Showing papers in "Journal of Applied Electrochemistry in 1990"

Particle size and structural effects in platinum electrocatalysis

Abstract: Of the several factors which influence electrocatalytic activity, particle size and structural effects are of crucial importance, but their effects and mechanism of interaction,vis-a-vis overall performance, have been, at best, vaguely understood. The situation is further aggravated by the use of a wide range of experimental conditions resulting in non-comparable data. This paper attempts systematically to present the developments to date in the understanding of these structural interactions and to point out areas for future investigation. The entire content of this review has been examined from the context of the highly dispersed Pt electrocatalyst, primarily because it has been examined in the greatest detail. In the first two sections a general idea on the correlations between surface microstructure and geometric model is presented. Subsequently, indicators of a direct correlation between particle size and catalyst support synergism are considered. The structural and particle size effect on electrocatalysis is examined from the point of view of anodic hydrogen oxidation and cathodic oxygen reduction reactions. The hydrogen and oxygen chemisorption effects, presented with the discussion on the anodic and cathodic electrocatalytic reactions, provide important clues toward resolving some of the controversial findings, especially on the dependence of particle size on the anodic hydrogen oxidation reaction. Finally, the effect of alloy formation on the cathodic oxygen reduction reaction is discussed, providing insights into the structural aspect.

Transition metal-based hydrogen electrodes in alkaline solution — electrocatalysis on nickel based binary alloy coatings

Abstract: Nickel-molybdenum, nickel-zinc, nickel-cobalt, nickel-tungsten, nickel-iron and nickel chromium binary alloy codeposits, obtained through electrodeposition methods on mild steel strips, have been characterized with the objective of qualitatively comparing and assessing their electrocatalytic activities as hydrogen electrodes in alkaline solution. It has been concluded that their electrocatalytic effects for the hydrogen evolution reaction rank in the following order: Ni-Mo > Ni-Zn (after leaching Zn in KOH) > Ni-Co > Ni-W > Ni-Fe > Ni-Cr > Ni plated steel. Further investigations on the alloy electrocatalysts have revealed that the cathodic overpotential contribution to the electrolysis voltage can be brought down by 0.3 V when compared with conventional cathodes. The best and most stable hydrogen evolving cathode, based on nickel-molybdenum alloy, exhibited an overpotential of about 0.18 V for over 1500 h of continuous electrolysis in 6m KOH at 300 mA cm−2 and 353 K. The salient features of the coatings, such as physical characteristics, chemical composition, crystal structure of the alloy phases and the varying effects of the catalytic activation method were analysed with a view to correlating the micro-structural characteristics of the coatings with the hydrogen adsorption process. The stability under open-circuit conditions, the tolerance to electrochemical corrosion and the long term stability of nickel-molybdenum alloy cathodes were very encouraging. An attempt to identify the pathway for the hydrogen evolution reaction on these alloy coatings was made, in view of the very low apparent activation energy values obtained experimentally.

Vanadium redox cell electrolyte optimization studies

Abstract: The stability of the positive electrolyte of the vanadium redox cell has been studied at various temperatures and at different solution compositions and solution state-of-charge (SOC). It has been found that at elevated temperatures for extended periods, V(V) can slowly precipitate from solution, the extent and rate of which being dependent on temperature, vanadium and sulphuric acid concentration as well as the SOC of the electrolyte. A H2SO4 concentration of 3–4m has been found to be more suitable than 2m, not only from the point of view of increased stability, but also because of the higher electrolyte conductivity which leads to increased voltage efficiencies during battery cycling.

The application of electrochemical impedance techniques to aluminium corrosion in acidic chloride solution

Abstract: Electrochemical impedance techniques are applied to the study of aluminium corrosion in hydrochloric acid solution in the passive region. Impedance spectra are obtained in stationary solution and in solution moving due to electrode rotation. The form of the spectra shows the importance of multistep dissolution, ionic migration through the oxide layer, relaxation effects and the influence of chloride ion.

Thin films of Co3O4 and NiCo2O4 obtained by the method of chemical spray pyrolysis for electrocatalysis III. The electrocatalysis of oxygen evolution

Abstract: The electrocatalytic properties of thin Co3O4 and NiCo2O4 films prepared on CdO-nonconductive glass by the method of chemical spray pyrolysis were investigated for oxygen evolution in varying KOH concentrations. The Tafel slopes were close to (2.3RT/F) V per decade for both oxides. The reaction order with respect to [OH−] was found to be approximately 1.3. It was observed that Co3O4 is more active than NiCo2O4 towards oxygen evolution. A mechanism for oxygen evolution involving the electrochemical adsorption of OH− as a fast step and the electrochemical desorption of OH forming an intermediate H2O2 as the rate-determining step has been suggested.

AC-Impedance measurements on aluminium in chloride containing solutions and below the pitting potential

Abstract: Impedance measurements were performed on aluminium in 0.5M NaCl in the frequency range 5×10−4-104 Hz and before the onset of pitting corrosion. The behaviour of the system was characterized by a high frequency capacitive loop related to the thickness of the oxide film, an inductive loop at medium frequency, which was interpreted on the basis of the dielectric relaxation model proposed by Dignam, and a second capacitive loop obtained at low frequencies which was ascribed to the film dissolution through the formation of a soluble chloride containing aluminium salt.

Electrodeposition of polypyrrole layers on aluminium from aqueous electrolytes

Abstract: A screening of twenty aqueous electrolytes for a film-forming electropolymerization of pyrrole with Al was performed. Electrodeposition of well adhering homogeneous polypyrrole layers on aluminium is possible from aqueous electrolytes containing 0.1–0.8M oxalic acid. Pretreatment of the metal by polishing (PD) or by anodic (galvanostatic) activation (GA) is an essential step. In all cases, the Al2O3 surface layer with pores, usually filled with electrolyte, is transformed to a Al2O3 layer with polypyrrole filled pores. The PPy-layers on Al allow an easy cathodic coating with metals like copper. The hydrogenoxalate doped layers exhibit the ususal redox capacity,y=0.3. The sandwich structure Al/Al2O3/PPy represents a condenser with an unusual composite dielectric with the electronically conducting ‘plates’ Al and PPy. According to our impedance measurements, the PPyin the pores is a high resistivity material due to overoxidation in the course of electropolymerization at high local current densities. The composite, Al2O3/overoxidized PPy, exhibits an unusually high permittivity, in the order of 103.

Nickel plating by pulse electrolysis : textural and microstructural modifications due to adsorption/desorption phenomena

Abstract: Nickel growth from an organic-free Watts bath working under d.c. plating conditions is governed by several interfacial inhibitors such as H2, Hads or Ni(OH)2. These inhibitors determine most of the structural or macroscopic properties of the nickel plates. Pulse electrolysis (p.e.) is thus a powerful means of perturbing the adsorption-desorption phenomena occurring at the nickel/electrolyte interface and hence offers an opportunity of preparing deposits exhibiting better properties. Through an analysis of the textural and microstructural changes produced by p.e., we show that molecular inhibitors desorb during the relaxation time, while conversely other inhibitors like Hads or anions are more strongly adsorbed and inhibit the nickel cathodic process.

Development of anodes for aluminium/air batteries -- solution phase inhibition of corrosion

Abstract: The discharge characteristics of aluminium in inhibited and uninhibited 4 M KOH at 50°C have been explored. The performance of pure aluminium as a fuel is compared with that for two leading alloy fuels that had been evaluated in our previous work, Alloy BDW (Al−1Mg−0.1In−0.2Mn) and Alloy 21 (Al−0.2Ga−0.1In−0.1Tl). The inhibitors employed in this study, SnO 3 2− , In(OH)3, BiO 3 3− , Ga(OH) 4 − , MnO 4 2− , and binary combinations thereof, are either present in Alloys BDW and 21 or have been investigated previously (SnO 3 2− ). We found that potassium manganate (K2MnO4) and Na2SnO3+In(OH)3 are effective inhibitor systems, particularly at high discharge rates (400 mA cm−2), but at low discharge rates only manganate offers a significant advantage in coulombic efficiency over the uninhibited solution. Alloy BDW exhibits a very low open circuit (standby) corrosion rate, but its coulombic efficiency under discharge, as determined by delineating the partial anodic and cathodic reactions, was found to be no better than that of aluminium in the same uninhibited solution. Alloy 21 was found to exhibit a comparable performance to Alloy BDW under open circuit conditions and a much higher coulombic efficiency at low discharge rates (100 mA cm−2), but the performance of this alloy under high discharge rate conditions was not determined. Alloy 21 has the significant disadvantage that it contains thallium.

Electrochemical impedance spectroscopy on porous electrodes

Abstract: Electrochemical d.c. and a.c. measurements have been carried out on porous Raney nickel in H2-saturated 0.1 M, 1 M, and 6 M NaOH solutions atT=303 K and 333 K using rotating-disc and static-plaar electrodes. For comparison, measurements were also performed on graphite-cloth and graphite-felt electrodes. From polarization curves and current transients obtained in the potential range of the hydrogen evolution and hydrogen oxidation reactions the dependence of the electrocatalytic activity of Raney nickel on the prepolarization conditions was studied. Impedance spectra in the frequency range 1 mHzf 10 kHz were used to determine the characteristic pore parameters and to identify the kinetic behaviour of the porous electrodes by application of transfer function analysis using non-linear fit routines.

The surface oxidation of pyrite in alkaline solution

Abstract: The collector-less flotation of pyrite has been studied by conventional techniques and is correlated to the electrochemical behaviour of pyrite in alkaline solution (1m NaClO4, pH 11). It was concluded that the initial oxidation of pyrite produces a hydrophobic sulphur rich surface together with hydrophilic iron hydroxide species. Also upon grinding, the surface is covered by hydrophilic species and therefore no significant flotation was obtained in the absence of a collector. However, collectorless flotation was readily obtained in an iron complexing solution like EDTA. This indicates that the remaining sulphur-rich layer is responsible for the floatability of pyrite under these conditions.

Electrochemical oxidation of carbon fibres in aqueous solutions and analysis of the surface oxides

Abstract: Carbon fibres with notably different surface oxides can be prepared by varying the electrochemical oxidation conditions. Through correlation of voltammetric analysis and mass spectroscopy, interpretation of the reduction peaks of the surface oxides on the basis of their potential and width is possible. Narrow voltammetric reduction peaks at strongly negative potential are indicative of the predominance of −COOH type groups, while wide peaks at more positive potential are indicative of the presence of an excess of type groups. A quantitative determination of the surface acidic groups (−COOH and ) is achieved by combination of Ag+ ion exchange and esterification of groups with 3,5-dinitrobenzoyl chloride. All results are confirmed by the independent method of −COOH group determination via their electrocatalytic behaviour in the reduction of azobenzene in methanol. The amounts of the −COOH and groups formed depend notably on the conditions of the electrochemical oxidation of the carbon fibres. The ratio between −COOH and groups coincides in all cases with that obtained by the mass spectrosopic data.

Investigation of anodic aluminium oxide layers by electrochemical impedance spectroscopy

Abstract: The effects of ac-electrochemical graining and anodizing of an aluminium substrate on the layer properties of both barrier and porous alumina layers are examined using electrochemical impedance spectroscopy (EIS) In order to show the capabilities of the technique for a quantitative determination, results based on impedance data are compared with complementary information from surface analytical techniques Though the results for the determination of barrier layer thickness and dielectric constant look promising, calculations are troubled by non-trivial dispersion phenomena This problem is treated using a fractal description of surface roughness of the substrate and of the layer thicknesses Information on pore structure of porous oxide films could not be obtained from the approach considered in this study

TiO2 catalysed reduction of Cr(VI) in aqueous solutions under ultraviolet illumination

Abstract: The photoassisted reduction of Cr(VI) to Cr(III) in aqueous suspensions of TiO2 under ultraviolet (UV) illumination has been studied by determining the amount of Cr(VI) photoreduced at different irradiation times, the mass of catalyst in suspension, the Cr(VI) initial concentration and the pH. Samples of wastes from metal-surface treatment industries have been used to test this photocatalytic procedure for Cr(VI) elimination in a real ease. It has been observed that the presence in these samples of certain species such as Fe(III) and Cr(III) at low concentrations can increase significantly the yield of Cr(VI) photoreduction. It is assumed that these cations act by maintaining the pH during the photoreduction process, preventing the alkalization by hydrolysis of the solution.

Mediated electrosynthesis with cerium (IV) in methanesulphonic acid

Abstract: Methanesulphonic acid has been found to solubilize the Ce(III)/Ce(IV) couple. This makes cerium mediated electrosynthesis practical for commercial production of several carbonyl compounds. Results are presented for the electrochemical generation of Ce(IV) in methanesulphonic acid and for naphthalene oxidation to 1,4-naphthoquinone using Ce(IV).

Electrosynthesis at oxide coated electrodes Part 1 the kinetics of ethanol oxidation at spinel electrodes in aqueous base

Abstract: The oxidation of ethanol at Ni/Co spinel coated electrodes in aqueous base has been investigated. It is shown that these electrodes are very stable and have a good catalytic activity for the conversion of ethanol to acetic acid (for example 24 mA cm−2 for the oxidation of 0.1 mol dm−3 ethanol), far superior to the nickel anodes previously used for the reaction in similar conditions. TheI-E curve for the oxidation of ethanol at a spinel shows a well formed wave prior to oxygen evolution at a potential where the spinel surface itself undergoes oxidation. The limiting current is partially mass transport controlled. The influence of the electrolysis conditions, the composition of the spinel coated electrodes and of parameters in their preparation, on the rate of ethanol oxidation is described.

Electrocatalytic oxidation of hydrogen, formic acid and methanol on platinum modified copolymer (pyrrole-dithiophene) electrodes

Abstract: Etude des proprietes electrocatalytiques d'une electrode de copolymere conducteur par inclusion d'une faible quantite de platine disperse. Application a l'electrooxydation de l'hydrogene, de l'acide formique et du methanol

The effects of antimony and glue on zinc electrowinning from Kidd Creek electrolyte

Abstract: The individual and combined effects of antimony and glue on zinc deposition current efficiency and polarization and on the morphology and orientation of 6h zinc deposits electrowon at 500 A m−2 and 38° C from Kidd Creek zinc electrolyte were determined. Glue increased zinc deposition polarization, reduced the deposit grain size, changed the preferred deposit orientation from basal to intermediate, but decreased the current efficiency. At concentrations above 0.02 mgl−1, antimony had a strong depolarizing effect on zinc deposition resulting in a basal deposit orientation and very low current efficiency. Certain combinations of antimony and glue, however, optimized zinc deposition current efficiency and consistently gave an intermediate 〈114〉 〈112〉 〈103〉 〈102〉 〈101〉 preferred deposit orientation. A correlation was observed between the current efficiency (CE) and the nucleation overpotential (NOP) for zinc deposition such that the CE was a maximum when the NOP of the initial cell electrolyte was 120–130 mV and when the NOP of the final cell electrolyte was 100–110 mV.

Electrodeposition of zinc-nickel alloy coatings: influence of a phenolic derivative

Abstract: The electroplating of Zn−Ni alloy films from a chloride bath has been studied under different plating conditions, both in the absence and presence of a phenolic derivative. Under the conditions examined, the electrodeposition of the alloys belonged to the anomalous type. The morphology and composition of the deposits varied with current density, temperature, bath composition and additive concentration. The results show that the additive modifies the structure and surface topography of the deposits to a large extent and produces smoother deposits. The corrosion resistance of the alloys has been analyzed by means of salt-spray tests.

Mechanistic and kinetic aspects of silver dissolution in cyanide solutions

Abstract: The factors influencing the dissolution kinetics of pure silver in cyanide solution have been analysed in terms of an electrochemical mechanism. A kinetic model is presented which incorporates coupled diffusion and charge transfer for the anodic branch, and combined diffusion, adsorption and charge transfer for the cathodic branch. The anodic oxidation of silver has been investigated using a silver rotating-disc electrode for concentrations between 10−3 and 10−1m NaCN. Oxygen reduction on silver has been studied at oxygen partial pressures between 0.104 and 1.00 atm. Mechanistic aspects of the oxygen discharge reaction are considered in explaining the kinetic differences between gold and silver dissolution in cyanide solution. It is shown that under conditions typical of conventional cyanidation gold dissolves measurably faster than silver.

The effect of pulsed reverse current on the polarization behaviour of acid copper plating solutions containing organic additives

Abstract: The polarization behaviour of acid copper solutions containing polyethers, sulphopropyl sulphides and chloride ions was studied using both direct and pulsed reverse current. The effect of these additives on the rest potentials of copper foils immersed in the electrolyte was also studied. Polyethers were found to have an inhibiting effect on the deposition of copper whereas the sulphopropyl sulphides produced a stimulating (i.e. depolarizing) effect. Chloride ion concentration was found to have an influence on the adsorption characteristics of the polyether. The use of pulsed reverse current in solutions containing both polyether and sulphopropyl sulphide was found to inhibit the adsorption/diffusion of the sulphopropyl sulphide at the cathode surface. Thus at higher current densities, above 2 A dm−2, the inihibiting effect of the polyether produced a shift in the cathodic polarization potential to more negative values as compared with an equivalent current density using direct current. At lower current densities, below 2 A dm−2, the depolarization effect of the sulphopropyl sulphide was still effective. This effect of pulsed current on additive containing solutions can improve dramatically the metal distribution in low current density areas on plated items. This was illustrated by plating Hull cell panels using both pulsed and direct current.

Limiting current in a high-temperature hydrogen pump with a SrCeO3-based proton conductor

Abstract: Using a SrCeO3-based proton conducting ceramic as a solid electrolyte, a high temperature hydrogen pump was constructed and the performance was examined at 650–800° C. The limiting diffusion current was measured as a function of hydrogen partial pressure at the anode by a potential sweep method with an automatic iR-compensation system. The limiting current density was a good measure of stable operating conditions for the hydrogen pump.

Electrolytic oxygen evolution on Ni-Co-P alloys

Abstract: Electrolytically obtained Ni−Co−P alloys exhibit amorphous structure. On heating these alloys to 773 K crystalline nickel and cobalt phosphide phases, and also nickel crystallites, are formed. After cathode-anode polarization of the amorphous and crystalline Ni−Co−P alloys in 5m KOH for 18 h, oxygen-cobalt compounds together with crystalline or amorphous nickel phosphides appear on the electrode surface. The evolution of oxygen from 5m KOH was studied on Ni−Co−P alloys prepared in this way. The Tafel parameters were determined and it was ascertained that the values of directional coefficients of the Tafel lines are comparable with those obtained for spinel NiCo2O4 oxides, while calculated values of the exchange currents for the oxygen evolution reaction are dependent on the chemical composition of the alloy. The rate of electrolytic oxygen evolution is virtually identical for amorphous and crystalline Ni−Co−P alloys of the same chemical composition. The highest rate of oxygen evolution was found for the alloy containing 15–20% Ni, 66–73% Co and 12–14% P.

Properties of Ni-Sx electrodes for hydrogen evolution from alkaline medium

Abstract: Active layers for hydrogen evolution were prepared electrolytically from nickel-plating baths containing various amounts of thiourea. The sulphur content of the deposited layers, which is dependent on the thiourea content of the electroplating bath, ranged between 10 and 20 wt%. X-ray photoelectron spectroscopic (XPS) analysis revealed that sulphur was present in the layers not only in the form of sulphide, but also in the forms of thiourea and sulphate entrained from the nickel-plating bath. X-ray diffraction analysis showed that the coatings were poorly crystalline. Nickel sulphide, which produces an X-ray diffraction pattern, was present only in coatings deposited from baths with higher amounts of thiourea. The surface layers of such coatings contained a maximum of 10 at % nickel in the form of Ni3S2. When the Ni−Sx electrodes were under load in 1m KOH over a long period, the hydrogen overpotential was found to vary with time. On the basis of XPS measurements, the initial decrease in hydrogen overpotential was ascribed to the washing-out of adsorbed residues of thiourea. The subsequent increase in the overpotential was due to the deposition of iron on to the surface of the layers from the electrolyte used.

Zinc deposition and passivated hydrogen evolution in highly acidic sulphate electrolytes : depassivation by nickel impurities

Abstract: In highly acidic sulphate electrolytes used for zinc electrowinning, the steady-state behaviour and the impedance of zinc electrodes have been analyzed under potentiostatic control. It is shown that zinc deposition takes place under conditions where hydrogen evolution remains passivated on the deposit surface. A depassivation of hydrogen evolution leading to the deposit corrosion can be provoked by an electrode excursion near the corrosion potential. The presence of nickel impurities facilitates this depassivation process even though the electrode potential remains cathodic. The depassivation becomes easier with the existence of a scratch on the substrate surface, because of bubbles retained on this defect.

Infrared reflectance absorption spectroscopy (IRRAS). Study of the thermal stability of perfluorinated sulphonic acid ionomers on Pt

Abstract: Infrared Reflectance Absorption Spectroscopy (IRRAS) has been used to study the thermal stability of Nafion (du Pont, equivalent weight=1100) and a similar perfluorinated sulphonic acid ionomer (PFSI) made by the Dow Chemical Company (equivalent weight=560). Purified aqueous PFSI electrolytes solutions were prepared, PFSI films (0.5–5 μm) were cast from these aqueous PFSI solutions onto Pt foil substrates for spectral study after heating at various temperatures. Spectra were recorded at room temperature in a vacuum after a film on Pt had been heated at a particular temperature in the presence of air. The temperature range was 22–300°C. Spectral changes for a PFSI film on Pt were not significant when heated at up to 200°C in an air atmosphere; however, when heated at 300°C there was a significant decrease in intensity for S-O related bands indicating substantial loss of the sulphonic acid groups from these ionomers.

On the mechanism of electroless plating. Part 3. Electroless copper alloys

Abstract: It has been shown that alloying copper with different metals has a pronounced effect on hydrogen evolution during anodic oxidation of formaldehyde and under electroless plating conditions. The codeposition of platinum, palladium, gold or nickel reduces the ratio between the amount of hydrogen evolved and the deposited metal. Diminishing the hydrogen evolution has a favourable effect on the structure of the deposited alloy. The reduced amount of hydrogen, as well as the synergetic effects observed during anodic oxidation of formaldehyde, can be explained by assuming a spillover of the oxidation intermediates.

Overall mass transfer in swirling decaying flow in annular electrochemical cells

Abstract: Mass transfer coefficients between an electrolyte solution and the inner cylinder of an annulus were determined experimentally, using the electrochemical technique, for both laminar and turbulent swirling flows of a liquid induced by means of a tangential inlet in the annular gap. The average mass transfer coefficients were determined as a function of the axial distance from the entrance, for various diameters of the inlet duct and thicknesses of the annular space, for a Reynolds number range of 100 to 5900. Enhancement of mass transfer up to 400% was achieved in comparison to that obtained in fully developed axial flow. Correlations of the experimental data, taking into account the geometrical and hydrodynamic parameters influencing the overall mass transfer, are presented.

Cathodic modification as a means of improving the corrosion resistance of alloys

Abstract: Cathodic modification is defined and its origin is discussed. The criteria necessary for cathodic modification are listed for the base alloy as well as the cathodic alloying component, and the principle of cathodic modification is treated in detail. The mechanism of cathodic modification is also described extensively. Various mechanisms that account for the redistribution of alloying elements on the corroded surface are compared and discussed, and the most likely one is pointed out. It is concluded that cathodic modification is a very powerful electrochemical means of improving the corrosion resistance of alloys, particularly of stainless steels and titanium-based alloys in non-oxidizing acid media.

Electrochemical studies with tin electrodes in citric acid solutions

Abstract: The electrochemical behaviour of tin in 0.5 M citric acid solution (pH=1.8) was studied by means of the potentiodynamic method. TheE-I profiles show an anodic current peak associated with a dissolution of the metal and the formation of a passivating film, and two cathodic current peaks which are related to the reduction of soluble Sn(II) species and reduction of the film. During the potential sweep in the cathodic direction, an anodic current peak can be observed and is interpreted in terms of a reactivation process. The data suggest that the passivation of tin in this medium occurs by a dissolution-precipitation mechanism. Depending on the potential sweep rate, the process is controlled either by mass transport or by the solution resistance in the pores of the film.