Eric Chainet

Nucleation and Growth of Zinc from Chloride Concentrated Solutions

The electrodeposition of metals is a complex phenomenon influenced by a number of factors that modify the rates of nucleation and growth and determine the properties of the deposits. In this work the authors study the influence of the zinc chloride (ZnCl{sub 2}) concentration on the zinc nucleation process on glassy carbon, in a KCl electrolyte under conditions close to those employed in commercial acid deposition baths for zinc. The electrochemical study was performed using cyclic voltammetry and potentiostatic current-time transients. The charge-transfer coefficient and the formal potential for ZnCl{sub 2} reduction were evaluated from cyclic voltammetry experiments. The nucleation process was analyzed by comparing the transients obtained with the known dimensionless (i/i{sub m}){sup 2} vs. t/t{sub m} response for instantaneous or progressive nucleation. The results show that the nucleation process and the number density of sites are dependent on ZnCl{sub 2} concentration. Scanning electron microscopy analysis of the deposits shows that the deposits are homogeneous and compact although a change in the morphology is observed as a function of ZnCl{sub 2} concentration. Evaluation of the corrosion resistance reveals the influence of the nucleation process on the subsequent corrosion resistance of the zinc deposits.

Local structure determination of the CoSi(111) interface by surface electron energy-loss fine-structure technique

Extended fine structures, similar to those observed with SEXAFS technique, have been detected in electron energy-loss spectra in the reflection mode. These structures are related to the local geometry (bond length) of the investigated system. We have applied this loss technique to study the CoSi(111) interface. Our structural results suggest the formation at room temperature of an initial silicide phase followed with a nearly pure cobalt film. Upon annealing at progressively higher temperatures, sequential silicide formation (Co2Si, CoSi, CoSi2) occurs, confirming most of our recent observations by other surface techniques.

Evidence of extended fine structures in the Auger spectra: A new approach for surface structural studies

Abstract We report for the first time on extended fine structures which we have observed above core-valence-valence Auger transitions on Cu and Co samples. We interpret these oscillating structures as originated from the same interference process which produces extended fine structures observed in the x ray absorption spectra (EXAFS). Using the same EXAFS analysis procedure, we deduce from the extended fine Auger structures (hereafter called EXFAS) the radial atomic distribution F(R) of Cu and Co samples. An excellent agreement is found as compared with results from synchrotron radiation EXAFS.

Effect of benzylideneacetone on the electrodeposition mechanism of Zn–Co alloy

The influence of benzylideneacetone (BA) on the mechanism of Zn–Co alloy electrodeposition onto AISI 1018 steel was studied in chloride acidic solutions. Results indicate that BA modifies the exchange current densities of zinc and cobalt such that the alloy is electrodeposited via a normal codeposition mechanism. Analysis of the deposits by Auger spectroscopy and X-ray diffraction shows that BA increases the cobalt concentration in the electrodeposited alloys and gives deposits with a constant concentration profile of both Zn and Co. BA also inhibits the formation of zinc hydroxide in the initial deposition stages, which supports the proposed mechanism of normal codeposition. Finally, it is shown that BA modifies the morphology of the deposits by inducing a reduction in the cluster size, leading to compact, smooth and shiny coatings.

Extraction of iridium(IV) from aqueous solutions using hydrophilic/hydrophobic ionic liquids

Anionic complexes of iridium(IV) were extracted from acidic solutions of HCl using ionic liquids (IL). The influence of the cationic structure of IL and of the aqueous phase pH on the distribution coefficients of IrCl62− was investigated. Liquid–-liquid extraction and precipitation of iridium(IV) were studied. Iridium(IV) was precipitated from aqueous solutions using water-soluble ionic liquids by forming a water-insoluble salt composed of two cations from the ionic liquid. Pyridinium cations appear to be the most efficient at precipitating iridium(IV). Iridium(IV) was found to be efficiently extracted using simple hydrophobic ionic liquids, exhibiting D values ranging up to 71. Investigations on the extraction mechanism tend to show that iridium(IV) is exchanged with two [NTf2]− anions from the IL phase. Finally, elution of iridium(IV) from ionic liquids towards aqueous solutions was carried out from [BMIM][NTf2], [OMIM][NTf2] or [OdMIM][NTf2] into solutions containing high amounts of hydrochloric acid or nitric acid.

Effects of organic additives on zinc electrodeposition from alkaline electrolytes

This work reports the effects of four organic compounds (referred to as levelers) on the electrodeposition of Zn on steel from alkaline free-cyanide electrolytes. The additives tested included polyvinylalcohol (PVA) and the condensation products of epichlorhydrin with amines, called polyamines (PAs), that were synthesized using an aliphatic amine (PA-I, from diethylamine and PA-II from diethylamine-triethylamine), and a heterocyclic quaternary imidazolium molecule (PA-Imid, from imidazole). These compounds were evaluated in the absence and in addition to a quaternary ammonium brightener, N-benzyl-3-carboxylpyridinium chloride (3NCP). The imidazole derivative-based polyamine (PA-Imid) causes greater inhibition of the zinc reduction process than the aliphatic polyamine, and more cathodic overpotential is necessary to promote massive metal deposition. The morphology of the deposits is modified when polyamines are added to the bath; more compact and smaller crystals are obtained with PVA as well as with polyamine PA-I. The addition of PA-II as well as PA-Imid yields crystals growing perpendicular to the substrate. The addition of 3NCP with PVA, PA, or PA-Imid increased the deposition overpotential and modified the morphology by diminishing the grain size. In the absence of additives, crystallographic orientation favored the basal Zn(002) with high atomic packing. The addition of the levelers favored the high-angle pyramidal Zn(101) with low atomic packing. The combination of the levelers with (3NCP) favored the prismatic Zn(100) crystallographic orientation. Additives decrease the size of zinc crystals and tend to increase the energy of lattice favoring the growth of pyramidal and prismatic planes.

Fine structure in electron energy-loss and Auger spectra: Applications to the local geometry determination of surfaces and interfaces

We review recent and main contributions to the extended fine structures observed in reflection mode electron energy-loss spectra. Those structures allow the local geometry determination of surfaces and interfaces like the well-known surface extended X-ray absorption fine structure technique (SEXAFS) usually performed with synchrotron facilities. The physical principles of the surface extended electron energy-loss fine structure (SEELFS) technique, its capabilities and limitations will be discussed. Several examples will be given and particular attention will be paid to the local order of clean surface, adsorbate monolayer on clean surface, initial stages of metal-semiconductor interface formation and metallic cluster growth on solid substrate. Furthermore, we obtain evidence of extended fine structures observed above core-valence-valence Auger transitions in Auger spectra of several materials. We tentatively suggest their underlying physical origins and analyse these oscillating structures following the same standard EXAFS formalism. The lattice parameters deduced from these extended fine Auger structures (EXFAS) are in good agreement with those deduced from synchrotron radiation EXAFS data, conferring therefore to the conventional Auger technique new capabilities as a surface sensitive local order probe.

Synthesis of Highly Porous Catalytic Layers for Polymer Electrolyte Fuel Cell Based on Carbon Aerogels

Recent studies of the PEMFC catalytic layer electrocatalystsupport showed that high surface area carbon blacks cannot beused efficiently (1) because they have necessarily small primarypores that yield high diffusion limitation for the oxygenreduction reaction (ORR). In the present paper, we firstsynthesized and characterized carbon aerogels which exhibit highsurface area, high porous volume and adjustable pore-sizedistribution. In that way, they enable to decouple betweensurface area and porosity, while they are also promisingelectrocatalyst supports (2). Second, we elaborated newcatalytic layers made from 2 different carbon aerogels with 2different Nafion loadings. Finally, we characterized thestructure of such catalytic layers, and evaluated their activitytowards the oxygen reduction reaction (ORR). Finally, anappropriate catalyst support should display large pore-size andhigh surface area in order to reduce the PEMFC cathode platinumloadings without loosing activity, following the reduction ofoxygen diffusion limitation.

Giant magnetoresistance in Co–Ag granular films prepared by electrodeposition

Preparation of granular magnetic Co–Ag films produced by pulsed electrodeposition from a chloride bath containing both cobalt ions and a low concentration of silver has been investigated. Deposition of cobalt on Ag is performed by a double pulse method. Combining in situ electrochemical and microgravimetric measurements, the kinetics of silver and cobalt reduction are presented. The thickness and deposition rates are monitored using an electrochemical quartz crystal microbalance (EQCM) during the growth of each material. Magnetic measurements have shown a superparamagnetic behavior in agreement with the existence of very small cobalt particles. Giant magnetoresistance (GMR) of ∼2% at room temperature is observed for the Co5Ag95 sample.

Effect of Quaternary Ammonium Compounds on the Electrodeposition of ZnCo Alloys from Alkaline Gluconate Baths

The effect of quaternary ammonium compounds on the electrodeposition of ZnCo alloys from an alkaline chloride-gluconate electrolyte was studied. A thermodynamic analysis was performed which indicated that Zn(OH) 2- 4 should be the predominant zinc species, whereas for cobalt the mixed hydroxy-gluconate complexes CoL 3 (OH) 2 and CoL(OH) 2 are expected to be prevalent. The electrodeposition of the alloy from this alkaline medium was found to be anomalous, but increasing the cobalt content in the bath caused the cobalt content in the deposit to increase. The effects of three quaternary ammonium compounds, N-benzyl-3-carboxylpyridinium chloride (3NCP), N-benzyl-triethylammonium chloride, and tetraethylammonium hydroxide, on the electrochemical behavior of ZnCo deposition were studied. The incorporation of these additives in the electrolytes modified the composition, morphology, and crystallographic structure of the ZnCo-alloy deposits. Cyclic voltammetry indicated the formation of several alloy phases as cobalt was added to these zinc baths. 3NCP had the greatest impact on the process, limiting the Co content and changing the structure of the deposit.