Oxygen-evolution reaction

Oxygen evolution reaction (OER) can be considered as one of the most relevant processes in electrochemistry involved in, for instance, chlor-alkali and ozone productions. One of the challenges is seawater electrolysis in order to store pure hydrogen and oxygen gases and prevent CI2 formation. The overall OER, 

FIGU RE 12.1 Hydrodynamic voltammograms at reticulated vitreous carbon in Oj-saturated 0.5 M H2SO4 at different rotation rates (a) 400, (b) 600, (c) 800, (d) 1000, (e) 1500, and (f) 2000 rpm. Potential scan rate 20 mV/s. (From Awad et al., 2008. J. Solid State Electrochem. 12, 251-258, with permission from Springer.) 

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Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

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. 

In this section we treat some electrochemical reactions at interfaces with solid electrolytes that have been chosen for both their technological relevance and their scientific relevance. The understanding of the pecularities of these reactions is needed for the technological development of fuel cells and other devices. Investigation of hydrogen or oxygen evolution reactions in some systems is very important to understand deeply complex electrocatalytic reactions, on the one hand, and to develop promising electrocatalysts, on the other. 

Damjanovic, A., Birss, V. I. and Boudreaux, D. S. (1991) Electron transfer through thin anodic oxide films during the oxygen evolution reactions at Pt... 

Ru losses can occur during electrolysis, as well as those due to shorting in mercury cell operations, from erosion, loss of Ru-based intermediates involved during the course of chlorine and oxygen evolution reactions and during shut-downs. 

The voltage of the above reaction under standard conditions is 1.35 V. A competing reaction at the anode is the oxygen evolution reaction according to the following equation ... 

The voltage of the oxygen evolution reaction under standard conditions (pH = 0) is 1.23 V. As the potential for oxygen evolution (Equation 16.2) is lower than the potential for chlorine evolution (Equation 16.1), oxygen evolution at the anode will always take place to some extent. 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

The overpotential for the oxygen evolution reaction on a silver anode in 0.1 N KOH was measured with respect to a reference electrode, at 25 °C. 

Fig. 13. (a) Schematic representation of the formation of mixed potential, M, at an inert electrode with two simultaneous redox processes (I) and (II) with formal equilibrium potentials E j and E2. Observed current density—potential curve is shown by the broken line, (b) Representation of the formation of corrosion potential, Econ, by simultaneous occurrence of metal dissolution (I), hydrogen evolution, and oxygen reduction. Dissolution of metal M takes place at far too noble potentials and hence does not contribute to EC0Ir and the oxygen evolution reaction. The broken line shows the observed current density—potential curve for the system. 

Finally, electrochemical pre-treatment is performed to obtain a reproducible surface. This is done mainly by cycling the applied potential over the entire potential window limited by the hydrogen and oxygen evolution reaction. Such a treatment has two functions first, removal of adsorbed species and, second, altering the microstructure of the electrode, the latter being caused by the repetitive dissolution and deposition of a metal mono-layer in the scanning procedure. 

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