The Cathode Half-Cell Reaction

If the pH of the electrolyte and the pressure of hydrogen are constant, the chemical potential of protons and hydrogen molecules are also constant. This means that the chemical potential (the Fermi level) of the electrons is constant, which defines a reference for the potential. When a bias is applied, the potentials are hereafter measured relative to this reversible hydrogen electrode reference, which evidentially is independent of the electrode material. 

The work function, however, influences the amount of transferred charge when the Fermi levels are aligned. This in turn determines the surface charge and the field over the narrow electrochemical interface. In general, it has to be tested to what degree the surface chemistry is dependent on the field. For the intermediates involved in water electrolysis, the adsorption energies are only very weakly influenced by the field, and normally, this effect can be neglected [6], 

At the cathode where the hydrogen is produced, one can think of three different reaction steps, Volmer, Tafel, and Herovsky  

Hydrogen evolution has been studied intensively using DFT [9, 10, 11], and new catalyst materials have been suggested [1, 12], However, the hydrogen evolution catalysis is very efficient on many noble metal surfaces, and in most cases, it is not the surface catalysis that limits the cathode reaction. 

From a catalysis point of view, the hydrogen evolution reaction can in principle be catalyzed perfectly, and in reality, it is seen that there is almost no overpotential on 

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The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

Entropy change in the cathodic half-cell reaction. 108... 

The Ca-Li alloy beads also react with dissolved CaCr04, forming a coating of Ca5(Cr04)3Cl. This Cr(V) compound is the same species that is formed in the cathodic half-cell reaction ... 

Anaerobic sites. Hydrogen evolution is a prime candidate for the cathodic half-cell reaction under anaerobic conditions. Corrosion rates therefore tend to increase with decreasing pH (increasing acidity levels). In the case of ground water saturated with calcium and carbonate, the corrosion product is mainly iron (II) carbonate, a milky white precipitate. On exposure to air this white product will revert rapidly to reddish iron (III) oxides. 

It is so universally applied that it may be found in combination with metal oxide cathodes (e.g., HgO, AgO, NiOOH, Mn02), with catalytically active oxygen electrodes, and with inert cathodes using aqueous halide or ferricyanide solutions as active materials ("zinc-flow" or "redox" batteries). The cell (battery) sizes vary from small button cells for hearing aids or watches up to kilowatt-hour modules for electric vehicles (electrotraction). Primary and storage batteries exist in all categories except that of flow-batteries, where only storage types are found. Acidic, neutral, and alkaline electrolytes are used as well. The (simplified) half-cell reaction for the zinc electrode is the same in all electrolytes ... 

Figure 9.3 The lead storage battery. The key to obtaining electrical energy from a redox chemical reaction is to physically separate the two half-cell reactions so that electrons are transferred from the anode through an external circuit to the cathode. In the process, electrical work is accomplished.

Electrode reactions are inner-sphere reactions because they involve adsorption on electrode surfaces. The electrode can act as an electron source (cathode) or an electron sink (anode). A complete electrochemical cell consists of two electrode reactions. Reactants are oxidized at the anode and reduced at the cathode. Each individual reaction is called a half cell reaction. The driving force for electron transfer across an electrochemical cell is the Gibbs free energy difference between the two half cell reactions. The Gibbs free energy difference is defined below in terms of electrode potential,... 

For the purposes of review. Figure 1 illustrates the basic function of the cathode in a solid oxide fuel cell. Whether acting alone or as part of a stack of cells, each cell consist of a free-standing or supported membrane of an oxygen-ion-conducting electrolyte, often yttria-stabilized zirconia (YSZ). Oxygen, which is fed (usually as air) to one side of the membrane, is reduced by the cathode to oxygen ions via the overall half-cell reaction... 

We suggest that these results arise from adsorption effects which are the cathodic complements to the anodic phenomena outlined above. The electrolysis half-cell reaction at the photocathode ... 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-cell reactions which control the rate of corrosion. 

D The anode receives electrons from the oxidation half-reaction (choice A) and the circuit conducts electron flow (choice C) to the cathode which supplies electrons for the reduction half-reaction. This flow of electrons from the anode to the cathode is relieved by a flow of ions through the salt bridge from the cathode to the anode (answer D). The salt bridge relieves the buildup of positive charge in the cathode half-cell (choice B is incorrect). 

Therefore for a given reaction to take place, the cell potential must be positive. The cell potential is taken as the difference between the two half-cell reactions, the one at the cathode minus the one at the anode. The half-cell potential exists because of the difference in the neutral state compared to the oxidized state, such as Fe/Fe + or, at the cathode, the difference between the neutral state and the reduced state, as in These reduction-oxidation (redox) potentials are measured relative to a standard half-cell potential. The chart shown in Table 2 lists potentials relative to the which is set as zero. 

A carbon (graphite) rod in the center of the dry cell serves as the cathode, but the reduction half-cell reaction takes place in the paste. An electrode made of a material that does not participate in the redox reaction is called an inactive electrode. The carbon rod in this type of dry cell is an inactive cathode. (Contrast this with the zinc case, which is an active anode because the zinc is oxidized.) The reduction half-cell reaction for this dry cell follows. 

Each electrode reaction, anode and cathode, or half-cell reaction has an associated energy level or electrical potential (volts) associated with it. Values of the standard equilibrium electrode reduction potentials E° at unit activity and 25°C may be obtained from the literature (de Bethune and Swendeman Loud, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, 1964). The overall electrochemical cell equilibrium potential either can be obtained from AG values or is equal to the cathode half-cell potential minus the anode half-cell potential, as shown above. 

Knowing that a given combination of anode and cathode half-cell reactions will proceed sponta-neonsly does not ensure that the electrode reaction rates will be sufficiently high for practical applications. Reaction kinetics at the anode and cathode and mass transfer of reactants/products to/from the electrodes may play important roles in an electrochemical cell and may influence the choice of cell design and operating conditions. These important points will be addressed later in this chapter. 

Interpretation of an experimentally determined polarization curve, including an understanding of the information derivable therefrom, is based on the form of the polarization curve, which results from the polarization curves for the individual anodic and cathodic half-cell reactions occurring on the metal surface. These individual polarization curves, assuming Tafel behavior in all cases, are shown in Fig. 6.2 (dashed curves) with Ecorr and the corrosion current, Icorr, identified. It is assumed that over the potential range of concern, the Iox x and Ired M contributions to the sum-anodic and sum-cathodic curves are negligible consequently, Uox = Iox M and Ured = Ired x. At any potential of the... 

The lead battery is used primarily in cars where they deliver current to the start engine. The reason for this widely use through many years is that lead batteries work well with good performance at typical outdoor temperatures. The anode in the lead battery is the lead electrode while the cathode typically consists of a lead electrode covered with lead oxide. Both electrodes are placed in an electrolytic solution of sulphuric acid. The following half cell reaction takes place at the anode ... 

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