Sulfonated Nano Bamboo Fiber Reinforced Quaternary Ammonia Poly

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Applications of Polymer, Composite, and Coating Materials

Sulfonated Nano Bamboo Fiber Reinforced Quaternary Ammonia Poly (ether ether ketone) Membranes for Alkaline Polymer Electrolyte Fuel Cells Yanqiu Peng, Ying Wang, Xing Wei, Jinping Zhou, Hanqing Peng, Li Xiao, Juntao Lu, and Lin Zhuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12637 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Applied Materials & Interfaces

Sulfonated Nano Bamboo Fiber Reinforced Quaternary Ammonia Poly (ether ether ketone) Membranes for Alkaline Polymer Electrolyte Fuel Cells

Yanqiu Peng1, Ying Wang2, Xing Wei1, Jinping Zhou1, Hanqing Peng1, Li Xiao1,*, Juntao Lu1, and Lin Zhuang1,2,*

1

College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China 2

The Institute for Advanced Studies, Wuhan University, Wuhan 430072, China

E-mails: [email protected], [email protected]

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of electrochemical devices that intrinsically enable the use of non-precious metal catalysts. As an important component of APEFCs, alkaline polymer electrolytes (APEs) have been a research focus in recent decades. To minimize the ohmic loss and to facilitate the water transport, the APE membrane should be as thin as possible, which generally requires a trade-off between the ionic conductivity and the mechanical robustness/dimensional stability of the membrane. Here we report a new reinforced APE membrane that can effectively disentangle such a trade-off. The quaternary ammonia poly (ether ether ketone) (QAPEEK) membrane is highly conductive but suffers from the over-uptake of water that leads to significant membrane swelling and weak mechanical strength. Upon reinforcing with sulfonated nano-bamboo fiber (s-NBF), the swelling degree decreases from 27.5% to 7.5% in 80oC water. The thickness of such an s-NBF/QAPEEK membrane can then be reduced to 15 µm, which diminishes the electrical resistance, very suitable for APEFC applications.

Keywords: alkaline polymer electrolytes; sulfonated nano-bamboo fiber; poly (ether ether ketone); ultrathin membrane; fuel cells

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Introduction Although proton exchange membrane fuel cells (PEMFCs) possess many advantages, such as the room-temperature startup, compact assembly, and high power density,1-3 their development has been hindered by a number of factors, in particular the dependence on Pt-based catalysts.4-6 Alkaline polymer electrolyte fuel cells (APEFCs) have been proposed to be a solution to address the Pt-dependence issue.5,7-11 Advantages of APEFCs over PEMFCs include faster kinetics of the oxygen reduction reaction (ORR) and more choices for cathode catalysts using abundant transition metals and carbon materials.12-15 Alkaline polyelectrolyte membranes (APEMs) are the enabling material for APEFCs. Since the performance of APEFCs hinges on the ohmic resistance of membrane electrode assembly (MEA), efforts have been devoted to development of ultrathin APEMs16-18. More importantly, ultrathin membranes can facilitate the water transport during fuel cell operations19-21. In APEFCs, water is produced at the anode, while it is a necessary reactant at the cathode. Driving the efficient transport of water from the anode to the cathode is pivotal to the stable cell operation, particularly at high current densities.22,23 The main challenge for making ultrathin APEMs is to balance the mechanical robustness/dimension stability and the ionic conductivity. In comparison to H+, the mobility of OH− is relatively low.24,25 An effective strategy to obtain high ionic conductivity is to improve the ion-exchange capacity (IEC). However, increasing IEC is accompanied with membrane swelling and weak mechanical strength.26 Although crosslinking is a common strategy to limit the swelling of membrane,27-31 it sacrifices the ionic conductivity at the same time. Another strategy is to make a composite membrane by impregnating the polyelectrolyte into a porous thin film such as poly (tetrafluoroethylene) (PTFE).16,31 However, it is not easy to find a hydrocarbon-based polyelectrolyte that is perfectly compatible with PTFE. The incompatibility leads to an exfoliation of polyelectrolyte from the composite membrane,32 especially at high temperatures. Introducing reinforcement materials to the APEMs is also a good choice except for the decrease of ionic conductivity.33-35 Our previous study has demonstrated an effective method to balance high ionic conductivity and low swelling degree.8 By designing a hydrophobic side chain into the APEM, an ion-aggregating structure can be constructed, which provides optimized channels for OH− conduction. Unfortunately,


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this strategy is not always applicable to all types of APEMs. In the present work, poly (ether ether ketone) (PEEK) is chosen to be the backbone of the APEM. It is an engineering polymer whose thermal and chemical stability, as well as the mechanical strength, meet well the requirements.36 However, the quaternary ammonia PEEK (QAPEEK) also faces the conductivity-swelling dilemma37, as those previously reported APEMs. To address this problem, we report here a new strategy of making composite membrane, namely, the sulfonated nano-bamboo fiber (s-NBF) reinforced QAPEEK membrane. Highly crystalline nano-bamboo fiber (NBF, Figure S1) is regarded as an excellent reinforcing component for polymers.38,39 There are abundant hydroxyl groups on the surface of these nanofibers, leading to a network formed by hydrogen bonds40. The hydrogen bonding demonstrates tremendous potential in managing the swelling polyelectrolyte membranes41-43. As for the choice of strengthening natural fibers, there is jute, cotton, bamboo and so on. Among them, bamboo fiber is one of the most attractive candidates because of its high strength.44 After hydrolysis by sulfuric acid, the nano-bamboo fiber which carried sulfonic acid groups, was well-dispersed.45 Moreover, the ionic cross-linking between the sulfonic acid group (SA) and the quaternary ammonia group (QA) further enforces the strength of the membrane.46 Consequently, the swelling degree of the s-NBF/QAPEEK membrane can be significantly reduced to 7.5% in 80oC water, even with a high IEC of 1.76 mmol/g. Due to the excellent dimension stability, ultrathin membranes (15 µm in thickness) can be made, which possesses a low area resistance (0.069 Ω⋅cm2 at 30oC, 0.0124 Ω⋅cm2 at 80oC), very suitable for APEFC applications.

Experimental section Materials. The synthesis of chloromethylated poly(ether ether ketone) (CMPEEK) was according to the report of Yan et al.36 Bamboo material (toothpick, made from phyllostachys pubescen), sulfuric acid (Sinopharm Chemical Reagent Co. Ltd., 98%), sodium hydroxide (NaOH, Sinopharm Chemical Reagent Co. Ltd., 96%), glacial acetic acid (Sinopharm Chemical Reagent Co. Ltd., 98%), sodium chlorite (NaClO2, Aladdin Chemistry Co. Ltd,


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80%), N,N-dimethylformamide (DMF, Sinopharm Chemical Reagent Co. Ltd., 99%), trimethylamine (Sinopharm Chemical Reagent Co. Ltd., 33% alcohol solution), n-propanol (Sinopharm Chemical Reagent Co. Ltd., 99%), potassium hydroxide (KOH, Sinopharm Chemical Reagent Co. Ltd., 85%), and hydrochloric acid (HCl, Sinopharm Chemical Reagent Co. Ltd., 37%) were used as received without further purification. Synthesis of QAPEEK. The procedure for producing QAPEEK is as follows: 0.2 g of CMPEEK was dissolved totally in DMF to form a 2 wt% solution, then a certain amount of trimethylamine was added, and a stir for 0.5 h at 40oC was needed for a complete reaction. (Scheme 1)

Scheme 1 The synthetic procedure of QAPEEK, s-NBF, and QAPEEK/s-NBF composite membrane.


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Preparation of sulfonated nano bamboo fiber (s-NBF). The preparation procedure is sketched in Scheme 1.47,48 The first step aimed to get rid of hemicellulose from bamboo material. Bamboo material (toothpick) was cut into small segments. 10 g toothpick segments were put into a 250 mL flask, then 100 mL 4 wt% aqueous NaOH were used to cook the toothpick at 80oC for 100 min with mechanical stirring. The resulting fibers were filtered and rinsed with large amounts of distilled water. It is necessary to treat the toothpick for four times with a repeat of cook-wash. After the pH of the residual water close to 7, the earth-yellow raw fibers were then air-drying. The second step was to remove lignin and render the fibers white. 10 g earth-yellow raw fibers were suspended in 300 mL water, 3.75 g NaClO2 and 2.5 mL glacial acetic acid were added to the suspension. The bleaching solution were stirred at 75oC for 60 min, then another part of 3.75 g NaClO2 and 2.5 mL glacial acetic acid were allowed to pour into the suspension, following by another 60 min stirring. The bleaching step was required to repeat five times. After the last treatment, the fiber turned into white. Then an operation of filtering and rinsing with distilled water was needed, and the blenching produce was dried for 24 h at 40oC. To produce sulfonated nano-sized bamboo fiber, further acid hydrolysis was conducted. Briefly, 1 g white bamboo fiber was suspended in 60 wt% sulfuric acid in water to form 4 wt% suspensions. The suspension was stirring at 50oC for 3 h, after that, some water was introduced to dilute the reaction suspension. A successive wash with water by centrifugation at 10,000 rpm was required until a turbid supernatant became visible, and the turbid supernatant were then collected. Three days’ dialysis using distilled water was carried out to ensure the dispersion is neutral. After that, 0.5 wt% s-NBF suspension was obtained. Preparation of s-NBF/QAPEEK membrane. A certain volume of DMF solution of QAPEEK was diluted with n-propanol in a 1:1 volume ratio, the s-NBF suspension was added subsequently in a weight ratio of 1:8 based on QAPEEK. The mixed suspension was stirred overnight at room temperature to form a homogenous solution. The mixed solution was cast onto a glass plate which is clean and flat, and dried at 65oC in oven overnight. To obtain the membrane with OH−, the s-NBF/QAPEEK membrane was immersed in KOH solution (1 mol/L) for 24 h. The KOH solution was changed every 6 hours to make sure complete


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replacement. Finally, the membrane was rinsed many times with deionized water until there was no residual KOH in the water. Fourier Transform Infrared (FTIR) Spectroscopy and 1H NMR. FTIR spectra of membranes were recorded on a Nicoler 6700 FTIR spectrometer. The record is in a range of 400 - 4000 cm-1, and with a wavenumber resolution of 4 cm-1. 1H NMR spectra of CMPEEK and QAPEEK were performed on a Bruker AVANCE III HD spectrometer at 400 MHz employing deuterated dimethylsulfoxide (DMSO) as the solvent with the internal reference tetramethylsilane (TMS). X-ray diffraction (XRD). The powder XRD patterns were obtained on a Shimadzu XRD-6000 X-ray diffractometer equipped with a Cu Kα radiation source, and it is operating at 40 kV and 30 mA. It was measured at 30oC, with a rate at 4o min-1. Diffraction profiles were acquired at a 2θ range of 6o – 40o. To calculate crystallinity index (CI) of s-NBF and other samples, the following Buschle-Diller-Zeronian equation is used: ூ

‫ܥ‬Iሺ%ሻ = ቀ1 − ூ ೘ ቁ × 100% మబబ

(1)

where Im represents the heights of the peak at 2θ of 18.6o, I200 represents the heights of the peak at 2θ of 22.6o. Transmission Electron Microscopy (TEM). A small amount of polymer solution was cast onto a Cu grid to form a thin film. Images were performed on an ultra-high-resolution trans-mission electron microscope (JEOL JEM-2010FEF) with an accelerating voltage of 200 kV. The samples were stained with phosphotungstic acid. Zeta potential (ζ). Zeta potential measurement was conducted with a Malvern Zetasizer Nano ZSP (Malvern Instruments, U.K.) at 25.0±0.5oC. The sample preparation is as follows: the s-NBF suspension, QAPEEK/DMF solution, and a certain ratio mixture of s-NBF and QAPEEK were diluted with DMF/n-Propanol mixed solution to a concentration of 10-4 g/mL. Mechanical strength. A tensile tester (WDW-05, Jinan dongfang Test Machine Co. Ltd. China) was applied to characterize the tensile stress-train behavior of QAPS membrane in OH- form. The constant crosshead speed was set to be 5 mm·min-2, and samples was cut into rectangles with 1 cm in width and 5 cm in length.


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Swelling degree (SD%) and water uptake (WU%). A measurement of of membrane were carried out to evaluate swelling degree (SD%) and water uptake (WU%). At first, a record of dimension and weight of totally dried membrane which are in Cl− form was taken, recorded as xdry(Cl−) and mdry(Cl−). In order to convert Cl− into OH−, a 24 hours immersion of membranes in 1 mol/L KOH solution was required. After that, the membranes were washed with deionized water several times until no free KOH remained. Then the membranes were wiped with filter paper before measuring dimension, xhyd(OH−), and the wet mass, mhyd(OH−). The SD% and WU% were calculated as: SD% = WU% =

௫೓೤ ೏ሺೀಹషሻ ି௫೏ೝ೤ሺ಴೗ష ሻ ௫೏ೝ೤ሺ಴೗ష ሻ

× 100

௠೓೤ ೏ሺೀಹషሻ ି௠೏ೝ೤ሺೀಹషሻ ௠೏ೝ೤ሺೀಹష ሻ

× 100

(2)

(3)

Ion-Exchange Capacity (IEC). A typical titration method was employed to determine the IEC of the membrane. A membrane (OH− form) was immersed in 30 mL 0.1 M HCl solution to exchange out the OH-. After equilibrating for 48 h, a titration was carried out with 0.1 M KOH solution. Finally, the APEM samples were washed with deionized water and dried under vacuum at 45oC for 24 h. Dry mass (mdry(Cl-)) was then weighed. The IEC of the membrane is obtained by: IEC =

௡౟൫ౄశ൯ ି௡౜൫ౄశ ൯ ௠ౚ౨౯ ሺେ୪ష ሻ

(4)

Where ni(H+) is the amount of H+ in the HCl solution before titration, nf(H+) is the amount of H+ in the final HCl solution after immersion for 48 h, and mdry(Cl-) is the weigh of the dried membrane in Cl- form. Area resistance (AR) and ionic conductivity (IC) of polyelectrolyte membranes. AC impedance spectroscopy (IviumStat) was applied to acquire the OH− conducting resistance and ionic conductivity of membranes in fully hydrated state. A membrane (OH− form) with an area of 2×2 cm2 was sandwiched between two electrodes. The testing condition was set as following: at open circuit, a frequency range is from 1 Hz to 1 MHz, oscillating amplitude is 5 mV. The ionic conductivity (σ) is calculated by:


8

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ߪ=

௟ ஺ோ

(5)

where l represents the thickness of APEMs, A is the electrode area, and R means the high-frequency resistance. Fuel cell tests. The procedure of fabricating membrane-electrode assemblies and fuel cell tests were the same as reported in the literature.16 The catalyst powder was dispersed in QAPEEK (Cl− as anion) ionomer solution by ultrasonic. The ink was then sprayed onto each side of a QAPEEK membrane or QAPEEK/s-NBF composite membrane (Cl− as anion) to fabricate the catalysts coated membrane (CCM). The area of all the electrodes is 4 cm2. The catalyst loading for PtRu/C in the anode and Pt/C in the cathode was set to be 0.4 mgmetal/cm2. The weight percentage of QAPEEK in the electrodes was controlled to be 20 wt%. Once the CCM was produced, it was immersed in 1.0 M KOH solution for 10 h to make sure the membrane and ionomer were all in OH- form. Finally, the CCM with OH− anion was repeatedly rinsed with deionized water in order to get rid of residual KOH. And then the CCM was assembled with two pieces of carbon paper (AvCard GDS3250) to form the membrane electrode assembly (MEA). The H2-O2 fuel cells were tested (850e Multi Range, Scribner Associates Co.) under a galvanic mode using humidified H2 and O2 gases. The cell temperature was set to 60oC, and the flow rate of both H2 and O2 gases was 200 mL/min with 0.1 MPa of backpressure.

Results and discussion Characterizations of APEM components. The chemical structure of CMPEEK and QAPEEK can be clearly identified using 1H NMR. As shown in Figure 1a, a peak is seen at 4.7 ppm, characteristic of -CH2Cl. The degree of chloromethylation (DC) of CMPEEK turned out to be 98%. The emergence of the peak at 3.1 ppm (Figure 1b), assigned to -CH3 on quaternary ammonium groups, confirms the successful attachment of the QA group. The C-N vibration can also be observed on FTIR spectroscopy (Figure 2) at around 975 cm-1 and 921 cm-1.


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Figure 1

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H-NMR spectra of CMPEEK (a) and QAPEEK (b)

Figure 2 FTIR spectra of s-NBF, QAPEEK, and QAPEEK/s-NBF membranes. The s-NBF (whose chemical structure is shown in Figure S1) was characterized using FTIR spectroscopy (Figure 2). The absorption around 3400 cm-1 is assigned to the -OH stretching vibration predominantly contributed by residue H2O. Another -OH stretching vibration at 3347 cm-1 is assigned to the hydrogen bond in s-NBF, which can barely been seen


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in the QAPEEK/s-NBF composite membrane. The absorption around 2902 cm-1 (C-H stretching vibration), 1431 cm-1 (C-H bending vibration), and characteristic absorption of carbohydrate structure at 1160 cm-1 and 1070 cm-1 are all contributed by the NBF. A peak at 1089 cm-1 (-SO3H) could be clearly observed which demonstrates the successful sulfonation of NBF.49 The sulfonation of NBF is important for its dispersion in membrane. Actually, a premise behind fabricating composite membrane is that the suspension of fillers. With sulfonic acid group grafted on the surface, s-NBF can disperse well in DMF/n-propanol solution to form a homogenous suspension. Hence, the QAPEEK/s-NBF composite membrane turned out to be homogeneous (Scheme 1). The FTIR spectrum of the QAPEEK/s-NBF composite membrane is mostly an addition of those of s-NBF and QAPEEK, No new signal is observed in the spectrum. The X-ray diffraction was employed to characterize the crystallinity of s-NBF (Figure 3a). The peaks are comprised of several different crystalline planes. The major diffraction peaks at 2θ =16.1o, 22.6o are assigned to the crystalline planes with Miller indices of (110) and (200) in the crystal structure of cellulose I.48 The crystallinity indexes (CI) of s-NBF is calculated to be 65.2%. In principle, the CI of s-NBF (whose hydrolysis was sufficient) should be more than 90%. It indicates that there are some amorphous regions in the s-NBF. The typical (200) lattice fringe distance of 0.387 nm was also verified by high-resolution TEM observation (Figure 3b).

Figure 3 XRD (a) and TEM (b) observation of the crystalline s-NBF.


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Figure 4 TEM image of (a) s-NBF, (b) QAPEEK, (C) QAPEEK/s-NBF composite membrane. More TEM images are presented in Figure S2. TEM was further used to observe the morphology of s-NBF and QAPEEK/s-NBF composite membrane. When the sample is prepared from diluted s-NBF suspension, no aggregations of s-NBF fibers can be found in TEM image (Figure 4a & S2). The length of s-NBFs are between 100 nm and 500 nm, and the diameter are less than 40 nm. The fibers with same size are observed in the QAPEEK/s-NBF composite membrane (Figure 4c), where the s-NBFs are covered with QAPEEK. It can be seen that fibers are well-dispersed in the composite membrane, which leads to an enhancement in the mechanical strength of the composite membrane. Reinforcement of the APEM. As aforementioned, the purpose of introducing the s-NBF component was to reduce the swelling degree. As shown in Figure 5a, the in-plane swelling degree of the s-NBF/QAPEEK membrane was only 7.5% at 30oC (with IEC = 1.76 mmol/g), even when the temperature was elevated to 80oC, the s-NBF/QAPEEK membrane showed a good dimensional stability, and the swelling degree remained essentially unchanged. On the other hand, the swelling degree of QAPEEK membrane was 20% at 30oC (with IEC = 1.70 mmol/g) and 27.5% at 80oC. The through-plane swelling degree, however, takes on a different trend. As shown in Figure 5b, the adding of s-NBF does not reduce the through-plane swell degree, indicating that the s-NBF is laid down, rather than standing in normal direction, inside the composite membrane. It is thus understandable that, due to the through-plane swelling, the water uptake of the composite membrane, albeit lower than that of the QAPEEK membrane, still increases with the temperature (Figure 5c). Nevertheless, the low in-plane swelling degree is key for fuel-cell assembly and operation, while the through-plane swelling will be restricted by the compact structure of the fuel cell.


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Figure 5 The temperature dependence of the swelling degree though-plane (b); and the water uptake (c).

in-plane (a) and

Not only the swelling degree, but also the mechanical strength, of the QAPEEK/s-NBF composite membrane is dependent on the weight ratio of s-NBF. As shown in Table 1, upon increasing the s-NBF ratio, the swelling degree decreases, and the tensile strength of the membrane increases from 8.2 MPa to 15.2 MPa. Obviously, after introducing s-NBF, the mechanical strength has been improved, when the weight ratio of QAPEEK and s-NBF


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reaches 8:1, the composite membrane shows a best mechanical strength. The mechanical strength of membrane is closely related to the in-plane swelling, it can be observed in Table 1. But further increasing the s-NBF ratio did not produce a positive effect, most probably because of the aggregation of NBF.38 To further explore the effect of s-NBF, we employed other two alkaline electrolytes to fabricate composite membrane. As showed in Table S1, after introducing s-NBF, all of the composite membranes are low in swelling degree. These results demonstrate that the s-NBF is indeed an excellent reinforcing filler for APEM. Table 1 Properties of QAPEEK membrane and QAPEEK/s-NBF composite membranes. Weigh ratio Thickness b mQAPEEK:ms-NBF (µm)

a

IEC a Tensile strength c (mmol/g) (MPa)

SD%b

IC (mS/cm)b

30oC

80oC

30oC

80oC

1:0

25

1.70

8.2

20.0%

27.5%

20.7

110

12:1

16

1.66

11.2

17.5%

22.5%

22.0

103

8:1

15

1.76

15.2

7.5%

7.5%

21.4

119

6:1

18

1.59

12.6

5.0%

5.0%

11.11

56.1

4.8:1

20

1.51

11.0

2.5%

2.5%

7.6

31.5

Determined by titration. b Measured in the OH- form. c Measured with fully hydrated membrane, in the OH- form.

The significant reduction in the in-plane swelling and the enhancement in mechanical strength should be a result of the ionic cross-linking between the sulfonic acid group (SA) on s-NBF and the quaternary ammonia group (QA) on QAPEEK. To verify this mechanism, zeta potential (ζ) measurements were carried out to observe the interaction between SA group and QA group. The magnitude of ζ reflects the degree of dispersion between adjacent electrostatic repulsions or similarly charged particles. Experimental results show that the ζ of s-NBF is -16.3 mV, while the ζ of QAPEEK is +42.9 mV, which means that the s-NBF are negatively charged and the QAPEEK carry a positive electric charge. After mixing QAPEEK and s-NBF (mQAPEEK : ms-NBF = 8:1), the resulting ζ is +34.3 mV. The decrease in ζ indicates the formation of aggregates, most probably due to the electrostatic attraction between the SA group and the QA group.50 Ion conduction and fuel cell performance. The high IEC and low swelling degree are two contradictory properties of APEMs. When the s-NBF is employed, the swelling degree is


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well controlled to a satisfactory level. To our knowledge, the non-conductive component, which occupies a certain ratio of the volume in the composite membrane, will hinder the conductivity of OH−. It can be demonstrated by experimental results shown in Table 1 as well. Once the amount of s-NBF increases, the ionic conductivity of composite membrane decreases gradually (Figure S3). But it does not mean that composite membrane will be inferior in fuel cell performance, since the thickness of membrane has been reduced, the overall area resistance (the resistance in the unit area) is actually lower than that of the pristine QAPEEK membrane, as shown in Figure 6. To further prove the significance of ultrathin membrane, Figure 6b shows an area resistance comparison of QAPEEK membrane and QAPEEK/s-NBF composite membrane in a same thickness of 35 µm. It is obvious that thicker membrane causes greater area resistance. In particular, when the weight ratio of QAPEEK and s-NBF is 8:1, the ionic conductivity of both membranes is nearly the same. Considering the swelling degree, the tensile strength and the ionic conductivity, the weight ratio of 8:1 is the optimum for QAPEEK/s-NBF composite membrane.

Figure 6 The temperature dependence of the area resistance of the QAPEEK membrane and the QAPEEK/s-NBF composite membrane. (a) QAPEEK membrane (25 µm in thickness, IEC = 1.70 mmol/g) and the QAPEEK/s-NBF composite membrane (15 µm in thickness, IEC = 1.76 mmol/g) with different thickness, (b) QAPEEK membrane (35 µm in thickness, IEC = 1.59 mmol/g) and the QAPEEK/s-NBF composite membrane (35 µm in thickness, IEC = 1.61 mmol/g) with same thickness. Membranes were in OH- form for measurement. To demonstrate the significance of using ultrathin membranes, APEFC single cells using s-NBF/QAPEEK membranes (15 µm in thickness, IEC = 1.76 mmol/g) and QAPEEK membranes (25 µm in thickness, IEC = 1.70 mmol/g) were tested. As seen in Figure 7, the open-circuit voltage is above 1.0 V, proving that the ultrathin s-NBF/QAPEEK membrane can


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well separate the H2 and O2 gas. At 60oC with 0.1 MPa of backpressure, the peak power density (PPD) with s-NBF/QAPEEK membrane is 930 mW/cm2, higher than the PPD with QAPEEK membrane (760 mW/cm2). Figure 7a shows the result of fuel-cell performance without backpressure. The composite membrane exhibits a PPD of 740 mW/cm2, higher than the QAPEEK membrane does (550 mW/cm2). In summary, the ultrathin s-NBF/QAPEEK membrane is superior over the pristine QAPEEK membrane (with the same IEC) in terms of fuel-cell performance, whether or not it is with backpressure (Figure 7a & 7b). When the membrane thickness was kept to be the same (28 µm for example), the APEFC with s-NBF/QAPEEK membrane is, to a small degree, lower in performance than the one with QAPEEK membrane (Figure 7c & 7d). Nevertheless, these results demonstrate that, upon reducing the membrane thickness down to 15 µm, the cell performance increases remarkably, mainly due to the enhancement in water transport from the anode side to the cathode side.

Figure 7 The cell performance of APEFC using QAPEEK membranes and the QAPEEK/s-NBF composite membranes. Ultrathin QAPEEK/s-NBF (15 µm, IEC = 1.76 mmol/g) and QAPEEK (25 µm, IEC = 1.70 mmol/g) at 60oC without backpressure (a) and at 60oC with 0.1 MPa backpressure (b). QAPEEK/s-NBF (28 µm, IEC = 1.85 mmol/g) and QAPEEK (28 µm, IEC = 1.89 mmol/g) at 60oC without backpressure (c) and at 60oC with 0.1 MPa backpressure (d).


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Alkaline stability tests. Sufficient quaternary ammonia groups are the essential requirement of ionic conduction. Hence, a test of the changes in IEC of QAPEEK membrane and QAPEEK/s-NBF composite membrane were carried out in 1 M NaOH solutions at 80oC for 15 days. As shown in Figure 8a, after 15 days’ immersion, the IEC of both membrane decreases to some degree, and the cation remaining degree of QAPEEK membrane was 81.4%, in comparison to 89.0% for the composite membrane. It turns out that the introduction of s-NBF is beneficial to the mechanical property of the APEM, but does not change much the alkaline stability, as also manifested by the ionic conductivity decline (Figure 8b). Note that the alkaline stability is a property of the APEM, but is not equal to the operation stability of the APEFC, which is not only related to the membrane, but also to the catalyst, the MEA structure, and the water management. Sine the subject of this paper is about the physicochemical properties of the APEM, an in-depth analysis on the fuel cell stability is beyond the scope of this paper. For readers’ reference, we provide the APEFC stability data in the supporting information (Figure S4 & S5).

Figure 8 Alkaline stability of QAPEEK membranes and QAPEEK/s-NBF composite membrane: (a) The changes in IEC and cation remaining degree of QAPEEK membrane and QAPEEK/s-NBF composite membrane after stability tests in 1 M NaOH solutions at 80oC for 15 days. ( IEC before test; IEC after test; Cation remaining degree); (b) The o changes in ionic conductivity, tested in 30 C.

Conclusion In this work, sulfonated nano bamboo fiber (s-NBF) was used to reinforce a typical APEM, QAPEEK membrane. The interaction between the SA group on s-NBF and the QA


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group on QAPEEK led to a significant enhancement in the mechanical robustness/dimension stability of the resulting composite membrane, and the swelling degree was substantially reduced to 7.5% in 80oC water. The application of ultrathin APEMs (15 µm in thickness) is thus practical, which can not only reduce the ion-conducting resistance, but also benefit the water transport across the membrane. APEFC single cells with the s-NBF/QAPEEK membrane exhibited a peak power density of 930 mW/cm2 at 60oC.

Supporting Information Available: Figs. S1-S5 and Table S1.

Acknowledgments This work was financially supported by the National Key Research and Development Program of China (2016YFB0101203) and the National Natural Science Foundation of China (91545205, 21573167, 21633008).

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Sulfonated Nano Bamboo Fiber Reinforced Quaternary Ammonia Poly

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