Christine Cachet-Vivier

Electrochemistry of powder material studied by means of the cavity microelectrode (CME)

The kinetic aspects of powder material electrochemistry can be studied using the cavity microelectrode (CME) as it allows carrying out voltammetry at scan rates between a few millivolts per second to several hundreds of volts per second. Thus, significant voltammogram characteristics-scan rate profiles can be drawn. Theoretical models suited to each material needs to be developed for their exploitation. First, we report significant results obtained with CME on powder materials. The materials studied were chosen for their wide variety of possible applications such as battery materials (polyaniline or Bi2O3, which modifies the electrochemical behavior of materials in which it is included), supercapacitor (carbon black), and for the electrocatalytic hydrogenation of organic compounds (PtO2). Secondly, we briefly describe the general action for establishing models to obtain a better understanding of the electrochemical processes.

Cyclic voltammetry study of bismuth oxide Bi2O3 powder by means of a cavity microelectrode coupled with Raman microspectrometry

An electrochemical study of the bismuth oxide Bi2O3 with a cavity microelectrode coupled in situ to Raman microspectroscopy allowed species transformations during the redox behavior of this oxide to be identified. It was shown that the electroactive species are probably bismuth oxy-hydroxides rather than Bi2O3 when a potential is applied, whereas they are Bi2O3 on open circuit voltage.

Electrochemical Study of Bi2 O 3 and Bi2 O 2 CO 3 by Means of a Cavity Microelectrode. I. Observed Phenomena and Direct Analysis of Results

The physical and chemical processes occurring for the electrochemical reduction and oxidation of Bi 2 O 3 and Bi 2 O 2 CO 3 can be determined through a direct analysis of the voltammograms recorded from the cavity microelectrode. The voltammogram shows an evolution for around 1 h, which corresponds to the first 80 cycles carried out at 50 mV s -1 . The evolution is due to the sequence of the following concurrent processes: (i) impregnation of the powder and its dissolution in the electrolytical solution, (ii) deposit of porous metal-Bi during the cathodic scan, and deposit of Bi(III) species during the anodic scan. These processes cause a reconfiguration of the architecture of the powder within the cavity by making the material grains smaller. For the second stage, the voltammograms do not vary any more during the cycling. For the first cathodic scan, the reduction process starts from the bottom of the cavity at the interface Pt/solution only, then it progresses toward the mouth. Contrarily, the oxidation takes place throughout all the grains. During the next cathodic scans, the reduction also starts throughout all the grains when the grains are not entirely oxidized from the preceding anodic scan.

Cavity microelectrode for studying battery materials: application to polyaniline powder

The cavity microelectrode (CME) offers a new tool for studies of powder materials. By comparison with a usual composite electrode, CME allows working without any additives (graphite, binder) which insures that the results are only ascribed to the material. In this study, polyaniline powder, taken as an example, was investigated with a CME in aqueous and non-aqueous electrolytes to evaluate its potentialities as a battery material in long cycle life performance. It was found that up to 85% of the initial capacity is recovered after one week of cycling (40 000 cycles) in aqueous electrolytes at various pH and 90% in non-aqueous solution.

Electrochemical study of LaNi3.55Mn0.4Al0.3Co0.75 by cavity microelectrode in 7 mol l-1 KOH solution

Abstract The electrochemical behavior of LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 material was investigated with the help of a cavity microelectrode. The cyclic voltammetry study versus the scan rate evidenced the formation of a passivation layer, which is characterized by a positive current during the cathodic sweep. The performances of the LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 compound as a negative electrode for nickel–metal hydride (Ni–MH) battery can also be determined with the cavity microelectrode. It was shown that even in drastic potential sweep conditions, the material exhibits a very good reversibility during several thousands of cycles.

Electrochemical proton insertion in manganese spinel oxides from aqueous borate solution

Abstract We report the proton insertion in LiMn2O4 and Li0.32Mn2O4 compounds with the spinel structure in borate buffer at pH 9.5. LiMn2O4 exhibits a quasi-voltage plateau near −0.2 V and a second one around −0.35 V/Hg/HgO. The charge process results in a single step in the voltage range 0.1/0.8 V. This first cycle leads to new electrochemical features for the subsequent cycles with a reversible behavior and S shaped discharge–charge curves (mean discharge potential=+0.1 V) in the voltage window 0.75/−0.5 V. The proton uptake involved during insertion corresponds to a faradaic yield of 0.45 F/Mn. This new system can be observed from the first cycle when H+ insertion directly proceeds in the chemically delithiated oxide Li0.32Mn2O4. In both cases, galvanostatic cycling experiments (C/25 rate) show a stable specific capacity of 100–110 Ah kg−1 is available near 0.1 V over 20 cycles.

A rapid evaluation of vanadium oxide and manganese oxide as battery materials with a micro-electrochemistry technique

Abstract The electrochemical behavior of vanadium oxide V 2 O 5 and bismuth doped manganese oxide Bi–MnO 2 as battery materials was studied with a cavity microelectrode (CME). This technique, which allows working on a very few amount of material, appears to be suitable when high scan rates are used. Thus, it is possible to perform many hundreds cycles in a short time. The performances of pure vanadium oxide as lithium insertion material in inorganic media, examined with a CME appear to be excellent whereas poor results were obtained with Bi doped MnO 2 , for which the electrochemical mechanism proceeds via dissolution.

First Evidence of Rh Nano-Hydride Formation at Low Pressure.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

Electrocatalytic Hydrogenation of C 2 H 4 on PtO2 Powder A Study of Surface Processes Occurring at Powder Materials by Cavity Microelectrode

Taking the electrocatalytic hydrogenation of ethylene as an example, we show that the eavity microelectrode allows the study of the surface processes occurring on powder materials. The electrocatalytic current can be observed at scan rate (v) up to a few tens of mVs -1 , while with a standard composite electrode, v must be lower than a few fractions of mV s -1 . The extent of the scan rate range is around two to three magnitudes. Therefore, it is possible to construct significant voltammogram characteristics (peak intensity, peak potential, and peak area) scan rate profiles, which are related to processes underlying the electrocatalysis. However, their quantitative exploitation needs a theoretical model, which can be built up by (i) applying the chemical and electrochemical kinetics to the various involved processes and (ii) taking into account the geometry proper to a composite electrode. Direct analysis of the model allows qualitatively explaining the variation of the ratio electrolytic-current/H chemisorption current observed on cavity microelectrode vs. the scan rate, when compared to that of the spherically shaped platinum electrode.

Cavity microelectrode for studying manganese dioxide powder as pH sensor

The potential-pH response of an electrolytic manganese dioxide is investigated by means of a cavity microelectrode (CME). The potential-pH curves show a complex evolution that could be explained by the disporportionation of MnOOH species, leading to the formation of Mn(2+) ions on the MnO(2) surface. Such a behaviour is not suited for pH sensor application. However when the tip of the electrode is coated by a Nafion membrane, the potential-pH evolution shows a unique slope close to -60 mV pH(-1). In addition, the sensor exhibits short time responses to pH variations, a good selectivity, and it can be easily renewed compared to classical sensors.