zwitterionic silica copolymer based crosslinked organic–inorganic hybrid

Zwitterionic silica copolymer based crosslinked organic–inorganic hybridpolymer electrolyte membranes for fuel cell applications{

Tina Chakrabarty, Ajay K. Singh and Vinod K. Shahi*

Received 27th May 2011, Accepted 19th November 2011

DOI: 10.1039/c1ra00228g

Zwitterionic (ZI) copolymers (consisting of sulfonic acid and amine groups) with plenty of –

Si(OCH)3 groups similar to stems, branches and fruits of vines from a bionic aspect, were

synthesized as a cross-linking agent. Organic–inorganic hybrid zwitterionic membranes (ZIMs),

with high flexibility, charge density and conductivity, was prepared using poly(vinyl alcohol)

(PVA). Developed ZIMs with dual acidic and basic functional groups, exhibited high stabilities,

water retention ability and cation selectivity. The ZIMs (especially Si–70%) were designed to

possess all the required properties (water uptake: 40.6%; ion-exchange capacity: 1.52 equiv. g21;

electro-osmotic flux: (2.34 6 1025 cm s21 A21); and conductivity: 9.67 6 1022 S cm21. ZIMs were

designed to possess all of the required properties of a proton-conductive membrane, namely,

reasonable swelling, good mechanical, dimensional, and oxidative strength, flexibility, and low

methanol permeability along with good proton conductivity due to zwitterionic functionality.

Moreover, Si–70% and a Nafion117 membrane exhibited comparable DMFC performance. Also,

investigation on a multi-ionic organic–inorganic hybrid ZIM as polymer electrolyte membranes

(PEMs) will give rise to a new developing field in materials and membrane science.

Introduction

PEMs with high conductivity, low methanol crossover and cost,

are greatly desired for direct methanol fuel cells (DMFCs) in

order to reduce ohmic losses and enhance their efficiencies

during operation.1–4 Perfluorosulfonic acid membrane (Nafion)

is a reference membrane for DMFC because of its high

electrochemical properties as well as excellent chemical resis-

tance.2,5 Nafion membranes show high methanol crossover, and

become dry under conditions of high temperature (above 80 uC)

or low humidity rather quickly due to the loss of water from the

membrane.5–13 Thus, widespread efforts were dedicated to

develop inorganic–organic composites based on a modified

Nafion membrane.14–17 PEMs with high proton conductivity at

intermediate temperatures under anhydrous or low-humidity

conditions, environmental affability with low methanol cross-

over, and production cost have attracted much interest recently

for problem solving in current technologies.7,18–22 For the

development of cheaper PEMs, fluorine-free materials with

properties comparable to those of Nafion, based on sulfonated

aromatic polymers, irradiation graft polymers, and cross-linked

and blend polymers, were successfully proposed.23–29

Only a few reports are available for ZI based hybrid

nanostructured PEMs.30–35 The acid–base composite PEMs

with high proton conductivity under anhydrous conditions, such

as poly(benzimidazole)phosphoric acid or sulfuric acid,36,37

poly(vinylphosphonic acid) heterocycle,38 and ionic liquids,39

were reported. However, the proton-conductive pathway can be

controlled by a suitable molecular assembly between acidic and

basic moieties by introducing these functional groups in the same

molecule for high proton conductivity, water retention, and low

methanol permeability.40–42

For developing ZIMs, organic–inorganic hybrid materials

are a suitable option, because of the synergistic advantages of

organic and inorganic segments (e.g., structural durability,

dielectric, ductility, processability, thermal and mechanical

stability).43–45 To enhance strength and compatibility,

preparation methods for organic–inorganic hybrid functional

materials were explored over the last decade.46,47

Incorporation of an inorganic ion exchange filler into an

organic matrix showed the leaching out due to the lack of

chemical bonding between two segments.47–49 In another sol–

gel process of alkoxysilane-functionalized polymers followed

by cross-linking with conventional agents, (aldehydes and

small alkoxysilanes), a low content of organic groups and lack

of any functional group are disadvantageous for homogenous

and conductive PEMs.50 To overcome these problems earlier

we reported 3-(3-(triethoxysilyl)propylamino)propane-1-sul-

fonic acid, a zwitterionomer cross-linking agent, which

contained one number of each sulfonic acid and amine groups

Electro-Membrane Processes Division, Central Salt and Marine ChemicalsResearch Institute, Council of Scientific & Industrial Research (CSIR),G. B. Marg, Bhavnagar, 364002, Gujarat, India.E-mail: [email protected]; [email protected];Fax: +91-278-2567562/2566970; Tel: +91-278-2569445{ Electronic Supplementary Information (ESI) available. See DOI:10.1039/c1ra00228g/

RSC Advances Dynamic Article Links

Cite this: RSC Advances, 2012, 2, 1949–1961

www.rsc.org/advances PAPER

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 1949–1961 | 1949

Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online / Journal Homepage / Table of Contents for this issue

http://dx.doi.org/10.1039/c1ra00228g



http://pubs.rsc.org/en/journals/journal/RA

http://pubs.rsc.org/en/journals/journal/RA?issueid=RA002005

per molecule.51 In our continuous effort, we tried to improve

numbers of functional groups (sulfonic acid and amine groups)

on the zwitterionomer for achieving highly stable and

conductive ZIMs with appreciable numbers of ion-exchange

sites in the membrane matrix.

Herein, a multi-ionic silicon based copolymer (3,3-dimethoxy-

7-(4-sulfonatobutyl)-2-oxa-7,10-diazonia-3-silatetradecane-

7,10-diium-14-sulfonate) was prepared as an organic–inorganic

hybrid zwitterionomer cross-linking agent using 3-(2-amino

ethyl amino)propyl trimethoxysilane (AAPTMS) and 1,4-

butanesultone via a ring opening reaction. Interestingly, the

structure of multi-ionic silicon based ZI cross-linking agent may

be similar to vines from a bionic aspect. The flexible main chain

is comparable with the stem of a vine, functional groups (sulfonic

and amine) are like fruit, and –Si(OCH3)3 groups look like

branches. A multi-ionic ZI precursor with plenty of –Si(OCH3)3

groups has the ability to cross-link with PVA in the presence of

TEOS. Structural features of prepared ZIM (long main chain

with many branched chains avoids organic–inorganic phase

separation and enhances membrane flexibility; plenty of acid and

basic groups balance membrane fixed charge concentration and

water retention ability) are expected to result in a highly stable

and conductive PEM.

Experimental section

Materials

AAPTMS, TEOS, 1,4-butanesultone (distilled under vacuum),

and Nafion117 (perfluorinated membrane) were purchased from

Sigma Aldrich Chemicals and were used as received.

Tetrahydrofuran (THF; Qualigens Fine Chemicals, Mumbai,

India) was distilled and kept dry over molecular sieves. PVA

(MW, 125 000; degree of polymerization, 1700; degree of

hydrolysis, 88%), and all other chemicals, reported in this

manuscript were obtained from SD Fine Chemicals (Mumbai,

India) of AR grade and used without further purification. In all

experiments, double-distilled water was used.

Preparation of zwitterionomer

Zwitterionomer precursor, 3,3-dimethoxy-7(4-sulfonatobutyl)-

2-oxa-7,10-diazonia-3-silatetradecane-7,10-diium-14-sulfonate,

was synthesized from 1,4-butanesultone and AAPTMS. In a

general synthesis method, AAPTMS in THF was stirred under a

nitrogen atmosphere, followed by the drop wise addition of 1,4-

butanesultone, (dissolved in THF) at 60 uC. The reaction mixture

was refluxed at 50 uC under stirring in a nitrogen atmosphere for 1

h, and then the solvent was evaporated. The obtained solid mass

was dried in a vacuum oven at 40 uC for 24 h, and yielded a dark

yellow coloured solid product.

CHNS: calcd (C, 38.85; H, 7.74; N, 5.66; S, 12.96); obsd (C,

38.81; H, 7.70; N, 5.64; S, 12.93).

Preparation of a cross-linked organic–inorganic hybrid ZIM

PVA (10 wt%) was dissolved in hot deionized water under

constant stirring. An appropriate amount of synthesized ZI

precursor was dissolved in deionized water at pHy2, separately,

and mixed with PVA solution, in the presence of a fixed amount

of TEOS (20 wt% to PVA). The obtained solution was stirred for

8 h at room temperature. The sol–gel process was achieved by

acid hydrolysis (pH: 2), and obtained gel was transformed as thin

film on a cleaned glass plate. Film was dried in ambient

conditions for 24 h and further at 70 uC for 12 h under IR lamp.

The obtained transparent membrane was peeled off for further

cross-linking with a formal solution (HCHO + H2SO4) for 3 h at

60 uC. Prepared membranes were designated as Si–X, where X is

the wt% of zwitterionomer in the membrane matrix, and varied

between 50–70 wt% of PVA content.

Fourier transform infrared (FTIR) and NMR characterization

FTIR spectra of dried membrane samples were obtained using

an attenuated total reflectance (ATR) technique with a Spectrum

GX series 49387 spectrometer in the range 4000–600 cm21. The

IR spectrum for a synthesized zwitterionomer was obtained by

the KBr pellet method. 1H NMR spectra was used to

characterize the synthesized zwitterionic material recorded by

FT NMR (Bruker, 200 MHz) Brucker DPX-200 in a d6-DMF

solvent.

Thermal and mechanical strength analysis

Thermal stabilities of the prepared zwitterionomer membranes

were obtained by thermo-gravimetric analysis (Mettler Toledo

TGA/SDTA851e with Stare software) under nitrogen atmo-

sphere with 10 uC min21 heating rate between 50 to 600 uC.

Differential scanning calorimetry (DSC) measurements were

achieved by Mettler Toledo DSC822e thermal analyzer with

Stare software. The dynamic mechanical strength of zwitterio-

nomer membranes were analysed by Mettler Toledo dynamic

mechanical analyzer (DMA) 861c instrument with Stare software

under nitrogen atmosphere with 10 uC min21 heating rate

between 30 to 300 uC. Details about the estimation of cross-

linking density is given in the ESI,{ Fig. S1.

Microscopic characterization

Silica distribution in the membrane phase was studied by a

JEOL 1200EX transmission electron microscope (TEM). The

JEOL 1200EX transmission electron microscope with a

tungsten electron source operated at an accelerating voltage

up to 120 kV. Scanning electron microscopy (SEM) images

were recorded by Leo microscope (Kowloon, Hong Kong)

after gold sputter coatings on dried membrane samples (1 and

0.1 Pa). Composition of silica and other elements, were

detected by energy-dispersive X-ray (EDX) measurements,

using a LEO VP1430 and an Oxford Instruments (Oxfordshire,

UK) INCA.

Ion-exchange capacity (IEC)

Detailed methods for estimation of membrane IEC is included in

the ESI,{ Fig. S2.

Water uptake, state of water, and membrane stabilities

Detail about methodology used for estimation of water uptake

and state of water is given in the ESI{(Fig. S3). Methods used for

dimensional, oxidative and hydrolytic stabilities studies of

developed membranes are included in the ESI{ (Fig. S4).

1950 | RSC Adv., 2012, 2, 1949–1961 This journal is � The Royal Society of Chemistry 2012

Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


Membrane conductivity studies

Conductivity of ZIMs (at different relative humidity (RH)) was

measured by four-probe Ac impedance spectroscopy using a

potentiostat/galvanostat frequency response analyzer (Auto Lab,

Model PGSTAT 30). Prior to measurement, the membrane

samples were soaked in deionized (DI) water for 24 h and rinsed

repeatedly to remove the last trace of free acid or base. The

membranes were mounted between two platinum electrodes (4.0

cm2), which were then placed in DI water to ensure 100% RH.

Direct current (DC) and sinusoidal alternating currents (AC)

were supplied to the respective electrodes for recording the

frequency at a scanning rate of 1 mA s21 within a frequency

range 106 to 1 Hz. Membrane conductivity (km) was estimated

from the following equation.

km ~d

R A(1)

Where d is the distance between electrodes, A is the cross-

sectional area of membrane and R is the membrane resistance

obtained from AC impedance data (the real part of the

impedance at high frequency, i.e. intercept of the impedance

semicircle with the Z-axis on the complex plane).

Electro-osmotic permeability measurements

Electro-osmotic permeability for different ZIMs was measured

in a two-compartment membrane cell (effective membrane area:

20.0 cm2), in equilibration of 0.01 M HCl solutions. Both the

compartments were kept under constant agitation by means of a

mechanical stirrer. A known potential was applied across the

membrane using Ag/AgCl electrodes, and subsequently volume

flux was measured by observing the movement of liquid in a

horizontally fixed capillary tube of known radius. The current

flowing through the system was also recorded with the help of a

digital multimeter. Several experiments were performed to obtain

reproducible values.

Methanol permeability

The methanol permeability of the composite membranes was

determined in a diaphragm diffusion cell, consisting of two

compartments (50 cm3) separated by a vertical membrane with a

20 cm2 effective area. The membrane was clamped between both

compartments, which were stirred during the experiments.

Before the experiment, the membranes were equilibrated in a

water–methanol solution for 12 h. Initially, one compartment

(A) contained a 30 or 50% (v/v) methanol–water mixture and the

other (B) contained double distilled water. Methanol flux arises

across the membrane as a result of the concentration difference

between the two compartments. The increase in the methanol

concentration with time in compartment B was monitored by

measuring the refractive index using a digital refractometer

(Mettler Toledo RE40D refractometer). The methanol perme-

ability (P) was finally obtained by the equation:

P ~1

A

CB (t)

CA (t { t0)VB l (2)

where A is the effective membrane area, l the thickness of the

membrane, CB(t) the methanol concentration in compartment B

at time t, CA(t 2 t0) the change in the methanol concentration in

compartment A between time 0 and t, and VB the volume of

compartment B. All experiments were carried out at room

temperature, and the uncertainty of the measured values was less

than 2%.

Membrane electrode assembly (MEA) and DMFC performance

Gas diffusion electrodes (three-layer structure) were prepared

by: (i) wet proofing of carbon paper (Toray Carbon Paper,

thickness: 0.27 mm, wet proofed with 15 wt% PTFE solution by

brush painting method); (ii) coating of gas diffusion layer

(GDL) on to the carbon paper; (iii) coating of catalyst layer on

to the GDL.27 The GDL (25 cm2) was fabricated by coating of

slurry (0.95 mg cm22) containing carbon black (Vulcan

XC72R) and PTFE dispersion on carbon paper. The anode

was made by coating slurry consisting catalyst (20 wt% Pt +

10 wt% Ru on carbon), 5 wt% Nafion ionomer solution,

isopropanol, and Millipore water (catalyst ink) on GDL had a

loading of 1 mg Pt and 0.5 mg Ru. The cathode was obtained by

coating the Pt catalyst ink with the same loading. Electrodes

were cold pressed with the membrane and cured at 60 uC for

12 h, followed by hot pressing at 130 uC for 3 min at 1.2 MPa.

Obtained MEA was clamped in single cell (FC25-01 DM fuel

cell).

The current–voltage polarization curves were recorded with

the help of MTS-150 manual fuel cell test station (ElectroChem

Inc., USA) with controlled fuel flow, pressure and temperature

regulation attached with electronic load control ECL-150

(ElectroChem Inc., USA). The measurements were performed

in the air mode of operation at 10 psi pressure with a fixed

concentration of methanol fed at the anode side with pressure

7 psi at 70 uC for a representative membrane.

Results and discussion

Synthesis of multi-ionic silicon based copolymer and ZIM

Multi-ionic ZI silica precursor was synthesized by 3-(2-amino

ethyl amino) propyl trimethoxysilane (AAPTMS) and 1,4-

butanesultone via a ring opening reaction under heating

conditions, Fig. 1. Because of the exothermic reaction, tempera-

ture was carefully monitored because of the volatile nature of the

solvent (THF). Bands observed at 1039 cm21 in the FTIR

spectrum confirmed sulfonate groups, while hydrated sulfonic

acid groups in multi-ionic ZI silica precursor absorbed at

1471–1163 cm21 (Fig. 2). This indicates that both functional

groups were present in the ZI silica precursor.52,53 A sharp peak

observed at 1643 cm21 and a broad absorbance between

3025–2500 cm21, also confirmed the presence of a substituted

quaternary ammonium group. The ZI silica precursor shows

strong intensity bands at 1211 cm21 which confirmed the

presence of –SiOR groups. The ZI silica precursor was

zwitterionic in nature confirmed by the presence of two sharp

peaks at 604 and 525 cm21. The ZI silica precursor was

characterized by 1H-NMR in D2O solution as shown in Fig. 3. A

triplet peak observed ca. 4.5 ppm indicates that the butanesul-

tone ring was opened and formed sulphonic acid groups.

Without ring opening this peak was not observed. The other


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


proton signals are clearly represented in Fig. 3. The ZI silica

precursor was further characterized by 13C NMR in D2O

solution and signals were observed at 10.56, 13.37, 21.01,

23.72, 24.23, 26.41, 31.68, 32.34, 48.38, 50.18, 51.63, 55.45,

60.59, 62.07 ppm and shown in (Fig. 4A). X-Ray diffraction

pattern data show that the ZI silica precursor is an amorphous

powder shown in (Fig. 4B). On the basis of spectral and

elemental analysis data the structure of ZI silica precursor was

confirmed and is shown in Fig. 1.

ZIMs were prepared by condensation polymerization of a ZI

silica precursor by an acid catalyzed sol–gel process in aqueous

media using PVA (Fig. 1). The obtained gel was transformed

into a thin film and cross-linked by HCHO in the presence of

acid at 60 uC for 3 h. Cross-linking occurs in two steps: (i)

formation of hemiacetal by reacting HCHO with –OH groups of

PVA; (ii) further reaction of hemiacetal with the –OH of PVA

and formation of acetal. Membranes lose their transparent

nature in wet conditions after cross-linking, but retain it in dry

conditions. The stable nature of ZIMs was achieved by

molecular level tailoring of the sol–gel process, in which organic

and inorganic segments were joined covalently.

The silica precursor and water forms a single phase solution in

the acid catalyzed sol–gel process, and solution-to-gel transition

was responsible for the silica (SiO2) network attached to the

organic matrix. The reaction mechanism proceeds through

bimolecular nucleophilic substitution and the nature of the

catalyst affects the membrane properties. Because of rapid

protonation of OR or OH substituents, directly attached to the

Si atom, sufficient numbers of interconnected Si–O–Si bonds

formed a three-dimensional cluster or a gel.54 Linear and weakly

cross-linked polymer clusters formed in acid catalyzed sol–gel

reaction because of steric crowding. The obtained membrane

showed good mechanical, thermal and electrochemical proper-

ties. Presence of –SO3H groups was confirmed because of a sharp

intensity absorption band near 1100–1037 cm21 and multi

headed bands in the region of 1460–1360 cm21 (Fig. 5).

Quaternary ammonium groups in ZIM were confirmed by peaks

between 1640–1540 and 3000–2800 cm21. A medium intensity

broad peak between 3400–3300 cm21 indicated the presence of

quaternary ammonium salt in the ZIM phase.53 Formation

of a cyclodiether part (–C–O–C–) and cross-linked membrane

structure was confirmed by an absorption band between 1170–

1030 cm21. Absorption band for about 1040–1150 cm21

confirmed Si–O–Si and Si–O–C groups in the membrane matrix,

as a result of condensation reaction between hydrolyzed silanol

(SiOH) groups.55,56

The structural features for multi-ionic ZI silica precursors are

similar to a vine from a bionic aspect. The main chains are

comparable to the stems of vine, while branched chains are like

branches of a vine, and are beneficial for the flexibility of the

Fig. 1 Scheme for the synthesis of ZI silica cross-linking agent and its structural similarity from a bionic aspect.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


Fig. 2 FTIR spectra for a multi-ionic ZI silica precursor.

Fig. 3 1H NMR of a multi-ionic ZI silica precursor.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


Fig. 4 Multi-ionic ZI silica precursor (A) 13C NMR in d6-DMSO (B) XRD.

Fig. 5 ATR-FTIR spectrum for Si–70% ZIM.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


cross-linked ZIMs. Multi-ionic ZI silica precursors contain

plenty of acidic and basic functional groups (–SO3H and

–N+(CH3)3Cl2). Functional groups were comparable to the

fruits of vine with controllable charge density, while –Si(OCH3)3

groups were similar to acetabulas of vines with high cross-linking

abilities.

Microscopic characterizations

High contents of organic moiety (PVA) and a relatively larger

amino propyl group were responsible for the formation of the

worm-like structure. In case of the membrane, 50 nm fine slices

were obtained with the help of a microtome cutter. The TEM

images show a worm like arrangement in membrane matrix and

silica particles are equally distributed in the membrane shown in

Fig. 6. SEM images of representative ZIMs (Si–50% and Si–70%)

are present in the ESI{ (Fig. S1). Excellent compatibility between

organic and inorganic phases suggests that the multi ionic ZI

silica precursor is an effective cross-linking agent for PVA,

because cross-linking occurred through covalent and hydrogen

bonds. Some aggregations on the membrane surface were

observed because accelerated hydrolysis of silica precursor, with

increase in ZI silica precursor content. Absence of phase

separation, cracks or holes on the membrane surface suggested

a homogeneous and dense nature of ZIM.

Thermal and mechanical stabilities

TGA curves for ZIMs in the H+ state show similar weight loss

patterns (Fig. S2, ESI{). Three weight loss steps attributed to

water loss (loose and bound) at 50–160 uC (5 to 8% of the initial

weight), defunctionalisation at 260–370 uC (12–18% of initial

weight), and membrane matrix degradation beyond 400 uC. Si–

70% membrane showed comparatively minimum weight loss, for

which a high degree of cross-linking was responsible. Further-

more, high thermal stabilities of developed ZIMs indicate

advantages of covalently bonded functional groups and cross-

linking.

DSC thermogram for ZIMs (Fig. S3, ESI{) showed 118.35,

117.78, 109.59 and 107.81 uC first endothermic peaks (Tg) and

second endothermic peaks at 174.1, 176.2, 179.4 and 231.4 uCfor Si–50%, Si–55%, Si–60%, Si–70% membranes, respectively.

Incorporation of a multi-ionic ZI silica precursor in the polymer

matrix had a profound effect on Tg values. Variation in Tg values

may be explained because of a plasticizing effect of the binder

(PVA) and alteration in its ordered arrangement with an increase

in ZI silica precursor content in the membrane matrix. This

observation indicates a highly cross-linked structure for ZIM at

elevated temperature. Here, it is interesting to note that Tg for

pristine PVA is 78 uC.57

Mechanical properties of the membranes (modulus and

strength) were analyzed by DMA curves (Fig. S4, ESI{).

Elongation of ZIMs increased with ZI silica precursor content,

and Si–50% showed maximum stress tolerance which may be

explained in terms of the enhanced cross-linked nature of Si–50%

membrane. Hydroxyl groups of PVA contributed towards

hydrogen bonding and stiffness of the polymer chain. With the

decrease in number of hydroxyl groups (because of its

involvement in either branching or cross- linking), the hydrogen

bonding was attenuated and thus the chain stiffness was reduced.

In addition, SiOH groups formed inter and intra molecular

cross-linked structures with PVA and a high content of ZI silica

precursor in polymer matrix produced agglomerates. Cross-

linking density, determined by DMA studies (Fig. 4S, ESI{), and

decreased with ZI silica content in the membrane matrix.

Variation in cross-linking density may be explained by formation

of cohesive domains in the ZIM matrix. This seems to be more

predominant than cross-linking with a plasticizer. Thus it is

necessary to optimize the ZI silica content in the membrane

matrix and cross-linking density for achieving better mechani-

cally stable ZIMs.

Oxidative, hydrolytic and acid–base stabilities

Stability and durability for membranes under stressed conditions

are important to explore their practical applications in electro-

chemical processes. Membrane oxidative stability was evaluated

by its treatment in Fenton’s reagent (3% aq. H2O2 + 3 ppm

FeSO4) at 80 uC for 1 h and recording oxidative weight loss

percent (Wox) (Table 1). The Si–50% membrane exhibited the

Fig. 6 TEM image of a ZI-60 membrane (a) low magnification (b) high magnification.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


lowest weight loss, which increased with ZI silica content in the

membrane matrix. Because of cross-linking, hydrophilic

domains turned compact and ?OH and ?OOH radicals (short

life time) cannot penetrate inside the siloxane containing

domains. Thus, Si–70% membrane with low cross-linking density

exhibited high weight loss. Methylene groups in PVA are

sensitive to free radical attack and cause chain degradation.

ZIMs were also subjected to an accelerated hydrolytic stability

test at 120 uC and 100% RH for 24 h. Hydrolytic stability test

results showed increased weight loss with a ZI silica precursor

(Table 1) and membranes retained their transparency, flexibility

and toughness.

Acid–base stability for developed ZIMs were tested for 5 h

equilibration in acid (HCl) and base (NaOH) solutions of

different concentrations and resulting weight loss data are

presented in (Fig. 5S, ESI{). Data confirmed the stabilities of

ZIMs in 5 M HCl and 7 M NaOH, as weight loss under these

conditions was less than 3%. Weight loss for the Si–70%

membrane was comparatively high under acidic and basic

conditions, and may be because of more ZI content and thus a

posses a high level of functionality.

Stability of ZIMs (containing sulfonic acid and amine groups)

in strong alkaline solution is highly concerning,58 because

quaternary ammonium groups are relatively unstable in alkaline

solutions due to direct nucleophilic substitution and Hofmann

elimination.59 Prepared ZIMs were immersed in 5 mol L21

NaOH solution for 100 h to investigate stability of alkaline

functional groups. Ratios of IECt (IEC of base treated ZIM at

time t) and IEC0 (IEC of untreated ZIM) were recorded, Fig. 7.

All lines drop rapidly in 0–25 h (first stage), then attain a level in

0–100 h (second stage). It seems ZIMs contain some quaternary

ammonium groups, which undergo Hofmann elimination under

the treatment with a strong base. Estimated from IEC0 and IECt

at 25 h, the percentage of unstable quaternary ammonium

groups was found to be 15.4%, 10.6%, and 2.9% for Si–70%, Si–

60%, and Si–50%, respectively. Presence of this group may be

attributed to insufficient cross-linking because of high swelling

or water uptake (Si–70% membrane). After the degradation of

these components, loss in membrane IEC was nearly constant.

This study reveals the membrane stability under strong alkaline

media against Hofmann degradation, and their suitability for

electro-membrane processes. The formation of base in the

cathode compartment occurs because of reductive water

splitting.

Water uptake, dimensional changes and water retention capability

Membrane water uptake has a profound effect on its proton

conductivity, mechanical and dimensional properties.60 Presence

of water molecules in the membrane phase facilitates the

dissociation of functional groups and is essential for high

membrane conductivity. On the other hand, high water volume

fraction in the membrane phase reduces its dimensional and

thermal stabilities along with ionic concentration in the

membrane phase. Water uptake values (Qw) increased with ZI

content for ZIMs (Table 1), because of the enhanced hydrophilic

nature of the membrane matrix. Water uptake profiles for

different ZIM at elevated temperature were also studied, and

increased linearly up to 80 uC. In this case, the extent of

plasticization and hydrophilic nature of the matrix play an

important role for water uptake. At low extent of plasticization

(high ZI content) the flexible polymer network permits larger

water uptake.61

Membrane water uptake values significantly depend upon the

void porosity of the membrane. Fig. 6S, ESI{ shows the effect of

void porosity on the water uptake properties. The results

revealed that with the increase in membrane void porosity water

uptake values increase. Total number of water molecules per

ionic site (lw) (Table 1) data also support a highly cross-linked

and hydrophilic membrane matrix at high ZI content. The lowest

number of water molecules per ionic site for Si–70% membrane

revealed its highly cross-linked structure with small void volume.

The water vapor sorption and water diffusion properties of

ZIMs affect their conductivity and applicability for electro-

chemical processes. Furthermore, the water retention capability

of membranes is helpful to assess their suitability for desired

applications. Water retention capability of ZIMs was illustrated

in (Fig. 8A) (Mt/Mo)–t (time) curves, and deswelling kinetics of

the developed membranes was calculated. The value of k

(constant) was derived from (Mt/Mo)–t1/2 curves (Fig. 8B) based

on Higuchi’s model for water desorption kinetics:

Table 1 Water uptakea (Qw), number of water molecules per ionic site (lw), swelling in water (Wv), oxidative, hydrolytic weight lossb (WOX & WHS

respectively), and water diffusion coefficient (D) values for different ZIMs

Membrane code Qw (wt%) lw/SO32 Wv (vol%) WOX (wt%) WHS (wt%) D/1026 (cm2 s21)

Si–50% 28.0 20.9 26.0 5.3 5.9 3.26Si–55% 31.4 17.8 28.0 7.8 8.4 2.42Si–60% 35.9 16.5 32.3 8.3 9.5 1.87Si–70% 40.6 14.9 34.9 9.3 10.9 1.31a Uncertainty in the measurements of 1 : 0.1%. b Uncertainty in the measurements of 2 : 0.01 mg.

Fig. 7 Alkaline resistance of different ZIMs: ratios IECt/IEC0 over the

immersion time in 5 mol L21 NaOH at room temperature.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


Mt=M0~ {kt

1=2z1 (3)

where M0 and Mt are the initial amount of water and remaining

water in polymer matrix at given time and k is a constant.

The obtained straight lines for different ZI contents were fitted

to Higuchi’s model, and suggested a diffusion controlled water

desorption mechanism. The water desorption rate was reduced

with ZI content, thus ZI acted as a water binder in the membrane

matrix and is responsible for formation of hydrophilic channels

and cross-linked chains. Water diffusion coefficients (D) across

ZIMs were also evaluated from a best-fit normalized mass

change (Table 1) and were found to decrease significantly with

ZI content in the membrane matrix. Thus ZI acted as a water

release barrier and enhanced the membrane water retention

capacity.

State of water

In a membrane matrix, the presence of water may be classified

into three types: (i) free water (with the same temperature and

enthalpy of melting as bulk water); (ii) freezing bound water

(weakly bound with polar or ionic groups of the polymer and

shows change in temperature and enthalpy in comparison with

bulk water and can be detected by DSC); and (iii) non-freezing

bound water (very strong interaction with polar or ionic groups

and shows no phase transition). According to Eikerlings

theory,62 the charged membrane possessed two types of water:

bound and bulk water. Bound water is necessary for the

solvation of ionic groups, whereas bulk water fills the void

volume. Low temperature DSC studies and water uptake values

were used to determine the state of water in the membrane phase.

DSC thermograms were recorded from 250 to +50 uC (Fig. 3S,

ESI{) as a broad endothermic peak of hydrated ZIMs. Enthalpy

of melting (DHm), melting temperature (Tm), the full width at

half-maximum of the melting peak (DTm), freezing water (lf),

bound water (lb) and bound water degree (x) were estimated

from DSC curves and presented in Table 2. Increase in DHm with

ZI content was attributed to low freezing and less bound water

percentage of the ZIMs.

Freezing water (lf) for ZIM was obtained by total melting

enthalpy by integrating melting curves peak area (Fig. S3, ESI{).

Also, bound water percentage (lb) was obtained by subtracting

freezing water content from total water in the membrane matrix.

The degree of bound water in percentage (x = lb/lw) was

estimated by the ratio of number of bound water molecules and

total water. Free water increased with ZI content in the

membrane matrix, while bound water reduced because of the

enhanced availability of ionic sites for binding. Reduction in

bound water with ZI content may be attributed to strong

interactions between ionic groups and water molecules which

effectively predominates over the siloxane network.

IEC and surface charge concentration

IEC indicates density of dissociable ionic groups in the

membrane matrix, which are responsible for ion conduction

across the membrane. ZIM contains both types of functional

groups: acidic (–SO32H+) and basic (quaternary ammonium),

which contribute towards IEC. Acidic and basic IEC values for

different ZIMs were estimated by the titration method (Table 3).

Fig. 8 Water desorption profile for ZIMs: (A) isotherm at 40 uC; (B)

Higuchi’s model fit of the deswelling behavior.

Table 2 State of water: enthalpy of melting (DHm), melting temperature (Tm), full width at half-maximum of the melting peak (DTm), glass transitiontemperature (Tg), total number of water molecules per ionic site (lt), number of free water molecules per ionic site (lf), number of bound watermolecules per ionic site (lb), and degree of bound water (x) for different ZIMs

Membrane DHm (J g21) Tma(uC) DTm

b(uC) Tg lt lf lb xc (%)

Si–50 5.35 25.3 9.99 118.35 20.9 1.20 19.70 94.26Si–55 7.81 24.1 12.33 117.78 17.8 1.32 16.48 92.58Si–60 10.74 23.4 14.1 109.59 16.5 1.48 15.02 91.14Si–70 14.72 21.1 14.8 107.81 14.9 1.62 13.28 89.73a Melting temperature of free and loosely bound water. b Full width at half-maximum of the melting peak. c Bound water degree x (%) = lb/lt .


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


Both types of IEC increased with ZI content in the membrane

matrix, and confirmed the good zwitterionomer nature of the

membrane. Here, it is interesting to note that the Si–70%

membrane showed 1.512 m equiv./gm IEC for acidic functional

groups (–SO32H+), while world-wide used acidic Nafion-117

membrane showed 0.910 m equiv./gm, IEC. Similarly, Si–70%

membrane showed 0.827 m equiv./gm alkaline IEC, whereas

Neosepta membrane showed 1.20–1.40 m equiv./gm alkaline

IEC.62 IEC studies confirmed the highly charged and zwitter-

ionic nature of developed ZIMs.

Surface charge concentrations (Xm) for different ZIMs were

estimated by the following equation.

X m~t IECð Þrd

Qw

(4)

Where t is membrane void porosity, rd is the density of the dry

membrane, and Qw is the volume fraction of water in the

membrane matrix. From acidic and basic IEC values, Xm values

for acidic and basic functional groups for different ZIMs were

also included in Table 3. Data support the schematic structure of

ZIM (Fig. 1) and confirmed the presence of strongly dissociable

functional groups (sulfonic acid and quaternary ammonium) in

the membrane matrix.

Conductivity of ZIMs

Conductivity of ZIMs (km) was measured under 100% relative

humidity at 30 uC (Table 4), increased with ZI content in the

membrane matrix and thus has a high concentration of

functional groups. With enhanced ZI content in the membrane

matrix, high Qw values affect hydrogen bonding and thus

membrane conductivity, because of increased surface charge

concentration. Results support that charged sites of ZIMs

enhanced the hydrophilic nature of the matrix and promoted

exchange of ions, which helped for conduction. Fig. 9 shows the

variation of km with enthalpy of melting (DHm) in equilibration

with 0.5 M NaCl solution. DHm depends on the free water

content in the membrane matrix responsible for membrane

conductivity. High extent of free water in the membrane phase

provides easy conduction of counter-ions. Thus, for highly

conductive membrane, designing a precursor with high extent of

free water is necessary. The designed ZI precursor contains two

acidic and basic functional groups in a molecule and provides a

highly charged matrix with appreciable free water content.

Knowledge of a membrane’s water retention capacity is also

helpful to assess the membrane conductivity at high temperature.

Membrane conductivity values at high temperatures (30 to

100 uC) under 100% RH are presented as an Arrhenius plot (Fig.

S5, ESI{) for the estimation of activation energy (Ea), using the

following expression,

ln km~{Ea

RT(5)

Where R is the universal gas constant (8.314 J mol21 K21),

and T is the absolute temperature. Ea values increased with the

ZI content in the membrane matrix (Table 4), and varied

between 3.64–4.29 kJ mol21. As a reference Nafion 117

membrane showed 6.52 kJ mol21 activation energy. It seems

ion transport across ZIMs followed a conduction mechanism

similar to Nafion 117 (Grotthus type conduction mechanism).

Membrane conductivity (Si–50% and Si–70%) depends on its

relative humidity (Fig. 10). Conductivity decreased with a

decrease in the RH (%) of membranes. At 100% RH, the Si–

70% membrane showed 0.0967 S cm21 conductivity, which was

reduced to 0.043 S cm21 at 25% RH. Furthermore, these ZIMs

(especially Si–70%) retained conductivity under relatively low

hydrated conditions.

Electroosmotic permeability studies

Electro-osmotic transport of mass (solvent) across charged

membranes revealed their mass electro-driven mass transport

properties and equivalent pore radius. Electro-osmotic flux

across ion exchange membranes occurred due to: (i) the

availability of ionic sites in the membrane matrix and (ii) the

existence of an electrical potential at the membrane–solution

Table 3 Acidic and basic IECs by titration methods (IECtit), and their surface charge concentration (Xm) for different ZIMs

Membrane Code

Acidic functional groups Basic functional groups

IECtita (m equiv./gm) Xm (m mol dm23) IECtit

a (m equiv./gm) Xm (m mol dm23)

Si–50% 0.743 0.931 0.424 0.531Si–55% 0.983 1.222 0.568 0.706Si–60% 1.212 1.606 0.734 0.972Si–70% 1.512 2.004 0.827 1.096a Uncertainty in the measurement was: 0.001 m equiv./g.

Table 4 Membrane conductivitya (km), electroosmotic permeability (JE), and activation energy (Ea), for different ZIMs

Membrane code km/1022 (S cm21)a JE/1025 (cm s21 A21)b Ea (kJ/mol) r (nm)

Si–50% 5.97 1.07 3.64 15.33Si–55% 7.01 1.16 3.79 16.88Si–60% 8.42 1.34 4.04 18.74Si–70% 9.67 2.34 4.29 19.23Nafion 117 9.56 14.01 6.52 —a Uncertainty in the measurement was: 0.01 6 1022 S cm21. b Uncertainty in the measurement was: 0.01 6 1025 cm s21 A21.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


interface called zeta potential.63,64 Knowledge of ion and

solvent transport rates across the membranes under electro-

driven conditions is necessary for intelligently designing a

better membrane for desired electro-membrane processes.65

Electro-osmotic flux across ZIMs in equilibration with 0.02 M

NaCl solution (Fig. S6, ESI{) was used for the estimation of

electro-osmotic drag (b) (implies that every coulomb of

electricity will exert a drag sufficient to carry b cm3 of water

through 1 cm2 of the membrane area). Equivalent pore radius

(r) for membranes was estimated from the Katchalsky and

Curran approach:63

r~8gFb

f 01w

� �1=2(6)

where F is the Faraday constant, g denotes the coefficient of

viscosity of the permeate, and f01w is the frictional coefficient

between the counter-ion and water in free solution, which can

be defined as f01w = RT/Di, (where Di is the diffusion coefficient

of the single ion (i) in the free solution, R is the gas constant,

and T is absolute temperature). The ionic diffusion coefficient

(Di) at a given electrolyte concentration was obtained from

ionic conductivity data.57 Equivalent pore radii (r) for ZIMs

suggest their dense nature and increased with ZI content in the

membrane matrix (Table 4). This information also supports our

earlier observations that in ZIMs, hydrophilic, charged and free

water content increased with ZI content. Thus it is necessary to

optimize the ZI content for desired electro-membrane applica-

tion for ZIMs.

Methanol permeability and selectivity parameter

ZIMs showed extremely low methanol permeability transmission

[(0.95–2.31) 6 1027 cm2 s21] compared with the Nafion117

membrane (13.10 6 1027 cm2 s21) (Fig. 11(A)). The mass-

transport behavior for a hydrated ZIM depends on its degree of

swelling, water uptake, and bulk microstructure. Methanol

permeability values increased with ZI content in the membrane

matrix. Incorporation of silica containing ZI precursor into theFig. 10 Conductivity of ZIMs (Si–70% and Si–50%) at different RH.

Fig. 9 Variation of km with DHm in equilibration with 0.5 M NaCl

solution for different ZIMs and (inset) variation in membrane

conductivity with different loading of zwitterionomer.

Fig. 11 (A) Methanol permeability (P) and (B) selectivity parameter (SP) values as function of ZI content in the membrane matrix for different ZIMs

and Nafion 117 membrane.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


polymer membranes dramatically altered the membrane trans-

port properties because of alteration in the free void volume and

ionic clusters. Thus, understanding the relationship between

the polymer structure and membrane performance, in terms

of permeability and selectivity, enables the tailoring of the

membrane structure for specific purposes. Permeation of liquid/

gas molecules through the polymer membrane occurs via the

diffusion mechanism, and the permeability of the penetrant

(methanol) is the product of its solubility and diffusivity. The

penetrant diffusivity is dependent on the free void volume in the

membrane, the size of the penetrant molecules, and the

segmental mobility of the polymer chain.

To directly compare the applicability of ZIMs for DMFC, the

ratio of the proton conductivity and methanol permeability (km/

P) data was used as the selectivity parameter (SP). SP values for

nanocomposite and N117 membranes are also presented in

Fig. 11(B). The Si–50 membrane exhibited the highest SP value

(6.28 6 105 S cm23 s) among the prepared ZIMs. The SP value

decreased with ZI content in the membrane matrix. In similar

conditions, the N117 membrane showed an SP value of 0.73 6105 S cm23 s. It was also noticed that, with an increase in the

operating temperature, SP values for ZIMs were also increased.

This observation may be attributed to the relatively low

methanol permeability of ZIMs. These results can be explained

on the basis of the lack of significant interactions between

methanol and functional groups. Ionized groups hydrate

strongly and excluded organic solvents (salting-out effect), which

is an essential feature of the polyelectrolyte membranes.

Furthermore, higher SP values of these membranes indicate a

great advantage for DMFC applications.

DMFC performance

DMFC performance for representative PEMs (Si–50% and Si–

70%) was tested in a single cell and compared with Nafion117

membrane at 70 uC (Fig. 12). The Si–70% membrane showed

0.95 V open circuit voltage (OCV), while Nafion exhibited

0.497 V, under similar experimental conditions. The current

densities at a potential of 0.20 V for Si–70%, and Nafion117 (79.5

and 73.6 mA cm22, respectively) indicated a comparable DMFC

performance of ZI and Nafion117 membranes. Good performance

of the Si–70% membrane was observed due to extremely low

methanol permeability in spite of its lower proton conductivity. Si–

70% membrane showed 16.1 mW cm22 maximum power density,

whereas for Nafion117showed 13.7 mW cm22. These results

indicated suitability of ZIMs for DMFC applications.

Conclusions

Multi-ionic silicon copolymer via ring opening reaction was

developed as cross-linking agent for preparing stable zwitterionic

organic–inorganic hybrid membranes. The prepared ZI silica

precursors contain sulfonic acid and amine groups (two number

each per molecule), which rendered its ZI nature because of

proton transfer. The structure of the ZI silica precursor is similar

to a vine from a bionic aspects with a long main chain, and

branched chains with large numbers of sulfonic acid, amine and

–Si(OCH3)3 groups. All these are desired properties for designing

a stable, homogeneous, and flexible ZIM by a sol–gel process in

aqueous media using a suitable water soluble plasticizer (PVA).

Developed membranes showed good stability, flexibility, water

uptake, and retention capacity. Incorporation of ZI in the

membrane matrix improved the thermal stability of ZIMs, which

were thermally stable up to 200 uC in a dry nitrogen atmosphere.

Among the developed ZIMs, Si–70 exhibited a higher acidic IEC

value (1.512 m equiv g21), water uptake (40.6%), and proton

conductivity (9.67 6 1022 S cm21). Comparable conductivity for

Si–70 membrane with Nafion 117 membrane, confirmed the

suitability of ZIMs for fuel cell applications. Reported data are

promising to design highly conducting acid–base composite

systems. Furthermore, relatively lower methanol permeability

and SP values of these membranes make them applicable for

DMFC. The acid–base composite 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.

Acknowledgements

Financial assistance received from the Department of Science

and Technology, New Delhi (Govt. of India), by sponsoring

project no. SR/S1/PC/06/2008 is gratefully acknowledged.

Instrumental support received from Analytical Science

Division, CSMCRI, is also gratefully acknowledged.

References

1 A. Siu, J. Schmeisser and S. Holdcraft, J. Phys. Chem. B, 2006, 110,6072.

2 F. Pereira, K. Valle, P. Belleville, A. Morin, S. Lambert and C.Sanchez, Chem. Mater., 2008, 20, 1710.

3 J. A. Kerres, J. Membr. Sci., 2001, 185, 3.4 J. Y. Kim, W. C. Choi, S. I. Woo and W. H. Hong, J. Membr. Sci.,

2004, 238, 213.5 P. Costamagna and S. Srinivasan, J. Power Sources, 2001, 102, 242.6 M. Khiterer, D. A. Loy, C. J. Cornelius, C. H. Fujimoto, J. H. Small,

T. M. McIntire and K. J. Shea, Chem. Mater., 2006, 18, 3665.7 M. A. Hickner, H. Ghassemi, Y. S. Kim, B. R. Einsla and J. E.

McGrath, Chem. Rev., 2004, 104, 4587.

Fig. 12 DMFC performance curves for MEAs made with different

membranes operated at 343 K for 20% (v/v) methanol used as fuel in air

mode with 10 psi pressure.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


8 T. H Yu, Y. Sha, W. G. Liu, B. V. Merinov, P. Shirvanian and W. A.Goddard, J. Am. Chem. Soc., 2011, 133, 19857.

9 T. Prabhuram, T. S. Zhao, Z. X. Liang, H. Yang and C. W. Wong, J.Electrochem. Soc., 2005, 152, A1390.

10 M. S. Kang, J. H. Kim, J. Won, S. H. Moon and Y. S. Kang, J.Membr. Sci., 2005, 147, 127.

11 Q. Li, R. He, J. O. Jensen and N. Bjerrum, Chem. Mater., 2003, 15,4896.

12 S. J. Paddison, Annu. Rev. Mater. Res., 2003, 33, 289.13 B. J. Wang, H. Zhang, X. Yang, S. Jiang, W. Lu, Z. Jiang and S. Z.

Qiao, Adv. Funct. Mater., 2011, 21, 971.14 M. Watanabe, H. Uchida, Y. Seki, M. Emori and P. Stonehart, J.

Electrochem. Soc., 1996, 143, 3847.15 J. A. Asensio, E. M. Sanchez and P. Gomez-Romero, Chem. Soc.

Rev., 2010, 39, 3210.16 H. Ni’mah, W.-F. Chen, Y.-C. Shena and Ping-Lin Kuo, RSC Adv.,

2011, 1, 968.17 J. Yanga, P. K. Shena, J. Varcoeb and Z. Wei, J. Power Sources,

2009, 189, 1016.18 B. P. Tripathi, M. Schieda, V. K. Shahi and S. P. Nunes, J. Power

Sources, 2011, 196, 911.19 V. K. Shahi, Solid State Ionics, 2007, 177, 3395.20 B. P. Ladewig, R. B. Knott, A. J. Hill, J. D. Riches, J. W. White, D. J.

Martin, J. C. Diniz da costa and G. Q. Lu, Chem. Mater., 2007, 19,2372.

21 B. P. Tripathi and V. K. Shahi, J. Phys. Chem. B, 2008, 112, 15678.22 B. Smitha, S. Sridhar and A. A. Khan, J. Power Sources, 2006, 152,

846.23 L. C. Cogoa, M. V. Batisti, M. A. Pereira-da-Silva, F. C. Nart and F.

Huguenin, J. Power Sources, 2006, 158, 160.24 B. Smitha, S. Sridhar and A. A. Khan, Macromolecules, 2004, 37,

2233.25 J. Wang, X. Zheng, H. Wu, B. Zheng, Z. Jiang, X. Haob and B.

Wang, J. Power Sources, 2008, 178, 9.26 Y. Wan, Y. Katherine, K. A. M. Creber, B. Peppley and V. Tam Bui,

J. Membr. Sci., 2006, 280, 666.27 A. Saxena, B. P. Tripathi and V. K. Shahi, J. Phys. Chem., 2007, 111,

12454.28 Chenxi Xu, Yuancheng Cao, Ravi Kumar, Xu Wu, Xu Wang and

Keith Scott, J. Mater. Chem., 2011, 21, 11359.29 Q. Li, J. O. Jensen, R. F. Savinell and N. J. Bjerrum, Prog. Polym.

Sci., 2009, 34(5), 449.30 H. Zou, S. Wu and J. Shen, Chem. Rev., 2008, 108, 3893.31 J. Liu, T. Xu and Y. Fu, J. Membr. Sci., 2005, 252, 165.32 J. Liu, T. Xu, M. Gong and Y. Fu, J. Membr. Sci., 2005, 260, 26.33 A. Narita, W. Shibayama, K. Sakamoto, T. Mizumo, N. Matsumi

and H. Ohno, Chem. Commun., 2006, 1926.34 H. S. Kim, S. J. Park, D. Q. Nguyen, J. Y. Bae, H. W. Bae, H. Lee,

S. D. Lee and D. K. Choi, Green Chem., 2007, 9, 599.35 P. Innocenzi and G. Brusatin, Chem. Mater., 2000, 12, 3726.36 A. Schechter and R. F. Savinell, Solid State Ionics, 2002, 147, 181.37 R. Bouchet and E. Siebert, Solid State Ionics, 1999, 118, 287.

38 H. G. Herz, K. D. Kreuer, J. Maier, G. Scharfenberger, M. F. H.Schuster and W. H. Meyer, Electrochim. Acta, 2003, 48, 2165.

39 A. N. Mondal, B. P. Tripathi and V. K. Shahi, J. Mater. Chem., 2011,21, 4117.

40 M. Yamada and I. Honma, Chem. Phys. Lett., 2005, 402, 324.41 S. R. Narayanan, S.-P. Yen, L. Liu and S. G. Greenbaum, J. Phys.

Chem. B, 2006, 110, 3942.42 M. Yamada and I. Honma, J. Phys. Chem. B, 2004, 108, 5522.43 E. Markovic, S. Clarke, J. Matisons and G. P. Simon, Macromolecules,

2008, 41, 1685.44 K. Tadanaga, H. Yoshida, A. Matsuda, M. Tsutomu and M.

Tatsumisago, Chem. Mater., 2003, 15, 1910.45 Q. G. Zhang, Q. L. Liu, A. M. Zhu, Y. Xiong and X. H. Zhang, J.

Phys. Chem. B, 2008, 112, 16559.46 P. G. Romero, Adv. Mater., 2001, 13, 163.47 R. K. Nagarale, G. S. Gohil, V. K. Shahi and R. Rangarajan,

Macromolecules, 2004, 37, 10023.48 X. J. Cui, S. L. Zhong and H. Y. Wang, J. Power Sources, 2007, 173,

28.49 V. V. Binsu, R. K. Nagarale and V. K. Shahi, J. Mater. Chem., 2005,

15, 4823.50 I. Honma, H. Nakajima, O. Nishikawa, T. Sugimoto and S. Nomura,

Solid State Ionics, 2003, 162–163, 237.51 B. P. Tripathi and V. K. Shahi, ACS Appl. Mater. Interfaces, 2009, 1,

1002.52 L. J. Bellami, The Infrared Spectrum of Complex Molecules, 2nd ed.;

Wiley: New York, 1958.53 G. Socrates, Infrared Characteristic Group Frequencies; Wiley: New

York, 1980.54 W. G. Klemperer, V. V. Mainz, S. D. Ramamurthi, C. J. Brinker,

D. E. Clark, D. R. Ulrich (ed.)Ulrich, Better Ceramics ThroughChemistry III, vol. 121, Materials Research Society, Pittsburgh, PA,198815–25.

55 F. Rubio, J. Rubio and J. L. Oteo, Spectrosc. Lett., 1998, 31, 199.56 P. Innocenzi, J. Non-Cryst. Solids, 2003, 316, 309.57 M. A. Vargas, R. A. Vargas and B. E. Mellander, Electrochim. Acta,

2000, 45, 1399.58 J. R. Varcoe, R. C. T. Slade, E. L. H. Yee, S. D. Poynton, D. J.

Driscoll and D. C. Apperely, Chem. Mater., 2007, 19, 2686.59 J. R. Varcoe and R. C. T. Slade, Fuel Cells, 2005, 5, 187.60 D. B. Spry, A. Goun, K. Glusac, D. E. Moilanan and M. D. Fayer, J.

Am. Chem. Soc., 2007, 129, 8122.61 P. M. Mangiagli, C. S. Ewing, K. Xu, Q. Wang and M. A. Hickner,

Fuel Cells, 2009, 9, 432.62 M. Eikerling, A. A. Kornyshev and U. Stimming, J. Phys. Chem. B,

1997, 101, 10807.63 A. Katchalsky, P. F. Curran, Nonequilibrium Thermodynamics in

Biophysics, Harvard Univ. Press, Cambridge, 1965.64 R. Parsons, Handbook of Electrochemical Constants, Butterworths,

London, 1959.65 N. Pismenskaya, E. Laktionov, V. Nikonenko, A. E. Attar, B.

Auclair and G. Pourcelly, J. Membr. Sci., 2001, 181, 185.


Dow

nloa

ded

on 2

1 M

arch

201

2Pu

blis

hed

on 0

6 Ja

nuar

y 20

12 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C1R

A00

228G

View Online


zwitterionic silica copolymer based crosslinked organic–inorganic hybrid

Documents