Electrochemical potential scale

Eq. (8)] represents by definition the zero point of the electrochemical potential scale (standard hydrogen electrode, often denoted SHE). 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

Fig. 2.1 The electrochemical potential scale and electrode potential conversion at 25°C.

Typically, the reference level for the solution redox potential is chosen to be the normal hydrogen electrode (NHE). Some tabnlations nse the saturated calomel electrode (SCE) as the reference level with the difference between these two scales well-known to be NHE = —0.2412 V versus SCE. The fundamental problem lies in the determination of the absolnte energy of the NHE relative to vacuum. Although a method to determine directly the absolute electrochemical potential of an NHE has not yet been described, a recent indirect measnrement has indicated that it is approximately 4.4 eV below the vacnum level. This value is often used to relate the solution electrochemical potential scale to the solid electrochemical potential scale. It provides the best approximation that is presently available to calculate the... 

For a doped semiconductor, the Fermi level position will be shifted from mid-gap, because the doping process will vary the tendency of the solid to either gain or lose electrons. For example, if donors are added to an intrinsic semicondnctor, the material will be more likely to lose electrons. The Fermi level of an n-type semiconductor will thus move closer to the vacuum level (i.e. will become more negative on the electrochemical potential scale) (Figure 9(b)). Similarly, if acceptors are added to an intrinsic material, the Fermi level will become more positive, because this phase will now have an increased tendency to accept electrons from another phase (Figure 9(c)). 

Figure 23. Schematic representation of the cathodic and anodic partial current densities at a metal electrode in contact with a simple redox system as a function of the over potential U - The bars on the vertical axis indicate one decade of current, the bars on the horizontal axis indicate 0.1 V (or 0.1 eV for the electrochemical potential scale).

Any cell reaction can be considered to be an electron transfer between two coupled half-cells. The measured potential corresponds to the difference of the electron energy. The arbitrary definition of a reference electrode raises the question of whether the electrochemical potential scale can be correlated with energy scales of electrons in surface physics. If measuring work functions or electron affinities, the reference value is the free electron in vacuum. Mehl and Lohmann calculated for the electron affinity of a hydrogen electrode —4.5 eV using the following Bom-Haber process... 

Figure 9.8 Band structure of some semiconductors on the electrochemical potential scale.

If the flat band potential U-, is known, one can relate the energy terms E of the semiconductor to the energy scale U of an electrochemical cell versus a suitable reference electrode. In solid state physics, the reference state of electron energies is usually the vacuum level, if one wants to compare different solids with each other. The electrochemical potential scale versus a reference electrode is fully in parallel with this absolute scale having only the opposite sign because of the negative charge of the electron. 

It is worth mentioning in this context that generally single potentials of electrochemical electrodes cannot be measured. Only the difference of two electrode potentials is measurable. For this reason, the potential of the standard hydrogen electrode (SHE) was defined as origin of the electrochemical potential scale, and all electrode potentials are referred to this electrode... 

The potential of that electrode was dehned as the origin of the electrochemical potential scale and was arbitrarily declared to be zero for all temperatures. Because of their complicated handling and some other disadvantages, the standard hydrogen electrode today is used only for very precise measurements, especially for the calibration and testing of pH buffer solutions. It is not trivial to realize exactly the experimental conditions that are necessary. Instead of the standard hydrogen electrode (SHE) more commonly the normal hydrogen electrode (NHE) is used, which contains an acid solution of the concentration 1 mol/L. 

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