Oxygen polarization curves

The oxygen polarization curve of Pd/C cubes is much more positive compared to those of octahedral and conventional Pd/C (Fig. 17.5a). This indicates that the Pd/C cubes were much more active for oxygen reduction. The specific ORR activities of Pd atoms in different samples were compared in the inset of Fig. 17.5a. The specific activities of cubic, octahedral, and conventional Pd/C were 0.31, 0.033, and 0.055 mA cm , respectively. The activity enhancement of Pd cubes was about... 

In addition to the standard polarization curve conducted on the intended oxidant, typically air, oxygen polarization curves are also commonly conducted. The oxygen polarization curve provides a measiue of the activation... 

Given that consensus is emerging regarding the fact of negligible anodic polarization and the pure kinetic control of the oxygen polarization curve, (2) reduces to... 

By proper calculation of E°(px, T), the oxygen reduction overpotential can be determined using Eq. 4 from the measured oxygen polarization curve, which follows an oxygen partial pressure dependent Butler-Volmer expression [121,122]. The theoretical Tafel line for air polarization should be just... 

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

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... 

The polarization curves for the oxygen evolution reaction are more complex than those for hydrogen evolution. Usually, several Tafel sections with different slopes are present. At intermediate CD their slope b is very close to 0.12 V, but at low CD it sometimes falls to 0.06 V. At high CD higher slopes are found at potentials above 2.2 V (RHE) new phenomena and processes are possible, which are considered in Section 15.6. 

FIGURE 15.5 Polarization curves for anodic oxygen evolution at a platinum electrode in perchloric acid solutions with various concentrations (1) 1.34 (2) 3 (3) 5 (4) 9.8 M. 

FIGURE 15.7 Polarization curves for anodic chlorine (1) and oxygen (2) evolution at a graphite electrode, and the current yields of chlorine as a function of potential (3). 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

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. 

The anodic evolution of oxygen takes place at platinum and other noble metal electrodes at high overpotentials. The polarization curve obeys the Tafel equation in the potential range from 1.2 to 2.0 V with a b value between 0.10 and 0.13. Under these conditions, the rate-controlling process is probably the oxidation of hydroxide ions or water molecules on the surface of the electrode covered with surface oxide ... 

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). 

Figure 6. Polarization curve of an air electrode operating with air and with pure oxygen.

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,...

Fig. 10-14. Energy levels and polarization curves (current vs. potential) for anodic transfer ofphotoexdted holes in oxygen reaction (2 HgO. -t- 4h O24 4 H. ) on a metal electrode and on an n-type semiconductor electrode j = anodic reaction current ep(02 20)- Fermi level of oxygen electrode reaction dCpi, = gain of photoenergy q = potential for the onset of anodic photoexdted ox en reacti . 4 pi, (=-Ae.. le) = shift of potential for the onset of anodic oxygen reaction from equilibrium oxygen potential in the negative direction due to gain of photoenergy in an n-type electrode Eib = flat band potential of an n-type electrode.

Fig. 10-16. Polarization curves for anodic oxygen and cathodic hydrogen redox reactions on an n-type semiconductor electrode of titanium oxide in the dark and in a photoex-cited state i = anodic current in the dark (zero) = anodic current...

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

Figure 10-32 shows the polarization curves for both the anodic oxygen evolution at an n-type anode and the cathodic hydrogen evolution at a p-type cathode. The anodic current (solid curve, n-SC ) of the photoexcited n-type anode occurs in the range of potential more cathodic (more negative) than the rai of potential for the anodic current (dashed curve n-SC ) of a p-type anode of the same semiconductor as the photoexcited n-type anode and the cathodic current (solid curve, p-SC ) of the photoexcited p-type cathode occurs in the range of potential more anodic (more positive) than the range of potential for the cathodic current (dashed curve, n-SC ) of an n-type cathode of the same semiconductor as the photoexcited p-type cathode. 

Fig. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

A mixed polarization diagram (where the polarization behavior of the two different electrodes is represented) for the sphalerite-hypersteel combination is given in Fig. 1.10 (Vathsala and Natarajan, 1989), in which the cathodic polarization curves for the sphalerite and the anodic polarization curves for the hypersteel ball material are seen to overlap. The active nature of the ball material is evident. The current values were observed to be lower in the absence of oxygen which indicated a lower anodic dissolution of the hypersteel grinding medium in the absence of oxygen. 

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.)... 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

---