Upper Potential Limit Research Articles

The electrochemical co-polymerization of p-aminodiphenylamine and aniline: effect of pH

The electrochemical oxidation of p-aminodiphenylamine (4ADPA) in the presence of aniline in phosphate medium of pH ranging from 1 to 10.5 has been investigated. Mixtures of both monomers can be co-polymerized to produce electroactive poly aniline- co-4ADPA (Pan-4adpa) films if the pH of the medium is kept, approximately, within 3 and 9 and the upper potential limit is lower than the oxidation potential of aniline. Pan-4adpa growth rates are pH dependent and are also affected by the amount of polymeric material deposited on the electrode surface. It has been suggested the existence of a certain inhibition effect of both H + and OH − on the polymerization reaction. Contrary to what is observed for those films synthesized at pH 5 and 7, the growth of Pan-4adpa at pH 8 seems to be an autocatalytic process until the inhibition of the polymerization reaction takes place.

Complications associated with kinetic studies of hydrous Ir oxide films

The kinetics of the oxidation and reduction of electrochemically formed Ir oxide films have been examined as a function of the potential limits used, oxide film thickness, solution pH and the degree of reversible aging (diminished kinetics) of the oxide. It is shown first that the oxidation kinetics of the principal Ir(III) to Ir(IV) redox reaction are diminished in acidic solutions if the lower potential limit is set so as to include the kinetically slow process which occurs in the anodic cyclic voltammetric pre-peak, A 0. Consistent with this, in alkaline solutions, little evidence for the prepeak is found, reversible aging does not occur and the kinetics do not depend significantly on the lower potential limit. Other results show that, if kinetic measurements are made employing a lower limit more positive than the A 0 pre-peak and an upper potential limit below that required to generate Ir(V) and/or Ir(VI) states, the kinetics of the Ir(III)/Ir(IV) process are independent of film thickness in acidic solutions. If anodic steps are made to 1.25 V or more in acidic solutions, the Ir(V)/(VI) states generated can mediate and, hence, accelerate, the oxidation of Ir(III) to Ir(IV), as seen by the unusual shapes of the j/ t transients.

An EQCM study of corrosion and complexation at electrode surfaces. Oxidation of silver in the presence of 4,4′-bipyridyl

The oxidation of Ag in the presence of 4,4′-bipyridyl (Bpy) has been studied by combined cyclic voltammetric and mass measurements (derived from the electrochemical quartz crystal microbalance). A Ag(I)—Bpy complex is formed at the electrode surface, but observed mass changes are found to result from the superposition of a mass loss (from release of Ag +) and a mass gain (from complex precipitation). Altering the conditions of the experiment (scan rate, upper potential limit and bipyridyl concentration) can switch the balance between the two processes and lead to a net mass loss or a net mass gain; at slow scan rates at a bulk concentration of 0.25 mM bipyridyl, the balance between the processes switches twice, even in the narrow potential range before complex reduction. The principal cause of these changes is the slow accumulation of Ag + in the diffusion layer as not all the Ag + released is complexed. At higher concentrations of bipyridyl (2 mM) mass changes due to complex formation are dominant until the potential is extended such that the surface concentration of bipyridyl is significantly depleted and electrode corrosion makes the principal contribution to the mass change.

The anion-specific effect in the overoxidation of polyaniline and polyindoline

The irreversible degradation (overoxidation) of polyaniline and polyindoline has been investigated in the presence of different anions incorporated in the films. In-situ conductivity, electron spin resonance and UV-Vis measurements together with cyclic voltammetry were performed to investigate the various properties of the two closely related polymers as a function of both the electrode potential and the nature of anions. When the upper potential limit is adjusted adequately to avoid irreversible degradation at higher potentials in polyaniline and polyindoline, the behavior of the polymers is not affected by the exchange of the anions ClO 4 −, SO 4 2−, Cl − and NO 3 −. However, beyond these potential limits, the polymer properties develop quite differently depending upon whether perchlorate or sulfate ions are available for charge compensation. In the presence of sulfuric, nitric and hydrochloric acid both polymers are subjected to rapid electrochemically induced irreversible degradation, whereas with perchloric acid the electroactive properties of the polymers are still present after the measurements have been performed. In this sense polyaniline and polyindoline have much in common, implying a similar structural configuration of the newly synthesized polyindoline compared with the well-known polyaniline. In view of the results achieved, a tentative explanation for the anion-specific effect is proposed. The effect seems to emerge from the anion-influenced interaction between the charge carriers of the polymers, identified primarily as bipolarons, and the different anions incorporated in the polymer matrix for charge compensation.

LEED spot profile analysis of the structure of electrochemically treated Pt(100) and Pt(111) surfaces

Abstract The structures formed by oxidation-reduction cycling of ordered Pt(100) and Pt(111) surfaces in aqueous electrolytes were studied using LEED spot profile analysis. Surfaces cycled to anodic potential limits above 1.0 V (RHE) and emersed at 0.5 V gave LEED spots which varied in width periodically with beam energy. The varying spot widths indicated a type of randomly stepped surface, and mean terrace widths were estimated by comparison with the previously published scattering calculations of Henzler and Lu and Lagally. LEED spots from surfaces cycled to potentials below 1.0 V (RHE) remained sharp at all beam energies. The restructuring resulted from the anodic formation of an amorphous surface oxide phaes involving place exchange of platinum and oxygen followed by a reduction process in which the platinum atoms do not all return to their original positions in the surface lattice. The observed spot profiles changed both with the the number of cycles and with the upper potential limit. Cycling to 1.28–1.58 V produced characteristically different structures than cycling to limits of 1.08–1.28 V. At the higher potentials, a randomly-stepped surface was formed whose mean terrace width and extent of vertical relief were functions of the total anodic charge. Cycling to the lower potential limits produced a type of structure which is intermediate between the classical two-level island and the multi-level randomly-stepped structure. A three-level structure with a high degree of correlation between the first (island-like) and third (hole-like) levels is proposed for the latter.

LEED spot profile analysis of the structure of electrochemically treated Pt(100) and Pt(111) surfaces

The structures formed by oxidation-reduction cycling of ordered Pt(100) and Pt(111) surfaces in aqueous electrolytes were studied using LEED spot profile analysis. Surfaces cycled to anodic potential limits above 1.0 V (RHE) and emersed at 0.5 V gave LEED spots which varied in width periodically with beam energy. The varying spot widths indicated a type of randomly stepped surface, and mean terrace widths were estimated by comparison with the previously published scattering calculations of Henzler and Lu and Lagally. LEED spots from surfaces cycled to potentials below 1.0 V (RHE) remained sharp at all beam energies. The restructuring resulted from the anodic formation of an amorphous surface oxide phaes involving place exchange of platinum and oxygen followed by a reduction process in which the platinum atoms do not all return to their original positions in the surface lattice. The observed spot profiles changed both with the the number of cycles and with the upper potential limit. Cycling to 1.28–1.58 V produced characteristically different structures than cycling to limits of 1.08–1.28 V. At the higher potentials, a randomly-stepped surface was formed whose mean terrace width and extent of vertical relief were functions of the total anodic charge. Cycling to the lower potential limits produced a type of structure which is intermediate between the classical two-level island and the multi-level randomly-stepped structure. A three-level structure with a high degree of correlation between the first (island-like) and third (hole-like) levels is proposed for the latter.

Computer Controlled Sweep Generator For Electrochemical Studies

SUMMARY A computer programmable sweep generator for use in voltammetric electro chemical studies has been constructed from a reversible binary counter and a digital-to-analog converter. The generator produces staircase ramps, triangular waves, and sawtooth waves. The initial potential, upper potential limit, sweep direction, and resolution (step size and sweep rate) can be controlled by a 12-bit computer word. This generator is characterized by low drift, high sweep rate accuracy, high reproducibility of its various settings regardless of sweep rate, and ease with which it can be controlled by a computer. In combination with the proper clock and potentiostat, it can be used to implement closed-loop, on-line computer control of electrochemical measurements.