Cyclic voltammetry study of arsenic in acidic solutions

An electrochemical investigation of arsenic species in acidic solutions, using cyclic voltammetry at a platinum rotating disk electrode, has been carried out. A well defined peak at about 1 V_SCE, with a large overpotential (>0.5 V) is attributed to the oxidation of As(III) to As(V). The reaction is under the mixed control of ionic transport and charge transfer. The corresponding reduction of As(V) is not observed in the potential range of the study (−0.6 to +1.2 V_SCE). As(V) is not electrochemically active in this range and it has no effect on hydrogen evolution. With As(III) in solution, hydrogen evolution on Pt is shifted towards more negative potentials and it takes place simultaneously with arsine production. A small cathodic peak at −0.2 V_SCE and a corresponding anodic peak at +0.3 V_SCE are attributed to the deposition of a thin layer of As by reduction of As(III) and its dissolution.     

Accelerated entry of hydrogen into iron from NaOH solutions at low cathodic and low anodic polarisations

Hydrogen entry into iron from NaOH solutions and from 0.05 M H₂SO₄ during voltammetric cycling was studied with the electrochemical permeation technique. Measurements were carried out on 35-μm thick membranes at 25 °C. Accelerated entry of hydrogen was revealed by peaks of hydrogen permeation rate (HPR) which occurred at low cathodic and low anodic polarisations in NaOH solutions, but not in 0.05 M H₂SO₄. In cathodic scans the HPR peak occurred at potentials of an oxide reduction, probably of Fe₃O₄ to Fe(II), whereas in anodic scans this peak appeared at potentials of the oxidation of iron to Fe(II) and to Fe₃O₄, and it was distinct especially at high scan rates. X-ray photoelectron spectroscopy (XPS) surface analysis indicated that the fraction of Fe₃O₄ in the surface film during cathodic scans was at the potential of HPR peak significantly larger than that at a nobler potential. It is suggested that the occurrence of HPR peaks can be due to a partial removal of surface layers and to the formation of species promoting the hydrogen entry, supposedly Fe₃O₄ without or with Fe(II). Possibly, these species might be effective by hindering the recombination of H_ads or by blocking adsorption sites.     

Cadmium cathodic deposition on polycrystalline p-selenium: Dark and photoelectrochemical processes

Cathodic reduction of Cd²⁺ on p-Se proceeds at low overpotential in the dark and results in bulk Cd, while the underpotential deposition is kinetically inhibited. Cadmium adlayer is photoelectrochemically deposited on illuminated electrode 0.7 V above E(Cd²⁺/Cd). The adlayer cathodic deposition under illumination proceeds with simultaneous formation of CdSe nanoparticles. Potentiodynamic electrochemical impedance spectroscopy has discriminated the two products of the photoelectrochemical reaction both by their potentials of anodic oxidation and by characteristic dependences of impedance on potential. Anodic oxidation of CdSe nanoparticles gives a sharp peak of real impedance in low frequencies close to the corresponding anodic current peak in cyclic voltammogram. The impedance peak appears below a threshold frequency f_t. The latter separates two modes of diffusion in anodic dissolution of CdSe nanoparticles. The diffusion proceeds independently at different particles above f_t and turns to cooperative mode below the threshold frequency. Due to this effect, information on spatial distribution of growing nuclei on electrode surface in early stages of electrodeposition can be obtained from potentiodynamic impedance spectra.     

Effect of anodic prepolarization on hydrogen entry into iron at cathodic potentials in 0.1 M NaOH without and with EDTA or sodium molybdate

Efficiency of cathodes for water electrolysis decreases after shut-downs due to corrosion at open-circuit potential. In the present work the effect of prepolarization at various potentials on hydrogen entry into iron during cathodic potential sweeps was studied by the measurement of the hydrogen permeation rate (HPR) through a 35-μm thick iron membrane in 0.1 M NaOH without and with EDTA or Na₂MoO₄ at 25 °C. Two types of the enhanced hydrogen entry at low cathodic polarizations were distinguished: one after prepolarization at low cathodic or low anodic potentials, and another after prepolarization at high anodic potentials. It is suggested that both types can be explained by acidification at the metal surface, the former due to anodic oxidation of iron, and the latter due to cathodic reduction of oxide layer (mainly of Fe₃O₄). XPS analysis revealed the presence of hydrated Fe-O species of unidentified valence. EDTA and Na₂MoO₄ increased the efficiency of hydrogen entry (j_H/j_c ratio), and molybdates also strongly increased cathodic currents of HER. Some of the effects of these additives can be explained in terms of their effect on surface layers.     

Cyclic voltammetric studies of electroless cobalt in NaOH

The electrochemical behaviour of electroless cobalt in 1N NaOH was studied at 35, 60 and 85°C by cyclic voltammetry. Three anodic peaks corresponding to the formation of Co(OH)₂/CoO, Co₃O₄, Co OOH were observed. The observed cathodic peaks were due to the reduction of the higher cobalt oxides to Co(OH)₂ and also due to the reduction of Co(OH)₂ to cobalt. The variation of peak potential with sweep rate was found to be appreciable only in the case of anodic peak (I). The total charge Q_T, for the passivation to occur was obtained by estimating the area under the first anodic peak at each sweep rate. It was found that the Q_T value decreases with increase of sweep rate. The increase of peak current with increase of sweep rate has been explained as due to the insufficient time available for the film growth at the reversible potential, and that the dissolution of the metal continues until a potential at which the rate of film growth exceeds that of active dissolution is reached. It has been suggested that the passivating film is CoO while the prepassive film is Co(OH)₂. The shift of the anodic peak (I) potentials to more anodic values compared to that for pure cobalt is probably due to the presence of phosphorus in the cobalt deposit.     

Change of mechanism of the hydrogen electrode reaction with overpotential—III. Potential dependence of the deuterium separation factor on Pt,Au and Ni cathodes

A relationship between the overall electrolytic deuterium separation factor of the hydrogen electrode reaction and the separation factors of the steps that constitute the reaction is formulated for the discharge-combination route, without assuming an overwhelming unique rate-determining step. This is utilized for the quantitative interpretation of sharp variation of the cathodic deuterium separation factor with overpotential observed on Pt and Au in aqueous H₂SO₄ and an insignificant variation on Ni in alkaline solutions. The variation on Pt and Au is interpreted in terms of change with cathodic overpotential of the mechanism from the rapid discharge—slow combination to the slow-discharge. The variation on Ni is expected to be significant only in the anodic region.     

Electrochemical investigation of the dimeric oxo-bridged ruthenium complex in aqueous solution and its incorporation within a cation-exchange polymeric film on the electrode surface for electrocatalytic activity of hydrogen peroxide oxidation

Electrochemical behavior of oxo-bridged dinuclear ruthenium(III) complex ([(bpy)2(H₂O)Ru^III-O-Ru^III(H₂O)(bpy)2]⁴⁺) has been studied in aqueous solution (KCl 0.5 mol L⁻¹) by both cyclic and rotating disk electrode (RDE) voltammetry in order to identify and elucidate the reaction mechanism. Modified electrode containing the oxo-bridged ruthenium complex incorporated into a cation-exchange polymeric film deposited onto platinum electrode surface was studied. Cyclic voltammetry at the modified electrode in KCl solution showed a single-electron reduction/oxidation of the couple Ru^III-O-Ru^III/Ru^III-O-Ru^IV. The modified electrode exhibited electrocatalytic property toward hydrogen peroxide oxidation in KCl solution with a decrease of the overpotential of 340 mV compared with the platinum electrode. The Tafel plot analyses have been used to elucidate the kinetics and mechanism of the hydrogen peroxide oxidation. The first at low overpotential region there is no significant change in the Tafel slope (∼0.130 V dec⁻¹) with varying peroxide concentration. The second region at higher overpotential the slope values (0.91–0.47 V dec⁻¹) were depended on the peroxide concentration. The apparent reaction order for H₂O₂ varies from 0.16 to 0.50 in function of the applied potential. The apparent reaction order (at constant potential) with respect to H⁺ concentration of 10⁻⁵ to 10⁻¹ mol L⁻¹ was 0.25. A plot of the anodic current vs. the H₂O₂ concentration for chronoamperometry (potential fixed = +0.61 V) at the modified electrode was linear in the 1.0 × 10⁻⁵ to 2.5 × 10⁻⁴ mol L⁻¹ concentration range.     

Black nickel electrodeposition from a modified Watts bath

Black nickel coatings were electrodeposited on to steel substrates from a Watts bath containing potassium nitrate. The best operating conditions necessary to produce smooth and highly adherent black nickel were found to be NiSO₄ · 6H₂O 0.63 M, NiCl₂ · 6H₂O 0.09 M, H₃BO₃ 0.3 M and KNO₃ 0.2 M at pH of 4.6, i=0.5 A dm⁻², T=25 °C and t=10 min. The modified Watts bath has a throwing power (TP) of 61%, which is higher than that reported, not only for nickel, but also for many other metals electrodeposited from different baths. The potentiostatic current–time transients indicate instantaneous nucleation. X-ray diffraction (XRD) analysis shows that the black nickel deposit is pure metallic nickel with Ni(111) preferred orientation.     

Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta₂O₅, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a “pancake” type and then a “coral reef” type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of “pancake” structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich “coral reef” structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.     

5,4′-dimethyl-2-methoxydiphenylether: a new product from the anodic oxidation of 4-methylanisole

Anodic oxidation of p-methylanisole (1a) in a solution of aqueous 0.05 m H₂SO₄/acetone cosolvent (1 : 1, v/v) on platinum gives 5,4-dimethyl-2-methoxydiphenylether (3), a compound hitherto unexplored, in 50% yield at a temperature below 15°C. Polymer products are formed in significant amounts (up to 60%) besides the diphenylether derivative (3) when the electrolytic oxidation proceeds in a solution containing AcOH as cosolvent at room temperature. The reaction mechanism includes both side chain anodic oxidation and ring 6e oxidation leading to a reactive intermediate, probably 3-methyl-5-methoxy-1,2-benzoquinone (5), whose polymeric products were also isolated.     

The corrosion of iron in sulphate solutions at pH = 0–2

The effects of pH and stirring rate on the kinetics and mechanism of pure iron corrosion in deaerated, argon saturated solutions at 25°C have been studied. The entire concentration of sulphate ions in the tested solutions was 2.0 M. The increase of pH in the tested range causes the change of the anodic process mechanism, which is manifested in the decrease of the Tafel anodic straight line slope from 0.06 V (pH ～ 0) through 0.04 V (pH ～ 1) to 0.03 V (pH ～ 2) and the increase of the order of reaction with respect to OH^? ion, which is 0, 1 and 2 respectively. At adequately low potentials there occurs a change (to a small degree dependent on pH) of the anodic process mechanism which corresponds to the anodic straight line slope being 0.12 V. For the hydrogen evolution reaction the cathodic Tafel slope has been found to be ?0.12 V and the order of reaction with respect to OH^? ion to be ?1. Stirring of the solution has no practical effect on either the corrosion rate or the run of the polarization curves above pH = 1. For lower pH values, at the decrease of the stirring rate there occurs the increase of the corrosion rate and the increase of the anodic and cathodic process rate which is clearly visible below the corrosion potential. The cause of this phenomenon may be the catalyzing of both electrode processes by small quantities (～ 10^?7 Ml^?1) of the H₂SO₄ reduction products. The effect of pH on the corrosion potential and current has also been found.     

Hydrogen evolution at nickel sulphide cathodes in alkaline medium

Hydrogen overpotentials at nickel sulphide cathodes have been measured in 1 N NaOH. The nickel sulphide catalysts have been examined for their structure and composition using scanning electron microscopy (SEM) and microprobe analyses. Nickel sulphide electrodes exhibit a transient behaviour during cathodic polarization. A significant increase in performance could be noticed accompanied with a clear change in the mechanism of the evolution reaction. Voltammetric measurements show a first anodic peak around ?0.95 C and a second broad peak around ?0.65 V. On the basis of galvanostatic potential curves, both anodic peaks have been attributed to oxidation reactions involving hydrogen; a strongly bonded hydrogen and a dissolved or weakly absorbed hydrogen. It is the formation of the strongly absorbed hydrogen structure which is believed to be the main cause of the observed transient behaviour of the nickel sulphide electrodes.     

Triangular potential sweep voltammetric study of porous iron electrodes in alkali solutions

The cyclic voltammograms of pure iron and sintered iron electrodes in 6.0m KOH solutions revealed a plateau and two anodic peaks in the forward direction and two cathodic peaks in the backward direction when polarized from –1.3 to –0.3 V vs Hg/HgO. In the forward scan the formation of Fe(OH)₂ and FeOOH occurs and these are subsequently reduced to Fe(OH)₂ and iron in the backward scan. The peak potential separation of the Fe/Fe(II) and Fe(II)/Fe(III) couples at zero sweep rate and the ratio of cathodic to anodic charges at zero sweep rates for the above two redox couples have been used to evaluate the reversibility of porous iron electrodes. Additions of LiOH, Na₂S, FeS, sulphur, Sb₂O₃ and As₂O₃ on the reversibility of these redox couples have been discussed. A suitable electrode fabrication condition has been suggested.     

Silver electrocrystallization from a nonpolluting aqueous leaching solution containing ammonia and chloride

Silver electrocrystallization from aqueous solutions at pH11, pC10 and pNH₃ – 0.2, where Ag(NH₃)₂ ^– is the dominant Ag(i) species, has been studied. In spite of the complexities of this medium, the experimental results can be satisfactorily described in terms of multiple nucleation and diffusion-controlled growth of hemispherical nuclei. Nucleation rates, A, and number densities of active sites on the electrode surface, N0, were determined from potentiostatic current transients as a function of overpotential. Saturation number densities of silver nuclei on the electrode surface obtained from the A and N0 values were found to be in excellent agreement with those obtained from the direct, microscopic observation of the electrode surface. Spatial distributions of nuclei were also analysed for silver electrodeposited at different potentials. It was found that nuclei were uniformly distributed when electrodeposited at low overpotentials, whereas inhibition of nucleation close to already established nuclei occurred at higher overpotentials. From the change of the true nucleation rate with overpotential, it was found that the critical nucleus is formed by a single atom within the –100 to –300 mV over-potential range.     

Overpotential Behavior of Carbon Monoxide Fuel in a Molten Carbonate Fuel Cell

The overpotential of carbon monoxide (CO) fuel was analyzed with a 100‐cm² class molten carbonate fuel cell. The overpotential at the anode was measured using the steady state polarization, inert gas step addition, and reactant gas addition methods. Then, the overpotential was compared between normal hydrogen fuel (H₂:CO₂:H₂O = 0.69:0.17:0.14 atm, inlet composition) and CO fuels (CO:CO₂:H₂O = 0.5:0.5:0 atm and 0.43:0.43:0.14 atm, inlet compositions). The CO fuel without H₂O showed a much greater overpotential at 150 mA cm^–2 than the CO fuel with H₂O. This implies that the water‐gas‐shift reaction prevails at the anode and humidification of CO fuel is an efficient way to reduce anodic overpotential. The anodic overpotential with CO:CO₂:H₂O = 0.43:0.43:0.14 atm was about 73% of that of the H₂ fuel at 150 mA cm^–2. The anode showed gas‐phase mass‐transfer limitations with CO fuels.     

Electrochemical behavior of pyrite in sulfuric acid solutions containing silver ions

A systematic electrochemical study of pyrite in H₂SO₄ solutions containing dissolved silver was undertaken to gain more information about the transfer of silver ions to pyrite and their role in enhancing the direct oxidation of pyrite. The results of cyclic voltammetry experiments provide additional evidence of the formation of metallic silver on the FeS₂ surface under open-circuit conditions. A pyrite electrode held at the open-circuit potential for 2 h in the presence of 10^–3 m Ag⁺ exhibits a large and sharp anodic peak at about 0.7V. The current associated with this peak is the result of the dissolution of metallic silver deposited during the initial conditioning period. There is no evidence of silver deposition without preconditioning until the potential drops below about 0.6V for Ag⁺ concentrations ranging from 10^–4 to 10^–2 m. However, subsequent silver deposition appears to be very sensitive to the dissolved silver concentration in this range. There is also evidence that the state of the pyrite surface has a pronounced influence on its interaction with silver ions. Agitation has also been found to have a significant effect on the electrochemistry of the Ag–FeS₂ system.     

The influence of oxygen additions to hydrogen in their electrode reactions at Pt/Nafion interface

The influence of oxygen gas added to hydrogen in their electrode reactions at the Pt/Nafion interface was investigated using ac impedance method. The electrochemical cell was arranged in either electrolytic (hydrogen enrichment) or galvanic (fuel cell) mode. The impedance spectra of the electrode reaction of a H₂/O₂ gas mixture were taken in each mode as a function of the gas composition, electrode surface roughness and the cell potential. The spectrum taken for the anodic reaction of electrolytic arrangement confirmed the anodic oxygen reduction reaction (AOR, the local consumption of hydrogen by the added oxygen) by showing an independent arc distinguishable from that for hydrogen oxidation. But the independent arc was not revealed in the spectrum taken on a smooth (low surface area) electrode or on a Pt/C anode of the galvanic cell. At any cell current density, the electrolytic mode showed its anodic overpotential much higher (nearly three times higher at the current density of 100 mA cm⁻²) than the potential registered in galvanic mode implying that the oxygen gas in the mixture engages more active and independent AOR at the anode of the electrolytic cell.     

An electrochemical investigation of graphite surfaces

Current-potential curves were determined during the application of a triangular potential sweep to a graphite electrode immersed in acid electrolyte. A maximum of 3 anodic and 3 cathodic peaks were observed in these curves depending on the experimental conditions. The anodic peaks in the potential ranges (vs standard hydrogen electrode potential) of 0·59V–0·91V, of 0·97V–1·15V and of 1·35V–1·45V were associated with cathodic peaks in the potential ranges of 0·47V–0.79V and of 0·83V–1·01V and at a potential of 0·25V, respectively. It is proposed that the anodic peaks were caused by the oxidation of hydroquinone-like groups in different environments on the graphite electrode surface to form quinone-like groups while the cathodic peaks were caused by the reduction of these quinone-like groups.     

Effect of some polymeric materials on the corrosion behaviour of tin in succinic acid solution

The anodic behaviour and corrosion of tin in various concentrations (0.05–0.7M) of succinic acid were studied using cyclic voltammetry. The potentiodynamic anodic polarization curves exhibit active/passive transition. The active dissolution of tin involves one anodic peak. The cathodic curve exhibits one cathodic peak corresponding to the reduction of the passive layer. The ratio of the anodic charge/cathodic charge is more than unity indicating that the passive layer is very thin and the dissolution products are mainly soluble species. Additions of some polyethylene glycols to the succinic acid solution decrease the anodic peak current and shift the peak potential in the negative direction. These changes depend on the concentration and molecular weight of the polyethylene glycol added. The effect of the inhibitors decreases in the following order: (PEG)₆₀₀₀ > (PEG)₄₀₀₀ > (PEG)₁₂₀₀. The inhibition efficiency decreases with increase in temperature, suggesting physical adsorption.     

Electrocristallisation et dissolution anodique d'un monocristal d'argent en milieu nitrate d'argent acidifie: aspects cinetiques et morphologiques

Silver electrodeposition and dissolution in acidified 1M AgNO₃ is investigated using several techniques: current-potential plots, scanning electron microscopy and impedance measurements down to low frequencies. For various NO^?₃ concentrations and single crystals of orientation (100), (110) or (111), it is observed that a low anodic or cathodic overpotential gives rise to a strong activation of the electrode surface which is blocked at the equilibrium. The deposit morphology depends on substrate orientation, current density and NO^?₃ concentration. Impedance diagrams are in agreement with a very fast electron transfer and reveal at low frequencies two time-constants independent on the value of anodic or cathodic current density, on NO^?₃ concentration and on substrate orientation. The results are discussed in terms of nucleation and propagation of edges.