Cathodes oxygen overvoltage

Since the oxygen overvoltage on a platinum electrode is equal to 0.45 V, the minimum potential required for oxygen evolution is (+ 0.813 + 0.45) V or 1.263 V. Since the voltage required to implement this reaction is lower than that for a normal chlorine electrode, it follows that oxygen will be evolved at the anode in preference to chlorine. In contrast to the two possible reactions at the anode, there are five reactions that are possible at the cathode as shown below ... 

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 ... 

To use this formula, the assumption has been made that the fuel consists of a binary mixture of hydrogen and water, while the cathodic gas is a binary mixture of oxygen and nitrogen. The diffusion coefficient for binary mixtures D y eff is estimated by the equation proposed by Hirschfelder, Bird and Spotz [12], and the Knudsen diffusion coefficient for species i is given by free molecule flow theory [11], Finally, combining Equations (6.15-6.18) the anodic and the cathodic concentration overvoltages are given by (see also Equations (A3.20) and (A3.21)) ... 

Iron is the usual material for cathodes operating at a current density between 10 to 20 A per sq. dm. Graphite or nickel is rarely used. Smooth platinum is the only suitable material for anodes as it has a sufficiently high oxygen overvoltage and so enables a high current density, of between 30 to 60 A per sq. dm, to be employed. 

Influence of C D. on Overvoltage.—Provided the conditions are such that the hydrogen is not removed by reaction with oxygen, or other oxidizing agent, or by diffusion away from the cathode, the overvoltage Cl) increases with increasing current density I in accordance with the equation... 

Also highly catalytically active Raney nickel electrodes have been developed. Their production is possible at remarkably low cost by cathodic deposition of a Ni/Zn alloy and subsequent activation by a treatment with hot K0H[6]. These electrodes are used as cathode and anode. Their oxygen overvoltage is below 200 mV and their hydrogen overvoltage less than 100 mV at current densities of 4000-6000 A m and 100°-120 C[7]. 

The addition of a small percentage of a noble metal to a base metal such as stainless steel or titanium can provide sites of low overvoltage for the cathodic reduction of dissolved oxygen or hydrogen ions. This permits larger currents and hence more positive potentials to be obtained at the anodic region, and promotes passivation under some circumstances . This effect has been demonstrated for stainless steels but has not been adopted in practice, since under other conditions the noble metal addition accelerates corrosion . 

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). 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

The coin has its reverse, however. The broadening of the potential window that is often bordered by the solvent electrochemical decomposition potentials (e.g. cathodic hydrogen evolution and anodic oxygen evolution from water) is due to an increase in reactions overvoltage. This may be caused by a diamond s lower electrochemical activity, as compared with the glassy carbon and like electrode materials. On the whole, this conclusion is corroborated by the kinetics studies on diamond electrodes... 

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