Thermodynamic 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...

Levy, R. M. and Gallicchio, E. 1998. Computer Simulations with Explicit Solvent - Recent Progress in the Thermodynamic Decomposition of Free Energies and in Modeling Electrostatic Effects , Annu. Rev. Phys. Chem., 49, 531. 

It has been pointed out by some authors CL, 2) that for a semiconductor having a thermodynamic decomposition potential, E in between Ec and E , a redox couple with a standard redox potential, E°, more negative than E is needed in order to operate the photoanode without decomposition. Then, the maximum photovoltage attainable is Us - Ej, which is often much lower than Eg -A-x. For GaP, this is only 0.8 V (4) (Fig. 11). S... 

At high temperatures or in an atmosphere with a low P02, oxide films are often eliminated by dissolution in the metal, by thermodynamic decomposition or by formation of volatile sub-oxides. Even in these conditions, oxygen can still affect wettability, for instance by adsorption at metal/oxide interfaces. This effect leads to contact angles lower than the nominal contact angle of the inert system. Some examples of these oxygen effects are given in Section 6.4. Therefore, when contact... 

Overall Energy Consumption. The voltage efficiency is the ratio of the thermodynamic decomposition voltage to the actual cell voltage ( cell) eiieigV efficiency (nener )> which is a product of the current and voltage efficiencies, can be expressed for a diaphragm cell as... 

These electrolyzers are operated at about 80°C, as opposed to the 95°C common in the industry. This lower temperature allows the use of vinyl ester resins and other plastics for cell construction. Moreover, the Dow diaphragm cell is optimized for low current density ( 0.5 kA/m2 ). It consumes less electrical energy per unit of production than others in the industry. The cell voltage at low current density is only 300—400 mV above the thermodynamic decomposition voltage. Dow does not license its diaphragm cell technology and operating data for these cells are not available in the open literature. 

Figure 2.9 Examples for thermodynamic decomposition energies ofTi02 and GaAs note that both semiconductors are anodically unstable butTi02 is cathodically stable (see text).

The CIP minimum obtained from both the site-site and 3D-RISM treatment is located at smaller distances as compared to the primitive model of continuum solvent. This shift is attributed to the entropy gain associated with the pairing of unlike charges being enhanced in a molecular solvent [84]. Figure 4.15 illustrates the thermodynamic decomposition of the PMF obtained from the 3D-RISM and site-site DRISM approaches for the Na+-Cl ion pair,... 

Conversely, if a solid is decomposed into another solid and a gas, as in many practical calcination processes, autocatalysis is frequently observed. The 6rst observation of this kinetic phenomenon goes back to G. N. Lewis, who in 1905 studied the thermodynamic decomposition pressure of solid silver oxide. Lewis noted that it took a long time for the equilibrium pressure of oxygen to establish itself. He further noted that, once oxygen started to be evolved, the process accelerated itself. He then conducted a classical kinetic study in which he showed that his data could be well represented at all... 

A general rule that is particularly useful in computing the thermodynamic decomposition voltage, E ev, of an electrolytic cell is ... 

The values of x for Q2, NaOH, and KOH are given in Table 4.4.1. rev in Eq. (139) refers to the thermodynamic decomposition voltage to produce the given product, which can be calculated from the free energy change involved in the overall reaction describing... 

The free energy change, AG, is related to the thermodynamic decomposition voltage, rev> as ... 

I. Thermodynamic Decomposition Voltage for Membrane and Diaphragm Cells. The primary anodic and cathodic reactions associated with the reaction scheme (140) are ... 

The thermodynamic decomposition voltage of a membrane cell can be calculated in a similar manner as that shown for the diaphragm cells. Typical operating conditions in a membrane cell are... 

Hence, the thermodynamic decomposition voltage of a mercury cell operating at SO C with 0.2% Na amalgam is equal to 3.058 V as shown below. 

The thermodynamic decomposition voltages calculated above are the equilibrium cell voltages when there is no applied current. When current passes, the system departs from equilibrium conditions to drive the reactions. The cell voltage will then he higher than the thermodynamic decomposition voltage because of the overvoltages associated with the anodic and cathodic reactions and the ohmic drops. 

---