Pre-removal of polybenzimidazole anion to improve flexibility of grafted quaternized side chains for high performance anion exchange membranes

Journal of Power Sources 451 (2020) 227813

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Pre-removal of polybenzimidazole anion to improve flexibility of grafted quaternized side chains for high performance anion exchange membranes Xiaozhou Wang a, Wanting Chen a, Xiaoming Yan b, Tiantian Li a, Xuemei Wu a, *, Yang Zhang a, Fan Zhang a, Bo Pang a, Gaohong He a, ** a

State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Panjin, 124221, China

b

H I G H L I G H T S

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

� PBI anion is essential to graft but binds quaternized cation to limit conductivity. � Non-cationic graft pre-removes PBI anion and then quaternization avoids ionic bond. � Flexible cation ended side chains greatly improve ability to microphase separation. � The de-anionic PBI achieves 6-fold con­ ductivity that of conventional grafted PBI. � Power density of H2/O2 cell (806 mW cm 2) is the highest report in PBI based AEMs. A R T I C L E I N F O

A B S T R A C T

Keywords: Anion exchange membranes Polybenzimidazole Conductivity Alkaline stability Fuel cell

Quaternized polybenzimidazole anion exchange membranes exhibit excellent stability, however, always suffer from poor hydroxide conductivity as compared with other polymer categories. In this work, it is found that polybenzimidazole anion transition state, though essential to provide grafting sites, has strong ionic interaction with the conventional quaternary ammonium cations containing grafting reagents. A novel grafting strategy is proposed to pre-remove polybenzimidazole anions by fully grafting of non-cationic ether-containing side chains, and then attaches quaternary ammonium cations to the end of pendent side chains. Further investigations with molecular dynamics simulation, TEM and SAXS indicate that by eliminating cation-anion binding, the flexible pendent quaternized cations exhibit excellent ability to aggregate into ionic clusters and transport hydroxide ions. The trimethylammonium grafting membrane shows conductivity of about 82.4 mS cm 1 at 80 � C, around 6fold that of the conventional grafted membranes. Grafting with alkaline stable piperidinium, 93.2% conductivity retention is achieved even soaking in 2 M KOH at 60 � C for 720 h. The H2/O2 single cell shows high peak power density of about 806.1 mW cm 2 at large current density of about 2025.0 mA cm 2, which is the highest value among the polybenzimidazole membranes reported in recent years.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Wu), [email protected] (G. He). https://doi.org/10.1016/j.jpowsour.2020.227813 Received 8 December 2019; Received in revised form 24 January 2020; Accepted 25 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.

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Journal of Power Sources 451 (2020) 227813

where the quaternized ammonium cation is located (either in polymer backbone or in flexible pendent side chains) [26,27]. According to recent works, Jheng et al. reported that a quaternized PBI AEM having imidazolium moieties in both the backbones and pendent side chains (msQPBI-OH) with an IEC value of about 1.49 mmol g 1 could only achieves a conductivity of about 27.2 mS cm 1 at 80 � C [22]. Xia et al. prepared a quaternized PBI AEM with pendant quaternary ammonium groups (QPBI-3/2) and an IEC value of about 1.68 mmol g 1, which exhibits an even lower conductivity of about 5 mS cm 1 at 80 � C [28]. The poor conductivity greatly restricts practical applications of the quaternized PBI AEMs, however, no specific study has systematically elucidated the reasons and proposed new strategies to make further improvement on conductivity. In the present work, a novel polybenzimidazole anion pre-removal (de-anionic) strategy is proposed to greatly improve conductivity of the quaternized PBI-based AEMs. By analyzing different grafting stra­ tegies, it is found that the quaternary ammonium cations in conven­ tional grafting reagents tend to form ionic bond with the benzimidazole anions in PBI, therefore have poor ability to aggregate into ionic clusters and lead to low conductivity in the conventional grafted PBI AEMs. In the novel de-anionic strategy proposed in this work, firstly, benzimid­ azole anions in PBI are thoroughly removed by the covalently bonding of non-cationic grafting reagents instead of the conventional quaternized grafting reagents. And then, quaternary ammonium cations are attached to the end of the grafted pendent side chains to promote flexible ag­ gregation of cations into ionic clusters. The ionic bond between qua­ ternary ammonium cations and benzimidazole anions can be avoided by the pre-removal of the anionic benzimidazole rings, together with a flexible ether-containing spacer design in the grafting reagent, these strategies greatly enhance abilities to aggregate into ionic clusters and transport hydroxide ions. The as-prepared de-anionic grafted PBI membrane exhibits high conductivity and the highest H2/O2 single cell power density as far as we know among the reported PBI based AEMs.

1. Introduction The increasing public awareness on the pollution caused by fossil fuels results in much attention on environmentally friendly power sources, such as fuel cell [1–3]. Among different fuel cell categories, alkaline anion exchange membrane fuel cell (AAEMFC) shows many advantages, such as fast oxygen reduction kinetics, feasible for non-noble metal catalysts and easy for water management [4,5]. Since the performance of AAEMFC highly depends on its key component of anion exchange membranes (AEMs), fabrication of AEMs with high conductivity and chemical stability has become the main challenge in developing long durable AAEMFCs [6,7]. It is increasingly apparent that chemical structure of polymer back­ bones plays a great role in dictating performance of AEMs [8,9]. Various commercialized polymers have been investigated in literatures, how­ ever, most of them have failed to demonstrate sufficient mechanical strength and chemical stability for AEM application. For instance, polystyrene based AEMs often suffer from poor thermal stability and mechanical strength [10], aryl ether-containing based AEMs, such as polyphenylene oxide (PPO) [11], polyetheretherketone (PEEK) [12], polyethersulfone (PES) [13], polysulfone (PSF) [14] et al. are lack of sufficient stability in alkaline environment because of quaternary car­ bon hydrolysis and ether hydrolysis, which are triggered by the qua­ ternary cationic groups connecting to the benzenes of the polymer backbones, and thus lead to performance collapse. Polybenzimidazole (PBI) has an unique rigid benzimidazole ring in the backbone, which provides not only excellent mechanical strength [15] but also active sites for grafting quaternized side chains to avoid formation of the alkaline unstable benzene-substitution quaternized structure [16], PBI could even be tolerant to concentrated alkaline so­ lution, e.g. 6 M KOH in the alkaline doping PBI AEMs [17,18], The N–H bonds in benzimidazole ring of PBI could partially dissociate proton (PBI pKa ¼ 5.5) [19] to form benzimidazole anion (N ) transition state, which could further reacts with quaternized reagents especially with the participation of alkaline (KOH, NaOH, K2CO3 or NaH et al.) [20] to prepare AEMs [21–25]. However, most of the quaternized PBI-based AEMs reported in literatures suffer from extremely poor conductivity as compared with AEMs based on other polymer backbones, no matter

Scheme 1. Synthesis process of (a) ClONþ; (b1) QPBI-CV1 and (b2) QPBI-CV2; (b3) PBI-Cl 100%, QPBI-DA, ImPBI-DA and PipPBI-DA. 2

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2. Experimental section

reagents and the long reaction time help to remove all the benzimidazole anions on PBI by grafting reaction. During the reaction, the color of solution was turning from reddish-purple into a bright yellow color, which indicated the reaction was complete. The mixture solution was precipitated by deionized water, washed with acetone and deionized water twice, and then dried at 30 � C under vacuum to fabricate the completely chlorinated side chains grafted PBI material PBI-Cl 100%. The PBI-Cl material with a grafted degree of about 90% was also pre­ pared as contrast by the similar method but a shorter reaction time, denoted as PBI-Cl 90%. As to the second step, trimethylammonium (TMA), imidazolium and piperidinium side chain grafted PBI AEMs (QPBI-DA, ImPBI-DA and PipPBI-DA) were prepared to verify the feasibility of the de-anionic strategy. 1 g PBI-Cl 100% material was dissolved in 60 ml DMSO, fol­ lowed by adding 0.5 g trimethylamine solution. After the mixture was sealed and kept stirring at 80 � C for 2 days, a white solid was obtained by pouring the reaction solution into ethyl acetate slowly. After washing with water and acetone for several times and vacuum drying at room temperature (RT) for 24 h, quaternized side chain grafted PBI material (QPBI-DA) was prepared by de-anionic strategy. The QPBI-DA mem­ brane was prepared by the similar method as QPBI-CV1 membrane, using DMSO as casting solvent. ImPBI-DA and PipPBI-DA material were prepared by the reaction of PBI-Cl 100% and 1-methylimidazole or 1methylpiperidine in DMSO solution, respectively. After precipitation by ethyl acetate and purification by water and acetone, the ImPBI-DA and PipPBI-DA membranes were then prepared by the similar method as QPBI-DA membrane.

2.1. Materials Poly(4,40 -diphenylether-5,50 -bibenzimidazole) (denoted as PBI) was purchased from Shanghai Shengjun Plastics Technology Co., Ltd. 1,2-Bis (2-chloroethoxy)ethane (ClOCl), ethanol, acetone, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), ethyl acetate, trimethyl­ amine solution (33 wt% in H2O), 1-methylimidazole (99%), 1-methylpi­ peridine (97%), potassium hydroxide (KOH), anhydrous potassium carbonate (K2CO3) were obtained commercially and used as received without further purification. Deionized water, which had been previ­ ously boiled to remove CO2, was used throughout this study. 2.2. Preparation of the quaternized side chain grafted PBI membrane (QPBI-CV1 and QPBI-CV2) by conventional strategies To explore the reason of the extremely low conductivity of the re­ ported quaternized side chain grafted PBI AEMs, conventional strategies from the literatures were performed, as shown in Scheme 1(a), (b1) and (b2). The synthetic route of the conventional cation grafting reagent, ClONþ, was shown in Scheme 1(a). 5 g 1, 2-bis (2-chloroethoxy)ethane (ClOCl) was dissolved in 50 ml ethanol, followed by 5.24 g trimethyl­ amine solution adding into the mixture dropwise and heating at 78 � C for 24 h. After the evaporation of the ethanol and the residual reagent, the crude product was dried in a vacuum oven at 90 � C to obtain the cation grafting reagents ClONþ dried product [29]. In conventional strategy (1), PBI is first reacted with acid binding agent to form benzimidazole anions, and then directly reacted with cation grafting reagent to prepare QPBI-CV1 [22]. In detail, 1 g PBI (5 mmol N–H) was added into 33 ml DMSO with stirring, after dissolving completely, 5 g K2CO3 powder and 1.22 g ClONþ (5 mmol) was added into the solution and the mixture was heated at 80 � C for 48 h. A white solid was obtained by pouring the reaction solution into ethyl acetate slowly. After washing with water and acetone for several times and vacuum drying at room temperature for 24 h, QPBI-CV1 material was prepared by conventional strategy (1). To prepared QPBI-CV1 mem­ brane, 0.1 g QPBI-CV1 was dissolved in 6 ml DMSO, and then casted onto the glass plate and dried at 60 � C for 3 days. The QPBI-CV1 membrane was soaked in 1 M KOH at room temperature for 2 days to obtain membrane in OH form. The surface residual KOH will be removed completely by washing the membrane with deionized water several times. The QPBI-CV1 membrane was soaked in deionized water for 48 h before characterization. In conventional strategy (2), PBI is directly reacted with the cation grafting reagent without alkaline as acid binding reagent [30,31]. In detail, QPBI-CV2 was prepared by dissolving 1 g PBI (5 mmol N–H) and 1.22 g ClONþ (5 mmol) into 33 ml DMSO without adding alkaline. After the mixture was heated at 80 � C for 48 h, a white solid was obtained by pouring the reaction solution into water slowly. After washing with water and acetone for several times and vacuum drying at RT for 24 h, QPBI-CV2 material was prepared by conventional strategy (2). Then the QPBI-CV2 membrane was prepared by the similar method as QPBI-CV1 membrane, using DMSO as casting solvent.

2.4. Characterization of the membranes The successful synthesis of QPBI-DA, QPBI-CV1 and QPBI-CV2 were confirmed by 1H NMR, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). 1H NMR was tested by Varian Unity Inova 500 spectrometer with DMSO‑d6 as solvent. FTIR was tested by Bruker EQUINOX55 FTIR spectrometer with a resolution of 0.5 cm 1 and a range of 400–4000 cm 1. XPS of membranes were performed on a Physical Electronics PHI5600 with X-ray source oper­ ated at 12 kV and 350 W. All membranes prepared in this work are free standing membrane. Mechanical properties of hydrated membranes were tested by SANS CMT8102 stretching tester with a stretch rate of 10 mm min 1. The morphology of membranes was investigated by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). Membranes were stained with I by immersing in 2.5 M NaI solution for 24 h at 80 � C, then washed several times with deionized water and dried completely [32]. TEM images were taken by a JEOL JEM-2000EX mi­ croscope with an accelerating voltage of 120 kV. Small angle X-ray scattering (SAXS) curves of membranes (hydrated and in OH form) was measured on PANalytical X’Pert Powder X-ray diffraction system with a scattering angle of 0.6–5� and the scattering rate is 1� min 1. The scattering vector was calculated as eqn. (1): q¼

4π sinθ λ

(1)

where λ ¼ 0.154 nm is the scattering wavelength and 2θ is the scattering angle. The Bragg spacing was calculated by eqn. (2):

2.3. Preparation of the quaternized side chain grafted PBI membrane by de-anionic strategy

d¼

As shown in Scheme 1(b3), there are two steps for the de-anionic strategy. First step is the preparation of chlorinated side chains grafted PBI (PBI–Cl). PBI (1 g, 5 mmol N–H protons) was dissolved in 100 ml DMAc, then mixed with the non-cationic grafting reagent, 1, 2-bis (2chloroethoxy)ethane (30 g, 160 mmol), and 5 g K2CO3. The mixture was kept stirring for 48 h at 80 � C. The extreme low PBI concentration help to avoid crosslinking reaction, while the excess non-cationic grafting

2π q

(2)

Ion exchange capacity (IEC) of membranes (in OH form) was measured by back titration. The membranes were dried in vacuum oven at 100 � C for 24 h first, then transfer into 25 mL 0.01 mol L 1 HCl so­ lution for 24 h with stirring. After all the OH in membranes was exchanged by Cl , the quaternary ammonium cations will be replaced by the strong acid to form N–H bond again, and then the solution was 3

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Journal of Power Sources 451 (2020) 227813

Fig. 1. 1H NMR spectra of (a) ClOCl, PBI, PBI-Cl 90%, PBI-Cl 100% and QPBI-DA (b) ClONþ, QPBI-CV1 and QPBI-CV2.

4

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back titrated by 0.01 mol L 1 NaOH with phenolphthalein as indicator. Each sample was tested three times and the testing values were aver­ aged. The IEC of membranes was calculated as eqn. (3): IEC ¼

C1 V1

C 2 V2

Alkaline stability was tested by monitoring the performances and structure variation of membranes in 60 � C, 2 M KOH solution for different time. After washed with deionized water to remove the remained alkaline thoroughly, the conductivity and IEC of membranes were measured at room temperature. 1H NMR was also tested to char­ acterize the change in chemical structure. The single cell performance of membranes was measured. The catalyst ink was created by adding deionized water into a mixture of Pt/ C catalyst (70 wt %) and ionomer (a homemade multi-cation side-chain grafted PBI prepared by the de-anionic strategy in this work) in ethanol with a weight ratio of H2O: Pt/C: ionomer about 625:20:1, followed by ultrasonically treating in ice bath for 1 h. The membrane with a thick of 10 μm was used to fabricate catalyst coated membrane (CCM). The catalyst ink was sprayed on both sides of the membranes with the catalyst loading of 0.5 mg cm 2. Two pieces of hydrophobic carbon paper was assembled on both sides of the catalyst layer to prepare the CCM with the active area of 4 cm2. Single fuel cell performance was tested by the 890e Multi Range fuel cell test station at 80 � C. H2 and O2 were fully humidified and supplied to cell with a flow rate 1 L min 1 with 0.1 MPa backpressure on both sides.

(3)

m

where C1 and V1 is the concentration and the volume of HCl solution, respectively; C2 and V2 is the concentration and the volume of NaOH solution, respectively; m is the dry membranes weight. Water uptake (WU) and swelling ratio (SR) of membranes were also tested. All membranes were first immersing in DI water at different temperature for 12 h, then the fully hydrated membrane was wiped with filter paper to remove surface water. The weight and size of hydrated membrane were measured before and after the membranes were dried at 100 � C under vacuum for 24 h. Each sample was tested three times and the testing values were averaged. The water uptake and swelling ratio were calculated as eqns. (4) and (5), respectively: Water uptake ð%Þ ¼

Wwet

Swelling ratio ð%Þ ¼

Wdry Wdry

Lwet

Ldry Ldry

� 100

(4)

� 100

(5)

2.5. Molecular dynamics (MD) simulation details

where Wwet and Wdry are the weight of fully hydrated membranes and dry membranes respectively; Lwet and Ldry are the average length of fully hydrated membrane and dry membrane, respectively, which can be calculated by Lwet¼ (Lwet1 � Lwet2)1/2, Ldry¼ (Ldry1 � Ldry2)1/2, where Lwet1, Lwet2 and Ldry1, Ldry2 are the length and width of fully hydrated membrane and dry membrane respectively. The number of water molecules per ionic group, hydration number λ, was calculated as eqn. (6): 1000 � WU IEC � 18

λ¼

Molecular dynamics (MD) simulations on QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes were carried out by the Supercomputer Center of Dalian University of Technology. The ionic bonds between benzimid­ azole anions and quaternary ammonium cations in the membrane is investigated, which has great relationship with the poor conductivity of conventional quaternized PBI AEMs. The QPBI-DA membrane (IEC ¼ 1.84 mmol g 1, WU ¼ 84.5%), QPBI-CV1 membrane (IEC ¼ 1.87 mmol g 1, WU ¼ 72.4%) and QPBI-CV2 membrane (IEC ¼ 2.21 mmol g 1, WU ¼ 11.7%) were simulated in the hydrated states at room temperature. The models in this work comprise five polymer chains which contains 10 functionalized repeat units. The models were treated by an annealing procedure to fully equilibrate the amorphous cells [35,36]. The micro­ structures and interactions in membranes are analyzed by radial distri­ bution function (RDF, g(r)) and coordination number (CN). RDF indicates the local probability density of finding B atoms around A atoms at a distance r, while CN is the integration of RDF and implies the number of B atoms around A atoms. RDF and CN within a radius shell are calculated as eqns. (10) and (11), respectively:

(6)

The water holding capacity of membrane was investigated through differential scanning calorimetry (DSC, TA Q20). The fully hydrated membrane sample was sealed in a DSC sample chamber and cooled to 80 � C at a rate of 2 � C min 1, and then heated at the same rate up to 20 � C. The quantity of freezable water Nfreez and non-freezable water Nnonfreez was calculated as eqns. (7) and (8), respectively: Nfreez ¼ Nnon

Mfreez �λ Mtotal

freez

¼λ

Nfreez

(7)

gA B ðrÞ ¼

(8)

Z

where Mtotal is the total mass of water absorbed in membrane, Mfreez is the mass of freezable water. The content of freezable water is defined as the endothermic peak area (Aendo) of the hydrated membrane divided by the melting endothermic heat of fusion of water (334 J g 1) [33,34]. The conductivity of membranes was characterized by a fourelectrode AC impedance analyzer (IviumTechnologies A08001) in AC impedance mode at a frequencies range of 1–105 Hz. The membranes were sandwiched in a homemade four-point cell, followed by com­ pressing and placing in deionized water at different temperatures or in different relative humidity (RH) at 80 � C for measurement. Stable RH environment was obtained by placing the test cell into an environmental chamber with 80 � C and certain RH (from 98% to 40% and stable 1 h for each RH). Each sample was tested three times and the testing values were averaged. The conductivity can be measured by eqn. (9):

σ¼

L RA

nB ðrÞ

ρB 4πr2 Δr

r

CN ¼ 0

ρB 4πr2 gA B ðrÞdr

(10) (11)

where nB(r) is the number of B atoms around an A atom within a distance of r, and ρB is the bulk number density of B atoms [37,38]. 3. Results and discussion 3.1. Chemical structure analysis with different graft strategies 3.1.1. Chemical structure confirm by 1H NMR Different chemical structure of PBI grafted AEMs fabricated by preremoval and conventional grafting strategies are firstly confirmed by 1 H NMR. Fig. 1(a) shows the 1H NMR spectra of ClOCl, PBI, PBI-Cl and QPBI-DA in the polybenzimidazole anion pre-removal strategy proposed in this work. Compared with the spectra of PBI-Cl 90% and PBI-Cl 100%, the characteristic signal corresponding to the proton of N–H in the original PBI (Hd, δ ¼ 13.2 ppm) gradually decreases until no longer being observed, indicating the complete removal of the N–H proton. The signal at 8.28 ppm for proton in benzene (He of PBI) gradually shifts upfield to 8.00 ppm (He’ of PBI-Cl 90% and 100%) indicating the grafted

(9)

where L denotes the distance between two potential electrodes, R de­ notes the membrane impedance and A is the cross-sectional area of the membrane. 5

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Scheme 2. The reaction route and chemical structure of QPBI-CV1, QPBI-CV2 and QPBI-DA.

electron-donating side chain has changed the chemical environment of the benzimidazole and nearby benzene in PBI. The characteristic peaks of the ether containing side chains can also be observed in the spectrum of PBI-Cl, especially the sign of H1 (δ ¼ 3.72 ppm in ClOCl) moving downshift to H1’ (δ ¼ 4.53 ppm in PBI-Cl 90% and 100%), indicating the electron withdrawing effect of benzimidazole ring on the grafted side chain. The grafted degree (GD) of PBI-Cl could be calculated by any way of the following: methylene protons of the side chains adjacent to the nitrogen of the benzimidazole ring: GD ¼ S(H1’)/S(Hf); benzene protons near benzimidazole ring in PBI backbone: GD ¼ S(He’)/S(Hf); residue N–H proton: GD ¼ 1-2S(Hd)/S(Hf), where S represents the integral areas of a certain H peak. Given the spectrum of PBI-Cl 90% and PBI-Cl 100%, all three calculations coincide well. After quaternization of PBI-Cl 100% with trimethylamine, the characteristic signals of trimethylammonium can be distinguished clearly (H700 -H900 at δ ¼ 2.98 ppm). The H5’ (δ ¼ 3.52 ppm) of the side chain shift to H5’’ (3.70 ppm), respectively due to the deshielding effect of the electron-withdrawing quaternary ammo­ nium. The quaternization degree (QD) of QPBI-DA can be calculated by QD ¼ S(H500 )/S(Hf) and gives value of about 81.9%. As comparison, quaternized PBI AEMs are also prepared following the conventional grafting strategies reported in the literature [22,30], denoted as QPBI-CV1 and QPBI-CV2. However, the 1H NMR results indicate different chemical structure as compared with QPBI-DA. As shown in the 1H NMR spectra in Fig. 1(b), quaternized grafting reagent (ClONþ) was firstly synthesized and confirmed by the characteristic peaks of both ClOCl (H1–H6 at δ ¼ 3.56–3.85 ppm) and trimethy­ lammonium (H7–H9 at δ ¼ 3.10 ppm). And then, QPBI-CV1 was syn­ thesized through the reaction between ClONþ and PBI with alkaline as acid binding agent to absorb the by-product HCl. The 1H NMR spectrum of QPBI-CV1 shows some contradictions on covalently grafting degree.

The signals of the quaternized side chain (H2’-H9’) and the partially signal upshift of He (δ ¼ 8.28 ppm) to He’ (δ ¼ 8.00 ppm) and signal downshift of H1 (δ ¼ 3.85 ppm) to H1’ (δ ¼ 4.53 ppm) provide evidence for successful grafting of the ClONþ group onto PBI backbone, and the grafted degree is about 75% calculated by GD ¼ S(H1’)/S(Hf) or GD ¼ S (He’)/S(Hf). However, unlike PBI-Cl 90%, the signal at δ ¼ 13.2 ppm (Hd) is disappeared in the spectrum of QPBI-CV1, indicating totally removal of N–H proton and the grafted degree calculated by GD ¼ 1-2S (Hd)/S(Hf) should be around 100%. There are also contradictions shown in the 1H NMR spectrum of QPBI-CV2, which is fabricated by another conventional grafting strategy without the participation of alkaline as acid binding agent. Although the signal of N–H proton (Hd) completely disappears and all the signal of ClONþ (H100 -H900 at δ ¼ 3.10–3.85 ppm) can be observed in the spectrum of QPBI-CV2, other evidence for covalently grafting is absent, i.e. there are no signal downshift from H100 to H1’ and signal upshift from He to He’, which indicates no grafting reaction occurs at all. Based on the 1H NMR results, different reaction mechanism and corresponding chemical structure of QPBI-CV1, QPBI-CV2 and QPBI-DA are deduced and shown in Scheme 2. As mentioned above, the N–H in benzimidazole ring of PBI could partially dissociate proton (PBI pKa ¼ 5.5) [19] to form benzimidazole anions (N ), which can be promoted by acid binding agent. In the conventional strategies (1) for QPBI-CV1, alkaline (KOH, NaOH, K2CO3 or NaH et al.) as an acid binding reagent could absorb the by-product HCl and promote the covalent grafting of ClONþ with benzimidazole anions N in PBI (as state I), which results in grafted degree of about 75% as shown in Fig. 1(b). But at the same time, some cations in the grafted side chains are inevitably trapped by the ionic bond of benzimidazole anions N (as state II), still some cations in the ClONþ grafting reagent merely have ionic bonds with benzimidazole 6

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Fig. 3. Molecular dynamics simulation of quaternized PBI AEMs, radial dis­ tribution functions (RDFs) and coordination numbers (CNs) of (Nþ N ) in the fully hydrated QPBI-CV1 and QPBI-CV2 membranes, (Nþ OH ) and (Nþ Nþ) in fully hydrated QPBI-DA and QPBI-CV1 membranes.

about 398.60 eV can be observed, indicating totally removal of the benzimidazole anion in QPBI-DA and a grafted degree of about 100% [41]. Oppositely, in the spectrum of QPBI-CV2, the peak of N at about 398.60 eV is observed and there is no peak of graphitic nitrogen appeared at 398.80 eV, indicating all the benzimidazole anions are forming ionic bond with the quaternary ammonium cations of ClONþ. While peaks of both the graphitic nitrogen at about 398.80 eV and the negatively charged nitrogen at about 398.60 eV appear in the spectrum of QPBI-CV1, indicating some benzimidazole anions are covalently grafted with ClONþ, others are forming ionic bond with the quaternary ammonium cations. The peak of pyridinic nitrogen (-N ¼ ) appears at about 400.60 eV, 400.45 eV and 400.35 eV in the QPBI-DA, QPBI-CV1 and QPBI-CV2 spectra, respectively. The gradually decreasing tendency might be caused by the increasing number of negatively charged nitro­ gen and the resonance structure of benzimidazole. As shown in Fig. 2(b), being different from the FTIR spectrum of QPBI-DA, the absorption peaks at about 1120 cm 1 and 1510 cm 1 can still be clearly found in the spectrum of QPBI-CV1, which are corre­ sponding to the out of plane bending and deformation of the ionic bond between the negatively charged nitrogen and the quaternary ammonium cation (N Nþ), respectively [41]. These two absorption peaks can be found even stronger in the spectrum of QPBI-CV2, indicating more ionic binding in QPBI-CV2. Both XPS and FTIR results provide a confirmation of the chemical structure of QPBI-DA, QPBI-CV1 and QPBI-CV2, as well as the existing ionc binding between benzimidazole anions and qua­ ternary ammonium cations in QPBI-CV1 and QPBI-CV2 membranes. Such an ionic binding can be completely removed by the pre-removal of polybenzimidazole anion strategy proposed in this work, which ensures a high degree of covalently grafting of the flexible Nþ ended side chain onto PBI backbones.

Fig. 2. The chemical structure of QPBI-DA, QPBI-CV1 and QPBI-CV2 mem­ branes confirmed by (a) the high resolution of N 1s XPS spectra and decon­ volution of N 1s spectra; (b) FTIR spectra.

anions (as state III) instead of covalently grafting. It gives a reasonable interpretation why the N–H proton signal disappears completely in the 1 H NMR spectrum of QPBI-CV1, but covalently grafted degree is only about 75%. As to the no-alkaline conventional strategy (2) for QPBI-CV2, the covalent grafting between chlorine of ClONþ and benz­ imidazole anions N could hardly be carried out without the promotion of alkaline, as a result, nearly all the benzimidazole anions are forming ionic bonds with the quaternary ammonium cations in ClONþ as state III. This is coincided well with the completely disappeared signal of N–H proton (Hd) but the unchanged signal of He (δ ¼ 8.28 ppm) and H1’’ (δ ¼ 3.85 ppm) in the 1H NMR spectrum of QPBI-CV2. Strong ionic interac­ tion between benzimidazole anions and quaternary ammonium cations is also evidenced by the reported blending and alkaline doping methods for PBI based AEMs [17,18,39–41], which could improve strength and alkaline uptake. Through the novel de-anionic strategy for QPBI-DA, all benzimidazole anions are pre-removed by 100% grafting of the non-cation ClOCl on PBI, which makes sure no anion-cationic binding occurs in the following quaternization reaction. As a result, all cationic groups are pendent at the end of the side chains (as state I), with well interpretation of all characteristic peaks in the 1H NMR spectrum of QPBI-DA.

3.2. Microstructure and morphology of the QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes

3.1.2. Anion-cationic binding confirm by XPS and FTIR Different chemical structure and anion-cationic binding of the qua­ ternized side chains grafted PBI prepared through different strategies can also be confirmed by XPS and FTIR. The N 1s corresponding XPS spectra of the QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes are shown in Fig. 2(a). The characteristic peak of nitrogen in the quaternary ammonium cations (Nþ) appears at around 403.05 eV in all of the three membranes [42,43]. In the spectrum of QPBI-DA, the characteristic peak of the graphitic nitrogen in the benzimidazole appears at about 398.80 eV and no characteristic peak of the negatively charged nitrogen (N ) at

3.2.1. Ion cluster aggregation ability simulation by molecular dynamics (MD) Different chemical structures and interaction between quaternary ammonium cations and PBI backbone by different grafted strategies greatly influence not only the covalent grafted degree but also the ability of quaternary ammonium cations to transport OH and to aggregate into ion clusters. The interaction between benzimidazole anions (N ), hy­ droxide (OH ) and quaternary ammonium cations (Nþ) in fully hy­ drated QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes are investigated through molecular dynamics (MD) simulation. As shown in Fig. 3, the 7

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Journal of Power Sources 451 (2020) 227813

Fig. 4. The TEM images of (a) QPBI-CV1, (b) QPBI-CV2 and (c) QPBI-DA; and (d) SAXS profiles of QPBI-DA (IEC ¼ 1.84 mmol g and QPBI-CV2 membranes (IEC ¼ 2.21 mmol g 1).

radial distribution functions (RDFs) of both QPBI-CV1 (Nþ N ) and QPBI-CV2 (Nþ N ) show a strong peak at radius of about 4.8 Å and 4.5 Å, respectively, indicating that the quaternary ammonium cation (Nþ) in QPBI-CV1 and QPBI-CV2 has a great tendency to be located near a benzimidazole anion (N ). The strong ionic bond between Nþ and N restricts the flexibility of the grafted side chains, as well as brings a large steric hindrance to the grafting reaction, which will restrict the grafted degree of QPBI-CV1. However, there is no peak shown around 4.5 Å for the de-anion QPBI-DA membrane because all the benzimidazole anions have been pre-removed by covalently grafting with the non-cationic reagent ClOCl, thus no cation-anion binding is formed in the further quaternization. These coincide with the fact that QPBI-CV1 and QPBICV2 often suffer from precipitation during grafting reaction, while QPBI-DA exhibits a good solubility due to the lack of cation-anion binding. The ionic binding of quaternized cations by PBI anions also leads to poor ability to micro-phase separation and OH transport. Smaller coordination number of QPBI-CV1(Nþ OH ) (CN, denoted as the number of OH atoms around one Nþ atom) as compared with that of QPBI-DA (Nþ OH ) suggests that less OH could be used to correlating quaternary ammonium cations and lower conductivity of membranes [35,44]. As comparison between QPBI-DA (Nþ Nþ) and QPBI-CV1 (Nþ Nþ), although both of them show a RDF peak at a radius of about 9.2 Å, the corresponding CNs (number of Nþ atoms around one Nþ atom) are quite different. The CN of the QPBI-DA (Nþ Nþ) (CN ¼ 3.6) is about 1.6-fold that of QPBI-CV1 (Nþ Nþ) (CN ¼ 2.3), indicating that the pendent quaternary ammonium cations in QPBI-DA are much more flexible and easier to aggregate into ionic clusters.

1

), QPBI-CV1 (IEC ¼ 1.87 mmol g

1

)

backbones maintain mechanical properties of AEMs [45]. Much better flexibility and OH transport ability for the cation-anion bond free Nþ pendent side chains endow a good micro-phase separation morphology in QPBI-DA membranes that will benefit for high conductivity. While the strong ionic interaction in QPBI-CV1 and QPBI-CV2 restricts the ability of quaternary ammonium cations to aggregate into ion clusters, result­ ing in poor micro-phase separation morphology. In this work, TEM and SAXS were tested to investigate the micro-phase separation in AEMs. The TEM images of QPBI-CV1, QPBI-CV2 and QPBI-DA membranes are shown in Fig. 4(a) through (c). Stained by I , the dark regions are made up of hydrophilic domains, whereas the bright regions consist of hy­ drophobic polymer backbones [32]. Only very small ion clusters (less than 1 nm) can be observed from the TEM images of QPBI-CV1 and QPBI-CV2 (Fig. 4(a) and (b), respectively), while many larger ion clus­ ters are scattering in TEM image of the QPBI-DA membrane (Fig. 4(c)), attributing to the flexible ether-containing cation ended side chains, and these indicate well-defined hydrophilic-hydrophobic micro-phase sep­ aration structure. It has been reported that an ionic peak in the SAXS spectrum of ionomer represents a characteristic ordered structure, which could be attributed to the hydrophilic/hydrophobic micro-phase separation [46]. As shown in Fig. 4(d), the maximum scattering peak (qmax) of QPBI-DA is about 0.67 nm 1, corresponding to an average inter-clusters distance of about 9.37 nm (calculated by eqn. (2)), while no obvious peaks can be observed from both QPBI-CV1 and QPBI-CV2 membranes, in line with the TEM results. Water absorbs in the hydro­ philic domains and improves micro-phase separation. Because TEM and SAXS were tested in dry and hydrated membrane, respectively, the size of ion cluster measured by SAXS is a little bigger than that measured by TEM. All results from the MD simulation, SAXS and TEM show a much better aggregation of ionic clusters and micro-phase separation morphology in the QPBI-DA AEM, which will facilitate high conductivity.

3.2.2. Membrane morphology investigation by TEM and SAXS In AEMs, cationic groups and non-ionic polymer backbones tend to aggregate respectively, forming hydrophilic-hydrophobic micro-phase separation. Hydrophilic ionic clusters provide interconnected ionic channels for hydroxide transport; while hydrophobic polymer 8

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Journal of Power Sources 451 (2020) 227813

Table 1 IEC and water absorption behavior of QPBI-DA, QPBI-CV1 and QPBI-CV2. AEMs

IECca (mmol g

QPBI-DA QPBI-CV1 QPBI-CV2

2.13 2.18 2.44

a b c d

1

)

IECmb (mmol g 1)

WUc (%)

Non-freezable waterd (%)

Hydration number, λ

Nfreez

Nnon-freez

1.84 � 0.05 1.87 � 0.03 2.21 � 0.06

84.5 � 4.8 72.4 � 6.2 11.7 � 0.3

80.24 80.96 99.83

25.5 18.5 2.9

5.0 3.5 –

20.5 15.0 2.9

The IEC calculated from the grafting degree. The measured IEC. Water uptake ratio at 30 � C. The non-freezable water content in total water uptake, measured by DSC.

Fig. 5. Water sorption and mechanical properties of QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes (a) water uptake and swelling ratio; (b) heating up DSC curves of QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes; (c) stress and strain of the hydrated membranes.

3.3. Electrochemical properties and single cell performance

weaken the ability of cations to draw water molecules. DSC was tested to investigate the water holding capacity of membrane, as shown in Fig. 5 (b) and Table 1. For all quaternized PBI AEMs, the freezable water content in total water uptake is lower than 20%, which indicates that most water in membrane is non-freezable water and has strong inter­ action with cations or PBI matrix [33,47]. All three quaternized PBI AEMs in this work have similar IECs, so the larger membrane water uptake, the larger hydration number (λ) and the lower freezable water content (Nfreez) will be. With the largest water uptake, QPBI-DA mem­ brane still shows a good water holding ability (80.24% of non-freezable water in total water uptake), which will help to improve the membrane conductivity and fuel cell performance, especially at low relative hu­ midity. As shown in Fig. 5(c), it is reasonable for QPBI-CV2 to exhibit high tensile strength because it has strong intramolecular force through the cation-anion bonds. With the increase of covalent grafting degree, effect of cation-anion bond decreases and water uptake increases, which will swell the membranes and decrease the intramolecular force in QPBI-CV1. Although the hydrated QPBI-DA membrane exhibits almost no ionic binding reinforce, it still shows good mechanical strength and

3.3.1. IEC, water uptake (WU), swelling ratio (SR) and mechanical properties The ion exchange capacity (IEC) of the QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes are determined by titration measurement and listed in Table 1. The result shows that both the QPBI-DA and QPBI-CV1 membranes have similar IEC of about 1.84 mmol g 1 and 1.87 mmol g 1, respectively, while IEC of the QPBI-CV2 membrane achieves a higher value of about 2.21 mmol g 1 because all N–H protons in PBI were replaced by the quaternary ammonium cations of ClONþ and the ionic binding between benzimidazole anions and quaternary ammonium cations is easy to be broken by the HCl back titration during the IEC measurement. The water uptake and swelling ratio of all the QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes increase slightly with the increase of tem­ perature, as shown in Fig. 5(a). QPBI-DA shows the lowest IEC but the highest water uptake, while the ionic binding in QPBI-CV1 and QPBICV2 will restrict the movement of quaternary ammonium cations and

Fig. 6. Electrochemical properties. (a) conductivity of QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes; (b) conductivity of QPBI-DA, QPBI-CV1 and QPBI-CV2 membranes as a function of relative humidity at 80 � C and (c) polarization curves of H2/O2 fuel cells using QPBI-DA and QPBI-CV1 membranes (10 μm in thick­ ness), respectively at 80 � C. H2 and O2 were fully humidified and supplied to cell with a flow rate 1 L min 1 with 0.1 MPa backpressure on both sides. 9

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Journal of Power Sources 451 (2020) 227813

Table 2 Properties of PBI-based AEMs reported in literatures. AEMs

Cation

IECma (mmol g 1)

HCb 30 � C/ 80 � C (mS cm 1)

SRc/WUd (%)

Tensile strengthe (MPa)

Elongation at breake (%)

OCV(V)/Peak power densityf (mW cm 2)

Alkaline stabilityg

Ref.

QPBI-DA

TMAr

1.84

23.1/82.4

18.8

82.9

1.0/806.1(80 � C)

ImPBI-DA

Imidazolium

2.09

26.4/86.1

23.0

96.3

1.0/825.5(80 � C)

PipPBI-DA

Piperidinium

2.03

28.1/54.8

21.3

88.5

1.0/661.8(80 � C)

QPBI-CV1

TMAr

1.87

5.1/14.8

34.5

153.3

1.0/142.1(80 � C)

QPBI-CV2

TMAr

2.21

1.2/5.3

17.4/ 84.5 23.4/ 69.0 23.8/ 127.5 13.2/ 72.4 4.9/11.7

47.1

52.3

–

81.4% (2 M KOH 60 � C 720 h) 37.3% (2 M KOH 60 � C 720 h) 93.2% (2 M KOH 60 � C 720h) 84.5% (2 M KOH 60 � C 720 h) –

sQPBIh

Imidazolium

1.42

3.1/10.7

4.0/41.0

20.4

12.5

–

This work This work This work This work This work [22]

Imidazolium

0.96

0.9/4.2

3.1/18.4

48.1

16.5

–

Imidazolium

1.49

5.1/27.2

4.1/43.8

19.2

8.0

–

QPBI-2/1k

Morpholin cation

1.94

13.0/56.0

45.0

6.0

0.93/3.1(30 � C)

QPBI-3/2l

Morpholin cation

1.68

1.0/5.0

66.0

12.0

–

50.0% (6 M NaOH 60 � C 7 days)

[28]

QPBI-1/1m

Morpholin cation Imidazolium

1.15

0.2/0.8

72.0

15.0

–

70.3(dry)

–

–

Coþ

1.92

4.4/6.0(60 � C) (Cl )s 12.3/38.1

99.0% (6 M NaOH 60 � C 7 days) –

[28]

2.08

34.0/ 85.0 (60 � C) 21.0/ 50.0 (60 � C) 6.7/28.0 (60 � C) -/17.6

38.3

55.0

–

Co

1.21

7.2/20.9

37.1

64.0

–

2.40

14.0(25 � C) /35.0 (Cl )s

26.8/ 40.2 18.2/ 27.1 41.4 (25 � C)/-

118.0

4.2

–

mQPBIi msQPBI

j

Me-PBI-C10(50):PBIOO(50)n MCp2CoþOH-PBIo MCp2Co 0.48OH B0.52-PBIp HMT-PMBI (SeriesIIIþ) 15%dxq þ

þ

Imidazolium

37.5% (1 M KOH 30 � C 4 days) 20.1% (1 M KOH 30 � C 4 days) 28.6% (1 M KOH 30 � C 4 days) 46.0% (6 M NaOH 60 � C 7 days)

76.0% (1 M KOH 80 � C 672 h) 75.0% (1 M KOH 80 � C 672 h) 15.0% (3 M KOH 80 � C 7 days) (Cl )s

[22] [22] [28]

[23] [24] [24] [25]

a

The measured IEC. Conductivity in water. c Swelling ratio at 30 oC. d Water uptake at 30 oC. e Fully hydrated membrane. f H2/O2 fuel cell assembled with AEM. g Alkaline stability: percentage of remained conductivity after soaking in alkaline solution. h Quaternized polybenzimidazole having imidazolium moiety in the side chains. i Quaternized polybenzimidazole having imidazolium moiety in the main-chains. j Quaternized polybenzimidazole having imidazolium moieties in both the main-chains and side chains. k PBI-based AEM with pendant quaternary ammonium groups and a grafting degree of 104%. l PBI-based AEM with pendant quaternary ammonium groups and a grafting degree of 84.2%. m PBI-based AEM with pendant quaternary ammonium groups and a grafting degree of 50%. n Methylated polybenzimidazole with an aliphatic chain in the backbone blended with PBI-OO in a ratio of 50:50. o Poly[2,20 -(1,10 -cobaltocenium)-5,50 bis-(N-methylbenzimidazole)]bicarbonate. p Poly[2-(1,10 -cobaltocenium)-20 -(1,10 -butyl)-5,50 bis-(N-methylbenzimidazole)] bicarbonate. q Increasing cross-linked in post functionalized partially methylated poly[2,20 (2,200 ,4,400 ,6,600 -hexamethyl-p-terphenyl-3,300 -diyl)-5,50 -bibenzimidazole]. r Trimethylammonium. s Cl conductivity. b

anti-swelling ability for AEMs application, with elongation at break of about 82.9%, tensile strength of about 18.8 MPa and swelling ratio lower than 25% at 80 � C, which could be attributed to the rigidity of PBI backbone and the good hydrophilic/hydrophobic micro-phase separa­ tion morphology. The excellent mechanical property of QPBI-DA makes it possible to prepare ultrathin membranes (for instance the thickness of all membranes fabricated in this work is around 10 μm), and thus greatly decreases ohmic resistance in fuel cell performance [48].

ammonium cations exists in QPBI-CV1 and QPBI-CV2. As a result, QPBIDA has achieved excellent micro-phase separation morphology and much higher conductivity than that of the QPBI-CV1 (around 6 folds) and QPBI-CV2 (around 15 folds) membranes, as shown in Fig. 6(a), reaching values of about 23.1 mS cm 1 at 30 � C and 82.4 mS cm 1 at 80 � C. Since all cations in QPBI-CV2 are trapped by ionic bonds, QPBI-CV2 only exhibits very low hydroxide conductivity through hydrogen bond in membrane. The conductivity of QPBI-DA is also much higher than that of QPBI-CV1 and QPBI-CV2 at different relative humidity at 80 � C, as shown in Fig. 6(b). H2/O2 single cell performance was tested to further evaluate the properties of the as-prepared quaternized PBI AEMs. As shown in Fig. 6(c), open circuit voltage with all as-prepared quater­ nized PBI membranes achieves values greater than 1.0 V, indicating an excellent gas-barrier ability of the membranes. The cell with QPBI-DA

3.3.2. Conductivity and single cell performance Conductivity is greatly influenced by structure of AEMs. Based on the chemical structural information provided by 1H NMR, the pendent cation ended flexible side chain is achieved in QPBI-DA, while the strong ionic interaction between benzimidazole anions and quaternary 10

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Journal of Power Sources 451 (2020) 227813

respectively) [49,50]. As comparison, previous reported quaternized PBI AEMs are listed in Table 2. Most of them exhibit good tensile strength and alkaline stability with stable cationic group. However, no matter the cationic groups are in PBI side chains [22,28] or main-chain [23–25], most quaternized PBI AEMs suffer from poor conductivity because the quaternary ammonium cations in the membrane are ionic bonded by polybenzimidazole anions through the conventional grafting methods. Thus, only extremely low or even no H2/O2 fuel cell performance could be obtained. The H2/O2 cell performance assembled with QPBI-DA membrane (806.1 mW cm 2 at 2025.0 mA cm 2) is the highest as far as we know among that assembled with quaternized PBI based AEMs reported so far, it is even much higher than that assembled with alkaline doped PBI AEMs, which suffers from a very fast power drop though conductivity is high (peak power density of 41.26 mW cm 2 at 60 � C or 544.4 mW cm 2 at 90 � C, lost all voltage within 5 h) [18,51]. By eliminating the ionic binding, flexibility and OH transport ability of the grafted quaternized side chain is greatly improved for high conductivity and cell performance. The de-anionic strategy is also proved to be universal, as listed in Table 2, all of the de-anionic quaternized PBI AEMs with different cations, i.e. trimethy­ lammonium (TMA), imidazolium and piperidinium cations (denoted as QPBI-DA, ImPBI-DA and PipPBI-DA, respectively), achieve excellent conductivity and H2/O2 fuel cell performance. This indicates a great potential of the quaternized PBI AEMs as prepared by de-anionic strat­ egy for AAEMFC application. 3.3.3. Alkaline stability Alkaline stability is an essential property of AEMs for the long du­ rable application under alkaline environment [52]. The highly alkaline stable PBI backbone will greatly benefit quaternized PBI AEMs [53]. The alkaline stability of the QPBI-DA and QPBI-CV1 membranes were studied by soaking the membranes in a 2 M KOH aqueous solution at 60 � C for 720 h and recording the residual conductivity, IEC and mechan­ ical properties with soaking time. As shown in Fig. 7(a), for both membranes, the conductivity and IEC decrease mainly in the first 120 h, then almost keep unchanged and still remain 81.4% and 85.7%, 84.5% and 87.4% after 720 h for QPBI-DA and QPBI-CV1, respectively. QPBI-CV1 exhibits a little better alkaline stability than QPBI-DA because there are less flexible cations. To confirm the chemical structure change of the AEMs, 1H NMR of QPBI-DA and QPBI-CV1 were examined before and after the alkaline treatment, as shown in Fig. 7(b). There is no obvious signal change for both membranes, except for a slight decrease of the signal of trimethylammonium at δ ¼ 2.98 ppm. The results confirm that no degradation has happened to the PBI backbone of QPBI-DA after 720 h hot alkaline treatment. As shown in Table 2, attached with a stable piperidinium [54], PipPBI-DA exhibits an excel­ lent alkaline stability and maintains 93.2% of the original conductivity after soaking in a 2 M KOH solution at 60 � C for 720 h. This indicates that the quaternized PBI AEMs prepared by the de-anionic strategy have a sufficient alkaline stability for practical application in alkaline fuel cell. Fig. 7. Alkaline stability of the QPBI-DA and QPBI-CV1 membranes (a) changes in conductivity and IEC as a function of immersion time in 60 � C, 2 M KOH (conductivity measured at 30 � C); (b) 1H NMR spectra for 0 and 720 h treat­ ment in 2 M KOH at 60 � C.

4. Conclusions This work mainly focuses on the root cause and solution to a poor conductivity of the quaternized PBI AEMs. The results from 1H NMR, XPS and FTIR investigations confirm the existence of strong ionic bond between benzimidazole anions in PBI and quaternary ammonium cat­ ions in grafting reagents in conventional quaternized PBI AEMs. The further analyses with molecular dynamics simulation, TEM and SAXS indicate that the strong ionic binding makes a great steric hindrance to perform covalent grafting, restricts the movement of quaternary ammonium cation ended side chains, and thus results in a poor microphase separation and conductivity. Accordingly, a novel de-anionic strategy to eliminate the ionic binding is proposed for the fabrication of quaternized side chain grafted PBI AEMs. Through de-anion of the

membrane shows a peak power density of about 806.1 mW cm 2 at very high current density of about 2025.0 mA cm 2, which is about 5.7 times higher than that of the fuel cell assembled with the QPBI-CV1 membrane (about 142.1 mW cm 2 at 300.2 mA cm 2). While the single cell with QPBI-CV2 membrane almost shows no performance because of extreme low conductivity of QPBI-CV2. The fuel cell performance of QPBI-DA is also much higher than that assembled with recently reported commer­ cial FAA-3 (IEC ¼ 2.3 mmol g 1) and AHA (IEC ¼ 1.1 mmol g 1) membranes (peak power density of 30.0 mW cm 2 and 2.5 mW cm 2, 11

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Journal of Power Sources 451 (2020) 227813

benzimidazole rings by introducing non-cationic grafting side chains, ionic binding restriction on the side chains is eliminated and welldefined hydrophilic-hydrophobic micro-phase separation is developed. Quaternized PBI AEMs with different cations (QPBI-DA, ImPBI-DA and PipPBI-DA) are prepared by the de-anionic strategy. QPBI-DA exhibits an excellent conductivity of about 82.4 mS cm 1 at 80 � C, which is around 6-fold that of QPBI-CV1 and QPBI-CV2 as prepared by the con­ ventional grafted strategies. QPBI-DA also shows a good balance be­ tween swelling and mechanical strength. Attached with the alkaline stable piperidinium, PipPBI-DA based membrane exhibits an excellent alkaline stability (with about a 93.2% conductivity retention after soaking in 2 M KOH at 60 � C for 720 h). Single cell performance with QPBI-DA membrane shows a peak power density of about 806.1 mW cm 2 at large current density of about 2025.0 mA cm 2, which is about 5.7-fold that of the cell assembled with QPBI-CV1 membrane. The deanionic strategy proposed in this work provides a universal and highly efficient approach to fabricating high conductive and alkaline stable quaternized PBI AEMs for the alkaline fuel cell.

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