No abstract

Composite film electrodes containing mechanically mixed Co 3 O 4 or CuCo 2 O 4 particles, carbon-black Vulcan XC-72R, and poly͑vinylidene fluoride-co-hexafluoropropylene͒ ͑PVdF-HFP͒ were formed on the glassy carbon disk surface of a rotating ring-disk electrode ͑RRDE͒ and studied for the oxygen reduction reaction ͑ORR͒ in O 2 -saturated 1 M KOH solution. The highest current densities were observed with CuCo 2 O 4 and they increased with the oxide content in the film, hence clearly demonstrating the excellent intrinsic electrocatalytic activity of CuCo 2 O 4 for this reaction. The results also showed that CuCo 2 O 4 is a better electrocatalyst than Co 3 O 4 with higher current densities and a greater number of electrons exchanged per O 2 molecule. It was found that the copper-cobalt spinel oxide component favors a total of 4e − in the oxygen reduction process. At the CuCo 2 O 4 -based composite electrode, direct reduction of O 2 into OH − ions ͑rate constant k 1 ͒ and the peroxide pathway ͑formation of HO 2− ions and their reduction into OH − ions, rate constants k 2 and k 3 ͒ are proceeding in parallel, with a k 1 /k 2 ratio that increases with the overpotential when the oxide content is greater than 23.5%. At the Co 3 O 4 -based composite electrode, k 1 is very weak with a k 1 /k 2 ratio that decreases rapidly with the overpotential.

normalCuxnormalCo3−xnormalO4 ( x=0 and 1) powders were prepared by a sol-gel method which favors high oxide specific surface areas with a larger value for normalCo3normalO4 , ascribed to a larger powder mesopore volume. X-ray diffraction measurements reveal that CunormalCo2normalO4 particles are less crystalline than normalCo3normalO4 with crystallite size 10 times smaller. The sol-gel method allows formation of spinel oxide particles that do not contain any resistive CuO phase. X-ray photoelectron spectroscopy analyses have shown that normalCo3normalO4 contains Co2+ and Co3+ species at the surface, tetrahedral Co2+ cations being predominant. In the case of CunormalCo2normalO4 , Cu+ , Cu2+ , and possibly Cu3+ cations are also detected, octahedral Cu2+ showing the highest concentration among the copper species. Composite film electrodes, based on mechanically mixed normalCo3normalO4 or CunormalCo2normalO4 particles, carbon black Vulcan XC-72R, and poly(vinylidene fluoride-co-hexafluoropropylene), were formed on a glassy carbon disk surface. The highest intrinsic electrocatalytic activity for the oxygen evolution reaction is obtained for the CunormalCo2normalO4 composite electrode containing the larger amount of oxide particles. Cyclic voltammetry experiments suggest that the surface normalCo2+∕normalCo3+ ratio is decreased when the electrodes are immersed into the KOH electrolyte, which may be associated to the formation of a superficial CoOOH layer.

In scanning electrochemical microscopy (SECM), an approach curve performed in feedback mode involves the downward displacement of a microelectrode toward a substrate while applying a bias to detect dissolved electroactive species at a diffusion-limited rate. The resulting measured current is said to be at steady state. In order to reduce the required measurement time, the approach velocity can be increased. In this paper, we investigate experimentally and theoretically the combination of diffusion and convection processes related to a moving microdisk electrode during feedback approaches. Transient modeling and numerical simulations with moving boundaries are performed, and the results are compared to the experimental approach curves obtained in aqueous solution. The geometry and misalignment of the microelectrode influence the experimental approach curves recorded at high approach velocities. The effects are discussed through the decomposition of the current into transient diffusional, radial convectional, and axial convectional contributions. Finally a ready-to-use expression is provided to rapidly evaluate the maximal approach velocity for steady state measurements as a function of the microelectrode geometry and the physical properties of the media. This expression holds for the more restrictive case of negative feedback as well as other modes, such as SECM approach curves performed at substrates displaying first order kinetics.

In developmental lexical-gustatory synesthesia, specific words (inducers) can trigger taste perceptions (concurrents) and these synesthetic associations are generally stable. We describe a case of multilingual lexical-gustatory synesthesia for whom some synesthesias were bidirectional as some tastes also triggered auditory word associations. Evoked concurrents could be gustatory but also tactile sensations. In addition to words and pseudowords, many voices were effective inducers, suggesting increased connections between cortical taste areas and both voice-selective and language-selective areas. Lasting changes in some evoked tastes occurred during childhood suggesting that some plasticity can be present after the initial learning of associations. Inducers were often linked to taste concurrents phonologically or semantically, but also through identifiable childhood episodes (persons or events). Several inducers were phonologically linked to episodic inducers suggesting a process of secondary acquisition for many inducers. Implications of these observations are discussed.