Electrode decomposition

The charge carriers may reduce or oxidize the semiconductor itself leading to decomposition. This poses a serious problem for practical photoelectrochemical devices. Absolute thermodynamic stability can be achieved if the redox potential of oxidative decomposition reaction lies below the valence band and the redox potential of the reductive decomposition reaction lies above the conduction band. In most cases, usually one or both redox potentials lie within the bandgap. Then the stability depends on the competition between thermodynamically possible reactions. When the redox potentials of electrode decomposition reactions are thermodynamically more favored than electrolyte redox reactions, the result is electrode instability, for example, ZnO, Cu20, and CdS in an aqueous electrolyte. 

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. 

While it is related to and sometimes dependent upon chemisorption effects, one pathway for electrode decomposition can be looked upon as a separate type of charge transfer phenomenon. Here, charge transfer generally occurs between species in the semiconductor near the surface and not across the interface to species in the electrolyte. This charge transfer can be highly... 

All these results can be explained in terms of the model proposed above (cf. Fig. 11). Namely, with ferrous oxalate having a standard redox potential E° (Ox/R) of —0.2 V (SCE), which is a little more negative than the E of the surface trapped hole located ca. 0.5 V above E , the surface trapped hole is effectively quenched by the rapid reduction, and the photoanodic current flows without decomposition. With ferrocyanide, having an E(0x/R) of 0.2 V (SCE), which is more positive than the E of the surface trapped hole, the surface trapped holes are accumulated to the extent that the surface potential created will level it down to the E(0x/R) of the redox couple. At this point, the rates of nu-cleophillic attack of H2O and OH to the surface trapped holes are still low and the electrode decomposition is prevented. 

To prevent photocorrosion and improve the cell efficiency one has to selectively increase rate of the "useful" electrochemical reaction in the redox couple at the semiconductor electrode as compared to side reactions (including the electrode decomposition, and current carriers recombination) competing for the photogenerated carriers. To achieve this aim, several methods have been worked out ... 

GC analysis is usually used for the detection of H2 and O2, as well as other gases that originate from undesired reactions or electrode decomposition. GC equipment consists of a gas injection component and a gas separation component with suitable column(s), followed by detector(s). A schematic image is shown in Fig. 9.2. 

As documented in numerous studies on device degradation behavior both the organic layers [129,130] and the low work function metal electrodes [131—134] were found to be prone to degradation upon exposure to oxygen or water, leading to the introduction of chemical defects into the conjugated backbone or metal electrode decomposition and ultimately delamination. 

In cases where the redox potentials of the electrode decomposition reactions are more thermodynamically favoured than the electrolyte redox reactions (oxidative decomposition potential more negative, reductive decomposition potential more positive, than the corresponding electrolyte redox reactions), the products of the electrolyte redox reactions have sufficient potential to drive the electrode decomposition reactions. Hence, this situation usually results in electrode instability, assuming that the electrode decomposition reaction is not kinetically inhibited. This is the case with ZnO, Cu20, and CdS in simple aqueous electrolytes, and these semiconductors are indeed unstable under these conditions. 

The reaction heat generation when the LIB is undergoing thermal runaway, mainly including solid electrolyte interface (SEI) decomposition, electrode reaction with electrolyte, and electrode decomposition. For the individual reaction, the heat generation can be expressed as Eq. (9) [7],... 

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

The cell is the basis of all electrolysis. The anode admits current into the electrolyte and the cathode serves as a means of exit for the electrical current. The electrical flow provides a definition for electrolysis the flow of current from the anode through the electrolyte and out of the cell through the cathode with ensuing decomposition of the electrolyte, with products being formed at the electrodes. 

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. 

Peroxodisulfuric acid, H2S2O8, is a colourless solid mp 65° (with decomposition). The acid is soluble in water in all proportions and its most important salts, (NH4)2S208 and K2S2O8, are also freely soluble. These salts are, in fact, easier to prepare than the acid and both are made on an industrial scale by anodic oxidation of the corresponding sulfates under carefully controlled conditions (high current density, T < 30°, bright Pt electrodes, protected cathode). The structure of the peroxo-disulfate ion [now preferably called hexaoxo-/r-peroxodisulfate(2-)]0 l is OaSOOSOa " with... 

Germanium In situ STM studies on Ge electrodeposition on gold from an ionic liquid have quite recently been started at our institute [59, 60]. In these studies we used dry [BMIM][PF<3] as a solvent and dissolved Gel4 at estimated concentrations of 0.1-1 mmol 1 the substrate being Au(lll). This ionic liquid has, in its dry state, an electrochemical window of a little more than 4 V on gold, and the bulk deposition of Ge started several hundreds of mV positive from the solvent decomposition. Furthermore, distinct underpotential phenomena were observed. Some insight into the nanoscale processes at the electrode surface is given in Section 6.2.2.3. 

Decomposition potential. If a small potential of, say, 0.5 volt is applied to two smooth platinum electrodes immersed in a solution of 1M sulphuric add, then an ammeter placed in the circuit will at first show that an appreciable current is flowing, but its strength decreases rapidly, and after a short time it becomes virtually equal to zero. If the applied potential is gradually increased, there is a slight increase in the current until, when the applied potential reaches a certain value, the current suddenly increases rapidly with increase in the e.m.f. It will be observed, in general, that at the point at which there is a sudden increase in... 

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 ... 

For the electrolysis of a solution to be maintained, the potential applied to the electrodes of the cell (Eapp ) must overcome the decomposition potential of the electrolyte (ED) (which as shown above includes the back e.m.f. and also any overpotential effects), as well as the electrical resistance of the solution. Thus, Eapp must be equal to or greater than (ED + IR), where / is the electrolysis current, and R the cell resistance. As electrolysis proceeds, the concentration of the cation which is being deposited decreases, and consequently the cathode potential changes. 

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

It must be emphasised that in evaluating the limiting cathode potential to be applied in the separation of two given metals, simple calculation of the equilbrium potentials from the Nernst Equation is insufficient due account must be taken of any overpotential effects. If we carry out, for each metal, the procedure described in Section 12.2 for determination of decomposition potentials, but include a reference electrode (calomel electrode) in the circuit, then we can ascertain the value of the cathode potential for each current setting and plot the current-potential curves. Schematic current-cathode potential... 

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