Page 235 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

2 12 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

where Me is the cation species (zirconium and yttrium): and y is the valency
associated with the cation. The temperature, the pressure and the different gas
flow rates are so chosen that the above reactions are thermodynamically and
kinetically favoured.
During the second stage of the reaction after the pores in the air electrode are
closed, electrochemical transport of oxide ions maintaining electroneutrality
occurs through the already deposited yttria-stabilised zirconia in the pores from
the high oxygen partial pressure side (oxygen/steam) to the low oxygen partial
pressure side (chlorides). The oxide ions, upon reaching the low oxygen partial
pressure side, react with the metal chlorides and the electrolyte film grows in
thickness. The flows of the metal chloride vapours are maintained above a
critical level to eliminate any gas-phase control of the EVD reaction.
Furthermore, the ratio of yttrium chloride to zirconium chloride is so chosen that
the electrolyte deposited contains about 10 mol% yttria.
The growth of the electrolyte film is parabolic with time and occurs by the
oxide ions diffusing through yttria-stabilised zirconia from the oxygen/steam
side to the chlorides side. The rate controlling step in this process is the electronic
transport (diffusion of electrons) through the electrolyte film. The
electrochemical vapour deposition process ensures the formation of a pore-free,
gas-tight, uniformly thick layer of the electrolyte over porous air electrode. A
representative micrograph of the electrolyte layer over porous air electrode is
shown in Figure 8.1 5.

Figure 8.1 5 Representative micrograph of the electrochemically vapour deposited YSZ electrolyte over a
porous air electrode.
The EVD technique to deposit the electrolyte is complex, capital-cost intensive,
and requires vacuum equipment that makes scaling it up to a cost-effective,
continuous manufacturing process for high volume SOFC production difficult if
not impossible. Fabrication of the YSZ electrolyte films by a more cost-effective
non-EVD technique such as plasma spraying followed by sintering, is being
investigated to reduce cell manufacturing cost.
The Ni/YSZ anode, 100-150 pm thick, is deposited over the electrolyte by a
two-step process. In the first step, nickel powder slurry is applied over the
electrolyte. In the second step, YSZ is grown around the nickel particles by
the same EVD process as used for depositing the electrolyte. Deposition of a