Olefin metathesis-crosslinked, bulky imidazolium-based anion exchange membranes with excellent base stability and mechanical properties

Journal Pre-proof Olefin metathesis-crosslinked, bulky imidazolium-based anion exchange membranes with excellent base stability and mechanical properties Xiaojuan Zhang, Yejie Cao, Min Zhang, Yiguang Wang, Hongying Tang, Nanwen Li PII:

S0376-7388(19)33286-7

DOI:

https://doi.org/10.1016/j.memsci.2019.117793

Reference:

MEMSCI 117793

To appear in:

Journal of Membrane Science

Received Date: 22 October 2019 Revised Date:

3 December 2019

Accepted Date: 26 December 2019

Please cite this article as: X. Zhang, Y. Cao, M. Zhang, Y. Wang, H. Tang, N. Li, Olefin metathesiscrosslinked, bulky imidazolium-based anion exchange membranes with excellent base stability and mechanical properties, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/ j.memsci.2019.117793. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

CRediT author statement Xiaojuan Zhang: Conceptualization, Methodology, Investigation, Writing - Original Draft Yejie Cao: Methodology, Formal analysis Min Zhang: Funding acquisition Yiguang Wang: Supervision, Project administration Hongying Tang: Visualization Nanwen Li: Writing - Review & Editing, Supervision, Project administration, Funding acquisition

Olefin metathesis-crosslinked, bulky imidazolium-based anion exchange membranes with excellent base stability and mechanical properties Xiaojuan Zhang1, Yejie Cao1, Min Zhang3, Yiguang Wang2*, Hongying Tang4* and Nanwen Li3* 1

Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an, 710072, China.

2

Institute of Advanced structure Technology, Beijing Institute of Technology, Beijing, 100081, China.

3

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China.

4

Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin, 300387, China.

* Corresponding author E-mail addresses: [email protected] (Y. Wang), [email protected] (H. Tang), [email protected] (N. Li)

Abstract: To improve the mechanical properties of anion exchange membranes (AEMs) with bulky imidazolium cations, a series of anion-conductive poly(2,6-dimethyl-phenylene oxide)s (PPO)s with crosslinkable terminal double bonds were synthesized by the Menshutkin reaction. Following crosslinking via olefin metathesis at room temperature catalyzed by a Grubb's second generation catalyst, tough, transparent, and flexible PPO-based AEMs were obtained. The crosslinked AEMs exhibited good mechanical properties (tensile strength at maximum load of 20.8–49.9 MPa and values of elongation at break of 1.5–3.0%) though their bulky imidazolium cation groups which could destroy the film-forming ability of polymer. And the obtained mechanical properties were considerably better than those of the non-crosslinked AEMs with a similar architecture, which broke into small pieces during the process of membrane fabrication using solvent casting. Furthermore, the crosslinked AEMs exhibited an extremely low water uptake (up to 13.9 wt% at 80 ºC) and minimal swelling (<7% at 80 ºC), attributed to the high-density crosslinking network. A high bromide conductivity (22.9 mS/cm at 80 ºC) was achieved despite the low water uptake for the crosslinked 1

AEM. Moreover, long-term alkaline stability testing in 1 M NaOH at 80 ºC, no obvious degradation of the imidazolium ring was observed, with the conductivity of aged crosslinked membranes remaining at ~100% after 960 h. It was assumed that the bulky substituents at the C2, C4, and C5 positions of imidazolium cations and the crosslinked architecture prevented H2O and/or OH– from attacking the cationic center. Moreover, the AEM fuel cell performance and durability was further explored for the membrane electrode assembly (MEA) with crosslinked and uncrosslinked bulky imidazolium-based ionomers in catalyst layers. An optimistic cell performance (a peak power density of 173 mW/cm2 at 410 mA/cm2) and improved short-term durability at a constant current density of 200 mA/cm2 were obtained for the crosslinked ionomers, and its lifetime was ca. 3 times longer than that of benzyltrimethylammonium functionalized PPO ionomer.

Keywords: Anion exchange membrane; Bulky imidazolium; Olefin metathesis; Mechanical properties; Alkaline stability

1. Introduction Alkaline exchange membrane fuel cells (AEMFCs) have attracted considerable attention due to their advantages over proton exchange membrane fuel cells, including reduced fuel penetration in batteries for the opposite conduction direction of ionic and fuel [1–3], the faster reaction kinetics of oxidation–reduction reactions in alkaline environments [4, 5], the lower production and operation cost due to the use of replaceable and inexpensive catalysts (Fe, Ni, etc.) rather than precious metal Pt catalysts [2, 6, 7], and the prolonged life-span of fuel cell stacks for the less corrosiveness of metal catalysts in alkaline environment than that in acidic medium [6, 7]. In AEMFCs, AEMs play a 2

critical role in separating the fuel and oxygen (or air) and performing anion transfer [7, 8]. Additionally, the properties of AEMs directly determine the final performance, energy efficiency, and service life of AEMFCs [6–8, 9]. Therefore, the preparation of AEMs with excellent thermal properties, chemical stability, mechanical strength, and the necessary ionic conductivity to tolerate long-term alkaline environments is urgently required.

Structurally, AEMs consist of two major components, namely the polymer skeleton and cationic groups. On the premise of excellent properties of selecting polymer skeletons, many cationic conductive groups have been extensively studied as candidate groups for AEMs, including quaternary ammonium [10–12], phosphonium [13, 14], cobaltocenium [15, 16], guanidinium [17, 18], and imidazolium groups [19–21]. Unfortunately, in a high pH environment at elevated temperatures, the commonly used benzyltrimethylammonium (BTMA) groups decompose via direct nucleophilic substitution, Hofmann or E2 elimination, or other degradation processes [12, 22–24]. Therefore, many efforts have been devoted to improving the stability of cationic groups in AEM materials. Notably, side-chain type AEMs (with an alkyl spacer between polymer backbones and organic cations) [25, 26] and comb-shaped AEMs (with a long alkyl side chain pendant to the N-centered cation) [27] exhibit high chemical stability under harsh alkaline conditions, whereby a steric hindrance effect protects the ammonium cations from attack by hydroxide ions and plays a critical role in the performance of AEMs during the aging tests [13, 28, 29]. Hickner et al. [30] first reported a ruthenium bis(terpyridine)-based AEM with steric hindrance around a complex cation that exhibited excellent stability in 1 M NaOH solution at room temperature over 180 days. Similarly, Coates et al. [13] explored a bulky phosphonium cation, tetrakis(dialkylamino)-phosphonium, synthesized by multi-step reactions, which was stable in 1 M KOH at 80 ºC for 22 days. 3

Subsequently, the same group synthesized multi-substituted imidazolium cations with bulky groups (aromatic substituents at the C2, C4 and C5 positions) with excellent alkaline stability (no cation degradation in 5 M KOH in methanol at 80 ºC for 30 days) [31]. Very recently, we designed a series of

AEMs

with

pendant

bulky

quaternary

phosphonium

groups

(based

on

tris(2,4,6-trimethoxyphenyl)phosphane) with excellent alkaline stability, even in a 10 M NaOH solution at 80 ºC for 200 h, derived from the efficient protection of the core organic cations against hydroxide attack [32].

Although the alkali resistance of AEMs can be greatly improved by incorporating organic cations with steric hindrance into the membrane, such a process may unfavorably affect the material’s film-forming performance, resulting in membrane fragility, especially when larger organic cations are directly grafted onto the polymer skeleton [32, 33]. This phenomenon can be explained by the steric hindrance provided by fixed organic cations, which reduces the entanglement interaction in polymer chains [32–34]. Especially in the case of a high ion exchange capacity (IEC), the trade-off between excellent alkaline stability (or high ionic conductivity) and good mechanical properties of AEMs is difficult to balance. Therefore, in order to solve this contradiction, the use of high-density grafted cationic groups with steric hindrance coupled with covalent crosslinking between polymer chains could not only significantly improve the mechanical properties of AEMs, but also afford controlled water uptake and excellent dimensional stability with high IEC values [35–37].

Herein, we combined the outstanding alkaline stability of bulky imidazolium groups with a Grubb's second generation catalyst catalyzed crosslinkable olefin metathesis reaction to produce a crosslinkable and anion-conductive poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). PPO was 4

selected as the polymer backbone of AEMs for its commercial availability as well as excellent physical-chemical stability [22, 23, 27]. Subsequently, the pendant alkene was further covalently crosslinked using a Grubb's second generation catalyst via olefin metathesis during membrane synthesis. Due to covalent crosslinking, the entanglement of polymer chains was improved, leading to the formation of robust, transparent, and flexible AEMs. Furthermore, the properties of the resulting crosslinked AEMs, such as mechanical strength, dimensional stability, water uptake, conductivity, and alkaline stability, were investigated in detail. In particular, the initial performance and durability of the H2/O2 fuel cell using the stable cationic bulky imidazolium-based PPO polymer as ionomers and crosslinked ionomers in the catalyst layer have been further demonstrated.

2. Experimental 2.1 Materials

Commercially available PPO was obtained from Sigma-Aldrich (Mn = 20,000) and used without further purification. N-bromosuccinimide, 2,2’-azo-bis-isobutyronitrile, benzil (98%), mesitaldehyde (95%), ammonium acetate (97%), monosodium phosphate (99%), 9-bromo-1-nonene (97%), and Grubb's second generation catalyst (C46H65Cl2N2PRu, 98%) were purchased from Energy Chemical (China). Other chemicals, including dimethyl sulfoxide-d6 (99.9%-D), methanol-d4 (99.8%-D), chloroform-d1 (99.8%-D + 0.03% V/V TMS + silver foil), methanol, acetonitrile, n-propanol, toluene, diethyl ether, sodium hydroxide, sodium bicarbonate, potassium bromide, hydrochloric acid (HCl), and hydrobromic acid were of analytical grade and purchased from J&K Scientific Ltd. The bromo-functionalized PPOs (BPPO-x, x is the degree of bromination) with three degrees of

5

functionalization (DF) (65%, 77%, and 100%) were synthesized according to our previous reports [27, 28].

2.2 Synthesis of 2-mesityl-4,5-diphenyl-1-nonene-imidazole (MDPN-Im)

The synthesis of MDPN-Im was divided into two steps. The first was the reaction of benzil, mesitaldehyde,

ammonium

acetate,

and

monosodium

phosphate

to

obtain

2-mesityl-4,5-diphenyl-1H-imidazole according to a previous report [38]. Subsequently, the mixture was added to a solution of 2-mesityl-4,5-diphenyl-1H-imidazole (5.00 g, 14.77 mmol, 1.00 eqv.) in THF (24 mL) and NaOH (2.96 g, 73.87 mmol, 5.00 eqv.) and was vigorously stirred for 1 h. 9-Bromo-1-nonene (3.03 g, 14.77 mmol, 1.00 eqv.) was then added to the above basic solution and the mixture was refluxed for 48 h. The mixture was then cooled, THF was removed with a rotary evaporator, and the residue was extracted three times with dichloromethane/water. Subsequently, the combined organic layer was washed twice with deionized water and then dried with MgSO4 overnight. The drying agent was filtered and the filtrate was concentrated, and the resultant product (6.81 g, ~100%, yellow oil) was obtained by dried at 50 ºC under vacuum for 24 h.

2.3 Crosslinkable quaternized PPO in the bromide form A crosslinkable quaternized PPO copolymer (PPO-Im-x, x is degree of grafting bulky imidazolium cations) in the bromide form with an unsaturated alkene pendant was synthesized as follows. Typically, BPPO with a DF of 0.77 (0.80 g, 3.42 mmol, 1.00 eqv.) was dissolved in 20 mL of N-Methyl pyrrolidone (NMP). Subsequently, 2-mesityl-4, 5-diphenyl-1-nonene-imidazole (6.33 g, 13.69 mmol, 4.00 eqv.) was added and the mixture was stirred for 120 h at 80 ºC. The reaction mixture was then precipitated into diethyl ether to obtain crosslinkable quaternized PPO-Im-77 6

copolymer. The polymer was filtered and further washed three times with diethyl ether. A brown powder was collected and dried in a vacuum for 12 h at room temperature to obtain the crosslinkable PPO in the bromide form with a yield of 93.0%. The above-mentioned procedures were also followed for the synthesis of BPPO with a DF of 0.65 and 1.00.

2.4 Membrane fabrication and cross-linking procedure

All membranes were prepared by solution-casting following a similar procedure: the crosslinkable quaternized PPO in the bromide form was dissolved in n-propanol/toluene (3:2 vol) to form an 8 wt% solution, and then the Grubb's second generation catalyst (4 wt% to the polymer) was added into the polymer solution. Subsequently, the solution was casted on a glass plate and covered with a watch glass to allow slow evaporation of the solvent. A crosslinked membrane was obtained by drying at room temperature for 24 h and then further drying in a vacuum oven at 80 ºC for 12 h. The resultant crosslinked membranes in the hydroxide (OH–) and bicarbonate (HCO3–) form were achieved by soaking in 1 M NaOH or NaHCO3, respectively, at 40 ºC for 60 h to exchange the Br form membranes, followed by thorough washing with deionized water to remove any residual salt. Finally, the samples were stored in degassed deionized water and blanketed with flowing argon prior to analysis. The thickness of the membranes was in the range of 30–40 µm.

2.5 Characterization 1

H-nuclear magnetic resonance (NMR) spectra (400 MHz) were recorded using a Bruker

DPX-400 spectrometer at room temperature using CDCl3 or DMSO-d6 as the solvents. The CDCl3 singlet at 7.26 ppm, or the DMSO-d6 singlet at 2.50 ppm was selected as the reference standard.

7

Fourier-transform infrared (FTIR) spectra of the samples were recorded on a Shimadzu Prestige-21 ATR spectrometer equipped with a ZnSe crystal by single reflection in the range 4000–400 cm–1 with a 4 cm–1 resolution in 64 scans using polymer thin films.

Thermogravimetric analysis (TGA) was conducted to explore the thermal stability of the samples using a TA SDT Q600 device. Prior to polymer degradation, the samples were preheated at 100 ºC for 40 min under a N2 atmosphere to remove the residual solvents and moisture. Samples of approximately 10 mg were placed in a ceramic cell and scanned from 50 to 700 ºC under a N2 atmosphere at a heating rate of 20 ºC/min.

The membrane morphologies were photographed with a digital camera. The content and distribution of various elements in the membranes were analyzed through energy-dispersive X-ray spectrum (EDX) obtained by scanning electron microscopy (SEM; Hitachi SU8020), and each sample was repeatedly scanned and tested five times.

The mechanical properties of membranes, including tensile strength (MPa), Young’s modulus (MPa), and elongation at break (%), were measured using the ASTM D-1708 standard. Long-strip specimens (50 mm × 10 mm overall size, 20 mm × 10 mm gauge area) were die-cut and loaded in cell of 100 N with a constant cross head speed of 5 mm/min (Instron 5866 universal tester). Under the same test conditions, six samples were tested to minimize error.

The alkali stability of the membranes (4 cm in diameter) was assessed by long-term immersion in a 1 M NaOH solution changed every three days at 80 ºC for time periods ranging from 0 to 960 h. The hydroxide ion conductivity of the treated membranes was measured at 25 ºC at certain times after complete removal of residual NaOH by washing by deionized water. 8

Water uptake The water uptake of the vacuum-dried membranes immersed in water for 24 h was measured at 20, 40, 60, and 80 ºC controlled by means of a thermostatic water bath. The weight percentage uptake (WU, %) was determined by the following equation:

Where Wwet and Wdry are the weight of the completely hydrated and the vacuum-dried membranes, respectively.

Swelling ratio and λ measurements

The swelling ratio, that is, the linear expansion ratio of a membrane, was determined by the difference between the dimensions of the dry membrane and that immersed in water at 20, 40, 60, and 80 ºC for 24 h. The calculation was based on the following equation:

Where Ldry is the length of dried membranes and Lwet is that of completely hydrated membranes.

Based on WU (%) and IEC, the number of bound water molecules per ammonium group (λ) was calculated as follows:

9

IEC measurements The experimental IEC (IECe) values of the membranes were measured in triplicate using a typical titration method. Accurately weighed samples in their OH– form were immersed into 25 mL of a 0.05 M HCl solution and equilibrated for 60 h under an Ar atmosphere, after which the HCl solution was back titrated with a 0.05 M NaOH solution using phenolphthalein as an indicator. The IEC values of the samples were calculated using the following equation:

Where n1·HCl and n2·HCl are the amounts (mmol) of HCl required before and after equilibrium, respectively, and Mdry is the mass (g) of the dried sample. The average value for three measurements calculated from the above equation was taken as the IEC value of the measured membrane.

Membrane ionic conductivity

The membrane ionic conductivity was measured by two-point alternating current impedance spectroscopy using an impedance/gain-phase analyzer (Bio-Logic VSP-300, FR) over the frequency range from 100 mHz to 100 kHz. Impedance was measured at 20 ºC and 30% RH. Membranes were cut into 1 cm × 4 cm strips, and set in a Teflon cell. Under the fully hydrated conditions, the OH– conductivity of the membrane was measured in the longitudinal direction by immersing the cell immersed in water previously degassed and blanketed with flowing Ar to avoid carbonation. Every sample was measured repeatedly to ensure reproducibility. The ionic conductivity (σ) (including OH– and HCO3–) was obtained as follows:

10

Where L is the length between sensor electrodes (cm), R is the real component of the impedance response at a high frequency (Ω), and A is the available cross-sectional membrane area for ion conduction (cm2).

Fabrication of Membrane/Electrode Assembly and Fuel Cell Testing

The catalyst-coated membrane (CCM) method was adopted to prepare the membrane electrode assemblies for fuel cell testing. The ionomer BTMA-40 and the LSCQA-40 AEMs (25 µm) were synthesized according our previous research [22] and showed in Scheme S1. A well-dispersed catalyst ink was prepared by mixing Pt/C (40 wt%, Johnson Matthey Co.) with deionized water, isopropanol, and ionomers solution (5 wt%, BTMA-40 or PPO-Im-100 polymer) using magnetic stirring and ultrasonication. As one of the controls, for the crosslinking of ionomers, the n-propanol solution of 4 wt% Grubb's second generation catalyst to PPO-Im-100 ionomers was added to the above well-dispersed catalyst ink and stirred sufficiently. Then the as-prepared ink was coated onto the membrane using an air spray gun to achieve the CCMs for the electrodes and the area of the electrodes was 5 cm2. The Pt loading and ionomer content in catalyst layer were calculated to be 0.5 mg/cm2 and 20 wt% respectively. In order to achieve full crosslinking of PPO-Im-x ionomer and complete solvent volatilization, the crosslinked membrane electrode for fuel cell testing was held under 50 ºC for another 30 min. Subsequently, the assembly of carbon paper (gas diffusion layer, HCP120, HESEN) and a CCM was used to obtain the membrane electrode assembly (MEA). The H2/O2 fuel cell performance was tested using a commercial detecting testing system (Smart2 PEM/DM, WonATech, Korea) at 60 ºC with a flow rate of 1000 sccm for both H2 and O2 under full 11

humidification (100% RH) with no back pressure. The MEA was activated at potentiostatic mode at 0.5 V for 30 minutes, and then it was activated at 0.1 V until the current density increased to a maximum and became constant for 30 min. Then, the polarization curve was obtained under galvanostatic mode after full activation. The durability test was conducted at 60 ºC with a flow rate of 200 sccm for both H2 and O2 under full humidification (100% RH) with no back pressure. The constant current density of 200 mA/cm2 was applied to the fuel cell, and the cell voltage as a function of time was recorded.

3. Results and discussion 3.1 Synthesis and characterization of crosslinkable bulky imidazolium-based PPO CH3

(a)

O

HN

N

N

3

Br

N

3

BPPO-x

CH3

CH3

O

O x

Br

CH3

CH3

O x

100-x

CH3

100-x

N Br N

MDPN-Im PPO-Im-x (b)

Grubbs 2nd Olefin Metathesis c-PPO-Im-x

:

Weak Chain Interaction

Strong Covalent Crosslinking

Scheme 1. (a) Synthesis of the crosslinkable PPO-Im-x precursor. (b) Schematic diagram for the fabrication of crosslinked AEMs via the olefin metathesis reaction. 12

To prepare robust AEMs with bulky imidazolium groups, the crosslinkable quaternized PPO polymers were first designed and synthesized in two main steps, namely (1) the preparation of 2-mesityl-4,5-diphenyl-1-nonene-imidazole (MDPN-Im) and (2) the grafting the MDPN-Im onto BPPO-x. Firstly, the condensation reaction of benzil with aldehyde in the presence of excess ammonium acetate yielded 2-mesityl-4,5-diphenyl-1H-imidazole (yield, 45.7%, Fig. S1) [38]. Subsequently, under alkaline conditions, MDPN-Im, with a crosslinkable double bond (Scheme 1a), was obtained by the reaction of 2-mesityl-4,5-diphenyl-1H-imidazole with 9-bromo-1-nonene. 1H NMR spectra results confirmed the chemical structure. The peaks at 12.34 ppm and 6.99–7.60 ppm were attributed to the N1 position proton in imidazole and phenyl groups at the C2, C4, and C5 positions in imidazole, respectively (Fig. 1 and Fig. S1). Following the substitution of N1, the appearance of new peaks at 4.94 ppm and 5.76 ppm, belonging to the terminal alkene groups, demonstrated the successful substitution reaction by 9-bromo-1-nonene [23, 39]. Finally, the crosslinkable, anion-conductive PPOs with DF values in the range 65–100% were prepared readily via the Menshutkin reaction [40–42]. Following quaternization, the new broad peaks at 6.80–8.00 ppm and 4.90 ppm, attributed to the phenyl protons at the C2, C4 and C5 positions of the imidazole ring and benzyl protons [38, 43, 44], respectively appeared (Fig. 1). The grafting degrees of crosslinkable PPOs were calculated to range from 65 to 100 mol% by the integration ratios of the isolated peaks at 1.23 ppm and 1.10 ppm. These values were in good agreement with the theoretical values according to the DFs of BPPO-x.

13

8 4

6 CDCl3 1-3

9

12

5

9

7 11

10

3

1

8

2

4

(a)

5 13

6

10

7

11, 12

14 DMSO-d6

15

16

8-10, 16

15, 6

1-4, 13, 14

(b) 8

7

6

11, 12

7

5

5

4

3

2

1

0

Chemical Shift (ppm) Fig. 1. 1H-NMR spectra of (a) 2-mesityl-4,5-diphenyl-1-nonene-imidazole (MDPN-Im) in chloroform-d1 and (b) PPO-Im-77 in dimethyl sulfoxide-d6.

Anion-conductive PPO AEMs with bulky imidazolium groups showed a very poor film-forming ability (Scheme 1b). Membranes broke into pieces after drying at elevated temperature, likely due to destruction of chain entanglements by the bulky cationic groups [45, 46]. In contrast, the crosslinked PPO via olefin metathesis reaction using Grubb's second generation catalyst showed an excellent film-forming ability. Transparent, robust, and crosslinked AEMs were obtained by casting a solution of ca. 8 wt% n-propanol/toluene (3:2 vol) with 4 wt% Grubb's second generation catalyst to polymer at room temperature (Scheme 1b). The crosslinked membranes could not be dissolved in water, methanol, or n-propanol, indicating the successful crosslinking. The gel fractions of crosslinked membranes were calculated to be above 55% from the ratio of the weight of the polymer before and after extraction from NMP at 80 ºC for 24 h. Moreover, following the cross-linking reaction, the 14

characteristic absorption bands at 998 cm–1 and 909 cm–1 from the terminal double bonds were obviously weakened due to incomplete crosslinking (Fig. 2), demonstrating the formation of a crosslinked architecture between the polymer chains [23, 39]. However, the characteristic absorption peaks at 1606 cm–1 and 732 cm–1, corresponding to the imidazolium ring [44], were maintained intact, suggesting the excellent stability of functional groups under the olefin metathesis reaction. Imidazolium ring: 1608, 732

Transmittance (%)

c-PPO-Im-77-aged

c-PPO-Im-77-pristine

PPO-Im-77

=C-H:998, 909

C=C: 1686 4000

3500

3000

1500

1000

500

-1

Wavenumbers (cm )

Fig. 2. FTIR spectra of PPO-Im-77 precursor and crosslinked c-PPO-Im-77 membranes before and after alkaline stability test in 1 M NaOH at 80 ºC for 960 h.

3.2 Thermal and mechanical properties The thermal stability of the membranes before and after crosslinking in their bromide ion form was evaluated by TGA under a N2 atmosphere. The corresponding differential thermogravimetry (DTG) curve (Fig. S2) were calculated by first-order derivative of the TGA curve of membranes before and after crosslinking (Fig. 3). By combining the number of peaks in Fig. S2 and the Fig. 3, the PPO-based polymers typically showed two major steps of thermal degradation. The first step was weight loss at ~200 ºC for the precursors before crosslinking, which was mainly due to the decomposition of grafted imidazolium group along with the release of bromoethane [19–21]. The second decomposition at ~280 ºC was attributed to the partial overlap between the degradation of 15

alkyl chains and the polymer backbone [39, 47]. Furthermore, the decomposition temperature of the polymer backbone was similar to its parent polymer BPPO-x, suggesting that the thermal decomposition of grafted alkyl chain did not trigger degradation of the polymer backbone [20, 48, 49]. In comparison, there was no obvious two-step degradation of the membranes after crosslinking, with the initial thermal degradation temperature of the crosslinked membrane being significantly delayed to ~250 ºC. The relatively improved thermal stability of the crosslinked membranes after crosslinking compared to prior crosslinking might be due to the densification effect of the crosslinking network [50]. PPO-Im-65 PPO-Im-100 c-PPO-Im-65 c-PPO-Im-100

100

Weight (%)

80

60

40

20

0 100

200

300

400

500

600

700

Temperature (°C)

Fig. 3. TGA curves of membranes before and after crosslinking from the PPO-Im-65, 100 precursors.

The mechanical properties of AEMs play a major role in their performance. Despite the bulky imidazolium cationic groups inducing a poor film-forming ability in the non-crosslinked copolymers, the crosslinked membranes exhibited excellent mechanical properties. Unlike in other crosslinked AEMs prepared with the typical quaternary ammoniums, in which crosslinking generally led to poor mechanical properties, e.g., brittleness of membranes, the covalent crosslinking of polymers with bulky pendant groups resulted in the entanglement of polymer chains, and thus in an excellent 16

film-forming ability [35, 36]. The crosslinked membranes showed a tensile strength at maximum load of 20.8–49.9 MPa and elongation at break of 1.5–3.0% (Table 1). Thus, the c-PPO-Im-x membranes were tough and flexible. Moreover, as the membrane was operated under full humidification in in H2/O2 fuel cell devices (100% RH), the mechanical properties of crosslinked membranes were tested at 100% RH. As shown in Table S1, the elongation at break of the crosslinked membranes at 100% RH was nearly twice than that of at 20% RH, and the mechanical strength (19.1–46.2 MPa) was almost unaffected despite the plasticizing effect of water. Table 1. Mechanical properties of crosslinked membranes before and after aging (20 ºC with 30% RH) Polymers

Gel fraction

Tensile strength (MPa)

Elongation at break (%)

(%)

Before aging

c-PPO-Im-65

66.2

20.8

9.4

1.5

0.9

c-PPO-Im-77

57.7

38.4

23.1

2.2

1.9

c-PPO-Im-100

59.6

49.9

39.9

3.0

2.6

After aging

Before aging

After aging

3.3 Ion exchange, water uptake, and swelling ratio In order to meet the requirements of fuel cell devices, in general, AEM materials need to undergo an exchange from halogen ions to hydroxide ions. So as to further explore the physical and chemical properties of membranes in hydroxide ions form, ion-exchange is commonly performed by immersing AEMs in 1 M NaOH solution for 48 h at room temperature. However, ion exchange is complicated herein by due to the bulky imidazolium cations within the crosslinked membranes. Therefore, several attempts at ion exchange were performed and the ionic conductivity was measured at different temperatures. Furthermore, the hydroxide and bicarbonate conductivity of the crosslinked membranes as a function of temperature were also recorded. As shown in Fig. S3, the lower hydroxide conductivities of c-PPO-Im-x membranes (5.1~10.2 mS/cm) were similar to those of the non-ion-exchange bromide ions (5.3~9.9 mS/cm) at room temperature. In addition, the IECe values 17

of the membranes in the range 1.03–1.11 meq./g were also obtained by acid-base back titration (Table 2). These values were obviously much lower than the theoretical values of the membranes ranging from 1.51 to 1.67 meq./g. To further confirm the incomplete ion exchange, we quantitatively assessed the Br and N atoms in the membrane before and after hydroxide ion exchange by SEM with EDX. The membranes were shown to have a uniform and compact structure both before and after hydroxide ion exchange (Fig. 4). Furthermore, N and Br elements in the membranes had a uniform distribution in the membrane, with the N content (~5.39%) after immersion in 1 M NaOH solution for 60 h at 40 ºC being very similar that of the crosslinked membranes (~5.73%) before ion exchange. Additionally, the Br atom concentration decreased from 10.73% to 1.51% following treatment with 1 M NaOH solution for 60 h at 40 ºC, suggesting that ~16.7% of Br ions were still not exchanged. This phenomenon might be related to the bulky hydrophobic substituents at the C2, C4, and C5 positions of the imidazolium cation, making the dissociation of anions and cations more difficult [33, 38]. Therefore, we focused on the properties of these crosslinked membranes in the Br form.

18

Bromine atomic percentage by EDX (%)

100

2.5 µm 2.5 µm

80

Br 60

Br 2.5 µm

40

20

Br

0 0

40

900

950

1000

Time (h) Fig. 4. The diagram of Br atomic percentage by EDX and the SEM images and EDX element mapping of Br in the cross-section of the c-PPO-Im-77 membrane with the prolongation of soaking time in 1 M NaOH at 80 ºC.

The trade-off effect between conductivity and water uptake is a barrier to high-performance AEM development. The mechanical properties and dimensional stability of AEMs would be negatively affected by excessive water uptake due to its plasticizing effect on ion-conducting membranes [39, 51]. The c-PPO-Im-65, 77, and 100 membranes showed a very low water uptake at 9.6–13.9 wt% even at 80 ºC (Table 2 and Fig. 5). The λ values in Br of these crosslinked c-PPO-Im-65–100 were in the range 0.56–0.69, much lower than that of AEMs with quaternary ammonium cations and similar to the comb-shape polymer with the bulky phosphonium cations. It was assumed that the lower water uptake and λ values of the crosslinked PPO-based AEMs could be attributed to the hydrophobicity of the bulky imidazolium cation and the crosslinking architecture [13, 33, 35]. A lower water uptake would result in a lower swelling ratio of the crosslinked AEMs; indeed, the swelling ratios of the crosslinked membranes at 20 ºC were lower than 4% due to their lower water uptake (Table 2). Even at an elevated temperature of 80 ºC, the swelling ratio of

19

crosslinked AEMs were still lower than 10% (Fig. 5). Furthermore, these values were much lower than that of the previously reported AEMs without crosslinking [32–34]. Table 2. The properties of the crosslinked PPO based-AEMs at 20 ºC IECt a

IECe b

IECt c (Br–)

WU

(meq./g)

(meq./g)

(meq./g)

(wt%)

c-PPO-Im-65

1.51

1.03

1.24

c-PPO-Im-77

1.58

1.07

c-PPO-Im-100

1.67

1.11

Polymers

a

σ(Br–)

SR (%)

λ (Br)

5.6

2.4

0.56

5.3

1.29

6.3

3.0

0.61

7.9

1.35

7.4

3.5

0.69

9.9

(mS/cm)

Theoretical IEC values (meq./g); b measured by titration (meq./g) at 20 ºC. (a)

(b)

6

12

Swelling ratio (%)

Water Uptake (wt%)

14

7

c-PPO-Im-65 c-PPO-Im-77 c-PPO-Im-100

10

8

c-PPO-Im-65 c-PPO-Im-77 c-PPO-Im-100

5

4

3 6 2 20

40

60

80

20

Temperature (°C)

40

60

80

Temperature (°C)

Fig. 5. (a) Water uptake and (b) swelling ratio of crosslinked c-PPO-Im-65, 77, and 100 membranes as a function of temperature.

3.4 Ionic conductivity The ionic conductivity of the membrane samples was assessed by two-point alternating current impedance spectroscopy using an impedance/gain-phase analyzer to measure the membrane impedance at a given temperature and relative humidity conditions. Surprisingly, the crosslinked AEMs showed reasonable Br conductivity in the range of 5.3–9.9 mS/cm despite their low water uptake at 20 ºC with the IEC (Br–) of 1.24–1.35 meq./g (Table 2). If the water uptake was taken into consideration, the λ normalized Br conductivities were calculated to range from 9.5 to 14.4 20

mS/cm. These values were much higher than that of previously reported AEMs [52, 53]. Additionally, the dependence of the Br conductivity on temperature was observed for these crosslinked membranes. The conductivities of Br were linearly dependent on the temperature in all the crosslinked membranes (Fig. 6a). The highest Br ion conductivity of 22.9 mS/cm was observed for the c-PPO-Im-100 membrane at 80 ºC. And as shown in Fig. S3, the highest hydroxide conductivity of c-PPO-Im-100 membrane was 21.9 mS/cm, which may be attributed to the incomplete ion exchange as revealed by EDX analysis. (a)

(b)

c-PPO-Im-65 c-PPO-Im-77 c-PPO-Im-100

c-PPO-Im-65 c-PPO-Im-77 c-PPO-Im-100

-3.6

20 -4.0

ln[σ σ (S/cm)]

Bromide conductivity (mS/cm)

24

16

12

Ea= 12.4 KJ/mol

-4.4

-4.8 8

Ea= 10.9 KJ/mol -5.2

4 20

40

60

80

Ea= 12.0 KJ/mol 2.8

3.0

3.2

3.4

-1

Temperature (°C)

1000/T (K )

Fig. 6. (a) Br conductivities of crosslinked c-PPO-Im-65, 77, and 100 membranes as a function of temperature; (b) Arrhenius plots of crosslinked c-PPO-Im-65, 77, and 100 membranes.

Furthermore, to further study the conductivity–temperature relationship, the activation energy (Ea) of the crosslinked membrane samples was also investigated by their linear fitted Arrhenius plots. The plot of lnσ (Br–) vs. 1000/T for the three crosslinked membranes and the Ea were calculated by the formula: Ea= −k×R, where k was the slope of the plot and R was the molar gas constant (8.314 J/(mol·K) (Fig. 6b). The Ea values of c-PPO-Im-65 and -77 membranes were found to range from 10.9 to 12.4 KJ/mol. The c-PPO-Im-100 membrane showed the highest Ea of 12.4 KJ/mol, consistent 21

with the law of Br conductivity and the upgraded IEC (Br–) resulted from the increased degree of bulky imidazolium cation substitution. These Ea values were close to those of other imidazolium-based AEMs [19, 21, 52, 53]. 3.5 Alkaline stability The alkaline stability of AEMs determines the durability of AEMFC devices. To further explore of the alkaline stability of membranes with bulky imidazolium cations under alkaline conditions at high temperatures, the crosslinked PPO-based membranes were immersed in 1 M NaOH at 80 ºC, and the change in ionic conductivity with time was recorded to evaluate their alkaline stability. The conductivities of c-PPO-Im-77 and c-PPO-Im-100 increased gradually and reached maximum values of 11.1 and 12.2 mS/cm following soaking in 1 M NaOH solution at 80 ºC for 60 h (Fig. 7). Notably, no apparent fluctuations were observed in the ionic conductivity over 960 h, with the conductivities of c-PPO-Im-77 and c-PPO-Im-100 remaining at ~100% over the test period. Additionally, there was no obvious change in the intensity of the characteristic absorption peaks of imidazolium ring at 1608 cm–1 and 732 cm–1 for the aged crosslinked membranes (Fig. 2). The excellent alkaline resistance of these crosslinked membranes with bulky imidazolium cations might be attributed to the hindered attack of hydroxide ions originating from the bulky steric hindrance of substituents at the C2, C4, and C5 of imidazolium cation positions and the dense crosslinking network structure, in line with the reported imidazolium monomers [33, 34] and better than other non-crosslinked imidazolium-AEMs with different substituent groups at the C1–C5 position (Fig. 8). In addition, in the operating cycle of fuel cells, the polymer electrolyte membrane is subjected to mechanical stability in a long-term, strong alkaline, and harsh chemically oxidizing–reducing

22

environment. The transparency of membranes decreased slightly, and there was no obvious change in the cross-section of the microstructure of the membrane during the earlier immersion process (Fig. 4). However, the cross-sections of the degraded membranes were rougher and had an increased porosity after soaking for up to 960 h in 1 M NaOH solution at 80 ºC. This was attributed to the cross-linking bonds being partially destroyed and to the slight precipitation of Grubb's second generation catalyst in the strong alkaline environment. Additionally, the slight strengthening of the out-of-plane bending vibration peaks of the terminal H-C= group at 998 cm–1 and 909 cm–1 (Fig. 2) could further confirm that the destroyed crosslinking of the covalent bond exposed the terminal double bond in the strong alkaline conditions. Surprisingly, the partly destroyed double bonds did not completely affect the mechanical properties of these crosslinked membranes, and the aged membranes retained their flexibility and robustness, with the tensile strength (39.9 MPa) and elongation at break (2.6%) of aged c-PPO-Im-100 membranes maintained at 80% and 86.7% of the values of the original membrane, respectively (Table 1). In conclusion, these results demonstrated that the combination of a crosslinked network via olefin metathesis reaction and the bulky steric hindrance at the C2, C4, and C5 positions of imidazolium was an effective strategy to maintain the good membrane physicochemical stability.

23

120

Conductivity remained (%)

100

80

@ 1 M NaOH, 80 °C 60

40

c-PPO-Im-77 c-PPO-Im-100

20

0 0

200

400

600

800

1000

Time (h)

Hydroxide conductivity remained (%)

Fig. 7. Alkaline stability tests of c-PPO-Im-77 and c-PPO-Im-100 AEMs in 1 M NaOH at 80 ºC.

100

80

60

40

20

This Work FPAES-Im-46 [54]

PES-Im1, 2 [58] CPES-Im0.95-TFH0.05 [59]

PAEK-PIL0.4 [55]

SAN70-[DMVIm] [OH]30 [60]

NMImPPO [43] DMImPPO [43] TMImPPO [43] AAEM-2 [34] 6BPAEK-BIm100 [56]

TrimPES-0.4 [53] 1.25-PAEK-QDM [61] poly(MEBIm-OH) [62] Im-PFEKS [44] polyfluorene ionomer [63]

11BPAEK-BIm100 [56]

Im-Am-PAEKS-3 [64]

[PDMVBIm] [OH] [57] PES-Im1 [58]

PSf-ImmOH-70 [65] TRP-100 [21]

0 0

200

400

600

900 1000

Time (h)

Fig. 8. Comparison of alkaline stability of reported imidazolium-based AEMs.

3.6 H2/O2 AEMFC performance and durability For practical evaluation of these materials, the anion-conductive PPO-based polymers with bulky imidazolium groups, the c-PPO-Im-x membranes were utilized as polymer electrolyte membranes in a single H2/O2 AEMFC. After numerous attempts, an open circuit voltage (OCV) of 1.0 V was observed for the cell with the crosslinked membrane (c-PPO-Im-100) having the highest 24

ionic conductivity (9.9 mS/cm at room temperature) as a separator in MEA, indicating the low gas permeability of crosslinked membrane for a separator [22, 48, 66]. However, polarization curves of the corresponding MEA with the crosslinked AEM cannot be obtained. The infeasibility to obtain the initial performance of AEMFCs may be attributed to the poor water uptake, so the worse migration of water at the interface between catalyst layer and membrane was not conducive to establish a effective phase boundary [22, 32]. Importantly, the ionomer in membrane electrode for alkaline fuel cell is directly contacted with the catalysts which undergo the most electrochemical stress in an operating fuel cell [28]. Therefore, the anion-conductive PPO-based polymers with bulky imidazolium groups, the PPO-Im-x polymers were used as ionomers in the catalyst layer to study their practical application in alkaline fuel cell for the well solubility in low boiling point solvent (methanol) and excellent alkaline stability. The home-made LSCQA-40 AEMs was fabricated into the membrane electrolyte assemble using BTMA-40, PPO-Im-100 ionomers and c-PPO-Im-100 crosslinked ionomers in the catalyst layers. Fig. 9 showed the polarization curves of H2/O2 AEMFC at 60 ºC with the H2/O2 flow rate of 1000 sccm. It was found that OCVs of the membrane electrolytes with these binders were all above 1.0 V, close to the theoretical value of 1.23 V [48], which indicated that the successful electrochemical reactions on both cathode and anode have been occurred with using the PPO-Im-100 ionomers and c-PPO-Im-100 crosslinked ionomers. The MEA with PPO-Im-100 ionomers displayed a peak power density of 185 mW/cm2 at current density of 438 mA/cm2, which closed to the initial fuel cell performance of crosslinked c-PPO-Im-100 ionomers (peak power density of 173 mW/cm2 at current density of 410 mA/cm2), and both of them were lower than the performance of BTMA-40 as ionomers in catalyst layers (274 mW/cm2 peak power density at a current density of 472 mA/cm2), This could be attributed to the higher water uptake at 20 25

ºC (~ 95 wt%) of BTMA-40-based AEMs compared with the c-PPO-Im-x AEMs (< 8 wt%), which made it difficult to use the crosslinked c-PPO-Im-x membrane as the separator in the AEMFCs. This phenomenon further emphasized that the higher water uptake accelerated the faster ion transfer in the catalyst layer, which in turn promoted the electrochemical reaction of the cathode since consumed 2 water molecules per four electron cathodes transferred [67-69]. 1.2

300

BTMA-40 PPO-Im-100 c-PPO-Im-100

1.0

250

Voltage (V)

150 0.6 100 0.4 50 0.2

Power density (mW/cm2)

200 0.8

0 0

100

200

300

400

500

600

2

Current density (mA/cm )

Fig. 9. Comparative polarization curves (solid symbols) and power density curves (hollow symbols) of AEMFCs at 60 ºC using LSCAQ-40 as AEMs with H2/O2 flow rate of 1000/1000 sccm (no backpressure).

In addition, the durability tests of the single fuel cells with three ionomers in the catalyst layers were evaluated respectively by the change of the voltage of the cell with the operation time at a constant current 200 mA/cm2. As shown in Fig. 10, the degradation of the membrane electrode under practical fuel cell conditions was reflected along with a rule of gradual decline in performance with operating time. Interestingly, although the MEA with BTMA-40 ionomers in the catalyst layer showed higher initial performance, it was much less resistant than PPO-Im-100 and c-PPI-Im-100 ionomers, and the voltage dropped by 79.2% within 6 hours. In comparison lifetime of cell with PPO-Im-100 ionomers in catalyst (retained 27.9% of voltage after 840 minutes), the crosslinked 26

c-PPO-Im-100 ionomers in catalyst layers showed more durable lifespan and remained 27.1% of voltage after about 1020 minutes, which may be attributed to the immobilization of ionomer crosslinking for the fabrication of durable catalyst layers, and the aggregation of Pt/C nanoparticles in the catalytic oxygen reduction reaction was partially inhibited [70]. 0.8

@ Current density 200 mA/cm2

Voltage (V)

0.6

c-PPO-Im-100 0.4

0.2

PPO-Im-100

BTMA-40 0

240

480

720

960

Time (min) Fig. 10. Durability tests of AEMFCs using LSCQA-40 AEMs at a current density of 200 mA/cm2. Test conditions: cell temperature of 60 ºC, H2/O2 flow rate of 200/200 sccm (no backpressure).

4. Conclusions In conclusion, a series of robust, transparent, and flexible PPO-based AEMs with bulky 2-mesityl-4,5-diphenyl-1-nonene-imidazolium cations have been designed and prepared by covalent crosslinking via the olefin metathesis using a Grubb's second generation catalyst (4 wt% to the polymer) at room temperature. The flexible and robust crosslinked AEMs exhibited good mechanical properties, with a tensile strength at maximum load of 20.8–49.9 MPa and values of elongation at break of 1.5–3.0%. The prepared tough, crosslinked membranes with IECt (Br–) of 1.24–1.35 meq./g exhibited a very low water uptake (9.6–13.9 wt%) and swelling (<7%), even at 80 ºC, attributed to 27

the high crosslinking density (>55%) and the hydrophobic bulky substitutes. Additionally, the Br– conductivity of the c-PPO-Im-100 membrane was 9.9 mS/cm at 20 ºC, but later increased to 22.9 mS/cm at 80 ºC. Furthermore, after 960 h of alkali stability testing, the conductivity remained at ~100% relative to the initial value and the tensile strength (39.9 MPa) and elongation at break (2.6%) of aged c-PPO-Im-100 membranes retained 80% and 86.7%. Moreover, the H2/O2 alkaline fuel cells using PPO-Im-100 and c-PPO-Im-100 ionomers in catalyst layer showed the peak power density of 185 and 173 mW/cm2 at the current density of 438 and 410 mA/cm2, and durability testing of fuel cell with c-PPO-Im-100 ionomers in catalyst layer showed a 72.9% performance loss after operating at a current density of 200 mA/cm2 for 1020 minutes. These results indicate that the combination of crosslinked network and bulky steric hindrance at C2, C4, and C5 positions of imidazolium could protect the imidazolium cations in highly alkaline solutions against hydroxide attack and thus enhance the alkaline stability required for the prolonged operation span of AEMFCs.

Acknowledgements The National Science Foundation of China (NO. 21835005, 51532003), Science and Technology Major Project of Shanxi Province (NO. 20181102019), the Autonomous Research Project of SKLCC, and the Hundred Talents Program of the Shanxi Province are gratefully acknowledged for their financial support.

28

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Highlights:

New AEMs with pendant sterically-protected imidazolium groups were crosslinked via olefin

metathesis. Tough and flexible AEMs with excellent mechanical properties were obtained rather than fragmentation. Conductivity of aged crosslinked membranes remained ~100 % after 960 h.

Improved short-term durability (1020 minutes) for the MEA with the crosslinked ionomers.

Declaration of interests ☒ 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.