Youjiang Chen

Electrochemical Oxidation of Hydroxylamine on Gold in Aqueous Acidic Electrolytes: An in Situ SERS Investigation

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.

Electrochemical and In Situ Optical Studies of Supported Iridium Oxide Films in Aqueous Solutions

Certain aspects of the dynamic behavior of electrochemically deposited hydrous Ir oxide IrOx 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 IrOx films supported on In-doped tin oxide on glass. The current transient response for IrOx 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.

The Oxidation of Hydroxylamine on Gold Electrodes in Mildly Acidic Aqueous Electrolytes: Electrochemical and In Situ Differential Reflectance Studies

The oxidation of hydroxylamine (HAM) on polycrystalline Au electrodes has been examined in aqueous acetate buffer (pH = 4) using electrochemical and in situ reflectance spectroscopic techniques. Cyclic voltammograms recorded under quiescent conditions in the potential region negative to the onset of Au oxidation were characterized by two clearly defined peaks centered at ca. 0.42 V and ca. 0.76 V versus SCE. Corresponding polarization curves obtained with a Au rotating disk electrode (RDE) as a function of rotation rate, ω, yielded two rather well defined plateaus. However, plots of the limiting currents, i lim , versus √ω, and particularly of i lim versus the concentration of HAM at fixed w, were found to be non-linear pointing to complexities in the reaction mechanism. Experiments involving dual electrode techniques, including rotating ring-disk electrodes, afforded evidence that the two sequential redox waves are associated primarily with the oxidation of HAM to nitrite and nitrate, respectively. Normal incidence differential reflectance spectroscopy measurements, ΔR/R, on a Au RDE performed either at high ω or large [HAM] failed to detect the presence of Au oxide during HAM oxidation in the potential region in which Au is known to undergo oxidation. This behavior is analogous with that found earlier in our laboratories for the oxidation of bisulfite on Au.

Oxygen Reduction on Supported Iridium Oxide Films in Neutral Phosphate Buffer: Implications for Neural Stimulation

Certain aspects of oxygen reduction on (hydrous) iridium oxide (IrO x ) films deposited galvanostatically on smooth Au and glassy carbon (GC) electrodes have been examined in aqueous O 2 -saturated neutral phosphate buffer (pH 7) solutions. Regardless of the nature of the substrate, the onset potential for O 2 reduction, E O2 onset , as determined by cyclic voltammetry, was ca. ―0.1 V vs saturated calomel electrode, which is very close to the corresponding E O2 onset found for both bare Au and GC electrodes. On this basis, the potential range in which the charge stored in the film is available for functional neural stimulation without generation of potentially harmful oxygen-derived species is on the order of 0.9 V. Studies involving a rotating Pt ring GC disk revealed that the reduction of dioxygen on IrO x |GC yields predominantly hydrogen peroxide. Further measurements performed with a rotating GC disk showed that IrO x | GC can oxidize hydrogen peroxide in the potential region associated with the more positive IrO x redox peak, a factor that might mitigate, at least in part, problems associated with the charge imbalance during the bipolar neural stimulation.

Supported Iridium Oxide Films in Aqueous Electrolytes: Charge Injection Dynamics as Monitored by Time-Resolved Differential Reflectance Spectroscopy

Dynamic aspects of the electrochemical charge-discharge of hydrated Ir oxide, IrC x (hyd), films in aqueous 0.5 M H 2 SO 4 have been investigated by simultaneous chronocoulometry and time-resolved, normal incidence (λ = 633 nm) differential reflectance spectroscopy, ΔR/R. Experiments were performed by applying potential steps between two judiciously selected values to IrO x (hyd) films electrodeposited on smooth Au disk microelectrodes to reduce the overall time constant of the cell. The analysis of the results obtained revealed that the rate at which charge is injected into or released from the film, as determined from the chronocoulometric response, is faster than the rate at which Ir sites in the lattice undergo redox transitions, as monitored by AR/R. This behavior appears consistent with the relatively high electronic conductivity of IrC x (hyd), which allows for charge to be stored on the highly convoluted surface of this structurally disorganized material, i.e., strictly capacitive, a process that occurs in parallel and at much higher rates than changes in the oxidation state of the Ir sites, i.e., pseudocapacitive.