energi applications of ionic liquids

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Energy applications of ionic liquids

James F. Wishart*

Received 30th March 2009, Accepted 1st May 2009

First published as an Advance Article on the web 11th May 2009

DOI: 10.1039/b906273d

Due to their unusual sets of properties, ionic liquids have many important applications in devices and

processes for the production, storage and efficient use of energy and other resources.

Introduction

Ionic liquids (ILs) are defined (somewhat arbitrarily) as molten

salts whose fusion temperatures are at or below 100 C. Although

their history extends back almost a century,1 they have received

a burgeoning amount of attention in the present decade. While it

may have been tempting a few years ago to view the interest in

the field as a passing fad, the literature on ionic liquids continues

to grow rapidly and to diversify into new areas. Ionic liquids are

here to stay.1

The attraction of ionic liquids (ILs) lies in their remarkable set

of properties when compared to conventional solvents. As salts

consisting of distinct anions and cations, ILs are inherently

binary (or higher order) systems. The anions and cations can be

independently selected to tune the ILs physicochemical prop-

erties (melting point, conductivity, viscosity, density, refractive

index, etc.) while at the same time introducing specific features

for a given application (hydrophobicity vs. hydrophilicity,

controlling solute solubility, adding functional groups for

catalysis/reactivity purposes, chirality, etc.) Examples of typical

IL anions and cations are shown in Fig. 1, however these are only

a sample of the infinite variety available. Generally speaking,

various tricks, such as using bulky organic ions, fluoroussubstitution, and low-symmetry structures, are employed to

prevent efficient ion lattice packing that would result in strong

coulombic interactions and high-melting solids instead of room-

temperature liquids.

Although the coulombic interactions in ILs are weakened by

design, in most cases they remain strong enough to make the ILs

vapour pressure negligible at room temperature. Herein lies one

of the major distinctions of ILs the low vapour pressure makes

them combustion resistant, evaporation-proof, and suitable for

vacuum applications, and the lack of a boiling point (below the

decomposition temperature) means that many ILs have very

wide temperature ranges in the liquid state. In some ILs the fluid

range extends all the way down to the glass transition tempera-

ture (typically 110 to 50 C). On the other hand, certain ILs

can be distilled if they are heated in high vacuum,2 or, as in the

case of protic ionic liquids, they are in equilibrium with volatile

neutral molecules via a proton transfer reaction.3

Ionic liquids are intrinsically conductive since they typically

contain on the order of 25 M electrolyte, however the viscosities

of ionic liquids tend to be higher than conventional solvents,ranging from 2030000 cP at room temperature. Nevertheless,

low-viscosity ionic liquids often have room-temperature

conductivities of 110 mS cm1, comparable to or larger than

that of 0.1 M aqueous KCl solution. The reciprocal relationship

between the viscosities and conductivities of ionic liquids, and

causes of non-ideality, have been carefully studied by Angell and

coworkers,4 as well as other groups.5

A large fraction of the reported ionic liquids are amphiphilic

due to the hydrocarbon side chains that festoon the cations or

anions. Molecular dynamics simulations6 have shown that

hydrophobic domains can be formed through hydrocarbon side-

chain aggregation, and if the chains are long enough the domains

Fig. 1 Typical cations and anions used to make ionic liquids.

Chemistry Department, Brookhaven National Laboratory, Upton, NY,11973, USA. E-mail: [email protected]; Fax: +1 631 344-5815; Tel: +1631 344-4327

Broader context

During the early phases of the rapid growth of interest in ionic liquids, their green and environmentally-friendly characteristics

were praised perhaps a little too enthusiastically. Ionic liquids, like any materials, are not intrinsically green; their environmental

benefits accrue through the ways they are employed. As our understanding of ionic liquids and their novel combinations of tunable

properties has grown, it is possible to see many opportunities for ionic liquids to contribute to the sustainable production, storage

and efficient use of energy as components of advanced devices and processes. This brief review provides a basic introduction to ionic

liquids and their interesting properties, and enumerates how these properties are being applied to improve the ways in which we

generate, store, and consume energy.

956 | Energy Environ. Sci., 2009, 2, 956961 This journal is The Royal Society of Chemistry 2009

MINIREVIEW www.rsc.org/ees | Energy & Environmental Science


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can merge into a continuous structure within the IL interwoven

with the polar domain. This results in the second major distinc-

tion between ILs and molecular solvents their ability to dissolve

and solubilize a wide range of polar and nonpolar materials that

would not be simultaneously soluble in a conventional molecular

solvent. Solutes can range from inorganic salts to hydrocarbons,

synthetic and bio-polymers, lipids, sugars, proteins and even

functional enzymes. ILs can dissolve wood and lignocellulosic

materials,7 and they even show promise as less hazardous fixa-tives for embalming tissues.8

Due to the electrostatic constraints of this ionic fluid, the polar

and nonpolar regions are forced to remain in intimate contact

and the liquid is heterogeneous on the molecular scale. Concepts

of polarity that are useful for understanding the nature of

conventional solvents are ill-defined in ionic liquids.9 Empirical

polarity measurements often give conflicting results, since probe

molecules sample local environments to which they have the

highest affinity, recalling the Indian fable of the blind men and

the elephant. Long-range structural order can persist in ionic

liquids as well, for example interfacial layering at charged

surfaces.10 These features can give rise to new reactivity patterns,

the preparation of hybrid materials, and the fabrication ofadvanced devices.

The introduction above provides only a brief overview of the

features of ionic liquids that motivate their energy-related

applications. Their utility manifests itself in wide-ranging aspects

of energy production, storage and utilization.

Ionic liquids and energy production

Ionic liquids will play many roles in the production of energy, for

example in the capture and conversion of solar energy into

electricity, the conversion of raw biomass or fossil resources

into cleaner fuels, and enabling advanced nuclear energy

technologies.

Solar photoconversion

Due to their conductivity, low volatility, and thermal stability,

ionic liquids have been extensively studied as electrolytes for

dye-sensitized solar cells (DSSCs).11 DSSCs use a metal complex

or organic dye absorbed onto a semiconductor, such as TiO2,

which has been applied to an electrode. Capture of a photon by

the dye causes the excited state to inject an electron into the TiO2.

The oxidized dye is reduced by a charge carrier within the elec-

trolyte, typically iodide anion. The electron and the oxidized

species (hole) recombine at the counter electrode, completing

the circuit.

Although ILs are conductive, mass transport of the holecarrier to the counter electrode can be a limiting factor for IL

DSSC performance.12 Iodide is the most utilized hole carrier

because I0 can dimerize to form I2 and pick up an iodide anion to

form I3. A Grotthus-type exchange mechanism transferring I2

between I3 and I has been invoked to explain the superior

charge transport in iodide-based DSSCs.13 A large body of

literature traces the improvement of IL-based DSSCs. The

present state of the art employs mixtures of IL cations and anions

(in addition to I and I3) to lower viscosity and thus optimize

hole transport. The energy efficiencies of IL-based DSSCs are

close to those of the best conventional solvent-based systems,

with substantial improvements in durability.14

Solar thermal conversion

Another emerging technology for conversion of solar energy

involves using arrays of reflectors to focus sunlight onto vessels

or pipes containing a heat transfer fluid. The heated fluid is then

conducted through a system that converts the heat into elec-

tricity, either through thermoelectric conversion or by powering

a generator. The heated fluid can be stored in an insulated vessel

for electric generation on demand. Fluids for these systems must

endure a wide range of temperatures, from night-time lows in

winter to daytime operating temperatures in excess of 200 C.

Many ILs are liquid and stable over that entire range, and several

studies15 have supported their suitability for this application. Inaddition to large-scale electricity installations, the technology is

being investigated to provide direct heat and air conditioning to

hospitals and resorts in Mediterranean countries.16

Biofuel production

Biofuels such as ethanol, butanol and biodiesel represent

a potentially sustainable, carbon-neutral transportation fuel

source if they can be obtained from non-food biomass grown and

harvested in an ecologically stable manner. One of the biggest

challenges is that the bulk of the energy available from such

biomass (corn stalks, switchgrass, softwood, etc.) is in the form

of lignocellulose, a tight, covalent- and hydrogen bond-linkedmatrix of the carbohydrate polymers cellulose and hemicellulose

and the phenolic polymer lignin. Plants owe their sturdiness to

the durability of lignocellulose and its recalcitrance to chemical

and biological attack. Breaking down lignocellulose into its

components so that they can be converted to biofuels requires

harsh chemical treatments or specialized organisms (e.g., fungi).

Happily, ionic liquids that have anions with good hydrogen

bond accepting properties, such as Cl, carboxylates, and

oxoanions, can dissolve cellulose (and lignocellulose) by dis-

rupting their networks of intramolecular hydrogen bonds.17 It is

therefore possible to dissolve wood and other plant material.7,18

James F: Wishart

James F. Wishart received a BS

in Chemistry from the Massa-

chusetts Institute of Technology

in 1979 and a PhD in Inorganic

Chemistry from StanfordUniversity in 1985 under the

direction of Prof. Henry Taube.

After a postdoctoral appointment

at Rutgers University he joined

the Brookhaven National Labo-

ratory Chemistry Department in

1987. His research interests

include ionic liquids, radiation

chemistry, electron transfer, and

new technology and techniques

for pulse radiolysis.

This journal is The Royal Society of Chemistry 2009 Energy Environ. Sci., 2009, 2, 956961 | 957


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The dissolved cellulose (or lignocellulose) can be recovered by

addition of water to the ionic liquid. The water breaks up the

celluloseanion hydrogen bonds and precipitates the cellulose in

a structurally disrupted form that is easier for enzymes to cleave

into fermentable sugars to make ethanol, for example.19 Several

groups are working on the adaptation of ionic liquid pre-treat-

ment of biomass for the production of ethanol and butanol. Ionic

liquids could possibly be used to extract butanol out of dilute

aqueous fermentation broths.20

Ionic liquids have been used to solubilize other natural

biopolymers in addition to cellulose, such as silk21 and wool

keratin,22 and to make hybrid biopolymer materials with exotic

items such as magnetic nanoparticles23 and carbon nanotubes.24

ILs have enabled the exploitation of cheap, renewable biopoly-

mers in many applications that used to be the province of fossil

fuel-derived plastics.

Biodiesel is another alternative fuel that has received a lot of

attention because of its compatibility with the present trans-

portation infrastructure. Several groups have used ionic liquids

to convert triglycerides to glycerol and biodiesel (fatty acid

methyl esters) through transesterification by immobilized lipase25

or homogeneous catalysis.26

More efficient production of cleaner fossil fuels

Although alternative energy sources will increase in capacity over

time, fossil fuels will remain a major source of energy for the

foreseeable future. Ionic liquids can make their production and

use cleaner. Desulfurization of petroleum feedstocks is vitally

important for the prevention of air pollution via SOx

aerosols

and the environmental destruction caused by acid rain, as well as

for the protection of antipollution catalysts and fuel cells. Stan-

dard methods such as hydrodesulfurization require elevated

temperatures and pressures, along with hydrogen gas.

Ionic liquids have been shown to be effective at extractingsulfur compounds from fuels under ambient conditions,

including compounds that are difficult to remove through

hydrogenation such as dibenzothiophene.27 Ionic liquid extrac-

tion has been coupled with oxidation by H2O2 to boost effi-

ciency,28 in some cases catalyzed by polyoxometallates.29 Ionic

liquids containing Lewis acidic metal complex anions also show

high performance.30

Ionic liquids are also effective for extracting aromatic hydro-

carbons from aromatic/aliphatic mixtures, and an engineering

analysis has shown significant cost savings and process benefits

could be obtained by ionic liquid-based removal of aromatics

from naphtha cracker feed streams.31 Regardless of the ability of

ILs to extract organic materials out of hydrocarbons, they showlimited solubility in hydrocarbons for the same Coulombic

reasons that make their vapour pressure very low. Consequently,

they are not washed away, even over many process cycles,

a bonus for their cost efficiency.

A cleaner, advanced nuclear fuel cycle

Nuclear power provides a significant fraction of the worlds

energy, and it is a major carbon-free source of continuous, base-

supply electrical power generation. Although the proportion of

renewable alternative energy production will continue to grow,

international recognition32 of the urgent need to reduce CO2emissions calls for the growth of nuclear power generation in the

near term to avoid an even larger increase in greenhouse gases

from fossil fuel-based generating stations. Still, serious technical

and political problems persist in the handling and ultimate

disposal of spent nuclear fuel and waste. Current approaches are

not sufficient for future needs, and work is underway to design an

advanced fuel cycle33 that recycles spent nuclear fuel to signifi-

cantly minimize waste, extracts more energy from existinguranium supplies, and burns up plutonium and other transuranic

elements so that they are no longer weapons proliferation risks

nor hazards to future generations.

Such a sea change in technology requires the development of

new, detailed spent fuel treatment and separations schemes that

present many challenges for chemists. Ionic liquids can play key

roles in advanced fuel separations systems because of their useful

properties, including low volatility and combustibility, amphi-

philicity, tuneable separations affinities including functionaliza-

tion, high conductivity, wide liquidus range and high thermal

stability. In addition, ionic liquids containing boron can

dramatically reduce the risk of criticality accidents in the pro-

cessing of fissile materials.34 Several groups are investigating theliquid/liquid extraction behaviour of ionic liquids for actinides

and fission products (lanthanides, Cs, Sr, Tc, etc.).35 Binnemans

recently published a review on lanthanides and actinides in ILs.36

ILs have been shown to be effective solvents for extraction, but in

some cases the operative mechanism is ion exchange instead of

true neutral extraction.37 Several studies have revealed the effect

of IL anion and cation hydrophobicity on the balance between

the mechanisms.38 In addition to liquid/liquid extraction, various

elements can be selectively electrodeposited in metallic form,

even if highly reactive, due to the high conductivity and wide

electrochemical windows of selected ILs.39 The use of ionic

liquids to electrodeposit a wide variety of metals and alloys has

industrial and commercial applications far beyond the nuclearfield.40,41

Instability under the radiation burden of spent nuclear fuel is

a common problem for nuclear separations systems based on

conventional solvents. Therefore it is important to understand

the chemistry produced by irradiation of ionic liquids and its

potential effects on separations processes. A review of this issue

has recently been published.42 Ionic liquid radiolysis product

studies have shown that ILs are generally resistant to radiation

damage, and product distributions have been characterized.43

EPR identification of radiation-induced radical species relates

initial transient species to the ultimate product distributions.44,45

Unlike conventional hydrocarbon solvents that direct damage

into the extractant molecules, thus reducing their efficiency, ionicliquids can be designed to direct damage away from the extrac-

tant, thereby increasing the durability of the separations

system.42,44 There are numerous reasons to believe that the future

of ionic liquids in advanced nuclear fuel recycling is bright.

Ionic liquids and energy storage

Batteries and supercapacitors

The strongest motivation for the development of ionic liquids in

the early years of the field was in their use as battery electrolytes,



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where their conductivity, wide electrochemical windows and

liquidus ranges, and low volatility made them attractive for high

performance applications such as thermally-activated batteries.46

The electrochemical properties of ionic liquids are intensively

studied worldwide and several reviews4750 and books41 have been

published on the subject, which includes many uses of ILs that

fall under other sections of this minireview. The field is so large

that it is not possible to do it justice within this space; interested

parties are urged to consult the reviews.Ionic liquids have been studied in nearly every battery system

imaginable, but their use in lithium batteries has received the

greatest attention.4749,51 Because of their chemical and

thermal stability, ILs are a good choice for development of

batteries with lithium metal anodes, although there are issues

with film formation at the IL/Li interface that require further

investigation.

Supercapacitors store energy through the formation of elec-

trochemical double layers formed by the adsorption of ionic

species on electrode surfaces.47,49,52 They are designed to have

very large electrode surface areas and they can store much more

charge than conventional dielectric capacitors. They can be

charged and discharged rapidly, and thus have higher powerdensities than batteries, although their energy density is lower.

Supercapacitors have many uses in mobile electrical systems and

power distribution systems (regenerative braking, accelerating

electric vehicles, load buffering, uninterruptible power supplies)

that can markedly improve energy conservation. Supercapacitors

can also enhance the performance characteristics of batteries

and fuel cells. Because of the ionic nature, conductivity and

durability of ILs, supercapacitors using them are actively being

developed.47,49,52,53

Ionic liquids and efficient energy utilization

Ionic liquids impact energy utilization most directly throughtheir application to fuel cells, but there are many other ways that

they can make positive contributions indirectly through

improved materials, devices and processes that conserve energy

or other resources during their manufacture or operation. The

example of metal electrodeposition has already been mentioned

above.

Fuel cells

As low-volatility conductive media with good thermal stability,

ionic liquids are finding applications as fuel cell electrolytes and

as proton conductors in proton exchange membranes, often in

combination with polymer gels.48,54 Typically, protic ionic

liquids, formed through the reaction of a strong acid with a base,

are used as the proton conductors. However, fuel cells typically

operate at temperatures over 100 C. Under those conditions,

reverse proton transfer to produce volatile neutral acid and base

molecules leads to slow loss of the ionic liquid through evapo-

ration of its molecular precursors. This problem can be avoided

through the use of very strong acids or bases.55

Advanced devices and processes

Ionic liquids can be combined with electroactive polymers to

make electrochromic windows or displays.56 Their durability,

chemical inertness and low volatility promise gains in reliability

and useful lifetime. In addition to changing color, the charging or

discharging of conductive polymers can be coupled to IL ion

migration and electrostriction to produce physical displace-

ments, creating electromechanical actuators or artificial

muscles.48,49,57 Turning signal transduction in the opposite

direction, ILs can be used to make sensors for the electro-

chemical detection of gases, ions (selective electrodes), biomole-

cules, and electroactive species.58 Similarly, the principles used todesign IL-based actuators could be applied to portable energy

harvesting systems59

Catalysis and industrial applications

The use of ionic liquids in catalytic systems has been studied

extensively1,60 Because many ILs are practically insoluble in

hydrocarbons and supercritical CO2,61 particular attention has

been paid to their use in phase-transfer catalysis, particularly

when the catalyst can be immobilized in the IL (to prevent

its loss and product contamination) by attaching a charged

functionality.

The solubility of gases in ionic liquids and their general lack ofvolatility make them useful for gas capture or separation appli-

cations, for example using supported ionic liquid polymer

membranes for separation of CO2, SO2, etc.,62 H2 purification

63

and storage of large gas volumes under mild conditions.64

Conclusion

Well over a dozen edited books, at least 14,000 articles, and

hundreds of patents concerning ionic liquids have been written.

This short minireview highlights the major ways that ILs can

impact energy technologies from generation to consumption.

Most of the examples mentioned are still in the research and

development stage, but commercial application of ILs continuesto move forward in many areas. It is hoped that this brief

introduction to ionic liquids will inspire new ideas for IL uses

among the readers of this review.

Acknowledgements

This work was supported by the U. S. Department of Energy,

Office of Basic Energy Sciences, Division of Chemical Sciences

under contract # DE-AC02-98CH10886.

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