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Evaluation of Water in Perfluorinated Anion Exchange Membranes With Different IEC Values Xundao Liu, Xiaohong Chen, Supeng Pei, Hong Li, and Yongming Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06289 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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The Journal of Physical Chemistry

Evaluation of Water in Perfluorinated Anion Exchange Membranes with Different IEC Values Xundao Liua, Xiaohong chena, Supeng Peib, Hong Lia,*, Yongming Zhanga, c* a

Shanghai Electrochemical Energy Devices Research Center, Shanghai Key Lab of Electrical Insulation and Thermal Aging, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240 (P. R. China)

b

School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 201418 Shanghai, (P. R. China)

ABSTRACT: Water is vital to the transport of ion through anion exchange membranes (AEMs). However, excessive water cause decline in stabilities of AEMs. To better understand the water effect on AEMs, two states of water, bulk-like water and nonbulk-like water, were identified quantitatively in perfluorinated anion exchange membrane with seven IEC values in the range of 0.9-1.89 meq·g-1. Water in the membranes goes through a redistribution process as IEC changes, where nonbulk-like water content increases almost linearly and steadily with IEC, instead bulk-like water content increases nonlinearly and sharply with IEC. Bulk-like water has a significant effect on the dimensional stability of the AEMs. It is an advisable strategy to increase IEC value from 0.90 to 1.45 meq·g-1 to improve the ion conductivity as well as reasonable dimensional stability of the AEMs.

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1. INTRODUCTION Anion exchange membranes fuel cells (AEMFCs) are attractive candidates as clean alternative power source for direct conversion of the chemical energy of a fuel into electricity by electrochemical reactions, because of facile oxidation of fuels, faster reduction kinetics of oxygen and relatively simple water management compared to proton exchange membrane fuel cells (PEMFCs).1-3 Since lower mobility of OH- (and HCO3-/CO32-) vs. H+, the conductivity of OH- is intrinsically lower than that of H+, AEMs often suffer from low OH- conductivity. A straightforward and practical strategy to improve the ionic conductivity is to increase the ion-exchange capacity (IEC).4-7 Higher IEC causes more ions groups on polymer chains and higher water content of membranes. Decrease in water content probably leads to a reduction of conductivity of AEMs, which might cause ohmic loss.8 However, excessive water sorption causes the membrane significantly swelling or even dissolution in water as well as reduction of conductivity. Recently, N-spirocyclic quaternary ammonium ionenes (spiro-ionenes) with excellent thermal and alkaline stability were synthesized for AEMs. However, the ionenes were soluble in water due to high IEC value (4.0~4.6 meq g-1). Although the ion conductivity of the blend membrane (containing 70 wt % spiro-ionene) reached the value of 120 mS cm-1 at 90℃, the water uptake of the blend membrane is as high as 900%.7 On the other hands, the ion conductivity of AEM with a mobile ion shuttle, a very low IEC of 0.68 meq g-1 as well as water uptake of 18.4% reaches 180 mS cm-1 at same temperature. Water molecules in the membrane aggregate to form “water pools” results in facile OH transport.9 −

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There have been progress in the understanding the nature of water inside proton exchange membrane (PEM) and its related effect on the membrane.10-15 It is commonly accepted that the existence of two states of water in PEM; one called bulk-like water interacts weakly or not at all with the membrane, and the second called nonbulk-like water is strongly bound to ionic groups or polymer backbone. The distribution of these two types of water in the membrane can be changed under certain condition, such as humidity, preparation methods, prestretching and supercritical carbon dioxide treatment and so on.11-12,16-17 Despite great efforts, correlation the two types of water and their effects on performance of membranes still remains a great challenge. For AEMs, quantitative investigation of the two types of water and their effects on morphology and properties of AEMs are limited. Herein, we identified the two types of water in the perfluorinated AEMs with seven different IEC values in the range of 0.90-1.89 meq·g-1 by Raman spectra and DSC. The two types of water go through a redistribution process as IEC changes. The correlation of the two water contents with morphology of the ionic clusters, dimensional stability and conductivity of the AEMs is discussed preliminary.

2. EXPERIMENTAL SECTION 2.1 Membrane preparation The PFSO2NH-MGMC-OH AEMs with different IEC values were synthesized according to our previous reports.18 The thickness of the membranes was about 30±3 µm.

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2.2 Characterization Raman spectra were taken on a Jobin Yvon Micro-Raman Spectroscopy (RamLab-010), equipped with a holographic grating of 1800 lines/mm and a He-Ne laser (532 nm) as the excitation source. Atomic force microscopy (AFM) was conducted on a CSPM5500 nanoscope scanning probe microscope (Benyuan Instruments, Guangzhou, China) in tapping mode. Gravimetric water uptake (WH2O %) was measured by Thermal gravimetric analysis (TGA) in the fully hydrated membrane, and WH2O%= mw / md（%）, where mw and md are the total mass of water in a wet membrane and the mass of dry membrane, respectively. Total water content (λtotal), donated as average number of water molecules per quaternary ammonium group. λtotal was calculated from the following equation:

×

λ = × .

(1)

DSC was recorded on a Q-2000 DSC (TA Instruments)，and the detailed procedure was as follows: after removing residual water from the fully hydrated the membrane surfaces, and then cooled from 2 to -70℃and then heated from -70 to 30℃at 5 ℃/ min-1 under a nitrogen atmosphere. Based on the integrated water melting enthalpy and enthalpy of fusion of water (314 J g-1), the weight percentage of bulk-like water (% bulk-like water) as compared to the total water content in the membrane, was calculated using the following equation 2; ∆" $

% bulk − like water = ∆"

#

× 100%

4


(2)

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Where ∆H is the experimental heat of fusion of water in the membrane determined by DSC; ∆Hf is the heat of fusion of bulk water (314 J/g). Therefore, bulk-like (λbulk) and nonbulk-like water content (λnonb) can be determined by the following equation:

λbulk = % bulk-like water×λtotal

(3)

λnonb =λtotal-λbulk

(4)

The ion-exchange capacity (IEC) of the membrane was measured using the typical back-titration method.6 IEC =

*+ ,-./01*2 ,-./0 *345

× 100

(5)

Where M0,NaOH and M1,NaOH are the amounts (mmol) of NaOH consumed in the titration without and with membranes, respectively, and Mdry is the mass (g) of the dried membrane. Ionic conductivity measurements under fully hydrated conditions at 60℃ were performed on a two electrodes AC impedance spectroscope using Autolab PGSTA302 electrochemical test system (Eco Chemie, Netherlands). To avoid CO2 contamination from ambient air, pure N2 was flowed through the system at a high rate. The frequency region from 1 Hz to 1 MHz was scanned, where the impedance had a constant value. The in-plane hydroxide conductivity（ 6, mS.cm-1） was calculated as follows: 7

6 = 8×9

:;:

× 1000

(6)

Where l is the length (cm) between two electrodes, and A is the cross-sectional 5



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area of the membrane available for ion conduction; Rmem is the membrane resistance value from the AC impedance data (Ω) at a high frequency.

The linear swelling ratio (LER %) was characterized by linear expansion ratio, which was determined by the difference between fully hydrated at room temperature and dry dimensions of a membrane sample: % =

?@ 1?3 ?3

× 100

(7)

Where Lw and Ld are the length of the fully hydrated membrane and dry membrane, respectively. The fixed charge concentration, Cfix, was calculated from the concentration of quaternary ammonium groups per volume of swollen polymer:

CABC = D ρE

2 ρF

$

(8)

Where ρw, ρp are the densities of water (taken as 1.0 g cm-3) and dry polymer （ρp=0.89 g cm-3, determine by hydrostatic weighing method19）, respectively.

3. RESULTS AND DISCUSSION 3.1 Water in the AEMs The AEMs (PFSO2NH-MGMC-OH) used here were synthesized in our lab. PFSO2NH-MGMC-OH owns (Figure1) a hydrophobic Teﬂon-like backbone with hydrophilic cationic side groups, which shows good ion conductivities and alkaline stability.18

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Figure 1. Molecular structure of PFSO2NH-MGMC-OH.

The Raman spectrum of PFSO2NH-MGMC-OH with IEC value of 1.45 meq g-1 in the finger print region (600 to 1500 cm-1) is shown in Figure 2A. Stretching vibrations arising from C-O-C, S-N, -N+- and CF appeared at 970, 1385, 1475, 1430 and 1197 cm-1, respectively（Figure 2A）. At higher wavenumbers (3000 to 3700 cm-1) in Figure 2B (black curve), the spectrum shows abroad response due to the OH stretching of water.11,20 To identify the contributions of different water species to the total OH stretching signal, we decomposed the total OH region using Gussian fit method to two components typical for nonbulk-like water which strongly interacts with either the polymer backbone or ionic groups and for bulk-like water which is completed surrounded by other water, respectively.11,20-21 The curve with peak maximum at 3520 cm-1 (blue line in Figure 2B) is associated with bulk-like water, and the curve with peak maximum at 3610 cm-1 (orange line in Figure 2B) is ascribed to nonbulk-like water. According to the relative integrated areas of these two curves over the entire range in the OH region, the fractional contribution of nonbulk-like water and bulk-like water to the total amount of water is about 45% and 55%, respectively. The corresponding contributions of the two types of water for other AEMs with various IEC were obtained by the same method, which are summed up in Figure 2C. 7



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It was noted that both the two contributions increase/decrease nonlinearly with IEC. Nonbulk-like water contribution with IEC value of the membranes showed a parabola profile. It first increases with IEC value, reaches a maximum of 45% at IEC value of 1.45 meq g-1, and then gradually decreases to 22% (IEC=1.89 meq·g-1). On the contrary, bulk-like water contribution first decreases with IEC value, reaches a minimum of 55% at IEC value of 1.45 meq g-1 and then rises up to 78%. Clearly, there is lowest bulk-like water contribution and highest nonbulk-like water contribution in the total absorbed water for the membranes with IEC of 1.45 meq·g-1.

Figure 2. The Raman spectrum of fully hydrated PFSO2NH-MGMC-OH samples (A); Relative contributions of bulk-like (blue) and nonbulk-like (orange) OH intensities to the overall OH stretching signal of the sample (black, IEC=1.45 meq g-1). Red: Gaussian fit to the overall OH signal (B); The nonbulk-like and bulk-like water contribution as an increase IEC (C). Errors are calculated at the 97% confidence level.

Furthermore, low-temperature DSC water-melting endotherms were used to identify different type of water in fully hydrated membranes. The exothermic and the corresponding endothermic peaks are due to freezing of bulk-like water and melting of bulk-like water, respectively.16-17 A thermogram of fully hydrated membrane sample at IEC value of 1.89 meq·g-1 was shown in Figure 3A. The melting 8


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temperature of bulk-like water is -3.0 ℃. The onset of fusion at subzero temperature illustrates the known supercooling phenomenon of water in the membranes.16 As shown in Figure 3B, melting transition of bulk-like water in membrane shifts to lower temperatures as IEC decreases (e.g., about -16 ℃ at IEC value of 0.90 meq·g-1). The depression in the water melting temperature may associate with increased local ion concentration because the amount of absorbed water is reduced as the IEC is lowered.16,22 Besides, the peaks of water-melting endotherms are not evident for the membranes with low IEC value. However, they became remarkable for the membranes with IEC value of 1.55~1.89 meq·g-1, indicating bulk-like water in the membranes increased dramatically.

Figure 3. (A) DSC thermogram of fully hydrated sample at IEC value of 1.89 meq·g-1. (B) Endothermic DSC thermograms of PFSO2NH-MGMC-OH membranes with various IEC.

Based on the TGA and DSC data, total, bulk-like and nonbulk-like water contents of the membranes with various IEC were calculated and shown in Figure 4A. Bulk-like water content (λbulk, determined from DSC data) increased greatly from 5.5 9



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to 14.1 with increasing IEC value while Nonbulk-like water content (λnonb, λnonb=λtotal-λbulk) only rises up a little from 1.9 to 3.7. The increase in the amount of total water is mainly from bulk-like water. λnonb increases linearly and steadily with IEC. Instead, λbulk increases nonlinearly with IEC. There is steady increase in the amount of bulk-like water in the membranes with IEC below 1.45 meq·g-1. However, the increase of λbulk becomes rapid for the membranes with IEC higher than 1.45 meq g-1, and the growth rate of λbulk is clearly far greater than that of λnonb. Figure 4B displays the curves of the percentage of nonbulk-like water content ratio (λnonb/λtotal %) and bulk-like water content ratio (λbulk/λtotal %) vs IEC. The general shape of the two curves is consistent with those in Figure 2C. The membrane with IEC of 1.45 meq g-1 has the maximum of λbulk/λtotal % and minimum of λnonb/λtotal %. The result agrees with that observed by Raman spectra. As we know, all water contents will decrease with relative humidity, however the majority of water lost is bulk-like water. And in the absence liquid-like water, proton conduction occurs along ion-line pore channels.16 So, among these AEMs, the membrane with IEC of 1.45 meq·g-1 presumably shows best ion conduction under low relative humidity.

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Figure 4. A：Total, bulk-like, and nonbulk-like water content of PFSO2NH-MGMC-OH samples as a function of IEC；B：The percentage of bulk-like water content ratio (λbulk/λtotal %) and nonbulk-like water content ratio (λnonb/λtotal %) with an increase of IEC.

The PFSO2NH-MGMC-OH membrane used here has perfluorinated sulfonic membrane-like structure except cation side groups. Thus, λtotal, λnonb, and λnonb/λtotal % of the PFSO2NH-MGMC-OH membrane with IEC of 0.9 are compared with those of perfluorinated PEMs with almost same IEC in the Table 1. It can be clearly seen that perfluorinated PEMs have higher λtotal, λnonb and λnonb/λtotal %. λnonb/λtotal % of the PEMs are about 2 times than that of the PFSO2NH-MGMC-OH membrane at the similar IEC. The higher λtotal and λnonb ratio is one of the reasons for the higher conductivity of the perfluorinated PEMs than that of the perfluorinated AEMs.

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Table 1. Comparison of water content among PFSO2NH-MGMC-OH and perfluorinated PEMs (NO. 1 and 2 are Nafion, No.3 is perfluorosulfonic acid (PFSA) membranes; NO.3 was tested at 100℃ and other samples were tested at room temperature ).

No.

λtotal

λnonb

λnonb ratio

IEC ( meq·g-1)

Reference

1

22

9

41%

0.97

17

2

20

11

55 %

0.97

16

3

35

16

46%

0.91

12

4

7

2

28%

0.90

This work

3.2 Effect of water on swelling ratio ， charge concentration, hydroxide conductivity

The linear swelling ratio (LER %) and λbulk as function of IEC were shown in Figure 5A. The general shapes of the two curves are similar, which does mirror the dependence of LER% on the λbulk. In the IEC range of 0.90~1.45 meq·g-1, λbulk was gradually increased from 5.5 to 5.7, LER% is controlled below 6%. However, in the IEC range of 1.55~1.89 meq·g-1, λbulk and LER% were increased sharply from 6.0 to 14.1 and from 7.2% to 17.5%, respectively. The value of LER% at IEC value of 1.89 meq·g-1 is 3.4 times than that at IEC value of 1.45 meq·g-1. Bulk-like water has a significant effect on the dimensional stability of AEMs. More bulk-like water weakens the interaction between polymer chains, enhances the mobility of polymer chains, and eventually induces larger swelling.

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Figure 5. (A) The change of linear swelling ratio (LER%) and bulk-like water content with IEC; Fixed charge concentration as function of IEC (B); the conductivity and nonbulk-like water content of various AEMs with IEC (C)

The charge concentration of the membrane as function of IEC was displayed in Figure 5B. The charge concentration of the membrane, which is the concentration of quaternary ammonium groups per volume of swollen polymer, is important for ion permselectivity and electrical resistance.23 The fixed charge concentration in the hydrated membranes with various IEC goes through a maximum of 1.23 meq cm-3 at IEC value of 1.55 meg g-1. Although IEC continues to rise up, large bulk-like water content swells the membrane and reduces the fixed charge concentration. As shown in Figure 5C, the conductivity of the membrane increased from 36.2 to 80.6 mS.cm-1 with IEC due to increase of the ion numbers and water content. It is worth noting that although bulk-like water content doubles for the membranes with IEC range of 1.55-1.89 meg cm-3 (Figure 5A), the conductivity only increases 10% since swelling of the membrane increases greatly and the charge concentration starts to decrease at this IEC range. So, considering good dimensional stability as well as the conductivity of the membrane, the membrane with IEC value of 1.45 meg cm-3 is the best choice for application. 13



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3.3 Morphology of the AEMs by AFM AFM characterization was used to investigate morphology of the AEMs. The phase images of the top wet membrane are shown in Figure 6. For the membrane with the low IEC value of 0.9 meq·g-1, the size of the ionic domains (darkened regions in Figure 6) was small and the most of them are less than 5 nm (Figure 6A). With increase of IEC, the ion domains became well-connected and the ionic domain dimension became greater (Figure 6B-D). In particular, both the size and distribution of the ionic domains were highly uniform in the IEC range of 1.25~1.45 meq·g-1, with the dimension of 5~15 nm. However, as the IEC further increased from 1.55 to 1.89 meq·g-1, the ionic domain was aggregated to form bigger domain with size of 20 nm, and most of them were separately.

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Figure 6. AFM phase images of wet membrane with increasing IEC value（The scale bar in all images is 20 nm）.

Based on the results above，we may correlate the morphological differences among the AEMs with different IEC value and water contents. A simplified schematic drawing presents the state of two type of water inside the AEMs (Figure 7) with low IEC (0.9-1.15 meq·g-1), medium (1.25-1.45 meq·g-1) IEC and high IEC (1.55-1.89 meq·g-1). Nonbulk-like water is distributed associated with the ionic groups to form the ion domains that are important to ion conduction, whereas bulk-like water is situated throughout the membrane. For the AEMs with low IEC and low nonbulk-like 15



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water content, the ionic domains exhibited small size and most of them distributed separately by a large amount of bulk-like water. At the medium IEC value, high nonbulk-like water content and more ionic groups derived the ionic groups connected efficiently and gathered to form bigger ion domains. These well-connected hydrophilic ion domains and nano-phase separations are favorable for the ion transportation and dimensional stability of the AEMs. However, at the high IEC, although the quantity of ionic groups reached a higher value, sharply increased bulk-like water caused some ion domains apart again and larger swelling of the polymer chains, finally reduced the fixed charge concentration of the membranes and the growth rate of the conductivity.

Figure 7. Schematic diagram illustrating two types of water in the hydrophilic pore of a membrane with increasing IEC value.

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5. CONCLUSIONS To summarize, quantification of bulk-like water and nonbulk-like water in perfluorinated AEMs with different IEC values was reported here, in addition water effects on properties of the AEMs and the morphology of ionic clusters. Values of bulk-like water content and nonbulk-like water content were estimated to be 6-14 and 2-4 for the fully hydrated AEMs with different IEC, respectively. Compared with perfluorinated sulfonic membranes with the same IEC value, the perfluorinated AEM with IEC of 0.9 meq·g-1 has much lower water contents. The two types of water in the membranes go through a redistribution process as IEC changes, where nonbulk-like water content increases almost linearly and steadily with IEC, instead bulk-like water content increases nonlinearly and sharply with IEC. The membrane with IEC of 1.45 meq·g-1 has minimum of bulk-like water content ratio and maximum of nonbulk-like water content ratio. Bulk-like water has a significant effect on the dimensional stability of the AEMs. It is an advisable strategy to increase IEC value from 0.90 to 1.45 meq·g-1 to improve the ion conductivity as well as reasonable dimensional stability of the AEMs.

AUTHOR INFORMATION Corresponding Author *E-mail:[email protected], [email protected].

Notes: The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge funding for this work provided by the National Nature Science Foundation of China (51573090), the Shanghai Natural Science Foundation (15ZR1422100) and Open Foundation from State Key Laboratory of Fluorinated Functional Membrane Material.

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Evaluation of Water in Perfluorinated Anion ... - ACS Publications

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