Oxide potentiodynamic

The UPD and anodic oxidation of Pb monolayers on tellurium was investigated also in acidic aqueous solutions of Pb(II) cations and various concentrations of halides (iodide, bromide, and chloride) [103]. The Te substrate was a 0.5 xm film electrodeposited in a previous step on polycrystalline Au from an acidic Te02 solution. Particular information on the time-frequency-potential variance of the electrochemical process was obtained by potentiodynamic electrochemical impedance spectroscopy (PDEIS), as it was difficult to apply stationary techniques for accurate characterization, due to a tendency to chemical interaction between the Pb adatoms and the substrate on a time scale of minutes. The impedance... 

The electrochemical behavior of single-crystal (100) lead telluride, PbTe, has been studied in acetate buffer pH 4.9 or HCIO4 (pH 1.1) and KOH (pH 12.9) solutions by potentiodynamic techniques with an RRDE setup and compared to the properties of pure Pb and Te [203]. Preferential oxidation, reduction, growth, and dissolution processes were investigated. The composition of surface products was examined by XPS analysis. It was concluded that the use of electrochemical processes on PbTe for forming well-passivating or insulating surface layers is rather limited. 

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

In total, the differences between Ci adsorbate stripping and COad stripping, in both potentiodynamic measurements and potentiostatic transients, can be qualitatively explained by the lower COad coverage after Ci adsorption. A quantitative comparison with a lower coverage CO adlayer reveals, however, that these coverage effects are not sufficient and that contributions from other effects, most likely related to the structure of the CO adlayer, are important for the different oxidation behavior of the respective adsorbates as well. 

Bulk Oxidation of Formic Acid, Formaldehyde, and Methanol Potentiodynamic Measurements... 

In this section, we present results of potentiodynamic DBMS measurements on the continuous (bulk) oxidation of formic acid, formaldehyde and methanol on a Pt/ Vulcan catalyst, and compare these results with the adsorbate stripping data in Section 13.3.1. We quantitatively evaluate the partial oxidation currents, product yields, and current efficiencies for the respective products (CO2 and the incomplete oxidation products). In the presentation, the order of the reactants follows the increasing complexity of the oxidation reaction, with formic acid oxidation discussed first (one reaction product, CO2), followed by formaldehyde oxidation (two reaction products) and methanol oxidation (three reaction products). 

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.

On the other hand, during potentiodynamic formaldehyde oxidation (solid line in the upper panel of Fig. 13.3b), there is only a small faradaic current at 0.6 V in the positive-going scan, in contrast to the much higher steady-state value (about 0.55 mA) attained in the potentiostatic experiment. 

Fig. 2.2. Cyclic voltammogram of a polished Pt electrode in 1CT2 M CH3OH/O.l M HCI04 solution, (full line) and potentiodynamic oxidation of methanol adsorbate after solution exchange with base electrolyte (dashed line). Sweep rate 60 mV/s, room temperature.

Fig. 3.1. Current (a), and mass intensity for 13COz production (b) during the potentiodynamic oxidation of methanol adsorbate (flow cell procedure, ad = 0.2 V RHE, see text). Scan rate 10 mV/s.

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

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. 

Potentiodynamic Technique. Adsorption of methanol on Pt in acid solution was studied by Breiter and Gilman (3) using a potentiostatic technique. The anodic sweep, with a sweep rate of 800 V/s, was started at rest potential and extended to 2.0 V with respect to a hydrogen reference electrode in the same solution. As shown in Figure 10.8, the current was recorded as a function of potential (time) in the absence (curve A) and in the presence (curve B) of methanol. The increase in current in curve B is due to oxidation of the adsorbed methanol on the platinum electrode. Thus, shaded area 2 minus shaded area 1 (Fig. 10.8) yields the change 2m (C/cm ) required for oxidation of the adsorbed methanol ... 

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