Electrochemical Aspects of Ionic Liquids

Hiroyuki Ohno

• Publisher: Wiley
• Year: 2011
• Print ISBN: 9780470647813
• eBook ISBN: 9781118003343
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

The second edition is based on the original book, which has been revised, updated and expanded in order to cover the latest information on this rapidly growing field. The book begins with a description of general and electrochemical properties of ionic liquids and continues with a discussion of applications in biochemistry, ionic devices, functional design and polymeric ionic liquids.

The new edition includes new chapters on Li ion Batteries and Actuators, as well as a revision of existing chapters to include a discussion on purification and the effects of impurities, adsorption of ionic liquids on interfaces and on the electrochemical double layer, among other topics.

1

IMPORTANCE AND POSSIBILITY OF IONIC LIQUIDS

HIROYUKI OHNO

1.1 IONIC LIQUIDS

Ionic liquids are salts with a very low melting temperature. Ionic liquids have been of great interest recently because of their unusual properties as liquids. Because these unique properties of ionic liquids have been mentioned in a few other books, we will not repeat them here but will summarize them in Table 1.1. Note that these are entirely different properties from those of ordinary molecular liquids. Also, every ionic liquid does not always show these properties. For electrochemical usage, the most important properties should be both nonvolatility and high ion conductivity. These are essentially the properties of advanced (and safe) electrolyte solutions that are critical to energy devices put in outdoor use.

TABLE 1.1. Basic and Possible Characteristics of Organic Ionic Liquids

Low melting point	• Treated as liquid at ambient temperature
	• Wide usable temperature range
Nonvolatility	• Thermal stability
	• Flame retardancy
Composed by ions	• High ion density
	• High ion conductivity
Organic ions	• Various kinds of salts
	• Designable
	• Unlimited combinations

Safety is a more important issue than performance these days, and it has been taken into account in the materials developed for practical use. Thus, more developments in ionic liquids are expected in the future. The nonvolatile electrolyte solution will change the shape and performance of electronic and ionic devices. These devices will become safer and have longer operational lives. They are composed of organic ions, and these organic compounds have unlimited structural variations because of the easy preparation of many different components. So there are unlimited possibilities open to the new field of ionic liquids. The most compelling idea is that ionic liquids are “designable” or “fine-tunable.” Therefore, we can easily expect explosive developments in fields using these remarkable materials.

1.2 IMPORTANCE OF IONIC LIQUIDS

Ionic liquids are salts that melt at ambient temperature. The principles of physical chemistry involved in the great difference between solution properties of molecular solvents and molten salts have already been introduced and summarized in several books. Thousands of papers have already been published on their outstanding characteristics and effectiveness for a variety of fields. Thus, as mentioned, in this book, we take the most important point that these salts are composed of organic ions and explore the unlimited possibility of creating extraordinary materials using molten salts.

Because ionic liquids are composed of only ions, they usually show very high ionic conductivity, nonvolatility, and flame retardancy. The organic liquids with both high ionic conductivity and flame retardancy are practical materials for use in electrochemistry. At the same time, the flame retardancy based on nonvolatility inherent in ion conductive liquids opens new possibilities in other fields as well. Because most energy devices can accidentally explode or ignite, for motor vehicles there is plenty of incentive to seek safe materials. Ionic liquids are being developed for energy devices. It is therefore important to have an understanding of the basic properties of these interesting materials. The ionic liquids are multipurpose materials, so there should be considerable (and unexpected) applications. In this book we, however, will not venture into too many other areas. Our concern will be to assess the possible uses of ionic liquids in electrochemistry and allied research areas.

1.3 POTENTIAL OF IONIC LIQUIDS

At present, most interest in ionic liquids is centered on the design of new solvents. Although the development of “new solvents” has led the development of possible applications for ionic liquids, there is more potential for development of electrochemical applications.

Electrochemistry basically needs two materials: electroconductive materials and ion conductive materials. Ionic liquids open the possibility of improving ion conductive materials. The aqueous salt solution is one of the best electrolyte solutions for electrochemical studies. However, because water is volatile, it is impossible to use this at a wide temperature range or on a very small scale. Many other organic polar solvents have been used instead of water to prepare electrolyte solutions. They, however, have more or less the same drawback, depending on the characteristics. The material known to be a nonvolatile ion conductor is the polymer electrolyte. Polymers do not vaporize but decompose at higher temperatures; the vapor pressure at ambient temperature is zero. Polymer electrolytes are considered a top class of electrolytes except for the one drawback: relatively low ionic conductivity.

One remarkable propertie of ionic liquids is the proton conduction at a temperature higher than 100 °C. Water-based proton conductors cannot be operated at such a high temperature because of vaporization of water. As mentioned in a later chapter, proton-conductive ionic liquids are the most expected materials.

Some literature has included statements that the ionic liquids are thermally stable and never decompose. This kind of statement has led to a misunderstanding that the ionic liquids are never vaporized and are stable even when on fire. Are the ionic liquids indestructable? The answer is “no.” However, although inorganic salts are entirely stable, the thermal stability of organic salts depends largely on their structure. Because ionic liquids are organic compounds, their degradation begins at the weakest covalent bond by heating. Nevertheless, ionic liquids are stable enough at temperatures of 200 °C to 300 °C. This upper limit is high enough for ordinary use.

Does it need more energy or cost to decompose ionic liquids after finishing their role? It is not difficult to design novel ionic liquids that can be decomposed at a certain temperature or by a certain trigger. It also is possible to design unique catalysts (or catalytic systems) that can decompose target ionic liquids. Some catalysts such as metal oxides or metal complexes have the potential to become excellent catalysts for the decomposition of certain ionic liquids under mild conditions. The post-treatment technologies of ionic liquids should therefore be developed along with the work on the design of ionic liquids.

At the present time there has been little progress in this area. Although post-treatment technologies are beyond the scope of this book, we do attempt to give ideas on the various future developments in ionic liquid technologies as well as in electrochemistry. This book is dedicated to introducing, analyzing, and discussing ionic liquids as nonvolatile and highly ion conductive electrolyte solutions. The astute reader will find the future prospects for ionic liquids between the lines in all chapters of this book.

2

PHYSICAL CHEMISTRY OF IONIC LIQUIDS: INORGANIC AND ORGANIC AS WELL AS PROTIC AND APROTIC

C. A. ANGELL, W. XU, M. YOSHIZAWA-FUJITA, A. HAYASHI, J.-P. BELIERES, P. LUCAS, M. VIDEA, Z.-F. ZHAO, K. UENO, Y. ANSARI, J. THOMSON, AND D. GERVASIO

2.1 CLASSES OF IONIC LIQUIDS

Ionic liquids in their high-temperature manifestations (liquid oxides, silicates, and salts) have been studied for a long time, using sophisticated methods, and much of the physics is understood. By contrast, the low-temperature ionic liquid (IL) field (<100 °C ILs), the subject of the present volume, is still under development. The many interesting studies on transport and thermodynamic properties of the <100 °C ILs have focused mainly on characterizing new systems for potential applications [1–5]. The task of placing this behavior within the wider phenomenology of liquid and amorphous solid electrolytes as well as in the context of the liquid state in general still has a long way to go. In this chapter, we review the current state of knowledge of physical properties of ionic liquids in an attempt to place them within this larger picture. We make an effort to emphasize the special status of the protic subclass of ionic liquids because these offer a degree of freedom not encountered in other branches of the solvent-free liquid state.

The first requirement of an ionic liquid is that, contrary to experience with most liquids consisting of ions, it must have a melting point that is not much higher than room temperature. The limit commonly suggested is 100 °C [1b]. Given the cohesive energy of ionic liquids (about which more will be said later on), ambient melting requires that the melting point occur at a temperature not too much higher than the glass transition temperature, T_g, which provides the natural base for liquid-like behavior. Ionic liquids nearly all melt within the range that we call the “low-temperature regime” of liquid behavior [6,7]. This means that in most cases, they will supercool readily and will exhibit “super-Arrhenius” transport behavior near and below ambient temperature—as is nearly always reported.

Such liquids come in different classes. The most heavily researched class is the aprotic organic cation class [1–4,8–15]. In this cation class, the low melting point is a consequence of the problem of efficiently packing large, irregular organic cations with small inorganic anions. More on this class is given in Section 2.3.

A second class [16] is one that may enjoy increased interest in the future because of the presence of one of its members in the first industrial IL process [1b], because of the new finding that its members can have aqueous solution-like conductivities [17] and can serve as novel electrolytes for fuel cells [18], and finally, because of the evidence that these liquids, in hydrated form, can be used as tunable solvents for biomolecules, on which stability against aggregation and hydrolysis may be provided under the right tuning [19]. This class is closely related to the first but differs in that the cation has been formed by transfer of a proton from a Brønsted acid to a Brønsted base. The process is reversible if the free energy of proton transfer is not too large. When the gap across which the proton must jump to reform the original molecular liquid is small, the liquid will have a low conductivity and a high vapor pressure. These properties are not of great interest in an ionic liquid, although the liquid may be fluid. If the gap is large, as in the case of ammonium nitrate, 87 kJ/mol (from data for HNO₃ + NH₃ → NH₄NO₃), then the proton will remain largely on the cation, and for many purposes, the system is a molten salt. If the acid is a strong acid like triflic acid, HSO₃CF_3, or a superacid like HTFSI [16], then the transfer of the proton will be energetic, and the original acid will not be regenerated on heating before the organic cation decomposes. Such liquids will not be easily distinguishable in properties from the conventional aprotic salts in which some alkyl group, rather than a proton, has been transferred to the basic site. This is particularly true of the protic ionic liquids (PILs) recently reported by Luo et al. [20] using superbases as proton acceptors. The stability of these systems has been characterized in terms of the relation between the boiling point elevation (or excess boiling point) over the linear (or average value) of the components [21], and the excess was shown to be a linear function of the difference in pK_a values determined in aqueous solutions.

This relation is shown in Figure 2.1. It seems to be free of exceptions when the base is a simple amine nitrogen. The protic ionic liquids as a class [16–18] are considerably more fluid than the aprotic ionic liqui...

Table of contents

• Cover
• Title page
• Copyright page
• PREFACE TO THE SECOND EDITION
• PREFACE TO THE FIRST EDITION
• ACKNOWLEDGMENTS FOR THE SECOND EDITION
• CONTRIBUTORS
• 1 IMPORTANCE AND POSSIBILITY OF IONIC LIQUIDS
• 2 PHYSICAL CHEMISTRY OF IONIC LIQUIDS: INORGANIC AND ORGANIC AS WELL AS PROTIC AND APROTIC
• Part I: BASIC ELECTROCHEMISTRY
• Part II: BIOELECTROCHEMISTRY
• Part III: IONIC DEVICES
• Part IV: FUNCTIONAL DESIGN
• Part V: IONIC LIQUIDS IN ORDERED STRUCTURES
• Part VI: GEL-TYPE POLYMER ELECTROLYTES
• Part VII: POLYMERIZED IONIC LIQUIDS
• Part VIII: CONCLUSION
• APPENDIX: STRUCTURES OF ZWITTERIONS
• Index