Hydrogen evolution overpotential

Hydrogen overvoltage — This expression is sometimes used as a synonym of hydrogen overpotential, hydrogen evolution reaction, however, this practice is discouraged. 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

It is recognized, that the overall process consists of the catalysis of reaction 1, an intermediate step being the reduction of water. Reaction 4 can be treated as hydrogen evolution on a compact silver electrode at negative potential. Silver has a small overpotential for evolution Of the two steps of which reaction 4 is composed... 

In the presence of iodide ions, the overpotential at a mercury electrode decreases, although the adsorption of iodide is minimal in the potential region corresponding to hydrogen evolution. The adsorption of iodide... 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

Therefore, criteria in the selection of an electrode reaction for mass-transfer studies are (1) sufficient difference between the standard electrode potential of the reaction that serves as a source or sink for mass transport and that of the succeeding reaction (e.g., hydrogen evolution following copper deposition in acidified solution), and (2) a sufficiently low surface overpotential and rate of increase of surface overpotential with current density, so that, as the current is increased, the potential will not reach the level required by the succeeding electrode process (e.g., H2 evolution) before the development of the limiting-current plateau is complete. 

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. 

Schuldiner (1959) studied the effect of H2 pressure on the hydrogen evolution reaction at bright (polished) Pt in sulphuric acid. The mechanism of the reaction was assumed to be as in equations (3.3) and (3.4). The step represented by equation (3.3) was assumed to be at equilibrium at all potentials and equation (3.4) represented the rate-determining step. The potentials were measured as overpotentials with respect to the hydrogen potential, i.e. the potential of the H +/H2 couple in the solution (0 V vs. RHE). 

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. 

One of the questions in the mechanism of hydrogen evolution, and ancillary topics connected with the surface hydrogen involved in corrosion, is the dependence of the surface hydrogen upon overpotential. This topic has importance not only because of the... 

Stray current dumpers (SCD) with proprietary coating applied in interlayers. The system and the coating composition is designed to promote hydrogen evolution at low overpotentials and to protect the titanium metal phase. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

Mercury, lead, cadmium and graphite are commonly used cathode materials showing large overpotentials for hydrogen evolution in aqueous solution. Liquid mercury exhibits a clean surface and is very convenient for small-scale laboratory use. Sheet lead has to be degreased and the surface can be activated in an electrochemical oxidation, reduction cycle [3, 22], Cadmium surfaces are conveniently prepared by plating from aqueous cadmium(ii) solutions on a steel cathode. 

Platinum and carbon are frequently used as counter electrode materials for both anode and cathode. Platinum is resistant to corrosion while carbon is cheap and can be discarded after use. Nickel is a suitable counter cathode material in aqueous solution because of the low overpotential for hydrogen evolution. Titanium coated with platinum and then over coated with mthenium dioxide is a stable counter anode material with a low overpotential for oxygen evolution. 

Jafarian et al. [130] have studied adsorption and absorption of hydrogen species on Ag electrodes in alkaline solution. Gurten etal. [131] have investigated the effect of alcohols MeOH, EtOH, and PrOH on hydrogen evolution reaction on Ag electrode in contact with 0.01 M NaCl solution. It was observed that the presence of alcohols increased the hydrogen yield and decreased anodic overpotential (for the Pt anode). 

---