Overpotential

The actual cell voltage achieved under a current load will be less than the theoretical equilibrium voltage due to voltage losses in the cell or polarization of the electrodes, where the potentials at each electrode approach each other or exhibit overpotential, thereby resulting in a reduced cell voltage. Contributions to the polarization include activation losses and concentration or mass transport losses. The additional cell loss includes ohmic loss due to current flowing through the cell and between the electrodes. 

V = Open circuit voltage - cathode activation losses - anode activation 

Illustrative fuel cell polarization with losses noted. (From Ye, S. 2007. Electrocatalysts for PEM fuel cells Challenges and opportunities. 14th National Meeting of Chinese Society of Electrochemistry, Yangzhou, China.) 

Kinetic processes in an electrochemical reaction must be considered when current is extracted from a battery, that is, when the current passed through an external circuit is different from zero. In this case, while current is removed from the battery, the equilibrium voltage or open circuit voltage falls because of electrode polarization or overvoltage [6,8,10,66,123,124], This electrode polarization effect occurs owing to kinetic limitations experienced by the reactions, and because of other processes taking place during the production of current flow. 

The Physical Chemistry of Materials Energy and Environmental Applications 

Similar to chemical kinetics, the mechanism of electrochemical reactions regularly requires a series of physical, chemical, and electrochemical steps, comprising charge-transfer and charge-transport reactions. The velocity of these individual steps controls the kinetics of the electrode reactions and, thus, of the cell reaction. In this sense, three diverse kinetics effects for polarization must be taken into account  

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Once current passes tluough the interface, the Galvani potential difference will differ from that expected from the Nemst equation above the magnitude of the difference is temied the overpotential, which is defined heiiristically as... 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

Iimnediately after the imposition of a large negative overpotential in a solution containing oxidized species,... 

The overpotential is defined as the difference between the actual potential of an electrode at a given current density and the reversible electrode potential for the reaction. 

The overpotential required for the evolution of O2 from dilute solutions of HCIO4, platinum electrodes is approximately 0.5 V. 

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

The mercury cell operates efficiently because of the higher overpotential of hydrogen on mercury to achieve the preferential formation of sodium amalgam. Certain trace elements, such as vanadium, can lower the hydrogen overpotential, however, resulting in the release of hydrogen in potentially dangerous amounts. 

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

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