A N-o-sulphonic acid benzyl chitosan (NSBC) and N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO) based multi-functional polymer electrolyte membrane with enhanced water retention and conductivity

Abstract

To achieve good proton conductivity even under dehydrated conditions, water retention is a pervasive issue for polymer electrolyte membranes (PEMs). Herein, we report a green and easy preparation procedure for multifunctional PEM grafted with –PO₃H₂ and –SO₃H groups based on a functionalized biopolymer (N-o-sulphonic acid benzyl chitosan (NSBC)) and N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO). Loading of NMPSGO in the NSBC matrix enhanced water retention, proton conductivity, stability and other desired properties of PEM relevant for DMFC applications. The reported NSBC/NMPSGO composite PEM was designed to promote internal self-humidification responsible for water retention properties, to promote proton conduction due to the presence of different acidic functional groups. It was hypothesized that strong hydrogen bonding between multi-functional groups due to the presence of inter-connected hydrophobic graphene sheets and organic polymer chains provides a hydrophobic–hydrophilic phase separation and suitable architecture for proton conducting channels. The most suitable PEM (NSBC/NMPSGO-8) exhibited 4.42% bound water content; 8.87 × 10⁻² S cm⁻¹ proton conductivity; 2.09 mequiv. per g ion-exchange capacity; and 16.93 × 10⁻⁷ cm² s⁻¹ methanol permeability. The proposed route offers a facile and generic strategy to design a variety of composite functional materials, in which both organic (NSBC) and inorganic (NMPSGO) were functionalized, with superior functional group molality, proton conductivity, water retention properties and stability.

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Introduction

Materials pertaining to water retention were initially developed for agricultural and medical areas.^1,2 However, recently water retention has been identified as serious concern for fuel cell PEMs.^3,4 In fuel cell applications, high water retention for PEMs is urgently required to achieve high performance and long-lifetime without any deterioration due to dehydration.^5–7 In general, PEM contains two types of water: bound water and bulk water.^8,9 A high bound water content in the membrane matrix provides sufficient proton carriers, and dissociates functional proton conductive groups by solvation, while slow water release reduces dehydration suitable for high proton conductivity under low relative humidity (RH) at intermediate temperatures. Generally, perfluorosulfonic acid PEMs such as Nafion, are reference membranes for the direct methanol fuel cell (DMFC) because of their desirable electrochemical properties as well as excellent chemical stability.^10–13 However, the reduced performance of Nafion above 80 °C, because of high methanol crossover, dehydration, and cost, is a serious concern.^4,9–13

Recently, tremendous efforts were rendered to improve the water retention capacity of PEM via physical or chemical modifications.^14–17 Water retention capacity of PEMs was enhanced by incorporating hydrophilic fillers, which can be characterized into inorganic or polymeric materials.^14–18 Different inorganics (e.g. silica and zeolites) were overwhelmingly utilized to improve the water uptake of the PEM, since they provide numerous hydrogen-bonding sites and hydrophilic domains in the membrane phase.^3,9,16 However, incorporation of these uncharged fillers deteriorates proton conductivity and water release was too rapid due to weak interactions between filler and water molecules. In addition, graphene oxide (GO) was also considered as attractive organic filler in PEM, because its incorporation enhanced proton conductivity, water retention and provides electron insulating environment.^19,20 GO is an attractive material and provide potential solution for many vital issues in energy research.²¹ Recently, significant efforts were rendered to improve the efficiency, processability, stability and cost effective-ness for fuel cells using graphene.^21–24 It was realized that proton conductivity of GO could be further improved after acid functionalization by grafting of –PO₃H₂ groups.

The best way to develop an ecofriendly and biocompatible PEM is to use naturally abundant biopolymers such as chitosan.^25–29 However, conductivity and performances of biopolymer based PEMs have not yet achieved the desirable target for practical applications. Grafting of multi-functional groups and desired level of cross-linking expected to exhibit high level of proton conductivity along with stability of the composite PEM.

In this manuscript, for the first time, we are reporting N-o-sulphonic acid benzyl chitosan (NSBC) and N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO) composite membrane with improved proton conductivity, water retention and mechanical properties for DMFC applications.

Results and discussion

Structural characterization of aminopropylsilane graphene oxide (APSGO) and NMPSGO

FT-IR spectrum of APSGO showed absorption bands at: ν ∼ 2927 cm⁻¹ (C–H stretching of methylene groups); ν ∼ 1115 and 694 cm⁻¹ (Si–O and Si–C stretching, respectively). Presence of free terminal primary amine groups in APSGO was confirmed by absorption bands at ν ∼ 3437, 1627, 1035, and 791 cm⁻¹ (–N–H stretch, –N–H bend, –C–N stretch, and –N–H wag of aliphatic primary amine, respectively) (Fig. 1a). Phosphorylation (1 : 1 (w/w) formaldehyde and phosphorous acid) of APSGO was carried out for preparing NMPSGO and confirmed by FT-IR and ³¹P-NMR spectra (Fig. 1b & 2). FT-IR spectrum of NMPSGO exhibited absorption band at ν ∼ 1388 cm⁻¹, due to the P=O stretch of organophosphate, while ³¹P-NMR spectrum showed a broad signal around δ = 7.47 ppm. Based on the spectral informations, schematic route for the preparation of N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO) has been described in Scheme 1.

Fig. 1 FT-IR spectra of: (a) APSGO, and (b) NMPSGO.

Fig. 2 Solid state ³¹P-NMR spectrum of NMPSGO.

Scheme 1 Schematic route for the preparation of N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO).

Transmission electronic microscopy (TEM) image of APSGO revealed relatively flat, exfoliated with wrinkled structure, while black dots on the surface of NMPSGO aroused due to the clustering of phosphonic acid groups (Fig. 3a & b). Presence of silica, nitrogen and oxygen in APSGO was confirmed by Energy dispersive spectroscopy (EDS) spectrum (Fig. 3c). The EDS spectrum of NMPSGO also showed the presence of phosphorous element (Fig. 3d).

Fig. 3 TEM Images of (a) APSGO, (b) NMPSGO, (c) EDS of APSGO, and (d) EDS of NMPSGO.

Characterization of the NSBC/NMPSGO composite membrane

NSBC/NMPSGO composite membranes were prepared by solution casting method followed by chemical cross-linking using glutaraldehyde. The FT-ATR spectrum of NSBC/NMPSGO-8 composite membrane showed absorption band at ν ∼ 1202 (sym. SO₂ stretch), ∼1291 cm⁻¹ (asym. SO₂ stretch) and confirmed presence of sulphonic acid groups (Fig. 4). Absorption band at ν ∼ 1672 cm⁻¹ and a weak absorption band at ∼1452 cm⁻¹ correspond to carbonyl stretching and O–H stretching of the –COOH group, respectively. Further, absorption band at ν ∼ 1335 cm⁻¹ (P=O stretch) confirmed the presence of –PO₃H groups. In Fig. 4, presence of peaks at ν ∼ 1056 cm⁻¹ and ∼744 cm⁻¹ indicated the presence of Si–O and Si–C groups in the membrane, respectively. Strong absorption band at ν ∼ 1123 cm⁻¹ observed due to the C–O stretch of large ring cyclic ethers, and confirmed the cross-linked structure of membrane matrix. Cross-sectional SEM image for NSBC/NMPSGO-8 membrane (Fig. 5a) revealed homogeneous distribution of NMPSGO in polymer matrix. Optical images of the pristine (NSBC/NMPSGO-0), and composite membrane (NSBC/NMPSGO-8) (Fig. 5b & c) also confirmed that membranes lost their transparency after incorporation of NMPSGO in the polymer matrix. NSBC contained –SO₃H group, while –PO₃H₂ and –COOH groups were present in NMPSGO along with hydrophobic carbon chains/sheets. This type of assembly for hydrophilic and hydrophobic segments expected to exhibit the phase separation and formation of proton conducting channels along with cross-linked carbon chains (Scheme 2).

Fig. 4 FT-ATR spectrum of NSBC/NMPSGO-8 membrane.

Fig. 5 SEM image: (a) NSBC/NMPSGO-8 (cross-section). Optical Images: (b) NSBC/NMPSGO-0 (with 0 wt% NMPSGO content), (b) NSBC/NMPSGO-8 (with 8 wt% NMPSGO content).

Scheme 2 Schematic reaction route for the preparation of NSBC/NMPSGO composite membranes.

The effect of NMPSGO content on the crystallinity of NSBC/NMPSGO composite membranes was investigated by WXRD (Fig. S1†). Diffraction peak for GO was observed at 11.80° (2θ), which was shifted at 2θ = 20° for APSGO (prepared by chemical modification of GO) and associated with the intricacy of an amorphous region. Further, NMPSGO, and NSBC/NMPSGO-8, also shows diffraction peak around 2θ = 20°. Increase in peak broadening revealed the reduction in crystallinity and strong interactions between NSBC, PVA and NMPSGO, responsible better compatibility between membrane forming materials and homogenous membrane matrix.

Thermal and mechanical properties

Thermal stability of NSBC/NMPSGO composite membranes was investigated by TGA and studied membranes exhibited three-step weight loss (Fig. S2†). First step weight loss (below 150 °C) was attributed to the evaporation of absorbed/bound water. Second step weight loss (∼200–400 °C) was observed due to decomposition of oxygen containing functional groups. Third step weight loss (>450 °C) was aroused due to the decomposition of polymer backbone. For different composite membranes, weight loss reduced with increase in NMPSGO content in the membrane matrix. NSBC/NMPSGO-0 and NSBC/NMPSGO-8 composite membrane showed about 29.67 & 39.04 wt% residues, respectively, while in case of unmodified GO incorporated composite membrane (NSBC/GO-8), 33.99 wt% residual mass confirmed the improved thermal stability of NMPSGO and prepared composite membranes.

The mechanical stability of prepared membranes was studied in terms of storage modulus via the dynamic mechanical analyzer (Fig. S3†). The storage modulus of pristine membrane (NSBC/NMPSGO-0) increased with the incorporation of NMPSGO and afforded significant mechanical reinforcement. Pristine membrane (NSBC/NMPSGO-0) exhibited ∼1651 MPa storage modulus, while NSBC/NMPSGO-8 composite membrane exhibited ∼2462 MPa storage modulus at 30 °C. These studies revealed the improved thermal and mechanical stability of composite membranes in compare with pristine membrane. It was observed that incorporation of NMPSGO (>8 wt%) in polymer matrix result aggregation of NMPSGO, this was further reduced the membrane performance and improve the brittle nature of the membrane.

Burst strength, oxidative and hydrolytic stabilities

Burst strength of composite membrane increased with NMPSGO content in the membrane phase and as a reference NSBC/NMPSGO-8 membrane exhibited 2.24 kg cm⁻² burst strength (Table S1†). In addition, oxidative and hydrolytic stabilities of composite membranes (wet condition) were extensively analyzed by recoding the weight loss under treatment with Fenton's reagent at 80 °C for 1 h, and pressurized steam at 140 °C for 24 h, respectively. In both conditions membranes were unbroken and showed about 0.48–3.38% weight loss (Table S1†). Oxidative and hydrolytic stable nature prepared composite membranes was attributed to the strong interactions between multi-functional groups in the membrane matrix.

Water uptake, swelling ratio, ion-exchange capacity (IEC), and state of water

Incorporation of non-ionic materials (such as TiO₂, SiO₂, graphene etc.) in PEM matrix generally improves the membrane stability but deterioration in ionic conductivity and IEC is serious problem. To avoid the deterioration in membrane properties, incorporation of functionalized GO (–PO₃H₂ acid grafted) in the polymer matrix was achieved. To assess the functional and hydrophilic character of PEM, water uptake, swelling ratio, and IEC are the important parameters. Total water uptake (bulk and bound water) depends on density of hydrophilic functional groups in the membrane matrix, and entrapment of water in void space via hydrogen bonding. Water uptake (WU) value for composite membranes was increased with NMPSGO content in the membrane matrix (Table 1). Presence of water molecules within PEM matrix is essential for proton conduction. Pristine NSBC/NMPSGO-0 and NSBC/GO-8 showed relatively low WU values and swelling ratio, which were further depended on NMPSGO content in membrane matrix for composite membranes. In this case, both membrane forming constituents, biopolymer (chitosan) and NMPSGO contained –SO₃H and –COOH/–PO₃H₂ groups, respectively. Membranes functional and hydrophilic character increased with NMPSGO content and relatively high charged density were attributed to the presence of multi-functional groups (carboxylic/sulphonic/phosphonic acid group). Thus, study on trade-off behavior of composite membranes with varied composition is necessary to understand the impact of functionalized biopolymer and functionalized graphene.

Table 1 Water uptake (WU), ion-exchange capacity (IEC), swelling ratio and number of water molecules per ionic site (λ) values for different membranes

Membrane	WU (%)	IEC (mequiv. per g)	Swelling ratio^a (%)	λ
a Recorded at 60 °C.
NSBC/NMPSGO-0	35.76	1.29	17.27	14.97
NSBC/NMPSGO-4	45.93	1.56	18.05	16.35
NSBC/NMPSGO-6	58.83	1.84	18.83	17.76
NSBC/NMPSGO-8	69.98	2.09	19.15	18.60
NSBC/GO-8	32.76	1.23	15.18	14.79
Nafion 117	41.60	0.90	15.05	25.68

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IEC is a measure of density of ionizable hydrophilic functionalities present in the membrane matrix, and responsible for the proton conduction. IEC values for composite membranes increased with NMPSGO content in the membrane matrix and prepared (NSBC/NMPSGO) composite membranes showed relatively high IEC value in compare with Nafion 117 membrane (0.90 mequiv. per g) (Table 1). Herein, biopolymer (NSBC) bears –SO₃H group, while inorganic graphene (NMPSGO) contains –COOH/–PO₃H₂ groups, responsible for high molality for ionizable groups. At room temperature and 100% RH, NSBC/NMPSGO-8 membrane exhibited 69.98 wt% WU corresponding to 18.60 water molecules per ionic site (λ). High water molality in the membrane matrix was responsible for the formation of ionic clusters in the membrane matrix and thus more facile migration of hydronium ions via vehicle mechanism.^20,30,31

Water retention ability of NSBC/NMPSGO composite membranes

Presence of water in the PEM matrix even at elevated temperature (>100 °C) is necessary for proton conduction, which alternatively depends on water retention ability of membrane. PEM contains two types of water: bound water and the bulk water.³ Bound water required for the solvation of the acidic groups, while bulk water fills the void volume. For simplicity, weight loss% for different membrane samples was fixed 100% at 100 °C (Fig. S4†). Water vapour sorption and diffusion properties of PEMs exhibit profound effect on their conductivity and suitability for fuel applications. (M_t/M_o)−t (time) curves (Fig. 6(a)) illustrates the water retention capability of NSBC/NMPSGO composite membranes. De-swelling kinetics of these composite membranes was further analysed by (M_t/M_o − t^1/2) curves (Fig. 6(b)) using Higuchi's model.³²

M_t/M_o = −kt^1/2 + 1 (1)

where M_o and M_t are amount of water present in polymer matrix, initial and at any given time, respectively, while k is a constant. For different composite membranes with varied NMPSGO content, obtained straight lines were fitted to Higuchi's model, and suggested diffusion controlled water desorption mechanism. NMPSGO possessed high density of functional groups, responsible for the formation of bound water filled ionic clusters in the membrane matrix. Because of more bound water, polymer matrix was less opting to dehydration. Thus, NMPSGO acted as barrier for water release and improves the water retention capacity even at higher temperature.

Fig. 6 Water desorption profile for different and composite membranes: (a) isotherm at 35 °C; (b) Higuchi's model fit of the deswelling behavior.

Bulk and bound water content for different composite membranes with varied NMPSGO compositions revealed their water retention ability (Table S2†). Bound water content in the membrane matrix is responsible for availability of water beyond 100 °C and necessary for proton conduction.⁹ It can be seen that bound water content or thus water retention capacity of composite membrane increased with NMPSGO content. Thus, high NMPSGO composition in the membrane matrix, improved the water holding capacity of PEM. Further, NSBC/NMPSGO-8 contained extremely low polymer (NSBC) and high NMPSGO content among the prepared composite membranes and showed 4 fold more bound water content than Nafion 117. High bound water content of composite membranes improved their water retaining ability and thus proton conductivity under low humid conditions. Bound water content enhanced because of the increased Availability of more water molecules per ionic sites improved the water binding with membrane matrix and thus bound water content. Reported composite PEM possessed enough binding sites in the membrane to constrain the water in polymer and modified GO network and resulted high bound water content.

Proton conductivity and methanol permeability

The proton conductivity (κ^m) of pristine (NSBC/GO) and composite (NSBC/NMPSGO) PEMs was measured at 30 °C and 100% relative humidity for hydrated membranes, and relevant data are presented in Table 2 κ^m values increased with NMPSGO content in the membrane matrix. Presence of different acidic groups (such as –SO₃H, –COOH, –PO₃H₂ etc.) in the membrane matrix afforded more facile proton transport, and improved the proton conduction. Proton conductivity of pristine membrane (NSBC/NMPSGO-0) (4.53 × 10⁻² S cm⁻¹) evidently improved to 8.87 × 10⁻² S cm⁻¹ (nearly equal value for Nafion 117 membrane) due to incorporation NMPSGO. In contrast, without phosphorylated NSBC/GO-8 composite membrane show small improvement in proton conductivity. Multi-functionalized nature of reported composite membranes was responsible for their high conductivity.

Table 2 Thickness, proton conductivity (κ^m)^a, methanol permeability (P)^b, selectivity parameter (SP)^a,b, and activation energy (E_a) values for different composite membranes and Nafion 117 membrane

Membrane	Thickness × 10^−2 (cm)	κ^m × 10^−2 (S cm^−1)	P × 10^−7 (cm^2 s^−1)	SP × 10^4 (S cm^−3 s^−1)	Ea (kJ mol^−1)
a Testing temperature: 30 °C.b Measured in 8 M methanol.
NSBC/NMPSGO-0	1.90	4.53	21.28	2.13	7.12
NSBC/NMPSGO-4	1.90	6.08	19.65	3.09	5.84
NSBC/NMPSGO-6	1.90	7.26	18.57	3.91	5.30
NSBC/NMPSGO-8	1.90	8.87	16.93	5.24	4.57
NSBC/GO-8	1.90	5.28	19.02	2.78	6.32
Nafion 117	1.88	9.56	25.6	3.73	5.45

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Activation energy (E_a) is the minimum energy required for proton conduction, and low activation energy can reduce the energy loss caused by the ionic resistance of the membranes. The temperature dependence (30–90 °C and 100% relative humidity) conductivity for prepared membranes showed consistency with the Arrhenius relationship (Fig. 7). Incorporation of NMPSGO in the membrane matrix moderately reduced the activation energy values (Table 2). As a reference, activation energy for NSBC/NMPSGO-8 membrane (E_a = 4.57 kJ mol⁻¹) was low in compare with Nafion 117 (E_a = 5.45 kJ mol⁻¹).³³

Fig. 7 Arrhenius plots of different composite membranes under 100% relative humidity.

One of the major hurdles for the commercialization of PEMFCs is huge fuel (methanol) crossover across the PEM. High methanol permeability (P) reduced power density along with electro-catalytic activity of cathode catalysts. The methanol permeability of the composite membranes and Nafion 117 is shown in Table 2. For composite membranes, methanol permeability decreased with increase in NMPSGO content may be due to reduction in void volume because of strong H-bonding between NMPSGO and functionalized NSBC. This further caused relatively slow diffusion of penetrates and thus low permeability. However, high methanol permeability in case of membrane without –PO₃H₂ groups (NSBC/GO-8), was attributed to the weak adhesion between polymer and additive (GO) matrix. This created the cavities and favoured molecular diffusion. Methanol permeability of the NSBC/NMPSGO-0 membrane for 8 M methanol at 30 °C (21.28 × 10⁻⁷ cm² s⁻¹) was reduced to 16.93 × 10⁻⁷ cm² s⁻¹ in case of NSBC/NMPSGO-8 composite membrane. Compared with Nafion 117, the methanol permeability of NSBC/NMPSGO-8 membrane was quite low, under similar experimental conditions.

The ratio of proton conductivity and methanol permeability κ^m/P was represented as selectivity parameter (SP) (Table 2). The SP values of NSBC/NMPSGO composite membranes increasing with NMPSGO content and NSBC/NMPSGO-8 membrane showed 2.5-fold greater SP than NSBC/NMPSGO-0 membrane. Compared with Nafion 117, NSBC/NMPSGO-8 membrane shows 1.4-fold greater SP. This may be attributed to improved proton conductivity, and reduced methanol permeability of NSBC/NMPSGO composite membrane.

Conclusions

We reported an easy and green procedure for the preparation of NMPSGO by grafting of –PO₃H₂ on GO also modified biopolymer (NSBC) with –SO₃H groups was prepared. NSBC/NMPSGO composite PEMs were fabricated and schematic structure is also presented. Loading of acidic functionalized GO (NMPSGO) in the NSBC matrix influenced different desirable membrane properties relevant for DMFC applications. Reported multi-functional PEM architected in such manner that its cross-linked structure and presence of functionalized GO improved the stability, while presence of multi-ionic clusters in the membrane matrix was responsible for creation of hydrophilic proton conduction channels. Functionalized GO was contributed towards mechanical stiffness electro-chemo-mechanical properties through cross-linking and strong interaction and bonding with free amines and –SO₃H groups of functionalized chitosan. Developed membranes showed good stability, flexibility, water uptake, and retention capacity. It was observed that the NMPSGO content in the membrane matrix improved the thermal stability of composite membranes. Among the developed membranes, NSBC/NMPSGO-8 exhibited a higher IEC value (2.09 mequiv. per g), proton conductivity (8.87 × 10⁻² S cm⁻¹) and 18.60 number of water molecules associated with per ionic site (λ).

Reported composite membranes were designed such as: to promote internal self-humidification responsible for water retention properties, to promote proton conduction due to the presence of multi-functional groups. Strong H-bonding between the functional groups and presence of hydrophobic graphene sheets and polymer chains provides a suitable architecture of proton conducting channels in the membrane matrix. Herein, data represent a promising starting point for the architecting highly conductive and stable composite systems. This type of functionalized biopolymer based material may have potential applications not only for the DMFC operated at intermediate temperatures under anhydrous (water-free) or extremely low humidity conditions but also for novel electrochemical devices, where water activity is not required.

Experimental

Materials

Graphene oxide (GO) was prepared from Graphite powder (Gt) (100 μm size, obtained from SD fine chemicals India).³⁴ Deacetylated chitosan (100% deacetylated), and 3-aminopropyltrimethoxysilane (APTMOS) were purchased from Sigma-Aldrich chemicals and used as received. Poly(vinyl alcohol) (PVA; MW, 125 000; degree of polymerization, 1700; degree of hydrolysis, 88%), tetrahydrofuran (THF), HCHO, acetone, acetic acid, sodium borohydride, gluteraldehyde, H₃PO₄, H₂O₂, H₂SO₄, HCl, NaOH, methanol, NaCl etc. of AR grade were obtained from SD fine chemicals India, and used with proper purification. Other chemicals are of commercial grade and used as received. In all experiments, milli-Q water was used.

Preparation of N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO)

Aminopropylsilane graphene oxide (APSGO) was prepared by condensation reaction between GO and APTMOS. In a typical synthetic procedure, highly oxygenated GO (50 mg) was dispersed in anhydrous THF (500 ml) and sonicated to obtain a homogeneous dispersion. Then, APTMOS (0.5 ml) was added, and reaction mixture was refluxed for 15 h. Finally, reaction mixture was cooled to room temperature, filtered and rinsed several times with THF, and dried under vacuum overnight.³⁵ Obtained APSGO was dispersed in 1 : 1 (w/w) formaldehyde and phosphorous acid solution in a closed vessel and stirred for 3 h at 70 °C. After cooling to room temperature, the obtained NMPSGO was filtrated, washed several times with water, and dried under vacuum overnight (Scheme 1).

Membrane preparation

N-o-sulphonic acid benzyl chitosan (NSBC) was prepared from deacetylated chitosan (100% deacetylated) according to previously reported method.³⁶ NSBC/NMPSGO composite membranes were prepared by solution casting method (Scheme 2). Different ratio of NMPSGO (0–8 wt%) to NSBC (60.0–55.2 wt%) was dispersed in deionized water (16 ml) and sonicated to generate a homogeneous dispersion, while poly(vinyl alcohol) (PVA) (40–36.8 wt%) was used as plasticizer. After complete dispersion, desired content of (1.6 g) was dissolved in same solution at 70 °C. Separately, NSBC (2.4 g) was dissolved in hot deionized water (60 ml) in presence of HCl (0.1 M) under constant stirring, to obtain a highly viscous solution. Both solutions were mixed under stirred conditions (12 h) at room temperature. Finally, obtained viscous solution was transformed into a thin film on PVC plate and dried under vacuum oven at 60 °C overnight. Resultant membranes were cross-linked with 10% gluteraldehyde solution in acetone in acidic medium (1 N HCl) for 20 h at 40 °C in closed vessels. Thus prepared membranes were designated as NSBC/NMPSGO-X, where X is the weight percentage of NMPSGO in the membrane phase. Pristine membrane (NSBC/GO-8) was prepared by same procedure using GO in place of NMPSGO.

Instrumentation

Detailed instrumental analysis such as infrared spectra, solid state ³¹P NMR spectra, WXRD, TEM, SEM, optical image, TGA, DMA, and burst strength are included in Section S1.† Bound water content was estimated from weight loss percentage obtained from TGA by sample heating between 100–150 °C with 10 °C min⁻¹ rate under nitrogen atmosphere.

Water uptake, ion-exchange capacity (IEC), and membrane stability studies

Detailed procedures for the determination of water uptake, swelling ratio and number of water molecules associated per ionic sites (λ) are included in the Section S2.† IEC was measured by the back-titration method and detailed procedure is included in the Section S3.† Procedures for studying oxidative and hydrolytic stabilities of the composite membranes are included in Section S4.†

Membrane conductivity and methanol permeability

Detailed procedure for proton conductivity measurement has been included in Section S5.† Methanol permeability of the composite membranes was determined in a diaphragm diffusion cell (Section S6†).

Acknowledgements

CSIR-CSMCRI registration number: 169/2014. Authors are grateful to Department of Science and Technology, New Delhi, for sponsoring project no. SR/S1/PC-62/2012. Instrumental support received from Analytical Science Division, CSIR-CSMCRI is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09581b

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