Oxygen reduction polarization curves

272 3 Transition Metal Macrocycles as Electrocatalysts for Dioxygen Reduction 

A better understanding of the structure of these pyrolyzed materials has been gained using an array of techniques, as will be illustrated in the following sections. 

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Oxygen/reduction polarization curves from RDE measurements for Ru,92MO(,Q8Se04, Ru/C, and Pt/C in 0.6 M H2SO4,60°C. (T. J. Schmidt et al.. Journal of the Electrochemical Society, 4 2620 (2000). Reproduced by permission of The Electrochemical Society.)... 

Fig. 15.25 Comparison of oxygen reduction polarization curves on (a) MnPc/C and MnPc Ag/C, (b) FePc/C and FePc Ag/C, (c) CoPc/C and CoPc Ag/C, and (d) NiPc/C and NiPc Ag/C with Ag/C and XC-72 catalysts at 2,500 rpm in 02-purged 0.1 M NaOH solutions. Scan rate 20 mV s ...

Figure 2. Normal Pulse (12 s step time 2 s pulse width) oxygen reduction polarization curves for the electrodes describedfor Fig. J.

Figure 4. Oxygen reduction polarization curves for our best electrodes containing PEDOT/PSS, polyiqnrole/PSS, and polyaniline supported catalysts. Pt loadings were 0.9, 0.8, and 0.2 mg cm, respectively.

The value of is 1.0, which has been widely reported in the literature [135-137]. For the value of, the literature presents two Tafel slopes for the oxygen reduction polarization curve [138-141]. The first case, where the slope is approximately 60 mV/decade at 25 °C, corresponds to a value of 2.0 for a ... 

Fig. 5.11 Schematic representation of relative positions of anodic metal, cathodic oxygen, cathodic hydrogen, and cathodic water reduction polarization curves. pH = 1. Po, = 0-2 atm. Curve M, anodic polarization curve for metal (e.g., Fe-18% Cr) curve H, cathodic polarization curve for H+ curve W, cathodic polarization curve for H20 curve O, cathodic polarization for 02...

Based on cathodic polarization curves, Dexter and Gao concluded that the increase of E for 316 stainless steel exposed to natural seawater was due to an increased rate of the cathodic reduction of oxygen at a given potential. It is not possible from Ecorr or polarization curves to decide whetlier the increase in Ecorr is due to thermodynamic effects, kinetic... 

FIGURE 22.2 Schematic polarization curves for spontaneous dissolution (a) of active metals (h) of passivated metals. (1,2) Anodic curves for active metals (3) cathodic curve for hydrogen evolution (4) cathodic curve for air-oxygen reduction (5) anodic curve of the passivated metal. 

Various carbon-based catalysts for the electrochemical oxygen reduction have been tested in the air gas-diffusion electrodes [7]. The polarization curves of the air electrodes were measured when operating against an inert electrode in 2 N NaCl-solution. The potential of the air electrodes was measured versus saturated calomel electrode (SCE). 

Fig. 14.19 Polarization curves of oxygen reduction on nitrogen-doped CNTs with different nitrogen content. Operating conditions 0.1 M KOH, 5 mVs-1 scan rate, 1600 rpm rotation speed,...

Figure 1, Polarization curve for the electrochemical reduction of oxygen coupled with the electrochemical oxidation of an unspecified organic impurity O.C, designates the open circuit potentials...

Figure 7. Electrode polarization curves for oxygen-containing solutions with a mixed potential resulting from oxygen reduction accompanied by Fe2+ oxidation. Curves are schematic but in accord with available data at significant points...

FIGURE 1.12 Polarization curves for the reduction of oxygen on a bare platinum disk in oxygen-saturated 0.5 M H2S04 at different rotation speeds. (From Ayad, A., et al., J. Power Sources, 149, 66, 2005.)... 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Figure 9 Polarization curve of carbon steel in deaerated, pH 13.5 solution at 65°C. Sample was initially held potentiostatically at —1.2 V(SCE) for 30 min before initiation of the potentiodynamic scan in the anodic direction at 0.5 m V/s. The cathodic loop results from the fact that the passive current density is only 1 pA/cm2, which is less than the diffusion-limited current density for oxygen reduction for the 0.5 ppm of dissolved oxygen present. (From Ref. 8.)...

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

The objective of the mass transport lab is to explore the effect of controlled hydrodynamics on the rate at which a mass transport controlled electrochemical reaction occurs on a steel electrode in aqueous sodium chloride solution. The experimental results will be compared to those predicted from the Levich equation. The system chosen for this experiment is the cathodic reduction of oxygen at a steel electrode in neutral 0.6 M NaCl solution. The diffusion-limited cathodic current density will be calculated at various rotating disk electrode rotation rates and compared to the cathodic polarization curve generated at the same rotation rate. 

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