Research Article www.acsami.org
High-Performance Semicrystalline Poly(ether ketone)-Based Proton Exchange Membrane Sinan Feng,†,‡ Jinhui Pang,‡ Xingwen Yu,† Guibin Wang,*,‡ and Arumugam Manthiram*,† †
Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ‡ Key Laboratory of Super Engineering Plastic of Ministry of Education, Jilin University, Qianjin Street 2699, Changchun 130012, P. R. China S Supporting Information *
ABSTRACT: A novel semicrystalline poly(ether ketone) (PEK)-based proton exchange membrane (semi-SPEK-x) has been developed. Through a one-step sulfonation and hydrolysis, a poly(ether ketimine) precursor transforms into PEK and ion-conducting groups are introduced. With an ionexchange capacity ranging from 1.49 to 2.00 mequiv g−1, the semi-SPEK-x polymers exhibit a semicrystalline feature in both dry and hydrated states. Owing to the semicrystalline domains inside the polymer, the obtained membrane exhibits low water uptake and low volume swelling ratio. More importantly, the semicrystalline structure lowers methanol permeability and, consequently, improves the overall performances of direct methanol fuel cells. KEYWORDS: semicrystalline polymers, poly(ether ketone), proton exchange membrane, fuel cell, low methanol permeability distributed hydrophilic SO3− groups normally prevent the backbone from forming a continuous hydrophobic domain.17−19 Thus, the ambiguous phase separation leads to ineffective proton conduction and poor water management inside the membrane. Many efforts have been attempted to construct ion channels by adjusting the distribution of sulfonic acid groups in a polymer.20−23 One important example is a comb-shaped poly(arylene ether sulfone) with a sulfonated graft chain synthesized by Guiver et al. The connecting nanochannels facilitate the transport of water molecules and maintain a desired hydration level under partially humidified environments.24 Another strategy is the development of a densely sulfonated polymer to enhance the hydrophobic− hydrophilic phase separation.25−31 This type of membrane exhibits good dimensional stability and high ionic conductivity because of the extended hydrophobic backbone and concentrated ion-conducting domains. However, the densely sulfonated polymers with a high ion-exchange capacity (IEC) tend to form large ionic clusters and may decrease the mechanical properties of the hydrated membrane.20,23 In addition, the extended hydrophilic domains facilitate the transport of methanol molecules in DMFC applications. To overcome the above issues and toward improving the stability of the hydrated membranes, it is crucial to enhance the
1. INTRODUCTION Fuel cells are generally perceived as an important part of the future solution to the energy crisis and the global sustainable development, providing clean electricity with virtually little or reduced emission.1−3 Among the numerous types of fuel cell systems/technologies under development, the proton exchange membrane fuel cells (PEMFCs) are in the forefront stage.4−6 As a key component in the PEMFCs, the polymer electrolyte membrane (PEM) transfers protons from the anode to the cathode and serves as a separator to prevent fuel crossover between the two electrodes.7−10 Although the Nafion-series membranes have been relatively well-developed and have become commercially available for about 50 years, limitations still exist with them. Major deficiencies of the Nafion membranes, such as (1) high cost, (2) high methanol permeability (in case of being applied in direct methanol fuel cells (DMFCs)), and (3) poor function at elevated temperatures (above 80 °C) or low humidity (below 80% relative humidity (RH)), have stimulated a vigorous research for alternative polymer membranes.1 Sulfonated poly(ether ether ketone) (SPEEK) has been considered as one of the most promising polymer materials to replace Nafion.11−15 However, in spite of its excellent stability and accessibility, the conventional SPEEK materials always exhibit excessive water swelling and inadequate proton conductivity.13 Moreover, the amorphous SPEEK materials display decreased physical stability due to the lack of crystalline domains.16 From a molecular structure point of view, the evenly © 2017 American Chemical Society
Received: March 15, 2017 Accepted: July 6, 2017 Published: July 6, 2017 24527
DOI: 10.1021/acsami.7b03720 ACS Appl. Mater. Interfaces 2017, 9, 24527−24537
Research Article
ACS Applied Materials & Interfaces
transferred into water for subsequent use. Nuclear magnetic resonance (NMR) spectrum of the synthesized PEKt-15 is provided in Figure S7. The syntheses of the other PEKt-x precursors (with different numbers of x) follow similar procedures as illustrated above, by adjusting the content of the diphenol monomer to render different number of x. 2.4. Sulfonation of PEKt-x. Dry PEKt (1.0 g) and concentrated sulfuric acid (15.0 mL) were added into a round-bottom flask (equipped with a magnetic stirrer). The sulfonation reaction was carried out at ambient temperature for 12 h. The solution was then slowly transferred into ice water. The precipitate was collected and washed with deionized (DI) water until the pH of the washing water reached 7. The obtained sulfonated polymer (semi-SPEK-x) was finally dried at 120 °C under vacuum for 12 h. 2.5. Membrane Casting and Acidification. The sulfonated polymer, semi-SPEK-x, was dissolved into a DCA solvent to form a 4.0% (w/v) solution, which was subsequently filtered using a 0.45 μm syringe. The filtered solution was then homogeneously spread onto a clean glass plate. After a preliminary dry process at 60 °C for 12 h in open air, the obtained polymer membrane was further dried at 100 °C for 24 h under vacuum. Acidification of the membranes was performed by immersing the dry membranes into a 1.0 M H2SO4 solution at 80 °C for 2 h. Then, the membranes were transferred into boiling water to wash out the residual H2SO4 on the surface. The acidified membranes were stored in DI water for further investigations. 2.6. Characterization. NMR spectra of the monomers and polymers were obtained with an Agilent MR (1H, 400 MHz) spectrometer. Fourier transform infrared (FTIR) spectra of the polymers were obtained with a Nicolet iS5 FTIR spectrometer (Thermo Scientific). The inherent viscosities of the polymers were determined with an Ubbelohde viscometer by employing a 0.5 g dL−1 DCA solution at 25 °C. Thermogravimetric analyses (TGAs) were carried out on a TA Q500 thermogravimetric analyzer. X-ray diffraction (XRD) patterns of the polymer membranes were collected with a Rigaku MiniFlex 600 instrument. The degree of crystallinity, Xc, was determined by
strength of the hydrophobic domain in polymers. It is generally believed that a crystalline structure could enhance the mechanical property of a polymer membrane through facile interactions between polymer chains.32 The above idea has recently inspired a few research studies on developing semicrystalline proton exchange polymer membranes. Holdcroft et al. developed a series of semicrystalline graft copolymers, sulfonated poly(vinylidene difluoride-co-chlorotrifluoroethylene)-g-polystyrene, based on which the effects of crystallinity on morphology, hydration, and ion-conduction behavior were investigated.33,34 It was found that the crystallinity of the copolymers was dependent on the graft length.33,34 Ion-conducting chains could also be attached to the semicrystalline substrate by a radiation-grafting method.35−37 Maekawa et al. successfully attached a styrene sulfonate monomer on a PEEK film with the radiation-induced graft polymerization approach.37 The semicrystalline substrate exhibited sufficient physical properties at high IEC values (above 3.0 mequiv g−1).37 In light of the above research and our previous endeavors with the concentrated sulfonated polymer membranes,28,30,38 herein, we present a strategical approach to improve the performances of the polymer membranes through introducing a crystalline moiety to the amorphous precursor. The semicrystalline poly(ether ketone) (PEK)-based PEM (semi-SPEKx) demonstrated in this study is synthesized by a sequence of strategical copolymerization and sulfonation procedures. First, a series of soluble amorphous poly(ether ketimine) (PEKt) precursors are synthesized. A subsequent sulfonation endows the polymers with ion-conducting groups. Meanwhile, hydrolysis of the Schiff base triggers the formation of a semicrystalline PEK backbone. With the synthesized semicrystalline PEK polymers, the influence of the crystalline structure on the overall properties of the polymer membrane is systematically investigated. The improved properties are also demonstrated by operating DMFCs with the synthesized semicrystalline PEK polymer membranes.
Xc =
Icr Icr + Iam
(1)
where Icr and Iam are the integrated values of the crystalline and amorphous peaks, respectively. The XRD patterns were analyzed by a curve-fitting method to separate the crystalline diffraction peaks from amorphous scattering to obtain the degree of crystallinity with a deconvolution function in Peakfit v4.12 software. 2.7. IEC. The theoretical IEC value was calculated based on the feed ratio of monomers, assuming that all of the phenyls on the diphenol monomer are fully sulfonated. The experimental IEC values of the sulfonated polymers were determined by a titration method. The weights of the dry (after being dried under vacuum) membranes were first recorded. Then, the dry samples were soaked in a 2.0 M NaCl solution for at least 48 h. The proton-exchanged NaCl solution was then titrated with a 0.05 M sodium hydroxide solution and with a phenolphthalein indicator. The IEC value was calculated on the basis of the consumed NaOH solution in accord with
2. MATERIALS AND METHODS 2.1. Chemicals and Materials. Sulfolane (99%; Alfa Aesar), toluene (99.5%; Alfa Aesar), dichloroacetic acid (DCA, 99%; Alfa Aesar), sulfuric acid (95−98%; Fisher Scientific), and anhydrous K2CO3 (99%; Alfa Aesar) were used as received. Aniline was purified before using by vacuum distillation. 4,4′-Difluorobenzophenone (99%; Acros), 4,4′-dihydroxybenzophenone (DHB, 98%; TCI America), and 4,4′-dihydroxydiphenylsulfone (98%; TCI America) were recrystallized from an ethanol−water mixture medium. Tetraphenylcyclopentadienone was synthesized in accordance with a previously reported method.39 The diphenol monomer was prepared according to our previous studies, as described in the Supporting Information (Sections 1−5).28,38 2.2. Synthesis of N-Phenyl(4,4′-difluorodiphenyl)ketamine. N-Phenyl(4,4′-difluorodiphenyl)ketimine was synthesized based on a previously presented procedure,40 as detailed in the Supporting Information (Section 6). The crude product was purified by recrystallization three times from a methanol medium. 2.3. Synthesis of PEKt-x. The synthesis details of PEKt-15 are given herein as an example. To a 50 mL three-neck flask (equipped with a mechanical stirrer and a Dean−Stark trap), 1.1732 g of the ketimine monomer, 0.7283 g of DHB, 0.5130 g of the diphenol monomer, 0.6081 g of K2CO3, 4.0 mL of toluene, and 6.0 mL of sulfolane were added. The mixture was heated under a N2 atmosphere to 140 °C. The temperature (140 °C) of the mixture was maintained for 3 h for the removal of water. Polymerization of PEKt-15 was carried out at 190 °C for 12 h. Afterward, the obtained product was
IEC =
VNaOH × C NaOH Wdry
(2)
where VNaOH and CNaOH are, respectively, the volume and the concentration of NaOH solution and Wdry is the weight of the dry membrane. 2.8. Water Uptake (WU) and Swelling Ratio (SW). Prior to WU and SW measurements, the polymer membranes were dried at 120 °C under vacuum for 12 h. The weights and dimensions of the dry membranes were recorded. Then, the polymer membrane samples were immersed into DI water to reach an equilibrium at assigned temperatures. Afterward, the weights and dimensions of the hydrated membranes were measured again. The WU was calculated in accord with 24528
DOI: 10.1021/acsami.7b03720 ACS Appl. Mater. Interfaces 2017, 9, 24527−24537
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ACS Applied Materials & Interfaces WU =
Wwet − Wdry Wdry
3. RESULTS AND DISCUSSION 3.1. Synthesis of PEKt Precursor and Semi-SPEK-x. To preserve crystallinity with the resulting polymer membrane, a straightforward method is to initially construct a semicrystalline polymer, followed by a selective functionalization process to introduce sulfonic acid groups. Generally, the semicrystalline PEK has previously been synthesized at a relatively high polymerization temperature (∼320 °C) to eliminate the solubility issue due to the crystallization of the oligomer. However, the higher temperature leads to various side reactions, resulting in undesired molecular weight distribution or even cross-linking.42 Another feasible route is to synthesize an amorphous polymer precursor, followed by a subsequent post-treatment process to achieve crystallinity. A few research groups have reported the utilization of monomers with bulky groups in the synthesis of soluble PEK precursors.43−45 It was previously reported that a N-phenyl(4,4′-difluorodiphenyl)ketimine could be used as a comonomer for the synthesis of polymer precursors.40,46 Because replacement of benzophenone with ketimine hinders the formation of crystalline domains in polymers, the polymerization can be conducted at relatively low temperatures (150−200 °C) without any solubility issues.46 In addition, an amorphous polymer precursor can easily transform into a semicrystalline structure via hydrolysis of a Schiff base under acidic conditions.40 Therefore, it provides a possibility of performing both sulfonation and hydrolysis in a one-step reaction to obtain sulfonated semicrystalline polymers. On the basis of the above ideas, we synthesized PEKt as a model polymer to investigate the possibility of the hydrolysis reaction with a sulfonating agent (concentrated sulfuric acid), as illustrated in Scheme 1. Upon post-treatment with
(3)
where Wdry (g) and Wwet (g) represent, respectively, the weights of the dry and hydrated polymer membranes. The SW of the membrane was calculated in accordance with SW =
Lwet − Ldry Ldry
(4)
where Ldry (g) and Lwet (g) represent, respectively, the lengths of the dry and hydrated polymer membranes. 2.9. Proton Conductivity (σ). Electrochemical impedance spectroscopy (EIS) spectrum of the hydrated polymer membranes was obtained with a frequency response analyzer/potentiostat (1260/1287; Solartron Analytical) from 0.1 Hz to 1 MHz. The measurements were carried out with an in-house-designed four-electrode test cell. During the impedance measurements, the test cell (with a piece of polymer membrane embedded) was submerged into a DI water bath or under constant-humidity conditions at assigned temperatures. The proton conductivity (σ, S cm−1) of polymer membranes was calculated according to
σ=
L RA
(5)
where L represents the distance between the electrodes in the testing cell and A represents the effective area of the polymer membrane sample. The resistance (R) is derived from the EIS data. 2.10. Fuel Cell Performance Test. Carbon-supported platinum (Pt/C) and carbon-supported platinum−ruthenium alloy (PtRu/C) were employed, respectively, as the cathode and anode catalysts in a DMFC. The electrodes were fabricated by depositing the Pt/C or PtRu/C catalyst onto a Toray carbon paper (TGP-H-090). The loading of the catalyst on the carbon paper was 2.0 mg cm−2. The membrane−electrode assembly was fabricated with a hydrated semicrystalline PEK polymer, a Pt/C cathode (5 cm), and a PtRu/C anode (5 cm2). The operation of the DMFC was controlled with a fuel cell test instrument (850 E; Scribner Associates Inc.). The cell was operated with aqueous methanol solutions (prepared with different concentrations) delivered to the anode. The flow rate of methanol fuel was controlled at 1.0 mL min−1. Oxygen was prehumidified before being delivered to the cathode. The flow rate of O2 was controlled at 200 mL min−1. Polarization curves of the DMFCs were measured with a current staircase mode. 2.11. Methanol (CH3OH) Permeability (P). CH3OH permeability of the membranes was assessed by a voltammetric method41 and performed with a DMFC (the same cell configuration for the fuel cell performance test). An aqueous CH3OH solution (prepared with different concentrations) was delivered into the anode of the fuel cell. Meanwhile, an inert gas (N2, prehumidified) was delivered to the cathode. CH3OH permeability could be evaluated by the measured limiting current (Jlim) obtained from the voltammetric profiles. The calculation of methanol permeability can be expressed by the equation below
P=
1 Jlim Lm KdlCm 6F
Scheme 1. Synthesis of Model Polymer PEKt and Hydrolysis of This Polymer with Sulfuric Acid
(6)
where Lm is the thickness of the polymer membrane, Cm is the concentration of CH3OH, kdl is the drag correction factor (0.739 for a 2 mol L−1 CH3OH solution),41 and F is the Faraday constant. 2.12. Mechanical Properties. The mechanical properties of the dried membranes were measured on a MTS 2.5 kN Universal Tester (Shimadzu Corp., Japan) at room temperature (RT) and 30% RH. The tensile test was performed at a strain rate of 2 mm min−1. 2.13. Oxidative Stability. Typically, the accelerated oxidative stability test for sulfonated polymers is carried out with Fenton’s reagent (3% H2O2 containing 2 ppm Fe2+). The membrane samples were soaked in Fenton’s reagent at 80 °C for 2 h, and the residual weight was recorded.
concentrated sulfuric acid (for 12 h), the resulting polymer did not dissolve in the conventional solvents (e.g., dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), or dimethylformamide (DMF)) any more. The untreated and the H2SO4-treated polymers were also analyzed with differential scanning calorimetry (DSC), as presented in Figure 1. Upon comparing the DSC curves of the two polymer samples, a shift in the glass-transition temperature could be observed (shifted from 172 to 161 °C). There also appeared a cold crystallization 24529
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DMSO). Therefore, we selected DCA as the solvent for membrane casting, as described in Section 2. The viscosities of semi-SPEK-x varied from 1.15 to 1.37 dL g−1 as IEC values increased from 1.49 to 2.00 mequiv g−1. Figure 2 shows the FTIR spectra of the precursor and the semi-SPEK-x polymers. The peaks at 1650 and 1590 cm−1 are attributed, respectively, to CO stretching and CN stretching. Upon sulfonation, a new peak at 1035 cm−1 appears in the profiles of all semiSPEK-x polymers, which can be assigned to SO stretching in the sulfonic acid group. In addition, the intensity decease in the CN stretching peak and the intensity increase in the CO stretching peak indicate the completion of the hydrolysis reaction. It should be noted that there is an expected overlap of the peaks of CN and CC (in phenyl) at ∼1590 cm−1. Therefore, the decrease in the peak intensity of the CN group is hard to separately and quantitatively estimate in the semi-SPEK-x. The IEC values of the polymers determined by an acid−base titration experiment (as described in Section 2) are summarized in the second and third columns of Table 1. The FTIR and IEC analyses results reveal that the polymers are successfully sulfonated and hydrolyzed during syntheses. 3.2. Thermal Properties. The thermal stability of the sulfonated polymers was evaluated by TGA. The results of the temperatures at 5 wt % weight loss (Td5%) are listed in the fourth column of Table 1. The Td5% values of all of the semiSPEK-x polymers are above 314 °C, indicating their excellent thermal stability. The TGA profiles of the semi-SPEK-x polymers (Figure 3) exhibit a typical two-stage decomposition feature of the sulfonated aromatic polymers.49 Detailed weight loss processes of the semi-SPEK-x polymers were analyzed with differential thermogravimetry (DTG), as presented in Figure 3. The first-stage weight loss occurring at about 350 °C is attributed to the cleavage of the SO3− groups. The second-stage weight loss at about 550 °C is due to the decomposition of the backbone of the polymer membrane. 3.3. Crystallinity Analysis with XRD. For the traditional SPEEK polymers, the crystallinity of the material usually decreases with the prolonged sulfonation process. The SPEEK polymers generally become amorphous when the IEC exceeds 1.0 mequiv g−1.16 In this study, the crystallinity of the semiSPEK-x polymers could be reobtained after a one-step sulfonation−hydrolysis reaction. XRD analysis reveals the crystalline nature of the semi-SPEK-x membranes, as displayed in Figure 4. All of the dry semi-SPEK-x membranes exhibit characteristic peaks of PEK47 at 19.1, 21.1, 23.5, and 29.5°, indicating the successful hydrolysis and the formation of PEK backbone. The degree of crystallinity (Xc) of each semi-SPEK-x was calculated by eq 1 (as listed in Table 1). Semi-SPEK-12 exhibited the highest Xc value in this series due to its high content of the PEK segment. However, as seen in Figure 4, all of the PEK characteristic peaks become smooth and broad as the IEC value increased from 1.49 to 2.00 mequiv g−1, indicating an increase in the amorphous moiety. As expected, the amorphous polymer (SPEKS-18) exhibits a typical broad peak due to the lack of crystalline domains. The crystallinity of the wet membranes was also investigated. When the membranes were fully hydrated, the water molecules diffusing into the polymers can disrupt the crystallization of the materials to some extent. Apparently, after being hydrated, the semiSPEK-18 membrane becomes fully amorphous because all of the characteristic peaks disappeared in the XRD pattern (Figure 4). However, with a relatively higher percentage of the semicrystalline backbone, the sharp characteristic peaks still
Figure 1. DSC curves of PEKt before and after hydrolysis (treated PEKt).
peak and a melting peak for the H2SO4-treated polymer, revealing its semicrystalline nature. However, the semi-SPEK-x is not soluble in most commonly used deuterated solvents. We performed an elemental analysis to confirm the complete conversion of the ketimine to ketone by considering the aniline as the leaving group during the hydrolysis reaction. The results from the elemental analysis (Table S1) confirmed the complete conversion of ketimine to ketone. These phenomena reveal that the hydrolysis and sulfonation can be achieved with a one-step reaction. Inspired by the above encouraging results, we designed a synthesis route for a series of highly sulfonated semicrystalline PEK-based polymers (semi-SPEK-x), as displayed in Scheme 2. The diphenol monomer was synthesized according to our previous method, as described in the Supporting Information. The densely arranged phenyls could be easily functionalized with sulfuric acid. They act as the ion-conduction moieties in the resulting polymer. The backbone was constructed with DHB and ketimine. The alternating carbonyl groups prevent the phenyls from being attacked by sulfuric acid during sulfonation. The ketimine transforms into ketone after the hydrolysis of the Schiff base, ensuring the formation of the PEK backbone. To investigate the influence of the crystallinity on the sulfonated polymers, an amorphous sulfonated poly(ether ketone sulfone) polymer (SPEKS-18) was employed here for a comparison. Amorphous-type SPEKS-18 was constructed with a poly(ether sulfone ketone) backbone (stable during sulfonation) instead of PEK with a crystalline domain. The resulting sulfonated polymer SPEKS-18 exhibits an amorphous feature because of the differences in the nature of the polymer structure.47 The difference in the bond angles of the sulfone linkage (105°) and the ether linkage (∼120°) hinders the formation of unit cells. In addition, the relatively more rigid sulfone groups decrease the mobility of the main chain, which inhibits the chain configuration during crystallization.48 The structural difference of SPEKS-18 and the semi-SPEK-x was then minimized. This way, the effect of the crystalline structure on the properties of the resulting membranes could be investigated. All semi-SPEK-x polymers (with different numbers of x) were successfully synthesized, as described in Section 2. However, the resulting sulfonated polymers are not soluble in the conventional polar solvents (such as DMF, NMP, and 24530
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ACS Applied Materials & Interfaces Scheme 2. Synthesis of the PEKt Precursor and Semi-SPEK-x Polymersa
a
The structure of amorphous SPEKS-18 is also provided.
3.4. WU and SW. Water management is a critical characteristic impacting the overall properties of polymer
exist for the hydrated semi-SPEK-12 polymer with an Xc value of 16.9%. 24531
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Figure 3. TGA and DTG curves of semi-SPEK-x polymers obtained in a N2 atmosphere.
Figure 2. FTIR spectrum of the semi-SPEK-x polymers and the PEKt precursor.
membranes. Both the hydrophilic and hydrophobic domains have influences on the hydration level of the polymer membranes.21 The IEC value determines the hydration behavior of a polymer membrane because it is directly related to the number of hydrophilic groups. The traditional sulfonated aromatic polymers usually show an excessive swelling behavior when the IEC value exceeds 2.0 mequiv g−1.1,49 On the other hand, the aggregation of the hydrophobic backbone usually forms continuous skeletons inside the polymer membrane, which will subsequently reduce water swelling. It has been reported that the introduction of fluorine functional groups into a polymer would aid in inducing a well-defined phase separation and would enhance the dimensional stability when the polymer was hydrated.50 In this study, the densely stacked semicrystalline domain in the semi-SPEK-x polymers enhances the stability of the hydrophobic backbone, restricting the polymer to absorb excessive water molecules. As a result, the semi-SPEK-x polymers show a relatively low hydration level. The results of the WU and SW of the semi-SPEK-x polymers are, respectively, presented in Figures 5 and 6. The relevant data derived from Figures 5 and 6 are summarized in Table 2. As we discussed in the last section, after being fully hydrated, the Xc values of semi-SPEK-12 and semi-SPEK-15 decreased, respectively, by 0.5 and 3.8%. However, semi-SPEK-18 did not show any crystalline peaks. With a relatively high Xc value, semi-SPEK-12 and semi-SPEK-15 are expected to have a higher dimensional stability than that of semi-SPEK-18. At a certain temperature, the WU and SW increase with an increase in the IEC value. At each temperature (25 or 80 °C), semi-SPEK-12 and semi-SPEK-15 exhibit relatively low SW (10−3 mS cm−1). 3.6. Methanol Permeability. Fuel crossover has generally been considered as one of the most serious issues of DMFCs. One important benefit of the semicrystalline SPEK membranes developed in this study is their low methanol permeability. Figure 9 shows the voltammetric profiles of the electro-
Figure 9. Voltammetric profiles at various cell temperatures for the oxidation of CH3OH permeating through semi-SPEK-x and SPEKS-18 polymer membranes being exposed to a 2.0 M methanol feed.
oxidation of the permeated methanol with an experimental platform described in Section 2. The experiments were conducted at both 25 and 60 °C and with 2.0 M CH3OH delivered to the anode. The calculated results regarding the CH3OH permeability through various polymer membranes based on the voltammetric profiles in Figure 9 and according to eq 6 are summarized in Table 2. The experimental results obtained with a 5.0 M methanol solution are provided in Figure S8. Because of the same diffusion pathways of methanol and proton, as seen in Table 2, the methanol permeability of polymer membranes shows a similar tendency to that of the proton conductivity. As the IEC values increase from 1.49 to 2.00 mequiv g−1, the methanol permeability of the semi-SPEK-x membranes increases from 1.9 × 10−7 to 4.5 × 10−7 cm2 s−1 at
Figure 10. Polarization curves and power densities of DMFCs prepared with the semi-SPEK-15, semi-SPEK-18, and SPEKS-18 membranes. The cells were operated with 2.0 M CH3OH delivered to the anode. 24534
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results of the DMFCs operated with a high-concentration (5.0 M) methanol solution are provided in Figure S9. Semi-SPEK18 shows overall better cell performances than amorous SPEKS-18 in terms of open circuit voltages and polarization losses at both low (25 °C) and high (60 °C) temperatures. The relatively better DMFC performances of semi-SPEK-18 are believed to be attributed to its low methanol permeability and moderate proton conductivity. However, as seen in Figure 10, the semi-SPEK-15 membrane shows a relatively high polarization loss regardless of its lower methanol permeability. This is likely attributed to the slightly low conductivity of semiSPEK-15. The highly crystalline domain in semi-SPEK-15 is capable of blocking methanol permeation. But it is also assumed to be hindering water diffusion. Therefore, from the discussion above, the content of crystalline domains inside the semi-SPEK-x membranes should be maintained at a reasonable level to balance between the methanol crossover and proton conductivity in DMFC applications. The semi-SPEK-18 membrane provides the best overall property. It should be noted that the DMFC operation conditions in this study were not optimized at the current stage. To confirm the advantages of the semi-SPEK-x polymers, Figure S10 compares the singlecell performances obtained with the Nafion-117 membrane and the semi-SPEK-18 membrane under the same conditions. The results clearly show the benefits of the semi-SPEK-18 membrane, demonstrating the potential application of semiSPEK-x materials in DMFCs. 3.8. Mechanical Properties and Oxidative Stability. The mechanical properties of the semi-SPEK-x membranes were measured at RT and 30% RH. The stress−strain curves are provided in Figure S11, and the data are summarized in Table 1. The tensile strength of the semi-SPEK-x membranes was between 41.6 and 49.9 MPa, and the value of the elongation at break was from 26.2 to 29.3%. The tensile strength of the semi-SPEK-x membranes decreases with an increasing IEC value and a decreasing Xc value. In addition, in comparison with SPEKS-18, all of the semi-SPEK-x membranes exhibit a higher tensile strength and a higher value of elongation at break, which are attributed to the existence of the crystalline domain in the semi-SPEK-x membranes. The oxidative stabilities of the polymers were evaluated by comparing the weight changes of membranes after soaking them in Fenton’s reagent at 80 °C for 2 h. The results are summarized in Table 1. All of the semicrystalline polymers exhibited a residual weight of over 90%, attributed to their densely stacked crystalline structure.
Research Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03720. Elemental analysis of PEKt and treated PEKt; synthesis details and NMR spectra of 4-bromo-2′,6′-difluorobenzophenone, 2,6-difluoro-4′-phenylethynyl benzophenone, 2,6-difluoro-4′-(2,3,4,5,6-pentaphenyl phenyl)benzophenone, 2,6-bis(4-methoxyphenyl)-4′-(2,3,4,5,6pentaphenyl phenyl)benzophenone, 2,6-bis(4-hydroxyphenyl)-4′-(2,3,4,5,6-pentaphenyl phenyl)benzophenone, and N-phenyl(4,4′-difluorodiphenyl)ketamine; NMR spectrum of PEKt-15; voltammetric profiles at various cell temperatures for the oxidation of methanol permeating through semi-SPEK-x and SPEKS18 polymer membranes being exposed to a 5.0 M methanol feed; polarization curves of DMFCs prepared with the semi-SPEK-15, semi-SPEK-18, or SPEKS-18 membranes (the cells were operated with 5.0 M methanol fed to the anode); stress−strain curves of semi-SPEK-x and SPEKS-18 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel: +86-0431-8516-8889. Fax: +860431-8516-8889 (G.W.). *E-mail:
[email protected]. Tel: +1-512-471-1791. Fax: +1-512-471-7681 (A.M.). ORCID
Arumugam Manthiram: 0000-0003-0237-9563 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Welch Foundation grant F-1254. One of the authors (S.F.) thanks the China Scholarship Council (No. 201506170095) for the fanatical support.
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REFERENCES
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4. CONCLUSIONS A series of semicrystalline PEK-based PEMs have been developed and employed as proton exchange membranes in DMFC applications. XRD analysis reveals that the membranes manifest crystalline nature in both dried and hydrated states. Introduction of crystalline structures suppresses excessive water absorption by the membranes and improves the dimensional stability of the hydrated membranes even at high IEC values. Moreover, the semicrystalline membranes exhibit reduced methanol permeability in comparison to that of the amorphous membrane. The improved single-cell performances (relative to those of the amorphous SPEKS) demonstrate this type of semicrystalline SPEK polymer membrane as a promising proton exchange membrane for DMFC applications. 24535
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