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Application of ionic liquids to the electrodeposition of metals

Andrew P. Abbott* and Katy J. McKenzie

Received 24th May 2006, Accepted 11th July 2006

First published as an Advance Article on the web 28th July 2006

DOI: 10.1039/b607329h

The electrodeposition of most of technologically important metals has been shown to be possible

from a wide range of room temperature molten salts, more commonly known today as ‘ionic

liquids’. These liquids are currently under intense scrutiny for a wide variety of applications some

of which have already been commercialized. Despite the fact that electrodeposition was the first

application studied in these liquids no metal deposition processes have as yet been developed to

an industrial scale. This review addresses the practical and theoretical aspects that need to be

considered when choosing ionic liquids for metal deposition. It details the current understanding

of the physical and chemical properties of these interesting fluids and highlights the areas that

need to be considered to develop practical electroplating systems. The effect of composition and

temperature on viscosity and conductivity are discussed together with the fundamental

approaches required to synthesise new liquids.

Introduction

Electrodeposition of metals is essential for a variety of in-

dustries including electronics, optics, sensors, automotive and

aerospace to name but a few. The main metals of interest

include Cr, Ni, Cu, Au, Ag, Zn and Cd together with a number

of copper and zinc-based alloys. The electroplating industry,

which dates back well over 100 years, is based solely on

aqueous solutions due to the high solubility of electrolytes

and metal salts resulting in highly conducting solutions. Water

does, however, suffer from the drawback that it has a relatively

narrow potential window, and hence the deposition of metals

with large negative reduction potentials such as Cr and Zn is

hindered by poor current efficiencies and hydrogen embrittle-

ment of the substrate. The main driving force for non-aqueous

electrolytes has been the desire to deposit refractory metals

such as Ti, Al and W.

High temperature molten salts have been extensively used

for the electrowinning of metals such as Li, Na, Ti and Al.1–3

It was the aim of developing lower temperature eutectics that

led firstly to the formation of LiCl/KCl/AlCl3 eutectics4 and

then in the 1950s to ethylpyridinium chloride/AlCl3 mixtures.5

Work by Osteryoung with the N-butylpyridinium cation,6 and

later by Hussey et al.7 using 1-ethyl-3-methyl-imidazolium as a

cation really led to the first concerted studies in the area of

ionic liquids. Wilkes and Zaworotko8 found that air- and

water-stable salts could be produced using tetrafluoroborate

or ethanoate anions. From these studies the arbitrary defini-

tion arose of an ionic liquid being an ionic material with a

melting point below 100 1C. This low melting point arises

because the salts have large, non-symmetrical organic cations

and hence low lattice energies although the relationship

between structure and phase behaviour is still not fully

understood.

The history and chemical properties of these liquids are

covered in several almost legendary reviews.9–12 While appli-

cations of ionic liquids have been proposed which vary from

fuel desulfurization13 to precious metal processing14 very few

have come to practical fruition although several are at pilot

scale. The main processes that use an ionic liquid are BASF’s

BASIL process15 and the Dimersols process.16 The former

uses the ionic liquid as a phase transfer catalyst to produce

alkoxyphenylphosphines which are precursors for the syn-

thesis of photoinitiators used in printing inks and wood coatings.

The phosphines are prepared by the reaction of phenyl-

chlorophosphines with alcohols and the imidazole acts as a

proton scavenger. The latter uses a Lewis acid catalyst for the

dimerization of butenes into C8 olefins which are usually

further hydroformylated giving C9 alcohols used in the

manufacture of plasticizers.

By far the largest academic activity has been with chemical

synthesis and metal deposition and while the lack of commer-

cialized processes naturally has some economic reasons many

of these will decrease as production scales are increased. In the

field of metal deposition there are still key issues to be

addressed to obtain a deposit with the correct morphology

and physical properties and it is in an endeavor to explain the

origin of these discrepancies that this review focuses on the

aspects involved in metal deposition from ionic liquids. It is

the aim of this review to look at the more fundamental aspects

of metal deposition and question the areas that need to be

addressed before practical use can be made of these interesting

systems.

Why use ionic liquids for electrodeposition?

Initially the main drive to use ionic liquids was the ability to

obtain high concentrations of aluminium in a highlyChemistry Department, University of Leicester, Leicester, UK LE17RH. E-mail: [email protected]; Fax: +44 116 252 3789

This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 4265–4279 | 4265

INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics

conducting aprotic medium for aluminium deposition.17 As

ionic liquids have developed, the key advantages of the liquids

have become:

� the wide potential windows,

� high solubility of metal salts,

� avoidance of water and metal/water chemistry and

� high conductivity compared to non-aqueous solvents.

Due to their low vapour pressures ionic liquids are well

suited to perform electrodeposition at a range of temperatures.

At elevated temperatures there is less concern about viscosity

and conductivity and phenomena such as nucleation, surface

diffusion and crystallisation associated with metal deposition

can be accelerated. The use of ionic liquids heralds not only

the ability to electrodeposit metals that have hitherto been

impossible to reduce in aqueous solutions but also the cap-

ability to engineer the redox chemistry and control metal

nucleation characteristics. It is the latter area that is only

now being addressed and will no doubt be the focus of

research over the forthcoming decade.

Several recent reviews have covered the use of ionic liquids

in electrochemistry17–21 and the most comprehensive of these

is the book by Ohno,22 which not only covers the fundamental

aspects but also a wide range of applications including lithium

batteries, photoelectrochemical cells, fuel cells and capacitors.

Reviews by Compton23 and Endres are also recommended for

their coverage of fundamental electrochemistry and electro-

deposition.20,24 For ionic liquids to be used efficiently, they

need not only be economically viable but also simple to handle

and recyclable. While it is impractical that ionic liquids will

compete with all aqueous solutions in large markets where the

current technology works effectively, it is realistic that they

should proffer an alternative for the deposition of metals that

can only currently be applied using vapour deposition. It is

also realistic that ionic liquids could have applications in such

fields as electroless deposition where the chemistry of aqueous

solutions limits the systems that can be studied. Ionic liquids

could also present an alternative to environmentally hazar-

dous processes such as the use of chromic acid for the

electrodeposition of chromium or the use of cyanide with

silver plating.

It is generally accepted that the cation is more important in

controlling the physical properties of the salt whereas the anion

has a greater effect upon the stability and chemical reactivity.

Cations

Table 2 shows that cationic structure and size affect the

viscosity and conductivity of the liquid and hence will control

mass transport of metal ions to the electrode surface. Cations

will also be adsorbed at the electrode surface at the deposition

potential and hence the structure of the double layer is cation

dependent. While phosphonium25,26 and sulfonium27 based

ionic liquids have been described, the vast majority of systems

used for metal deposition have been based on nitrogen-based

cations and these will be the main ones considered in this

review. Table 1 shows the range of nitrogen-based cations that

have been studied. Imidazolium based cations have been

favoured due to their superior fluidity and conductivity and

of these 1-butyl-3-methylimidazolium is the most preferred

due to its high conductivity.28,29 Much of the preliminary work

was performed with pyridinium cations.30,31 A range of pyr-

rolidinium based ionic liquids have been described recently

and while these have not yet been extensively utilised for

electrochemistry their wide potential windows will certainly

make them useful.32–34

Ionic liquids with alternative cations such as those derived

from biodegradable imidazoles,35 lactams,36 amino acids37

and choline38 have been prepared although it is only the last

of these which has been used for metal deposition. These

alternatives may eventually present viable alternatives for

larger scale applications due to either simplicity of synthesis

or availability of starting materials, however issues such as

narrower potential windows and lower conductivities will have

to be overcome. From an environmental viewpoint, choline

based salts are the most promising since choline chloride is

made quantitatively in a one-pot, solvent-less process using

HCl, ethylene oxide and trimethylamine.

The other important role that the cation probably plays in

electrodeposition is in controlling the structure and most

importantly the Helmholtz layer thickness. This area is prac-

tically unstudied although Endres39 has shown that the cation

appears to control the morphology of aluminium deposited

from various triflamide based ionic liquids.

Anions

Anions will also affect the conductivity and viscosity of the

ionic fluids (see below). The anion can also affect the coordi-

nation geometry around the metal ion, which affects the

reduction potential, reduction current and nucleation. This

area is almost unstudied although Katayama40 has collated the

results for various metal salts in chloroaluminate melts. It is

imperative that systematic studies are also carried out into the

thermodynamics of metal ion reduction in ionic liquids with

nominally non-coordinating anions.

Ionic liquids can be divided into two major classes; those

based on discrete anions and those with anionic complexes.

These two classes of compounds have quite different charac-

teristics, which will lead to different applications. Both types of

liquid are tuneable and the variables are largely common to

both systems.

Discrete anions

Table 1 shows a range of discrete anions that have been used.

It is an interesting observation that despite their short history

there are more ionic solvents that have been reported and

characterised than molecular ones.

The large range of anion–cation combinations possible

means that the physical and chemical properties can be

tailored to the specific application and this is the subject of a

corresponding review by Endres.24 The ability to combine an

inert cation and anion by simple metathesis is viewed as being

a key advantage of these types of liquids. The ionic liquids

with optimum conductivity and viscosity are generally highly

fluorinated ensuring good shielding of the charge from the

cation.

4266 | Phys. Chem. Chem. Phys., 2006, 8, 4265–4279 This journal is �c the Owner Societies 2006

Anions such as BF4� and PF6

� were initially used quite

extensively because of their wide potential window,41,42 how-

ever, slow hydrolysis by water, yielding HF, has led to an

increase in the use of water stable anions such as

(CF3SO2)2N�.43 These have even larger potential windows

and have higher conductivities and lower viscosities than the

corresponding BF4� and PF6

� salts. Other anions such as

(CN)2N�,44 p-toluenesulfonate and methylsulfonate32 have

also recently been described but have not been extensively

used for metal deposition.

One issue associated with the use of ionic liquids with

discrete anions is that they are new materials and one barrier

to commercialisation could be the registration costs of these

compounds. The large-scale production of some imidazolium

compounds has meant that some of the more exotic com-

pounds are now available for electrochemical applications.

Some of these new materials show such significant electro-

chemical stability that application to smaller-scale devices, like

batteries and capacitors, may nullify current economic bar-

riers. Some ionic liquids have potential windows in excess of

Table 1 A selection of cations and anions used to make ionic liquids

Nitrogen based cations Discrete anions

Name Abbreviation

1-Butyl-3-ethylimidazolium [BEIm]1-Butyl-3-methylimidazolium tetrafluoroborate [BMIm]BF4

1-Butyl-3-methylimidazolium hexafluorophosphate [BMIm]PF6

1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIm]Tf2N1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMP]Tf2N1-Butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BMMIm]Tf2N1-Butyl-3-methylimidazolium trifluoromethane sulfonate [BMIm]TfO1-Ethyl-3-methylimidazolium tetrafluoroborate [EMIm]BF4

1-Ethyl-3-methylimidazolium chloride [EMIm]Cl1-Ethyl-3-methylimidazolium dicyanamide [EMIm][dca]1-Ethyl-3-methylimidazolium hexafluorophosphate [EMIm] PF6

1-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [OMIm] TF2N1-Propyl-1-methylpyrrolidinium [PMP]Trifluoroacetate TFABis(trifluoromethylsulfonyl)imide Tf2NTrifluoromethanesulfonate TfO


5 V although this is strongly dependent on water content and

electrode material.22

Complex anions

Eutectic based ionic liquids can be expressed in terms of the

following general formula R1R2R3R4N+ X. z Y.

The ionic liquids described can be subdivided into three

types where R1R2R3R4N+ is for example a choline-like cation

such as HOC2H4N+(CH3)3, X is generally a halide anion

(usually Cl�). They are based on equilibria set up between X�

and a Lewis or Brønsted acid Y, z refers to the number of Y

molecules which complex X�.

e.g. 3AlCl3 + 2Cl� 2 AlCl4� + Al2Cl7

� or Cl� + 2

urea 2 Cl�.2urea.

Eutectic Type 1 Y=MClx, M= Zn,45,46 Sn,38 Fe,47 Al,17

Ga48

Eutectic Type 2 Y=MClx � yH2O, M=Cr,49 Co, Cu, Ni,

Fe

Eutectic Type 3 Y = RZ, Z = CONH2,50 COOH,51 OH

The relative proportions of anionic species depend on the

ionic liquid composition. The ability to vary the composition

of Lewis or Brønsted acid adds an additional dimension to the

tuneability of the eutectic-based ionic liquids. The anionic

species have been identified for Type I, II and III based

eutectics where Al,52 Sn,53 Zn,53 Cr49 and urea50 are the

complexing agents and some studies have quantified the

proportion of species present.52,53 They are also clearly useful

for electroplating if the metal of interest falls in the category

defined above under Type I or Type II as the metal ion

concentration can be as high as 10 mol dm�3.

There is some controversy over the classification of complex

anions as ionic liquids. It could, however, be argued that

discrete anions are just a subset of complex anions where the

Lewis acid and Lewis base are present in equimolar amounts

and the equilibrium constant lies considerably over to the right

e.g. R4N+ F� + BF3 2 R4N

+ BF4�

The term ionic liquids can be used to describe a wide range

of materials with similar properties. Authors such as Angell54

have shown that systems such as Ca(NO3)2 � 4H2O and

LiCl � 5.8H2O have some properties similar to ionic liquids

which opens the debate as to where ionic liquids finish and

solutions start. Inevitably practical plating systems might need

to have a significant molecular component either as a bright-

ener, diluent or complexing agent.

Synthesis of ionic liquids

While the number of ionic liquids has expanded almost

exponentially over the past 20 years it is evident that new

classes of salts will be developed in the coming years. It is

evident that just like molecular solvents where classes of

liquids such as alcohols or alkanes have specific applications,

so ionic liquids such as triflamides will have uses such as

deposition of metals with large negative reduction poten-

tials.55,56 The compounds listed in Table 1 are by no means

exhaustive and it has been estimated that there are more than

1018 ionic liquids.57

The synthesis of ionic liquids usually begins with the

quaternisation of an imidazole, pyridine, phosphine or amine,

for example, to produce the cationic component.58 The qua-

ternisation is carried out usually using an alkylhalide. Pre-

dominantly the halides are made by this method although

other salts such as CF3SO3 can also be made in this way.59 The

halide can then be exchanged for the desired anion by reaction

with a metal salt (most frequently silver e.g. R4N Cl + Ag

Tf2N - R4N Tf2N + AgClk), Brønstead acid or an ion

exchange resin. In industry Tf2N based liquids are commonly

made by acid/base titration from R4NOH and H-Tf2N or by

metathesis reaction with LiTf2N and R4NCl.

Synthesis of complex anions is trivial and involves

simple mixing of the quaternary ammonium halide with

a Lewis or Brønstead acid, generally with moderate

heating. An additional advantage of using complex anions

is the fact that they are simple mixtures of two components

so can be split to give the original components for recycling

or disposal.

Table 2 Physical properties of various ionic liquids

Salt g/mN m�1 R+/A R�/A P(r 4 R) Z/103 Pa s T/K

EMIm BF4 46.7 3.31 2.50 38 298EMIm TfO 39.2 3.31 2.97 45 293EMIm Tf2N 39.6 3.31 3.62 34 293EMIm (CN)2 42.6 3.31 2.66 16 298BMIm BF4 46.6 3.55 2.50 3.06 � 10�5 219 298BMIm PF6 48.8 3.55 2.78 7.85 � 10�5 450 298BMIm Tf2N 37.5 3.55 3.62 1.20 � 10�4 69 298HMIm PF6 43.4 3.81 2.78 1.06 � 10�4 585 298OMIm PF6 36.5 4.03 2.78 3.20 � 10�3 682 303Acetylcholine Tf2N 38.6 3.70 3.62 4.85 � 10�5 240 298BzCOOC2H4N(CH3)3 Tf2N 37.71 3.96 3.62 1.76 � 10�5 6570 298Me2N(Bz)(C2H4OH) Tf2N 39.51 3.72 3.62 3.28 � 10�5 762 298Me2N(C2H4Br)(C2H4OH) Tf2N 40.02 3.59 3.62 4.52 � 10�5 626 298(C4H9)4N picrate 26.4 4.13 3.66 5.29 � 10�3 51 364PhCH2mim Tf2N 40.8 3.73 3.62 1.99 � 10�5 110 298NaCl 98 1.02 1.81 0.836 0.7 1273Water 71.8 1.87 2.74 � 10�2 1.00 298CH2Cl2 26.54 2.62 0.141 0.39 303


Potential window

Before deciding which type of ionic liquid is most suitable for

the deposition of a given metal it is important to consider the

reduction potential of the metal and the potential window of

the liquid. This is not always trivial as reduction potentials are

significantly affected by the coordination of the anion to the

metal. The redox potentials for most metals have been collated

for chloroaluminate40 based liquids but insufficient data is

available for other anion systems. Preliminary evidence in

Type II eutectics shows that the relative redox potentials of

metals are related to the coordination geometry of the metal

centre. It has been shown that the difference in the reduction

potentials of metals with similar co-ordination geometry, e.g.

Pb(II) and Zn(II) are very similar to the difference in the

standard aqueous redox potentials.60 The reduction potentials

of metals with different co-ordination geometries e.g. Ag(I)

and Zn(II) are significantly different. Metals with octahedral

coordination geometries do not tend to give reversible electro-

deposition characteristics, however this depends on the ionic

liquid and the available anions as to whether a deposited metal

can be stripped away from the electrode surface.

The redox limits are related to the stability of the anion and

cation. In general most quaternary ammonium cations will

have relatively similar reduction potentials, although direct

comparison of literature values are complicated by issues of

purity, electrode material and reference electrodes. The reduc-

tion of quaternary ammonium cations produces a very reac-

tive neutral radical which can further decompose to

alkylamines, alkanes and a range of other products.61 Matsu-

moto62 showed that for various Tf2N� salts using a glassy

carbon electrode, the negative reduction limit decreased in the

order PrMe3N+ 4 PhMe3N

+ 4 EMI 4 BP whereas the

positive oxidation limit remained constant. Protogenic ionic

liquids such as EtNH3NO3 evolve hydrogen at relatively low

over-potentials and may therefore be less useful for most

electrodeposition experiments. The electrolytes have, however,

been found to be useful electrolytes for fuel cells.62,63

Discrete anions such as BF4�, PF6

� and Tf2N� are stable to

reduction. The largest potential window reported to date is

that of N-methyl N0-propyl piperidinium triflamide which was

found to be stable over a 5.8 V window.64 Most imidazolium-

based salts with discrete anions have potential windows in the

range 4 to 5 V and are stable down to potentials of �2.2 V vs.

Fc/Fc+,65 which is clearly advantageous for the deposition of

metals such as Al, Ti, Li and Na. The addition of water to

these systems, however, decreases the potential window be-

cause the water molecules will aggregate making hydrogen

evolution easier at the electrode surface. The effect of water

added to imidazolium salts is given by Schroeder et al.66 and

Matsumoto65 where decreases in the cathodic limit of ca. 2 V

are reported.

Katase et al. studied the deposition of Li, Mg, Ni, Zn, Al,

La, and Dy in trimethyl-n-hexylammonium bis(trifluoro-

methyl)sulfonyl)amide (TMHA-Tf2N), and found that despite

the wide electrochemical window (5 V) the presence of small

amounts of water caused some problems in metal electrode-

position.67 For Type I and II eutectics it is usually the

reduction of the metal that defines the negative potential limit.

Type I eutectics have been prepared with Zn, Sn, Fe, Al, Ge,

Ga and Cu halides. The more Lewis acidic the metal halide the

more negative is its reduction potential. Because the negative

reduction potential is related to the Lewis acidity of the liquid,

the relative proportions of metal and quaternary ammonium

salts will also affect the potential window. Type I haloalumi-

nate mixtures were some of the original ionic liquids made and

these systems have been studied in considerable detail. They

have the most negative reduction limits and potential limits

down to �2.6 V vs. Fc/Fc+ have been reported68 using ethyl

methyl imidazolium salts on a tungsten electrode in Lewis

basic chloroaluminate melts (xAlCl3= 0.4). While they have

clearly been studied for aluminium deposition Fe, Co, Ni, Cu,

Zn, Ga, Pd, Ag, Cd, In, Sn, Sb, Te, Pt, Au, Hg, Tl, Pb and Bi

have been deposited as pure metals andMg, Sr, Ti, Cr, Nd and

La have been deposited as alloys.64 The deposition of these

metals is mostly of academic interest since the water sensitivity

of the chloroaluminate melts will not make them a realistic

alternative. The deposition of alloys is, however, of consider-

able use because aluminium alloys are difficult to form parti-

cularly given the difference in reduction potentials between

aluminium and most alloying metals. A more comprehensive

review of chloroaluminate potential windows is given by

Matsumoto.65

The oxidative limit for Type I and II eutectics is generally

the evolution of chlorine although this is usually electrode

specific as many metal electrode materials will oxidise in

preference. Type II eutectics were developed to extend the

range of metals that could be included in ionic liquids.49

Relatively little is known about these systems and to date

the only metal that has been shown to be successfully depos-

ited from this type of liquid is chromium. The waters of

hydration play an important role in the stability and fluidity

of these liquids. The waters do not behave as bulk water and

the potential window is limited by the reduction of metal in the

chromium case rather than water. The metal can be reduced

with high current efficiency and this is unaffected by the

addition of up to 10 wt% water suggesting that the water is

strongly associated with the chloride anions or the metal

centre.

In general, eutectic based ionic liquids are considerably less

susceptible to the addition of water and unpublished data

from a number of workers in the field actually shows that

the presence of small amounts of water is actually beneficial

to deposit morphology. Even the chloroaluminate systems

are tolerant of moderate amounts of water.69,70 This is clearly

an important issue for the development of practical electro-

plating systems but the effect of water has not yet been

quantified.

For ionic liquids with large halide concentrations break-

down of metal oxides on electrode surfaces is prevalent. This

means that few metals are actually inert in these liquids and for

the vast majority of eutectic systems the anodic limit is not

governed by the liquid but rather by the oxidation of the

electrode material. Even metals such as Pt, Au, Al and Ti can

be oxidised in ionic liquids as the oxides are broken down in

high ionic strength liquids. This is more prevalent in eutectic-

based ionic liquids where the chloride ions act as good ligands

for the dissolving metal ions. This has advantages for metal


plating as it means that soluble anodes can be used for almost

all metals. This is particularly useful for aluminium as it

ensures that aluminium concentrations in solution remain

approximately constant and it minimises the handling of

moisture sensitive compounds. It also means that the over-

potential required to drive the deposition process is small. The

same has been shown for chromium deposition on a 500 ml

scale using chromium rod anodes, but further scale up would

necessitate the design of a different type of anode as chromium

sheet and rod are not available on a commercial scale. The

ease of metal dissolution means that most electrodes can be

pre-etched in situ in the ionic liquid such that subsequent

deposition leads to better adherence to the substrate. Endres

et al.71 have recently shown that the adhesion of aluminium to

mild steel is greatly enhanced by etching the electrode in situ in

the ionic liquid prior to deposition. While this is a well-known

technique in aqueous electrolytes it has seldom been applied to

ionic liquids. The technique could be used for substrates that

are difficult to electrodeposit onto, such as aluminium, and

while we have shown qualitatively that this is possible it

appears to be ionic liquid specific and the etch time and

potential are not trivial to optimise. The instability of anodic

materials will however present difficulties for systems that

require inert anodes e.g. for alloy deposition or electro-win-

ning applications and this is an important matter that needs to

be urgently addressed.

The electrodissolution of metals in ionic liquids has been

utilised for electropolishing of stainless steel.72,73 This was

performed using a type III eutectic where the hydrogen bond

donor is ethylene glycol. The process is essentially the same as

the sulfuric and phosphoric acid process currently operated

but the process is considerably more current efficient (ca. 90%)

and the electrolyte is non-corrosive and non-toxic. This has

been scaled up to 1.3 tonnes in a pilot plant, thus demonstrat-

ing that ionic liquids have significant potential for metal

finishing applications. It may also be that other metal finishing

applications such as anodising or chromating may lend them-

selves to this sort of approach.

Systems studied

To date the majority of technologically important metals have

been electrodeposited from ionic liquids with both discrete and

complex anions. The deposits have mostly been amorphous or

nano-crystalline. Examples of the metals studied are shown in

Table 3. A comprehensive review is given by Endres in a recent

article20 and aluminium17,18 eutectics are reviewed in a number

of other articles.22 The metals studied have been dependent on

the discovery of new ionic liquids. In the 1960s and 1970s the

study of ionic liquids was dominated by the deposition of

aluminium and some aluminium alloys, although the main

advances were made in the 1990s. Table 3 shows that the

majority of transition block elements have been studied in

chloroaluminate liquids and while they may not be practically

applicable to the deposition of pure metals the deposition of

aluminium based alloys would be technologically very impor-

tant if the processes could be optimised and scaled up to

obtain bright crystalline deposits of the desired composition.

Of significant note are studies of Al/Ti and Al/Cr alloys which

should offer good corrosion and wear resistance.74,75

The introduction of discrete anions in the 1990s8 heralded a

wider range of metals that could be electrodeposited and the

air and moisture stability of these systems makes them a great

technological advance. The reduction of viscosity to ca. 10 cP

(0.01 Pa s) has also highlighted them as potential candidates

for deposition of reactive metals such as Li and Ti.55,56

Conductivities of 10 mS cm�1 still represent a difficulty for

industrial plating applications particularly if dimensionally

stable anodes are used for the electrodeposition of metals with

large negative reduction potentials as it will necessitate the use

of large over-potentials with high current densities and this

will result in significant ohmic loss.

Studies of deposition in ionic liquids still need to address the

issue of material nucleation and growth and the identification

of suitable brighteners/levellers. Endres and Freyland have

used scanning tunnelling microscopy to investigate the funda-

mental issues of metal nucleation in a variety of ionic

liquids.76–78 Electrocrystallisation of Ni and Co and their

respective alloys with Al were studied in AlCl3/bmimC- and

bmimPF6. Clear differences were observed in the 2-D phase

formation of the Ni monolayer vs. Co island formation. This

was ascribed to the difference in interfacial free energies of

these two metals. Electrodeposition of the aluminium alloys

with Ni and Co was found to be very similar. In both cases,

codeposition starts at potentials positive of the pure alumi-

nium redox potential and as the Al content increases the grain

size of the alloy decreases.

Table 3 Examples of metals deposited from ionic liquids

Ionic liquid type

Discrete anions BF4� Cd,115 Cu,116 Sb,117 In,118 Sn,119 Pd/In,120 Pd,120 Au,121 Ag122,123,Pd/Au,121 In/Sb,119 Cd/Te,124 Pd/Ag125

PF6� Ag,123 Ge,76–78,126

(F3CSO2)2N� Li,127–129 Mg,130 Ti,131 Al,39 Si,132,133 Ta,134 La,135 Sm,135 Cu,136 Co,137 Eu,138 Ag,139–141 Cs142 Ga,143

Ga/As,144 In/Sb,145 Sn,146 Nb/Sn,147–149

Type I Eutectics AlCl3 Al,17,18 Fe,113,150–152 Co,152,153 Ni,152 Cu,152,154,155 Zn,156 Ga,157–158 Pd,113,159 Au,160,161 Ag,152,162,163

Cd,164 In,165 Sn,166 Sb,167 Te,168 Ti,169 Cr,170 Hg,171 Na,172 Li,173 Tl,174 La,175 GaAs,176,177 Pb178 andBi179

Al alloys with Ag,180,181 Fe,182 Mg,183 Mn,184 Ni,152,185,186 Cu,187 Co,188 Sr,189 Ti,190 Cr,191 Nb,192,193

Nd194,195 and La175,194,195

ZnCl2 Zn,45,196 Sn,197 Zn/Fe,198 Zn/Pt,199 Zn/Co,200 Zn/Sn,197,200 Cd,201 Zn/Cd,201 Zn/Cu,45 Zn/Te202

Zn/Ni,203Zn/Co/Dy204

Type II Eutectics CrCl3.6H2O Cr49

Type III Eutectics Urea Zn,106 Zn/Sn,106 Sn106 Cu,105 Zn/Pb,104 Ag,105

Ethylene Glycol Zn,106 Sn,106 Zn/Sn106


Very few studies have addressed the physical properties of

the films such as hardness and roughness, which are vital. One

report shows that nanocrystalline aluminium has improved

hardness over bulk aluminium but the number of such studies

needs to be increased to demonstrate the potential advantages

for metal deposition offered by ionic liquids.79

While Type II and III eutectics may appear to address

metals that can already be deposited from aqueous systems,

the ability to replace environmentally hazardous chemicals

such as CrO3 and KCN together with the ability to fine tune

the composition and morphology of alloys are clearly

advantageous to already mature market products. Before

attempting to optimise ionic liquids for electrodeposition

applications it is important to appreciate the ways in which

ionic liquids differ from aqueous solutions. The principle

factors are viscosity, conductivity, speciation, potential win-

dow, double layer structure, redox potentials and metal hy-

droxide/oxide chemistry. All of these factors will affect the

rate at which metal ions diffuse to the electrode surface and

the thermodynamics and kinetics of the metal reduction

process. These in turn control the nucleation and growth

mechanisms and affect the material morphology and it is the

fundamental mechanism of material growth that needs to be

understood. There are numerous parameters that can be

systematically varied to change the deposition characteristics.

These are:

� Temperature

� Cation� Anion

� Electrolytes� Metal salts

� Hydrogen bond donor

� Diluents

� Anode material

� Brighteners� Pre-treatment protocol.

� Complexing agents

Viscosity

One of the main differences between ionic liquids and aqueous

solutions is the comparatively high viscosity of the former.

Table 2 shows that viscosities are typically in the range 10–

500 cP (0.01–0.50 Pa s) and this has a concomitant effect upon

the diffusion coefficients not only of the metal species to the

electrode but also the counter ions and complexants away

from the diffusion layer. Fig. 1 shows the viscosity of various

ionic liquids with discrete and complex anions. It can be seen

that cation structure has a significant effect but the type of

anion is equally if not more important in governing mass

transport.

A number of groups have studied the effect of viscosity on

diffusion coefficient. By comparing the diffusion coefficients

obtained from conductivity and NMR studies some groups

have claimed to obtain information about the ion–ion inter-

actions.80 a more detailed analysis of the effect of diffusion

coefficient on voltammetry is given by Compton.81

Most new liquids characterise the viscosity as a function of

temperature and the majority find that it varies in an Arr-

henius manner with temperature, viz;

ln Z ¼ ln Z0 þEZ

RTð1Þ

where EZ is the activation energy viscous flow and Z0 is a

constant. Others have found that it obeys a Vogel–

Tamman–Fulcher relationship.82 A comprehensive study of

viscosity is that of VanderNoot82 and there are several collec-

tions of viscosity data in recent reviews.9,22,83

One method of analyzing the viscosity of ionic liquids is

using hole theory.84 This was developed for molten salts but

has been shown to be very useful for ionic liquids. We have

recently shown that the values of EZ are related to the size of

the ions and the size of the voids present in the liquid.51

The viscosity of room temperature ionic liquids is several

orders of magnitude higher than high temperature molten salts

due partially to the difference in size of the ions, but also due

to the increased void volume. At elevated temperatures, how-

ever, the viscosity decreases by more than one order of

magnitude.

It has been shown85 that hole theory can be applied to both

ionic and molecular fluids to account for viscosity. The

Fig. 1 Viscosity of various ionic liquids as a function of temperature

(A) discrete anions: triethylsulfonium, 1-butyl-3-methylimidazolium,

1-butyl-1-methyl pyrrolidinium and acetylcholine bistriflimide (data

taken from ref. 82) and (B) complex anions: choline chloride (ChCl)

with urea,50 ethylene glycol73 and CrCl3.6H2O49 and benzyl tri-

methylammonium chloride with AlCl3 (ref. 82).


viscosity of a fluid, Z, can be modeled by assuming it behaves

like an ideal gas, but its motion was restricted by the avail-

ability of sites of the ions/molecules to move into. Hence it was

shown that

Z ¼ m�c=2:12sPðr4RÞ ð2Þ

Where m is the molecular mass (for ionic fluids this was taken

as the geometric mean), �c is the average speed of the molecule

(= 8kT/pm)1/2) and s is the collision diameter of the molecule

(4pR2). The probability of finding a hole of radius, r, greater

than the radius of the solvent molecule, R, in a given liquid,

(P(r 4 R)) is given by integration of the following expres-

sion.84

Pdr ¼ 16

15ffiffiffipp a7=2 r6 e�ar

2

dr ð3Þ

where a = 4pg/kT and g is the surface tension. The good

correlation obtained between the calculated and measured

viscosities showed that it is valid to think of the viscosity of

fluids as being limited by the availability of holes. It is evident

from eqns (1) and (2) that decreased viscosity can be obtained

by decreasing the surface tension of the liquid i.e. increasing

the free volume, or by decreasing the ionic radius.

This explains why the ionic liquids with the lowest viscosity

have highly fluorinated anions. Comparatively little surface

tension data has been reported but work by Branco et al. and

Huddleston et al. showed that ionic liquids with fluorinated

anions have low surface tensions and low viscosities.55,86,87

The effect of the cation is less clear since in the case of the

imidazolium cation (I) as the length of the R group increases

the viscosity should initially decrease as the ion–ion interac-

tions and hence the surface tension decrease. However, as the

alkyl group increases in size its mobility will decrease due to a

lack of suitably sized voids for the cations to move into and

this explains the data presented by Tokuda et al. who showed

a minimum in viscosity for ethyl methyl imidazolium salts.29

Conductivity

The conductivity of an ionic liquid is also strongly dependent

upon temperature and in an analogous manner to viscosity it

is found to change in an Arrhenius manner

ln s ¼ lns0 �EL

RTð4Þ

where EL is the activation energy for conduction and s0 is a

constant. It has been noted that the empirical Walden rule84

(LZ = constant) is applicable to ionic liquids, where L is the

molar conductivity and Z is the viscosity. Deviations from the

Walden rule have previously been used to explain ionic

association in proton transfer ionic liquids.88,89 The Walden

rule is normally only valid for ions at infinite dilution where

ion–ion interactions can be ignored which is clearly not the

case in ionic liquids.

Since viscosity is limited by the availability of holes then

charge mobility can be considered as being equal to the

migration of holes in the opposite direction to that of the

ions. Since the fraction of suitably sized holes in ambient

temperature ionic liquids is very low (ca. 10�6) the holes can be

assumed to be at infinite dilution and migration should be

described by a combination of the Stokes–Einstein and

Nernst–Einstein equations.

l+ = z2Fe/6pZR+ (5)

where z is the charge on the ion, F is the Faraday constant and

e is the electronic charge. This would explain why so many

studies of conductivity in ionic fluids have noted that the

empirical Walden rule is valid. Since the Stokes–Einstein

equation is valid for both ions then the conductivity of the

salt can be determined since

L = l+ + l� (6)

an expression can be written for the conductivity, k

k ¼ z2Fe

6pZ1

Rþþ 1

R�

� �rMw

ð7Þ

where r is the density and Mw is the molar mass of the ionic

fluid. Hence all of the theories developed for limiting molar

conductivities in molecular solvents are also applicable to ionic

liquids where there is an infinite dilution of suitably sized holes.90

High temperature molten salts do not obey eqn (7) since the

fraction of suitably sized holes is much larger (ca. 0.5) and

hence ionic activity becomes an important factor and because

the ions are smaller than in the high temperature analogues the

ionic association constant will be significant.

Using eqns (2) and (7) it is possible to postulate the

minimum viscosity and conductivity that an ionic liquid can

achieve. To obtain an optimum conductivity and viscosity the

ion size and surface tension of the liquid need to be as small as

possible. It is therefore evident why the highest reported

conductivity for an ionic liquid to date is that of

EtNH3+NO3

� (ca. 150 mS cm�1 at 298 K).54 It is difficult

to foresee a system that could have smaller ions that would still

be a liquid with the obvious exception of protogenic liquids

such as sulfuric, formic and acetic acids which could be

thought of as extreme ionic liquids.

Added electrolytes

Aqueous electroplating solutions typically have conductivities

in the region of 100–500 mS cm�1 because they are mostly high

strength aqueous acids.91 This allows high current densities to

be applied with only limited ohmic loss. Ionic liquids have

significantly lower conductivities at room temperature and

besides increasing the temperature one way to increase the

conductivity could be to add a small inorganic cation such as

Li+ that could have an increased mobility compared to the

large organic cation. This could have an effect upon the

viscosity and freezing point of the liquid as the small cation

will be strongly associated with the anions, but the presence of

a small amount should be negligible. This has been attempted

by a number of groups particularly those developing lithium

ion batteries, but the effect on the conductivity has not been as

significant as expected.92,93 Other salts such as Na+ and K+

have negligible solubility in most ionic liquids. The addition of

electrolytes is clearly an area that requires considerable in-

vestigation in the future.


The addition of lithium ions will also have a considerable

effect on the structure of the double layer. Few studies have

been carried out on the structure of the double layer in an ionic

liquid but those that have tend to suggest that the models used

for aqueous solutions are inappropriate in ionic liquids.94,95

One study on imidazolium salts such as triflate, (F3CSO2)2N�

and BF4� suggested that a layered model of alternating anions

and cations was appropriate to describe the structure.95 Work

by Baldelli96,97 which probed the electric field at the ionic

liquid-electrode interface using sum frequency generation

spectroscopy and electrochemistry came to the conclusion that

the double layer is one-ion-layer thick. The double layer

capacitance in an ionic liquid is considerably smaller than in

an aqueous solution and less than that predicted by having a

perfect Helmholtz layer at the interface. One possible explana-

tion of this could be that ion pairs are present on the electrode

surface at all potentials.

Assuming that most metals are reduced at potentials nega-

tive of the potential of zero charge then the electrode must be

coated with a layer of cations at least 6–7 A thick. Since most

metal species are anionic these must be incorporated in or

close to the Helmholtz layer. Adding small ions such as Li+ to

an ionic liquid will decrease the Helmholtz layer thickness

considerably and should make metal ion reduction easier. This

should enhance nucleation and it has been shown qualitatively

to be the case for the deposition of chromium from an eutectic

mixture of chromium chloride and choline chloride. The

incorporation of up to 10 mol% LiCl led to a change of

deposit morphology from microcrystalline to nano-crystalline

and a change in visual appearance from metallic to black.98

Endres also studied the effect of LiF on the nucleation of

tantalum in 1-butyl-1-methylpyrrolidinium bis(trifluoro-

methylsulfonyl)imide. It was found that dense, thick, corro-

sion resistant coatings could be achieved from this electrolyte

system.99

An additional issue that has not been addressed is the

structure of the diffusion layer during the electrolysis process.

Silva100 has proposed that when the metal nucleates at the

electrode surface the layer close to the electrode surface

becomes depleted of metal complex and rich in counter anions.

In principle this could affect the coordination chemistry of

metal ions approaching the electrode–ionic liquid interface.

For instance, in the deposition of aluminium from a Lewis

acidic chloroaluminate liquid the diffusion layer will become

more Lewis basic and the aluminium species present52 could

change from Al2Cl7� to AlCl4

�. This would have a clear effect

upon the growth characteristics and may explain the tendency

for ionic liquids to form nano-crystalline deposits i.e. as the

Lewis acidity changes the rate of growth may decrease and the

rate of nucleation may increase in comparison. A study using

quartz crystal microbalance with probe beam deflection101

analysis could quantify the depletion of the diffusion layer of

metal and the build up of anions. There are still many

unanswered questions on nucleation and on the role of the

rigid and diffuse double layer, therefore many more experi-

ments are required.

An additional complexity will be that the cation will affect

the surface energy of the growing metal nucleus and adsorp-

tion could affect the relative energetics of the nucleation and

growth processes which would have a concomitant effect on

deposit morphology. Endres has found that deposition of

aluminium from Tf2N� salts shows a marked effect of the

cation on deposit morphology.39 This could be due to an

adsorption phenomenon that either limits nucleation or pre-

vents growth. This is another reason why capacitance mea-

surements are urgently required to understand the growth

mechanism in these liquids.

Thermal properties

In general ionic liquids with discrete anions have significantly

wider liquid ranges than the eutectic based liquids. Some imide

based salts are liquid over a 400 to 500 K range. Type I

eutectics have smaller liquid ranges but are still relatively

robust and can withstand temperatures up to ca. 250 1C

without significant decomposition. Type II eutectics are ex-

tremely sensitive to water content and hence temperature.

They are hydrophilic and absorb water from the atmosphere,

but this is also the case for aqueous solutions such as the

chromic acid solution used for chromium plating. It has been

shown that these types of eutectics98 function best in the range

40–60 1C where the water content remains approximately

constant. Type III eutectics depend upon the thermal proper-

ties of the hydrogen bond donor but generally the upper limit

to which they can be used is ca. 150–200 1C.

An additional issue that has not been sufficiently addressed

is the heat capacity of the liquids. The relatively low electrical

conductivity of ionic liquids can lead to ohmic heating when

significant currents are passed through the electrolytes. This

can mean that the liquids actually have to be cooled rather

than heated during large-scale electrolysis. To some extent

heating the plating liquid will not cause it to boil, but it can

affect nucleation and deposit morphology. Waliszewski

et al.102 studied the heat capacity of imidazolium and pyridi-

nium salts and found it had values ranging from 300 to 600

J K�1 mol�1. Hence ionic liquids are easier to heat than

aqueous solutions but alternatively they may suffer to a

greater extent from ohmic heating. These results were con-

firmed by Crosthwaite et al. who studied a wider range of

liquids and found that the heat capacities increased linearly

with the molar mass of the ionic liquid.31

Metal salts

Probably the key issue still to be addressed with the design of

ionic plating systems is the coordination chemistry and con-

centration of the metal complex. It affects both the thermo-

dynamics and kinetics of metal ion reduction and hence is a

major contributory factor to deposit morphology and proper-

ties. To understand the issues it is necessary to make a

comparison with current water-based plating systems. Table

4 lists some constituents of the most commonly used aqueous

electroplating solutions. It is apparent that many have strong

ligands such as oxides or cyanides or oxidising agents present,

which significantly affect the coordination geometry of the

metal complexes. In addition a cocktail of brighteners are

added to most commercial electroplating systems which affect

adsorption, mass transport and coordination chemistry.91


Most of the ionic liquid systems listed in Table 3 have been

performed using chlorides, and the metal complexes formed

are dependent upon the Lewis acidity of the metal and Lewis

basicity of the ionic liquid. In aqueous plating solutions

chlorides are rarely used as they tend to yield black powdery

deposits. This is thought to be due to the ease of nucleation

leading to large numbers of small nuclei formed at the

electrode surface. This is therefore an issue that needs to be

addressed in future ionic liquids design. Type I eutectic based

ionic liquids necessarily have high metal concentrations and

will tend to promote nucleus formation. The addition of a

strong complexing agent to hinder nucleation may not be

trivial as it will also affect the charge on the metal centre

and affect the interaction between the metal centre and the

halide anion of the ammonium salt. Hence some brighteners

can affect the phase behaviour e.g. the addition of ethylene

diamine to a choline chloride: zinc chloride eutectic mixture

causes the system to freeze despite both components being

liquid.

A number of oxides have been found to dissolve in high

concentrations in Type III eutectics.60 While these have been

studied in terms of electrowinning of metals from ores, the

potential exists to use them for metal plating.103 One issue that

arises is the speciation of the oxide following dissolution.

FAB-MS data have shown that some metals particularly with

acid based hydrogen bond donors revert to the halometalate

complexes e.g. CuO gives CuCl3�.104 Urea based liquids give

complexes when the oxygen is still attached to the metal centre

e.g. ZnO gives [ZnOCl � urea]�.60Fig. 2 shows the first example of the anion effect on the

reduction potential for the Cu2+/Cu+ and Cu+/Cu couples

for various copper salts in a Type III eutectic of

HOC2H4N(CH3)3 Cl: 2urea.105 It is surprising that the redox

potential is so significantly affected since the counter ion is

present in such a small concentration (ca. 20 mM) compared

to the chloride ion (41 M) which shows that the anion

remains bound to the metal upon dissolution. It can also be

seen that the anion not only shifts the redox potential by a

significant amount but also that the trend in Cu2+/Cu+ redox

couples does not match that of the Cu+/Cu couples. The

redox potentials can be changed by up to 100 mV in this way.

Bulk deposition of copper from all of these ionic liquids leads

to black powdery deposits over a wide current density region.

The addition of well known complexing agents such as ethy-

lene diamine and EDTA can have a more significant effect on

redox potentials as shown in Fig. 1B where the position of a

stripping potentials can be shifted by over 250 mV. The

complexing agents are clearly hindering the ability of the

metal to be reduced and therefore hindering nucleation. Bulk

deposition from ionic liquid containing just CuCl2 produces

black, powdery deposits whereas the addition of a complexing

agent can lead to lustrous copper deposits.105

A further issue that has been largely overlooked in ionic

liquids is the importance of the concentration of metal ions. In

systems such as the Type I and II liquids this is interlinked

with all of the physical properties of the liquid since the metal

salt is the anionic component of the liquid. In many cases the

working concentration equates to 5 to 10 mol dm�3 which

although seemingly high is not overly different to many

Table 4 Typical concentrations of metal salts used in commercialaqueous electroplating solutions91

Metal ElectrolyteConcentration ofmetal salts/g dm�3

Chromium CrO3, H2SO4 250–450Copper CuSO4, Potash alum, H2SO4 60–200Nickel NiSO4, NiCl2, Boric acid 200–350Silver Ag salts (various), KCN 80–200

Fig. 2 Cyclic voltammogram of a Pt electrode (diameter 1 mm) in ChCl: 2 urea eutectic containing (A) 20 mM of the following copper salts

Cu(OAc)2 (dashed line), CuCO4 (thin solid line) CuCl2 (dotted line) and CuCl2 + 5 wt.% LiSO4 (thick solid line) (B) 20 mM CuCl2 with ethylene

diamine (thick solid line) 1,10-phenathroline (dashed line) acetonitrile (dotted line) and Na2EDTA (thin solid line).


aqueous plating solutions as shown in Table 3. Of more

importance is the effect of metal ion concentration on ionic

liquids with discrete anions and Type III eutectics. In these

cases the metal speciation is strongly controlled by the anion in

the former case and the hydrogen bond donor in the latter.

An additional complication is that in all ionic liquids the

metal complex will tend to dissolve to form anionic species and

these will have a significant effect on viscosity and mass

transport and hence the effect of concentration of metal ions

on reduction current will not be linear. Added to this is the fact

that the anions of the metal salt will further affect the viscosity

leading to an extremely complicated effect of the metal salt on

the deposition characteristics. None of these factors have yet

been quantified or even highlighted.

Hydrogen bond donor

In Type III eutectics the presence of a Brønstead acid changes

the redox potential of the metal and this could act as a built in

brightener. It has previously been shown that changing the

hydrogen bond donor (HBD) from an alcohol to an amide can

change the redox potentials of metals.105 This is partly because

the HBD has a marked effect on viscosity and conductivity

(see above) but also it will change the potential window of the

liquid and the stability/reduction potential of the metal com-

plex. This affects not only the morphology of the metals

deposited from these media but also the composition of alloys

formed. Recent work106 on Zn/Sn alloys has shown that

changing the HBD can change the alloy formed from a 2

phase alloy to a homogeneous alloy.

Diluents

The idea of adding molecular diluents to ionic liquids may

seem counter productive but it is perfectly logical that the

addition of small molecules will have two effects: they decrease

the surface tension of the liquid, increasing the molar free

volume and facilitating easier ion movement. Secondly, the

diluent molecules become part of the mobile species and

lubricate the ion movement. The most appropriate diluent will

depend upon the metal being deposited and the ionic liquid

being used. Several groups have investigated the addition of

small amounts of a variety of co-solvents to ionic liquids and

they have been shown to have a significant effect on the

viscosity of the mixtures.107 This has the advantage that only

a small amount of diluent is required to decrease viscosity and

increase conductivity.

Hussey et al. studied Lewis acidic AlCl3-emimCl ionic

liquids [450 mol% AlCl3] and found that although aluminum

could be electroplated from the neat liquid, the quality of the

electrodeposit is greatly enhanced by the addition of benzene

as a cosolvent.108 The same group studied the deposition of

Ag/Al alloys from the same melt and found that the addition

of benzene hindered the adsorption of silver on the electrode

surface. Attempts to deposit Ni and Ni alloys from chloro-

aluminate melts found that the addition of benzene (20 wt%)

produced a bright deposit at low current densities but a black,

finely-divided powder was produced at higher current densi-

ties.109 Lin and Sun110 studied the analogous chlorozincate

melts and found that the addition of benzene hindered zinc

nucleation. Moy and Emmenegger111 suggested that while

aromatic diluents were effective at producing bright metallic

deposits, substituted aryl fluorides and chlorides were more

stable in Lewis acidic chloroaluminate melts. Hence the effect

of diluents is not fully understood and considerably more

work is needed to understand this variable.

The most suitable diluent and the optimum concentration

will be specific to each metal and ionic liquid. Water may also

be a suitable diluent in some cases acting as both a ligand and

viscosity improver. It has been shown that it is beneficial for

the deposition of metals such as Cr. In some chloride based

eutectics (Type I–III) the presence of water does not signifi-

cantly decrease the potential window until approximately

10 wt.% water is added.98

Anode material

Most work to date has either used soluble anodes or has not

considered the anodic reaction. Even metals such as Cr and Al

have been used as soluble anodes as they can be readily

oxidised in ionic liquids. This is naturally important as it

means that the over-potential for the overall process is small.

No pure metals have been found that do not anodically

dissolve and even Pt, Au and Ti can be made to dissolve in

eutectic based liquids.105 Graphite has been used, but it

fragments following electrolysis at high over-potentials leaving

a black powdered residue at the base of the cell. While anode

dissolution may be advantageous for some metal deposition

applications, for others it may prove problematic e.g. for alloy

deposition or for electrowinning applications. In some cases,

e.g. chromium, it may be impossible to obtain electrodes

because the metals are not commercially available in a suitable

form. The anodic processes occurring in the ionic liquids

containing discrete anions have not been well characterised.

They will be extremely complex as the fluorinated anions tend

to be very stable and act as poor ligands. This means that both

metal dissolution and solution oxidation will both be difficult.

If inert anodes, e.g. iridium oxide coated titanium, are used

then it is difficult to envisage what the anodic process will be

and this is important to determine as the systems will have to

operate at relatively high current densities.

Brighteners

Brighteners are essential to most electroplating systems and

are thought to function by two mechanisms in aqueous

solutions; by complexing the metal ions and decreasing their

reduction potential to make it more difficult to nucleate metal

clusters or by adsorption of an organic species on the electrode

surface blocking nucleation and hindering growth. No sys-

tematic studies have been carried out in ionic liquid using the

types of brighteners used in aqueous solutions and this is

clearly an area that needs to be addressed to see if the bright-

eners function in the same way as they do in water.

Brighteners that involve a complexation with a solution-

based species will depend upon the comparative strength of the

ionic liquid–metal interactions. It would therefore be logical to

suppose that ionic liquids with discrete anions would be likely

to work directly with brighteners used in aqueous solutions as

the interaction between the metal salt and the anion will be


considerably weaker than those between the metal salt and the

brightener. In eutectic based ionic liquids the chloride anions

act as strong Lewis bases and could decrease the relative

interaction between the metal salt and the brightener.

Brighteners which rely on electrostatic or hydrophobic

interactions may function in ionic liquids but their efficacy is

likely to be surface and cation/anion specific. As with other

solutes in ionic liquids, the general rule of like dissolving like is

applicable i.e. ionic species will generally be soluble as will

species capable of interacting with the anion. Aromatic species

tend to exhibit poor solubility in ionic liquids consisting of

aliphatic cations and vice versa.

The principle of using brighteners in ionic liquids has not

really been demonstrated but recent unpublished results by

Caporali112 showed that semi-bright aluminium could be

obtained using 5 g l�1 phenanthroline as a brightener in a

[BMIm]Cl/AlCl3 ionic liquid. We have also studied the use of

brighteners in choline-based ionic liquids105 and Fig. 3 shows

the effect on the deposit morphology of adding acetonitrile as

a brightener to a ChCl: urea eutectic containing copper(II)

chloride. Visually the deposit formed without acetonitrile is

black and powdery whereas the one with a brightener has a

mirror finish. A range of brighteners have been added to

choline-based ionic liquids and Fig. 4 shows that suitable

compounds have been developed for the deposition of silver,

copper, nickel, zinc and zinc alloys. Endres studied the use of

nicotinic acid for the deposition of Pd and Al/Mn alloys from

an AlCl3-1-butyl-3-methylimidazolium chloride ionic liquid

and showed that in contrast to producing a brighter surface

finish it aided the formation of nanocrystalline deposits.113

Pre-treatment protocols

There is very little difference between the pre-treatment pro-

tocols in aqueous solutions and those in ionic liquids other

than in the latter the work-piece needs to be dried before

insertion into the plating solution. A number of methods have

been studied but by far the best adhesion is obtained by

degreasing in a chlorinated solvent, followed by an aqueous

pickle, rinse, dry and then anodic etch in the ionic liquid prior

to deposition. Anodic etch potentials and times are dependent

on the substrate and the ionic liquid used. The main issue is the

removal of the oxide film. Metals such as Al and Mg will

require a larger anodic pulse for a longer period than other

metals such as Cu or Ni. Metal oxide dissolution is easier in

ionic liquids containing a metal that is a good oxygen scaven-

ger. Endres has shown that the adhesion of aluminium to mild

steel is greatly enhanced by an anodic pulse prior to deposi-

tion. It was shown that this alloy was formed between the

substrate and the coating metal.71

Concluding remarks

In conclusion, it has been shown that the majority of metals

can now be electrodeposited from ambient temperature ionic

liquids. It is most likely that ionic liquids will be task specific

with discrete anions being used for metals that cannot be

electrodeposited from aqueous solutions such as Al, Li, Ti, V

and W. Type I eutectics will probably be the most suitable for

Al, Ga and Ge. Type II eutectics are most suitable for Cr and

Type III are most suited to Zn, Cu, Ag and associated alloys.

Type III will also find application in metal winning, oxide

recycling and electropolishing.

Before this technology can find large scale application a

number of fundamental aspects have to be addressed. Firstly

Fig. 3 SEM images of copper deposited from ChCl: 2 urea eutectic

containing 20 mM CuCl2 (A) without and (B) with acetonitrile added

as a brightener.

Fig. 4 Photograph of brass samples coated with metals from various

choline-based ionic liquids.


the double layer structure needs to be more thoroughly

analysed and the effects of electrolytes, brighteners, tempera-

ture and composition need to be quantified. Next, the effect of

complexing agents on the redox properties of metals needs to

be ascertained as this is vital to the development of novel alloy

plating techniques. It is likely that diluents will have to be

studied to improve mass transport properties, increase con-

ductivity and improve nucleation characteristics. Finally the

subject of brighteners will have to be addressed. It will

probably start by the application of the systems currently used

in aqueous solutions but will inevitably rely on the informa-

tion gained from double layer studies to develop material with

the correct interfacial properties.

The future for ionic liquid based technology in metal

finishing is extremely bright but more fundamental aspects

of metal growth will have to be studied so that the subject

becomes a practical reality instead of an academic curiosity. A

concerted effort is now taking place through an European

project involving 33, mostly industrial, partners to focus on

the aspects addressed in this review and so it is likely that

many of the issues highlighted above will be answered in the

course of the next 3 years.114

Acknowledgements

The authors would like to acknowledge the EU under the FP6

programme for funding this work through the IONMET

project.

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Application of ionic liquids to the electrodeposition of ...€¦ · Electrodeposition of metals is...

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