Effect of “bridge” on the performance of organic-inorganic crosslinked hybrid proton exchange membranes via KH550

Journal of Power Sources 340 (2017) 126e138

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Effect of “bridge” on the performance of organic-inorganic crosslinked hybrid proton exchange membranes via KH550 Hailan Han a, Hai Qiang Li a, Meiyu Liu b, Lishuang Xu a, Jingmei Xu a, Shuang Wang b, Hongzhe Ni a, b, **, Zhe Wang a, b, * a b

College of Chemical Engineering, Changchun University of Technology, Changchun 130012, People's Republic of China Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, People's Republic of China

h i g h l i g h t s
SiO2 particles were fixed in polymer chains through cross linker.
We prepared novel crosslinked hybrid proton exchange membranes.
The membrane exhibited high proton conductivity and low methanol permeability.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 June 2016 Received in revised form 4 November 2016 Accepted 10 November 2016

A series of novel organic-inorganic crosslinked hybrid proton exchange membranes were prepared using sulfonated poly(arylene ether ketone sulfone) polymers containing carboxyl groups (C-SPAEKS), (3aminopropyl)-triethoxysilane (KH550), and tetraethoxysilane (TEOS). KH550 acted as a “bridge” after reacting with carboxyl and sulfonic groups of C-SPAEKS to form covalent and ionic crosslinked structure between the C-SPAEKS and SiO2 phase. The crosslinked hybrid membranes (C-SPAEKS/K-SiO2) were characterized by FT-IR spectroscopy, TGA, and electrochemistry, etc. The thermal stability, mechanical properties and proton conductivity of the crosslinked hybrid membranes were improved by the presence of both crosslinked structure and inorganic phase. The proton conductivity of C-SPAEKS/K-SiO2-8 was recorded as 0.110 S cm1, higher than that of Nafion® (0.028 S cm1) at 120  C. Moreover, the methanol permeability of the C-SPAEKS/K-SiO2-8 was measured as 3.86  107 cm2 s1, much lower than that of Nafion® 117 membranes (29.4  107 cm2 s1) at 25  C. © 2016 Elsevier B.V. All rights reserved.

Keywords: Crosslinked hybrid membranes SiO2 Proton exchange membrane Fuel cells

1. Introduction Proton exchange membrane fuel cells (PEMFCs) are widely used in portable devices and mobile power appliances. They are also promising power sources for vehicle power owing to their numerous advantages, such as high efficiency, low or none pollutes, and energy saving. To address these applications, the proton exchange membranes (PEMs), a key factor in FCs performance, require few prerequisite conditions, including relatively high proton conductivity, excellent mechanical properties, and low alcohol permeability (e.g., methanol or ethanol). The most extensive commercially available membranes are Nafion® due to their higher

* Corresponding author.. ** Corresponding author. E-mail addresses: [email protected] (H. Ni), [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jpowsour.2016.11.066 0378-7753/© 2016 Elsevier B.V. All rights reserved.

proton conductivity. However, Nafion® has an operating temperature limited to ~80e120  C. On the one hand, as the glass transition temperature of Nafion® is 110e115  C, so improving the operating temperatures will affect the mechanical properties [1]. On the other hand, because Nafion® is over-reliant on water content, a sharp decline in its proton conductivity was observed at temperatures above 80  C, which hindered its large-scale applications [2]. It's well known that PEMFC operation at elevated temperatures has many benefits, such as simplified electrochemical kinetics and higher CO-rich reformed hydrogen [1]. Therefore, finding new PEMs with higher proton conductivities at medium high temperatures (80e120  C) is desirable [3e10]. During the past years, alternative materials to Nafion® have been developed and tested with non-perfluorinated polymers that have elevated mechanical properties and superior chemical stabilities. This includes sulfonated poly(arylene ether), poly(styrene

H. Han et al. / Journal of Power Sources 340 (2017) 126e138

sulfonic acid), sulfonated polyether sulfone, sulfonated poly(ether ether ketone), and sulfonated poly(arylene ether ketone sulfone), among others [11e17]. The sulfonated poly(arylene ether ketone sulfone) is one of the most promising new polymer materials due to its excellent mechanical and thermal properties, higher proton conductivity, and low methanol permeability. The degree of sulfonation (DS) is responsible for its properties. Though elevated DS values would certainly contribute to improved proton conductivities, the high swelling ratio would reduce the mechanical properties, and hence affect the PEM efficiency. To overcome these limitations, several approaches have been tested using inorganic particles crosslinked through acid-base interactions. It is worth noting that incorporation of inorganic fillers may contribute to the improvement of thermal stability, proton conductivity, and mechanical properties. A number of particles have been investigated in the construction of organic-inorganic hybrid membranes for PEM, including TiO2, SiO2, ZrO2, and carbon nanotube [18]. For example, Sacca et al. [19] prepared a composite membrane containing TiO2 for medium temperature PEMFC and they found that the composite membrane was able to function at 110  C to yield a power density of 0.514 W cm2 at 0.56 V. However, agglomeration occurred due to the poor compatibility between the dispersed components. To prevent agglomeration, a number of studies were performed by modifying organic particles on polymer chains. For instance, Wang et al. [20] designed a series of PEMs prepared by embedding dopamine-modified SiO2 (DSiO2) nanoparticles into SPEEK. The amino groups at the surface of DSiO2 reacted with the sulfonic acid groups in SPEEK. This affected the SPEEK chain packing and inhibited the nanophase separation. When DSiO2 content reached 15 wt%, a proton conductivity up to 4.52  103 S cm1 was obtained at 120  C. However, the proton conductivity of the membranes was overall relatively low due to the interaction between the amino and the sulfonic acid groups. The SiO2 based materials are very attractive due to their relevant chemical and thermal properties, substantial specific area, and low cost. The present study was, thus, devoted to doping SiO2 into a polymer matrix through the sol-gel method. To prevent agglomeration and phase separation, the SiO2 was fixed on the polymer chains through cross-linking. Because of the large specific surface area of SiO2, the binding force of polymer for water was enhanced and water retention capacity improved, thereby increasing the proton conductivity at medium high temperatures. Moreover, the crosslinker that played the role of a “bridge”, strengthens attachment of the SiO2 into the branched chain of the polymer, hence improving the interaction between polymer backbone and inorganic components. Moreover, the crosslinked structure made the membranes more compact, improving its mechanical properties, and reducing the methanol permeability of the membranes. A series of organic-inorganic crosslinked hybrid membranes were prepared by the sol-gel method, including C-SPAEKS, KH550, and TEOS. The KH550 could react with the carboxyl groups and sulfonic acid groups of C-SPAEKS to attach the SiO2 to the main chains containing carboxyl groups, inducing a partial loss of sulfonic acid groups. This approach did not only induce the organicinorganic structure and crosslinked network structure but also helped in uniform dispersion of the inorganic particles in the polymer matrix. It is expected that the crosslinked hybrid membrane will have a higher proton conductivity at medium high temperatures.

127

4,40 -dichlorodiphenyl sulfone (SDCDPS) were synthesized according to the literature [21e23]. Bisphenol A (AR grade) was obtained from Tianjin Guangfu Chemical Reagent Company, China. 4,40 Difluorobenzophenone (DFB) (99% purity) was purchased from Yanbian Longjing Chemical Company. N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), toluene, and anhydrous potassium carbonate (AR grade) were obtained from Beijing Chemical Reagent Company. (3-Aminopropyl) triethoxysilane (KH550) and tetraethyl orthosilicate (TEOS) were supplied by Aladdin Chemistry Co. All the other reagents were used without further purification. 2.2. Synthesis of the C-SPAEKS A series of C-SPAEKS were synthesized as described in the literature and a schematic representation of the procedure was shown in Scheme 1 [21]. Typically, 4C-PH, SDCDPS, Bisphenol A and DFB were added using the nucleophilic substitution polycondensation method. The mole ratio of 4C-PH and Bisphenol A in C-SPAEKS were constant. However, the degree of sulfonation (DS), defined as the number of sulfonic groups repeating unit of polymer determined through the ratio between SDCDPS and DFB, was variable. The DS in C-SPAEKS-20, C-SPAEKS-40 and C-SPAEKS-60 were 20%, 40%, and 60%, respectively. The detailed procedure of CSPAEKS-40 synthesis was as follows: The reagents 4C-PH (1.38 g), Bisphenol A (5.472 g), SDCDPS (2.946 g), DFB (5.232 g), K2CO3 (5.175 g), toluene (15 mL) and tetramethylene sulfone (30.67 mL) were added to a 100 mL three-neck round-bottom flask equipped with a heating jacket, nitrogen inlet, mechanical stirrer, and reflux condenser. The mixture was then refluxed at 130  C for 4 h to remove the water by azeotropic distillation. After evaporation of toluene, the temperature of the mixture was raised to 180  C and maintained for 11 h. After completion of the reaction, the solution was cooled to 25  C and poured into deionized water. The obtained polymers were cut into pieces, washed several times with deionized water and dried at 60  C for 24 h. The monomer ratios of C-SPAEKS polymers are listed in Table 1. 2.3. Preparation of C-SPAEKS/K-SiO2 and C-SPAEKS/SiO2 hybrid membranes

2. Experimental

The C-SPAEKS was first dissolved in DMAc to form a 10 wt% solution, then KH550 and TEOS at 1:4 M ratio or TEOS alone were added to the solution. The pH value of the solution was adjusted to 4.0 by adding 0.1 M HCl solution. The mixture was then stirred for 12 h at room temperature, cast onto glass plates, and placed in a vacuum oven at different temperatures: 60  C for 12 h, 80  C for 12 h, and 120  C for 12 h. Afterward, the hybrid membranes were acidified with a 2.0 M HCl solution for 24 h, followed by several washing steps with deionized water to remove the excess acid. The thickness of the resulting membranes ranged from 60 mm to 70 mm and the SiO2 content in the mixture varied from 0, 4, 6, and 8 wt%. The crosslinked hybrid membranes were denoted as C-SPAEKS/K-SiO2-x (with KH550) and C-SPAEKS/SiO2-x (without KH550), where x represent the weight percentage of SiO2 in the C-SPAEKS. The reaction process of the C-SPAEKS/K-SiO2-6 crosslinked hybrid membrane was shown in Scheme 2. The dispersion of SiO2 in C-SPAEKS/K-SiO2 crosslinked hybrid membrane and C-SPAEKS/SiO2 hybrid membrane was depicted in Scheme 3. The specific proportion of C-SPAEKS/SiO2 and C-SPAEKS/K-SiO2 polymers was listed in Table 2.

2.1. Materials

2.4. Characterization

4-Carboxyphenyl hydroquinone (4C-PH) and 3,30 -disulfonated

Fourier transform infrared (FT-IR) absorption spectroscopy was

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Scheme 1. Synthesis of C-SPAEKS-40 polymers.

2.5. Measurements The water uptake and swelling ratio of the membranes were estimated according to Eqs. (1) and (2):

Water uptake ð%Þ ¼

Wwet  Wdry  100% Wdry

(1)

Swelling ratio ð%Þ ¼

Twet  Tdry  100% Tdry

(2)

The weights and lengths values (Wdry, Tdry) were measured 5 times using different parts of the same membrane after being dried in an oven at 80  C for 24 h. Typically, the films were immersed in deionized water at different temperatures for 24 h, then the films surfaces were rapidly dried using a filter paper and the Wwet, Twet values were repeatedly measured.

Table 1 Monomer ratios of C-SPAEKS. Sample

DS

4C-PH

Bisphenol A

SDCDPS

DFB

C-SPAEKS-20 C-SPAEKS-40 C-SPAEKS-60

20% 40% 60%

2 2 2

8 8 8

1 2 3

9 8 6

At a constant temperature, the water content of the membrane changed with time, which is defined by the water desorption coefficient. This was measured 3-times using different membranes analyzed by TGA at 80  C for 1 h [24]. The water desorption coefficient was calculated following eq. (3):

1 2  Mt Dt ¼4 M∞ Pl2 =

conducted using a Vector-22 spectrometer (Bruker, Germany). Samples were measured in the range 4000e300 cm1 for 128 times at a resolution of 4 cm1. Thermogravimetric analysis (TGA) was carried out using a Pyris 1 TGA (Perkin Elmer) instrument running under a nitrogen atmosphere heated from 40 to 600  C at a ramp rate of 10  C min1. Transmission electron microscopy (TEM) images were obtained with a JEM-1011 electron microscope. The membranes were stained by soaking in a 0.1 M silver nitrate solution at room temperature for 1 day, then rinsed with deionized water and dried overnight under vacuum at room temperature. The stained membranes were cut into small pieces and embedded in a Spurr's epoxy resin then cured overnight at 70  C. The samples were cut into slices of 100 nm thickness using a Leica EM UC6 ultramicrotome and placed on copper grids. The surface morphology and cross-section of the membranes were then investigated by scanning electron microscope (SEM) (Shimadzu SSX-550). The membranes were sprayed with a gold layer before SEM analysis.

(3)

where Mt/M∞ is the change in water weight with time, D is the water desorption coefficient of the membrane, and l is the thickness of the membrane. The ion exchange capacity (IEC) values of the membranes were determined by classical acid-base titration method. A dried membrane was soaked in 1.0 M NaCl solution for 48 h to undergo a complete exchange ion process of Hþ with Naþ. The resulting solutions were titrated with a 0.01 M NaOH solution using phenolphthalein as the indicator. The IEC values (mmol/g) of the membranes were measured according to Eq. (4):

IEC ¼

VM Wdry

(4)

where V is the volume of the NaOH solution used in titration, M is the concentration of NaOH solution, and Wdry is the mass of dried membranes. The mechanical properties of the membranes were measured using Instron 5965 equipment. The size of the membranes was 15  4 mm2 and the samples were measured using a programmed elongation rate of 1 mm min1. The sample was cut into rectangular shape with a size of 2  1 cm2 and then immersed in Fenton's reagent (3% H2O2 solution containing 4 ppm Fe2þ) at 80  C. After 1 h, the oxidative stabilities of the samples were evaluated from the weight change. The proton conductivity s (Scm1) was evaluated using a fourelectrode AC impedance spectrometer (Salton 1260) in the frequency range from 100 kHz to 0.1 Hz at 10 mV AC perturbation using a Princeton Applied Research Model 2273A Potentiostat. [25]. s can be obtained using Eq. (5):

s¼

L RS

(5)

H. Han et al. / Journal of Power Sources 340 (2017) 126e138

129

Scheme 2. Synthesis of C-SPAEKS/K-SiO2-6 membrane.

where s is the proton conductivity (S cm1), R is the resistance (U), S is the effective surface area (cm2) of the membrane, and L is the

distance (cm) between the electrodes. Before the test, each sample was cut to form small samples of 4  1 cm2 and to fully hydrate the

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constantly stirred during the testing period. The methanol concentrations in the water cell were determined using a Shimadzu GC-8A gas chromatograph. The methanol permeability coefficient of the membranes was evaluated at temperatures of 25  C and 60  C, using Eq. (6) [26]:

CB ðtÞ ¼

A DK C ðt  to Þ VB L A

(6)

where DK (cm2 s1) is the methanol diffusion coefficient, VB (mL) represents the volume of the permeated reservoirs, A (cm2) is the effective area, and L (cm) is the thickness of the membranes. CA and CB (mol m3) are the methanol concentrations in both the methanol and water reservoirs, respectively. The fuel cell performance of the prepared membranes was studied by using Fuel Cell Test Station (Green Light Company). The catalyst layer (40 wt % Pt/C, JM Co.) was transferred on both side of the membrane in order to form the catalyst coated membrane (CCM). The Pt loading in the anode and cathode were both 0.2 mg cm2. The Toray carbon paper composed of PTFE and carbon power (TGP-H-060, Toray) was used as a gas diffusion layer (GDL). The GDL was placed on the two sides of the CCM. The membrane electrode assembly (MEA) was fabricated by hot-pressed method. The active area was 25 cm2. The flow rate of H2 and O2 were 2 dm3 min1. H2 and O2 fuel cell testing was conducted at 100  C at 100% relative humidity. All measurements performed in this study were subject to statistical analysis, where each group of five data was averaged to obtain the mean and final value used for subsequent calculations. 3. Results and discussion 3.1. Structural characterization

Scheme 3. The dispersion of SiO2 in the C-SPAEKS/SiO2 hybrid membrane and CSPAEKS/K-SiO2 crosslinked hybrid membrane.

membranes, the samples were immersed in deionized water for 48 h. The methanol permeability was measured using a custom iron diffusion cell separated into two compartments by a membrane. One of them was filled with 190 mL deionized water and the other with 190 mL methanol solution (10 M). Both liquids were

The FT-IR spectrum of the C-SPAEKS-40 membrane is shown in Fig. 1. The broad bond located at 3443.78 cm1 was assigned to the eOH stretching vibration absorption. The characteristic sharp bands recorded at 1024.45 and 1079.65 cm1 were assigned to the O]S]O asymmetric and symmetric vibrations of the sulfonic acid groups. The C-SPAEKS/SiO2 hybrid membrane displayed a novel characteristic peak at 427.61 cm1 due to the SieOH stretching vibration. The shift in the peak position of the C-SPAEKS/SiO2 membrane at 3413.78 cm1 was related to the presence of eOH of the SiO2 surface, demonstrating the successful preparation of C-SPAEKS/SiO2 membranes. The novel characteristic peak recorded at 1724.37 cm1 gave further evidence of reaction between the carboxyl groups of CSPAEKS-40 and amino groups of KH550, which produced a new functional group eNHeCOe in the C-SPAEKS/K-SiO2 membranes. Comparison with the C-SPEKS-40 membrane also revealed the presence of two new bands at 459.00 and 854.19 cm1, corresponding to SieOeSi bending and SieOH stretching vibrations. The change in the peak position of the C-SPAEKS/K-SiO2 membrane at 3413.78 cm1, 1029.87 and 1080.30 cm1 might be related to the covalent interaction between carboxyl groups and amino groups, as

Table 2 The DS and a specific proportion of C-SPAEKS/SiO2 and C-SPAEKS/K-SiO2 polymers. Sample

DS

C-SPAEKS (g)

KH550 (mL)

TEOS (mL)

0.1 M HCl (mL)

C-SPAEKS/SiO2-4 C-SPAEKS/SiO2-6 C-SPAEKS/SiO2-8 C-SPAEKS/K-SiO2-4 C-SPAEKS/K-SiO2-6 C-SPAEKS/K-SiO2-8

20 40 60 20 40 60

1.057 1.057 1.057 1.057 1.057 1.057

e e e 0.048 0.070 0.094

0.180 0.268 0.356 0.180 0.268 0.356

0.32 0.25 0.22 0.32 0.25 0.22

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131

Fig. 1. FT-IR spectrum of C-SPAEKS-40, C-SPAEKS/SiO2, and C-SPAEKS/K-SiO2 membranes.

well as the ionic interaction between sulfonic acid groups and amino groups. These findings suggest the successful synthesis of the CSPAEKS/K-SiO2 membranes. 3.2. Thermal stability TGA was employed to evaluate the thermal stability of the prepared membranes and typical TGA curves of the C-SPAEKS-40, C-SPAEKS/SiO2, and C-SPAEKS/K-SiO2 are shown in Fig. 2. It will be noted that both hybrid membranes exhibited similar trends with three decomposition stages. The first weight loss recorded at around 90e200  C was caused by the loss of water and solvent molecules because both hybrid membranes could store more water due to the hygroscopic effect of SiO2 [27]. Table 3 showed that the temperature leading to a 5% weight loss (Td5%) of membranes [28] was significantly higher for the C-SPAEKS/K-SiO2 membrane than that of C-SPAEKS-40. This was attributed the crosslinked structure, which increased the limitation of the rigid polymer backbones and effectively improved the thermal stability of the crosslinked hybrid membranes [29]. The second weight loss starts at around 260  C could be assigned to the degradation of the sulfonic acid groups. The weight loss observed at 470  C was related to the decomposition of the main chain forming the membranes. However, a novel weight loss was observed in the crosslinked hybrid membranes at around 340  C that might be attributed to: i) decomposition of the alkyl segment of KH550, or/and ii) incomplete condensation of the Si-OH that induced further dehydration condensation at higher

temperatures [30]. The weight residues of both the hybrid membranes were estimated to more than 63% of the original weight at 600  C, which is higher than that of the C-SPAEKS-40 membrane. Hence, it might be concluded that the thermal stability of the hybrid membranes was improved due to crosslinked structure and presence of the inorganic phase.

Fig. 2. TGA curves of C-SPAEKS-40, C-SPAEKS/SiO2-6, and C-SPAEKS/K-SiO2-6 membranes.

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Table 3 The properties of Nafion® 117, C-SPAEKS-40, C-SPAEKS/SiO2and C-SPAEKS/K-SiO2 membranes. Membrane

®

Nafion 117 C-SPAEKS-40 C-SPAEKS/SiO2-4 C-SPAEKS/SiO2-6 C-SPAEKS/SiO2-8 C-SPAEKS/K-SiO2-4 C-SPAEKS/K-SiO2-6 C-SPAEKS/K-SiO2-8

l (mm)

177.8 62 65 64 63 67 66 65

Td5% (οC)

e 160.26 183.06 250.2 251.4 241.72 244.12 294.48

Water uptake (%)

Swelling ratio (%)

25οC

80οC

25οC

80οC

18.53 19.43 18.02 16.48 15.53 17.66 15.98 14.72

29.20 31.18 28.68 26.38 25.15 27.44 26.20 25.05

14.85 9.91 9.15 8.56 8.32 9.10 8.47 8.18

22.70 7.53 7.01 6.42 5.86 6.87 6.33 5.67

IEC (mmol/g)

water desorption coefficient (1011 cm2/s)

0.90 0.88 0.84 1.03 1.38 0.86 1.12 1.43

e 10.60 8.35 7.64 3.12 7.92 3.70 2.46

Td5%: The temperature which is leading to a 5% weight loss of membranes.

3.3. IEC, water uptake, and swelling ratio The IEC is a critical parameter in membranes performance, which is responsible for protons conduction and provides a reliable approximation of proton conductivity. IEC is generally defined as the number of moles of fixed hydrophilic functional groups per gram of polymer. The IEC values of the membranes are listed in Table 3. The IEC of the crosslinked hybrid membranes increased from 0.86 to 1.43 mmol/g as the silica ratios rose from 4 to 8 wt%. Thus, these membranes exhibited improved IEC when compared to C-SPAEKS-40. The elevated recorded values of IEC were due to the increased DS and presence of more hydrophilic Si-OH functional groups in the crosslinked hybrid membranes. The latter was expected to enhance the water uptake and proton conductivity of the membranes. It is well known that the water uptake of sulfonated polymers has a significant effect on the mechanical properties and proton conductivities [31]. Because water is considered as a transmission medium of protons, elevated water retention capacity yields higher proton conductivities [32]. However, too high water uptake could result in deterioration of the membrane-catalyst interface and possible performance failure of fuel cells [26]. For PEM materials, water uptake may be rationalized into: bulk water and bound water. Bulk water occupies the space in the membrane matrix and provides better connectivity for proton transfer. Bound water, an indicator of the water retention capacity [33], is responsible for the formation of hydrophilic clusters that facilitate the proton transfer pathway. The water uptake of a series of membranes with different SiO2 contents is illustrated in Table 3. The increase in temperature raised the water uptake of all membranes. The water uptake of the CSPAEKS-40 membrane was 19.43% at 25 οC and 31.18% at 80 οC, respectively. However, both hybrid membranes exhibited lower uptake than C-SPAEKS-40, which declined as the SiO2 contents rose. The water uptake of C-SPAEKS/K-SiO2-8 was recorded as 14.72%, which was lower than that of C-SPAEKS/K-SiO2-4 (17.66%) at 25 οC. This might be explained by: i) decreased percentage of eSO3H membranes as content of SiO2 particles increased, and ii) the SiO2 particles reduced the size of channels for water and proton transportation [2]. Also, it was noticed that the water uptake of CSPAEKS/K-SiO2 membranes was inferior to those of C-SPAEKS/SiO2 membranes. It is well-known that crosslinked structures hindered chains mobility and increase interaction of polymers, which reduces the free volume available to hold water molecules. In general, moderate swelling yields larger available space for continuous proton transfer through the membranes [33]. A trend showing a decrease in the swelling ratio as SiO2 loading increased from 0 to 8 wt% at a different temperature is depicted in Table 3. A similar trend was observed for the water uptake. The swelling ratio of C-SPAEKS/K-

SiO2-8 was recorded as 8.18%, which is lower than 9.91% obtained with C-SPAEKS-40 at 25 οC. The highest swelling ratio of all hybrid membranes at 80 οC was registered as 7.01%, which is also lower than that of Nafion® 117 (22.70%). For the same content of SiO2, the swelling ratio of C-SPAEKS/K-SiO2 crosslinked hybrid membranes was slightly lower than that of the C-SPAEKS/SiO2 hybrid membrane. The introduction of the crosslinked structure into the polymer matrix seemed to limit the swelling volume and increased the dimensional stability. These findings indicated that addition of inorganic fillers and crosslinked structures were favorable for the membrane size stability [20]. 3.4. Water retention capacity The water retention capacity is important to membrane because water plays an essential role in high proton conductivity. The water retention capacity was obtained by measuring the water desorption coefficient of the membranes using TGA at 80  C for 1 h. Afterward, the water desorption coefficient of the membranes was estimated using the Fick's diffusion law [25]. Fig. 3 and Table 3 depict the results of water diffusion coefficients of the membranes. The introducing of SiO2 brought better water retention. At the same silica content, the water desorption coefficient of C-SPAEKS/K-SiO2 was lower than C-SPAEKS/SiO2. Therefore, the water retention capacity of both the hybrid membranes was significantly improved if compared to C-SPAEKS-40 (Fig. 3). The hygroscopic nature of silica accommodated water molecules in the void spaces of membrane matrix and played an important role in proton transfer [33].

Fig. 3. The water diffusion coefficient of C-SPAEKS-40, C-SPAEKS/SiO2, and C-SPAEKS/ K-SiO2 membranes.

H. Han et al. / Journal of Power Sources 340 (2017) 126e138

3.5. Morphology TEM was used to investigate the morphology of the membranes and the results of C-SPAEKS/K-SiO2 at different percent loadings are shown in Fig. 4. It could be seen that silica particles were dispersed uniformly throughout the matrix at both lower and higher filler concentrations. The average size of the silica particles was measured as 20 nm. The covalent and ionic bonding present in SiO2 particles, as well as the functionalized polymers C-SPAEKS-40, promoted a better distribution of the particles and improved the compatibility between the organic and inorganic phases [34]. The surface section morphologies of both C-SPAEKS-40 and C-SPAEKS/ K-SiO2 crosslinked hybrid membranes were depicted in Fig. 5 (I). It is clear that the surface of the C-SPAEKS-40 membrane was smoother than those of the crosslinked hybrid membranes. The miscibility of the polymer and filler was checked by recording the homogeneity of the membranes. The SiO2 particles were evenly dispersed on the crosslinked hybrid membranes without phase separation and formation of cracks [35]. The latter indicated that the organic and inorganic components were blended at the molecular level. The average size of the silica particles was also recorded as 20 nm. The influence of the introduced crosslinker into the hybrid membranes was studied through cross-section SEM morphology measurements and the results of C-SPAEKS/SiO2 and C-SPAEKS/KSiO2 membranes at the different silica contents are presented in Fig. 5 (II). At moderate silica contents, the particles were very well dispersed in the C-SPAEKS/SiO2 membranes (Fig. 5 (II) a-c). However, many aggregates formed on the membranes at elevated SiO2 contents but the dispersion of C-SPAEKS/K-SiO2 seemed quite regular (Fig. 5 (II) d-e). The silica was attached to the polymer

133

matrix through the dual role of covalent and ionic bonding between the polymer and KH550. As a result, there is no apparent aggregation even at increased silica contents. 3.6. Mechanical properties For fuel cells application, it is necessary for PEMs to possess adequate mechanical properties to withstand the membrane electrode assembly [36,37]. The crosslinked network structure affected the mechanical properties of the membranes, which is reflected in their stress-strain behaviors (Fig. 6). The C-SPAEKS/SiO2 membranes displayed excellent mechanical properties with a tensile strength of ~31 MPa, along with an elongation at break ranged from 2% to 4%. Compared to C-SPAEKS/SiO2 membranes, the maximum tensile strength of the C-SPAEKS/K-SiO2 hybrid membranes increased to 49 MPa and elongation at break was 11%. The results indicated that the crosslinked network reduced the free volume in the system and increased forces between the polymers, thus improving the mechanical properties of the membranes [38,39]. The young's modulus of C-SPAEKS/K-SiO2 crosslinked hybrid membranes with 4, 6 and 8 wt% SiO2 were recorded as 1647.72 MPs, 1775.13 MPa, and 1144.64 MPa, respectively (Table 4). Meanwhile, the respective tensile strength of the crosslinked hybrid membranes were 53.64 MPa, 53.68 MPa, and 49.75 MPa. The improvement in both parameters, young's modulus, and tensile strength, was structurally originated from both the fine dispersion of SiO2 in the matrix and the strong interface between the organic and inorganic phases [35]. The presence of silica particles limited the motion of the polymer chains, leading to higher young's modulus and tensile strength values. The additional reinforcing effect of the

Fig. 4. TEM images of: (a) C-SPAEKS/K-SiO2-4, (b) C-SPAEKS/K-SiO2-6, and (c) C-SPAEKS/K-SiO2-8 membranes.

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Fig. 5. SEM images (I) surface morphologies of: (a) C-SPAEKS-40, (b) C-SPAEKS/K-SiO2-4, (c) C-SPAEKS/K-SiO2-6 and (d) C-SPAEKS/K-SiO2-8 membranes; (II) cross-section images of: (a) C-SPAEKS/SiO2-4, (b) C-SPAEKS/SiO2-6, (c) C-SPAEKS/SiO2-8, (d) C-SPAEKS/K-SiO2-4, (e) C-SPAEKS/K-SiO2-6 and (f) C-SPAEKS/K-SiO2-8.

crosslinked hybrid membranes might result from the interracial interaction between the polymer chains and silica particles [40]. The high surface area of silica particles creating an elevated interface with the polymer matrix was also obtained [41]. At the highest concentration, the excessive silica content weakened young's modulus and tensile strength, of the crosslinked hybrid membranes. This could be related to the microstructure of crosslinked

hybrid membranes that started to change due to aggregation of the silica particles, hence resulting in the decline of mechanical properties [42]. The elongation at break parameter also decreased as silica content rose in both hybrid membranes. A slight reduction in the elongation may be induced by the increased silica content in the crosslinked hybrid membranes. In other words, higher crosslinking is produced by higher silica loading. Therefore, it will

H. Han et al. / Journal of Power Sources 340 (2017) 126e138

135

content changed from 4 wt% to 8 wt%, the oxidative stability of the crosslinked hybrid membranes increased from 93.7% to 96.2%. These values were higher than those obtained with hybrid membranes (changed from 92.8% to 95.6%). The latter was caused by the crosslinked structure in the membranes, which makes the polymer chains less likely to be attacked by free radicals. Moreover, the oxidative stability of C-SPAEKS/K-SiO2-8 was determined as 96.2%, which is higher than those of all the other membranes. The excellent oxidative stability of C-SPAEKS/K-SiO2-8 membranes was associated with not only the crosslinked structure but also by the formed organic-inorganic hybrid structure. Because of the presence of SiO2, the radicals produced from Fenton's reagent could not penetrate through the SiO2 particles to attack the polymer chains [48,49].

3.8. Proton conductivity Fig. 6. Stress versus strain curves for dry C-SPAEKS-40, C-SPAEKS/SiO2, and C-SPAEKS/ K-SiO2 membranes at 25  C.

increment the rigidity and reduce the flexibility of membranes [42e45]. 3.7. Chemical stability The oxidative stability is one of the important factors for assessing the lifetime of PEMs under the stringent fuel cell conditions [46]. The Fenton's reaction is a well-known process for evaluating the radical oxidative stability of PEMs. The formation of free radicals, such as hydroxyl and hydroperoxy, occur during the redox processes, which eventually attack the proton exchange membranes to result in continuous degradation of the polymer chains [33]. The oxidative stability of the membranes was determined by measuring changes in their weights after using the Fenton's reagent at 80  C for 1 h and the results are shown in Table 5 [47]. As the SiO2

The proton conductivity is one of the key properties for predicting PEM suitability [50]. To test the proton conduction, all membranes were washed and fully hydrated with deionized water prior to measurements [51]. Fig. 7 shows the Arrhenius plot of conductivity vs. temperature of the hybrid membranes at different temperatures. In general, there are two primary conductive mechanisms describing the proton diffusion through the membranes: Vehicular mechanism and Grotthuss mechanism [29]. It was suggested that free water could participate via the Vehicular mechanism and the involvement of bound water takes part through Grotthuss mechanism [52e56]. In Vehicular mechanism, Hþ combines with water molecules to form hydrophilic regions þ where ions such as H3Oþ, H5Oþ 2 , and H9O4 are transported between the clustered sulfonic acid groups [54]. For Grotthuss mechanism, the proton Hþ in the form of H3Oþ ion jumps to the neighboring lone electron pair of the water molecule [55]. The measurements indicated that the proton conductivity of CSPAEKS/K-SiO2-8 was 0.110 S cm1, which was higher than that of C-SPAEKS/SiO2-8 (0.082 S cm1) and Nafion® 117 (0.028 S cm1) at 120  C (Table 5). The introduction of SiO2 into the hybrid

Table 4 The mechanical properties of C-SPAEKS-40, C-SPAEKS/SiO2, and C-SPAEKS/K-SiO2 membranes. Membrane

Young's modulus (MPa)

C-SPAEKS-40 C-SPAEKS/SiO2-4 C-SPAEKS/SiO2-6 C-SPAEKS/SiO2-8 C-SPAEKS/K-SiO2-4 C-SPAEKS/K-SiO2-6 C-SPAEKS/K-SiO2-8

1487.78 1490.54 1872.87 1520.29 1647.72 1775.13 1144.64

± ± ± ± ± ± ±

Tensile strength (MPa)

52.08 52.16 56.18 45.60 44.48 46.15 32.05

39.39 39.46 31.72 41.04 53.64 53.68 49.75

± ± ± ± ± ± ±

Elongation at break (%) 2.75 ± 0.06 3.63 ± 0.09 2.81 ± 0.06 2.29 ± 0.04 10.15 ± 0.23 8.34 ± 0.18 7.18 ± 0.16

0.85 0.98 1.59 1.72 2.41 2.43 1.95

Table 5 Proton conductivity, methanol permeability coefficient, oxidative stability, and Ea of membranes. Membrane

®

Nafion 117 C-SPAEKS-40 C-SPAEKS/SiO2-4 C-SPAEKS/SiO2-6 C-SPAEKS/SiO2-8 C-SPAEKS/K-SiO2-4 C-SPAEKS/K-SiO2-6 C-SPAEKS/K-SiO2-8

Proton conductivity (S cm1)

Methanol permeability coefficient (107 cm2 s1)

Ea (KJ mol1)

Oxidative stability (%)

25  C

80  C

120  C

25  C

60  C

25e80  C

80e120  C

0.081 0.021 0.026 0.036 0.046 0.030 0.038 0.047

0.053 0.072 0.085 0.109 0.131 0.093 0.110 0.132

0.028 0.043 0.052 0.067 0.082 0.061 0.085 0.110

29.4 5.63 4.57 4.53 3.94 4.55 4.52 3.86

e 9.52 7.82 7.75 6.71 7.78 7.74 6.69

e 19.40 18.89 18.24 17.29 18.62 17.54 16.37

e 15.15 14.35 14.21 13.80 12.15 7.41 5.18

99.0 92.3 92.8 94.4 95.6 93.7 95.4 96.2

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H. Han et al. / Journal of Power Sources 340 (2017) 126e138

Fig. 7. Arrhenius plot of proton conductivity vs. temperature of (a) C-SPAEKS/SiO2 and (b) C-SPAEKS/K-SiO2 membranes.

membranes formed the crosslinked structure, which induced a decrease of free volume in the network. This, in turn, reduced the free water in the membrane. However, crosslinked structures held more bound water, compensating for some loss of free water. The bound water facilitated the proton transport through Grotthus mechanism, which contributed to elevate the proton conductivity [42]. The hydrophilic SiO2 particles improved the proton migration by boosting the number of the hydrophilic cluster channels within the crosslinked hybrid membranes [18]. Moreover, silica particles have the ability in retaining water even at a medium high temperature as reported elsewhere [57]. Therefore, the proton conductivity of the crosslinked hybrid membranes improved as SiO2 content became substantial at medium high temperatures [58]. The activation energy (Ea) is defined as the minimum energy required for the proton conduction across the PEM. Ea is obtained from the slope at the linear fit of Arrhenius formula described by Eq. (7) [42].

Ea ¼ b  R

(7)

where R is the gas constant (8.314 J K1 mol1) and b is the slope of 1 1 the linear regression of lnm k (S cm ) vs. 1000/T (K ) plots. When the SiO2 content reached 8 wt%, the Ea of C-SPAEKS/SiO28 was measured as 13.80 kJ mol1 and that of C-SPAEKS/K-SiO2-8 was 5.18 kJ mol1 at medium high temperatures (Table 5). Ea declined as the SiO2 content increased.

was lower than C-SPAEKS/SiO2 under the same silica content. Moreover, the methanol permeability coefficient of C-SPAEKS/KSiO2-8 was measured as 6.69  107 cm2 s1, which is inferior to that of C-SPAEKS/SiO2-8 (6.71  107 cm2 s1) at 60  C. This was probably caused by the methanol transport channels that underwent considerable narrowing due the silica particles. The SiO2 particles acted as a “barrier” to hinder the permeation of methanol. Moreover, the crosslinked structure formed in C-SPAEKS/K-SiO2 membranes blocked the penetration of methanol. This trend was similar to that of water uptake [60]. 3.10. Fuel cell performance Fig. 8 shows the polarization curves and power density of single cells assembled with C-SPAEKS-40, C-SPAEKS/SiO2-8, and CSPAEKS/K-SiO2-8 membranes under 100% RH condition at 100  C. The peak power density of the fuel cell based on C-SPAEKS-40, CSPAEKS/SiO2-8 and C-SPAEKS/K-SiO2-8 membranes was 339, 380 and 478 mW cm2, respectively. And the open circuit voltage (OCV) of these membranes was 0.89, 0.90 and 0.92 V, respectively. The single cells performance of the crosslinked hybrid membranes was superior to the C-SPAEKS-40 and hybrid membranes, which agrees

3.9. Methanol permeability The methanol crossover is a key point affecting the fuel cell efficiency and represents membrane selectivity [59]. The methanol permeability coefficients of the membranes were measured at 25  C and 60  C, and the values are displayed in Table 5. It will be noted that the methanol permeability coefficients of the crosslinked hybrid membranes were lower than the values obtained with Nafion® (29.4  107 cm2 s1) and C-SPAEKS-40 (5.63  107 cm2 s1) at 25  C. The methanol permeability coefficients of C-SPAEKS/SiO2-4, C-SPAEKS/SiO2-6 and C-SPAEKS/ SiO2-8 were 4.57  107 cm2 s1, 4.53  107 cm2 s1 and 3.94  107 cm2 s1, respectively at 25  C. With the corresponding, the values of C-SPAEKS/K-SiO2-4, C-SPAEKS/K-SiO2-6 and CSPAEKS/K-SiO2-8 were 4.55  107 cm2 s1, 4.52  107 cm2 s1 and 3.86  107 cm2 s1, respectively. The value of C-SPAEKS/K-SiO2

Fig. 8. Polarization curves and power density of the membranes at 100  C under 100% RH condition.

H. Han et al. / Journal of Power Sources 340 (2017) 126e138

well with the proton conductivity results. The relatively elevated proton conductivities obtained with the crosslinked hybrid membranes in comparison with those of C-SPAEKS-40 and hybrid membranes were related to SiO2 and the crosslinked structure. For organic-inorganic hybrid membranes, the hydroxyl groups of the oxides improved the proton conductivity of the membrane at higher temperatures according to the Grotthuss mechanism. In wet conditions, a membrane contains water molecules which effectively promote the transfer of protons. The eOH groups of SiO2 and water molecules encapsulated by the crosslinked structure created a path for proton transport in the polymer matrix [61]. Compared to the C-SPAEKS membranes, the introduction of SiO2 and KH550 improved the fuel cell performance obviously. 4. Conclusions A series of novel organic-inorganic crosslinked hybrid proton exchange membranes were successfully synthesized by sol-gel method using C-SPAEK, KH550, and TEOS. The crosslinked structure and the inorganic phase were introduced into the same membrane yielding the combining effects of both components without macroscopic phase separation. The crosslinked hybrid membranes exhibited excellent mechanical properties and dimensional stability due to the crosslinked structure. The proton conductivity increased by introducing inorganic phase. Overall, these findings indicated that these crosslinked hybrid membranes hold promise for application in medium high temperatures in proton exchange membrane fuel cells.

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Acknowledgments [22]

The authors would like to thank the National Natural Science Foundation of China (Grant No. 51273024, Grant No. 51303015 and Grant No. 51673030), Jilin Province Development and Reform Commission (Grant No. 2014Y129), Jilin Provincial Science & Technology Department (Grant No. 20160519020JH and Grant No. 20160520138JH) and Education Department of Jilin Province (Grant no. 2016330) for financial support.

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Nomenclature DS: Degree of sulfonic groups repeating unit of polymer Wdry: Weights of dried membranes (g) Wwet: Weights of wet membranes (g) Twet: Thickness of wet membranes (cm) Tdry: Thickness of dry membranes (cm) l: Thickness of membrane (cm) D: Water desorption coefficient of membrane L: Distance between the electrodes (cm) S: Effective surface area of membrane (cm2) R: Membrane resistance (U) CA: Methanol concentration in the methanol reservoir (mol m3) CB: Methanol concentration in the water reservoir (mol m3) DK: Methanol diffusion coefficient (cm2 s1) L: Effective thickness of membrane (cm) A: Effective thickness of membrane (cm2) VB: Volume of permeated reservoirs (mL) t: time (s) T: Temperature (K) Greek symbols

s: Proton conductivity (S cm1)