Highly Proton-Conducting Membranes Based on Poly(Arylene Ether)S with Densely Sulfonated and Partially Fluorinated Multiphenyl for Fuel Cell Applications

membranes

Article Highly Proton-Conducting Membranes Based on Poly(arylene ether)s with Densely Sulfonated and Partially Fluorinated Multiphenyl for Fuel Cell Applications

Tzu-Sheng Huang 1 , Tung-Li Hsieh 2 , Chih-Ching Lai 1, Hsin-Yi Wen 3, Wen-Yao Huang 1,* and Mei-Ying Chang 1,*

1 Department of Photonics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan; [email protected] (T.-S.H.); [email protected] (C.-C.L.) 2 General Education Center, Wenzao Ursuline University of Languages, Kaohsiung 80793, Taiwan; [email protected] 3 Department of Green Energy and Environmental Resources, Chang Jung Christian University, Tainan City 71101, Taiwan; [email protected] * Correspondence: [email protected] (W.-Y.H.); [email protected] (M.-Y.C.)

Abstract: Series of partially fluorinated sulfonated poly(arylene ether)s were synthesized through nucleophilic substitution polycondensation from three types of diols and superhydrophobic tetra- trifluoromethyl-substituted difluoro monomers with postsulfonation to obtain densely sulfonated ionomers. The membranes had similar ion exchange capacities of 2.92 ± 0.20 mmol g−1 and favorable mechanical properties (Young’s moduli of 1.60–1.83 GPa). The membranes exhibited considerable dimensional stability (43.1–122.3% change in area and 42.1–61.5% change in thickness at 80 ◦C) and ox- −1 idative stability (~55.5%). The proton conductivity of the membranes, higher (174.3–301.8 mS cm ) than that of Nafion 211 (123.8 mS cm−1), was the percent conducting volume corresponding to

Citation: Huang, T.-S.; Hsieh, T.-L.; the water uptake. The membranes were observed to comprise isolated to tailed ionic clusters of Lai, C.-C.; Wen, H.-Y.; Huang, W.-Y.; size 15–45 nm and 3–8 nm, respectively, in transmission electron microscopy images. A fuel cell Chang, M.-Y. Highly Proton- containing one such material exhibited high single-cell performance—a maximum power density of − Conducting Membranes Based on 1.32 W cm2 and current density of >1600 mA cm 2 at 0.6 V. The results indicate that the material is a Poly(arylene ether)s with Densely candidate for proton exchange membranes in fuel cell applications. Sulfonated and Partially Fluorinated Multiphenyl for Fuel Cell Applications. Keywords: poly(arylene ether)s; ionomers; proton exchange membranes; fuel cell Membranes 2021, 11, 626. https:// doi.org/10.3390/membranes11080626

Received: 28 July 2021 1. Introduction Accepted: 12 August 2021 Published: 15 August 2021 The fuel cell is a type of energy conversion device, and the prototype fuel cell, which converted chemical energy into electrical energy through an electrochemical mechanism,

Publisher’s Note: MDPI stays neutral was produced at the beginning of the 19th century [1–3]. Due to the world’s thirst for with regard to jurisdictional claims in energy, fuels such as coal, oil, and gas are constantly consumed, and the generation of published maps and institutional affil- greenhouse gases has become an environmental concern. Hydrogen energy—a clean iations. energy resource, producing little pollution (no CO2), small emissions, and low noise—is being developed by countless researchers in this era of the pursuit of green energy [4]. On the other hand, hydrogen is produced following the opposite principle of fuel cells, according to which it is produced from water splitting and used as a carrier for energy storage. Additionally, the water electrolysis unit can be used in combination with a fuel Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. cell unit to store intermittent or overflow energy (ex. solar, wind, waste heat, or nuclear). This article is an open access article In addition, fuel cells can be used for energy storage applications, which is how they differ distributed under the terms and from, or have advantages over, lithium-ion batteries [5] and redox flow batteries [6]. In conditions of the Creative Commons addition to the basic ion selectivity differences, membrane performance needs to be able to ◦ Attribution (CC BY) license (https:// perform well at extreme temperatures (i.e., −40 to 60 C) in order to ensure environmental creativecommons.org/licenses/by/ sustainability. Correspondingly, they may be affected during cold/hot shock cycling 4.0/). and charge/discharge cycling to maintain good ionic conductivity, mechanical properties,

Membranes 2021, 11, 626. https://doi.org/10.3390/membranes11080626 https://www.mdpi.com/journal/membranes Membranes 2021, 11, 626 2 of 20

electrochemical and thermal stability [5,6]. However, their future development will depend on functional molecular designs while meeting the key economic and technical drivers of cost, power density, efficiency, and durability. Among them, proton-exchange membrane (PEM) fuel cell using functional polymer materials as the core has been widely developed. In addition to having high proton conductivity and favorable mechanical properties, PEMs meet various performance requirements—low sensitivity to humidity, low oxidant permeability, and adequate electrochemical, chemical, thermal, and dimensional stability—If a satisfactorily low-cost and functional polymer material is to be manufactured [7–10]. PEMs have four major types— perfluorinated sulfonic acid (PFSA), fluorinated, hydrocarbon (HC)-based, and functional PEMs, all of which are widely researched. PFSA polymer materials (such as the Nafion, Flamion, Aciplex, and Dow series membranes) have excellent performance and are widely used in commercial applications [11]. However, the high pollution and external costs due to perfluorinated compounds have prompted many research teams to investigate alternatives. By contrast, HC-based PEMs are the most environmentally friendly because they are com- pletely free of halogens, but they rarely combine high proton conductivity and favorable mechanical or other crucial properties. Functional sulfonated poly(arylene ether)s have higher thermostability and increased proton conductivity at temperatures above 80 ◦C and in low humidity. Poly(arylene ether)s have sulfonated derivatives of various forms, such as poly(arylene ether ketone)s [12–15], poly(arylene sulfide)s [16,17], polyimides [18–20], and polybenzimidazoles [21–23], which have been extensively investigated because of their high thermal stability, favorable chemical stability, appropriate mechanical properties, and low production cost, as well as the easy adjustment of their molecular structure [24,25]. Countless efforts have been made to synthesize new polymer architectures with conductive components (moieties); nevertheless, the influence on the morphology at many scales is not clear; therefore, PEMs have high proton conductivity, proper hygroscopicity, and sufficient mechanical properties, but at the same time, are still under study [9]. In general, sulfonated HC-based polymers have a wide diversity of morphology types because hydrophobic and hydrophilic moieties are multiscale distributed by structural effects. These are microphase separations to form proton transport channels. As reported widely, the chemistry and architecture of PEMs have effects on proton conductivity, they can adjust the structural design of polymer composites to improve by the ion exchange capacity and microphase separation [26]. We recently reported a series of poly(arylene ether)s with a high-free-volume multiarylbenzene (MAB) structure in the polymer backbone and a trifluoromethylphenyl side chain; this series has high proton conductivity, mechanical and dimensional stability, and film-forming ability [27–29]. Adamski et al. reported two classes of HC-based PEMs, sulfonated phenylated poly(phenylene) biphenyl and sulfonated poly(arylene ether), with branching multiphenylated structures. They described the relationship between densely sulfonated polymers and water uptake and transport, aiding in the design of next-generation solid polymer electrolytes [30]. Lee et al. synthesized a series of ethynyl-terminated sulfonated–fluorinated poly(arylene ether) random copolymers, including benzenesulfonate, hexafluoroisopropylidene, and perfluoroben- zene derivatives; molar volume per charge (MCV), percent conducting volume (PCV), and derivative parameters were introduced for evaluation of the membrane properties and comparison of cross-linked poly(arylene ether)s with Nafion membranes [31]. The hydrophilic and hydrophobic substituents of the hydrophobic polymer backbone strongly influenced the microphase separation. With dense sulfonation, a superhydrophilic domain can be produced, and local halogenation or perfluoroalkane substitution can create a superhydrophobic domain in the structure. Furthermore, the trifluoromethyl-substituted group is a strongly electron-withdrawing group and can deactivate the nucleophilic aromatic substitution (SNAr) reaction of the sulfonating agent, creating favorable localization for hydrophobicity [13,32,33]. However, compatibility challenges may occur when the polarities of the components are extremely different, regardless of whether the monomers are polymerized through physical blending or preacidification. Membranes 2021, 11, x FOR PEER REVIEW 3 of 19

tivate the nucleophilic aromatic substitution (SNAr) reaction of the sulfonating agent, creating favorable localization for hydrophobicity [13,32,33]. However, compatibility challenges may occur when the polarities of the components are extremely different, regardless of whether the monomers are polymerized through physical blending or preacidification. Herein, we report a series of novel poly(arylene ether)s synthesized using a multiphenylated difluoro monomer with a trifluoromethylphenyl side chain and three types of multiphenylated bisphenol monomer. Sulfonated poly(arylene ether)s were prepared through treatment with chlorosulfuric acid, with the trifluoromethyl (-CF3) substituent expected to inhibit the SNAr reaction in specific moieties, effectively dividing the local, Membranes 11 2021,densely, 626 sulfonated moieties. After sulfonation, bipolar domains form in the3 ofmolecules, 20 which become partially fluorinated sulfonated poly(arylene ether)s with highly efficient

proton transport.Herein, Furthermore, we report athe series free of novelvolume poly(arylene effect and ether)s hydrogen synthesized bond using cohesion a multi- of bisphenol monomersphenylated with difluoro different monomer degrees with a trifluoromethylphenyl of sulfonation provide side chain corresponding and three types water sorption abilityof multiphenylated and preserve bisphenol high monomer.mechanic Sulfonatedal stability. poly(arylene The ether)ssynthesis were preparedof sulfonated through treatment with chlorosulfuric acid, with the trifluoromethyl (-CF ) substituent poly(arylene ether)s was confirmed using proton nuclear magnetic resonance3 (1H NMR) expected to inhibit the SNAr reaction in specific moieties, effectively dividing the local, and Fourier transformdensely sulfonated infrared moieties. (FTIR) After spectroscopy. sulfonation, bipolar PEMs domains were form prepared in the molecules, through solution casting withwhich dimethyl become partially sulfoxide fluorinated (DMSO), sulfonated and poly(arylene their properties—water ether)s with highly uptake, efficient dimen- proton transport. Furthermore, the free volume effect and hydrogen bond cohesion of sional stability,bisphenol mechanical monomers strength, with different proton degrees conductivity, of sulfonation providemorphology, corresponding and wa-single-cell performance—wereter sorption determined. ability and preserve high mechanical stability. The synthesis of sulfonated poly(arylene ether)s was confirmed using proton nuclear magnetic resonance (1H NMR) 2. Materials and Fourier Methods transform infrared (FTIR) spectroscopy. PEMs were prepared through solution casting with dimethyl sulfoxide (DMSO), and their properties—water uptake, dimen- 2.1. General Methodssional stability, mechanical strength, proton conductivity, morphology, and single-cell performance—were determined. 2,5-Bis(4-bromophenyl)-3,4-bis(3-(trifluoromethyl)phenyl)cyclopenta-2,4-dienone (DTF-EO), 2″2.,3 Materials″,5″,6″-Tetraphenyl-[1,1 and Methods ′:4′,1″:4″,1′′′:4′′′,1′′′′-quinquephenyl]-4,4′′′′-diol (9B- DO), 4-Bromo-42.1. General′-(4-bromophenyl-3 Methods ′,5′-diphenyl-1,1′;2′1″-terphenyl (6B-DB), and 3′,6′- 2,5-Bis(4-bromophenyl)-3,4-bis(3-(trifluoromethyl)phenyl)cyclopenta-2,4-dienone bis(4-bromophenyl)-1,1(DTF-EO), 200,3′:200,5′00,1,6″00-terphenyl-Tetraphenyl-[1,1 (5B-DB)0:40,100:400 ,1were000:4000,1 0000synthesized,-quinquephenyl]-4,4 as described0000-diol (9B- previously [27,29].DO), Toluene 4-Bromo-4 and0-(4-bromophenyl-3 dichloromethane0,50-diphenyl-1,1 (DCM) were0;20100-terphenyl dried over (6B-DB), CaH and2 and 30,6 0P- 2O5, re- 0 0 00 spectively, andbis(4-bromophenyl)-1,1 then freshly distilled:2 ,1 -terphenyl under a (5B-DB) N2 atmosphere were synthesized, and deoxygenated as described previ- through ously [27,29]. Toluene and dichloromethane (DCM) were dried over CaH2 and P2O5, purging with N2 for 30 min prior to use. Other reagents and solvents were purchased from respectively, and then freshly distilled under a N2 atmosphere and deoxygenated through Alfa Aesar (Haverhill,purging with MA, N2 for USA), 30 min prior Aldrich to use. Chem Other reagentsical Co. and (St. solvents Louis, were MI, purchased USA), from Fisher Sci- entific (Hampton,Alfa Aesar NH, (Haverhill, USA), Merck MA, USA), (Darmstadt Aldrich Chemical, Germany), Co. (St. Louis, or Tokyo MI, USA), Chemical Fisher Scien- Industry tific (Hampton, NH, USA), Merck (Darmstadt, Germany), or Tokyo Chemical Industry Co. Co. (Tokyo, Japan)(Tokyo, Japan)and andused used without without furtherfurther purification. purification. All reactions All reactions were performed were under performed under repurifiedrepurified N2 atmosphere. N2 atmosphere. The monomer monomer reaction reaction steps are steps presented are presented in Scheme1. in Scheme 1.

Scheme 1. Synthesis of monomers. Scheme 1. Synthesis of monomers.

Membranes 2021, 11, 626 4 of 20

2.2. 30,60-Bis(4-bromophenyl)-40,50-diphenyl-3,300-bis(trifluoromethyl)-1,10:20,100-terphenyl (6F7B-DB) (1) DTF-EO (8.10 g, 11.94 mmol), diphenyl acetylene (2.55 g, 1.20 eq.) diphenyl ether (8.00 g) were placed in a 3-necked round-bottomed flask with condenser and thermometer ◦ ◦ under N2 atmosphere at 40 C. The reaction temperature was then increased to 220 C for 20 h. After being cooled to room temperature, the crude mixture was poured into methanol and deionized water (DI water), and extraction was subsequently performed using ethyl acetate and DI water; dewatering was then performed using MgSO4. Finally, the resultant product was obtained through reduced pressure distillation and recrystallization from tetrahydrofuran (THF)/n-hexane, producing a crystal in a 54.3% yield. 1H-NMR (500 MHz, ◦ CDCl3, 25 C): d (ppm) 7.15 (d, 1H), 7.09 (s, 1H), 6.96–7.07 (m, 4H), 6.88–6.93 (m, 3H), 6.82–6.95 (m, 2H), 6.64–6.70 (m, 2H). Matrix-assisted laser desorption ionization–tandem time-of-flight mass spectrometry MALDI-TOF/TOF MS (m/z): Calculated for C44H26Br2F6: 828.48, found 828.30.

2.3. 4,40000-Difluoro-3,30000-bis(trifluoromethyl)-200,300,500,600,4,40000-difluoro-3,30000-bis(trifluoromethyl) -200,300,500,600-tetra (trifluoromethyl)phenyl-[1,10:40,100:400,1000:4000,10000-quinquephenyl] (12F9B-DF) (2) For the aforementioned reactor, 6F7B-DB (10.00 g, 12.1 mmol) and 4-fluoro-3-(trifluoromethyl) phenylboronic acid (5.00 g, 36.18 mmol) were dissolved in toluene (600 mL) under N2 atmosphere. Subsequently, 2.0 M K2CO3 (aq.) (5.00 g, 28.94 mmol) was added to ta the reactor. The mixture was boiled, (A- Phos)2PdCl2 (catalyst, 100 mg, 0.14 mmol) was added, and the mixture was then stirred for 2 days. After being cooled to room temperature, toluene and saturated saline were used to extract the organic layer. Any impurities in the mixture were removed using activated carbon, and the mixture was filtered using MgSO4. Finally, the crude product was purified through recrystallization from ethyl acetate/n- 1 ◦ hexane, producing a crystal in 80.1% yield. H-NMR (500 MHz, CDCl3, 25 C): d (ppm) 7.60 (d, 1H), 7.52–7.56 (m, 1H), 7.10–7.18 (m, 5H), 7.05–7.09 (m, 1H), 7.03 (d, 1 H), 6.98 (d, 1H) 6.82–6.95 (m, 7H). MALDI-TOF/TOF MS (m/z): Calculated for C58H32F14: 944.85, found 994.66.

2.4. 4,40000-Dimethoxy-200,300,500-triphenyl-1,10:40,100:400,1000:4000,10000-quinquephenyl (8B-DMO) (3) Briefly, 6B-DB (5.00 g, 8.11 mmol) and 4-methoxyphenylboronic acid (3.10 g, 8.11 mmol) were dissolved in toluene (800 mL) under the N2 atmosphere. Subsequently, 2.0 M K2CO3 (aq.) (4.50 g, 32.6 mmol) was added to the mixture. The mixture was boiled, ta (A- Phos)2PdCl2 (catalyst, 100 mg, 0.14 mmol) was added, and the mixture was then stirred for 2 days. Toluene and saturated saline were used to extract the organic layer. Any impurities in the mixture were removed using activated carbon, and the mixture was filtered using MgSO4. Finally, the crude product was purified through recrystallization from ethyl acetate/n-hexane, producing a crystal in 85.1% yield. 1H-NMR (500 MHz, ◦ CDCl3, 25 C): d (ppm) 7.62 (s, 1H), 7.48 (d, 2H), 7.40 (d, 2H), 7.12–7.21 (m, 9H), 7.70 (d, 2H), 6.92–6.96 (m, 5H), 6.85−6.91 (m, 9H), 6.80–6.83 (m, 2H), 3.83 (s, 3H), 3.81 (s, 3H). MALDI-TOF/TOF MS (m/z): Calculated for C50H38O2: 670.84, found 670.37.

2.5. 200,300,500-Triphenyl-[1,10:40,100:400,1000:4000,10000-quinquephenyl]-4,40000-diol (8B-DO) (4)

All glassware used must be dried rigorously and assembled under the N2 atmosphere. 8B-DMO (6.00 g, 4.47 mmol) was dissolved in dry DCM (120 mL). Boron tribromide (5.1 mL, 20.4 mmol) was then injected at −78 ◦C under cryogenic storage dewar, and the solution was stirred for 1 day. DI water was slowly added to stop the reaction. Extraction was performed using ethyl acetate and DI water, and dewatering was performed using MgSO4. Finally, the resultant product was obtained through reduced pressure distillation and recrystallization from THF/n-hexane, producing a crystal in 47.0% yield. 1H-NMR ◦ (500 MHz, CDCl3, 25 C): d (ppm) 7.62 (s, 1H), 7.43 (d, 2H), 7.34–7.37 (m, 4H), 7.21–7.12 (m, 10H), 6.93–6.95 (m, 3H), 6.85–6.90 (m, 9H), 6.80–6.83 (m, 4H). MALDI-TOF/TOF MS (m/z): Calculated for C48H34O2: 642.78, found 642.39. Membranes 2021, 11, x FOR PEER REVIEW 5 of 19

(500 MHz, CDCl3, 25 °C): d (ppm) 7.62 (s, 1H), 7.43 (d, 2H), 7.34–7.37 (m, 4H), 7.21–7.12 (m, 10H), 6.93–6.95 (m, 3H), 6.85–6.90 (m, 9H), 6.80–6.83 (m, 4H). MALDI-TOF/TOF MS (m/z): Calculated for C48H34O2: 642.78, found 642.39.

2.6. 4,4′′′′-Dimethoxy-2″,3″-diphenyl-1,1′:4′,1″:4″,1′′′4′′′1′′′-quinquephenyl (7B-DMO) (5) Briefly, 5B-DB (6.00 g, 11.1 mmol) and 4-methoxyphenylboronic acid (5.06 g, 33.3 mmol) were dissolved in toluene (700 mL) under the N2 atmosphere. Subsequently, 2.0 M K2CO3 (aq.) (4.50 g, 39.1 mmol) was added to the mixture. The mixture was boiled, (A-ta Phos)2PdCl2 (catalyst, 100 mg, 0.14 mmol) was added, and the mixture was stirred for 2

Membranesdays.2021 Moreover,, 11, 626 toluene and saturated saline were used to extract the organic layer.5 of Any 20 impurities in the mixture were removed using activated carbon, and the mixture was filtered using MgSO4. Finally, the crude product was purified through recrystallization from 2.6. 4,40000-Dimethoxy-200,300-diphenyl-1,10:40,100:400,100040001000-quinquephenyl (7B-DMO) (5) ethyl acetate/n-hexane, producing a crystal in a 63.9% yield. 1H-NMR (500 MHz, CDCl3, 25 °C): d (ppm) 7.56 (s, 1H),Briefly, 7.48 5B-DB (d, (6.00 2H), g, 11.1 7.35 mmol) (d, and2H), 4-methoxyphenylboronic 7.16 (d, 2H), 6.92–6.95 acid (5.06 (m, g, 33.3 5H), mmol 6.86–) were dissolved in toluene (700 mL) under the N2 atmosphere. Subsequently, 2.0 M K2CO3 6.83 (m, 2H). MALDI-TOF/TOF(aq.) (4.50 g, 39.1MS mmol) (m/z): was Calculated added to the for mixture. C44H34 TheO2: mixture594.74, was found boiled, 594.24. (A-ta Phos)2PdCl2 (catalyst, 100 mg, 0.14 mmol) was added, and the mixture was stirred for 2 days. Moreover, toluene and saturated saline were used to extract the organic layer. 2.7. 2″,3″-Diphenyl-[1,1Any′:4 impurities′,1″:4″,1 in′′′ the:4 mixture′′′,1′′′ were-quinquephenyl]-4,4 removed using activated′′′′-diol carbon, (7B-DO) and the mixture (6) was As regards the aforementionedfiltered using MgSO 4reactor. Finally, theand crude cooling product bath, was purified7B-DOM through (4.00 recrystallization g, 6.73 mmol) from ethyl acetate/n-hexane, producing a crystal in a 63.9% yield. 1H-NMR (500 MHz, was dissolved in DCM (150 mL).◦ Boron tribromide (3.90 mL, 15.6 mmol) was then injected CDCl3, 25 C): d (ppm) 7.56 (s, 1H), 7.48 (d, 2H), 7.35 (d, 2H), 7.16 (d, 2H), 6.92–6.95 (m, at −78 °C, and the solution5H), 6.86–6.83 was (m,stirred 2H). MALDI-TOF/TOF for 1 day. DI water MS (m/ zwas): Calculated slowly for added C44H34 Oto2: stop 594.74, the reaction. Extraction wasfound performed 594.24. using ethyl acetate and DI water, and dewatering was performed using MgSO2.7. 2004.,3 00Finally,-Diphenyl-[1,1 the0:4 resultant0,100:400,1000:4 000product,1000-quinquephenyl]-4,4 was obtained0000-diol (7B-DO)through (6) reduced pressure distillation andAs regardsrecrystallization the aforementioned from reactor THF/n-hexane, and cooling bath, 7B-DOMproducing (4.00 g,a 6.73crystal mmol) in was dissolved in DCM (150 mL). Boron tribromide (3.90 mL, 15.6 mmol) was then injected 92.1% yield. 1H-NMR (500 MHz, CDCl3, 25 °C): d (ppm) 7.56 (s, 1H), 7.44 (d, 2H), 7.34 (d, at −78 ◦C, and the solution was stirred for 1 day. DI water was slowly added to stop the 2H), 7.16 (t, 2H), 6.92–6.96reaction. (m, Extraction 3H), was6.83–6.88 performed (m, using 4H). ethyl MALDI-TOF/TOF acetate and DI water, and MS dewatering (m/z): Calcu- was lated for C42H30O2: 566.69,performed found using 566.22. MgSO4. Finally, the resultant product was obtained through reduced pressure distillation and recrystallization from THF/n-hexane, producing a crystal in 92.1% 1 ◦ yield. H-NMR (500 MHz, CDCl3, 25 C): d (ppm) 7.56 (s, 1H), 7.44 (d, 2H), 7.34 (d, 2H), 2.8. General Procedure7.16 for (t, the 2H), Synthesis 6.92–6.96 (m, of 3H),Polymers 6.83–6.88 (m, 4H). MALDI-TOF/TOF MS (m/z): Calculated for C H O : 566.69, found 566.22. Polymerization reactions42 30 2 were conducted in a three-necked 100-mL flame-dried flask equipped with a stirring2.8. General bar and Procedure Dean–Stark for the Synthesis apparatus of Polymers fitted with a condenser under N2 Polymerization reactions were conducted in a three-necked 100-mL flame-dried flask atmosphere. The flask was charged with K2CO3 (3.61 mmol), dimethylacetamide (DMAC; equipped with a stirring bar and Dean–Stark apparatus fitted with a condenser under 25 mL), dry tolueneN (152 atmosphere. mL), difluoro The flask mo wasnomer charged 12F9B-DF with K2CO 3(1.50(3.61 mmol),g, 1.51 dimethylacetamide mmol), and bisphenol monomer (1.51(DMAC; mmol). 25 mL), Three dry toluene types (15 of mL), polymers difluoro monomer were used: 12F9B-DF 9B-DO, (1.50 g, 8B-DO, 1.51 mmol), and 7B-DO. The polymerand reaction bisphenol steps monomer are (1.51presented mmol). Three in Scheme types of polymers 2. were used: 9B-DO, 8B-DO, and 7B-DO. The polymer reaction steps are presented in Scheme2.

Scheme 2. Synthesis of sulfonatedScheme ionomers. 2. Synthesis of sulfonated ionomers. The one-pot reaction was dried through azeotropic distillation at 130 ◦C for 2 h; then, The one-pot reactionthe reaction was temperaturedried through was increased azeotropic to 160 ◦distillationC for 36 h. After at 130 being °C cooled for 2 to h; room then, temperature, the crude mixture was poured into methanol and DI water to precipitate a the reaction temperaturebeige ﬁbrous was polymer.increased The polymerto 160 was °C ﬁltered, for 36 washed h. After several being times with cooled water andto room hot temperature, the crudemethanol, mixture and dried was in poured a vacuum in atto 80 ◦methanolC for 8 h. and DI water to precipitate a beige fibrous polymer. The polymer was filtered, washed several times with water and hot methanol, and dried in a vacuum at 80 °C for 8 h.

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2.9. General Procedure for Sulfonation To a solution of the polymer (1.20 g) in DCM (75 mL) at room temperature, chlorosulfuric acid in DCM was added dropwise. The reaction mixture was stirred for 24 h and then poured into water. The polymer precipitate was ﬁltered, washed thoroughly with DI water until the pH became neutral, and then dried in a vacuum at 60 ◦C overnight to obtain the sulfonated polymer. The polymers were sulfonated to different extents by using the aforementioned procedure and adding 4 or 12 mL of a sulfonating agent, respectively. The sulfonated polymer was readily soluble in polar aprotic solvents such as dimethylfor- mamide, DMAC, DMSO, and n-methyl-2-pyrrolidone (NMP). The ionomer reaction steps are presented in Scheme2.

2.10. Measurements 1H NMR spectroscopy was performed using a 500-MHz Varian Unity Inova 500 spectrometer using CDCl3 or DMSO-d6 as the solvent. MALDI-TOF mass spectra were obtained using a Bruker Daltonics Autoflex III TOF/TOF with 2,5-dihydroxybenzoic acid employed as the matrix. The FTIR spectra of the polymer membranes were obtained using a Bruker VERTEX 70 FTIR spectrometer. Gel permeation chromatography analysis was conducted using a Viscotek 270 Max with a refractive index detector and THF used as the eluent at a flow rate of 1 mL min−1. For calibration, a polystyrene standard (molecular weight between 75 and 117 kDa) was used. Stress–strain curves were obtained for the film specimens (length, 10.0 mm; width, 1.0 mm; and thickness, 30–40 µm) by using the PerkinElmer Pyris Diamond TMA at 25 ◦C. Membranes with various thicknesses may possess different physical–chemical properties, including mechanical strength, proton conductance (not electrical conductivity), water uptake, and resistance [34]. Accordingly, the thickness was controlled between 30 and 40 µm to match the comparable range of the selected standard, Nafion 211 (25.4 µm). For sufficient ion cluster growth time, it is necessary to have both high-concentration dissolution characteristics and appropriate solvent evaporation rate under film-forming temperature conditions. Therefore, this study uses high-boiling DMSO as the casting solvent.

2.11. Stability The thermal stability of the polymers was evaluated through thermogravimetric analysis (TGA) performed using a PerkinElmer Pyris 1 from 50 to 800 ◦C at a heating rate ◦ −1 of 10 C min under N2 atmosphere. Before the analysis, the membranes were dried in ◦ the TGA furnace at 130 C under N2 for 1 h to remove moisture. The membranes were weighed and soaked in Fenton’s reagent (3% H2O2 aqueous solution containing ferrous sulfate at 2.0 ppm) at 80 ◦C for observation after 24 h. Oxidative stability (OS) was evaluated using the change in weight of the membranes after exposure to Fenton’s reagent.

2.12. Water Uptake and Dimensional Stability The water uptake (WU) of the membranes is gauged by comparing the weights of the dry and the wet membrane samples by means of Equation (1). The dry membrane ◦ weight (Wdry) is obtained by vacuum drying the sample at 80 C for 24 h immediately before weighing it. The weight of the corresponding membrane in wet conditions (Wwet) is obtained by immersing the membrane sample in DI water at a specified temperature for 24 h, wiping off the surface moisture with filter paper, and then quickly weighing it. The final WU is obtained from the average of three experiments.

Wwet − Wdry Water uptake (WU) = × 100%, (1) Wdry Membranes 2021, 11, 626 7 of 20

The swelling ratio was calculated using the following Equations (2) and (3):

Twet − Tdry In plane selling ratio (∆T%) = × 100% (2) Tdry

Awet − Adry Through plane selling ratio (∆A%) = × 100% (3) Adry

where Awet and Adry are the areas, and Twet and Tdry are the thickness of the wet and dry membranes, respectively.

2.13. Ion Exchange Capacity The ion exchange capacity (IEC) of the membranes was determined through acid- base titration. Each dried membrane was weighed and then immersed in 1.0 M HCl (aq.) for 24 h to protonate the acid groups; it was then washed thoroughly with DI water to reach a neutral pH. Subsequently, the membrane was immersed in a 1.0 M NaCl (aq.) for 24 h to replace the protons of the sulfonic acid groups with Na+ ions. The solution was titrated using 0.01 M NaOH (aq.), and phenolphthalein was used as an indicator. The IEC (mmol g−1) was calculated from the titration results by using the following Equation (4):

−1 IEC (mmol g ) = (VNaOH × MNaOH)/Wdry (4)

where VNaOH and MNaOH are the volume and concentration of the NaOH solution, respectively, and Wdry is the weight of the dry membrane.

2.14. Hydrated Molar Volume per Charge The molar volume per charge (MVC) is an estimate of equivalent volume (cm3 per ionomer or the mol equivalent of acid groups) based on the summation of molar volume subunits rather than true volume measurements, as shown in Equation (5) [31,35] as follows: MVC = ∑ niVi (5) i

where Vi is the volumetric contribution of structural group i, which appears ni times per charge. The hydrated molar volume per charge (MVC(wet)) was the sum of the molar volumes of the component atoms or water uptake of the polymers, which is given by Equation (6) [31,35] as follows:

= + × MVC(wet) ∑ niVi VH2O λ (6) i

3 −1 where VH2O is the molar volume of water, 18 (cm mol )., and λ is the number of water molecules per charge based, as shown in Equation (7) as follows:

[H O] 10 × WU λ = 2 = (7) − M × IEC SO3 H2O

where MH2O is the molar mass of water.

2.15. Percent Conducting Volume Percent conducting volume (PCV), which is the ratio of the water uptake volume to the acid volume concentration, was calculated using the following Equation (8) [31,35]:

VH O × λ PCV = 2 (8) MVC(wet) Membranes 2021, 11, 626 8 of 20

Recently, research teams have employed this equation to better compare the proton conductivity of sulfonated polymers in PEMs in various states of WU [31,35,36].

2.16. Proton Conductivity The membranes’ proton conductivity was measured using a frequency response analyzer (Solartrom 1260) along the in-plane direction over the frequency range 10 MHz to 100 Hz at a voltage amplitude of 100 mV. Conductivity was measured after clamping a 10.0 mm × 5.0 mm sample between the two platinum electrodes of a conducting cell. The test cell was placed in an Espec SH-241 environmental chamber to measure its conductivity at 80 ◦C and various values of relative humidity (RH). Proton conductivity (σ) was then calculated using the equation σ = L/RA, where L (cm) is the distance between the electrodes, R (Ω) is the membrane’s resistance, and A (cm2) is the cross-sectional area of the sample.

2.17. Microstructure Analysis Transmission electron microscopy (TEM) was performed using a JEM-2100 (high- resolution) TEM instrument operated at an accelerating voltage of 200 kV. The acid form of the s-P12FmB-X membranes was dyed and converted into Ag+ ions through overnight immersion in 1.0 M AgNO3. Subsequently, the membranes were washed thoroughly with DI water and vacuum dried at 80 ◦C for 12 h. These dyed membranes were placed in an enclosure of epoxy resin and ultramicrotome under cryogenic conditions to obtain samples with a thickness of 30 nm.

2.18. Single-Cell Performance Catalyst ink was prepared by mixing Pt/C (HiSPEC 4000, Alfa Aesar, Haverhill, MA, USA) with a 5 wt% Nafion D520 binder. The ink was sprayed onto both sides of the membranes. The active surface area was 5 cm2 with an overall Pt loading of 0.4 mg cm−2. A membrane electrode assembly (MEA) was obtained by sandwiching the s-P12FmB- X membranes between two gas diffusion layers (GDL 24 BC, Hephas Energy Co., Ltd., Hsinchu, Taiwan). An MEA featuring a Nafion 211 membrane was fabricated using the same procedure to serve as a reference. The anode and cathode were supplied with hydrogen at a flow rate of 0.2 L min−1 and oxygen at a flow rate of 0.4 L min−1, respectively. The fabricated cell was activated for 4 h with hydrogen and oxygen at 80 ◦C and 0.5 V.

3. Results and Discussion 3.1. Synthesis and Characterization of the Monomers and Polymers A novel tetra-trifluoromethyl-substituted hexaarylbenzene (HAB) derivative difluoro monomer was successfully synthesized in three steps, as illustrated in Scheme1. Using the method discussed in our previous report, DTF-EO was synthesized from 1,3- bis(4-bromophenyl)propan-2-one and 1,2-bis(3-(trifluoromethyl)phenyl)ethane-1,2-dione through aldol condensation. DTF-EO was then reacted with diphenylacetylene through the Diels–Alder reaction to obtain monomer1 (6F7B-DB). To obtain the leaving for the condensation reaction, monomer 2 (12F9B-DF) was prepared from 6F7B-DB and 4-fluoro- 3-(trifluoromethyl)phenyl)boronic acid through Suzuki coupling. To obtain the diol monomers, a series of volume-related MAB guides—nB-DB, where m is the number of Benzene Rings and synthesized, as described previously. Then, nB-DMO series were synthesized from three guides and 4-methoxyphenylboronic acid through Suzuki coupling. Finally, diol monomers (nB-DO) were prepared from nB-DMOs through BBr3 demethyla- tion. All the monomer structures were confirmed through 1H NMR and MALDI-TOF-MS spectroscopy (Figures S1–S12). The polymers were synthesized through SNAr with a difluoro monomer (12F9B-DF) and diol (nB-DO); these polymers are denoted P12F99B, P12F98, and P12F97B, respectively, as shown in Scheme2. The polymers were readily soluble in common solvents DMAC, DMSO, NMP, THF, and chloroform. The P12FmB series polymers had molecular weights of −1 75–117 kDa and polydispersity indices of 2.0–2.7 mmol g . These results reveal that a short Membranes 2021, 11, x FOR PEER REVIEW 9 of 19

increase the substitution activity of halophenyl enough to obtain high-molar-mass poly(arylene ether)s during polymerization [37,38]. Postsulfonation of the P12FmB polymers was conducted using chlorosulfonic acids Membranes 2021, 11, 626 9 of 20 of various concentrations to obtain s-P12FmB-X ionomers, where m indicates the composition of phenyl rings and X indicates the IEC. The structures were confirmed through 1H perfluoroalkylNMR spectroscopy. group could contribute The NMR sufficient signals solubility, of and ionome the tetra-trifluoromethylrs and polymers are presented in Figures substitution in the phenyl provided a strong electron-withdrawing group to increase the substitution1, S13, activityand S14, of halophenyl respectively. enough to obtainAfter high-molar-mass polycondensation, poly(arylene ether)s the NMR signal of the P12FmB se- duringries polymerizationpolymers [comprised37,38]. a single sharp peak (labeled 12) and was shifted to 7.70 ppm Postsulfonation of the P12FmB polymers was conducted using chlorosulfonic acids of variousbecause concentrations of a change to obtain s-P12FmB-Xin the chemical ionomers, where environment m indicates the compositionof the trifluoromethyl aromatic ring at of phenyl rings and X indicates the IEC. The structures were confirmed through 1H NMR spectroscopy.the junction. The NMR For signals instance, of ionomers inP12F98B, and polymers as are trif presentedluoromethyl in Figure1, passivates the strong electron- Figureswithdrawing S13 and S14, respectively. and deactivating After polycondensation, aromatic the NMR ring, signal the of thesubstituent P12FmB of the sulfonated group is series polymers comprised a single sharp peak (labeled 12) and was shifted to 7.70 ppm becauseoften of not a change in inthe the chemicaltrifluoromethyl environment of substituted the trifluoromethyl phenyl aromatic group. ring at The new signal at 7.59–7.77 theppm junction. is assigned For instance, inP12F98B,to the proton as trifluoromethyl next to passivatesthe sulfonated the strong electron- pendant benzene ring; the chemical withdrawing and deactivating aromatic ring, the substituent of the sulfonated group is oftenshift not inis thesimilar trifluoromethyl to that substituted reported phenyl in group. our The previous new signal atwork 7.59–7.77 [27,28,39]. ppm is assigned to the proton next to the sulfonated pendant benzene ring; the chemical shift is similar to that reported in our previous work [27,28,39].

Figure 1. 1H NMR spectrum of P12F99B (top) and s-P12F99B (bottom) from 6.6 ppm to 8.1 ppm. Figure 1. 1H NMR spectrum of P12F99B (top) and s-P12F99B (bottom) from 6.6 ppm to 8.1 ppm. To conﬁrm the sulfonation of the polymers, FTIR spectroscopy was performed to determine the structural composition of the untreated polymer and polymers with different degrees ofTo sulfonation. confirm After the thesulfonation sulfonation treatment, of the apolymers broad absorption, FTIR band spectroscopy was was performed to de- generated from 3749 to 2425 cm−1 (–OH stretching) and was accompanied by changes in thetermine absorption the peaks structural in ﬁngerprint composition regions, as shown of in Figurethe un S15a,b,treated which polymer reveals a and polymers with different slightdegrees shift of theof peaksulfonation. corresponding After to symmetrical the sulfonation stretching of C=C treatment, absorption froma broad absorption band was gen- −1 −1 1492 to 1478 cm . Larger absorption peaks appear−1 at 1350 and 1010 cm and correspond toerated symmetrical from stretching 3749 and to in-plane 2425 bending cm of (–OH the O=S=O stretching) functional group and [40– was42]. The accompanied by changes in the appearanceabsorption of an enlarged peaks shoulder in fingerprint at 1225 cm−1 isregions, ascribed to as asymmetric shown stretching in Figure [43]. S15a,b, which reveals a slight The peak corresponding to the absorption of aromatic ethers (Ar–O–Ar) on the main chain remainsshift atof 1035 the cm peak−1 [42,44 corresponding]. to symmetrical stretching of C=C absorption from 1492 to 1478 cm−1. Larger absorption peaks appear at 1350 and 1010 cm−1 and correspond to symmetrical stretching and in-plane bending of the O=S=O functional group [40–42]. The ap- pearance of an enlarged shoulder at 1225 cm−1 is ascribed to asymmetric stretching [43]. The peak corresponding to the absorption of aromatic ethers (Ar–O–Ar) on the main chain remains at 1035 cm−1 [42,44].

3.2. Thermal Stability The s-P12FmB-X series arylene ether ionomers obtained through sulfonation were dried to remove moisture and then kept at 120 °C for 30 min before their thermal stability was evaluated to guarantee the high stability of the cell during operation. Compared with the untreated polymers, the s-P12FmB-X series ionomers exhibited less than 2 wt% thermogravimetric loss due to loss of residual moisture before 120 °C. The trend, similar for all of the sulfonated polymers, consisted of a drying stage and two degradation stages. The thermal degradation curve obtained at 200–400 °C was attributed to the decomposition of sulfonic acid groups [45]. The secondary degradation curve obtained above 600 °C

Membranes 2021, 11, 626 10 of 20

3.2. Thermal Stability Membranes 2021, 11, x FOR PEER REVIEW 10 of 19 The s-P12FmB-X series arylene ether ionomers obtained through sulfonation were dried to remove moisture and then kept at 120 ◦C for 30 min before their thermal stability was evaluated to guarantee the high stability of the cell during operation. Compared with the untreated polymers, the s-P12FmB-X series ionomers exhibited less than 2 wt% approximatelythermogravimetric corresponded loss due to loss of to residual the degradation moisture before 120of ◦theC. The polymers’ trend, similar main chain. Figure 2 for all of the sulfonated polymers, consisted of a drying stage and two degradation stages. presentsThe thermal the degradation degradation curve of obtained polymers, at 200–400 an◦dC their was attributed 5% weight to the decomposition loss temperature (Td5%) is listed ◦ in ofTable sulfonic 1. acidAllgroups the untreated [45]. The secondary polymers degradation exhibited curve obtainedexcellent above thermal 600 C stability, with Td5% higherapproximately than 589 corresponded °C. This was to the attributed degradation ofto the th polymers’e multiple main phenyl chain. Figure ionomer2 because rigid ar- presents the degradation of polymers, and their 5% weight loss temperature (Td5%) is omaticlisted ingroups Table1. are All theknown untreated to provide polymers exhibited strong excellentresistance thermal to thermal stability, with degradation [14]. ◦ Td5% higher than 589 C. This was attributed to the multiple phenyl ionomer because rigid aromatic groups are known to provide strong resistance to thermal degradation [14].

Figure 2. Thermogravimetric analysis curves of the P12FmB polymers and after sulfonation. Figure 2. Thermogravimetric analysis curves of the P12FmB polymers and after sulfonation. Table 1. Thermal stability and mechanical properties of the polymers and ionomers.

Table◦ 1. Thermal stabilitya and mechanical propertiesa of the polymers anda ionomers. Membrane Td5% ( C) Young’s Modulus (GPa) Tensile Strength (MPa) Elongation at Break (%) P12F99B 589.0 1.69 86.2 25.6 Td5% Young’s Tensile Strength a Elongation at s-P12F99B-2.89 265.4Membrane 0.51 68.3 74.9 s-P12F99B-3.23 230.8 0.45(°C) Modulus 48.7 a (GPa) 57.7(MPa) Break a (%) P12F98B 610.1 1.83 80.3 34.9 s-P12F98B-2.61 262.6P12F99B 0.59589.0 70.7 1.69 75.6 86.2 25.6 s-P12F98B-3.00 232.2 0.53 56.2 66.9 P12F97B 608.9s-P12F99B-2.89 1.60 265.4 97.1 0.51 40.3 68.3 74.9 s-P12F97B-2.84 251.0s-P12F99B-3.23 1.03 230.8 76.6 0.45 95.8 48.7 57.7 s-P12F97B-2.92 234.7 0.73 59.6 82.7 Naﬁon 211 -P12F98B 0.28610.1 34.2 1.83 101.5 80.3 34.9 s-P12F98B-2.61a Measured at 25262.6◦C and 40% RH. 0.59 70.7 75.6 3.3.s-P12F98B-3.00 Mechanical Properties 232.2 0.53 56.2 66.9 P12F97BThe mechanical properties608.9 of the P12FmB (innate 1.60 form, solid symbol lines 97.1 in Figure 3) 40.3 ands-P12F97B-2.84 s-P12FmB-X (acid form,251.0 hollow symbol lines in 1.03 Figure 3) series membranes 76.6 and of 95.8 the control sample (PFSA, Naﬁon 211) were measured; the detailed results are shown ins-P12F97B-2.92Table1 . The innate-form 234.7 membranes were discovered 0.73 to have Young’s 59.6 moduli of 82.7 1.60–1.83Nafion GPa 211, a tensile strength - of 80.3–97.1 MPa, elongation 0.28 at break of 25.6–40.3%, 34.2 and 101.5 excellent source characteristics. The toughness of poly(arylene ether)s is derived from a Measured at 25 °C and 40% RH.

3.3. Mechanical Properties The mechanical properties of the P12FmB (innate form, solid symbol lines in Figure 3) and s-P12FmB-X (acid form, hollow symbol lines in Figure 3) series membranes and of the control sample (PFSA, Nafion 211) were measured; the detailed results are shown in Table 1. The innate-form membranes were discovered to have Young’s moduli of 1.60– 1.83 GPa, a tensile strength of 80.3–97.1 MPa, elongation at break of 25.6–40.3%, and excellent source characteristics. The toughness of poly(arylene ether)s is derived from the contribution of rigid aromatic groups and their van der Waals forces, giving the full range of polymers similar strength and generally low elongation at break as a more brittle characteristic. The elongation at break P12F97B > P12F98B > P12F97B can be attributed to the architecture effect of the polymer chains. P12F97B has fewer side chains on the main chain, which allows it to stack better and obtain slightly stronger properties. After treatment, the degree of acidification was higher, and the strength was lower, conforming to the characteristics of strongly hydrophilic group implantation [8,46,47]. Moisture acted similar to a plasticizer to enhance the ductility of the membranes (elongation at break, 57.7–95.8%) but combined with the rigid structure of the poly(arylene ether)s to yield sufficient toughness.

Membranes 2021, 11, x FOR PEER REVIEW 11 of 19

Although a significant increase in elongation at break was obtained after the sulfonation treatment, a decrease in strength was observed in Table 2. The sustained increase in IEC does not allow for better toughness of the membranes. Consequently, Young’s moduli and tensile strength of the acid-form membranes were 0.34–0.57 GPa and 48.7 MPa–82.8 MPa, respectively. This result demonstrated that the mechanical properties of the s- P12F97B-X series of membranes were comparable to those of Nafion 211 membranes (Young’s modulus, 0.28 GPa; tensile strength, 34.2 MPa). Regarding the elongation at break, s-P12F97B-2.92 > s-P12F98B-3.00 > s-P12F97B-2.89, which is attributed to the following two points: 1. the enhancement of IEC is accompanied by the enhancement of WU, and the effect of self-wetting increases the plasticizer content and enhances the ductility; 2. the stacking surface, which mainly contributes to van der Waals forces, changes with the substitution of sulfonic acid groups, decreasing van der Waals forces. Since the active Membranes 2021, 11, 626 zone of sulfonation substitution (diol-monomers, Scheme 2) is also11 of 20 located at the difference of the three polymer structures, the increase of IEC also means that the force domi-

nancethe contributionchanges from of rigid van aromatic der Waals groups and forces their vanto hydrogen der Waals forces, bonding. giving theIn fullthe presence of hydro- genrange bonding of polymers and similar plasticizer strength and(moisture), generally low the elongation ductility at break is increased as a more brittle and the strength is decreased,characteristic. allowing The elongationfor proper at break toughness. P12F97B >The P12F98B detailed > P12F97B hydration can be attributed behavior will be discussed to the architecture effect of the polymer chains. P12F97B has fewer side chains on the subsequently.main chain, which allows it to stack better and obtain slightly stronger properties. After treatment, the degree of acidification was higher, and the strength was lower, conforming Table 2. IEC, water uptake,to the swelling, characteristics and ofoxidative strongly hydrophilic stability of group the implantations-P12FmB-X [8, 46and,47]. Nafion Moisture 211 acted membranes. similar to a plasticizer to enhance the ductility of the membranes (elongation at break, IEC a 57.7–95.8%WU) but ( combined%) with the rigidΛ b structure of the poly(aryleneΔA (%) ether)s to yieldΔT suffi- c (%) OS (%) Membrane cient toughness. Although a significant increase in elongation at break was obtained after (mmol g−1the) sulfonation30 °C treatment, 80 °C a decrease 30 °C in strength 80 °C was 30 observed °C in 80Table °C2 . The 30 sustained °C 80 °C 80 °C increase in IEC does not allow for better toughness of the membranes. Consequently, s-P12F99B-2.89 2.89 Young’s 96.6 moduli and114.3 tensile strength18.6 of the28.0 acid-form membranes64.3 were100.10.34–0.57 35.9 GPa and 61.5 - s-P12F99B-3.23 3.23 48.7 MPa–82.8 106.7 MPa142.9, respectively. 18.3 This result26.0 demonstrated 76.5 that the122.3 mechanical 35.5 properties - - of the s-P12F97B-X series of membranes were comparable to those of Nafion 211 mem- s-P12F98B-2.61 2.61 branes 65.4 (Young’s modulus,88.5 0.2813.9 GPa; tensile18.8 strength, 34.236.9 MPa). Regarding52.0 the29.7 elongation 54.1 55.9 s-P12F98B-3.00 3.00 at break, 66.7s-P12F97B-2.92 103.3 > s-P12F98B-3.0012.3 >19.1 s-P12F97B-2.89, 33.7 which49.4 is attributed 31.9 to the fol- 56.0 61.2 s-P12F97B-2.84 2.84 lowing 40.0 two points:74.6 1. the enhancement 7.8 of IEC 14.6 is accompanied28.6 by the45.1 enhancement 26.3 of WU, 42.1 50.0 and the effect of self-wetting increases the plasticizer content and enhances the ductility; s-P12F97B-2.92 2.92 2. the stacking 39.3 surface,78.6 which mainly 7.5 contributes 14.9 to van32.4 der Waals forces,43.1 changes29.1 with the 42.5 55.0 Nafion 211 0.91 substitution17.0 of sulfonic29.0 acid groups,7.5 decreasing22.0 van der Waals- forces.- Since the active- zone - 88.6 of sulfonation substitution (diol-monomers, Scheme2) is also located at the difference of the a b c IEC determined by acid-base titration.three polymer λ calculation structures, the from increase WU. of IECand also IEC. means Change that the in force membrane dominance length changes (ΔL) and thickness (ΔT). from van der Waals forces to hydrogen bonding. In the presence of hydrogen bonding and plasticizer (moisture), the ductility is increased and the strength is decreased, allowing for proper toughness. The detailed hydration behavior will be discussed subsequently.

◦ FigureFigure 3. 3. Stress–strainStress–strain curves curves of MEAs of atMEAs 25 C andat 25 40% °C RH. and 40% RH. 3.4. Hydration Behavior 3.4. HydrationFuel cells Behavior are operated under high humidity, and the water management characteristics of PEMs must not be neglected. Excellent water uptake (WU) for the proton carrierFuel may cells be necessary,are operated and its unde powerfulr high inﬂuence humidity, on fuel celland performance the water [2 ,9management,48]. character- isticsHowever, of PEMs the hygroscopic must not swelling be neglected. of polymers Excelle is accompaniednt water by dimensional uptake (WU) changes, for the proton carrier microphase transitions, and even polymer corrosion [9]. The WU (Figure4), hydration maynumber, be necessary, dimensional and stability, its powerful and oxidative influence stability (OS) on of fuel the s-P12FmB cell performance series mem- [2,9,48]. However, branes were investigated, and the results are listed in Table2. The Ave designation in the s-P12FmB-Ave implies the average characterization of this series of ionomers. Since the

Membranes 2021, 11, 626 12 of 20

Membranes 2021, 11, x FOR PEER REVIEW 12 of 19

IEC represented by X cannot be mathematically averaged to express its signiﬁcance, the average characterization and distribution are expressed as s-P12FmB-Ave.

Table 2. IEC, waterthe uptake, hygroscopic swelling, and oxidativeswelling stability of ofpolymers the s-P12FmB-X is and accompanied Naﬁon 211 membranes. by dimensional changes, mi- IECcrophasea transitions,WU (%) and evenΛ b polymer∆A corrosion (%) [9].∆T Thec (%) WU OS(Figure (%) 4), hydration num- Membrane (mmolber, g−1 )dimensional 30 ◦C 80 ◦stability,C 30 ◦C and 80 ◦oxidativeC 30 ◦C stability 80 ◦C 30(OS)◦C of 80 the◦C s-P12FmB 80 ◦C series membranes s-P12F99B-2.89 2.89 96.6 114.3 18.6 28.0 64.3 100.1 35.9 61.5 - s-P12F99B-3.23 3.23were investigated, 106.7 142.9 and 18.3 the results 26.0 76.5are listed 122.3 in Table 35.5 2. -The Ave - designation in the s- s-P12F98B-2.61 2.61P12FmB-Ave 65.4 implies 88.5 the 13.9 average 18.8 characterization 36.9 52.0 29.7 of this 54.1 series of 55.9 ionomers. Since the IEC s-P12F98B-3.00 3.00 66.7 103.3 12.3 19.1 33.7 49.4 31.9 56.0 61.2 s-P12F97B-2.84 2.84represented 40.0 by 74.6X cannot 7.8 be mathematically 14.6 28.6 45.1 averaged 26.3 to 42.1express 50.0 its significance, the aver- s-P12F97B-2.92 2.92age characterization 39.3 78.6 and 7.5 distribution 14.9 32.4 are expressed 43.1 29.1 as s-P12FmB-Ave. 42.5 55.0 Naﬁon 211 0.91 17.0 29.0 7.5 22.0 - - - - 88.6 a IEC determined by acid-base titration. b λ calculation from WU. and IEC. c Change in membrane length (∆L) and thickness (∆T).

FigureFigure 4. 4.WaterWater uptake uptake as a as function a function of temperature of temperature for s-P12FmB-Ave for s-P12FmB-Ave and Nafion 211 membranes. and Nafion 211 membranes. As expected, the WU of the membranes increased with an increase in temperature, IEC,As and expected, the number the of arms WU of of the the phenyl membranes group substituent. increased The amount with of WUan ofincrease the in temperature, s-P12FmB series membranes was 24–67% at 30 ◦C but increased to 74–143% at 80 ◦C. The IEC,highest and WU,the fornumber the s-P12F99B-2.89 of arms of membrane, the phenyl was twice group that ofsubstituent. the others at 80 The◦C. Inamount of WU of the s-P12FmBaddition, theseries slightly membranes higher IEC of thewas polymer 24–67% was attributedat 30 °C to but the numerous increased sulfonated to 74–143% at 80 °C. The highestactive sitesWU, and for larger the free s-P12F99B-2.89 volume on the structure. membrane, Within the was range twice of comparable that of IECs, the others at 80 °C. In s-P12F98B-3.00, which had an asymmetric notch in its structure, had the highest initial addition,WU (60%) the at low slightly temperatures, higher and IEC this uptake of the was po greaterlymer than was that of attributed Nafion 211.The to the numerous sul- fonatedeffective active WU properties sites and are expectedlarger tofree provide volume an increase on the in the structure. proton conductivity Within of the range of compa- rablethe IECs, film. Typically, s-P12F98B-3.00, water is required which and had generated an asymme during thetric operation notch of in a PEM its structure, fuel had the high- cell. As a result, membranes with excessive swelling rates may shrink under repeated cell est on/offinitial (charge/discharge WU (60%) at cycling)low temperatures, operations, resulting and in this creep uptake and the riskwas of greater catalytic than that of Nafion 211.layer The peeling. effective In this study,WU theproperties drawbacks ofare obtaining expected films withto provide acceptable an swelling increase rates in the proton con- need to be verified by subsequent rigorous fuel cell lifetime studies. ductivityIn this of system,the film. the Typically, hydration number water (λ ),is therequired number ofand equivalent generated carriers during that the operation of a PEMparticipate fuel cell. in carrying As a ionsresult, that aremembranes calculated using with the experimentalexcessive valueswelling of WU, rates was may shrink under repeatedequivalent cell to theon/off number (charge/discharge of water molecules per cycling) sulfonic acidoperations, group. As theresulting result, the in creep and the risk s-P12F99B-X series membranes had higher λ (average 27.0) under operating conditions of catalytic(80 ◦C; RH, layer ~99%). Thispeeling. may be In because this thestudy, free space the and drawbacks the distribution of ofobtaining sulfonic acids films with acceptable swellingin the structure rates need are relatively to be concentrated. verified by On subseq the otheruent hand, rigorous the λ of the fuel s-P12F98B-Ave cell lifetime studies. In this system, the hydration number (λ), the number of equivalent carriers that par- ticipate in carrying ions that are calculated using the experimental value of WU, was equivalent to the number of water molecules per sulfonic acid group. As the result, the s- P12F99B-X series membranes had higher λ (average 27.0) under operating conditions (80 °C; RH, ~99%). This may be because the free space and the distribution of sulfonic acids in the structure are relatively concentrated. On the other hand, the λ of the s-P12F98B-Ave and s-P12F97B-Ave membrane were approximately 19.0 and 14.8, respectively; the λ value decreases as the number of arms decreases. In this case, the equivalent carrying capacity of the s-P12F99B-Ave membrane may be better. Under similar IEC conditions, λ was discovered to increase with an increase in the number of sulfonyl phenyl arms, indicating that the densely hydrophilic concentration affects the hydration behavior. The swelling of the s-P12FmB series membranes resulted in an approximate 30–120% change in area and 30–60% change in thickness at 80 °C, as illustrated in Figure 5. Notably, the s-P12F97B series membranes exhibited excellent dimensional stability, with an average area change of 43.8 ± 10.3% and thickness change of 42.4 ± 3.6%. The s-P12F99B-3.23 membrane had the largest dimensional change at low temperature because of its lowest

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and s-P12F97B-Ave membrane were approximately 19.0 and 14.8, respectively; the λ value decreases as the number of arms decreases. In this case, the equivalent carrying capacity Membranes 2021, 11, x FOR PEER REVIEW 13 of 19 of the s-P12F99B-Ave membrane may be better. Under similar IEC conditions, λ was discovered to increase with an increase in the number of sulfonyl phenyl arms, indicating that the densely hydrophilic concentration affects the hydration behavior. The swelling of the s-P12FmB series membranes resulted in an approximate 30–120% trifluoromethylchange in area and content 30–60% and change highest in thickness water at 80 absorp◦C, as illustratedtion capacity, in Figure which5. No- were crushing at 80 °C. tably,This the result s-P12F97B can seriesbe attributed membranes exhibitedto the vari excellentable dimensional free volume stability, and with rigidity of the multi- an average area change of 43.8 ± 10.3% and thickness change of 42.4 ± 3.6%. The phenylateds-P12F99B-3.23 structure; membrane thehad sulfonated the largest dimensional territory change allows at lowwater temperature to occupy be- it [49,50], and the strongcause hydrophobic of its lowest triﬂuoromethyl phase (–CF content3) improves and highest the water solubility absorption capacity,and acceptable which swelling rate of were crushing at 80 ◦C. This result can be attributed to the variable free volume and rigidity the ofmembrane the multiphenylated [27,51]. structure; the sulfonated territory allows water to occupy it [49,50], and the strong hydrophobic phase (–CF3) improves the solubility and acceptable swelling rate of the membrane [27,51].

Figure 5. Swelling as a function of temperature for s-P12FmB-Ave. Figure 5. Swelling as a function of temperature for s-P12FmB-Ave. The resistance of the membranes to oxidation was tested using Fenton’s reagent, and all membranes exhibited excellent OS at 80 ◦C for 1 h; the results are listed in Table2. AfterThe 16 resistance h, continuous of swelling the membranes was observed, to and oxidatio only the s-P12F99B-3.23n was tested membranes using Fenton’s reagent, and all membranescompletely degraded. exhibited The degradation excellent of OS the at s-P12F99B-Ave 80 °C for 1 filmh; the was results attributed are to itslisted in Table 2. After high WU, which caused collapse and dissolution and increased the reaction area. Until 16 h,24 h,continuous the asymmetric swelling s-P12F98B-Ave was stillobserved, had higher and oxidative only stability the s-P12F99B-3.23 than s-P12F97B- membranes com- pletelyAve— degraded.58.6 and 52.5 wt%,The respectively. degradation Inevitably, of the the s-P1 long-term2F99B-Ave stability of film the prepared was attributed to its high WU,membranes which caused is not that collapse optimistic and or promising, dissolution compared and to increased Nafion membranes. the reaction It is area. Until 24 h, recommended to introduce functional groups that can effectively form electron-deficient the structuresasymmetric in the s-P12F98B-Ave polymer structure to still improve had the higher OS of membranesoxidative [stability37]. Although than s-P12F97B-Ave— 58.6the and trifluoromethyl 52.5 wt%, group respectively. can effectively Inevitably, form an electronic the long-term trap structure, stability the OS was of the prepared mem- branesexpected is not to increase that optimistic as the volume ratioor promising, of trifluoromethyl compared molecules into the Nafion repeating membranes. unit It is recom- was increased [52,53]. Up to 24 h, the stability of the s-P12F98B-Ave was similar to that of mendedthe s-P12F97B-Ave— to introduce58.6 functional and 59.5 wt%, groups respectively. that This can implies effectively a threshold form degree electron-deficient of struc- turesintensive in the sulfonation polymer as structure a function ofto the improve number of the arms OS of of phenyl membranes group substituents, [37]. Although the trifluo- romethyland this thresholdgroup can can beeffectively used as a design form reference an electron for subsequentic trap research. structure, the OS was expected to increase3.5. Proton as Conductivitythe volume ratio of trifluoromethyl molecules in the repeating unit was in- creasedProton [52,53]. conductivity Up to 24 is a h, principal the stability indicator of in the evaluation s-P12F98B-Ave of the mass was transfer similar to that of the s- characteristics of an MEA, which usually involve multiple transport mechanisms and hy- P12F97B-Ave—58.6dration behaviors on multiple and 59.5 scales wt%, [9,26,32 respectively.,37,54,55]. This article This describes implies space-related a threshold degree of inten- sive sulfonation as a function of the number of arms of phenyl group substituents, and this threshold can be used as a design reference for subsequent research.

3.5. Proton Conductivity Proton conductivity is a principal indicator in the evaluation of the mass transfer characteristics of an MEA, which usually involve multiple transport mechanisms and hydration behaviors on multiple scales [9,26,32,37,54,55]. This article describes space-related hydration factors such as IEC, λ, MVC(wet), and PCV; the characteristics are summarized in Table 2. The proton conductivity of the s-P12FmB series membranes was measured as a function of RH at 80 °C and compared with that of the iconic standard Nafion membrane (Figure 6). Compared with the Nafion membrane (123.8 mS cm−1) under the same meas- urement conditions, the s-P12FmB membranes (>173.9 mS cm−1) had superior proton conductivity due to the dense sulfonated structure. The conductivity of the s-P12FmB membranes decreased with a decrease in the RH but was higher than that of the Nafion at all RHs. The proton conductivity of the s-P12FmB-Ave membranes with comparable IECs increased gradually with an increase in the number of phenyl groups in the structure; this increase may be attributed to the higher concentration of local sulfonate groups. Due to the existence of trifluoromethyl groups, the probability of sulfonation of the benzene ring

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hydration factors such as IEC, λ, MVC(wet), and PCV; the characteristics are summarized in Table2. The proton conductivity of the s-P12FmB series membranes was measured as a function of RH at 80 ◦C and compared with that of the iconic standard Nafion membrane Membranes 2021, 11, x FOR PEER REVIEW(Figure 6). Compared with the Nafion membrane (123.8 mS cm −1) under the same mea- 14 of 19 surement conditions, the s-P12FmB membranes (>173.9 mS cm−1) had superior proton conductivity due to the dense sulfonated structure. The conductivity of the s-P12FmB membranes decreased with a decrease in the RH but was higher than that of the Nafion at all RHs. The proton conductivity of the s-P12FmB-Ave membranes with comparable IECs canincreased be reduced gradually [13], with and an increase the concentration in the number of phenylof localized groups in thesulfonate structure; groups this on the polymer chainincrease can maybe further be attributed increased. to the higher Accordingly, concentration ofs-P12F99B-2.89 local sulfonate groups. had Duehigh to proton conductivity (294.1the existence mS cm of−1) trifluoromethyl at low IEC, groups, whereas the probability s-P12F98B-3.00 of sulfonation and of s-P12F97B-2.92 the benzene ring had lower proton can be reduced [13], and the concentration of localized sulfonate groups on the polymer −1 conductivitychain can be further (219.0 increased. and 186.6 Accordingly, mS cm s-P12F99B-2.89, respectively) had high at proton high conductivityIEC. (294.1 mS cm−1) at low IEC, whereas s-P12F98B-3.00 and s-P12F97B-2.92 had lower proton conductivity (219.0 and 186.6 mS cm−1, respectively) at high IEC.

FigureFigure 6. 6. ProtonProton conductivity conductivity of the of s-P12FmB-Ave the s-P12FmB-Ave and Nafion 211 an membranesd Nafion as 211 a function membranes of as a function of relative humidity. relative humidity. Due to the nature of the monomer components, the molecular volumes of their repeat unitsDue were to different, the nature and the of IECthe was monomer introduced; components, the MVC values the of the molecular polymers in volumes the of their repeat dry state were calculated. To approximate the operating conditions of a fuel cell, the λ unitsunder were high different, humidity and and temperature the IEC was was measured introduced; to obtain the MVC MVC(wet) valuesin this case. of the polymers in the dryAlthough state were the actual calculated. volume was To not approximate obtained, when the saturated, operating the estimated conditions volume of a fuel cell, the λ change was assumed to be mainly due to the filling effect caused by the ion clusters (wet) underabsorbing high water humidity and swelling and under temperatur high humiditye was [9,31 ,measured36,52]. Incidentally, to obtain we can noticethe MVC in this case. Althoughthat the conductivity the actual of s-P12F97B-Xvolume was membranes not obtain undered, low humiditywhen saturated, conditions has the a estimated volume changecontradictory was assumed behavior. It to does be not mainly comply withdue the to rising the filling trend of conductivityeffect caused with theby the ion clusters ab- increase of IEC. However, the results from MVC to MVC(wet) can be observed to be very sorbingsimilar, water which means and thatswelling the equivalent under space high provided humidity by the concentration[9,31,36,52]. of sulfonateIncidentally, we can notice that[–SO the3H] conductivity is similar. With theof increases-P12F97B-X of RH, the memb master controlranes of under conductivity low returnshumidity to conditions has a contradictoryIEC and returns behavior. to the normal It trenddoes at not high comply RH conditions. with Accordingly, the rising we trend believe of that conductivity with the the counterintuitive behavior of s-P12F97B-X membranes under low humidity conditions increasehad an acceptableof IEC. marginHowever, of error. the results from MVC to MVC(wet) can be observed to be very similar,To which determine means the influence that the of hydration equivalent relative spac to conductivity,e provided the by influence the concentration of free of sulfonate volume was focused on the hydration domain (water-rich areas in the membrane), and the 3 [–SOhydrationH] is ratiosimilar. of ionomers With under the increase fully hydrated of conditionsRH, the wasmaster determined control as the of PCV;conductivity returns to IECthe and result returns is listed into Table the3 .normal With the changetrend inat the high number RH of conditions. side-chain arms, Accordingly, the differ- we believe that the counterintuitive behavior of s-P12F97B-X membranes under low humidity conditions had an acceptable margin of error. To determine the influence of hydration relative to conductivity, the influence of free volume was focused on the hydration domain (water-rich areas in the membrane), and the hydration ratio of ionomers under fully hydrated conditions was determined as the PCV; the result is listed in Table 3. With the change in the number of side-chain arms, the difference in IEC was insufficient to demonstrate the effect of the hydration zone on conductivity, whereas the PCV clearly differed. Consequently, even if the IEC was different, the PCV exhibited by ionomers with the same number of arms in the system under full hydration was similar. For example, even if the λ value of the s-P12F99 membranes was different, the proportion of domains that were hydrated was similar (PCV ≈ 0.71), and the conductivity was also similar. Conversely, as the PCV decreased, the conductivity decreased considerably, such as for s-P12F98B-2.61 and s-P12F98B-3.00. Although s- P12F98B-2.61 had a higher MCV(wet) (562.8 and 543.8 cm2 [eq. mol.]−1, respectively), its PCV was low (0.60 and 0.63, respectively); the ratio of the volume of the hydrophilic phase to that of the hydrated membrane was relatively small; therefore, the conductivity was low (255.1 and 212.6 cm2 [eq. mol]−1, respectively). s-P12FmB series membranes decreased PCV with an increase in the number of arms that were sulfonated. Notably, the membrane had low sensitivity to humidity, and the conductivity remained higher than 100 cm2 (eq. mol)−1

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ence in IEC was insufficient to demonstrate the effect of the hydration zone on conductivity, whereas the PCV clearly differed. Consequently, even if the IEC was different, the PCV exhibited by ionomers with the same number of arms in the system under full hydration was similar. For example, even if the λ value of the s-P12F99 membranes was different, the proportion of domains that were hydrated was similar (PCV ≈ 0.71), and the conductivity was also similar. Conversely, as the PCV decreased, the conductivity decreased considerably, such as for s-P12F98B-2.61 and s-P12F98B-3.00. Although s-P12F98B-2.61 had 2 −1 a higher MCV(wet) (562.8 and 543.8 cm [eq. mol.] , respectively), its PCV was low (0.60 and 0.63, respectively); the ratio of the volume of the hydrophilic phase to that of the hydrated membrane was relatively small; therefore, the conductivity was low (255.1 and 212.6 cm2 [eq. mol]−1, respectively). s-P12FmB series membranes decreased PCV with an increase in the number of arms that were sulfonated. Notably, the membrane had low sensitivity to humidity, and the conductivity remained higher than 100 cm2 (eq. mol)−1 and was comparable with the Nafion membrane (39–65 cm2 [eq. mol]−1) at low humidity (RH, 60–80%). The low-humidity sensitivity is beneficial because humidity often varies during cell operation.

Table 3. Proton conductivity, MVC, and PCV of the s-P12FmB-X membranes at 80 ◦C.

◦ ◦ Proton Conductivity at 80 C (mS/cm) MVC MCV (Wet) at 80 C PCV Membrane λ ◦ 40%RH 60%RH 80%RH 95%RH (cm3 eq.−1 mol−1) (cm3 eq.−1 mol−1) at 80 C s-P12F99B-2.89 15.8 55.8 134.2 294.1 28.0 151.8 713.5 0.71 s-P12F99B-3.23 19.5 60.2 151.0 301.8 26.0 139.5 658.4 0.71 s-P12F98B-2.61 11.3 42.7 103.3 212.6 18.8 161.6 562.8 0.60 s-P12F98B-3.00 17.2 55.8 128.0 255.1 19.1 145.1 543.8 0.63 s-P12F97B-2.84 14.2 43.8 100.9 173.9 14.6 142.6 458.1 0.57 s-P12F97B-2.92 11.2 40.7 101.6 187.3 14.9 139.7 460.3 0.58 Naﬁon 211 14.6 38.8 64.5 123.8 14.8 - - -

3.7. Fuel Cell Performance A single-MEA fuel cell with suitable mechanical characteristics and excellent proton conductivity was fabricated using s-P12FmB-X series and Naﬁon 211 membranes with overall Pt loading of 0.4 mg cm−2 on both sides of the membrane. The fuel cell was examined at 80 ◦C at full hydration. Figure8 shows the polarization and power density Membranes 2021, 11, x FOR PEER REVIEW 15 of 19

and was comparable with the Nafion membrane (39–65 cm2 [eq. mol]−1) at low humidity (RH, 60–80%). The low-humidity sensitivity is beneficial because humidity often varies during cell operation.

Table 3. Proton conductivity, MVC, and PCV of the s-P12FmB-X membranes at 80 °C.

MCV (wet) at Proton Conductivity at 80 °C (mS/cm) MVC PCV Membrane λ 80 °C at 80 °C 40%RH 60%RH 80%RH 95%RH (cm3 eq.−1 mol−1) (cm3 eq.−1 mol−1) s-P12F99B-2.89 15.8 55.8 134.2 294.1 28.0 151.8 713.5 0.71 s-P12F99B-3.23 19.5 60.2 151.0 301.8 26.0 139.5 658.4 0.71 s-P12F98B-2.61 11.3 42.7 103.3 212.6 18.8 161.6 562.8 0.60 s-P12F98B-3.00 17.2 55.8 128.0 255.1 19.1 145.1 543.8 0.63 s-P12F97B-2.84 14.2 43.8 100.9 173.9 14.6 142.6 458.1 0.57 s-P12F97B-2.92 11.2 40.7 101.6 187.3 14.9 139.7 460.3 0.58 Nafion 211 14.6 38.8 64.5 123.8 14.8 - - -

3.6. Microstructure Analysis The s-P12FmB series membrane had clear phase separation, including main isolated clusters (size, 3–8 nm) and mesoscale leopard-like clusters (15–45 nm), as visualized in the Membranes 2021dry, 11 ,state 626 by using TEM (Figure 7). In Figure 7, the shadowy areas16 of 20 represent hydrophilic ionic clusters, whereas the brighter areas represent the hydrophobic polymer matrix. The expansion ofcurves the ofshadowy the MEA containing area the implies s-P12FmB-X the and Nafionexpa 211nsion membranes. of the The proton s-P12F97B- transmission channel. 2.92 cell exhibited an excellent current density of more than 1670 mA cm−2 (at 0.6 V) and The aforementionedpower density characteristics similar to that of the MEA are containing observ Nafionable 211 withto varying identical graphite degrees in the cross-sec- bipolar plate and other fuel cell operating conditions. As shown in Figure8, the cell tional TEM withimages the s-P12F97B-2.92 of the membranes-P12FmB demonstrated series the highestmembranes. maximum power The density overall shape of the s- P12F99B-2.89(1.32 membrane W cm−2) and had was superior gyroid. power density In to thatthe containing microphase Nafion 211. Theseparation, fuel cell isolated clusters containing the s-P12F98B-3.00 membrane performed well (current density > 1650 mA cm−2 tended to be atconnected 0.6 V), but greater to ohmic condensate region loss induced subreg powerions density with that was low inferior sulfonate to that for density and a diam- Nafion 211 (>2000 mA cm−2). On the other hand, all s-P12FmB-X series membranes are eter of approximatelyknown to have reached45 nm, the realm as shown of overhydration in Figure (PCV > 0.35) 7a. from In PCV, the which image means of the s-P12F98B-3.00 membrane, thethat thetendency hydrophilic phaseto connect may extend toappears satiation. The to original be reduced, TEM images usedforming to a metastable state evaluate the ion channels will need to be discussed in terms of their opposites. The gyroid between the fromgyroid the ion and cluster, cylindrical which was expected types, to be beneficialas shown for proton in transport,Figure instead 7e. Under similar IEC, the has a detrimental effect on the structural support of the membrane. In addition to the number of sidemacroscopic chains swelling passing of the membrane through under the high aromatic temperature andgroup high RH decreased, conditions and the density and morphologyduring of large cell operation, clusters water crossover changed and fuel until permeation round may also isolated occur. As a result,clusters formed in the s- the s-P12F99B-2.89 membrane, which has the highest proton conductivity characteristics, P12F97B-2.92exhibited membrane, cells performance as revealed that were not in that Figure expected. The7f. optimizationThus, the of membranetrend in the microphase sep- permeation modulation IEC may be the direction of research to obtain improved cell aration patternperformance. could Encouragingly, be successfully this study has changed been successful by in controlling improving cell performance the number of aromatic side through a slight structural design (different number of phenyl substitution arms and dense chains. Predictably,sulfonation), whichthe generated is also expected tooperatin provide a referenceg groups for subsequent produced researchers. during the cell process form the mainAccordingly, path for the highfurther performance growth of the cellwhile containing being the s-P12F97B-2.92 transported. membrane may have been due to its high dimensional stability and sufficient proton conductivity.

Figure 7. TEM image of the (a) s-P12F99B-2.89 membrane at 20k and (b) 100k magnification; (c) s-P12F98B-3.00 membrane at 20k and (d) 100k magnification; (e) s-P12F97B-2.92 membrane at 20k and (f) 100k magnification.

Membranes 2021, 11, x FOR PEER REVIEW 16 of 19

Figure 7. TEM image of the (a) s-P12F99B-2.89 membrane at 20k and (b) 100k magnification; (c) s- P12F98B-3.00 membrane at 20k and (d) 100k magnification; (e) s-P12F97B-2.92 membrane at 20k and (f) 100k magnification.

3.7. Fuel Cell Performance A single-MEA fuel cell with suitable mechanical characteristics and excellent proton conductivity was fabricated using s-P12FmB-X series and Nafion 211 membranes with overall Pt loading of 0.4 mg cm−2 on both sides of the membrane. The fuel cell was examined at 80 °C at full hydration. Figure 8 shows the polarization and power density curves of the MEA containing the s-P12FmB-X and Nafion 211 membranes. The s-P12F97B-2.92 cell exhibited an excellent current density of more than 1670 mA cm−2 (at 0.6 V) and power density similar to that of the MEA containing Nafion 211 with identical graphite bipolar plate and other fuel cell operating conditions. As shown in Figure 8, the cell with the s- P12F97B-2.92 membrane demonstrated the highest maximum power density (1.32 W cm−2) and had superior power density to that containing Nafion 211. The fuel cell containing the s-P12F98B-3.00 membrane performed well (current density > 1650 mA cm−2 at 0.6 V), but greater ohmic region loss induced power density that was inferior to that for Nafion 211 (>2000 mA cm−2). On the other hand, all s-P12FmB-X series membranes are known to have reached the realm of overhydration (PCV > 0.35) from PCV, which means that the hydrophilic phase may extend to satiation. The original TEM images used to evaluate the ion channels will need to be discussed in terms of their opposites. The gyroid from the ion cluster, which was expected to be beneficial for proton transport, instead has a detrimental effect on the structural support of the membrane. In addition to the macro- scopic swelling of the membrane under high temperature and high RH conditions during cell operation, water crossover and fuel permeation may also occur. As a result, the s- P12F99B-2.89 membrane, which has the highest proton conductivity characteristics, exhibited cells performance that were not that expected. The optimization of membrane permeation modulation IEC may be the direction of research to obtain improved cell performance. Encouragingly, this study has been successful in improving cell performance through a slight structural design (different number of phenyl substitution arms and dense sulfonation), which is also expected to provide a reference for subsequent researchers. Accordingly, the high performance of the cell containing the s-P12F97B-2.92 membrane may have been due to its high dimensional stability and sufficient proton conduc- Membranes 2021, 11, 626 tivity. 17 of 20

Figure 8. Fuel cell performance of s-P12F99B-2.89, s-P12F98B-3.00, s-P12F97B-2.92 and Nafion Figure211 membranes 8. Fuel cell at 80 performance◦C. of s-P12F99B-2.89, s-P12F98B-3.00, s-P12F97B-2.92 and Nafion 211 membranes at 80 °C. 4. Conclusions In summary, we introduced a series of novel sulfonated poly(arylene ether)s that con- 4. Conclusionstain superhydrophobic tetra-trifluoromethyl-substituted HAB and three types of densely sulfonated phenylated MAB structures. The polymers were prepared through nucleophilic substitution polycondensation, and postsulfonation was performed using chlorosulfuric acid. The electron-withdrawing pendant groups of trifluoromethyl activated polymerization and improved solubility to yield high-molecular-weight polymers. The results indicate that trifluoromethyl inhibits the sulfonation activity of local phenyl substitutions and postsulfonation can achieve more precise positioning in polymer while maintaining the distribution of hydrophobic and hydrophilic moieties. Furthermore, all of the membranes exhibited high thermal stability, favorable dimensional stability, and conductivity that was not sensitive to humidity. The fuel cell containing the s-P12F97B-2.92 membrane demonstrated excellent performance with a current density of more than 1670 mA cm−2 at 0.6 V and a maximum power density of 1.32 W cm2 at 80 ◦C at full hydration, properties that are superior to those of Nafion 211. In addition, the s-P12F97B membranes exhibited tensile stress at a maximum load of 59.6–76.6 MPa and elongation at break of 82.8–95.8%. The combination of high thermal stability, acceptable dimensional stability, high proton conductivity, and excellent single-cell performance makes s-P12F97B-2.92 attractive as a PEM material for fuel cell applications.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/membranes11080626/s1, Figure S1: 1H NMR spectrum of 30,60-bis(4-bromophenyl)-40,50- diphenyl-3,300-bis (trifluoromethyl)-1,10:20,100-terphenyl (1) (6F7B-DB), Figure S2: 1H NMR spectrum of 4,40000-difluoro-3,30000-bis(trifluoromethyl)-200,300,500,600,4,40000-difluoro-3,30000-bis(trifluoromethyl)- 200,300,500,600,-Tetra (trifluoromethyl)phenyl-[1,10:40,100:400,1000:4000,10000-quinquephenyl] (2) (12F9B-DF), Figure S3: 1H NMR spectrum of 4,40000-Dimethoxy-200,300,500-triphenyl-1,10:40,100:400,1000:4000,10000- quinquephenyl (3) (8B-DMO), Figure S4: 1H NMR spectrum of 200,300,500-Triphenyl-[1,10:40,100:400,1000: 4000,10000-quinquephenyl]-4,40000-diol (4) (8B-DO), Figure S5: 1H NMR spectrum of 4,40000-Dimethoxy- 200,300-diphenyl-1,10:40,100:400,1000:4000,1000-quinquephenyl (5) (7B-DMO), Figure S6: 1H NMR spectrum of 200,300-Diphenyl-[1,10:40,100:400,1000:4000,1000-quinquephenyl]-4,40000-diol (6) (7B-DO), Figure S7: MALDI-TOF spectrum of 6F7B-DB, Figure S8: MALDI-TOF spectrum of 12F9B-DF, Figure S9: MALDI- TOF spectrum of 8B-DMO, Figure S10: MALDI-TOF spectrum of 8B-DO, Figure S11: MALDI-TOF spectrum of 7B-DMO, Figure S12: MALDI-TOF spectrum of 7B-DO, Figure S13: 1H NMR spectrum of P12F99B (top) and s-P12F99B (bottom), Figure S14: 1H NMR spectrum of P12F97B (top) and s-P12F97B (bottom), Figure S15: FTIR spectra of the P12FmB polymers and after sulfonation from (a) −1 −1 650 to 3800 cm and (b) 700 to 1700 cm . Membranes 2021, 11, 626 18 of 20

Author Contributions: T.-S.H. and C.-C.L. conducted the experiment. T.-S.H. wrote the manuscript and with the help of H.-Y.W. and T.-L.H. T.-S.H. generated the figures and tables. M.-Y.C. and W.-Y.H. supervised the project. M.-Y.C. and W.-Y.H. proposed the idea for this project. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Science and Technology of Taiwan, Grant Number MOST 108-2221-E-110-035. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available in a publicly accessible repository. Acknowledgments: The authors are grateful to the Ministry of Science and Technology, Taiwan (MOST 108-2221-E-110-035), for the financial support of this work. This manuscript was edited by Wallace Academic Editing. We also gratefully acknowledge Brightness & Green Technology Co., Ltd. for the high-performance liquid chromatography measurements. Conflicts of Interest: The authors declare no conflict of interest.

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