PROCEDIMIENTOS PARA LA CONTRATACION DE PERSONAL A HONORARIOS Ley N° 19.896, artículo 5°

We did not intend to go into the review of any particular published papers on the electrode kinetics of RTILs, having focussed the previous sections of this chapter on certain general features of the most elementary electron transfer processes at electrodes, as such ’quantum electrochemistry’ might appear to be useful in rationalization of the simplest electrochemical reactions in RTILs. We nevertheless will briefly discuss below a very limited selection of published, predominantly experimental papers.

Lagrost et al558 _{have studied four different ionic liquids, based on}

1-alkyl-3-methylimidazolium or quaternary ammonium cations as reaction media for several typical electrochemical reactions of oxidation of organic molecules (anthracene, naphthalene, durene, 1,4-dithiafulvene, and veratrole). They recover the reaction mechanisms from the analysis of cyclic voltammetry curves and determine the thermodynamic and kinetics pa- rameters of the corresponding reactions. Most strikingly they come to a conclusion that the uncovered mechanisms seem to be almost unchanged in ionic liquids, as compared with conventional organic media. However, the overall one order of magnitude decrease of the electron-transfer rates between the studied aromatic molecules and the electrode, observed for all the studied molecules, indicated at a higher solvent reorganization energy than they could be in ordinary organic solutions. Reorganization of dense ionic atmospheres in RTILs can indeed be larger than the reorganization in a polar solvent, so these results are quite encouraging for application of the theory. These reactions, however, are certainly needed but not the simplest one as they involve both first and second order reactions of cation radicals. Interesting results were obtained for oxidation of dissolved hydrogen gas on platinum electrodes in different ionic liquids.559 _{Studies of similar processes in protic ionic liquids} have also been reported.560

Matsumiya et al561 _{studied electrochemical ferrocene-ferriceniurn electron transfer reac-} tions in ammonium-imide RTILs , via the analysis of cyclic voltammogrammes over a po- tential range -0.3 to +0.5 V versus I-/I3- reference electrode in a wide enough temperature range of 298-373 K. The Fc/Fc+ reaction was found to be a single-electron transfer pro-

cess (as expected) and diffusion controlled. The electron transfer rate constant and transfer coefficient have been retrieved by electroanalysis.562 _{Both the rate constant and the diffu-} sion coefficients for Fc/Fc+ were studied as a function of temperature, exhibiting Arrhenius dependencies.

Efforts have been invested in optimizing reference electrodes for voltammetry in RTILs.563 In this work both the diffusion and the kinetics of electron transfer across the ionic liq- uid/electrode interface were studied using cyclic voltammetry and scanning electrochemical microscopy. In another work in this area564 _{the authors studied the anodic oxidation of} several arenes and anthracenes in RTILs. The authors of this work made conclusion about substantial deviation from the outer-sphere Marcus-type behavior of these compounds in contrast to their behavior in traditional organic solvents, in terms of correlations of the rate constant with molecular size and solvent static dielectric constant. For a number of processes in a series of RTILs the electron-transfer kinetics was found to be independent of the solution viscosity.

In Ref.565 _{ferrocene was used as a redox probe and the electrochemical properties of} a series of RTILs were studied using voltammetric methods and scanning electrochemical microscopy. The effect of RTIL viscosity on mass transfer dynamics within each RTIL was studied, as well the heterogeneous electron transfer rate constant, was determined.

The classical example of rather complicated electron transfer reactions - the oxygen re- duction reaction (ORR) - has been studied in Ref.566 _{at Pt surfaces in a protic RTIL using} the method of rotating disk electrode. Water content measurements suggested that the ORR proceeded in the ionic liquid via a 4-electron reduction to water. A Tafel analysis of the ro- tating disk voltammetry data revealed a change in the ORR Tafel slope which seemed to be related with the change of the exchange current density the applied potential.

The hydrogen evolution reaction has been explored at gold, molybdenum, nickel, tita- nium and platinum electrodes. Significant differences in electrochemical rate constants were observed between the different metals. Most importantly, the authors came to a conclusion that the reaction mechanism was consistent at all five metals in the studied RTIL, in contrast to the known variations in a series of these metals in aqueous systems.567 _{Hydrogen evolu-} tion was also investigated in various RTILs in Ref.568 _{at Pt electrode. This work revealed a}

strong effect of the nature of anion, suggesting significant interaction between protons and anions. H+ reduction on a palladium microelectrode in RTIL was studied in Ref.569_showing signatures of Pd/H layer formation under the studied conditions.

Kinetics of Li/Li+ was studied in Refs570,571 _{for a large set of RTILs.}

That list could be continued, but all in all the experiments are far ahead of a molecular level theory (which is just to be developed), and many of the drawn examples are more complicated than the simplified one-stage processes sketched in the formalism described in this chapter above. The synergy will come after theorists will be able to formulate some striking predictions of quantum electrochemistry for experimentalists to test them. It has taken many years to get such tests done for some spectacular but elementary heterogeneous electron transfer reactions in ordinary electrolytes,554,555 _{and it will also probably take some} time for this to happen in RTILs.

7

RTILs in confined geometry

The electrochemical performance of an electrode-RTIL interface is generally ‘amplified’ by its area, whether it is used for energy storage (capacitors, batteries) and generation (solar cells, fuel cells), or electrocatalysis. In this respect porous and in particular nanoporous electrodes may provide the maximal enhancement of the interfacial area, as long as the electrodes are well wetted by RTIL. This justifies the interest in understanding RTIL performance in nano-confinement.

Before we go into details of molecular-scale physical-chemical phenomena in RTILs in confined geometry, we would like to note that the phenomena discussed in this section are interesting for various applications. However, due to space limitations, we will dwell on only one of those applications, as an example, namely the EDL supercapacitors.