Synergistic Effect of 2-Acrylamido-2-Methyl-1-Propanesulfonic Acid on the Enhanced Conductivity for Fuel Cell at Low Temperature

membranes

Article Synergistic Eﬀect of 2-Acrylamido-2-methyl- 1-propanesulfonic Acid on the Enhanced Conductivity for Fuel Cell at Low Temperature

Murli Manohar * and Dukjoon Kim *

School of Chemical Engineering, Sungkyunkwan University, Suwon, Kyunggi 16419, Korea * Correspondence: [email protected] (M.M.); [email protected] (D.K.)

Received: 3 November 2020; Accepted: 10 December 2020; Published: 15 December 2020

Abstract: This present work focused on the aromatic polymer (poly (1,4-phenylene ether-ether-sulfone); SPEES) interconnected/ cross-linked with the aliphatic monomer (2-acrylamido -2-methyl-1-propanesulfonic; AMPS) with the sulfonic group to enhance the conductivity and make it ﬂexible with aliphatic chain of AMPS. Surprisingly, it produced higher conductivity than that of other reported work after the chemical stability was measured. It allows optimizing the synthesis of polymer electrolyte membranes with tailor-made combinations of conductivity and stability. Membrane structure is characterized by 1H NMR and FT-IR. Weight loss of the membrane in Fenton’s reagent is not too high during the oxidative stability test. The thermal stability of the membrane is characterized by TGA and its morphology by SEM and SAXS. The prepared membranes improved 1 proton conductivity up to 0.125 Scm− which is much higher than that of Naﬁon N115 which is 1 0.059 Scm− . Therefore, the SPEES-AM membranes are adequate for fuel cell at 50 ◦C with reduced relative humidity (RH).

Keywords: 2-acrylamido-2-methyl-1-propanesulfonic; proton-exchange membrane; conductivity; cross-linking; temperature

1. Introduction Recently, lots of polymer electrolyte membranes have been prepared from the sulfonation of aromatic polymers such as poly(arylene ether sulfone) [1–4] and modified poly(arylene ether sulfone) [5–7] for the application of electrodialysis and fuel cells as their rigid-rod backbone structures are basically quite stable in thermal and mechanical aspects. To optimize the membrane properties for fuel cell, huge efforts have been additionally given for their chemical and physical modifications such as grafting, [8] cross-linking, [9] degree of sulfonation, [10] surface modification [11–17], and composites [18–22]. Even though those sulfonated aromatic polymers have shown quite promising results [1–7] some critical shortcomings in membrane properties limited their wide commercialization. The proton conductivity of aromatic polymer membranes is generally lower than that of Nafion, a typical commercial polymer electrolyte membrane, because the aromatic polymer membranes usually form relatively smaller ion cluster dimensions for water flow in association with the lower acidic ion cluster environment. While the proton conductivity is possibly enhanced by an increment of the degree of sulfonation, it could sacrifice the mechanical and dimensional stability, as the membrane loses its toughness and accommodates too much water when the degree of sulfonating is beyond a certain level. As one of the promising aromatic polymers, SPEES has been extensively studied for the application of the proton exchange membranes based on its outstanding thermal, mechanical, and chemical properties. However, as it also showed similar drawbacks, the achievement of higher proton

Membranes 2020, 10, 426; doi:10.3390/membranes10120426 www.mdpi.com/journal/membranes Membranes 2020, 10, 426 2 of 17 conductivity crucially deteriorates other membrane properties. To short out these outcomes, we propose the AMPS introduced into SPEES in this study, because the highly concentrated sulfonic acid groups in the aliphatic AMPS are expected to enhance the conductivity and ﬂexibility of the membrane [23–25]. As an aliphatic polar monomer, AMPS has been recently used for the preparation of polyelectrolyte for a few applications [22–25], because of its good thermal, chemical, and ﬂexible properties. Polymerization of ionic liquid-AMPS (IL-AMPS) with a reactive surfactant produced polymers with high ionic conductivity suitable for the sensor, fuel cell, and battery applications [25–34]. The pristine PAMPS derivatives are, however, generally too weak in mechanical strength in water, illustrating hydrogel properties. The swelling ratio of poly(AMPS-co-MAA) hydrogels touches quite a high level with an increase of AMPS concentration because the sulphonic acid groups in AMPS are strongly hydrophilic [27–29]. In this term, we synthesized the hybrid aromatic-aliphatic polymer electrolyte using AMPS and SPEES. When such an AMPS was introduced to the slightly sulfonated SPEES, it will give an opportunity to optimize the synthesis of polymer electrolyte membranes with tailor made combinations of conductivity due to the distribution of AMPS in uniform way after the cross-linked with SPEES. In this study we synthesized the SPEES-AMs to investigate a variety of membranes properties including proton conductivity, thermal, chemical properties, water uptake, and swelling ratio. Fuel cell operation at low temperature will become a great deal in the PEMFCs with reducing the thermal energy for the application of low temperature fuel cell.

2. Materials and Methods

2.1. Materials 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS), poly(1,4-phenylene ether-ether-sulfone) (PEES), TiCl AA and AlEt Cl ([25] wt% in toluene) were purchased from Sigma-Aldrich 3· 2 (St. Louis, MO, USA). Methanol, acetone, N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), sulfuric acid (H2SO4), were purchased from Daejung Reagents & Chemicals (Korea).

2.2. Synthesis of SPEES and Introducing of Thionyl Chloride into SPEES

The SPEES was synthesized [3,30], through the sulfonation of PEES at 50 ◦C for 8–9 h as shown in Scheme1. The reaction product was precipitated in deionized (DI) water, and then washed several times with DI water to remove excess H2SO4. SPEES polymer was dried at room temperature in a vacuum oven for storage. The yield of the ﬁnal SPEES product was 99%. To convert it to sulfonamide form [30,31], the required amount of SPEES was dissolved in DMAc and reacted with SOCl2 at 80 ◦C for 5–6 h to form the SPEES-Cl (Scheme1). The product, SPEES-Cl, stems according to the above method and procedures. Membranes 2020, 10, 426 3 of 17 Membranes 2020, 10, x FOR PEER REVIEW 3 of 19

SO H O O 3 H SO S O O 2 4 S O O n o n O 50 C O PEES SPEES HO3S Cl S Cl DMAc, 80oC O

O Cl SO H S O 3 O O S O O S O O n n-1 O O O HO3S S O SPEES-Cl Cl CH 3 O TiCl , 70oC 3 o S OH CH3 O DMAc, 60 C O N x H CH3 O AlEt2Cl S OH 12 h CH O N 3 O H CH3 O O N O S OH SO H S CH3 O 3 O O O S O O S O O n n' O O HO3S O S O O H C N SPEES-AM O 3 x S HO CH3 O SchemeScheme 1. Synthesis 1. Synthesis scheme of scheme 2-acrylamido-2-methyl- of 2-acrylamido-2-methyl-1-propanesulfonic1-propanesulfonic (AMPS)-based (AMPS)-based SPEES-AM membraneSPEES-AM. membrane. 2.3. Membrane Preparation 2.3. Membrane Preparation SPEES-Cl (1.0 g) was dissolved in DMAc (10.0 mL) under continuous magnetic stirring at 60 C SPEES-Cl (1.0 g) was dissolved in DMAc (10.0 mL) under continuous magnetic stirring at 60 °C◦ and AMPS was also dissolved in the same solvent. The AMPS solution was dropwise added to and AMPS was also dissolved in the same solvent. The AMPS solution was dropwise added to SPEES-Cl solution at different weight ratio of 1:0.1, 1:0.5, and 1: 1 AMPS pendant with benzene SPEES-Cl solution at different weight ratio of 1:0.1, 1:0.5, and 1: 1 AMPS pendant with benzene (SPEES-AM) under magnetic stirring for 12 h at 55–60 C. The solution color was turned from colorless (SPEES-AM) under magnetic stirring for 12 h at 55–60◦ °C. The solution color was turned from to light pale, and the color of pale increased with the AMPS volume. colorless to light pale, and the color of pale increased with the AMPS volume. Proton exchange membranes based on the aliphatic AMPS modified SPEES were synthesized Proton exchange membranes based on the aliphatic AMPS modified SPEES were synthesized using thermal copolymerization of AMPS and SPEES through sulfonamide formation, as shown in using thermal copolymerization of AMPS and SPEES through sulfonamide formation, as shown in Scheme1. After addition of TiCl 3 AA and AlEt2Cl (25 wt% in toluene), the self-polymerization of Scheme 1. After addition of TiCl3·AA· and AlEt2Cl (25 wt% in toluene), the self-polymerization of AMPS [25–29] was conducted at 70 C for 90 min, and then the reaction mass was quenched into AMPS [25–29] was conducted at 70 °C◦ for 90 min, and then the reaction mass was quenched into methanol [32,33]. The product was filtrated and washed with methanol for several times and then methanol [32,33]. The product was filtrated and washed with methanol for several times and then dried at room temperature. The dry self-polymerized AMPS and SPEES were disloved in DMAc dried at room temperature. The dry self-polymerized AMPS and SPEES were disloved in DMAc seperately. The AMPS solution was drop-wise added into SPEES solution and stirrered for 12 h at 60 C. seperately. The AMPS solution was drop-wise added into SPEES solution and stirrered for 12 h at◦ For membrane preparation, solution was casted onto the clean plane glass surface at 50 C for 5–6 h. 60 °C. For membrane preparation, solution was casted onto the clean plane glass surface ◦at 50 °C for 5–62.4. h. Instrumental Characterization

1 2.4. InstrumentalH NMR analysisCharacterization of SPEES and SPEES-AM was performed using (NMR, Unity Inova Varian, Palo Alto, CA, USA) equipped with a 500 MHz high-resolution NMR console (S/N: S010002), 1 a 51H mm NMR Bore analysis Oxford of super SPEES conduction and SPEES-AM magnet was (S/N: performed 70418) using using dimethyl (NMR, sulfoxideUnity Inova (DMSO-d6) Varian, Paloas the Alto, solvent. CA, USA) Fourier equipped transform with infrared a 500 MHz spectroscopy high-resolution (FT-IR, NMR Bruker, console Germany) (S/N: was S010002), used for a 51 the mm Bore Oxford super conduction magnet (S/N: 70418) using dimethyl sulfoxide (DMSO-d6)1 as the functional group investigation under the IR frequency range from 4000 to 600 cm− . FE-SEM analysis solvent. Fourier transform infrared spectroscopy (FT-IR, Bruker, Germany) was used for the functional group investigation under the IR frequency range from 4000 to 600 cm−1. FE-SEM analysis has been tested for different membranes (surface, morphology) with JEOL, JSM 7000 F,

Membranes 2020, 10, 426 4 of 17 has been tested for diﬀerent membranes (surface, morphology) with JEOL, JSM 7000 F, Tokyo, Japan at 15.00 kV. Thermogravimetric analysis was performed with a thermogravimetric analyzer (TGA, TG7300, SEICO INST) for the thermal stability of the prepared membranes. The microstructure of the AMPS cross-linked membranes was analyzed by small-angle X-ray scattering spectroscopy (SAXS, Anton Paar®, Graz, Austria).

3. Physio-Chemical Characterization

3.1. Water Uptake and Swelling ratio and Ion-Exchange Capacity (IEC)

Membranes were dried in the oven at 120 ◦C until a constant weight marked as Wdry, and then immersed in H2O at room temperature for 24 h to reach the ﬁnal dilution, the surface water was wiped with a ﬁlter paper to be immediately weighted, marked as Wwet. The water content (%) was calculated as the following Equation (1) Wwet Wdry WU (%) = − 100 (1) Wdry × SIEC was calculated according to the previous report [3]. The swelling ratio (SR) was calculated with the Ldry and Lwet where Ldry is the weight and length of dry membrane and Lwet weight and length of wet membrane. Lwet Ldry SR (%) = − 100 (2) Ldry ×

3.2. Oxidative, Chemical Stability, and Proton Conductivity

Oxidative stability was tested with Fenton’s reagent (3% H2O2 and 3 ppm FeSO4) for our prepared membranes at 80 ◦C in a closed vial for 72 h. Chemical stability was measured in the 3 M KOH and 3 M H2SO4 at 25 ◦C for one week. (Before all measurements, all samples dried under the vacuum oven at 80 ◦C for 3 days.) The proton conductivity (σ) measured by the Equation (3):

L σ = (3) R W T × × where, R is the ohmic resistance, and L (= 0.425 cm) is the distance between the anode and cathode electrodes. W is the width, and T is the thickness of the membrane sample.

3.3. Membrane Electrode Assembly (MEA) MEA was prepared for fuel cell performance. The catalyst ink was sonicated by a sonicator (Sonomasher, SL Science, Seoul, Korea) by adding 0.1 g of Pt/C (40%), 1 mL of DI water, 0.66 g of Nafion ionomer (5 wt% in IPA), and 8.042 g of IPA. The prepared solution was sprayed onto a gas diffusion layer (GDL) using a sprayer gun (Model GP-1, Tokyo, JAPAN 21701) in 5–10 min intervals. The MEA was prepared by pressing the catalyst-coated membrane using a heating press (Ocean Science, 2 Seoul, Korea) at 100 ◦C and 5 MPa for 3 min. The active area of MEA was 6.25 cm and the Pt loading 2 amount of both anode and cathode was 0.30 mg cm− each. The H2/O2 fuel cell was operated at 50 ◦C under 15% RH. The polarization curve for MEA fabricated with each membrane was obtained using 3 1 a unit cell station (SPPSN-300) provided by CNL Energy (Seoul, Korea) at 200 cm min− H2 and O2 flow rate.

4. Results and Discussion

4.1. Strutucture Characterization Generally, the sulfonated aromatic-based polymer electrolyte membranes have a major disadvantage of the brittleness if the ionic site is directly attached to the polymer matrix (low sulfonation;Membranes Scheme2020, 10 ,2a). 426 These membranes exhibited low conductivity and 5 ofpoor 17 fuel cell performance and if the degree of sulfonation increased, membrane matrix deteriorates (Scheme 2b). The detailedScheme schematic2a). These illustration membranes exhibited is presented low conductivity in Scheme and poor2. Here fuel cell in performancethis present and work, if the we applied degree of sulfonation increased, membrane matrix deteriorates (Scheme2b). The detailed schematic the aliphaticillustration monomer is presented (AMPS) in Scheme to reduce2. Here the in this problem present work, of low we appliedsulfonation the aliphatic degrees monomer (Scheme 2a), as presented (AMPS)in Schemes to reduce 1 theand problem 2c. of low sulfonation degrees (Scheme2a), as presented in Schemes1 and2c.

Scheme 2. Schematic illustration of sulfonation of PEES (a) low degree of sulfonation (b) high degree Scheme 2. Schematic illustration of sulfonation of PEES (a) low degree of sulfonation (b) high of sulfonation and (c) low degre of sulfonation with apliphatic monomer. degree of sulfonation and (c) low degre of sulfonation with apliphatic monomer. 1H NMR analysis confirmed the structure of SPEES and SPEES-AM. In Figure1, the aromatic 1H NMRprotons analysis (Ar–H) areconfirmed observed around the structure= 7–8 ppm for of SPEES, SPEES while and the SPEES-AM. aliphatic protons In are Figure observed 1, at the aromatic δ= 1.5 ppm and 1.90 ppm, ((CH3)2 a) and another aliphatic –CH2-S at 2.80–2.90 ppm and CH=CH2 protons (Ar–H)(multiplet, are 5.50–6.20) observed for SPEES-AM.around = The 7–8 peak ppm observed for SPEES, at 4.5 ppm while from N–H the of aliphatic AMPS (SPEES-AM-01) protons are observed at δ= 1.5 ppmwas shiftedand 1.90 toward ppm, upfield ((CH (c, 3.7–4.003)2 a) and PPM) another (SPEES-AM-03 aliphatic (FTIR –CH spectra2-S for at SPEES 2.80–2.90 and SPEES-AM ppm and CH=CH2 1 (multiplet,are 5.50–6.20) depicted in Figurefor 2.SPEES-AM. The aromatic C–C The stretching peak vibration observed was observedat 4.5 at 1596–1471ppm from cm − N–Hwith of AMPS medium to high intensity and the aromatic C–H, O=S=O (sulfone) group. The O–H group is observed (SPEES-AM-01) was shifted1 toward1 upfield (c, 3.7–4.001 PPM) (SPEES-AM-03 (FTIR spectra for at 3145–3025 cm− , 1184–1139 cm− , and 3250–3650 cm− (broad peak), respectively. The –SO3H group SPEES andin SPEESSPEES-AM is observed are at 1247–1181 depicted cm -1inin FigureFigure2. The 2. cross-linking The aromatic was confirmed C–C fromstretching the presence vibration was 1 observed atof 1596–1471 secondary sulfonamide cm−1 with group medium (1300–1350 to high cm− ). intensity For SPEES-AM, and Cthe=C aromatic stretching vibration C–H, O=S=O was (sulfone) observed at 1657 cm 1 and C=O was at 1727 cm 1. After AMPS grafting, the intensity of the O–H − − −1 −1 −1 group. Thebroad O–H band group decreased. is observed at 3145–3025 cm , 1184–1139 cm , and 3250–3650 cm (broad peak), respectively. The –SO3H group in SPEES is observed at 1247–1181 cm-1 in Figure 2. The cross-linking was confirmed from the presence of secondary sulfonamide group (1300–1350 cm−1). For SPEES-AM, C = C stretching vibration was observed at 1657 cm-1 and C = O was at 1727 cm−1. After AMPS grafting, the intensity of the O–H broad band decreased.

Membranes 2020, 10, 426 6 of 17

Membranes 2020, 10, x FOR PEER REVIEW 6 of 19

Membranes 2020, 10, x FOR PEER REVIEW 7 of 19 Figure 1.Figure1 H NMR1. 1 H NMR for for SPEES, SPEES, SPEES-AM-01,SPEES-AM-01, and SPEES-AM-03. and SPEES-AM-03.

SPEES

SPEES-AM-03 % T SPEES-AM-02

SPEES-AM-01

1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm )

Figure 2. FT-IRFigure 2. for FT-IR SPEES, for SPEES, SPEES-AM-01, SPEES-AM-01, SPEES-AM-02, SPEES-AM-02, and SPEES-AM-03. and SPEES-AM-03.

4.2. Morphological Structure While the SPEES membrane was transparent, SPEES-AM discolored the transparency into white haziness as shown in Figure 3A–D. All prepared membranes have been compatible with AMPS. In Figure 3D, SPEES-AM-03 membrane is still foldable after pressurizing without losing its primary stage. Figure 4 shows the SEM images of SPEES and SPEES-AMs.

Membranes 2020, 10, 426 7 of 17

4.2. Morphological Structure While the SPEES membrane was transparent, SPEES-AM discolored the transparency into white haziness as shown in Figure3A–D. All prepared membranes have been compatible with AMPS. In Figure3D, SPEES-AM-03 membrane is still foldable after pressurizing without losing its primary stage.Membranes Figure 2020,4 10 shows, x FOR thePEER SEM REVIEW images of SPEES and SPEES-AMs. 8 of 19

Membranes 2020, 10, x FOR PEER REVIEW 8 of 19

Figure 3. Optical images for (A) SPEES, (B) SPEES-AM-01, (C) SPEES-AM-02, and (D) Figure 3.SPEES-AM-03.3.Optical Optical images images for (Afor) SPEES, (A) SPEES, (B) SPEES-AM-01, (B) SPEES-AM-01, (C) SPEES-AM-02, (C) SPEES-AM-02, and (D) SPEES-AM-03. and (D) SPEES-AM-03.

Figure 4.FigureFE-SEM 4. FE-SEM for( forA) ( SPEES,A) SPEES, (B (B)) SPEES-AM-01, SPEES-AM-01, (C) ( CSPEES-AM-02,) SPEES-AM-02, (D) SPEES-AM-03. (D) SPEES-AM-03.

Figure 4. FE-SEM for (A) SPEES, (B) SPEES-AM-01, (C) SPEES-AM-02, (D) SPEES-AM-03.

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It can be seen that the SPEES membrane (Figure 4A) exhibits a smooth and tight surface while the SPEES-AMMembranes membrane2020, 10, 426 (Figures 4A–D) exhibits an uneven and stiff surface, indicating8 of 17 that the phase-separated APMS domains are interconnected with SPEES phases. This difference is related to the different molar ratio of AMPS to SPEES in the membrane, and the obvious course and It can be seen that the SPEES membrane (Figure4A) exhibits a smooth and tight surface while microporousthe SPEES-AM morphology membrane can be (Figure observed4A–D) exhibits in the an image uneven of and the sti membranesff surface, indicating (Figure that 4C,D). the The images showedphase-separated to be homogenous APMS domains without are interconnected any cracks with and SPEES holes phases. in the This membrane difference is phase. related toAn AMP has higherthe hydrophilic different molar nature ratio of beca AMPSuse to SPEESof higher in the ionic membrane, concentration and the obvious than course slightly and sulfonated microporous SPEES. The sulfonicmorphology groups can can be observedproduce in larger the image electrostati of the membranesc repulsive (Figure forces4C,D). Thethan images carboxylic showed togroups be [25– homogenous without any cracks and holes in the membrane phase. An AMP has higher hydrophilic 27], and thus the ion cluster size increased with increasing AMPS/SPEES molar ratio. Besides, the nature because of higher ionic concentration than slightly sulfonated SPEES. The sulfonic groups can alkyl groupproduce of AMPS larger electrostatic is hydrophobic, repulsive which forces than can carboxylic form hydrophobic groups [25–27 ],micro-domains and thus the ion cluster to decrease size the hydrogenincreased bonding with interaction increasing AMPS between/SPEES hydrophilic molar ratio. Besides, polymeric the alkyl chains group and of AMPS polymer is hydrophobic, backbone [29]. Element whichcomposition can form in hydrophobic the membrane micro-domains matrix towas decrease analyzed the hydrogen by the bonding EDS (Figure interaction 5). betweenEDS confirms the presencehydrophilic of C, polymeric N, O, and chains S and via polymer elementa backbonel mapping [29]. Element (Figure composition 5a) and in also the membrane confirmed the matrix was analyzed by the EDS (Figure5). EDS confirms the presence of C, N, O, and S via elemental homogeneous distribution of all presented elements, which suggest the successful synthesis of mapping (Figure5a) and also confirmed the homogeneous distribution of all presented elements, SPEES-AMwhich and suggest also presented the successful the synthesis EDS spectra of SPEES-AM in Figure and also5b. presented the EDS spectra in Figure5b.

Figure 5. Figure(a) Elemental 5. (a) Elemental mapping mapping images images for for C, C, O, O, N, N, and and S;S; and and (b ()b EDS) EDS spectrum spectrum of SPEES-AM-02. of SPEES-AM-02.

SAXS measurements employed for the micro-phase separation of SPEES and SPEES-AMs. Since the SAXSlocation measurements of the SAXS peakemployed is related for to thethe inter-cluster micro-ph distance,ase separation the average of dimensionSPEES and of the SPEES-AMs. ionic Since the clusterslocation in theof membranes,the SAXS peak which is constitute related the to ionicthe pathsinter-cluster for proton distance, migration inthe the average PEMFCs, dimension can be of the ionic deducedclusters from in the the SAXSmembranes, data [7]. Figurewhich6 showsconsti thetute SAXS the patternsionic paths of the plainfor proton SPEES membranemigration in the PEMFCs,and can the be SPEES-AM deduced membranes.from the SAXS The averagedata [7]. dimension Figure of6 shows ionic clusters the SAXS is calculated patterns with of the the plain SPEES membranegiven Equation and (4): the SPEES-AM membranes. The average dimension of ionic clusters is d = 2π/q (4) calculated with the given Equation (4): d =2π/q (4) where, q is the scattering vector, which is equal to 44π/λsinθ with 2θ being the scattering angle and λ the X-ray wavelength. As we can see that, in case of SPEES there is no micro-phase separation observed while in the incorporation of AMPS into SPEES increases the average dimension of the ionic clusters membranes showed distinct peaks at q = 3.2 nm−1, corresponding to a characteristic distance of d = 1.96 nm. All SPEES-AMs with more dense ionic groups tend to display a more profound scattering peak, which suggests larger spacing of ion-conducting channels. Generally, the

Membranes 2020, 10, 426 9 of 17 where, q is the scattering vector, which is equal to 44π/λsinθ with 2θ being the scattering angle and λ the X-ray wavelength. As we can see that, in case of SPEES there is no micro-phase separation observed while in the incorporation of AMPS into SPEES increases the average dimension of the 1 ionic clusters membranes showed distinct peaks at q = 3.2 nm− , corresponding to a characteristic distanceMembranes of 2020d =, 101.96, x FOR nm. PEERAll SPEES-AMsREVIEW with more dense ionic groups tend to display a more profound10 of 19 scattering peak, which suggests larger spacing of ion-conducting channels. Generally, the polymeric polymeric electrolyte membranes form the ionic clusters from the counterbalance between the electrolyte membranes form the ionic clusters from the counterbalance between the electrostatic energy electrostatic energy released by ion-dipole interactions and the elastic free energy attributable to the released by ion-dipole interactions and the elastic free energy attributable to the deformation of deformation of backbone chain. This, combined with a higher concentration of –SO3H, provides backbone chain. This, combined with a higher concentration of –SO H, provides membranes with high membranes with high proton conductivity, makes us believe th3at the AMPS cross-linked SPEES proton conductivity, makes us believe that the AMPS cross-linked SPEES membranes with reduced membranes with reduced size of ionic clusters can exhibit fascinating properties for PEMFC size of ionic clusters can exhibit fascinating properties for PEMFC applications at low temperature applications at low temperature with reduced RH. with reduced RH.

Figure 6. SAXS patterns of SPEES and SPEES- AM membranes. Figure 6. SAXS patterns of SPEES and SPEES- AM membranes. 4.3. Thermal and Mechanical Properties 4.3. Thermal and Mechanical Properties Thermal analysis of the prepared membranes showed three-step weight losses in the temperature Thermal analysis of the prepared membranes showed three-step weight losses in the range between 30 ◦C and 500 ◦C in Figure7. All membranes were quite stable up to 100 ◦C. temperature range between 30 °C and 500 °C in Figure 7. All membranes were quite stable up to Beyond 100 ◦C, the ﬁrst weight loss was observed by the evaporation of bound water in the membrane 100 °C. Beyond 100 °C, the first weight loss was observed by the evaporation of bound water in the via ionic charge of –SO3H. SPEES-AM-03 showed the maximum weight loss in this region because of themembrane presence via of moreionic boundcharge water of –SO associated3H. SPEES-AM-03 with more showed functional the groups maximum in it. Theweight second loss weightin this region because of the presence of more bound water associated with more functional groups in it. loss was observed around 200–350 ◦C because of the decomposition of the sulfonic acid groups in The second weight loss was observed around 200–350 °C because of the decomposition of the SPEES-AM membranes. Weight loss up to 350 ◦C for the SPEES-AM-03 membrane was appreciably highersulfonic than acid those groups for thein SPEES-AM SPEES-AM-01 membranes. and SPEES-AM-02 Weight loss membrane up to 350 because °C formore the SPEES-AM-03 sulfonic acid groupsmembrane were was present appreciably inside it. higher The third than weight those for loss the step SPEES-AM-01 was attributed and to SPEES-AM-02 the decomposition membrane of the because more sulfonic acid groups were present inside it. The third weight loss step was attributed polymer backbone around 350 ◦C in SPEES-AM membranes. to the decomposition of the polymer backbone around 350 °C in SPEES-AM membranes.

Membranes 2020, 10, 426 10 of 17 Membranes 2020, 10, x FOR PEER REVIEW 11 of 19

Figure 7. TGA for SPEES-AM membranes. 4.4. Water Uptake and Swelling Ratio and IEC 4.4. Water Uptake and Swelling Ratio and IEC The water uptake and swelling behavior of the membranes are key factors that significantly The water uptake and swelling behavior of the membranes are key factors that significantly affect the proton conductivity, and mechanical properties of membranes. It can be clearly seen that affect the proton conductivity, and mechanical properties of membranes. It can be clearly seen that the water uptake increased with the increment of AMPS content because of more sulfonic groups the water uptake increased with the increment of AMPS content because of more sulfonic groups incorporated within the SPEES-AM membranes [27–29]. The effect of AMPS content on water uptake, incorporated within the SPEES-AM membranes [27–29]. The effect of AMPS content on water the swelling degree, and IEC was studied and summarized in Table1 and the solubility test is also uptake, the swelling degree, and IEC was studied and summarized in Table 1 and the solubility test presented in Table2. is also presented in Table 2. Table 1. Proton conductivity (σ), water uptake (WU), swelling ratio (SR), acid-base test (AT-BT), and IEC Table 1. Proton conductivity (σ), water uptake (WU), swelling ratio (SR), acid-base test (AT-BT), of SPEES and SPEES –AM Membranes at 30 ◦C. and IEC of SPEES and SPEES –AM Membranes at 30 °C. Membrane σ WU (%) SR (%) AT (%) BT (%) IEC (meq g 1) IEC· − MembraneSPEES 0.031σ WU 16 (%) 20 SR (%) 3.25 AT (%) 4.11 BT (%) 1.75 (meq·g−1) SPEES-AMPS-01 0.047 20 27 0.11 0.21 1.87 SPEES-AMPS-02SPEES 0 0.058.031 2616 3520 1.223.25 0.454.11 1.75 2.01 SPEESSPEES-AMPS-03-AMPS-01 0 0.071.047 3020 4127 3.780.11 2.550.21 1.87 2.17 N115 0.021 11 13 SPEES-AMPS-02 0.058 26 35 1.22 0.45 2.01 SPEES-AMPS-03 0.071 30 41 3.78 2.55 2.17 Table 2. Solubility data. N115 0.021 11 13 Solvent PEES SPEES SPEES-AMPS Table 2. Solubility data. CH3OH Insoluble Insoluble CH3OCH3 Insoluble Insoluble Solvent PEES SPEES SPEES-AMPS DMAc Insoluble Soluble at <40 ◦C Soluble at 25 ◦C DMFCH3OH Insoluble Insoluble Soluble Insoluble at < 40 ◦C Soluble at 25 ◦C THFCH3OCH3 Insoluble Insoluble Insoluble Insoluble Soluble at 45 ◦C NMPDMAc Insoluble Insoluble Soluble Soluble at <40 at 50 °C◦C Soluble Soluble at 25 at °C 25 ◦C DMSO Insoluble Soluble at 50 ◦C Soluble at 25 ◦C DMF Insoluble Soluble at <40 °C Soluble at 25 °C H2SO4 Soluble Soluble Soluble THF Insoluble Insoluble Soluble at 45 °C NMP Insoluble Soluble at 50° Soluble at 25 °C DMSO Insoluble Soluble at 50° Soluble at 25 °C

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Membranes 2020, 10, 426 H2SO4 Soluble Soluble Soluble 11 of 17

As the concentration of AMPS increases the swelling ratio increases because of the higher As the concentration of AMPS increases the swelling ratio increases because of the higher concentration of -SO3H groups which are surrounded by a large number of water molecules concentration of -SO3H groups which are surrounded by a large number of water molecules thereby thereby increasing the swelling capacity. It can be seen that SPEES-AM-01 exhibited minimum increasing the swelling capacity. It can be seen that SPEES-AM-01 exhibited minimum water uptake water uptake and swelling at low temperature of 30 °C in comparison with SPEES-AM-02 and 03. and swelling at low temperature of 30 ◦C in comparison with SPEES-AM-02 and 03. IEC value also IEC value also increased with increasing amount of functional group from 1.75 to 2.17 (meq·g−1). increased with increasing amount of functional group from 1.75 to 2.17 (meq g 1). · − 4.5.4.5. Oxidative Oxidative and and Chemical Chemical Stability Stability TheThe oxidative oxidative stability stability of of SPEES-AM SPEES-AM membranes membranes ha hass been been measured measured as as an an important important property property of esteemed membranes related to durability in Fenton’s reagent (3% H2O2 and 3 ppm FeSO4). of esteemed membranes related to durability in Fenton’s reagent (3% H2O2 and 3 ppm FeSO4). FigureFigure 8 deﬁnesdefines thethe radicalradical degradationdegradation ofof AMPS-based AMPS-based membranes membranes measured measured in in Fenton’s Fenton’s reagent reagent at at 60 °C. The degradation during the oxidative testing of the membranes is estimated by the weight 60 ◦C. The degradation during the oxidative testing of the membranes is estimated by the weight loss, loss,conductivity conductivity loss, loss, and mechanicaland mechanical loss. Asloss. shown As shown in Figure in 8Figure, the membranes 8, the membranes show good show oxidative good oxidativedurability durability in Fenton’s in solution. Fenton’s The solution. SPEES-AM-01 The SPEES-AM-01 membrane wasmembrane observed was to loseobserved approximately to lose approximately ~1.5% weight after being immersed in a 3% H2O2 solution for 120 h. An initial sharp ~1.5% weight after being immersed in a 3% H2O2 solution for 120 h. An initial sharp decrease in decreaseweight percentage in weight ofpercentage SPEES-AM of membraneSPEES-AM between membrane 0.42 between and 0.53% 0.42 was and observed 0.53% was after observed 72 h of testing after 72in h Fenton’s of testing reagent. in Fenton’s It can reagent. be clearly It seencan be that clearly the weight seen reducedthat the weight with the reduced increase with of AMPS the increase content, ofindicating AMPS content, that oxidation indicating attacks that on oxidation the membrane attacks by on radical the membrane species occurs by radical mainly onspecies AMPS occurs units. mainlyChemical on stabilityAMPS units. is an essentialChemical requirement stability is an for essential long-term requirement durability and for maximumlong-term weightdurability % loss and in maximumacid (AT) 3.78weight and % in loss base in (BT) acid 2.55(AT) % 3.78 (Table and1) in for base polymer (BT) 2.55 electrolyte. % (Table In 1) addition, for polymer we also electrolyte. studied Inproton addition, conductivity we also studied after the proton chemical conduc stabilitytivity test. after the chemical stability test.

1.2 SPEES-AM-01 SPEES-AM-02 1.0 SPEES-AM-03

0.8

0.6 % Weight loss Weight % 0.4

0.2

0.0 20 40 60 80 100 120 140 Time (h)

Figure 8. Oxidative stability test for SPEES-AM-based membranes at 60 C. Figure 8. Oxidative stability test for SPEES-AM-based membranes at 60 °C.◦ 4.6. Proton Conductivity and Activation Energy

+ +, Immobile sulfonic acid groups may dissociate and the hydronium ions (e.g., H3O ,H5O2 + and H9O4 ) were formed via hydrogen bonding around the sulfonic acid groups from sulfonated polymer electrolytes in a hydrated condition [30,31]. In swollen state, free protons easily transport

Membranes 2020, 10, x FOR PEER REVIEW 13 of 19

4.6. Proton Conductivity and Activation Energy

Immobile sulfonic acid groups may dissociate and the hydronium ions (e.g., H3O+, H5O2+, and MembranesH9O4+) were2020, 10formed, 426 via hydrogen bonding around the sulfonic acid groups from sulfonated12 of 17 polymer electrolytes in a hydrated condition [30,31]. In swollen state, free protons easily transport from sulfonated polymer electrolyte membranes along with the hydrogen-bonded ionic network so fromthat the sulfonated proton conductivity polymer electrolyte will be membranesclearly enhanced along by with the the water hydrogen-bonded uptake in the membrane ionic network matrix. so thatProton the conductivity proton conductivity is one of will the be most clearly import enhancedant properties by the water for PEMFC uptake inmembranes. the membrane The matrix.proton Protonconductivity conductivity is discussed is one with of the the most following important mechanism properties [25], for presented PEMFC in membranes. Scheme 3a TheGrotthus proton or conductivity“jump” mechanism is discussed which with can the be following idealized mechanismas protons [25being], presented passed down in Scheme the 3chaina Grotthus of water or “jump”molecules mechanism and ion whichexchange can sites; be idealized and (b) as vehicle protons mechanism being passed which down assumes the chain that of protons water molecules combine and ion exchange sites; and (b) vehicle mechanism which assumes that protons combine with solvent with solvent molecules to yield complexes like H3O+ and then diffuse as a whole across the + moleculesmembrane. to We yield observed complexes that like the H 3highO and conductivity then diﬀuse of as SPEES-AM a whole across membranes the membrane. is due Weto the observed linked thationic the cluster high structure conductivity as proved of SPEES-AM by SAXS membranes and SEM analysis is due toformed the linked in the ionic membrane. cluster structureIonic cluster as provedsuch as by SPEES-AM SAXS and have SEM analysisnot been formed found inin thethe membrane. SPEES membrane Ionic cluster to allow such H as+ SPEES-AMto jump from have one not – been found in the SPEES membrane to allow H+ to jump from one –SO H group to another through SO3H group to another through the channel in the presence of water. In3 the SPEES-AM membranes, the channel in the presence of water. In the SPEES-AM membranes, on the other hand, the H+ transfer on the other hand, the H+ transfer had to be facilitated with the tertiary (3°) amine and –SO3H hadgroups to be to facilitated form ionic with bonds. the tertiary (3◦) amine and –SO3H groups to form ionic bonds.

Scheme 3. Proton transfer mechanism in SPEES and SPEES-AM membranes: (a) Grotthus mechanism Scheme 3. Proton transfer mechanism in SPEES and SPEES-AM membranes: (a) Grotthus (b) vehicle mechanism. mechanism (b) vehicle mechanism. As presented in Figure9, the proton conductivity of SPEES-AM membranes are dependent on the waterAs uptake presented and sulfonic in Figure acid 9, group the proton content conduc in thetivity membrane of SPEES-AM sites. It should membranes be noted are that dependent the proton on the water uptake and sulfonic acid group content1 in the membrane sites. It should be noted1 that the conductivity of SPEES-AM-01 showed 0.047 Scm− while SPEES-AM-03 showed 0.071 Scm− in acid proton conductivity of SPEES-AM-01 showed 0.047 Scm−1 while1 SPEES-AM-03 showed 0.071 Scm−1 form at room temperature and Nafion N 115 showed 0.021 Scm− under the same measurements. In this −1 workin acid the form observable at room point temperature is the increase and of Nafion AMPS monomerN 115 showed because 0.021 of the Scm enhancement under the of watersame uptakemeasurements. and proton In this conductivity. work the Itobservable can be clearly point seen is the that increase the conductivity of AMPS monomer SPEES-AM-01 because exhibited of the enhancement1 of water uptake and proton 1conductivity. It can be clearly seen1 that the conductivity ~ 0.09 Scm− , SPEES-AM-02 ~ 0.094 Scm− , and SPEES-AM-03 ~ 0.13 Scm− which are relatively SPEES-AM-01 exhibited ~ 0.09 Scm−1, SPEES-AM-02 ~ 0.094 Scm−1, and SPEES-AM-03 ~ 0.13 Scm−1 comparable in proton conductivity with temperature increased at 80 ◦C and 90% RH. SPEES-AM-02 which are relatively comparable in proton conductivity with temperature increased at 80 °C and and SPEES-03 were not stable up to 80 ◦C due to high water-swelling in membrane matrix on the effect 90% RH. SPEES-AM-02 and SPEES-03 were not stable up to 80 °C due to high water-swelling in of high ionic sites (SO3H). membraneIn polymer matrix electrolyte on the effect membranes, of high ionic the activationsites (SO3H). energy is an important parameter for proton conduction that takes the minimum energy part for proton transfer. The activation energy for proton conduction must be reduced which can reduce the energy loss produced that would be favorable for refining energy consumption during fuel cell operation. The activation energy is obtained based on the proton conductivity dependent on time and temperature. Protons move rapidly during high temperatures in a solids material, which follows a simple Arrhenius law and is shown in Figure 10. The activation energy (Ea) was determined from the temperature dependence of ion conductivity applying the Arrhenius Equation (5).

Ea ln σ = ln σ0 (5) − RT Membranes 2020 10 Membranes 2020, 10, ,x FOR, 426 PEER REVIEW 13 of14 17 of 19

0.12

0.10

0.08 ) -1 0.06 Scm σ ( 0.04 N115 SPEES SPEES-AM-01 0.02 SPEES-AM-02 SPEES-AM-03

0.00 30 40 50 60 70 o Temperature ( C)

Membranes 2020Figure, 10, x FOR 9. Proton PEER REVIEW conductivity of Naﬁon (N115), SPEES and SPEES-AM membranes. 15 of 19 Figure 9. Proton conductivity of Nafion (N115), SPEES and SPEES-AM membranes.

In polymer electrolyte membranes, the activation energy is an important parameter for proton conduction that takes the minimum energy part for proton transfer. The activation energy for proton conduction must be reduced which can reduce the energy loss produced that would be favorable for refining energy consumption during fuel cell operation. The activation energy is obtained based on the proton conductivity dependent on time and temperature. Protons move rapidly during high temperatures in a solids material, which follows a simple Arrhenius law and is shown in Figure 10. The activation energy (Ea) was determined from the temperature dependence of ion conductivity applying the Arrhenius Equation (5). 𝐸 σ σ 𝑎 (5) ln = ln − 𝑅𝑇

Figure 10. Arrhenius plot of Naﬁon (N115), SPEES and SPEES-AMs. Figure 10. Arrhenius plot of Nafion (N115), SPEES and SPEES-AMs.

The proton conductivity is also checked at constant temperature (45 °C) after acid stability up to an hour in Figure 11 and all membranes observed that the conductivity is also constant. We have seen that all membranes perform well and there is no one membrane that loses its conductivity property. Therefore, SPEES-AMs are completely sufficient for fuel cell operations at low temperature with low RH. Continuity is not seen beyond 65 °C conductivity and has little fluctuation and also affects the backbone of the membrane. The water uptake of SPEES-AM membranes upward with the increasing AMPS content, which loosens the compact structure of membrane, and therefore some sulfonic acid groups leach into the water during the measurement. It should be mentioned that none of the membranes were broken into small pieces, and all samples remained in a good membrane form after the 120 h test period.

Membranes 2020, 10, 426 14 of 17

The proton conductivity is also checked at constant temperature (45 ◦C) after acid stability up to an hour in Figure 11 and all membranes observed that the conductivity is also constant. We have seen that all membranes perform well and there is no one membrane that loses its conductivity property. Therefore, SPEES-AMs are completely sufficient for fuel cell operations at low temperature with low RH. Continuity is not seen beyond 65 ◦C conductivity and has little fluctuation and also affects the backbone of the membrane. The water uptake of SPEES-AM membranes upward with the increasing AMPS content, which loosens the compact structure of membrane, and therefore some sulfonic acid groups leach into the water during the measurement. It should be mentioned that none of the membranes were broken into small pieces, and all samples remained in a good membrane form Membranes after2020,the 10, 120x FOR h test PEER period. REVIEW 16 of 19

0.10

0.09

0.08

0.07

0.06 ) -1 0.05 (Scm

σ 0.04

0.03 SPEES-AM-01 SPEES-AM-02 0.02 SPEES-AM-03 SPEES 0.01

0.00 0 102030405060 Time (m) Figure 11.Figure Proton 11. conductivityProton conductivity of SPEES of SPEES and andSPEES-AM SPEES-AM me membranesmbranes atat 4545◦ °CC for for an an hour hour after after acid acid stability. stability. 4.7. Fuel Cell Performance

4.7. Fuel Cell ThePerformance H2/O2 fuel cell performance of SPEES-AM membranes is shown in Figure 12. As shown in Figure 12 SPEES-AM-01 and SPEES-AM-02 exhibited the open-circuit voltage (OCV) of 0.90 V and The H2/O2 fuel cell performance of SPEES-AM membranes is shown in Figure 12. As shown in 0.98 V respectively. The fuel cell performance has been tested at 50 ◦C under 15% RH. This research Figure 12will SPEES-AM-01 create a new achievement and SPEES-AM-02 fuel cell operation exhibited at low the temperature open-circuit with RH voltage and utilization (OCV) of of water 0.90 V and 0.98 V respectively.molecule generated The fuel at thecell cathode performance section. has SPEES-AM-02 been tested showed at 50 a °C maximum under power15% RH. density This of research 2 2 will create92 mWcma new− achievementat a current density fuel of cell 159 mAoperatio cm− , whilen at SPEES-AM-01low temperature showed with a peak RH power and density utilization of of 2 water molecule75 mWcm generated− . Therefore, at itthe seems cathode that the section. fuel cell operationSPEES-AM-02 at low temperature showed a can maximum facilitate operation power density and energy in order to reduce the energy consumption and can facilitate in the ﬁeld of energy. of 92 mWcm−2 at a current density of 159 mA cm−2, while SPEES-AM-01 showed a peak power density of 75 mWcm−2. Therefore, it seems that the fuel cell operation at low temperature can facilitate operation and energy in order to reduce the energy consumption and can facilitate in the field of energy.

Membranes 2020, 10, 426 15 of 17 Membranes 2020, 10, x FOR PEER REVIEW 17 of 19

1.0 100

0.9

0.8 80 Power density (mWcm

0.7

0.6 60

0.5

0.4 40 Cel voltatage (V) voltatage Cel -2

0.3 )

0.2 SPEES-AM-02 20 SPEES-AM-01 0.1

0.0 0 0 50 100 150 200 250 Current density (mAcm-2)

Figure 12.12. Fuel cell performance ofof SPEES-AMSPEES-AM membranesmembranes inin HH22/ OO22 single cell PEMFC test atat 5050 ◦°CC (under(under 15%15% RH).RH). 5. Conclusions 5. Conclusions A hybrid concept based on aromatic and aliphatic polymer electrolyte membranes with synthesized A hybrid concept based on aromatic and aliphatic polymer electrolyte membranes with SPEES and electrolyte monomer AMPS exhibits elevated proton conductivity of 0.125 Scm 1 at 70 C synthesized SPEES and electrolyte monomer AMPS exhibits elevated proton conductivity− of 0.125◦ (100% RH). The presence of AMPS in the SPEES domain enables to provide high electrolyte. The water Scm−1 at 70 °C (100% RH). The presence of AMPS in the SPEES domain enables to provide high uptake and swelling behavior of the membranes are key factors that significantly affect the proton electrolyte. The water uptake and swelling behavior of the membranes are key factors that conductivity, and mechanical properties of the membranes. The AMPS-based membrane deteriorated significantly affect the proton conductivity, and mechanical properties of the membranes. The at high temperature (above 65 C) because of high functional charge with sulfonic, tertiary amine AMPS-based membrane deteriorated◦ at high temperature (above 65 °C) because of high functional and ketone. All these membranes have been shown to have constant conductivity during constant charge with sulfonic, tertiary amine and ketone. All these membranes have been shown to have temperature (45 C) and can be of great benefit at low temperatures under low RH for fuel cell constant conductivity◦ during constant temperature (45 °C) and can be of great benefit at low applications with cost-effectiveness. temperatures under low RH for fuel cell applications with cost-effectiveness.

AuthorAuthor Contributions:Contributions: Conceptualization,Conceptualization, M.M.;M.M.; methodology,methodology, M.M.;M.M.; software,software, M.M.;M.M.; validation, M.M.; formal analysis, M.M.; investigation, M.M.; resources, M.M.; data curation, M.M.; writing—original formal analysis, M.M.; investigation, M.M.; resources, M.M.; data curation, M.M.; writing—original draft draft preparation, M.M.; writing—review and editing, M.M.; visualization, D.K.; supervision, D.K.; projectpreparation, administration, M.M.; writing—review D.K.; funding acquisition,and editing, D.K. M.M All.; authorsvisualization, have readD.K.; and supervision, agreed to theD.K.; published project versionadministration, of the manuscript. D.K.; funding acquisition, D.K. All authors have read and agreed to the published version of the manuscript. Funding: This work was sponsored by National Foundation of Korea, Republic of Korea, (NRF2018M3D1A1058624).Funding: This work was sponsored by National Foundation of Korea, Republic of Korea, Conﬂicts(NRF2018M3D1A1058624). of Interest: The authors declare no conﬂict of interest. Conflicts of Interest: The authors declare no conflict of interest References

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