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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/290452976

Ionic liquids and their solid-state analogues asmaterials for energy generation and storage

 ARTICLE · JANUARY 2016

DOI: 10.1038/natrevmats.2015.5

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12 AUTHORS, INCLUDING:

Mega Kar

Monash University (Australia)

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Jennifer M Pringle

Deakin University

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Hiroyuki Ohno

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Masayoshi Watanabe

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Ionic liquids (ILs) possess unique properties thatmake them highly attractive for a range of applications(BOX 1). As solvent media for materials synthesis, theirhigh thermal stability and their ability to dissolve awide range of metal and organic compounds providenew preparative directions. In the context of electro-chemical applications and devices, these properties,coupled with high intrinsic ion conductivity, makethem attractive as high-stability electrolyte materials.Their strong electrostatic interactions render them vir-tually non-volatile and therefore of low flammability.On the other hand, the structural variety in their com-ponent molecular ions provides an enormous range of

solvency properties, which are unparalleled in otherliquid media.

In this Review, we highlight recent developments inthe application of ILs and their solid-state analogues,organic ionic plastic crystals (OIPCs), in the field ofenergy materials. Without question, sustainable gener-ation and storage of energy is one of the great challengesof our time. There are many alternative technologiesunder active development, in particular for ‘distributed’generation and storage at the community and house-hold levels. These developments demand electroactivematerials and electrolytes with high energy-storageand power-delivery capacity, and safety of operation.

In this context, ionic materials have much to offer, andthe breadth of their ‘designable’ properties creates newpossibilities in materials synthesis and design.

We focus on recent developments in the use of ILsas media for the preparation of electroactive materialsand on the electrochemical application of ILs in a rangeof sustainable energy technologies. Because the litera-ture is voluminous, we focus our attention on importantdevelopments in the period since 2012; readers inter-ested in delving further into earlier work can accessthe literature via these recent papers. Several reviewshave appeared in recent years, and we refer the readerto these for a more historical account of each field1–3. A

glossary of nomenclature abbreviations and molecularstructures of the compounds described in this Reviewis provided in TABLE 1.

Space limitations preclude discussion of two impor-tant areas of energy technology in which ILs have thepotential to have a considerable impact: CO

2 capture

and nuclear fuel processing. Both of these require spe-cific and unique functionality in the IL and are there-fore most appropriately discussed in dedicated reviews.We also set aside the interesting, but distinctly sepa-rate, topic of ILs used as salts in solvent-based media,in which the IL nature of the salt is often insignificant,and behaviour is dominated by the solvent properties.

1 ARC Centre of Excellence for

Electromaterials Science,

School of Chemistry, Monash

University, Clayton, Victoria

3800, Australia.2 ARC Centre of Excellence for

Electromaterials Science,

Institute for Frontier Materials,

Deakin University, Burwood,

Victoria 3125, Australia.3Helmholtz Institute Ulm,

Helmholtzstraße 11, 89081

Ulm, Germany and Karlsruhe

Institute of Technology, PO

box 3640, 76021 Karlsruhe,

Germany.4Functional Ionic Liquid

Laboratories (FILL), Graduate

School of Engineering, Tokyo

University of Agriculture and

Technology, 2-24-16

Nakacho, Koganei, Japan.5Department of Chemistry and

Biotechnology, Yokohama

National University, 79–5

Tokiwadai, Hodogaya-ku,

Yokohama 240–8501, Japan.6 Department of Polymer

Science and EngineeringCollege of Chemistry,

Chemical Engineering and

Materials Science, Soochow

University, Suzhou, 215123,

China.7 Department of Materials

Chemistry, Nankai University,

Tianjin, 300071, China.

Correspondence to D.R.M.

douglas.macfarlane@

monash.edu

Article number: 15005 

doi:10.1038/natrevmats.2015.5

Published online 12 Jan 2016

Ionic liquids and their solid-stateanalogues as materials for energygeneration and storageDouglas R. MacFarlane1, Maria Forsyth2, Patrick C. Howlett 2, Mega Kar 1,

Stefano Passerini 3, Jennifer M. Pringle2, Hiroyuki Ohno4, Masayoshi Watanabe5,

Feng Yan6 , Wenjun Zheng 7 , Shiguo Zhang 5 and Jie Zhang 1

Abstract | Salts that are liquid at room temperature, now commonly called ionic liquids, have

been known for more than 100 years; however, their unique properties have only come to lightin the past two decades. In this Review, we examine recent work in which the properties of

ionic liquids have enabled important advances to be made in sustainable energy generation

and storage. We discuss the use of ionic liquids as media for synthesis of electromaterials,

for example, in the preparation of doped carbons, conducting polymers and intercalation

electrode materials. Focusing on their intrinsic ionic conductivity, we examine recent reports

of ionic liquids used as electrolytes in emerging high-energy-density and low-cost batteries,

including Li-ion, Li–O2, Li–S, Na-ion and Al-ion batteries. Similar developments in electrolyte

applications in dye-sensitized solar cells, thermo-electrochemical cells, double-layer

capacitors and CO2 reduction are also discussed.

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Electrode materials synthesis in IL media

In this section, we review the IL-based synthesis of elec-troactive materials, including conducting polymers,doped carbons, battery anode and cathode intercalationcompounds, and electrocatalysts. Although the ‘green’credentials of ILs as reaction media have been widelydiscussed, our emphasis is on the special properties ofthe resultant materials that can be obtained owing totheir IL origins.

Conducting polymers. Conducting polymers (CPs) areunique materials in that their synthesis and electrochem-ical cycling (from the neutral, non-conductive state to thecharged, ‘doped’ conductive state) rely on ion insertionor de-insertion from the electrolyte. Thus, the nature ofthe ions, their high concentration and the extent of inter-calation into the polymer, can influence properties such

as the morphology, electrochemical activity, conductivityand stability 4. Interestingly, the anions that are effectivein forming ILs, such as [NTf 

2]−, [OTf]−, [PF

6]−, [fsi]− and

[fap]−, are also those that dominate the family of inter-calating ions (also known as dopants) for CPs; this com-monality stems from the fact that some properties, suchas charge delocalization and electrochemical stability,are key to both families of materials. The advantages ofsynthesizing CPs in ILs often include improved mono-mer solubility, and access to higher monomer oxidationpotentials and higher polymer n- and p-doping potentials.CP films electrodeposited from ILs are also commonlysmoother than those from molecular solvents4. A detaileddiscussion of electro-polymerization of CPs in IL mediahas recently been published5.

Poly(3,4-ethylenedioxythiophene), PEDOT, has acentral role in applications including actuators, electro-chromics, organic light-emitting diodes and organicsolar cells. The nature of both the anion and cation ofthe IL electrodeposition medium from which PEDOTis synthesized can influence its resistance, as recently

demonstrated for deposition onto a Pt(111) surface6;the resistance of the IL/PEDOT interface for the oxi-dized film and the total resistance of the reduced filmwere observed to follow the trend [C

4mmim][NTf 

2] >

[C4mim][OTf] > [C

2mmim][NTf 

2] > [C

2mim][NTf 

2],

whereas the effect on the ion-exchange rate duringredox switching followed the trend [C

4mmim][NTf 

2] <

[C2mmim][NTf 

2] < [C

4mim][OTf] < [C

2mim][NTf 

2].

This study also demonstrated the presence of two poly-mer redox processes and the high doping levels that canbe achieved by growth in ILs. Alternatively, the propertiesof PEDOT can be enhanced by chemical functionaliza-tion with an IL moiety, such as an imidazolium group, orby combination with polymerized ILs to form more sta-ble IL–CP materials7. PEDOT, with good catalytic activityfor the oxygen reduction reaction (ORR), has recentlybeen electrochemically deposited onto flexible carboncloth from [C

4mpyr][NTf 

2] using a sandwich-cell con-

figuration8, producing better polymer film coverage andimproved performance compared with analogous filmsgrown in acetonitrile.

The synthesis of CPs in ILs is also a viable route toenhancing optical and electrical properties. For exam-ple, poly(pyrrole) and PEDOT have recently beencopolymerized in [C

4mim][BF

4] at different ratios to

alter the resultant morphologies and spectro-electro-chemical behaviour9, which is pertinent to applica-

tion in electrochromics. New monomers continue tobenefit from electro-polymerization in ILs, as exem-plified by the asymmetric sulfur analogue of PEDOT,poly(thieno[3,4-b-]-1,4-oxathiane) — grown in [C

4mim]

[PF6] — which shows very promising electrochromism10.

The monomer oxidation potential is advantageouslylower in the IL, and the polymer displays a smoother andmore uniform morphology than that grown from dichlo-romethane, as well as higher electrochemical activity,conductivity, doping levels and stability (10,000 cycles).

Inevitably, given the burgeoning field of nanotech-nology, attention is turning to the development of CPswith nanometre-scale morphologies. For example,

Box 1 | Turning a salt into a liquid

 

120°187°

115 pm

131 pm

N –

N

N+ N

N

a b

Ionic liquids (ILs), also known as room-temperature molten salts, are a large family

of recently discovered liquid salts usually comprising organic cations and various

anions, such as I−, BF4

− and CF3SO

3−. They have many features in common with

simple, often very-high-melting-point salts, such as NaCl. However, the separation

between the large molecular ions in an IL weakens and shields the electrostaticforces between the ions, lowering the melting point. Another molecular feature

that assists in lowering the melting point is delocalized charges. For example, in the

dicyanamide ion (see the figure, panel a) the bond lengths and angles determined

from crystal structures indicate that the negative charge is spread over all three

nitrogen atoms, further weakening the electrostatic interactions. Similarly, the

positive charge in the imidazolium cation (see the figure, panel b) is delocalized

across the N–C–N bonds of the five-membered ring.

The combination of these two ions produces a very well-known IL, ethylmethylim-

idazolium dicyanamide ([C2mim][dca]), which has a melting point of −21 °C. The

weakening of the electrostatic interactions between the ions by the charge

delocalization effect also increases the mobility of the ions in the liquid state;

[C2mim][dca] is thus one of the most fluid ILs known. Low symmetry, or preferably a

complete lack of symmetry, in one or both of the ions is also a common feature of

IL-forming ions, as this prevents efficient packing into a low-energy crystal

structure, thereby further lowering the melting point. The rich variety of N- andP-based cations available, coupled with an equally vast family of inorganic and

organic anions, creates a potentially enormous family of ILs, with the different ions

producing a wide range of tunable properties.

As liquid salts, ILs typically exhibit significant ion conductivity that makes them

ideal as electrolytes. They also exhibit very low vapour pressure and low

flammability. The mix of electrostatic, hydrogen-bonding and hydrophobic types of

interaction that exist in an IL create powerful and tunable solvency properties; they

can be water soluble or miscible, or strongly water repellent. Equally, they can be

excellent solvents for inorganic salts and for proteins.

For applications in which a solid form of the IL is more desirable, several soft

material versions of ILs have been developed, including poly(ILs) and gels,

sometimes known as ionogels. Moreover, solid-state analogues, so-called organic

ionic plastic crystals (OIPCs), are an emerging alternative (BOX 2).

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graphitization and conductivity 24,25. Interestingly, a sim-ple relationship between the N/C molar ratio and theN content of the doped carbons was observed, and theupper limit of N content was found to be strongly relatedto the carbonization temperature26.

IL-based carbonaceous materials have found applica-tion as ORR electrocatalysts24,27,28. The high N content isbelieved to provide abundant electrocatalytically activesites, as the N-dopants can induce charge delocalizationand change the chemisorption mode, which effectivelyweakens the O–O bonding and facilitates the ORR 29.The excellent activity is reflected by onset and half-wavepotentials, and kinetic current density comparable tothose of commercial Pt/C fuel-cell materials24,27,28.

Electroactive materials by ionothermal synthesis in ILs. Since the concept of ‘ionothermal’ synthesis of materi-als using an IL solvent was proposed by Morris30, themethod has been extensively used for the fabrication ofinorganic nanomaterials and IL–inorganic hybrids31–35;Eshetu et al.33 recently provided a useful review of the

field. In general, the IL acts as a reaction medium withhigh thermal and chemical stability 36, and it can also actas a structure-directing agent owing to its highly struc-tured architecture37. Recently, open-framework fluoridematerials for Li and Na battery cathodes were prepared bythis method38, whereby the dehydration process was facil-itated by infiltration of, and capping by, the IL [C

4mim]

[BF4], resulting in FeF

3·0.33H

2O with a conversion yield

of 100%. Jana et al.39 prepared flowerlike hierarchicalarchitectures of layered SnS

2 using [C

2mim][BF

4] as the

reaction medium to produce nanosheet-petals of SnS2.

Mali et al.40 synthesized TiO2 with a nanoflower morphol-

ogy using an imidazolium IL, achieving a high degree ofcrystallization even at mild annealing temperatures. Theuse of ultrasonic irradiation for materials synthesis in ILshas also been reviewed recently 41.

Another excellent example of the tunable morpholog-ical and electrochemical features made possible by thissynthetic approach, Choi et al.42 reported the [C

4mim]

[BF4]-based synthesis of IL–Co(OH)

2 hybrid electrode

materials with tailored geometric morphology for appli-cation in supercapacitors. The Co(OH)

2 hybrids exhib-

ited surface areas of up to 400 m2 g−1 (with a mesoporesize of 4.8 nm) and retained a surface adsorbed layer ofIL; this layer enhances the pseudo-capacitance of theelectrode, which allowed specific capacitance as high as859 F g−1 to be achieved. Density functional theory (DFT)calculations were used to probe the effect of the IL onthe pseudo-capacitive redox reactions of Co(OH)

2 at

the molecular level; under anodic potentials, the processwas found to involve the loss of a f irst electron–hydro-gen pair and then a second pair, ultimately producingCoO

2. The optimized structures of [C

4mim][BF

4] on the

Co(OH)2 surface are shown in FIG. 1; the [C

4mim]+ cation

facilitates hydrogen desorption from the surface. Thehydrogen desorption energies are smaller than that of abare Co(OH)

2 surface, thus lowering the energy barrier

to the pseudo-capacitive process. Such hybrid materialsproduced by ionothermal synthesis hold considerablepromise for enhanced electrochemical properties.

Electrocatalyst deposition from IL media. ILs are widelyrecognized as media for the electrodeposition of met-als and semiconductors. The general methodologyand advantages of electrodeposition in IL media havebeen reviewed elsewhere5. Deposition of various semi-conducting materials has been explored, in particularby the Endres43 group, as exemplified by their recentdescription of the preparation of crystalline Ga-dopedGe and Si

x Ge

1−x  from an IL medium. IL media have also

been used for the electrodeposition of a broader rangeof electrocatalytic materials; for example, birnessite-typeMnO

x  has been used as a water oxidation catalyst 44,45.

Similarly, ILs have recently been used to deposit highlycatalytically active forms of Pd46,47 for the ORR in fuelcells and MoS

x  for the hydrogen evolution reaction48.

CoNi@Pt core–shell mesoporous nanorods with veryhighly active surfaces for methanol electro-oxidationhave also been developed using IL electrodepositionmethods49. Thus, IL-based synthesis is opening up newapproaches to prepare electroactive materials with newmorphologies and excellent electrochemical properties.Further understanding of the roles of the ILs in these pro-cesses via systematic studies and theoretical simulationswould greatly facilitate progress in this field.

ILs as electrolyte materials

With their intrinsic ion conductivity, high metal-saltsolubility and high electrochemical stability, some IL

a

b

c

Box 2 | Organic ionic plastic crystals as solid-state ionic liquid analogues

Most ionic liquids (ILs) will, eventually, at some sufficiently low temperature, form a

crystalline solid. In certain families of organic salts, multiple crystal structures form as

a function of temperature, and the higher-temperature structures exhibit soft, plastic

mechanical properties and significant ion conductivity. These are often referred to as

organic ionic plastic crystals (OIPCs). The molecular structure of the ions is often

closely related to IL-forming ions; for example, [C4mpyr]+ often forms ILs, whereas the

smaller and more symmetrical [C2mpyr]+ forms OIPCs. The mechanical and transport

properties of OIPCs originate in rotational motions of one or both of the ions in the

crystal (see the figure); in other words, the ions can exhibit some of the motional

properties that are characteristic of the liquid without losing their 3D ordered

structure. This can produce ion conduction in the material, which thereby becomes

interesting and useful as a solid-state electrolyte.

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types are a natural fit to most battery electrochemistries,as long as the cost is not prohibitive. Several excellentreviews highlight the significant role that ILs can havein high-energy-density batteries, including Li-ion50,Na-ion51, Li–S (REF. 52,53) and Li–air54,55 systems; Karet al.56 have recently reviewed the broad application ofILs in metal–air battery systems.

Lithium batteries. Although ILs have not achieved large-scale application in commercial Li batteries, after morethan a decade of intense research, interest in their prop-erties as safe electrolytes continues unabated50. This hasbeen encouraged by growing recognition of the needto improve the charge–discharge cycling performanceof Li metal electrodes to enable the next generation ofhigh-energy-density batteries, such as Li–S or Li–air(also written as Li–O

2) batteries54,55. In Li–air configura-

tions, a high-specific-capacity anode is needed to matchthe enormous capacities of the air cathode; Bruce et al.54 provide an excellent review of this area. Research con-tinues to focus on small quaternary ammonium cations,particularly cyclic pyrrolidinium and piperidinium cat-ions, owing to their high chemical and electrochemicalstability (higher than [C

nmim]+) and relatively high con-

ductivity. For example, Elia et al.57 recently reported aLi–air cell based on [C

4mpyr][NTf 

2] that exhibited an

82% charge–discharge energy efficiency.Enhancing the oxidative stability of the IL will ena-

ble the use of new high-voltage cathodes55. For example,Wu et al.58 recently reported stable cycling of a range ofhigh-voltage cathodes (notably Li

1.2Ni

0.2Mn

0.6O

2at 4.8 V)

in [C4mpip][NTf 

2] using additives to stabilize both the

anode and cathode solid electrolyte interphases (SEIs).The SEI is typically a layer of breakdown products,which allows facile transport of Li ions, but otherwiselimits further breakdown reactions, thereby stabilizingthe interphase and increasing the cycle life of the bat-tery. The SEI may form entirely as a result of trace-levelimpurities (for example, water, oxygen and nitrogen),

but it may also involve the electrolyte and electrode com-ponents. Stable cycling of a 4.7-V LiNi

0.5Mn

1.5O

4 cell59 

and a 4.5-V LiFe0.1

Co0.9

PO4 cell60 using [C

4mpyr][NTf 

2]

electrolytes has also been reported. Recent reports ofsmall phosphonium cation ILs61,62 exhibiting propertiescomparable or superior to the best N-based electrolytesshould prompt a considerable expansion of studies usingthese cations. Theoretical investigations are also begin-ning to provide insights into the origins of the superiorconductivity and fluidity of these ILs63.

Anion choice is largely dominated by [NTf 2]− and

[fsi]−, again owing to their stability and conductivity,as well as their well-known SEI-forming properties.However, the cost of some anions is a limiting factor,especially for [NTf 

2]−. Recent work 64 has shown that

Li cycling can be achieved in [dca]− ILs by doping witha small amount of water as an SEI-forming component.These ILs, which contain only H, C and N atoms, areintrinsically cheaper and also have superior transportproperties.

The [fsi] anion has attracted considerable attention as

a potentially less expensive ion65, with properties compa-rable or superior to those of [NTf 

2]− (REF. 66). Matsui et al.67 

demonstrated charge–discharge cycling of high-volt-age LiNi

1/3Mn

1/3Co

1/3O

2  cathodes in [C

2mim][fsi].

The benefits of high Li-ion concentrations have also beenrevealed in reports of high-performance Li cycling ininorganic–organic salt mixtures based on [C

3mpyr][fsi]

(REF. 68) or [P1,1,1,i4

][fsi] (REF. 61), in which the Li-salt con-centration can exceed 60 mol% (3.8 mol kg−1). Theseapproaches to the goal of a ‘liquid lithium salt’ supportthe notion that extremely high Li-salt concentrationscan be used to manipulate Li-ion transport properties69 to produce higher-rate charging than previously thoughtpossible (FIG. 2). At present, the [fsi] anion is unique inallowing the creation of these ambient-temperatureinorganic–organic IL mixtures; further investigationinto the reasons for this is required and may lead to thediscovery of other anions with similar properties.

Interest has also been driven by reports of enhancedLi-battery cycling performance in ILs compared withconventional electrolytes70. This is further highlightedby reports of inadequate Li-metal cycling performancein the ether-based electrolytes used for most Li–airstudies71. Grande et al.70 recently performed a carefulexamination of [C

4mpyr][NTf 

2] in this context, reveal-

ing once again that the SEI-forming properties of the ILdictate the long-term cycling performance. Piper et al.72 

demonstrated the stable cycling (77% capacity reten-tion after 100 cycles) of a high-capacity Si nanocom-posite anode architecture in a [C

3mpyr][fsi] electrolyte.

Theoretical simulations and surface analysis indicatedthe key role of the [fsi] anion in the formation of a sta-ble SEI, which promoted the structural integrity of thenanocomposite anode.

A key property in the context of Li–S cells is theability of the IL to limit the solubility of polysulfides,thereby reducing capacity loss due to the formation ofhigh-impedance phases and redox shuttle mechanisms.Excellent summaries of electrolytes used for Li–S cellshave recently been published73,74. The solubility of

N  H O

O

FF

F

Box 3 | Mixing acids and bases to make protic ionic liquids

Protic ionic liquids (ILs) are liquid

organic salts prepared by the

neutralization of an organic base with

an acid. The acids can be any of the

very common (and inexpensive) acids

such as nitric, sulfuric or phosphoric

acid. Their ease of preparation and

inexpensive starting materials make

them some of the most easily utilized

ILs. A well-known example is

ethylammonium nitrate prepared by

the mixing of ethylamine and nitric

acid. The transferred proton remains chemically active and can participate in

synthetic and electrochemical applications, such as the hydrogen fuel cell and water

splitting. The proton can also be highly mobile, as desired in these applications and,

in some cases, it is transported via a water-like Grotthus mechanism. Proton acidity

tends to be lower in protic ILs, resulting in lower proton transfer than would be

expected on the basis of aqueous behaviour. The dynamic nature of the proton

transfer equilibrium also makes these ILs slightly volatile and potentially distillable.

Proton transfer between trifluoroacetic acid and methyl pyrrolidine, depicted in the

figure, produces another well-known example of a protic IL.

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redox-active sulfur species (S8 and Li

2S

m) is reported to

be mainly dependent on the anion, with low solubilityresulting from the relatively weak coordination abilityof anions such as [NTf 

2]− (REF. 75). Studies of SEI for-

mation in Li–S cells with [C4mpyr][NTf 

2] also indicate

the importance of [NTf 2]− in maintaining a stable SEI,

although it is made more resistive by the presence of anouter, polysulfide-derived layer76.

Recently, ‘solvate’ ILs77, based on glyme–Li-saltmixtures, have attracted attention as safe, inexpen-sive electrolytes and have been shown to support thereversibility of the cathodic reaction in Li–air cells78.Blurring the lines further are reports of conceptuallysimilar, high-concentration ‘solvent-in-salt’ systems,which suppress polysulfide solubility in Li–S cells whileexhibiting excellent cycling performance79. For example,a 4 mol l−1 Li[fsi] in dimethoxyethane electrolyte exhib-its high Coulombic efficiencies and an excellent rateperformance of over thousands of cycles80.

New forms of Li-IL electrolytes include nanopar-ticle hybrids (cations tethered to SiO

2 nanoparticles).

For example, enhanced Li cycling was achieved when11 wt% [C

3mpip][NTf 

2]–nanoparticle hybrid was added

to a standard electrolyte81. Solid electrolytes based on[C

3mpyr][NTf 

2] confined inside a SiO

2–polymer matrix

also exhibit comparable performance to that of the pureIL in a Li–LiFePO

4 cell82.

Several ILs that exhibit OIPC phases have beenshown to support cycling in solid-state Li–LiFePO

4 

cells83–85. These materials are solid-state analogues ofILs (BOX 2), in which there is substantial ion conductiv-ity due to crystal defects. These solid electrolytes offerconsiderable manufacturing advantages compared withliquid electrolytes in high-volume battery manufactureand are attracting increasing attention for this reason.

Sodium batteries. Room-temperature Na and Na-ionbatteries have recently attracted attention as a low-costenergy-storage technology 86. Researchers have typicallyused carbonate electrolytes to evaluate the electrodes87;however, these cause corrosion of the Na-metal anode,resulting in unstable cycling performance87. To avoidthis problem, IL-based electrolytes have been inves-tigated, with the additional benefit of their improvedsafety characteristics. Early work focused on inorganicsystems based on K[fsi] and Na[fsi] mixtures (m.p. ≈65 °C); cells using this electrolyte could be operatedat 90 °C using NaCrO

2 and Na

2FeP

2O

7 cathodes with

capacities close to 100 mAh g−1 (REF. 88). More recently,IL electrolytes composed of NaNTf 

2 in [C

nmpyr][NTf 

2]

or Na[fsi] in [C3mpyr][fsi] have been shown to facili-

tate stable Na deposition and stripping at temperaturesof up to 100 °C (REFS 89–91). Chagas et al.92 demon-strated a Na

0.45Ni

0.22Co

0.11Mn

0.66O

2 cathode operating

Figure 1 | Ionothermal synthesis of hybrid IL–Co(OH)2 capacitor materials. Optimized structures42 of free [C

4mim]

[BF4] (panel a) and [C

4mim][BF

4] on the Co(OH)

2 surface (panel b). Labels 1–4 in panel b represent the surface hydrogen

atoms beneath the [C4mim]+ cation or [BF

4]− anion. Adapted with permission from REF. 42, American Chemical Society.

 

a b

Side view

Top view

N F

Co

O H

CB

4

3

2

1

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at 4.6 V versus Na/Na+ with a Na[NTf 2]/[C

4mpyr][fsi]

electrolyte; this showed superior cycling stability incomparison with a carbonate-based electrolyte.

Similar to the high-concentration Li[fsi]-based sys-tems discussed above, excellent performance has beenreported for a Na-ion battery based on high-concentra-tion Na[fsi]–[C

3mpyr][fsi] electrolytes at 90 °C (REF. 93).

Current work indicates that the marked enhancement inNa electrochemistry arises from an unexpectedly high

transference number of Na ions90. Once again, such con-centrated metal–ion systems, in which [M+] > [IL

cation],

show great promise for energy-storage devices.There is also some evidence that mixed anions may

provide additional benefits in Na electrolyte systems94,95.Wang et al.75 investigated the effect of adding NaBF

4,

NaClO4, Na[NTf 

2] or NaPF

6 to [C

4mpyr][NTf 

2] and dis-

covered that NaBF4 produced remarkably higher ionic

conductivities than the other salts; however, the deviceperformance with a Na

0.44MnO

2 cathode was best with

NaClO4 and worst with NaPF

6, which correlated with

measurements of the interfacial resistance. Conversely,the mixture with NaBF

4 was optimal with a NaFePO

4 

cathode95. Current understanding of these phenomena isthat metal speciation (the way in which the metal ion iscoordinated to the anions in the electrolyte) is crucial indetermining the ion transport, and the electrochemicaland interfacial behaviour at both the anode and cathode.Speciation has been explored in NaNTf 

2/[C

nmim][NTf 

2]

electrolytes, revealing that the coordination of Na isdifferent from Li, with sixfold coordination by three

anions96. Recent molecular dynamics simulations of aphosphonium PF

6− OIPC also demonstrated significant

differences between Na and Li coordination and theirresultant conduction mechanisms. Na+ has a preferencefor a higher coordination number in PF

6− electrolytes

compared with Li+, and rearrangement of this solvationenvironment dominates metal–ion transport97.

 Magnesium, aluminium and zinc batteries. The depo-sition of Al and Mg from ILs has gained attention inthe past decade as the basis for emerging rechargeablemetal battery technologies. Both metals have very neg-ative reduction potentials (−2.3 and −2.4 V versus thestandard hydrogen electrode (SHE), respectively) andare cheaper and safer than Li or Na as battery materials.Based on the early work of Wilkes98,99 on chloroalumi-nate-based ILs such as AlCl

3/[C

2mim]Cl, Lin et al.100 

recently developed an ‘ultrafast’ rechargeable Al battery.It demonstrated high currents (4 A g−1; more than 7,500cycles) at an energy density of 40 Wh kg−1, which is com-parable to lead–acid batteries. Unfortunately, these ILsare highly moisture sensitive and relatively expensive.Use of air-stable ILs that incorporate cations such as4-propylpyridine101 could increase the stability of thesesystems. Despite their high cost, these ILs show greatpromise for Al electrochemistry.

Investigation of ILs for Mg electrochemistry is at a

relatively early stage. Considerable work has been doneon establishing an efficient Mg primary cell, but furtherwork is required to develop rechargeable Mg batteries.Kakibe et al.102 demonstrated Mg cycling by the substi-tution of alkyl groups on cations such as [C

2mim]+ to

avoid unwanted reactions of the Mg surface with the C2proton on the imidazolium ring. By mixing the ILs withadditives103, Mg cycling can be enhanced, and undesir-able side reactions, such as hydrogen formation, can besuppressed. Studies on Mg electrochemistry using molec-ular solvents have been useful in the design of suitableILs. Recent work by Mohtadi et al.104 and Shao et al.105 has shown, in the case of Mg[BH

4]

2 in ethereal solvents,

Figure 2 | High rate cycling of lithium. a | Schematic of ionic liquid (IL)-based Li-ion

battery cell. b | LiCoO2 cells in an inorganic–organic IL at different charging rates from

1C (1-hour rate) to 5C (12-minute rate) for [C3mpyr][fsi] electrolyte (blue lines) and

organic electrolyte (red lines). The [C3mpyr][fsi] electrolyte comprises 50 mol%

(3.2 mol kg−1) Li[fsi] in [C3mpyr][fsi] and the organic electrolyte comprises 1 M LiPF

6 in

EC:DMC = 50:50 vol%. Panel b reprinted with permission from REF. 68. Copyright (2013),

The Electrochemical Society.

 

+

–

a

    V   o    l   t   a   g   e    d   u   r    i   n   g   c    h   a   r   g    i   n   g    (    V    )

Charging capacity (mAh g–1)

4.20

4.15

4.10

4.05

4.00

4.95

0 20 40 60 80 100 120   140

b

Constant current cut-off: 4.2 V

[C3mpyr][fsi] electrolyte

Organic electrolyte

Hermeticallysealed case

Li metal anode

Ionic-liquid-impregnatedseparator membrane

LiCoO2 cathode layer

1C

2C

3C

4C

5C

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that the electrochemical reversibility of Mg is improvedwhen THF is replaced by dimethoxyethane. This suggeststhat the presence of multiple ether oxygens, which candisplace the anion from metal-centred associated spe-cies, may be important for Mg deposition and stripping.Preliminary electrochemical studies have already demon-strated Mg cycling from ether-functional ILs106. The workof Mohtadi et al.104 also indicates that the anion must beweakly coordinating to Mg2+ to enable its release at theelectrode; earlier studies showed that NTf 

2− complexes

with Mg2+ and ultimately degrades to form a passivatingfilm on Mg that hinders anodic dissolution107. Thus, ILswith very reductively stable but weakly coordinating ani-ons are a promising avenue of investigation in this area.

Although the reduction potential of Zn is not asnegative as Al or Mg (−0.76 V versus SHE), its electro-chemistry is still of interest owing to its low cost andhigh abundance. Much progress has been made onlong-term cycling of Zn in [N(CN)

2]− (REFS 108–110) 

and [NTf 2]−-based111 ILs in the presence of co-solvents

such as water and dimethyl sulfoxide. Purpose-designedalkoxy-functionalized cations also hold promise asoptimized electrolytes for rechargeable Zn batteries,with more than 800 charge–discharge cycles havingbeen demonstrated112. Fluidic Energy Inc. has patenteda rechargeable Zn–air battery based on IL electrolytesand is currently commercializing these for large-scaleapplications113,114.

Dye-sensitized solar cells and thermocells

Dye-sensitized solar cells (DSSCs) are attractive photo-electrochemical devices because of their relatively highlight-to-electric conversion efficiencies and low pro-duction costs. In 2014, a record conversion efficiencyof 13% was achieved using a porphyrin-based dye in a

liquid electrolyte (acetonitrile)115. The non-volatility ofILs makes them ideal candidates for improving the long-term stability of DSSCs. Various ILs, especially from theimidazolium family, have been extensively studied; forexample, [C

2mim][B(CN)

4] and [C

2mim][N(CN)

2]

have exhibited efficiencies of more than 8%, althoughthe long-term stability still needs to be improved116.ILs based on triazolium cations have been studiedrecently 117, and the potential of ILs to operate effectivelywith porphyrin dyes has also been demonstrated118.

Recent developments in the application of ILs inDSSCs include functionalization of redox electrolytes.Among the alternative couples to the classic iodide/triiodide couple, cobalt complexes have attracted greatattention119; however, their limited solubility hindersapplication. Functionalization of CoII/III(bpy)

3  with

imidazolium cations improves both the redox poten-tial and solubility in IL electrolytes, with an overallpower conversion efficiency of 7.3% reported120 in a[C

3mim]I/[C

2mim][SCN] mixture at 1 sun (where

1 sun corresponds to standard illumination at AM1.5, 

100 mW cm–2). Imidazolium-cation-functionalized2,2,6,6-tetramethylpiperidine-N -oxyl (TEMPO) wasalso synthesized and applied in DSSCs; this dual-redox-couple electrolyte gives a higher efficiency thanthose with a single redox couple121.

Devices based on liquid electrolytes generally suffer

from leakage issues. Incorporating ILs into a poly(IL)material or physically gelling them in inorganic nano-materials to form quasi-solid electrolytes has the poten-tial to eliminate leakage122–124. In addition, OIPCs havebeen shown to perform well, with efficiencies exceed-ing ~5%123–126 (FIG. 3). However, effective sealing is stillnecessary to ensure long-term stability, especially inhumid environments for hydrophilic OIPCs. Yanet al.124 recently reported water-resistant, solid-state DSSCsbased on a hydrophobic OIPC, [C

2mpyr][NTf 

2],

 which

produced >5% efficiency at 1 sun; the unsealed deviceexhibited long-term stability (>800 h) at 100% relativehumidity at 50 °C. Thus, looking to the future in this

Figure 3 | OIPC-based dye-sensitized solar cells. a | Current–voltage profiles of

various organic ionic plastic crystal (OIPC) materials, the optimum being [C1mpyr]

[N(CN)2] at around 5% solar efficiency. b | Differential scanning calorimetry (DSC)

thermogram and crystal structure of [C2mpyr][NTf 

2] in phases IV and III, indicating a

greater extent of rotatory motion at the higher temperatures (the melting point of

this compound is 91 °C, at which point it becomes a highly conductive ionic liquid

(IL)). Adapted with permission from REF. 125, the Royal Society of Chemistry.

 

100 200 300 400 500 700 900 1,000

0

8

10

6

4

2

Voltage (mV)

    C   u   r   r   e   n   t    d   e   n   s    i   t   y    (   m    A

   c   m  –    2    )

[C1mpyr][N(CN)

2]

[C1mpyr][I]

[Et4N][N(CN)

2]

[C2mpyr][BF

4]

[C1mpyr][PF

6]

[C1

mpyr][NTf 2

]

[C1mpyr][SCN]

a

b

–100 50–50 100 1500

Phase IV

Phase IV

Phase III

Phase III

Phase II Phase I Melt

Temperature (°C)

600 800

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field, IL gels and OIPCs offer considerable advantages,not only in improving the long-term stability of DSSCs,but also in offering the possibility of fabricating flexibledevices via reel-to-reel printing.

ILs are also enabling advances in thermo-electrochem-ical cells (TECs). TECs are devices that generate a voltagebetween two electrodes operating at different tempera-tures. Typically, these involve a redox couple in an elec-trolyte, similar to the DSSC electrolytes discussed above;in this case, it is the Seebeck coefficient, S

e = dE / dT , of

the redox couple (where E is the associated equilibriumpotential) that is the key thermodynamic property. Wasteheat-harvesting from industrial power stations is anattractive application; however, the typical temperatureregime (T 

hot ≈ 130–150 °C; T 

cold ≈ ambient temperature)

rules out most solvent-based electrolyte systems. Recentwork 127 has shown that IL electrolytes are well suited tothis application. S

e relates to the entropy change in the

redox reaction, which involves both the reaction entropyand the solvent reorganization entropy. The latter dependson the nature of the IL; S

e for the same redox couple can

range from 1.54 to 1.82 mV K−1 for CoII/III(bpy)3, indicat-ing a relatively subtle effect of the ion structure on thereorganization entropy 128–130. Effects of longer-range ILstructures may also contribute to this and are worthy offurther investigation. Similar to the other device appli-cations discussed, the transport properties, especially atT 

cold, and thermal conductivity of the IL are crucial in

determining the overall efficiency 131; transport-propertylimitations at low temperatures can be overcome by rais-ing T 

cold to 50–70 °C. Finally, mixtures of ILs with high-

boiling-point solvents such as methoxypropionitrile canenhance the transport properties. Under these conditions,with T 

hot = 130 °C, a power output as high as 800 mW m−2 

was obtained in an IL–solvent mixture132.

Electric double layer capacitors

ILs are also under intense investigation as safety- andenergy-enhancing electrolytes for electric double layercapacitors (EDLCs). This type of high-power devicestores energy in the electric double layer at the electrode/electrolyte interface, with the electrodes usually consist-ing of high-surface-area activated carbon. The electro-chemical stability imparted by ILs could considerablyincrease the operating voltage and thus the energy storagedensity (proportional to the square of the voltage)133,134.Conventional electrolytes for EDLCs predominantly con-sist of organic salts (for example, N

2,2,2,2BF

4) dissolved in

low-viscosity solvents, such as acetonitrile. Thus, as forthe other devices discussed above, the low flammabil-ity and negligible vapour pressure of ILs are attractiveproperties. The high ionic concentration of ILs is alsobeneficial in terms of weight-specific performance135.Unlike battery systems, the formation of SEIs is avoidedto improve internal resistance136; hence the voltage rangemust stay within the window of electrolyte stability: forexample, with [C

4mpyr][NTf 

2], EDLCs are limited to

3.7 V (REF. 135), whereas, as discussed in the previous sec-tions, this IL can be used with cathode materials reaching4.6 V versus Na/Na+ (REF. 92). It has also become clear thatthere are capacitance-reducing ‘sieving’ effects with ions

that are too large for the pores of the activated carbon134;hence, there is considerable scope to tune the carbonmaterial to suit the chosen ions. ILs have also beensuccessfully used with high-volumetric-energy-densitygraphene in EDLCs137,138.

Mainly imidazolium and pyrrolidinium cations havebeen investigated for low-viscosity and cathodically sta-ble IL-based electrolytes134. More recently, sulfonium andazepenium cations have been considered — the formerfor its physicochemical properties and the latter owingto the reduced cost of the raw materials139,140. In mostcases, these cations are combined with fluorinated ani-ons such as [NTf 

2]−, [fsi]−, [BF

4]− and [PF

6]−. As a result

of their increased anodic stability and size compatibilitywith activated carbons, high-performance EDLCs withhigh operating voltages between 3.0 and 3.5 V can berealized134,141. For cost and environmental reasons, theperformance of low viscosity, [dca]-based ILs was exam-ined, and it was found that these ILs exhibited highercapacity at increased current densities than the more

 viscous [NTf 2]-based ILs142.

Carbon dioxide reduction and water splitting

Chemical or electrochemical conversion of CO2 is a

potentially effective means of energy conversion andstorage. By mimicking photosynthesis in nature, it isseen as part of a suite of globally important ‘artificialphotosynthesis’ technologies143,144. However, despitethe worldwide research effort to develop commercially

 viable technologies for CO2 utilization, little progress

has been made because of the high kinetic stability ofCO

2 and the complexity of CO

2 conversion reactions. In

protic media, some routes for CO2 reduction are ther-

modynamically reasonable via proton-coupled elec-tron-transfer pathways. However, because hydrogenevolution is often thermodynamically and kineticallyfavoured, the Faradaic efficiency of CO

2conversion can

be very low. Thus, finding efficient and highly selectivecatalyst and media combinations for CO

2 reduction

is widely recognized as one of the grand challenges ofthe twenty-first century 145. A wide range of electrolytemedia with different levels of proton availability has beeninvestigated for CO

2 reduction; however, ILs have been

relatively little explored despite their attractive prop-erties (for example, high CO

2 solubility)146. ILs based

on the imidazolium cation have been the most widelystudied owing to the unique interaction between theelectroreduced form of imidazolium (that is, carbene)

and CO2 (or between imidazolium and CO2•−

) throughthe formation of a carbene–CO

2 complex147,148. Masel

and co-workers148 reported the electrocatalytic reduc-tion of CO

2 to CO in a [C

2mim][BF

4]–water mixture

with Faradaic efficiency greater than 96%. They postu-lated the formation of a more easily reducible carbenecomplex between CO

2•− and [C

2mim]+, with [C

2mim]+ 

acting as a co-catalyst for the reduction of CO2. An

in situ investigation of the reaction mechanism usingsum frequency generation spectroscopy 149 suggestedthat surface-adsorbed [C

2mim]+ plays an important

role via the formation of an adsorbed carbene–CO2 

complex and by suppressing the hydrogen evolution

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reaction. The role of [C2mim]+ has also been investi-

gated by Sun et al.147 in acetonitrile at a lead electrode.

It was found that the major products for CO 2 reductionwere CO and an imidazolium carboxylate species when[C

2mim][NTf 

2] was used as the supporting electrolyte.

By contrast, the onset potential for CO2 reduction shifted

negatively and oxalate was the main product when[N

2,2,2,2][ClO

4] was used. Mechanisms for CO

2 reduction

(FIG. 4) have been proposed to explain the differences inproducts obtained; these are supported by the findings ofWatkins and Bocarsly 150 who studied the electrochemicalreduction of CO

2 at post-transition metal electrodes in

[C2mim][tfa]. No voltammetric process associated with

1,3-dimethylimidazolium carboxylate was detectedin the potential region in which CO

2 reduction was

observed. It was concluded that imidazolium carboxy-late does not contribute directly to the CO

2 reduction

current observed in [C2mim][tfa]. In other words, the

C2 position of the imidazolium ring is not catalyticallyactive under the conditions of the experiment. By con-trast, well-defined reduction processes associated withimidazolium carboxylates were reported by Cheeket al.151 in [C

4mpyr][OTf] at a Pt electrode. Clearly, the

medium and electrode materials affect the role of theimidazolium C2 position in the catalytic reduction ofCO

2, and further studies are required to understand and

optimize the role of the IL cation.During CO

2electroreduction, an accompanying and

sustainable oxidation reaction is needed. Water oxida-tion to O

2 is ideal because the amount of O

2 produced

would be negligible compared with atmospheric O2;

oxidation of water also produces the protons needed toreduce CO

2. However, in comparison with water oxida-

tion in aqueous media, this reaction has not been widelyexplored in ILs. It has been suggested that this processmay require a lower activation energy in ILs owing to

the disruption of the hydrogen bonding network oth-erwise present in liquid water 152. Investigation of wateroxidation in protic IL–water mixtures, based on butylammonium cations and various anions, has shown thatwater oxidation in these mixtures is diverted from itsusual (and energetically costly) four-electron pathwayinto the two-electron pathway that leads to H

2O

2 at

lower overpotentials44,153 (FIG. 5). Thus, this energeticallyefficient process offers an important direction for cost-effective CO

2 reduction. H

2O

2 is a valuable chemical,

but, if necessary, could be decomposed into water andoxygen to produce a completely sustainable process. Thismechanism also considerably enhances the photoactivityof the MnO

x  

catalyst45.It is also noteworthy that ILs have been used as reac-

tion media for electrosynthesis reactions using CO2 as

a carbon source154,155. However, an important drawbackof using ILs in these applications is their relatively lowmass-transport rates. Miniaturized devices may offer asolution; for example, a gas-diffusion electrode with aprotic-IL membrane156 can be used for CO

2 conversion or

water splitting157. To optimize protic ILs for this purpose(that is, enhancing the proton conductivity), extensivemechanistic studies are needed158.

Fuel cells

Proton-conducting electrolyte materials are much sought

after for proton exchange membrane (PEM) fuel cells.Diaz et al.159 recently provided a useful review of this area.Conventional hydrated-Nafion membranes are limitedby the volatility of the water, which is crucial to their con-ductivity. This could be circumvented in protic-IL-basedmembranes, which would allow access to higher temper-atures. The labile protons typically create high protonconductivity, even in an anhydrous state, which enablestheir function in fuel cells158. It has been suggested thatsome cations, such as the protonated imidazolium cation,may also allow movement of protons via the water-like,Grotthuss conduction mechanism156. Various electrolytemembranes that incorporate ILs are being investigated;

Figure 4 | Reaction pathways for the electrochemical reduction of CO2 at a lead

electrode in acetonitrile. a | Reduction of CO2 in the absence of [C

2mim][NTf 

2].

b | Catalytic mechanism based on the involvement of the imidazolium cation.

c | Non-catalytic mechanism based on the production of an imidazolium carboxylate

zwitterion147. Adapted with permission from REF. 147, American Chemical Society.

Desorption

Desorption

Oxalate desorption

 

N

N

N

N

N

N

N

NO

CO

NN

O ONN

N

N

N

N

CO C

O

O

O

C

O

NN

OO

N

N

O O

OO

a

b

c

Lead electrode2CO

2

 + 2e–

CO2

 + e–

CO2  adsorption

CO2  dimerization to oxalate

2H+

+ e–

H2O

0.5H2

Carboxylate formation

between CO2  and C

2mim

O

C

O O

C

C C

O

OC

O

CO2  coordination

with C2mim

• –

CO2  coordination

with C2mim

• –

• –

• –

CO2

 + e–

• –

CO

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for example, protic IL–polymer composites that useNafion or alternative polymers as a support can reachpower densities as high as 400 mA cm−2 (REFS 156,160).Efficient application of OIPCs in fuel cells has also beenreported recently 161. Further advances, particularly inimproving performance and reducing costs, are stillrequired to realize commercial devices. Nonetheless,there is a potentially bright future for IL-based materialsin PEM-based fuel cells.

There are several different strategies being pursued toproduce practical biofuel cells based on microbial spe-cies or enzymes as the electrocatalytic species. A par-ticularly interesting example is a microbial biofuel cell

that operates in wastewater to simultaneously generateenergy and treat the wastewater162 (FIG. 6). SupportedIL-based membranes have been explored in this contextas alternatives to Nafion as the separator between thetwo aqueous chambers163.

ILs are also being explored as candidate reactionmedia for enzyme-based biofuel cells owing to theirstrong enzyme-stabilizing effect. For example, Fujitaand co-workers164–168 have explored ‘hydrated’ ILsbased on [Ch][H

2PO

4]–water mixtures as excellent

solvents for proteins. The electron-transfer activity ofcytochrome C dissolved in this medium was maintainedfor at least one year164. This exceptionally long shelf life

Figure 5 | Photo-driven water oxidation in butylammonium protic ILs. a | Schematic of the formation process of

H2O

2. b | Oxidation currents in protic ionic liquid (IL)/water electrolytes.  c |  Ab initio calculation of the protic IL solvation

environment stabilizing H2O

2 (in this case, ethylammonium cations). d | Photocurrent dependence on potential on

MnO x-coated fluorine-doped tin oxide (FTO) electrodes. Organic electrolyte, 1 M butylammonium nitrate. SCE,

standard calomel electrode; SHE, standard hydrogen electrode. Panel b adapted from REF. 153 with permission of

the Royal Society of Chemistry. Panel d adapted from REF. 45 with permission of the Royal Society of Chemistry.

e–

e–

e–

h+

2H2

4H+

O2+2H+

H2O

2+2H+

2H2O

7

6

5

4

3

2

1

0

0.6 0.7 0.8 0.9 1.0 1.1 1.2

Borate buffer, pH 9

Organic electrolyte, pH 9

Borate buffer/organic

electrolyte, pH 9

Potential vs SCE (V)

    N   e   t   p    h   o   t   o   c   u   r   r   e   n   t    (   m    A   c   m    –       2        )

FTO glass

Anodea

b

c

 

    C   u   r   r   e   n   t    d   e   n   s    i   t   y    (   m    A   c   m     –

       2        )

1.4 120

100

80

60

40

20

0

1.2

1.0

0.8

0.6

0.4

0.2

0

1.2 1.61.00.80.6 1.4Potential vs SHE (V)

1 M butylammonium sulfate

1 M butylammonium

methanesulfate

d

1 M diethylammonium sulfate

1 M butylammonium

ethanesulfate

Cathode

F  ar  a d  ai   c effi ci   en c  y (   % )  

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demonstrates the applicability of this IL in enzyme-based biofuel cells, as confirmed recently for the oxida-tion of cellobiose by cellobiose dehydrogenase169. Theeffectiveness of hydrated [Ch][H

2PO

4] in solubilizing

and stabilizing enzymes is thought to result from itsstructural similarity to the surface of cell membranes165;hence, the medium is, to an extent, biomimetic withrespect to certain enzymes. The optimal water contentfor these mixtures has been explored and suggestedto be around seven water molecules per ion-pair forassured biological activity 167.

Future directions and challenges

As the field of energy storage and conversion contin-ues to grow, both in global importance and in tech-nological sophistication, it is apparent that IL-basedsystems can provide a platform for safe, durable andhigh-energy-density materials. In addition, alterna-tive chemical and electrochemical mechanisms thatare often supported by IL systems provide opportu-nities for new directions in these fields. For example,the concentrated-metal-ion/organic-salt systems arean interesting new approach to enhance performance.

Similarly, solvate–IL systems are likely to have an effecton post-Li-ion battery applications.

There is much scope to better understand howstructure and dynamics in IL electrolytes correlate withelectrochemical and transport behaviour. Despite thefact that millions of combinations of ILs are possiblewith the currently available cations and anions, onlya handful have been studied in any great depth; in thefield of energy storage, this focus has been mainly onfluorinated anions owing to their stability. Given thepower of theoretical modelling, ab initio methodsshould provide further insights into the molecular fea-tures that lead to high (electro)chemical stability and

potentially help design new, perhaps non-fluorinatedanions. Although ab initio and DFT techniques havegreatly helped us to understand the origins of the struc-tures of ILs and how they develop into heterogeneity onthe nanometre scale, a detailed approach to predictingtransport properties ab initio remains elusive. This isparticularly important for electroactive ions that areoften present as solutes in the IL; details of their solva-tion, speciation and mobility are invaluable, and com-putational approaches could have an important role inoptimization of their transport properties in particular.This approach will support the power demands of newapplications and large-scale devices, for which theseproperties need to be substantially improved.

The ‘green’ credentials of ILs are widely discussed;however, these claims are often not well substantiated.In the context of energy science and technology, con-

 ventional electrolytes or synthetic media are often usedas the sole basis for comparison; in many cases, ILscompare favourably. However, it is also necessary toquantitatively assess toxicity, recyclability and biodeg-radability, for which there is, in most cases, scant infor-mation available. We commend this task to researchers

with the requisite skills to derive such information forthese important materials.

Cost is another issue for most applications of ILs,especially for those that have high electrochemicalstability because of the need for fluorinated anions.Recent work has detailed new IL-based electrolyte sys-tems that avoid fluorinated anions by providing alter-native pathways to SEI formation and that representan important area for future development. The abilityto maintain high performance and thermal stabilityeven in the presence of small amounts of volatile addi-tives provides tremendous scope to tune SEI formationusing these additives, while at the same time improving

Figure 6 | Wastewater-driven microbial fuel cell. a | Fuel cell design162. b | Practical multi-element immersion design16.

PEM, proton exchange membrane. Reproduced with permission from REF. 162, Elsevier.

H+

Microorganisms

Catalyst

e– e–

a b

Anode

PEM

Effluent

Influent

Air

Cathodic chamber Separator Anodic chamber

Organic matter

Oxygen

 

Cathode: 6O2

 + 24H+ + 24e–

 12H2O

Anode: C6H

12O

6

 + 6H2O 

 6CO2

 + 24H+ + 24e–

Cathode

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the transport properties. More broadly, there is hugepotential to optimize properties of IL-based mixtures,and we encourage researchers to pursue this researchdirection. A better understanding of the properties ofmixtures of ILs, as well as mixtures with molecular sol-

 vents and high concentrations of inorganic salts, willopen up possibilities for further development.

Our vision of this field for the next decade featuresan expanding base of materials that retain the fun-damentally unique suite of properties offered by ILand OIPC salts. These will be increasingly optimizedand tuned for a widening range of applications andpotentially lead to entirely new directions in energygeneration and storage.

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 AcknowledgementsD.R.M. and M.F. thank the Australian Research Council for

support from the Australian Laureate Fellowship programme

and J.M.P. and P.C.H. for support from the Discovery Projects

program and the Australian Centre for Electromaterials

Science. This work was supported in part by the National

Natural of Science Foundation of China (Grant Nos

21371101, 21421001), 111 Project (B12015) and MOE

Innovation Team (IRT 13022) of China. H.O. acknowledges

the financial support of the Grant-in-Aid for ScientificResearch from the Japan Society for the Promotion of Science

(KAKENHI, No. 26248049).

Competing interests statementThe authors declare no competing interests.

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