Reversible SOFC thermodynamics

The first and second laws of thermodynamics describe the reversible SOFC. The reactants (fuel and air) supply a total enthalpy lln.H.. With total enthalpy leaving the fuel cell, the change in enthalpy is AH = - Hn H.. Heat has to be extracted from the fuel cell and the reversible work w-p delivered (Figure 1-16). 

Thermodynamic considerations are applied to understand the processes of energy conversion in SOFCs. The reversible work of a fuel cell, represented by the Nemst voltage, can be calculated by the Gibbs free enthalpy of the reaction. The consideration of the electrical effects shows that the molar flow of the spent fuel is proportional to the electric current and that the reversible work is proportional to the reversible voltage. A coupling between the thermodynamic data and the electrical data is only possible by using the quantities power or heat flow and not by using work and heat. 

A solid oxide fuel cell (SOFC) is an electrochemical device that converts chemical energy of a fuel and an oxidant gas (air) directly into electricity without irreversible oxidation. It can be treated thermodynamically in terms of the free enthalpy of the reaction of the fuel with oxidant. Hydrogen and oxygen are used to illustrate the simplest case in the early part (Section 3.2) of this chapter. This treatment allows the calculation of the reversible work at equilibrium for the reversible reaction. Heat must also be transferred reversibly to the surrounding environment in this instance. 

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