Page 294 - Biofuels Refining and Performance

Fuel Cells 273

is in a molten (liquid) state and is a good ionic conductor. The composi-
tion of salts in the electrolyte may vary but usually consist of
lithium/potassium carbonate (Li CO /K CO , 62–38 mol%) for operation
2
2
3
3
at atmospheric pressure. For operation under pressurized conditions,
lithium/sodium carbonate (LiCO /NaCO , 52–48 or 60–40 mol%) is used
3
3
as it provides improved cathode stability and performance. This allows for
the use of thicker Li/Na electrolyte for the same performance, resulting
in a longer lifetime before a shorting caused by internal precipitation.
The composition of the electrolyte has an effect on electrochemical activ-
ity, corrosion, and electrolyte loss rate. Li/Na offers better corrosion
resistance but has greater temperature sensitivity. Additives are being
developed to minimize the temperature sensitivity of the Li/Na elec-
trolyte. The electrolyte has a low vapor pressure at the operating tem-
perature and may evaporate very slowly; however, this does not have any
serious effect on the cell life. The electrolyte is suspended in a porous,
) matrix. The ceramic
insulating, and chemically inert ceramic (LiAlO 2
matrix has a significant effect on the ohmic resistance of the electrolyte.
It accounts for almost 70% of the ohmic polarization. The electrolyte
management in an MCFC ensures that the electrolyte matrix remains
completely filled with the molten carbonate, while the porous electrodes
are partially filled, depending on their pore size distributions.
Electrode. The anode is made of a porous chromium-doped sintered
Ni-Cr/Ni-Al alloy. Because of the high temperatures resulting in a fast
anode action, a large surface area is not required on the anode as com-
pared to the cathode. Partial flooding of the anode with molten carbon-
ate is desirable as it acts as a reservoir that replenishes carbonate in
the stack during prolonged use. The cathode is made up of porous lithi-
ated nickel oxide. Because of the high operating temperatures, no noble
catalysts are needed in the fuel cell. Nickel is used on the anode and
nickel oxide on the cathode as catalysts. Bipolar plates or interconnects
are made from thin stainless steel sheets with corrugated gas diffusion
channels. The anode side of the plate is coated with pure nickel to pro-
tect against corrosion.
Performance. At the high operating temperatures of an MCFC, CO is not
a poison but acts as a fuel. In the MCFC, CO has to be added to oxygen
2
(air) stream at the cathode for generation of carbonate ions. The anode
reaction converts these ions back to CO , resulting in a net transfer of
2
two ions with every molecule of CO . The need for CO in the oxidant
2
2
stream requires that CO from the spent anode gas be separated and
2
mixed with the incoming air stream. Before this can be done, any resid-
ual hydrogen in the spent fuel stream must be burned. Systems devel-
oped in the future may incorporate membrane separators to remove the
hydrogen for recirculation back to the fuel stream to increase efficiency.