Synthesis of submicrometric dendritic iron particles in an Electrochemical and Vibrating Hele-Shaw cell: study of the growth of ramified branches

The purpose of this study is to explore a new synthesis way for the production of iron nanoparticles exploiting the nanometric structure of long ramified iron branches formed by electrodeposition in a Hele-Shaw cell. After the growth, these branches are fragmented by the action of a vibrating element (piezoelectric disk) integrated into the cell. The emphasis is put on the growth of the ramified iron branches which is performed by galvanostatic electrodeposition in a stagnant electrolyte (FeCl2) inside the Hele-Shaw cell (50 μm deep). The competition between the co-formation of H2 bubbles (H+ reduction) and the growth of ramified iron branches (FeII reduction) is analyzed by varying both the applied current density j and the FeCl2 concentration. Two regimes, depending mainly on j, are highlighted: below a threshold current density of 8 mA/cm2 only H2 bubbles are formed, while above this threshold, iron branches grow accompanied by the formation of H2 bubbles which nucleate and grow at the top of the branches during their formation. The H2 bubbles influence the branches growth especially at low j (24 mA/cm2), the branches follow a columnar growth with a constant front velocity, well predicted by the theory. Scanning Electron Microscopy (SEM) of the iron branches shows a dendritic structure constituted of nanometric crystallites, whose size depends on the local growth velocity: increasing the growth velocity from 3.6 μm/s to 40 μm/s leads to a decrease in the crystallites size, from ∼1 μm to ∼10 nm. Using the acoustic vibrations (4 kHz) of the piezoelectric disk, these fragile branches are successfully fragmented into submicrometric fragments of dendrites exhibiting high specific surfaces S/V (equivalent to the S/V of nanoparticles of 30 nm diameter). Advantages/Drawbacks compared to other synthesis ways as well as the optimization of the proposed synthesis are discussed

Enhancement of the electrochemical activity of a commercial graphite felt for vanadium redox flow battery (VRFB), by chemical treatment with acidic solution of K2Cr2O7

The novel thermal–chemical activation method is proposed to prepare the graphite felt with highly functionalized surface and enhanced active surface area, to be used as the positive half-cell electrode for the vanadium redox flow batteries (VRFBs). The prescribed treatment consists of boiling the felt within K2Cr2O7 solution in 5 M H2SO4, at different temperatures and time durations. It is time efficient, effective and durable. The cyclic voltammetry analysis enable to optimize the operating conditions i.e. boiling at 140 °C for 2 h. The activation treatment creates different surface oxygen functional groups, as characterized by FTIR, that act as catalytic sites toward the positive half-cell redox couple reaction (VO2+⇌VO2+). Moreover, the improvement of various other properties of the graphite felt, resulted due to activation are also quantified, such as the surface roughness of fibers, the wettability toward electrolyte, the adsorption affinity against the vanadium and the surface energy of the graphite felt. The improved reversibility of VO2+/VO2+ redox couple reaction against graphite felt is also evaluated and confirmed by the increase of intrinsic heterogeneous electronic transfer constant by 2.2 and 5.5 folds for the oxidation and reduction reactions, respectively. The performance evaluation of electrode at stack level by charge–discharge cycles, shows the improvement of the voltage efficiency and faradaic yield, as well as, the consistency of the performance over three cycles (∼16 h)

Facile chemical activation of graphite felt by KMnO4 acidic solution for vanadium redox flow batteries

The activation method is proposed to enhance the electroactivity of the graphite felt (GF), to be used as positive half-cell electrode for vanadium redox flow batteries (VRFBs). Two following activation approaches are used 1) functionalization of electrode surface by the chemical treatment with acidic solution of the KMnO4 and 2) deposition of KMnO4 derived particles on the surface of GF. The intrinsic heterogeneous electronic transfer constant ( of V(V)/V(IV) system is enhanced by 2.6 and 6.1 times for the oxidation (V(IV) → V(V)) and reduction (V(V) → V(IV)) reactions, respectively. The performed electrolysis in the half-cell shows the improved reversibility of the electrode. The treated electrode is also evaluated on the stack level by using 100 cm2 GF in each electrolytic section of the battery, by charge and discharge cycles. The results clearly indicate the lowering of anodic and cathodic overvoltage, i.e. the average charging voltage decreases from 1.86 V to 1.76 V, while the obtained voltage during discharge increases from the 0.59 V to 0.8 V. The proposed activation method is operationally simple; and activated electrode shows the promising performance as positive half-cell electrode in VRFBs

One-flow feed divided electrochemical reactor for indirect electrolytic production of hypochlorite from brine for swimming pool treatment-experimental and theoretical optimization

A ‘two-compartment’ asymmetric electrochemical reactor, operating without electrodes polarity inversion, was designed and optimized for the chlorination of swimming pools. Gaseous chlorine, produced at the anode and absorbed in the alkalinized catholyte, provides the hypochlorite solution. Empiric equations providing the chloride concentration dependence on the initial current density magnitude were established. Experimental optimization of the effect of the various operating parameters allows a chloride conversion close to 50% with faradic yields higher than 80%, and a chlorine production of 1 kmol/day/m2 to be achieved. Macroscopic mass balance, was performed and the obtained theoretical results correlate with the experimental one

Investigation of dental amalgam electrode behaviour for the long term monitoring of nuclear waste disposals

International audienc

Exploitation of the nanostructure of electocrystallized ramified branches for an alternative synthesis of colloidal metallic nanoparticles

We explore an alternative synthesis of colloidal metal nanoparticles (cMnP) by electrocrystallization. The principle is to make grow long metallic ramified branches by galvanostatic electrolysis of a stagnant metal salt aqueous solution inside a Hele-Shaw (thin gap) cell. In the absence of supporting electrolyte, the constraint of electroneutrality forces the deposit to grow as ramified branches. These branches grow by successive nucleation / growth events that leads to a fine branches structure made up of metal crystals whose size can be lower than 100 nm. The objectives of this work are to study the formation mechanism of such metal nanocrystals assemblies and elaborate a device able to dissociate and release them to propose an alternative synthesis of cMnP

Influence of electrode material and roughness on iron electrodeposits dispersion by ultrasonification

This study relates the sonoelectrochemical production of metallic particles and nanoparticles. The emphasis is on the influence of electrode material and roughness on the morphology of iron electrodeposits and their dispersion from the electrode by ultrasonification. Ultrasonification is either applied during cyclic voltammetries with solution stirring or after galvanostatic iron electrodeposition; no dispersion was observed when using a gold electrode, whereas dispersion was always observed when using vitreous carbon (VC) substrates. Scanning Electron Microscopy (SEM) imaging of the electrodeposits shows higher iron coverage on gold than on VC electrodes. Iron spreads more on gold than on VC. The values of both the interfacial energy of the iron/electrode interface and the work of adhesion of iron on the electrode are in agreement with the previous observations. Dispersion kinetics on VC were found to be dependent on the electrode surface roughness. Results suggest that dispersion follows a first order kinetics, which is coherent with the constant action of cavitation bubbles in the vicinity of the electrode surface. Enhancement of mass-transfer by ultrasound has also been observed

Crossover between Re-Nucleation and Dendritic Growth in Electrodeposition without Supporting Electrolyte

This work deals with the formation of dendritic structures by electrodeposition of Cu2+ and Ag+ without supporting electrolyte in Hele-Shaw cells. The transition between the two main patterns, ramified branches and dendrites, is specifically addressed at the scale of branch microstructure using careful SEM observations. Ramified branches, composed only of grain assemblies, are obtained at low current densities because of a re-nucleation process induced by space charge dynamics (Fleury, Nature, 1997). For current densities higher than a given threshold, ramified branches are also formed by re-nucleation but another growth mode, the dendritic growth, is also observed while, at the macro-scale, the pattern remains fractal and isotropic. This shows that 1) pattern transition originates from a morphological transition at microstructure scale and 2) the re-nucleation process enables a freedom in local growth direction allowing the pattern to be fractal at the macro-scale. The onset of the dendritic growth mode, from shape instability of the grains, is considered with Mullins & Sekerka model. This latter disagrees with the observations by predicting that the grains are always unstable. It is proposed that the space charge plays a key role by controlling the shape stability and thus the transition between the two growth modes

Remediation of Chlorinated Organic Compounds: Single- and Multi-Component Approaches

International audienceChlorinated organic compounds (COCs) represent a major concern and are widespread distributed in soil and groundwater. Due to their strong hydrophobicity and their density higher than water, these COCs infiltrate through aquifers and form DNAPL pools. A large part of DNAPL can be removed by physical technologies (mainly pumping), but an important part will remain trapped and adsorbed in the aquifer matrix. In situ remediation technologies have been then developed in order to destroy in situ the remaining COCs.This study aims at characterizing the chemical reductive dechlorination of a mixture of COCs, mainly composed of hexachlorobutadiene (HCBD) and hexachloroethane (HCA). Many studies have shown the great efficiency of bimetallic Pd/Fe particles for the remediation of COCs (Colombo et al., 2015; Kim and Carraway, 2003; Lien and Zhang, 2007). Preliminary studies have been conducted in order to select the most appropriate reactants, which are Pd/Fe microparticles dispersed in a polyacidic hydrophobic matrix (Rodrigues et al., 2015).First, HCBD and HCA were individually investigated in a monophasic single-component system, i.e. in presence of one dissolved pollutant in deionized water:methanol (99.9:0.1% v/v) solutions. Several analytical parameters were studied: pollutant/reactant ratios, temperatures and presence or absence of surfactants. Similar experiments were performed in a monophasic multi-component system, containing dissolved HCBD and HCA, to characterize the impact of a hydrophobic mixture.Then, as most part of the pollutant is present as a DNAPL in groundwater, HCBD and HCA were investigated in a polyphasic single-component system, i.e. in presence of one pure pollutant in deionized water. This second approach combines both reduction reactions and transport processes, especially solubilization. The aim is to understand the influence of temperature and the presence of a surfactant in a polyphasic system to define the rate-determining step of the global remediation process. Finally, polyphasic multi-component systems were performed in presence of a mixture of HCBD and HCA, and in presence of a DNAPL taken from a polluted site.These two approaches allowed the determination of degradation pathways and kinetic laws for the two compounds individually and in mixture.ReferencesColombo, A., Dragonetti, C., Magni, M., Roberto, D., 2015. Degradation of Toxic Halogenated Organic Compounds by Iron-Containing Mono-, Bi- and Tri-Metallic Particles in Water. Inorganica Chim. Acta 431, 48–60.Kim, Y.H., Carraway, E.R., 2003. Reductive Dechlorination of TCE by Zero Valent Bimetals. Environ. Technol. 24, 69–75.Lien, H.-L., Zhang, W.-X., 2007. Nanoscale Pd/Fe Bimetallic Particles: Catalytic Effects of Palladium on Hydrodechlorination. Appl. Catal. B Environ. 77, 110–116.Rodrigues, R., Betelu, S., Garnier, F., Colombano, S., Joubert, A., Cazaux, D., Masselot, G., Tzedakis, T., Ignatiadis, I., 2015. SILPHES – Investigation of Chemical Treatments for the Remediation of Recalcitrant Chlorinated Solvents. In: 13th International UFZ-Deltares Conference on Groundwater-Soil-Systems and Water Resource Management