Thermodynamic decomposition potential

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... 

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,... 

Fig. 5.10 Relative band edge diagram for FeS2 and the energy position of some electron donor species. The thermodynamic reactions corresponding to corrosion processes at the anodic and cathodic sides are indicated as decomposition potentials due to holes, fip dec, and to electrons, n,dec> respectively. r]c and are the cathodic and anodic overpotentials, respectively, for the decomposition reaction of pyiite crystals in acid medium. (Reproduced from [159], Copyright 2009, with permission from Elsevier)...

The reactions that are more favored thermodynamically tend to be also favored kineti-cally. Semiconductor electrodes can be stabilized by using this effect. For this purpose, redox couples in the electrolyte are established with the redox potential more negative than the oxidative decomposition potential, or more positive than reductive decomposition potential in such a manner that the electrolyte redox reaction occurs preferentially compared to the electrode decomposition reaction. 

Figure 4. Relative positions of hole decomposition potentials for the semiconductor and desired redox potentials for a photoelectrocheniical cell using n-type semiconducting electrodes. All examples are thermodynamically unstable, but (b) is kinetically more stable than (a).

Gerischer(16), Bard and Wrighton(17) have recently discussed a simple model for the thermodynamic stability of a range of photoelectrodes. As has been discussed previously, except for the rare case where the anodic and cathodic decomposition potentials lie outside the band gap, the electrode will be intrinsically unstable anodically, cathodically, or both.(16) It is the relative overpotential of the redox reaction of interest compared to that of the appropriate decomposition potential which determines the relative kinetics and thus stability of the electrode as illustrated in Figure 4. The cathodic and anodic decomposition potentials may be roughly estimated by thermodynamic free-energy calculations but these numbers may not be truly representative due to the mediation of surface effects. 

Hence the basic (reversible) thermodynamic standard potential ( therm) of decomposition (as also the overpotentials) decreases as the temperature increases. 

In many cases, the electrode reactions are not reversible and the decomposition potential is observed to be in excess of the thermodynamically calculated value. The excess voltage, referred to as an overvoltage, is found to vary with the nature and surface area (e.g., roughness) of the electrodes, impurities in the solution, and the actual current density passing through the solution. The relation between current density Id and overvoltage E was investigated by Tafel, who proposed the very successful empirical equation... 

Fluorine is produced by electrolysis of molten salts on carbon anodes including KF-21TF at about 100°C, potassium bifluoride at about 250°C, and fluoride salts at about 1000°C. The decomposition potential of molten potassium bifluoride is 1.75 V at 250°C, a value close to that estimated thermodynamically [80]. The kinetics of the anodic process is characterized by a Tafel slope of 0.56 V per decade, j), = 1 x 10 A/cm [81], and by a complex reaction mechanism involving the formation of fluorine atoms on carbon. During the electrolysis, C-F surface compounds on the carbon anode are formed via side reactions. Intercalation compounds such as (CF) contribute to the anodic effect in the electrochemical cell, which can be made less harmful by addition of LiF. 

Thermodynamic and over potential region For a terminal voltage smaller than the water decomposition potential Ud (Ud 2 V), no significant electrolysis happens and no current flows between the electrodes. 

The importance of kinetic factors in the photocorrosion of semiconductors has become increasingly clear during the past year. Of course, the thermodynamic calculation of decomposition potentials serves as a useful guide to the equilibrium situation, but detailed kinetic and mechanistic considerations are more immediately relevant to the problem of long-term photoelectrode stabilization. Gerischer has reviewed thermodynamics and kinetics of photodecomposition, and Cardon et have developed a detailed kinetic treatment. Similar... 

When photoelectrochemical solar cells became popular in the 1970s, many reports appeared concerning the stability, dissolution, and flat-band potential of semiconductors in solutions. These papers investigated parameters such as the energy level of the band edges, which is critical for the thermodynamic stability of the semiconductor and how to determine the potential for the onset of the (photo) electrochemical etching [38-40]. The criterion for thermodynamic stability of a semiconductor electrode in an electrolyte solution is determined by the position of the Fermi level with respect to the decomposition potential of the electrode with either the conduction band electrons or valence band holes E. Under illumination, the quasi-Fermi level replaces the Fermi level. The Fermi level is usually found within the band gap of the semiconductor and its position is not easily evaluated (especially the quasi-Fermi level of minority carriers). Therefore it was found more practical to use the conduction band minimum (Eq) and valence band maximum (Ey) as criteria for electrode corrosion. Thus, a semiconductor will be corroded in a certain electrolyte by the conduction band electrons if its... 

Fig. 1 Typical correlation between energy positions of band edges and decomposition potentials, controlling thermodynamic stability against photodecomposition, (a) stable, (b) unstable, (c) unstable against anodic decomposition, (d) unstable against cathodic decomposition (from Ref 39).

Fig. III.11 gives a summary for a number of semiconductors for which the position of the band edges and the decomposition potentials are known. One sees that apparently, at least in contact with aqueous solutions, none of these semiconductors is thermodynamically stable against anodic photodecomposition, while some are indeed stable against cathodic decomposition.

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