Deakin Research Online
This is the published version:
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.
Available from Deakin Research Online:
http://hdl.handle.net/10536/DRO/DU:30031077
Reproduced with the kind permission of the copyright owner.
Copyright : 2010, The Royal Society of Chemistry
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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, Victoria
3800, 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 of
chemical, physical and biological processes. Electrochemical processes form an important subset of these
that are particularly of interest, since ILs tend to be good electrochemical solvents and exhibit other
properties which make them very useful as electrolytes in electrochemical devices. It is important
therefore 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 the
product of their organic ionic nature. This perspective examines a range of electrochemical reactions in
ionic liquids, in many cases in the context of real world applications, to highlight the phenomena as far as
they are understood and where data gaps exist. The important areas of lithium and conducting polymer
electrochemistry are discussed in detail.
Professor Doug MacFarlane leads the Monash Ionic Liquids Group at
Monash University and the Energy Program in ACES. His research
interests include the chemistry and properties of ionic liquids and solids
and their application in a wide range of technologies from
electrochemical (batteries, fuel cells, solar cells and corrosion
prevention), to biotechnology (drug ionic liquids and protein
stabilisation) and biofuel processing.
Douglas R. MacFarlane
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Dr Jenny Pringle received her degree and PhD, on ionic liquids, at The
University of Edinburgh in UK before moving to Monash University in
2002. In 2008 she started an ARC QEII Fellowship, in association with
the ARC Centre of Excellence for Electromaterials Science (ACES),
which focuses her research on ionic liquids and plastic crystals into the
area of dye-sensitized solar cells.
Jennifer M. Pringle
Dr Patrick Howlett received his chemistry degree from Curtin University
in Perth in 1992. He then held various positions in industry before joining
CSIRO in 1999, and received his PhD jointly with CSIRO Energy
Technology and Monash University in 2004. He is now a Senior Research
Fellow within ACES, investigating corrosion resistant coatings, batteries
etc. and also developing advanced surface characterization methods,
including synchrotron techniques, to look at the formation of ionic liquid
interphases on reactive metal surfaces.
Patrick C. Howlett
Professor Maria Forsyth is the Associate Director of ACES. She has been
involved in the field of electro-materials science since graduating from
her PhD at Monash University in 1990 and taking up a Fulbright
fellowship at Northwestern University (USA) in the field of lithium
battery electrolytes. Her current research focuses on developing an
understanding of charge transport at metal/electrolyte interfaces and
within electrolyte materials. Overall, she has co-authored over 230
refereed publications.
Maria Forsyth
1. Introduction
The charge-rich medium dominated by electrostatic forces that ILs represent potentially has a significant
effect on the thermodynamics and kinetics of many processes. While the world of solution chemistry has
well developed concepts to deal with differences in polarity, hydrophobicity and hydrogen bonding, ionic
liquids add a new dimension to these property variables – charge density. In this dimension processes that
involve charge creation, separation, or reduction may be strongly influenced by an IL medium, as
compared to more traditional solvents. This would include many, if not most, electrochemical processes
and 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 chemistry
provides a substantial starting point for this endeavor,1 however the organic nature of the IL medium
introduces a much wider range of intermediate properties and allows for a much more extensive
exploration 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 their
early development, in particular in various device and materials processing applications,2,3 for example
electrodeposition.4 The reasons for the interest in these applications stem from the unique set of properties
that certain ILs offer as electrolytes. The fact that they are intrinsically ion conductors and, in the aprotic
cases, 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 wide
electrochemical windows of stability, reaching in some cases to below Li/Li+ potential in the reductive
region and to well above +2 V vs. Ag/Ag+ in the oxidative region. Such electrochemical stability then
makes these liquids of interest as electrolytes for lithium batteries and also for the electrodeposition of
important metals such as aluminium and titanium. The same properties have also allowed very stable
electrochemical cycling of the intrinsically conductive polymer electrodes that are being investigated for
applications including batteries and actuators.
With these important applications in mind it is of some significance to investigate in more detail the
influence the ionic liquid may have on the electrochemical process of interest. Our goal here is therefore
to address questions such as: “How do electrochemical processes in ionic liquids compare with those in
other 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 arises
because the field is generating an ever increasing suite of ionic liquids of quite divergent properties. The
organic nature of the molecular ions allows, in some cases, nanoscale heterogeneity to be present in the
liquid state separating highly charged regions from distinctly hydrocarbon regions. Equally, ion pairing and
aggregation can, in some cases, produce ionic liquids of distinctly low “ionicity”;5,6 in other words the ions
are not able to move and act independently such that measures of free ion concentration fall well short of
100%. Such examples can therefore be thought of as intermediate in properties between true ionic liquids
and true molecular solvents.7
This paper will consider these questions as applied to a number of electrochemical phenomena of
importance in electrochemical applications, using some recent examples as a context. We begin by
examining the influence of the ionic liquid medium on the thermodynamics of the processes as manifested
in their redox potentials. Rigorous data is scant but sufficient for some trends to be apparent. In some
cases where a metal ion is involved in the process of interest this requires us the consider the metal-ion
speciation that is present in the ionic liquid. We then proceed to discuss these basic questions as applied to
a number of practically important electrochemical deposition reactions where surface film formation and
deposit morphology are important aspects of the processes. Thus we examine ionic liquid effects in
examples including lithium electrochemistry, CdS semiconductor deposition and intrinsically conductive
polymer formation. In each of these cases there is direct chemical involvement of one or both of the ionic
liquid ions and thus we emphasise this active role and the extent to which it is dependent on the chemistry
of the ions involved.
Our approach here is to provide some perspectives on the phenomena involved, based to an extent on
examples from our own laboratories. Our aim is not to provide a thorough review of these topics since that
would require a very large article; rather we aim to provide an overview of the effects and some views as
to 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 represents
the “solvent” for that species. Given the distinctly different solvent environment that an ionic liquid
represents, as compared to an aqueous or non-aqueous solution, it is of fundamental importance to inquire
into possible differences in redox potentials of otherwise well understood redox reactions. The question
amounts to asking whether there are differences in the “solvent” environment, and hence free energy of
reactants and products, that could produce a significant shift in redox potential overall. The greatest
likelihood 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 of
these questions via simulation studies and concluded that Marcus theory applies equally well to the ionic
liquids studied and that solvent reorganization free energies were quite similar to those calculated for
CH3CN, 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 study
carried out by Zhang et al.10 on the redox behaviour of the polyoxometallate species Įí[S2W18O62]4í which
undergoes a series of stepwise reversible oxidation reduction reactions (Fig. 1) to eventually reach the í10
anionic species. A careful study in a variety of solvents, including several ionic liquids, was carried out
and showed a very clear and strong solvent dependence that became more pronounced as the charge on
the 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 sí1 of 5
mM(Bu4N)4[S2W18O62] in [C4mim][PF6] at 20 °C
using a GC electrode (from ref. 10). Numbers
indicate individual steps in the reduction of the -4
species, eventually to the -10 species.
Table 1 Comparison of reversible potentials of the various stages of reduction of the Įí[S2W18O62]4í anion
in Ionic Liquids and various molecular solvents
Solvent/electrolyte
E°ƍ/V vs. Fc+/Fc
4-/55-/66-/77-/88-/99-/100.02
í0.257
í0.72
í1.005
í1.373
í1.628
[C4mim][PF6]
í0.001
í0.249
í0.701
[C2mim][NTf2]
0.023
í0.263
í0.778
í1.120
[C4mpyr][NTf2]
í0.01
í0.287
í0.736
í1.000
[C4mim][BF4]
í0.437
í0.802
í1.357
í1.745
DCM/Bu4NPF6
í0.245
í0.628
í1.199
í1.595
í2.094
í2.464
CH3CN/Bu4NPF6
0.02
í0.2
í0.64
í0.89
í1.08
í1.20
H2O/Et4NCl
For example, E0ƍ for the í7/í8 redox process ranged from í0.89 V vs. Fc+ /Fc for an aqueous solution
to í1.75 V vs. Fc+/Fc for a dichloromethane solution†. This is a substantial range corresponding to almost
100 kJ molí1 in reaction free energy. The E0 values for the ionic liquid based solutions were clustered
fairly tightly around í1.0 to í1.1 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 solution
data is due to the much more effective shielding of the additional charge that the process introduces into
solution. The ionic liquid result, though concordant with the computational results of Lynden-Bell
discussed above, is slightly perplexing since the dielectric constants of ionic liquids11,12 are not thought to
be particularly high and certainly less than that of acetonitrile. However, this illustrates the fact that
dielectric shielding is not a useful concept in this respect when an ionic liquid is involved.9 In the ionic
liquid a local reorganisation is also possible in the immediately surrounding shell of ions13 in response to
the change in state of charge that takes place in the redox reactions of Fig. 1. Lattice Madelung constant
calculations on a regular lattice of the sort recently discussed by Izgorodina et al.14 also suggest that the
electrostatic component of free energy depends on a Madelung factor that is determined by the size of the
IL ions and their state of association. Thus it is possible for ionic liquids to “shield” additional charged
species to an extent that is as effective as the dielectric shielding in aqueous solutions. It is important to
note, however, that the effect is not dipolar in its origins.
When redox processes between much lower charge species such as the I3í/Ií couple are involved, the
effects are much smaller. This particular redox process is of considerable applied interest in the context of
dye sensitized solar cells where it provides the redox shuttle between the counter electrode and the
oxidized dye species. As far as we are aware a detailed investigation of the dependence of the redox
potential of the I3í/Ií couple in ionic liquids has not been published. Many effects involving the electrolyte
in the solar cell influence the potential and hence the effect of the redox couple cannot be resolved from
full cell studies; careful comparative measurements on the cathode reaction are very much needed.
The abovementioned theoretical and experimental studies have involved redox reactions carefully
chosen to not interact in any chemical way with the IL. However in the case of many metal ions there is
strong likelihood of coordination type interactions between the metal ion and the anion, even those
considered to be very weakly basic.15 This metal ion speciation becomes a very significant matter in
understanding ionic liquid electrochemistry. A recent example of such a shift in redox potential can be
seen in work16 where the reduction of Cd(II) in a phosphonium tosylate ionic liquid, into which CdCl2 was
dissolved, was observed at í0.45 V vs. Ag/AgCl at 100 °C. In aqueous solution the Cd(II) reduction
process would be expected at í0.2 V vs. Ag/AgCl. This apparent stabilization of the metal cation could
be envisaged to result from Cd(II) complex ion formation in the ionic liquid utilizing the IL anion to form
species such as [CdCl2tos2]2í. These species will have distinctly different redox potentials compared to the
aquo ion. To the extent that the complex ion in the ionic liquid is more stable than the corresponding aquo
ion, the reduction potential will be more negative. Of course, similar shifts in redox potential are also
possible in aqueous solutions involving coordinating species.
The existence and impact of such speciation was investigated in both our laboratories17 and those of the
Endres group18 recently in respect of the Al(III) ion in NTf2 ionic liquids. In this case a variety of complex
ions of the type shown in Fig. 2 were identified by 27Al NMR, Raman spectroscopy and ab initio studies
and were proposed as having a significant impact on the electrodeposition of Al from the mixture. In some
cases the nature of the complex renders the species not reducible within the range of potentials accessible
in the IL electrolyte. Equally they have a strong effect on the phase behaviour of mixtures of AlCl3 with
the NTf2 IL, in some cases a liquid–liquid phase separation occurring, with one of the phases being richer
in the reducible Al(III) species. The existence and role of such complex ion species in ionic liquids is a
theme 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 ion
species of overall stoichiometry AlCl(NTf2)3 formed
in NTf2 IL solutions of AlCl3 (from ref. 17 ESI).
Similarly, the Compton group19 has provided some useful perspectives on the practically very important
Li+(IL) + eí = Li(s) reaction, showing that the Li+/Li couple has a formal potential of í3.2 V vs. Fc+/Fc
(approx í3.9 V vs. NHE) in NTf2 ionic liquids. This is only slightly less negative than in THF which is
known to coordinate strongly to Li+, hence suggesting quite significant complexation of the Li+ in this
ionic liquid by the anion.20 However, the role of surface layers in lithium electrochemistry is also very
important, and may impact significantly on the redox potential of the overall electrochemical process, as
discussed in detail below.
In summary, then, it seems that ionic liquids can have a number of effects on redox potentials. By
shielding charges very effectively, the ionic liquid can serve to stabilize charged species, especially when
the charges are large, and hence alter the overall thermodynamic driving force in the reaction. In cases
where complex-ion species can form, for example with metal cations dissolved in the IL, the ionic liquid
ions can be significantly involved in the metal speciation, mimicking the effect of ligand complexation of
metal ions in aqueous solution. The presence of such species can have a substantial effect on the observed
redox 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 of
great practical interest. Such films have implications for a wide range of applications including
electrodeposition, energy storage, energy conversion, electrocatalysis, tribology and corrosion.
Furthermore, the opportunity to study a molten salt interacting with a surface (a charged metal surface or
indeed any other surface) at room temperature, and hence with a far greater range of materials and
conditions than was previously accessible in traditional molten salts, has spurred substantial interest, both
fundamental and practical.21–24
Initial interest in surface film formation arose from studies of IL electrolytes for lithium metal
electrodes. It is widely accepted that lithium electrochemistry is dominated in most cases by the presence
of 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 SEI
has variously been argued to be necessary25or problematical26 to the establishment of a lithium metal based
battery. The study of IL electrolytes for use with active metal electrodes such as lithium, raises interesting
questions with regard to the formation and action of an SEI in ILs. The following section will summarise
current 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 been
those based on quaternary ammonium cations with a NTf2 anion.27–30 The first report of the observation of
SEI formation in these ILs was presented by Katayama31 and we subsequently reported similar
observations 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 dominated
by LiF, Li2S, Li2O, Li2S2O4etc. These findings were not surprising in light of the extensive reports of
Aurbach who had previously investigated LiNTf2 electrolytes in a range of aprotic solvents and had
presented a detailed analysis of the anion decomposition and the resulting SEI properties.33,34 There have
now been many reports of the formation of these types of salt films on electrode or active metal
surfaces35–43 and their presence and importance seems to be widely accepted. However, there are still
questions regarding the mechanism of formation and the origins of the species making up the film, which
are important both fundamentally and in application. Aurbach presented the following reaction scheme to
account for the decomposition of the NTf2 anion in the presence of lithium.33,34
LiN(SO2CF3)2 + neí + nLi+ ĺ Li3N + Li2S2O4 + LiF + C2FxLiy
LiN(SO2CF3)2 + 2eí + 2Li+ ĺ Li2NSO2CF3 + CF3SO2Li
Li2S2O4 + 6eí + 6Li+ ĺ 2Li2S + 4Li2O
Li2S2O4 + 4eí + 4Li+ ĺ Li2SO3 + Li2S + Li2O
Based on our findings and the reports of Aurbach et al.,33,34 we undertook to confirm and elucidate the
mechanism of SEI formation via NTf2 anion decomposition in the IL.44 Using electrochemical and
spectroscopic techniques, along with supporting ab initio calculations, it was indeed shown3,45,46 that the
decomposition of the NTf2 anion was a possible mode of film formation. Furthermore, the rate of
decomposition and the amount of product formed was shown to be strongly influenced by the electrode
substrate and the presence and concentration of contaminant levels of H2O.
A further complication arises when the activity of substrate oxide/hydroxide surface species is
considered. As already shown,44 there is a marked dependence of the decomposition process on the type
of substrate e.g., Pt, Ni or Cu. Thus, there is an open question as to whether these processes would occur
on an ‘active’ substrate (e.g., Li with its ubiquitous, native surface film), even in the complete absence of
water and oxygen. In fact, we have recently shown the strong interaction of a number of ILs with
hydroxyl functionalized ceramic surfaces (e.g., SiO2 and MgO/OH2 particles) using multinuclear
solid-state MAS NMR.47,48
Most recently, Passerini et al.49,50 have suggested another possibility for the origins of the surface
species 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 be
removed by heating to decomposition and then washing in anhydrous acetonitrile. This is an attractive
possibility given that it is apparently relatively easily solved and it seems to account for all of the observed
behaviour. It is also possible that the same impurity has been present in the majority of NTf2 based
electrolyte systems studied in laboratories around the world, since the NTf2 ion has normally been
obtained, by researchers and ionic liquid producers alike, from a single source.51 Further study and
exploration 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, recent
work with ‘ultra-pure’ ionic liquid electrolytes by Endres et al.,52,53 and by Passerini et al.,22,54,55 of the
effects of O2 and H2O (and other) contaminants has challenged some of the original reaction mechanisms
proposed by Aurbach. This recent work has shown that, under conditions of negligible concentrations of
O2 and H2O contaminants, no anion decomposition occurs at the lithium reduction potential. This would
indicate that if the level of atmospheric contaminant can be maintained at sufficiently low levels, then
anion decomposition can be avoided allowing many important electrodeposition applications to be
realized. Thus it seems that appropriately pure ILs of this type now offer the possibility of studying the
lithium electrodeposition/dissolution reaction in the absence of an SEI. This represents a much sought
after and important opportunity to understand the role of the SEI properties in the problem of lithium
dendrite formation.
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Another advantage that may be gained if the SEI can be eliminated lies in the reduction in ohmic
resistance in the cell that could be obtained, given the large impedance contribution produced by these
films, e.g., resistivities of 107–108 ȍ cm at 25 °C in a C3mpyrNTf2 ionic liquid.32 However, it is important
to note that the magnitude of this contribution is questioned, with some authors postulating a larger
resistive contribution from the compact double layer in these materials produced by the arrangement of
positive IL cations at the charged surface; these present a barrier or screen which may retard the transport
of ions to and from the electrode surface.56,57 The physical detection of these layers using an AFM tip at a
negatively charged surface supports this hypothesis.58 Similarly, Sugimoto et al.59,60 discuss the relative
ease or difficulty of desolvation from an ionic cage as a source of interfacial impedance. Unfortunately it
is difficult to deconvolute these various contributions to the overall impedance response of an active
electrode 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 the
generated surface film to protect metal surfaces from corrosion. This is effectively the role of the SEI in
lithium cells. Corrosion in general is, of course, an electrochemical phenomenon whereby the metal is
oxidized by its environment. Traditional approaches to solving or mitigating this enormously costly (both
in economic and energy terms) problem usually involve the use of some sort of protective surface film that
inhibits or alters the redox processes. Magnesium alloys were chosen as a focus in our work because of
their important engineering properties as light alloys for aircraft and vehicle components, but considerable
surface reactivity and hence poor corrosion resistance. After examining a range of NTf2 salts, it was found
that 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 the
corrosion current density by more than one order of magnitude compared to an untreated sample and
producing a cathodic shift of the equilibrium corrosion potential by 500 mV. Analysis of the film
composition (Fig. 3) revealed the presence of similar chemical components to those found on a Li surface
exposed 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 characterization
data indicates that the NTf2 anion decomposition process occurring on the Mg surface is similar to that
occurring on Li as discussed above,48,62–64 indicating a generic behaviour of this anion when exposed to
strongly negative potentials and the presence of oxygen and H2O.
Fig. 3 Depth profile plot of normalised ToF-SIMS
peak intensities acquired during gallium ion etching
of 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 for
film formation on magnesium alloys.65 This was motivated by literature reports of phosphating treatments
for Mg alloys66,67 and also by the attractive prospect of a range of inexpensive ILs which could be
systematically 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 with
addition of a small amount, 6 wt%, of water.68 The water was postulated to hydrate the native oxide
layer on the alloy surface to produce a more reactive surface for film deposition. Control of the film
properties and morphology has been made difficult by the varying surface features resulting from the
microstructure of the alloys. In many cases the corrosion mechanisms in these alloys are dominated by the
alloy microstructure.69 Surface preparation, for example use of a mechanical polishing or an
electropolishing technique, modifies the corrosion resistance of the alloy and was also shown to
subsequently modify the effectiveness of the IL surface treatment.70
Our present understanding of how these films develop and interact with the alloy is most clearly
illustrated by a recent publication48 which examines the formation of phosphonium NTf2 and
diphenylphosphate (dpp) surface films on Mg alloy ZE41 (a Mg–Zn-rare earth-Zr based alloy). ZE41
consists of the Į-Mg matrix containing finely distributed particles, thought to be ȕ-phase particles, and the
T-phase (Mg7Zn3RE) distributed at the grain boundaries (Fig. 4). It was shown that the Zr rich particles
within the grains dominated the early stages of corrosion in an aqueous NaCl solution.69 Thus, the
morphology of the alloy surface has regions of distinct compositional difference and the accompanying
oxide film also reflects these differences. On this surface it was found that the IL film deposition was
influenced 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., relatively
inert 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 exhibiting
stronger interactions, particularly with the dpp anion. In this case it is also evident that the adsorbed anion
causes the concomitant binding of the phosphonium cation via electrostatic attraction.47 Thus, at present
we consider these films to arise from chemical reactions that take place on the reactive metal surface and
also from strong interactions of the IL species (particularly the anions) with the surface active native
oxide film. In this respect, our picture of these films is one of a chemically modified and augmented oxide
film which generally has similar dimensions to the original native oxide surface.
Fig. 4 SEM image of the grain boundary region of a
ZE41 Mg alloy treated with P6,6,6,14dpp IL for 24 h,
which suggests that there is more IL interaction
next to the grain boundary areas (from ref. 48).
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Fig. 5 31P NMR spectra for P6,6,6,14dpp ionic liquid
pure and adsorbed onto inorganic powders; the
resonance at 32 ppm (associated with the cation) is
little altered in all cases while that at í11 ppm
broadens significantly in the case of Mg(OH)2,
indicating a strong, probably chemical, interaction
with the surface.
More recently we have found that improved surface film deposition on AZ31 and ZE41 in these ionic
liquids can be achieved under an applied potential.71,72 The interphase layer appears more resistive, as
shown 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 less
importance 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 surface
hydroxide which then may interact with the anion to form metal phosphate esters, or it is possible that the
application of a potential bias (negative or positive) may favour electrochemical processes such as metal
oxidation (e.g. Mg ĺ Mg2+ + 2eí) or IL oxidation/reduction that then facilitates the deposition of an
insoluble metal salt on the alloy surface. This observation opens up the possibility of IL film deposition on
not only magnesium alloys, but other reactive metals such as aluminium and titanium alloys. This could
lead to improved durability of these alloys and hence allow their use in a variety of engineering
applications in the infrastructure, automotive, aerospace, electronic and biomedical industries; areas in
which 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 to
be a useful medium for the preparation and growth of semiconductor films. The Endres group have very
much led the way in this field.73–76 The ionic liquid in this context often represents a stable, ion-conducting
medium for operation at elevated temperatures, with usefully controllable solvation properties. However
the IL may also have a more direct influence on the chemical process involved, as exemplified by the
recent work from our laboratories with CdS.16 CdS and its analogues are semiconductors of great interest
in photovoltaic solar cells because their bandgaps and band energy levels are well suited to this
application. However high quality CdS films are relatively difficult to obtain at low cost. Electrochemical
methods have been investigated in aqueous and protic non-aqueous media, but have struggled to produce
films of high quality, in part because of the limited temperature range available in these media. In general
deposition temperatures above 100 °C tend to produce more defect-free films. Contamination of the film
with the solvent and/or its reductive breakdown products also degrades the semiconductor properties. In
this case an IL medium of sufficient thermal and reductive stability, but relatively low cost, in the form of
a 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 is
relatively soluble in the tosylate ionic liquid, probably as a result of complex ion formation involving one
or more tosylate ligands, as discussed above. This serves to solubilize and stabilize the ion. To produce
CdS thin film deposits on the ITO electrode at high efficiency with respect to the Cd requires that either
the Cd(II) or S be generated at the electrode in a controlled fashion. In our case the reduction of S2O32í in
solution via S2O32í + 2eí ĺ S2í + SO32í 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 S2O32í to S2í
occurs at a slightly less negative potential than reduction of sulfur; the latter occurs close to the potential
of Cd(II) reduction and hence resulted in films contaminated with Cd(0). However, useful rates of S2O32í
reduction were possible without interference from Cd(0) and very high quality films were thus
produced.16Fig. 7 shows the degree of thin film control achievable from the process, the film thickness
being directly proportional to the charge passed in the coulometric experiment. Of particular note in terms
of semiconductor performance was that the electrodeposit was extremely well ordered, showing strong
signs of preferential ordering in the XRD patterns. Morphological effects such as this have been observed
in a variety of IL based electrodeposition processes, including the conducting polymer materials discussed
below, and seem to be a feature of the IL as a medium in this type of electrochemical reaction. In the case
of eletrodeposition reactions involving metal species this may be a result of the complexation of the metal
ion in solution and thereby the need to dissociate this complex at the electrode surface. This inserts a
potentially rate determining step in the overall process, mitigating the effects of diffusion which can
otherwise produce rough and dendritic deposits. The effect of complexing agents in influencing deposit
morphology in metal electrowinning generally is well known. Such effects underscore the need for more
detailed studies of complex ion equilibria and kinetics in ionic liquids.
Fig. 6 Electrodeposition of CdS from an
P1,4,4,4Tosylate ionic liquid. The process is based on
the reduction of the S2O3 anion in the ionic liquid
(black line). The reduction to sulfide appears to be
direct 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 thickness
electrodeposited at 130 °C from P1,4,4,4Tosylate ionic
liquid by reduction of the thiosulfate anion in the
presence of Cd(II) (from ref. 16).
Conducting polymers
ILs have repeatedly been shown to be extremely effective electrolyte media for the electrodeposition of
conducting polymers. Their wide electrochemical windows in some cases allows access to monomers that
are outside the range attainable using conventional molecular solvent systems, for example those
monomers containing electron-withdrawing groups such as 3-chlorothiophene,77 or those that are
extremely sensitive to moisture.78 For example, the synthesis of poly(paraphenylene) is normally restricted
to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely
anhydrous. 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 The
solubility of monomers in ionic liquids can also be an advantage.80 Murray et al.81 have used an ionic
liquid for the electrodeposition of poly(terthiophene) incorporating anionic dyes, for use in photovoltaic
devices, 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 is
probably one of the key benefits. Our early work showed that the use of ionic liquids as electrolytes in a
range of electrochemical devices utilizing conducting polymers that had been synthesized in conventional
solvent systems resulted in significantly improved lifetimes.82 Since then, a number of authors have
demonstrated that use of an ionic liquid for both the synthesis and utilization of the conducting polymer
can significantly benefit the electrochemical stability.83–86 For example, Deepa et al.87 have demonstrated
an increased electrochromic cycle life cycle life for poly(pyrrole) grown in an IL compared to those grown
in acetonitrile/LiNTf2, in addition to a higher conductivity (due to a longer conjugation length), better
coloration efficiency and charge storage capacity. The synthesis of PEDOT in an ionic liquid has also
been reported to yield significant improvements in stability, in addition to currents seven times greater
than for films synthesized in an acetonitrile electrolyte, making them of interest for electrochemical
capacitor applications.85
While there are increasing reports in the literature regarding the use of ionic liquids for the synthesis of
conducting polymers, there is still a lack of understanding regarding the fundamental influences that these
new media have on the pertinent redox processes and, thus, insufficient conclusive evidence regarding the
benefits of using ILs as the deposition media. An observed increased rate of polymerization in an ionic
liquid compared to a molecular solvent has been attributed to the high viscosity of the ionic liquid, which
improves 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 in
the morphology of films grown in ionic liquids is now a common observation;86–91 We have observed
significantly smoother poly(pyrrole) films grown in an ionic liquid compared to a molecular solvent.92 Our
studies of EDOT electropolymerization in two different ionic liquids compared to two conventional
acetonitrile-based electrolytes suggest that the process is initially much slower in the latter. The data
suggests a different mechanism of deposition; the current transient in acetonitrile/Bu4NClO4 is indicative
of progressive nucleation, with a slower growth rate, whereas the current transients in the ionic liquids
suggest instantaneous nucleation and increased growth rate, thus indicating a strong influence of the anion
on polymer growth.89
Electrodeposition of conducting polymers by cyclic voltammetry commonly reveals significant
differences in the growth CVs in the different systems,80,88,89,91–94 although the interpretation of these
results vary. The post-polymerisation CVs of the polymers can also be significantly different to those of
films grown in molecular solvents,80,85,87,89 and the stability can be greatly increased.83–85 However, it is
important also to note that the nature of the cycling electrolyte can have an effect on the redox response
of the polymer—as suggested in the introduction, the ionic liquid is not merely passive, but is actively
involved in the various processes. Redox cycling of a conducting polymer results in counter-ion
incorporation or expulsion and in a conventional molecular solvent/electrolyte system anion movement
dominates 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 involve
either the cation or the anion, and hence the nature of both of these species can significantly influence the
electrochemical 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 mechanism
likely depends on the relative mobility of the component ions,96 while the viscosity and conductivity of the
medium, and its ability to swell the polymer film, can also have an impact. EQCM studies have shown that
for 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 electrochemical
activity upon cycling in an acetonitrile solution compared to an ionic liquid, suggesting better swelling of
the polymer and thus faster transport of ionic species into and out of the polymer during cycling. On
return to the ionic liquid there was a rapid return to the lower charge capacity regime, but there was then a
progressive increase in current with cycling in the ionic liquid, as also observed by Randriamahazaka et
al.98 In agreement with other authors,99 we observed no memory effect upon cycling the films in these
different solvents.
Poly(pyrrole) can also be electrodeposited from ILs onto iron, a corrosion-susceptible electrode,86 and
although relatively high potentials (1.3 V vs. a Ag wire quasi-reference electrode) were required, the
polymer was electroactive and conducting, indicating negligible polymer overoxidation problems
compared 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 films
prepared 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 the
electrodeposition and redox cycling behaviour of conducting polymers in a number of ways, although the
exact mechanisms are not yet fully understood. The morphology of the polymers can be significantly
smoother; this is influenced by both the viscosity of the ionic liquid and also the nature of the ions. The
rate of polymerization may be altered; in an ionic liquid the availability of the anion, which becomes
incorporated in the polymer as the counterion, is not limited by diffusion, as it can be in molecular solvent
systems, and thus the polymer growth rate is only limited by the availability of the monomer. The higher
viscosity of the IL may trap oligomers near the electrode and thus improve redox–redox coupling, but
ultimately the rate of polymer growth will be limited by diffusion of the monomer and we have observed
that growth of thick polymer films can be significantly slower in an ionic liquid. There is little evidence to
suggest 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 electrochemical
processes of the rather unique chemical environment that the ionic liquid medium represents. With
molecular solvents as one extreme and classical molten salts as the other, ionic liquids lie in various
positions between these extremes with some measure of electrostatic charge density as the yardstick. The
properties of such coulombic media can impact on electrochemical processes in a number of ways
including (i) on the redox potentials of electrochemical reactions taking place in the IL, (ii) by allowing
access to domains of electrochemical potential and/or temperature not accessible in other solvents and
thereby novel reaction processes, and (iii) via a significant involvement in the process of interest by direct
chemical participation, for example by forming metal-ion complexes. All of these effects are of interest in
the various applications currently being investigated for ionic liquids in the electrodeposition field such as
advanced lithium batteries, electrodepositon of metals and semiconductors. They also have an indirect
importance in applications such as tribology where the extreme pressure regime of metal surface
lubrication almost certainly involves the redox chemistry of the metal coupled with the electrochemical
and thermal breakdown of the ionic liquid.
Nonetheless, there is much yet to be understood about these effects and it is the hope of the authors
that 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 redox
potentials. As discussed in section 2 the variation in redox potential can correspond to as much as 100 kJ
molí1 difference in reaction energy between ionic liquids and other aprotic solvents. That the redox
potentials of the reaction processes studied tend to cluster around similar values irrespective of the ionic
liquid suggests that there is a generic, though not necessarily IL independent, shielding effect of the
electrostatic environment on the activity of the redox active species. Further investigation of these effects
in model systems by standard electrochemical methods is clearly needed. Additionally, factors that control
reactive 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 just
beginning to appear and more of this type of work is to be strongly encouraged, especially where multiple
electrochemical and spectroscopic techniques can assist in deconvoluting the effects. The role and
properties of the electrochemical double layer have not been discussed in any detail here since this will be
reviewed elsewhere in this volume. There is little known about how the presence of the compact double
layer at the ionic liquid—electrode interface influences electrochemical processes. Certainly one can
expect that the activity of complex ion species involving the ionic liquid anion, such as those postulated to
be involved in the Cd, Al and Li examples discussed above, will be strongly influenced by the varying
composition of the double layer region. All of these fundamental issues require much further in-depth
exploration and we commend these matters to those with the tools and skills to undertake the appropriate
investigations and add to this important, growing field.
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Footnote
† In a discussion such as this which attempts to make broad observations about the effect of a wide range of ionic liquid media
and other solvents, reliable and meaningful reference potential scales become a substantial issue. Bond et al.101 have discussed
reference 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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