Elaboration of liquid and gel electrolytes for supercapacitor applications over a wide temperature range

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Elaboration of liquid and gel electrolytes for

supercapacitor applications over a wide temperature range

Laure Dagousset

To cite this version:

Laure Dagousset. Elaboration of liquid and gel electrolytes for supercapacitor applications over a wide

temperature range . Chemical Sciences. Université Paris-Seine, 2016. English. �tel-01499312�

THESE

Présentée en vue de l’obtention du grade de

DOCTEUR DE L’UNIVERSITE PARIS-SEINE

Spécialité : Chimie des Polymères - Science des Matériaux Soutenue publiquement le 13 juillet 2016

par Laure DAGOUSSET

Elaboration of liquid and gel electrolytes for supercapacitor applications over a wide temperature range

Mise au point d’électrolytes pour applications aux supercondensateurs fonctionnant sur une gamme de température étendue

Devant le jury composé de :

Rapporteurs Fouad Ghamouss PCM2E, U-Tours

David Mecerreyes Polymat, Espagne

Examinateurs Jean Le Bideau IMN, U-Nantes

Fabien Miomandre ENS Cachan

Directeur de thèse Pierre-Henri Aubert LPPI, UCP, Cergy-Pontoise Co-encadrants Tran-Minh Giao Nguyen LPPI, UCP, Cergy-Pontoise Frédéric Vidal LPPI, UCP, Cergy-Pontoise Grégory Pognon Thales Research &

Technology, Palaiseau Sébastien Jus Thales Research &

Technology, Palaiseau Ecole doctorale : Sciences et Ingénierie, Université de Cergy-Pontoise

Unité de recherche : LPPI, Laboratoire de Physicochimie des Polymères et Interfaces, EA 2528

Pour commencer, je souhaite remercier Fabien Miomandre, David Mecerreyes, Fouad Ghamouss et Jean Le Bideau pour avoir accepté de faire partie des membres du jury au beau milieu de l’été. Leur intérêt et implication sur le thème de cette thèse ont donné lieu à une séance de questions/remarques des plus intéressantes à la suite de mon exposé.

Le travail présenté dans ce mémoire est le résultat d’une thèse CIFRE, financée par THALES et l’Association Nationale de la Recherche et de la Technologie (ANRT). J’ai donc été amenée à travailler régulièrement avec deux laboratoires : le Laboratoire de Chimie et des Matériaux Multifonctionnels (LCMM) au centre de Thales Research & Technology à Palaiseau, ainsi que le Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI) à l’Université de Cergy- Pontoise. Cela fait beaucoup de monde à remercier, et je vais essayer de le faire dans l’ordre chronologique pour n’oublier personne.

Avant de commencer cette thèse, j’ai découvert TRT en faisant un stage sur les céramiques piézoélectriques sous la direction d’Elodie Leveugle et de Mai Pham-Thi. Ce milieu m’a beaucoup plu, et je souhaitais rester à TRT. Quelques jours avant la fin de mon stage, j’ai discuté avec le chef du laboratoire de Chimie, Pierre Le Barny, ainsi que Christophe Galindo, futur chef du LCMM, d’un sujet de thèse concernant la mise au point d’électrolytes pour supercondensateurs. Quelques mois plus tard, le LPPI donnait son accord pour une collaboration sur le sujet. Pierre-Henri Aubert devenait mon directeur de thèse, Giao Nguyen et Fréderic Vidal co-encadrants de thèse. Pierre et Christophe ont quitté TRT deux mois et un an respectivement après mon arrivée, mais j’ai beaucoup appris durant cette première partie de ma thèse. Le départ de Christophe s’est accompagné de l’arrivée de Grégory Pognon, devenu mon encadrant, puis de Sébastien Jus, notre nouveau chef de laboratoire. Toutes les personnes que je viens de citer ont contribué de manière considérable au bon déroulement de ma thèse et je les remercie très chaleureusement pour tous les conseils qu’ils m’ont prodigué.

Côté LCMM, Christophe est un chef impartial, débordant d’énergie et donnant envie de se lancer des défis (comme la rédaction de thèse en anglais). Grégory, mon encadrant, ami et co-bureau, serait capable de faire comprendre l’électrochimie à n’importe qui…

Côté LPPI, je tiens à remercier P-H, Giao et Fred pour m’avoir aidée lorsque j’en avais le plus besoin, c’est-à-dire lors des rédactions de publications, et surtout du manuscrit de thèse. J’aimerais remercier particulièrement Giao pour ses conseils sur la synthèse de gels électrolytiques. Par ailleurs, votre importante implication tout au long de ma thèse m’a beaucoup aidé lors des changements d’encadrants TRT.

La bonne ambiance d’un laboratoire est essentielle pour travailler dans de bonnes conditions et

j’aimerais remercier toutes les personnes ayant contribué à cela. Je ne peux pas citer tout le monde,

mais j’aimerais remercier les personnes suivantes :

coup de marteau sur le démarreur de la R21!), Yves avec son accueil tout en bonbons et chocolats et son superbe labo de polissage, Françoise S (« ♫ Je vais bien, tout va bien ♫ ») et Françoise G., Claude (merci pour tes conseils en électronique et merci pour la planche à voile !!) , Shaima, Renato, Marc E. (Merci pour la super visite privée de Thales Helios !), ainsi que les doctorants, apprentis et stagiaires : Nicolas D. qui nous a tous convertis à la pole, Nicolas G. et sa bonne humeur quotidienne, Lilia, Romain, Rémy et nos sessions piano avec le 4 mains de Khatchatourian, Joffrey, Phong, Coline, Amélie…

Au LPPI : P-H, Giao, Fred, Cédric P. et Cédric V., Isabelle L., ainsi que Florent, Bruno (Oui oui, je viendrai te voir à Lima ! ☺ ), Thomas, Adrien, Houssam, Adelyne et Adeline, Vincent, Sébastien, Marie…

Une thèse, ce n’est pas seulement un projet de 3 ans, mais c’est également l’occasion de rencontrer de nombreuses personnes, généralement passionnées par les sciences. C’est pourquoi j’ai voulu m’investir dans l’association Young Employee Society (YES) de TRT. Organiser les visites de différents sites Thales était extrêmement enrichissant et je remercie notamment Florian (qui, en plus, a pris du temps pour me créer un super programme qui calcule les capas et me trace des courbes), Baptiste, Godefroy, Nicolas D., Lilia, Romain qui m’ont accompagné dans cette aventure. Je remercie également Cédric Demeure et Bertrand Porte de leur soutien pour YES.

Les conférences sont également l’occasion de créer un réseau autour d’un sujet commun, et je remercie Volker Presser et François Béguin, avec qui j’ai eu des conversations très intéressantes concernant les supercondensateurs.

Pour finir je souhaite remercier les personnes qui me rendent heureuse et auxquelles je tiens particulièrement. Jerem, évidemment, que j’aime et qui me fait rire quotidiennement, ainsi que mes parents, beaux-parents et grand-parents, et ma fratrie adorée : Marc et Noémie, Marie et Antoine, et Martin. Votre soutien m’est très précieux et fait ma force.

Je remercie également mes amis, dont certains finissent actuellement leur thèse (courage !). Faustine, Sidonie, Céline, Laure, Terry, Samy et Muriel, Charles et Aude, C. Ménard, Antoine B. et mon covoitureur Antoine J., Lucie et JC, Sambo, Sopheap, Hicham, ainsi que Marine et nos amis passionnés de voile.

Merci à tous !!

Tout est poison, rien n’est poison : seule la dose fait le poison.

Paracelse, Médecin (1493-1541)

TABLE OF CONTENTS

G

ENERAL INTRODUCTION

... 1

C

HAPTER

I: B

IBLIOGRAPHY

... 4

I. Supercapacitors ... 5

I.1. From capacitors to supercapacitors ... 5

I.2. The electrochemical double layer model and comparison to capacitors ... 6

I.3. Development of electrochemical double layer capacitors ... 8

I.4. Comparison between different types of energy storage systems ... 10

I.5. Supercapacitor applications ... 11

I.6. Components of supercapacitors ... 13

II. Ionic liquids ... 20

II.1. Brief history of ionic liquids ... 20

II.2. Ionic liquids applications ... 23

II.3. ILs organization in bulk and at interfaces with the electrodes ... 25

III. IL based electrolytes for wide temperature range applications... 28

III.1. IL mixtures as electrolytes ... 28

III.2. IL/organic solvent mixtures as electrolytes ... 30

References ... 33

C

HAPTER

II: E

XPERIMENTAL

... 39

Abbreviations of chemicals ... 40

I. Electrodes ... 41

I.1. Synthesis of bucky papers ... 41

I.2. Determination of the electrical conductivity ... 41

II. Electrolytes ... 42

II.1 Determination of the water content ... 42

II.2. Differential Scanning Calorimetry ... 42

II.3 Ionic conductivity ... 44

II.4. Viscosity ... 44

II.5. Density ... 45

II.6. Electrochemical characterizations ... 45

III. Gel electrolytes ... 47

III.1. Synthesis of gel electrolytes ... 47

III.1.1 Synthesis of polyethylene oxide (PEO) single network ... 48

III.1.2. Nitrile butadiene rubber (NBR) single network ... 48

III.1.3. Synthesis of NBR/PEO semi-interpenetrating polymer networks (semi-IPNs) and IPNs ... 49

III.2 Physical characterizations of gel electrolytes ... 51

III.2.1. Differential Scanning Calorimetry ... 51

III.2.2. Ionic conductivities ... 51

III.2.3. Electrochemical measurements ... 51

C

HAPTER

III: P

HYSICO

-

CHEMICAL AND ELECTROCHEMICAL PROPERTIES OF NEAT

IL

S AND

IL/GBL

MIXTURES OVER A

WIDE TEMPERATURE RANGE

... 52

Introduction ... 53

I. Generalities on physico-chemical properties of ionic liquids ... 53

I.1. Thermal properties ... 53

I.2. Viscosity and ionic conductivity ... 56

I.3. Ionicity ... 58

I.4. Electrochemical stability ... 60

II. Choice of the electrolyte ... 61

II.1. Selection of an organic solvent ... 62

II.2. Selection of an Ionic Liquid ... 62

III. Physico-chemical characterization of neat ILs and IL.GBL binary mixtures ... 64

III.1. Determination of the water content in IL and IL/GBL mixtures ... 65

III.2. Thermal properties ... 65

III.3. Ionic coductivity ... 67

III.4. Viscosity ... 69

III.5. Walden Plot approach ... 70

III.6. Electrochemical stability of ILs and IL/GBL electrolytes ... 71

IV. Conclusion ... 76

References ... 79

C

HAPTER

IV: E

LECTROCHEMICAL CHARACTERISATIONS AND ACCELERATED AGEING TESTS OF SUPERCAPACITORS BASED ON

IL/GBL

ELECTROLYTES AND SINGLE

-

WALL CARBON NANOTUBES ELECTRODES

... 82

Introduction ... 83

I. Determination of the electrochemical window at room temperature with a three-electrodes system ... 84

I.I Method ... 84

I.2. Results ... 86

II. Determination of the electrochemical window at room temperature with a two-electrodes system ... 88

II.1. Method ... 89

II.2. Results ... 90

III. Electrochemical stability of supercapacitors at extreme conditions ... 90

III.1. Electrochemical techniques... 91

III.2. Galvanostatic cycling and floating tests at 100°C on supercapacitors ... 94

III.3. Thermal stability of supercapacitors at 100°C... 98

III.4. Ageing of supercapacitors at -50°C ... 99

IV. Conclusion ... 100

References ... 104

C

HAPTER

V:

S

YNTHESIS AND PHYSICO

-

CHEMICAL CHARACTERIZATIONS OF NBR

/

PEO ELECTROLYTES AND THEIR USE

AS SEPARATORS FOR SUPERCAPACITOR APPLICATIONS

. ... 106

Introduction ... 107

I. State of the art of Solid State Electrolytes ... 108

I.I. Polyelectrolytes ... 108

I.1.1. Dry polyelectrolytes ... 108

I.1.2. Polyelectrolyte gels and ionogels ... 109

I.2. Solid Polymer Electrolytes ... 111

I.2.1. Classical solid polymer electrolytes ... 111

I.2.2. Solid Polymer Gel electrolytes ... 112

I.3. Interpenetrating polymer networks (IPNs) and semi-IPNs ... 113

II. Synthesis of SGEs based on PEO, NBR single networks and NBR/PEO (semi-)IPN ... 115

II.1. Single networks ... 115

II.1.1. PEO based gel electrolytes synthesis ... 115

II.1.2. Nitrile butadiene rubber (NBR) polymer electrolytes synthesis ... 116

II.2. Synthesis of SGEs based on NBR/PEO IPNs and semi-IPNs ... 117

II.2.1. IPN synthesis description ... 118

II.2.2. Semi-interpenetrating netwoks ... 118

II.3. Optimization of gel electrolytes based on NBR/PEO (semi-)IPN ... 119

II.3.1. IPN versus semi-IPN: ionic conductivities ... 119

II.3.2. Electrolyte content ... 120

III. Physical and electrochemical properties of NBR/PEO semi-IPNs ... 123

III.1. Thermal analysis ... 123

III.2. Effect of the scan rate on the capacitance evolution ... 124

III.3. Determination of the electrochemical windows at different temperatures ... 126

III.4. Thermal ageing of systems containing NBR/PEO semi-IPN gel electrolytes and BPSWCNT electrodes ... 127

III.4.1. Capacitance evolution during thermal and floating ageing ... 127

III.4.2. Opening of coin cells after thermal and floating ageing ... 129

Conclusions ... 131

References ... 133

G

ENERAL CONCLUSION

... 137

P

ERSPECTIVES

... 139

A

PPENDIX

... 140

A.I. Representative cations and anions used in aprotic ionic liquids ... 141

A.II. Relation between the pseudo-activation energy involved in conductivity and viscosity ... 142

A.III. Vogel-Tammann-Fulcher equation parameters for IL and IL/GBL ionic conductivity measurements from -50°C to 100°C ... 143

A.IV. Measurement of the water content in IL and IL/GBL mixtures by Karl Fisher titration ... 143

A.V. Thermal analysis of IL and IL/GBL electrolytes in BPSWCNTs electrodes ... 144

A.VI. Electrochemical impedance spectroscopy ... 145

A.VII. Mechanisms of radical polymerization ... 147

References ... 147

R

ESUME

... 148

S

UMMARY

... 149

1

G ENERAL INTRODUCTION

Global warming, pollution and geopolitical concerns are among the problems connected with the dependence of modern societies on fossil fuels. Efforts are being made around the world in order to fight against these issues. In that perspective, the European Council of 11 and 12 December 2008 definitively adopted the European plan on climate change involving the 20-20-20 targets: by 2020, reduce by 20% the emissions of greenhouse gases, increase by 20% the energy efficiency in the EU and to reach 20% of renewables in total energy consumption in the EU [1].

To deal with the intermittent nature of renewable energies such as wind and solar power, Energy Storage Systems (ESSs) with high power and high energy densities have been developed.

Among them, supercapacitors are promising ESSs capable of managing high power rates compared to batteries (the power densities are 10-20 kW.kg

^-1

and ~1 kW.kg

^-1

respectively).

However, they are not able to store the same amount of charge as batteries (energy densities are

~ 1-15 Wh.kg

^-1

and ~100-150 Wh.kg

^-1

respectively). Consequently, supercapacitors are suitable for applications in which power bursts are needed but high energy storage capacity are not required.

Due to their properties, supercapacitors, are often associated to batteries in order to increase their lifetime by insuring the peak of demand that impair batteries. This couple [battery/supercapacitor] is of particular interest in transport and its use is spreading all over the world. For example, the Group Bolloré produces the “Bluecar”, part of the Autolib’ carsharing in Paris since 2011. It is a 100% electric vehicle that possess a 30kWh lithium polymer battery coupled to a supercapacitor, that can run for 250km without charging. In the same perspective of producing all electric vehicles, Bolloré produces the Bluetram, the Bluebus or the Blueboat [2].

Buses equipped with supercapacitors are also expanding in China. They charge at bus stops while passengers are getting on and off the bus. Another advantage of supercapacitors is that the energy released during braking of trains or racing cars can be stored and then released when a power burst is needed. That operation can be performed for a high number of charge/discharge as their lifetime is extremely high, with up to a million cycles without a decrease of their performances.

The general context of this thesis is governed by Thales Avionics, which seeks the replacement of

electrolytic capacitors by supercapacitors in planes. Indeed, as the specific energy of

supercapacitors is higher than that of electrolytic capacitors, at equivalent performances, the

volume and weight occupied by supercapacitors can be much smaller than the one of electrolytic

2 as:

 Safety (no leakage, use of green and thermally stable electrolytes)

 Broad operating temperature range (from -50°C to 100°C)

 Resistance to vibration

This thesis has been focusing on the two first requirements, i.e the elaboration of safe electrolytes, with good thermal and electrochemical stabilities from -50°C to 100°C. Ionic liquids (ILs) are candidates that perfectly match these properties. Indeed, their extremely low volatility renders them safer than electrolytes based on organic solvents. Moreover they have wider electrochemical stability windows (4-6V) than aqueous electrolytes (~1V) or electrolytes based on salts in organic solvents (~2.7V).

However, ILs exhibit high viscosities and their melting points lie within the desired temperature operating range. Supercapacitors power performances rely on the ability of ions to move quickly to the electrodes upon charging. It is therefore necessary to decrease ILs’ viscosities to allow better ion mobility. To that purpose, addition of an organic solvent upon ILs would lead to a decrease of the viscosity, as well as a decrease or supression of the melting temperature, hence extending the operating conditions down to lower temperatures.

The first chapter is devoted to a state–of-art of supercapacitors. A brief history of their development and applications will be presented. Then, its individual parts (electrode material, separator and electrolyte) will be described. Since this thesis has been focused on supercapacitors working over a wide temperature range, and that designing new electrolytes is the best way to achieve this goal, it was decided to orient our work toward electrolytes based on ionic liquids/organic solvent mixtures. Therefore, a bibliography on ionic liquids and organic solvents was established and will be presented. It allowed us to select a category of ionic liquids:

aprotic ionic liquids (AILs), and an organic solvent: -butyrolactone.

The second part of this work will consist in choosing a selection of three ionic liquids as electrolytes among AILs. Then, the thermal behavior as well as the physico-chemical properties such as viscosity and ionic conductivity of neat ILs and ILs/organic solvent mixtures will be determined from -50°C to 100°C. They will also be characterized electrochemically in this temperature range in order to determine their electrochemical stability window.

The third part of this work will be focused on the performances of supercapacitors based on bucky

paper single wall carbon nanotubes (BPSWCNT) electrodes and ILs or ILs/organic solvent

electrolytes. Classical ageing test will also be performed.

3

device, the fourth and last part of this work will be devoted to the elaboration of solid-state supercapacitors. The synthesis and physical characterization of gel electrolytes will be detailed.

They will be constituted of a Nitrile Butadiene Rubber / Poly(ethylenoxide) semi-interpenetrating polymer network retaining an ethyl-methyl imidazolium bistrifluoromethansulfonylimide/

-butyrolactone electrolyte.

4

C ^HAPTER I: B IBLIOGRAPHY

5

I. Supercapacitors

I.1. From capacitors to supercapacitors

The first capacitor was invented in 1746 by Pieter van Musschenbroek at the University of Leyden in Holland. The “Leyden jar” (Figure 1) consists of a glass jar half-filled with water, coated inside and outside with a thin layer of tin or silver foil, and usually a metal wire or chain is driven through a cork in the top of the jar. The glass acts as a dielectric. The outer foil is grounded while the inner foil and can be charged through the chain with an electrostatic generator. Once charged, the jar can produce a strong electrical discharge by connecting the brass rod to the outside tin foil.

Figure 1: Schematic of the Leyden Jar [3]

Benjamin Franklin improved the Leyden jar by replacing the jar by flat glass, and years later

Michael Faraday contributed to the development of capacitors by trying to store unused electrons

produced during the experiment. His work led to the first usable capacitor made from large oil

barrels, and his achievements in the field of electricity lead to the use of “Farad” as the unit of

capacitance. One Farad is defined as the capacitance of a capacitor across which, when charged

with one coulomb of electricity, there is a potential difference of one volt, knowing that one

coulomb is equal to the amount of charge ( 6.241×10

¹⁸

electrons) produced by a current of one

ampere (A) flowing for one second, 1C = 1 A.s.

6

I.2. The Electrochemical Double Layer (EDL) model and comparison to capacitors

Conventional capacitors are composed of two electron-conducting plates, separated by a dielectric, i.e an insulator that can store energy by becoming polarized such as glass, ceramic, plastic film or paper. When a difference of potential is applied between the plates, an electric field develops across the dielectric and the positive charges accumulate on one plate (Q+) and the negative charges accumulate on the other one (Q-) (See Figure 2).

Figure 2: Schematic of charged conventional capacitor

The difference of potential between the electrodes is the voltage (V). The ratio of the charge Q (Coulomb) to the voltage gives the capacitance (Farad) value of the capacitor as shown in Equation 1:

= ^{Equation 1}

The capacitance also depends on the surface area of the plates as well as the thickness of the dielectric, according to the following equation:

= ^{  Equation 2}

Where C (F) is the capacitance, A the area of the plates (m²), 

0

the vacuum permittivity (

0

=8.854.10

^-12

F.m

^-1

) and 

r

the relative permittivity of the dielectric material, and d (m) the distance between the plates.

Electrochemical double layer capacitors (EDLCs) are often compared to conventional capacitors

because they both rely on electrostatic energy storage. More than that, ELDCs behave like two

capacitors in series: one capacitor for each electrode/electrolyte interface. In the equivalent circuit

presented in Figure 3, each capacitor is represented by a capacitance (C

DL

) in parallel with a

resistance (R

i

). Two other resistances are involved: the ionic resistance of the electrolyte Re, and

the interface resistance between electrodes and current collectors Rc.

7

When a difference of electric potential is applied between the electrodes, electrons in the negatively polarized electrode are balanced by an equal number of positive cations at the electrode/electrolyte interface, while the holes stored at the positively polarized electrode are electrically balanced by anions.

Figure 3 : Representation of the equivalent circuit of an EDLC (left). The equivalent series resistance (ESR,

 is the sum of the resistances involved in the device: ionic resistance of the electrolyte Re, leakage current R

l

and the interface resistance between electrodes and current collectors Rc. Representation of a charged state symmetric EDLC (right).

For a closer look at the double layer, it is worth mentioning the study of the electrochemical double layer concept by electrochemists. In 1853, Helmholtz [4] defined the double layer (DL) as a molecular dielectric that electrostatically stores charge. It is composed of the layer of positive or negative charge of the electrode, and the layer of adsorbed ions of opposite charge on the electrode. Then Gouy and Chapman [5],[6] created a model that takes into account the distribution of ions close to the adsorbed layers, and called it the diffuse layer. In 1924, Stern [7]

improved this model by associating Helmholtz and Gouy-Chapman models, as well as the finite size of ions (Figure 4).

Figure 4: From left to right: Helmholtz, Gouy-Chapman, and Stern model where Ψ is the potential and Ψ

0

the electrode potential [8].

C

_dl+

C

_dl-

R

_l+

R

_l-

R

_e

R

_c

R

_c

8

The capacitance of the double layer (C

DL

) can be represented as the equivalent capacitance of two capacitors in series, C

S

for the Stern layer and C

D

for the diffuse layer. The capacitance C

DL

is given by Equation 3:

= + Equation 3

Similarly, double layers on both electrode/electrolyte interfaces of a supercapacitor are equivalent to two capacitors in series, and the capacitance of the entire system can be expressed according to Equation 4:

= + ^{Equation 4}

Where C

DL+

, C

DL-

and C are the capacitance (F) of the positive electrode, the negative electrode, and the resulting device, respectively.

As for a conventional capacitor, the capacitance of each double layer can be written as C=(





r

.A)/d, where 

r

is the relative permittivity of the dielectric electrolyte, d is the effective thickness of the Helmholtz double-layer (i.e molecular size, a few Angstroms) and A is the surface area. The very small d values account for the very high capacitance of EDLCs and the use of their superlatives names such as “Supercapacitors” or “Ultracapacitors”.

The use of specific gravimetric capacitance is generally found in the literature for symmetric systems, composed of electrodes similar by their composition, size and weight. The gravimetric capacitance of the single electrode, C

E

(F.g

^-1

), is given by Equation 5

= . Equation 5

Where C

^E

is the capacitance of a single electrode and m the total weight of active material on both electrodes.

I.3. Development of EDLCs

The first electrical device using the double-layer charge storage was reported in 1957 by H.I.

Becker of General Electric, but the invention of the device as we currently know is attributed to

Robert A. Rightmire, a chemist at the Standard Oil Company of Ohio (SOHIO). Since then,

SOHIO researches lead to many patents but in 1971 the company had to cut many long-term

development programs for a time because of debts. Nippon Electric Company (NEC) then

contributed to the development and commercialization of “Supercapacitors” in 1978 under license

from SOHIO. Initially, they were used as back-up power devices for volatile clock chips and

complementary metal-oxide- semiconductor (CMOS) computer memories. One unique feature of

9

the NEC capacitor design was bipolar construction. NEC developed processes to stack six or eight cells in series in a bipolar arrangement and successfully sealed the entire device. This is important because this simple construction eliminates the need for cell interconnects. NEC reported development of much larger double-layer capacitors in 1991, using an activated carbon composite electrode. In the meantime, many companies emerged all around the world in order to develop EDLCs. Following a list of well-known EDLCs companies [9]:

Panasonic (1978, Japan). Panasonic manufactured ‘Goldcap” double layer capacitors in order to replace unreliable coin cell batteries used in memory back-up. A non-aqueous electrolyte was used in the cell construction and offered the advantage of higher unit cell operating voltage (6 cells in series for 5.5 V in NEC versus 3 cells in series for 5.4 V in Panasonic). In 1999, Panasonic introduced the well-designed

‘UpCap’ capacitor, rated at 2,000F and 2.3V for transport applications including Hybrid-Electric Vehicles (HEVs).

ELIT (1988, Russia): ELIT (former named as MP Pulsar) is the first company to develop asymmetric electrochemical capacitors, based on a nickel oxyhydroxide positive electrode, potassium hydroxide electrolyte, and an activated carbon negative electrode. Those devices were designed to power wheelchairs and children’s cars. In 1990 ELIT shifted to symmetric capacitors with potassium hydroxide electrolyte and carbon electrodes.

ECOND Corporation (1991, Russia). ECOND ‘PSCap’ are large capacitors used to start 3,000 horsepower diesel locomotive engines. They use bipolar construction with an aqueous electrolyte.

Equivalent series resistances of modules are typically in the milliohm range and module voltages up to 200 volts are common.

Maxwell (1991, California): Before 1991, Maxwell technologies had developed a broad line of high- voltage capacitors used in high-energy-density applications like laser flash-lamp power supplies. Then, the company was awarded a contract by the U.S Department of Energy (DoE) and Maxwell had to orient researches toward required specifications. The key to their study was to reduce the electronic resistance by working on the shape and nature of the electrodes (aluminum-carbon cloth), as well as on the current collector. Those developments lead to the production of Bootscaps supercapacitors, from 5F cells, to 3,000F, 2.7V rated tubular cells.

Other manufacturers emerged with a variety of supercapacitors that offers energy densities ranging from

1 to 15 Wh.kg

^-1

and power densities up to 40 kW.kg

^-1

[10]. Among them : ESMA, ELTON (Russia),

CSIRO, CAP-XX (Australia), Nesscap (1998, Korea),Hosen Corporation and Nippon Chemicon, NCC

(1995, Japan) , Saft, Bollore Technologies (2001, France), Skeleton Technologies (Estonia) and

FastCAP Systems (U.S.A) [9].

10

The development of supercapacitors over the last 30 years has been focused on the increase of energy and power densities by studying asymmetric and symmetric capacitors with aqueous, organic or ionic liquid based electrolytes, as well as the good sealing of the packaging.

I.4. Comparison between different types of energy storage systems (ESS)

In 1968, D. Ragone suggested using a logarithmic diagram representing the specific energy (or pnergy density) (Wh.kg

^-1

) versus the specific power (or power density) (W.kg

^-1

) to compare the performance of batteries, and the use of the resulting Ragone plot (Figure 5) was latter extended to many energy storage devices such as capacitors, supercapacitors or fuel cells. For example, batteries are known to be devices containing a high energy density but having a low power density (they can deliver energy for a long time but they slowly charge and discharge): they lie on the top left of the plot. On the opposite, capacitors have a high power density but a poor energy density: they lie on the bottom right of the plot. The Ragone plot testifies to the wide variety of ESSs, which allows tailoring their use for all types of applications depending on their demand of high energy or power, but also assessing their complementary nature in hybrid equipment.

Figure 5: Ragone plot : Energy density (Wh.kg

^-1

) versus Power density (W.kg

^-1

) for various ESSs.

For a supercapacitor, the maximum stored specific energy (Wh.kg

^-1

) is given by Equation 6:

= _{. .} ^{Equation 6}

11

where C(F) and m (kg) are the capacitance and the mass of the supercapacitor respectively, and V

max

(V) is the maximum voltage that can be applied without deteriorating the electrolyte.

The normalization to the supercapacitor mass provides a basis for comparison with other devices.

The maximum specific power (W.kg

^-1

) also depends on the maximal applicable voltage and is given by Equation 7:

= _{. .} ^{² Equation 7}

where ESR () is the equivalent series resistance of the system i.e the sum of the resistances involved in the device: ionic resistance of the electrolyte, leakage current and the interface resistance between electrodes and current collectors.

As seen on the Ragone plot (Figure 5), supercapacitor performances are between those of batteries and capacitors, and offer a large amount of energy that they can deliver quickly.

Besides, they exist in many different types so they cover an expansive region of the Ragone plot and can be used for various applications.

I.5. Supercapacitor applications

Supercapacitors modules exist in different forms: cylindrical, prismatic (Figure 6), but also in different sizes, so they can be used for different applications. They usually complement a primary energy source that cannot repeatedly provide quick bursts of power, such as an internal combustion engine, fuel cell or battery. Besides, such association with supercapacitors extends the lifetime of these devices.

Figure 6 :Cylindrical (left) and prismatic (right) types of supercapacitors [11]

12 Following the main application fields for supercapacitors:

Trains and Hybrid-electric vehicles (HEVs): regenerative systems

In regenerative systems, energy is captured and re-used at a later time. The energy capture comes from the vehicle kinetic energy reconversion system (KERS) and is subsequently used during acceleration steps. These events are typically short in duration but very high in power, thus supercapacitors are particularly adapted to this kind of applications. Ex: racing car Formula 1 (2009), Toyota Prius, N700 series of the Shinkansen (Japan 2009), fourth generation TGVs (France, 2010), German ICE 3 trains (Germany, 2012) [12]

Cars: start/stop technology

Start/stop technology enables the engine in a vehicle to shut down when it is in a traffic jam or at a red light. As soon as the driver depresses the clutch or brake pedal, the system automatically restarts the engine. Ex: implementations in HEVs are made by Peugeot SA in e-HDi micro-hybrid technology.

Buses and Trams

Electric buses and trams have been put into use in different countries. Bolloré started the production of the BlueTram in 2014 in France. Equipped with 30 supercapacitor modules, these buses can charge for 90 seconds and then drive for 1.5km. More recently (July 2015) a bus equipped with CSRCAP supercapacitors from the Ningbo CSR New Technology company appeared in China, and can charge in ten seconds which allows driving for 5 km. The lifetime of those supercapacitors is close to 12 years and they can undergo a million charge/discharge.

Aeronautics:

In 2006, 16 supercapacitors were implemented on Airbus A380 airplane for the opening of emergency doors in case of power failure.

Wind turbine pitch control:

Supercapacitors are used in blade pitch systems and help increase reliability and stability to the energy grid. The pitch control system in a wind turbine is located in the rotor hub and controls the rotor blades. The orientation of the rotor blades influences how much wind energy is captured.

The main purpose for the pitch system is to orient the rotor blades in order to capture the maximum wind energy or to protect the turbine against high speed winds.

Forklifts and cranes:

Supercapacitors capture energy and provide power peaks to assist in lifting operations and ease

the stress on the battery pack which runs the forklift.

13

I.6. Components of supercapacitors

Electrodes materials

Supercapacitors can be symmetrical, asymmetrical (two different electrode materials), hybrid (i.e one of the electrodes is similar to an EDLC electrode and the other is a battery-like electrode), or they can also rely on pseudo-capacitance (Figure 7). Most of commercial supercapacitors are symmetrical and based on activated carbon while the others are still at stage of research. Hybrid systems and pseudo-capacitors are interesting because they have a higher energy density than EDLCs which rely only on a capacitor behavior. However, faradaic reactions involved in those devices are not entirely reversible and limit their lifetime.

Figure 7 : Different types of supercapacitors and the nature of their electrodes

The most common materials used for supercapacitor electrodes are activated carbon, carbon nanotubes and graphene. All of these materials have different advantages and drawbacks, and this is why it can also be interesting to combine them [13]. Ideal electrode materials have a large surface area, a good electronic conductivity, and optimized pore size as well as pore distribution.

Main features of these materials are presented below:

Activated carbon (AC)

Activated carbon is the most widely used active material for supercapacitor electrodes due to its

high surface area (up to 3000 m².g

^-1

) and relatively low cost. They are most often obtained from

coconut shells woods, fossil fuel and their derivative or even polymers, by performing a heat

treatment under inert atmosphere (carbonization) and then activation (oxidation treatment to

create porosity). It is possible to control partially the surface area obtained by modifying the

14

oxidation process, which can be physical (heating treatment under oxidizing atmosphere) or chemical (KOH, NaOH, K

2

CO

3

…) [14]. However, the pore size distribution is much dispersed and many pores are too narrow (micropores, <2nm) to allow electrolyte ions inside of them: the surface area of AC is underused. Still, the great surface area of AC leads to very high capacitances (up to 300. F.g

^-1

[15]) and consequently high energy densities. To make the electrodes, it is common to mix AC with carbon black to improve the electron conduction, and to use a binder such as PTFE [16].

Graphene

Graphene is one-atom thick sheet made of sp² bonded carbon atoms in a polyaromatic honeycomb crystal lattice. It is the basic structural element of other allotropes such as graphite, charcoal, carbon nanotubes and fullerenes. It was first observed in 1962 but it is only in 2004 that it could be isolated and characterized by A.Geim and K.Novoselov [17]. This work resulted in the two winning Nobel Prize in physics in 2010. The outstanding properties of electrical conductivity as well as chemical and thermal stability made graphene attractive in the field of semiconductor, electronic and of course energy storage devices such as batteries and supercapacitor. However, graphene sheets tend to re-stack because of - interactions between the layers, which lead to a decrease in the surface area and the capacitance of the system. Consequently, graphene has been mixed with different materials such as metal oxides [18], polyaniline[19], polypyrrole[20] or carbon nanotubes [21] in order to limit the  stacking. In the same idea, Y.B. Tan et al.[22] and Q.

Ke et al. [23] published a review on graphene for supercapacitor applications.

Ningbo CSR New Energy Technology, a subsidiary of the Chinese company CSR Zhuzhou Electric Locomotive and Skeleton technology are working on the development of graphene based supercapacitors [24].

Carbide derived carbon (CDC)

CDCs are prepared by high-temperature extraction of metals from carbides such as TiC or SiC serving as precursors. The most common methods for CDC production are high-temperature chlorination or vacuum decomposition [25][26]. They are interesting for supercapacitor applications because they allow a fine-tuning of porous network and a better control than AC over surface area (S

A

~2000m².g

^-1

) or functional groups [27]. For example, the pore size usually increases with an increase of temperature treatment, which is generally between 1000°C (carbonization temperature) and 1300°C (the structure collapses for higher temperatures).

Regarding supercapacitors performances, the capacitance depends on the CDC structure

(usually within the range 70-220 F.g

^-1

) [25][28] and the applicable rate depends on starting

carbide. CDC can be adapted for high energy or high power applications by changing the

synthesis temperature. Skeleton Technologies produces also supercapacitors based on CDC

electrodes.

15 Carbon nanotubes (CNTs)

Carbon nanotubes can be found under different forms: single wall (SWCNT) [29], multi-wall (MWCNT) [30] or vertically aligned (VACNT) [31]. They are prepared by catalytic decomposition of certain hydrocarbons like ethylene, toluene and only their external surface area is accessible to the electrolyte ions [32][33]. The surface area is between 250 and 400m².g

^-1

and the pore size distribution is controlled, with a majority of mesopores (2-50nm). The specific capacitance (20- 80F.g

^-1

) [16] is rather low compared to AC mainly because of a difference in surface area and also the hydrophobic property of CNTs. It is possible to increase the capacitance to about 130F.g

^-1

by chemical or heating treatments: it increases the surface area and creates functional groups that increase the hydrophilicity or even participate to pseudo-capacitance [14][34][35].

However, if the conditions of temperature or acidification are too harsh it can damage the nanotubes. CNTs can be shaped into electrodes without the use of a binder and therefore allows high power densities 20 kW.kg

^-1

in potassium hydroxide [36]. The American company FastCap Systems is currently developing supercapacitors based on VACNTs [37].

Onion-like carbon

Onion-like carbons (OLCs) are quasi-spherical nanoparticles with diameter of 6-7 nm, consisting of concentric graphitic shells, and were first produced in 1994 by Vladimir Kuznetsov by vacuum annealing of nanodiamond precursors [38]. Their average specific area is close to 500m².g

^-1

, and this surface is fully accessible to ion adsorption because there is no porous network inside the particles. Carbon onions debuted as a material for supercapacitors in 2006-2007 in both aqueous and organic electrolytes. A few years later, D. Pech et al. [39] studied micro-supercapacitors, produced by the electrophoretic deposition of a several–micrometre-thick layer of nanostructured carbon onion. This process does not require the use of an organic binder. They showed that such systems lead to very high power performances (close to 1kW.cm

^-3

, and roughly ten times higher than for activated carbon), certainly because ions can easily access the active material. However, the gravimetric capacitance is about one-third that of activated carbon.

Main features of carbon materials used in commercialized supercapacitors are presented in

Figure 8, and newest supercapacitors on the market and their data are available in Table 1.

16

Figure 8 : Different carbon material used for supercapacitor applications and their main features [38][40].

Table 1 : Recent developments in supercapacitors

Company Electrode type Capacitance

(F) Applicable

voltage (V) Energy density

(Wh.kg

^-1

) Power density (kW.kg

^-1

) Ningbo CSR New

Energy Technology

Graphene and

AC 12 000 3.0 ^{11.7 19.0}

Ningbo CSR New Energy

Technology

Nano-hybrid

graphene 30 000 2.8 21.0 2.0

Skeleton

Technologies CDC ^{4 500 2.9 9.6 19.5}

Maxwell

BCAP3400 unspecified 3400 2.85 7.4 14

Maxwell K2 series

unspecified 3000 3 3.75 16

Values provided by the websites of the three different companies

17 Separator

The separator is a physical barrier between the two electrodes of the supercapacitor with a dual-acting of preventing electrical shorting and letting ions move freely between the electrodes. It can be a gelled electrolyte [41][42], a microporous polymeric film (polypropylene, polytetrafluoroethylene) or other porous inert material, such as cellulose, filled with electrolyte.

Properties such as thickness, wettability, porosity and thermal or electrochemical stability are vital in maintaining a low ESR.

Many polymeric-based separators have been developed and are now commercially available:

 Cellulose-based separator (paper)

 Glass-fiber-based separators: fibers size:1-4 µm, porosity: 70-90%, thickness: >30µm. This type of separator is useful for devices that undergo cycling at extreme temperatures.

 Porous propylene films: Ex: Celgard (See Figure 9) developed for Li-ion applications, but is not porous enough to maintain a low ESR in supercapacitors

 Polytetrafluoroethylene (PTFE) Ex: Gore (See Figure 9)

 Polyvinylidene fluoride (PVdF): porosity ~80%, good mechanical properties

 Polyimide for high temperature applications, expensive

 Polyamide-based porous films with TiO

2

for higher heat resistance

 Thermoplastic resin and pulp fiber: excellent mechanical strength in addition to liquid permeability, expensive.

T.L.Wade [43] carried out an interesting work on supercapacitor separators in order to evaluate the effect of their structural features on supercapacitor performances. Those features are the pore size, pore size distribution, porosity, thickness and tortuosity. Four different materials were selected: polypropylene, cellulose, polyethylene and polytetrafluoroethylene.

Figure 9 : SEM images of commercial separators created using a variety of materials: Celgard 2500 (Polypropylene)

(top left), NKK TF40 (Cellulose) (top right), Solupor 14P01A (Ultrahigh Molecular Weight Polyethylene) (bottom left),

Gore 11367985-3 (Polytetrafluoroethylene) (bottom right).

18 They conclude that:

 The separator is a major contributor to the resistance of high power supercapacitors, causing greater than 30% of the total resistance for the four commercial separators investigated (Figure 9).

 The supercapacitor resistance is directly proportional to the thickness while being inversely proportional to the porosity of the separator. Pore size or pore size distribution have no effect on the resistance.

 Supercapacitor resistance is a function of voltage.

Electrolytes

Electrolytes used in supercapacitors are aqueous, organic, or liquid salts (commonly known as room temperature ionic liquids). The two main criteria for the selection of an electrolyte are the electrochemical stability window and the ionic conductivity. The first is important to maximize the specific energy values, while the second is correlated to the ESR and therefore has a major influence on the specific power. It is also important to have a high ion concentration (usually higher than 0.2mol/L) in order to avoid depletion problems during the charge of the supercapacitor [8].

Aqueous electrolytes

Aqueous electrolytes are generally acid-based (H

2

SO

4

) or alkali (KOH) and exhibit the highest conductivities among electrolytes (up to 10

³

mS.cm

^-1

), giving a high power performance to the system. However, the narrow electrochemical stability window (EW) of the water (EW=1.23V at neutral pH) limits the applicable voltage to 1V, which is the lowest EW found among electrolytes and limits both the energy density and the power density. However, Khomenko et al. [44]

demonstrated that it is possible to increase the operating voltage up to 1.6V for H

2

SO

4

electrolytes by using different optimized carbons as positive and negative electrodes, and/or balancing the mass of the electrodes. The corrosion of current collectors usually made of Aluminum alloys leads to the development of neutral pH electrolytes that would prevent such deterioration.

Organic electrolytes

Organic electrolytes are composed of salts in organic solvents. The main advantage of these

electrolytes compared to aqueous ones is their wider electrochemical stability window, close to

2.7V. Nevertheless, they exhibit an higher ESR and a lower ionic conductivity, which lead to a

reduced power capability. Capacitance is also generally lower than for aqueous electrolytes

because of lower concentrations and wider ionic radius. The most widely used solvents are

acetonitrile and propylene carbonate, and classical salts are tetraethylammonium

tetrafluoroborate (TEABF

4

) or tetrabutylammonium hexafluorophosphate (TBAPF

6

). Acetonitrile

19

leads to good performances but it is toxic and its low boiling point (82°C à 760mmHg) limits the maximum operation temperature to about 70°C. Ue et al. [45] classified 0.65M TEABF

4

/organic solvents solutions depending on their viscosity, ionic conductivity, phase transition temperatures and electrochemical stability (Table 2).

Table 2 : Physical properties (dielectric constant (

r

), viscosity ()), melting temperature (Tm) and boiling temperature (Tb), and electrolytic conductivities () of common organic solvents. Limiting reduction (Ered) and oxidation (Eox) potentials of their organic liquid electrolytes containing 0.65 mol.L

^-1

TEABF

4

at 25°C.[45],[46]

Intrinsic properties of solvents TEABF

4

solutions

Solvents 

r

Tm Tb   Ered Eox

 (°C) (°C) mPa.s) mS.cm

^-1

 (V vs.SCE) Propylene carbonate

(PC)

65 -49 242 2.5 10.6 -3.0 +3.6

Butylene carbonate (BC)

53 -53 240 3.2 7.5 -3.0 +4.2

-Butyrolactone (GBL)

42 -44 204 1.7 14.3 -3.0 +5.2

-Valerolactone (GVL)

34 -31 208 2 10.3 -3.0 +5.2

Acetonitrile (ACN)

36 -49 82 0.3 49.6 -2.8 +3.3

Glutaronitrile (GLN)

37 -29 94 5.3 5.7 -2.8 +5

Adiponitrile (ADN)

30 2 108 6.0 4.3 -2.9 +5.2

Methoxyacetonitrile (MAN)

21 -35 120 0.7 21.3 -2.7 +3.0

3-methoxypropionitrile (MNP)

36 -57 165 1.1 15.8 -2.7 +3.1

Sulfolane (TMS)

43 28 120 10 2.9 -3.1 +3.3

Dimethylsulfoxide (DMSO)

47 19 189 2 13.9 -2.9 +3.5

Trimethylphosphate (TMP)

21 -46 197 2.2 8.1 -2.9 +3.5

In this table we can see that several solvents combine desired properties of a wide liquid range

temperature, good ionic conductivity and wide electrochemical stability window. Among them,

butylene carbonate, -butyrolactone and -valerolactone appear to be very promising with ionic

20

conductivities close to 10mS.cm

^-1

, electrochemical windows wider than 7V and at liquid state over temperature range wider than [-40°C, 200°C].

Ionic liquids (ILs)

The attraction of ILs in energy storage applications is due to numerous desired properties such as:

 A high thermal stability that is correlated to a wide liquidus range: unlike molten salt composed of small ions, ILs ions often have large and asymmetric shapes. As a result, Coulombic interactions are weaker in the range of their action. Moreover, when cations are associated to charge-delocalized anions, it is difficult for the system to acquire a stable ordered crystal structure [47].

 A low volatility which improves the safety of electrochemical devices: electrostatic interactions between ions strongly keep them together, preventing them from forming ions pairs that can enter in vapour phase. Thus, the boiling point of ILs is often higher than their decomposition temperature, Td > 200-300°C.

 A wide electrochemical window (close to 5V) which increases the device efficiency.

 The “green” label. They are considered as green solvents for their ability to be recycled almost indefinitely with no or minimal use of volatile 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 [47]. However, although ILs based on fluoride anions are known to be water-stable [48], they can decompose under conditions used for their preparation or during acidic reactions, releasing toxic and corrosive HF[49][50]. Thus, new fluorine-free ionic liquids have been investigated [51][52].

Our work is entirely based on the use of ILs and ILs/organic solvents as electrolytes for supercapacitor applications over a wide temperature range. Therefore, the following part of this chapter is devoted to the development of ILs over the last century and its use in different applications, and particularly in supercapacitors. More specifically, the ions organization in bulk and at the interface with electrodes will be detailed.

II. Ionic liquids

II.1. Brief history of ionic liquids

The interest toward ionic liquids is relatively recent but their discovery happened around a

century ago. The most popular definition is “salts with a melting temperature below 100°C”. It was

first brought up by P. Walden in his paper on ethylammonium nitrate (EAN) [53], and then it was

codified in a NATO workshop in Crete in 2000.Many publications keep this definition in spite of the

criticized decision to arbitrary choose the boiling temperature of the water as the melting temperature

for ILs.

21

N N

The development of ionic liquids is divided in three generations, summarized in Table 3:

Table 3: Development of ionic liquids

Year Researchers Molecule Properties

1914 P.Walden[53] (EtNH

3+

, NO

3-

) ethylammonium nitrate

Tm=12°C

Sensitive toward air and water

First generation of ionic liquids 1948 F. Hurley and

T. Wier[54]

AlCl

4-

ButylPyridiniumchloroaluminate

Sensitive to hydrolysis, release hydrochloric acid with a bad impact on the process and the environment

1978 B.Osteryoung, Hussey [55][56]

AlCl

4-

Tetraalkylammoniumchloroaluminate

Sensitive to hydrolysis, release hydrochloric acid with a bad impact on the process and the environment

Second generation of ionic liquids 1982 Wilkes AlCl

4-

1-ethyl-3-

methylimidazolium chloroaluminate

Cation hydrolytically stable

1992 Wilkes, Zawarotko

[51][52] 1-ethyl-3-methylimidazolium

with tetrafluoroborate (BF

4-

), hexafluorophosphate (PF

6-

), sulfate (SO

4-

) and nitrate (NO

3-

)

Anions stable toward water.

However long term exposure to humidity can affect their

physico-chemical and

electrochemical properties, and anions can decompose releasing toxic and corrosive hydrogen fluoride [49][50].

1992 Cooper and O’Sullivan

trifluoromethanesulfonate (CF

3

SO

3-

) methanesulfonate (CH

3

SO

3-

) and tris-

Stable in aqueous medium

N N

22

[57] (trifluoromethanesulfonyl)methide [(CF

3

SO

2

)

3

C

^-

] with imidazolium cations

Task specific ILs

1996 Bonhôte [58] Triflate (TfO

^-

), Nonaflate (NfO

^-

) bis(trifluoromethylsulfonyl)imide (TFSI

^-

), Trifluoroacetate (TA

^-

), Heptafluorobutanoate (HB

^-

) with imidazolium cations

Hydrophobic anions

>2000 phospinates , sulfate, sulfonate and dicyano groups [45][60][61], carboranes ([CB

x

H

y

X

z

]

^-

) and orthoborates

Fluorine free anions, green chemistry

Given the wide variety of ionic liquids, multiple labels are often appropriate for a specific IL, depending whether the cation, the anion or functional groups are considered. However protic and aprotic ILs are two well-known categories of ILs that possess different chemical and electrochemical properties and therefore target different applications.

Protic ionic liquids

T. L. Greaves and C.J. Drummond published two reviews on applications and physico-chemical properties of protic ionic liquids [62][63]. They are used for many applications such as chromatography, biological systems, liquid-liquid extractions, catalysis and organic synthesis, or energy storage. They are especially used with polymer electrolyte membranes for fuel cells non- humid conditions.

They are formed by proton transfer from an equimolar combination of a Brønsted acid and a Brønsted base. This process is reversible, and there is equilibrium between neutral forms of the acid and base, with the anion and cation as following:

HA + B  [BH]

⁺

+ A

^-

For example, ethyl ammonium nitrate is a protic ionic liquid:

CH

3

CH

2

NH

2

+ HNO

3

-> CH

3

CH

2

NH

3+

+ NO

3-

23

Protic ionic liquids are generally cheaper and simpler to prepare than other ILs as there is no by- product. The first protic ionic liquids have been synthesized in the laboratory of H. Ohno in Japan [64][65]. The labile proton creates donor and acceptor proton sites and can lead to hydrogen bonding [66]. Proton hopping between cations adds to the number of cations that entirely move to participate in the ionic conductivity. Labile proton hopping between ions is a general process described for the first time in 1806 by Theodor Grotthuss suggesting a theory on the water conductivity[67]. As a result, these ILs generally exhibit a “superionic” behavior. However aggregation or the formation of ion complexes also can happen to prevent complete proton transfer, creating neutral species and limiting the ionicity (i.e the ability of the ions to be dissociated) of protic ionic liquids [68].

Aprotic ionic liquids

Aprotic ionic liquids are more complicated and expensive to prepare than protic ionic liquids and usually require multi-step reactions. However they are often thermally and electrochemically more stable than corresponding protic ionic liquids. Some of them are hydrophobic, which is convenient, knowing that humidity can affect the electrochemical window of the electrolyte. Consequently, they are widely studied for supercapacitors or lithium batteries applications [47],[48],[70], and were chosen for our studies. The most widely studied aprotic ionic liquids are based on dialkyl imidazolium or pyrrolidinium cations with anions such as BF

4-

, PF

6-

, TFSI

^-

or FSI

^-

(see Appendix A.I).

II.2. Ionic liquids applications

There are a lot of anions and cations combinations that can lead to 10

⁶

different ILs with extremely tunable properties [70]. Indeed, it is possible to modify the shape, the symmetry and the protic/aprotic nature of the ions. In addition, it is possible to mix ionic liquids with each other or ILs with organic solvents in order to tailor the properties of the electrolytes. The choice of anions and the nature or the length of alkyl groups on cations has an impact on the melting temperature, the viscosity, or the ionic conductivity, with a steady advantage: a vapor pressure extremely lower than organic solvents.

Rogers, Welton [71], MacFarlane [47][72], Ohno [73], Angell [48],[74]–[76] and many others participated in the development of task-specific ionic liquids (TSILs). Galinski et al. [77] published in 2006 a review with physico-chemical, thermal and electrochemical properties of many ILs. They appeared as an alternative to organic solvents in many fields such as organic or inorganic synthesis, nanomaterials, biosensing, catalysis, extraction, separation methods or energy storage [78].

According to the web source “ISI Web of Knowledge” around 9,050 papers were published on

the subject “Ionic liquids” in 2015 and more than 75,000 between 2010 and 2015. The extraordinary

interest in those electrolytes leads to their use in many fields. Following is a schematic and a list of

their mains applications:

24 Figure 10 : ILs applications according to Seddon et al. [79]

Electrolytes for electrochemical devices such as supercapacitors, batteries, dye sensitized solar cells, actuators or fuel cells

ILs are particularly interesting for these applications for safety concern. Indeed, energy storage devices haven’t been widely used in vehicle applications because they currently contain organic electrolytes that can be responsible for fire or explosions. The use of thermally stable ILs would prevent such events [80]. In the case of supercapacitors their wide electrochemical stability leads to higher power and energy densities as those properties are proportional to the square of the applicable voltage.

Solvents for homogeneous and surface catalysis, and synthesis.

Solvents are generally classified by their polarity. A qualitative definition would be that a polar solvent

is one that will dissolve and stabilize dipolar or charged solutes. Under this definition, ionic liquids

would be highly polar solvents. The solvent properties of ILs allow them to be involved in organic

reactions (Diels-Alder) transition-metal-mediated catalysis (hydrogenation reactions,

hydroformylation, Heck reaction), or organometallic reactions [71][81].

25

Media for electrochemical reactions, electrocatalysis and electrodeposition

The use of ILs for electrodeposition allows extreme conditions like a high temperature or operating voltage. It is particularly interesting for the electrodeposition of metals such as aluminum, manganese or nickel that require very low potentials [47], or potentials that can conflict with water decomposition. Moreover it is possible to influence the morphology and the crystal size of deposits depending on the selected cations [82][83]. The favorite anion is the dicyanamide (CN)

2

N

^-

as it is stable at reducing potentials and able to complex (thus dissolve and stabilize) the metal cation [47].

II.3. ILs organization in bulk and at interfaces

For supercapacitor applications, it is crucial to understand the organization of IL molecules both in bulk and at interfaces electrode/electrolyte. Indeed many experiments are undergone to answer questions such as “What kind of ILs are more likely to form ion pairs? Why?” The addition of an organic solvent upon ILs is known to disrupt the organization and self-assembly of ions by decreasing their coulombic interactions, but “What is the influence of solvation on the structural and capacitive properties of electrical double layer capacitors?” [84]. “What happens between the surface of the pore material and the first layer of ions? Is desolvation possible? Under what conditions?”

Many researchers use different techniques in order to understand those phenomena by using in situ infrared spectroelectrochemistry [85], or in situ-NMR spectroscopy [86].

Robert Hayes et al. recently published a review on the “Structure and nanostructure in ILs” [70] that offers a complete bibliography on the matter over the last decade.

IL organization in bulk

The organization of ions in ILs can be investigated by molecular dynamics simulation, or solid-state NMR [86],[87] and absorption spectroscopy or dielectric spectroscopy [70]. ILs are liquids entirely composed of ions and the molecule organization is driven by numerous interactions, starting with the predominant ones: Coulombic interactions, roughly one order of magnitude stronger than any other ion-ion interactions. In bulk, they ensure the electroneutral distribution of charges, thus decreasing the system energy and lead to a quasi-lattice structure. However, other contributions must be taken into account such as hydrophobic interactions which increase with the ion alkyl chain length, or hydrogen bonding.

It was demonstrated by dielectric spectroscopy measurements that the H-bond formation in

ethylammonium nitrate(EAN) stabilized ion pairs by shielding the long-range Coulombic forces with

8% of ion-pairs with an extremely short lifetime of 10

^-10

s at 25°C[88]. The same technique, was used

to evaluate the ion-pair formation of aprotic ionic liquids (imidazolium, pyrrolidinium, pyridinium,

tetraalkylammonium and triethylsulfonium) and protic ionic liquids (EAN,propylammonium nitrate

PAN). It revealed no signature of ion pair formation, or if they existed their lifetime would be shorter