The Oxygen Evolution Reaction

As a demanding reaction, it is very sensitive to the structural and compositional details of the anode materials. For this reason, research on anodes for O2 evolution calls for close characterization of electrocatalysts, especially from the point of view of materials chemistry and physics. 

The species that are oxidized in the first step of O2 formation are Fl20 molecules in acidic solution  

The intermediate energy depends on the interaction with the electrode surface. Materials binding OHads weakly as a rule show high overpotentials for O2 evolution. Their mechanism is dominated by step (7.25) as the rate-determining step and the Tafel slope is close to 120 mV. 

If OHads is adsorbed strongly, its formation is fast and successive steps of OHads desorption become rate determining. Under similar conditions, the Tafel slope is lower and electrode materials are electrocatalytically more active. Along the same conceptual lines of volcano curves, if the adsorption of OHads is too strong, its removal becomes very difficult and the overpotential increases again. Thus, in principle, a volcano curve should be expected as in the case of H2 evolution. 

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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. 

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

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. 

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

In this chapter, the focus will be on trends in electrocatalysis of the water-splitting reaction or the oxygen evolution reaction (OER), which is the reaction at the anode side in an electrolysis cell. Furthermore, simple framework for addressing OER applying DFT simulations will be presented. For further reading, there are two previous book chapters where the approach has been reviewed [3, 4],... 

Furthermore, BDD anodes have a high overpotential for the oxygen evolution reaction compared with the platinum anode (Fig. 1.3). This high overpotential for oxygen evolution at BDD electrodes is certainly related to the weak BDD-hydroxyl radical interaction, what results in the formation of H202 near to the electrode s surface (1.14), which is further oxidized at the BDD anode (1.15) ... 

Class 1 anodes, or active anodes, have low oxygen evolution overpotential and consequently are good electrocatalysts for the oxygen evolution reaction ... 

Recent Work on TiCh on Photosplitting of Water or on the Oxygen Evolution Reaction... 

An increasing amount of attention is being given to oxides as possible anodes for oxygen evolution because of the importance of this reaction in water electrolysis. In this connection, numerous studies have been carried out on noble metal oxides, spinel and perovskite type oxides, and other oxides such as lead and manganese dioxide. Kinetic parameters for the oxygen evolution reaction at a variety of single oxides and mixed oxides are shown in Table 3. 

Kinetic parameters for the oxygen evolution reaction on various oxides... 

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