Irreversible electrode potentials activation overpotential

If a substance is reduced or oxidized reversibly, then its half-wave potential wiU be near the standard potential for the redox reaction. If it is reduced or oxidized irreversibly, the mechanism of electron transfer at the electrode surface involves a slow step with a high energy of activation. Therefore, extra energy must be applied to the electrode for the electrolysis to occur at an appreciable rate. This is in the form of increased applied potential and is called the activation overpotential. Therefore, Exa will be more negative than the standard potential in the case of a reduction, or if will be more positive in the case of an oxidation. An irreversible wave is more drawn out than a reversible wave. Nevertheless, an S-shaped wave is still obtained, and its diffusion current will be the same as if it were... 

In the case of irreversible reactions, the polarographic half-wave potential also depends on the standard potential (formal potential) however, the kinetics of the electrode reaction lead to strong deviation as an overpotential has to be applied to overcome the activation barrier of the slow electron transfer reaction. In the case of a totally irreversible electrode reaction, the half-wave potential depends on the standard rate constant ks of the electrode reaction, the transfer coefficient a, the number e- of transferred electrons, the diffusion coefficient T>ox, and the drop time t [7] as follows ... 

Whereas Pt in an acidic solution saturated with H2 acquires the reversible potential of the hydrogen electrode, this is not the case for the same Pt electrode in an acidic solution saturated with O2. This is related to the high activation energies involved in breaking and forming chemical bonds. Thus the O2 reaction is known to be highly irreversible. In particular, a Pt electrode in 02-saturated solution acquires a potential 0.9V (SHE) rather than 1.23 V. Hence an overpotential of >0.3 V can already be expected from an analysis of the equilibrium conditions. 

The operation of the cell is associated with various irreversibilities and leads to various potential losses. In the case of electrodes the total resistance comprises of the internal resistance, contact resistance, activation polarization resistance, and concentration polarization resistance. Internal resistance refers to the resistance for electron transport, which is usually determined by the electronic conductivity and the thickness of the electrode structure. Contact resistance refers to the poor contact between the electrode and the electrolyte structure. All resistive losses are functions of local current density. However, one can minimize the overpotential losses by appropriate choice of electrode material and controlling the micro-structural properties during manufacturing process. 

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