Some twenty-five years after they first came to prominence as alternative electrochemical solvents, room temperature ionic liquids (RTILs) are currently being employed across an increasingly wide range of chemical fields. This review examines the current state of ionic liquid-based electrochemistry, with particular focus on the work of the last decade. Being composed entirely of ions and possesing wide electrochemical windows (often in excess of 5 volts), it is not difficult to see why these compounds are seen by electrochemists as attractive potential solvents. Accordingly, an examination of the pertinent properties of ionic liquids is presented, followed by an assessment of their application to date across the various electrochemical disciplines, concluding with an outlook viewing current problems and directions.

The electrochemical reduction of oxygen is reported in four room temperature ionic liquids (RTILs) based on quaternary alkyl -onium cations and heavily fluorinated anions in which the central atom is either nitrogen or phosphorus. Data were collected using cyclic voltammetry and potential step chronoamperometry at gold, platinum, and glassy carbon disk electrodes of micrometer dimension under water-free conditions at a controlled temperature. Analysis via fitting to appropriate theoretical equations was then carried out to obtain kinetic and thermodynamic information pertaining to the electrochemical processes observed. In the quaternary ammonium electrolytes, reduction of oxygen was found to occur reversibly to give stable superoxide, in an analogous manner to that seen in conventional aprotic solvents such as dimethyl sufoxide and acetonitrile. The most significant difference is in the relative rate of diffusion; the diffusion coefficients of oxygen in the RTILs are an order of magnitude lower than in common organic solvents, and for superoxide these values are reduced by a further factor of 10. In the quaternary phosphonium ionic liquids, however, more complex voltammetry is observed, akin to that expected for the reduction of oxygen in acidified organic media. This is shown to be consistent with the occurrence of a proton abstraction reaction between the electrogenerated superoxide and quaternary alkyl phosphonium cations following the initial electron transfer.

No abstract

The cyclic voltammetry response of partially blocked electrodes is modeled using finite difference simulations and a method presented for determining currents at electrode surfaces which have a well-defined geometric blocking pattern. Peak current and peak separation data are presented for six decades of scan rates, blocking coverage values between 0.1 and 0.9 and between the limits of reversible and irreversible electrochemistry. The validity of the simulation approach employed is verified by data obtained experimentally from purpose-built partially blocked gold film electrodes, with either a cubic or hexagonal geometric array of electroinactive disks uniformly distributed on the electrode surface. Comparison of theory with experiment suggests that the modeling of hexagonally distributed blocking systems is superior to that of the cubically arranged ones.

Measurements on the diffusion coefficient of the neutral molecule N,N,N',N'-tetramethyl-para-phenylenediamine and the radical cation and dication generated by its one- and two-electron oxidation, respectively, are reported over the range 298-348 K in both acetonitrile and four room temperature ionic liquids (RTILs). Data were collected using single and double potential step chronoamperometry at a gold disk electrode of micrometer dimension, and analysed via fitting to the appropriate analytical expression or, where necessary, to simulation. The variation of diffusion coefficient with temperature was found to occur in an Arrhenius-type manner for all combinations of solute and solvent. For a given ionic liquid, the diffusional activation energies of each species were not only closely equivalent to each other, but also to the RTIL's activation energy of viscous flow. In acetonitrile supported with 0.1 M tetrabutylammonium perchlorate, the ratio in diffusion coefficients of the radical cation and dication to the neutral molecule were calculated as 0.89 +/- 0.05 and 0.51 +/- 0.03, respectively. In contrast, amongst the ionic liquids the same ratios were determined to be on average 0.53 +/- 0.04 and 0.33 +/- 0.03. The consequences of this dissimilarity are considered in terms of the modelling of voltammetric data gathered within ionic liquid solvents.

The electrochemistry of microdroplets, shown to be nearly monodisperse, of N,N,N‘,N‘-tetraalkyl-para-phenylenediamine oils (TRPD, R = n-butyl, n-hexyl, n-heptyl, and n-nonyl) immobilized on a basal plane pyrolytic graphite electrode and immersed into aqueous electrolyte solution is studied using cyclic voltammetry. Upon oxidation of the TRPD droplet to the cation radical TRPD+•, anion uptake from, or cation loss into the aqueous solution takes place, so as to maintain electroneutrality within the oily deposit. The former process is shown to produce an ionic liquid, with the anion insertion taking place at the triple phase boundary of electrode |TRPD oil| aqueous electrolyte; the latter process, in contrast, takes place at the interface between the two immiscible liquids, and with two-thirds-order kinetics. The possibility of a chemical reaction taking place between the electrogenerated and inserted ions at the three-phase junction, viz. redox-catalysis or otherwise, is illustrated via reference to two systems (azide and iodide).

No abstract

No abstract