Deakin Research Online30031077/howlett... · cases, generally of very low vapour pressure makes them immediately attractive as device electrolytes. Quite a large number of ionic liquids,

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MacFarlane, Douglas R., Pringle, Jennifer M., Howlett, Patrick C. and Forsyth, Maria 2010, Ionic liquids and reactions at the electrochemical interface, Physical chemistry chemical physics, vol. 12, no. 8, pp. 1659-1669.

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Copyright : 2010, The Royal Society of Chemistry

Phys. Chem. Chem. Phys., 2010, 12, 1659-1669 DOI:10.1039/B923053J (Perspective)

Douglas R. MacFarlane, Jennifer M. Pringle, Patrick C. Howlett and Maria Forsyth

Australian Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria3800, Australia. E-mail: [email protected]

Received 3rd November 2009, Accepted 17th December 2009

First published on the web 14th January 2010

Ionic liquids (ILs) represent a fascinating, and yet to be fully understood, medium for a variety ofchemical, physical and biological processes. Electrochemical processes form an important subset of thesethat are particularly of interest, since ILs tend to be good electrochemical solvents and exhibit otherproperties which make them very useful as electrolytes in electrochemical devices. It is importanttherefore to understand the extent to which electrochemical reactions and processes behave in a relatively“normal”, for example aqueous solution, fashion as opposed to exhibiting phenomena more uniquely theproduct of their organic ionic nature. This perspective examines a range of electrochemical reactions inionic liquids, in many cases in the context of real world applications, to highlight the phenomena as far asthey are understood and where data gaps exist. The important areas of lithium and conducting polymerelectrochemistry are discussed in detail.

Douglas R. MacFarlane

Professor Doug MacFarlane leads the Monash Ionic Liquids Group atMonash University and the Energy Program in ACES. His researchinterests include the chemistry and properties of ionic liquids and solidsand their application in a wide range of technologies fromelectrochemical (batteries, fuel cells, solar cells and corrosionprevention), to biotechnology (drug ionic liquids and proteinstabilisation) and biofuel processing.

Ionic liquids and reactions at the electrochemical interface - Physical ... http://pubs.rsc.org/en/Content/ArticleHtml/2010/CP/B923053J

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Jennifer M. Pringle

Dr Jenny Pringle received her degree and PhD, on ionic liquids, at TheUniversity of Edinburgh in UK before moving to Monash University in2002. In 2008 she started an ARC QEII Fellowship, in association withthe ARC Centre of Excellence for Electromaterials Science (ACES),which focuses her research on ionic liquids and plastic crystals into thearea of dye-sensitized solar cells.

Patrick C. Howlett

Dr Patrick Howlett received his chemistry degree from Curtin Universityin Perth in 1992. He then held various positions in industry before joiningCSIRO in 1999, and received his PhD jointly with CSIRO EnergyTechnology and Monash University in 2004. He is now a Senior ResearchFellow within ACES, investigating corrosion resistant coatings, batteriesetc. and also developing advanced surface characterization methods,including synchrotron techniques, to look at the formation of ionic liquidinterphases on reactive metal surfaces.

Maria Forsyth

Professor Maria Forsyth is the Associate Director of ACES. She has beeninvolved in the field of electro-materials science since graduating fromher PhD at Monash University in 1990 and taking up a Fulbrightfellowship at Northwestern University (USA) in the field of lithiumbattery electrolytes. Her current research focuses on developing anunderstanding of charge transport at metal/electrolyte interfaces andwithin electrolyte materials. Overall, she has co-authored over 230refereed publications.

1. Introduction

The charge-rich medium dominated by electrostatic forces that ILs represent potentially has a significanteffect on the thermodynamics and kinetics of many processes. While the world of solution chemistry haswell developed concepts to deal with differences in polarity, hydrophobicity and hydrogen bonding, ionicliquids add a new dimension to these property variables – charge density. In this dimension processes thatinvolve charge creation, separation, or reduction may be strongly influenced by an IL medium, ascompared to more traditional solvents. This would include many, if not most, electrochemical processesand hence one of the longer term goals in the field of the physical chemistry of ionic liquids must be to


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understand and, where possible, quantify these effects. To some extent the field of molten salt chemistryprovides a substantial starting point for this endeavor,1 however the organic nature of the IL mediumintroduces a much wider range of intermediate properties and allows for a much more extensiveexploration of chemistry and electrochemistry than has ever been possible in molten salts.

Electrochemical processes have also been an important application area for ionic liquids since theirearly development, in particular in various device and materials processing applications,2,3 for exampleelectrodeposition.4 The reasons for the interest in these applications stem from the unique set of propertiesthat certain ILs offer as electrolytes. The fact that they are intrinsically ion conductors and, in the aproticcases, generally of very low vapour pressure makes them immediately attractive as device electrolytes.Quite a large number of ionic liquids, notably those based on the NTf2, BF4 and PF6 anions, exhibit wideelectrochemical windows of stability, reaching in some cases to below Li/Li+ potential in the reductiveregion and to well above +2 V vs. Ag/Ag+ in the oxidative region. Such electrochemical stability thenmakes these liquids of interest as electrolytes for lithium batteries and also for the electrodeposition ofimportant metals such as aluminium and titanium. The same properties have also allowed very stableelectrochemical cycling of the intrinsically conductive polymer electrodes that are being investigated forapplications including batteries and actuators.

With these important applications in mind it is of some significance to investigate in more detail theinfluence the ionic liquid may have on the electrochemical process of interest. Our goal here is thereforeto address questions such as: “How do electrochemical processes in ionic liquids compare with those inother solvent types?”, “Is the ionic liquid only passive, or is it actively involved in the process” and also“What variation do we observe between ionic liquids of different types?”. The latter question arisesbecause the field is generating an ever increasing suite of ionic liquids of quite divergent properties. Theorganic nature of the molecular ions allows, in some cases, nanoscale heterogeneity to be present in theliquid state separating highly charged regions from distinctly hydrocarbon regions. Equally, ion pairing andaggregation can, in some cases, produce ionic liquids of distinctly low “ionicity”;5,6 in other words the ionsare not able to move and act independently such that measures of free ion concentration fall well short of100%. Such examples can therefore be thought of as intermediate in properties between true ionic liquidsand true molecular solvents.7

This paper will consider these questions as applied to a number of electrochemical phenomena ofimportance in electrochemical applications, using some recent examples as a context. We begin byexamining the influence of the ionic liquid medium on the thermodynamics of the processes as manifestedin their redox potentials. Rigorous data is scant but sufficient for some trends to be apparent. In somecases where a metal ion is involved in the process of interest this requires us the consider the metal-ionspeciation that is present in the ionic liquid. We then proceed to discuss these basic questions as applied toa number of practically important electrochemical deposition reactions where surface film formation anddeposit morphology are important aspects of the processes. Thus we examine ionic liquid effects inexamples including lithium electrochemistry, CdS semiconductor deposition and intrinsically conductivepolymer formation. In each of these cases there is direct chemical involvement of one or both of the ionicliquid ions and thus we emphasise this active role and the extent to which it is dependent on the chemistryof the ions involved.

Our approach here is to provide some perspectives on the phenomena involved, based to an extent onexamples from our own laboratories. Our aim is not to provide a thorough review of these topics since thatwould require a very large article; rather we aim to provide an overview of the effects and some views asto properties that remain to be investigated and understood.

2. Redox potentials

Here we consider a redox species dissolved into the ionic liquid such that the ionic liquid then representsthe “solvent” for that species. Given the distinctly different solvent environment that an ionic liquidrepresents, as compared to an aqueous or non-aqueous solution, it is of fundamental importance to inquireinto possible differences in redox potentials of otherwise well understood redox reactions. The questionamounts to asking whether there are differences in the “solvent” environment, and hence free energy ofreactants and products, that could produce a significant shift in redox potential overall. The greatestlikelihood for this is in redox reactions that involve solid reactants or products, since in these cases only


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one side of the reaction is influenced strongly by the environment. Lynden-Bell8,9 has investigated some ofthese questions via simulation studies and concluded that Marcus theory applies equally well to the ionicliquids studied and that solvent reorganization free energies were quite similar to those calculated forCH3CN, in part because long range interactions were screened very effectively by both types of solvents.

A useful experimental comparison of IL vs. molecular solvent behaviour is the comprehensive studycarried out by Zhang et al.10 on the redox behaviour of the polyoxometallate species [S2W18O62]4 whichundergoes a series of stepwise reversible oxidation reduction reactions (Fig. 1) to eventually reach the 10anionic species. A careful study in a variety of solvents, including several ionic liquids, was carried outand showed a very clear and strong solvent dependence that became more pronounced as the charge onthe species increased (Table 1). The ionic liquids showed fairly uniform behaviour amongst those studied,whilst an increasing divergence was seen amongst the molecular solvents.

Fig. 1 Cyclic voltammograms at 0.1 V s 1 of 5mM(Bu4N)4[S2W18O62] in [C4mim][PF6] at 20 °Cusing a GC electrode (from ref. 10). Numbersindicate individual steps in the reduction of the -4species, eventually to the -10 species.

Table 1 Comparison of reversible potentials of the various stages of reduction of the [S2W18O62]4 anionin Ionic Liquids and various molecular solventsSolvent/electrolyte E° /V vs. Fc+/Fc

4-/5- 5-/6- 6-/7- 7-/8- 8-/9- 9-/10-[C4mim][PF6] 0.02 0.257 0.72 1.005 1.373 1.628[C2mim][NTf2] 0.001 0.249 0.701[C4mpyr][NTf2] 0.023 0.263 0.778 1.120[C4mim][BF4] 0.01 0.287 0.736 1.000DCM/Bu4NPF6 0.437 0.802 1.357 1.745CH3CN/Bu4NPF6 0.245 0.628 1.199 1.595 2.094 2.464H2O/Et4NCl 0.02 0.2 0.64 0.89 1.08 1.20

For example, E0 for the 7/ 8 redox process ranged from 0.89 V vs. Fc+ /Fc for an aqueous solutionto 1.75 V vs. Fc+/Fc for a dichloromethane solution†. This is a substantial range corresponding to almost100 kJ mol 1 in reaction free energy. The E0 values for the ionic liquid based solutions were clusteredfairly tightly around 1.0 to 1.1 V, this being closer to the aqueous solution value. The reason that the


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aqueous solution value is substantially less negative than the low dielectric constant non-aqueous solutiondata is due to the much more effective shielding of the additional charge that the process introduces intosolution. The ionic liquid result, though concordant with the computational results of Lynden-Belldiscussed above, is slightly perplexing since the dielectric constants of ionic liquids11,12 are not thought tobe particularly high and certainly less than that of acetonitrile. However, this illustrates the fact thatdielectric shielding is not a useful concept in this respect when an ionic liquid is involved.9 In the ionicliquid a local reorganisation is also possible in the immediately surrounding shell of ions13 in response tothe change in state of charge that takes place in the redox reactions of Fig. 1. Lattice Madelung constantcalculations on a regular lattice of the sort recently discussed by Izgorodina et al.14 also suggest that theelectrostatic component of free energy depends on a Madelung factor that is determined by the size of theIL ions and their state of association. Thus it is possible for ionic liquids to “shield” additional chargedspecies to an extent that is as effective as the dielectric shielding in aqueous solutions. It is important tonote, however, that the effect is not dipolar in its origins.

When redox processes between much lower charge species such as the I3 /I couple are involved, theeffects are much smaller. This particular redox process is of considerable applied interest in the context ofdye sensitized solar cells where it provides the redox shuttle between the counter electrode and theoxidized dye species. As far as we are aware a detailed investigation of the dependence of the redoxpotential of the I3 /I couple in ionic liquids has not been published. Many effects involving the electrolytein the solar cell influence the potential and hence the effect of the redox couple cannot be resolved fromfull cell studies; careful comparative measurements on the cathode reaction are very much needed.

The abovementioned theoretical and experimental studies have involved redox reactions carefullychosen to not interact in any chemical way with the IL. However in the case of many metal ions there isstrong likelihood of coordination type interactions between the metal ion and the anion, even thoseconsidered to be very weakly basic.15 This metal ion speciation becomes a very significant matter inunderstanding ionic liquid electrochemistry. A recent example of such a shift in redox potential can beseen in work16 where the reduction of Cd(II) in a phosphonium tosylate ionic liquid, into which CdCl2 wasdissolved, was observed at 0.45 V vs. Ag/AgCl at 100 °C. In aqueous solution the Cd(II) reductionprocess would be expected at 0.2 V vs. Ag/AgCl. This apparent stabilization of the metal cation couldbe envisaged to result from Cd(II) complex ion formation in the ionic liquid utilizing the IL anion to formspecies such as [CdCl2tos2]2 . These species will have distinctly different redox potentials compared to theaquo ion. To the extent that the complex ion in the ionic liquid is more stable than the corresponding aquoion, the reduction potential will be more negative. Of course, similar shifts in redox potential are alsopossible in aqueous solutions involving coordinating species.

The existence and impact of such speciation was investigated in both our laboratories17 and those of theEndres group18 recently in respect of the Al(III) ion in NTf2 ionic liquids. In this case a variety of complexions of the type shown in Fig. 2 were identified by 27Al NMR, Raman spectroscopy and ab initio studiesand were proposed as having a significant impact on the electrodeposition of Al from the mixture. In somecases the nature of the complex renders the species not reducible within the range of potentials accessiblein the IL electrolyte. Equally they have a strong effect on the phase behaviour of mixtures of AlCl3 withthe NTf2 IL, in some cases a liquid–liquid phase separation occurring, with one of the phases being richerin the reducible Al(III) species. The existence and role of such complex ion species in ionic liquids is atheme to which we will return at several points in the discussion throughout this article.


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Fig. 2 Ab initio structures of Al(III) complex ionspecies of overall stoichiometry AlCl(NTf2)3 formedin NTf2 IL solutions of AlCl3 (from ref. 17 ESI).

Similarly, the Compton group19 has provided some useful perspectives on the practically very importantLi+(IL) + e = Li(s) reaction, showing that the Li+/Li couple has a formal potential of 3.2 V vs. Fc+/Fc(approx 3.9 V vs. NHE) in NTf2 ionic liquids. This is only slightly less negative than in THF which isknown to coordinate strongly to Li+, hence suggesting quite significant complexation of the Li+ in thisionic liquid by the anion.20 However, the role of surface layers in lithium electrochemistry is also veryimportant, and may impact significantly on the redox potential of the overall electrochemical process, asdiscussed in detail below.

In summary, then, it seems that ionic liquids can have a number of effects on redox potentials. Byshielding charges very effectively, the ionic liquid can serve to stabilize charged species, especially whenthe charges are large, and hence alter the overall thermodynamic driving force in the reaction. In caseswhere complex-ion species can form, for example with metal cations dissolved in the IL, the ionic liquidions can be significantly involved in the metal speciation, mimicking the effect of ligand complexation ofmetal ions in aqueous solution. The presence of such species can have a substantial effect on the observedredox potential of the metal ion involved.

3. Electrodeposition processes and film formation in ionic liquids

Li and other reactive metals

Processes in ionic liquids at an electrochemical interface that lead to the formation of a surface film are ofgreat practical interest. Such films have implications for a wide range of applications includingelectrodeposition, energy storage, energy conversion, electrocatalysis, tribology and corrosion.Furthermore, the opportunity to study a molten salt interacting with a surface (a charged metal surface orindeed any other surface) at room temperature, and hence with a far greater range of materials andconditions than was previously accessible in traditional molten salts, has spurred substantial interest, bothfundamental and practical.21–24

Initial interest in surface film formation arose from studies of IL electrolytes for lithium metalelectrodes. It is widely accepted that lithium electrochemistry is dominated in most cases by the presenceof a unique surface film, termed the Solid Electrolyte Interphase (SEI), which allows lithium ion transport,but prevents further corrosion of the substrate by its passive behaviour. The presence or absence of an SEIhas variously been argued to be necessary25or problematical26 to the establishment of a lithium metal basedbattery. The study of IL electrolytes for use with active metal electrodes such as lithium, raises interestingquestions with regard to the formation and action of an SEI in ILs. The following section will summarisecurrent issues and understanding in this field and its implications for related applications in corrosion.

In recent times, the principle IL systems of interest that support lithium electrochemistry have beenthose based on quaternary ammonium cations with a NTf2 anion.27–30 The first report of the observation ofSEI formation in these ILs was presented by Katayama31 and we subsequently reported similarobservations along with an analysis of the film morphology and composition.28,32 The film was


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demonstrated to be composed mainly of components arising from the NTf2 anion; hence it was dominatedby LiF, Li2S, Li2O, Li2S2O4etc. These findings were not surprising in light of the extensive reports ofAurbach who had previously investigated LiNTf2 electrolytes in a range of aprotic solvents and hadpresented a detailed analysis of the anion decomposition and the resulting SEI properties.33,34 There havenow been many reports of the formation of these types of salt films on electrode or active metalsurfaces35–43 and their presence and importance seems to be widely accepted. However, there are stillquestions regarding the mechanism of formation and the origins of the species making up the film, whichare important both fundamentally and in application. Aurbach presented the following reaction scheme toaccount for the decomposition of the NTf2 anion in the presence of lithium.33,34

LiN(SO2CF3)2 + ne + nLi+ Li3N + Li2S2O4 + LiF + C2FxLiy

LiN(SO2CF3)2 + 2e + 2Li+ Li2NSO2CF3 + CF3SO2Li

Li2S2O4 + 6e + 6Li+ 2Li2S + 4Li2O

Li2S2O4 + 4e + 4Li+ Li2SO3 + Li2S + Li2O

Based on our findings and the reports of Aurbach et al.,33,34 we undertook to confirm and elucidate themechanism of SEI formation via NTf2 anion decomposition in the IL.44 Using electrochemical andspectroscopic techniques, along with supporting ab initio calculations, it was indeed shown3,45,46 that thedecomposition of the NTf2 anion was a possible mode of film formation. Furthermore, the rate ofdecomposition and the amount of product formed was shown to be strongly influenced by the electrodesubstrate and the presence and concentration of contaminant levels of H2O.

A further complication arises when the activity of substrate oxide/hydroxide surface species isconsidered. As already shown,44 there is a marked dependence of the decomposition process on the typeof substrate e.g., Pt, Ni or Cu. Thus, there is an open question as to whether these processes would occuron an ‘active’ substrate (e.g., Li with its ubiquitous, native surface film), even in the complete absence ofwater and oxygen. In fact, we have recently shown the strong interaction of a number of ILs withhydroxyl functionalized ceramic surfaces (e.g., SiO2 and MgO/OH2 particles) using multinuclearsolid-state MAS NMR.47,48

Most recently, Passerini et al.49,50 have suggested another possibility for the origins of the surfacespecies formed in these electrolytes. The presence of an unidentified impurity in the ‘as received’ LiNTf2

salt is suggested to account for the formation of the observed surface species. The impurity can beremoved by heating to decomposition and then washing in anhydrous acetonitrile. This is an attractivepossibility given that it is apparently relatively easily solved and it seems to account for all of the observedbehaviour. It is also possible that the same impurity has been present in the majority of NTf2 basedelectrolyte systems studied in laboratories around the world, since the NTf2 ion has normally beenobtained, by researchers and ionic liquid producers alike, from a single source.51 Further study andexploration of the identity and possible role of this impurity is of obvious importance.

Whilst a clear picture of how the SEI formation process occurs in the IL seems to be emerging, recentwork with ‘ultra-pure’ ionic liquid electrolytes by Endres et al.,52,53 and by Passerini et al.,22,54,55 of theeffects of O2 and H2O (and other) contaminants has challenged some of the original reaction mechanismsproposed by Aurbach. This recent work has shown that, under conditions of negligible concentrations ofO2 and H2O contaminants, no anion decomposition occurs at the lithium reduction potential. This wouldindicate that if the level of atmospheric contaminant can be maintained at sufficiently low levels, thenanion decomposition can be avoided allowing many important electrodeposition applications to berealized. Thus it seems that appropriately pure ILs of this type now offer the possibility of studying thelithium electrodeposition/dissolution reaction in the absence of an SEI. This represents a much soughtafter and important opportunity to understand the role of the SEI properties in the problem of lithiumdendrite formation.


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Another advantage that may be gained if the SEI can be eliminated lies in the reduction in ohmicresistance in the cell that could be obtained, given the large impedance contribution produced by thesefilms, e.g., resistivities of 107–108 cm at 25 °C in a C3mpyrNTf2 ionic liquid.32 However, it is importantto note that the magnitude of this contribution is questioned, with some authors postulating a largerresistive contribution from the compact double layer in these materials produced by the arrangement ofpositive IL cations at the charged surface; these present a barrier or screen which may retard the transportof ions to and from the electrode surface.56,57 The physical detection of these layers using an AFM tip at anegatively charged surface supports this hypothesis.58 Similarly, Sugimoto et al.59,60 discuss the relativeease or difficulty of desolvation from an ionic cage as a source of interfacial impedance. Unfortunately itis difficult to deconvolute these various contributions to the overall impedance response of an activeelectrode and this remains an important outstanding issue in the understanding of ionic liquid electrolytes.Experiments which probe the individual processes in model cases are sorely needed.

Corrosion protection

Considering the SEI film formation process in another light led us to examine the possibility of using thegenerated surface film to protect metal surfaces from corrosion. This is effectively the role of the SEI inlithium cells. Corrosion in general is, of course, an electrochemical phenomenon whereby the metal isoxidized by its environment. Traditional approaches to solving or mitigating this enormously costly (bothin economic and energy terms) problem usually involve the use of some sort of protective surface film thatinhibits or alters the redox processes. Magnesium alloys were chosen as a focus in our work because oftheir important engineering properties as light alloys for aircraft and vehicle components, but considerablesurface reactivity and hence poor corrosion resistance. After examining a range of NTf2 salts, it was foundthat the large trihexyl(tetradecyl)phosphonium (P6,6,6,14) cation provided the most protective surface.61,62

The IL treatment was found to be protective to both pure magnesium and the alloy AZ31 (nominally 3%Al and 1% Zn) and the film properties were dependent on the treatment duration and temperature.Optimum treatment conditions produced significant reductions in the corrosion rate, reducing thecorrosion current density by more than one order of magnitude compared to an untreated sample andproducing a cathodic shift of the equilibrium corrosion potential by 500 mV. Analysis of the filmcomposition (Fig. 3) revealed the presence of similar chemical components to those found on a Li surfaceexposed to the same IL, although a large alkyl carbon content indicated that the cation was also present.The film thickness was found to be in the range of hundreds of nanometres. Overall, our characterizationdata indicates that the NTf2 anion decomposition process occurring on the Mg surface is similar to thatoccurring on Li as discussed above,48,62–64 indicating a generic behaviour of this anion when exposed tostrongly negative potentials and the presence of oxygen and H2O.

Fig. 3 Depth profile plot of normalised ToF-SIMSpeak intensities acquired during gallium ion etchingof a AZ31 Mg alloy treated in P6,6,6,14NTf2 for 20 h.


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To expand this area of application, a range of phosphate-based phosphonium ILs were investigated forfilm formation on magnesium alloys.65 This was motivated by literature reports of phosphating treatmentsfor Mg alloys66,67 and also by the attractive prospect of a range of inexpensive ILs which could besystematically investigated through changes to the anion structure. A commercially available IL,P6,6,6,14bis(2,4,4-trimethylpentyl)phosphinate, was found to provide good protection to Mg alloy AZ31 withaddition of a small amount, 6 wt%, of water.68 The water was postulated to hydrate the native oxidelayer on the alloy surface to produce a more reactive surface for film deposition. Control of the filmproperties and morphology has been made difficult by the varying surface features resulting from themicrostructure of the alloys. In many cases the corrosion mechanisms in these alloys are dominated by thealloy microstructure.69 Surface preparation, for example use of a mechanical polishing or anelectropolishing technique, modifies the corrosion resistance of the alloy and was also shown tosubsequently modify the effectiveness of the IL surface treatment.70

Our present understanding of how these films develop and interact with the alloy is most clearlyillustrated by a recent publication48 which examines the formation of phosphonium NTf2 anddiphenylphosphate (dpp) surface films on Mg alloy ZE41 (a Mg–Zn-rare earth-Zr based alloy). ZE41consists of the -Mg matrix containing finely distributed particles, thought to be -phase particles, and theT-phase (Mg7Zn3RE) distributed at the grain boundaries (Fig. 4). It was shown that the Zr rich particleswithin the grains dominated the early stages of corrosion in an aqueous NaCl solution.69 Thus, themorphology of the alloy surface has regions of distinct compositional difference and the accompanyingoxide film also reflects these differences. On this surface it was found that the IL film deposition wasinfluenced by the chemical reactivity of the Mg alloy (i.e., an electronegative surface at 2.7 V vs. SHE)which caused decomposition of the IL anion. Furthermore, the nature of the oxide surface (e.g., relativelyinert ZrO2vs. more reactive MgO/(OH)2 surface species) was shown using solid-state MAS NMR (Fig. 5)to substantially impact the surface film morphology, with the more reactive MgO/(OH)2 species exhibitingstronger interactions, particularly with the dpp anion. In this case it is also evident that the adsorbed anioncauses the concomitant binding of the phosphonium cation via electrostatic attraction.47 Thus, at presentwe consider these films to arise from chemical reactions that take place on the reactive metal surface andalso from strong interactions of the IL species (particularly the anions) with the surface active nativeoxide film. In this respect, our picture of these films is one of a chemically modified and augmented oxidefilm which generally has similar dimensions to the original native oxide surface.

Fig. 4 SEM image of the grain boundary region of aZE41 Mg alloy treated with P6,6,6,14dpp IL for 24 h,which suggests that there is more IL interactionnext to the grain boundary areas (from ref. 48).


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Fig. 5 31P NMR spectra for P6,6,6,14dpp ionic liquidpure and adsorbed onto inorganic powders; theresonance at 32 ppm (associated with the cation) islittle altered in all cases while that at 11 ppmbroadens significantly in the case of Mg(OH)2,indicating a strong, probably chemical, interactionwith the surface.

More recently we have found that improved surface film deposition on AZ31 and ZE41 in these ionicliquids can be achieved under an applied potential.71,72 The interphase layer appears more resistive, asshown by impedance measurements, and also more protective in aggressive chloride solutions.Furthermore, the issues of a heterogeneous structure of the underlying metal appear to be of lessimportance under these conditions. It is not clear at present as to the origin of the improved surface film;for example it may be that a more negative potential leads to a more uniform distribution of surfacehydroxide which then may interact with the anion to form metal phosphate esters, or it is possible that theapplication of a potential bias (negative or positive) may favour electrochemical processes such as metaloxidation (e.g. Mg Mg2+ + 2e ) or IL oxidation/reduction that then facilitates the deposition of aninsoluble metal salt on the alloy surface. This observation opens up the possibility of IL film deposition onnot only magnesium alloys, but other reactive metals such as aluminium and titanium alloys. This couldlead to improved durability of these alloys and hence allow their use in a variety of engineeringapplications in the infrastructure, automotive, aerospace, electronic and biomedical industries; areas inwhich the high strength to weight ratio of these light alloys can be utilized.

Semiconductors

For many of the same reasons discussed above in respect of reactive metals, ionic liquids have proven tobe a useful medium for the preparation and growth of semiconductor films. The Endres group have verymuch led the way in this field.73–76 The ionic liquid in this context often represents a stable, ion-conductingmedium for operation at elevated temperatures, with usefully controllable solvation properties. Howeverthe IL may also have a more direct influence on the chemical process involved, as exemplified by therecent work from our laboratories with CdS.16 CdS and its analogues are semiconductors of great interestin photovoltaic solar cells because their bandgaps and band energy levels are well suited to thisapplication. However high quality CdS films are relatively difficult to obtain at low cost. Electrochemicalmethods have been investigated in aqueous and protic non-aqueous media, but have struggled to producefilms of high quality, in part because of the limited temperature range available in these media. In generaldeposition temperatures above 100 °C tend to produce more defect-free films. Contamination of the filmwith the solvent and/or its reductive breakdown products also degrades the semiconductor properties. Inthis case an IL medium of sufficient thermal and reductive stability, but relatively low cost, in the form ofa methyl tri-isobutyl phosphoniumtosylate allowed a number of alternate reaction pathways to be explored


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towards the preparation of CdS that would not be possible in most other types of medium. CdCl2 isrelatively soluble in the tosylate ionic liquid, probably as a result of complex ion formation involving oneor more tosylate ligands, as discussed above. This serves to solubilize and stabilize the ion. To produceCdS thin film deposits on the ITO electrode at high efficiency with respect to the Cd requires that eitherthe Cd(II) or S be generated at the electrode in a controlled fashion. In our case the reduction of S2O3

2 insolution via S2O3

2 + 2e S2 + SO32 was the approach that proved most productive from a number of

alternatives including direct reduction of dissolved sulfur. As shown in Fig. 6, the reduction of S2O32 to S2

occurs at a slightly less negative potential than reduction of sulfur; the latter occurs close to the potentialof Cd(II) reduction and hence resulted in films contaminated with Cd(0). However, useful rates of S2O3

2

reduction were possible without interference from Cd(0) and very high quality films were thusproduced.16Fig. 7 shows the degree of thin film control achievable from the process, the film thicknessbeing directly proportional to the charge passed in the coulometric experiment. Of particular note in termsof semiconductor performance was that the electrodeposit was extremely well ordered, showing strongsigns of preferential ordering in the XRD patterns. Morphological effects such as this have been observedin a variety of IL based electrodeposition processes, including the conducting polymer materials discussedbelow, and seem to be a feature of the IL as a medium in this type of electrochemical reaction. In the caseof eletrodeposition reactions involving metal species this may be a result of the complexation of the metalion in solution and thereby the need to dissociate this complex at the electrode surface. This inserts apotentially rate determining step in the overall process, mitigating the effects of diffusion which canotherwise produce rough and dendritic deposits. The effect of complexing agents in influencing depositmorphology in metal electrowinning generally is well known. Such effects underscore the need for moredetailed studies of complex ion equilibria and kinetics in ionic liquids.

Fig. 6 Electrodeposition of CdS from anP1,4,4,4Tosylate ionic liquid. The process is based onthe reduction of the S2O3 anion in the ionic liquid(black line). The reduction to sulfide appears to bedirect since the reduction of dissolved S (red line)lies beyond the potential used (from ref. 16).


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Fig. 7 Dependence of CdS film thicknesselectrodeposited at 130 °C from P1,4,4,4Tosylate ionicliquid by reduction of the thiosulfate anion in thepresence of Cd(II) (from ref. 16).

Conducting polymers

ILs have repeatedly been shown to be extremely effective electrolyte media for the electrodeposition ofconducting polymers. Their wide electrochemical windows in some cases allows access to monomers thatare outside the range attainable using conventional molecular solvent systems, for example thosemonomers containing electron-withdrawing groups such as 3-chlorothiophene,77 or those that areextremely sensitive to moisture.78 For example, the synthesis of poly(paraphenylene) is normally restrictedto media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completelyanhydrous. However, using a very dry (<3 ppm water) and an anodically stable ionic liquid,poly(paraphenylene) can be electrodeposited onto platinum as a coherent, electroactive film.78,79 Thesolubility of monomers in ionic liquids can also be an advantage.80 Murray et al.81 have used an ionicliquid for the electrodeposition of poly(terthiophene) incorporating anionic dyes, for use in photovoltaicdevices, and the films produced were more robust than those obtained using DMF.

The stability that ionic liquids impart to conducting polymers has been well demonstrated and isprobably one of the key benefits. Our early work showed that the use of ionic liquids as electrolytes in arange of electrochemical devices utilizing conducting polymers that had been synthesized in conventionalsolvent systems resulted in significantly improved lifetimes.82 Since then, a number of authors havedemonstrated that use of an ionic liquid for both the synthesis and utilization of the conducting polymercan significantly benefit the electrochemical stability.83–86 For example, Deepa et al.87 have demonstratedan increased electrochromic cycle life cycle life for poly(pyrrole) grown in an IL compared to those grownin acetonitrile/LiNTf2, in addition to a higher conductivity (due to a longer conjugation length), bettercoloration efficiency and charge storage capacity. The synthesis of PEDOT in an ionic liquid has alsobeen reported to yield significant improvements in stability, in addition to currents seven times greaterthan for films synthesized in an acetonitrile electrolyte, making them of interest for electrochemicalcapacitor applications.85

While there are increasing reports in the literature regarding the use of ionic liquids for the synthesis ofconducting polymers, there is still a lack of understanding regarding the fundamental influences that thesenew media have on the pertinent redox processes and, thus, insufficient conclusive evidence regarding thebenefits of using ILs as the deposition media. An observed increased rate of polymerization in an ionicliquid compared to a molecular solvent has been attributed to the high viscosity of the ionic liquid, whichimproves redox–redox coupling, further oxidation of oligomers and deposition of the polymer by


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accumulation of the products near the electrode surface due to slow diffusion conditions.88 Differences inthe morphology of films grown in ionic liquids is now a common observation;86–91 We have observedsignificantly smoother poly(pyrrole) films grown in an ionic liquid compared to a molecular solvent.92 Ourstudies of EDOT electropolymerization in two different ionic liquids compared to two conventionalacetonitrile-based electrolytes suggest that the process is initially much slower in the latter. The datasuggests a different mechanism of deposition; the current transient in acetonitrile/Bu4NClO4 is indicativeof progressive nucleation, with a slower growth rate, whereas the current transients in the ionic liquidssuggest instantaneous nucleation and increased growth rate, thus indicating a strong influence of the anionon polymer growth.89

Electrodeposition of conducting polymers by cyclic voltammetry commonly reveals significantdifferences in the growth CVs in the different systems,80,88,89,91–94 although the interpretation of theseresults vary. The post-polymerisation CVs of the polymers can also be significantly different to those offilms grown in molecular solvents,80,85,87,89 and the stability can be greatly increased.83–85 However, it isimportant also to note that the nature of the cycling electrolyte can have an effect on the redox responseof the polymer—as suggested in the introduction, the ionic liquid is not merely passive, but is activelyinvolved in the various processes. Redox cycling of a conducting polymer results in counter-ionincorporation or expulsion and in a conventional molecular solvent/electrolyte system anion movementdominates when the anion is small and mobile and cation movement dominates when the anions are large,such as for polyelectrolyte anions. In an ionic liquid, redox cycling of the conducting polymer can involveeither the cation or the anion, and hence the nature of both of these species can significantly influence theelectrochemical behaviour. We have shown by solid state NMR that even the relatively large [P6,6,6,14]cation can be incorporated into poly(pyrrole) films during growth and cycling.95 The cycling mechanismlikely depends on the relative mobility of the component ions,96 while the viscosity and conductivity of themedium, and its ability to swell the polymer film, can also have an impact. EQCM studies have shown thatfor poly(p-phenylene), cation exchange becomes more important at higher scan rates.97

We have observed that PEDOT films grown in ionic liquid scan show an increase in electrochemicalactivity upon cycling in an acetonitrile solution compared to an ionic liquid, suggesting better swelling ofthe polymer and thus faster transport of ionic species into and out of the polymer during cycling. Onreturn to the ionic liquid there was a rapid return to the lower charge capacity regime, but there was then aprogressive increase in current with cycling in the ionic liquid, as also observed by Randriamahazaka etal.98 In agreement with other authors,99 we observed no memory effect upon cycling the films in thesedifferent solvents.

Poly(pyrrole) can also be electrodeposited from ILs onto iron, a corrosion-susceptible electrode,86 andalthough relatively high potentials (1.3 V vs. a Ag wire quasi-reference electrode) were required, thepolymer was electroactive and conducting, indicating negligible polymer overoxidation problemscompared to those associated with using aqueous systems. It has also been reported that poly(pyrrole)deposited onto glassy carbon from an ionic liquid not only shows better electrochemical activity than filmsprepared in aqueous solutions but also that the films are more selective to the detection of dopamine,which is of interest in the development of conducting polymer-modified electrodes as biosensors.100

Thus, in summary, the use of ionic liquids rather than molecular solvents appears to influence theelectrodeposition and redox cycling behaviour of conducting polymers in a number of ways, although theexact mechanisms are not yet fully understood. The morphology of the polymers can be significantlysmoother; this is influenced by both the viscosity of the ionic liquid and also the nature of the ions. Therate of polymerization may be altered; in an ionic liquid the availability of the anion, which becomesincorporated in the polymer as the counterion, is not limited by diffusion, as it can be in molecular solventsystems, and thus the polymer growth rate is only limited by the availability of the monomer. The higherviscosity of the IL may trap oligomers near the electrode and thus improve redox–redox coupling, butultimately the rate of polymer growth will be limited by diffusion of the monomer and we have observedthat growth of thick polymer films can be significantly slower in an ionic liquid. There is little evidence tosuggest that the doping levels of the polymers are significantly increased through use of an ionic liquid,although this would be desirable. However, the stability of the polymers can be significantly enhanced,and it is this effect that is driving much of the interest in this area.


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4. Conclusions and directions for the future

The discussion and examples above have attempted to survey and describe the impact on electrochemicalprocesses of the rather unique chemical environment that the ionic liquid medium represents. Withmolecular solvents as one extreme and classical molten salts as the other, ionic liquids lie in variouspositions between these extremes with some measure of electrostatic charge density as the yardstick. Theproperties of such coulombic media can impact on electrochemical processes in a number of waysincluding (i) on the redox potentials of electrochemical reactions taking place in the IL, (ii) by allowingaccess to domains of electrochemical potential and/or temperature not accessible in other solvents andthereby novel reaction processes, and (iii) via a significant involvement in the process of interest by directchemical participation, for example by forming metal-ion complexes. All of these effects are of interest inthe various applications currently being investigated for ionic liquids in the electrodeposition field such asadvanced lithium batteries, electrodepositon of metals and semiconductors. They also have an indirectimportance in applications such as tribology where the extreme pressure regime of metal surfacelubrication almost certainly involves the redox chemistry of the metal coupled with the electrochemicaland thermal breakdown of the ionic liquid.

Nonetheless, there is much yet to be understood about these effects and it is the hope of the authorsthat the role of an article such as this is to expose gaps in our understanding and stimulate further work.There is much yet to understand about the thermodynamic aspects of reactive species in ionic liquids,including the factors that control their speciation, thermodynamic activity and therefore their redoxpotentials. As discussed in section 2 the variation in redox potential can correspond to as much as 100 kJmol 1 difference in reaction energy between ionic liquids and other aprotic solvents. That the redoxpotentials of the reaction processes studied tend to cluster around similar values irrespective of the ionicliquid suggests that there is a generic, though not necessarily IL independent, shielding effect of theelectrostatic environment on the activity of the redox active species. Further investigation of these effectsin model systems by standard electrochemical methods is clearly needed. Additionally, factors that controlreactive species mobility and therefore their electrochemical kinetics require much further investigation.Studies of complex ion equilibria, and the formation and dissociation kinetics of these species, are justbeginning to appear and more of this type of work is to be strongly encouraged, especially where multipleelectrochemical and spectroscopic techniques can assist in deconvoluting the effects. The role andproperties of the electrochemical double layer have not been discussed in any detail here since this will bereviewed elsewhere in this volume. There is little known about how the presence of the compact doublelayer at the ionic liquid—electrode interface influences electrochemical processes. Certainly one canexpect that the activity of complex ion species involving the ionic liquid anion, such as those postulated tobe involved in the Cd, Al and Li examples discussed above, will be strongly influenced by the varyingcomposition of the double layer region. All of these fundamental issues require much further in-depthexploration and we commend these matters to those with the tools and skills to undertake the appropriateinvestigations and add to this important, growing field.

References

1 S. Z. El Abedin and F. Endres, Acc. Chem. Res., 2007, 40, 1106–1113 Article ChemPort .2 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009, 8, 621–629

Article ChemPort .3 D. R. Macfarlane, M. Forsyth, P. C. Howlett, J. M. Pringle, J. Sun, G. Annat, W. Neil and E. I.

Izgorodina, Acc. Chem. Res., 2007, 40, 1165–1173 Article ChemPort .4 F. Endres, A. P. Abbott, and D. R. MacFarlane, Electrodepostion from Ionic Liquids, Wiley, 2009.5 H. Tokuda, S. Tsuzuki, M. Susan, K. Hayamizu and M. Watanabe, J. Phys. Chem. B, 2006, 110,

19593–19600 Article ChemPort .6 D. R. MacFarlane, M. Forsyth, E. I. Izgorodina, A. P. Abbott, G. Annat and K. Fraser, PCCP, 2009,

11, 4962–4967.7 K. J. Fraser, E. I. Izgorodina, M. Forsyth, J. L. Scott and D. R. MacFarlane, Chem. Commun., 2007,

3817–3819 RSC Article .


14 of 18 8/11/2010 10:35 AM

8 R. M. Lynden-Bell, J. Chem. Phys., 2008, 129, 204503 Article ChemPort .9 R. M. Lynden-Bell, J. Phys. Chem. B, 2007, 111, 10800–10806 Article ChemPort .

10 J. Zhang, A. M. Bond, D. R. MacFarlane, S. A. Forsyth, J. M. Pringle, A. W. A. Mariotti, A. F.Glowinski and A. G. Wedd, Inorg. Chem., 2005, 44, 5123–5132 Article ChemPort .

11 S. Schrodle, G. Annat, D. R. MacFarlane, M. Forsyth, R. Buchner and G. Heftner, Aust. J. Chem.,2007, 60, 6–8 Article .

12 S. Schrodle, G. Annat, D. R. MacFarlane, M. Forsyth, R. Buchner and G. Hefter, Chem. Commun.,2006, 1748–1750 RSC Article .

13 E. I. Izgorodina, M. Forsyth and D. R. MacFarlane, PCCP, 2009, 11, 2452–2458.14 K. Izgorodina, U. L. Bernard, P. M. Dean, J. M. Pringle and D. R. MacFarlane, Cryst. Growth Des.,

2009, 9, 4834–4839 Article .15 D. R. MacFarlane, J. M. Pringle, K. M. Johansson, S. A. Forsyth and M. Forsyth, Chem. Commun.,

2006, 1905–1917 RSC Article .16 A. Izgorodin, O. Winther-Jensen, B. Winther-Jensen and D. R. MacFarlane, PCCP, 2009, 11,

8532–8537.17 N. M. Rocher, E. I. Izgorodina, T. Ruther, M. Forsyth, D. R. MacFarlane, T. Rodopoulos, M. D.

Horne and A. M. Bond, Chem.–Eur. J., 2009, 15, 3435–3447 Article ChemPort .18 P. Eiden, Q. X. Liu, S. Z. El Abedin, F. Endres and I. Krossing, Chem.–Eur. J., 2009, 15,

3426–3434 Article ChemPort .19 R. Wibowo, S. E. W. Jones and R. G. Compton, J. Phys. Chem. B, 2009, 113, 12293–12298

Article ChemPort .20 P. M. Bayley, G. H. Lane, N. M. Rocher, B. R. Clare, A. S. Best, D. R. MacFarlane and M. Forsyth,

PCCP, 2009, 11, 7202–7208.21 C. Aliaga, G. A. Baker and S. Baldelli, J. Phys. Chem. B, 2008, 112, 1676–1684 Article ChemPort .22 S. Baldelli, Acc. Chem. Res., 2008, 41, 421–431 Article ChemPort .23 M. D. Bermudez, A. E. Jimanez and G. Martinez-Nicolls, Appl. Surf. Sci., 2007, 253, 7295–7302

Article ChemPort .24 J. M. Gottfried, F. Maier, J. Rossa, D. Gerhard, P. S. Schulz, P. Wasserscheid and H. P. Steinrück, Z.

Phys. Chem., 2006, 220, 1439–1453 Article ChemPort .25 E. Peled, D. Golodnitsky and J. Penciner, The anode/electrolyte interface, 1999.26 N. Munichandraiah, L. G. Scanlon and R. A. Marsh, J. Power Sources, 1998, 72, 203–210

Article ChemPort .27 H. Sakaebe and H. Matsumoto, Electrochem. Commun., 2003, 5, 594–598 Article ChemPort .28 P. C. Howlett, D. R. MacFarlane and A. F. Hollenkamp, Electrochem. Solid-State Lett., 2004, 7,

A97–A101 Article ChemPort .29 J. H. Shin, W. A. Henderson and S. Passerini, J. Electrochem. Soc., 2005, 152.30 S. Seki, Y. Ohno, H. Miyashiro, Y. Kobayashi, A. Usami, Y. Mita, N. Terada, K. Hayamizu, S.

Tsuzuki and M. Watanabe, J. Electrochem. Soc., 2008, 155, A421 Article ChemPort .31 Y. Katayama, T. Morita, M. Yamagata and T. Miura, Electrochemistry, 2003, 71,

1033–1035 ChemPort .32 P. C. Howlett, N. Brack, A. F. Hollenkamp, M. Forsyth and D. R. MacFarlane, J. Electrochem.

Soc., 2006, 153.33 D. Aurbach, A. Zaban, Y. Ein-Eli, I. Weissman, O. Chusid, B. Markovsky, M. Levi, E. Levi, A.

Schechter and E. Granot, J. Power Sources, 1997, 68, 91–98 Article ChemPort .34 D. Aurbach, I. Weissman, A. Zaban and O. Chusid, Electrochim. Acta, 1994, 39, 51–71

Article ChemPort .35 J. Hassoun, A. Fernicola, M. A. Navarra, S. Panero and B. Scrosati, J. Power Sources, 2010, 195,

574–579 Article ChemPort .


15 of 18 8/11/2010 10:35 AM

36 V. Baranchugov, E. Markevich, G. Salitra, D. Aurbach, G. Semrau and M. A. Schmidt, J.Electrochem. Soc., 2008, 155, A217 Article ChemPort .

37 E. Markevich, V. Baranchugov, G. Salitra, D. Aurbach and M. A. Schmidt, J. Electrochem. Soc.,2008, 155, A132 Article ChemPort .

38 V. Borgel, E. Markevich, D. Aurbach, G. Semrau and M. Schmidt, J. Power Sources, 2009, 189,331–336 Article ChemPort .

39 I. A. Profatilova, N. S. Choi, S. W. Roh and S. S. Kim, J. Power Sources, 2009, 192, 636–643Article ChemPort .

40 S. K. Martha, E. Markevich, V. Burgel, G. Salitra, E. Zinigrad, B. Markovsky, H. Sclar, Z.Pramovich, O. Heik, D. Aurbach, I. Exnar, H. Buqa, T. Drezen, G. Semrau, M. Schmidt, D.Kovacheva and N. Saliyski, J. Power Sources, 2009, 189, 288–296 Article ChemPort .

41 P. Reale, A. Fernicola and B. Scrosati, J. Power Sources, 2009, 194, 182–189 Article ChemPort .42 S. Caporali, F. Ghezzi, A. Giorgetti, A. Lavacchi, A. Tolstogouzov and U. Bardi, Adv. Eng. Mater.,

2007, 9, 185–190 Article ChemPort .43 A. Lewandowski and A. Swiderska-Mocek, J. Appl. Electrochem., 2009, 1–10.44 P. C. Howlett, E. I. Izgorodina, M. Forsyth and D. R. MacFarlane, Z. Phys. Chem., 2006, 220,

1483–1498 Article ChemPort .45 P. C. Howlett, E. I. Izgorodina, M. Forsyth and D. R. MacFarlane, Zeitschrift Fur Physikalische

Chemie-International Journal of Research in Physical Chemistry & Chemical Physics, 2006, 220,1483–1498.

46 P. C. Howlett, N. Brack, A. F. Hollenkamp, M. Forsyth and D. R. MacFarlane, J. Electrochem.Soc., 2006, 153, A595–A606 Article ChemPort .

47 M. Forsyth, T. F. Kemp, P. C. Howlett, J. Sun and M. E. Smith, J. Phys. Chem. C, 2008, 112,13801–13804 Article ChemPort .

48 M. Forsyth, W. C. Neil, P. C. Howlett, D. R. Macfarlane, B. R. W. Hinton, N. Rocher, T. F. Kempand M. E. Smith, ACS Appl. Mater. Interfaces, 2009, 1, 1045–1052.

49 E. Paillard, Q. Zhou, W. A. Henderson, G. B. Appetecchi, M. Montanino and S. Passerini, J.Electrochem. Soc., 2009, 156, A891 Article ChemPort .

50 G. B. Appetecchi, M. Montanino, A. Balducci, S. F. Lux, M. Winterb and S. Passerini, J. PowerSources, 2009, 192, 599–605 Article ChemPort .

51 S. Atchia, W. Gorecki, M. Armand and D. Deroo, Electrochim. Acta, 1992, 37, 1743–1745Article ChemPort .

52 A. Shkurankov, S. Zein El Abedin and F. Endres, Aust. J. Chem., 2007, 60, 35–42 Article ChemPort .53 F. Endres, S. Zein El Abedin and N. Borissenko, Z. Phys. Chem., 2006, 220, 1377–1394

Article ChemPort .54 S. Randstrom, G. B. Appetecchi, C. Lagergren, A. Moreno and S. Passerini, Electrochim. Acta,

2007, 53, 1837–1842 Article .55 S. Randstrom, M. Montanino, G. B. Appetecchi, C. Lagergren, A. Moreno and S. Passerini,

Electrochim. Acta, 2008, 53, 6397–6401 Article .56 M. Mezger, H. Schoder, H. Reichert, S. Schramm, J. S. Okasinski, S. Schoder, V. Honkimiki, M.

Deutsch, B. M. Ocko, J. Ralston, M. Rohwerder, M. Stratmann and H. Dosch, Science, 2008, 322,424–428 Article ChemPort .

57 Y. Lauw, M. D. Horne, T. Rodopoulos and F. A. M. Leermakers, Phys. Rev. Lett., 2009, 103,117801 Article ChemPort .

58 R. Atkin, S. Z. El Abedin, R. Hayes, L. H. S. Gasparotto, N. Borisenko and F. Endres, J. Phys.Chem. C, 2009, 113, 13266–13272 Article ChemPort .

59 T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono and M. Ishikawa, J. Power Sources, 2008, 183,436–440 Article ChemPort .


16 of 18 8/11/2010 10:35 AM

60 T. Sugimoto, Y. Atsumi, M. Kikuta, E. Ishiko, M. Kono and M. Ishikawa, J. Power Sources, 2009,189, 802–805 Article ChemPort .

61 M. Forsyth, P. C. Howlett, S. K. Tan, D. R. MacFarlane and N. Birbilis, Electrochem. Solid-StateLett., 2006, 9, B52–B55 Article ChemPort .

62 N. Birbilis, P. C. Howlett, D. R. MacFarlane and M. Forsyth, Surf. Coat. Technol., 2007, 201,4496–4504 Article ChemPort .

63 M. Forsyth, P. C. Howlett, S. K. Tan, D. R. MacFarlane and N. Birbilis, Electrochemical and SolidState Letters, 2006, 9, B52–B55.

64 P. C. Howlett, S. K. Tan, S. Zhang, J. Efthimiadis, D. R. MacFarlane and M. Forsyth, Corrosion andPrevention, 2005, Paper 079, 1–6.

65 J. Sun, P. C. Howlett, D. R. MacFarlane, J. Lina and M. Forsyth, Electrochim. Acta, 2008, 54,254–260 Article ChemPort .

66 L. Kouisni, M. Azzi, M. Zertoubi, F. Dalard and S. Maximovitch, Surf. Coat. Technol., 2004, 185,58–67 Article ChemPort .

67 C. S. Lin, C. Y. Lee, W. C. Li, Y. S. Chen and G. N. Fang, J. Electrochem. Soc., 2006, 153.68 P. C. Howlett, S. Zhang, D. R. MacFarlane and M. Forsyth, Aust. J. Chem., 2007, 60, 43–46

Article ChemPort .69 W. C. Neil, M. Forsyth, P. C. Howlett, C. R. Hutchinson and B. R. W. Hinton, Corros. Sci., 2009,

51, 387–394 Article ChemPort .70 P. C. Howlett, W. Neil, T. Khoo, J. Z. Sun, M. Forsyth and D. R. MacFarlane, Isr. J. Chem., 2008,

48, 313–318 Article ChemPort .71 J. Efthimiadis, private communication, 2010.72 P. Howlett, Electrochim. Acta, 2009 DOI:10.1016/j.electacta.2009.11.080.73 S. Z. El Abedin and F. Endres, ChemPhysChem, 2006, 7, 58–61 Article .74 R. Al-Salman, S. Z. Abedin and F. Endres, PCCP, 2008, 10, 4650–4657.75 R. Al-Salman and F. Endres, J. Mater. Chem., 2009, 19, 7228–7231 RSC Article .76 X. D. Meng, R. Al-Salman, J. P. Zhao, N. Borissenko, Y. Li and F. Endres, Angew. Chem., Int. Ed.,

2009, 48, 2703–2707 Article ChemPort .77 Y. Pang, H. Xu, X. Li, H. Ding, Y. Cheng, G. Shi and L. Jin, Electrochem. Commun., 2006, 8,

1757–1763 Article ChemPort .78 S. Zein El Abedin, N. Borissenko and F. Endres, Electrochem. Commun., 2004, 6, 422–426

Article ChemPort .79 T. Carstens, S. Z. El Abedin and F. Endres, ChemPhysChem, 2008, 9, 439–444 Article ChemPort .80 J. M. Pringle, M. Forsyth, D. R. MacFarlane, K. Wagner, S. B. Hall and D. L. Officer, Polymer,

2005, 46, 2047–2058 Article ChemPort .81 P. S. Murray, S. F. Ralph, C. O. Too and G. G. Wallace, Electrochim. Acta, 2006, 51, 2471–2476

Article ChemPort .82 W. Lu, A. G. Fadeev, B. Qi, E. Smela, B. R. Mattes, J. Ding, G. M. Spinks, J. Mazurkiewicz, D.

Zhou, G. G. Wallace, D. R. MacFarlane, S. A. Forsyth and M. Forsyth, Science, 2002, 297, 983–987Article ChemPort .

83 W. Lu, A. G. Fadeev, B. Qi and B. R. Mattes, Synth. Met., 2003, 135–136, 139–140Article ChemPort .

84 J. D. Stenger-Smith, C. K. Webber, N. Anderson, A. P. Chafin, K. Zong and J. R. Reynolds, J.Electrochem. Soc., 2002, 149, A973–A977 Article ChemPort .

85 K. Liu, Z. Hu, R. Xue, J. Zhang and J. Zhu, J. Power Sources, 2008, 179, 858–862 Article ChemPort .86 A. M. Fenelon and C. B. Breslin, J. Electrochem. Soc., 2005, 152, D6–D11 Article ChemPort .87 M. Deepa and S. Ahmad, Eur. Polym. J., 2008, 44, 3288–3299 Article ChemPort .88 K. Sekiguchi, M. Atobe and T. Fuchigami, J. Electroanal. Chem., 2003, 557, 1–7 Article ChemPort .


17 of 18 8/11/2010 10:35 AM

89 K. Wagner, J. M. Pringle, S. B. Hall, M. Forsyth, D. R. MacFarlane and D. L. Officer, Synth. Met.,2005, 153, 257–260 Article ChemPort .

90 M. C. Li, C. A. Ma, B. Y. Liu and Z. M. Jin, Electrochem. Commun., 2005, 7, 209–212Article ChemPort .

91 P. G. Pickup and R. A. Osteryoung, J. Am. Chem. Soc., 1984, 106, 2294–2299 Article ChemPort .92 J. M. Pringle, J. Efthimiadis, P. C. Howlett, J. Efthimiadis, D. R. MacFarlane, A. B. Chaplin, S. B.

Hall, D. L. Officer, G. G. Wallace and M. Forsyth, Polymer, 2004, 45, 1447–1453 Article ChemPort .93 P. Damlin, C. Kvarnstrom and A. Ivaska, J. Electroanal. Chem., 2004, 570, 113–122

Article ChemPort .94 K. Sekiguchi, M. Atobe and T. Fuchigami, Electrochem. Commun., 2002, 4, 881–885

Article ChemPort .95 J. M. Pringle, D. R. MacFarlane and M. Forsyth, Synth. Met., 2005, 155, 684–689 Article ChemPort .96 H. Randriamahazaka, C. Plesse, D. Teyssie and C. Chevrot, Electrochim. Acta, 2005, 50,

4222–4229 Article ChemPort .97 O. Schneider, A. Bund, A. Ispas, N. Borissenko, S. Z. El Abedin and F. Endres, J. Phys. Chem. B,

2005, 109, 7159–7168 Article ChemPort .98 H. Randriamahazaka, C. Plesse, D. Teyssie and C. Chevrot, Electrochem. Commun., 2004, 6,

299–305 Article ChemPort .99 H. Randriamahazaka, C. Plesse, D. Teyssie and C. Chevrot, Electrochem. Commun., 2003, 5,

613–617 Article ChemPort .100 A. Zhang, J. Chen, D. Niu, G. G. Wallace and J. Lu, Synth. Met., 2009, 159, 1542–1545

Article ChemPort .101 S. K. Sukardi, J. Zhang, I. Burgar, M. D. Horne, A. F. Hollenkamp, D. R. MacFarlane and A. M.

Bond, Electrochem. Commun., 2008, 10, 250–254 Article ChemPort .102 A. Lewandowski, L. Waligora and M. Galinski, Electroanalysis, 2009, 21, 2221–2227

Article ChemPort .

Footnote

† In a discussion such as this which attempts to make broad observations about the effect of a wide range of ionic liquid mediaand other solvents, reliable and meaningful reference potential scales become a substantial issue. Bond et al.101 have discussedreference electrodes and concluded that the reduction of the cobaltocenium ion is particularly useful. Lewandowski et al.102

have shown that Fc+/Fc is reliably 0.53 V vs. (Ag/Ag+ in acetonitrile) in a number of aprotic ionic liquids.

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Deakin Research Online30031077/howlett... · cases, generally of very low vapour pressure makes them immediately attractive as device electrolytes. Quite a large number of ionic liquids,