Preparation of anion exchangeable titanate ...

Materials and Design 154 (2018) 63–72

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Preparation of anion exchangeable titanate nanotubes and their effect on anion exchange membrane fuel cell Vijayakumar Elumalai, Dharmalingam Sangeetha ⁎ Department of Mechanical Engineering, Anna University, Chennai 600025, India

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Titanate nanotubes were synthesized and functionalised with quaternary ammonium. • High conductivitiy composite membranes were fabricated using functionalised filler and quaternised polysulfone. • Using the synthesised membrane as electrolyte showed a power density of 285 mW/cm2.

a r t i c l e

i n f o

Article history: Received 22 September 2017 Received in revised form 10 May 2018 Accepted 12 May 2018 Available online 14 May 2018 Keywords: Functionalised titanate nanotubes Anion exchange filler Anion exchange membrane Composite anion exchange membrane Alkaline fuel cell performance

a b s t r a c t In the present study, Titanate Nano Tubes (TNT) were synthesised by hydrothermal method and functionalised with quaternary ammonium group via chloromethylation followed by amination. Quaternary ammonium functionalised TNT (QTNT) were characterized by FTIR, solid state CP/MAS 13C and CP/MAS 29Si for their successful chemical modification and their nano tube morphology was confirmed by XRD, SEM and TEM analyses. The synthesised QTNT was then incorporated into Quaternary ammonium functionalised Polysulfone (QPSU) at various wt% (1, 3, 5 and 7) to form high density ion exchange (OH−) composite membranes. The morphology of the synthesised membranes were also characterized by SEM and XRD. Additionally, the membranes were tested for water uptake, Ion Exchange Capacity (IEC) and hydroxyl conductivity with respect to alkaline fuel cell application. The Membrane Electrode Assembly (MEA) consisting of Pt anode, Ag cathode and various QTNT composite membranes were fabricated and tested in an in-house built fuel cell setup. From the performance study, it was inferred that the membrane with 5% QTNT showed the maximum power density of 285 mW/cm2 at 60 °C. The experimental results corroborate that addition of QTNT has the propensity to increase the performance of the anion exchange membrane significantly. © 2018 Elsevier Ltd. All rights reserved.

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (D. Sangeetha).

https://doi.org/10.1016/j.matdes.2018.05.024 0264-1275/© 2018 Elsevier Ltd. All rights reserved.

Anion Exchange Membranes (AEM) have been used as anion (hydroxyl) conductive solid electrolytes in many electrochemical energy

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V. Elumalai, D. Sangeetha / Materials and Design 154 (2018) 63–72

T N T

OH OH OH

Si(OMe)3(CH2)3Cl

T N T

O O O

TEA Si(CH2)3Cl

T N T

O O O

Si(CH2)3

N

Cl

Fig. 1. Schematic representation of QTNT.

technology devices such as fuel cells, redox flow batteries, water electrolysers and electro dialysis etc., [1,2] where their fundamental roles were to transfer the anions from cathode to anode and act as separator between the electrodes [3]. AEM carry the potential to substitute Proton Exchange Membranes (PEM), whose working function depends on noble metal electrodes [4]. AEMs basically consist of a stationary cation head group and mobile anions(OH−) grafted on the polymer matrix [4]. Recently, AEM's utilisation in the field of fuel cell has considerably increased. Apart from the use of non-noble metal catalysts, they also provide additional features like (i) faster reaction kinetics on both the electrodes (ii) reduced fuel cross-over (iii) easier water management by preventing the possibility of water-flooding (iv) minimized risks of corrosion in alkaline media, thus guaranteeing the durability of the catalyst [5–7]. The only major drawback in the AEM is its lower anionic conductivity compared to proton conductivity of PEM, due to the larger size of anion and lesser hydration behaviour of the cation head group resulting in lower fuel cell performance [8,9]. To develop an AEM, comprising of high ionic conductivity along with physico-chemical (thermal, mechanical and chemical) stabilities, is an important challenge in the field of Anion Exchange Membrane Fuel Cell (AEMFC) for their improved efficiency and durability [10–13]. The conduction of hydroxyl ion in AEMs takes place via two mechanisms 1.Hopping mechanism (through ion exchange group), 2. Diffusion or Grotthus mechanism (through water molecule) [14,15]. So, AEMs with high ion conductivity can be attained by increasing the number of ion exchange groups and water molecules in the membranes [11,16]. But excessive grafting of ion exchange groups in the polymers leads to loss in the mechanical and thermal stability of the membranes [17]. Improvement in the anion conductivity via raising water uptake of the membranes can be achieved by incorporating hydrophilic inorganic fillers to form a hybrid membrane [18–20]. Several inorganic fillers such as SiO2 [21], ZrO2 [22], TiO2 [23], bentonite [24], montmorillonite [25] have been employed to fabricate hybrid membranes and to prove the significant improvements in the conductivity of AEM with overall stabilities. Additionally, they also prevent dehydration and reduce permeation of reaction gases in the fuel cell operating condition [26]. Amongst these inorganic nano fillers, TiO2 possess superior hydrophilic nature and good compatibility with solvents used for membrane casting. This behaviour makes them stable with homogeneous dispersion in the polymer matrix devoid of agglomeration formation. In the advancement of the composite membranes, sulphonated form of TiO2 nano particles has also been used by Rhee et al. [27] which showed higher power density compared to the commercial membrane Nafion. TiO2 based anion exchange composite membranes reported by Patrick et al. [23] proved that the TiO2 has an ability to enhance the performance of the AEM. These results encourage to choose TiO2 as filler particle for the fabrication of a composite AEM with Quaternary ammonium functionalised polysulfone (QPSU). Polysulfone (PSU) was chosen due to its high mechanical and thermal stability in addition to its capacity to get functionalised easily with Quaternary Ammonium (QA) groups on its aromatic rings by simple reaction pathways. From the various study carried out it is concluded that ammonium ions are the most stable than the other onium ions such as sulfonium and

phosphonium ions [28]. Hence in the present study ammonium group functionalised PSU was used as ionomer for the preparation of membranes. In the present work, anatase form of TiO2 was converted into two dimensional nanostructures of Titanate Nano Tubes (TNT) by hydrothermal method, and subsequently functionalised with quaternary ammonium group for the first time and used as filler in AEM. TNT has many advantages over TiO2 nano particles such as 1. They possess high surface area (200–300 m2/g) hence they can accommodate more number of functional groups on its surface. 2. The hollow tube morphology retains large amount of water molecules inside it. The Quaternary ammonium functionalised TNT (QTNT) based nano composite membranes were fabricated using QPSU. The effects of addition of different wt% of QTNT content were optimised to achieve maximum properties of nanocomposite AEMs. Quaternising the TNT provides additional ion exchange sites for the conduction of hydroxyl ion for the improvement of the AEM.

2. Materials and methods Preparation of QPSU was carried out as per our previous work [29]. TiO2 anatase was acquired from Sigma Aldrich. Other chemicals such as chloroform, methanol, triethylamine, dimethylformamide, paraformaldehyde, chlorotrimethylsilane and stannic chloride were purchased from Merck (India). Vulcan XC-72 (20% of Platinum in carbon support) was purchased from Arora-Mathey. Carbon cloth was obtained from Cobat Carbon Inc. 3-Chloropropyltrimethoxysilane 97% (Alfa Aesar) and Pt/C 20% (Arora-Mathey) were used as received without further purification. Double distilled water was used throughout the experiments.

Fig. 2. FTIR spectra of TNT(a), Cl-TNT(b) and QTNT(c).


2.1. Synthesis of titanate nano tubes Titanate nanotubes (TNT) were synthesised by hydrothermal method [30]. 4.4 g of TiO2 powders were added to 200 ml of 10 M

65

NaOH solution and was stirred magnetically for 1 h. It was further sonicated for 30 mins. The reaction was maintained at 160 °C for 24 h in a teflon lined autoclave. The obtained reaction mass was neutralised with 1 M HCl and then washed with distilled water for several times.

Fig. 3. (i). 13C CP/MAS NMR spectrum of QTNT. (ii). 29Si CP/MAS NMR spectrum of QTNT.

66


TNT powders were obtained by vacuum filtration and dried at 120 °C using air circulating oven. The obtained TNT powders were ball milled for 1 h to obtain fine powders.

2.2. Chloromethylation of TNT 2.5 g of TNT was dispersed in 200 ml of ethanol and sonicated for 30 mins. To this, 25 ml of 3-Chloropropyltrimethoxysilane was added drop-wise with an additional funnel and the reaction was carried out at 100 °C under nitrogen atmosphere for 48 h under constant stirring [31]. The reaction crude was filtered using Whatman filter paper followed by washing with dichloromethane and acetone to remove the un-reacted contents and to yield pure Chloromethylated TNT (ClTNT).

2.5. Preparation of membrane electrode assembly (MEA) and fuel cell performance The procedure for preparation of electrodes [Pt anode (0.25 mg/cm2) and Ag cathode (0.375 mg/cm2)] were followed as per our previous work [32]. The composite membrane was sandwiched between the two electrodes (anode and cathode) and was hot pressed at 120 °C with a pressure of 1.5 kN for 2 mins. The MEA performance was obtained from the lab-made fuel cell with an active surface area of 25 cm2 at 60 °C. Highly pure hydrogen (99.99%) and oxygen (99.99%) gases were used as fuel and oxidant which flow through the anode and cathode under controlled flow rate of 600 SCCM and 300 SCCM respectively under 80% relative humidity. [13].

2.6. Instrumental characterization 2.3. Quaternary ammonium functionalised TNT Cl-TNT (2 g) was dispersed in 200 ml of ethanol and sonicated for 30 mins. To the above solution triethylamine (20 ml) was added drop wise and the reaction was carried out under nitrogen atmosphere for 24 h at 60 °C [13]. Purification was employed similar to the procedure followed for Cl-TNT (Fig. 1).

2.4. Preparation of QTNT/QPSU composite membrane The QPSU (Cl−) was dissolved completely using DMF to get a homogeneous solution. Then different weight percentages of QTNT (1%, 3%, 5% and 7%) were added in portion wise and allowed to stir for 24 h for uniform mixing of QTNT and QPSU matrix. The solution was sonicated for 30 min before casting on a clean dry petri dish and it was vacuum dried at 60 °C to form the membranes. The membranes were washed with deionised water for several times to remove the residual solvent and dried at 70 °C for 8 h. Finally, membranes were submerged in KOH (1 M) to get OH– conducting membranes [32].

The successful functionalisation of TNT to Cl-TNT and QTNT was confirmed by Fourier Transformer Infrared Red (FTIR) Spectrometer (Alpha Bruker, USA) in the wave number range from 4000 to 400 cm−1. Cross Polarised/Magic-Angle Spinning (CP/MAS) 13C Nuclear Magnetic Resonance (NMR) and CP/MAS 29Si NMR of the QTNT were recorded using BRUKER 400 MHz NMR (USA) spectrometer to detect the covalent bonding of quaternary ammonium group on the TNT surface. The crystalline nature (X-ray diffractometer (XRD) patterns) of the composite membranes was studied with “X” Pert Pro diffractometer (United Kingdom) at 2 theta range of 10–80° with a scanning rate of 2° per minute. Nitrogen sorption analysis of the samples were analysed using Autosorb Automated Gas Sorption System (M/s. Quantachrome, USA) with liquid Nitrogen as the adsorbate. The surface area was calculated using Brunauer–Emmett–Teller (BET) method whereas pore size was calculated using Barrett-Joyner-Halenda (BJH) method. The morphology of the as-synthesised TNT, functionalised TNT and composite membranes was analysed using Scanning Electron Microscope (SEM) (JEM5600LV, USA). Before SEM analysis, all the samples were subjected to gold ion sputter coating at the pressure range of 1 to 0.1 Pa. HighResolution Transmission Electron Microscopy (HRTEM) images of the

Fig. 4. SEM images of TNT (a & b), QTNT(c), TEM images of TNT (d & e) and QTNT (f).


67

Fig. 5. SEM images of bare QPSU (a), 1% QTNT QPSU (b), 3% QTNT QPSU(c), 5% QTNT QPSU(d), 7% QTNT QPSU(e).

TNT and the modified TNT were captured using HRTEM Tecnai T30 (USA) using copper grid. The mechanical strength of the QTNT composite membranes were recorded using Hounsfield universal testing machine (USA) with a crosshead speed of 10 mm/min under 100% relative humidity. Membranes of rectangular shapes with 50 mm length and 5 mm width [33] were used to obtained the stress-strain curves. Differential Scanning Calorimetry (DSC) analysis of the membranes were carried out in NETZSCH STA 449F3 Jupiter (USA) at a heating rate of 10 °C/min.

group of the chloropropylsilane. This confirms that the TNT surface has been modified with the chloromethyl group. In the QTNT (Fig. 2c), the symmetric and asymmetric stretching vibrations of the C\\N appeared at 1403 cm−1 and 1452 cm−1 respectively [13]. Thus, the FTIR analysis clearly confirms the formation QA group in the TNT. This has also been proved evidently in the solid state NMR section.

500

3. Results and discussion

The FTIR spectra of TNT(a), Cl-TNT(b) and QTNT(c) are shown in Fig. 2. Pure TNT showed three major peaks. The broad peak around 3500–3000 cm−1 and another at around 1620 cm−1 can be attributed to the stretching and bending vibrations of the surface hydroxyl (O\\H) groups and absorbed water molecules. The third broad and intense band positioned at 900–500 cm−1 could be ascribed to Ti\\O and Ti\\O\\Ti stretching of TNT (Fig. 2a) skeleton [34]. In Cl-TNT (Fig. 2b), a new peak arises at 2929 cm−1 corresponding to the C\\H stretching of the propyl chain and a peak for C\\Cl around 700 cm−1 which could not be resolved separately as it was merged with the Ti\\O\\Ti stretching. Additionally, a peak that appeared at 940 cm−1 due to the Ti\\O\\Si group [35] pronounces that the condensation reaction has taken place between the hydroxyl group of TNT and methoxy

3 Volume Absorbed(cm )

3.1. FTIR

400

(a) TNT (b) QTNT (c) TiO2 (a)

300

(b) 200

100

(c)

0

0.0

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 6. Nitrogen adsorption-desorption isotherm of TNT (a), QTNT (b) and TiO2 (c).

68


Table 1 Texture Properties of TNT and QTNT. Sample

BET surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

TNT QTNT

242 178

0.98 0.65

7.5 5.2

revealing the bond formation that occurred between methoxy silane and TNT. The three peaks obtained at the values of −49.91 ppm, −58.89 ppm and − 68.44 ppm correspond to 29Si atoms of the T1[CH2-Si(OR)2(OTi≡)], T2[-CH2-Si(OR)(OTi≡)2] and T3[-CH2-Si(OTi≡)3] sites respectively [36]. From the peak intensities, it can be inferred that the dimethoxy condensation was predominant, followed by trimethoxy and monomethoxy during the chloromethylation step and were retained in the quaternization step [37].

3.2. Solid state NMR 3.3. Morphology The effective functionalization of QA groups on TNT was acutely confirmed by using solid state CP/MAS NMR studies. Fig. 3(i) shows the 13C CP/MAS NMR spectrum of QTNT which shows three signals and their chemical shifts (ppm) were allotted as per numbering shown in insert figure. The chemical shifts of the “a” labelled carbon which is adjacent to the silicon atom appeared in the up-field region at 9.47 ppm and the “b” labelled carbon and “c” labelled carbon appeared in the downfield at 26.65 ppm and 46.28 ppm respectively [13]. The covalent bond formation between the TNT and silane group were confirmed by 29 Si CP/MAS NMR spectrum (Fig. 3(ii)) which provides the evidence about the presence of organic substituted silicon atoms (T sites) [32]. QTNT showed three well resolved peaks in the down-field region,

It can be clearly seen from the scanning electron microscopy (SEM) (Fig. 4(a) &(b)) images that all the raw TiO2 particles were converted to nanotubes. The as-synthesised TNT showed cylindrical rod like morphology [38]. Most of the rods were not isolated from each other and appeared as bundles [39]. The length of the TNT ranged in several micro meters. After functionalization, the QTNT (Fig. 4(c)) tends to isolate from one another and showed a cloudy appearance on its surface, which may be due to the presence of organic group. In order to perceive the hollow nature of TNT, the TEM images were captured and shown in Fig. 4(d) & (e). From the TEM images, it is observed that the TNTs were hollow and most of the TNTs were open at both ends with diameter of

Fig. 7. (i). XRD pattern of TNT (a) and QTNT (b). (ii) XRD patterns QPSU (a), 1% QTNT/QPSU (b), 3% QTNT/QPSU (c), 5% QTNT/QPSU (d), 7% QTNT/QPSU (e).


0.5

-0.5

DSC (mW/mg)

3.4. Nitrogen sorption analysis

(a) QPSU (b) 1% QTNT/QPSU (c) 3% QTNT/QPSU (d) 5% QTNT/QPSU (e) 7% QTNT/QPSU

0.0

(d) (c)

-1.0

(e) (b)

-1.5 -2.0

(a)

-2.5 -3.0 -3.5 50

100

150

69

200

250

300

Temperature (°C)

The nitrogen adsorption-desorption isotherms of the TNT (Fig. 6(a)) and QTNT are reported in Fig. 6(b). QTNT showed type IV isotherm with H3 hysteresis loops at a relative pressure P/Po ≥ 0.8 which are the characteristics of slit-shaped pores with nanotube structures and free from aggregates of particles. The hydrothermal treatment converts the nonporous TiO2 particles (Fig. 6(c)) into porous materials. The BET surface area, pore volume and pore diameter are listed in Table 1. The obtained surface area pure TNTs were found to be 6 times higher than the TiO2 particles (surface area 37 m2/g) used for the hydrothermal synthesis. After grafting of QA group into the TNT, i.e., QTNT also showed the same type of isotherm which concludes that the tubular structure was maintained but the surface area and pore volume was relatively lesser due to the filling of the QA groups in the pores of the TNT. 3.5. X-ray diffraction and differential scanning calorimetry (DSC)

Fig. 8. DSC curve of QPSU (a), 1% QTNT/QPSU (b), 3% QTNT/QPSU (c), 5% QTNT/QPSU (d), 7% QTNT/QPSU (e).

the nanotubes varying between 8 and 12 nm [39]. Each of the tube has a constant diameter along its length and samples have the nanotube structure entirely [39]. Moreover, the interference fringe spacing of the nanotube was found to be 0.34 nm which are consistent with the interplanar distances of the TNT [39]. From the TEM image of QTNT (Fig. 4(f)), it can be confirmed that the tube morphology was retained even upon chemical modification of its surface. Further these structures with tube morphologies have the capacity to retain the water inside it and are highly appreciated for fuel cell working under humid conditions [40]. Surface morphology of the various QTNT (1%, 3%, 5% and 7%) composite membranes was examined by SEM and the images are reported in Fig. 5. The bare QPSU membrane (Fig. 5(a)) exhibits a very flat surface without pores and cracks [41]. The QTNT/QPSU (Fig. 5(b)–(e)) membranes also showed relatively uniform smooth surface and dispersion of TNT particles has taken place homogeneously throughout the QPSU matrix. Since both these materials are hydrophilic in nature, an outstanding compatibility behaviour of TiO2 with the polymer matrix was observed [42]. Hence all the prepared membranes were well suited to be used as electrolyte in AEMFC. It was observed that in 1%, 3% and 5% QTNT/QPSU, the particles were not associated with any agglomeration, i.e. upto to 5 wt% QPSU has accommodated the QTNT in a well dispersed manner [43]. However, the 7% QTNT/QPSU showed some larger lumps on its surface due to the agglomeration of QTNT in higher loading. The penetration level of the QTNT into the QPSU matrix was highly influenced by the amount of fillers used. High concentration of the fillers, leads to a high viscous casting solution wherein high interaction takes places within the polymer matrix. This leads to decreasing chances of penetration of QTNT into the polymer chains, resulting in the agglomeration [44]. It is also worth to note from the SEM images of the composite membranes that the particle size of the fillers increases with increasing loading level [45].

The crystalline phase characteristics of hydrothermally synthesised TNT was studied using the X-ray analysis and the results are presented in Fig. 7(i). In Fig. 7(i)(a) of the XRD pattern of nano-TNT, the prominent peaks appeared at 10.58°, 23.788°, 28.58°, 37.90° and 48.438° [46] corresponding to (200), (110), (310), (0 0 4) and (020) lattice planes respectively were attributed to the TNT ([H2Ti3O7]) [47]. A high intensity peak at 10.58° of TNT reveals the formation of trititanate layer. The intensity of the plane (2 0 0) decreased in QTNT (Fig. 7(i) (b)) due to the replacement of the protons by the larger QA group, which occupied between the titanate layers. In addition to this, other plane intensities also decreased with a slight-broadening. From the XRD studies, it was confirmed that trititanate nanotube morphology was retained after it was modified by QA group and it is in good agreement with TEM analysis (Fig. 4f). The X-ray diffraction patterns of the composite and plain membranes are shown in Fig. 7(ii). The plain QPSU showed a broad and intense peak around 2θ of 19–20° corresponding to the polymer backbone confirming that QPSU is a semi-crystalline polymer [48]. Upon the addition of QTNT, the intensity of the polymer peak decreased indicating that the membranes become amorphous than the plain QPSU [49]. The amorphous membranes are more favoured in conduction of ions through it than the crystalline membranes [50]. The observed amorphous nature of the composite membranes may be attributed to the penetration of the fillers in the crystal region of the polymer chains and disturbs the structured arrangement to become more disordered. The 5% QTNT composite membranes showed more amorphous character due to the maximum dispersion (penetration) occurred (SEM Fig. 5d). In case of 7% QTNT composite membrane the crystallinity increased due to the agglomeration, but it was lower than that of the plain QPSU. This observation was in compliance with the other nano composite membranes [51,52]. Differential scanning calorimetry (DSC) curves of neat QPSU along with other composites at the temperature range from 30 °C to 300 °C are presented in Fig. 8 The crystallization peak was observed around 190 °C and area of the peak is directly proportional to the crystallinity

Table 2 Properties of QTNT/QPSU composite membranes. Membrane

AMI-7001a QPSU 1% QTNT/QPSU 3% QTNT/QPSU 5% QTNT/QPSU 7% QTNT/QPSU a

Thickness (μm)

450 ± 2.5 101 ± 3 97 ± 4 98 ± 5 95 ± 4 99 ± 5

Commercial AEM.

Before acceleration test

After acceleration test

Water absorption (%)

IEC (meq/g)

Conductivity (S/cm) × 10−2

Water absorption (%)

IEC (meq/g)

Conductivity (S/cm) × 10−2

17 5.75 9.34 12.75 16.17 14.37

1.3 0.76 1.32 1.51 1.73 1.68

1.72 0.78 1.38 1.62 1.95 1.74

13 5.2 8.43 11.35 14.43 12.04

1.05 0.67 1.04 1.33 1.45 1.50

1.56 0.66 1.15 1.36 1.54 1.41

70


40

Tensile Strength (MPa)

35 30

(e)

(a) QPSU (b) 1% QTNT/QPSU (c) 3% QTNT/QPSU (d) 5% QTNT/QPSU (e) 7% QTNT/QPSU

(d)

(c) (b) (a)

25 20 15 10 5 0 2

4

6

8

10

12

Strain (%) Fig. 9. Stress-strain curves of QPSU (a), 1% QTNT/QPSU (b), 3% QTNT/QPSU (c), 5% QTNT/ QPSU (d), 7% QTNT/QPSU (e).

of the material and denoted by heat of fusion (ΔHc). The pure QPSU showed a board peak which then decreased upon the addition of QTNT revealing the decrease in crystalline nature. The crystalline pattern of the composite membranes was found to be similar to the results of XRD studies. 3.6. Ion exchange capacity (IEC), water uptake (Wup) and conductivity (σ) The testing procedure for Ion Exchange Capacity (IEC), Water uptake (Wup) and conductivity (σ) was described in our previous work [53] and the values are shown in Table 2. IEC is the measure of tendency of the insoluble material to exchange the ions. The IEC values of the all composite membranes were higher than the plain QPSU. Even on 1% addition of QTNT the value of IEC increased from 0.76 to 1.36 meq/g because the large surface area of the TNT could accommodate high amount of QA (Ion Exchange Group [IEG]) on its surface which could also assist in the ion exchange process in addition to QPSU. On increasing the filler content, the IEC similarly increases for 3 and 5% QTNT composite membranes and reaches the maximum value of 1.73 meq/g which is 2.5 times higher than the plain membrane. However on the

7% QTNT/QPSU there is a slight dip in the value of IEC and it may be due to the agglomeration of fillers in the QPSU matrix [51]. In this case some of IEGs might have been blocked, thereby not permitting them to participate in the ion exchange process. Wup is also considered as one of the important parameter in the membrane based fuel cell since the conduction of ion (H+ or OH−) also takes places through water molecules by Grotthus mechanism [54]. From the Table 1, it was found that the Wup increases gradually with increase in filler content up-to 5% which suggests that a linear relationship exists up to this composition. The increase in Wup is mainly due to the tubular morphology of the TNT molecules. Moreover the increase in IEC causes the filler to be more hydrophilic which facilitates better retention of water molecules inside the membranes during the hydration process. Hydroxyl conductivity is the pivotal factor on which the performance of the fuel cell depends upon. Conductivity of the AEM depends on the concentration of IEG and the volume of water inside the membrane. The conduction of OH– proceeds by two mechanisms namely Hopping and Grotthus. In former the OH– conduction takes place via IEG and in later it takes place through the water molecules. QTNT possessed capability of increasing both the water uptake (tubular morphology) and IEG (functionalised with QA). As expected, the conductivity of the membranes increased upon the addition of this functionalised filler. The plain QPSU showed only 7.8 S/cm2 whereas all the composite membranes showed higher values of conductivity. This was attributed to the high density of ion exchange sites, proper connectedness of the ion transfer channels and appropriate water uptake in the composite membranes, making them less resistant to the hydroxyl ion [55] when they passed through resulting in high hydroxide conductivity. The 5% QTNT composite membrane showed a maximum conductivity of 1.95 S/cm2. However, after acceleration stability test the values of IEC, Water uptake and conductivity were reduced (b10% loss) due to the conversion of some QA groups into ylide which cannot act as a IEG [56] but the losses were in the acceptable level. 3.7. Mechanical properties The stress-strain curves of QPSU and QTNT composite membranes measured under fully hydrated state are shown in Fig. 9. The composite membranes exhibited low-dimensional changes to the applied stresses compared to pure QPSU membrane, a property which would be beneficial while operating in a fuel cell. The membrane electrode assembly in a fuel cell undergoes stress due to following reasons: tensile/compressive

Fig. 10. Polarization curve of QPSU (a), 1% QTNT/QPSU (b), 3% QTNT/QPSU (c), 5% QTNT/QPSU (d), 7% QTNT/QPSU (e).

V. Elumalai, D. Sangeetha / Materials and Design 154 (2018) 63–72 Table 3 Comparison of various AEMs reported in literature. Membrane

5% QTNT/QPSU QA-POSS/QPSU QPSU/ILSBA-15 QPSU/QSBA-15 QPSU/MWCNT QPSU/SiO2 QPSU/ZrO2 PAES/ZrO2 QAPS/PTFE

IEC, Conductivity, meq/g S/cm 1.73 2.03 1.86 1.743 0.792 1.37 0.921 1.82 1.27

1.95 −2

2.11 × 10 1.89 × 10−2 1.80 × 10−2 0.84 × 10−2 1.63 × 10−2 1.51 × 10−2 0.23 × 10−2 0.31 × 10−2

Catalyst Anode cathode

Reference Power density mW/cm2

Pt

Ag

285

Pt Pt Pt Pt Pt Pt

Ag Ag Pt Pt Pt Pt

Pt

Pt

321 278 298 190 252 250 – 315

Present work [53] [32] [13] [58] [59] [29] [22] [60]

forces, hydration of the membrane due to water present in the vapor and liquid phase and cycling temperatures [57]. Such stresses can lead to changes in the performance of the MEA as they are related more to the membrane than the other components of the MEA. From the curves, it was clear that the addition of QTNT increased the tensile strength of the membranes. The observed increase in the tensile strength was due to the dispersion of the QTNT materials in the QPSU matrix that reinforces the membrane effectively. The better dispersion can be due to the weak bonding forces that arises between the QTNT and QPSU matrix and compatibility nature of QTNT, that can efficiently diffuse into the polymer matrix at the molecular level. Maximum dispersion of QTNT was observed in the 5% composite membrane (SEM Fig. 5(d)), that exhibited the maximum tensile strength around 40 MPa. Whereas, the 7% QTNT composite membrane showed slightly lesser strength (35 MPa) due to the agglomeration in the membrane which increases the crystallinity. A similar trend was observed in the DSC and XRD also. 3.8. Fuel cell performance Fig. 10 shows the performance curves of plain QPSU and various composite membranes in an in-house built fuel cell station at 60 °C using Pt as anode and Ag as cathode catalyst. High Open Circuit voltages (OCVs) were observed for the composite membranes 0.75, 0.83, 0.92 and 0.86 V for 1, 3,5 and 7% QTNT/QPSU respectively whereas the plain QPSU showed an OCV of 0.66 V. The increase in OCVs were due to composite membranes which were denser than the plain QPSU which may lower the fuel cross-over. The maximum power density of QPSU (a), 1% QTNT/QPSU (b), 3% QTNT/QPSU (c) 5% QTNT/QPSU (d) and 7% QTNT/QPSU (e) were found to be 158, 205, 241, 285 and 263 mWcm−2 respectively. From the polarization curves, it was also inferred that the current density of QPSU (a), 1% QTNT/QPSU (b), 3% QTNT/QPSU (c) and 5% QTNT/QPSU (d) and 7% QTNT/QPSU (e) were 425, 610, 680 and 720 mA/cm−2 respectively. The higher hydroxide conductivity of the composite membranes due to the presence of additional ion exchange groups (QTNT) and some degree of water uptake inturn lead to the higher fuel cell performance. The observed maximum power density (285 mWcm−2) value for 5% filler containing composite membrane was considerably higher and on par with earlier reports (Table 3). 4. Conclusion In summary, TNTs with tubular morphology were prepared by hydrothermal method and confirmed by TEM and XRD studies. Subsequently the TNT was functionalised with quaternary ammonium group by simple two-step process and it was analysed by FTIR, CP/ MAS NMR for the successful functionalization. Composite membranes containing various wt% of QTNT nano-fillers and QPSU were fabricated by solution casting method. The composite membranes showed an impressive enhancement in terms of the IEC, Water uptake, conductivity and tensile strength. The results exhibited by the membranes are

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favourable to utilize in alkaline fuel cell application. From the abovementioned studies, 5 wt% of QTNT composite membrane was found to exhibit the maximum electrochemical properties, with an OCV of 0.92 V and a maximum power density of 285 mW/cm2 at 60 °C operating temperature. These results showed that functionalised TNT composite membranes are promising for potential application in alkaline membrane fuel cells. This approach proves to be successful in achieving a high IEC anion exchange membranes using functionalised inorganic fillers. Acknowledgement The authors thank Council of Scientific and Industrial Research (CSIR), New Delhi, India (Vide letter No. 01(2452)/11/EMR-11, letter dated 16.05.2011) and Science and Engineering Research Board (SERB), New Delhi, India for the financial support (Vide file No. EMR/2016/ 005615). References [1] J. Cheng, G. He, F. 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