Reversible cell potential

One main advantage of such a power source is the direct transformation of the chemical energy of methanol combustion into electrical energy. Hence, the reversible cell potential, can be calculated from the Gibbs energy change, AG, associated with the overall combustion reaction of methanol (1), by the equation ... 

Reversible cell potentials have been the source of much thermodynamic data on aqueous electrolytes. In recent years, this technique has been extended to nonaqueous solutions and to molten salt systems. Its use for aqueous solutions, relative to other techniques, has decreased. Various ion specific electrodes have been developed in recent years. These are used primarily in analytical chemistry and have not produced much thermodynamic data. 

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. 

An analysis by Cairns and Liebhafsky (3) for a H2/air fuel cell shows that a change in the gas composition that produces a 60 mV change in the reversible cell potential near room temperature corresponds to a 300 mV change at 1200°C (2192°F). Thus, gas composition changes are more significant in high temperature fuel cells. 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

The improvement in cell performance at higher pressure and high current density can be attributed to a lower diffusion polarization at the cathode and an increase in the reversible cell potential. In addition, pressurization decreases activation polarization at the cathode because of the increased oxygen and water partial pressures. If the partial pressure of water is allowed to increase, a lower acid concentration will result. This will increase ionic conductivity and bring about a higher exchange current density. The net outcome is a reduction in ohmic losses. It was reported (33) that an increase in cell pressure (100% H3PO4, 169°C (336°F)) from 1 to 4.4 atm (14.7 to 64.7 psia) produces a reduction in acid concentration to 97%, and a decrease of about 0.001 ohm in the resistance of a small six cell stack (350 cm electrode area). 

The dependence of reversible cell potential on pressure is evident from the Nemst equation. For a change in pressure from Pi to P2, the change in reversible potential (AVp) is given by... 

Thus, a tenfold increase in cell pressure corresponds to an increase of 46 mV in the reversible cell potential at 650°C. 

Table 6-4 Equilibrium Composition of Fuel Gas and Reversible Cell Potential as a...

From the basic principles we can make preliminary design estimates. Inefficiencies in a system arise because of voltage losses and because all of the current does not enter into the desired reactions. The minimum potential required to perform an electrolytic reaction is given by the reversible cell potential, a thermodynamic quantity. Additional voltage that must be applied at the electrodes represents a loss that is manifested in a higher energy requirement. The main causes of voltage loss are ohmic drops and overpotentials. The applied potential is equal to the sum of the losses plus the thermodynamic requirement ... 

The reversible potential ofBr2 + 2e —> 2 Br is 1.08 V atm. The pH of sea water is near 7 and 25 °C hence the reversible potential of 2H+ + 2e —> Hj is about -0.42 V. The reversible cell potential would be then about 1.5 V. The idea here is to reduce the potential below the thermodynamically reversible value by photoillumination ofthe anode (photoassisted water decomposition Szklarczyk, 1983). 

The open circuit voltage (OCV) of a fuel cell is identical to the reversible cell potential, provided that the cell is in thermodynamic equilibrium. That is usually achieved, especially at high temperatures. There are some causes which... 

The reversible cell potential for a MCFC depends on the gas composition at the anode (partial pressures of H2, H20 and C02) and at the cathode (partial pressures of 02 and C02),... 

Erev (reversible cell potential). Erev can be expressed as... 

The reversible cell potential in 30% KOH is 1.29 V. The electrical energy produced per mole is given by... 

Table 1 Thermodynamics and theoretically reversible cell potential for fuel cell reactions... 

The theoretical efficiency in such systems is close to unity, but in practice, the practical efficiency is significantly lower. This is attributable to the different mechanical and thermal losses in the system but also to the direct chemical reactions between the reactants and secondary electrochemical reactions in the cell. The voltage efficiency of the cell, q, is defined as the quotient between the cell voltage, V. at a given cell current, I, and the cell voltage at open circuit, (the maximum value of the cell voltage, equivalent, if the cell is in equilibrium, to the reversible cell potential, Ej) ... 

Observation shows that the value of S depends on the current drawn in the external circuit. The limiting value of S measured as the current goes to zero is called the electromotive force of the cell (the cell emf) or the reversible cell potential,... 

To avoid a cumbersome notation we will suppress the subscript, rev, on the cell potential we do so with the understanding that the thermodynamic equality (as distinct from the inequality) holds only for the reversible cell potentials (cell emfs). 

Measurement of Reversible Cell Potential with Liquid Junction Potential 55... 

MEASUREMENT OF REVERSIBLE CELL POTENTIAL WITH LIQUID JUNCTION POTENTIAL... 

Fig. 2.11 Schematic of the experimental setup for measuring reversible cell potential.

In most practical situations, the choice of metal is such that the reversible cell potential is seldom much in excess of 0.5 V. Even so, because of the large variations in exchange currents and mechanisms for the two electrode processes which are possible, the corrosion rate for different metals and environments can still vary by many orders of magnitude. 

---