A method is herein described that allows spatially resolved capacitive currents at electrode|electrolyte interfaces to be recorded in situ by monitoring the ohmic drop in the electrolyte, Δφsol, using two microreference electrodes. Measurements were performed in 0.1 M H2SO4 aqueous solutions using Au and Pt either as single or dual working electrodes, that is, connected to one another, in an otherwise conventional three-electrode electrochemical cell. Plots of Δφsol versus E, the potential of the dual Au|Pt electrode with respect to the main reference electrode, recorded during voltammetric cycles, yielded curves bearing features characteristic of the Au|0.1 M H2SO4 or Pt|0.1 M H2SO4 interface depending on whether the microreference electrodes were placed close to the Au or Pt electrode surface, respectively. Extensions of this methodology to measurements in the micrometer domain are discussed briefly.

The electrooxidation of hydroxylamine (HAM) on roughened Au electrodes has been examined in aqueous buffered electrolytes (pH 3) using in situ surface-enhanced Raman scattering (SERS). Two distinct spectral features were observed at potentials, E, within the range in which HAM oxidation was found to ensue, centered at 803 cm(-1) for 0.55 < E < 0.8 V and at 826 cm(-1) for 1.0 < E < 1.40 V versus SCE, attributed, respectively, to adsorbed nitrite and adsorbed NO(2). Similar experiments performed in solutions containing nitrite instead of HAM under otherwise identical conditions displayed only the peak ascribed to adsorbed nitrite over the range of 0.1 < E < 0.8 V versus SCE with no additional features at higher potentials. These observations strongly suggest that under the conditions selected for these studies the oxidation of HAM on Au proceeds at least in part through a pathway that does not involve nitrite as a solution-phase intermediate.

An electrode incorporating two distinct heterogeneous electrocatalysts acting in series was specifically designed to promote the reduction of nitrate beyond the nitrite stage in weakly buffered aqueous solutions (pH = 3) containing Cd2+. This novel interface consists of Au nanoparticles, Au(np), on which underpotentially deposited Cd reduces nitrate predominantly to nitrite, dispersed on a hemin-modified glassy carbon (GC) surface, Hm|GC, where nitrite is further reduced to yield hydroxylamine as the only product detected using a rotating Au(ring)|Hm|Au(np)|GC (disk) electrode. Additional evidence in support of this series mechanism was obtained from numerical simulations in which the bifunctional electrode was regarded as a hexagonal, closed-packed array of coplanar concentric Cd|Au(np) disks and Hm|GC rings (with no insulating gap), using rate constants determined independently from rotating Au and Hm|GC disk electrodes in solutions containing either nitrate or nitrite, respectively.

Certain aspects of the dynamic behavior of electrochemically deposited hydrous Ir oxide ͑IrO x ͒ films supported on Au microelectrodes during charge and discharge have been investigated by a combination of chronocoulometry and simultaneous in situ normalized reflectance spectroscopy techniques in aqueous solutions. Correlations between the reflectance spectra and the optical properties of the films in its various states of oxidation were sought from in situ transmission measurements for IrO x films supported on In-doped tin oxide on glass. The current transient response for IrO x ͉Au microelectrodes following a potential step, within the voltage region in which the films display pseudocapacitive characteristics, was found to exhibit a well-defined peak, as opposed to a monotonic decay reported by other groups. Some features of this behavior can be attributed to changes in the conductivity of the film as a function of its state of charge, as has been proposed for electronically conducting polymers. Also presented in this work are data collected over the pH range 0.3-13, which confirm the much faster charge-discharge dynamics in basic compared to acidic media. A primitive model based on proton conductivity within the hydrated oxide lattice is presented which accounts grossly for this pH-induced effect.Electrochemical capacitors are expected to play a key role in meeting some of the challenges associated with energy storage and electrical power management in a variety of technological areas including transportation and microelectronics. 1 Particularly attractive are electrode materials that display high specific capacities and fast rates of charge and discharge, such as the hydrated oxides of Ru and Ir in aqueous electrolytes. 1 Both of these metals are expensive and, in the case of Ir, very scarce; hence, it is doubtful they will find wide use in large-scale devices. However, hydrated Ir oxide, to be denoted hereafter as IrO x , is currently being regarded as a viable material for functional neural stimulation applications because of its biocompatibility and outstanding electrochemical characteristics. [2][3][4] With some notable exceptions, 5-12 most of the studies on IrO x have focused on electrochemical techniques aimed at gaining insight into various aspects associated with the charge and discharge of filmtype electrodes in aqueous 13-19 and, to a far lesser extent, nonaqueous electrolytes. 20,21 From a general perspective, fundamental studies of fast heterogeneous redox dynamics require careful design of the experimental setup, as the physical size of the electrodes and the electrolyte conductivity largely determine the resistance-capacitance ͑RC͒ constant of the cell and thus, the rate of the processes that can be reliably measured. 22 The tactic herein employed for the in situ study of supported IrO x films involves a combination of microelectrodes and normal incidence normalized reflectance spectroscopy, 23 where R͑E s ͒ and R͑E ref ͒ are proportional to the light collected at the detector for the electrode...

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