Showing papers on "Overpotential published in 1978"

Semiconducting oxides: effects of electronic and surface structure on dissolution kinetics of nickel oxide

Abstract: The effect of decreasing dissolution rate of nickel oxide per unit surface area in acid solution with increasing prior annealing temperature (700–1450°C) has been shown to occur over a wide range of pH. The linear dependence of log (rate) on pH may be explained on a model of non-oxidative dissolution in which the pH variation changes the overpotential at the surface. Electron microscopy shows a different mode of attack at pH 200 fold) for all prior annealing temperatures but the oxide annealed at 1450°C is still the slowest to dissolve in the presence of cobaltic ions. This is believed to be because it has the lowest kink site density, the role of the Co3+ being hole injection into the p-type semiconductor at kink sites. The general conclusion of the work is that the supply of the majority carriers (the holes) may be rate limiting in the dissolution process.

The comparison of galvanostatic and potentiostatic copper powder deposition on platinum and aluminium electrodes

Abstract: Experiments have been carried out on copper powder electrodeposition by constant overpotential and constant current on aluminium, graphite, platinum and copper electrodes. It is shown that for one and the same quantity of electricity, electrode material, electrode surface area, electrolyte, temperature and time of deposition, different particle size distribution curves, current efficiency and specific energy consumption are obtained in galvanostatic and potentiostatic deposition.

The effect of pulsating overpotential on the morphology of electrodeposited silver powder particles

Abstract: The effects of overpotential amplitude, frequency and pulse-to-pause ratio in silver deposition by squarewave pulsating overpotential on the morphology of powdered particles were investigated. These results were compared with those obtained in constant overpotential electrolysis. The possibility of obtaining powder particles, with different properties, depending on conditions of electrolysis was demonstrated.

Electrodeposition of zinc on copper from alkaline zincate solutions

Abstract: Experiments have been carried out on the deposition of zinc by constant and pulsating overpotential from alkaline zincate solutions. In pulsating overpotential deposition the amplitude values of overpotential used were smaller than the critical overpotential for dendritic growth. The transformation of the form of the zinc deposit from spongy to dendritic in constant overpotential electrodeposition is shown. The possibility of obtaining smooth deposits in pulsating overpotential electrodeposition is also shown.

Catalyst modification by metal deposition: the deposition of silver on platinum at underpotential and overpotential

Abstract: It is demonstrated that platinum catalysts can be modified by electrochemical means. The deposition of silver is described at both underpotential and overpotential. Although the deposition reactions can be quantified, the electrode kinetics cannot be investigated because of the simultaneous formation and reduction of platinum oxides.

Aluminium corrosion studies. IV. Pitting corrosion

Abstract: Measurements are reported for the variation of the open-circuit potential, Er, of aluminium in oxygen-saturated sodium salt solutions. The value of Er was independent of SO42- and NO3- concentrations and similar to the value obtained for water (0.04 (s.h.e.)). Er was a function of chloride concentration given by Er = -0.475-0.060log[Cl-] V (s.h.e.) at 25°C. There was a less well defined relationship between Er and NO2-, I- and Br-, and a complex relationship with F-. The potentiodynamic characteristics are reported for aluminium in 1-0.01 mol l-1 Cl- oxygen-saturated solutions. Functional relationships were found for E0, Ep, Es and E0' with chloride activity at 5, 25, 50 and 75°C. Hysteresis effects are reported. The experimental results are interpreted in terms of a thermodynamic equilibrium condition between the surface oxide and soluble aluminium chloride. As the system oscillates across the equilibrium conditions the surface will passivate or pit. A critical bulk solution chloride concentration is necessary to maintain the growth of the pit; the experimental value was 1.6 mol l-1 Cl- and the corresponding open-circuit potential was Ecrit = -0.48 V (s.h.e.). The pitting potential, Ev, was interpreted as an overpotential, ηp, given by η = Ep,- Ep-Ecrit.

Electrochemical Determination of Glue in Copper Refinery Electrolyte

Abstract: A method has been developed for determination of the active glue concentration in copper electrorefinery tankhouse electrolyte. The procedure is carried out in two major steps: i) separation of the active glue fraction from the tankhouse electrolyte containing other organic additives by means of ultrafine molecular filtration and ii) its subsequent quantitative determination by cathodic overpotential measurements. In contrast to earlier approaches, this method is insensitive to varying composition of the tankhouse electrolyte, and a single calibration curve can be used for interpolation of active glue concentrations from the overpotential measurements.

Reduction of steel cathode overpotential

Abstract: The hydrogen overpotential of a steel cathode for use in the electrolysis of aqueous alkali metal solutions in diaphragm electrolytic cells is reduced by depolarization in a highly alkaline electrolyte The reduction results in an appreciable reduction of the overall electrical energy required to effect electrolysis of said solutions

Selection and evaluation of materials for advanced water electrolyzers

Abstract: The mechanism of dissolution of ruthenium anodes and of the time variation of overpotential on such electrodes have been elucidated using combined electrochemical-ellipsometric studies. Attempts are being made to stabilize ruthenium based electrocatalysts for SPE water electrolyzers by investigation of mixed oxides with the requisite characteristics for highly efficient oxygen electrodes. The following materials have been identified as the most attractive for advanced alkaline water electrolyzers operating at 120 to 150/sup 0/C: Anode Electrocatalyst--High Surface Area Nickel Cobalt Oxide; Cathode Electrocatalyst--High Surface Area Nickel Boride; Separator--Teflon Bonded Potassium Titanate or Nafion; and Cell Frame--Teflon or Teflon Based. The tests on barrier materials, electrodes and cell components have so far been carried out on a static basis in a pressure vessel. Teledyne Energy Systems (TES) has designed and fabricated an advanced alkaline water electrolysis test rig (5-cell stack). In a joint BNl--TES--University of Virginia program, the most promising materials will be tested for performance and/or life in this test-rig.

Chemical and electrochemical kinetics of CuII reduction in chloride media

Abstract: The deposition and dissolution of copper from copper sulphate solutions have been studied by galvanostatic measurements. From current overpotential relations it is concluded that the rate limiting step in the electrochemical reduction or oxidation at pH 1 is the reaction Cu2++ e ⇌ Cu+. In the presence of chloride and sulphite ions, the CuCl–2 complex is formed which stabilises the CuI state. The rate limiting step is now the reduction of this complex. Anodically the slow step is the oxidation of CuCl–2.The addition of sulphite ions chemically reduces CuII to CuI, but the rate is much slower than the electrochemical reduction at an overpotential of –100 mV.At pH 1 chemical dissolution of the copper deposited at the cathode occurs.

The passivation phenomenon of platinum in fused lithium chloride + potassium chloride eutectic: Part II. Chlorine overpotential and superpassivation of platinum

Abstract: Platinum is known to have a very high chlorine overpotential, about 0.8 V, in fused lithium chloride + potassium chloride eutectic. The high overpotential can be ascribed to the formation of the thick passivation film of platinum chlorides. The high chlorine overpotential was decreased by the addition of alkali metal oxides and a reversible chlorine evolution was revealed in a similar manner as the graphite electrode. The reversible chlorine evolution was ascribed to the formation of the oxide passivation film. The chlorine overpotential at the oxide film was increased stepwise as the applied potential was made more positive. The stepped transitions of the chlorine overpotential was ascribed to the valence change of the oxide film. Platinum shows a typical N-shaped passivation at +0.65 V versus Ag/AgCl(0.1) which has been ascribed to the dissolution of platinum into Pt(II) ions and following formation of the passivation film of supersaturated Pt(II) chloride. Platinum was found to show another passivation phenomenon at high temperatures, above 450°C. The N-shaped current-potential curve was observed at +1.8 V which was far more positive than the potential of the standard chlorine electrode. The dissolution of platinum prior to the passivation was found to occur due to the formation of high valence platinum ions such as Pt6+ and Pt8+.

The Effect of the Charge of the Developer Ion on Development Rates

Abstract: The effect of the charge of the developer ion on the induction time ean be ascribed to a change in the value of the heterogeneous rate constant which essentially generates an overpotential. Thi eff...