95) than the other investigated amines. Moreover, the imidazolium-based anion exchange membrane (MImPPO) was degraded during the process of converting Br– into OH– by soaking in a 1 M NaOH solution for 2 days to some amount because of the lower basicity of N-methylimidazole (pKa = 6.95), which leads to lower ionic conductivity.

Figure 6

Hydroxide conductivity of the prepared AEMs as a function of temperature.

Alkaline Stability Test

It was probed by soaking the prepared AEMs into 1 M NaOH for 30 days at 60 °C. The change in OH– conductivity was calculated at 30 °C. Figure depicts the alkaline stability of the prepared anion exchange membrane bearing different quaternary ammonium groups. The attained results showed that OH– conductivity decreased from the original one. The OH– conductivities of MImPPO, QPPO, MPiPPO, and MPyPPO decreased to 59, 52, 44, and 42%, respectively. Therefore, MImPPO showed the lowest alkaline stability among the prepared anion exchange membranes due to the lowest basicity of N-methylimidazole (pKa = 6.95). On the contrary, MPyPPO represents higher alkaline stability among the prepared AEMs due to the higher basicity of N-methyl pyrrolidine (pKa = 10.32). Moreover, the pyrrolidinium cation exhibits excellent electrochemical stability because of its nonaromatic character.[50] The alkaline stability of the prepared anion exchange membranes followed the order MPyPPO > MPiPPO > QPPO > MImPPO.

Figure 7

Hydroxide conductivity of the pristine and alkaline-treated membranes.

Conclusions

In summary, homogeneous AEMs containing different quaternary ammonium were developed. The synthesis of AEMs was proved by FTIR and 1H NMR tests. They showed higher thermal, chemical, and mechanical stabilities. Moreover, they showed a homogeneous morphology. Among the prepared AEMs, MPyPPO exhibited a higher hydroxide conductivity of 34.5 mS cm–1 at 20 °C and 84 mS cm–1 at 80 °C. Moreover, the prepared anion exchange membrane MPyPPO also showed higher alkaline stability among the prepared AEMs. Therefore, the pyrrolidinium group proves to be a promising quaternary ammonium group to attain highly conductive and alkaline stable BPPO-based AEMs.