Potential Step Technique Research Articles

ChemInform Abstract: A FURTHER STUDY OF DIFFUSION AND ITS ROLE IN ELECTRODE REACTIONS, THE DEVELOPMENT OF A NOVEL DOUBLE POTENTIAL STEP TECHNIQUE

Das beschriebene neue Doppelpotentialstufenverfahren beruht auf dem Konzept, daß das Produkt eines Elektrodenprozesses von der Oberfläche abdiffundiert und mit anderen potentiell elektroaktiven Spezies reagiert.

Electrochemical nucleation from molten salts—I. Diffusion controlled electrodeposition of silver from alkali molten nitrates

The electrodeposition of silver on platinum from solutions in the molten nitrate eutectic NaNO 3—KNO 3 has been studied using chronopotentiometry, cyclic voltammetry and the potential step technique. In each case, the early stages of metal deposition are shown to be controlled by a nucleation process. Similar results were also obtained from studies of electrodeposition from aqueous solution, ie of silver on platinum and graphite and of mercury on graphite. Unless allowance is made for the effects of nucleation, anomalous diffusion coefficients will be obtained. The potentiostatic growth transients are in accord with a process of instantaneous nucleation and subsequent hemispherical growth, although the observed concentration dependence is at variance with that predicted by existing theories.

ChemInform Abstract: SOME ASPECTS OF THE ELECTROCHEMICAL BEHAVIOUR OF THE IRON ELECTRODE IN ALKALINE SOLUTIONS

Abstract The anodic oxidation of iron in strong KLH solutions has been examined by Faradaic impedence and potential step techniques. The conclusions indicate that the rate determining film growth process is modified by the presence LI+ and S2− in the solution.

A further study of diffusion and its role in electrode reactions: The development of a novel double potential step technique

A novel double potential step technique is described. It is based on the concept that the product of an electrode process diffusing away from the surface may react with another potentially electroactive species; the fluxes of the two species are balanced during the first pulse and this leads to a rising i-t response during the second pulse at a potential where only the latter species is electroactive. The experiments have been simulated and the shape and timescale of the transient have been shown to depend on the nature and rate of the chemical reaction. These concepts are tested with three systems.

Some aspects of the electrochemical behaviour of the iron electrode in alkaline solutions

The anodic oxidation of iron in strong KLH solutions has been examined by Faradaic impedence and potential step techniques. The conclusions indicate that the rate determining film growth process is modified by the presence LI + and S 2− in the solution.

Properties of fused polysulphides—V. voltammetric studies on sulphur and polysulphides in fused KSCN and LiCl-KCl eutectic

The oxidation of alkali metal polysulphides in fused KSCN and LiCl/KCl eutectic has been studied by cyclic voltammetry and potential step techniques. Sulphur was found to be the final oxidation product. For solutions in KSCN at 210°C only one oxidation peak was observed, involving a common, one-electron electrochemical process for all the polysulphides in the composition range M 2S 3-M 2S 6. It is proposed that the common electroactive species leading to the formation of the sulphur deposit is S − 2 or S − 3. The anodic formation of sulphur was found to be diffusion controlled, and no inhibition due to the formation of a poorly-conducting liquid phase was apparent. The cathodic dissolution of sulphur was studied in both solvents. It resembles the anodic dissolution of a metal, giving rise to characteristics stripping peaks in the cyclic voltammograms.

Electrocrystallisation and the potential step method

A result pertaining to potential step technique and the surface-diffusion model is presented. The interesting aspect of result lies in its invariant nature with respect to distribution of growth sites on surface. This generalises result of Schnittler. Following Burton et al. 1 there have been several attempts to find role that surface diffusion of adatoms plays in determining rates of electrocrystallisation. Recognising a mass transport step on surface implies non-homogeneity of surface (in both microscopic and macroscopic sense) and any attempt to obtain quantitative effects of surface diffusion requires a knowledge of geometry of surface. In almost all cases considered mathematically thus far, this demand has been met by an assumption of a parallel-step structure for surface. Hence results also, at least one suspects so, lean heavily on this assumption. Although certain types of averageing and randomness can be introduced in such models, conclusions that can be asserted (even if somewhat apologetically) as being independent of (or invariant with) details of surface configuration are always welcome. One such attempt was made by Schnittler 2 concerning results for potential-on and -off experiments in electrocrystallisation. He pointed out invariance (to detailed distribution of growth sites Sx, $2 . . . . ) of transient when potential is switched off after having been maintained at level q~ until a steady state is reached. By a simple mathematical argument, we shall show (Appendix) in this paper how a corresponding result exists for more general case of changing t/I to q~ instead of equilibrium. This, apart from generalising Schnittler's result, provides an extra parameter viz. thi at input stage.

Monolayer Formation in the Reduction of Nickel and Silver Ions from the LiCl ‐ KCl Eutectic

The deposition of nickel and silver on platinum from the eutectic at 400°C has been studied using the chronopotentiometric, cyclic voltammetric, and potential step techniques. An observed predeposition of nickel on platinum was observed to take place at 0.5V anodic to the equilibrium potential between nickel in bulk and the solution of its ions. The predeposition process was studied by an a‐c impedance method and delineated as the formation of an adsorbed monolayer. A similar but less well‐defined predeposition step was observed with silver at 0.1V anodic to the reversible potential. Notwithstanding the predeposition phenomena, diffusion coefficients for both metal ions were determined by each of the voltammetric techniques and found to be in reasonable agreement with each other and with previously reported values.

Specular reflectivity studies of the adsorption of halide anions on gold electrodes in 0.2 M HClO 4

The specular reflectivity of gold has been measured in 0.2 M HClO4 with and without halide ions present. The reflectivity potential curves depend on the halide concentration at potentials where halide ions are adsorbed. Increasing halide ion concentration decreases the reflectivity with a limiting value approached at higher halide ion concentrations. The decrease in reflectivity produced by anion adsorption also is potential dependent and approaches a limiting value at more anodic potentials. Hysteresis is evident in the reflectivity-potential curves recorded with cyclic linear potential sweeps when the sweep rates and halide ion concentrations are such that diffusion of halide ions to and from the electrode is expected to be rate controlling. Potential-step techniques and simultaneous reflectivity measurements have been used to demonstrate diffusion control of the adsorption kinetics at low halide concentrations.

Specular reflectivity studies of the adsorption of halide anions on gold electrodes in 0.2. M HClO4

The specular reflectivity of gold has been measured in 0.2 M HClO4 with and without halide ions present. The reflectivity potential curves depend on the halide concentration at potentials where halide ions are adsorbed. Increasing halide ion concentration decreases the reflectivity with a limiting value approached at higher halide ion concentrations. The decrease in reflectivity produced by anion adsorption also is potential dependent and approaches a limiting value at more anodic potentials. Hysteresis is evident in the reflectivity-potential curves recorded with cyclic linear potential sweeps when the sweep rates and halide ion concentrations are such that diffusion of halide ions to and from the electrode is expected to be rate controlling. Potential-step techniques and simultaneous reflectivity measurements have been used to demonstrate diffusion control of the adsorption kinetics at low halide concentrations. The factors involved in the change of the reflectivity of the gold-electrolyte solution interface with potential and anion adsorption are discussed. The change in the surface electron charge density appears to be particularly important. The reflectivity and voltammetry data have been used to determine the “adsorption potential” for the I−, Br−, and Cl− anions, (corresponding to the potential of the voltammetry peak or the maximum in the slope of the reflectivity-potential curve). These adsorption potentials have been found to fit a linear dependence on the third power of the ionic radius reasonably well as is to be expected if the principal factor responsible for the difference in adsorptivity of these three ions is their polarizability.

The Transient Behavior of Graphite‐Silver Iodide and Platinum‐Silver Iodide Interfaces in a Solid‐State System

The electrochemical characteristics of the interface formed by solid silver iodide in contact with either platinum or graphite have been studied by linear sweep voltammetry, chronopotentiometry, and the potential and current step techniques. The results indicate that the graphite‐silver iodide interface exhibits pure double‐layer behavior over the potential range of , with no frequency dispersion effects observed up to approximately 1 MHz. In contrast, pure double‐layer behavior could not be isolated for the platinum‐silver iodide case under the same conditions. It is suggested that, for cathodic potentials, this is due to the rapid discharge of silver atoms onto the platinum surface at subunit activity, and at anodic potentials to liberation of iodine. Comparison of the two electrode systems in terms of recent ideas concerning molten salt double‐layer behavior indicates that in both of these systems, the double layer is essentially detemined by the structure of solid . The graphite interface behaves very much in an ideally polarized manner while the platinum interface behaves very much as an ideally reversible system because of the rapid faradaic reactions prevalent throughout the entire accessible potential range.

The Proton Diffusion Coefficient for the Nickel Hydroxide Electrode

The diffusion coefficient for protons moving in the hydrated nickel hydroxide lattice has been determined using a potential step technique. It was found to have the value about during charging and during discharging for the nickel hydroxide structure usually found in the nickel electrode of the nickel‐cadmium battery. The temperature dependence of the coefficient has been determined, and it has been concluded that the proton moves in the lattice by thermal diffusion. The enthalpy of diffusion was found to be 2.2 kcal deg−1 mole−1 during charging and 2.3 kcal deg−1 mole−1 during discharging of the electrode.

The Electrochemical Reduction of Meta‐Dinitrobenzene in Dimethylsulfoxide

The electrochemical reduction of meta‐dinitrobenzene (m‐DNB) in solutions of dimethylsulfoxide was studied on platinum electrodes. The rotating disk electrode and the potential step technique were used. The diffusion coefficient of m‐DNB in the electrolyte solution was found to be cm2/sec. The kinematic viscosity of the test solutions was also determined. The electrochemical reduction in the absence of a proton donor takes place in two steps which correspond to the formation and reduction of the m‐DNB radical anion. When a proton donor is added, homogeneous reaction takes place with the radical anion in the solution near the electrode resulting in an effective increase in the number of electrons transferred to four. This corresponds to the reduction to m‐nitrophenylhydroxylamine.

Electrode Mounting For Potentiostatic Anodic Polarization Studies

Abstract : Samples of an arc melted, 10Cr- 10Ni- Fe alloy were mounted with the Stern-Makrides compression gasket assembly. Some electrodes were mounted directly, others were partially encased or coated with various resins prior to mounting. Potentiostatic anodic polarization measurements were performed in hydrogensaturated, normal sulfuric acid at 25C, using an identical potential step technique for each specimen. Polarization behavior in the active region is unaffected by mounting material. However, all resins except heat-cured epoxy and alkyd types, markedly increase apparent passive current density. This is due to the presence of fine crevices at the electrode-resin interfaces. Although the area within crevices may be small, current density is high, and the effect on measured passive currents is pronounced. The results indicate that crevices in, or adjacent to, electrodes must be avoided during potentiostatic anodic polarization measurements. Specimens should be mounted with the Stern-Makrides compression gasket, in heatcured epoxy resin, in alkyd varnish or other methods which ensure freedom from microscopic crevices. (Extracted)