Decomposition anodic

Partially platinized titanium impressed current anodes were chosen because contamination of the feed water by anodic decomposition products had to be avoided. Four pure zinc reference electrodes were installed in the tank to control and regulate the potential. The supports for the anodes were of polypropylene, which can operate for short periods up to 100°C, in contrast to the usual PVC supports used in cold water. 

Small amounts of lithium phenolate or dilithium-2,2 biphenyldiolate, whose anodic decomposition starts at about 3.2 V versus Li and thus before anodic decomposition of the borates begins, prevent the anodic decomposition of benzenediolatoborate anions in PC. The behavior of the solution is then determined by the additive. 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

P. Lemasson and J. Gautron (1981) On the anodic decomposition of the zinc selenide electrode. J Electroanal Chem 119 289-299... 

Dimitrijevic NM, Kamat PV (1987) Transient photobleaching of small CdSe colloids in acetonitrile, anodic decomposition. J Phys Chem 91 2096-2099... 

Concerning the competition between redox process and anodic decomposition one can define a stability factor s by... 

According to these results it is favorable to apply semiconductor electrodes which exhibit a large overvoltage concerning the anodic decomposition but not for the oxidation of a redox couple. Unfortunately, there are only few examples which fulfil this condition. Another example was found with n-GaAs in the presence of Eu ... 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

Often, there is a potential regime where the process represented by (8) is completely dominant compared to the anodic decomposition of the semiconductor. In some cases, e.g. 

The auxiliary electrode, which is normally a mercury pool, must be positioned in a compartment separate from the working electrode. Such a separation compromises the desired symmetric disposition of the electrodes. Normally, the compartments of a macroelectrolysis cell are separated by sintered glass frits, such that the catholyte and the anolyte are not mixed. In fact, if the working electrode is involved, for example, in a cathodic process, the auxiliary electrode will act as an anode. This implies that the auxiliary electrode will produce oxidized material (by anodic decomposition of the solvent itself, of the supporting electrolyte, of mercury-contaminated products or of electroactive residues diffused at the auxiliary electrode) that may subsequently be reduced at the working electrode, contaminating and falsifying the primary process. 

The nonactivated CH bond in aliphatic hydrocarbons is oxidized at a potential that lies mostly more anodic than 2.5 V [vs. SCE (saturated calomel electrode)] [5, 6]. This necessitates electrolytes with high anodic decomposition potentials. 

On the basis of their previous experiences with lithium borates coordinated by substituted ligands. Barthel and co-workers modified the chelatophos-phate anion by placing various numbers of fluorines on the aromatic ligands. Table 13 lists these modified salts and their major physical properties. As expected, the introduction of the electron-with-drawing fluorines did promote the salt dissociation and reduce the basicity of phosphate anion, resulting in increased ion conductivity and anodic stability. The phosphate with the perfluorinated aromatic ligands showed an anodic decomposition limit of 4.3 V on Pt in EC/DEC solution. So far. these modified lithium phosphates have attracted only academic interest, and their future in lithium ion cell applications remains to be determined by more detailed studies. 

Fig. 7.7 Percentage of photogenerated holes that contribute to anodic decomposition versus O2 evolution from naked (no catalyst), Pt-coated, and polymer-Pt coated n-CdS photoanode in 0.5 M Na2S04 solution (pH = 8.6) [14].

In order to find out in any particular case whether a semiconductor is liable to anodic or cathodic decomposition (both in darkness and under illumination), it is convenient to use the energy diagram (Fig. 15), which plots the energies of band edges and electrochemical potential levels for decomposition reactions. Various situations are possible here, as is schematically shown in Fig. 15. A semiconductor is stable with respect to anodic decomposition if the electrochemical potential level for the corresponding reaction... 

Fig. 15. Diagram illustrating the thermodynamic stability of a semiconductor against corrosion and photocorrosion (a) semiconductor is absolutely stable, (b) stable against cathodic decomposition, (c) stable against anodic decomposition, and (d) unstable. [From Gerischer (1977a).]...

Consider now the processes caused by the formation of quasilevels. As was noted above, the shift of Fn relative to F is very small for majority carriers (electrons) and can usually be neglected precisely, this was done in constructing Fig. 16b. But for minority carriers (holes) the shift of Fp can be very large. The shifts of both Fnx F and Fp increase with the growing intensity of semiconductor illumination, so that for a certain illumination intensity Fp may reach the level of the electrochemical potential of anodic decomposition Fdec, p, and Fn—the level of a certain cathodic reaction (for example, reduction of water with hydrogen evolution FHljH20). These reactions start to proceed simultaneously, and their joint action constitutes the process of photocorrosion. 

This oxoacidity is described by p02 = - log a(02 ), where a(02 ) is the oxide ion activity [6]. Not surprisingly, the anodic and cathodic potential limits of carbonate melts are very dependent upon the oxoacidity. Anodic decomposition of the melt is assumed to arise from the oxidation of oxide provided by the thermal decomposition of CO ... 

N-type semiconductors can be used as photoanodes in electrochemical cells Q., 2, 3), but photoanodic decomposition of the photoelectrode often competes with the desired anodic process (1 4 5). When photoanodic decomposition of the electrode does compete, the utility of the photoelectrochemical device is limited by the photoelectrode decomposition. In a number of instances redox additives, A, have proven to be photooxidized at n-type semiconductors with essentially 100% current efficiency (1, 2, 3, 6>, ], 8, 9). Research in this laboratory has shown that immobilization of A onto the photoanode surface may be an approach to stabilization of the photoanode when the desired chemistry is photooxidation of a solution species B, where oxidation of B is not able to directly compete with the anodic decomposition of the "naked" (non-derivatized) photoanode (10, 11, 12). Photoanodes derivatized with a redox reagent A can effect oxidation of solution species B according to the sequence represented by equations (1) - (3) (10-15). 

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. 

The results of this kinetic analysis have been included in Table I. It can be seen that, if both the anodic decomposition of the semiconductor and the anodic oxidation of the competing reactant would occur by irreversible hole-capture steps ((L)(H)(I) or (M)(H)(1)), as was hitherto generally accepted, the stabilization should be independent of light intensity, in contradiction with the results described above. The mechanism in which the reducing agent reacts by donating an electron to a localized surface hole ((L)(X)) leads to an expression in which s is a function of the variable (y/j) only. The three other mechanisms considered lead to the relationship of the type (18), in which s is a function of (y2/j). 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

Figure 8 shows (C1") as a function of ip. The ability of chloride ions to suppress the anodic decomposition of the electrode... 

The anodic oxidation of iodide, bromide and chloride ions at illuminated ZnO electrodes, which occurs in competition with the anodic decomposition of the electrode itself, was studied as functions of halide ion concentration, illumination intensity and solution pH in order to investigate factors which affect the degree of competition. The reactivity of halide ions, obtained under fixed conditions, was in the order of I >Br >Cl, reflecting the importance of the redox potential in determining the reactivity. 

The above FIA systems are based on monitoring the anodic decomposition of hydrogen peroxide at a platinum electrode set at 600-700 mV vs a Ag/AgCl electrode. However, at such high potentials other electroactive species, notably ascorbic acid, uric acid and hypoxanthine, will also be oxidised unless appropriate sample pre-treatment is taken. 

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