Page 298 - Biofuels Refining and Performance

Fuel Cells 277

For methane-rich fuels, this reforming reaction is

CH H O → CO 3H 2
2
4
This reaction is generally not in chemical equilibrium, and the CO
shift reaction takes place to provide more hydrogen. The overall cell
reaction is
1
H O → H O
2
2
2
2
Electrolyte. The use of a solid electrolyte in the SOFC eliminates the
electrolyte management problems associated with the liquid electrolyte
fuel cells and also reduces corrosion considerations to a great extent. In
2
the SOFC, it is the migration of oxygen ions (O ) through the elec-
trolyte that establishes the voltage difference between the anode and
2
cathode. Therefore, the electrolyte must be a good conductor of O ions
and a bad conductor of electrons; it must also be stable at the high oper-
ating temperature. Some ceramics possess these properties and there-
fore are good candidates for this application. With the help of modern
ceramic technology and solid-state science, many ceramics can be tai-
lored for electrical properties unattainable in metallic or polymer mate-
rials. These tailored ceramic materials are termed electroceramics, and
one group is known as fast ion conductors or superionic conductors.
These superionic conductors when used as a solid electrolyte allow easy
passage of ions from the cathode to the anode in an SOFC. The material
generally used as an electrolyte in the SOFC is dense yttria-stabilized
zirconia. It is an excellent conductor of negatively charged oxygen ions
at high temperatures (1000 C), but its conductivity reduces drastically
with the drop in temperature. Other materials such as scandia-stabilized
zirconia (ScSZ), which shows good ionic conductivity at a lower temper-
ature (800 C), are also being investigated, but the electrolyte developed
with ScSZ-based materials is very expensive and they degrade very fast.

Electrode. The anode is made of metallic Ni and Y O -stabilized ZrO 2
3
2
(YSZ). Ytrria-stabilized zirconia is added in Ni to inhibit sintering of the
metal particles and to provide a thermal expansion coefficient close to
those of the other cell materials [26]. Nickel structure is normally
obtained from NiO powders; therefore, before starting the operation for
the first time, the cell is run with hydrogen in an open-circuit condition
to reduce the NiO to nickel. The anode structure is fabricated with a
porosity of 20–40% to facilitate mass transport of the reactant and prod-
Sr MnO , x
uct gases. The Sr-doped lanthanum manganite (La 1 x x 3
0.10–0.15; known as LSM) is most commonly used for the cathode mate-
rial. LSM is a p-type semiconductor. Similar to the anode, the cathode
is also a porous structure that permits rapid mass transport of the reac-
tant and product gases.