Cathodic-reactant half-cell

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,... 

The sources of polarization in PAFCs (with cathode and anode Pt loadings of 0.5 mg Pt/cm, 180°C, 1 atm, 100% H3PO4) have been discussed in Section 2 and were illustrated as half cell performances in Figure 2-3. From Figure 2-3, it is clear that the major polarization occurs at the cathode, and furthermore, the polarization is greater with air (560 mV at 300 mA/cm ) than with pure oxygen (480 mV at 300 mA/cm ) because of dilution of the reactant. The anode exhibits very low polarization (-4 mV/100 mA/cm ) on pure H2, and increases when CO is present in the fuel gas. The ohmic (iR) loss in PAFCs is also relatively small, amounting to about 12 m at 100 mA/cm. ... 

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

The coulombic efficiency of a cell is defined as Qp/Qj. It is often more useful to determine the capacity of each half-cell separately, since for operational reasons, most practical batteries do not have an equal number of equivalents of anodic and cathodic reactants. 

Discharge curves for typical cathodic half-cells are shown in Fig. 2.23. In curve (a) reactants and products are both in liquid or solid solution, while curve (b) represents the case where they form two distinct solid phases. In Fig. 2.24, a two-stage discharge takes place, where all three electroactive components form separate solid phases and the electrolytic phase remains virtually invariant, e.g. 

Fig. 2.23 Discharge curves for typical cathodic half-cells, (a) Reactants and products in solution phase, e,g, Fe3 (aq) and Fe2+(aq) (b) reactants and products both form solid phases, e,g. Ag20(s) and Ag(s)...

The shorthand for the cathode half-cell includes both reactant (Fe3+) and product (Fe2+) as well as the electrode (Pt). The two ions Fe3+(aq) and Fe2+(aq) are separated by a comma rather than a vertical line because they are in the same phase. 

We can obtain the cell half-reactions simply by reading the shorthand notation. To find the balanced equation for the cell reaction, add the two half-reactions after multiplying each by an appropriate factor so that the electrons will cancel. The shorthand notation specifies the anode (on the extreme left), the cathode (on the extreme right), and the reactants in the half-cell compartments. 

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... 

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. 

Knowledge of the parameters of the individual electrode reactions permits writing expressions for the individual oxidation or reduction curves (see the section Complete Polarization Curves for a Single Half-Cell Reaction in Chapter 3). Thus, the expression for the cathodic-reactant reduction reaction ... 

In a concentration cell, each half-cell houses the same half-reaction, but the concentrations of reactants are different. During operation, the solution in the anode half-cell spontaneously becomes more concentrated and that in the cathode half-cell becomes less concentrated until their concentrations are equal. 

Therefore, each half-cell consists of inactive electrodes immersed in an electrolyte solution that contains all the reactant species involved in that half-reaction (Figure 21.6). In the anode half-cell, 1 ions are oxidized to solid I2. The electrons that are released flow into the graphite anode, through the wire, and into the graphite cathode. From there, the electrons are consumed by Mn04 ions, which are reduced to Mn " " ions. (A KNO3 salt bridge is used.)... 

The factor 0.0592 is valid when T = 298K. A concentration cel is a voltaic cell in which the same half-reaction occurs at both the anode and cathode but with different concentrations of reactants in each half-cell. At equilibrium, Q = K and = 0. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

As stated above, the membrane acts as the proton transporting medium, is an electrical insulator, and separates the reactant gases from direct chemical reaction. On either side of this membrane are placed two electrodes. The anode at which hydrogen is consumed in the hydrogen oxidation reaction (HOR) and the cathode in which oxygen from air is consumed in the oxygen reduction reaction (ORR). The two half-cell reactions and the overall reaction are shown below. 

Oxidation-reduction (redox) reactions involve the movement of electrons from one reactant to another. The half-reaction method of balancing redox reactions separates the overall reaction into two half-reactions, which mimics the actual separation of an electrochemical cell into two half-cells. Two types of electrochemical cells are distinguished by whether they generate electrical energy (voltaic) or use it (electrolytic). In both types of cell, electrodes dip into an electrolyte solution, the oxidation half-reaction occurs at the anode, and the reduction half-reaction occurs at the cathode. (Section 21.1)... 

Since cell potential depends not only on the half-reactions occurring in the cell, but also on the concentrations of the reactants and products in those half-reactions, we can construct a voltaic cell in which both half-reactions are the same, but in which a difference in concentration drives the current flow. For example, consider the electrochemical cell shown in Figure 18.12 , in which copper is oxidized at the anode and copper ions are reduced at the cathode. The seeond part of Figure 18.12 depicts this cell under nonstandard conditions, with [Cu ] = 2.0 M in one half-cell and [Cu ] = 0.010 M in the other ... 

Describe the role of non-fVwork in electrochemical systems. Define the roles of the anode, cathode, and electrolyte in an electrochemical cell. Given shorthand notation for an electrochemical cell, identify the oxidation and reduction reactions. Use data for the standard half-cell potential for reduction reactions, E°, to calculate the standard potential of reaction E. Apply the Nernst equation to determine the potential in an electrochemical cell given a reaction and reactant concentrations. 

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