Cell potential thermodynamic function

Each reactant and product appears in the Nemst equation raised to its stoichiometric power. Thermodynamic data for cell potentials have been compiled and graphed (3) as a function of pH. Such graphs are known as Pourbaix diagrams, and are valuable for the study of corrosion, electro deposition, and other phenomena in aqueous solutions.Erom the above thermodynamic analysis, the cell potential can be related to the Gibbs energy change... 

Consequently, a wealth of information on the energetics of electron transfer for individual redox couples ("half-reactions") can be extracted from measurements of reversible cell potentials and electrochemical rate constant-overpotential relationships, both studied as a function of temperature. Such electrochemical measurements can, therefore, provide information on the contributions of each redox couple to the energetics of the bimolecular homogeneous reactions which is unobtainable from ordinary chemical thermodynamic and kinetic measurements. 

A number of detailed thermodynamic comparisons of half-cells containing alkali metal and alkali metal amalgams are available. For example, Cogley and Butler examined cell potentials as a function of amalgam concentration for the cell shown below [22]. 

Equations (47)-(50) indicate how thermodynamic quantities can be obtained from cell potentials measured under standard conditions. However, standard states are hypothetical states (e.g., infinitely dilute behavior at 1.0 m concentration), which cannot be prepared in the cell. As a result, an extrapolation procedure is used to find 8° from measured cell voltages as a function of concentration. From Eq. (47), we write the dependence of 8 on the concentration of the electrolyte in the form... 

Professor S. Srinivasan and his team have studied the effect of pressure and characteristics of the current-potential relations in a hydrogen-oxygen fuel cell with a proton exchange membrane (Y. W. Rho, O. A. Velev, S. Srinivasan, and Y. T. Kho,./. Electrochem. Soc. 141 2084, 2089, 1994). In this problem, it is proposed to study the applicability of the theoretical dependence of the cell potential as a function of pressure. The temperature is 25 °C and it may be assumed that the pressure of the gas in each of the compartments, i.e., the anodic compartment (hydrogen) and the cathodic compartment (oxygen), are the same, Pn =Po P- For the formation of water in its standard state, the relevant thermodynamic quantities are ... 

This chapter is concerned primarily with the computation of potentials of a cell using the hydrogen electrode as a probe for studying ionic equilibrium processes in mixed-organic-aqueous solvent systems. Computation of a number of other thermodynamic functions of the ionic process under investigation or of the solvent used is rather straightforward once the standard potential of the measuring cell has been calculated. 

Figure 2. displays cell potential measured from the electrodialysis cell as a function of operation temperature. As operation temperature increased from 35 to 90°C, the cell potential decreased from 0.68 to 0.41 V at the applied current of 2 A. This is because high temperatures reduce the amount of electrical energy required to concentrate HI molarity from a thermodynamic standpoint and hence electrical energy demand AG for electrodialysis of Hix solution decreases with increasing temperature... 

Sometimes it is convenient to analyze interfacial thermodynamic data at constant electrode charge density Om rather than at constant cell potential difference E. Once the interfacial tension data at a given concentration have been differentiated with respect to to obtain one may calculate the function which is given by... 

The standard cell potential is a thermodynamic state function. How are E° values treated similarly to AH°, AG°, and 5° values How are they treated differently ... 

Briegleb G and Bieber A, Dissociation constants of substituted benzoic acids at different temperatures and thermodynamic functions of acid dissociation. Potential measurements in cells without liquid junction potenHals, Z. Elektrochem., 55,250-259 (1951) (4.212). 

NB High quality work done with electrometric cells widt liquid junction potentials. Thermodynamic ionization functions calculated. 

The interest in the analysis of the dependencies of equilibrium potential on composition of cathode materials for lithium-metal cells appeared in the late-1970s [2-8] where phase composition and phase transitions of oxides and hal-cogenides of transient metals upon lithiation were discussed. The usefulness of the simultaneous scrutiny of the equilibrium potential together with its tanpera-ture coefficient was first proved in several works [9-13] published soon after. The approach to the calculation of kinetic parameters using the thermodynamic data, which is the subject of this chapter, has been proposed [14-16] later. In early 2000, new interest in the method has arisen, both in the thermodynamics of the processes within the electrodes for lithium-ion cells [17-22] and in the connection between thermodynamic functions and kinetic parameters [23]. In the series of recent works, M. Bazant [24] described the development of the fundamental theory of electrochemical kinetics and charge transfer applied to lithium iron phosphate (LFP). 

As we have seen, the value of the standard cell potential gg of a cell reaction has useful thermodynamic applications. The value of °gjj for a given cell reaction depends only on temperature. To evaluate it, we can extrapolate an appropriate function to infinite dilution where ionic activity coefficients are unity. 

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. 

The thermodynamically stable oxidation state of a metal in a given environment is a function of the prevailing oxidation potential. The value of the potential is given by the Nemst equation, which is described in Chapter 5 for the generic reduction half-cell ... 

Fig. 22.6. Redox potentials (mV) of various half-cell reactions during mixing of fluid from a subsea hydrothermal vent with seawater, as a function of the temperature of the mixture. Since the model is calculated assuming 02(aq) and H2(aq) remain in equilibrium, the potential for electron acceptance by dioxygen is the same as that for donation by dihydrogen. Dotted line shows currently recognized upper temperature limit (121 °C) for microbial life in hydrothermal systems. A redox reaction is favored thermodynamically when the redox potential for the electron-donating half-cell reaction falls below that of the accepting half-reaction.

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. 

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