Study of High Performance Sulfonated Polyether Ether Ketone Composite Electrolyte Membranes

Article Study of High Performance Sulfonated Polyether Ether Ketone Composite Electrolyte Membranes

Gwomei Wu 1,2,*, Sheng-Jen Lin 1, I-Chan Hsu 1, Juin-Yih Su 2 and Dave W. Chen 2,*

1 Institute of Electro-Optical Engineering, Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan 333, Taiwan 2 Chang Gung Memorial Hospital, Keelung 204, Taiwan * Correspondence: [email protected] (G.W.); [email protected] (D.W.C.); Tel.: +886-3-211-8800 (G.W.)

Received: 19 June 2019; Accepted: 11 July 2019; Published: 12 July 2019

Abstract: In this study, high performance composite electrolyte membranes were prepared from polyether ether ketone polymeric material. An initial sulfonation reaction improved the membrane hydrophilicity and its water absorbability and thus enhanced the ionic conductivity in electrochemical 4 2 1 cells. Protonic conductivity was improved from 10− to 10− S cm− with an increasing sulfonation time from 72 to 175 h. The eﬀects of blending nano SiO2 into the composite membranes were devoted to improve thermal and mechanical properties, as well as methanol permeability. Methanol permeability was reduced to 3.1 10 7 cm2 s 1. Finally, a further improvement in ionic conductivity was carried × − − out by a supercritical carbon dioxide treatment under 20 MPa at 40◦C for 30 min with an optimum SiO2 blend ratio of 10 wt-%. The plasticizing eﬀect by the Lewis acid-base interaction between CO2 and electron donor species on polymer chains decreased the glass transition and melting temperatures. The results show that sulfonated composite membranes blended with SiO2 and using a supercritical carbon dioxide treatment exhibit a lower glass transition temperature, higher ionic conductivity, lower methanol permeability, good thermal stability, and strong mechanical properties. Ionic conductivity was improved to 1.55 10 2 S cm 1. The ion exchange capacity and the degree of × − − sulfonation were also investigated.

Keywords: composite electrolyte membrane; polyether ether ketone; sulfonation; nano SiO2; supercritical carbon dioxide

1. Introduction Due to rising costs in natural energy resources and growing concerns for environmental protection, modern society has been in great demand for more efficient energy storage and consumption technologies [1–4]. However, the production of usable energy from the combustion of fuels is an inefficient process due to intrinsic thermodynamic principles. A modern electric power plant is unable to harness more than 35–40% of the potential energy from oil, coal, or natural gases. A gasoline engine has an efficiency of only about 25–30%. The rest of the energy is inevitably lost to the surroundings as heat waste. An additional cooling system is thus required to solve this problem. Fuel cells are electrochemical cells that consume fuels under nearly thermodynamically reversible conditions [5,6]. The energy conversion efficiencies are thus much higher. Fuels can be fed continuously to produce power. This operation is not only more efficient but also more economically feasible. Therefore, many researchers are making large amounts of effort to find the best materials and processes to elevate the electrochemical performance of fuel cells for different applications [7–10]. In order to obtain higher energy and power density for fuel cells, a solid polymer electrolyte membrane (SPEM) can be used to replace the heavy-weight aqueous electrolytes, such as non-woven polypropylene/polyethylene, Nafion®, and Flemion®. Nafion® has been used as a separator in some

Polymers 2019, 11, 1177; doi:10.3390/polym11071177 www.mdpi.com/journal/polymers Polymers 2019, 11, 1177 2 of 13 fuel cells, but its cost is quite high [11]. For the practical application of an SPEM in direct methanol fuel cells, it is necessary to have low methanol permeability, high ionic conductivity, good mechanical strength, good thermal stability, and lower cost [12,13]. In the last decade, polyether ether ketone (PEEK) has exhibited great potential to be developed as an SPEM due to its balanced combination properties of good mechanical strength, high thermal stability, and superior chemical resistance. The hydrophobic property and the high glass transition temperature (Tg) of PEEK are intrinsic and need to be improved for SPEM preparation. For direct methanol fuel cell and proton exchange membrane fuel cell applications, it is beneficial to be more hydrophilic. As such, more water electrolytes can be sustained in the free volume of the matrix. This will allow for easier ionic transport during electrochemical cell discharge. Some studies have focused on blendings with second polymers, such as polyether sulfone, polyphenyl sulfone, polyphenylene sulfide, and polyvinyl pyrrolidone [14,15]. Further improvements are still desired for their applicability. On the other hand, it has been reported that polyaryl ether ketones under sulfonation—instead of perfluorinated polymers—were quite durable under fuel cell conditions [16,17]. PEEK has many unique characteristics that could have great potential for future use in polymer electrolyte fuel cells [18,19]. This study reveals an improved method to increase the hydrophilicity of PEEK by suitable sulfonation in concentrated sulfuric acid at a long time and a high temperature. The sulfonated PEEK (sPEEK) polymer can be incorporated with nano particles to improve mechanical strength and ionic conductivity [20,21]. A supercritical CO2 (scCO2) fluid treatment can be further adapted to reduce its Tg. The CO2 molecules have a plasticizing effect by the Lewis acid-base interaction between the CO2 and the electron donor species on the polymer chains [22,23]. Both the Tg and melting temperature (Tm) of the polymer can be reduced. scCO2 treatment in polyethyleneoxide-LiCF3SO3 has been studied for use in lithium-salt polymer electrolytes and has shown significant improvement in ionic conductivity [24,25]. In this paper, we prepared high performance sulfonated PEEK composite electrolyte membranes with high ionic conductivity, low methanol permeability, good mechanical strength, and good stability. The high hydrophilicity and water absorbability of sPEEK was introduced by sulfonation that reacted in concentrated sulfuric acid solution. The ion exchange capacity (IEC) and the degree of sulfonation (DS) were also investigated. The sPEEK membranes were modified by blending with nano scale SiO2 particles at different weight ratios from 1 to 20 wt-%. The sPEEK and sPEEK/SiO2 composite membranes were further treated in supercritical carbon dioxide under the condition of 20 MPa at 40 ◦C. In addition, the methanol permeability for the composite electrolyte membranes was also carefully investigated.

2. Materials and Methods Polyether ether ketone (PEEK, Victrex US, Inc., Grade 450G, M.W. 34,000, West Conshohocken, PA, USA) was dried and used for a sulfonation reaction. The sulfonation reaction was carried out by dissolving the PEEK granules in 98% concentrated sulfuric acid (Aldrich, St. Louis, MO, USA). The polymer/H2SO4 ratio was 0.05 g/mL. The reaction was conducted in continuous mechanical stirring at 90◦C from 72 h to 175 h. After the reaction, the maroon color solution was quickly quenched in iced, de-ionized water. The white-color precipitate of sulfonated PEEK then appeared. The sPEEK precipitate was rinsed in fresh de-ionized water for several times until the water became neutral. The sulfonated sample was dried in a vacuum oven at 60 ◦C for more than 1 day. Figure1 shows the chemical structures of PEEK and sPEEK, while the x denotes the extent of the sulfonation reaction. The dried sPEEK polymer was ﬁrstly dissolved in a N-methyl-2-pyrrolidone (NMP, Aldrich) 2 solvent. The appropriate weight of nano SiO2 (Aldrich, 10 nm, 640 m /g) was also added to the NMP solvent in another beaker under ultrasonic vibration for at least 90 min. The completely dissolved solution of sPEEK/NMP was then mixed with SiO2/NMP under continuous mechanical agitation for 36 h. The mixture solution was ultrasonically vibrated for 120 min before its casting on a glass plate. The sample was then dried in a vacuum oven at 90◦C for 2 days to remove the residual solvent. Polymers 2019, 11, 1177 3 of 13

The completely dried sPEEK/SiO2 composite membrane was further treated in supercritical CO2 ﬂuid 1 using the Apeks Supercritical extraction system with a ﬂow rate of 2 mL min− in the reactor at 40 ◦C and 20 MPa for 30 min. All the samples were then slowly cooled to the ambient temperature to obtain thePolymers transparent 2019, 11, x compositeFOR PEER REVIEW membranes. 3 of 13

Figure 1. Chemical structures of polyether ether ketone (PEEK) and sulfonated PEEK(sPEEK) in the Figure 1. Chemical structures of polyether ether ketone (PEEK) and sulfonated PEEK(sPEEK)in the sulfonation reaction. sulfonation reaction. The glass transition temperatures of the membranes were analyzed by differential scanning The dried sPEEK polymer was firstly dissolved in a N-methyl-2-pyrrolidone (NMP, Aldrich) calorimetry (DSC, TA 2010, New Castle, DE, USA) purged by N2 gas, and the temperature was solvent. The appropriate weight of nano SiO2 (Aldrich, 10 nm, 640 m2/g) was also added to the NMP increased to 250◦C at a heating rate of 10◦C/min. The AC impedance method was employed to measure ionicsolvent conductivity. in another Thebeaker sPEEK under polymeric ultrasonic composite vibration electrolyte for at least membranes 90 min. wereThe completely sandwiched dissolved between SS316solution stainless of sPEEK/NMP steel electrodes was then with mixed the surface with SiO area2/NMP of 0.785 under cm2 continuous. The AC impedance mechanical measurements agitation for were36 h. carriedThe mixture out by solution AutoLab was from ultrasonically Eco Chemi vibrated with a computer for 120 min program. before its The casting frequency on a glass range plate. was fromThe sample 100 Hz was to 100 then kHz, dried and in an a excitationvacuum oven signal at 90°C of 10 for mV 2 wasdays used. to remove In addition, the residual a convection solvent. oven The wascompletely set up to dried control sPEEK/SiO the testing2 composite temperature. membrane The following was further equation treated was usedin supercritical to calculate CO the2 ionicfluid −1 conductivityusing the Apeks in this Supercritical study. extraction system with a flow rate of 2 mL min in the reactor at 40 °C and 20 MPa for 30 min. All the samples were thenL slowly cooled to the ambient temperature to σ = (1) obtain the transparent composite membranes. Rb A The glass transition temperatures of the membra× nes were analyzed by differential scanning where L and A are the thickness and area of the composite membranes, respectively. Rb was derived calorimetry (DSC, TA 2010, New Castle, DE, USA) purged by N2 gas, and the temperature was from the Nyquist plot of the AC impedance analysis at a low intersection of the high frequency increased to 250°C at a heating rate of 10°C/min. The AC impedance method was employed to semi-circle on a complex impedance plane on the z axis. measure ionic conductivity. The sPEEK polymeric composite electrolyte membranes were The methanol permeability of the sPEEK polymer composite membranes was measured using a sandwiched between SS316 stainless steel electrodes with the surface area of 0.785 cm2. The AC glass diffusion cell. The cell consisted of two compartments with a circular hole of 3 cm in diameter. impedance measurements were carried out by AutoLab from Eco Chemi with a computer program. One compartment was filled with a 35 mL solution of 3 M methanol, and the other compartment was The frequency range was from 100 Hz to 100 kHz, and an excitation signal of 10 mV was used. In filled with 35 mL of de-ionized water. The solution in each compartment was continuously stirred addition, a convection oven was set up to control the testing temperature. The following equation to keep uniform concentration, and the methanol concentration of each cell was measured by gas was used to calculate the ionic conductivity in this study. chromatography (GC, Varian, Palo Alto, CA, USA). The measurement was performed in a water bath 2 at 30 ◦C, and the area of the membrane was controlledσ = L at 5.3 cm . For the analysis of water absorption, the fully dried× composite membrane samples with an area(1) of Rb A 3 3 cm2 were first weighed (W ) and then immersed in de-ionized water for 2 days. The soaked × 0 membraneWhere surfaceL and A was are wiped the thickness with filter and paper, area andof the the composite wet membrane membranes, weight respectively. was obtained R (Wb was1). Thederived water from absorption the Nyquist (A%) wasplot calculatedof the AC from impedanc the followinge analysis equation: at a low intersection of the high frequency semi-circle on a complex impedance plane on the z axis. W W The methanol permeability of the sPEEKA% polyme= 1 −r composite0 membranes was measured using(2) a glass diffusion cell. The cell consisted of two compartmentsW0 with a circular hole of 3 cm in diameter. One Thecompartment ion-exchange was capacityfilled with (IEC) a 35 of mL the solution s-PEEK of composite 3 M methanol, electrolyte and the membranes other compartment was analyzed was byfilled a titration with 35 method.mL of de-ionized It was determined water. The by solution equilibrating in each the compartment membranes was in a continuously 1.0 M HCl solution stirred for to 24keep h so uniform that the concentration, charge group of and the membranethe methanol was concentration in H+ form. The of membraneeach cell was was thenmeasured washed by with gas de-ionizedchromatography water (GC, free ofVarian, HCl before Palo equilibrationAlto, CA, USA). in 0.1 The M measurement NaCl for 24 h. was Finally, performed the ion-exchange in a water bath at 30 °C, and the area of the membrane was controlled at 5.3 cm2. For the analysis of water absorption, the fully dried composite membrane samples with an area of 3 × 3 cm2 were first weighed (W0) and then immersed in de-ionized water for 2 days. The soaked membrane surface was wiped with filter paper, and the wet membrane weight was obtained (W1). The water absorption (A%) was calculated from the following equation:

𝑊 −𝑊 A % = (2) 𝑊

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The ion-exchange capacity (IEC) of the s-PEEK composite electrolyte membranes was analyzed by a titration method. It was determined by equilibrating the membranes in a 1.0 M HCl solution Polymersfor 24 2019h so, 11that, 1177 the charge group of the membrane was in H+ form. The membrane was then washed4 of 13 with de-ionized water free of HCl before equilibration in 0.1 M NaCl for 24 h. Finally, the capacityion-exchange was determined capacity was by determined the back titration by the method back titration with a 0.1method M NaOH with solution.a 0.1 M NaOH The degree solution. of sulfonationThe degree (DS) of sulfonation was also determined (DS) was also from determined the IEC data: from the IEC data:

M w, p IEC x = Mw,pIEC x = − (3) 11 MM w, fIECIEC − w, f where Mw,p is the molecular weight of the nonfunctional polymer repeat unit and Mw,f is the where Mw,p is the molecular weight of the nonfunctional polymer repeat unit and Mw,f is the molecular molecular weight of the functional group with the counter ion (–SO3Na). weight of the functional group with the counter ion (–SO3Na).

3.3. Results Results and and Discussion Discussion

3.1.3.1. IEC IEC and and DS DS Analysis Analysis FigureFigure2 shows2 shows the IECthe andIEC DSand analysis DS analysis results ofresults the sPEEK of the membranes sPEEK membranes with di fferent with sulfonation different reactionsulfonation times reaction in the times range in of the 72–175 range h. of It72–175 is clearly h. It indicatedis clearly indicated that both that the both IEC andthe IEC DS valuesand DS continuouslyvalues continuously increased increased with the with increasing the increasing sulfonation sulfonation time. However, time. However, the increase the rate increase appeared rate toappeared slow down to slow when down over 150when h, indicatingover 150 h, the indica approachingting the saturationapproaching of thesaturation reaction. of Nevertheless, the reaction. 1 −1 theNevertheless, IEC and DS the reached IEC and up DS to 1.9reached meq g up− andto 1.9 68%, meq respectively. g and 68%, The respectively. effects of nanoThe effects SiO2 content of nano forSiO sPEEK2 content composite for sPEEK membranes composite with membranes the DS fixed with at the 68% DS are fixed provided at 68% in are Figure provided3. While in theFigure SiO 23. 1 contentWhile the increased SiO2 content from 0 increased to 20 wt-%, from the 0 IEC to 20 value wt-%, decreased the IEC fromvalue 1.90 decreased to 1.74 meqfrom g1.90− . Thisto 1.74 can meq be −1 attributedg . This can tothe be interactionattributed to of the hydrogen interactio bondingn of hydrogen between bonding the nano between SiO2 particles the nano and SiO the2 sulfonicparticles groupsand the of sulfonic sPEEK. groups The ion of exchange sPEEK. The capability ion exch wasange thus capability reduced was accordingly. thus reduced accordingly.

Figure 2. Ion exchange capacity (IEC) and degree of sulfonation (DS) analysis results of the sPEEK Figure 2. Ion exchange capacity (IEC) and degree of sulfonation (DS) analysis results of the1 sPEEK membranes with diﬀerent sulfonation reaction times at 90◦C. At 175 h, the IEC was 1.9 meq g− and DS membranes with different sulfonation reaction times at 90°C. At 175 h, the IEC was 1.9 meq g−1 and was 68%. DS was 68%.

Polymers 2019, 11, x FOR PEER REVIEW 5 of 13 PolymersPolymers2019 2019, ,11 11,, 1177 x FOR PEER REVIEW 5 ofof 1313

Figure 3. Effects of nano SiO2 content on composite membrane IEC. The DS was 68% for the sPEEK Figure 3. Effects of nano SiO2 content on composite membrane IEC. The DS was 68% for the Figurepolymer. 3. Effects of nano SiO2 content on composite membrane IEC. The DS was 68% for the sPEEK sPEEK polymer. polymer. 3.2.3.2. IR IR Analysis Analysis 3.2. IR Analysis TheThe Fourier-transform Fourier-transform infrared infrared (FTIR, (FTIR, Jasco) Jasco) spectra spectra for for the the sulfonated sulfonated PEEK PEEK samples samples with with didifferentfferentThe DSFourier-transform DS values values from from 0% 0% toinfrared to 68% 68% are (FTIR,are shown shown Jasco) in in Figure Fispectragure4. There4. for There the are aresulfonated three three characteristic characteristic PEEK samples absorption absorption with differentbands of DS –SO values3H groups from 0%that to appear 68% are in shownthe wave in Finumbergure 4. regionThere areof 1020three1 cm characteristic−1, 12501 cm −absorption1, and 16501 bands of –SO3H groups that appear in the wave number region of 1020 cm− , 1250 cm− , and 1650 cm− . −1 Thesebandscm . absorption ofThese –SO 3absorptionH groups bands can thatbands be appear assigned can bein theassigned to thewave S= Otonumber stretch,the S=O region the stretch, asymmetric of 1020 the asymmetriccm O−1=, S1250=O stretch,cm O=S=O−1, and and stretch, 1650 the cmand−1. theThese symmetric absorption stretching bands canvibration be assigned of O=S=O to th duee S=O to thestretch, –SO 3theH groups, asymmetric respectively O=S=O [26].stretch, The symmetric stretching vibration of O=S=O due to the –SO3H groups, respectively [26]. The substitution chemicalandsubstitution the symmetric reaction chemical took stretching reaction place in vibration took one ofplace the of inO=S=O four one positions of due the to four the of positions the –SO aromatic3H groups, of the rings aromatic respectively between rings the[26]. between ether The groups.substitutionthe ether This groups. chemical was causedThis reaction was by caused the took relatively placeby the in relatively lower one of electron the lower four densityelectron positions of density of the the other ofaromatic the two other aromatic rings two between aromatic rings intherings the ether polymer’s in groups.the polymer’s monomer This was monomer structure.caused by st Theructure.the relatively electron-attracting The electron-attractinglower electron nature density from nature the of the neighboring from other the two neighboring carboxylaromatic groupringscarboxyl in played the group polymer’s a role played [16, 18 monomera ].role The [16,18]. IR st analysisructure. The IR results, Theanalysis electron-attracting in conjunctionresults, in conjunction with nature the previousfrom with thethe IEC neighboringprevious analysis, IEC havecarboxylanalysis, confirmed group have thatconfirmedplayed the sulfonica role that [16,18]. the functional sulfonic The groupsIR functional analysis were results,groups indeed quantitativelyinwere conjunction indeed quantitativelyintroduced with the previous to introduced the PEEK IEC aromaticanalysis,to the PEEK polymerhave aromatic confirmed molecules. polymer that the molecules. sulfonic functional groups were indeed quantitatively introduced to the PEEK aromatic polymer molecules.

Figure 4. FTIR spectra for the sulfonated PEEK samples with diﬀerent degrees of sulfonation: (a) 0%, Figure 4. FTIR spectra for the sulfonated PEEK samples with different degrees of sulfonation: (a) 0%, (b) 41%, (c) 49%, (d) 57%, (e) 66%, and (f) 68%. Figure(b) 41%, 4. FTIR(c) 49%, spectra (d) 57%, for the (e) sulfonated66%, and ( fPEEK) 68%. sample s with different degrees of sulfonation: (a) 0%, (b) 41%, (c) 49%, (d) 57%, (e) 66%, and (f) 68%. 3.3. Water Absorption and Ionic Conductivity 3.3. Water Absorption and Ionic Conductivity

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3.3. Water Absorption and Ionic Conductivity Table1 summarizes the e ffects of sulfonation time on water absorption, ionic conductivity, and the degree of sulfonation for the sPEEK membrane samples. The water absorption increased with the increasing sulfonation time. This was due to the improvement in hydrophilicity by chemically incorporating the sulfonic groups into the polymer main chains. Therefore, the water content in sPEEK membranes became higher. This induced water channel in the free volume of the polymer matrix among the sulfonic groups, which then provided a smooth pathway for ionic transport. The ionic conductivity was thus significantly improved to 4.68 10 3 S cm 1 for the 68% degree of sulfonation × − − (DS68) sPEEK membrane sample, and the water absorption was 45.3%. On the other hand, both the effects of the nano SiO2 blend ratio and the scCO2 treatment results on the polymer composite electrolyte membranes are shown in Table2. The sPEEK blends with low SiO 2 contents were unable to elevate the water absorbability and the ionic conductivity. This was shown by the high hydrogen bonding between the SiO2 and –SO3H groups, and the SiO2 particles occupied a finite free volume which reduced the space for water content. However, while the nano SiO2 content increased to more than 10 or 20 wt-%, more SiO2 nano-particles could loosen the tight polymer chains and sustain more water content. Thus, ionic conductivity and water absorbability could be elevated. When the sPEEK/SiO2 composite membranes were conducted in a supercritical CO2 fluid treatment, the interaction between SiO2 and the sulfonic groups was destroyed, and the SiO2 particles apparently loosened the tight polymer chains and largely increased the free volume to contain more water. Therefore, the water absorption and ionic conductivity for the composite membranes (sample SCF-DS68-S10) were both improved significantly to 63.1% and 1.55 10 2 S cm 1, respectively. × − − Table 1. Effects of sulfonation time on water absorption, ionic conductivity, and degree of sulfonation for the sPEEK membrane samples.

Membrane Properties Sample Sulfonation Ionic Conductivity DS (%) Absorption (%) Time (h) ( 10 3 S cm 1) × − − DS41 72 41.0 18.2 0.05 DS49 96 48.7 24.9 0.26 DS57 120 57.2 32.0 1.29 DS66 144 65.8 36.1 2.62 DS68 175 68.0 45.3 4.68

Table 2. Eﬀects of nano SiO2 blend ratio and supercritical CO2 ﬂuid treatment on the 68% degree of sulfonation (DS68) sPEEK composite electrolyte membranes.

Properties Sample Sulfonation SiO2 Absorption Ionic Conductivity Supercritical CO2 Time (h) Content (%) (%) ( 10 3 S cm 1) Fluid Treatment × − − DS68-S0 175 0 45.3 4.68 No DS68-S1 175 1 44.7 3.37 No DS68-S5 175 5 42.0 3.18 No DS68-S10 175 10 52.6 6.46 No DS68-S20 175 20 59.2 7.34 No SCF-DS68-S0 175 0 56.5 9.14 Yes SCF-DS68-S10 175 10 63.1 15.47 Yes Polymers 2019, 11, 1177 7 of 13 Polymers 2019, 11, x FOR PEER REVIEW 7 of 13

3.4.membranes, DSC and TGA a two-step Thermal thermal Analysis degradation process was observed. There was a weight loss of about Polymers 2019, 11, x FOR PEER REVIEW 7 of 13 10–20%The thermalat a temperature gravimetric range analysis of 300–380°C (TGA) thermographswhen the degree of of the sulfonation sPEEK membrane was increased samples to 68%. are shownHowever, in Figure this 5was. The close original to the PEEK melting polymer temperature exhibited one-stepof PEEK degradation (Tm = 350 °C). without In addition, the sulfonation. Figure 6 membranes, a two-step thermal degradation process was observed. There was a weight loss of about Theshows derivative the TGA scan thermographs had a peak lossof the at aroundsPEEK/SiO 6002 C.composite For the highlymembranes sulfonated while sPEEK the DS membranes, was fixed at 10–20% at a temperature range of 300–380°C when◦ the degree of sulfonation was increased to 68%. a68%. two-step For the thermal higher degradation nano SiO2 content process in was sPEEK observed. from 1% There to 20%, was the a weight weight loss loss of at about this temperature 10–20% at However, this was close to the melting temperature of PEEK (Tm = 350 °C). In addition, Figure 6 range increased to about 30%. The later decomposition temperature was observed from the peak loss a temperature range of 300–380◦C when the degree of sulfonation was increased to 68%. However, shows the TGA thermographs of the sPEEK/SiO2 composite membranes while the DS was fixed at of the derivative scan at 500–600°C. Therefore, the sulfonation and nano SiO2 in sPEEK was able to this was close to the melting temperature of PEEK (Tm = 350 ◦C). In addition, Figure6 shows the TGA 68%. For the higher nano SiO2 content in sPEEK from 1% to 20%, the weight loss at this temperature thermographsslightly reduce of thethe sPEEK thermal/SiO degradationcomposite membranes resistance whilefor the the sPEEK DS was membrane ﬁxed at 68%. after For theprolonged higher range increased to about 30%. The2 later decomposition temperature was observed from the peak loss treatment. nano SiO2 content in sPEEK from 1% to 20%, the weight loss at this temperature range increased to of the derivative scan at 500–600°C. Therefore, the sulfonation and nano SiO2 in sPEEK was able to aboutslightly 30%. reduce The laterthe decompositionthermal degradation temperature resistance was observedfor the sPEEK from the membrane peak loss ofafter the prolonged derivative scantreatment. at 500–600 ◦C. Therefore, the sulfonation and nano SiO2 in sPEEK was able to slightly reduce the thermal degradation resistance for the sPEEK membrane after prolonged treatment.

Figure 5. Thermal gravimetric analysis (TGA) thermographs of the original PEEK and sPEEK membrane samples.

Figure 5. Thermal gravimetric analysis (TGA) thermographs of the original PEEK and sPEEK Figure 5. Thermal gravimetric analysis (TGA) thermographs of the original PEEK and sPEEK membrane samples. membrane samples.

Figure 6. TGA thermographs of the sPEEK/SiO2 composite membranes with a 68% degree of sulfonation

(DS68).Figure The6. TGA nano SiOthermographs2 content varied of the at 1%sPEEK/SiO (S1), 5%2(S5), composite 10% (S10), membranes and 20% (S20).with a 68% degree of

sulfonation (DS68). The nano SiO2 content varied at 1% (S1), 5% (S5), 10% (S10), and 20% (S20). The typical DSC curve for the PEEK membrane is displayed in Figure7. PEEK is a polymer with semi-crystallineFigureThe typical 6. TGA structure, DSC thermographs curve and thefor Tofthem the canPEEK sPEEK/SiO be seenmembrane at2 340composite◦ C.is Thedisplayed membranes Tg onset instarts Figure with at a about7. 68%PEEK 145degree is◦C. a In polymerof order sulfonation (DS68). The nano SiO2 content varied at 1% (S1), 5% (S5), 10% (S10), and 20% (S20). with semi-crystalline structure, and the Tm can be seen at 340°C. The Tg onset starts at about 145°C. In order to verify the Tg for sPEEK composite samples with the different nano SiO2 contents, we The typical DSC curve for the PEEK membrane is displayed in Figure 7. PEEK is a polymer with semi-crystalline structure, and the Tm can be seen at 340°C. The Tg onset starts at about 145°C. In order to verify the Tg for sPEEK composite samples with the different nano SiO2 contents, we

PolymersPolymers2019 2019,,11 11,, 1177 x FOR PEER REVIEW 88 of 1313 Polymers 2019, 11, x FOR PEER REVIEW 8 of 13 narrowed down the temperature scan range to 100–225°C. From Figure 8, it can be clearly observed tonarrowedthat verify the T the gdown for T gsPEEKfor the sPEEK temperature with compositea 68% scan DS samples(samplerange to DS68-S0) with100–225°C the diwa.ff Fromerents about Figure nano 190°C. SiO 8, it 2It cancontents, was be much clearly we highernarrowed observed than downthatthat theof the Ttheg temperature for original sPEEK withPEEK scan a range polymer.68% toDS 100–225 (sample With ◦C.the DS68-S0) From incorporation Figure was 8about, it canof 190°C.the be clearlynano It was SiO observed much2 particles higher that thein than Ttheg 2 forthatcomposite sPEEK of the with samplesoriginal a 68% (DS68-S1~DS68-S20),PEEK DS (sample polymer. DS68-S0) With the wasthe Tg wasincorporation about slightly 190◦C. further Itof was the increased much nanohigher SiO to 195°Cparticles than thatdue in ofto thethe g originalcompositeinteraction PEEK samples of hydrogen polymer. (DS68-S1~DS68-S20), Withbonding the incorporationbetween the SiO T2 ofwashydration the slightly nano cluster SiO further2 particles and increased the insulfonic the to composite 195°C groups. due samplesFigure to the 9 2 (DS68-S1~DS68-S20),interactionshows the effectsof hydrogen of the the superbonding Tg was critical slightly between CO further2 fluidSiO increased hydration(SCF) treatment to cluster 195◦C using and due tothesPEEK the sulfonic interaction with groups.a DS of of hydrogen Figure68% and 9 2 bondingshowsthe SiO the2 betweencontent effects ofof SiO the102 hydrationwt-%.super criticalIt is cluster clearly CO and fluidevidenced the (SCF) sulfonic treatmentthat groups.the T usingg was Figure sPEEKsignificantly9 shows with thea decreasedDS e ffofects 68% of fromand the 2 g superthe195°C SiO critical(sample content CO DS68-S10) of2 fluid 10 wt-%. (SCF) to183°C It treatment is clearly(sample usingevidenced SCF-DS68-S10). sPEEK that with the The a T DS wasinteraction of 68%significantly and between the SiOdecreased SiO2 content2 and from the of 2 10195°Csulfonic wt-%. (sample groups It is clearly DS68-S10) was evidencedefficiently to183°C thatdestroyed, (sample the Tg was and SCF-DS68-S10). significantlythe SiO2 particles decreased The couldinteraction from just 195loosen between◦C (sample the tightSiO DS68-S10) polymerand the to183sulfonicchains,◦C thus (samplegroups lowering was SCF-DS68-S10). efficiently the Tg. destroyed, The interaction and the between SiO2 particles SiO2 and could the sulfonic just loosen groups the wastight e ffipolymerciently destroyed,chains, thus and lowering the SiO theparticles Tg. could just loosen the tight polymer chains, thus lowering the T . 2 g

Exo up

) -1 Exo up

) -1

Heat flow g (W Heat flow g (W Heat Flow (W/g) Heat Flow (W/g)

100100 150 200 250 300 350350 400400 o 100100 150 200Temperature( 250 o C)300 350350 400400 Temperature (oC) Temperature( o C) Temperature ( C)

Figure 7. Typical differential scanning calorimetry (DSC) curve for the original PEEK. Tm was 340°C, Figure 7. Typical diﬀerential scanning calorimetry (DSC) curve for the original PEEK. T was 340 C, Figureand the 7. T Typicalg onset starteddifferential at about scanning 145°C. calorimetry (DSC) curve for the original PEEK. Tmm was 340°C,◦ and the Tg onset started at about 145°C. 145◦C.

) (a)

-1

) (a) -1 (b) (b)

DS68-S0(a) DS68-S0 (c) DS68-S0(a)DS68-S1(b) DS68-S0 DS68-S1

Heat flow g (W Heat Flow (W/g) DS68-S5(c)DS68-S10(d) DS68-S5 DS68-S10 (d)

Heat Flow (W/g) DS68-S10(d)DS68-S20(e) DS68-S10DS68-S20 (d) DS68-S20(e) DS68-S20 (e) (e) 100100 125 150150 175175 200200 225 100100 125 150150 175175o 200200 225 Exo up Temperature(Temperature (oC)C) o o Exo up Temperature(Temperature ( C)C)

FigureFigure 8.8.DSC DSC curves curves for thefor sPEEKthe sPEEK composite composite membranes membranes with diﬀ erentwith SiOdifferent2 contents. SiO2 Thecontents. sulfonated The

sPEEKFiguresulfonated exhibited8. DSC sPEEK curves much exhibited higher for themuch Tg sPEEKthan higher the composite original Tg than PEEK themembranes original polymer. PEEK with The incorporationpolymer.different TheSiO ofincorporation2 contents. nano particles The of

increasedsulfonatednano particles T gsPEEKeven increased more.exhibited Tg even much more. higher Tg than the original PEEK polymer. The incorporation of nano particles increased Tg even more.

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PolymersPolymers 20192019,, 1111,, 1177x FOR PEER REVIEW 99 of 13

) (a) -1

) (a) -1 (b)

(bc)

Heat flow (W g (W flow Heat (a) DS68-S0 DS68-S0

(b)DS68-S10 DS68-S10

Heat Flow (W/g) (c)SCF-DS68-S0 SCF-DS68-S0 (c) (d) scCO2-DS68-S0 Heat flow (W g (W flow Heat (a) DS68-S0 DS68-S0 (d)SCF-DS68-S10 SCF-DS68-S10 scCO (b)DS68-S10 DS68-S102-DS68-S10

Heat Flow (W/g) (c)SCF-DS68-S0 SCF-DS68-S0 (d) scCO2-DS68-S0 100 120(d) SCF-DS68-S10 140140 160160 180 200200 220 scCO SCF-DS68-S102-DS68-S10 o Exo up Temperature(o C) Temperature ( C) 100 120 140140 160160 180 200200 220 o Figure 9. Effects of supercritical CO2 fluid (SCF) treatment oon sPEEK/SiO2 composite membranes. Tg Exo up Temperature(Temperature ( C)C) has been decreased on the DSC curves. Figure 9. EffectsEﬀects of of supercritical supercritical CO CO2 2fluidﬂuid (SCF) (SCF) treatment treatment on on sPEEK/SiO sPEEK/SiO2 composite2 composite membranes. membranes. Tg 3.5. MechanicalThasg has been been decreased Properties decreased on onthe the DSC DSC curves. curves. 3.5. MechanicalThe mechanical Properties strength and elongation properties were determined by a tensile stress-strain 3.5. Mechanical Properties testing system (Instron) at room temperature, and the results are shown in Figures 10 and 11. In The mechanical strength and elongation properties were determined by a tensile stress-strain general,The themechanical tensile strength strength of and the elongation polymer copropertimpositees membranewere determined increased by awith tensile increasing stress-strain SiO2 testing system (Instron) at room temperature, and the results are shown in Figures 10 and 11. In general, testingcontent, system while the(Instron) ultimate at elongationroom temperature, at break decrand easedthe results with it. are The shown highest in tensileFigures strength 10 and of11. 10.3 In the tensile strength of the polymer composite membrane increased with increasing SiO2 content, general,MPa was the found tensile with strength 10 wt-% of SiO the2 contentpolymer in co themposite polymer membrane composite increased membrane. with The increasing pure sPEEK SiO2 while the ultimate elongation at break decreased with it. The highest tensile strength of 10.3 MPa was content,membrane, while without the ultimate SiO2 content, elongation exhibited at break a decrlowereased tensile with strength it. The highest of 6.5 MPatensile but strength with a of good 10.3 found with 10 wt-% SiO2 content in the polymer composite membrane. The pure sPEEK membrane, MPaelongation-at-break was found with value 10 wt-% of 76%. SiO However,2 content in the the te polymernsile strength composite was reduced membrane. to 8.3 The MPa pure when sPEEK the without SiO2 content, exhibited a lower tensile strength of 6.5 MPa but with a good elongation-at-break membrane,SiO2 content without was as SiOhigh2 content,as 20 wt-%. exhibited Its elongati a loweron-at-break tensile strength was also of further 6.5 MPa down but withto 28%. a goodThis value of 76%. However, the tensile strength was reduced to 8.3 MPa when the SiO2 content was as elongation-at-breakcould be attributed valueto the of interaction 76%. However, of the the hydrogen tensile strength bonding was between reduced SiO2 to 8.3 and MPa the when sulfonic the high as 20 wt-%. Its elongation-at-break was also further down to 28%. This could be attributed SiOgroups,2 content and thewas tight as highpolymer as 20 chains wt-%. were Its elongatipartiallyon-at-break destroyed. wasThe interfacealso further played down some to role28%. in This the to the interaction of the hydrogen bonding between SiO2 and the sulfonic groups, and the tight couldmechanical be attributed properties. to Inthe addition, interaction it was of notedthe hydrogen that, after bonding the sulfonation, between the SiO2 mechanical and the strengthsulfonic polymer chains were partially destroyed. The interface played some role in the mechanical properties. groups,decreased and due the totight the polymer hydrophilicity chains wereof the partially sulfonic destroyed. groups. The interfaceabsorbed played water someplasticized role in the In addition, it was noted that, after the sulfonation, the mechanical strength decreased due to the mechanicalpolymeric membranes properties. [27].In addition, it was noted that, after the sulfonation, the mechanical strength hydrophilicity of the sulfonic groups. The absorbed water plasticized the polymeric membranes [27]. decreased due to the hydrophilicity of the sulfonic groups. The absorbed water plasticized the polymeric membranes [27].

Figure 10. Eﬀects of SiO2 content on the mechanical tensile strength of composite membranes at Figure 10. Effects of SiO2 content on the mechanical tensile strength of composite membranes at room temperature. room temperature. Figure 10. Effects of SiO2 content on the mechanical tensile strength of composite membranes at room temperature.

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Figure 11. Elongation at-break results of the composite membrane samples at room temperature. Figure 11. Elongation at-break results of the composite membrane samples at room temperature. 3.6. Methanol Permeability 3.6. Methanol Permeability Figure 12 shows the methanol permeability analysis results through the sPEEK composite electrolyteFigure membranes12 shows the as amethanol function permeability of SiO2 content analysis at 25◦ C.results The DSthrough was atthe 68%. sPEEK The composite methanol 6 2 1 permeabilityelectrolyte membranes clearly decreased as a function with the of increasingSiO2 content SiO at2 content.25°C. The It wasDS was 0.47 at 1068%.− cm Thes −methanolfor the 6 2 1 × −6 2 −1 sPEEKpermeability membrane clearly (DS68-S0, decreased without with the SiO increasing2) and 0.31 SiO210 content.− cm sIt− wasfor the0.47composite × 10 cm membranes for the × −6 2 −1 (DS68-S20,sPEEK membrane SiO2 content (DS68-S0, 20 wt-%). without The SiO methanol2) and 0.31 permeability × 10 cm data s werefor the all muchcomposite lower membrane than that ® 6 2 1 of(DS68-S20, the Naﬁon SiO2 content117 membrane, 20 wt-%). which The methanol were calculated permeability at about data 2 were10 allcm muchs lower[28]. than The sPEEKthat of × − − exhibitedthe Nafion a® higher 117 membrane, mechanical which strength, were and calculated the rigid benzene at about rings 2 × in 10 the−6 cm polymer2 s−1 [28]. chains The provided sPEEK aexhibited complex a routehigher for mechanical methanol strength, permeation. and the With rigid the benzene SiO2 incorporation rings in the inpolymer the sPEEK chains membrane, provided thea complex more complicated route for methanol route reduced permeation. methanol With permeationthe SiO2 incorporation and was indeed in the verysPEEK beneﬁcial membrane, for the directmore complicated methanol fuel route cell reduced and proton meth exchangeanol permeation membrane and fuel was cell indeed applications. very beneficial The lower for methanolthe direct permeabilitymethanolPolymers 2019 fuel, 11 of , cellx the FOR sPEEKand PEER proton REVIEW composite exchange electrolyte membrane membranes fuel cell could applications. prolong the The cycle lower life of methanol fuel11 cells. of 13 permeability of the sPEEK composite electrolyte membranes could prolong the cycle life of fuel cells.

3.7. EDS Analysis

The dispersion of nano SiO2 particles in the sPEEK membranes was observed by using energy dispersive X-ray spectroscopy (EDS). The peak intensity of the Si atom increased with increasing nano SiO2 content from 1 wt-% to 20 wt-%. From the EDS mapping (Figure 13), it can be suggested that the nano SiO2 particles were well dispersed in the sPEEK membranes. Though a slight aggregation phenomenon of nano particles occurred as the SiO2 content increased to 20 wt-%, the uniform dispersion of SiO2 was still observed when the content was lower than 10 wt-%. The ultrasonic vibration for the mixing of SiO2 in sPEEK during the preparation step was necessary to warrant the well dispersion results. The uniformity of electrolyte membrane properties could thus be maintained for fuel cell applications.

Figure 12. Methanol permeability data of the sPEEK/SiO2 composite electrolyte membranes with Figure 12. Methanol permeability data of the sPEEK/SiO2 composite electrolyte membranes with diﬀerent SiO2 contents at 25 ◦C. different SiO2 contents at 25 °C.

Figure 13. Energy dispersive X-ray spectroscopy(EDS) mapping micrographs based on the Si atom

for the composite membranes with different nano SiO2 contents: (a) 0%, (b) 1%, (c) 10%, (d) 20%.

4. Conclusions We presented a simple method to prepare a novel proton solid polymer electrolyte with a high ionic conducting membrane and low methanol permeability. This was conducted by using a polyether ether ketone material via sulfonation and then blended with nano SiO2, along with a further supercritical CO2 fluid treatment to improve its characteristics. An infrared study verified the proper incorporation of sulfonic acid groups. The degree of sulfonation was further evidenced by the ion-exchange capacity determined by titration. The characteristic properties of the composite electrolyte membranes were carefully studied using DSC, TGA, EDS, an Instron tester, and AC impedance spectroscopy. The sulfonation process improved water absorption and ionic

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3.7. EDS Analysis

FigureFigure 13. Energy dispersivedispersive X-rayX-ray spectroscopy(EDS) spectroscopy(EDS) mapping mapping micrographs micrographs based based on on the the Si atomSi atom for the composite membranes with different nano SiO contents: (a) 0%, (b) 1%, (c) 10%, (d) 20%. for the composite membranes with different nano SiO2 2 contents: (a) 0%, (b) 1%, (c) 10%, (d) 20%. 4. Conclusions 4. Conclusions We presented a simple method to prepare a novel proton solid polymer electrolyte with a high ionic We presented a simple method to prepare a novel proton solid polymer electrolyte with a high conducting membrane and low methanol permeability. This was conducted by using a polyether ether ionic conducting membrane and low methanol permeability. This was conducted by using a ketone material via sulfonation and then blended with nano SiO2, along with a further supercritical polyether ether ketone material via sulfonation and then blended with nano SiO2, along with a CO2 fluid treatment to improve its characteristics. An infrared study verified the proper incorporation further supercritical CO2 fluid treatment to improve its characteristics. An infrared study verified of sulfonic acid groups. The degree of sulfonation was further evidenced by the ion-exchange the proper incorporation of sulfonic acid groups. The degree of sulfonation was further evidenced capacity determined by titration. The characteristic properties of the composite electrolyte membranes by the ion-exchange capacity determined by titration. The characteristic properties of the composite were carefully studied using DSC, TGA, EDS, an Instron tester, and AC impedance spectroscopy. electrolyte membranes were carefully studied using DSC, TGA, EDS, an Instron tester, and AC The sulfonation process improved water absorption and ionic conductivity. The SiO2 incorporation impedance spectroscopy. The sulfonation process improved water1 absorption and ionic also influenced ionic conductivity, and it was improved to 0.0073 S cm− . This improvement was attributed to both loosening in rigid polymer chains and the more amorphous characteristic of the membranes. With a higher SiO2 content, a more complicated route was able to reduce methanol permeation and become very beneficial for direct methanol fuel cell and proton exchange membrane fuel cell applications. The composite sample with 68% DS and 10 wt-% SiO2 showed a low methanol permeability of 0.38 10 6 cm2 s 1 but a high mechanical strength of 10.3 MPa. The super critical × − − Polymers 2019, 11, 1177 12 of 13

CO2 fluid treatment was a useful process to improve the electrochemical characteristics for the composite electrolyte membranes. It was able to partially destroy the interaction between SiO2 and the sulfonic groups, which improved the ionic conductivity and water absorbability significantly to 1.55 10 2 S cm 1 and 63.1%, respectively. Consequently, it became possible to achieve a good balance × − − between ionic conductivity and other important application properties. Further developments are still desired to meet requirements for different electrochemical devices.

Author Contributions: Investigation, G.W., I.-C.H. and S.-J.L.; resources, J.-Y.S. and D.W.C.; writing—original draft preparation, I.-C.H. and S.-J.L.; writing—review and editing, G.W.; funding acquisition, J.-Y.S.; supervision, D.W.C. Funding: This research was funded in part by the Ministry of Science and Technology under research grants MOST105-2221-E182-059-MY3 and MOST108-2221-E182-020-MY3, and CGMH financial support of CMRPD3G0062 and CMRP246. Conflicts of Interest: The authors declare no conflict of interest.

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