materiais liquido-ionicos para os desafios eletroquimicos do futuro

materiais liquido-ionicos para os desafios eletroquimicos do futuro

(Parte 1 de 3)

nature materials | VOL 8 | AUGUST 2009 | w.nature.com/naturematerials 621 review article Published online: 24 july 2009 | doi: 10.1038/nmat2448

It was with simple triethylammonium nitrate that a pure low-melting salt — an ionic liquid — was first identified more than a century ago. In the 1930s, a patent application described cellulose dissolution using a molten pyridinium salt above 130 °C. It was the need for a sturdy medium for nuclear fuel reprocessing that prompted the study of low-melting-point chloroaluminates. Among the ‘onium’ cations with positive nitrogen(s), those derived from the imidazolium ring proved to be the best choice in terms of melting points and electrochemical stability.

At roughly the same time, the need for new anions for organic polymer electrolytes based on polyethylene oxide led to the concept of a ‘plasticizing anion’, that is, an anion having a delocalized charge and multiple conformations differing only marginally in energy. The archetype of such anions is the bis(trifluoromethylsulphonyl)amide

(CF3SO2–N–SO2CF3) ion, also known as NTf2, in which the extremely electron-withdrawing CF3SO2− groups are conjugated and linked by flexible S–N–S bonds. When combined with an imidazolium cation, such as the ethylmethylimidazolium cation, this anion produces a fluid ionic liquid (melting point –15 °C) with an ion conductivity comparable to that of the best organic electrolyte solutions; it shows no decomposition or significant vapour pressure up to ~300–400 °C (ref. 1). Surprisingly, it is not miscible with water (~1,0 p.p.m. in equilibrium with liquid H2O), and thus defies the conventional wisdom that states polarity is synonymous with hydrophilicity.

Ionic liquids (Box 1) then developed rapidly, with a reinvestigation of ions, for example quaternary ammonium cations, that had been avoided previously by organic chemists because of unsymmetrical shapes that hindered easy purification through crystallization. The organic chemistry community had earlier engaged in research of media with controllable Lewis acidity (chloroaluminate ionic liquids), but the modern era of ionic liquids has produced numerous ‘neutral’ ionic liquids, that is, those based on ions which are unreactive towards acids or bases, be they Lewis or Brønsted. As a result, it is now difficult to name an organic reaction that has not been performed successfully in these potentially ‘green’ solvents, which can be recycled almost indefinitely with no or minimal use of volatile ionic-liquid materials for the electrochemical challenges of the future michel armand1, Frank endres2, douglas r. macFarlane3, hiroyuki ohno4 and bruno scrosati5*

Ionic liquids are room-temperature molten salts, composed mostly of organic ions that may undergo almost unlimited structural variations. This review covers the newest aspects of ionic liquids in applications where their ion conductivity is exploited; as electrochemical solvents for metal/semiconductor electrodeposition, and as batteries and fuel cells where conventional media, organic solvents (in batteries) or water (in polymer-electrolyte-membrane fuel cells), fail. Biology and biomimetic processes in ionic liquids are also discussed. In these decidedly different materials, some enzymes show activity that is not exhibited in more traditional systems, creating huge potential for bioinspired catalysis and biofuel cells. Our goal in this review is to survey the recent key developments and issues within ionic-liquid research in these areas. As well as informing materials scientists, we hope to generate interest in the wider community and encourage others to make use of ionic liquids in tackling scientific challenges.

organic compounds. Most products made in ionic liquids can be distilled off, in the case of small molecules, or extracted with water or hydrocarbon solvents, at least one of which is usually immiscible with the ionic liquid.

It is this unique solvent potential that makes ionic liquids key materials for the development of a range of emerging technologies. The advent of ionic liquids has made viable processes that fail, or are even impossible, with conventional solvents. Watersensitive metals or semiconductors that previously could not be deposited from conventional water baths can now, by turning to ionic liquids, be directly electroplated. Energy devices, such as the polymer-electrolyte-membrane fuel cells, lithium batteries and supercapacitors presently under development to address the challenges of increasing energy costs and global warming, may greatly benefit from a switch to low-vapour-pressure, non-flammable, ionic-liquid-based electrolytes. The significant dissolution power of ionic liquids also extends to macromolecules, as best illustrated by the solutions of up to 25 wt% cellulose that can be obtained; this has great consequences for the textile industry and for the only ethically acceptable source of biofuels, polyosides from wood. Until it was realized that cellulose was soluble in ionic liquids, the only known solvent for cellulose was an explosive organic compound. Ionic liquids also have applications in biology and biomimetic processes, as their enzymes can be remarkably active, opening a vast field for bioinspired catalysis and biofuel cells.

The scientific and technological importance of ionic liquids today spans a wide range of applications. New types of lubricants and seals, and fluids for thermal engines and adsorption refrigeration have been suggested on the basis of the diverse and unique properties of ionic liquids. We have focused this review on some of the most significant examples of recent key developments in ionic liquid applications made possible by their unique solvent capabilities (Fig. 1 and Box 1). These include electrodeposition, energy management, bioscience and biomechanics. Our main objective is to show how the progress recently achieved in these crucial fields could not have been possible without the advent of ionic liquids.LRCS CNRS 6007, Université de Picardie Jules Verne, F-80039 Amiens, France, Institute of Particle Technology, Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany, School of Chemistry and ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia, Department of Biotechnology, Tokyo University of Agriculture and Technology, Tokyo 184-8588, Japan, Department of Chemistry, University Sapienza, 00185 Rome, Italy. *e-mail: bruno.scrosati@uniroma1.it

622 nature materials | VOL 8 | AUGUST 2009 | w.nature.com/naturematerials review articleNATure mATerIAls doi: 10.1038/nmat2448 electrodeposition Ionic liquids are superior media for the electrodeposition of metals and semiconductors, and have an unprecedented potential to revolutionize electroplating. Key advantages that enable them to overcome the limits imposed by common aqueous or organic media are their wide electrochemical window, spanning up to 6 V in some cases; extremely low vapour pressures, which allow deposition at temperatures well above 100 °C; and numerous, only partly understood, cation/anion effects that make it possible to influence the morphology and crystal size of deposits2–4.

Processes that are impossible in water baths become viable if ionic liquids are used. These include the direct electroplating of water-sensitive metals such as aluminium and possibly even magnesium, as well as many other metals having a deposition potential conflicting with water decomposition. Aluminium is an important metal for corrosion protection of steel as, in contrast to industrially used zinc coatings, it is self-passivating in air and is much more sustainable in terms of known ore resources. However, with an electrode potential of −1.7 V relative to the normal hydrogen electrode, it cannot be deposited from aqueous media, and the wellknown SIGAL process5,6 involving alkylaluminium compounds in aromatic solvents is hazardous owing to the high flammability of the aluminium precursors used, requiring the strict exclusion of oxygen. The electro deposition of aluminium from ionic liquids has been discussed several times in the literature7–9 and is sufficiently advanced that it could be introduced to industry within the next five years as a commercial process. Nonetheless, although ionic liq- uids are air stable and some are hydrophobic, the presence of AlCl3 solute species in the ionic-liquid solution as aluminium precursors still requires the exclusion of moisture, and this is a challenge for the next generation of aluminium-plating baths. AlF4−, which is less prone to releasing HF than AlCl3 is to releasing HCl, would be a good choice; however, it disproportionates into AlF63− at room temperature. Stabilizing such species in ionic liquids would be very promising.

This process has shown an interesting cation effect: in one ionic liquid, the deposited aluminium is nanocrystalline, with grain sizes of around 50 nm; with a different cation the grain size is, under practically the same conditions, around 5 μm (ref. 10). It is remarkable that just variation of the ionic liquid leads to different morph ologies, without the addition of any brightener or grain refiner. This effect results from the solvation layers forming at the interface between the electrode and the ionic liquid; these vary from liquid to liquid and depend upon the metal salt dissolved. Ionic liquids have thus been proven to be excellent media for deposition of a range of

Figure 1 | Ionic liquids, that is, salts with melting points below 100 °C, are potential candidates to be new ‘green’ reaction media with a number of important properties. They can be exploited in various applications, which range from energy storage and conversion to metal deposition, and can be used as reaction media and in chemistry, biochemistry and even biomechanics.

Ion conductive materials for electrochemical devices

Solvents for chemical reaction

Solvents for bioscience

Possible properties of ionic liquids Variation of ion structure

Ionic liquids

Thermal and chemical stabilityNegligible volatility Flame retardancy

Moderate viscosity High polarity

Low melting point

High ionic conductivity Solubility (a nity) with many compounds

Ionic liquids are low-temperature molten salts, that is, liquids composed of ions only. The salts are characterized by weak interactions, owing to the combination of a large cation and a charge-delocalized anion. This results in a low tendency to crystallize due to flexibility (anion) and dissymmetry (cation). The archetype of ionic liquids is formed by the combination of a 1-ethyl-3-methylimidazolium (EMI) cation and an N,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combination gives a fluid with an ion conductivity comparable to many organic electrolyte solutions and an absence of decomposition or significant vapour pressure up to ~300–400 °C. Ionic liquids are basically composed of organic ions that may undergo almost unlimited structural variations because of the easy preparation of a large variety of their components. Thus, various kinds of salts can be used to design the ionic liquid that has the desired properties for a given application. These include, among others, imidazolium, pyrrolidinium and quaternary ammonium salts as cations and bis(trifluoromethanesulphonyl)imide, bis(fluorosulphonyl)imide and hexafluorophosphate as anions.

Box 1 | structure of the archetype ionic liquid.

CH3

EMI cation TFSI anion

On the basis of their composition, ionic liquids come in different classes that basically include aprotic, protic and zwitterionic types, each one suitable for a specific application.

Box 2 | Designs of ionic liquids for specific applications.

n H

Aprotic Protic Zwitterionic

Suitable for lithium batteries and supercapacitors

Suitable for fuelcells

Suitable for ionic-liquid-based membranes

nature materials | VOL 8 | AUGUST 2009 | w.nature.com/naturematerials 623 review articleNATure mATerIAls doi: 10.1038/nmat2448 other metals including manganese, nickel, tin and copper; these and presumably many others can easily be deposited from liquids with the dicyanamide anion (CN)2N, which combines charge delocalization and stability at reducing potentials with an ability to complex the metal cation and therefore dissolve and stabilize it11. A further advantage over aqueous plating baths is that many ionic liquids are benign media without toxicity issues in comparison with the cyanides that are involved in many aqueous systems (for example

[Ag(CN)2]–, which is used in silver plating). It could be argued that, owing to the very wide electro chemical windows, it should be trivial to deposit reactive elements such as titanium, tantalum, molybdenum and other transition metals that, from the thermodynamic point of view, should be reduced at electrode potentials similar to that of aluminium or even less negative. The main difficulty with these transition elements is the multiplicity of redox states resulting in the electrochemical equivalent of short circuits. From the thermodynamic point of view, the electrochemical window of many ionic liquids is wide enough to allow the deposition of many reactive elements including titanium (from titanium(i)), which lies very close to the deposition potential for lithium. The electrodeposition of thin tantalum layers is possible within a narrow set of experimental parameters, and it has thus been successfully deposited from ionic liquids at elevated temperature, for example to coat biomedical implants12,13. On the other hand, attempts to deposit titanium from titanium halides have hitherto been unsuccessful owing to the formation of titanium subvalent halides, which redissolve into the electrolyte, instead of elemental titanium14.

As a proof of principle, it was shown that nanoparticles of chromium, molybdenum and tungsten could be made from their respective carbonyl compounds as the source of metal in the bath15. Once again, a dependence of the particle size on the type of ionic liquid was observed. It seems to be much easier to deposit these elements as alloys; accordingly, the deposition of a ternary aluminium– molybdenum–titanium alloy has recently been described16.

Ionic liquids also have the potential to be the basis of important approaches in nanoscience. There is great interest in making well-defined metal nanowires, for example using template-assisted techniques. Iron, cobalt and nickel nanowires are, owing to their ferro magnetic properties, interesting for fundamental research. These elements can be obtained in aqueous solution, but as a result of hydrogen coevolution the nanowires are usually of limited quality. This problem does not exist in ionic liquids, as there is no hydrogen evolution, and the template-assisted deposition of silver17, cobalt18, germanium and silicon19 has been reported to deliver compact nanowire mats of these metals; see Fig. 2 for a typical example.

Furthermore, ionic liquids are of importance in semi conductor electrodeposition; recently it was shown that photoluminescent

SixGe1−x with a bandgap of at least 1.5–3.2 eV can be made by electro deposition from an ultrapure ionic liquid containing silicon and germanium halides (Fig. 3). During deposition, different colours, from orange to green, are observed in the visible spectrum; these are due to a quantum size effect of the semiconductor particles with sizes between 2 and 20 nm (ref. 20). These results show that the ionic liquid allows deposits so pure that photoluminescence effects can be seen. In addition, the material absorbs visible light and may open the way to a simple electrochemical fabrication of inexpensive solar cells.

However, although the feasibility of metal and semiconductor deposition from ionic liquids has been demonstrated in a variety of cases, there are some fundamental issues that still have to be clarified. The solubility of precursor compounds departs appreciably from that in conventional solvents. The basic electrochemistry of the processes is yet to be fully understood and the structure of the double layer, which, owing to the formation of solvation layers, can obviously not be well described with the models developed for aqueous solutions, must be clarified. The role of the large ions in influencing the diffusion kinetics and the mechanism of deposit nucleation and growth are other aspects requiring further investigation.

energy management Ionic liquids are also likely to be important in the field of energy. Global concerns about present energy policy, which relies on fossil fuels and suffers the associated economical and ecological problems, call for the increased use of renewable energy sources in households and industry and for the large-scale replacement of internal-combustion-engine vehicles by those with zero or low emissions. This in turn requires the availability of suitable systems both to store the energy in power stations harnessing intermittent sources (solar or wind) and to power electric or hybrid vehicles. Electrochemical systems, such as batteries, are ideal for this purpose21. In particular, lithium batteries, which have the highest energy efficiency of all known electrochemical storage systems and which have reached an advanced state of technological development, seem to be the power source of choice. Lithium batteries can have

Figure 2 | Processes that are impossible in water baths become viable if an ionic liquid solvent is used. The use of ionic liquid baths also improves the quality and the morphology of the deposits. The figure shows a highresolution scanning electron microscope image of germanium nanowires made by electrodeposition from ionic liquids. Figure reprinted from ref. 19, © 2008 RSC.

Figure 3 | Deposits of photoluminescent semiconductors, for example Gesi obtained from ionic liquid baths. Different colours are seen in the visible spectrum, owing to quantum size effects of the semiconductor particles. This is a clear demonstration that ionic liquids allow deposits of particle size in the nanometre range that are pure enough to make visible the photoluminescence effect. The panels correspond to potentials of −2.62 V (a), −2.68 V (b), −2.74 (c), −2.78 (d), −2.83 V (e), and −2.85 V (f). Figure reprinted from ref. 20, © 2008 RSC.

1 µm de f bc

624 nature materials | VOL 8 | AUGUST 2009 | w.nature.com/naturematerials review articleNATure mATerIAls doi: 10.1038/nmat2448 different configurations (Fig. 4). However, their implementation for energy storage and in vehicle applications has been slowed by safety concerns surrounding the use of large-scale lithium cells. Undesired reactions between the battery components and the liquid organic electrolyte, triggered by unpredictable events such as short circuits or local overheating, lead to an exothermic reaction of the electrolyte with the electrode materials, producing a rapid increase of the battery temperature and, eventually, to fire or explosion.

Again, the unique properties of ionic liquids may help to solve the problem as they are practically non-flammable, which is a significant safety asset. The replacement of the conventional, flammable and volatile, organic solutions with ionic-liquid-based, lithiumion-conducting electrolytes may greatly reduce, if not prevent, the risk of thermal runaway. This provides the lithium battery with the level of safety that is required for their large-scale application in important and strategic markets. Extensive work in this direction is in progress, and the testing of ionic liquids as new electrolyte media for future, highly safe, lithium batteries is under way in many industrial and academic laboratories22–25. However, the results vary widely, as can be expected from the wide variety of ionic liquids investigated. Key aspects to be understood are, among others, the thermodynamic and kinetic stabilities of the ionic liquids with respect to the electrode materials. It would be naive to expect the organic ionic-liquid cations to be stable at the extremely negative potentials of alkali-metal deposition; in other words, the average electro negativity of an ion based on carbon, hydrogen or nitrogen cannot be as low that of Li+ or K+. However, the kinetic metastability of the cation and/or protective, passivating-layer formation on the electrode have been welcome observations26. The structure of the interfaces and the nature of the electrode-protecting films are other aspects that need particular attention.

These aspects will be analysed in this review to evaluate their effective influence on the progress of lithium battery technology. The electrolytes that are under study are solutions of a lithium salt in an ionic liquid. The high solubility of lithium salts, even though it requires low-lattice-energy salts (for example LiBF4 or Li[NTf2]), is quite surprising as, in contrast to organic polar solvents, ionic liquids do not possess free electron pairs to provide for a solvation interaction with the cation, other than those on the anion. The resultant structures, probably involving polyanionic species such as [LiXn](n−1) still need to be better understood. In principle, ionic liquids are only metastable at the potential of lithium reduction; the cations, despite often being resonance stabilized, are sensitive to electron injection, with the subsequent formation of a radical, or proton elimination. For instance, the archetypal ionic-liquid cation, ethylmethylimidazolium, possesses an acidic C2 proton that limits its stability below 1.3 V relative to Li+/Li0 (−1.7 V in comparison with H+/H2). Yet more and more reports seem26–28 to show that in half-cells or full batteries, selected ionic-liquid electrolytes behave well, not only with the conventional graphite negative electrode, but also for Li0 plating. The latter had been abandoned in organic electrolytes because of uneven plating of the metal during cycling (that is, dendrite formation), with short circuits and dramatic runaway reactions. Graphite was thus selected for ‘Li-ion technology’ but is still a weak point in terms of safety, as solvents can forcefully cointercalate and exfoliate the graphite with large release of heat.

This behaviour does not seem to occur with ionic liquids, as the solvation is provided by anions that have no tendency to co-inter- calate at such potentials. The very high-capacity LixSi (0 < x < 4.4) alloying electrode (capacity reaching 2,0 mA h g−1) also behaves quite well in contact with ionic liquids. The NTf2 electrolytes have been shown to cycle well with Li0 (ref. 26), offering a dramatic increase in possible energy densities and a simpler technology for high-performance batteries. The chemical manipulation of the cation (quaternary ammonium instead of ethylmethylimidazolium) and of the anion [(FSO2)2N]− instead of [CF3SO2)2N]−) allows the fine-tuning of the interactions at the interface and passivating-layer formation. Owing to their high anodic stability, ionic liquids function without problem at the positive electrode, even with highly oxidizing materials (for example LiMn1.5Ni0.5O4 and LiMnPO4). The main problem is technological, as the only acceptable (sustainable) current collector in practical batteries is aluminium, which tends to become corroded at such high voltages. The battery electrolytes based on ionic liquids are thus serious contenders in the design of safe batteries for electric and hybrid vehicles. There is also hope, in the longer term, of building lithium batteries using oxygen from air as the oxidizer at the cathode, which would produce a great increase in capacity, even if the end product of oxygen reduction is Li2O2, that is, only two electrons per O2 instead of the four possible29. The fact that many of the ionic liquids of interest in this area are hydrophobic is an asset; however, when a lithium salt is dissolved, the electrolyte becomes very water sensitive, and new salts must be designed to counter this problem.

The development of a zero-emission, hydrogen-powered car has been a highly desirable goal for some time. Although the concept is very appealing, the development of hydrogen cars is still restricted to a few models operating in limited geographical regions. The core of the hydrogen car is the hydrogen fuel cell. Fuel cells have been known for more than a century, but some of the issues that have

Figure 4 | In their basic structure, lithium batteries are formed by two lithium-exchanging electrodes separated by a lithium-ion-conducting electrolyte. The most common version uses an electrode combination based on a graphite negative electrode with a lithium cobalt oxide positive electrode connected either by a liquid solution of a lithium salt in an organic solvent mixture or by the same solution trapped in a gel. The electrochemical process is the cycling of lithium ions from one electrode to the other. These lithium-ion batteries are commercially available and dominate the consumerelectronics market. However, their use in more demanding applications, such as in sustainable transport, is prevented by a number of drawbacks, among which safety is the main concern. Safety can be improved by excluding liquids, that is, by using solvent-free, polymer electrolytes. The safe operation of this solid-state version, which replaces the graphite with lithium metal at the negative side, has been demonstrated. However, these batteries operate well only at temperatures higher than 50 °C, which limits their range of application. The real breakthrough in the field may come by using ionicliquid-based gel or polymer electrolytes in combination with revolutionary electrode materials: the high thermal stability and low vapour pressure of the ionic liquids ensure reliability by preventing thermal runaway and pressure build-up, and the innovative electrode structure is expected to enhance the capacity and energy density of the battery.

(Parte 1 de 3)

Comentários