We experimentally observed for the first time a bell-shaped (convex parabolic) differential capacitance versus potential (C dl -E) curve, which is expected according to the theory of Kornyshev given for the electrical double layer (EDL) of metal electrode/ionic liquid (IL) interface, at platinum and gold electrodes in four different [quaternary ammonium, imidazolium, and pyrrolidinium cations and bis(trifluoromethanesulfonyl)imide anion-based] ILs with cations and anions of similar sizes. The C dl -E curves measured at a glassy carbon (nonmetallic) electrode in the same set of ILs were found to be U-shaped, in contrast to those obtained at platinum and gold electrodes. The present study corroborates the so-called Kornyshev's model of the EDL at metal electrode/IL interfaces and at the same time demands a theoretical model for the nonmetallic electrode/ IL interface. The EDL formation in ILs is discussed.

An extensive study has been done for the first time on the structure of the electrical double layer (EDL) at polarized glassy carbon (GC) and gold (Au) electrode interfaces in a series of room-temperature ionic liquids (RTILs) via the measurement of capacitance-potential curves. The parabolic capacitance-potential curves similar to those observed in high-temperature inorganic molten salts were obtained at GC electrode in all of the RTILs studied. Potential of zero charge (PZC) at GC electrode in imidazolium-based RTILs depends significantly on the electrochemical pretreatment of the electrode surface: Electrochemical oxidation pretreatment generates the oxide surface on GC electrode, which results in a favorable adsorption of positively charged imidazolium cations on the electrode surface and in turn shifts the PZC to the positive direction of potential, whereas at the electrochemically reduced GC electrode, on which the adsorption of the imidazolium cations is less favorable, PZC shifts to the negative direction of potential. Such an effect of electrochemical pretreatment was not observed at the highly oriented pyrolytic graphite electrode. The hump on the anodic side of the capacitance-potential curves at Au electrode in imidazolium-based RTILs results from the π-electronic interaction of the imidazolium ring with the metal electrode, which was substantiated by using nonmetallic electrode and varying the ions of the RTILs. Such an enhanced interaction of the imidazolium ring with a gold electrode, as in the case of anion adsorption, shifts the PZC to the negative direction of potential. Such a hump as that observed at the gold electrode was not observed at the GC electrode. Similarly to the case in high-temperature inorganic molten salts, capacitances at PZC increase with increasing temperature. Different aspects of the obtained capacitance-potential curves are interpreted satisfactorily based on the hitherto proposed concept of the EDL structures.

Differential capacitances were measured at Hg/room-temperature ionic liquids (RTILs) interfaces as a function of potential with the aim of getting an insight of their interfacial structures. Capacitance−potential curve measured at Hg in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) resembles well the inner layer capacity at the Hg/aqueous solution interface containing nonspecifically adsorbing electrolyte. In both cases, the hump decreases with an increase in temperature which is discussed in the light of the previous theory. Both the alkyl group and the charged moiety of the cation of 1-alkyl-3-methylimidazolium based RTILs are found to interact concurrently with the Hg surface with the possible change of their orientation in response to the applied potential, and the appearance of a shallow minimum in the capacitance−potential curve related to potential of zero charge (PZC) depends on the extent of their interaction. PZC shifts to the negative direction of potential with increasing the chain length of alkyl residue of the cationic moiety because of the constraint in the orientational change needed for the interaction of positively charged imidazolium ring with Hg surface. Electrocapillary curves were also measured to determine the PZC. Throughout this study, a minimum of the capacitance−potential curve is designated as the PZC in agreement with the maximum of the corresponding electrocapillary curve. Different aspects of the capacitance−potential curves are interpreted satisfactorily on the basis of the hitherto proposed concept of electrical double layer structure.

The stability of superoxide ion (O(2)(*-)) generated chemically by dissolving KO(2) in dried dimethyl sulfoxide solutions containing imidazolium cation [e.g., 1-ethyl-3-methylimidazolium (EMI(+)) and 1-n-butyl-2,3-dimethylimidazolium (BMMI(+))] based ionic liquids (ILs) was investigated with UV-visible spectroscopic, NMR, and voltammetric techniques and an ab initio molecular orbital calculation. UV-visible spectroscopic and cyclic voltammetric measurements reveal that the O(2)(*-) species reacts with BMMI(+) and EMI(+) cations of ILs to form hydrogen peroxide. The pseudo first order rate constant for the reaction of BMMI(+) and O(2)(*-) species was found to be about 2.5 x 10(-3) s(-1). With a molecular orbital calculation, the O(2)(*-) species is understood to attack the 2-position (C-2) of the imidazolium ring (i.e., BMMI(+)) to form an ion pair complex in which one oxygen atom is bounded to C-2 and the other to the hydrogen atom of -CH(3) group attached to C-2. Eventually, the ion pair complex of BMMI(+) cation and O(2)(*-) species undergoes a ring opening reaction as evidenced with (1)H NMR measurement.

This study describes the causes of a U-like capacitance-potential (C-E) curve observed at nonmetallic electrode/ionic liquids (ILs) interfaces, in contrast to those observed at metal electrodes and expected according to the Kornyshev theory (Kornyshev, A. A. J. Phys. Chem. B 2007, 111, 5545). Several C-E curves were measured at glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) electrodes in three different ILs with inherent ionic concentrations of 6.4, 3.3, and 1.7 M. The minimum capacitance value (2.2 µF cm -2 ) at the HOPG electrode is significantly lower than those at GC or metal electrodes (>10 µF cm -2 ). The degree of curvature of the "U-like" curve measured at the GC electrode decreases in the ILs with low inherent ionic concentrations. This observation is in agreement with the theoretical curves deduced by considering both semiconductor and Kornyshev theories and the inherent properties (concentrations, sizes of ions, dielectric constant, etc.) of ILs used. The capacitance at the GC electrode exhibits a complex potential dependence, being different from those at HOPG and metal electrodes that were explained using semiconductor and Kornyshev theories, respectively. Depending on the characteristics of ILs, both concepts of semiconductor theory and Kornyshev's model may be required to explain the C-E curves at the GC electrode.

The ion-pairing phenomenon during the dioxygen (O2) reduction reaction (ORR) in a room-temperature ionic liquid (RTIL) was recognized and extensively studied using a voltammetric technique. Cyclic voltammetric two one-electron ORRs at a gold electrode in dimethyl sulfoxide (DMSO) solution containing 0.1 M tetraethylammonium perchlorate and in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) RTIL were carried out, and the obtained results were compared. Two cathodic peaks at −0.71 and −1.35 V versus Ag|AgCl|NaCl (sat.) and a well-defined anodic peak at −0.71 V and two ill-defined anodic peaks at potentials more positive than −0.2 V were observed in EMIBF4. The first and second cathodic peak potentials obtained in EMIBF4 were by 0.07 and 0.65 V more positive than those observed for the reductions of O2 to superoxide (O2 •-) and O2 •- to peroxide (O2 2-) in DMSO solution, respectively. Such a potential shift in the ORR was considered to be due to the ion-paring phenomenon as observed in the redox reaction of 1,4-benzoquinone in DMSO in the presence of EMIBF4. A 57 mV positive shift of the midpoint potential of the O2/O2 •- redox couple with a 10-fold increase of EMIBF4 in DMSO solution was obtained. The association constant and free-energy change (ΔG) of the ion-pairing were determined to be 13.5 M-1 and −6.4 kJ mol-1, respectively. The value of ΔG for the comproportionation reaction of O2 2- and O2 to form O2 •- in EMIBF4 solution (−61.7 kJ mol-1) was one-half of that in DMSO solution (−117.7 kJ mol-1), suggesting a favorable stability of O2 2- in EMIBF4.

Interfacial structures at Au(111) electrode in N2-saturated 1-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) have been studied by the measurements of differential capacitances and cyclic voltammograms. Capacitance–potential (C–E) curves are found to vary significantly with changing the direction of potential scan and temperature. C–E curves measured by sweeping the potential from negative to positive direction always have lower capacitance values compared to the curves measured by reversing the direction of potential scan, and this difference is larger for the RTIL with longer alkyl group. Special arrangement of the ions at the interface in response to the starting potential, constraint in orientation, and change in the ion electrode and ion–ion interactions are presumed to be the causes for this variation. Temperature has a dramatic effect on the Au(111)|EMIBF4 interface. Capacitance around the potential of zero charge decreases with increasing the temperature at the Au(111)|EMIBF4 interface, whereas at the Au(111)|BMIBF4 and Au(111)|OMIBF4 interfaces, it increases, which is discussed on the basis of the effect of temperature on the so-called solidlike crystallinity of RTILs.

Structures of the electrical double layer at Hg|room-temperature ionic liquid (RTIL) interfaces were studied by measuring the differential capacitance and electrocapillary curves as a function of potential. Maxima of the electrocapillary curves measured at the Hg|1-hexyl-3-methylimidazolium tetrafluoroborate (HMIBF4) and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIBF4) interfaces demonstrate an unusual broadness on the anodic side of the potential of zero charge (PZC), which is significantly different from those obtained at Hg in RTILs containing shorter alkyl chains or in conventional molecular solvents containing electrolytes. This broadness of the electrocapillary curve was found to depend on the crystal structure and spatial heterogeneity of the RTILs containing larger alkyl groups, which impede the charged moieties from being in contact with the electrode surface within a certain potential range. Cleaving of the liquid crystal structure by the dilution of OMIBF4 with dimethyl sulfoxide, which is reflected on the electrocapillary and surface charge density versus potential curves, supports the above reasoning. This is the first report on the dependence of the interfacial structure at the Hg electrode on the structure of the RTIL itself. A schematic model of the structure of the electrical double layer is also given.