Fabrication and characterization of supported dual acidic ionic liquids for polymer electrolyte membrane fuel cell applications

2.1 Materials and methods

Melt-processed semi-crystalline ETFE film with thickness of 50 μm were commercially obtained from Good fellow (Cambridge, England). The dense film is highly hydrophobic with water permeability of 170 × 10⁻¹³ cm³·cm cm⁻² s⁻¹ Pa⁻¹ at 38 °C. 4-VP (>95% purity) and 1,4-butane sultone (≥99% purity) were purchased from Aldrich and used without any further purification. Acetonitrile was pre-dried by shaking over type 4 Å molecular sieve particles followed by distillation over calcium hydride. Other chemicals including iron (II) sulfate, sulfuric acid, hydrogen peroxide, hydrochloric acid and potassium hydroxide were analytical grade and used as received. Deionised (DI) water (18 MΩ resistivity) was used in all experiments and was produced using a NANO pure DIamond™ water purifier.

2.2 Preparation of poly(4-VP) grafted ETFE films (membrane precursor)

The poly(4-VP) grafted ETFE films were prepared using pre-irradiation method started by irradiation with electron beam (EB). ETFE films (5 cm × 5 cm) were washed with ethanol and dried under vacuum oven prior to recording their weight and keeping them in thin polyethylene bags sealed under vacuum. The films were irradiated to a total dose of 100 kGy with an EB using a universal accelerator (NHV-Nissin High Voltage, EPS 3000, Cockroft Walton type, Japan) operated at an acceleration voltage of 1 MeV, beam current of 2 mA and 10 kGy dose per pass.

In the second step, grafting of 4-VP onto irradiated ETFE films was performed by introducing O₂ free monomer solution diluted with water obtained by bubbling with purified N₂ gas for 30 min to an evacuated glass ampoule containing the irradiated ETFE film. The ampoule was sealed and heated in a water bath at 60 °C to start the grafting reaction. After the reaction was completed, the grafted film was removed, washed with methanol under sonication for several hours to remove the unreacted monomer and/or homopolymers. The grafted film was dried under vacuum at 60 °C overnight and the weight of the grafted film was determined. The grafted film was denoted as ETFE-g-poly(4-VP). The degree of grafting (DOG) was determined from the following equation and confirmed by TGA.

2.3 Immobilization of IL onto grafted ETFE films

The ETFE-g-poly(4-VP) films were treated with 1,4-butane sultone to introduce sulfonic acid alkyl pyridinium IL. The reaction was carried out by immersing the ETFE-g-poly(4-VP) films in a dry acetonitrile in two-neck 250 mL round flasks under inert atmosphere. The mixture was cooled to 0 °C in an ice bath and 1,4-butane sultone/acetonitrile 2% (v/v) was added. The mixture was heated to 85 °C under continuous stirring for two days to obtain the corresponding zwitterion denoted as ETFE-g-poly(4-VP)-SO₃ that was washed with diethyl ether (3 × 25 mL) and dried under vacuum. The obtained zwitter ion was then immersed in a 1 M H₂SO₄ solution under stirring for 24 h at 60 °C to obtain supported dual acidic IL [ETFE-g-poly(4-VP)-SO₃H]HSO₄ membranes, which were then extracted with deionised water and rinsed in water and ethanol solutions before drying under vacuum at 80 °C for 48 h.

The number of sulfonic acid molecules per alkyl substituted pyridine rings of the grafted matrix or degree of sulfonation was calculated and found to be 50% of the total poly(4-VP) grafted on ETFE films. The reaction was repeated in a higher concentration of the 1,4-butane sultone and longer reaction time to increase the acid functionalization level per polymer repeating unit. A complete sulfonation (i.e., all alkyl pyridine groups in the membranes were fully sulfonated) was achieved with the increase in the 1,4-butane sultone/acetonitrile concentration to 5% (v/v) and the time to 3 days. Membranes with high IEC (ETFE-d-IL3.2 and ETFE-d-IL3.4 with DOG of 46 and 59%, respectively) were chosen for further evaluation of the properties.

2.4 Evolutions of properties of [ETFE-g-poly(4-VP)-SO₃H]HSO₄ membranes

Micrographic images of the obtained membranes were obtained using a Philips XL30 field emission scanning electron microscope (FE-SEM) after coating with 5 nm Au. Energy dispersive X-ray spectroscopy (EDS) was performed with Zeiss GeminiSEM 500. FTIR coupled with attenuated total reflection (FT-IR-ATR) analysis was performed using an Agilent Cary 660 spectrometer. XRD diffraction patterns were obtained by a Philips X’Pert 1 X-ray diffractometer with graphite monochromatised Cu Kα radiation (λ = 1.5401 Å) at a scanning rate of 2° min⁻¹ over range of 2θ = 4–80°. X-ray photoelectron spectroscopy (XPS) was performed with PHI Quantera II Scanning XPS Microprobe with a monochromatic radiation source of Al Kα anode (1486.6 eV) at a pressure <1 × 10⁻⁸ bar at room temperature. Peak integration was carried out followed by deconvolution of the C 1s peaks by assuming a Gaussian line shape. TGA measurements were performed on a Perkin Elmer TGA 7 under N₂ atmosphere in a temperature range of 20–700 °C at a heating rate of 10 °C min⁻¹. The thermal properties of membrane samples were measured by a Perkin Elmer, Pyris-1 DSC. The degree of crystallinity (X_c) has also been calculated from the ratio of enthalpy of melting (ΔH_f) of polymer electrolyte to the enthalpy of melting for a fully crystalline polymer (ΔH₀ = 113.4 Jg⁻¹ for ETFE). $$X_c=\frac{\mathrm{\Delta }{H}_f}{\mathrm{\Delta }{H}_0}\times 100\%$$

TMA measurements were carried out with a TA Instrument 2940 in a tensile film mode using sample dimensions of 20 mm × 5 mm × 0.075 mm under N₂ atmosphere. The experiments were run at a temperature ranging from 30 to 350 °C with a temperature ramp rate of 5 °C min⁻¹.

IEC was determined by the back-titration method. The membrane in the acid form was immersed in 50 mL of deionized water containing 5 mL of 0.1 N NaOH for 48 h in order to release H⁺. The back-titration was accomplished by titrating the excessive NaOH remaining in solution with 0.1 M HCl solution. The IEC value was obtained by subtracting the added volume of 0.1 M HCl from the initial NaOH volume. Each data point of IEC measurements reported is an average of three measurements. Mohr titration method was used to determine Cl⁻ per g of dried membrane samples as reported elsewhere (Abouzari-lotf et al., 2017a).

In-plane ionic conductivity of the membranes was measured using DC source meter (Keithley 2400 from Keithley Instruments Inc., Cleveland, OH, USA) attached to a four-point probe of Bekk-Tech conductivity cell (BT-112) and controlled by a Lab view software. Membrane strips with dimensions of 5 mm × 25 mm were assembled between the platinum electrodes and placed in conductivity cell. The potentiostat was set to apply a specific voltage between the two inner probes and measured the resulting current. The slope of the data from current versus voltage measurements was applied to determine the resistance (R) and proton conductivity (σ) obtained according to the following equation:

The water uptake and swelling of the membranes were obtained by recording the changes in the weights and dimensions. The membrane samples were dried under high vacuum for 24 h and weighed prior to equilibrate in de-ionised water for 24 h at various temperature. The samples were then taken out, blotted dry and the weights and dimensions were quickly measured. The gravimetric water uptake was calculated using the following equation:

In the same way, the swelling in-plane (S_in) and through-plane (S_tr) of the membranes in de-ionised water were calculated using the following equation:

The chemical stability of the membranes was evaluated by Fenton agent containing 3% H₂O₂ and 2 ppm FeSO₄. Pre-weighed dry membranes were soaked in a Fenton aqueous solution at 30 °C for periods up to 120 h and the stability was evaluated by recording the weight retained in the membranes after complete drying. The ability of the membranes to retain their ionic character (i.e. durability) was tested by evaluating the proton conductivity under different time intervals up to 120 min.

3.1 Chemical composition of the membranes

FT-IR spectra in Fig. 4 shows the characteristic absorption bands of ETFE based ionic liquid electrolyte membrane in comparison with their corresponding poly(4-VP) grafted and original ETFE film. The spectrum of original ETFE film is identified by the presence of a number of strong bands in the range of 1000–1400 cm⁻¹ representing C—F of CF₂ groups. The grafted films displayed characteristic bands representing C⚌N and C⚌C of pyridine ring stretching vibrations at 1600, 1558, and 1418 cm⁻¹. The bands in the range of 720–850 cm⁻¹ is related to C—H bending of pyridine ring. The [ETFE-g-poly(4-VP)-SO₃H]HSO₄ membrane showed a abroad band at 3200–3600 cm⁻¹ region, which is associated with —OH stretching vibration of the sulfonic acid group in the membrane. Moreover, the quaternary salt of pyridinium in IL immobilized ETFE showed a band at 1640 cm⁻¹. This band can be assigned to C⚌N vibrations characteristic of the quaternary nitrogen atom in the pyridine ring.

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Fig. 4

FTIR spectra of (a) original ETFE film (blue), ETFE-g-poly(4-VP) film with 46% DOG (black) and corresponding supported IL membrane (red) and (b) expanded IR spectra in the region of 500–1700 cm⁻¹. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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To further elucidate the chemical structure and uniform ILs functionalization at the surface and core of ETFE substrates, XPS and EDS analysis were applied on the membrane surface and cross-section, respectively. Both techniques are highly sensitive to the surface with few nanometers of attenuation depths. Fig. 5 shows a survey scan and high-resolution narrow scans of C 1s and S 2s spectra. Compared to the two peaks in C 1s spectra of pristine ETFE at 286.0 eV (due to —CH₂—) and 290.5 eV (due to —CF₂—), (Gajos et al., 2016; Yamamoto et al., 2008), C 1s of IL functionalized ETFE was deconvoluted into 9 peaks corresponding to different C atoms in the structure of IL loaded membrane. They include aromatic and aliphatic —CH₂— (C—C^∗—C) at 284.5 eV, main chain —CH₂— at 286.0 eV, C—S at 287.2 eV, C⚌N⁺ at 288.4 eV, C—N⁺ at 289.3 eV, grafted IL units onto —CF— at 290.5 eV, main chain —CF₂— at 290.8 eV, and two peaks at higher binding energy of 292.2 and 293.1 eV which are likely to be due to the shake-up satellites of carbons or possible changes (e.g. crosslinking and unsaturation) in the ETFE backbones upon irradiation.

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Fig. 5

XPS spectra: wide scan (a) and narrow scan with deconvoluted C 1s peaks (b) of supported IL membrane.

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The chemical composition and uniformity of membranes were further confirmed with EDS analysis of surface and cross-section and the obtained images are presented in Fig. 6. The membrane was proven to contain elemental oxygen, sulfur and fluorine. Moreover, mapping of such elements confirms the presence of a uniform distribution of ionic liquid groups in the membrane surface and cross-section. These observations clearly confirm that the incorporation of poly(4-VP) grafts providing the host for ionic liquid took place homogenously across the membrane during the grafting reaction.

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Fig. 6

SEM images of membrane with 46% DOG: cross-section (top) and membrane EDX mapping with analysis of F, O, and S atoms of cross-section and surface views (bottom).

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3.2 Morphology of the membranes

The SEM images of the original ETFE, grafted and IL immobilized ETFE films are shown in Fig. 7. It was found that ETFE control membrane (Fig. 7a) was dense, uniform and defect free. The surface morphology of the grafted ETFE film (Fig. 7b) displayed white dots which represent 4-VP uniformly distributed on the surface of the polymer matrix. Upon incorporation of ILs in the membrane, numerous short-rang and roomy channels can be clearly distinguished in the image of supported IL membrane (bright colour in Fig. 7c), which can be attributed to the aggregation of ILs within the formed ionic clusters. In fact, these irregular channels emerged as a result of the dissociation of the hydrophobic ETFE polymer and hydrophilic pyridinium alkyl sulfonate groups (Che et al., 2015a; Wu et al., 2016).

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Fig. 7

SEM images of (a) original ETFE film, (b) ETFE-g-poly(4-VP) film with 46% DOG and (c) supported IL membrane and related photographs (d).

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3.3 Thermal properties of the membranes

Fig. 8a shows TGA thermograms of original ETFE film, poly(4-VP) grafted film and supported IL membrane. The original ETFE film shows a single-step degradation pattern starting at 440 °C while ETFE-g-poly(4-VP) film exhibits a two-step degradation pattern. The first step represents the decomposition of poly(4-VP) grafts started at 320 °C and the second one began at 440 °C is assigned for the decomposition of ETFE backbone. Interestingly, the exact value of DOG could be also calculated from the TGA thermograms of grafted films. Typically, the DOG value for this sample was found to be around 43%, which is in good agreement with the calculated data based on the gravimetric changes. For [ETFE-g-poly(4-VP)-SO₃H]HSO₄ film, three-step degradation pattern was observed at temperatures of 280, 320 and 440 °C. The first weight loss at around 280 °C belongs to the decomposition of butyl sulfonic acid groups, whereas the second and third steps (320 and 440 °C) are due to the decomposition of grafted poly(4-VP) and ETFE backbone, respectively. The observed weight loss that started at a temperature below 100 °C and continues to 150 °C in the grafted film and IL membrane is due to the loss of water molecules involved in hydrogen bonds. From the TGA results, it can be concluded that supported IL membranes have a thermal stability suitable for PEMFC operation at high temperature (>100 °C).

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Fig. 8

TGA (a), DSC (b) and TMA (c) thermograms of original ETFE film, ETFE-g-poly(4-VP) film with 46% DOG, and corresponding supported IL membrane.

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Fig. 8b shows DSC thermograms of the supported IL membrane and their corresponding grafted and original ETFE films. The strong endothermic peak at 269 °C depicts the melting temperature (T_m) and the area under the peak represents the crystalline fraction of the original ETFE film. Grafting of poly(4-VP) shifted T_m to 265 °C and decreased the total area under the melting peak suggesting a reduction in the crystalline fraction of the ETFE. This is caused by the dilution of the crystalline phase with the incorporation of amorphous poly(4-VP) grafts. The reduction of melting peak area caused by incorporation of IL results a partial disruption along with the plasticization effect of IL which conforms the increase in the amorphous phase. This observation is going well with the calculated X_c, which showed a reduction from 36.5% in the original ETFE film to 26.2% and 25.1% in the grafted film and supported IL membranes, respectively. Nevertheless, the membrane maintains a reasonable level of X_c despite the dilution effect and minor disruption in its crystalline structure.

The thermomechanical behaviour of the original ETFE, grafted ETFE film and supported IL membrane was examined by TMA to determine the T_g as shown in Fig. 8c. The TMA thermograms showed that there is a decrease in T_g of the supported IL membrane compared to those of the original and the grafted ETFE films. Such decrease was expected as the T_g of a polymer is directly related with the mobility of polymeric chains.

3.4 Structural properties of the membranes

X-ray diffraction measurements were carried out to monitor the structural changes took place in the ETFE matrix by the incorporation of poly(4-VP) grafts and subsequent IL grafted ETFE as depicted in curves presented in Fig. 9. The original ETFE film shows a typical diffraction curve resembling a semicrystalline polymer with a crystalline fraction peak at 19.6° and amorphous halo at 40.2°. The XRD curve of the grafted ETFE film shows a similar crystallinity peak but with a reduced intensity without any obvious shift in the crystalline peak position. This indicates that there is a decrease in the degree of crystallinity under the influence of dilution effect caused by incorporation of the amorphous poly(4-VP) grafts in the polymer matrix. It can be seen that IL grafted ETFE film consists of less intense and broader halos. The broadening of the peaks and reduction in intensity of crystalline phase is due to increase in amorphicity of polymeric membrane caused by the plasticization effect of IL.

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Fig. 9

XRD curves of original ETFE film, ETFE-g-poly(4-VP) with 46% DOG and supported IL membrane.

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3.5 IEC, water uptake, dimensional stability of the membranes

The values of IEC of the membranes are presented in Table 1. The IEC increased from 3.20 (in ETFE-d-IL3.2 membrane) to 3.41 mequiv g⁻¹ (in ETFE-d-IL3.4 membrane) with the rise in DOG from 46 in the former to 59% in the latter. This observation can be attributed to the increase in the content of poly(4-VP) grafts that can host more sulfonic acid groups. The IEC values of the new membranes are almost 3 times higher than that of Nafion 112 and this is likely due to the presence of large number of dual ionic linkage in their molecular structure. On the other hand, Mohr titration method resulted in the values which are almost half of the IEC values which clearly confirms the presence of dual acidic groups.

The water uptake and swelling are two important parameters for PEMs, which represent their water retention and dimensional stability, respectively. According to the vehicle mechanism of proton transport, an appropriate amount of water content in PEMs is necessary for achieving high proton. On the other hand, excessive water content in membranes could deteriorate dimensional stability and even results in serious loss of mechanical properties (Kreuer, 2001). Therefore, a promising PEM should balance the two properties and thus achieved high proton conductivity as well as excellent dimensional stability. The water uptake and swelling were measured at the temperatures ranging 30 and 80 °C and the results are displayed in Table 1. Compared to the negligible water uptake values for the pristine ETFE film, IL functionalized membranes showed significant interaction with the water due to the inclusion of hydrophilic ionic groups. As shown, for the IL membranes, the water uptake increased with sulfonation degree and temperature. On the other hand, ETFE-d-IL3.4 showed the higher water uptake than that of ETFE-d-IL3.2 at the same temperature due to higher content of sulfonic acid groups.

The dimensional changes associated with water swelling in the supported IL membranes and Nafion 112 at 30 and 80 °C are also presented in Table 1. All the membranes demonstrated increased swelling ratios with the temperature increase. However, the swelling ratio of the supported membranes were interestingly lower than that of Nafion despite their higher IECs. In addition to the inherent stability of base ETFE film, it seems that the grafted dual acidic ILs have restricted volumetric expansion during swelling which favours a higher IEC.

3.6 Proton conductivity of the membranes

Fig. 10a shows the variation of proton conductivity with the temperature for two supported IL membranes and Nafion 112 at 100% relative humidity. The proton conductivity showed a temperature dependant trend in all membranes. However, the increasing of conductivity with temperature was rather continuous in Nafion 112 membrane throughout the temperature range (30–95 °C) whereas it took place in two consecutive stages in the supported IL membranes. For instance, the increase in conductivity of both IL membranes was lower than that of Nafion 112 in the temperature range of 30–80 °C (first stage) beyond which (second stage) it jumped to greater values reaching 223 mS cm⁻¹ for ETFE-d-IL3.2 and 259 mS cm⁻¹ for ETFE-d-IL3.4 at 80–95 °C compared to 160 mS cm⁻¹ for Nafion 112 membrane at the same temperature. Such interesting temperature-dependent conductivity trend can be attributed to the mobile protons produced in the dissociation of HSO₃ groups. The polymeric membranes are formed by pyridinium cations with sulfonic groups as side chains and HSO₃⁻ anions that is essentially a strong acid. Thus, when exposed to water and temperature, the sulfonate groups present in both counter-ions dissociate into SO₃⁻ and free mobile protons (Kataoka et al., 2015). As TGA thermogram showed, these supported membranes contain high amounts of retained water inside their chemical structure, there are plenty of mobile protons that can easily jump through the H-bonded network formed by ions and water molecules (Wojnarowska et al., 2014). On the other hand, it looks like that the jumping in conductivity values between 80 and 95 °C is due to some kind of phase transition. Since water uptake values also changes intensely at this temperature window (comparing the values for 70 and 80 °C in Table 1), it could be concluded that the transition is from crystalline to amorphous.

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Fig. 10

Variation of proton conductivity of: (a) supported IL membranes and Nafion 112 with temperature and (b) the corresponding Arrhenius plots.

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The dependence of the proton conductivity of the membranes on the temperature (T) was investigated and the data were fitted to Arrhenius equation given below: $$\ln (\sigma )=-\frac{E_a}{R}\left(\frac{1}{T}\right)+\ln {\sigma }_0$$ where σ is the proton conductivity, σ₀ is the pre-exponential factor (mS/Kcm), E_a is the activation energy, T is the temperature in K and R is the universal gas constant (8.314 J mol⁻¹ K⁻¹). The obtained data were plotted in Fig. 10b in which 2-region linear trends were observed for both IL membranes resembling a similar trend to that discussed in Fig. 10a. In the first region where the temperature increased from 30 to 80 °C, the data were fitted to a straight line suggesting a proton hopping mechanism in association with high protons and alkyl chains mobilities. In the region above 80 °C, the ionic conductivity increased sharply reaching approximately four times higher values than lower temperature while maintaining a linear trend up to 95 °C. The facilitated mobility of proton hopping sites within membrane due to the phase transition could be the main factor for this increase. This was evident from the lower activation energy (E_a) of the two IL membranes recorded at the temperature range of 80–95 °C (E_a = 18.6 and 21.4 kJ mol⁻¹) compared to that for the range 30–80 °C (E_a = 19.6 and 22.1 kJ mol⁻¹ for membranes I and II respectively). These observations suggest that the supported IL membranes obtained in this study possess high proton conductivity suitable for operating PEMFC above 80 °C.

The stability of proton conductivity of the developed membranes was evaluated in a fully hydrated state at 95 °C over a period of 50 h and the obtained data was plotted in Fig. 11. The prolonged proton conductivity in the two supported IL membranes was found to slightly decrease yet remained stable over a period of 50 h. For instance, the conductivity slightly decreased from 259 to 242 mS cm⁻¹ in ETFE-d-IL3.4 and from 223 to 215 mS cm⁻¹ in ETFE-d-IL3.2 after 5 h and remained unvaried for 50 h. This trend indicates that the supported dual acidic IL membranes are stable enough for future testing in PEMFC.

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Fig. 11

Variation of proton conductivity of supported IL membranes with time.

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3.7 Chemical stability of the membranes

The accelerated oxidative degradation test of supported IL membranes was investigated in Fenton’s reagent (3% H₂O₂ aqueous solution containing 2 ppm FeSO₄) at 30 °C and the obtained data are presented in Table 2. It was interestingly found that only 1 and 4% weight losses could be observed in both supported IL membranes after 5 h compared to ∼9% weight loss in Nafion 112 under same conditions. Since the pristine ETFE was stable in the similar test condition, it could be concluded that degradations in the first 5 h were mainly occurring in the grafted regions. Prolonging the time to 24 and 48 h led to equivalent weight losses in all membranes reaching about 10 and 15%, respectively. Comparing to the initial degradations in the pristine ETFE, weight losses after 24 and 48 h could be arising from degradations in both of the grafted regions and ETFE backbone. Nevertheless, the supported IL membranes retained their physical properties such as flexibility and transparency and resisted dissolution in Fenton’s reagent after 120 h period as Nafion 112. These results suggest that supported IL membranes obtained in this study have desired high chemical stability. Such enhanced chemical stability can be attributed to the durable ETFE backbone with covalently grafted pyridine ring and covalently immobilization of ionic groups in the membrane.