Article 1 : Influence of the Conductivity and Viscosity of Protic Ionic Liquids

Ionic Liquids Electrolytes on the Pseudocapacitance of RuO

Electrodes

Ce premier chapitre de la discussion est un article qui a été publié en 2009 dans le volume 113 de la revue The Journal of Physical Chemistry C aux pages 1632–1639. En plus d'avoir contribué de façon importante à la rédaction de cet article, l'ensemble des résultats et des expériences qui y sont réalisées sont le fruit de mes travaux.

Influence of the Conductivity and Viscosity of Protic Ionic

Liquids Electrolytes on the Pseudocapacitance of RuO

Electrodes

Laurence Mayrand-Provencher and Dominic Rochefort

Département de chimie, Université de Montréal

C.P.6128 Succ. Centre-ville, Montréal, Québec, Canada H3C 3J7

Abstract

Several new protic ionic liquids (PILs) were prepared by mixing trifluoroacetic acid (TFA) with various heterocyclic amines using different base:acid ratios (1:1 and 1:2). Their specific conductivities have been measured by electrochemical impedance spectroscopy and were found ranging from 0.71 to 9.07 mS cm-1 at 27.0 °C. In all cases, the conductivity of a given ionic liquid increased with a higher proportion of acid (1:2 ratio) which is explained mainly the smaller viscosity obtained in these conditions. Indeed, in most PILs, the dynamic viscosity (10.3–484 cP at 27.0 °C) is decreased by a higher acid proportion. Both parameters were used to construct a Walden plot in order to evaluate the quality of these protic ionic liquids and compare them with common room temperature (RT) molten salts. The physicochemical properties of the PILs were then compared with the specific capacitance values obtained with a thermally prepared RuO2 electrode in these ionic

liquid electrolytes. This work brings evidence that pseudocapacitance is involved in the energy-storage mechanism of the PILs studied here and shows that PILs could eventually be used as electrolytes in metal oxide-based supercapacitors.

1. Introduction

Electrochemical capacitors (ECs), also known as supercapacitors, are devices having higher specific energy densities than conventional dielectric capacitors and that possess higher specific powers than most batteries. ECs can be combined with batteries as a complementary energy source for specific applications such as electric vehicles (EVs) and hybrid electric vehicles.1-3 When coupled to a battery, ECs can allow sufficient power bursts to be released for the needs of EVs4 which a battery alone cannot provide.5 _{There are several other applications of ECs that takes}

advantage of their short-term, high-power outputs. Many portable apparatus containing digital electronic devices use ECs to achieve their very specific power needs. As an example, digital communication devices, typically needing high current outputs for a short period of time followed by a long recovery period, could advantageously employ capacitors for a more efficient use of energy.6

In ECs, two kinds of energy-storage mechanisms have been used: the double- layer, where the charge storage is electrostatic in nature, and the pseudocapacitance, where the charge storage implies a faradaic process, or a partial faradaic transfer, such as electrosorption or a redox process involving the electroactive materials.1, 7 Most commonly used ECs rely on high-surface area carbons as capacitive electrode material due to its low cost, its high electronic conductivity, and its extremely good reversibility.8 However, some metal oxide-based ECs, such as many made from RuO2, have been shown to provide much higher specific capacitance (Cs) values than

those obtained with carbon.3 This increased capacitance, which is found to be the highest in RuO2, is due to the important contribution of pseudocapacitance in

conducting metal oxide electrodes. In RuO2 for example, the redox processes giving

rise to pseudocapacitance involve proton coupled reactions that implies a shift of oxidation state from Ru4+, to Ru3+ and Ru2+, accordingly to eq. 3.1.9 Another interesting aspect of RuO2-based ECs is that the proton-coupled faradaic reactions

have an excellent reversibility, giving mirrorlike cyclic voltammograms with carefully chosen electrolytes.7, 10

RuO2 electrodes as an application for ECs have been prepared in several

different ways, and one of the best performing methods was developed by Zheng et al.10 It involves hydrous RuO2 preparation by a sol-gel process, and they reported Cs

values as high as 720 F g-1 for a powder formed at 150 °C. Variants of this procedure have also been studied, yielding even higher Cs (e.g., 977 F g-1 for Liu et al.3), while

many other different methods, such as electrochemical development of RuO2

films9, 11 or thermal decomposition in air of RuCl3,11-13 have also been used.

Most pseudocapacitance studies of metal oxide-based ECs have been limited to aqueous solutions because of the need for protons in the electrolyte to achieve the faradaic reactions leading to pseudocapacitance (eq. 3.1). However, contrarily to most organic electrolytes, aqueous electrolytes can only be used on a relatively narrow potential domain which is a factor limiting the maximum energy stored by the device.2 _{Also, another constraint for energy-storage with aqueous electrolyte-based}

devices is that they can only be operated in a limited range of temperatures. One strategy to overcome these limitations could be to replace the aqueous electrolytes with protic ionic liquids, which can be stable on a large variety of potential windows, have boiling points that can greatly exceed the ones of aqueous electrolytes, and can even remain in a liquid state at very low temperatures.14 Aprotic ionic liquids, such as those based on alkylimidazolium,15 aliphatic quaternary ammonium16 and phosphonium salts,17 have been greatly studied for use in double layer capacitors. Because of the required presence of protons, these aprotic ionic liquids (AILs) cannot be used as electrolytes for metal oxide-based pseudocapacitors. Protic ionic liquids are a category of ionic liquids obtained by a proton transfer from a Brønsted acid to a Brønsted base, and the term “ionic liquid” (IL) is used here as it was first introduced;18 ILs are all considered to be fused salts that melt at temperatures lower than 100 °C. In energy-related research, PILs have been mostly investigated as electrolytes in fuel cells, where they are being sought after for their stability at high temperatures and their proton conductivity. Although the potential of heterocyclic amines and some other bases to support proton transport has been demonstrated,19-21 no appropriate PIL has been found for use as an application in fuel cells so far.22

Recently, we reported for the first time that pseudocapacitance can arise on a metal oxide electrode (RuO2) in a PIL electrolyte composed of 2-methylpyridine and

trifluoroacetic acid.7 We showed the presence of redox waves on the cyclic voltammograms for RuO2 that were attributed to the faradaic processes responsible

for pseudocapacitance in this material. The specific capacitance of the electrode was comparable to that obtained in 0.1 M H2SO4. Although this finding could result in

new research directions and perspectives for EC development, work remains to be done to determine how the properties of the PIL affect the proton-coupled faradaic reactions in metal oxide electrodes and to develop more efficient PILs for pseudocapacitors. Thus, understanding the fundamental principles involved in the properties of PILs used as electrolytes is of primary importance to push further their development and to make them appropriate electrolytes not only for our metal oxide- based ECs but also for nonaqueous fuel cells.

In this work, the physical properties of new PILs, consisting of a variety of heterocyclic amines and of TFA as the acid, are compared to the electrochemical behavior of a RuO2 electrode in such electrolytes. The specific capacitance of these

PILs has been determined with a crystalline, thermally prepared RuO2 electrode,

which is physically robust and chemically stable in the studied PILs, and the values obtained highlight the fact that pseudocapacitance is involved in the energy-storage process for the PILs mentioned in this work. The fundamental results obtained help to orient further research in the domain of metal oxide-based ECs using PILs as proton- rich organic electrolytes.