Enhanced Conductivity of Anion-Exchange Membrane by

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Energy, Environmental, and Catalysis Applications

Enhanced Conductivity of Anion Exchange Membrane by Incorporation of Quaternized Cellulose Nanocrystal Xia Cheng, Jianchuan Wang, Yunchuan Liao, Cunpu Li, and Zidong Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05298 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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ACS Applied Materials & Interfaces

Enhanced Conductivity of Anion Exchange Membrane by Incorporation of Quaternized Cellulose Nanocrystal Xia Cheng,ξ Jianchuan Wang,*, ξ, Ϯ Yunchuan Liao,ξ Cunpu Li,ξ and Zidong Wei*, ξ ξ School of Chemistry & Chemical Engineering, Chongqing University, Chongqing 400044, P. R. China Ϯ State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu610065, Sichuan, P. R. China KEYWORDS: anion exchange membrane, cellulose, ion conductivity, cell performance, composite membrane

ABSTRACT: High ion conductivity of anion exchange membrane is essential for the operation of alkaline anion exchange membrane fuel cell. In this work, we demonstrated an effective strategy to enhance the conductivity of anion exchange membrane (AEM), by incorporation of quaternized cellulose nanocrystal (QCNC) for the first time. Morphology observation demonstrated a uniform distribution of QCNC within QPPO matrix, as well as a clear QCNC network, which led to significant enhancement in hydroxide conductivities of composite

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membranes, e.g. 2 wt% QCNC/QPPO membrane possessed a conductivity of 160 % (60 mS cm1

, @80 oC) of that of QPPO. Furthermore, H2/O2 cell performance of membrane electrode

assembly (MEA) based on 2 wt% QCNC/QPPO AEM showed an excellent peak power density of 392 mV cm-2 at 60 oC without back pressure, while that of neat QPPO AEM was only 270 mV cm-2.

1. Introduction As the demands for clean energy growing, fuel cell has attracted more and more attentions. Especially the alkaline anion exchange membrane fuel cell (AEMFC), which allows the use of non-noble metal as catalysts, has advantages of fast oxygen reduction kinetics, minimized corrosion problems under alkaline conditions etc.,1 make it suitable for power sources of future vehicles and other portable electric devices. However, the development of AEMFCs is hindered by the poor performance of anion exchange membrane (AEM). As a key component of AEMFC, AEM should possess high ion (usually hydroxide) conductivity and adequate chemical & mechanical stabilities.2 Unfortunately, there are challenges in achieving those goals. To improve the conductivity, one way is to increase the fixed cation concentration within the membrane, i.e., to increase the ion exchange capacity (IEC). Whereas, the mechanical stability compromises due to the excessive absorption of water. In such situation, crosslinking is an effective way to prepare robust AEMs under high IEC, for the immobilization of molecular chains by the cross-linkers.3-10 Nonetheless, the ion conduction path may be blocked by cross-


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linking networks, thus the crosslinking degree and conductivity should be carefully balanced. Except for increasing IEC, there is a different approach to promote ion conduction by microphase separation.11-20 In this ideal, hydrophilic phase with only a small amount of cation groups forms a continuous phase (low IEC), leading to a very efficient ion conduction channel in AEM. Therefore, high conductivity and mechanical stability can be obtained simultaneously. Difficulties in this way are the synthesis and phase control of block copolymer. Another facile route to enhance the conductivity of AEMs is fabricating hybrid membranes.21, 22 By incorporation of other materials, the comprehensive performance of AEMs could be improved. Triptycene Poly(Ether Sulfone)s，23 SiO2,24-27 TiO2,28, 29 α-Al2O3,30 Bentonite,31 ZrO2,32, 33 montmorillonite,34 POSS35 and graphene36-38 have been reported to be the inorganic fillers of AEMs. Cellulose is a renewable material produced from the biosphere, it has been widely used in filtration as membrane. By filling porous cellulose nonwovens with ionomer, cellulose supported AEM for fuel cell had been fabricated.39 Besides, with reactive hydroxyl groups, cellulose can be further functionalized with cationic groups for ionic transportation and thus applied in AEMs.40-42 Furthermore, by hydrolysis with hydrochloric acid, cellulose nanocrystals (CNC) can be obtained.43 CNC has high surface area, exceptional mechanical properties and surface reactive hydroxyl group which enables it to be surface functionalized44, 45. Thus, with the inherent of hydrophilicity and further surface functionalization, one may expect the enhancement of AEM performance by adding of CNC. In this paper, CNC was surface modified with quaternary ammonium groups, and added into quaternized poly(phenylene oxide) (PPO) to make composite membranes. Ion conductivity, mechanical properties and alkaline stability were investigated as well as IEC, water uptake etc., compared with neat PPO.


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2. Experimental 2.1. Materials CNC (diameter 10-20 nm, longitude100-300 nm) was purchased from Tianjin Woodelfbio Cellulose Co., Ltd. Silane coupling agent (Si-702) (≥ 97.0 %) was purchased from Nanjing Union Silicon Chemical Co., Ltd. Iodomethane (RG, ≥ 98 %, Adamas), N-bromosuccinimide (NBS) (RG, ≥ 99 %, Adamas), poly(2,6-dimethyl-1,4-phenylene oxide (PPO) (1.06 g cm-3, Aldrich) and 2,2-Azobis(2-Methylpropionitril) (AIBN) (98%+, Damas-Beta), trimethylamine (2.0 mol L-1 in THF, Aladdin) were purchased from Shanghai Titan Science & Technology Co., Ltd. Other solvents like N-Methylpyrrolidone (NMP) (AR, ≥ 99.0 %), toluene (AR, ≥ 99.5 %), chlorobenzene (AR, ≥ 99.0 %) and dimethyl sulfoxide (DMSO) (anhydrous, ≥ 99.0 %) were used as received. 2.2 Preparation of QCNC At first, CNC was de-sulfated by concentrated NaOH (aq) to remove the sulfate ester groups on the surface of CNC.45 The de-sulfated CNC was added to a mixed solvent (deionized water/methanol = 20/80), ultrasonicated for 1 h until a well dispersion was achieved. Meanwhile, certain amount of silane coupling agent were dissolved in another beaker with mixed solvent (deionized water/methanol = 20/80), stirred for 2 h to achieve the hydrolysis of silane. Then, the hydrolyzed silane solution was added to the CNC suspension, stirred for another 2 h at room temperature. Next, the suspension was filtered and dried, further heat treated at 100 oC to facilitate the dehydration. At last, the resultant CNC powder was dispersed in NMP, added with excessive iodomethane in order to converted amino groups to quaternary ammonium groups.


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Finally, the resultants were washed with ethanol and water for serval times to remove the impurity as much as possible. As a consequence, QCNC was prepared. 2.3 Preparation of QPPO QPPO was synthesized according to a typical procedure.46 Briefly, PPO (12 g) was dissolved in 100 mL of chlorobenzene to form a homogenous solution, then NBS (5.34 g) and AIBN (0.62 g) was added. The mixture was reacted at reflux conditions (135 oC) for 3 h. After cooling, the resulted solution was poured into 10-fold of ethanol to precipitate the product. The brominated PPO was filtered and washed several times, then dried under at 60 oC. Following, brominated PPO was dissolved in NMP and then added with excessive trimethylamine, stirred for 24 h at room temperature. The solution was poured into 10-fold excess of toluene, the precipitates (QPPO in Br- form) were washed and collected. 2.4 Preparation of QCNC/QPPO composite membranes The required amount (Table S1) of QCNC was dispersed in 15 mL of DMSO respectively, ultrasonicatied for 30 min until well suspension was obtained. Then added into QPPO solution (15 mL), stirred at 70 oC for 3 h. At last, the homogeneous solutions were poured onto glass plate, evaporated at 110 oC for 12 h to form the composite membranes. Furthermore, the membranes were dried in vacuum at 60 oC for 24 h. For neat QPPO membrane, the same procedure was adopted. 2.5 Characterizations 1

H-NMR spectra was recorded on an Agilent 400-MR compact NMR system at 400 MHz. X-ray

Photoelectron Spectroscopy (XPS) analysis was conducted on a PE PHI-5400 spectrometer


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equipped with a monochromatic Al X-ray source (Al KR, 1.4866 keV). Zeta-potential was measured with a Zetasizer Nano Z system (Malvern ZS 90). Thermal gravimetric analysis (TGA Q500, TA Instruments) was performed under N2 flow, and the samples were heated from room temperature to 600 oC at a rate of 10 oC min−1. The morphologies of QCNC and composite membranes were observed with a scanning electron microscope (SEM, Nova NanoSEM 450 or JEOL JSM-5900LV) at an accelerating voltage of 5 kV. Transmission Electron Microscope (TEM) images were obtained with a JEOL JEM-200CX TEM, using an acceleration voltage of 200 KV, samples were microtome sectioned to ultrathin slices, 0.5 wt% QCNC/QPPO, 1 wt% QCNC/QPPO, 3 wt% QCNC/QPPO and 4 wt% QCNC/QPPO were stained with phosphotungstic acid, 2 wt% QCNC/QPPO was unstained. The mechanical properties of AEMs were carried out with a tensile testing machine (MTS, E44.104), all the samples were fully hydrated, tested at a rate of 5 mm/min. For chemical stability test, the samples were soaked in 1 mol/L NaOH at 80 oC, then the conductivities were recorded every other 24 h (tested at 20 oC). For water uptake (WU) and swelling ration (SR), samples (1 cm in length and 1cm in width) were dried under vacuum at 45 oC for 24 h and then the mass, length and thickness were measured. The membranes were immersed in DI water for 24 h to achieve full hydration, then the samples were removed from the DI water and wiped with tissue paper, quickly measured the mass, length and thickness. The WU of the membranes was calculated as follows: W WU =

−W wet dry × 100% W dry

(1)

Where Wwet and Wdry are the mass of hydrated sample and dried sample, respectively. The SRl and SRh was calculated based on the following equations:


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SRl =

SRh =

l wet − l dry l dry

hwet − hdry hdry

× 100%

(2)

× 100%

(3)

Where SRl and SRh are the swelling ratio along length direction and thickness direction, respectively. lwet and ldry are the length of hydrated sample and dried sample, respectively. hwet and hdry are the thickness of hydrated sample and dried sample, respectively. The IEC of AEM samples (about 0.2 g) were measured by a typical titration method. Firstly, AEMs were immersed in aqueous NaOH (1 mol L-1) for 24 h to convert the anion to OHcompletely. Then AEM samples in OH- form were was washed with DI water to remove the residual NaOH, followed by being immersed into 30 mL of 0.1 mol L-1 HCl and equilibrated for 48 h, after which the residual HCl was back titrated by aqueous NaOH (0.1mol L-1) with phenolphthalein as indicator. At last, the titrated AEM samples in Cl- form were weighed after dried under vacuum at 45 oC for 24 h. The IEC of QCNC was determined by the same way, except for that dialysis was adopted to purify QCNC. IEC values of the samples were calculated as follows: V IEC =

HCl

×c

HCl

−V W

NaOH

×c

NaOH

(4)

dry

Where VHCl and cHCl are the volume and concentration of the HCl solution, respectively. VNaOH and cNaOH are the volume and concentration of the NaOH solution, respectively; Wdry represents the weight of the dry membrane.


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Hydroxide and bicarbonate conductivities (σ) of AEMs were conducted with an electrochemical interface (Solatron 1287) in combination with an impedance/gain-phase analyzer (Solatron 1260), at a frequency range from 1 to 106 Hz. The two probe method was applied. For hydroxide conductivity test, samples were converted to OH- form with sodium hydroxide thoroughly before test, then washed with DI waterand measured in fully hydrated state (immersed in DI water, DI water was boiled and cooled with N2). For bicarbonate conductivity test, samples were converted to bicarbonate form with sodium bicarbonate for 48 h, then washed and tested. The ohmic resistance was then obtained from the associated Nyquist plot (interception with the real axis). The ion conductivity was calculated as follows:

σ =

L RA

(5)

Where R is the membrane resistance, L is the distance between the electrodes, and A is the crosssectional area of the membrane samples. For H2/O2 fuel cell tests, Pt/C or PtRu/C catalysts (60 wt% in metal content) were mixed with QPPO (50 % quaternization degree, Figure S3) ionomer solution, ultrasonicated to yield inks consisting of 20 wt% ionomer and 80 wt% catalyst. Then, Pt/C ink and PtRu/C ink were sprayed to the cathode and anode sides of AEM (20 µm thick) respectively, fabricating a catalyst-coated membrane (CCM). The metal loading in both anode and cathode was 0.2 mg cm-2, and the electrode area was 5 cm2. The prepared CCM was immersed in 1 mol/L NaOH for 24 h to be converted to OH- form, followed by positioned between two carbon papers to make membrane electrode assembly (MEA). Single cell fuel tests were carried out with a FEL60-10-10 fuel cell test station (Fuel Cell Technologies, INC.) at 60 oC, pure hydrogen and oxygen were supplied to the anode and cathode at a flow rate of 400 mL min-1 without back pressure.


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3. Results and discussions

Figure 1. 1H NMR spectrum of QPPO. QPPO was chosen as the membrane matrix, for it is the most commonly studied AEM. The chemical structure of QPPO was analyzed by 1H NMR spectrum.14 As shown in Figure 1, a peak at 4.3 ppm was observed, which is ascribed to the protons at position 6 of the methylene group. In addition, a peak at 3.0 ppm which is assigned to the protons at position 7, suggesting the existence of methyl on quaternary ammonium groups. Thus, 1H NMR verified the successful synthesis of QPPO. Although the products in each step were purified as much as possible, there were still some impurities, which might come from NBS, AIBN and trimethylamine derivatives. However, the small content of impurities won’t influence the properties of composite membranes. Furthermore, the quaternization degree (%) of QPPO was calculated, by the comparison of the integration of methylene at 4.3 ppm with methyl at 2.0 ppm. A relative low quaternization degree of 16 % of PPO was acquired, which resulted in a low theoretical IEC of 1.18 mmol/g and compromised ion conductivity of as-made QPPO.


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Figure 2. Schematic of the quaternization of CNC. In order to realize good compatibility with QPPO and facilitate the ion conduction, CNC was surface functionalized with quaternary ammonium groups. The general procedure was shown in Figure 2, original CNC was surface esterified with negative charged sulfate groups by hydrolysis with sulfuric acid, thus it was subjected to sodium hydroxide aqueous solution to remove the sulfate groups at first.45 Then, the alkoxy groups of amino silane hydrolyzed and reacted with the hydroxyl of CNC (dehydration condensation reaction), resulted in surface functionalization with amino groups. Finally, amino groups were converted to quaternary ammonium groups by iodomethane, hence QCNC with surface cationic groups was obtained.


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Figure 3. SEM pictures of CNC and QCNC. Figure 3 showed the morphology of CNC and QCNC at the same magnification. CNC was rodlike nanomaterials with diameter of 10-20 nm and longitude of 100-300 nm, respectively. Compared with CNC, the surface morphology of QCNC was different, implying the coating of silane. Furthermore, because QCNC was washed with ethanol and water to remove the unreacted sliane and iodomethane, the silane should be covalently bonding to the surface of CNC, rather than physical absorption. Thus, the distinction on surface morphologies of CNC and QCNC proved the successful surface functionalization of CNC with silane coupling agent.


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Figure 4. (A) XPS spectra of CNC and QCNC, (B) TGA curves of CNC and QCNC, under N2 atmosphere, (C) Zeta potential curves of CNC and QCNC.


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What’s more, the successful functionalization of quaternary ammonium groups on QCNC was confirmed by XPS, TGA and zeta potential test. As shown in Figure 4 (A), the XPS spectrum of QCNC showed a small peak at 399.15 eV and 101.46 eV, which were assigned to N1S and Si2p respectively. On the contrary, there was no peak of N1S and Si2p emerged from the spectrum of CNC. The atom content of N and Si were 1.6 % and 2.2 % respectively, implying the existence of quaternary groups on the surface of QCNC. Figure 4 (B) demonstrated the TGA curves of CNC and QCNC, CNC possessed a main weight loss around 260 oC, which was ascribed to the unstable hydroxyl groups removed from the main chain. The higher weight loss from 350 oC was attributed the decomposition of polymer main chain. For QCNC, the first weight loss started from 210 oC, which was 50 oC lower than that of CNC, suggesting more unstable groups exist on the QCNC, i.e., amino groups from silane. Note that both CNC and QCNC showed a slight weight loss below 100 oC, because of the water evaporation. Figure 4 (C) demonstrated the apparent zeta potential of CNC and QCNC, the apparent Zeta-Potential of original CNC was 19.4 mV (induced by sulfate groups), while that of QCNC changed to +5.6 mV, suggesting quaternary ammonium groups exist on the surface of QCNC. The content of quaternary ammonium groups in QCNC was roughly estimated to be 8.85 wt%, by calculation from TGA curves and the C and N atom percentages from XPS result (Figure S2). With ionic group functionalized, QCNC possessed a IEC value of ca.1.4 mmol g-1 (by titration).


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Figure 5. TEM images of QCNC/QPPO composite membranes, with different QCNC loadings of 0.5 wt%, 1 wt%, 2 wt%, 3 wt% and 4 wt%. Samples with 0.5 wt%, 1 wt%, 3 wt% and 4 wt% QCNC loading were stained with phosphotungstic acid, sample 2 wt% was unstained. The facture surface of AEMs were studied by SEM (Figure S1), all membranes in this work were dense membrane without pore, which satisfied H2/O2 fuel cell. From the fracture surface, one could hardly observe QCNC aggregation even any single QCNC, indicating a well dispersion of QCNC in QPPO matrix. Further investigation on the morphology of QCNC in the composite membrane was carried out with TEM (Figure 5). For QCNC loadings lower than 2 wt% (including 2 wt%), well dispersion of QCNC in QPPO matrix was achieved. Although a few aggregations were observed, most of the nanomaterials were isolated as single ones, with diameter of approximately 10-20 nm and length of 100-300 nm which was in accordance with


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SEM result in Figure 3. For dispersion of nanomaterials in polymer nanocomposites, such an outcome suggested a good dispersion of QCNC in our membrane. Two main reasons were attributed the good dispersion: 1) with organic groups grafted on the surface, QCNC can disperse in DMSO to form an excellent homogenous suspension. 2) QCNC possessed the same quaternary ammonium groups as QPPO, the good compatibility (meant good interaction between QCNC and QPPO, proved by FTIR spectrum with a prominent red shift in Figure S4) facilitated the dispersion of QCNC in QPPO matrix. For higher QCNC loading at 3 wt% and 4 wt%, obvious aggregations emerged, which would degenerate the membrane properties. What’s more, one can clearly identify a network forming with QCNC loading increasing. At low QCNC loading of 0.5 wt%, it was very hard to observe abundant QCNC, only a few single QCNC scattered in the image. As QCNC loading increased to 1 wt%, plenty of QCNC rods were observed, which started to connect with each other and formed a network. At higher loading of 2 wt%, a well-connected network was completely formed. At much higher loadings of 3 wt % and 4 wt%, severe aggregations appeared, which led to the destruction of QCNC network. The well connected QCNC network within the membrane (2wt%) was expected to improve the ion conductivity, and the results would be shown in the latter paragraph.


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Table 1. IEC, water uptake, swelling ratio, hydroxide conductivity and tensile strength of QPPO and composite membranes Sample

IEC a

WU b

SRl b

SRh b

σb

Tensile strength c

(meq g-1)

(wt%)

(%, length)

(%, thickness)

(mS cm-1)

(MPa) (wet)

QPPO

1.00

15.1±1.1

2.2±0.1

12.7±3.4

16.7±0.2

28.3±0.9

0.5wt%QCNC/QPPO

1.01

18.0±2.2

2.4±0.4

12.8±2.1

19.3±0.6

28.9±2.5

1 wt% QCNC/QPPO

1.06

17.7±1.4

2.2±0.2

10.4±0.6

21.3±0.6

28.6±0.6

2 wt% QCNC/QPPO

1.05

16.9±0.4

2.6±0.3

16.5±5.1

28.0±0.1

30.9±0.5

3 wt% QCNC/QPPO

1.00

16.8±0.2

2.4±0.0

16.4±2.9

20.5±0.3

22.8±2.8

4 wt% QCNC/QPPO

1.04

17.6±0.8

2.3±0.4

14.9±0.4

13.9±0.7

20.2±2.7

a

Determined by titration in the Cl- form

b

Measured in the OH- form at 20 oC

c

Measured with fully hydrated membranes, in the OH- form

IEC, water uptake and swelling ratio of the AEMs were demonstrated in Table 1. As a consequence of low DS, QPPO possessed a relative low IEC of only 1.0 mmol g-1 (by titration). With QCNC (IEC = ca. 1.4 mmol g-1) added, the IEC values of 0.5 wt% QCNC/QPPO, 1 wt% QCNC/QPPO, 2 wt% QCNC/QPPO, 3 wt% QCNC/QPPO and 4 wt% QCNC/QPPO were almost the same, for the loading of QCNC was only a small amount (up to 4 wt%), which won’t contribute too much to the IEC. Because the IEC of all AEMs were nearly the same, the water uptake and dimensional swelling ratio of composite membranes showed hardly change, compared with QPPO membrane. Neat QPPO and QCNC/QPPO composite membranes possessed water uptakes between 15 wt% to 18 wt%. The swelling of membranes was anisotropic, it had a preferred orientation along the thickness. For all AEMs, the swelling ratios along the length direction were all around 2 %, while the swelling ratios along the thickness direction were relatively high, from 12.7-16.9 %. The relatively low water uptakes and swelling


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ratios will benefit the mechanical properties of AEMs, thus quite thin films (20 µm) with low ohm resistance could be adopted to make membrane electrolyte assembly (MEA).

Figure 6. Hydroxide (A) and bicarbonate (B) conductivities of QPPO and composite samples with a function of temperature, Hydroxide (C) and bicarbonate (D) conductivities of QPPO and composite samples with a function of QCNC loading.


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Table 2. Membrane properties of this work and other composite anion exchange membranes -1

-1

Membrane

Fillers

Matrix

IEC (mmol g )

σ (mS cm )

WU (%)

Ref.

QPPO/QCNC(2%)

QCNC

QPPO

1.05

60 (80 oC)

17 (20 oC)

This work

QPSF/1% GO

Graphene oxide

QPSF

1.17

11 (60 oC)

24.7 (60 oC)

36

BPPO-TEA-1TiO215IL

Ionic liquid coated nanoTiO2

QPPO

1.47

22 (30 oC)

8.9 (30 oC)

48

QPPO-Im-3% LDH

layered double hydroxide

PPO-Im

1.94

15 (30 oC)

112 (30 oC)

49

HPSf-GGO-30%

Guanidiniumfunctionalized GO

HPSF

0.90

4.0 (30 oC)

~ 40 (30 oC)

37

TMA-PSU/LDH

layered double hydroxide

QPSF

1.67

3.7 (25 oC)

14 (25 oC)

50

QPSU-0.5%-QPbGs

Quaternized polymer brushfunctionalized graphenes

QPSU

1.77

52 (25 oC)

7.0 (25 oC)

51

Poly(PhTPY)-Zr-TPY

ZrO(ClO4)2

Poly(PhTPY)

0.76

10

20.5

52

AMSi

Functionalized SiO2

QPSF

1.45

45 (25 oC)

5 (25 oC)

53

QPSF/5% MMT-1

CTAC modified montmorillonite

QPSF

̶

24 (60 oC)

~ 40 (60 oC)

34

QPSF/1% MGO

P(VBC-co-St)GO

QPSF

̶

13 (60 oC)

20.8 (60 oC)

54

PSf-Im-76%/12% SiO2-Im

SiO2-Im

PSf-Im

32 (20 oC)

~ 50 (20 oC)

55

1.32

For H2/O2 alkaline anion exchange membranes, hydroxide conductivity is a critical property. As shown in Figure 6 (A), the hydroxide conductivity of neat QPPO was 16.7 mS cm-1 at 20 oC, and rose to 38 mS cm-1 at 80 oC. This insufficient conductivity suffered from the low IEC of only 1.0 mmol g-1 (Table 1). To one’s excitements, for the same QPPO with 0.5 wt%, 1 wt%, 2 wt% and 3 wt% QCNC, the conductivities were higher than those of QPPO in all temperature range. Especially for 2 wt% QCNC/QPPO, the conductivity at 20 oC dramatically increased to 28 mS


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cm-1. Meanwhile, a conductivity of over 60 mS cm-1 at 80 oC was attained, which was almost 160 % of that of QPPO. One might notice, 4 wt% QCNC/QPPO was inferior to neat QPPO, which was ascribed to the over-aggregation of QCNC in composite membranes. Obviously, QCNC loading made a difference to the conductivity of AEMs. Therefore, the hydroxide conductivities as a function of QCNC loading were draw in Figure 6 (C). For all temperatures, with the loading of QCNC increased, hydroxide conductivity increased to a maximum value at 2 wt% loading, then dropped after 3 wt%. Although we tried our best to weaken the effect of carbon dioxide, including boiling the DI water and cooling with N2, bicarbonate conductivities of membranes were measured as well in order to eliminate the influence of carbon dioxide. As shown in Figure 6 (B) and (D), bicarbonate conductivities of all membranes were much lower than hydroxide conductivities, due to the diffusion coefficient of bicarbonate was lower than that of hydroxide. Identically, 2 wt% QCNC/QPPO showed the highest bicarbonate conductivities over all temperature range. This phenomenon might be explained by the threshold theory: 47 at lower loading, QCNC were isolated and scattered in the membrane, conductivity of AEMs showed limited improvement; at threshold value (ca. 1wt%- 2 wt%), QCNC started to form network, thus the conductivity improved; when a well-connected & efficient QCNC network formed (2 wt% in this paper), the conductivity reached the maximum value; at much higher loadings, QCNC over-aggregated, led to frustrated drop in conductivity. To compare our membrane with reported ones, the properties of some other composite AEMs were listed in Table 2.


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Figure 7. (A) Illustration of hydrophilic channels in composite AEMs induced by QCNC, (B) TEM image of composite membrane, sample was stained with phosphotungstic acid, the dark region was hydrophilic area. To explain the conductivity improvement by incorporation of QCNC, the influence of IEC can be excluded first, because the IEC of QPPO and QCNC/QPPO samples were nearly the same (see Table 1). As analyzed in the last paragraph, the loading of QCNC greatly affected the conductivity of composite membranes, which was traced to the structure evolution within membranes. Thus the unique QCNC network structure in composite membranes was the only explanation for the improvement on conductivity. As depicted in Figure 7 (A), at appropriate loading (which was 2 wt% in this paper), QCNC interconnected with each other and formed a network in AEM. Since there were abundant hydroxyl and cationic groups on the surface, QCNC were highly hydrophilic which attracted water molecules around them, thus hydrophilic channels formed simultaneously in the AEMs, which facilitated the ion conduction and enhanced the conductivities of AEMs56, 57. The hydrophilic ion channel was further confirmed by TEM image


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(Figure 7 (B)). Sample was stained with phosphotungstic acid, so as to observe the ion exchange groups (dark dots in TEM image) scattering. As shown in Figure 7 (B), the ion exchange groups around QCNC were much more concentrated, thus one can easily identify a distinguished hydrophilic ion channel (the dark area).

Figure 8. Mechanical properties of QPPO and composite membranes, measured with fully hydrated membranes, in the OH- form. Good mechanical properties are essential for outstanding AEMs, since AEM plays the role of isolating the fuel (anode) from oxidant (cathode). Hence, tensile strength and elongation at break of all AEM samples were measured at fully hydrated state, the results were shown in Figure 8. PPO is a strong aromatic polymer with poor ductility. Generally, incorporation of QCNC didn’t reinforce AEMs strikingly, as the tensile strength slightly increased from 28 MPa (neat QPPO) to 31 MPa (2 wt% QCNC/QPPO). For higher loadings of QCNC at 3 wt% and 4 wt%, the tensile


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strengths decreased sharply, which was due to the over-aggregation of QCNC. Regarding to the elongation at break, with the increase loading of QCNC, composite membranes showed slight decrease. Overall, the composite membranes retained the essential good mechanical properties of QPPO, thus membranes as thin as 20 µm could be fabricated, which satisfied the operation of MEA.

Figure 9. Alkaline stability (1 mol/L NaOH, 80 oC) of QPPO and composite membrane, conductivity was tested at 20 oC. AEM works at both strong basic circumstance and high temperature. It is well known that benzyltrimethylammonium group is quite unstable under alkaline circumstance, the degradation mechanism of this cationic group is mainly via direct nucleophilic substitution (displacement), but appears to be more complex. This had been well summarized in a review paper.2 Therefore, alkaline stability test was performed at 1 mol/L NaOH & 80 oC. As Figure 9 showed, the conductivity of QPPO decreased quickly from 30.8 mS/ cm-1 to 6.1 mS/ cm-1, only 24 % of


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original conductivity was remained after 120 h. Unfortunately, the conductivity of QCNC/QPPO membrane also declined, a conductivity of only 5.0 mS/ cm-1 was left for 2 wt% QCNC/QPPO after 120 h. Overall, QCNC couldn’t improve alkaline stability, nor speeded up the degradation, the degradation mechanism is the same for both neat QPPO membrane and QCNC/QPPO composite membrane.

Figure 10. H2/O2 fuel cell performance at 60 oC without back pressure. Solid square and hollow square are the power density and cell voltage of 2 wt% QCNC/QPPO based MEA, solid circle and hollow circle are the power density and cell voltage of neat QPPO based MEA. CCM method was adopted to make MEA, PtRu/C and Pt/C were coated on anodes and cathodes of AEM (20 µm) respectively, both with QPPO as ionomer. Single cell performance test was carried out to evaluate the practical properties of AEM in cell system. Both AEMs were the same thickness of 20 µm, MEAs were prepared strictly with the same method and conditions. As shown in Figure 10, MEA based on neat QPPO AEM exhibited


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a peak power density of 270 mW cm-2, at cell voltage of 0.5 V and current density of 0.551 A cm-2. To one’s excited, MEA based on 2 wt% QCNC/QPPO AEM showed a dramatically increase in cell performance. At cell voltage of 0.85 V and current density of 0.45 A cm-2, its peak power density elevated to 392 mW cm-2, which was 145 % of that of QPPO based MEA. The much higher peak power density was attributed to the enhanced ion conductivity of 2 wt% QCNC/QPPO AEM, compared with QPPO AEM. This convincible cell performance result was consistent with that of conductivity test.

4. Conclusions CNC was surface functionalized with quaternary ammonium groups and added into QPPO to fabricate QCNC/QPPO composite membranes. There was good compatibility between QCNC and QPPO, for they both possessed the same functional groups, which led to a good distribution of QCNC in QPPO matrix. With the QCNC loading increasing, a distinct QCNC network can be observed, which contributed to the enhancement of ion conductivity of composite membranes. For example, the conductivity of 2 wt% QCNC/QPPO membrane was 160 % of that of neat QPPO. Cell performance confirmed the enhancement of conductivity, a much high peak power density of 392 mW cm-2 was achieved with 2 wt% QCNC/QPPO AEM, while MEA based on QPPO only exhibited a value of 270 mW cm-2.

ASSOCIATED CONTENT Supporting Information


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Table of QCNC and QPPO proportion in membranes preparation; SEM pictures of the fracture surfaces of membranes; TGA curves of CNC and QCNC; 1H NMR of QPPO for ionomer; FTIR spectrum of CNC, QCNC and 2 wt% QCNC/QPPO. AUTHOR INFORMATION Corresponding Author *Jianchuan Wang, E-mail: [email protected] *Zidong Wei, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We would like to express our great thanks to the financial supports by the Major Program of the National Natural Science Foundation of China (Grant No. 91534205), National Natural Science Foundation for young scientists of China Grant No. 21706020) and the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (Grant No. sklpme20174-04). REFERENCES (1) Wang, Y.-J.; Qiao, J.; Baker, R.; Zhang, J., Alkaline Polymer Electrolyte Membranes for Fuel Cell Applications. Chemical Society Reviews 2013, 42 (13), 5768-5787. (2) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.;


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Table of Content


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Enhanced Conductivity of Anion-Exchange Membrane by

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