Hydrogen overpotentials

The mercury cell operates efficiently because of the higher overpotential of hydrogen on mercury to achieve the preferential formation of sodium amalgam. Certain trace elements, such as vanadium, can lower the hydrogen overpotential, however, resulting in the release of hydrogen in potentially dangerous amounts. 

As is well known, high-purity zinc corrodes much less rapidly in dilute acids than commercial purity material in the latter instance, impurities (particularly copper and iron) are exposed on the surface of the zinc to give local cathodes with low hydrogen overpotentials this result is of practical significance only in the use of zinc for sacrificial anodes in cathodic protection or for anodes in dry cells. In neutral environments, where the cathodic... 

The stability of tin over the middle pH range (approximately 3-5-9), its solubility in acids or alkalis (modified by the high hydrogen overpotential), and the formation of complex ions are the basis of its general corrosion behaviour. Other properties which have influenced the selection of tin for particular purposes are the non-toxicity of tin salts and the absence of catalytic promotion of oxidation processes that may cause changes in oils or other neutral media affecting their quality or producing corrosive acids. 

Hydrogen Overpotential (Overvoltage) the displacement of the equilibrium (or steady-state) electrode potential of a cathode required for the discharge of hydrogen ions at a given rate per unit area of electrode. 

This technique is applied to mixtures of metal ions in an acidic solution for the purpose of electroseparation only the metal ions with a standard reduction potential above that of hydrogen are reduced to the free metal with deposition on the cathode, and the end of the reduction appears from the continued evolution of hydrogen as long as the solution remains acidic. Considering the choice of the cathode material and the nature of its surface, it must be realized that the method is disturbed if a hydrogen overpotential occurs in that event no hydrogen is evolved and as a consequence metal ions with a standard reduction potential below that of hydrogen will still be reduced a classic example is the electrodeposition of Zn at an Hg electrode in an acidic solution. 

Termination of the plateau at a sufficiently high overpotential. The potential at which a consecutive electrode reaction sets in (e.g., hydrogen evolution in cathodic reactions) is determined by the composition of the electrolyte (specifically, the pH) and by the nature and state of the electrode surface (hydrogen overpotential). The reduction of ferricyanide in alkaline solution on nickel also provides a better-defined plateau in this respect than the deposition of copper in acid solution. 

The competition by hydrogen evolution in the C02 reduction reaction has been minimised in two main ways (i) in aqueous solutions by employing metals with large hydrogen overpotentials (e.g. Pb, Hg, etc.) as cathodes, and (ii) by employing aprotic solvents. In this section, we will consider the latter approach, with particular respect to the reduction of C02 at Pt and Au. 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

Zinc is a nontoxic, relatively inexpensive, and abundant material. It is the most electropositive metal which is fully compatible with aqueous electrolytes. Its low (negative) electrode potential and its high hydrogen overpotential make it a very suitable negative electrode material for use in aqueous electrolytes. ... 

Since their discovery in 1866, it has been known that sulphoxides are reducible by zinc and acid to the conesponding sulphide [63], fhe equivalent electrochemical process cannot be characterised because sulphoxides also decrease the hydrogen overpotential [64], Dialkyl sulphoxides are not reduced in absence of protons and dimethyl sulphoxide is used as a solvent for electrochemical reduction. Phenyl methyl sulphoxide gives a single two-electron wave on polarography in both ethanol (E./, = -2.17 V vs. see) and dimethylformamide (E./, = -2.32 V vs. see), forming phenyl methyl sulphide [65],... 

Esaki H, Namhu T, Morinaga M, Udaka M, Kawasaki K (1996) Development of low hydrogen overpotential electrodes utilizing metal ultra-fine particles. Int J Hydrogen Energy 21 877-881... 

The similarity of the reduction potentials for H+ to hydrogen, and for C02 to various products, explains the issue of competing hydrogen evolution in aqueous solution. Thus, there has been much interest in high hydrogen overpotential metals that provide slow kinetics for H+ reduction. 

Similarly, the kinetics and mechanism of C02 reduction to formic acid has been studied by Vassiliev et al. over a wide range of metal electrodes with high or moderate hydrogen overpotentials (e.g., Cu, Sn, In, Sb, Bi, In, Zn, Pb) [48]. The best... 

As with other metals with a high hydrogen overpotential (e.g., In, Pb, Hg, Cd), tin shows a very good selectivity and, under suitable conditions, produces formate in very high yields. Thus, Li and Oloman [88] have investigated the development of a continuous reactor at atmospheric pressure with a cathode constructed from a copper net onto which tin had been electrodeposited. Experiments with this three-dimensional cathode on the influence of process variables on yields and selectivity towards formate provided promising results, such that the system was scaled-up and subsequently monitored initially in a small pilot plant [89],... 

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