Noble metals polarization resistance

Oxygen Electrocatalytic Properties Oxygen Reduction. Figure 8 compares steady-state polarization curves for the electroreduction of Op on a typical pyrochlore catalyst, Pb2(Rui.42Pbo.53)06.5 15 w/o platinum on carbon. The latter was considered representative of conventional supported noble metal electrocatalysts. The activities of both catalysts are quite comparable. While the electrodes were not further optimized, their performance was close to the state of the art, considering that currents of 1000 ma/cm could be recorded, at a relatively moderate temperature (75 C) and alkali concentration (3M KOH). Also, the voltages were not corrected for electrolyte resistance. The particle size of the platinum on the carbon support was of the order of 2 nanometers, as measured by transmission electron microscopy. 

Electrochemical oxidation of organic conpounds requires an anode that is stable under anodic polarization, and that hs a low catalytic efficiency for oxygen evolution. Noble metals, such as platinum, are resistant to oxidation, but they have hi catalytic efficiencies for oxygen evolution and are prone to fouling (d, 7). Dimensionally stable anodes, such as titanium coated widi active or inactive catalysts, are less prone to oxygen evolution and fouling, but ftiey do suffer from leaching of the catalyst from e electrode surface. 

Measurement of corrosion rate by Tafel. Another method used to determine the corrosion rate as well as polarization resistance is the extrapolation method. This method consists of the exploration of the linear segments of potential-current density curves. In this case, the metal is initially made to act as a cathode. The cathodic potential-current density curve is measured by applying a potential range, defined by the variation between the corr and some potential active to corr, for example c, Figure 19-4. The cathodic current density approaches zero in the corr- Increasing the applied potential in the noble direction, the metal behaves as an anode, and the other part of the curve can be obtained. 

The fused carbonate cells operate between 500 and 800°C. In these temperatures the main feature that limits the current density at a given potential is not activation polarization or diffusion current, but the internal resistance of the electrolyte. Another persistent problem is the extreme corrosive nature of the electrolyte, limiting the choice of electrodes to those which resist corrosion, i.e. the noble metal ones. 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

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