Electrode Treatment

It has already been stressed that the electrode surface should be clean and very smooth for efficient ED to occur. Even brand new electrodes can sometimes function less than optimally due to the conditions of storage once manufactured. Manufacturers quote various optimal methods for cleaning electrodes. A new HDV should be constructed after any 

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K. M. Colbow, J. J. Zhang, and D. P. Wilkinson. Electrode treatment method for improving performance in liquid feed fuel cells. US Patent 6,153,323 (2000). 

Figure 18.16 SEM pictures of PTFE surfaces (a) treated by argon RF plasma at sample positions of 10 in. (inside glow) from copper coil electrode. Treatment conditions are 2 seem argon, lOOmtorr, 7.0W RF power input, 2min treatment, (b) treated by argon LTCAT for 2 min. Treatment conditions are 1500 seem argon, 9 in. sample position, 6.0 A arc current.

Bodalbhai and Brajter-Toth [655] undertook a closer investigation of the effect of electrode treatment on small adsorbing biological molecules, such as 2,6-dia-mino-8-purinol (DAPOL), in order to .shed light on the relative importance of. . . adsorption in their electrochemical behavior. On the basis of effects such as the pH dependence of adsorption rate, they concluded that the chemistry of the surface may be important. ... 

It would not be superfluous to emphasize the significance of maintaining well-defined, similar conditions in such studies. After all, electrocatalytic research has followed catalytic investigations faithfully, even into generating a hiatus of conflicting data by use of obscure procedures and electrode treatments. It is essential that fundamental studies for unambiguous unraveling of electrocatalytic principles be conducted with electrodes of known structure and history, at several potentials and under well-controlled experimental conditions. 

Weber, K. Creager, S.E. Voltammetry of redox-active groups irreversibly adsorbed onto electrodes. Treatment using the marcus relation between rate and overpotential. Anal. Chem. 1994, 66, 3164. 

Kyriacouand, G. and A. Anagnostopoulos (1992). Electroreduction of CO2 on differently prepared copper electrodes The influence of electrode treatment on the current... 

A series of electrode treatments (e.g., blocking free-COOH groups and pinholes, washing, and inert gas spraying) were used to reduce the nonspecific adsorption of the labeled species... 

In most electrode preparation schemes, the electrode surface is treated before modification to provide reproducible surface attributes. These procedures often involve conventional electrode treatments (see Chapter 5), with adaptations or additions where needed. The pretreatment is critical the electrochemical behavior of a derivatized electrode is often sensitive to small variations in the substrate properties (e.g., crystallinity, roughness, chemical functionahty). Thus, preparation procedures must frequently be refined through trial and error to achieve the desired performance. On the other hand, the function of the electrode can often be fine-tuned by changing pretreatment variables, enhancing the ver-satihty of a modification scheme. 

Procedures for the determination of traces of polonium by deposition into platinum electrodes from 2 N nitric acid at —0.8 V (presumably vs. NHE) have been given by Abramova and Ziv (190). An earlier study by Coche (191) indicated the importance of electrode treatment in such determinations. 

After a heat treatment of several hours the electrodes are deposited by sputtering a 50 nm base layer of Ni/Cr or NiAVi followed by 1.5 pm Au-layer generated by galvanization. 

The treatment may be made more detailed by supposing that the rate-determining step is actually from species O in the OHP (at potential relative to the solution) to species R similarly located. The effect is to make fi dependent on the value of 2 and hence on any changes in the electrical double layer. This type of analysis has permitted some detailed interpretations to be made of kinetic schemes for electrode reactions and also connects that subject to the general one of this chapter. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

The ohmic drop across the electrolyte and the separator can also be calculated from Ohm s law usiag a modified expression for the resistance. When gas bubbles evolve at the electrodes they get dispersed ia and impart a heterogeneous character to the electrolyte. The resulting conductivity characteristics of the medium are different from those of a pure electrolyte. Although there is no exact description of this system, some approximate treatments are available, notably the treatment of Rousar (9), according to which the resistance of the gas—electrolyte mixture, R, is related to the resistance of the pure electrolyte, R ... 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

Platiaum and its alloys are also used as biomedical electrodes, eg, platiaum—indium wires for permanent and temporary pacemaker leads and defibrillator leads. Electrophysiology catheters, which contain platinum electrodes and marker bands, have been used to map the electrical pathways of the heart so that appropriate treatment, such as a pacemaker, can be prescribed. 

The specification requirements for electrode binder pitch, eg, high C/H ratio, high coking value, and high P-resin content, effectively ruled out pitches from gasworks or low temperature tars. The cmde tar is distilled to a medium-soft pitch residue and then hardened by heating for several hours at 385—400°C. This treatment increases the toluene-insoluble content and produces only a slight increase in the quinoline-insoluble (Ql) material, the latter by the formation of mesophase. 

An innovative technology called the "lasagna" process is based on the electrokinetic phenomenon called electro osmosis. The lasagna process was created to treat difficult wastes in low permeabiUty, sdt- and clay-laden soils (40). The lasagna process is so named because it consists of a number of layered subsurface electrodes and treatment zones. These layers can be constmcted either horizontally where contaminants are forced to more upward or in vertical position where lateral contaminant movement is desired. 

Electrode kinetics lend themselves to treatment usiag the absolute reaction rate theory or the transition state theory (36,37). In these treatments, the path followed by the reaction proceeds by a route involving an activated complex where the element determining the reaction rate, ie, the rate limiting step, is the dissociation of the activated complex. The general electrode reaction may be described as ... 

In the thermodynamic treatment of electrode potentials, the assumption was made that the reactions were reversible, which implies that the reactions occur infinitely slowly. This is never the case in practice. When a battery deUvers current, the electrode reactions depart from reversible behavior and the battery voltage decreases from its open circuit or equiUbrium voltage E. Thus the voltage during battery use or discharge E is lower than the voltage measured under open circuit or reversible conditions E by a quantity called the polari2ation Tj. 

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