Electrodeposition of carbon films from molten alkaline fluoride media

The electrodeposition of carbon films from carbonate ions (CO₃²⁻) in molten alkaline fluorides (LiF/NaF) was investigated in the 700-800 °C temperature range using cyclic voltammetry and chronopotentiometry. The cathodic peak in the cyclic voltammogram indicates that CO₃²⁻ ions are reduced in a one-step process: CO₃²⁻+4e⁻→C+3O²⁻. Deposits of amorphous carbon were obtained by potentiostatic electrolysis and analysed by several physical techniques: X-ray diffraction, Raman spectroscopy, scanning electron microscopy coupled with energy dispersive spectroscopy.     

The electrochemical reduction of oxygen on zinc corrosion films in alkaline solutions

Kinetics of the O₂ reduction has been characterized on Zn corrosion films by Pt/Zn rotating ring-disc electrode (RRDE) and EIS methods. On zinc-oxide films a two-step reduction was identified in various buffer solutions of pH 10.5, while a small quantity of H₂O₂ intermediate could be detected. On the basis of results obtained from Pt/Zn and Pt/Pt RRDE experiments in solutions containing H₂O₂, it was further confirmed that the HO₂⁻ was reduced to OH⁻ through the zinc-oxide corrosion layer. Capacitance data of the zinc-oxide/electrolyte interface calculated from steady-state impedance diagrams measured at various cathodic potentials indicate the presence of a space charge layer of the semi-metallic ZnO. The solid-state reaction mechanism of HO₂⁻ disproportion with participation of Zn_i⁺ interstitials, oxygen ion vacancies of the non-stoichiometric Zn-oxide, and chemisorbed HO₂⁻ is discussed.     

碳钢在Na2CO3-NaHCO3溶液中的阳极极化行为

使用动电位极化技术研究了碳钢在Na₂CO₃-NaHCO₃溶液中的阳极极化行为，测试结果表明，阳极极化曲线上存在着两个电流峰和两个钝化区.与在NaHCO3溶液中相比，碳钢在Na₂CO₃-NaHCO₃溶液中的极化曲线上的第一个钝化区变得更宽.当增加NaHCO₃浓度后，两个峰值电位均往正方向移动，表明碳钢的钝化能力降低.研究发现，第二个电流峰对应的电位与NaHCO₃浓度的对数呈良好的线性关系.根据铁在Na₂CO₃-NaHCO₃溶液中的腐蚀热力学，推测该峰处发生的反应可能是：3FeCO₃+4H₂O---Fe₃O₄+3HCO^-₃+5H⁺+2e.XPS分析表明铁电极表面主要由二价铁和三价铁的氧化物组成，进一步支持了以上反应.     

Electrochemical reforming of CH4−CO2 mixed gas using porous Gd-doped ceria electrolyte with Cu electrode

This paper reports on the composition and flow rate of outlet gas and current density during the reforming of CH₄ with CO₂ using three different electrochemical cells: cell A, with Ni−GDC (Gd-doped ceria: Ce_0.8Gd_0.2O_1.9) cathode/porous GDC electrolyte/Cu−GDC anode, cell B, with Cu−GDC cathode/ porous GDC electrolyte/Cu−GDC anode and cell C, with Ru−GDC cathode/ porous GDC electrolyte/ Cu−GDC anode. In the cathode, CO₂ reacts with supplied electrons to form CO fuel and O²⁻ ions (CO₂+2e⁻→CO+O²⁻). Too low affinity of Cu cathode to CO₂ in cell B reduced the reactivity of the CO₂ with electrons. The CO fuel, O²⁻ ions and CH₄ gas were transported to the anode through the porous GDC mixed conductor of O²⁻ ions and electrons. In the anode, CH₄ reacts with O²⁻ ions to produce CO and H₂ fuels (CH₄+O²⁻→2 H₂+CO+2e⁻). The reforming efficiency at 700−800 °C was lowest in cell B and highest in cell A. The Cu anode in cells A and C worked well to oxidize CH₄ with O²⁻ ions (2Cu+O²⁻→Cu₂O+2e⁻, Cu₂O+CH₄→2Cu+CO+2H₂). However, a blockage of the outlet gas occurred in all the cells at 700−800 °C. The gas flow is inhibited due to a reduction in pore size in the cermet cathode, as well as sintering and grain growth of Cu metal in the anode during the reforming.     

Indirect cathodic electrocatalytic degradation of dimethylphthalate with PW11O39Fe(III)(H2O) and H2O2 in neutral aqueous medium

Cyclic voltammetry and degradation of dimethylphthalate (DMP) revealed that the iron-substituted heteropolytungstate anion PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ is an excellent indirect cathodic oxidative electrocatalyst in the presence of H₂O₂. PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ can electrocatalyze the reduction of H₂O₂ to hydroxyl radicals via an inner-sphere electron transfer mechanism, which cause oxidative decomposition of DMP. Almost complete DMP removal and ca. 30% mineralization were obtained in less than 120 min in a mixed phosphate solution at pH 6.86 containing 0.1 mM DMP. MS analyses of the intermediates and final products suggested that glyoxal, oxalic acid and acetic acid are the main ring-opening products, besides some unstable hydroxylated aromatic intermediates. The effects of added H₂O₂ concentration, applied cathodic potential and DMP initial concentration on the degradation of DMP were also investigated. A concentration of 1.0 mM H₂O₂ and cathodic potential of −0.3 V were optimal conditions for DMP degradation in our experiments. At higher initial DMP concentrations degradation also occurred, but at a slower decay rate compared to lower initial concentrations. The present system thus represents a possible method to use PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ as an indirect cathodic oxidative electrocatalyst in water and wastewater treatment.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

Double channel electrode flow cell application to the study of HO2 production on MnxCo3−xO4 (0 ≤ x ≤ 1) spinel films

We conducted a study on the electroreduction of O₂ in alkaline solution at room temperature on pure thin oxide electrodes of composition Mn_xCo_3−xO₄ (0 ≤ x ≤ 1) using the double channel electrode flow cell (DCEFC). The oxides were prepared at 150 °C and deposited by spray pyrolysis onto titanium substrates. The oxygen reduction reaction (orr) occurs through “interactive” and “parallel” pathways, and the ratio of O₂ molecules reduced to OH⁻ ions with respect to those reduced to HO₂⁻ ions depends on the oxide stoichiometry and on the applied overpotential. The formation of HO₂⁻ increases when the manganese concentration increases. The results obtained for the orr show that the number of electrons transferred per O₂ molecule decreases from 3 to 2 and the ratio k₁/k₂ (the rate constants for direct reduction to OH⁻ and indirect reduction to HO₂⁻) increases, respectively, in the overpotential studied range (−0.05 to −0.6 V). The Mn³⁺ ions placed in the B-sites of the spinel structure seem to be the active centres, where hydrogen peroxide is formed.     

Oxygen reduction on NixCo3−xO4 spinel particles/polypyrrole composite electrodes: hydrogen peroxide formation

The oxygen reduction reaction (orr) was studied on composite electrodes GC/PPy/PPy(Ni_xCo_3−xO₄)/PPy with x=0.3 and 1, in 2.5×10⁻³ M KOH+0.8 M KCl at room temperature. The orr takes place on the oxide particles with formation of hydrogen peroxide (H₂O₂), which diffuses through the polymer layers to reach the bulk of electrolyte solution. The amount of H₂O₂ produced, determined indirectly by iodine spectrometry, depended strongly on the oxide stoichiometry. The effects of the orr and of the generation of H₂O₂ on the conductivity of PPy and on the behaviour of the embedded oxide particles are discussed.     

Modelling the transport of carbonic acid anions through anion-exchange membranes

Electrodiffusion of carbonate and bicarbonate anions through anion-exchange membranes (AEM) is described on the basis of the Nernst-Planck equations taking into account coupled hydrolysis reactions in the external diffusion boundary layers (DBLs) and internal pore solution. The model supposes local electroneutrality as well as chemical and thermodynamic equilibrium. The transport is considered in three layers being an anion exchange membrane and two adjoining diffusion layers. A mechanism of competitive transport of HCO₃⁻ and CO₃²⁻ anions through the membrane which takes into account Donnan exclusion of H⁺ ions is proposed. It is predicted that the pH of the depleting solution decreases and that of the concentrating solution increases during electrodialysis (ED). Eventual deviations from local electroneutrality and local chemical equilibrium are discussed.     

The behaviour of iron electrode in CO2− saturated neutral electrolyte—I. Electrochemical study

The properties of iron electrode in CO₂-saturated sodium chloride supporting electrolyte were investigated by cyclic voltammetry of static and rde-Ferrovac electrode in the potential region from ?1.1 to ?0.3 V (nhe). In addition to the reduction of H₃O⁺ ions formed due to the protolysis of carbonic acid, the direct (activated) reduction of both H₂CO₃ molecules and HCO^?₃ ions was observed. It is proposed that catalytic, E_rC′_i mechanism accounts for the occurrence of the latter processes.     

Electrodeposition of CdS from acidic aqueous thiosulfate solution—Invesitigation of the mechanism by electrochemical quartz microbalance technique

Electrochemical quartz crystal microbalance was used to study mechanism of cathodic electrodeposition of CdS from acidic aqueous solutions containing 0.01 M Cd(ClO₄)₂ and 0.1 M Na₂S₂O₃ as a source of sulfur. Experiments were performed by means of cyclic voltammetry and potentiostatic method. A comparison of gravimetric and current responses at pH 3 and 4 allowed for determination of the potential range in which side reactions of reduction of SO₃²⁻ and H⁺ ions compete most strongly with formation of CdS. The film thickness was determined by means of two methods: from AFM profiles and EQCM measurements.     

The anodic dissolution of SIMFUEL (UO2) in slightly alkaline sodium carbonate/bicarbonate solutions

The corrosion of nuclear fuel under waste disposal conditions is likely to be influenced by the bicarbonate/carbonate content of the groundwater since it increases the solubility of the U^VI corrosion product, [UO₂]²⁺. As one of the half reactions involved in the corrosion process, the anodic dissolution of SIMFUEL (UO₂) has been studied in bicarbonate/carbonate solutions (pH 9.8) using voltammetric and potentiostatic techniques and electrochemical impedance spectroscopy. The reaction proceeds by two consecutive one electron transfer reactions (U^IV → U^V → U^VI). At low potentials (≤250 mV (vs. SCE) the rate of the first electron transfer reaction is rate determining irrespective of the total carbonate concentration. At potentials >250 mV (vs. SCE) the formation of a U^VIO₂CO₃ surface layer begins to inhibit the dissolution rate and the current becomes independent of potential indicating rate control by the chemical dissolution of this layer.     

Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system

This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H₂ fuel and circulate CO₂ gas in the production process of electric power. A large electric power was measured in the H₂–O₂ gas system and the CO–O₂ gas system at 1073 K. The formation process of O²⁻ ions in the endothermic cathodic reaction (1/2O₂+2e⁻→O²⁻) controlled the cell performance in both the gas systems. The electric power of the H₂–CO₂ gas system, which allowed to change CO₂ gas into a CO fuel (H₂+CO₂→H₂O+CO) in the cathode, was 1/31–1/11 of the maximum electric power for the H₂–O₂ gas system. This result is related to the larger endothermic energy for the formation of O²⁻ ions from CO₂ molecules at the cathode (CO₂+2e⁻→CO+O²⁻) than from O₂ molecules. The CO–H₂O gas system and the H₂–H₂O gas system was expected to produce a H₂ fuel in the cathode (CO+H₂O→H₂+CO₂, H₂+H₂O→H₂+H₂O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H₂O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.     

Synthesis optimization of Li1 + x[Mn0.45Co0.40Ni0.15]O2 with different spherical sizes via co-precipitation

Li_1 + x[Mn_0.45Co_0.40Ni_0.15]O₂ spherical cathode materials with different sizes (about 2 and 5 μm) were fabricated by calcining uniform spherical metal carbonate, [Mn_0.45Co_0.40Ni_0.15]CO₃ with lithium hydroxide at high temperature. The precursor of spherical metal carbonate, [Mn_0.45Co_0.40Ni_0.15]CO₃, was obtained via co-precipitation method at room temperature, which was significantly dependent on synthetic conditions, such as the reaction temperature, the concentration of NH₄HCO₃, and stirring speed, etc. The optimized condition resulted in [Mn_0.45Co_0.40Ni_0.15]CO₃, of which the particle size distribution was uniform and the particle shape was spherical. The final products, Li_1 + x[Mn_0.45Co_0.40Ni_0.15]O₂, had a well-ordered layered structure and uniform homogeneity. Raman spectroscopy analysis showed the Raman-active species E_g and A_1g modes were observed at 488, 473 cm^− 1 and 597, 590 cm^− 1, respectively, for the obtained spherical cathode materials.     

Correlation between ion-exchange properties and swelling/shrinking processes in hexasulfonated calix[6]arene doped polypyrrole films: ac-electrogravimetry and electrochemical atomic force microscopy investigations

Electrogenerated polypyrrole films doped with hexasulfonated calix[6]arenes were subjected to ac-electrogravimetry and electrochemical atomic force microscopy (EC-AFM) studies in aqueous potassium nitrate solutions. The former technique reveals that these films are mainly cation exchangers although solvent molecules (H₂O) and anions (NO₃⁻) are also exchanged, in much lower amounts, in the course of the doping/undoping process. Unexpectedly, within the potential range encompassing this process, K⁺ cations were found to be exchanged for more cathodic potentials whereas H₃O⁺ are exchanged for more anodic potentials. EC-AFM investigations revealed substantial shrinking and swelling during the oxidation (doping) and reduction (undoping) processes respectively. An obvious correlation can easily be built between these observations: the oxidation of the polymer films provokes an expulsion of the cations, as expected from cation exchanger polymer films, and therefore a decrease of the volume (and thickness) of these films whereas their reduction causes an insertion of cations and an increase of their volume (and thickness). This electromechanical mechanism is amplified by the simultaneous exchange of free water molecules. Suggestions based on these observations, on structural characteristics of polypyrrole films, and on complexation ability of hexasulfonated calix[6]arenes incorporated in the films are discussed to explain (i) the change of the identity of the exchanged cations as a function of the potential, (ii) the exchange of free water molecules and, (iii) the exchange of small amounts of nitrate ions.     

Investigation of the tetracarbonatozirconate ion by electrospray mass spectrometry in aqueous media

Reported here is the study of zirconium (IV) aqueous solutions in carbonate media by electrospray ionisation mass spectrometry (ESI–MS). Spectra analysis lead to consider the existence in the spray of the initial pseudomolecular labile species [Zr(CO₃)₄(HCO₃)]⁵⁻, xH⁺, (6−x) K⁺ (or Na⁺). The existence of this species confirms the associative exchange mechanism of carbonate ions with [Zr(CO₃)₄]⁴⁻ ion.     

<Emphasis Type="Italic">In situ</Emphasis> microobservation of the cathode in molten carbonate fuel cells

In molten carbonate fuel cells (MCFC), the wettability of the electrode and the electrolyte distribution are very important factors influencing the active reaction area. We have observed the molten carbonate behaviour directly on the cathode (porous NiO) and the electrolyte plate (LiAlO₂) under various gas conditions and at controlled potentials using an environmental scanning electron microscope (ESEM) equipped with a hot stage. We estimated the liquid electrolyte distribution in the cathode and measured the contact angles on NiO and LiAlO₂ in the electrolyte. Moreover, the electrolyte movement in the reaction CO₂ + O₂ + 2e^– = CO₃ ^2– was observed on the surface of the porous NiO in a CO₂/O₂ atmosphere. The reaction CO₃ ^2– + 2e^– = CO + 2 O^2– of the gas generation was observed in a H₂O atmosphere. The active reaction points on the electrode are the areas where the electrolyte film is thin.     

Metastable and stable pitting events on Al induced by chlorate and perchlorate anions—Polarization, XPS and SEM studies

The metastable and stable pitting events of Al were studied in 0.075 M deaerated acidic NaClO₃ and NaClO₄ solutions (pH 3) using potentiodynamic anodic polarization and potentiostatic measurements, complemented with SEM and XPS examinations of the electrode surface. Metastable pits (appeared here as oscillations in current in the nA range) form at potentials close to the pitting potential (E_pit). SEM examinations of the electrode surface showed that the current oscillations resulted in observable pits on sample surface. The repassivated metastable pitting sites are prone to become preferential sites for following metastable pits to nucleate, resulting in accumulated corrosion damages on the surface. Results showed that Cl⁻ ions produced in solution via the reduction of ClO₃⁻ and ClO₄⁻ anions at sufficiently negative cathodic potentials as well as their decomposition at high anodic potentials. Chloride production induced via the reduction of perchlorates is much slower than induced by chlorates. XPS examinations of the electrode surface showed that the amount of ClO₄⁻ and Cl⁻ anions detected on the electrode surface increases with both cathodic and anodic polarizations (even above E_pit). Experimental results revealed that addition of Cl⁻ ions to the ClO₄⁻ solution accelerates pitting corrosion, indicating that these anions cooperate together in passivity breakdown and initiation of pitting. The role of ClO₃⁻ and ClO₄⁻ ions, despite their large size, in pitting process is also discussed here. A point defect model (PDM) is employed to explain passivity breakdown induced by pitting corrosion as a result of the aggressive attack of Cl⁻ ions.     

A perovskite-type KNbO3 nanoneedles based biosensor for direct electrochemistry of hydrogen peroxide

A KNbO₃ nanoneedles (KNs) based hydrogen peroxide (H₂O₂) biosensor was first proposed. Perovskite-type KNs can directly catalyze H₂O₂. The mechanism can be explained by Molecular Orbital Principles, with the formation of σ-bonding between the e_g orbital of surface niobium ions and surface adsorbed oxygen-related intermediate species. Direct electron transfer between the Horseradish peroxidase (HRP) and electrode surface was achieved. Co-catalyst system of both HRP and KNbO₃ was introduced to the oxidation of H₂O₂, thus the as-prepared biosensor exhibited high sensitivity (750 μA mM⁻¹ cm⁻²) and ultrafast response (1–2 s) to H₂O₂. Therefore, KNs provide a promising material for enzymes assembly and sensing application.     

Role of Cl in breakdown of Cu-Ag alloys passivity in aqueous carbonate solutions

The electrochemical behaviour of two Cu-Ag alloys was studied in 0.1 M Na₂CO₃ solution containing different concentrations of Cl⁻ ions using linear polarization and current/time transients under the effect of different variables of Cl⁻ ions concentration, scan rate and applied anodic potentials. In Cl⁻ free solutions, the anodic voltammogram consists of two potential regions I and II. The potential region I exhibits three anodic peaks A₁, A₂ and A₃ that correspond to the formation of Cu₂O, Cu(OH)₂ and CuO, respectively. The potential region II exhibited four anodic peaks A₄, A₅, A₆ and A₇ due to the formation of AgO⁻, Ag₂O, Ag₂CO₃ and Ag₂O₂. In the presence of Cl⁻ ions, the anodic voltammograms depends considerable on the concentration of Cl⁻ ions. Increasing the amounts of Cl⁻ ions up the 0.02 M (alloy I) or 0.006 M (alloy II) the heights of all the anodic peaks were decreased and their peak potentials were shifted to less negative values. The existence of pitting was confirmed by SEM micrograph. The pitting potential E_pit was shifted towards more active potential values as the concentration of Cl⁻ ions in the solution was increased. When the scan rate is high, initiation of the pitting can be noticed only at more positive potentials, corresponding to a sufficiently short pit incubation time. The potentiostatic current/time transients show that the incubation time decreases with increasing the applied anodic potential and the Cl⁻ ion concentration and the pitting corrosion can be described in terms of instantaneous three-dimensional growth under diffusion control.