Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Electrochemical reduction of nitrate in weakly alkaline solutions

The electrocatalytic activity of several materials for the nitrate reduction reaction was studied by cyclic voltammetry on a rotating ring disc electrode in solutions with different concentrations of sodium bicarbonate. Copper exhibited highest catalytic activity among the materials studied. Nitrate reduction on copper was characterized by two cathodic shoulders on the polarization curve in the potential region of the commencement of hydrogen evolution. In this potential range an anodic current response was observed on the Pt ring electrode identified as nitrite to nitrate oxidation. This indicates that nitrite is an intermediate product during nitrate reduction. These conclusions were verified by batch electrolysis using a plate electrode electrochemical cell. Copper and nickel, materials representing the opposite ends of the electrocatalytic activity spectra, were used in batch electrolysis testing.     

Electrochemical reduction of nitrates and nitrites in alkaline nuclear waste solutions

Sodium nitrate and nitrite are major components of alkaline nuclear waste streams and contribute to environmental release hazards. The electrochemical reduction of these materials to gaseous products has been studied in a synthetic waste mixture. The effects of electrode materials, cell design, and other experimental parameters have been investigated. Lead was found to be the best cathode material in terms of current efficiency for the reduction of nitrate and nitrite in the synthetic mix. The current efficiency for nitrite and nitrate removal is improved in divided cells due to the elimination of anodic oxidation of nitrite. Operation of the divided cells at high current densities (300–600 mA cm^–2) and high temperatures (80°C) provides more efficient reduction of nitrite and nitrate. Nearly complete reduction of nitrite and nitrate to nitrogen, ammonia, or nitrous oxide was demonstrated in 1000 h tests in a divided laboratory electrochemical flow cell using a lead cathode, Nafion® 417 cation exchange membrane, and oxygen evolving DSA® or platinum clad niobium anode at a current density of 500 mA cm^–2 and a temperature of 70° C. Greater than 99% of the nitrite and nitrate was removed from the synthetic waste mix batch in the 1000 h tests at an overall destruction efficiency of 55%. The process developed shows promise for treating large volumes of waste.     

Electrochemical reduction of thick oxide film on platinum electrode in alkaline solutions

The electrochemical reduction of the thick oxide film formed on Pt electrode by severe preanodization has been studied in LiOH, NaOH and KOH solutions of different concentrations (0.001 ～ 1.0 M) using a galvanostatic technique.An outermost monolayer oxide and an inner multilayer oxide of the thick oxide film exhibit different potential behaviors in the cathodic reduction. In dilute solution, both the oxides are completely reduced in a potential range of 0.6-0.4 V (vs rhe in the test solution) in a single step. As the concentration is increased, however, the reduction potential of the inner oxide only shifts rapidly into a H-electrosorption potential region and the amount of the oxide reduced at this potential decreases. The remaining oxide is slowly reduced at H₂-evolution potential. The retardation of the reduction of the inner oxide is related to cations adsorbed on Pt electrode. This retardation effect increases in the order, K⁺ < Na⁺ < Li⁺.     

Electroreduction of oxygen on gold-supported nanostructured palladium films in acid solutions

The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm on polycrystalline Au substrate (Pd/Au) was studied. The Pd films were prepared by electron beam evaporation and oxygen reduction was studied in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions using the rotating disk electrode (RDE) method. The surface morphology of Pd overlayers was examined by scanning tunnelling microscopy (STM). O₂ reduction predominantly proceeds through 4e⁻ pathway on all Pd/Au electrodes. The specific activity (SA) of oxygen reduction was lower in H₂SO₄ solution and decreased slightly with decreasing the Pd film thickness. In HClO₄, the SA was higher and not significantly dependent on the film thickness. The Tafel slope values close to −60 mV at low current densities and −120 mV at high current densities were found for all electrodes.     

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.     

Electrochemical oxygen reduction on oxide catalysts

The process of oxygen electroreduction and adsorption in alkaline medium on nickel and cobalt oxides has been studied by the potentiodynamic and the disc-ring electrode methods. The oxide films were formed on the surface of metal specimens by thermal or electrochemical oxidation.After oxidation, the surface was examined by X-ray diffraction and electron diffraction. It was found that, depending on the oxidation conditions, simple or complex oxides are formed on the surface of metal specimens. Nickel and Cobalt oxide films obtained by thermal oxidation at t° ?300°C are more active than pure metals. The oxides of spinel structure Co₃O₄ and NiCo₂O₄ have an optimum activity. It is shown that oxygen adsorbed during the heat treatment of the specimens accelerates the process of the molecular oxygen reduction. On the oxides of spinel structure the oxygen reduction reaction proceeds to OH^?, whereas on simple nickel oxides the final product of oxygen reduction is HO^?₂.     

Electrochemical and oxygen reduction behaviour of solid silver-bismuth/antimony electrodes in KOH solutions

Oxygen reduction behaviour at silver-low bismuth/antimony electrodes was investigated by electrochemical methods in KOH solutions. The electrochemical redox characteristics such as peak multiplicity, peak currents and peak potentials on these electrodes under both potentiostatic steady state and cyclic voltammetric conditions were obtained. The anomalies associated with the redox peak potential values both in the anodic and cathodic branches of the cyclic voltammograms were attributed to the distinct electrochemical behaviour of bismuth/antimony on the electrode surface along with silver. The behaviour of these electrodes for the electrocatalytic cathodic reduction of oxygen was also investigated both by potentiostatic steady state and cyclic voltammetric techniques under rotating conditions. The kinetic parameters corresponding to the electrochemical reduction of oxygen on these electrodes were obtained. These electrodes were ranked with respect to their electrocatalytic activity both for the oxygen reduction and evolution reactions.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

Electrochemical behavior of Au nanoparticle deposited on as-grown and O-terminated diamond electrodes for oxygen reduction in alkaline solution

Au nanoparticle was electrochemically deposited on both as grown and oxygen-terminated (O-terminated) boron-doped diamond (BDD) films. The surface coverages of Au nanoparticle were 0.07 and 0.18 corresponding to the areas of Au 0.012 and 0.029 cm², respectively, as noted from linear sweep voltammetry. The SEM studies indicated different morphologies of Au deposition such as random distribution of small spherical particles at both the grain boundaries and the facets on the as grown diamond film and clusters principally on the cross edges of two facets on the O-terminated diamond. The electrochemical behavior for oxygen reduction was examined using differential pulse voltammetry (DPV), which confirmed the higher catalytic efficiencies of Au deposited as grown and O-terminated BDD electrodes when compared to a polycrystalline Au electrode. Moreover, the mechanism of Au nanoparticle deposited BDD films for the oxygen reduction was investigated by ac impedance and hydrodynamic voltammetric methods.     

Electrochemical hydrogen evolution on polypyrrole from alkaline solutions

Thin films of conductive polypyrrole (PPy) were formed electrochemically from aqueous sulfuric acid. The films showed good electrocatalytic properties towards hydrogen evolution (h.e.r) from alkaline solutions on planar and packed-bed iron electrodes. Current–potential relations were measured at various temperatures and KOH concentrations on Fe, Ni and Fe/PPy planar electrodes. The current was found to be constant during 40 h of operation. SEM micrographs showed no difference in the morphology before and after this period. The activation energy for h.e.r. was found to be 50.2, 58.5 and 33.4 kJ mol^–1 for Fe, Ni and Fe/PPy planar electrodes, respectively. The results showed that Fe/PPy can be used to produce hydrogen at both ambient and relatively high temperatures 70 C. The polypyrrole coating on iron screens was found to reduce the potential required to sustain a specific rate of hydrogen generation and, hence, the energy consumed during the process.     

Electrochemical behaviour of iron in alkaline sulphide solutions

Reactions of the SH^? anion on iron electrodes in deoxygenate alkaline solutions were studied using constant potential, constant current and acyclic voltammetric techniques.In the absence of sulphide, a dual oxide layer is formed. When sulphide is present, oxide growth is inhibited. At sufficiently anodic potentials the oxide layer is attacked, leading to a breakdown of passivity and the formation of sodium ferric sulphide, identified by its characteristic visible spectrum. Oxidation of the sulphide ion leads to repassivation of the electrode by a deposit of elemental sulphur. The effectiveness of this repassivation is dependent on the sulphide concentration since excess sulphide dissolves the passivating sulphur deposit to form polysulphides. Polysulphides were identified by colour and by their uv — visible spectrum.The voltammetric data were analysed according to the theory for film formation under Ohmic control. Agreement with this theory suggests that sulphide attacks the oxide in a similar manner to Cl^? (ie pitting attack) and that the repassivation is due to the local plugging of these pits with electrochemically deposited sulphur.     

Electrochemical formation of mackinawite in alkaline sulphide solutions

The growth of mackinawite, an iron-rich sulphide, on an iron electrode at constant potential in alkaline sulphide solutions is preceded by the fast growth of a thin imperfect film of magnetite. Nucleation of mackinawite patches on the electrode starts at fault sites in the oxide film and the rate of coverage is controlled by the rate of oxide removal. Sulphide film formation occurs when the proton concentration at the fault site is high enough to reduce the local concentration of OH⁻ significantly. The protons are released when sulphide ions are oxidised yielding polysulphide species and an elemental sulphur deposit. The mackinawite films are highly stressed and crack readily. This leads to substantial currents on the sulphide covered surface.     

Electrochemical behaviour of iron in alkaline sulphate solutions

The electrochemical behaviour of pure iron in alkaline sulphate solutions was studied using cyclic voltammetry atT = 298 K. It has been found that the results depend on the polarization pre-treatment of the electrode and the activation state of its surface. At starting potentials in the range of hydrogen evolution and at maximum activation conditions, the voltammograms show three anodic current maxima and two cathodic ones. A correlation between these different maxima is given. The electrode processes, which may take place at these maxima, are discussed in terms of possible iron dissolution mechanisms.     

Anodic behaviour of arsenopyrite and cathodic reduction of ferrate(VI) and oxygen in alkaline solutions

This paper presents the results of an electrochemical study of the anodic characteristics of arsenopyrite in strongly alkaline solutions and of the cathodic reduction of ferrate(VI) and of dissolved oxygen at an arsenopyrite surface at potentials which are relevant to the oxidation reactions. Cyclic voltammetry at both arsenopyrite disc and arsenopyrite disc/platinum ring electrodes has shown that arsenic(III) is the main product of the anodic process at potentials in the region of the rest potential during oxidation by either ferrate(VI) or oxygen. Evidence for partial passivation of both the anodic and cathodic reactions has been obtained from potentiostatic current–time transients. The initial stage of oxidation by ferrate(VI) has been shown to be mass-transport controlled and this is also true of the oxidation by oxygen in dilute solutions of sodium hydroxide.     

Kinetics of oxygen reduction on carbon materials in alkaline solution

Oxygen reduction in alkaline solution on a wide range of carbon blacks, active carbons, and graphites has been studied using the ultra thin electrode technique. Results were obtained as a function of pH and KOH concentration, oxygen partial pressure, and temperature. Under all conditions, the pH dependence of the reaction (vs nhe) at constant cd is zero, as pH (or KOH concentration) increases. In concentrated alkali, a change in mechanism is noted. The pH region for this change increases as BET area of the carbon increases. A mechanism is suggested to account for these observations, which do not involve a reversible O₂/O₂H^? couple. For carbons exhibiting the second (～2RT/3F Tafel slope) mechanism in 6 N KOH, activity is proportional to available surface area.     

间硝基苯甲酸在硫酸介质中的电还原特性

采用循环伏安法研究了硫酸介质中电极材料、扫描速度、反应温度和添加剂等对间硝基苯甲酸(mNBA)电解还原的影响。结果表明：在该体系中最合适的阴极材料为铜电极;在电还原过程中,随着扫描速度和温度增大,mNBA电还原反应的峰电流不断增大,其电极过程受扩散控制,电还原反应伴有不可逆随后化学反应;在溶液中加入无机盐SnCl2对mNBA的电解还原有明显的助催化作用。     

Electrochemical reduction of 4-(2′-thienyl)quinazoline in acid solutions

The electrochemical behaviour of 4(2′-thienyl)quinazoline (4-TQ) was studied in acid solutions of varying ionic strengths using polarography, cyclic voltammetry and controlled-potential electrolysis. 4-TQ is reduced in two single electron steps to give the dihydro-4-TQ product. The two polarographic waves were found to be diffusion-controlled, the first wave being reversible and the second wave irreversible. On the basis of the results obtained, a mechanism is suggested for 4-TQ reduction in which a dimerization reaction goes in parallel with the electron-transfer step.