Cathodes hydrogen overvoltage

In an alkali-chlorine cell a saturated (about 6 N) solution of sodium chloride is electrolyzed at ordinary temperatures, between a steel cathode (hydrogen overvoltage 0.2) and a graphite anode (oxygen overvoltage 0.6 volt chlorine overvoltage negligible). The nature of the electrode process. Explained ... 

The presence of certain alloying elements in titanium that decreases the cathodic (hydrogen) overvoltage, thereby depolarizing the cathodic process. 

A conductor with a hydrogen overvoltage exceeding that of smooth platinum can be used for the cathode. 

Mercury Cells. The cathode material ia mercury cells, mercury [7439-97-6] Hg, has a high hydrogen overvoltage. Hydrogen evolution is suppressed and sodium ion reduction produces sodium amalgam [11110-32-4J, HgNa. 

Figure 21-6 shows the possibility of reducing the overvoltage of cathodic hydrogen evolution. One can also reduce restrictions in the O2 reduction hy using copper in lead alloys. Such alloying elements can be very effective because they... 

It has been suggested that the role of nickel (as NiAlj) is to provide sites of low hydrogen overvoltage, where cathodically liberated hydrogen may be liberated without disrupting the protective oxide . The distribution of such sites is apparently critical however, since high corrosion resistance is associated with a fine dispersion of the second phase, while the electronic conductivity of the film is probably also important . 

Cathodic inhibitors are passivating inhibitors they raise the hydrogen overvoltage, thus impeding the reduction of hydrogen ions at the cathode. They are suitable for inhibiting sulfuric or phosphoric acids. Examples are benzotriazole (at low concentrations) and alkylamines. 

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be reahzed at many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by severaf orders of magnitude between metals. In one of the first theories of hydrogen evofution, the recombination theory of hydrogen overvoltage, the rate of this reaction was finked directfy to the rate of the catalytic... 

Pressure at elevated pressures, the value of the hydrogen overvoltage changes only very slightly but at lower pressures, it increases sharply on certain metals, as for example in the case of copper, nickel, and mercury cathodes. 

Cathode material the hydrogen overvoltage value varies from one cathode material to another. It can be quite high at certain electrodes made of lead and tin. 

Cathode surface the hydrogen overvoltage is smaller on rough unpolished surfaces than on smooth, shining, polished surfaces. 

Krishtalik, L. I., Hydrogen overvoltage and adsorption phenomena, Part III, Effect of the adsorption energy of hydrogen on overvoltage and the mechanism of the cathodic process, AE, 7, 283 (1970). 

The electrodes are the typical and most important components of an electrochemical cell - especially the working electrode - which usually decide about the success of an electroorganic synthesis. Electrode materials need a sufficient electronic conductivity and corrosion stability as well as, ideally, a selective electrocat-alytic activity which favors the desired reaction. The overvoltages for undesired reactions should be high, for example, for the decomposition of the solvent water by anodic oxygen or cathodic hydrogen evolution. But, additionally, the behavior of electrodes can show unexpected and incomprehensible effects, which will cause difficulties to attain reproducible results. 

Cathode Nickel may be an alternative for platinum metals in alkaline solutions due to its low hydrogen overvoltage and catalytic activity. The activity is especially high at the very fine dispersed Raney nickel , which is available from a layer of a nickel alloy on the cathode surface by dissolving the alloy metal (aluminum or zinc) in alkaline solution prior to use (e.g. [23, 24]. Raney nickel usually is not stable against oxygen and self-ignition in air may be possible). 

Cathode Iron is a very cheap cathode material with a relatively low hydrogen overvoltage (e.g. [25]). It is of interest for industrial applications. In order to avoid corrosion during interruption of the current, stainless steel may be suitable, especially in laboratory cells where the increased electrical resistance and hydrogen overvoltage are irrelevant. 

Cathode The hydrogen overvoltage of carbon is relatively high. Thus, if the cathode is the working electrode, a lot of reduction reactions is enabled. On the other hand, the catalytic activity for hydrogenation reactions is low. This is advantageous if the cathode is the counter electrode cathodic side-reactions are avoided besides hydrogen evolution. 

Cathodic surfaces of finely divided platinum, palladium and nickel have a low hydrogen overvoltage and the dominant electrochemical reaction is the generation of a layer of hydrogen atoms. The electrocatalytic hydrogenation of aldehydes and ketones can be achieved at these surfaces. Cathodes of platinum or palladium black operate in both acid solution [203] and in methanol containing sodium methoxide [204], The carbonyl compound is converted to the alcohol. Reduction of 4-tert-butylcyclohexanone is not stereoselective, however, 1,2-diphenylpropan-l-one is converted to the / reo-alcohol. 

The Electrodeposition of Metals.—If hydrogen were evolved at a cathode at its reversible potential under all conditions no metal higher in the electromotive series than hydrogen could be deposited electro-lytically from an aqueous solution. Due to hydrogen overvoltage, however, it is possible to obtain deposits of these metals as high in the... 

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