The Effect of Micro-Morphology on Alkaline Polymer Electrolytes

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The Effect of Micro-Morphology on Alkaline Polymer Electrolytes Stability Juanjuan Han, Jing Pan, Chen Chen, Ling Wei, Yu Wang, Qiyun Pan, Nian Zhao, Bo Xie, Li Xiao, Juntao Lu, and Lin Zhuang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09481 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Applied Materials & Interfaces

The Effect of Micro-Morphology on Alkaline Polymer Electrolytes Stability Juanjuan Han* (1), Jing Pan (3), Chen Chen (3), Ling Wei (2), Yu Wang (1), Qiyun Pan (1), Nian Zhao (1), Bo Xie (1), Li Xiao (2), Juntao Lu (2), Lin Zhuang (2) 1. Institute for Advanced Materials, Hubei key Laboratory of Pollutant Analysis & Reuse Technology, Hubei Normal University, Huangshi 435002, P. R. China 2. College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, P. R. China 3. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States

*Corresponding author, Juanjuan Han; Email address, [email protected]

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Abstract Recent studies demonstrated that the chemical stability of alkaline polymer electrolytes (APEs) could be improved by reducing the inductive effect between cations and backbones. Therefore,

pendent

cations

were

recommended.

However,

micro-phase

separated

morphologies would be generated by elongating the spacer between cations and backbones, which have a significant influence on the chemical stability of APEs too. In order to analyze how the patterns of micro-morphology affect the chemical stability of the materials, in the present work, four APEs (a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS) with different lengths of side chain between polysulfone and quaternary ammonium are synthesized. The longer of the side chain is, the more obvious of the micro-phase separation for the ax-QAPS membranes is observed. After immersing in a hot alkaline solution (80 °C, 1 M KOH) for 30 days, a5-QAPS exhibits the highest chemical stability. The IEC and ionic conductivity of a5-QAPS film are reduced by 10.0% and 10.5%, respectively. The weight loss of a5-QAPS membrane is 8.0%, which is similar with the value of the pristine backbone. The increased chemical stability can be ascribed to the suitable micro-morphology constructed in a5-QAPS sample. Besides, a5-QAPS membrane shows a high conductivity of 75.5 mS cm-1, while the swelling ratio is limited to 15.0% in liquid water at 80 °C. And a peak power density of 339.1 mW cm-2 is obtained by applying a5-QAPS as the APE to the H2-O2 fuel cell at 60 °C. Keywords: chemical stability, pendent cation, micro-morphology, cation-backbone interaction, alkaline polymer electrolyte

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Introduction In comparison to proton exchange membrane fuel cells and alkaline fuel cells, alkaline polymer electrolyte fuel cells (APEFCs) are recognized as a new kind of energy conversion systems for stationary and mobile applications.1-4 Due to protons are substituted by hydroxides and liquid electrolytes are replaced by solid polymer electrolytes, APEFCs possess the advantages of high power density, high CO2 tolerance, fast cathode reaction kinetics and free from precious metals catalysts.5-8 Therefore, the study of APEFCs technology has drawn an increased attention in the recent decades.9-11 Alkaline polymer electrolytes (APEs) play the role of transferring OH- and separating two electrodes,12-15 how to acquire APEs with good swelling behavior, high ionic conductivity, excellent mechanical property and robust chemical stability is one of the most important topics in this field.16-18 In the past decade, considerable studies were focused on how to develop the physical properties of APEs, especially ionic conductivity and swelling behavior.19-21 At present, the strategy of constructing broad and connective ion channels in APEs has been accepted as an effective method to alleviate the trade-off between high ionic conductivity and low swelling degree of the materials.22-25 After the ionic conductivity and mechanical property of APEs were greatly improved, how to increase their chemical stability has become an urgent problem to be solved. Recent reports have demonstrated that the aromatic ether backbone degraded in a hot alkaline solution when cations were tethered closely on the main chain.26-28 That's because the cation groups in the APEs would delocalize the nearby electronic structure, resulting in polarization of polar groups, such as quaternary carbons and ether groups in the polysulfone backbone, which in turn would make these sites very attractive for nucleophilic attack. Besides, the stability of cations is also affected by the types of backbones. Nuñez's work29 found that the electron-withdrawing effect of the sulfone moiety in polysulfone could enhance the electrophilic character of the nearby QA groups, which increased the susceptibility of the cations to be nucleophilic attacked. Hence, in order to avoid the alkaline instability of APEs that caused by the induction effect, pendent cations were proposed.30-32 3



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In our previous work, we found that the stability of aromatic ether backbone based APEs could be effectively enhanced by attaching pendent cations to backbones.30 The results unambiguously demonstrated that both of the backbones and cations stabilities of the APEs could be effectively improved by using a long side chain to separate the fixed cations from the backbone to weaken the cation-backbone interaction. However, hydrophilic and hydrophobic micro-phase separation morphologies were formed in the APEs. The micro-mophology is another significant factor that has been verified to affect the chemical stability of APEs.33 In order to explore the influence of micro-morphology on APEs stability, systematic study of the relationship was done in the present work. Four cations with different lengths of side chain were grafted to the polysulfone backbone via an amine group to produce four polysulfone based APEs, namely, a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS (Figure 1) with 1, 3, 5 and 7 atomic spacers between the quaternary ammonium group and the polysulfone

backbone,

respectively.

Gradually

enhanced

micro-phase

separation

morphologies were observed by elongating the length of the side chain. In the current work, the chemical stabilities of these alkaline polymer electrolytes under a hot alkaline solution were highlighted. Besides, the physical properties of the APEs were measured, such as ionic conductivity, water uptake and swelling degree. Finally, power densities of H2-O2 fuel cell based on the electrolytes were tested to evaluate their performance as applied APEs.

Experimental Section Materials: Bis(4-fluorophenyl) sulfone (99%), 2,2′-diallylbisphenol A (95%), N-bromosuccinimide (99%), azobis (isobutyronitrile) (80%), trimethylamine (33% alcohol solution), glycidyl trimethyl ammonium chloride (95%), ammonia solution (25~28wt%), Chloromethyl methyl ether (99%), Chlorobenzene (99.5%), 1,2-dichloroethane (99%), chloroform

(99%),

N,N-Dimethylacetamide

(DMAC,

99%),

toluene

(99.5%),

N,N-dimethylformamide (DMF, 99.5%), dimethyl sulfoxide (DMSO, 99%), methanol (99.5%), ethanol (99.7%), potassium hydroxide (85%), potassium carbonate (99%), trifluoroacetic acid (99%), zinc powder (95%), hydrochloric acid (37%) and Polysulfone (Udel P-3500, Mn=33.4 kg/mol, Mw Mn-1=1.76) were purchased commercially. All reagents 4


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were used as received unless otherwise noted. Syntheses: Diallyl polysulfone (DAPS). All reactants were dried before use. Bis(4-fluorophenyl) sulfone (2.59 g), 2,2′-Diallylbisphenol A (3.23 g), potassium carbonate (13 g), toluene (80 mL) and DMAC (80 mL) were added to a three-neck round-bottomed flask that equipped with a mechanical stirrer, a Dean–Stark trap and a nitrogen gas inlet and outlet. The reaction was heated at 120 °C under reflux, after 12h, toluene was vaporized at 140 °C, and the produced water was removed from the mixture at the same time. Then the temperature was increased to 160 °C, a viscous mixture was obtained after 4h. After cooling down the resultant mixture solution to room temperature, methanol (200 mL) was added to generate a precipitated crude product. After washing with deionized water and drying in a oven at 80 °C, the rough polymer was dissolved in chloroform (100 mL). In order to remove the suspended solids, diatomite was applied to filter the obtained solution. Finally, the white diallyl polysulfone product was acquired by pouring the filtrate into methanol again and drying the precipitate at 60 °C under vacuum oven. Mn=31.5 kg/mol, Mw Mn-1=1.94. Brominated diallyl polysulfone (BDAPS). DAPS (2 g) was dissolved in 1, 2-dichloroethane (60 mL) in a 150mL three neck round-bottomed flask that equipped with a mechanical stirrer to form a solution. After the temperature of the solution was elevated to 130 °C, benzoyl peroxide (0.12 g) and N-bromosuccinimide (2.0 g) were added slowly, the reaction was stirred at 135 °C for 3 h under reflux. After cooling down, the solution was poured into methanol, a crude product was obtained. The final brown product was acquired by precipitating the crude product in methanol again and drying the precipitate at 60 °C under vacuum drying oven after washing it with deionized water. Mn=28.8 kg/mol, Mw Mn-1=2.19. Chloromethyl polysulfone (CMPS). The preparation of CMPS was accorded to a procedure that we reported previously.34 The product of CMPS has a Mn of 30.0 kg/mol and Mw Mn-1=2.24. Quaternary ammonium with single long side chain. Glycidyl trimethylammonium chloride (2 g) was added into deionized water (40 mL) to form a dilute and homogeneous solution at 40 °C by magnetic stirring. Then the solution was added dropwise to an excessive ammonia solution (15.0 mL) via a pressure equalizing funnel. The temperature was kept at 40 5



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°C and maintained for 4h, the final product was obtained by evaporating the excess reactant and solvent of the mixture on a rotary vacuum evaporator. Electrospray

ionization-mass

spectrometry

(ESI-MS):

The

signal

of

[NH2CH2CH(OH)CH2N(CH3)3]+ with a relative abundance of 100 was observed at m/z = 133.1. 1H

NMR (300 MHz, in d6-DMSO): [NH2CH2CH(OH)CH2N(CH3)3]+ Cl-, NH2-CH2- δ

2.6-2.8 ppm, 2H; NH2-CH2-CH(OH)- δ 4.0-4.2 ppm, 1H; NH2-CH2-CH(OH)-CH2- δ 3.4-3.5 ppm 2H; NH2-CH2-CH(OH)-CH2-N+(CH3)3 δ 3.26 ppm 9H. Elemental analysis (EA): The measured weight percentages of the element N, H, and C were 16.62%, 10.16%, and 42.71%, respectively, which corresponded to the calculated values. a1-QAPS. The synthesis procedure of a1-QAPS was followed to our previous work.34 Dried CMPS and DMF were added to a round-bottom flask to form a homogeneous solution and the concentration was 10 wt %. Then an alcohol solution of trimethylamine was added, after warming the solution to 40 °C and stirring for 0.5h, a1-QAPS membrane was obtained by pouring the solution onto a glass Petri dish and vaporizing the solvent at 55 °C. In order to make sure all the solvents were taken away, a1-QAPS membrane was dried at 80 °C for another 10 h in a vacuum oven. The method to obtain an APE with OH- type was same to the procedure that we reported previously.35 a3-QAPS. Dried BDAPS and DMSO were mixed in a round-bottom flask to generate a dilute solution of 10 wt %, an alcohol solution of trimethylamine was added later. Then the solution was warmed to 40 °C and stirred for 0.5h to yield the product. The procedure of preparing the membrane and obtaining an APE with OH- type were same as above. a5-QAPS. Dried CMPS and DMSO were added to a round-bottom flask to produce a solution of 10 wt %. After warming the solution to 50 °C, the single long side chain quaternary ammonium was added dropwise to the flask. After stirring at the temperature for 3h, a5-QAPS was obtained. The procedure of preparing the membrane and obtaining an APE with OH- type were same as above. a7-QAPS. Dried BDAPS and DMSO were added to a round-bottom flask to form a 6


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homogeneous solution and the concentration was 10 wt %. After warming the flask to 50 °C, the single long side chain quaternary ammonium was added dropwise. After stirring at the temperature for 3h, a7-QAPS was produced. The procedure of preparing the membrane and obtaining an APE with OH- type were same as above. Measurements: Detailed measurement methods of 1H NMR spectra, elemental analysis (EA), electrospray ionization mass spectrometry (ESI-MS), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), UV-vis spectra and Gel permeation chromatography (GPC) are described in the Supporting Information (Section S1). Detailed measurement and calculation methods of ion exchange capacity (IEC), ionic conductivity (IC), swelling degree (SD), water uptake (WU), hydration number (λ), effective diffusion coefficient (D) of the mobile ions(D), ion concentration (c) of membranes and ion diffusivity in an infinite dilute aqueous solution (D0) are described in the Supporting Information (Section S2). The detailed method of stability test and fuel cell test are described in Section S3 and Section S4 of the Supporting Information, respectively.

Results and Discussion Preparation of ax-QAPS Membranes. In order to explore how the micro-morphology of APEs affects their chemical stability, four polysulfone backbone based APEs with different lengths of pendent QA(s) were prepared. As exhibited in Figure 1a, a1-QAPS and a5-QAPS membranes

were

obtained

by

tethering

trimethylamine

and

1-amino-2-hydroxy-3-trimethylammonium propane chloride to CMPS, respectively. 1H NMR spectra were employed to ascertain the structure of the samples. As exhibited in Figure 2a, the signal appeared at 3.1 ppm was for the H atoms in quaternary ammonium, demonstrating a1-QAPS was synthesized successfully. The signal of H atoms of aromatic rings was observed at chemical shifts (δ) that between 6.5 and 9.0 ppm. And the H atoms of the benzyl group were appeared at δ = 4.5 ppm. In comparison to a1-QAPS sample, the 1H NMR spectrum of a5-QAPS membrane (Figure 2b) appeared new signals which corresponded to H atoms of the alkyl groups of the side chain. 7



Figure 1. Overview of the synthesis of the APEs: a, a1-QAPS and a5-QAPS, b, a3-QAPS and a7-QAPS. Figure 1b depicted the preparation procedure of a3-QAPS and a7-QAPS membranes. The DAPS backbone was prepared via a nucleophilic substitution reaction and its structure was verified by 1H NMR (Figure 2c). The H signals of the benzene rings were appeared at δ from 6.5 to 9.0 ppm. The 1H NMR peaks of the allyl groups were observed at δ between 3.8 and 6.5 ppm. The peak at δ = 1.6 ppm corresponded to the H atom in the isopropyl group of DAPS backbone. In the bromination step, a more stable conjugated structure between benzyl and double bond was formed by a rearrangement step.36 The appearance of a 1H NMR peak at a δ = 4.2 ppm (Figure 2d) clearly indicated that the bromination reaction occurred on the allyl groups. The membranes of a3-QAPS and a7-QAPS were produced by attaching trimethylamine and the pendent TMA+ to BDAPS via a Menshukin reaction, respectively. As shown in Figure 2e and 2f, the 1H NMR signals of the QA ionic groups in a3-QAPS and a7-QAPS samples were observed at a δ = 3.1 ppm, which demonstrated that the two APEs were synthesized successfully.37

8


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Figure 2. 1H NMR spectra of a1-QAPS(a), a5-QAPS(b), DAPS(c), BDAPS(d), a3-QAPS(e) and a7-QAPS(f). Ionic Conductivity and Swelling Behavior. In order to study how the length of the side chain of the APEs affects their swelling behavior, the swelling ratio and water uptake as a function of different temperatures of a1-QAPS (IEC=1.13 mmol g-1), a3-QAPS (IEC=1.12 mmol g-1), a5-QAPS (IEC=1.11 mmol g-1) and a7-QAPS (IEC=1.12 mmol g-1) samples were recorded. As shown in Figure 3a, with increasing the temperature, the swelling ratio for all the membranes was increased. The dimensional stability of the APEs was remarkably enhanced (Table 1) by elongating the length of the side chain. Typically, at room temperature, the 9



swelling ratios of a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS were 20.0%, 9.0%, 5.0% and 0, respectively. The swelling degree of a1-QAPS could not be measured at 80 °C due to over swelling, under the same temperature, the swelling degrees of a3-QAPS, a5-QAPS and a7-QAPS were changed to 40.0%, 15.0% and 8.0%, respectively. Low swelling degree is accompanied by low water uptake,38,39 as depicted in Figure 3b, the water uptakes of a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes were 65.0%, 45.0%, 35.0% and 30.0% at 30 °C and increased to 165.0% (70°C), 105.0%, 56.0% and 42.5% at 80 °C, respectively.

Figure 3. The APEs (a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS) swelling degree (a) and water uptake (b) as a function of temperature. Generally, low water uptake leads to low ion conduction efficiency in APEs. However, as illustrated in Figure 4, a5-QAPS exhibited the highest ionic conductivity (28.0 mS cm-1 at 30 °C and 75.5 mS cm-1 at 80 °C) rather than that of a1-QAPS sample (20.1 mS cm-1 at 30°C 10


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and 38.4 mS cm-1 at 70 °C). Specially, a7-QAPS membrane showed the lowest ionic conductivity of 12.8 mS cm-1 at 30 °C and 38.8 mS cm-1 at 80 °C. Due to all the ax-QAPS membranes were composed by polysulfone backbone and QA cations and based on similar IECs, such an intriguing relationship between the swelling behavior and ionic conductivity could be ascribed to their different patterns of micro-morphologies which triggered by the self-aggregation of the pendent cations.

Figure 4. Ionic conductivity of a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes measured at different temperatures.

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Table 1. Properties of the ax-QAPS APEs Membranes

IECsa (mmol g-1)

IECsb (mmol g-1)

OH- conductivitiesc (mS cm-1)

Swelling ratiosc (%)

Water uptakesc (%)

λsc

a1-QAPS

1.18

1.13

20.1

20.0

65.0

31.9

a3-QAPS

1.16

1.12

24.0

9.0

45.0

22.3

a5-QAPS

1.17

1.11

28.0

5.0

35.0

17.5

a7-QAPS

1.19

1.12

12.8

0

30.0

14.9

a Experimental

IECs were calculated from 1H NMR. b Experimental IECs were tested by titration. c Measured at room temperature in water.

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Ion Transport and Morphology. In order to explore the relationship between an APE structure and its ion transport, the metric of normalized ionic diffusion coefficient (D/D0) for OH- transport was introduced to illustrate the performance of intrinsic ion conduction. Due to a hydrated environment is critical for the solvation and transportation of hydroxides, APEs with low water uptakes always result in low λ(s) and low D/D0(s). As shown in Table 1 and Figure 5, with increasing the length of the pendent side chain, the λ of the APEs decreased gradually (Figure 5a), while the ion transport in the APEs was enhanced except for a7-QAPS (Figure 5b). As shown in Figure 5b, a5-QAPS membrane exhibited the highest D/D0 of 0.135 at a relatively low λ of 17.5. With the spacer lengths of 1, 3 and 7, the D/D0 values of a1-QAPS, a3-QAPS and a7-QAPS were 0.12, 0.125 and 0.058 at the λ values of 31.9, 22.3 and 14.9, respectively. In addition to the water uptake, the micro-morphology is another important factor to affect the value of D/D0. In order to explain the above results, TEM analyses of the alkaline membranes were employed. Due to the samples were stained with iodine ions before the test, the dark domains in the following TEM images represent the hydrophilic clusters.

13



Figure 5. The λ (a) and D/D0 (b) as a function of the length of pendent chain for the APEs at room temperature. As identified in Figure 6, there was no obvious micro-phase separation in a1-QAPS (Figure 6a), while a gradual enhancement in hydrophilic/hydrophobic micro-phase separation caused by increasing the side chain length of the ax-QAPS membranes was unambiguously observed (Figure 6b-6d). For a5-QAPS, the resulting broad ion channels in the hydrated membrane have served as a highway for increasing the OH- transport efficiency. However, the ion clusters in a7-QAPS were over assembled and separated, which hampered the transport of ions, leading to inferior OH- conductivity. For a1-QAPS, although high water uptake facilitated ion transport, the short and narrow ion channels in the structure restrained the efficient transport of OH-, resulting in low ionic conductivity. Obviously, in the present work, the changes of D/D0 values of the ax-QAPS membranes were in line with their micro-morphology analyses. Besides, the hydrophobic matrix of the micro-morphology of the membranes effectively restricted the swelling degree and water uptake of the APEs. 14


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In addition to TEM, SAXS patterns were also applied to characterize the micro-morphology of the APEs. More detailed information about the ion clusters were provided. As depicted in Figure 6e, for a1-QAPS membrane, no peak was observed in its SAXS curve, which was in line with its TEM result. Specifically, no ionic peak was appeared in a3-QAPS sample. A sharp and narrow scattering peak of 1.50 nm-1 was observed in a5-QAPS sample, indicating the ionic clusters in the membrane were uniform and corresponded to a Bragg spacing of about 4.19 nm. For a7-QAPS, a scattering peak emerged at 0.64 nm-1, corresponding to a Bragg spacing of about 9.8 nm.

Figure 6. TEM results for a1-QAPS (a), a3-QAPS (b), a5-QAPS (c) and a7-QAPS (d). Small-angle X-ray scattering (SAXS) patterns of the four APEs (e). Alkaline Stability. At present, how to improve the chemical stability of APEs is the most concerned property for their application in fuel cells.40,41 Due to the molecular weight of the APEs would affect their alkaline stabilities, GPC analyses were conducted firstly for PS, CMPS, DAPS and BDAPS polymers (The molecular weight of the ax-QAPS membranes could not be measured due to their poor solubility in THF). As shown in Table S1, the Mn and polydispersity index (PDI) for the polymerized DAPS were 31.5 kg/mol and 1.94, respectively, which were similar to those of the commercialized PS-3500 (Mn=33.4, PDI=1.76). The Mn values of CMPS (Mn=30.0 kg/mol) and BDAPS (Mn=28.8 kg/mol) were similar to those of PS and DAPS, respectively, indicating that the process of obtaining functionalized polymers affected negligible changes of Mn. Therefore, it could be concluded that the Mn values of the four APEs were similar, demonstrating that the effect of the Mn 15



values of the APEs on their alkaline stabilities could be ignored. To evaluate the backbone and cation alkaline stabilities of a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes, the APEs were immersed in a hot alkaline solution for 30 days. During the experiment, the backbone degradation of the APEs was evaluated by their weight loss, and the weight loss was detected by UV-vis spectroscopy. The degradation curves of the APEs were illustrated in Figure 7. During the 30 days test, both of the weight losses of PS and DAPS were less than 5.0 wt %, indicating the two pristine polymers were quite stable under the measurement condition. However, once the polymer was attached with QA groups, the backbone began to degrade. The sample of a1-QAPS exhibited the worst backbone stability, after the test, the weight loss of a1-QAPS was more than 45% and the membrane was broken into pieces. The backbone stability of the APEs was enhanced by increasing the spacer between the cations and backbones, after the stability test, the weight losses of a3-QAPS, a5-QAPS and a7-QAPS membranes were 25.0%, 8.0% and 17.0%, respectively. The membrane of a5-QAPS showed the greatest backbone stability rather than a3-QAPS or a7-QAPS sample, such an interesting property can be explained by its cationic induced effect and micro-morphology. The cation-inducing effect and micro-phase separation in the APEs are two important factors that affect the stability of the backbone. It's known that the cation-inducing effect will disappear when the distance between two polar groups exceeds 3~5 atoms, and the micro-phase separation will be more obvious by elongating the length of the side chain. Since there was no obvious micro-phase separation in a3-QAPS, the decreased cation-inducing effect was the critical factor to improve the backbone stability of the membrane. For a5-QAPS, both of the weakened electronic effect and appropriate micro-morphology did a good job in enhancing the chemical stability of the membrane. APEs with a well-defined micro-morphology exhibited suppressed dimensional swelling and water uptake, which can improve their chemical stability because the OH- ions were located in the hydrophilic domains. In addition, the hydrophobic domains in the ordered micro-phase separation morphology can reduce the susceptibility of the backbone to be attacked by OHions and thereby the chemical stability of the backbone was maintained. For a7-QAPS, the negligible cation inducing effect played a positive role in increasing the stability of the 16


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backbone, however, the over aggregated and isolated hydrophilic/hydrophobic micro-phase separation was formed in the APE, which resulted in a decrease of mechanical property, thereby causing a physical degradation of the APE during the stability test. Obviously, when the length of the side chain reached 7, the micro-morphology was the main factor affecting the stability of the APE.

Figure 7. The backbone stability test of a1-QAPS, a3-QAPS, a5-QAPS, a7-QAPS and pristine backbones conducted in 1 mol L-1 KOH solution at 80 °C. The cation chemical stability of the ax-QAPS membranes was evaluated by IEC and IC. Compared to the backbone stability, in addition to induction effect and micro-morphology, the factor of steric effect should be considered when evaluating the stability of alkyltrimethylammonium cations. Pivovar and co-workers identified that Hofmann elimination was the most vulnerable degradation pathway for alkyltrimethylammonium cations, but due to the steric interference, the barrier of Hofmann elimination increased remarkably as the length of the alkyl chain was extended from 2 to 4 carbons and then dropped gradually and became stable from 4 to 6 carbons. The SN2 barriers in n-alkylTMA+ cations for n = 3, 5 and 6 were similar, which indicated that the differences in the steric effects among the three structures could be ignored.42 Besides, due to the inductive effect between polysulfone backbone and QA was mutual, which will disappear when the distance between two polar groups exceeded 3~5 atoms, the micro-morphology become the main factor to decide the chemical stability of a3-QAPS, a5-QAPS and a7-QAPS membranes. As shown in Figure 8, the tendency of cations degradation was in line with those of the 17



backbones. After 30 days of immersion, a5-QAPS exhibited the highest cation stability, the losses of the IEC and IC were 10.0% and 10.5%, respectively. In contrast, the IEC of a1-QAPS decreased from 1.13 to 0.68 mmol g-1 (loss of 40.0%) and the change of the IC could not be recorded due to over swelling of the membrane under the test condition. Compared to a1-QAPS, a3-QAPS sample showed a higher cation stability, which was due to the decreased cation-inducing effect and increased steric effect. The retentions of IEC and IC of a3-QAPS were 74.1% and 71.5%, respectively. Compared to a5-QAPS, a7-QAPS membrane exhibited a lower cation stability, which was caused by the physical degradation.

Figure 8. Cations stability test of a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS in 1 mol L-1 KOH solution at 80oC: the corresponding dependence of the (a) IEC and (b) OH- conductivity on the testing time. A severer condition (6M KOH at 80oC) was conducted to investigate the alkaline stability of the APEs. As demonstrated in Figure S1 and Figure S2, the degradation of the 18


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membranes showed minor changes in 1M and 6M KOH at 80 oC after 30days, indicating that increasing the alkaline concentration from 1M to 6M had no significant effect on the alkaline stability of the materials. A similar result was found in our previous work.43 Due to the stability test was performed in a 250 mL of alkaline solution and the solution was refreshed every 3 days, the amount of hydroxide during the test was significantly excessive even in the 1M KOH. Besides, as shown in Figure 7, the degradation rate was much slower after 15 days, which indicated the test time of 30 days was long enough to enter the degradation plateau of the APEs. Therefore, in comparison to the testing results in 1M KOH solution, after 30 days, the degradation of the APEs in 6M KOH solution may not cause significant changes in the losses of IECs, ICs and weights. Fuel Cell Performance. The fuel cell performance equipped with each APE membrane with similar thickness (50±3 μm) was evaluated. In the CCM, each electrode was loaded with Pt catalyst of 0.4 mg cm-2 and ionomer of 20 wt%, the area of each electrode was 4 cm2. As shown in Figure 9, the corresponding dependence of the cell voltage and power density of the single fuel cells on the current density based on a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes at 60oC were summarized. The higher the ionic conductivity of the APE, the better its fuel cell performance. At 60 oC, the ionic conductivities of a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes were 33.8, 42.0, 58.5, 28.8 mS cm-1, respectively. The peak power densities of corresponding membranes were 136.8, 194.5, 339.1 and 70.5 mW cm-2 at the current densities of 352.7, 433.7. 800.7 and 175.7 mA cm-2, respectively. In addition to APEs, many factors affect the performance of the APEFCs, such as types of catalysts and ionomers, water management, gas flow and MEA fabrication procedures. Therefore, in the present work, we have not focused our main attention to optimize the performance of the APEFCs. Besides, the stability of the APEFCs is also affected by the free radicals that generated in the fuel cell system. More work about optimizing the performance and long-term stability of the APEFCs will be taken in our subsequent work.

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Figure 9. Fuel cell performance of the studied APEs (a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS) under H2/O2 conditions.

Conclusions Although it is an effective method to improve the alkaline stability of APEs by elongation the length of the side chain between backbones and cations, different patterns of micro-morphologies would be constructed at the same time, which have a significant influence on the alkaline stability of APEs too. Therefore, the factor of micro-morphology need to be considered when designing a high stable APE. In the present work, we have compared four PS-based APEs with different lengths of pendent QA(s) to evaluate how the micro-morphologies affected their chemical stabilities. The results found that not all patterns of the micro-phase separation morphology could effectively improve the chemical stability of the membranes. The membrane of a5-QAPS showed the highest chemical stability rather than a3-QAPS and a7-QAPS membranes. After the 30 days test (1M KOH, 80 oC), the losses of the weight, IEC and IC of a5-QAPS were 8.0%, 10.0% and 10.5%, respectively. Due to induction effect is negligible when the distance between two polar groups exceeds 3 to 5 atoms and the differences of steric effect among a3-QAPS, a5-QAPS and a7-QAPS membranes could be ignored, the micro-morphology became the main factor to decide the chemical stability of a3-QAPS, a5-QAPS and a7-QAPS membranes. The sample of a5-QAPS membrane showed a more ordered micro-morphology than a3-QAPS membrane. After the test, the losses of the weight, IEC and IC of a3-QAPS were 25.0%, 25.9% and 20


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28.5%, respectively. However, once over hydrophilic/hydrophobic micro-phase separation was formed, the mechanical property of APEs will be deteriorated, resulting in physical degradation. Therefore, a7-QAPS showed a lower stability than a5-QAPS. After the test, the losses of the weight, IEC and IC of a7-QAPS were 17.0%, 19.7 and 22.0%, respectively. Due to strong cation-inducing effect was existed between PS backbone and QA(s) and no micro-phase separation was constructed in a1-QAPS, the sample exhibited a poor chemical stability. After the test, the weight loss of a1-QAPS was more than 45% and the loss of the IEC was 40%. Employing a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes as the APEs, the APEFCs with a peak power density of 136.8, 194.5, 339.1 and 70.5 mW cm-2 at 60 °C were achieved, respectively.

Supporting Information Supporting Information Available: [Detailed measurement methods of 1H NMR, EA, ESI-MS, TEM, SAXS, UV-vis and GPC. Detailed measurement and calculation methods of IEC, IC, SD, WU, λ and D. Detailed method of stability test and fuel cell test. The IECs and ICs for a1-QAPS, a3-QAPS, a5-QAPS and a7-QAPS membranes before and after the stability tests in 1 M and 6 M KOH solutions at 80 °C for 30 days. The values of weight remaining of the APEs before and after the stability tests in 1 M and 6 M KOH solutions at 80 °C for 30 days. GPC results of PS, CMPS, DAPS and BDAPS polymers.]

Acknowledgments The authors acknowledge the sponsors. This work was financially supported by the National Natural Science Foundation of China (Grant 21802038, 51871091, 91545205), the Natural Science Foundation of Hubei Province (Grant 2018CFB329), Jiangsu Key State Laboratory Cultivation base for Photovolatic Engineering Science (Grant SKLPST201704) and Hubei key Laboratory of Pollutant Analysis & Reuse Technology (Grant PA20170202).

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The Effect of Micro-Morphology on Alkaline Polymer Electrolytes

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