Page 117 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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

30

10

1
30 25 20 15 10 5 0
-log(p / atm)
02
Figure4.22 Totalelectrical conductivity of Ceo.8Smo,201,s-sasafunctionofoxygenpartialpressure.

4.5 Fabrication of Zr02- and Ce02-Based Electrolyte Films

The electrolyte film can be fabricated by a number of processes depending
upon the configuration of the cells. For tubular SOFCs, an electrochemical
vapour deposition (EVD) technique was developed by Westinghouse Electric
Corporation (now Siemens Westinghouse Power Corporation) in 1977 [40] to
fabricate gas-tight thin layers of doped zirconia. This EVD process involved
growing a dense oxide layer on a porous substrate at elevated temperatures and
reduced pressures, as described in Figure 4.12 [4 11.

MeCI, VAP
r3

.POROUS
SUBSTRATE

' H,O __+ '-b H,
Figure4.12 Principle of electrochemical vapourdeposition (EVD).

Under the operating conditions of the EVD process, the oxide exhibits both
oxide ion and electronic conductivity. Thus, the oxide ion flux during the oxide
growth is balanced by an electron flux, thereby preserving the electroneutrality
of the oxide. The growth rate of the oxide is commonly described by the classical