CYCLIC POLARIZATION CURVES

While Fig. 10.14a, b contain 30% and 10% of Pt3Ni cubes (with the remaining particles being truncated-octahedrons), respectively, Fig. 10.14c contains only tmncated-octahedrons. The particle size is on the order of 5 to 7 nm. Only two types of facets are exposed of aU the nanocrystals, i.e., the 111 and 100. The fractions of the 111 surface area over the total surface area could be calculated based on the geometries of the shapes and the population statistics. The ORR kinetics of the nanocrystals were studied on RDEs in 02-saturated 0.1 M HCIO4, at room temperature, at 1,600 rpm, with a potential scan rate of 10 mV/s. Figure 10.15 shows comparison of polarization curves, cyclic-voltammetry curves, mass activities, and specific activities of the Pt3Ni nanocrystals to the standard TKK Pt/Vulcan carbon catalyst. As shown in Fig. 10.15d, almost-linear correlations were obtained for both mass activities and specific activities versus the fi action of the (111) surface area over the total surface area. A tabulated kinetic activity comparison is shown in Table 10.1. The mass activity and specific activity comparisons were made at 0.9 V versus RHE. 

Figure 7. Cyclic voltammetry polarization curves for MEA made with different Pt-Ru/C catalysts [25], 3M (Pt/Ru = 1 1), 3M (Pt/ Ru = 1 2) and 3 M (Pt/Ru = 2 1) represent the catalysts prepared using the unprotected metal nanoclusters as building blocks E-tek (Pt/ Ru = 1 1) represents the commercially available catalyst (C14-30). All the catalysts have the same total metal loading of 30wt.%.

Figure 6.8 S-shaped polarization curve observed in the CO oxidation model (for the exact model parameters, see Koperetal. [2001]). The thin line shows the cyclic voltammetry observed at a low scan rate of 2 mV/ s.

Figure 8.9 Polarization curves for a PtSn/C catalyst recorded by a rotating disk electrode in 0.5 M H2SO4 saturated with either pure hydrogen, a H2/2% CO mixture, and pure CO (the arrow points to the onset of CO oxidation) at 60 °C with 1 mV/s and 2500 rev/min the dashed curve is the cyclic voltammogram (in arbitrary units) in an argon-purged solution at 60 °C with 50 mV/s. (Reprinted with permission from Aienz etal. [2005]. Copyright 2005. Elsevier.)...

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

From polarization curves of the type shown in case 3, three important parameters can be determined ECOSI, Ebth and In the literature there exists a nearly infinite number of variations of nomenclature, many of which are shown in Table 2. The interpretation of cyclic polarization curves has been and continues to be a subject of great controversy. The classic interpretation of case 3 would be that the potential of a material must exceed EM for new pits (or localized corrosion sites) to nucleate, but that at potentials between EM and En existing pits can propagate. At potentials below En all localized corrosion sites repassivate. Thus, from a design or material selection perspective, a material will perform well if its Econ is kept below This criterion can be met by environment... 

Controversy concerning the interpretation of cyclic polarization curves has raged for many years. Of particular interest is which (if either) of the two potentials can be used for material selection and mitigation strategy decisions. The classic interpretation is that a material s potential must exceed Ehl[ in order to initiate pits, but if flaws were introduced into the surface in any way, they could propagate at all potentials above Ew. Thus Eq, could be used in design as a protection potential. 

Figure 42 Cyclic polarization curve for Type 302 stainless steel in 1,000 ppm NaCl. Note the definition of the breakdown and repassivation potentials, the vertex current density, and the appearance of metastable pits.

In this first experiment you will mn a standard cyclic polarization curve on stainless steel. 

When the system is operated under galvanostatic conditions or when a sufficiently large series resistance is added, pronounced oscillations are observed around the positively sloped branch in the steady-state polarization curve. A cyclic voltammogram exhibiting current oscillations is reproduced in Fig. 12. Typical time series under galvanostatic control, close to the onset of oscillatory behavior, are shown in Fig. 13(A). Their... 

Fig. 7.15 Schematic cyclic polarization curve fora metal showing susceptibility to pitting. Pitting is initiated at Eb jtand propagation stops...

Fig. 1.9 (a) The cyclic voltammetry for Au(lOO) in 0.1 M KOH along with the polarization curve for CO oxidation in CO-saturated solution (continuous curve) ... 

F ie 7.9 Schematic representation of a cyclic anodic polarization curve of an active-passive material in a chloride-containing environment pitting potential ( pu) and protection potential ( p ) are identified [1]... 

Fig. 8 Schematic cyclic polarization curve for a system exhibiting localized corrosion.

A correlation has been between pitting potential and pitting in the field the tendency to form pits naturally at open circuit increases as the experimentally determined pitting potential decreases [51]. The difference between E-p and E, which is related to the extent of hysteresis in a cyclic potentiodynamic polarization curve, has also been considered to be a measure of the susceptibility to localized corrosion [3,52]. Issues associated with this interpretation have been reviewed [53]. 

Cyclic potentiodynamic polarization used in determining pitting potential consists of scanning the potential to more anodic and protection potentials during the forward and return scans and compare the behavior at different potentials under identical conditions. The polarization curve of an alloy (with or without coating showing active-passive behavior may be obtained in a chosen medium as a function of chloride concentration). E, or Ep represent pitting potential or breakdown potential,... 

Kar] Dilatometry, cyclic polarization curves, optical micrography 18 mass% Cr, < 3.3mass% N, mechanical alloying (Fe-Cr -l- Cr2N)... 

Fig. 17.6 Comparison of polarization curves for ORR on Pd2Co/C in oxygen-saturated 0.1 M HCIO4 solution with and without 0.1 M methanol rotation rate 900 rpm sweep rate 10 mV s . The inset is the cyclic voltammetry of Pd2Co/C nanoparticles m nitrogen-saturated 0.1 M HCIO4 + 0.1 M methanol solutions sweep rates 20 mV s [19]...

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