Mixed potential battery electrode

If the two electrodes are short-circuited together, the cathodic process of the positive combines with the anodic process of the negative as shown in Figure 11. The battery now has a singular potential - the short circuited potential. This clearly is a mixed potential or could be viewed as a corrosion potential of the system. 

The battery electrode mixed potential brings attention to the corrosion engineer s problem of controlling either the cathodic or anodic process to minimize the corrosion current. The problems of surface passivation, the question of identifying the distinctive spatial locations of the reaction processes are frequently present in practical situations - what is cathodic to an anodic region or vice versa, what are useful ways to modify the surface processes ... 

Note Actually not the true equilibrium voltage but only the open circuit voltage can be measured with lead-acid batteries. Due to the unavoidable secondary reactions of hydrogen and oxygen evolution and grid corrosion, mixed potentials are established at both electrodes, which are a little different from the true equilibrium potentials (cf Fig. 1.18). But the differences are small and can be ignored. 

The self-discharge process in a lead-acid battery is initiated by the mixed potentials of the negative and positive electrodes, and E , at open circuit. The open-circuit potential of the electrodes is determined by the two partial reactions on each negative and positive electrode. 

Elemental fluorine is produced commercially by the electrolysis of KF-2HF (mp 72 °C) using carbon anodes and steel cathodes. The voltage of the cells is 8-12 V (standard electrode potential for F2 = 2.85 V) and current is as high as 15kA, depending on cell size. The cathode and anode are separated by skirts to prevent mixing of F2 with H2 formed at the cathode. The western world production of fluorine is ca. 2-3 x 10 ta , of which about 55% is used for production of UFe and 40% is for SFe. Other uses include production of CF4, NF3, and fluorinated graphite for batteries. 

Although water is used preferentially as a medium for electrode reactions, there is growing interest in the use of nonaqueous solvents. This is for several reasons first, there are compounds which exhibit very limited solubility in water. Second, some species may not be stable in aqueous media. Third, the range of available potentials, relatively narrow in water, may be wider on both the cathodic and the anodic side in an aptly chosen solvent. Also, some processes of industrial or technical importance are sometimes carried out in nonaqueous or mixed solvents. For instance, in recent years different types of batteries, especially those with lithium electrodes, have been developed and further improved. They are based on the application of nonaqueous solvents. These applications frequently result from the fact that thermodynamic and kinetic parameters of various electrode reactions are greatly affected by the reaction medium. 

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