Investigating the usefulness of chitosan based proton exchange membranes tailored with exfoliated molybdenum disulfide nanosheets for clean energy applications

Carbohydrate Polymers 208 (2019) 504–512

Contents lists available at ScienceDirect

Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Investigating the usefulness of chitosan based proton exchange membranes tailored with exfoliated molybdenum disulfide nanosheets for clean energy applications

T

Kumar Divyaa, Dipak Ranab, Subbiah Alwarappanc, ⁎ Meenakshi Sundaram Sri Abirami Saraswathia, Alagumalai Nagendrana, a b c

Polymeric Materials Research Lab, PG & Research Department of Chemistry, Alagappa Government Arts College, Karaikudi, 630 003, India Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada CSIR-Central Electrochemical Research Institute (CSIR-CECRI), Karaikudi, 630003, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Chitosan Nanocomposite membrane Proton conductivity Methanol permeability Exfoliated molybdenum disulfide Direct methanol fuel cells

Chitosan based proton exchange membranes (PEMs) has been synthesized by a facile solution casting strategy using two-dimensional exfoliated molybdenum disulfide (E-MoS2) nanosheets. The prepared PEMs are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field-emission scanning electron microscopy (FESEM) with Energy dispersive X-ray spectroscopy (EDX), water uptake, Thermogravimetric analysis (TGA), AC impedance spectroscopy and cyclic voltammetry. In comparison with pure chitosan membrane, E-MoS2 nanosheets incorporated membranes exhibit excellent water absorbing capacity, ion-exchange capacity and proton conductivity. Moreover, the changes in roughness of nanocomposite membranes is investigated by atomic force microscopy (AFM) and the results confirm that the E-MoS2 nanosheets content enhances the surface roughness as well as provide good mechanical and thermal resistivity to the chitosan/E-MoS2 membranes. Chitosan membranes with 0.75% E-MoS2 nanosheets demonstrated higher proton conductivity of 2.92 × 10−3 Scm−1 and membrane selectivity of 8.9 × 104 Scm−3 s with reduced methanol permeability of 3.28 × 10−8 cm2 s−1. Overall, results evidenced that the chitosan/E-MoS2 nanocomposite membranes will be an alternate to Nafion in direct methanol fuel cells (DMFCs).

1. Introduction Till date there are extensive efforts to provide a clean energy and to minimize the concern for energy demand. Globally, majority of the power production is based on fossil fuel that pollutes the atmosphere and thereby affects the environment intensely. So, there is a critical need to look for alternative resources for the power generation (Sorrell, 2015). Proton exchange membrane fuel cells (PEMFCs) including direct methanol fuel cells (DMFCs) are considered as the most efficient alternate energy conversion strategies as they employ clean fuels and also deliver high energy (Deluca & Elabd, 2006; Wee, 2006). Recently, DMFCs are widely preferred for portable energy devices related applications. The functioning of DMFC depends on the use of polymer electrolyte material (Das, Dutta, Hazra, & Kundu, 2015; Dutta, Das, & Kundu, 2016; Dutta, Das, & Kundu, 2016; Dutta, Das, & Kundu, 2015). Often, perfluorinated material such as Nafion®, are employed for the design of proton exchange membrane (PEM) due to the existence of

⁎

high proton conductivity and excellent chemical resistance. On the other hand, Nafion® membrane is expensive and its conductivity is poor at high temperatures (Neelakandan et al., 2014; Velayutham, Kaushik, Rajalakshmi, & Dhathathreyan, 2007). Therefore, less expensive synthetic sulfonated hydrocarbon or bio polymer based PEM that offer excellent proton conductivity and stability at high temperature and minimizes the methanol permeability are widely preferred (Divya, Sri Abirami Saraswathi, Subbiah et al., 2018; Uma Devi, Divya, Kaleekkal, Rana, & Nagendran, 2018; Uma Devi, Divya, Rana, Sri Abirami Saraswathi, & Nagendran, 2018; Yang, Tu, Li, Shang, & Tao, 2010). At present, bio polymers such as starch, cellulose, chitosan are being used as an alternative PEM in fuel cells due to their biocompatibility and their minimal cost. Of various natural polymers, chitosan is the second most abundant biopolymers in environment and it is obtained by the de-acetylation of chitin (Ma & Sahai, 2013). Moreover, chitosan have been considered as the widely preferred material and employed in a variety of applications such as water treatment system, food and

Corresponding author. E-mail address: [email protected] (A. Nagendran).

https://doi.org/10.1016/j.carbpol.2018.12.092 Received 31 October 2018; Received in revised form 28 December 2018; Accepted 29 December 2018 Available online 30 December 2018 0144-8617/ © 2018 Elsevier Ltd. All rights reserved.

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

tissue engineering due to is non-toxicity, excellent hydrophilicity and its cost is very less (Wafiroh, Widati, Setyawati, & Buono, 2014). Recently, chitosan is employed as PEM and investigated for DMFC applications (Purwanto et al., 2016). However, the hydrophilicity of chitosan increases the membrane swelling and the resulting membrane is not stable. Moreover, the proton conductivity is very poor and thereby restricts its applications. In order to improve the stability, proton conductivity and reinforcement, nanofillers are dispersed effectively onto the chitosan matrix during membrane fabrication (Caetano et al., 2013; Sahu et al., 2011). At present, 2D layered nanomaterials such as graphene, MoS2, WS2 are preferred for plethora of applications due to their excellent physicochemical properties (Geim & Grigorieva, 2013; Toth et al., 2016). Moreover, these materials are used in many potential applications such as solid lubricant, transistors and as a cathode material in lithium ion batteries (Chianelli et al., 2006; Feng et al., 2009; Hernandez et al., 2008; Radisavljevic, Radenovic, Brivio, Giacometti, & Kis, 2011; Soon & Loh, 2007; Polcar & Cavaleiro, 2011). It is well known that the interaction between bulk MoS2 and polymer matrix will not be efficient since the absence of reactive groups in bulk MoS2 and thereby affects the dispersion of nanomaterial in the surface of polymer matrix. Besides, the bulk MoS2 is not having much attraction towards polymers and hydrophilic/hydrophobic additives as well as difficult to insert them into its galleries. So the bulk MoS2 material is preferably exfoliated and then used in the fabrication of nanocomposite membranes. During exfoliation of bulk MoS2 via ultrasonication, it is separated as single layers due to the weak van der Waals forces are held in between Mo and S layers. Exfoliated MoS2 (E-MoS2) nanosheets are effectively employed as fillers for the fabrication poly (methyl methacrylate) (Zhou et al., 2014) and poly(vinyl alcohol)based nanocomposite membranes (Zhou et al., 2012). Herein, we report the fabrication of chitosan/E-MoS2 nanocomposite PEMs by a facile solution casting method and its usefulness in clean energy applications.

Table 1 Chitosan and E-MoS2 composition of nanocomposite membranes. Membrane code

Composition (10 wt.%)

Chitosan Chitosan-0.5 Chitosan-0.75 Chitosan-1 Chitosan-1.5

Chitosan

E-MoS2

100 99.5 99.25 99 98.5

0 0.5 0.75 1 1.5

and stored at room temperature (Feng et al., 2014). 2.4. Characterization of chitosan/E-MoS2 nanocomposite membranes 2.4.1. FT-IR and XRD Chitosan/E-MoS2 nanocomposite membranes are analyzed by FT-IR and XRD using Shimadzu spectrophotometer (IRTracer-100, USA) and Bruker, D8 Advance X-ray powder diffractometer, Germany respectively. 2.4.2. FESEM-EDX and AFM The surface morphology together with elemental composition of membranes are analyzed by FESEM-EDX (VEGA3 TESCAN and BRUKER Nano Gmbh, Germany), and membrane surface roughness profile is probed by AFM (BT 02218, Nanosurf, Switzerland). 2.4.3. Water uptake, swelling ratio and contact angle In order to measure water uptake, the membrane samples are initially dried in an oven and weighed to know their dry weight, then the membrane sample is immersed in deionized water for 24 h and again the hydrated membranes are weighed to know their wet weight. A similar protocol is adopted to measure the swelling ratio, however here only the thickness of the membranes is measured before and after immersion in water. Finally the water uptake and swelling ratio are calculated using Eqs. (1) and (2) respectively

2. Experimental section 2.1. Materials High molecular weight chitosan is obtained from Sigma Aldirch, Saint Louis, USA (Product No. 419419; 75% deacetylated and viscosity of 800–2000 cps in 1% acetic acid). Sodium hydroxide, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and glacial acetic acid are purchased from Merck Millipore, India. Bulk MoS2 powder is received from Alfa Aesar. Sodium chloride and methanol are purchased from Loba Chemie, Pvt. Ltd. (Mumbai, India), and Sisco Research Laboratories (SRL), India respectively.

Water uptake(%) =

Wwet − Wdry Wdry

× 100

(1)

Where Wwet and Wdry are the weight of the membranes before and after absorption of water respectively

Swelling ratio(%) =

dwet − d dry d dry

× 100

(2)

Where d wet and ddry are the thickness of dry and wet membrane respectively. The surface hydrophilic character of the prepared membranes is probed by Goniometer (Rame-Hart Instrument, 250-F1, USA).

2.2. Preparation of E-MoS2 nanosheets E-MoS2 nanosheets are synthesized from bulk MoS2 as described in our previous study (Divya, Sri Abirami Saraswathi, Rana et al., 2018). Initially, the bulk MoS2 powder is dispersed in the exfoliation medium (DMF) and ultrasonicated for 3 h at 3000 rpm. The final product obtained is filtered and dried at 50 °C for 24 h in an oven.

2.4.4. Ion exchange capacity, proton conductivity and lambda value Ion exchange capacity of the nanocomposite membranes is determined by titration (Muthumeenal, Neelakandan, Kanagaraj, & Nagendran, 2016). Initially the membrane samples are cut into squares and subsequently immersed in 3.0 M NaCl for 24 h for protonation via the replacement of Na+ by H+. Later, the protonated solution is titrated against the standard 0.1 M NaOH using phenolphthalein indicator. Finally, the ion exchange is calculated from the following equation.

2.3. Membrane design Chitosan and E-MoS2 nanocomposite membranes are designed by a simple solution casting method and the composition of polymer and nanoadditive are shown in Table 1. Initially, E-MoS2 nanosheets are dispersed in 20 mL deionized water under ultrasonication for 1 h. Following this, the mixture is poured into another beaker that contains chitosan at different concentrations in 2 wt. % acetic acid. The resulting mixture is stirred at room temperature for 24 h. Finally, the homogenous solution obtained is transferred into the glass plate and allowed to dry at 50 °C for 24 h. The dried film is then neutralized by immersing the film in NaOH solution for 2 h. Following this, the film is peeled off

IEC=

VNaOH − CNaOH Wdry

(3)

Here VNaOH refers to the volume of NaOH consumed, CNaOH concentration of NaOH, and Wdry is the weight of dried membrane sample Proton conductivity measurement is conducted at room temperature and at 80 °C using AC impedance spectroscopy. At first, the sample to be tested is immersed in dilute sulphuric acid for 24 h. Consequently, the 505

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Fig. 1. (a) FT-IR spectra of chitosan and chitosan/E-MoS2 nanocomposite membranes (b) XRD spectrum of chitosan and chitosan-0.75% nanocomposite membranes.

pre-treated membranes are placed between two stainless steel electrodes and then appropriate frequency is applied (range 1 MHz to 10 Hz with 10 mV amplitude using Biologic SP200 electrochemical workstation). The resistivity is measured from the high-frequency intercept point of the impedance with the real axis and the conductivity of membranes is calculated from the following equation.

σ=

L RA

PCH3OH = m×

VB L × A CA

(5)

Here, PCH3OH is denoted as the methanol permeability of the membrane, VB and CA represents the volume of compartment ‘B’ and concentration of methanol in compartment ‘A’ respectively. ‘L’ is the thickness of the membrane with the active surface area (A) and ‘m’ is the slope obtained from the plot of concentration vs time.

(4)

Where, σ, R, L and A refers to the conductivity, resistivity, thickness and cross-sectional area of the membrane samples respectively (Kumar, Dutta, Das, & Kundu, 2014).

2.4.7. Membrane selectivity Membrane selectivity is described as the ratio between the proton conductivity and the methanol transport through the membranes and is calculated from the equation (Mondal, Soam, & Kundu, 2015)

2.4.5. Mechanical, thermal and oxidative stability of membranes The thermal and mechanical stability of prepared nanocomposite membranes are tested by Universal Testing Machine and Themogravimetric analyzer respectively. In TGA analysis, 10 mg of the membrane sample is analyzed in the temperature range 40 °C–950 °C (scan rate at 100C/min) using SDTQ600 (TA Instruments, New Castle, DE). The mechanical stability of the membrane sample is analyzed at room temperature and the sample is cut into rectangles (6 cm × 1 cm) Oxidative stability of the membrane sample is determined using Fenton’s reagent containing 3 wt.% H2O2 containing 4 ppm FeSO4. Initially, the membrane sample is immersed in this solution for 5 h and the sample was taken out and the resulting weight is measured (Jana et al., 2015).

Membrane selectivity(δ) =

Proton conductivity (σ) Methanol permeability(τ)

(6)

3. Results and discussion 3.1. XRD and FT-IR The chemical composition and the intercalation of E-MoS2 nanosheets in chitosan matrix are confirmed by FT-IR and XRD analysis, as shown in Fig. 1(a) and (b). In FT-IR, the peak that appears at 1575 cm−1 and 1651 cm−1 are attributed to the stretching vibration of C]O in eNHCOe group and NH bending of eNH2 group of chitosan respectively. The broadband at 2908 cm−1 and 3312 cm−1 represents the −CH2e stretching vibration and symmetric stretching of −OH group of chitosan. In comparison with pure chitosan, FT-IR of chitosan/ E-MoS2 membranes shows progressive disappearance of NH2 absorption peak at 1575 cm-1. This change is due to the synergistic electrostatic interaction between the negative charges on the E-MoS2 nanosheets and polycationic chitosan as well as hydrogen bonding between chitosan and hydroxyl groups of E-MoS2 (Jiang, Sun, Zhang, & Hou, 2018). The intercalation of E-MoS2 nanosheets on chitosan matrix is also evident from XRD and is shown in Fig. 1(b). The peak at 19.3° in 2θ angle represents the amorphous nature of chitosan while the chitosan0.75% membrane exhibit peaks at 18.21° and 29.9° that corresponds to the (100) and (103) plane. These peaks are due to the strong intercalation of nanosheets in chitosan matrix that effectively shielded the predominant (002) peak of E-MoS2 nanosheets and due to the sandwich structure of chitosan between the layered structure of E-MoS2 nanosheets resulting in the close packed arrangement (Feng et al., 2014; Saada & Bissessur, 2012).

2.4.6. Methanol uptake and permeability Membrane samples are immersed in 5.0 M methanol for 24 h, subsequently the samples are taken out from the solution and the reduction in concentration of methanol is measured to determine the methanol uptake. Methanol permeability through the membrane samples are measured using two compartment diffusion cell system and the methanol concentration at feed (Compartment A) and permeate (Compartment B) compartments are measured by cyclic voltammetry (Biologic SP-200 electrochemical workstation, potential range −0.2 V to 1.2 V, scan rate: 100 mVs−1, Pt, Pt wire and Ag/AgCl as the working, counter and reference electrodes respectively in sulfuric acid). In the beginning, the PEM sample to be tested is treated with distilled water for 24 h to attain a fully hydrated condition. Then the pre-treated PEM is placed between the diffusion cell compartments. Feed part of the compartment filled with 5.0 M methanol and the permeate compartment filled with deionized water. Subsequently, the whole setup is closed and stirred at room temperature. Finally, the methanol diffusion on water compartment is measured at 5 h intervals and the methanol permeability is calculated by the following equation (Jiang, Kunz, & Fenton, 2005) 506

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Fig. 2. FESEM images of chitosan (a) and chitosan-0.5% (b) membranes, FESEM with EDX spectrum of chitosan (c, c') and chitosan-0.75% (d, d') membranes.

morphology, while E-MoS2 incorporated PEMs demonstrate rough surface. In general, rougher surface of the membranes improve the hydrophilicity and thereby enhances the proton conductivity. As depicted in Table 3, the line (Ra and Rq) & area (Sa and Sq) roughness parameters of the chitosan-0.5% and chitosan-0.75% PEMs are augmented when E-MoS2 nanosheets are added. AFM images of nanocomposite membranes clearly show that the nodular (peak) structure with increase in the roughness parameters. The results are in good agreement with the FESEM reports.

3.2. FESEM/EDX and AFM The surface morphology and roughness of pure and modified chitosan membranes are shown in Figs. 2 and 3. It can be seen that the pure chitosan membrane possesses smooth surface with low roughness in comparison with the chitosan/E-MoS2 membranes as shown in FESEM and AFM images respectively. FESEM images clearly indicate that the EMoS2 nanosheets are distributed uniformly within the chitosan matrix and thus facilitate the surface roughness along with the uniform dispersion (Baby Suneetha, 2018). The existence of E-MoS2 nanosheets within the chitosan matrix is also confirmed by the FESEM with EDX analysis and is shown in Fig. 2c, c' and d, d'. EDX spectrum evidently shows the elemental peaks such as Mo, S, and N and proves the presence of E-MoS2 nanosheets in chitosan matrix. Composition of elements such as C, O, N, S and Mo are presented in Table 2. AFM images of chitosan and chitosan/E-MoS2 PEMs are examined to investigate their surface roughness and its influence on the performance. As shown in Fig. 3, chitosan PEM show flat and uniform surface

3.3. Water uptake, swelling ratio and contact angle Water uptake influences the performance of PEMs in terms of improving proton conducting pathway. The water uptake and swelling ratio of pure and chitosan nanocomposite membranes are listed in Table 4 and shown in Fig. 4(a). The results show that the water uptake values gradually increases from 21.0 to 25.7% for pure chitosan and chitosan-0.75% membranes respectively. It is due to the unsatisfied 507

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Fig. 3. AFM images of (a) chitosan, (b) chitosan-0.5% and (c) chitosan-0.75% membranes.

coordination sites between the molybdenum and the sulfur atom of EMoS2 nanosheets that allowed water molecule to occupy the vacant site and thus enhance the hydrophilicity of chitosan matrix (Divya, Sri Abirami Saraswathi, Rana et al., 2018; Divya, Sri Abirami Saraswathi, Subbiah et al., 2018). Swelling ratio of PEM reflects its suitability for practical applications in DMFCs. In general, higher the water uptake of PEM, swelling nature also increases. The swelling ratio of the pure chitosan is 6.3% and it increases to 11.4% for chitosan-0.75% membrane. This enhancement is due to the presence of hydrophilic hydroxyl groups in the E-MoS2 nanosheets. This result is in good agreement with the observed contact angle values of the PEMs (Li, Zhang, Zhang, & Huang, 2015) Surface hydrophilicity of the PEMs can be studied using contact angle measurements. In general, lower contact angle of membrane surface be a sign of higher surface hydrophilicity. As shown in Table 3 and Fig. 4(b), pure chitosan PEM shows the higher contact angle of 78.2° and it decreases to 73.4° and 67.7° for chitosan-0.5% and chitosan-0.75% membranes respectively. Due to the presence of polar hydroxyl groups in the E-MoS2 nanosheets there is a gradual increase in hydrophilicity and decrease in contact angle values of nanocomposite membranes.

Table 2 Elemental composition of chitosan nanocomposite membranes. Membrane

Elemental composition

Chitosan Chitosan-0.75%

C

O

N

S

Mo

59.68 48.18

29.69 33.47

10.04 6.69

– 3.72

– 7.94

Table 3 Surface roughness and contact angle values of pure and chitosan/E-MoS2 nanocomposite membranes. Membrane

Chitosan Chitosan-0.5% Chitosan-0.75%

Surface roughness (nm)

Contact angle (°)

Line roughness

LA

RA

78.2 73.4 67.7

78.7 73.3 69.0

Area roughness

Ra

Rq

Sa

Sq

83.7 96.4 897.2

99.5 135.5 1046.1

127.2 229.9 671.4

169.7 321.7 986.8

Table 4 Water uptake, swelling ratio, IEC, oxidative stability and tensile strength of chitosan/E-MoS2 nanocomposite membranes. Membrane

Water uptake (%)

Swelling ratio (%)

IEC meq g−1

Oxidative stability (%)

Tensile strength (MPa)

Chitosan Chitosan-0.5% Chitosan-0.75% Chitosan-1% Chitosan-1.5% Nafion-212

21.0 22.4 25.7 23.7 20.4 10.7

6.3 ± 0.2 8.4 ± 0.2 11.4 ± 0.1 9.3 ± 0.3 5.5 ± 0.4 9.9

0.31 0.42 0.51 0.47 0.44 0.73

94.5 95.3 96.6 92.7 90.2 –

30 55 74 62 45 25

± ± ± ± ±

0.1 0.3 0.4 0.2 0.3

508

± ± ± ± ±

0.3 0.4 0.3 0.2 0.1

± ± ± ± ±

1.1 2.3 2.0 1.6 1.4

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Fig. 4. Water uptake, swelling ratio, IEC (a), contact angle (b) and TGA (c) of chitosan and chitosan/E-MoS2 nanocomposite membranes.

3.4. Thermal stability

3.5. Ion-exchange capacity (IEC) and proton conductivity

Chitosan and chitosan/E-MoS2 nanocomposite PEMs showed excellent thermal stability. Usually inorganic 2D layered nanosheets exhibit high mechanical and thermal stability as they minimize the conduction of heat as well as restrict the polymer chain segmental mobility. As shown in Fig. 4(c), there are three stages of weight loss. First weight loss noticed up to 200 °C is due to the loss of absorbed water molecule, second weight loss between 250 and 400 °C is due to the decomposition of the surface or edges on the chitosan molecules and final weight loss between 400 and 650 °C is possibly due to the decomposition of intercalated E-MoS2 nanosheets. The results demonstrated that the intercalated E-MoS2 nanosheets on chitosan system extend the weight loss from 40 to 76% due to their strong interfacial interaction by means of hydrogen bonding and thus enhance the thermal stability of the PEMs.

IEC is directly associated with the availability of the proton conducting sites on the membrane matrix. IEC of pure and nanocomposite membranes are listed in Table 4. As shown in Table 4, the addition of EMoS2 increased the IEC of 0.51 meq g−1 for chitosan-0.75% membrane. It indicates that the increase in E-MoS2 nanosheets increase the IEC values. It is due to the presence of hydrophilic hydroxyl groups at the EMoS2 nanosheets that provided conducting ionic sites. However, the IEC values of PEMs start to decrease beyond 0.75% addition of E-MoS2 nanosheets is possibly due to the agglomeration of nanosheets. Proton conductivity of the PEMs is associated with its IEC, water uptake, surface hydrophilicity and morphology. As shown in Table 5, chitosan/E-MoS2 nanocomposite PEMs exhibits increase in proton conductivity up to the incorporation of 0.75% of E-MoS2 nanosheets at room temperature and 80 °C. Chitosan-0.75% PEM shows the maximum 509

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Table 5 Proton conductivity, methanol permeability and selectivity of chitosan/E-MoS2 nanocomposite membranes. Membrane

Chitosan Chitosan-0.5% Chitosan-0.75% Chitosan-1% Chitosan-1.5% Nafion -212

Proton conductivity × 10−3Scm−1 At Room Temperature

At 80 °C

Methanol permeability ×10−8 cm2 s−1

0.62 2.36 2.92 2.63 2.18 40

1.48 3.22 3.61 3.53 3.02 –

5.20 4.14 3.28 3.57 4.02 550

proton conductivity value of 2.92 × 10−3 and 3.61 × 10−3 Scm-1 at room temperature and 80 °C respectively. Beyond this, the proton conductivity is found to decrease due to the agglomeration of nanosheets that aggravate the boundary effect. The augmentation in proton conductivity of chitosan/E-MoS2 nanocomposite PEMs possibly due to the presence of −OH group on E-MoS2 nanosheets may provide the anionic proton conducting framework. Further the interaction between chitosan and −OH group develops a continuous hydrophilic proton conducting channels which allows the transfer of proton via vehicular mechanism in the form of hydronium ions. In addition, the size of the bonded atom also enhances the polarizability. In the case of E-MoS2 nanosheets, S2- is larger than O2- and has polarizable framework that possibly increased the proton mobility. Further, the electrostatic interaction between polycationic chitosan and plenty of −OH groups containing E-MoS2 nanosheets can also be a reason for the increased proton mobility. Therefore, proton hopping may also occur via ionic and hydrogen bonds. Thus it is evident that the proton conductivity of chitosan/E-MoS2 nanocomposite PEMs is the result of both vehicular and Grotthuss mechanisms (Feng, Tang, & Wu, 2013; Li et al., 2015; Vijayalekshmi & Khastgir, 2018)

Methanol uptake (%)

Membrane selectivity ×104 Scm−3 s

23.3 19.4 16.3 16.7 17.2 –

1.2 5.7 8.9 7.4 5.4 0.72

± ± ± ± ±

0.3 0.2 0.1 0.1 0.3

Espinosa-Medina, & Sebastián, 2003). From the results, it is concluded that the chitosan-0.75% membrane possesses high methanol resistance than pure chitosan and Nafion membranes. 3.7. Oxidative and mechanical stability Durability of the PEMs is assessed by the oxidative stability. When fuel cell is functioned, the peroxyl and hydroxyl radicals were generated and attack the membrane matrix harshly which reduces the stability of the membranes. Herein the oxidative stability of chitosan and chitosan/E-MoS2 nanocomposite was evaluated using Fenton’s reagent and the values are displayed in Table 4. The results show that the chitosan-0.75% nanocomposite membrane shows the highest oxidative stability of 96.6%. The enhancement in oxidative stability of tailored nanocomposite PEMs is ascribed by the stable dispersion of E-MoS2 nanosheets makes the membrane more rigid. Mechanical stability of the chitosan and chitosan/EMoS2nanocomposite PEMs are mentioned in Table 4. E-MoS2 incorporated chitosan PEMs demonstrated higher mechanical stability of 74 MPa upon comparision with pure chitosan PEM (30 MPa). The increase in mechanical stability is possibly due to the fine dispersion of EMoS2 nanosheets that created strong interfacial interaction with chitosan matrix and resulted in the reinforcement of membrane (Zhou et al., 2012)

3.6. Methanol uptake and permeability Methanol uptake behaviour of pure chitosan and chitosan/E-MoS2 nanocomposite PEMs are tabulated in Table 5. It is evident that methanol uptake of PEMs decreases from pure chitosan to nanocomposite membranes up to the incorporation of 0.75% of E-MoS2 nanosheets. This can attributed to the excellent network structures and bonding between chitosan and E-MoS2 nanosheets which restricts the methanol uptake or permeability (Shaari et al., 2018). Methanol permeability of chitosan and chitosan/E-MoS2 nanocomposite membranes is shown in Table 5. From the table it is observed that the chitosan-0.75% membrane shows low methanol permeability of 3.28 × 10−8 cm2s-1 in comparison with pure chitosan and Nafion membranes. Fig. 5 is the representative cyclic voltammogram corresponding to the methanol oxidation after 24 h at methanol compartment (a) and water compartment (b) when separated by pure chitosan and chitosan/E-MoS2 nanocomposite membranes. It can be seen that the oxidation of methanol on Pt electrode was associated from 0.2 to 0.8 V with the upward and downward scan for 0.5 V and 0.4 V respectively which was found in methanol permeability test (Cho et al., 2004). The observed results indicate that the pure chitosan membranes possess the highest anodic peak current value of Ia = 0.074 mAcm−2 on water side and the lowest peak current value of Ia = 0.036 mAcm-2 on methanol side. On the other hand, the chitosan-0.75% membranes possess the least current value of Ia = 0.008 mAcm-2 on water side and the highest peak current value of Ia = 0.074 mAcm-2 on methanol side in comparison with pure chitosan membranes. It indicates that the chitosan membranes tailored with E-MoS2 nanosheets strongly suppressed the methanol transport as a result of their ionic channels which increased the tortures path that restricted the flow of methanol from methanol side to water side via membrane matrix (Smit, Ocampo,

3.8. Membrane selectivity The selectivity of PEMs is the direct indication of its performance in DMFCs. As shown in Table 5, E-MoS2 incorporated PEMs possess higher selectivity up to 0.75% composition in comparison with pure chitosan membranes and the order of selectivity is found to be chitosan < chitosan-0.5% < chitosan-0.75%. Beyond this concentration, the selectivity starts decreasing due to the agglomeration of E-MoS2 nanosheets creates the boundary effect. The results clearly show that the chitosan/E-MoS2 nanocomposite PEMs displayed excellent selectivity for DMFC applications. 4. Conclusions Chitosan/E-MoS2 nanocomposite PEMs are successfully fabricated by solution casting technique. FT-IR, XRD, and FESEM results confirmed the successful introduction of E-MoS2 into the chitosan matrix. Addition of E-MoS2 enhanced the hydrophilic nature of the chitosan PEMs which indicate higher proton conduction pathway. The hydrogen bonding between chitosan and E-MoS2 nanosheets enhanced the oxidative, mechanical and thermal stability of the nanocomposite membranes. AFM results confirmed the increase in both the surface area and surface roughness of the Chitosan/E-MoS2 nanocomposite PEMs. The methanol permeability of PEMs is found to gradually decrease from chitosan to chitosan-0.75% PEMs. Overall, results demonstrated that the chitosan/E-MoS2 nanocomposite PEMs exhibited higher conductivity and selectivity as well as restricted methanol permeability in comparison with pure chitosan and Nafion 212 membranes. 510

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Fig. 5. Cyclic voltammogram of methanol oxidation after 24 h at methanol compartment (a) and water compartment (b) separated by pure chitosan and chitosan/EMoS2 nanocomposite membranes at a scan rate of 100 mV.

Acknowledgements

Nanotechnology, 3, 563–568. Jana, K. K., Charan, C., Shahi, V. K., Mitra, K., Ray, B., Rana, D., et al. (2015). Functionalized poly(vinylidene fluoride) nanohybrid for superior fuel cell membrane. Journal of Membrane Science, 481, 124–136. Jiang, R., Kunz, H. R., & Fenton, M. J. (2005). Investigation of membrane property and fuel cell behavior with sulfonated poly(ether ether ketone) electrolyte: Temperature and relative humidity effects. Journal of Power Sources, 150, 120–128. Jiang, X., Sun, Y., Zhang, H., & Hou, L. (2018). Preparation and characterization of quaternized poly(vinyl alcohol)/chitosan/MoS2 composite anion exchange membranes with high selectivity. Carbohydrate Polymers, 180, 96–103. Kumar, P., Dutta, K., Das, S., & Kundu, P. P. (2014). Membrane prepared by incorporation of cross-linked sulfonated polystyrene in the blend of PVdF-co-HFP/Nafion: A preliminary evaluation for application in DMFC. Applied Energy, 123, 66–74. Li, J., Zhang, Y., Zhang, S., & Huang, X. (2015). Sulfonated poly imide/S-MoS2 composite membrane with high proton selectivity and good stability for vanadium redox flow battery. Journal of Membrane Science, 490, 179–189. Ma, J., & Sahai, Y. (2013). Chitosan biopolymer for fuel cell applications. Carbohydrate Polymers, 92, 955–975. Mondal, S., Soam, S., & Kundu, P. P. (2015). Reduction of methanol crossover and improved electrical efficiency in direct methanol fuel cell by the formation of a thin layer on Nafion 117 membrane: Effect of dip-coating of a blend of sulphonated PVdFco-HFP and PBI. Journal of Membrane Science, 474, 140–147. Muthumeenal, A., Neelakandan, S., Kanagaraj, P., & Nagendran, A. (2016). Synthesis and properties of novel proton exchange membranes based on sulfonated polyethersulfone and N-phthaloyl chitosan blends for DMFC applications. Renewable Energy, 86, 922–929. Neelakandan, S., Rana, D., Matsuura, T., Muthumeenal, A., Kanagaraj, P., & Nagendran, A. (2014). Fabrication and electrochemical properties of surface modified sulfonated poly (vinylidenefluoride-co-hexafluoropropylene) membranes for DMFC application. Solid State Ionics, 268, 35–41. Polcar, T., & Cavaleiro, A. (2011). Review on self-lubricant transition metal dichalcogenides nanocomposite coatings alloyed with carbon. Surface & Coatings Technology, 206, 686–695. Purwanto, M., Atmaja, L., Mohamed, M. A., Salleh, M. T., Jaafar, J., Ismail, A. F., et al. (2016). Biopolymer-based electrolyte membranes from chitosan incorporated with montmorillonite - crosslinked GPTMS for direct methanol fuel cells. RSC Advances, 6, 2314–2322. Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V., & Kis, A. (2011). Single-layer MoS2 transistors. Nature Nanotechnology, 6, 147–150. Saada, I., & Bissessur, R. (2012). Nanocomposite materials based on chitosan and molybdenum disulfide. Journal of Materials Science, 47, 5861–5866. Sahu, A. K., Bhat, S. D., Pitchumani, S., Sridhar, P., George, C., Chandrakumar, N., et al. (2011). Bio-composite membrane electrolytes for direct methanol fuel cells. Journal of the Electrochemical Society, 158, B1319–B1328. Shaari, N., Kamarudin, S. K., Basri, S., Shyuan, L. K., Masdarand, M. S., & Nordin, D. (2018). Enhanced proton conductivity and methanol permeability reduction via sodium alginate electrolyte-sulfonated graphene oxide bio-membrane. Nanoscale Research Letters, 13, 82. Smit, M. A., Ocampo, A. L., Espinosa-Medina, M. A., & Sebastián, P. J. (2003). A modified Nafion membrane with in situ polymerized polypyrrole for the direct methanol fuel cell. Journal of Power Sources, 124, 59–64. Soon, J. M., & Loh, K. P. (2007). Electrochemical double-layer capacitance of MoS2 nanowall films. Electrochemical and Solid-State Letters, 10, A250–A254. Sorrell, S. (2015). Reducing energy demand: Are view of issues challenges and approaches. Renewable and Sustainable Energy Reviews, 47, 74–82. Toth, P. S., Velický, M. J., Bissett, M. A., Slater, T. J., Savjani, N., Rabiu, A. K., et al.

AN thanks Science and Engineering Research Board (SERB), Government of India for the grant EMR/2016/007670. References Baby Suneetha, R. (2018). Spectral, thermal and morphological characterization of biodegradable graphene oxide – chitosan nanocomposites. Journal of Nanoscience and Technology, 2, 342–344. Caetano, C. S., Caiado, M., Farinha, J., Fonseca, I. M., Ramos, A. M., Vital, J., et al. (2013). Esterification of free fatty acids over chitosan with sulfonic acid groups. Chemical Engineering Journal, 230, 567–572. Chianelli, R. R., Siadati, M. H., De la Rosa, M. P., Berhault, G., Wilcoxon, J. P., Bearden, R., et al. (2006). Catalytic properties of single layers of transition metal sulfide catalytic materials. Catalysis Reviews – Science and Engineering, 48, 1–41. Cho, K. Y., Eom, J. Y., Jung, H. Y., Choi, N. S., Lee, Y. M., Park, J. K., et al. (2004). Characteristics of PVdF copolymer/Nafion blend membrane for direct methanol fuel cell (DMFC). Electrochimica Acta, 50, 583–588. Das, S., Dutta, K., Hazra, S., & Kundu, P. P. (2015). Partially sulfonated poly (vinylidene fluoride) induced enhancements of properties and DMFC performance of Nafion electrolyte membrane. Fuel Cells, 15, 505–515. Deluca, N. W., & Elabd, Y. A. (2006). Polymer electrolyte membranes for the direct methanol fuel cell: A review. Journal of Polymer Science Part B: Polymer Physics, 44, 2201–2225. Divya, K., Sri Abirami Saraswathi, M., Rana, D., Subbiah, A., & Nagendran, A. (2018). Custom-made sulfonated poly (ether sulfone) nanocomposite proton exchange membranes using exfoliated molybdenum disulfide nanosheets for DMFC applications. Polymer, 147, 48–55. Divya, K., Sri Abirami Saraswathi, M., Subbiah, A., Nagendran, A., & Rana, D. (2018). Sulfonated poly (ether sulfone)/poly (vinyl alcohol) blend membranes customized with tungsten disulfide nanosheets for DMFC applications. Polymer, 155, 42–49. Dutta, K., Das, S., & Kundu, P. P. (2015). Synthesis, preparation, and performance of blends and composites of π-conjugated polymers and their copolymers in DMFCs. Polymer Reviews, 55, 630–677. Dutta, K., Das, S., & Kundu, P. P. (2016a). Polyaniline nanowhiskers induced low methanol permeability and high membrane selectivity in partially sulfonated PVdF-coHFP membranes. RSC Advances, 6, 107960–107969. Dutta, K., Das, S., & Kundu, P. P. (2016b). Highly methanol resistant and selective ternary blend membrane composed of sulfonated PVdF‐co‐HFP, sulfonated polyaniline and nafion. Journal of Applied Polymer Science, 133, 43294. Feng, K., Tang, B., & Wu, P. (2013). Selective growth of MoS2 for proton exchange membranes with extremely high selectivity. ACS Applied Materials & Interfaces, 24, 13042–13049. Feng, X., Wang, X., Xing, W., Zhou, K., Song, L., & Hu, Y. (2014). Liquid-exfoliated MoS2 by chitosan and enhanced mechanical and thermal properties of chitosan/MoS2 composites. Composites Science and Technology, 93, 76–82. Feng, C. Q., Li, J., Ma, H., Zeng, R., Guo, Z. P., & Liu, H. K. (2009). Synthesis of molybdenum disulfide (MoS2) for lithium ion battery applications. Materials Research Bulletin, 44, 1811–1815. Geim, A. K., & Grigorieva, I. V. (2013). Van der Waals heterostructures. Nature, 499, 419–425. Hernandez, Y., Nicolosi, V., Lotya, M., Blighe, F. M., Sun, Z. Y., De, S., et al. (2008). Highyield production of graphene by liquid-phase exfoliation of graphite. Nature

511

Carbohydrate Polymers 208 (2019) 504–512

K. Divya et al.

Wafiroh, S., Widati, A. A., Setyawati, H., & Buono, G. P. (2014). Synthesis and characterization of hybrid zeolite, a chitosan sulfonated membrane for proton exchange membrane fuel cell (PEMFC). Journal of Chemical and Pharmaceutical Research, 6(9), 71–76. Wee, J. H. (2006). Which type of fuel cell is more competitive for portable application: Direct methanol fuel cells or direct borohydride fuel cells. Journal of Power Sources, 161, 1–10. Yang, X., Tu, Y., Li, L., Shang, S., & Tao, X. (2010). Well-dispersed chitosan/graphene oxide nanocomposites. ACS Applied Material Interfaces, 2, 1707–1713. Zhou, K., Liu, J., Wang, B., Zhang, Q., Shi, Y., Jiang, et al. (2014). Facile preparation of poly(methyl methacrylate)/MoS2 nanocomposites via in situ emulsion polymerization. Materials Letters, 126, 159–161. Zhou, K., Jiang, S., Bao, C., Song, L., Wang, B., Tang, G., et al. (2012). Preparation of poly (vinyl alcohol) nanocomposites with molybdenum disulfide (MoS2): Structural characteristics and markedly enhanced properties. RSC Advances, 2, 11695–11703.

(2016). Asymmetric MoS2/graphene/metal sandwiches: Preparation, characterization, and application. Advanced Materials, 28, 8256–8264. Uma Devi, A., Divya, K., Kaleekkal, N. J., Rana, D., & Nagendran, A. (2018). Tailored SPVdF-co-HFP/SGO nanocomposite proton exchange membranes for direct methanol fuel cells. Polymer, 140, 22–32. Uma Devi, A., Divya, K., Rana, D., Sri Abirami Saraswathi, M., & Nagendran, A. (2018). Highly selective and methanol resistant polypyrrole laminated SPVdF-co-HFP/PWA proton exchange membranes for DMFC applications. Materials Chemistry and Physics, 212, 533–542. Velayutham, G., Kaushik, J., Rajalakshmi, N., & Dhathathreyan, K. S. (2007). Effect of PTFE content in gas diffusion media and microlayer on the performance of PEMFC tested under ambient pressure. Fuel Cells, 7, 314–318. Vijayalekshmi, V., & Khastgir, D. (2018). Fabrication and comprehensive investigation of physicochemical and electrochemical properties of chitosan-silica supported silicotungstic acid nanocomposite membranes for fuel cell applications. Energy, 142, 313–330.

512