zwitterion functionalized titania–silica hybrid membranes with improved proton conductivity

zwitterion functionalized titania–silica hybrid membranes with improved proton conductivity

Journal of Membrane Science 469 (2014) 355–363 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

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Journal of Membrane Science 469 (2014) 355–363

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Fabrication of chitosan/zwitterion functionalized titania–silica hybrid membranes with improved proton conductivity Yongheng Yin a,b, Tao Xu a,b, Xiaohui Shen a,b, Hong Wu a,b,c,n, Zhongyi Jiang a,b a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 18 March 2014 Received in revised form 21 June 2014 Accepted 1 July 2014 Available online 8 July 2014

Based on the discovery that acid–base pairs can construct efficient proton conduction channels, a binary titania–silica inorganic dopant functionalized with carboxyl groups and amino groups (denoted as TiC– SiN) is introduced into chitosan (CS) to fabricate novel hybrid membranes with proton-conducting and methanol-rejecting properties. The titania precursor and the silica precursor are prehydrolyzed, and the mixed sol is functionalized with –COOH and –NH2 groups successively by the facile chelation method to obtain TiC–SiN sol. The hybrid membranes are prepared by mixing the TiC–SiN sol with CS followed by in situ so–gel process. The membranes are characterized in terms of thermal property, water uptake, proton conductivity, etc. The results show that incorporation of hygroscopic inorganic phase increases the water uptake ability of the membranes. Moreover, the zwitterionic groups provided by the TiC–SiN dopants construct new proton pathways, which can enhance the proton conductivity of the membranes. Particularly, incorporating 7 wt% TiC–SiN affords the hybrid membrane a proton conductivity of 0.0408 S cm  1 at room temperature, which is 4 times higher than that of the pure CS membrane. Overall, the highest selectivity of the hybrid membranes is 4.85  104 S s cm  3, which is nearly 3 times higher than that of the pure CS membrane. & 2014 Elsevier B.V. All rights reserved.

Keywords: Binary titania–silica in situ sol–gel Hybrid membrane Zwitterion Proton conductivity

1. Introduction Direct methanol fuel cells (DMFCs) have been attracting considerable interests as a novel kind of portable power source owing to its compactness, high energy density, and convenient recharge of fuels [1,2]. Proton exchange membranes (PEM), which controls the cost and durability of DMFCs, is the crucial part in DMFCs. The proton conductivity of PEMs is one of the key factors limiting the performance of DMFCs [3,4]. Currently, Nafion-based membranes are widely used as PEMs in fuel cells operating from 60 to 80 1C and show good proton conductivity in a humid environment owing to the continuous, ordered nanochannels [5,6]. However, many limitations still exist with these membranes, such as high methanol permeability, a tendency to disintegrate in the presence of hydroxyl radicals (an intermediate in the cathode reaction), high cost and so on. Such drawbacks significantly lower the fuel efficiency and cell performance, thus impede the commercialization of Nafion-based

n Corresponding author at: School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. Tel./fax: þ 86 22 23500086. E-mail address: [email protected] (H. Wu).

http://dx.doi.org/10.1016/j.memsci.2014.07.003 0376-7388/& 2014 Elsevier B.V. All rights reserved.

PEMs. Moreover, under high temperature and low humidity conditions, the proton conductivity of Nafion drops dramatically due to the severe water loss and nanochannel deformation [7]. So the development of PEMs with affordable proton conductivity in broad applications remains a great challenge. In recent years, the proton-transfer mechanism of acid–base pairs has been extensively studied [8,9]. It was reported that the acid–base pairs can promote the proton migration via Grotthuss mechanism in three ways [10,11]: (i) tight acid–base complexes, in this manner, the proton donors and acceptors are linked directly and provide an ultrafast proton transfer; (ii) loose complexes, where the proton donors and acceptors are linked by water bridges. Here, the proton is dissociated from acid, diffused by the water bridge and then trapped by base to complete the protolysis; (iii) encounter pairs, formed by acid and base molecules separated remotely, this way usually happens at a low base concentration. Inspired by these findings, many researchers used fillers with zwitterionic groups to improve the proton conductivity of membranes [12,13]. Leem et al. [14] impregnated amino acid-functionalized silica into porous polyethylene terephthalate to fabricate hybrid membranes, the carboxyl groups and amino groups in the pores act as a nanowire for proton conduction. Gu et al. [15] introduced zwitterion-coated dendrimer

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into polybenzimidazole to fabricate membranes with acid–base pairs, which is in favor of proton transfer by hydrogen bonds forming between dendrimer and water molecules. Wang et al. [16] also found that the microcapsules bearing both carboxylic acid and imidazole show better proton conduction ability than those bearing only acidic or basic groups in sulfonated poly(ether ether ketone), the prepared membrane with zwitterionic groups exhibit a good proton conductivity of 0.034 S cm  1 at 40 1C. The hybrid membranes are usually fabricated by either mixing inorganic fillers with polymer solutions directly, or by in situ formation of inorganic phase using a sol–gel method. In particular, the process via sol–gel chemistry can adjust the bulk properties of membranes at a molecular level. This technique produces particles with well controlled size by tuning preparation variables such as concentration of precursor, pH of mixture, reaction temperature, etc, and the prepared hybrid membranes are highly homogeneous [17–19]. Inorganic particles such as ZrO2 [20], SiO2 [21,22], TiO2 [18,23], CeO2 [24] and so on have already been introduced into different matrixes through an in situ sol–gel method. The proton conductivity of these hybrid membranes is improved mainly due to the water sorption capacity of the inorganic phase. Some literatures also focus on synthesizing inorganic dopant with sulfonate groups, phosphate groups, imidazole groups or zwitterionic groups through the in situ sol–gel process to enhance the proton conductivity [17,25– 29]. However, the binary inorganic dopant with tunable mole ratio of acidic groups to basic groups has not been synthesized and used in hybrid membranes up to date. The major objective of this study is to incorporate zwitterion functionalized titania–silica (TiC–SiN) into chitosan through a facile in situ sol–gel method. This method endows the hybrid membranes with homogeneous structure and desirable proton conductivity. The mole ratio of acidic groups to basic groups and the content of inorganic dopant are adjusted to investigate the influence of acid–base pairs on membrane properties. The proton migration mechanism in the hybrid membranes is discussed.

2. Experiment 2.1. Materials and chemicals Chitosan (CS), with a deacetylation degree of 91%, was purchased from Golden-Shell Biochemical Co., Ltd. (Zhejiang, China). Tetraethylorthosilicate (TEOS), 3-(3,4-dihydroxyphenyl) propionic acid and 3-aminopropyltrimethoxysilane (APTMS) were purchased from Alfa Aesar. Titanium tetrachloride (TiCl4), sulfuric acid (H2SO4), hydrochloric acid, acetic acid and ethanol were purchased from Guangfu Fine Chemical Research Institute (Tianjin, China). All the reagents were used as received without any further purification. Water used in all the experiments was deionized water. 2.2. Synthesis of TiC–SiN sol The TiC–SiN sol with zwitterionic groups was prepared by a consecutive chelation of carboxyl groups and amino groups as shown in Fig. 1(a). First, the silica sol was obtained by prehydrolyzing TEOS in 2 M HCl solution at 60 1C for 2 h. Simultaneously, the titania sol was prepared by mixing TiCl4 with water–ethanol solution (v:v¼ 1:2) at room temperature. Then the silica sol and titania sol were mixed at various ratios, followed by stirring at room temperature for 3 h to get the pure titania–silica sol. Next, as a carboxylic group generator, 3-(3,4-dihydroxyphenyl) propionic acid was added to the pure titania–silica sol and the chelation took 1 h. Then desired amounts of APTMS were added into the above solution, followed by stirring at 60 1C for 5 h to get the sol aminofunctionalized. Finally the maroon functionalized TiC–SiN sol with

different mole ratio of acidic –COOH groups to basic –NH2 groups was obtained.

2.3. Preparation of hybrid membranes The process of preparing hybrid membranes is depicted in Fig. 1(b). 1.0 g CS was dissolved in 35 g aqueous acetic acid solution (2.0 wt%) under constant stirring at 80 1C for 2 h. Then premeasured amounts of TiC–SiN sol were mixed with CS solution and the resultant mixture was under ultrasonic treatment for 15 min to ensure complete mixing. After degassing by filtration and standing, the mixture was cast onto a clean glass plate and dried at room temperature for 36 h to obtain the hybrid membrane. The membrane was immersed in 2 M H2SO4 solution for 24 h to accomplish the cross-linking, then it was washed thoroughly with water to remove the residual H2SO4 and dried under vacuum at 25 1C for 24 h. The as-prepared hybrid membranes were designated as CS/XTiC–YSiN-Z, where X:Y represents the mole ratio of TiCl4 to TEOS and Z is the weight ratio of inorganic titania–silica dopant to CS. As a comparison, pure CS membrane was also prepared using the above procedure without the addition of TiC– SiN sol. The thickness of all the membranes was in the range of 110–120 μm.

2.4. Characterizations The cross-sectional morphologies of the membranes were observed using field emission scanning electron microscope (FESEM, Nanosem 430) operated at 10 kV. The cross-sections were prepared by freeze-fracturing samples in liquid nitrogen and then coated with a thin layer of sputtered gold. Fourier transform infrared spectra (FTIR) of the sol and membranes were recorded on a spectrometer (Nicolet MAGNA-IR 560) in the wavenumber range of 4000–400 cm  1. The crystalline structures of the membranes were characterized with an X-ray diffractometer (XRD, RigakuD/max2500v/Pc) using Cu Kα radiation. The scanning angle ranged from 21 to 551 with a scanning rate of 21 min  1. The thermal stability of the membranes was characterized by a thermogravimetric analyzer (TGA, Perkin-Elmer Pyris) over the temperature range of 20–800 1C at a heating rate of 10 1C min  1. Each sample was vacuum-dried at 100 1C for 24 h prior to the measurement and measured under nitrogen flow.

2.5. Water uptake and swelling degree To investigate the swollen property of the membranes, their water uptakes and swelling degrees were measured. Prior to measurement, each membrane ( 4  4 cm2) was dried at 60 1C for 24 h and then weighed (Wdry, g) and measured (Adry, cm2). The dried sample was immersed in water at 25, 40, 60 and 80 1C for 24 h, after the surface water on the fully swollen membrane was removed, the membrane was immediately weighed (Wwet, g) and re-measured (Awet, cm2). The above measurement was repeated three times and the water uptake and swelling degree were calculated by Eqs. (1) and (2), respectively. The standard deviation was within 74.0%. Water uptake ð%Þ ¼

W wet  W dry  100 W dry

ð1Þ

Awet Adry  100 Adry

ð2Þ

Swelling degree ð%Þ ¼

Y. Yin et al. / Journal of Membrane Science 469 (2014) 355–363

TiCl4

CS

HAc (2.0 wt%)

357

TEOS

HCl (2M)

H2O : EtOH (v:v=1:2)

Transparent SiO2 sol

Transparent TiO2 sol

CS solution

Mixing Modified Mixing Casting membrane Cross-linking by H2SO4 CS/TiC-SiN hybrid membrane

Fig. 1. Illustration for synthetic process of TiC–SiN sol (a) and preparation process of hybrid membranes (b).

2.6. Methanol permeability The methanol permeability (P) of the membranes was determined at room temperature by using a two compartment diffusion cell (including a water compartment and a methanol compartment) [30]. The methanol permeability was calculated from the following equation: P¼S

V Bl AC A0

The proton conductivity (σ) of the membranes was measured by a two-point-probe method. All the membrane samples were immersed in room-temperature water for 24 h prior to conductivity measurement. The membrane impedance was measured using a frequency response analyzer (FRA, IVIUM Tech.). The samples were treated at room temperature and elevated temperature (60–100 1C) by water vapor, the relative humidity was kept at 100% during measurements. The proton conductivity of the membranes was calculated according to

ð3Þ

where VB is the volume of water compartment, S is the slope of the straight line of concentration versus time, l, A, and CA0 are the membrane thickness, effective membrane area and feed concentration in methanol compartment, respectively. The standard deviation of measurements was within 74.0%. 2.7. Ion-exchange capacity, proton conductivity and selectivity The ion-exchange capacity (IEC) of the membranes was determined by classical acid–base titration method as described in literature [31].

σ¼

l AR

ð4Þ

in which l is the membrane thickness, A is the effective membrane area and R is the membrane resistance derived from the FRA. The measurement was repeated three times and the standard deviation was within 75.0%. The overall membrane performance was evaluated by selectivity (Φ), which is defined as the ratio of proton conductivity to methanol permeability and calculated by [32]

Φ¼

σ P

ð5Þ

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3. Results and discussion 3.1. Characterization of the TiC–SiN sol Fig. 2 shows the FTIR spectra of pure titania–silica sol and functionalized TiC–SiN sol. The sharp band at 1620 cm  1 and the broad band at 3400 cm  1 correspond to the bending vibration and the stretching vibration of O–H bond in adsorbed water molecules, Pure 1Titania-1Silica Functional 1TiC-1SiN Functional 1TiC-2SiN

3.2. Characterizations of the hybrid membranes

1430

Transmittence

1620

450

1100

4000

3500

3000

2500

2000

1500

respectively. The absorption bands at 450, 950 and 1100 cm  1, which are assigned to Ti–O–Ti, Ti–O–Si and Si–O–Si asymmetric stretching respectively [33], clearly confirm the presence of titanoxane and siloxane network in titania–silica sol. After functionalization of the pure titania–silica sol, the characteristic peak of the –COOH groups at 1430 cm  1 appears, which verifies successful grafting of –COOH groups onto the titania–silica sol. The N–H stretching vibration band of the functionalized TiC–SiN sol is overlapped by the O–H stretching band. Additionally, the band at 1100 cm  1 of the functionalized 1TiC–2SiN sol is more intensive compared with that of the functionalized 1TiC–1SiN sol, suggesting a higher silica content.

950

1000

500

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of the pure titania–silica sol and the functionalized TiC–SiN sol.

The cross-section images of the CS/TiC–SiN hybrid membranes are shown in Fig. 3. The images reveal that the hybrid membranes possess relatively dense and uniform structure. The inorganic phase is finely dispersed in the CS polymer without any observable defects between the inorganic domain and the CS polymeric matrix. This uniform and defect-free morphology is attributed to the advantage of the in situ sol–gel method. The in situ sol–gel process endows the TiCl4 and TEOS a low hydrolysis rate, which favors the homogenous growth of inorganic phase. Moreover, the CS polymer supplies the nanoscale space for the hydrolysis and condensation reaction of the inorganic precursors, thus constraining the obvious generation of particles. As shown in Fig. 3a–d, the cross-section morphology becomes rougher, indicating more TiC– SiN inorganic dopants are incorporated into the hybrid membrane.

Fig. 3. FESEM images of the cross-sections of the hybrid membranes: CS/1TiC–2SiN-1 (a), CS/1TiC–2SiN-3 (b), CS/1TiC–2SiN-5 (c), CS/1TiC–2SiN-7 (d), CS/1TiC–1SiN-5 (e), and CS/2TiC–1SiN-5 (f).

Y. Yin et al. / Journal of Membrane Science 469 (2014) 355–363

4000

CS/1TiC-2SiN-3 CS/1TiC-2SiN-5

CS/1TiC-2SiN-7

10

20

30

b

CS/1TiC-2SiN-5 CS/1TiC-1SiN-5

1529

a---- CS b---- CS/1TiC-2SiN-1 c---- CS/1TiC-2SiN-3 d---- CS/1TiC-2SiN-5 e---- CS/1TiC-2SiN-7 3500

CS/2TiC-1SiN-5

10

3000

2500

2000

1500

1000

d f

Transmittence

30

40

50

a 1637 3340

1529

a---- CS d---- CS/1TiC-2SiN-5 f---- CS/1TiC-1SiN-5 g---- CS/2TiC-1SiN-5

2500

2000

1500

Fig. 5. XRD patterns of pure CS membrane and hybrid membranes with various 1TiC–2SiN content (a) and different TiC–SiN inorganic dopants (b).

another important reason why inorganic phase is homogeneously dispersed in CS polymer. Fig. 5 illustrates the XRD patterns of pure CS membrane and hybrid membranes. It can be seen that the pure CS membrane has four characteristic peaks (2θ ¼11.21, 18.01, 20.91 and 26.01) [36], indicating a semicrystalline structure. The XRD patterns after incorporating different TiC–SiN dopants (Fig. 5b) have no new diffraction peaks, showing that the synthesized inorganic phase through in situ sol–gel method is probably amorphous. As the 1TiC–2SiN content increases, the intensity of four characteristic peaks in CS becomes weaker and the peak positions are shifted slightly (see Fig. 5a). When the 1TiC–2SiN load is 7 wt%, the peaks at 2θ of 18.01 and 26.01 nearly disappear. Such a result is reasonable due to the new interactions formed between TiC–SiN inorganic dopant and CS chains, which influence the ordered packing of CS polymer.

g

3000

20

2 (degrees)

Wavenumber (cm )

3500

50

pure CS

1637

3340

40

2 (degrees)

a

-1

4000

CS/1TiC-2SiN-1

Intensity

Transmittence

e d c

pure CS

Intensity

The FTIR spectra of pure CS membrane and CS/1TiC–2SiN hybrid membranes (Fig. 4a) indicate the specific interactions between the CS polymer and the inorganic phase. For the pure CS membrane, the absorption bands at 3340, 1637 and 1529 cm  1 are attributed to the vibration of hydroxyl, amide I and amide II groups, respectively [34]. After the incorporation of TiC–SiN, the intensities of the CS characteristic peaks decrease remarkably. It is due to the strong, abundant hydrogen bonds as well as the electrostatic attractive forces generated between –OH, –NH2 groups in CS and –NH2, –COOH groups in TiC–SiN [31]. It is worth noting that the –NH2, –COOH groups in inorganic dopants are weak acidic and basic groups, so the moderate electrostatic interaction between TiC–SiN and CS results in homogeneous casting solution without forming insoluble complexes as described in literature where strong acidic dopants were mixed with chitosan [35]. The intensities of the three characteristic peaks for the hybrid membranes decrease as the 1TiC–2SiN content increases, suggesting the interaction between CS matrix and 1TiC–2SiN dopant is enhanced. The FTIR spectra of CS/1TiC–2SiN, CS/1TiC–1SiN and CS/2TiC–1SiN membranes with the same filler content (Fig. 4b) also show decreased intensities of the three characteristic peaks compared with the pure CS membrane. This is

359

1000

Wavenumber (cm-1) Fig. 4. FTIR spectra of pure CS membrane and hybrid membranes with various 1TiC–2SiN content (a) and different TiC–SiN inorganic dopants (b).

3.3. Thermal stability of the hybrid membranes Fig. 6 presents the thermal stability of the membrane samples, which is crucial for the lifetime of PEMs [37]. The hybrid membranes as well as the pure CS membrane exhibit similar

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12

2 -1

80 70 60 50 40 30 20

c

b d a

10

CS/1TiC-1SiN CS/1TiC-2SiN CS/2TiC-1SiN pure CS

70

Water uptake

66

68

9

64

Methanol permeability

8

62

7

60

e 58

6 0

100

200

300

400

500

600

700

800

0

900

2

3.4. Water uptake, swelling degree and methanol permeability of the hybrid membranes Water uptake, which is determined by both free volume properties and hydrophilicity, affects the methanol permeability and proton conductivity of PEMs. In this study, the water uptake and methanol permeability of membranes at room temperature are measured and plotted in Fig. 7. Compared with the pure CS membrane, both the water uptake and methanol permeability of the hybrid membranes increase after the introduction of TiC–SiN inorganic dopants. The water uptake and methanol permeability increase with the increased TiC–SiN content. One reason for this phenomenon is that the inorganic materials, silica and titania, are hygroscopic and more likely to absorb water, another reason is that more hydrophilic groups (–COOH, –NH2) in TiC–SiN are also helpful to attracting water. The order of the water uptake of hybrid membranes is CS/2TiC–1SiN 4CS/1TiC–2SiN 4CS/1TiC–1SiN, so it can be inferred that the hydrophilic –COOH groups and Si–OH groups play the most important roles in water retention. For the methanol permeability, all the hybrid membranes possess higher values compared with the pure CS membrane, this is because the TiC–SiN inorganic phase is hygroscopic and enlarge the ion channels inside the membranes. Although the methanol permeability of the hybrid membrane is slightly higher than the CS membrane, it is much lower than that of the commercialized Nafions membrane (4 1  10  6 cm2 s  1) [40,41]. The water uptake and dimensional swelling properties of the membranes at different temperatures are shown in Fig. 8. Both the

8

Fig. 7. Water uptake and methanol permeability of pure CS and hybrid membranes at room temperature.

100

pure CS CS/1TiC-1SiN-7 CS/1TiC-2SiN-7 CS/2TiC-1SiN-7

90

Water uptake (%)

degradation processes. The first weight loss from 50 to 210 1C is attributed to the evaporation of adsorbed water and residual solvent. The sharp weight loss from 210 to 310 1C is the result of degradation of the CS main chains. The third weight loss region from 310 to 800 1C corresponds to further degradation of CS. Compared with the pure CS membrane, the presence of inorganic phase retards the oxidative degradation of hybrid membranes, leading to an improvement in the membrane thermal stability. This improvement also indicates that the hydrogen bonds and the electrostatic interactions between CS and TiC–SiN restrain the movement of CS molecules. Among all the hybrid membranes with 5 wt% filler content shown in Fig. 6, CS/1TiC–2SiN-5 membrane exhibits the best thermal stability, indicating that the hydrogen bonds formed by –NH2 groups in chitosan and inorganic dopants contribute most to the improvement in thermal stability. However, all the hybrid membranes display an onset of degradation at 210 1C, which means that they are sufficiently stable at desired operating temperatures (o 100 1C) for most DMFCs [38,39].

6

Filler content (%)

Temperature (°C) Fig. 6. TGA curves of pure CS membrane and hybrid membranes.

4

50

40

Swelling degree 80 30

Water uptake 70

60

Swelling degree (%)

a- -CS b- -CS/1TiC-2SiN-1 c- -CS/1TiC-2SiN-5 d- -CS/2TiC-1SiN-5 e- -CS/1TiC-1SiN-5

11

Water uptake (%)

90

-7

Methanol permeability (10 cm s )

Weight (%)

100

20

10 20

30

40

50

60

70

80

Temperature (°C) Fig. 8. Water uptake and swelling degree of pure CS and hybrid membranes at different temperatures.

water uptake and the swelling degree increase with the increasing temperature. However, the swelling degree of the hybrid membrane is reduced by the incorporation of inorganic dopants compared with the pure CS membrane. This phenomenon is particularly more evident as the temperature is elevated. All the hybrid membranes display lower swelling degrees than CS membrane at 80 1C. The above results indicate that the incorporation of inorganic dopants into CS polymer matrix leads to the formation of electrostatic forces between the functional groups in TiC–SiN and the amino groups in polymer, thus reducing the swelling degree but keeping the water-absorbing capacity. 3.5. Ion-exchange capacity The ion-exchange capacity (IEC) values of all the membranes are listed in Table 1. The higher IEC values of all the hybrid membranes than that of the pure CS membrane (0.274 mmol g  1) suggests that the incorporation of inorganic dopants endows the membrane with more acid sites and dissociable H þ ions. The IEC value increases with the inorganic filler content. Since the CS/2TiC–1SiN membranes possess more carboxylic acid groups than the other two kinds of hybrid membranes, CS/2TiC–1SiN membranes show the highest IEC values (0.356–0.587 mmol g  1). The CS/1TiC–2SiN-1 membrane has a similar IEC value (0.278 mmol g  1) to pure CS membrane because of the low content of acidic groups. It can be concluded from the IEC

Y. Yin et al. / Journal of Membrane Science 469 (2014) 355–363

361

Table 1 IEC values, proton conductivity, methanol permeability and selectivity of pure CS and hybrid membranes. IEC (mmol g  1)

Membrane

CS CS/1TiC–1SiN-1 CS/1TiC–1SiN-3 CS/1TiC–1SiN-5 CS/1TiC–1SiN-7 CS/1TiC–2SiN-1 CS/1TiC–2SiN-3 CS/1TiC–2SiN-5 CS/1TiC–2SiN-7 CS/2TiC–1SiN-1 CS/2TiC–1SiN-3 CS/2TiC–1SiN-5 CS/2TiC–1SiN-7

σ (10  2 S cm  1) 25 1C

0.274 0.323 0.409 0.433 0.510 0.278 0.343 0.392 0.434 0.356 0.433 0.451 0.587

1.11 1.80 2.68 3.31 3.42 2.60 3.41 3.80 3.92 2.87 3.57 3.95 4.08

P (  10  7 cm2 s  1)

Φ (  104 S s cm  3)

25 1C, 2 M methanol

25 1C, 2 M methanol

6.64 6.72 7.10 7.57 8.18 6.98 7.35 7.83 8.69 7.37 7.84 8.31 9.39

1.67 2.68 3.77 4.37 4.18 3.72 4.64 4.85 4.51 3.89 4.55 4.75 4.35

0.14

pure CS CS/1TiC-1SiN-7 CS/1TiC-2SiN-7 CS/2TiC-1SiN-7

0.12

-2

Proton conductivity (S cm-1)

-1

Proton conductivity (10 S cm )

4

3

CS/1TiC-1SiN CS/1TiC-2SiN CS/2TiC-1SiN

2

0.10

0.08

0.06

0.04

1 0

2

4

6

8

Filler content (%) Fig. 9. Proton conductivity of pure CS membrane and hybrid membranes at room temperature.

values that the carboxyl groups contribute most to the enhanced ionexchange capacity, which is expected to improve the proton conductivity of the membranes. 3.6. Proton conductivity and selectivity of the hybrid membranes The proton conductivity of the membranes measured at room temperature and 100% RH is displayed in Fig. 9. Obviously, the pure CS membrane shows a proton conductivity of 0.011 S cm  1 and the proton conductivity of all the hybrid membranes is higher than the pure CS membrane. As the TiC–SiN content increases, the proton conductivity of hybrid membrane increases. Among all the hybrid membranes, the CS/2TiC–1SiN-7 membrane displays the highest proton conductivity (0.0408 S cm  1) at 25 1C, which is 4 times higher than that of pure CS. The proton conductivity of pure CS and three as-prepared hybrid membranes at different temperatures ranging from 60 to 100 1C is shown in Fig. 10. The proton conductivity of all the membranes increases with the increasing temperature. After the incorporation of inorganic dopants, the proton conductivity of the hybrid membranes is enhanced further than that of pure CS membrane at elevated temperatures. The CS/2TiC–1SiN-7 membrane exhibits the highest proton conductivity among all the membranes at both room temperature and elevated temperatures since abundant acid–base pairs in favor of proton migration are generated inside the membrane. The proton conductivity of the hybrid membranes follows the order of CS/2TiC–1SiN 4CS/1TiC–2SiN 4CS/1TiC–1SiN,

0.02 60

70

80

90

100

Temperature (°C) Fig. 10. Proton conductivity of pure CS and hybrid membranes at different temperatures ranging from 60 to 100 1C.

which is consistent with the water uptake results shown in Fig. 7. It can be deduced that the protons are also transferred with the help of water molecules, i.e. migrate via the Vehicle mechanism. Based on the above analysis, the proton transfer mechanism in CS/TiC–SiN hybrid membranes is presented in Fig. 11. The Vehicle mechanism (route a in Fig. 11) and Grotthuss mechanism (route b in Fig. 11) both exist in the hybrid membranes: (i) the incorporation of the TiC–SiN inorganic phase increases both the free water and bound water contents in the hybrid membranes, which facilitates the protonation of water to form hydronium ions diffusing through the continuous nanochannels. Nevertheless, the nanochannels also lead to the diffusion of methanol molecules; and (ii) the zwitterion functionalized TiC–SiN dopants are homogeneously dispersed within CS matrix, which creates new proton transfer pathways with the aid of acid–base pairs. In this mechanism, acidic –COOH groups and basic –NH2 groups are linked by the intermediate water bridges formed by hydrogen bonds. The proton first interacts with the acidic –COOH group, then transfers along the water bridges to arrive at the basic –NH2 group, finally moves out of the acid–base pair to undergo another transfer. Accordingly, protons will transfer via the Grotthuss mechanism continuously by acid–base pairs and hydrogen bonds network formed and broken alternately. As shown in Table 1, the IEC value of CS/2TiC–1SiN-7 membrane is  2 times higher than pure CS; however, the proton conductivity of CS/2TiC–1SiN-7

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Fig. 11. Schematic representation of proton transfer mechanism in hybrid membranes: proton transfer via Vehicle mechanism (a) and proton transfer via Grotthus mechanism with acid–base pairs (b).

synthesized by a facile chelation method and the resultant TiC– SiN inorganic phase are dispersed uniformly in CS matrix via an in situ sol–gel process. After the incorporation of TiC–SiN, the functional groups interact with CS matrix and improve the thermal stability of hybrid membranes. Meanwhile, the hygroscopic inorganic silica, titania and the hydrophilic –NH2, –COOH groups lead to an increase in water absorption, which facilitates the proton transfer via Vehicle mechanism. Most importantly, the zwitterion functionalized TiC–SiN dopant containing both acidic groups as proton donors and basic groups as proton receptors, which are linked by hydrogen-bonded water molecules, contribute to the proton migration via Grotthuss mechanism. Overall, the CS/TiC–SiN hybrid membranes possess higher selectivity than pure CS membrane.

6

CS CS/1TiC-1SiN CS/1TiC-2SiN CS/2TiC-1SiN

4

Selectivity (10

4

S s cm -3 )

5

3

2

1

0

0

1

3

5

7

Filler content (%) Fig. 12. Selectivity of pure CS membrane and hybrid membranes at room temperature.

membrane is  4 times higher than the pure CS membrane. It indicates that the increase of proton conductivity of the hybrid membranes is mainly attributed to the synergetic effect of acid– base pairs. Thus, after the incorporation of zwitterion functionalized inorganic dopants, the proton conductivity of the hybrid membranes is improved evidently. The ideal PEMs for DMFCs should possess high proton conductivity and low methanol permeability, so the selectivity is often used to describe the overall performance of the membranes. The proton conductivity, methanol permeability and selectivity of all the membranes are listed in Table 1 and the selectivity is more visually given in Fig. 12. Due to the evident increase in proton conductivity of hybrid membranes, the selectivity of all the hybrid membranes is improved after the introduction of TiC–SiN, indicating the enhanced comprehensive performance. Particularly, CS/1TiC–2SiN-5 membrane possesses the highest selectivity of 4.85  104 S s cm  3, which is 3 times higher than that of pure CS membrane.

4. Conclusions In the present study, a kind of binary titania–silica sol functionalized with carboxyl groups and amino groups (TiC–SiN) is

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