What is Oxygen Evolution Reaction(OER)?

What is Oxygen Evolution Reaction(OER)?

The oxygen evolution reaction (OER) takes place in many industrial processes, such as water electrolysis, electrowinning, cathodic protection and electro-organic synthesis. Unlike hydrogen and chlorine evolution on metals or metal oxides, the OER represents low reversibility. Understanding the complicated kinetics of the OER presents a challenge to electrochemists. Metal oxides are most frequently used as anodes in acidic media, although most transition metal oxides such as nickel, cobalt and manganese undergo corrosion under these conditions. The cations of these metals are known to poison the membrane by attaching to the sulphonic acid groups, thereby reducing the conductivity of the membrane. These cations also strongly adsorb to the active sites of the platinum cathode, further deactivating the electrode. Platinum group metal oxides such as RuO2 and IrO2, and some transition metal oxides such as PbO2 and SnO2, have been found to be more stable during the OER. The standard potential for OER is 1.23 VRHE (RHE, reversible hydrogen electrode), which falls above the standard potential of almost all solid materials, explaining why only a few materials are stable under the OER.

The OER is a complex reaction and involves pathways of high activation energy and energetic intermediates. Oxygen species cover the surface of bare metals by underpotential deposition (UPD), by the discharge of water before the liberation of oxygen, and since the M–O bond strength (M: metal) is always stronger than the O–O dissociation energy, the OER always takes place at a metal oxide surface. In acidic media, the mechanism of the OER involves two parallel reaction paths. First, there is the discharge of the water molecules at the metal oxide surface, MOx, to form adsorbed hydroxyl radicals:
MOx + H2O → MOx (· OH) + H+ + e-

A second step depends on the nature of the interaction between the metal oxide and the electrogenerated hydroxyl radicals. They distinguished two limiting cases, i.e. oxygen evolution via physisorbed hydroxyl radicals and via chemisorbed intermediates. In the first case, the physically adsorbed hydroxyl radicals are electrochemically oxidized to form oxygen, involving hydrogen peroxide as an intermediate: MOx (· OH) → MOx + H+ + ½ O2 + e-


In the second case, the chemisorbed hydroxyl radicals react with the oxide to form the higher oxide: MOx (· OH) → MOx+1 + H+ + e-

The higher oxide then decomposes to regenerate the lower oxidation state and evolves oxygen: MOx+1 → MOx + ½ O2

A more general scheme for the mechanism of the OER can be written as follows:
S + OH- → SOH + e-
SOH + OH- → SO- + H2O
SO- → SO + e-
2SO → 2S + O2 (g)  
where S represents a catalytically active site for the reaction.