Thermodynamic cell voltage

AE is the thermodynamic cell voltage depending on the nature of the electrode reactions, rj is the total overpotential and represents the surplus of electrical energy required to drive the process at a practical rate and to overcome mass transfer resistances. AFn = IR is the ohmic drop in the interelectrode gap, the electrode... 

Energy Consumption In electrorefining, the thermodynamic cell voltage is zero. An applied voltage is only required to polarize the electrodes and overcome ohmic resistances. The typical voltage components in the copper electrorefining cell are shown in Table 6. 

As shown, this overall electrochemical reaction has a thermodynamic cell voltage ( °eu) of 1.229 V at reversible conditions. Accordingly, the standard free enei change of the overall reaction (AG°g[[ = —nFEggy, where n is the electron transfer number of the overall reaction (here n = 2), and F is the Faraday s constant, 96,745 C mol ). If the AG°g j is negative, the corresponding cell reaction should have spontaneity in standard conditions. 

The actual operating domain in the fuel cell mode is a cell voltage Vhetween zero and the thermodynamic cell voltage Vq, i.e., F > 0, or and the current density i... 

For n eleetrons, the electrical work performed is nFE, where F is Faraday s constant and E is the cell voltage. This represents the action of moving an electrical charge through an electrical potential field. Under reversible conditions, where no net current flows, the maximum electrical woric is performed (nEE°), where E° corresponds to the thermodynamic cell voltage. When maximmn work is performed, this corresponds to the change in free energy (AG) for the reaction, and consequently ... 

In an Hj-Oj fuel cell, typical cell voltage EP°) under standard conditions or under normal temperature and pressure conditions is typically 1.23 V, and the thermodynamic cell voltage E° ) is about 1.48 V. For almost all fuel cell reactions, one can expect the reaction entropy AS) to be less than zero. This means the amount of heat generated in the surrounding areas will be a product of temperature T) and reaction entropy AS). With the conversion of the chemical energy into the... 

Figure 3.12 Fuel cell voltages and current levels as a function of various parameters involved. (AS reaction entropy T temperature electrolyte resistance ff- thermal cell voltage P thermodynamic cell voltage f cell voltage under load i load current tj overvoltage at the electrodes n number of cells f configuration factor.)...

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

Table 7. Thermodynamic Decomposition Voltage of Chlor—Alkali Cells at 25°C...

The temperature dependence of the equilibrium cell voltage forms the basis for determining the thermodynamic variables AG, A//, and AS. The values of the equilibrium cell voltage A%, and the temperature coefficient dA< 00/d7 which are necessary for the calculation, can be measured exactly in experiments. 

During charging and discharging of the cell, the terminal voltage U is measured between the poles. It should also be possible to calculate directly the thermodynamic terminal voltage from the thermodynamic data of the cell reaction. This value often differs slightly from the terminal voltage measured between the poles of the cell because of an inhibited equilibrium state or side reactions. 

As the cell is discharged, Zn2+ ions are produced at the anode while Cu2+ ions are used up at the cathode. To maintain electrical neutrality, SO4- ions must migrate through the porous membrane,dd which serves to keep the two solutions from mixing. The result of this migration is a potential difference across the membrane. This junction potential works in opposition to the cell voltage E and affects the value obtained. Junction potentials are usually small, and in some cases, corrections can be made to E if the transference numbers of the ions are known as a function of concentration.ee It is difficult to accurately make these corrections, and, if possible, cells with transference should be avoided when using cell measurements to obtain thermodynamic data. 

Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

These relationships can be used to obtain thermodynamic data otherwise difficult to get. Vice versa they can be used to calculate the temperature coefficient of a cell voltage respectively an electrode potential based on known thermodynamic data. 

The pressure dependency of the cell voltage (and correspondingly the electrode potential) can also be derived using standard thermodynamic equations... 

In redox mediation, to have an effective electron exchange, the thermodynamic redox potentials of the enzyme and the mediator have to be accurately matched. For biocatalytic electrodes, efficient mediators must have redox potentials downhill from the redox potential of the enzyme a 50 mV difference is proposed to be optimal [1, 18]. The tuning of these potentials is a compromise between the need to have a high cell voltage and a high catalytic current. Furthermore, an obvious requirement is that the mediator must be stable in the reduced and oxidized states. Finally, for operation of a membraneless miniaturized biocatalytic fuel cell, the mediators for both the anode and the cathode must be immobilized to prevent power dissipation by solution redox reactions between them. 

Thus measuring the cell voltage at equilibrium vs charge passed between the electrodes is equivalent to measuring the chemical potential as a function of x, the Li content of a compound like Li Mo Seg. Thermodynamics requires that p increase with concentration of guest ions, and so E decreases as ions are added to the positive electrode. 

Another kinetic aspect is observed if a component other than the electroactive species is predominantly mobile. The electroactive species are in this case made available to the electrolyte by the motion of the other components in the opposite direction. In a binary compound this does not make a difference to the electrode performance. But in the case of a compound with more than two components the composition is changed to values which are not expected from a thermodynamic point of view for the variation of the concentration of the electroactive species. Other phases are formed which may provide a lower or higher activity of the electroactive species than that expected thermodynamically. This has an influence both on the current and the cell voltage. Upon discharging and charging a galvanic cell, the composition of the electrode at the interface with the electrolyte may follow very different compositional pathways (Weppner, 1985). 

Fig. 8.10 Principles of GITT for the evaluation of thermodynamic and kinetic data of electrodes. A constant current Iq is applied and interrupted after certain time intervals t until an equilibrium cell voltage is reached. The combined analysis of the relaxation process and the variation of the steady state voltage results in a comprehensive picture of fundamental electrode properties.

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