Page 81 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

58 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

The oxidation of hydrogen follows the equation
1
H2 +-02 + H20.
2

This equation is independent of the process itself. The reaction path in a SOFC
depends on the anode and the cathode reactions. Hydrogen is adsorbed at the
anode, ionised and the electrons are removed by the connection to the electrical
load where the electrical work is used. Oxygen is adsorbed at the cathode
connected with the load and ionised by the arriving electrons. The oxide ion is
conducted by the electrolyte to the anode. The hydrogen ions (protons) and the
oxide ion form a molecule of water. The first reactionQon the anode is

H2 + 2H+ + 2e-. (8)

The reaction @ on the cathode is

1
-02 + 2e- --+ 02-. (9)
2

The oxide ion 02- is conducted through the electrolyte and arrives at the
anode. At the anode, water forms @ according to the reaction

2H+ + 02- + H20. (10)

As Figure 3.2 shows, the product H20 is mixed with the anode gas and its
concentration increases with increasing fuel utilisation U? The fuel utilisation Uf
is the ratio of the spent fuel flow and the inlet fuel flow and is defined by

Uf=l--. ~FA,O
mFI

where kFIis the fuel mass flow at the cell’s inlet and ljlFAnO is the fuel mass flow at
the outlet of the anode. A similar definition can be made with the molar flow.
Because these mixing effects are irreversible processes they produce entropy and
a reversible SOFC operation is only possible as the limiting process of the real
process with Uf --f 0. Equation (8) shows that the molar flow of the electrons is
twice that of the molar flow of hydrogen, thus

The electric current Iis a linear function of the molar flow riel of the electrons or
the molar flow of the spent fuel - in this example the molar flow of hH2 the spent
hydrogen
I = riel. (-e) . NA = - F = -2n~2 . E. (13)