Oxidation, steady-state current-potential

Fig. II.4.1 Steady-state current-potential curve for oxidation of a reactant (R) at an electrode. At E practically no current flows, at E2 the current is limited by mass transport...

Fig. 61. Steady-state current-potential curves of formic acid oxidation on platinized platinum in XM HCOOH-1-0.5 M HjSO at 25°C.

Fig. 69. Steady-state current-potential curves for the oxidation of unsaturated hydrocarbons on platinized platinum in 0.5 M H2SO4 at 80°C.

Fig. 93. Steady-state current-potential curves of the methanol oxidation in 6 M KOH -h 4 M CH3OH at 40 °C on different porous carbon electrodes impregnated with platinum (curves a, b, c, e, f) and on platinized platinum (curve d)...

From steady-state current-potential and cyclic voltammetry studies, complex mechanisms comprising rate-determining chemical steps were proposed for the cathodic reduction of ethyl halides and anodic oxidation of Na[Al(C2H5)4], Mg(C2H5)2, and C2H5MgBr at lead electrodes in THF. The anodic processes depend strongly on the state of the electrode surface. Complete poisoning of the lead electrode eventually occurs In THF solution [646]. 

A series of studies was made by Roscoe and co-workers on the adsorption and electrochemical oxidation mechanisms of the amino acids glycine, a- and ) -alanine, and a-, and y-aminobutyric acid at a platinum electrode in order to determine the role that the position of the amino group plays in the surface adsorption properties and subsequent oxidation of these amino acids. The investigations were made in aqueous solutions at pH 1,7, and 13 using steady-state current-potential measure-... 

Hydrogenase based enzyme electrode was not inhibited, when CO content in the mixture was less than 0.1 %. In the presence of 1 % CO the rate of hydrogen oxidation was decreased by 10 % and zero-current potential was shifted positively for 30 mV. The steady-state Currents were achieved in a few minutes [10], An important advantage of the hydrogen enzyme electrode is completely reversible nature of inhibition by CO. Like the soluble hydrogenase the enzyme electrode recovered 100 % of its initial activity as soon as the atmosphere of pure carbon monoxide was changed back to hydrogen. 

The electrode gave a steady-state current when the electrode potential was maintained at +0.35 V vs. Ag/AgCl in 0.1 M phosphate buffer. Addition of ethanol to the buffer solution resulted in an increase in the anodic current, which was attributed to the oxidation of membrane-bound NADH. A steady response was obtained within 40 sec. The increase in the anodic current was linearly correlated with the concentration of ethanol. 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

This is the steady-state current which is theoretically predicted if stage 1 is the rate-determining step in the sub-stages sequence represented in Equations 4.8 1.12. An important parameter to compare both in theory and experimentally is the Tafel slope or the transfer coefficient which results from it. Therefore, Equation 4.30 has to be written in a form that contains only one exponential term. Since the considered I-E curve is an oxidation wave, the effect of the reduction (second term in the right-hand part of Equation 4.30) will be negligible with potentials that are situated sufficiently far away from the equilibrium potential, and for the anodic current the following applies ... 

Figure 15. Typical steady-state current response for a ferrocene-ethylene oxide-siloxane polymer (I) / glutamate oxidase / graphite rod electrode at an applied potential of +350 mV vs. SCE.

Below 0.45 V the chemisorbed intermediates formed on methanol adsorption are stable on any smooth platinum surface, with the steady-state current for methanol oxidation being extremely small. Above this potential, oxidation of methanol takes place at a rate that increases exponentially with potential, with the product being primarily C02. In addition, above potentials of approximately 0.6 V, the surface is steadily stripped of adsorbed carbon-containing species, with the loss of such species being complete near 0.8 V. It would seem likely on most surfaces that it is oxidation of COads or =C-OH in a sequential reaction pathway that leads to C02, but more active intermediates, such as CO adsorbed at less stable sites, such as those at the edges... 

Wang et al. [64] proposed the use of CNTPE for the detection of homocysteine. Voltammetric experiments of 160 pM homocysteine at CNTPE showed a well defined signal at 0.28 V that reached a maximum at 0.64 V. This peak current depended linearly with the square root of the scan rate. On the contrary, at CPE only a slight increase in the oxidation current was obtained at 0.40 V, with no peak current definition. A linear relationship between the voltammetric current and homocystein concentration at 0.64 V was observed between 20 and 180 pM, with a detection limit of 17.3 pM. Amperometric experiments were also performed at a potential of 0.70 V and a linear relationship between steady-state currents and homocystein concentration was obtained... 

In this study we applied these diagnostic criterion to evaluate whether the oxide films on tungsten are anion conducting in oxidizer solutions. As expected from the potentiodynamic polarization experiments, which show a constant current passivation regime, the steady state current density was found to be independent of potential in passive region using potentiostatic measurements. 

In an earlier study we had reported the XPS analysis of tungsten oxides formed during anodic polarization experiments. It was determined that even at high applied potentials, the oxide thickness values are less than the mean free path of electrons in the oxides (generally assumed to be between 30 to 50 A ). Clearly the oxide growth in tungsten is a slow process. However, despite the relatively small thickness vsilues, the steady state current density during anodic polarization is restricted to a few tens of microamperes. 

It is interesting to estimate the effective tip radius immersed in the water layer, which is responsible for a tip current of 1 pA at 1.5 V bias. As shown in Fig. 11, a polyurethane-coated W tip behaves as a microelectrode. A sigmoidal diffusion-limited current superimposed on the linear background current was obtained for the reduction of 1 mM Ru(NH3)g+ in 10 mM NaC104 solution. An effective radius estimated from the nearly steady-state current is 3 /xm. Also shown in Fig. 11 is the anodic background current due to the oxidation of W at potentials positive of 0.4 V versus SCE (curve b). From the data shown in curve c of Fig. 10 and curve b of Fig. 11, if one assumes that similar effective tip radius is responsible for both anodic and cathodic redox processes, an estimated effective contact radius of 3 nm can be obtained for a background current flow of 1 pA at a bias voltage of 1.5 V. 

With the substrate biased at a potential slightly more positive than E° of A/B couple, B is oxidized to form A for both DISP1 and ECE mechanisms. However, in the latter case the reduction of C also occurs at the substrate. The numerical solution of corresponding diffusion problems (see Ref. 38 for problem formulations) yielded several families of working curves shown in Fig. 12 (DISP1 pathway) and Fig. 13 (ECE pathway). In both cases the tip and the substrate currents are functions of the dimensionless kinetic parameter, K = ka2/D. The normalization of the iT and is for two-electron processes is somewhat problematic. In Ref. 38 both quantities are normalized with respect to the one-electrode steady-state current, which flows at infinite tip/substrate separation (iT,ie.=o = 4FDac°). However, this value is not equal to experimentally measured tip current at d — which... 

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