This article was published as part of the

Hybrid materials themed issue

Guest editors Clment Sanchez, Kenneth J. Shea and Susumu Kitagawa

Please take a look at the issue 2 2011 table of contents to

access other reviews in this themed issue

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 907925 907

Cite this: Chem. Soc. Rev., 2011, 40, 907925

Ionogels, ionic liquid based hybrid materialsw

Jean Le Bideau,a Lydie Viaub and Andre Vioux*b

Received 30th July 2010

DOI: 10.1039/c0cs00059k

The current interest in ionic liquids (ILs) is motivated by some unique properties, such as

negligible vapour pressure, thermal stability and non-ammability, combined with high ionic

conductivity and wide electrochemical stability window. However, for material applications, there

is a challenging need for immobilizing ILs in solid devices, while keeping their specic properties.

In this critical review, ionogels are presented as a new class of hybrid materials, in which the

properties of the IL are hybridized with those of another component, which may be organic

(low molecular weight gelator, (bio)polymer), inorganic (e.g. carbon nanotubes, silica etc.) or

hybrid organicinorganic (e.g. polymer and inorganic llers). Actually, ILs act as structuring

media during the formation of inorganic ionogels, their intrinsic organization and

physicochemical properties inuencing the building of the solid host network. Conversely, some

eects of connement can modify some properties of the guest IL, even though liquid-like

dynamics and ion mobility are preserved. Ionogels, which keep the main properties of ILs except

outow, while allowing easy shaping, considerably enlarge the array of applications of ILs. Thus,

they form a promising family of solid electrolyte membranes, which gives access to all-solid

devices, a topical industrial challenge in domains such as lithium batteries, fuel cells and dye-

sensitized solar cells. Replacing conventional media, organic solvents in lithium batteries or water

in proton-exchange-membrane fuel cells (PEMFC), by low-vapour-pressure and non ammable

ILs presents major advantages such as improved safety and a higher operating temperature range.

Implementation of ILs in separation techniques, where they benet from huge advantages as well,

relies again on the development of supported IL membranes such as ionogels. Moreover,

functionalization of ionogels can be achieved both by incorporation of organic functions in the

solid matrix, and by encapsulation of molecular species (from metal complexes to enzymes) in the

immobilized IL phase, which opens new routes for designing advanced materials, especially

(bio)catalytic membranes, sensors and drug release systems (194 references).

1. Introduction

Ionic liquids (ILs) are organic salts which exhibit low melting

temperature, by convention below 100 1C. They consist ofonly ions; their properties are strikingly dierent from those of

molecular liquids. One of the most important properties is

negligible vapour pressure (and generally thermal stability and

non-ammability). Actually, the physicochemical properties

of ILs can be tuned by the choice of the anion-cation pair,

which opens millions of possibilities.1,2 ILs have been widely

used by electrochemists for a long time, as a result of their high

ionic conductivity (within 104 to 8.102 S cm1 around roomtemperature) and their wide electrochemical potential window

(which can be as high as 5.7 V between Pt electrodes).3,4 A

renewed interest for such electrolytes, which prove to be

high-performance over a wide range of temperatures (typically

up to 200300 1C), has been triggered by the topical demand inadvanced electrochemical devices, such as actuators, lithium

batteries, electric double-layer capacitors, dye-sensitized solar

cells and fuel cells.5

Moreover, the unique combination of non-volatility and

favourable solubilising characteristics of ILs has motivated a

growing interest as a replacement for organic solvents, and more

generally of volatile organic compounds (VOCs), in many

industrial applications like catalysis6,7 and extraction.8 Actually,

negligible vapour pressure allows easy retrieval of the nal

products by distillation or by using biphasic chemical processes,

without degradation or loss of solvent by evaporation and,

consequently, an easy recycling. Thus, the use of ILs appears

sustainable and they have been claimed (maybe too loosely) as

a Institut des Materiaux Jean Rouxel (IMN),Universite de Nantes - CNRS, 2 rue de la Houssinie`re, BP 32229,F44322 Nantes, France. E-mail: Jean.LeBideau@cnrs-imn.fr;Tel: +33 240 373 919

b Institut Charles Gerhardt de Montpellier,UMR 5253 CNRS-UM2-ENSCM-UM1, Universite Montpellier 2,Place Euge`ne Bataillon Montpellier, cc1701, F34095, France.E-mail: Andre.Vioux@univ-montp2.fr; Tel: 33 4 67 14 39 70

w Part of the themed issue on hybrid materials.

Chem Soc Rev Dynamic Article Links

www.rsc.org/csr CRITICAL REVIEW

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908 Chem. Soc. Rev., 2011, 40, 907925 This journal is c The Royal Society of Chemistry 2011

green solvents. To what extent ILs are actually environment

friendly is in debate nowadays. Nevertheless benign ILs have

been recently synthesized from biomolecules.9,10 Anyway,

considerable developments have been carried out in organic

chemistry, especially in the eld of homogeneous catalysis by

organometallic complexes and biocatalysis, as in many cases

ILs bring about some improvement of catalyst eciency and

selectivity.1113

On the other hand, although ILs easily dissolve inorganic

species as well, their use as solvents in inorganic chemistry is

much more recent. Actually, they have proved to be an

innovative reaction media, especially in metal electro-

deposition,14 ionothermal syntheses (ILs enable reaction

temperatures above 300 1C without autoclaving)15,16 and

solgel.17 There have been numerous studies on the controlled

growth of nanoparticles in ILs, especially metal nanoparticles

in relation to their catalytic activity in organic reactions.18 The

use of ILs in the preparation of a wide range of inorganic (and

hybrid) materials (including silicas, organosilicas, metal

oxides, metal chalcogenides, metal salts, open-framework

structures, nanostructured metals and alloys etc.) has been

recently reviewed.1921

The growing use of ILs in the materials eld originates from

their unique structural and physicochemical features. The

local organisation of ILs (or ILs containing solutes) involves

uxional ion aggregates that are stabilized by ionic inter-

actions as well as hydrogen bonds and van der Waals forces.22

A short overview on physicochemical properties of ILs, which

sets the background of all applications based on ILs, is given

in the rst part of the article in the form of a tentative

outline of the current understanding of structure-properties

relationships.

There is currently a challenging need for immobilizing ILs in

solid devices, while keeping their unique properties. To this

purpose, one possibility is to impregnate a support, the IL

being used as an adsorbed lm. This approach, which is

beyond the scope of this article, is well illustrated by the

current development of supported IL catalysis (SILC), in

which metal catalysts are immobilized within IL lms

supported on silica beads, resulting in free owing powders

that are particularly suitable for continuous xed-bed reactors.

SILC has been widely reviewed.23,24 Another way of

immobilization involves the formation of a three-dimensional

network which percolates throughout the IL and is responsible

for the solid-like behaviour of the resulting material, called

ionogel (or ion gel). Actually, if any gel may be regarded as

solid and liquid phases percolating throughout each other,

here the specicity is the negligible vapour pressure and the

thermal stability of the liquid phase, which makes ionogels

stable materials over time and allows for using them up to

temperatures much higher than ambient.

Jean Le Bideau (born 1965)

obtained his Materials Science

Degree from the Universite de

Nantes (France), and then

joined the Institut des

Materiaux de Nantes under

the supervision of Professor

Jean Rouxel, where he

received his PhD in 1994. After

two years stay at Michigan

State University (USA) under

the supervision of Professor

Daniel G. Nocera, he moved

to the Universite de Montpellier

(France) as Associate

Professor, and returned in 2008 to the Universite de Nantes as

full Professor. His research is aimed at building low dimensional

organicinorganic materials with specic connement eects.

Since 2003, he focuses his research on solgel chemistry in ionic

liquids, more specically on the properties of the resulting

conned ionic liquids.

Jean Le Bideau

Lydie Viau earned a degree in

chemical engineering in 2000

from the Ecole Nationale

Superieure de Procedes et

dIngenierie Chimiques dAix-

Marseille (France) and a PhD

degree in organometallic

chemistry in 2003 under the

supervision of Dr Hubert Le

Bozec at the University of

Rennes. She spent one year at

the University of Canberra

(Australia) and one year at

the University of Nantes

(France) before joining the

group of Professor Andre Vioux at the Institut Charles Gerhardt

in Montpellier (France). Her current research interest mainly

focus on the development of ionogels based on silica and

polymers suitable for catalysis, drug delivery, electrolyte

membranes.

Lydie Viau

Andre Vioux studied chemistry

at the University of Montpellier

and completed his PhD in

coordination chemistry in 1981

under the supervision of Prof.

Robert Corriu. In 198384 he

worked at the University of

Paris 6 on solgel chemistry

under the supervision of Prof.

Jacques Livage. In the late

80s, he developed some new

approaches to out-of-equilibrium

ceramics by thermal conversion

of silicon-containing polymer

resins at the University of

Montpellier, where he has been full Professor since 1992. Over the

last decades he has focused his research on new ways to inorganic

organic hybrid materials and on solgel processing in unusual media.

Andre Vioux

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 907925 909

The various types of ionogels may be separated into

physical and chemical gels. In physical gels, the internal 3-D

network is cross-linked through weak (reversible) interactions

(e.g. hydrogen bonds, hydrophobic interactions, crystallite

junctions etc.), whereas in chemical gels the cross-linkage

results from covalent bonding. Physical gels can be obtained

by using an organic gelator (molecular species of low

molecular mass or polymers as well) or a divided solid (e.g.

fumed silica particles, carbon nanotubes) which coagulate the

whole assembly. As a matter of fact, free-standing membranes

can be prepared by incorporating an IL into a polymer.25

However, physical gels are often in the form of jellies, slurries

or pastes, or at the least, present a limited mechanical solidity.

Another possibility is to create chemical gels, by conning the

IL in a covalently interconnected network. Typically, the

chemical cross-linking of the above mentioned polymers

results in mechanically resistant organic matrices. Alternatively,

chemical gels based on inorganic oxide matrices may be

prepared by solgel synthesis from an alkoxide precursor.

Interestingly, the structural and textural properties of the

inorganic matrix may be strongly inuenced by the presence

of the IL.17 A third way, which has just emerged, involves

nanocomposite matrices in which the polymer or biopolymer

network is cross-linked via covalently bonded oxide nanollers

generated in situ by solgel.26,27 The dierent ways of obtaining

organic, inorganic or nanocomposite ionogels are reviewed in

the second part of the article.

It is worth underlining that the intrinsic hybrid character of

ionogels relies on the intimate combination of an IL and a

solid-like network. The properties of ionogels are expected to

derive both from those of the IL and those of the component

forming the solid-like network. As mentioned above, this

component may be organic (polyethylene oxide, cellulose

etc.), inorganic (carbon nanotubes, silica etc.) or organic

inorganic (silsesquioxane etc.). However, properties may be

aected by the nanometre scale assembly. This is the case in

some ionogels based on organic polymers, in which only one

glass transition is observed at a temperature (Tg) lower than

that of the pure polymer and higher than that of the pure IL.28

This behaviour is typical of plasticizing eects.29 The eects of

connement that are observed in inorganic ionogels are much

more specic. Actually, physicochemical properties of the

IL, as phase transitions and molecular dynamics, may be

signicantly modied by the connement in the nanopores

of the host matrix. These specic connement eects are

presented in the third part of the article.

Ionogels, which keep the main properties of ILs except

outow, considerably enlarge their array of applications.

Some of these applications, which range from solid electrolytes

to drug release to catalysis (Fig. 1), are reviewed in the fourth

part of the article. They are generally based on the intrinsic

properties of ILs, such as ionic conductivity, drug activity and

solvent ability. These properties can be adjusted to specic

tasks by the choice of the anion-cation combination. Blending

adds a further possibility of tuning. Moreover, the encapsulation

of designed molecules (catalysts, sensing molecules, uorescent

metal complexes etc.) in the immobilized IL phase opens

innite possibilities of functionalization. The host matrix

and its interactions with IL ions bring their own contribution

to the whole functionality. Finally, polymer tractability as well

as versatility of solgel in shaping allows easy adaptations to a

wide range of devices.

2. Relationships between structure andphysicalchemical properties of ILs

ILs can be dened as aprotic or protic, depending on the

nature of cation. The former class arises from quaternization

reactions. Typically aprotic ILs involve organic cations as

imidazoliums, pyridiniums and phosphoniums (Fig. 2). Protic

ILs arise from a proton transfer to the same site occupied by

an alkyl group in aprotic ILs. Note that this transfer between a

Brnsted acid (AH) and a Brnsted base (usually an amine) is

equilibrated, which implies the presence of some residual

neutral species.30 However, the displacement of equilibrium

towards ion formation may be maximised using 2 : 1 or 3 : 1

acidbase ratios. Indeed the oligomeric anionic species

(A2H or A3H2

) that formed are weaker bases than themonomeric anion (A), because of solvation by the additionalacid molecules.31,32 The uidities, and attendant conductivities,

of protic ILs tend to be much higher than those of aprotic

ILs.33 Within a given family of ILs, the physicochemical

properties can be tuned by the choice of the anion

(e.g. hydrophilicity, Fig. 2). Typically halide anions can be

Fig. 1 Ionogels, multi-purpose hybrid materials.

Fig. 2 Some examples of cations and anions used in the formation of

aprotic ILs and changes of water miscibility with anion type.

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910 Chem. Soc. Rev., 2011, 40, 907925 This journal is c The Royal Society of Chemistry 2011

exchanged by metathesis reaction or converted into complex

anions by Lewis acid addition (Fig. 3).34

However, even if the preparation of ILs involves quite

simple organic chemistry, their purication is far from being

straightforward, since both distillation and crystallization can

not be easily used. Then, small amounts of impurities

have been shown to dramatically inuence their properties.

Accordingly, since the development of purication methods

of water-stable and air-stable ILs is relatively recent,

reliable data on their physicochemical properties are still

incomplete, despite a huge number of reports. A comprehensive

overview of physicochemical properties of ILs is far beyond

the scope of this article. The only purpose here is to give the

non-specialist reader a glance at the current approaches

to rationalize the fundamental physicochemical properties

of ILs.

Among the interactions taking place in ILs, Coulombic

interactions are predominant, insuring their high cohesive

energy and, therefore, their extremely low vapour pressure.

There are evidences of ionic association in ILs and,

consequently, all ions could not be available to contribute to

conductivity. Typically, ion pairs, if suciently long lived,

appear neutral in the electric eld. A fundamental point relies

in the balance between anion-cation interaction and their

respective volume.35,36

The empirical Waldens rule connects conductivity per mole

of charge carrier L= sVe (where s is the specic conductivityin S cm1 and Ve is the volume per equivalent) to the uidity(i.e. the reciprocal viscosity Z1) of the electrolyte medium,stating that LZ = C, where C is a temperature dependentconstant. A way of evaluating the concept of ionicity in ILs

(i.e. the eective fraction of ions available to contribute to

ionic conduction) would be to compare the Walden plot of ILs

(i.e. the logarithmic plot of L vs. Z1) to that of a so-calledideal electrolyte solution (where the ions are assumed to be

fully dissociated), typically a 0.01 M aqueous KCl solution.

However, one has to keep in mind that the Waldens rule

applies to innitely diluted electrolyte solutions. Nevertheless,

quite unexpectedly, the Walden plot of ILs, which are the very

opposite extreme from diluted electrolyte solutions, discloses

excellent correlation between viscosity and conductivity.35

Deviations from the Walden rule (Fig. 4),37 have been

discussed in term of ionicity (from good to poor ILs

and non-ILs),35 as a decrease in ionicity of ILs should bring

about an increasing deviation from the so-called ideal KCl line

(slope 1 through the origin).

Actually, the Walden plot represents a qualitative approach.

Other authors proposed a more quantitative approach by

dening the self-dissociativity of ILs as the molar conductivity

ratio (Limp/LNMR), calculated from the molar conductivitymeasured by the electrochemical impedance method

(Limp, L in the Walden plot approach) and that estimated byuse of pulse-eld-gradient spin echo (PGSE) NMR, ionic

self-diusion coecients LNMR.38 The molar conductivity

(LNMR) can also be calculated from the ionic self-diusioncoecients (D+ and D), using the NernstEinstein equation,which for a 1 : 1 salt can be written as:

LNMR NAe2

kTD D 1

where NA is the Avogadro number, e is the electron charge on

each ion carrier, k is the Boltzmann constant, and T is the

absolute temperature. This statement is based on the assumption

that every diusing species detected by PGSE-NMR contributes

to the molar conductivity. On the other hand, Limp is relatedto the migration of charged species in an electric eld. Thus,

the Limp/LNMR ratio would reect the ionicity. Notethat the diusion coecients of each ion are given by the

StokesEinstein equation:

D kT6pZr

2

where r is the eective ionic radius. Interestingly, if one

substitutes these diusion coecients into the above Nernst

Einstein equation, one obtains a relationship in the form:

L constant Z1 1r

1r

3

Actually, this relationship provides a renement of the Walden

plot which takes into account the radius (i.e. the volume) of

the conductive species. Ionicity was estimated from the

vertical distance of the data points to the reference KCl line

for a range of aprotic ILs. A relative good agreement with

Limp/LNMR was obtained.39

Finally, a measurement of the ionicity of ILs would be

provided by the eective ionic concentration (Ce), which is

the product of Limp/LNMR and the molar concentration. For1-alkyl-3-methylimidazolium ILs [CnMIm][TFSI], Ce was

found to decrease from 3.1 to 1.5 103 mol cm3 as thenumber of carbon atoms n in the alkyl chain increases from

1 to 8. This decrease in ionicity cannot be explained in terms

of Coulombic interactions, but it can be understood considering

the interactions between the alkyl chains. Actually, small to

wide angle X-ray scattering (SWAXS) evidenced the

occurrence of nanoscale structural heterogeneities whose sizes

depend on the length of the alkyl chain and are related to

chain segregation into nano-domains.40 Interestingly, computer

simulations predicted structuring in a manner that is analogous

to microphase separation between polar and nonpolar

domains.41 Thus, the short-range structuring of ILs arises

from a balance of long-range (Coulombic) and short-range

(hydrogen bonding, van der Waals, dipoledipole, pp)interactions. As a consequence, ILs may be regarded as

solvents, but also as supramolecular networks in which the

introduction of other species takes place with the formation of

Fig. 3 Typical preparations of imidazolium ILs by metathesis

reaction and Lewis acid addition.

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 907925 911

inclusion-type compounds between the guest molecules and

the host pre-organised medium (ILs).22 Note that the presence

of H-bonded nanostructured networks, with polar and

non-polar regions, may be responsible for the stabilisation of

enzymes supported in ILs, which can maintain their

functionality under very extreme denaturative conditions.13

Discussing the concept of ionicity is beyond the scope of this

review. However, it is worth underlining here that Abbott

proposed a model in which charge transport is governed by the

mobility of holes at innite dilution, which explains why the

Walden rule has been found to be applicable to ILs, and why

the application of the StokesEinstein equation allows accurate

prediction of their conductivity.42 Actually, the ions are not

closely packed, allowing some cavity volumes to exist, although

at a very low (about 106) molar fraction. At a giventemperature, these holes are of random size and location and

undergo constant ux. An ion will be able to move only if there is

a hole of suitable dimensions adjacent to it. The assumption is

that this occurrence is rate-limiting for the ionic conduction.

3. Preparation of ionogels

Synthetic routes to ionogels fall into three categories depending

on the nature of the solid-like network, which may be organic

(using low molecular weight gelators or polymers), inorganic

(typically using oxide nanoparticles, carbon nanotubes or

oxide networks arising from solgel), or hybrid organic

inorganic (typically polymers reinforced with inorganic llers)

(Fig. 5).

3.1 Organic ionogels

3.1.1 Use of low molecular weight gelators. Low molecular

weight gelators (LMWGs) are organic molecules, which are

added in small quantities to liquids at elevated temperature

and then induce physical gelation upon cooling. Actually

LMWGs are able to self-assemble in solution through

supramolecular bonding such as hydrogen bonding, pp inter-actions or electrostatic interactions.

Fig. 4 (a) Walden plot of log(molar conductivity, L) against log(reciprocal viscosity, Z1), which includes the classication for ILs proposed byAngell et al.35 and (b) a close-up view of the region occupied by typical aprotic ILs. The solid line indicates the ideal line for a completely

dissociated strong electrolyte aqueous solution (KClaq.). Reprinted with permission from ref. 37.

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912 Chem. Soc. Rev., 2011, 40, 907925 This journal is c The Royal Society of Chemistry 2011

Thus, Kimizuka et al. studied the gelation of ether-containing

ILs by the addition of L-glutamic acids or carbohydrates such

as b-D-glucose, a-cyclodextrin.43 Formation of brous nano-structures was observed due to strong interactions between

carbohydrates and ether groups. Shinkai used a cholesterol

based compound known to aggregate by one-dimensional

stacking of the cholesterol moieties.44 However, this molecule

turned out to be poorly soluble in ILs; accordingly acetone

co-solvent was needed. Yanagida and co-workers reported

ionogels arising from an imidazolium IL and N-benzyloxy-

carbonyl-l-isoleucyl aminooctadecane as a gelator. They

applied these electrolyte gels to the fabrication of a dye-

sensitized solar cell (DSSC) which showed light-to-electricity

conversion eciency of 5% and high temperature durability.45

Hanabusa et al. synthesized clyclo(dipeptides) gelators from

aspartame which allowed gel formation of a wide variety of

ILs, including imidazolium, pyridinium, pyrazolidinium,

piperidinium, morpholinium and ammonium salts, by using

less than 1 wt% of gelator.46 Recently the rst organo-

metallic gelator, a pyridine-bridged bis(benzimidazolylidene)-

palladium complex, proved ecient in the gelation of a variety

of ILs.47 Gelation was shown to involve p-stacking, van derWaals interactions and metal-metal bonding. Dong et al. used

bis(4-acylaminophenyl)methane and bis(4-acylaminophenyl)-

ether with varied acyl chains to gelify imidazolium ILs.48 The

phase transition temperatures of the gels increased with an

increase of the acyl chain length of the gelators, whereas the

conductivities of ionic-liquid gels decreased. However, the

dierences in conductivities between the gels and corresponding

bulk ILs were within one order of magnitude.

3.1.2 Polymer gels. The use of polymers to immobilize ILs

in the form of free-standing membranes which combine the

mechanical exibility of a polymer and the characteristic

conductivity of ILs has been widely developed, particularly

as materials for electrochemical devices. Many gelled systems

are obtained simply by swelling a polymer in an IL49 or mixing

the polymer and the IL together with a co-solvent which is

subsequently removed. Another route is the polymerization of

monomers in an IL used as a solvent, but the loss of miscibility

above a certain degree of polymerization may be a

limitation.50

Of course, the key point is the miscibility between the IL and

the polymer.29 However, numerous ILs have been reported to

act as plasticizers, by lowering the glass transition temperature

(Tg) which thus provides exibility. Thus, the outstanding

compatibility of imidazolium salts with poly(methyl

methacrylate)s (PMMA) has been reported.28,50,51 Fig. 6

illustrates the improvement in thermal stability of PMMA

ionogels compared with that of PMMA itself, despite

of a decrease in the Tg with the incorporation of

IL ([C4MIm][TFSI]). However, poly(ethylene oxide)s (PEO)

and uoropolymers and copolymers, as sulfonated tetra-

uoroethylenes (Naons) and poly(vinylidene uoride-co-

hexauoropropylene)s (PVdF-HFP), have attracted the most

interest as host polymers in electrolyte membranes for lithium

batteries, fuel cells and dye-sensitized solar cells.52,53 Even in

the case of complete miscibility,29 there is a limitation

associated with the plasticizing eect which entails the loss

of mechanical resistance on increasing the IL loading.

Cross-linking permits the improvement of mechanical stability

without losing signicant conductivity.5456

The limitation related to non miscibility is well illustrated by

the connement of ILs in epoxy-based networked polymers

performed by curing epoxy resins with an amine in the

presence of an IL.57 The materials obtained exhibited

threshold behaviour at 40 wt% loading of IL, moving from

insulating with a high Youngs modulus to ion conducting

with a low Youngs modulus, which reected the transition

from discrete microdroplets to a percolating phase of IL in the

polymer network. Accordingly, the IL was easily extracted

with acetone by means of Soxhlet extractor at high IL loading

(4 40 wt%), whereas it was retained at lower IL loading.The obstacle of non miscibility is hard to bypass (e.g. by

using the polymerization of IL-based microemulsions),58 as

the long-term operation of the membranes remain subjected to

the risk of some leakage of IL. On the other hand, in some

cases cross-linked resins with permanent macroporosity could

be prepared by using the IL as a porogen solvent.59 Another

interesting way to take advantage from non miscibility is the

use of some polymer-IL binary systems that exhibit phase

separations depending on external stimuli (as temperature) to

access to smart ionogels. Thus, poly(benzylmethacrylate)

(PBzMA) and its copolymers exhibit lower critical solution

temperature (LCST)-type phase separation in common hydro-

phobic ILs.60 Moreover, cross-linked PBzMA gels show

reversible and discontinuous volume phase transition in

[C2MIm][TFSI] with the change in temperature. Lodge et al.

reported the self assembly of tri-block copolymers that

alternated soluble and insoluble polymer blocks and yielded

transparent thermoreversible ionogels (Fig. 7).61,62 The recent

application of ionogels derived from tri-block copolymers as

gate dielectric in polymer thin-lm transistors evidences high

potential in exible electronics.63 The key point is that

ionogels are both printable and have tremendous specic

capacitances, in excess of 10 mF cm2, which is 500 times ashigh as many typical dielectrics and 10 times as high as some

other recently reported dielectric layers.

Special mention has to be made of the use of biopolymers,

as gelatin and polysaccharides, which provides sustainable

materials, furnished with biomolecular functions.43,64,65 It is

worth stressing here that the dissolution of high molecular

weight polysaccharides, especially cellulose, is a challenge of

great importance in sustainable chemistry context.66 ILs do

Fig. 5 Dierent types of ionogels.

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oer a route to such an aim, thanks to the ability of some

anions, as chloride at high concentration, to break the original

intra- and inter-chain hydrogen bonding of polysaccharides.67

Typically, cellulose completely dissolves in water-soluble

1-butyl-3-methylimidazolium chloride on microwave pulse

heating (120170 1C).68 Solvation involves hydrogen bondingbetween the carbohydrate hydroxyl protons and the

chloride ions in a 1 : 1 stoichiometry, as demonstrated by

Fig. 6 (a) TG curves for MMA network polymers with dierent mole fractions of dissolved [C2MIm][TFSI] and [C2MIm][TFSI] bulk at a heating

rate of 10 1C min1. From ref. 50. (b) DSC thermograms for MMA network polymers with dierent mole fractions of dissolved [C2MIm][TFSI]and [C2MIm][TFSI] bulk at a heating rate of 10 1C min

1. Reprinted with permission from ref. 50.

Fig. 7 An ABA triblock copolymer with soluble B block (blue) and insoluble A blocks (red) (left) self-assembles in the presence of an IL

(+ and symbols) to form an ion gel (center). Suitable choice of A block enables thermoreversible gelation when the A blocks become soluble at a

higher temperature (right). Reprinted with permission from ref. 62.

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914 Chem. Soc. Rev., 2011, 40, 907925 This journal is c The Royal Society of Chemistry 2011

13C and 35/37Cl NMR relaxation measurements.69 Cellulose

can be regenerated in water.68 Otherwise, gelation simply

occurs by keeping the IL solution at room temperature for a

few days under ambient humidity.70 Gels are similarly

obtained from chitosan,71 agarose,72 and starch73,74 or by

mixing a solution of cellulose with starch or carrageenans.75,76

3.2 Inorganic ionogels

3.2.1 Bucky gels. Carbon nanotubes (bucky tubes)

consist of a rolled-up graphene sheet with heganonally

arranged sp2-hybridized carbon atoms and provide good

electronic and optical properties. CNTs possess extraordinary

mechanical, electrical and thermal properties, however one

major drawback is that they are hard to process. One

approach to increase the solubility and processability is to

absorb organic molecules onto their surfaces.

In 2003, Aida and Fukushima found that single walled carbon

nanotubes (SWNTs) could be easily dispersed in imidazolium

ILs, where heavily entangled bundles of carbon nanotubes were

exfoliated to give highly dispersed, much ner bundles.77

Gelatinous materials were obtained by grounding the IL and

SWNTs, followed by centrifugation to remove the excess of IL.

Phase transition and rheological properties suggested that the

gels are formed by physical cross-linking of the nanotube

bundles, mediated by local ordering of the ILs rather than by

entanglement of the nanotubes. The bucky gels thus obtained

are easy to process into any shape. Another method included the

heating and ultrasonication of CNTs in the presence of the IL at

90 1C.78 The dispersion mechanism for SWNTs in ILs is stillunclear. A possible cationp interaction between the surface ofSWNTs and the imidazolium IL was postulated. However,

recent experiments and simulation studies inferred that the IL

interact with SWNTs through weak van der Waals interactions,

cationp or electrostatic interactions.79 Anyway, bucky gelsopened a wide range of applications, as they combine electro-

conductive CNTs and ion conductive ILs.80

3.2.2 Silica-based ionogels. Silica-based ionogels can be

obtained either by dispersion of silica nanoparticles into ILs

or by solgel processing.

Dispersion of silica nanoparticles. Numerous studies

reported a striking stabilization of metal nanoparticles in

ILs, even in the absence of any stabilizers, such as surfactants

or polymers.20 Nevertheless, bare silica colloids were shown to

be unstable in ILs, leading to the formation of a close packed

particulate network. This gelation of ILs in the presence of

silica nanoparticles was rst used by Graetzel to prepare

thermally stable quasi-solid electrolytes.81 Later on, Honma

studied the ionic conductivities of gels obtained using 10 wt%

of [C4MIm][TFSI] and evidenced the importance of particles

size on gelation.82 Studies on the mechanism responsible for

the colloidal aggregation demonstrated a reaction-limited

cluster aggregation, suggesting a moderate repulsive-

interaction between the particles, which was not enough to

stabilize the particles due to extremely high ionic strength and

the resulting surface-charge screening.83

Solgel processing. The rst solgel synthesis in an IL

solvent was reported in 2000 by S. Dai et al., which prepared

mesoporous silicas with high surface areas from a

mixture of tetramethoxysilane (TMOS) and formic acid in

[C4MIm][TFSI].84 Actually, the IL, which was removed at the

end of the synthesis, was used as a template. Since this

pioneering work, numerous solgel syntheses involving IL

templates were reported.17,85,86 ILs proved to have signicant

inuence on the structure of the resulting silica-based materials.

The eect of ILs as drying control chemical additives was

ascribed to the formation of a non volatile liquid lm on the

walls of pores which could protect them from the interface

strains associated with the formation of a meniscus on

evaporation.87 This would permit the pore walls to strengthen

on ageing, before removing the IL by means of Soxhlet

extraction or calcination thus giving access to highly porous

structure.

Anyway, this approach is out of the scope of this review.

Solgel processes involving silicon alkoxide precursors, in

which the interstitial IL phase rst present between colloidal

particles is then entrapped as condensation cross-linking

extends, should result in more intimate biphasic systems than

the simple impregnation of oxide particles. In this view, Deng

and co-workers reported the immobilization of a catalyst IL in

silica gels prepared in the presence of HCl aqueous

solutions,88,89 while some of us developed high ionic conducting

solid membranes by adapting the non-aqueous solgel method

of Dai.9092 The latter way provided monolith ionogels, cast as

pellets or rods, which turned out to be endowed both with the

transparency and mechanical properties of silica, and the

conductivity performances of ILs (about 3 102 S cm1 at200 1C).91 Recently, monolithic silica ionogels endowed withboth ionic conductivity and electronic conductivity: were

obtained by incorporating multiwalled carbon nanotubes

(MWNT).93 It is worth noting that the MWNT percolation

was reached with a rather low loading (i.e. 3.6 wt%), and

exhibited an electronic conductivity of 1.31 102 S cm1 atroom temperature.

Alternatively, the IL could be extracted from ionogels by

polar solvents such as acetonitrile, then exchanged for a new

IL phase.94 Care was taken to keep the solvent of extraction

inside the material to avoid cracking and nally the solvent

was replaced by the new IL. This method makes it possible to

incorporate any (even sensitive to formic acid) metal complex

within the ionogel, opening the route to many applications as

sensors and display devices. Thus, an ionogel showing a very

intense and highly monochromatic red photoluminescence

under UV irradiation, was obtained by doping an europium(III)

tetrakis b-diketonate complex into an imidazolium IL and byimmobilizing the IL in a silica-like network.95,96

Derived from the above working up, ionogels could also be

obtained by synthesizing the silica gel in a classical molecular

solvent, then exchanging this solvent for an IL. Recently, this

method was successfully applied by Taubert using a basic

catalyzed solgel reaction.97,98 Dynamics and physical

properties of the IL nanophase conned in silica-like matrices

will be developed in next section.

Inorganic matrices other than silica. Quasi-solid systems

arising from the mechanical mixing of inorganic particles as

titania or caesium hydrogen sulfate and ILs have been used as

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electrolyte materials.99,100 Parallel to silicon alkoxides, solgel

reactions of metal alkoxides in ILs were also investigated.

They turned out to be an interesting route to oxide particles,

with control of morphology and crystalline form.101 Thus,

Dionysiou systematically studied the synthesis of highly

porous titanium oxide particles with anatase crystalline structure

from a titanium isoproxide precursor by hydrolytic solgel

reaction at room temperature in the presence of water

immiscible 1-butyl-3-methylimidazolium tetrauorophosphate

[C4MIm][PF6].102 However, anatase monoliths with a

worm-like pore structure and large surface areas were

prepared using [C4MIm][BF4] as a templating agent by a

classical solgel method associated with a peptization process

in the presence of an aqueous solution of HCl.103 Actually, the

only report relevant to ionogels was the recent preparation of

TiO2 gels from amixed emulsion of 1-butyl-3-methyl-imidazolium

bis(triuoromethylsulfonyl)imide [C4MIm][TFSI], TiCl4,

methanol and formic acid, which was ultra-sonicated at

25 1C, then aged and dried at 80 1C.104 These electrolytematerials showed stable ion conductivities over a wide range of

temperatures, including the intermediate region of 100200 1C,and thermal stability at 275 1C (conductivity of 102 S cm1 at275 1C). Tin dioxide translucent monolith ionogels wereobtained by using a b-diketonato stabilized tin precursor.105

A major advantage of this precursor over commercially

available SnCl4 was the possibility to use a wide range of

ILs ([C4MIm][BF4], [C4MIm][PF6], [C4MIm][Br]), thanks to

the chelating ligand on the tin precursor which permitted to

control the polycondensation rate.

3.3 Composite (hybrid organicinorganic) ionogels

To our knowledge, the association of molecular organogelators

and inorganic nanoparticles has been hardly explored.

However it is worth mentioning the latent chemically-cross-

linked gel electrolyte precursors for quasi-solid dye sensitized

solar cells (DSSC) reported by Kato et al.106 The gel electro-

lyte precursors consisted of silica or titania nanoparticles and a

dicarboxylic acid (typically with a C16 chain) as the latent

gelators. The viscosity of the precursor was low at rst,

but when the precursor was baked at 80 1C, it solidiedimmediately.

By contrast, the incorporation of inorganic nanoparticles in

polymer ionogels is widely used, as it is well known to improve

the mechanical properties. Accordingly, it is a way of counter-

balancing the plasticizing eect of high IL loadings, while

preserving exibility and tractability of the resulting

nanocomposite materials. It is worth mentioning here the

use of imidazolium ILs as compatibilizers between apolar

polymer chains and polar inorganic ller, as clays107 and

silica108 (see also the specic case of carbon nanotubes109).

Although this use does not enter the scope of the review, it

sheds light on the role that may be played by ILs in some

ternary polymer/inorganic ller/IL systems. Moreover,

nanocomposite polymer electrolytes (NCPEs) have demonstrated

enhanced ion conducting properties.110114 Actually, inorganic

oxide nanollers have large specic surface areas covered with

various Lewis acidic or basic groups. The interactions between

these surface groups and the ionic species are most likely

involved in the observed conductivity enhancement. Typically

the creation of a large IL/oxide interface is thought to promote

ion-pair dissociations (for instance by giving rise to specic

interactions with the anion or the cation) and to provide some

free volume for ions to diuse. Moreover, mesoporous silica

llers lead to higher lithium transference numbers in electro-

lyte membranes for Li-ion batteries.114 Recently, an in situ

solgel synthesis of silica nanollers has been successfully used

to prepare transparent and exible electrolyte membranes.27

Indeed, the use of modied poly(methyl methacrylate)s

(PMMA) bearing pendant trimethoxysilane groups and tetra-

ethoxysilane (TEOS) allowed immobilizing high loadings of

IL, while silica nanollers covalently bonded to the polymer

chains provided mechanical strength. Note that the inorganic

functionalisation of cellulose with siloxane groups, which can

subsequently undergo polycondensation, was shown to

similarly yield ionogels with hybrid organicinorganic host

matrices.26

4. Connement eects

The connement of ILs, where ion-wall interactions become

important relatively to ionion interactions, can infer some

changes in their physicochemical behaviour. The rst insight

into the eect of connement is given by the thermal

behaviour. Typical phase transitions disclosed on bulk ILs

by DSC analysis (on heating after a fast cooling) comprise

glass-transition temperature (Tg) (the compound passed

from the glass state to a subcooled liquid phase), cold

crystallization (Tcc; exothermic), and melting (Tm; endothermic).

In silica ionogels, while glass transition is observed whatever

the degree of connement, no hint of any cold crystallization

nor melting is observed for the highest degrees of connement

(i.e. the smallest space for the IL).91,92 Moreover, rst-order

transitions are clearly shifted. In ionogels, melting (Tm@pore)

is shifted to lower temperatures, although the signal

simultaneously broadens.115 Such a displacement of phase

transition has been reported for many conned solvents, but

the lowest shift observed with ILs is greater than the highest

observed with molecular liquids (conversely with the viscosity

behavioursee below).116 This shift is commonly related to

the melting temperature of the conned species (Tm), the pore

diameter (d), the molar volume (Vm), the enthalpy of fusion

DHf, the surface tensions at solid-substrate gss and liquid-substrate gls interfaces on the basis of the GibbsThomsonequation Tm Tm@pore = a(TmVm/DHf)(gss gls)/d.91,115 Thedependence on the chemical nature of the host matrix is thus

expected. Actually, it appears that there is a strong dependence

of thermal behaviour on the chemical nature of both pore

walls and IL ions.98 Note that CNTs oered an opportunity to

observe unique crystalline organisation due to specically

strong CH F contact with the uorinated anion in thecrystal phase induced by the strong connement degree.78,117

It is also noteworthy that related simulation studies of

connement of [C1MIm][Cl] conned between hydrophobic

graphene sheets showed a transition at much higher temperatures

than expected for the bulk IL, moreover at a distance of the

wall of no less than 1.1 nm.118 This evidences the particularly

important eect of the liquid-substrate anity on transition

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temperature in the case of ILs (attractive forces increase the

melting point of conned species; conversely, repulsive forces

decrease it), concurrently with the degree of connement

(volume of conned species vs. volume of connement) which

decreases the melting point.119 Moreover, it is noteworthy that

recent rheology studies on conned ILs between mica walls

(SFA apparatus) showed strong IL-surface interaction below

around 10 nm, i.e. below around 13 IL organized layers

(depending on the nature of the IL, gures are given here

for [C4MIm][TFSI]).120 As referred to the above mentioned

DSC studies, this appears to be a limit below which connement

shows strong eects. Other recent rheological studies conclude

at a lower viscosity enhancement for connement of ILs as

compared to that of connement of non-polar molecular

liquids,116 or more precisely to a strong lowering of the

viscosity for [BMIm] based ILs conned between methylated

mica walls compared to non-methylated parent walls.121

Solid state NMR spectroscopy gives further information on

the molecular dynamics of conned ILs. From a general point

of view, when increasing the temperature, the IL system

transforms from the rigid lattice to more dynamic phenomena

associated with the diusive state: this change is accompanied

by a line narrowing of the NMR signals, which normally takes

place at the melting point in case of crystalline salts and at the

glass transition temperature in case of amorphous materials.

So, as a rst approach, following the line width gives insights

into the dynamics of IL. Whatever in polymer gels or silica

conned ILs, connement usually results in decreasing the

temperature at which the line narrowing occurs (1H and 19F

nuclei).91,122 This shows that the connement studied did not

inhibit ion dynamics.1H NMR spectra of conned IL showed a relatively narrow

signal without spinning on monolithic silica ionogels

(Fig. 8a).123 Unusual for solids, this evidenced that liquid-like

mobility was still enough to average the dipoledipole and

chemical shift anisotropy (csa) which were responsible for line

broadening. A spinning rate as low as 400 Hz was enough to

recover liquid-like resolution, although the signal was not as

narrow as for bulk IL due to susceptibility gradient at the

wall neighbourhood. Other studies on similar systems (weak

interactions between conned IL and pore wall) pointed out

an even better resolution, concluding to the absence of a

noticeable wall eect.97 From the spectra observed without

spinning and at a spinning rate of 400 Hz, it was concluded

that the dynamics of the conned ionic liquid experienced only

a slight slowing down. It is noteworthy that the above

mentioned MAS NMR experiments probe short-range

correlations, which is consistent with the distribution of

chemical shifts at the wall neighbourhood. Relaxation-time

measurements were used to probe longer spatial range correlations

(Fig. 8b). 1H T1 relaxation times measured above the melting

temperature showed similar T1 for conned and bulk IL, thus

illustrating similar dynamics. Nevertheless, below the

temperature of crystallisation of bulk IL, T1 parameter of

conned IL exhibited values in the same range as above the

crystallisation temperature, thus revealing that liquid-like

behaviour was gained toward lower temperatures on connement.

Further proton and uorine nuclear magnetic relaxation

dispersion (NMRD) studies, widening space and time

scales probed to 20 nm and 5 ms, respectively, suggested atranslational diusion of this conned cation and anion at the

proximity of the at pore surface. From a model, translational

time (time during which an ion is correlated with the surface)

and escape time (time during which the ion is not correlated to

the surface) were measured and showed a decrease of the

translational correlated time along with an increase of the

ratio escape time/translational time when increasing the

temperature.124 PFG NMR studies showed also that conning

an IL or conning an IL-based electrolyte (i.e. with addition of

lithium salt) does not entail the ionic mobility.125

The ionic conductivity of silica ionogels with rigid porosity

is of course of interest. While complex impedance spectro-

scopy (CIS) allows ionic conductivity measurements, ions

transportations throughout the ionogel has been demonstrated

with lithium battery prototypes as well as with solar cells.126

By means of CIS, the ionogels showed conductivity of the

same order of magnitude as that of bulk IL. Typically, the

conductivity of a monolithic ionogel was found to be roughly

half that of the conned IL, for the same shape factor, which

means very similar conductivities. The Arrhenius plots

recorded between ca. 30 1C and +220 1C showed breakslopes as reported by several studies.9092,125 The origin of this

Fig. 8 (a) 1H NMR room-temperature spectra of the pristine IL ([C4MIm][TFSI]) and a monolithic silica ionogel (IL/SiO2 = 0.5) at zero, 400 Hz

and 3kHz spinning rates (asterisks indicate spinning sidebands). (b) 1H relaxation times vs. temperature of pristine IL (open symbols) and of a

monolithic silica ionogel (IL/SiO2 = 0.5; black squares). Reprinted with permission from ref. 123.

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break slope is not completely elucidated and could be related

either to ion pairs dissociation or to the disappearance of

wall-to-ions interactions, as seen by NMRD.124

Infra-red spectroscopy on silica based ionogels indicated no

striking dierence between the bulk and conned ILs.97

Nevertheless, in the case of imidazolium ILs with long alkyl

chains, the shifts of the bands assigned to ~n(CH2) disclosedsome changes in the conformational order of the alkyl chain,

which were ascribed to a more disordered conformation

adopted on connement compared with the bulk crystalline

solid IL at room temperature.94 Similarly, Raman spectra

disclosed that, after being conned in mesoporous silica,

[C4MIm][Cl] was in the liquid form as early as 25 1C, whereasit was expected to melt at 70 1C.94 These results, which areconsistent with DSC and NMR data, conrm that steric

constraints at the wall neighbourhood prevent solid state

organization of the IL at low temperature.83 Nevertheless, it

has to be emphasized that the conclusions from the spectro-

scopic studies presented just above refer to connement at no

less thanB6 nm but that a uorescence red shift was observedrecently in one study for smaller pore diameters, i.e. higher

connement.127

Luminescence spectroscopy was informative too. An

unexpected connement eect was observed specically with

a dicyanamide-based IL. Fluorescence emissions were

recorded with dierent IL loadings; the emission intensity of

a 25 wt% [C4MIm][N(CN)2] ionogel was found to be about

200 times stronger than that of bulk [C4MIm][N(CN)2]. This

was tentatively attributed to the presence of strong ppconjugations.94 It could be inferred that a specic arrangement

of the ions at the wall neighbourhood led to a specic

anionanion arrangement, which in turn led to this unexpected

uorescence enhancement. Another indirect way to probe the

physical feature of a conned species was to compare the

luminescence spectrum of an europium(III) complex dissolved

in an IL conned in a silica matrix with that of the same

europium(III) complex dissolved in the bulk IL. No dierence

in crystal-eld ne structure or in relative intensities could be

observed in the spectra of the ionogel and the IL solution. This

indicated that connement of the europium(III)-doped IL had

negligible inuence on the structure of the europium(III)

complex. The decay time turned out to be similar in the bulk

(0.55 ms) and conned IL (0.52 ms).95 The luminescence decay

times of others lanthanide(III) cations (samarium, neodymium,

ytterbium, erbium, terbium) encapsulated in ionogels were

similarly recorded. No signicant dierence between the decay

times (conned IL or bulk) was observed, evidencing again the

liquid-like behaviour of the conned IL.128

5. Applications

5.1 Electrolytic membranes

ILs are of considerable interest to implement advanced

electrochemical devices, specically energy devices, whose

development is expected to address the topical challenges

of renewable energy and global warming, as lithium

batteries, fuel cells and solar cells. Actually, replacing

conventional media, organic solvents in batteries or water in

proton-exchange-membrane fuel cells (PEMFC), by low-

vapour-pressure and non ammable ILs presents major

advantages such as, among others, improved safety and

operating temperatures higher than 100 1C. ImmobilizingILs within solid membranes gives access to all-solid devices,

which are an industrial challenge for practical applications.

Lithium-ion batteries. Lithium batteries are expected to nd

a prominent role as electrochemical storage systems in

renewable energy plants, as well as power systems for sustainable

vehicles, such as hybrid and electric vehicles. However, scaling

up the lithium battery technology for these applications is still

problematic since issues such as costs, wide operational

temperature and safety are still to be resolved. The latter point

attracts a special attention since a too rapid discharge of a

lithium battery or a short-circuit can result in overheating,

rupture, and even explosion. The current development of

all-solid IL-based lithium batteries may be regarded as a real

breakthrough in the eld (Fig. 9).129

Actually, IL-polymer systems, commonly referred to as gels

in the literature, showed performances in term of number of

cycling, rate of cycling and solid electrochemical interface

formation which were similar to those obtained with non

conned IL-based electrolytes.130 The connement of a mixed

salt (IL with lithium salt) within a poly(vinylidene uoride)

hexauoro propylene (PVdFHFP) copolymer matrix has

been intensively studied.131 It is here of importance to obtain

exible, thin (lowering the ohmic drop), mechanically resistant

(avoiding short-cuts upon pressure between the positive and

the negative electrode) membranes. It was shown that the

conductivity in these polymeric matrices, e.g. PVdFHFP

membranes loaded with an electrolyte solution of LiTFSI

in N-butyl-N-ethylpiperidinium N,N-bis(triuoromethane)-

sulfonimide [PP24][TFSI], was on the order of 104 S cm1

at room temperature, which resembled that of the parent

bulk electrolyte under the same conditions. However, an

Fig. 9 Toward all-solid IL based lithium battery. Reprinted with

permission from ref. 129.

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unexpected but potentially signicant enhancement in Li

transference number was observed in passing from the liquid

to the membrane electrolyte system. Another polymer

which was intensively studied for hosting lithium salt or ILs

containing lithium salt is polyethylene oxide (PEO), in relation

to its ability to solvate Li cation. The addition of IL to

P(EO)20-LiTFSI systems resulted in an increase of more than

one order of magnitude in ionic conductivity at room

temperature (moving from salt-in-polymer to polymer-in-

salt systems).132 In this case, with a N-butyl-N-methyl-

pyrrolidinium [PY14] to lithium cations ratio of 3.2, the

ionic conductivity of the polymer electrolyte reaches

2 104 S cm1 at 40 1C. Moreover, a low temperatureconductivity enhancement of the ILs containing-PEO

electrolytes was pointed out, which was attributed to the

formation of amorphous phases instead of PEO-lithium

crystalline phases.

Another approach is the solgel hydrolysis-polycondensation

of silicon alkoxide directly onto a porous electrode. This

allows a full wetting of the porosity of the composite electrode

as well as the simultaneous formation of a solid electrolytic

separator.126 The concept allows single-step preparation of the

assembly of the electrode and solid electrolyte separator and

can be applied without specic adaptation to current thick

electrode technology prepared by the widespread tape casting

technique. Here the polarization, i.e. the voltage dierence

between charge and discharge, was less important with the

ionogel electrolyte than with genuine IL-based electrolyte,

which was attributed to the good wetting of the composite

electrode by the ionogel. Moreover for LiNi1/3Mn1/3Co1/3O2based electrodes, the discharge capacities obtained were

B110 mAh g1 and B105 mAh g1 with the ionogeland IL based electrolytes resp., whereas with classical

carbonated based electrolytes the capacities were found

around 125 mAh g1. Nevertheless, up to now the maindrawback remains the low cycling rate (C/20 at RT). It is also

interesting to note that the capacity of a battery with LiV3O8as positive electrode and with PEO-PVdF conned IL-based

electrolytes exhibited a (slight) increase, as referred to those

observed for this cathode material in standard electrolytes.133

This result was attributed to a lower solubility of the electrode

material in the IL-polymer electrolyte.

Fuel cells. Protic ILs can serve as proton carriers in fuel cell

electrolytes (Fig. 10), thus giving access to conductivities

exceeding 10 mS cm1 under fully anhydrous conditions.134,135

For instance, ammonium salts can be used as protic

electrolytes for fuel cells running above 80 1C.136 Actually,conventional PEMFCs require medium temperature operation

in order to provide more ecient power generation and longer

lifetime.137 Moreover, PEMFCs also require membranes

eciently separating both fuels. In this prospect, providing

membranes with proton conducting ionic liquids (PCILs), non

volatile and non degradable in the needed range of temperature,

is the focus of intense research.138

Most recent research has been devoted to PVdF-HFP,

Naons and sulfonated polyimide ionogel-membranes. The

latter ones have been recently studied with diethylmethyl-

ammonium triuoromethanesulfonate for FC operating under

nonhumidied conditions at intermediate temperatures. Using

an IL weight loading as high as 80%, these membranes

showed thermal stability (4300 1C), ionic conductivity(4102 S cm1at 120 1C), mechanical strength and gaspermeation comparable to those of hydrated Naons.139

Naons membranes were probably the most studied for FC

applications. In these membranes the choice of the IL and the

version of Naons are important. The sensitivity of the choice

of the PCIL is exemplied by the comparison of two ILs with

the same diethylmethylammonium cation associated with

either triuoromethanesulfonate or TFSI anion: the rst one

showed good activity for FC electrodes reaction as well as

relatively low ohmic drop, leading to open circuit voltage

(OCV) of 1.03 V at 150 1C, whereas the second showed lowerelectrode activities, leading to an open circuit voltage of ca.

0.7 V.140 Modied Naons were also shown to be of

interest. Thus connement of triuoromethanesulfonate

triethylammonium in Naons neutralized with triethyl-

ammonium resulted in lower water uptake and improved

thermal and mechanical stability as compared to the reference

Naons membrane.141 The key role of ionomer-IL inter-

actions on the proton transference number has been recently

emphasized.135,142 Polymer membranes based on PVdF-HFP

were also developed. A marked decrease in conductivity,

larger than what could be expected from the dilution of the

conducting IL by the insulating polymer matrix, was ascribed

Fig. 10 IL based polymer electrolytes fuel cells. Reprinted with permission from ref. 140.

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to the morphology of the membrane and the interactions

between the polymer matrix and the IL.143 Superacids

like triuoromethanesulfonic acid or N,N-bis(triuoro-

methanesulfonyl)imide were used to enhance proton

conductivity. Unfortunately, strong interaction between

HTFSI and parent IL (with the same anion) increased the

viscosity and accordingly decreased the ionic conductivity.144

Also based on a uorinated polymer, uorohydrogenate ionic

liquid based membranes showed OCV around 1.0 V at 130 1Cwith interesting conductivity at 34.7 mS cm1 at 130 1C.145

Hybrid inorganicorganic nanocomposites were also

investigated. Biopolymer composites were fabricated, aiming

at increasing proton transport by blending a basic chitosan

exchange membrane and basic oxide llers such as CaO.146

Filling Naons with SiO2 particles led to an unexpected

segregation into Naons domains with helical chains and

hydrophilic domains where SiO2 particles were shown to be

covered by a shell of PCIL.147

Finally, a recent work was reported on electrolyte

membranes fully based on silica-derived ionogels.148

Tetraethoxysilane/ethyl-triethoxysilane/trimethylphosphate and

1-butyl-3-methylimidazolium-bis(triuoromethylsulfonyl)imide

were used as precursors. The sol was then cast onto one

side of the Pt-loaded carbon serving as electrode. The

resulting membrane showed good hydrogen permeability

(2.7 1012 mol cm1 s1 Pa1 for the 40 wt% [C4MIm][TFSI]at 150 1C). Nevertheless, the main drawback of such amembrane still resides in the occurrence of cracks which would

short-cut the gas path.

Solar cells. Still in the sustainable development context,

harvesting solar energy with a high yield is a major challenge

because of the clean, inexhaustible and ready availability of

the resource.149 In the ionogel prospect, physical gels obtained

by mixing silica nanoparticles as gelator to solidify IL-based

electrolytes (with addition of KI and I2) resulted in enhanced

performances of dye sensitized solar cells (DSSCs) (Fig. 11).

As a matter of fact, such ionogels were applied to DSCCs

based on a ruthenium polypyridyl photosensitizer leading

to 7% eciency at AM 1.5 sunlight.81 Ionogels provide

considerable benets over bulk ILs since they enable the

fabrication of exible, all solid-state devices, free of leakage

and available in varied geometries. At the borderline of

ionogel, it is worth mentioning DSSCs in which the IL-based

electrolyte wetted an array of TiO2 nanotubes perpendicular to

a Ti foil, thus enhancing electron transfer. The photovoltaic

conversion eciency was of 3.6% under simulated AM 1.5

sunlight.150 Such an approach was similar to that carried out

with IL lling the straight nanopores of alumina sandwiched

between a counter electrode and a TiO2 electrode.151 Even

though the electrochemically inactive Al2O3 wall occupies

50% of the electrolyte layer, the solar cell performances were

the same as those with IL gelled with Al2O3 nanoparticles,

where random ion paths were expected. Actually, the increase

in performances was explained by straight ion diusion and

Grotthuss-type diusions fabricated on Al2O3 walls.152

Similar devices based on ZnO nanoparticles and CNT layers

at the electrolyte interface (thus creating a good continuous

electrolytic junction) were tested.153 Fully organic ionogel

electrolytes were also used, based on poly(vinylpyrrolidone),

and showed an overall light-to-electricity energy conversion

eciency of 5.41% under irradiation of AM 1.5.154 Some

studies were reported with the same objective with organic

gelators, synthetic or natural, but the eciency was lower than

those already reported.155,156 It is worth noting that previous

DSSCs based on volatile solvents showed the highest eciency

at 11%, but suered from intrinsic volatility. Replacement of

volatile solvents by ILs decreased eciency down to 7% and

showed long-time instability. However, the assemblies of

eutectic ternary mixtures of iodide ILs with nanocrystalline

TiO2 particles at one of the DSSC interfaces led to eciency of

8.2% under AM 1.5 illumination.157 It is remarkable for our

subject that association with oxide particles permitted to reach

high eciency as well as excellent stability, thus opening way

to low cost DSSCs.

Electrochemical sensors and biosensors. Coll et al. described

for the rst time the preparation of PVC membranes containing

[C4MIm][PF6] and polyazacycloalkane as ionophore for use as

anion-selective electrodes and found a remarkable selective

response to the highly hydrophilic anion sulfate.158 Pletnev,

using the same concept, described the use of PMMA and PVC

membranes containing bis(triuoromethanesulfonimidate)

salts of 1-butyl-2,3-dimethylimidazolium and dodecylethyl-

diphenylphosphonium, [BDMIm][TFSI] and [DEDPP][TFSI],

respectively. These ion-selective membranes demonstrated

good and extremely stable responses to both cations and

anions (including surfactants).159

Modied electrodes with bulky gels are characterized by a

large surface area, excellent chemical and physical stabilities

which make them ideal candidates for constructing sensors

with high performance. Zhu was able to selectively detect

dopamine even in the presence of ascorbic acid and uric acid

using a glassy carbon electrode modied with a MWCNTs/

[OMIm][PF6] gel.160 An electrode modied with a gel

containing ferrocene lled with SWCNTs has been reported

which eciently catalyzed oxidation of H2O2 to O2 and

reduction to H2O.161

ILs are known for their good biocompatibility with

biomolecules and enzymes and they improve their reusabilityFig. 11 All-solid dye sensitized solar cell. Reprinted with permission

from ref. 157.

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and stability. Thus, catalytically active proteins and enzymes

were also conned for biosensors applications in order to

achieve direct electron transfer.162 Some examples are based

on the immobilization of enzyme-IL systems in chitosan163 or

Naons.164 These membranes yielded suitable matrix for

direct electrochemistry of HRP and hemoglobin (Hb). New

hybrid organicinorganic siloxane-based hydrogels entrapping

IL have been recently developed for similar uses.26

Interestingly, an amperometric biosensor was obtained by

coating the surface of a glassy carbon electrode with a

silica ionogel lm doped with HRP and ferrocene

(IL:[C4MIm][BF4]).165 The resulting electrode had faster

response in sensing hydrogen peroxide than with classic

solgel lm, with a detection limit for H2O2 of 1.1 mM. Amore sophisticated system based on silica/RTIL/Naon/

carbon electrode was described where Hb could be adsorb

onto the surface. This biosensor shows a stable, sensitive and

fast response to oxygen.166

Bulky gels based electrodes can also be modied by

catalytically active proteins like Hb,167 HRP168 or glucose

oxidase.169 Closely related works using carbon nanotubes

composites with gold170 or AuPt nanoparticles171 were also

reported.

Actuators. CNT nanollers are known to improve the

mechanical properties of polymers and to endow them with

electronic conductivity provided percolation threshold was

reached. In 2005 Aida et al. reported the synthesis of the rst

generation of bucky-gel actuators, which worked in air,

without any support or external electrolytes, and without

deposition of a metallic electrode layer. The lm was prepared

through layer-by-layer casting of the electrode from a mixture

of a uorinated copolymer (PVdF-HFP), 4-methyl-2-pentanone,

an IL and SWCNTs arising from high pressure CO conversion.172

However, the performances were limited by the robustness of

the polymer, which also lowered the electrical conductivity

and capacitance of the electrode layers. A second generation of

actuators was reported by the same authors in 2009.173 These

actuators comprised polymer-free CNT electrodes obtained

using super-growth SWNTs arising from a water assisted

chemical vapour deposition. Performances were increased, so

that a displacement as large as 5 cm was observed at a

frequency of 1 Hz, that is a ten fold increase to the performance

of the rst generation. Recently, elastic conductors were

achieved from millimetre-long SWNTs, a uorinated copolymer

and a compatible IL, which could be made printable.

Conductivity of more than 100 S cm1 and stretchability ofmore than 100% were obtained. A rubber-like stretchable

active-matrix display that showed good luminance characteristics

was constructed from integrated printed elastic conductors,

organic transistors and organic light-emitting diodes.174

5.2 Separation membranes

Besides their use as electrolytes, ILs are also well known for

their ability to dissolve a wide range of species, from organic

compounds to metal salts. The solubility of gases in ILs is of

prior importance for using them as solvents for reactions

(hydrogenation, hydroformylation etc.) or for the storage of

gases.175 Emphasis was put on the outstanding ability to

dissolve carbon dioxide.176 The use of ILs to separate gases

benets from huge advantages, as the absence of contamination

of the gas stream (in relation to the low vapour pressure of

ILs) and the possibility to work at higher temperatures than is

in conventional absorption solvents.

Actually, the specic properties of ILs considerably broaden

the range of separation applications, from metal ions

separations to chromatography. The design of task-specic

ions and the choice of the cationanion combination permit

one to tune the high or selective solvation ability and non

miscibility in water or other solvents. However, the challenge

to implement ILs in separation techniques is also to

develop supported IL membranes, either by immobilizing

ILs within polymer membranes or in porous inorganic

supports.24

The operational stability of supported IL membranes based

on hydrophilic polymers has been demonstrated.177 The

nature of the supporting polymer membrane strongly aected

the transport (i.e. permeation) rates and selectivity in the

separation of organics.178,179 Ion selective polymer membranes

involving ILs (typically phosphonium salts) as ion exchangers

were successfully used in potentiometric sensors.159,180 In

those cases, the hydrophobicity of the polymer (typically

PMMA or poly(vinyl chloride)) could prevent spontaneous,

non-specic ion extraction from the aqueous sample,

while the IL acted as a plasticizer, improving the solubility

and migration of analytes. The ability of ILs (typically

phosphonium based ILs like Cyphos 104) to extract metalions may nd application in ionogels as well. Some recent

examples include silica based ionogels for the extraction

of Y(III),181 and alginate based ionogels for the extraction of

Pd(II).182

In gas separation, ionogel polymer membranes were shown

to increase selectivity compared with conventional polymer

membranes (including those arising from polymerisable ILs).

However, the IL was likely to be displaced at low dierential

pressure. This problem was overcome by using a combination

of polymerisable IL with 20 mol% free RTILs to increase the

compatibility between the liquid and solid phase while keeping

good gas permeativity. These tailored membranes allowed gas

separation with a CO2 permeability increased ofB400%, andan improvement of CO2/N2 and CO2/CH4 selectivity with

respect to analogous membranes arising from only poly-

merisable ILs.183 It is worth stressing the ecient separation

of CO2 from H2 at temperatures around 100 1C and the topicalinterest for applications as hydrogen production from syngas

(an industrial mixture of CO and H2 which can be reacted with

water to produce CO2 and more H2).184 Another recent result

is the use of LMWG as 12-hydroxystearic acid to form gels

which exhibit liquid-like CO2 gas transport ability as they are

composed of 98.5% of IL.185

Hybrid organicinorganic membranes which involve the

dispersion of inorganic particles (typically zeolite) in the

polymer matrix combine superior selectivity with excellent

fabrication properties of polymer membranes.186 However,

limited compatibility between inorganic and organic

components may result in a lack of adhesion. The addition

of IL both increased the permeability and proved to be an

excellent compatibilizer, yielding defect-free membranes.

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5.3 Catalytic membranes

SILC, where the IL (which contains the catalyst) is immobilized

as a thin lm on high surface area supports, has been widely

developed to circumvent viscosity and recyclability problems.23,24

In this case, reactants and products diuse through the

residual voids in pores which are coated with the liquid

catalyst lm.

Silica based ionogels were shown to provide alternative

nanoreactors systems.24 The presence of two interpenetrated

networks (silica and IL) allows free transport of reactants and

products through the liquid phase, while pore diameters are

small enough to prevent IL leaching. Actually, Deng et al.

reported the enhanced catalytic performance of the carbonylation

of aniline into ureas and carbamates in the presence of

ionogels loaded with Rh(PPh3)3Cl and Pd(PPh3)2Cl2.89 The

reactions were carried out without additional organic solvent

in an autoclave at 135180 1C under 5 MPa CO pressure. Pdsalts were also encapsulated in silica based ionogels and apply

as an eective catalytic system without leaching for Heck

coupling reaction in the presence of toluene.187

ILs have recently demonstrated attractive performances as a

reaction media in biocatalysis.13 The activity of biomolecules

in ILs has been found to be comparable or higher than in

conventional solvents, and enhanced enzyme stability has been

reported. The rst reports of applications of ionogels to

biocatalysis seem promising. Thus, the activity of horseradish

peroxidase (HRP) immobilized in ionogels was shown to be

about 30-fold greater than that in silica gel without IL, with an

excellent thermal stability.188 HRP was immobilized in iono-

gels prepared by acidic hydrolysis of TEOS in [C4MIm][BF4].

The enzyme was thought to be protected by the IL from the

detrimental eect of ethanol during the synthesis, while the

interconnected pore structure of ionogels would facilitate the

internal diusion of the substrate.

5.4 Drug release

Drug release has just emerged as a new application of ionogels.

It is dealt separately as, contrary to the previous elds of

applications, it is based on ionogels that are able to lose

gradually the IL. In other words, whereas all the other

applications require the complete stability of ionogels under

the operating conditions, drug release involves the in situ

delivery of the IL loading.

A common problem that exists with many active

pharmaceutical ingredients (APIs) is their low solubility. As

a result, much research is conducted on nding ways to

improve drug solubility and availability. It has been demon-

strated that ILs can dissolve compounds which are insoluble

or sparingly soluble in water and organic solvent including

cellulose and compounds having pharmaceutical activity.

These studied were however limited by the toxicological issue

of ILs. Some eorts have been conducted recently to obtain

nontoxic and biodegradable ILs.189,190 Some ion constituents

of ILs have biologically activity, such as ammonium cations

which are well-known as antibacterial agents. Recently, new

ILs have been created from APIs, some of them combining

anionic and cationic APIs with biological activities that

complement each other. These new IL drugs present promising

advantages over usual solid APIs, which can suer from

polymorphic conversion, low solubility and low bioavailability

associated with the solid form.

Actually, the availability of APIs is a key issue. A tremendous

eort is currently being undertaken by the pharmaceutical

industry to develop drug delivery systems in which the release

of the active substance is controlled on demand. Drug

carriers being considered as promising candidates include

soluble polymers, microparticles made of insoluble or

biodegradable natural and synthetic polymers, microcapsules,

cells, lipoproteins, liposomes, and micelles. Silica matrices

show high biocompatibilitybiodegrability and resistance to

microbial attack and they exhibit higher mechanical strength,

enhanced thermal stability and negligible swelling in organic

solvents, compared to most organic polymers. All these

features make silica supports promising carriers for the drug

delivery systems formulation. Several research groups have

investigated both drug storage and delivery properties of

ordered mesoporous silica such as MCM-41 and SBA-15

and of solgel silica materials.191 The use of ionogels for

biological applications is very limited to-date. Lin was the

rst to report on the synthesis of mesoporous materials

containing long alkyl chains antibacterial ILs and their use

as controlled release delivery device.192 Recently some of us

reported the synthesis of a new IL containing drug composed

of an imidazolium cation and ibuprofenate anion.193 We