Voltage decomposition

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

Table 7. Thermodynamic Decomposition Voltage of Chlor—Alkali Cells at 25°C...

Molten halides are liquid electrolytes in many instances, and their decomposition may be canned out in principle to produce the metal and halogen, usually in the gaseous state. The theoretical decomposition voltage, E°, is calculated from the Gibbs energy of formation tlrrough the equation... 

I.D. Efros, M.F. Lantratov, About decomposition voltage of potassium fluorotantalate in molten salts solutions, Metallurgiya, Moscow, 1965 p. 284 (in Russian). 

Overpotential. It has been found by experiment that the decomposition voltage of an electrolyte varies with the nature of the electrodes employed for the electrolysis and is, in many instances, higher than that calculated from the difference of the reversible electrode potentials. The excess voltage over the calculated back e.m.f. is termed the overpotential. Overpotential may occur at the anode as well as at the cathode. The decomposition voltage ED is therefore ... 

E°(h is the decomposition voltage of water. Above this value, water is not stable... 

The 1-E plots may be extended to higher voltages in order to obtain the decomposition voltage of the sample. 

In practice, for a ternary system, the decomposition voltage of the solid electrolyte may be readily measured with the help of a galvanic cell which makes use of the solid electrolyte under investigation and the adjacent equilibrium phase in the phase diagram as an electrode. A convenient technique is the formation of these phases electrochemically by decomposition of the electrolyte. The sample is polarized between a reversible electrode and an inert electrode such as Pt or Mo in the case of a lithium ion conductor, in the same direction as in polarization experiments. The... 

Before Nernst had put forward his theory, Bodlander (Zeitschr. physik. Chcm., 27, 55, 1898) had been able to calculate the solubility of a salt by the measurement of its decomposition voltage, and had found that where the reaction occurring is the dissociation of a solid salt into solid uncharged atoms, the work done to split up the salt into its ions, and discharge these at the electrodes, is very nearly equal to the heat of formation. 

A definite decomposition voltage occurs for the following reason. As soon as there is a potential difference between the electrodes, H+ ions move to the cathode and Cl ions to the anode. The ions are discharged, forming layers of adsorbed gas on the inert metal surfaces. This essentially amounts to having a hydrogen electrode and a chlorine electrode in place of the two platinum electrodes. The outcome is a typical chemical cell ... 

The curve in Figure 6.19 (C) conveys the fact that there is a small current through the cell even below the decomposition voltage. This is called the diffusion current. There is a slow diffusion of ions away from the polarized electrodes, and a small current of ions must stream toward the electrodes at any applied voltage in order to keep them under polarized condition. 

As far as conductometry is concerned, there remain a few complications caused by processes at the electrodes, e.g., electrolysis above the decomposition voltage of the electrolyte with some liberation of decomposition products at the electrode, or apparent capacitance and resistance effects as a consequence of polarization of the electrode and exchange of electrons at its surface. In order to reduce these complications the following measures are taken ... 

LijN has a low decomposition potential, 0.445 V, which limits its usefulness as a practical solid electrolyte in batteries. Various Li3N-derivative phases have been synthesised, such as cubic Li5Nl2 with an antifluorite-derivative structure and a related phase in the system, LijN-Lil-LiOH. Some of these have high conductivity and the latter materials in particular are comparable to those of H-doped LijN. They also have a much higher decomposition voltage, 1.5V, but do suffer from increased sensitivity to chemical attack. 

According to P. Pascal,0 fluorine is diamagnetic the specific magnetic susceptibility is —3447 X10 j and the atomic susceptibilitv calculated from the additive law of mixtures for organic compounds is —63X10-1. Ionic fluorine is univalent and negative. The decomposition voltage required to separate this element from its compounds is 1 75 volts.7 The ionic velocity (transport number) 8 of fluorine ions at 18° is 46 6, and 52-5 at 25° with a temp, coeff. of 0 0238. 

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