Cathodic reduction of oxygen on copper and brass

In this investigation, the electrochemical reduction of oxygen on copper and brass has been studied using the ring-disc electrode technique in solutions containing chloride, sulphate and nitrate anions and ammonium cation. The E—I curves obtained in the forward and reverse direction of polarization for copper and brass in NaCl, Na₂SO₄ and NH₄Cl solutions are similar in nature and show two waves. However in (NH₄)₂SO₄ and NH₄Cl solutions are similar in nature and show two waves. The rate of oxygen reduction is the highest in ammonium sulphate for both copper and brass whereas it is the lowest in NH₄Cl in the case of copper. In Na₂SO₄ and NaCl solutions, the rate of oxygen reduction is higher on copper than on brass. Apparent Tafel slopes for oxygen reduction obtained for copper and brass vary from 65 mV to 240 mV depending upon the medium.The two steps observed with copper disc electrode have been identified as due to the reduction of oxygen to hydrogen peroxide and reduction of H₂O₂ to OH^? ion or water depending upon the pH of the solution. In acid chloride and sulphate media no H₂O₂ was detected, which suggests direct reduction to H₂O. The diagnostic plots of I_d/I_rυs ω^{?$\text{1}\text{2}$} employed by Bockris et al indicate that in Na₂SO₄ the reduction of oxygen to H₂O₂ takes place in a parallel reaction whereas in (NH₄)₂SO₄ both direct reduction of O₂ to water (or OH^? ion) and the reduction through intermediate H₂O₂ occur.     

Electrochemical behaviour of lead chloride (PbCl2) in dimethylsulphoxide (DMSO)—II. a chronopotentiometric study

In our earlier electrode kinetic studies (chronopotentiometric) of lead chloride (PbCl₂) in dimethylsulfoxide (DMSO), the points in the plot of E, vs log (τ^1/2 ? t^1/2) were linear in the form of a wave. This situation was similar to that observed by Delahay in the chronopotentionmetric reduction of chromate ion (aqueous medium) and thought to be due to a prekinetic step. To verify the presence of a prekinetic step in our case a survey of iτ^1/2vs i for a wider range of lead chloride concentration was started. But a distortion observed in the initial part of the curve (from 4·0 mM PbCl₂) was another point of similarity with that of chromate ion. This led us to consider the polarographic and chronopotentiometric reduction behaviour of CrO^2?₄ in two different electrolytes sodium hydroxide and sodium carbonate. Such considerations justify the presence of a prekinetic step in electrochemical reduction of chromate. Such a prekinetic step, probably the dissociation of PbCl₂, is verified and Kk^1/2₂ values for this coupled chemical reaction are determined according to the transition time expressions of Delahay and Dracka.     

Effect of electrolyte on the supercapacitive behaviour of copper oxide/reduced graphene oxide nanocomposite

Cupric oxide/reduced graphene oxide (CuO/rGO) nanocomposites were synthesized through a chemical reduction method using hydrazine hydrate as the reducing agent. The morphology, elemental composition, and bonding network of the CuO/rGOnanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy respectively. The XRD results reveal lattice spacing and lattice strain from 3.371 to 3.428 Å and 1.05 × 10⁻³to 5.44 × 10⁻³ respectively, with the increasing ratio of rGO: CuO from 1:1 to 1:5. The cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS)and galvanostatic charge-discharge (GCD) studyofCuO/rGOas the electrode material showed excellent super-capacitive behavior in H₂SO₄ over Na₂SO₄ electrolytes. Moreover CuO/rGO nanocomposites exhibited better capacitance retention in H₂SO₄(75.69%) compared to Na₂SO₄(12.06%).     

Influence de la composition et de la temperature sur les reactions electrochimiques et le domaine de stabilite electrochimique des borates de sodium fondus

The oxidation and reduction reactions, limits of the electrochemical stability range, were determined by cyclic voltammetry in molten sodium borates 3 Na₂OB₂O₃ (I), Na₂O2B₂O₃ (II) and Na₂O4 B₂O₃ (III) between 750°C and 1000°C.These reactions are: on gold electrode the oxidation of O^2? ions to oxygen and the reduction of borate ions to elemental amorphous boron; on platinum electrode the oxidation and passivation of the metal and the reduction of borate ions to form platinum borides Pt₃B and Pt₂B.The electrochemical stability ranges are respectively in solvent I 0.67 V (750°C), 0.97 V (850°C), 0.92 V (1000°C); in solvent II 1.94 V (750°C), 1.92 V (850°C), 1.85 V (1000°C); in solvent III 1.85 V (850°C), 1.75 V (1000°C).     

Sodium niobates and protonic niobates nanowires obtained from hydrothermal synthesis: Electrochemical behavior in aqueous electrolyte

Niobium-based oxides can be used in several applications due to a diverse set of properties. In this work, Na₂Nb₂O₆.H₂O sodium niobate nanowires were obtained by hydrothermal synthesis at low temperature. Dehydrated sodium niobate (Na₂Nb₂O₆) and sodium niobate with perovskite structure (NaNbO₃) were obtained by submitting Na₂Nb₂O₆.H₂O to heat treatment (350 °C and 500 °C, respectively). To obtain protonic niobates, sodium niobates were immersed in nitric acid in order to promote ion exchange reactions. From this procedure, protonic niobates (H₃O)₂Nb₂O₆.H₂O and (H₃O)₂Nb₂O₆ were obtained. The sample NaNbO₃ did not undergo any transformation. Cyclic voltammetry tests carried out in a neutral aqueous solution 1 M Na₂SO₄ showed a wide potential window for both niobates (sodium and protonic). However, the protonic niobate samples (H₃O)₂Nb₂O₆.H₂O and (H₃O)₂Nb₂O₆ presented much higher current density values than the sodium niobates. This result can be related to a structural rearrangement that allowed a significant increase in the intercalation of sodium Na ⁺ ions from the electrolyte into the structure of these protonic niobates, when polarized. Therefore, in this work, it was demonstrated that it is possible to obtain protonic niobates from sodium niobates, as well as, it was verified the distinct electrochemical behavior between these materials.     

Electrochemical insertion of sodium into graphite in molten sodium fluoride at 1025 °C

The electrochemical insertion of sodium into graphite was studied in molten sodium fluoride at 1025 °C. The results obtained evidenced two mechanisms for sodium insertion into graphite: sodium intercalation between the graphite layers and sodium sorption into the porosity of the material. Subsequent internal rearrangement of inserted sodium occurred, via transference from the pores towards the intercalation sites. In addition, the intercalation compound was found to undergo a fast decomposition process (k = 2.55 × 10⁻⁹ mol s⁻¹). X-ray diffraction analysis was used to confirm the formation of a high stage compound (Na_0.1C₈), the composition of which was consistent with compositions observed in the case of chemical vapor and electrochemical insertion of sodium, during experiments in the sodium perchlorate-ethylene cabonate electrolyte.     

Carbonate and sulphate green rusts—Mechanisms of oxidation and reduction

The oxidation and reduction of carbonate, GR(CO₃), and sulphate, GR(SO₄), green rusts (GR) have been studied through electrochemical techniques, electrochemical quartz crystal microbalance (EQCM), FTIR, XRD and SEM. The used samples were made of thin films electrodeposited on gold substrate. The results from the present work, from our previous studies and from literature were compiled in order to establish a general scheme for the formation and transformation pathways involving carbonate or sulphate green rusts. Depending on experimental conditions, two routes of redox transformations occur. The first one corresponds to reaction via solution and leads to the formation of ferric products such as goethite or lepidocrocite (oxidation) or to the release of Fe^II ions into the solution (reduction) with soluble Fe^II-Fe^III complexes acting as intermediate species. The second way is solid-state reaction that involve conversion of lattice Fe²⁺ into Fe³⁺ and deprotonation of OH groups in octahedra sheets (solid-state oxidation) or conversion of lattice Fe³⁺ into Fe²⁺ and protonation of OH groups (solid-state reduction). The solid-state oxidation implies the complete transformation of GR(CO₃) or GR(SO₄) to ferric oxyhydroxycarbonate exGRc-Fe(III) or ferric oxyhydroxysulphate exGRs-Fe(III), for which the following formulas can be proposed, Fe^III₆(OH)_(12−2y)(O)_(2+y)(H₂O)_(y)(CO₃) or Fe^III₆(OH)_(12−2z)(O)_(2+z)(H₂O)_(6+z)(SO₄) with 0 ≤ y or z ≤ 2. The solid-state reduction gives ferrous hydroxycarbonate exGRc-Fe(II) or ferrous hydroxysulphate exGRs-Fe(II), which may have the following chemical formulas, [Fe^II₆(OH)₁₀(H₂O)₂]·[CO₃, 2H₂O] or [Fe^II₆(OH)₁₀(H₂O)₂]·[SO₄, 8H₂O].     

Polarization measurements on solid platinum—molten sodium sulphate—sodium chloride interfaces

Electrochemical studies in sodium sulphate—sodium chloride melts at 900° are reported. The limiting reactions which occur during electrolysis of fused sodium sulphate are suggested to be the SO₃ reduction to S^2? and O^2? and the SO^2?₄ oxidation to SO₂ and O₂ at the cathode and anode, respectively. Additions of sodium chloride into the sulphate do not reduce its stability range.A relevant finding is that platinum is anodically passivated in sulphate melts. Voltammetric scans made to elucidate its passivation mechanism show that the anodically polarized platinum is continuously modified and passes through three states of passivation. Moreover, it is observed that the platinum samples used as cathodes are attacked by the sulphate reduction products.     

Reduction of nitric oxide at a flow-through mercury plated nickel electrode

The electrochemical reduction of nitric oxide at a flow-through mercury-plated nickel gauze electrode in sulphuric acid was investigated. The current efficiencies of hydroxylamine, nitrous oxide and of hydrogen formation were determined. The main experimental results are: 1. The ratio between the NH₂OH and N₂O formation depends on the cd and on the flowrate of the electrolyte through the electrode, but does not depend on the H₂SO₄ concentration in the investigated range from 0.25 to 2.0 M and likewise not on the temperature. 2. The rate of the reduction of nitric oxide to NH₂OH and N₂O increases with increasing cd up to a maximum value, thereafter this rate decreases with increasing cd. 3. The ratio between the current efficiency of the NH₂OH formation and the current efficiency of the N₂O formation increases slowly with increasing cathodic potential.It seems that at low cd (much lower than the cd where the rate of the reduction of NO reaches its maximum) the reduction of NO is affected by both the electrochemical parameters and by the transport of NO to the electrode surface. However, at high current densities the reduction is dominated by mass-transport of NO only. NOH is an intermediate for both the NH₂OH and the N₂O formation.     

Potentiometric SO2 gas sensor based on a thick film of Ca2+ ion conducting solid electrolyte

A planar miniaturized SO₂ sensor based upon a thick film of Ca²⁺ ion conductor-CaO·0.6MgO·6Al₂O₃ (CMA) with a Na₂SO₄ auxiliary electrode and a Pt/O₂ reference electrode was fabricated and tested. The thick film was fabricated by screen-printing CMA ink on an alumina substrate and then fired at 1823 K. The solid electrolyte was interfaced with a sodium sulphate auxiliary phase containing Pt paste and the sensor showed a good SO₂ response at 873–1073 K. The electromotive force (emf) values obtained were linearly dependent upon the logarithm values of SO₂ concentration in a range of 10–500 ppm. Both the electrodes were exposed to the same test gas thus eliminating the need to separate the electrode chambers.     

Reversible Na+-extraction/insertion in nitrogen-doped graphene-encapsulated Na3V2(PO4)2F3@C electrode for advanced Na-ion battery

NASICON-structured sodium vanadium fluorophosphate has caused widespread concern for sodium energy conversion and storage because of its high voltage platform and high theoretical energy density. However, the inferior electrical conductivity is still a big problem, which greatly prevent the applications of Na₃V₂(PO₄)₂F₃ material. Herein, the nitrogen-doped graphene-encapsulated Na₃V₂(PO₄)₂F₃@C (NG-NVPF@C) has been prepared using the sol-gel approach. The physical and electrochemical performances for the resulted NG-NVPF@C composite have been systematically characterized and compared with that of Na₃V₂(PO₄)₂F₃@C (NVPF@C) in this study. The electrochemical tests demonstrate that the as-fabricated NG-NVPF@C displays higher capacity, superior rate property and better cyclic life than NVPF@C. It displays the discharge capacity of 108.6 mAh g⁻¹ at 5C. Moreover, it also possesses the high capacity of 101.6 mAh g⁻¹ at 10C over 300 cycles with the capacity retention of about 96.5%. The improved properties of NG-NVPF@C electrode are assigned to the constructed conductive network by nitrogen-doped graphene, which can modify the conductivity of Na₃V₂(PO₄)₂F₃.     

Comparison of oxidation efficiency of disperse dyes by chemical and photoelectrocatalytic chlorination and removal of mutagenic activity

Degradation of Disperse Orange 1, Disperse Red 1 and Disperse Red 13 dyes has been performed using electrochemical oxidation on Pt electrode, chemical chlorination and photoelectrochemical oxidation on Ti/TiO₂ thin film electrodes in NaCl or Na₂SO₄ medium. 100% discoloration was obtained for all tested methods after 1 h of treatment. Faster color removal was obtained by photoelectrocatalytic oxidation in 0.1 mol L⁻¹ NaCl pH 4.0 under UV light and an applied potential of +1.0 V (vs SCE reference electrode), which indicates also values around 60% of TOC removal. The conventional chlorination method and electrochemical oxidation on Pt electrode resulted in negligible reduction of TOC removal. All dyes showed positive mutagenic activity in the Salmonella/microsome assay with the strain TA98 in the absence and presence of S9 (exogenous metabolic activation). Nevertheless, there is complete reduction of the mutagenic activity after 1 h of photoelectrocatalytic oxidation, suggesting that this process would be good option to remove disperse azo dyes from aqueous media.     

Formation characteristics of sodium calcium silicate compounds based on the solid-state reaction

The formation thermodynamics, phase transition and stability of sodium calcium silicate compounds under different calcination parameters in the Na₂O–CaO–SiO₂ system were studied using XRD, FTIR and SEM-EDS methods. As the Na₂O/SiO₂ ratio increases from 0.3 to 0.7 when the CaO/SiO₂ ratio is 1.0, the formation sequence of sodium calcium silicate compounds is Na₂Ca₃Si₂O₈→Na₆Ca₃Si₆O₁₈→Na₂Ca₂Si₂O₇→Na₂CaSiO₄; as the CaO/SiO₂ ratio increases from 0.3 to 1.2 when the Na₂O/SiO₂ ratio is 0.5, the formation sequence is Na₆Ca₃Si₆O₁₈→Na₂Ca₂Si₂O₇→Na₂Ca₃Si₂O₈. As the most stable sodium calcium silicate compound, Na₆Ca₃Si₆O₁₈ forms by the solid-state reaction of preformed Na₂SiO₃ with CaO and SiO₂, while increasing the calcination temperature and holding time can promote its crystal stability. The decomposition of Na₆Ca₃Si₆O₁₈ in sodium aluminate solution follows the mixed control of the film diffusion and chemical reaction, and the corresponding activation energy is between 40 and 41 kJ/mol.     

Preparation and optimization of ZrO2 modified P2-type Na2/3Ni1/6Co1/6Mn2/3O2 with enhanced electrochemical performance as cathode for sodium ion batteries

Surface stabilization is necessary for cathode materials to gain a long-term cycling stability because of unfavorable side reactions and exfoliation caused by corrosive environment. To improve the cyclic stability of P2-type ternary cathode Na_2/3Ni_1/6Co_1/6Mn_2/3O₂ for sodium ion batteries, we prepare a ZrO₂-coated Na_2/3Ni_1/6Co_1/6Mn_2/3O₂ through a simple wet chemical method. The coating layer is distributed homogeneously on the surface, and the fraction of ZrO₂ (1 wt-%, 2 wt-%, 3 wt-%, 4 wt-%, 5 wt-%) helps control the thickness of the coating layer. It turns out that all the materials exhibit pure P2 structure without any impurities. The material with a 2 wt-% ZrO₂ coating exhibits the best electrochemical performance in rate capability and long-term cyclic stability. It delivers a superior initial discharge capacity of 140 mA h·g⁻¹ between 2 and 4.5 V at 20 mA g⁻¹. Even cycles at high current density (100 mA g⁻¹), it shows 106 mA h·g⁻¹ reversible discharge capacity with 88% capacity retention after 300 cycles. The improvement in electrochemical performance is attributed to the segregation of cathode materials from the corrosive electrolyte by the nano-sized ZrO₂ layer. The EIS results confirm that a thin ZrO₂ coating layer can effectively protect the electrode from dissolution and stabilize the SEI film. This study can be used to develop the electrochemical performance of cathode materials for sodium ion batteries by surface modification via ZrO₂.     

Comparative studies of chemical and electrochemical preparation of artificial bronze patinas and their protection by corrosion inhibitor

The bronze artefacts of cultural heritage are often covered with patina, a layer of corrosion products, which confers their aesthetic and also protects the substrate bronze. Due to the increasing atmospheric pollution these layers are often dissolving when exposed in urban environment. In this work we propose the use of an innoxious imidazole compound as a corrosion inhibitor for patinated bronze. On a Cu-6Sn (wt%) bronze, three types of patinas were synthesized: two by chemical methods (in a sulphate solution and a chloride one) and one by an electrochemical process (in a sulphate/carbonate solution). A blue-green patina was obtained in all three cases, and their morphological and structural characterization was performed by SEM, EDS and Raman spectroscopy. It was found that the sulphate patina is composed essentially of brochantite, the chloride patina of atacamite, and the electrochemical patina of malachite. All three patinas have also a smooth part of surface consisted of cuprite. As corrosion inhibitor 4-methyl-1-(p-tolyl) imidazole was used on all patinas, in a solution of 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ acidified to pH 5 which simulates acid rain in urban environment. The results have shown that the inhibitor improves the stability of all three kinds of patinas and can be recommended for protection of works of art.     

Thermodynamic modeling of CO2 solubility in aqueous solutions of NaCl and Na2SO4

A thermodynamic model based on the electrolyte NRTL activity coefficient equation and PC-SAFT equation-of-state is developed for CO₂ solubility in aqueous solutions of NaCl and Na₂SO₄ with temperature up to 473.15 K, pressure up to 150 MPa, and salt concentrations up to saturation. The Henry's constant parameters of CO₂ in H₂O and the characteristic volume parameters for CO₂ required for pressure correction of Henry's constant are identified from fitting the experimental gas solubility of CO₂ in pure water with temperature up to 473.15 K and pressure up to 150 MPa. The NRTL binary parameters for the CO₂-(Na⁺, Cl⁻) pair and the CO₂-(Na⁺, SO₄²⁻) pair are regressed against the experimental VLE data for the CO₂-NaCl-H₂O ternary system up to 373.15 K and 20 MPa and the CO₂-Na₂SO₄-H₂O ternary system up to 433.15 K and 13 MPa, respectively. Model calculations on solubility and heat of solution of CO₂ in pure water and aqueous solutions of NaCl and Na₂SO₄ are compared to the available experimental data of the CO₂-H₂O binary, CO₂-NaCl-H₂O ternary and CO₂-Na₂SO₄-H₂O ternary systems with excellent results.     

Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions

Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na₂SO₄ solutions of different concentrations at 25 ± 0.2 °C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that i_corr and i_c increases with increasing either Cl⁻ or SO₄²⁻ concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl⁻ or SO₄²⁻ concentration.     

The influences of sodium sources on the structure evolution and electrochemical performances of layered-tunnel hybrid Na0.6MnO2 cathode

Manganese (Mn) based oxide materials are regarded as promising cathodes for sodium ion batteries (SIBs) due to their high energy density, low-cost and environmental benignity. Here, we focus on the influences of various sodium sources on the structure diversity and electrochemical performances changes of layered-tunnel hybrid Na_0.6MnO₂ cathode. The Na_0.6MnO₂ cathodes were prepared by precipitation method followed by grinding with different sodium sources and annealing in air. The XRD results evidenced that the mass ratio of layered and tunnel components would be markedly influenced by sodium source. Electrochemical test results also demonstrate distinctive performances of Na_0.6MnO₂ cathodes with various sodium sources. Na_0.6MnO₂ cathode with Na₂C₂O₄ exhibited the best performances with 90 mAh g⁻¹ retained after 100 cycles at 1.0C. Superior rate performance with average discharge capacities of 180, 159, 143, 126, 112 and 93 mAh g⁻¹ at 0.1, 0.5, 1.0, 2.0, 4.0 and 8.0C was also observed. Furthermore, the EIS demonstrate that Na_0.6MnO₂ cathode with Na₂C₂O₄ displayed smaller charge transfer and fast Na⁺ diffusion rate, which indicated enhanced electrochemical reaction kinetics. The excellent electrochemical performance of Na_0.6MnO₂ with Na₂C₂O₄ is mainly due to the appropriate proportion of layered-tunnel component and their synergistic effects, which are influenced by sodium sources.     

Electrochemical behaviour of copper in a NaF-AlF3-BaCl2 ternary melt at 750° C

Voltammetric and chronopotentiometric methods were used to study the electrochemical behaviour of copper in the NaF-AlF₃-BaCl₂ ternary melt at 750° C. Copper, graphite and platinum were used as electrode materials. It was shown that the electrochemical reduction of copper ions is a single step process, with the reversible exchange of one electron at a copper electrode. The value of the diffusion coefficient of the cuprous ion, determined by means of chronopotentiometry, isD =(2.8±0.3)× 10^–5 cm² s^–1.     

Anion dominated electrochemical process of poly(N-methylpyrrole)

The electrochemical behavior of poly(N-methylpyrrole) (PNMP) has been studied in aqueous solution by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The electrochemical process of PNMP (TsO^-) in TsONa solution or PNMP(NO₃^-) in NaNO₃ solution showed good chemical reversibility. The process only involved the anion doping and dedoping. PNMP(TsO^-) kept good electrochemical activity in NaNO₃ or Na₂SO₄ solution. But PNMP(NO₃^-) became less active electrochemically in Na₂SO₄ solution and inactive in TsONa solution. PNMP(NO₃^-) in NaCl solution showed similar electrochemical behavior to that in NiCl₂ solution. © 1996 John Wiley & Sons, Inc.