Temperature dependence of the electrochemical behavior of the 690 Ni-base alloy between 25 and 325 °C

International audienceThe electrochemical behavior and the chemical composition of passive films formed on the Alloy 690 at room temperature in borate buffer solution (pH = 9.0) was studied with different techniques for two surface finishings. XPS and quantum yield measurements showed the presence of Ni and Cr oxides and hydroxides for passive films formed on both the as received and the mechanically polished one, whereas the presence of mixed spinel type Ni (1-x) Fe x Cr 2 O 4 was only observed on the as received material. Additionally, Electrochemical Impedance Spectroscopy (EIS) highlighted a higher corrosion resistance for the as received alloy in comparison with the mechanically polished alloy, which was linked to the chemical composition of the oxide film. Electrochemical measurements were performed before, during, and after oxidation of the Alloy 690 of Steam Generator (SG) tube of Pressurized Water Reactor (PWR) at high temperature and high pressure in the simulated primary circuit of PWR. At high temperature, the interface becomes electrochemically active yielding the precipitation of the corrosion products which form a few tens of nm thick diffusion barrier for the released metal cations. This overlayer is built on the top of a few nm thick, Cr rich inner layer at the alloy/oxide interface which was found to behave similarly to that initially formed at ambient temperature. It is concluded that high temperature oxidation in the static condition of an autoclave at 325°C does not promote a better passivation state than the one already existing initially

Influence of normal and radial contributions of local current density on local electrochemical impedance spectroscopy.

A new tri-electrode probe is presented and applied to local electrochemical impedance spectroscopy (LEIS) measurements. As opposed to two-probe systems, the three-probe one allows measurement not only of normal, but also of radial contributions of local current densities to the local impedance values. The results concerning the cases of the blocking electrode and the electrode with faradaic reaction are discussed from the theoretical point of view for a disk electrode. Numerical simulations and experimental results are compared for the case of the ferri/ferrocyanide electrode reaction at the Pt working electrode disk. At the centre of the disk, the impedance taking into account both normal and radial contributions was in good agreement with the local impedance measured in terms of only the normal contribution. At the periphery of the electrode, the impedance taking into account both normal and radial contributions differed significantly from the local impedance measured in terms of only the normal contribution. The radial impedance results at the periphery of the electrode are in good agreement with the usual explanation that the associated larger current density is attributed to the geometry of the electrode, which exhibits a greater accessibility at the electrode edge

Recent advances in understanding the capacitive storage in microporous carbons

This article presents a review of our recent work on capacitance of carbide-derived carbons. Specific capacitance as high as 14 &F/cm² or 160 F/g was achieved using carbide derived carbons with tailored subnanometer pore size, which is significantly higher than 6 &F/cm² or 100 F/g) for conventional activated carbons. Such high capacitance was obtained in several types of organic electrolytes with or without solvent. A maximum is obtained for the carbons with the mean pore size close to the bare ion size, ruling out the traditional point of view that mesoporosity is highly required for maximum capacitance. Surprisingly, carbons with subnanometer porosity exhibit high capacitance retention, since only a 10% loss is measured when 6 A/g discharge is drawn. These findings show the importance of fitting the ion size with the mean pore size. The double layer theory falls short to explain such charge storage mechanisms at the nanometer scale, thus atomistic modeling is required to find out an alternative charge storage model

Development of cavity microelectrode devices and their uses in various research fields

International audienceThe cavity microelectrodes (CMEs) have been introduced in the 1990s and since then have been employed for the characterization of various types of materials ranging from materials used for energy storage, biological applications, catalysis characterization, or corrosion studies. This technique takes advantage of working on small amount of pure electroactive materials (few hundreds nanograms) using usual electrochemical techniques (cyclic voltammetry, electrochemical impedance spectroscopy, etc.) and allows a rapid screening of the electrochemical behavior of new materials. The CME development and its main applications for the electrochemistry of powder materials are reviewed and discussed

Copper-Palladium Nanoelectrocatalysts for Carbon Dioxide Reduction, International Symposium on Electrocatalysis

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Insights into the Voltammetry of Cavity Microelectrodes Filled with Metal Powders: The Value of Square Wave Voltammetry

International audienceCavity MicroElectrodes (CMEs) offer a valuable platform to evaluate the electrocatalytic performance of micro-and nano-particulate materials. The technical factors and physicochemical processes affecting the electrochemical response at CMEs are to be recognized, specifically, the accessibility of redox species to the electrocatalyst surface. With this aim, the voltammetric response of CMEs is investigated through a joint experimental and theoretical approach including a comparative study of cyclic and square wave voltammetry (SWV). Experiments reveal a capacitive distortion of the response that increases with the powder surface area, but with a faradaic response analogous to that of recessed or inlaid microdisks, i.e. with electrochemical reactions occurring essentially on the first layer of the powder load. We show that SWV is well suited to discriminate Faradaic processes at CMEs and we present accurate mathematical expressions to describe it. These results provide guidelines for the design and analysis of CME voltammetric measurements

Nanocatalyseurs Cu100-xPdx pour la réduction électrochimique du CO2

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