Potentiodynamic oxide formation

Figure 1.38 Potentiodynamic oxide formation cyclic voltammograms for a polycrystalline Nb electrode and two different single crystal Nb surfaces in addition to the corresponding reciprocal capacitance curves 0.5 M H2S04, 100 mV s-1 [18].

Figure 1.45 Potentiodynamic oxide formation on Ta and simultaneously recorded capacitance. 0.5 M H2S04, with neutral (borate buffer) and alkaline (NaOH) electrolyte as dashed curves. dU/dt = 25 mVs-1 (source Refs. [11, 13]).

In the lower part of Fig. 16, the potentiodynamic oxide formation according to Eq. (39) is shown on a pure aluminum (99.99%) sample in an acetate buffer of pH = 6.0. The sweep rate in these experiments was dU/dt = 100 mV s . Simultaneously the electrode capacity C was recorded with a lock-in amplifier at 1013 Hz and is shown in the upper part of the figure. The U(t) diagram used is shown as an insertion. The initial film thickness do in this experiment was 1.0 nm. 

By today s standards of surface preparation, Will s procedures for surface preparation were crude, the surface structures were not characterized by use of surface analytical instrumentation (Which was neither widely available nor well developed at that time), and he employed extensive potentiodynamic cycling through the "oxide" formation potential region prior to reporting the quasi-steady state voltammetry curve, i.e., the potentiodynamic I-V curve. The studies employing surface analytical methods made a decade or more later were... 

Typical anodization curves of silicon electrodes in aqueous electrolytes are shown in Fig. 5.1 [Pa9]. The oxidation can be performed under potential control or under current control. For the potentiostatic case the current density in the first few seconds of anodization is only limited by the electrolyte conductivity [Ba2]. In this respect the oxide formation in this time interval is not truly under potentiostatic control, which may cause irreproducible results [Ba7]. In aqueous electrolytes of low resistivity the potentiostatic characteristic shows a sharp current peak when the potential is switched to a positive value at t=0. After this first current peak a second broader one is observed for potentials of 16 V and higher, as shown in Fig. 5.1a. The first sharp peak due to anodic oxidation is also observed in low concentrated HF, as shown in Fig. 4.14. In order to avoid the initial current peak, the oxidation can be performed under potentiodynamic conditions (V/f =const), as shown in Fig. 5.1b. In this case the current increases slowly near t=0, but shows a pronounced first maximum at a constant bias of about 19 V, independently of scan rate. The charge consumed between t=0 and this first maximum is in the order of 0.2 mAs cnT2. After this first maximum several other maxima at different bias are observed. 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

Fig. 24. Potentiodynamic polarization curve of Cu in 0.1 M KOH with anodic and cathodic current peaks and the related reactions of oxide formation or reduction dissolution of cations and the indication of the stability ranges of the CU2O and duplex oxide layer, z ph at CII indicates oscillating photocurrent due to a chopped light beam [86],...

The typical electrochemical oxide formation on Ta is shown in Figure 1.45. Again simultaneously taken potentiodynamic cyclic voltammograms (upper part) and... 

Because of the imposed potential variation, the potentiodynamic technique presents similar uncertainties as the galvanostatic method. Possible desorption of the adsorbate, owing to potential change, can complicate the results. Oxide formation in certain potential regimes may be more important in the potentiodynamic than in the galvanostatic method. Uncertainties from potential and concentration variations within porous electrocatalysts can be... 

The iodide ion is adsorbed at the open circuit on palladium from 1 mM KI, pH 10 for 180 s. In this process, iodide is oxidized, forming a chemisorbed monolayer of zero-valence iodine atoms, while H+ or water molecules are reduced to produce molecular hydrogen. It can be observed from the potentiodynamic profile of palladium that the H-atom adsorption region is totally inhibited and the onset potential for the oxide formation is shifted toward more positive values. Besides, a large anodic peak current is developed as a result of the iodine electrooxidation to iodate. Complete iodine desorption can be accomplished by the application of negative potentials at pH 10. Thus, after 5 min of potential holding at —1.0 V in pH 10 and several cycles within the potential limits of water stability, the repetitive current potential profile of iodine-free palladium can be obtained. 

A Survey on Oxide Formation from CVs Potentiodynamic measurements of oxide formation are useful to get a survey on the influence of potential U on i ox, irreversibility of oxide formation, electrode or oxide capacity C, and thickness d from the oxide formation charge qox-... 

A potentiodynamic kinetic model of Pt oxide formation that is consistent with CV data over a wide range of scan rates and for different catalyst surface structures is an important step in understanding Pt oxide formation and reduction. Evaluation of the model against a range of electrochemical and spectroscopic data and comparison with theoretical calculations of reaction pathways and energetics could help furnishing details of reaction mechanisms. 

Figure 13.3 Potentiodynamic electrooxidation of (a) formic acid, (b) formaldehyde, and (c) methanol on a Pt/Vulcan thin-film electrode (7 xgpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH (a), HCHO (b), or CH3OH (c). The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p-L s at room temperature). The top panels show the faradaic current (solid lines), the partial currents for Ci oxidation to CO2 (dashed lines) and for formic acid formation (dash-dotted line), calculated from the respective ion currents, and the difference between the measured faradaic current and the partial current for CO2 oxidation (formic acid oxidation (a), formaldehyde oxidation (b)), or the difference between faradaic current and the sum of the partial currents for CO2 formation and formic acid oxidation (methanol oxidation, (c)) (dotted line). The solid lines in the lower panels in...

Figure 13.4 Current efficiency plots for the potentiodynamic electro-oxidation of formaldehyde (a) and methanol (h positive-going scan c negative-going scan) on a Pt/Vulcan thin-fihn electrode (data from Fig. 13.3a, h) dashed lines, current efficiency for CO2 formation dash-dotted fines, current efficiency for HCOOH formation dotted fines, current efficiency for HCHO formation.

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