Overpotential for hydrogen evolution

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. 

One of the most important reasons for the application of mercury to the construction of working electrodes is the very high overpotential for hydrogen evolution on such electrodes. Relative to a platinum electrode, the overpotential of hydrogen evolution under comparable conditions on mercury will be -0.8 to -1.0 V. It is therefore possible in neutral or (better) alkaline aqueous solutions... 

Anodic Ni oxides catalyze H2 evolution if they are not formed at too high potentials, otherwise they may depress the activity [93, 385, 449, 456]. Insulating layers are normally inefficient for hydrogen evolution [457, 458]. It is interesting to note that semiconductors can reduce the overpotential for hydrogen evolution on Hg and the effect increases as the semiconductor band gap decreases [459]. This is in line with the observation that a passivated Nb electrode is not an efficient electrocatalyst... 

For reductions, hanging mercury drop electrodes or mercuryfilm electrodes are frequently used owing to their microscopic smoothness and because of the large overpotential for hydrogen evolution characteristic for this electrode material. Mercury film electrodes are conveniently prepared by the electrochemical deposition of mercury on a platinum electrode from an acidic solution of an Hg2+ salt, e.g. the nitrate. However, the oxidation of mercury metal to mercury salts or organomercurials at a low potential, 0.3-0.4 V versus the saturated calomel electrode (SCE), prevents the use of these electrodes for oxidations. 

The DME presents special features derived from its homogeneous and isotropic drops, small size, and periodical renewed surface so that the current on each drop rises from zero to its maximum value toward the end of the drop life. Moreover, it is well known that mercury has the highest overpotential for hydrogen evolution, which enables polarization of the electrode to very negative potentials. 

The advantages of the liquid surface and large overpotential for hydrogen evolution make mercury the material of choice for cathodic processes, unless the use of mercury is specifically contraindicated by some incompatibility with the system. Incompatibility can arise from strong specific absorption, as with some sulfur-containing compounds, or in high-temperature systems such as fused salts because of the low boiling point of mercury (356.6°C). 

In the electroflotation method, the bubble size is critical to the efficiency of the phase separation and depends on the electrode metal. The bubble sizes formed on cathodes with small overpotentials for hydrogen evolution (such as Pd, W, and Ni) are larger than those formed on high... 

In Section 8.2 the basics of pulsed dectrodeposition (PED) will be described for the case of aqueous electrolytes which allow the deposition of comparatively noble metals like Cu, Ni, Pd, or Au less noble metals like Fe or Zn can still be electrodeposited from aqueous electrolytes because they exhibit a comparatively large overpotential for hydrogen evolution. However, the main limitation of aqueous dectrolytes, of course, is their narrow electrochemical window which adversely affects the electrodeposition of metals like A1 or Ta. Therefore, recently, the PED technique has been extended to ionic liquids as electrolytes. General electrochemical aspects of ionic liquids can be found in Ref. [44] here, in Section 8.3, we will only address the technical aspects with respect to PE D. Examples of nanometals and nanoalloys electrodeposited from chloroaluminate-based ionic liquids are given in... 

We have just mentioned that one reason for a limited range of potentials in a particular SSE is the reactivity of the components of the SSE toward oxidation and reduction. It is also obvious that the limiting cathodic process in protic solvents, nos 1-9 in Table 4, must be reduction of protons or the equivalent, the proton donor. The unfavourable cathodic limit for reduction of protons can, however, be vastly improved by the use of mercury as the cathode material and a tetraalkylammonium salt as SSE (nos. 1 and 3). The reason for mercury being such a favourable material is its large overpotential (see Section 10) for the reduction of protons (hydrogen evolution reaction). We have already commented (p. 24) on the fact that the reduction of protons occurs many orders of magnitude faster on certain metals than on others, and this manifests itself by the overpotential, i.e., in order to make the reaction go at a measurable rate one has to increase the electrode potential from the equilibrium potential. Table 6 shows overpotentials for hydrogen evolution and... 

In fact, carbon and graphite exhibit good electrochemical activities for oxygen reduction, high overpotential for hydrogen evolution, and low catalytic activity for hydrogen peroxide decomposition (Do and Chen 1994a, b Ponce-de-Leon and Pletcher 1995). 

Metals and alloys are usually selected as cathodes for hydrogen evolution in EF. For most cases, stainless steel is a good choice because it is cheap and readily available. Nickel is known to have low overpotential for hydrogen evolution. Therefore, usage of a nickel cathode can save energy consumption. Titanium is expensive, but it is very stable. Therefore, this metal can be selected as a cathode material in treatment of corrosive wastewaters. 

In the first part of this century, electrochemical research was mainly devoted to the mercury electrode in an aqueous electrolyte solution. A mercury electrode has a number of advantageous properties for electrochemical research its surface can be kept clean, it has a large overpotential for hydrogen evolution and both the interfacial tension and capacitance can be measured. In his famous review [1], D. C. Grahame made the firm statement that Nearly everything one desires to know about the electrical double layer is ascertainable with mercury surfaces if it is ascertainable at all. At that time, electrochemistry was a self-contained field with a natural basis in thermodynamics and chemical kinetics. Meanwhile, the development of quantum mechanics led to considerable progress in solid-state physics and, later, to the understanding of electrostatic and electrodynamic phenomena at metal and semiconductor interfaces. 

Coming back to the activation overpotential for hydrogen evolution It is quite clear that if it is diminished, an explosive chlo-rine/hydrogen mixture will be produced. This will be the case immediately, if ions of heavy metals or other metals are precipitated at the mercury surface. So the brine must be thoroughly purified, as described in Sect. 5.2.3.4.1. This holds especially if potassium- or lithium-brine is used as a feed, because the reversible potentials of these species are more negative. 

Regarding suitable cathodes for the electrocatalytic reduction of carbon dioxide to methanol, indium, and tin have good prospects because each has a high overpotential for hydrogen evolution, the chief competitor in aqueous solution for carbon dioxide reduction to methanol ... 

In most cases, additional metal ions are present, mainly those with a positive standard potential, which are deposited preferentially at the cathode and have a low overpotential for hydrogen evolution. On the other hand, zinc sponge is deposited even at relatively low current densities during electrolysis of ZnSO solutions in the presence of Cu, As or Sb ions these are nobler than zinc and are deposited at the maximum rate (i.e. at their limiting current) in a powdery form, and the zinc deposited on them "copies" their powdery structure. Powdered metals (Fe, Cu, Mi) are usually deposited at the cathode at current densities higher than the limiting one with simultaneous evolution of hydrogen. 

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