Polyetherimide based anion exchange membranes for alkaline fuel cell: Better ion transport properties and stability

Colloids and Surfaces A 588 (2020) 124348

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Polyetherimide based anion exchange membranes for alkaline fuel cell: Better ion transport properties and stability

T

Vikrant Yadava,b, Abhishek Rajputa,c, Prem P. Sharmaa,b, Prafulla K Jhad, Vaibhav Kulshresthaa,b,c,* a

CSIR- Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Gijubhai Badheka Marg, Bhavnagar, 364002, Gujarat, India Academy of Scientific and Innovative Research, Ghaziabad, 201002, India c Department of Physics, The MK Bhavnagar University, Bhavnagar, 364002, Gujarat, India d Department of Physics, The M S University of Baroda, Vadodara, Gujarat, India b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyetherimide Alkaline anion exchange membranes Ionic conductivity Mechanical stability

Alkaline anion exchange membrane (AAEM) is the key component for alkaline exchange membrane fuel cells (AEMFCs). AAEMs with good ionic conductivity, excellent thermo-mechanical and alkaline stability were synthesized by chloromethylation and quaternization followed by alkalization of polyetherimide. Chloromethylation of polyetherimide, a key step in the synthesis of anion exchange membranes based on polyetherimide, frequently involves cancer-causing chemicals. Here, an approach towards the use of non-carcinogenic reagent has been implemented for the synthesis of AAEMs. Physiochemical properties of prepared membranes are tuned by varying tertiary amines. Derivatives during the membrane synthesis are characterized by proton NMR spectroscopy. FTIR spectroscopy confirms the successful quaternization of polyetherimide, and dense nature of membranes is cross-checked by SEM imaging. Synthesized anion exchange membranes show ionic conductivity in order of 2.68–3.22 × 10−2 S/cm complemented by ion exchange capacity of 1.58–2.07 meq/g. Under strong alkaline conditions 1 M KOH at 60 °C and 5 M KOH at room temperature, membranes are stable without losing their integrity. Based on preliminary studies, it is anticipated that functionalized polyetherimide may be a suitable candidate as an anion exchange membrane in fuel cell applications.

⁎ Corresponding author at: CSIR- Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Gijubhai Badheka Marg, Bhavnagar, 364002, Gujarat, India. E-mail address: [email protected] (V. Kulshrestha).

https://doi.org/10.1016/j.colsurfa.2019.124348 Received 29 October 2019; Received in revised form 14 December 2019; Accepted 15 December 2019 Available online 23 December 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental

Over the past decades, research on synthesis and applications of ion exchange membranes increases rapidly because of energy and water crisis. Ion exchange membranes are key component in fuel cells, redox flow battery, and electrodialysis (ED). A fuel cell is a clean and energyefficient technology. In fuel cell and redox flow battery, ion exchange membranes (IEM) act as solid polymer electrolyte, whereas in electrodialysis ion exchange membranes are responsible for the movement of counter ions [1–4]. Cost and stability are the two main challenges in the preparation of IEMs for electrochemical devices. Most efforts have been directed towards the development of proton exchange membranes (PEMs), which exhibit good chemical, mechanical, and thermal stability, along with high ionic conductivity [5]. There are several disadvantages with the use of PEMs in fuel cell which includes poisoning of expensive non-reusable Pt or Pt-based electrocatalyst with carbon monoxide at low temperature, slow electrode kinetics, high membrane cost, and fuel permeability, which restricts the commercialization of PEM fuel cells [6,7]. Nafion is a widely used proton exchange membrane in the fuel cells having excellent mechanical and chemical stability along with extraordinary proton conductivity. High methanol cross over, temperature stability and cost are the drawbacks of Nafion [8–10]. To overcome the hurdles associated with PEMs, more emphasis is being given to developing alkaline anion exchange membranes with comparable properties [6–11]. The alkaline medium in alkaline fuel cell provides noncorrosive electrolyte which facilitates the use of low cost, non-noble metal as catalyst [12–15]. Low methanol cross over is another advantage of AAEMs in fuel cells. Use of AAEMs in fuel cell reduces the polymer degradation by oxidative radicle mechanism at higher pH [16–18]. During past years development of AAEMs increases significantly based on polysulfone, poly(arylene ether ketone), poly(2, 6-Dimethyl-1, 4-diphenylene oxide), copolymers of vinyl benzyl chloride and styrene, polyphenylene, etc [19–25] These polymer backbones are quaternized by chloromethylation or bromination reactions via radical mechanism followed by replacement reaction (Menshutkin reaction) of halide group with tertiary amines to get quaternary ammonium group, responsible for ionic conduction [21,22]. Chloromethyl methyl ether is commonly used chloromethylating agent, in spite of their high reactivity, cancer-causing, and harmful to human health [26,27]. Bromination reaction on active methyl group of aromatic polymers via radical mechanism is very brisk and leads to the degradation of polymer backbone [28]. Polyetherimide (PEI), a thermoplastic polymer to be in the right place because of their high glass-transition temperature, chemical resistance, thermomechanical properties, and oxidative stability over a wide range of temperature [29–32]. Polyetherimide has an ether (–O–) and isopropylidene (–C(CH3)2–) group in its structure, which not only provides outstanding thermomechanical stabilities and chemical resistance but also makes it an attractive candidate for ion exchange membranes [7,33–35]. Low cost of polyetherimide attracted attention towards itself due to its applicability in a wide variety of applications including membranes for gas separation at room temperature, sensors, host polymer for composites, ion exchange membranes synthesis, polymer blends, etc [36–47]. In the current study chloromethylation of PEI is carried out with cost-effective, eco-friendly, and highly reactive chloromethyl ethyl ether (CMEE) instead of hazardous and carcinogenic chloromethyl methyl ether. The main focus of this study was to investigate the effect of different reaction parameters on chloromethylation of PEI and effect of different tertiary amines on properties of anion exchange membranes. Herein, we demonstrated different quaternizing approaches to improve membrane ionic conductivity and overall water content.

2.1. Materials Polyetherimide, N-methyl morpholine (NMM), N, N-dimethyl hexylamine (DMHA), tin (IV) chloride, chloromethyl ethyl ether were supplied by Sigma Aldrich India. Formaldehyde solution, methanol, and chloroform were obtained from Finar chemicals. Phosphorous trichloride was purchased from Spectrochem Pvt. Ltd. Triethylamine (TEA) was supplied by Loba Chemie. Potassium hydroxide, sodium chloride, and other reagents are commercially available and used as received without further purification. De-ionized water was used throughout the experiment.

2.2. Preparation of Chloromethyl ethyl ether The procedure for the preparation of Chloromethyl ethyl ether has involved mixing of formaldehyde solution and ethanol (3:2 V/V) in a round bottom flask with the help of a magnetic stirrer for an hour. On complete mixing, 33 ml phosphorous trichloride was added to the resultant solution in a dropwise fashion with continuous stirring in an ice bath. The addition of phosphorus trichloride to reaction mixture is a highly exothermic reaction, results in a drastic change in reaction temperature. Stirring continues for 2 h, and then the solution was kept for 30 min, the solution was separated into two layers. The upper solution was the desired product and separated by a separating funnel. The overall equation for the preparation follows: CH2O + C2H5OH + 1/3 PCl3 + H2O → ClCH2OC2H5 + 1/3 H3PO3 (1)

2.3. Chloromethylation of polyetherimide and membrane synthesis Polyetherimide was chloromethylated by synthesized chloromethyl ethyl ether. In a typical synthesis, a two neck round bottom flask was charged with 5 g polyetherimide and 200 ml of chloroform, on complete mixing 2 wt. % anhydrous tin chloride (SnCl4) in relation to reactants was added carefully to a homogenous solution dropwise. Next, freshly prepared CMME was added slowly to the above solution, and the mixture was refluxed for a predetermined time (Table S–1). The resulting solution was precipitated in methanol and soaked in methanol for 12 h, filtered and washed several times with methanol. The resulting chloromethylated polyetherimide (CMPEI) was dried under vacuum for 24 h and stored in a vacuum for further use. Alkaline anion exchange membranes synthesis involves two phases: (i) chloromethylation of an active phenyl ring of PEI and (ii) successive quaternization by the Menshutkin reaction between halomethyl group and tertiary amine followed by alkalization, via simple solution casting method. The schematic process for preparation of the quaternized membrane is depicted in Scheme 1. Three different quaternization approaches were performed to get quarternized PEI using N, N-dimethyl hexylamine, triethylamine, and N-methyl morpholine. In brief purified CMPEI was dissolved in N, N-dimethyl acetamide, after getting a homogenous solution, tertiary amine was added dropwise to mixture and stirred for 8 h at room temperature, and the resultant solution was cast on a glass plate using doctor’s blade followed by drying in vacuum for 24 h at 50 °C. Quaternized polyetherimide membranes were designated as QPEI-HN, QPEI-EN and QPEI-MN based on tertiary amine N, N-dimethyl hexylamine, triethylamine, and N-methyl morpholine respectively. Quaternized polyetherimide (QPEI) membranes were washed with distilled water several times and soaked into 1.0 M KOH solution for 24 h, to convert all the charged site into OH−. Before use alkalitreated membranes were washed with DI water several times to remove unreacted alkali. 2

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Scheme 1. Chemical structure of polyetherimide (PEI) based alkaline anion exchange membranes.

chloromethyl ethyl ether. Chloromethylation of polyetherimide is the key step in the synthesis of polyetherimide based AAEMs. Chloromethylated polyetherimide undergoes informal chemical modifications because of the presence of highly reactive chloromethyl group tethered on the polymer backbone. The number of chloromethyl group present per unit repeating chain of polymer determines the degree of quaternization, which in turn ultimately determines the fate of ionic conductivity and mechanical stability of prepared anion exchange membranes. Chloromethylation of polymer backbone occurs via electrophilic attack of highly reactive chloromethyl ethyl ether on phenyl ring, present adjacent to isopropyl group, instead of those which are adjacent to imide group, because of electron-withdrawing nature of later one. Chloromethylation could easily cause gel formation during reaction, leading to the lower yield of chloromethylated polymer. It is perceived that cross-linking takes place via attack of active phenyl ring on chloromethylated group of another polymer chain in Friedel-crafts alkylation manner, resulting in a crosslinked inter or intra-polymer architecture [48]. Several parameters affect the successful chloromethylation. To mitigate such kind of

2.4. Physiochemical, structural characterizations and stability The membrane samples have been characterized by means of chemical and structural properties. Thermal and mechanical stabilities of the membrane samples are evaluated by the TGA and UTM, respectively. Water Uptake behavior of membranes is determined by recording the weight gain after equilibrating in water for 24 h. The ion exchange capacity (IEC) of membranes was estimated by the classical Mohr’s method. Ion conductivity of the membranes was measured on potentiostat/galvanostat (CH 608E). Details of the experiments are given in the Electronic Supplementary Information section. 3. Results and discussion 1

H NMR spectra of prepared chloromethyl ethyl ether and commercially available chloromethyl ethyl ether is presented in Fig. S–1. Chemical shift values at δ = 1.26 ppm (t), δ = 3.76 ppm (q) and δ = 5.5 ppm (s) are found correspond to CH3, CH2 and CH2Cl groups, respectively and well-matched with commercially available 3

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Triethylamine quaternized CMPEI shows chemical shift value 4.51 ppm associated with −CH2-N+-, 2.38 and 1.09 ppm assigned to CH2 and CH3 group of triethylamine respectively. N-methyl morpholine quaternized CMPEI membrane having a chemical shift value of 4.52 ppm arises as a result of protons associated with a carbon near to quaternized nitrogen and 3.12 ppm arises from CH2 group of morpholine near to highly electronegative oxygen atom. 1H NMR spectra of different quaternized membrane show disappearance of a chemical shift value 4.62 ppm, and new broad chemical shifts around 4.52 ppm proves the successful quaternization of CMPEI. FT-IR spectra of PEI, CMPEI, and QPEI membranes is presented in Fig. 3. From spectra of PEI characteristic peaks of imide group absorption at 1780 and 1720 cm−1 associated to asymmetric and symmetric stretching of imide carbonyl group, 1356 and 745 cm−1 are of CeN stretching and bending, 1605 cm−1 assigned as aromatic carbon C]C and absorption peak at 1233 was due to aromatic ether (CeOeC) [32]. New absorption peaks at 780 and 1475 cm−1 are replanted to stretching vibration of C-Cl bond and scissoring vibration of CH2Cl group in chloromethylated polyetherimide (CMPEI), confirms the successful Chloromethylation of PEI [49]. The new absorption peak at 1117 cm−1 in quaternized PEI membranes is due to the stretching vibration of CeN bond. Also, the disappearance of the absorption peak at 780 cm−1 confirms the transformation of the chloromethyl group to the quaternary ammonium group [50]. IR imaging was also performed to confirm the uniform distribution of functional groups within the membrane matrix and shown in Fig. S–2. Surface morphology and cross-sectional views of prepared membranes are studied through SEM analysis and presented in Fig. S3. Micro pores, holes, and cracks are not observed in the cross-section image, which in turn suggests the homogenous nature of prepared membranes as well as good compatibility of tertiary amines with CMPEI. Quaternary ammonium group in polymer matrix widely distributed and makes membrane dense and is suitable for electrochemical applications. From digital photographs of prepared membranes, it is concluded that prepared membranes are flexible and transparent. The variation in total nitrogen content in quaternized membranes in relation to PEI was analyzed by elemental analysis in CHNS analyzer taking sulfanilamide as a reference standard and presented in Table 1, indicate the increment in nitrogen content in quaternized membranes compared with polyetherimide membrane. Suitability of a membrane for any electrochemical processes can be accessed in terms of their ionic conductivity, water retention capacity, and amount of functional group present. The water content of the membrane has an insightful effect on its ion transport parameters and dimensional stability. IEC is an indication of the density of fixed ionic sites in the membrane thus provide an assessment of ionic conductivity. The ion exchange capacity of synthesized membranes QPEI-HN, QPEIEN, and QPEI-MN are very close to Neosepta® AMX and Neosepta ACM and found to be 1.58, 1.95 and 2.07 meq/g respectively, whereas for AMX and ACM it is 1.4–1.7 and 1.5 meq/g, respectively [50,51]. Water uptake has an intense effect on the ion exchange membrane for the migration of ions during electrochemical processes. Membrane with higher ion exchange capacity should have higher water uptake and vise-versa, while higher water uptake reduces the stability of membrane [52]. Water uptake for synthesized membranes QPEI-HN, QPEI-EN, and QPEI-MN is also correlated with AMX and ACM are found to be higher, helpful for ion transport (Table 2). Two types of water present in IEMs: bound water and free water. Bound water is more responsible for the transport of ions, calculated by weight loss (%) by TGA analysis between 100–150 °C. Free water is the difference of total water uptake to bound water and occupies voids present in the polymer membrane matrix [53,54]. More bound water in membrane results in higher water retention ability. Dimensional changes directly related to water content as well as mechanical stability of membranes, lesser will be the dimensional changes, higher will be mechanical stability and vice versa. Dimensional changes calculated for synthesized membranes presented

alkylation reaction, we studied the effect of reaction parameters viz: concentration of chloromethylating reagent, reaction temperature, and reaction time. 1 H NMR spectroscopy is used as an investigating tool for the determination of tethered chloromethyl group, and gel formation is determined by the gravitational method. Number of tethered chloromethyl group obtained by integrating peak area of chloromethyl group present on phenyl ring and isopropyl group and presented in Table S–1, using the following formula:

− CH2 Cl tethered =

3 × ACH2 Cl AC (CH3)2

(2)

Where ACH2Cl is peak area of CH2Cl protons with a chemical shift of 4.4–4.6 ppm and AC(CH3)2 corresponds to peak area of C(CH3)2 protons at 1.6–1.8 ppm in 1H NMR spectrum. The effect of chloromethylating reagent “chloromethyl ethyl ether” on reaction is studied at constant reaction temperature and time of 70 °C for 60 min, respectively. From Table S–1, it is observed that with an increase in the amount of CMEE (wt. % concerning PEI weight), number of tethered chloromethyl groups increases. 1H NMR spectra of chloromethylated PEI obtained by treatment with different concentrations of chloromethyl ether was presented in Fig. 1(A). The number of tethered chloromethyl groups varies from 0.17 to 1.14 per repeating unit with varying CMEE amounts from 0.5 wt. % to 3.0 wt. % with PEI amount. Fig. 1 (A), clearly shows that peak intensity significantly increases for protons corresponds to chloromethyl group. We can observe from the structure of polyetherimide that two activated phenyl rings can be chloromethylated. Temperature can also play an important role in a reaction and speed up the reaction by giving adequate activation energy or promote other unwanted side reactions, which are not dominant at low temperature. Variation of temperature on the reaction was studied from 30 °C to 90 °C for a fix time of 60 min with PEI to CMEE ratio 1:3. Fig. 1(B) and Table S–1, evident that the higher temperature promotes the chloromethylation reaction and the number of tethered −CH2Cl, increased from 0.16 to 1.53 from 30 °C to 90 °C. Chloromethylation of PEI requires temperature to trigger although catalyst is present in reaction media. From the results, it is clear that 90 °C is the optimum temperature for PEI to get the highest yield with a plenty number of highly reactive CH2Cl group. The effect of time is also studied on chloromethylation of PEI at reaction temperature 90 °C with PEI to CMEE ratio, 1:3. From Fig. 1(C) and Table S–1, it is perceived that longer reaction time results in a higher number of chloromethyl group but as we increase the reaction time from 120 min to 150 min, the number of chloromethyl group increased per repeating unit combined with 12.4 % gel formation, which reduces the over yield of chloromethylated product. The CMEE to PEI ratio, reaction temperature and reaction time of 1:3, 90 °C and 120 min, respectively are the most fragile parameter to obtain maximum −CH2Cl group per repeating unit of the polyetherimide. It is concluded that elevated temperature, longer reaction time, and high PEI/CMEE ratio are favorable for chloromethylation reaction. Structural information of PEI, CMPEI, and QPEI was derived from 1 H NMR spectra and presented in Fig. 2. 1H NMR analysis designated that chloromethylation of parent polyetherimide occurs at the activated position of the phenyl ring in a repeating unit. PEI shows chemical shift values lying in the range of 7.0–7.8 ppm are due to multi hydrogens of the phenyl group and 1.74 ppm associated with CH3 group. Additionally, a chemical shift value of 4.58 ppm associated with −CH2Cl group is found in chloromethylated PEI that reveals successful Chloromethylation of PEI.7 CMPEI quaternized with N, N-dimethyl hexylamine with a significant and broad chemical shift value of 4.53 ppm associated with protons near to quaternized nitrogen (−CH2-N+-). Chemical shift values at 0.9–1.3, 2.39 and 2.21 ppm are assigned for multi hydrogen on long alkyl chain of N, N-dimethyl hexylamine. 4

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Fig. 1. A). Effect of concentration on chloromethylation of PEI at constant reaction temperature and time. The inset shows the chemical shift between 4.2–5 ppm. B). Effect of reaction temperature on chloromethylation of PEI at constant CMEE concentration and reaction time. C). Effect of reaction time on chloromethylation of PEI at constant CMEE concentration and reaction temperature.

water content. Ionic conductivity of synthesized membranes in hydroxide form was evaluated in water and presented in Table 2 Higher water content results in the formation of a hydrogen bond with hydroxyl ions and these water molecules serve as a vehicle for their conduction across membrane. Ion exchange membranes have two states of water: bound water and free water. Out of two, bound water mainly responsible for ionic conductivity. It is noticeable from Table 2, QPEIMN with highest bound water exhibits highest ionic conductivity among all. The ionic conductivity for QPEI-MN is 3.22 × 10−2 S/cm that is 5 % and 20 % higher than 3.07 × 10-2 S/cm and 2.68 × 10-2 S/ cm exhibited by QPEI-EN and QPEI-HN, respectively. Temperature also

in Table 2. Hydration number is responsible for the formation of ionic clusters and significantly increases with an increase in fixed charge density. Ion exchange capacity in combination with water uptake used for the estimation of surface charge density of membrane (χfix). Surface charge density has an intense effect on IEC, with the increase in charge density, IEC value for the membrane significantly increases, as shown in Table 2. Higher water uptake, IEC, and higher number of water molecules per unit ionic site are observed for QPEI-MN among all three. Ionic conductivity of synthesized membranes was evaluated by using a Nyquist plot, as shown in Fig. S–4. Ionic conductivity of the membrane directly proportional to fixed charge concentration and total 5

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Fig. 2. 1H NMR of Polyetherimide, chloromethylated polyetherimide, and quaternized polyetherimide membranes.

Fig. 4. Arrhenius plot of different quaternized PEI membranes. Table 3 Comparison of ion exchange capacity (IEC), ionic conductivity (ɸ) and water uptake (Wt) of synthesized membranes with literature. Membrane

IEC (meq/g)

Φ ×10−2 (S/cm)

Wt (%)

Reference

QPIENPC PAEK-API 2.0 PVA:[DimL][OH] = 1:25 5 % IL-TnT/QAPSU PEAK-MmOH-25 PPO-ASU 30 PPO-DMP 30 H-PBFB-QA-26 PV/CS-HDT-20 wt% PV-HDT/CS-TPTZ20 wt% QPEI-HN QPEI-EN QPEI-MN

1.28 1.43 1.35 1.93 1.16 1.60 1.65 1.34 0.70 0.60 1.58 1.92 2.07

0.34 – 1.48 2.08 – 3.01 1.99 2.30 0.46 0.39 2.68 3.07 3.22

27.21 36.80 – 15.97 44.10 72.00 57.10 25.10 – – 36.95 40.98 42.84

[56] [57] [58] [59] [60] [61] [61] [62] [63] [63] Present study Present study Present study

Fig. 3. FTIR-ATR spectra of polyetherimide, chloromethylated polyetherimide, and different quaternized polyetherimide membranes.

increment in ionic conductivity is observed [14]. The highest ionic conductivity achieved for QPEI-MN is 11.2 × at 90 OC that is three-fold greater than 3.22 × 10-2 S/cm obtained at 30 °C. The activation energy (Ea) for ion transport through AAEMs based on the Grotthuss mechanism [55] is also calculated, and the corresponding Arrhenius type plot was presented in Fig. 4. The calculated Ea for membranes varies from 7.64 to 11.39 kJ mol-1 and presented in Table 2, confirm the role of temperature in ion conduction. Ion transport parameters (hydroxyl ion conductivity, ion exchange capacity) and water associated properties (water uptake) of synthesized membranes are compared with reported data and presented in Table 3. Synthesized membranes show strong competitiveness with previously reported works, and evaluations make them a suitable candidate for fuel cell [56–63]. Thermogravimetric (TG) analysis and DTGA thermographs for different membranes along with chloromethylated polyetherimide are presented in Fig. 5. For chloromethylated polyetherimide three

Table 1 CHN analysis of PEI and prepared membranes. Membrane

C%

N%

H%

PEI QPEI-HN QPEI-EN QPEI-MN Sulfanilamide

70.01 68.52 65.32 67.82 41.85

3.81 5.03 5.27 5.59 16.26

4.61 4.75 5.48 4.74 4.68

plays an important role in ion conduction, as the temperature increases, the ionic conductivity for synthesized membranes increases remarkably (Fig. S–5). It is evidenced that, as temperature increases, the free volume of the membrane increases thus ion migrating channels become dilated, which significantly promotes the overall movement of ions and

Table 2 Ion exchange capacity (IEC), ionic conductivity (Φ), activation energy for ion conduction (Ea), fixed charge density (χfix), water uptake (Wt), bound water (Wb), free water (Wf), hydration number (λ) and linear expansion ratio (LER) for prepared membranes. Membrane

IEC (meq/g)

Φ ×10−2 (S/cm)

Ea (kJ/ mol)

χfix (mmol/dm3)

Wt (%)

Wb (%)

Wf (%)

λ

LER (%)

QPEI-HN QPEI-EN QPEI-MN Neosepta®AMX Neosepta ACM

1.58 1.92 2.07 1.4–1.7 1.50

2.68 3.07 3.22 – –

11.39 9.47 7.64 – –

0.90 0.99 1.12 – –

36.95 40.98 42.84 25-30 15.00

0.26 0.34 0.44 – –

36.69 40.64 42.40 – –

10.67 11.80 12.45 – –

30.95 21.24 19.46 – –

6

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Fig. 5. Thermal analysis of chloromethylated and quaternized polyetherimide membranes (inset showing DTGA for the same).

Fig. 7. Alkaline stability evaluation of synthesized membranes in 1 M KOH at 60 °C.

significant weight loss traces are perceived: (1) weight loss of 15.97 % between 170–220 °C is due to HCl produced from crosslinking of chloromethyl group with aromatic rings, [64] (2) weight loss of 18.49 % in between 500–530 °C was due to partial degradation of polymer backbone and (3) final a weight loss of 63 % in between 620–660 °C due to degradation of polymer chain. While QPEI membranes show a fourstep degradation process, first is due to the removal of absorbed water around 100 °C, the second one in between 220–240 °C is probably due to removal quaternary ammonium group [65]. Third weight loss in between 500–520 °C attributed to partial degradation of polymer chain while the fourth one weight loss of nearly 74 % at a temperature range of 620–640 °C was due to degradation of the polymer chain. Mechanical properties of polymers are indicated by the elastic modulus, tensile strength, and flexibility to tolerate external forces. Figs. 6, and S–6 represents the nominal mechanical stability parameters for the membrane samples in dry and wet conditions, respectively. The elastic modulus of membranes was presented in Fig. S–7 and measured from the linear part where the membrane shows elastic behavior. Elastic modulus value for QPEI-HN, QPEI-EN, and QPEI-MN were found to be 6.51, 8.20, and 11.50 MPa with dry membrane samples and 11.35, 13.13 and 17.40 MPa with fully hydrated membrane samples, respectively. It is observed that ductility is higher in wet conditions, and tensile strength is more in the case of dry samples. The presence of water in the membranes shows the plasticizing nature and enhances the elasticity in the membranes. These results show that the synthesized membranes are stable mechanically as well as thermally [66]. Degradation process in AAEMs under strong alkaline conditions occur via cationic moiety degradation or polymer backbone

disintegration. To explore the reliability of synthesized membrane samples against fuel cell, we surveyed the stability of synthesized membranes in 1 M KOH alkaline solution at 60 °C for a duration of 240 h. The change in hydroxide conductivity of membrane samples as a function of time is presented in Fig. 7. It is clear from the figure that hydroxide conductivity slightly fluctuated with immersion time and holds nearly the same value when time reaches up to 240 h, this slight decrease in conductivity of samples attributed by loss of quaternary ammonium head from polyetherimide backbone. Remarkably, all the membrane samples retain their integrity and mechanical strength, demonstrating the absence of polymer chain disintegration. Moreover, all the membranes experienced a minor loss in weight and ion exchange capacity (Table 4 and Fig. S–8) over the testing period. To further verify the alkaline stability of prepared membranes, membrane samples were subjected to higher alkali concentration (5 M KOH) at 30 °C for 48 and fluctuations in hydroxide conductivity, IEC and weight were examined over the test period. Table 4 and Fig. S–9 infers that under strong alkaline conditions a slight decrease in weight and IEC were experienced by membrane samples and reduction in weight and IEC of membranes found in order of 1.6–3.3% and 4.68–5.69 %, respectively. Furthermore reduction in conductivity at higher concentrations is due to degradation of the quaternary ammonium group. A widely quoted concern is the displacement of the quaternary ammonium group by hydroxyl ion via nucleophilic displacement reaction. A significant loss in fixed conducting ammonium group will cause a strong reduction in OH ̶ ions in the membrane subsequently lowers ionic conductivity. Fig. 8, indicates a loss in hydroxide conductivity of membrane samples is less than 6 % after treatment with 5 M KOH for 48 h. The above results reveal that corresponding membranes are stable under strong alkaline environment and suitable for the fuel cell. The synthesized membranes under hydrolytic conditions (100 °C for 24 h) retained their transparency, flexibility, and toughness. Especially, membrane samples were not dissolved in hot boiling water and showed a slight weight loss in between 2.6–3.15%. Data reveals that Table 4 Alkaline stability of prepared membranes samples. Membrane

%Weight lossa

% Weight lossb

QPEI-HN QPEI-EN QPEI-MN

6.1 5.9 6.4

3.3 1.6 2.4

a weight loss (%) of membrane samples after 240 h immersion in 1 M KOH at 60 °C. b weight loss (%) of membrane samples after 48 h immersion in 5 M KOH at 30 °C.

Fig. 6. The mechanical property of quaternized PEI membrane samples in dry condition. 7

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Kulshrestha, WHITE GRAPHENE based composite proton exchange membrane: improved durability and proton conductivity, International J. Hydrogen Energy 43 (2018) 21683–21689. [5] A.K. Mandal, S. Bisoi, S. Banerjee, Effect of Phosphaphenanthrene Skeleton in sulfonated polyimides for proton exchange membrane application, ACS Applied Polymer Materials 1 (2019) 893–905. [6] M. Osinska-Broniarz, D. Waszak, M. Kopczyk, Ion exchange membrane for low temperature fuel cells, Chemik 67 (2013) 793–800. [7] G. Wang, Y. Weng, D. Xie, D. Chu, R. Chen, Preparation of alkaline exchange membrane based on functional poly (ether imide) polymers for potential fuel cell applications, J. Memb. Sci. 326 (2009) 4–8. [8] G. Rambabu, S. Sasikala, S.D. Bhat, Nanocomposite membrane of sulfonated poly (pthlalizinone ether ketone)-Sulfonated-Graphite nanofibers as electrolyte for direct methanol fuel cell, RSC Adv. 6 (2016) 107507–107518. [9] H. Park, Y. Kim, W.H. Hong, Y.S. Choi, H. 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Fig. 8. Hydroxide conductivity of test samples before and after alkali stability test in 5 M KOH for 48 h at 30 °C.

membranes are stable under elevated temperatures. 4. Conclusion A simple approach to synthesized AAEMs has been established in the present manuscript. The influence of chloromethyl ethyl ether concentration, reaction time, and temperature on chloromethylation of polyetherimide was scrutinized. The synthesized membranes were highly conductive with an order of 2.68–3.22 × 10−2 S/cm. Moreover, prepared membranes exhibit ion exchange capacity in a range of 1.58–2.07 meq/g with water uptake 36.95–42.84%. All the membranes are stable thermally as well as mechanically. Effect of alkaline concentration and temperature are investigated in terms of loss in ion exchange capacity, ionic conductivity and weight loss. The result confirms the stability and consistency of membranes under strong alkaline conditions and on hydrolytic treatment. Among three, QPEI-MN performs better in terms of ionic conductivity and stability under harsh conditions, might be a much suitable candidate for fuel cell application. Eventually, results are in good agreement with the desired properties of an ideal alkaline exchange membrane, infers the potential of synthesized membranes in anion exchange membrane fuel cell application. Author’s contribution Vikrant Yadav: Synthesis of membranes, their characterization and writing of manuscript Abhishek Rajput: Chemical characterization of Membranes Prem P. Sharma: Synthesis of membranes and their thermal characterization P K Jha: Analysis of results Vaibhav Kulshrestha: Designing of experiments and overall supervision Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Author V. Kulshrestha is thankful to IUC, DAE CSR Indore for providing financial support. Analytical Discipline and Centralized Instrument facility CSMCRI, Bhavnagar is greatly acknowledged for instrumental support. 8

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