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The growth of electrolytic copper dendrites between two coplanar copper discs, 250 μm in diameter, was investigated in oxalic acid medium containing copper sulphate by a technique based on the coupling of electrochemical measurements and microvideo optical in situ observations. The influence of the oxalic acid concentration, the potential difference applied between the two microelectrodes, and a laminar flow perpendicular or parallel to the electric field was investigated on the growth rate of the dendrites and on the short circuit time. Contrarily to solutions of oxalic acid alone, the activation current was diffusion-limited. The growth rate of the dendrites was proportional to the electric field. At the end, the technique was used to study the dendrite growth on a tantalum cathode where a morphology transition was evidenced.

The growth of electrolytic copper dendrites between two coplanar copper disc electrodes, 250 μm in diameter, was investigated in oxalic acid medium by a technique based on the coupling of electrochemical measurements and optical in situ observations. The current transients observed when a potential difference was applied between the two microdiscs were recorded. Simultaneously, the length of the dendrite was measured in real time thanks to a microvideo equipment. After some image processing, the growth rate of the dendrite was calculated by considering the increase of its length for a time increment. It was shown that the current transients exhibited a steep increase when the dendrites reach the anode and then formed a short circuit between the two copper discs. Depending on the experimental conditions, two types of dendrite morphology were observed: a filamentary growth and hierarchically branched thick structures. A strong relationship between the current transients, the change of the growth rate with time and the morphology of the dendrites was demonstrated. Low deposition currents are associated with a constant growth rate and filamentary dendrites whereas higher currents are associated with increasing growth rate with respect to time and hierarchical dendritic morphology.

The growth of electrolytic copper dendrites between two coplanar copper discs, 250 μm in diameter, was investigated in oxalic acid medium by a technique based on the coupling of electrochemical measurements and microvideo optical in situ observations. The influence of the oxalic acid concentration, the potential difference applied between the two microelectrodes, and a laminar flow perpendicular or parallel to the electric field was investigated on the growth rate of the dendrites and on the short circuit time. It was shown that during the incubation time, the copper ions dissolved from the anode are transported to the cathode mainly by migration. The growth rate of the filamentary dendrites increases with the oxalic acid concentration, activation current, and potential. Then, various techniques able to eliminate, or at least to slow, the dendrite development were studied. The addition of a supporting electrolyte, buffered solutions, or inhibitors slows or eliminates the deposit growth. At the end, the influence of the size of the anion and of the presence of dissolved oxygen in the solution was examined. In particular, the absence of oxygen in the solution prevents the deposit growth at the time scale of the experiment as there is practically no cathodic reaction available during the incubation time to compensate the anodic dissolution current.

An impinging jet cell was used to investigate the influence of the convection on the kinetics of scale deposition in a carbonically pure water. CaCO 3 crystallized due to the increase of the local pH obtained by the reduction of dissolved oxygen. The working electrode was the transparent electrode of an electrochemical quartz crystal microbalance which records versus time the mass of the deposit and the microscope image of the electrode/solution interface. The chronoelectrogravimetric curves showed that the rate of the CaCO 3 deposition increased with the convection until a limiting value of the Reynolds number Re while the current of oxygen reduction continues to increase following a Re 1/2 law. This result was confirmed by the measurement of the growth rate of the two CaCO 3 crystal forms obtained by using the optical method. On the contrary, the nucleation rate was not influenced by the flow rate. It was shown that the kinetics of scale deposition was directly linked to the interfacial pH; a theoretical approach gives the interfacial pH with respect to the Re number.

International audience; The friction stir welded (FSW) assembly of 2050-T8 and 7449-T79 was studied by local electrochemical techniques, namely SECM and LEIS, under non-corrosive and low corrosive conditions. During the FSW process, it generates different well defined microstructural zones giving rise to a galvanic coupling at their interfaces. SECM and LEIS measurements were carried out to investigate the local electrochemical behavior around these interfaces. SECM results clearly showed a change of the electrochemical kinetic on the passive layer between both materials in the different microstructural zones of the welded assembly whereas the LEIS techniques allowed the investigation of the interface on a larger scale.

The effects of a superimposed magnetic field B on the structure of nickel electrodeposits prepared from a quiescent Watts solution were studied by scanning electron microscope and transmission electron microscopy investigations. It was observed that B can induce a change of the surface morphology and of the preferential growth direction of the nickel grains. In the absence of organic inhibitor, it is shown that these effects result from an inhibition of nickel electrocrystallization in relation to the mass-transport enhancement of H + ions promoted by the magnetic field. The phenomenon is much more important in the presence of a strong inhibitor such as 2-butyne-1,4-diol, the activity of which is also under mass-transport limitation. The presence of the magnetically induced convective flow is directly illustrated by the perturbations of the relief of the deposit in the vicinity of attached bubbles. These results demonstrate that most structural modifications of nickel electrodeposits observed in the presence of a magnetic field are consequences of convection phenomena induced by a magnetohydrodynamic effect.

International audience; The study of galvanic coupling between two metals is still a relevant topic, particularly in aerospace andautomotive industries. The development of local electrochemical techniques leads to a better understandingof the phenomena occurring at the interfacial zone. Galvanic coupling between carbon steel and zincwas investigated by local electrochemical impedance spectroscopy (LEIS) in a 0.06 g L−1 NaCl solution.Voltammetry and conventional electrochemical impedance experiments were also performed to betteranalyze the local impedance data. Local measurements carried out at a fixed frequency showed that zincdissolution was more significant at the steel/zinc interface. A particular shape of the local impedance diagramwas observed above the zinc surface with the presence of a large inductive loop in the low-frequencyrange. Numerical simulations, performed by using finite element method, allowed the local experimentaldiagrams obtained over the zinc sample to be validated taking into account the radial contribution of thecurrent between both materials in galvanic coupling.

International audience; Electrodeposition of CaCO3 was studied under the influence of magnesium ions present in a carbonically pure water. The investigation was performed using electrochemical, gravimetric and optical methods. The chronoamperometric measurements confirmed the inhibiting property of Mg2+ when its concentration is higher than 120 mg L−1 in a solution containing 160 mg L−1 of Ca2+. The optical technique led the nucleation-growth process to be accessed by means of an optical microscope positioned behind a transparent electrode. The increase of Mg2+ concentration changed drastically the calcite morphology. At 360 mg L−1 of Mg2+, the calcite morphology was optically amorphous but the Raman spectrum confirmed its structure. The crystal growth was recorded in situ and the image analysis software characterised the nucleation process as well as the growth rate of the crystals. It allowed the influence of the Mg2+ ions on the crystallisation process of CaCO3 to be quantified.

Lithium intercalation hosts are a key point to the energy density of the largely used LiCoO2 (even if of high cost and toxicity) as well as of manganese oxides which have been investigated most extensively. Iron oxides are attractive electrode materials for low-voltage rechargeable lithium batteries from both cost and environmental standpoints. However, search for iron oxides of conventional crystalline structures and micrometer particle sizes as lithium intercalation cathodes, has been greeted with disappointing results. Here we report on the synthesis, characterizations, electrochemical study and electrochemical impedance spectroscopy (EIS) of a nanocrystalline γ-Fe2O3 that simultaneously exhibits high lithium insertion capacity and good capacity retention upon cycling. These properties reveal thermodynamics of the nanocrystalline material inherently different from those of its microcrystalline counterpart. Moreover, EIS showed that the intercalation process of the lithium ion occurs according to two processes involving first the reduction of the surface Fe3+ with concomitant charge neutralization by Li+ ions onto the surface defects of the nanoparticle followed by the reduction of the core Fe3+ with insertion of the Li+ deeper in the particle.

International audience; A novel technique allowed EIS and microscopical observation of the interface between electrode and solution to be coupled in situ. The electrochemical reduction of oxygen was investigated by using this device when the electrode surface was progressively covered by an insulating layer of scale deposit. It was shown that two time domains can be distinguished from the initial time of the experiment corresponding to a bare electrode up to a totally blocked surface. In the first part of the experiment, EIS showed one time constant which characterizes the charge transfer process of water reduction. When the percentage of coverage reached more than 80%, this latter electrochemical contribution to EIS was negligible and the reduction process of oxygen was highlighted by two time constants observed from impedance diagrams. The high frequency response characterized the charge transfer process and the low frequency loop identified the diffusion process of oxygen. An electrochemical model was proposed to explain the shape of the impedance spectra plotted with respect to the time of coverage. The good correlation between the experimental and the fitted model curves led to the change versus time of the different parameters of the equivalent circuit. It notably showed that the double layer capacity was directly proportional to the active surface measured by the microscope observation until the surface was totally covered, according to the expectations in the experimental conditions used here.

An original technique allowed three in situ measurements to be obtained simultaneously, i.e. electrochemical and mass measurements and a microscope image of the interface between electrode and solution. The nucleation-growth process of CaCO3 deposition was investigated by varying different parameters which influenced the supersaturation coefficient, the Ca2+ concentration, pH and convection of the carbonically pure water near the substrate. The results showed that the variation of the scaling rate was due, for the two first cases, to an influence of the nucleation process whereas for the last case, the kinetics of scale deposition were modified by a shift of the growth rate.

Nanoscale materials offer the advantage of combining structural effects (inside grain structure) with surface effects or grain-boundary effects. Therefore, the electrochemistry of this type of materials is very different from that of traditional microcrystalline ones mainly due to the contribution of the 'surface defects' allowing strong coulombic interactions between the inserted lithium ions and the surface ions called 'electrochemical grafting'. When electrochemical grafting is the first electrochemical step to take place, it can favor the power density and the cycling life of electrode materials. As illustrative examples, electrochemical behaviors of nanocrystalline oxides such as LiMn 2 O 4 , γ-Fe 2 O 3 , and of nanohybrid inorganic-organic materials such as γ-Fe 2 O 3 /PPY (PPY = polypyrrole) are presented.

A conductive transparent electrode is used to observe and quantify the nucleation-growth process of scale deposition. The deposition is accelerated by increasing the interfacial pH induced by an electrochemical reaction on the electrode surface. Three measurements were simultaneously obtained with respect to time. The chronoelectrogravimetric and chronoamperometric curves showed that an increase of the supersaturation in the vicinity of the electrode increased the rate of scale deposition. This result was highlighted in the first instants of deposition when overlapping of crystals was not predominant. The third measurement was the in situ observation by a microscope through the transparent electrode. It allowed the count of crystals and the surface of the individualized crystals to be measured vs. time. It was shown that the increase of the local concentration of calcium and carbonate ions had no influence on the growth rate of both varieties of crystal present on the electrode surface, namely, vaterite and calcite. On the contrary, the count of the particles showed that the increase of the kinetic of scale deposition was only due to the enhancement of the nucleation rate. Thus, for the lowest investigated supersaturation, the number of nuclei increased linearly with time, whereas the highest supersaturation promoted a faster nucleation rate and a higher proportion of calcite. © 2003 The Electrochemical Society. All rights reserved.

This paper is devoted to the description of magnetic field effects on zinc electrodeposition from basic media. The major effect of the magnetic field stems from a slight magnetohydrodynamic forced convection located close to the cathodic interface. Classical impedance analysis reveals that the reaction mechanism is governed by two diffusing species, Zn(II) and OH-, the contributions of which have been discerned. This leads to a good agreement between experimental and theoretical impedance diagrams and shows, once again, that the magnetic field exerts no effect upon the transfer kinetics.

A very recent paper, referred to below as Paper I, had been devoted to the description of magnetic field effects on the structural features of nickel electrodeposits grown from a Watts solution, either pure or in the presence of small amounts of a primary brightener and leveling agent such as 2-butyne-1,4-diol (BD). The conclusions of Paper I stressed the major effect of the magnetic field which was supposed to come down to a magnetohydrodynamic forced convection close to the cathodic interface. This convection enhances those among the cathodic reactions which are likely to be mass-transport controlled, which is the case for the hydrogen electrode reaction as well as for the catalytic hydrogenation undergone by BD on the cathodic surface. The aim of this paper is to describe the results of electrochemical studies which have been carried out with the same system so as to check the validity of our assumption.