Oxygen evolution overpotential

In the case of chromium in 1 N H2SO4 transpassivity occurs at about 1 1 V (below the potential for oxygen evolution, since the equilibrium potential in acid solutions at pH 0 is 1 23 V and oxygen evolution requires an appreciable overpotential) and is associated with oxidation of chromium to dichromate anions ... 

According to Sato et al.,6,9 the barrier-layer thickness is about 1.5 to 1.8 nm V-1, and increases to 3 nm around the oxygen-evolution potential. In Fig. 5, the scale of the electrode potential, Vrhe, is that of the reversible hydrogen electrode (RHE) in the same solution. The electrode potentials extrapolated from the linear plots of the potentials against the film thickness suggested that the potential corresponding to the barrier thickness equal to zero is almost equal to 0.0 V on the RHE scale, independent of the pH of the solution, and approximately agrees with the equilibrium potential for the oxide film formation of Fe or Fe. Therefore it is concluded that the anodic overpotential AE applied from the equilibrium potential to form the oxide film is almost entirely loaded with the barrier portion. 

The conceptual development of limiting-current measurement was advanced substantially by Agar and Bowden (A2), who investigated the current-overpotential relationship for oxygen evolution at nickel electrodes in fused sodium hydroxide. Here water transport is the limiting step ... 

The presence of iron in nickel oxyhydroxide electrodes has been found to reduce considerably the overpotential for oxygen evolution in alkaline media associated with the otherwise iron free material.(10) An in situ Mossbauer study of a composite Ni/Fe oxyhydroxide was undertaken in order to gain insight into the nature of the species responsible for the electrocatalytic activity.(IT) This specific system appeared particularly interesting as it offered a unique opportunity for determining whether redox reactions involving the host lattice sites can alter the structural and/or electronic characteristics of other species present in the material. 

Smooth platinum, lead dioxide and graphite are anode materials commonly used in electrooxidation processes. All show large overpotentials for oxygen evolution in aqueous solution. Platinum coated titanium is available as an alternative to sheet platinum metal. Stable surfaces of lead dioxide are prepared by electrolytic oxidation of sheet lead in dilute sulphuric acid and can be used in the presence of sulphuric acid as electrolyte. Lead dioxide may also be electroplated onto titanium anodes from lead(Il) nitrate solution to form a non-porous layer which can then be used in other electrolyte solutions [21],... 

Platinum and carbon are frequently used as counter electrode materials for both anode and cathode. Platinum is resistant to corrosion while carbon is cheap and can be discarded after use. Nickel is a suitable counter cathode material in aqueous solution because of the low overpotential for hydrogen evolution. Titanium coated with platinum and then over coated with mthenium dioxide is a stable counter anode material with a low overpotential for oxygen evolution. 

Fig. 7.111. Current density (based on real surface area) for oxygen evolution on perovskites at an overpotential of 0.3 V vs. M—OH bond strength. The transition-metal ions (M) in perovskites are indicated with different symbols. (Reprinted from J. O M. Bock-ris and T. Ottagawa, J. Electrochem. Soc. 131 2965,1984. Reproduced by permission of The Electrochemical Society, Inc.)...

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. 

Correlation of the activation energy or overpotential at a given current and for a particular rds on different substrates with relative measures of the heat of chemisorption of intermediates have been successfully made for hydrogen evolution [138], oxygen evolution [139], and oxygen reduction [76], etc. 

Figure 2. Oxygen evolution reaction current density (in units of A/gpt) with respect to the overpotential applied to the cathode electrode. The OER current density is based on the measured O2 concentration at the exit of a 50-cm2 cell using a GC, assuming 4e-/C>2 molecule. The cell is operating with 10% H2/He (150 kPaabs, varying T and RH) at various potentiostatic conditions.

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. 

Aqueous solutions all evolve H2 when the cathode potential is made sufficiently negative. However, it may be possible to have an aqueous solution that contains inexpensively dissolved substances (e.g., S02) that become oxidized at potentials much less than that of water itself. Then, looked at thermodynamically, the reversible potential of the reaction in the cell would be less than that of water. In addition, the i0 value for oxygen evolution (i0 1010 A cm-2 at 25 °C) is particularly low and the anode overpotential particularly high. Substitution of, e.g., S02 oxidation could be achieved at a lesser overpotential than with 02 evolution. 

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 ... 

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