Oxygen evolution overvoltage

These improvements are ascribed to the enhancement of the oxygen evolution overvoltage and lower electrochemical impedance, as indicated by CV and EIS. The results suggest that the use of Na2W04 may be an effective way to improve the high temperature performance of nickel-metal hydride batteries (116). 

This anode half reaction is highly irreversible and is accompanied by an appreciable overvoltage usually the potential of oxygen evolution is about 0 5 to 0 7 V higher than E . 

Metallurgical palladium-rathenium alloys are of interest insofar as at a component ratio of 40% Ru and 60% Pd, hydrogen is adsorbed as on pure Pd, and oxygen is adsorbed as on pure Ru. The overvoltage for hydrogen evolution at this alloy is just as low as it is at Pd, that for oxygen evolution is as low as it is at Ru. 

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

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.

Allowing for the overvoltage (0.6 volt) the oxygen evolution potential resulting from the discharge of OH- ions is 0.81 0.6 = 1.41 volt and hence discharge of chloride ions and the formation of chlorine... 

The coin has its reverse, however. The broadening of the potential window that is often bordered by the solvent electrochemical decomposition potentials (e.g. cathodic hydrogen evolution and anodic oxygen evolution from water) is due to an increase in reactions overvoltage. This may be caused by a diamond s lower electrochemical activity, as compared with the glassy carbon and like electrode materials. On the whole, this conclusion is corroborated by the kinetics studies on diamond electrodes... 

Molten salts are favored electrolytes in cases where aqueous solutions cannot be used because the decomposition voltage of water is lower than that of the salt in question. Although high overvoltages for hydrogen and oxygen evolution allow us to extend the use of aqueous electrolytes somewhat beyond the limits set by thermodynamics, there are many substances which cannot be electrowon or plated from aqueous solutions. 

In a study of a series of iron-based spinels, Iwakura et al. [313] did not observe a correlation of chlorine evolution overvoltage with the magnetic properties of the oxide. In contrast, a dependence of overpotential on magnetic properties was observed for oxygen evolution. 

The data summarized in this paper have established that the oxide pyrochlores under discussion substantially reduce the activation energy overvoltages associated with oxygen electrocatalysis. Specifically, it is found that these catalysts, in aqueous alkaline media near ambient temperature, are superior to any other oxygen evolution catalyst and are equal in performance to the best known oxygen reduction catalysts. As bidirectional oxygen electrocatalysts, they appear to be unmatched. 

When the E-value for (i) is computed as 0.82 V for pH 7.0, as in (a), oxygen evolution takes precedence. [In practice, overvoltage or polarization is more difficult to avoid in the case of reactions involving gases (O2) as compared with liquids and dissolved solutes, so that electrolysis of NiBr2 at most electrodes would probably lead to Br2 formation.]... 

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