Ultra-stable and high ion-conducting polyelectrolyte based on six

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Ultra-stable and high ion-conducting polyelectrolyte based on six-membered N-spirocyclic ammonium for hydroxide exchange membrane fuel cell applications Nanjun Chen, Chuan Long, Yunxi Li, Chuanrui Lu, and Hong Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02884 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Applied Materials & Interfaces

Ultra-Stable and High Ion-Conducting Polyelectrolyte Based on Six-Membered N-spirocyclic Ammonium for Hydroxide Exchange Membrane Fuel Cell Applications Nanjun Chen, Chuan Long, Yunxi Li, Chuanrui Lu, Hong Zhu* State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Department of Organic Chemistry, School of Science, Beijing University of Chemical Technology, Beijing, 100029, P. R. China Tel:+86-10-64444919; *E-mail: [email protected].

Abstract In response to prepare high stable and ion-conducting polyelectrolyte for hydroxide exchange membrane (HEM) applications, we present an ultra-stable polyelectrolyte based on six-membered heterocyclic 6-azonia-spiro [5.5] undecane (ASU) and polyphenyl ether (PPO). A series of ASU-functionalized PPO polyelectrolytes (ASU-PPO), which can be easily dissolved in low-boiling pointing solvent, have been successfully synthesized by a remote-grafting method. The ASU precursor is stable in 1 M NaOH/D2O at 80 oC for 2500 h as well as in 5 M NaOH/D2O at 80 oC for 2000 h, and the predicted half-life of the ASU precursor would exceed 10000 h, even higher in the future. Besides, these remote-grafting ASU-PPO polyelectrolytes are stable in 1 M NaOH(aq) at 80 oC for 1500 h. Robust and pellucid segmented ASU and triple-ammonium-functionalized PPO based HEMs attach OH− conductivity of 96 mS/cm at 80 °C and realize maximal power density of 178 mW/cm2 under current density of 401 mA/cm2. Keywords: Anion exchange membrane, polyelectrolyte, fuel cells, N-spirocyclic 1

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ammonium, alkaline stability, ion conductivity Introduction Recently, polymer electrolyte fuel cells (PEFCs) have attracted a surge in research interest as highly efficient energy conversion devices.1-3 Especially, alkali membrane fuel cells (AMFCs) have gained prominence and considered as a primary succedaneum for next-generation and low-cost fuel cell systems since the early 21st century due to utilizing non-noble metal catalysts and having the faster cathode reaction kinetics.4-8 However, hydroxide exchange membrane (HEM), which is a key material for AMFCs, is prone to suffer from the Hofmann degradation in alkali media, resulting in severely damaging the HEMs conductivity.9-12 This problem in HEMs has been primary limitation to realize the commercial application of HEMs used in actual devices. Numerous cationic polyelectrolytes have been developed in HEMs to improve the lifetime and ion conductivity of the HEMs.13-18 Nowadays, the ion conductivity of HEMs, which is comparable with perfluorosulfonate (Nafion) membrane in PEMFCs, has been prominently improved by different strategies. HEMs with suitable micro-phase separation have been proved to possess good membrane conductivity and chemical stability.19 Hickner and coworkers reported multi-cations side chain HEMs which distinctly improves the hydroxide conductivity and chemical stability.20, 21 In terms of alkaline stability in HEMs, researchers have developed many different cation systems

in

recent

years.

Yan

and

coworkers

have

reported

a

tris(2,4,6-trimethylphenyl) phosphonium and a permethyl cobaltocenium,22, 2


methyl 23

and

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both of two cationic groups show the excellent alkaline stability. Nevertheless, it is quite difficult to graft these cationic groups in polymer backbone by an efficient method, limiting their applications in HEMs. Holdcroft and coworkers have reported a poly(arylene-imidazoliums) based polyelectrolyte which is stable in 10 M KOH(aq) at 100 oC for 168 h.24 Even different organometallic cations,25-27 which haven’t suffered from Hofmann degradation in alkali media, have been explored in HEMs. However, these HEMs still can’t combine the high ion conductivity and good durability. These large-hindrance cationic groups are still limited by the complicated preparation process and relatively-low ion conductivity to their actual applications. Marino and Kreuer28 systematically studied the alkali stability of quaternary ammonium (QA) groups and found that QA groups with N-heterocyclic structure show superb alkaline stability. Especially, 6-azonia-spiro [5.5] undecane (ASU) groups containing two six-membered rings exhibit the highest half-life of 110 h in 6 M NaOH at 160 oC. Notably, the half-life of ASU is much higher than that of tetramethylammonium (TMA) benchmark (61.9 h). The steric conformation and steric hindrance effect of six-membered rings effectively hinder the elimination and substitution reactions in ASU rings. Compared to five-membered 5-azonia-spiro [4.4] nonane (ASN), the half-life of the ASN only reaches 28.4 h, indicating that N-spirocyclic QA with two six-membered rings is much better than five-membered or other-membered rings. In contrast, the benzyltrimethylammonium (BTMA) merely reaches a half-life of 4.2 h. At present, researchers have reached a consensus that the benzylic-ammonium structure should be avoided in HEMs, which would accelerate 3



the degradation of QA groups under alkaline conditions.29-32 Jannasch et al

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33

has

reported N-spirocyclic QA polyelectrolytes containing five- and six-membered rings are stable in 1 M KOD/D2O at 80 °C for 1800 h. Besides, this polyelectrolyte exhibits a good hydroxide conductivity and already shows a promising application in HEMs. In addition, according to their recent reports,34, 35 any additional heteroatoms should not appear in N-spirocyclic rings, otherwise the electron-withdrawing effect of heteroatoms would accelerate the degradation of the N-spirocyclic rings. Nowadays, the ASU cationic groups have not been successfully utilized in HEMs with an effective method due to lacking of the grafting side in ASU rings. Therefore, the appropriate ASU precursor needs to be developed to prepare the high-stable HEMs for AMFC applications. Herein, we present a reliable and ultra-stable ASU precursor to effectively utilize the ASU group in HEMs. A piperidine-containing ASU (P-ASU) precursor was designed and prepared by a ring-closing reaction (as shown in Scheme 1), On the one hand, considering the electronic effect of the substituent groups in ASU, the electron-donating alkyl chain was designed to attach with the ASU ring, which would increase the steric hindrance of the ASU ring and improve the alkaline stability. On the other hand, the piperidine groups in P-ASU precursor are used to efficiently graft the ASU groups with halogenated polymers. In this work, the P-ASU precursor is grafted with brominated poly-2,6-dimethyl-1,4-phenyl ether (BPPO) to fabricate ASU functionalized PPO polyelectrolytes (ASU-PPO), and the ASU groups are remotely swung on PPO backbone. The alkaline stability of the ASU precursor 4


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and ASU-PPO polyelectrolytes was evaluated in different concentrations of NaOH at 80 oC for a long time. The ASU-PPO polyelectrolyte was fabricated into a membrane by an electrostatic-spraying method. However, the ASU-PPO membranes with high ASU-functionalized degree show a little brittleness due to the rigidness of ASU ring. Therefore, a little of triple-cation (TC) precursors were jointly embed into the ASU-PPO polyelectrolyte to fabricate ASU-TC-PPO segmented membranes so to improve the flexibility. The electrochemical performance, chemical stability, and mechanical property of ASU-TC-PPO membranes were test to assess their practical applications in fuel cells. Scheme 1. Structure of ASU precursor and ASU-PPO polyelectrolyte.

Experiment 2.1 Materials 1,3-di(piperidin-4-yl) propane, poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) (dried at oven), 1, 5-dibromopentane, and dibenzoyl peroxide (BPO) were purchased from

Sigma-Aldrich.

Trimethylamine

solution,

N,N,N,N-tetramethyl-1,6-hexanediamine, and butanimide (NBS) were purchased 5



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from J&K Company. Tetrahydrofuran (THF), chloroform, N, N-dimethylformamide (DMF), potassium carbonate (K2CO3), and sodium hydroxide are purchased from Aladdin and used as received. 2.2 Preparation of 3-(3-(piperidin-4-yl) propyl)-6-azaspiro [5.5] undecan-6-ium (P-ASU) The synthesis of P-ASU was performed by successive quaternarization reactions between 1,3-di(piperidin-4-yl) propane and 1,5-dibromopentane in THF solution. 4 g of 1,3-di(piperidin-4-yl) propane was dissolved in 120 mL THF solution, and then 2.59 mL of 1,5-dibromopentane and 1.31 g of K2CO3 were added. A white solid was observed after continuously stirring the above solution at 50 oC for 12 h. After complete reaction, the white solid was filtrated and carefully washed by ethyl acetate for three times to remove any reagents. Subsequently, the white precipitate was dissolved in ethanol/methanol to remove the insoluble inorganic salt. Finally, the pure P-ASU precipitate was obtained after evaporating the solvent with a 72% yield. 1

HNMR and

13

CNMR spectra of the P-ASU are shown in Figure S1 and Figure S2.

Mass spectroscopy (MS) analysis with an electrospray ionization source is shown in Figure S3. 2.3 Preparation of ASU-PPO polyelectrolyte. Preparation

of

(5-bromopentyl)trimethyl

ammonium

bromide,

1-(N’,N’-dimethylamino)-6-(N,N’-dimethylammonium)-11-(N,N’,N’’-trimethyl ammonium) undecane bromide (DMTMUB), and brominated PPO (BPPO) were according to our previous works

36-38 1

. HNMR spectra of (5-bromopentyl)trimethyl 6


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ammonium bromide, DMTMUB, and BPPO are shown in Figure S4 and Figure 1. BPPO with different brominated degrees were prepared by a free-radical bromide method. BPPO with 60 % brominated degree at benzyl position was dissolved in DMF solution, then the P-ASU precursor and K2CO3 were added to the polymer solution for the Menschutkin reaction. The reaction was continuously stirred at 65 oC for 72 hours. Subsequently, the reaction solution was poured into CCl4 to obtain a viscous yellow precipitate. The precipitate was carefully washed by deionized water for several times. Finally, the precipitate was dried in an oven at 60 o

C for 24 hours to obtain the ASU-PPO polyelectrolyte. Finally, the ASU-PPO

membrane was fabricated via spraying the ASU-PPO polyelectrolyte on silicon wafer. 1

HNMR spectrum of the ASU-PPO polyelectrolyte is shown in Figure 1(c).

2.4 Synthesis of ASU-TC-PPO membrane. As description in section 2.4, 40%, 50%, 60%, and 70% functionalized degree of ASU-PPO polyelectrolytes were prepared in DMF solution, and then additional 10% functionalized degree of DMTMUB was added to the solution for another 72 hours. After complete reaction, the solution was precipitated in CCl4 to obtain a yellow solid. Then, the solid was purified by deionized water for three times. Finally, the solid was dissolved in ethanol to form a yellow solution. The ASU-TC-PPO/ethanol solution is sprayed onto the surface of a smooth silicon wafer with a heating process, and then the ASU-TC-PPO membrane was peeled from the silicon wafer to obtain a dense and transparent membrane. The picture of the ASU-TC-PPO membrane is shown in Scheme 2. Finally, the ASU-PPO and ASU-TC-PPO membranes were exposed in 1 M 7



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NaOH for 48 hours, and then the membrane samples were soaked in deionized water to purify excessive salts. 2.5 Characterization 2.5.1 Structural Characterization The structure of the cationic precursors and polyelectrolytes is determined by 1

HNMR and mass spectrometry. A Bruker AV 400 NMR spectrometer was used to

record the 1HNMR spectra. CDCl3, DMSO-d6, and D2O were used as solvents. 2.5.2. Physical properties The ion exchange capacity (IEC) of the ASU-TC-PPO membranes was determined by a back-titration method. A membrane sample in OH− form was immersed in a 0.01 M HCl solution for 48 h to complete ion exchange. Subsequently, the HCl solution was titrated by a 0.01 M NaOH solution, and a Mettler toledo pH meter was used to accurately detect the titration end-point. The dry mass of the membrane sample was measured in Cl− form. The IEC was calculated as follow:

IEC=

Cl V1 -C2 V2

(1)

mdry

C1 and C2 signify the concentration of the standardized HCl and NaOH solution, and V1 and V2 are the volume of the standardized HCl solution and NaOH solution, respectively. The mdry is the dry mass of the membrane sample in Cl− form. The water uptake (Wu) and swelling ratio (Sr) of the membrane sample were determined in OH− form. Typically, a membrane sample was soaked in deionized water for 12 hours. Then the filter paper was used to wipe the water on the surface of the membrane, and then the wet mass (mwet) and one-direction size (length or width) 8


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(L1) were measured. Next, the sample was dried to constant weight, and the dry mass (mdry) and directional size (L2) of the membrane were measured. The calculation of the Wu and Sr is listed as follow.

Wu %=

wet-mdry

mdry

Swelling %=

L1 -L L2

(2)

×100%

(3)

×100%

The hydration number (λ), defined as the number of water molecules per QA group, as follow:

λ=

Wu*1000

(4)

2.5.3 Membrane Conductivity The ion conductivity of the ASU-PPO and ASU-TC-PPO membranes is measured in both OH− and Cl− forms. In detail, the membrane sample in OH− form was carefully purified by deionized water for several times. The ion conductivity in the

longitudinal

direction

was

performed

on

a

Zahner

Ennium

electrochemical workstation at elevated temperatures under fully hydrated state. The two-electrode AC impedance method was used in measurements, and the frequency ranged from 1 Hz to 100 kHz. The high-frequency impedance was chose to calculate the ion conductivity, as follow:

=

(5)

d represents the membrane thickness, A is the electrode area, and R signifies the membrane resistance.

9



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2.5.4 Thermal stability and mechanical properties. A TG209C thermogravimetric analyzer (TGA) was used to detect the thermal decomposition of the ASU-TC-PPO. TGA measurement is operated under a N2 atmosphere with a temperature ranges from 30 to 800 °C at a heating rate of 10 °C. min−1. The ASU-TC-PPO membrane was cut into a dumbbell-shape (1×5 cm), and then an Instron Model 1185 instrument was used to evaluate the mechanical properties of the membrane at 100% humidity with a crosshead speed of 10 mm·min−1. 2.5.5 Morphology Characterization A Zeiss Supra 55 scanning electron microscopy (SEM) was used to observe the morphologies of the ASU-TC-PPO membrane with a 15 kV operated condition. Atomic force microscopy (AFM, DI Multimode V, Bruker Co.) and small angle X-ray scattering (SAXS) were used to investigate the micro-phase morphology of the ASU-TC-PPO in Br− form. 2.5.6 Alkaline Stability The alkaline stability test of the P-ASU precursor, ASU-PPO polyelectrolyte, and ASU-TC-PPO membrane was evaluated by 1HNMR and

13

CNMR spectroscopy.

Specifically, the P-ASU precursor was dissolved both in 1 M NaOH/D2O at 80 oC for 2500 h and 5 M for 2000 h to evaluate the alkaline stability. The ASU-PPO polyelectrolyte was soaked in 1 mol/L NaOHaq at 80 oC for 1500 h. Then the polyelectrolyte was re-dissolved in d6-DMSO for 1HNMR analysis. Moreover, the 10


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changes in OH− conductivity of the ASU-TC-PPO was performed by soaking the membrane in 1 M NaOH/H2O at 80 oC for 720 hours in N2 atmosphere, and the base solution was updated every 5 days. 2.5.7 Single Cell Performance. The ASU-TC-PPO-50 membrane (IEC=2.35 mmolg−1, 100 µm) was assembled with gas diffusion layer (GDL) and catalyst layer into MEA. A HephasMini-L100 fuel cell system was used to evaluate the cell performance. The humidified gas was controlled at 300 mL/min for both H2 and O2, and the cell was carried out at 60 oC with at a backpressure of 100 kPa. The preparation procedure of MEA was according to our early report.30 In detail, a catalyst ink was prepared by dispersing the commercial Pt/C catalyst (40%, Johnson Matthey Co.) and ASU-PPO ionomer in ethanol/methanol/water solution with a weight ratio of 5:1 (catalyst to ratio). Next, a catalyst coated membrane (CCM) was fabricated after spraying the catalyst ink on the ASU-TC-PPO membrane. Area in both anode and cathode is 5 cm2 with a Pt loading of 0.5 mg Pt cm−2. Finally, the CCM was pressed with GDL to fabricate MEA. Scheme 2. Synthesis route of the ASU-PPO polyelectrolyte and ASU-TC-PPO membrane. (a) Synthesis of P-ASU, (b) synthesis of ASU-PPO, and (c) synthesis of TC precursor.

11



3. Results and discussion 3.1 Polyelectrolyte synthesis and characterization. Fabrication of the ASU-TC-PPO membrane is divided into three steps. (1) Synthesis of the cationic precursors, (2) synthesis of the ASU-PPO polyelectrolyte, (3) fabrication of the ASU-TC-PPO membrane. Firstly, an ASU precursor (P-ASU) with a terminal piperidine group was designed to graft the ASU groups with halogenated polymers. The P-ASU was prepared by continuous nucleophilic substitutions between 3-di(piperidin-4-yl) propane and 1,5-dibromopentane. THF was chosen as the solvent to efficiently synthesize the P-ASU groups, so to eliminate the di-substituted ASU (as shown in Figure S8) product because the P-ASU would immediately precipitate in THF once it formed. The synthesis of the P-ASU precursor can be easily repeated by 12


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other researchers with a high yield. As shown in Figure 1(b), the peaks at 3.61, 3.52, 3.34, and 3.23 ppm are assigned to the -CH2-N+ groups in P-ASU, and the peaks at 2.51 and 2.92 ppm correspond to the protons in sec-amine groups. The protons of -CH2-N+ groups in P-ASU ring show a slight chemical shift due to the steric configuration of the N-heterocyclic ring. Besides, the integral area ratio of amine and ammonium groups is 2:1, indicating the P-ASU is only mono-substitution ASU. MS was used to analysis the molecular weight of the P-ASU. As shown in Figure. S3, two m/z signals at 140 and 279.5 are observed, and both of the signals are assigned to the molecular ion peak of the P-ASU because the secondary amine in P-ASU are easily protonated and doubled the P-ASU charge, so to halve the mass charge ratio. The synthesis of the triple-cations precursor is according to our previous report.38 In Figure S4, the peaks at 3.12 ppm and 2.92 ppm belong to the ammonium and amine groups, indicating the successful synthesis of the triple-cations precursor. Secondly, the ASU-PPO polyelectrolyte was synthesized by the substitution reaction between P-ASU and BPPO. The steric hindrance of the piperidine ring in P-ASU would hinder the further substitution between the P-ASU and BPPO under given temperatures, thus avoiding the formation of benzylammonium structure which has been proved to be detrimental to the alkaline stability of the polymer backbone and QA groups. In Figure 1, the chemical shift of the proton in -CH2Br group (4.41 ppm) is disappeared in Figure 1(c) while a new peak at 3.61 ppm is observed, indicating that -CH2Br groups are completely replaced by piperidine groups. Besides, the chemical shift of the proton in benzene ring in ASU-PPO polyelectrolyte didn’t 13



shift to a low field, indicating that the benzyl ammonium structure doesn’t form in ASU-PPO polyelectrolyte. The solubility of the ASU-PPO polyelectrolyte was studied. As shown in Table S1, the ASU-PPO polyelectrolyte shows a good solubility, and it can be easily dissolved in low boiling point solvent, such as methanol, ethanol, and isopropanol. Therefore, the ASU-PPO polyelectrolyte can be used as a superb ionomer for AMFC applications. The ASU-PPO polyelectrolyte is attempted to fabricate into a membrane by a casting method. However, the ASU-PPO membrane shows a little fragile due to the rigidness of the N-heterocyclic rings in P-ASU, limiting the application of ASU-PPO membrane. Thirdly, to improve the film-forming property of the ASU-PPO membrane, a small amount of triple-cation precursors are introduced to the side chain of the ASU-PPO polyelectrolyte so to prepare the ASU and triple-cations side-chained PPO membrane (ASU-TC-PPO). Different functionalized ASU-PPO polyelectrolytes were prepared to explore the optimal proportion of the P-ASU in PPO backbone, and then 10% functionalization of triple-cations groups were grafted with redundant -CH2-Br groups in ASU-PPO polyelectrolytes.

1

HNMR spectrum of the ASU-TC-PPO

membrane is shown in Figure 2, DMSO/D2O was chose as the mix solvent to eliminate the effect of H2O peak (3.34 ppm) on the polyelectrolyte sample by H-D exchange. The appearance of new peak at 5.12 ppm belongs to the triple-cations groups, indicating triple-cations precursor is successfully grafted on PPO backbone.

14


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Figure 1. 1HNMR spectra of (a) BPPO, (b) P-ASU, and (c) ASU-PPO polyelectrolyte.

15



Figure 2. 1HNMR spectra of (a) TC-QAPPO and (b) ASU-TC-PPO polyelectrolyte.

3.2 Thermal stability and mechanical properties. TGA was used to study the thermal decomposition of the ASU-PPO and ASU-TC-PPO membranes. As shown in Figure 3, all the membrane samples show a similar degradation regular. The initial weight loss below 150 oC is attributed to the evaporation of residual water. The second stage between 190 oC and 300 oC corresponds to the degradation of quaternary ammonium groups, indicating that the ASU-PPO and ASU-TC-PPO membrane are available for AMFC applications. The last stage above 360 oC is assigned to the decomposition of the PPO backbone. The tensile strength (Ts) and elongation at break (Eb) of ASU-TC-PPO-25 membrane at different relative humidity are shown in Figure S5. Humidity strongly 16


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influences the mechanical properties of the ASU-TC-PPO membrane. At low humidity, the ASU-TC-PPO exhibits a high Ts of 26 MPa and low Eb of 23%. However, with the increase of humidity, the ASU-TC-PPO membrane becomes more flexible. Therefore, the Ts of the ASU-TC-PPO membranes is weakened and Eb is increased at high humidity.

Figure 3. TGA curves of the ASU-TC-PPO membranes.

3.3 Physical properties. IEC signifies the number of exchangeable ions, which significantly affects the Wu and membrane conductivity. In general, high IEC of HEM exhibits a higher ion conductivity, but leads to the excessive Wu and Sr. Herein, pH meter was used to measure the IEC of the ASU-PPO and ASU-TC-PPO membranes, and the pH meter has been disclosed more useful and precise than the traditional chemical indicator in our recent works. As shown in Table 1, the ASU-remote-grafted ASU-PPO membrane 17



shows the IEC of 1.84 mmol/g which is closed to the theoretical IEC, implying the P-ASU is sufficiently functionalized with BPPO. The IEC values of ASU-TC-PPO membranes range from 2.23 to 2.49 mmol/g, and the increasing IEC in ASU-TC-PPO is attributed to the existence of triple-ammonium side chain. Wu and Sr crucially influence the dimensional stability of HEMs. In Table 1, the Wu and Sr of the ASU-PPO and ASU-TC-PPO membranes show a same rule with IEC. The Wu of the ASU-PPO membrane shows a moderate value for HEM applications. The sufficient Wu for HEM is beneficial to accelerating the ion dissociation and inducing the ion transport channels. The in-plane swelling ratio of the ASU-PPO and ASU-TC-PPO ranges from 15% to 21%. The temperature dependence of Wu and Sr is shown in Figure. 4. The Wu and Sr of the ASU-PPO and ASU-TC-PPO membranes show a little increasing tendency at elevated temperatures. Compared to the previous ammonium-based PPO membrane,39-44 the Wu and Sr of the ASU-PPO membrane are relatively lower due to the rigid N-heterocyclic rings in P-ASU precursor. On the other hand, the λ values of the ASU-TC-PPO membranes didn’t show a distinct variation, and the increasing ASU-functionalized degree shows a slight improvement in the λ values.

18


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Figure 4. Water uptake and swelling ratio as a function of temperature. Table 1: Physical properties of membrane samples at room temperature

a

b

Membrane

IEC (meq g−1)

IEC (meq g−1)

Wu (%)

Sr (%)

λ

Ref

ASU-PPO-60

2.24

1.84

62±3

8±2

28

√

ASU-TC-PPO-40

2.81

2.23

94±4

15±2

27

√

ASU-TC-PPO-50

2.92

2.35

101±3

17±1

26

√

ASU-TC-PPO-60

3.02

2.41

113±3

20±2

28

√

ASU-TC-PPO-70

3.10

2.49

121±5

21±2

29

√

J10PPO

2.22

2.08

228±20

31

56

[42]

T20NC6NC5N

2.74

2.52

135±5

18

27

[20]

D30NC6NC6

2.74

2.47

130±3

25

26

[21]

a

IEC: theoretical value . b IEC measured by back-titration. √: This work.

3.4 Membrane morphology SEM was used to investigate the morphology of the ASU-TC-PPO. As shown in 19



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Figure 5, the surface and cross-section of the ASU-TC-PPO membrane are tight and dense which are beneficial to avoiding the gas permeation in fuel cell process. The thickness of the ASU-TC-PPO membrane is about 90 µm which can be disclosed in cross section. The hydrophilic/hydrophobic phase separation has been demonstrated to be benefit to construct the ion transport channels which are significant to fabricate the high-efficient HEM. AFM and SAXS were used to study the micro-phase morphology of the ASU-TC-PPO membrane. AFM images of the ASU-TC-PPO membranes are shown in Figure 6, both of the ASU-TC-PPO membranes in Br− form show a distinct hydrophilic/hydrophobic phase separation. The white regions represent the hydrophobic phase which corresponds to the polymer backbone, and the black regions signify the soft hydrophilic phase which is assigned to the ionic clusters in side chains. The flexible alky side chains in ASU-TC-PPO can easily swing in aqueous environment and promote the aggregation of the hydrophilic phases. Therefore, the continuous hydrophilic domains are aggregated with each other to construct the ion transport channels, which has been proved to be significantly improved the ion conductivity in previous reports.45,

46

Furthermore, the

ASU-TC-PPO-60 with a higher IEC shows larger hydrophilic/hydrophobic phase domains compared with the ASU-TC-PPO-40. SAXS patterns of the ASU-TC-PPO membranes are shown in Figure 7, the interdomain spacing (d, nm) between the ionic domains can be calculated from the Bragg’s equation: d=2π/q, where q signifies the ionomer peak which is assigned to interparticle scattering.47 All of the ASU-TC-PPO membranes show a scattering peak around 0.47 nm, and the d values range from 20


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13.02 to 13.56 nm, which are almost in agreement with AFM calculation. The ASU-TC-PPO membrane with high IEC shows a lower scattering peak and a higher interdomain spacing, which also can be clearly seen in Figure 6(b) and (d). In a word, the AFM and SAXS analysis forcefully disclose the clear micro-phase separation in ASU-TC-PPO membranes.

Figure 5. SEM images of the ASU-TC-PPO membrane, (a) plane section, (b) cross section.

21



Figure 6. AFM images of ASU-TC-PPO membrane, (a) and (b) ASU-TC-PPO-40, (c) and (d) ASU-TC-PPO-60.

Figure 7. SAXS patterns of the ASU-TC-PPO membranes.

3.5 Ion conductivity 22


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Membrane conductivity is one of the crucial performance of HEM, which determines the internal resistance of the HEMs. In past few years, the hydroxide conductivity of HEM has been prominently boosted by researchers using a phase-separation strategy. HEM containing pendant cationic groups has been proved to be benefit to the separation of hydrophilic/hydrophobic phase, thus improving the ion conductivity. The Cl− and OH− conductivities of the ASU-remote-grafting ASU-PPO and ASU-TC-PPO membranes at elevated temperatures are investigated to assess their performance. In Figure 8, the ion conductivities of ASU-PPO and ASU-TC-PPO membranes are increased at elevated temperatures due to the accelerating mobility of ion. The ASU-PPO membrane with a low IEC reaches maximum Cl− and OH− conductivities of 33 mS/cm and 68 mS/cm at 80 oC, respectively. The ASU-TC-PPO membranes display higher ion conductivities due to the excessive triple-ammonium groups on their side chain, and the Cl− and OH− conductivities of the ASU-TC-PPO membrane range from 17~43 mS/cm and 36~96 mS/cm, respectively. Notably, the actual OH− conductivity of the ASU-TC-PPO is higher than current values due to the inevitable effect of CO2. These ion conductivity results achieve a moderate level in current researches.48-50 Especially, compared to the large-hindrance-cation HEM systems, the ASU-TC-PPO membranes show their advantages in ion conductivity. On the other hand, the balance of the hydroxide conductivity and swelling ratio in these ASU-PPO membranes at elevated temperatures is pretty harmonious. The ASU-TC-PPO membranes didn’t show an excessive swelling ratio at high hydroxide conductivity, which is a precondition to the 23



practical application of the ASU-TC-PPO membrane used in AMFCs.

Figure 8. (a): Cl− conductivity and (b): OH− conductivity of the ASU-PPO and ASU-TC-PPO membranes as a function of temperature.

3.6 Alkaline stability The alkaline stability of HEMs determines their lifetime in AMFCs. Recently, N-heterocyclic ammonium groups have attracted prominent attention due to a significant improvement in alkaline stability. Especially, in the past three years, ASU groups have been considered as one of the highest stable ammonium groups. However, how to efficiently utilize ASU groups in practical HEMs has been an urgent issue for researchers. Lillocci and coworkers51 have demonstrated that the degradation rate of N-heterocyclic ammonium groups is related to the ring strain, and the alkaline stability of six-membered ASU is significantly higher than that of the 5- and 7-membered rings. Ammonium groups are prone to suffer from the Hofmann degradation and SN2 substitution in alkali media. On the one hand, the Hofmann degradation undergoes an elimination in a trans-coplanar intermediate. However, the 24


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N-heterocyclic structure is difficult to form a coplanar structure, thus decreasing the possibility of elimination. On the other hand, the SN2 substitution is triggered by a back-attacking reaction from nucleophile (such as OH−, CH3O−). Nevertheless, the N-heterocyclic structure shows a prodigious steric hindrance and hinders the SN2 substitution. The stereo structure of ASU is shown in Scheme 3. As disclosed in recent report,34 an electron-withdrawing group in ASU ring would significantly weaken the stability of the ASU due to accelerating the degradation pathway. Therefore, the electron-withdrawing groups should be avoided in ASU or kept away from the rings. Herein, we present an ultra-stable P-ASU precursor for HEM applications. The electron-donating alkyl chain is designed to near the ASU ring to avoid the withdrawing effect. Besides, the alkyl chain would increase the steric hindrance of the ASU ring and improve the alkaline stability. In order to assess the alkaline stability of P-ASU, 1HNMR and 13CNMR spectroscopy were simultaneously used to monitor the P-ASU structural variation after dissolving it in 1 M NaOH/D2O at 80 oC for 2500 h. In Figure 9, 1HNMR spectra of the P-ASU precursors didn’t show any degradation signals after 2500 h. Besides, as shown in Figure S7, nothing changed has been observed in

13

CNMR spectra of the P-ASU after alkaline treatment as well. For

accelerating the degradation, we increase the alkali concentration to 5 M, and notably, the concentration of NaOH in D2O is difficult to exceed 5 M because the NaOH would affect the solubility of P-ASU. Similarly, as shown in Figure S6 and Figure S7, no degradation signal was detected in 1HNMR and

13

CNMR spectra after alkaline

stability test of the P-ASU in 5 M NaOH/D2O at 80 oC for 2000 h, only a little 25



insoluble white crystals were observed. Up to now, the alkaline-stability evaluation of the P-ASU is still proceeded, and we predict the half-life of the P-ASU will exceed 10000 hours in 1 M NaOH, even higher in the future. The P-ASU precursor exhibits an ultra-stable alkaline stability and is promising to solve the lifetime problem of HEMs. Moreover, the alkaline stability test of the ASU-PPO polyelectrolyte is performed in 1 NaOH/H2O at 80 oC for 1500 h, and then the ASU-PPO powder was filtrated and re-dissolved in d6-DMSO by ultrasonication for 1HNMR analysis. As shown in Figure 10, the ASU-PPO polyelectrolyte still shows a super alkaline stability after 1500 hours. A big H2O peak at 3.34 ppm is shown in the ASU-PPO spectra and overlaps ASU ring signals. In order to clearly observe the variation of ASU in ASU-PPO polyelectrolyte. D2O is added to the d6-DMSO solution to H-D exchange so to eliminate the effect of H2O. After H-D exchange, the characteristic peaks of the ASU rings are still clearly shown in 1HNMR spectrum. Only the integral area of benzene ring slightly decreases after accelerating experiment, indicating that the slight degradation in ASU-PPO polyelectrolyte is assigned to the PPO polymer backbone. As early report,

52

PPO attached with electron-withdrawing groups (such as,

ammonium, hydroxyl, methoxyl groups) would decrease the alkaline stability. In this work, tertiary amine is designed to attach with PPO so to efficiently graft ASU and decrease the adverse effect on PPO backbone at the same time. 1

HNMR spectroscopy was used to assess the alkaline stability of the

ASU-TC-PPO polyelectrolyte by directly dissolving the polyelectrolyte in 1 M 26


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NaOH/D2O/DMSO solution. The NaOH/D2O/DMSO system has been demonstrated to possess much higher basicity than in NaOH/D2O system according the early reports.53 As shown in Figure 11, the ASU-TC-PPO polyelectrolyte shows a clearly degradation after 96 h along with new peaks at 5.01, 5.72, 2.31, and 2.75 ppm which belong to vinyl groups and tertiary amine. The main degradation pathway in the ASU-TC-PPO polyelectrolyte is attributed to the Hofmann degradation in triple-ammonium groups (as shown in Figure 11(b)). Moreover, the changes in hydroxide conductivity in ASU-TC-PPO membrane was used to evaluate the actual membrane stability in 1 M NaOH/H2O at 80 oC. As shown in Figure 12, the hydroxide conductivity of the ASU-TC-PPO membrane shows a 15.7% degradation after 720 h, which is mainly ascribed to ammonium groups degradation. While the ASU-PPO membrane only shows a slight drop in hydroxide conductivity (3.3%). Nowadays, we are focusing on chemically-stable aromatic polymer backbone and investigating the effect of the number of alkyl chains on the performance of the ASU-PPO polyelectrolyte. Scheme 3. Model of P-ASU and its degradation pathway.

27



Figure 9. 1HNMR spectra of P-ASU before and after alkaline treatment. The P-ASU is dissolved in 1 M NaOH/D2O at 80 oC for various times.

28


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Figure 10. (a) 1HNMR spectra of ASU-PPO polyelectrolyte before and after alkaline treatment in 1 M NaOH/H2O at 80 oC for various times. (b) 1HNMR spectra of ASU-PPO polyelectrolyte after H-D exchange experiment after alkaline treatment for 1500 h. (c) The structure of the ASU-PPO polyelectrolyte.

Figure 11. (a) 1HNMR spectra of ASU-TC-PPO polyelectrolyte before and after alkaline treatment: the ASU-TC-PPO polyelectrolyte is directly dissolved in 2 M NaOH/D2O/d6-DMSO at 80 oC for various times. (b) Hofmann degradation of the ASU-TC-PPO polyelectrolyte.

29



Figure 12. The changes in hydroxide conductivity of ASU-PPO-60 and ASU-TC-PPO-60 membranes after alkaline treatment in 1 M NaOH/H2O at 80 oC for various times.

3.7 Fuel cell performance. The ASU-TC-PPO-50 membrane (IEC=2.35 mmolg-1, 100 µm) was used to evaluate the single cell performance. The single cell measurement was performed in humid H2-O2 feeds at 60 °C. The power density and voltage curves of the ASU-TC-PPO membrane are shown in Figure 13. the ASU-TC-PPO membrane achieves a maximum power density (Pmax) of 178 mW/cm2 under 401 mA/cm2 current density, the Pmax only reaches a general level in current researches.15,20,45 However, the ASU-TC-PPO membrane still displays their availability in fuel cells. We are confident to obtain a better fuel cell performance after we optimize the relevant methods and conditions.

30


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Figure 13. Single cell performance of the ASU-TC-PPO-50 membrane in H2-O2 condition at 60 oC.

4. Conclusion In summary, ultra-stable P-ASU precursor and ASU-PPO polyelectrolyte are present in this work. Especially, the P-ASU is stable in 1 M NaOH/D2O at 80 oC for 2500 h. The P-ASU shows a promising application in preparing highly stable HEMs. Besides, the ASU-PPO polyelectrolyte still exhibits a superb alkaline stability, which is stable in 1 M NaOH/H2O at 80 oC for 1500 h. Furthermore, the ASU-PPO and ASU-TC-PPO membranes also exhibit a superior ion conductivity. The highest OH− conductivity of the ASU-TC-PPO-70 attaches to 96 mS/cm at 80 oC, and the Pmax of the ASU-TC-PPO achieves a 178 mW/cm2. We are focusing on chemically-stable polymer backbone and optimizing the fabrication method of MEA to improve the fuel cell performance.

Acknowledgments 31



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We gratefully appreciate the financial support from the National Key Research and Development Program of China (No. 2016YFB0101203), the National Natural Science Foundation of China (No. 21776014 and U1705253), the International S&T Cooperation Program of China (No. 2013DFA51860), and the Fundamental Research Funds for the Central Universities (No. JC1504).

Supporting Information Available 1

HNMR spectra of 1,3-di(piperidin-4-yl) propane and P-ASU precursor;

13

CNMR spectrum of P-ASU; Mass spectrum of the P-ASU; 1HNMR spectra of

(5-bromopentyl) trimethylammonium bromide and DMTMUB; Solubility of ASU-PPO and ASU-TC-PPO polyelectrolytes; Ts and Eb of ASU-TC-PPO-25 membrane at different humidity. 1HNMR spectra of P-ASU precursor before and after alkaline treatment in 5 M NaOH at 80 oC; 13CNMR spectra of P-ASU precursor before and after alkaline treatment under different alkaline conditions; Possible by-products in P-ASU synthesis process.

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Ultra-stable and high ion-conducting polyelectrolyte based on six

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