Normal hydrogen scale

The dissolution reaction is Pt - Pt2+ + 2e and the value of its reversible thermodynamic potential is 1.2 V on the normal hydrogen scale. The evolution of O2 in acid solution at a current density of, say, 100 mA cm, needs an overpotential on platinum of nearly 1.0 V, i.e., the electrode potential would be >2.0 V. It follows feat at these very anodic potentials platinum would tend to dissolve, although its dissolution would be slowed down by fee fact feat it forms an oxide film at fee potentials concerned. Nevertheless, fee facts stated show feat fee alleged stability of Pt may be more limited than is often thought. This is an important practical conclusion because dissolved Pt from an anode may deposit on fee cathode of fee cell, and instead of having fee surface one started wife as fee cathode, it becomes in fact what is on its surface, platinum. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

In the reduced state, poly-3-methyl thiophene is an intrinsic semiconductor with an energy gap of 1.95 eV. (a) What would be the value in electron volts of its Fermi level (b) Show that the concentration of electrons in the conduction band would be negligible. However, from 0.45 to 1.1V on the normal hydrogen scale, this polymer shows metallic conductivity, (c) What change in electronic structure has occurred ... 

Corrosion occurs wherever there is a metal, the thermodynamic potential of which (on the normal hydrogen scale) is negative to that of some partner reaction. This chapter contains many graphic (and some unexpected) examples of practical corrosion, e.g., drops of moisture condensing on a metal surface tend to stimulate corrosion in the metal underneath the drop. 

There are some six pathways for C02 reduction. At pH 7 and on the normal hydrogen scale of potential, these reactions are as follows ... 

Langmuir,2 in considering the importance of contact potentials for electrolytic cells, pointed out in 1916 that there is a general parallelism between the thermionic work function and the standard electrode potentials. This is shown in Table XVI, where the last column gives the difference between the electrode potential, on the normal hydrogen scale, and the work function. This difference varies much less than the values of either the work function, or the electrode potentials, separately. 

At the beginning of this chapter it was pointed out that aqueous and many nonaqueous electrochemical systems suffer from the small size of the potential range in which solutes dissolved in them can be examined. This is because (for pH = 0, say) if the potential of an electrode immersed therein is more negative than 0.00 V on the normal hydrogen scale (see Section 4.8.3), the water itself in the solution begins to decompose to form H2. On the other hand, at a potential more positive than 1.23 V on the same scale, the aqueous solvent tends to decompose to form Oj. 

Apart from platinum s intermediate nature on bonding, another point in platinum s favor is availability platinum can be purchased in various suitable forms at a reasonable price some noble metals are difficult to find and purchase. The word noble means here stable and of course that is a first point one wants in an electrocatalyst. It must be a catalyst, not enter into the reaction. It is meant to accelerate the reaction. It must itself be stable, thermally and electrochemically. On the last point, platinum is only fairly good because oxide-free platinum does start itself to dissolve around 1.0 V on the normal hydrogen scale. By using it in anodic reactions in a potential range anodic to 1.0 V, Pt(II) is likely to get into the solution and may be deposited on the cathode. 

Commercially pure nickel contains at least 99% Ni (Table 2-1). Nickel is more noble than iron but less noble than copper (Pourbaix, 1974). Thermodynamically (Pourbaix diagram), nickel is shown to dissolve as Ni " at pH <9, to form nickel (II) oxide (NiO) at pH 9-12, and to dissolve as HNiOj at pH >12. Nickel (II) oxide is shown to be stable approximately between -0.5 and -1-0.3 V on the normal hydrogen scale (NHE). In practice it is found that nickel has good corrosion resistance in environments such as cold and hot aqueous caustic solutions. Nickel is also highly tolerant of... 

In these reactions, (2) is the process taking place at the reference electrode which therefore determines the potential scale. In practice other reference electrodes, such as the saturated calomel electrode are frequently used but the data are normally expressed on the hydrogen scale. 

The Vacuum Reference The first reference in the double-reference method enables the surface potential of the metal slab to be related to the vacuum scale. This relationship is determined by calculating the workfunction of the model metal/water/adsorbate interface, including a few layers of water molecules. The workfunction, — < ermi. is then used to calibrate the system Fermi level to an electrochemical reference electrode. It is convenient to choose the normal hydrogen electrode (NHE), as it has been experimentally and theoretically determined that the NHE potential is —4.8 V with respect to the free electron in a vacuum [Wagner, 1993]. We therefore apply the relationship... 

Figure 12.2 Position and width of energy bands of several illuminated semiconductors, with respect to the electrochemical scale (NHE = normal hydrogen electrode).

ECb. Evb. Ef. ancl Eg are, respectively, the energies of the conduction band, of the valence band, of the Fermi level, and of the band gap. R and O stand for the reduced and oxidized species, respectively, of a redox couple in the electrolyte. Note, that the redox system is characterized by its standard potential referred to the normal hydrogen electrode (NHE) as a reference point, E°(nhe) (V) (right scale in Fig. 10.6a), while for solids the vacuum level is commonly used as a reference point, E(vac) (eV) (left scale in Fig. 10.6a). Note, that the energy and the potential-scale differ by the Faraday constant, F, E(vac) = F x E°(nhe). where F = 96 484.56 C/mol = 1.60219 10"19 C per electron, which is by definition 1e. The values of the two scales differ by about 4.5 eV, i.e., E(vac) = eE°(NHE) -4-5 eV, which corresponds to the energy required to bring an electron from the hydrogen electrode to the vacuum level. 

A scale showing the potential values of the most commonly used reference electrodes is given in Figure 2. The potentials are quoted both with respect to the normal hydrogen electrode and the saturated calomel electrode. 

The relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36 ... 

Fig. 3.5 Band position of anatase Ti02, bandgap = 3.2 eV, in the presence of a pH = 1 aqueous electrolyte. The energy scale is indicated in electron volts (eV) using either normal hydrogen electrode (NHE) or vacuum level as reference showing the condition for water splitting.

Fig. 1-7.—Curves showing the energy of interaction of two normal hydrogen atoms. The scale for the internuclear distance Tab is based on the unit a0 = 0.530 A.

The potentials are referred to the normal hydrogen electrode (NHE). The energy levels for the oxidation and reduction of water at pH 7 are shown by horizontal lines. Energy scale in volts. 

The hydrogen electrode. The hydrogen electrode is discussed first because it is the primary reference electrode used to define an internationally accepted scale of standard potentials in aqueous solution. By convention, the potential of an electrode half-reaction that is measured with respect to the normal hydrogen electrode (NHE also written as SHE, standard hydrogen electrode) is defined as the electrode potential of the half reaction. This convention amounts to an arbitrary assignment for the standard potential of the hydrogen electrode as zero at all temperatures. Thus, there is in effect a separate scale of electrode potentials at each temperature level. 

In electrochemistry we have customarily employed, instead of the absolute electrode potential / abs scale, a relative scale of the electrode potential, E yila scale, referred to the standard or normal hydrogen electrode potential E m at which the hydrogen electrode reaction, 2H + 2e dox = H2(gas), is at equilibrium in the standard state unit activity of the hydrated proton, the standard pressure of 101.3 kPa for hydrogen gas, and room temperature of 298 K. Since Eniie is + 4.44 V (or + 4.5 V) in the absolute electrode potential scale, we obtain Eq. 9.9 for the relation between abs scile and [Refs. 4 and 5.] ... 

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