Chemistry Reference
In-Depth Information
Table 20.2
Summary of recent technology used for the main types of fuel cells
Property
AFC
SPFC
PAFC
MCFC
SOFC
Operating temperature
333-363 K
353-373 K
473 K
923 K
1073-1273 K
Anode
Pt (
8
0%)
Pt/C, 4mgPtcm
-
2
Pt/C on PTFE,
Ni-Cr/Ni-Al
Ni/ZrO
2
Pd (20%)
0.1 mg Pt cm
-
2
Cathode
Au (90%)
Pt/C, 4 mg Pt cm
-
2
Pt/C on PTFE,
NiO-Li
LaMnO
3
(doped)
Pt (10%)
0.5 mg Pt cm
-
2
Electrode support
Graphite
Graphite
Carbon paper
LiAlO
2
—
Electrolyte
KOH
Perfluorinated
H
3
PO
4
, 100%
Li
2
CO
3
(62%)
ZrO
2
with Y
2
O
3
(35-45%)
sulfonic acid
K
2
CO
3
(3
8
%)
(
8
mol.%)
polymers
Electrolyte support
Metals
—
SiC on PTFE
c-LiAlO
2
, a-LiAlO
2
—
Cell interconnect
Graphite
Graphite
Graphite
Ni on Incoloy
LaCrO
3
(doped)
OH
-
H
+
H
+
CO
3
2
-
O
2
-
Charge carrier
3.2 Solid polymer fuel cell
were used as the electrolyte, however it was found
that hydrated membranes also could act as the elec-
trolyte [11]. Perfluorinated sulfonic acid polymers
are used as the membrane due to their relatively
high thermal tolerance and high chemical stability
towards strong acids and alkalis. The membrane is
sandwiched between two platinum-impregnated
porous electrodes backed with a coating of poly-
tetrafluorethene to provide a path for gas diffusion
to the catalyst layer. A critical issue for the success-
ful operation of an SPFC is water management
within the membrane. The ionic conductivity of the
membrane depends on the water content and is
highest when the membrane is fully saturated. Water
is produced from the cell reactions as liquid and it
will have to be removed from the cell. Too much
water in the cell will result in flooding of the elec-
trodes and a reduction in the gas diffusion to the
electrodes. Too little water in the cell will result in
dehydration of the membrane and a reduction in
ionic conductivity. The membrane also will shrink on
dehydration, causing it to lose contact with the elec-
trodes and because there is no liquid electrolyte to
fill the gap electrical contact will be lost. Dehydra-
tion can be caused by a range of factors such as high
reactant flow and low humidity of the reactants.
Operating the SPFC at higher temperatures, such as
approaching 373 K, and lower pressure also will con-
tribute to dehydration. Another factor to take into
account in the water balance is the effect of water
drag through the cell as the ion species are trans-
ported across the cell, which is represented more
The solid polymer fuel cell (SPFC) is typified by high
power density, low weight and volume, low cost, low
corrosion, low operating temperature and relative
ease of achieving gas-tight seals. An SPFC may be
started up quickly and reach full power within a
short time. It can also respond very fast to changes
in demand for power. These properties make an
SPFC the preferred fuel cell for vehicle power. The
disadvantages of an SPFC result from the low oper-
ating temperature, which means that the electrode
kinetics are slow and that expensive noble metal
catalysts are required. These catalysts are poisoned
by carbon monoxide, which means that certain
fuels, such as reformed hydrocarbon fuels, must be
pretreated to lower the carbon monoxide, carbon
dioxide and hydrocarbons to acceptable levels. This
may be achieved by membrane separation, metha-
nation, pressure swing adsorption or selective oxi-
dation. Another disadvantage of the low operating
temperature of an SPFC is that no heat is available
for external use in combined heat and power appli-
cations, in contrast to the MCFC and SOFC types.
Methanol would be a useful fuel for vehicular use
[9] although hydrogen, despite the logistical and
safety issues, has been developed extensively and
the partial oxidation of gasoline also has been
investigated.
A cation-exchange membrane was introduced into
fuel cells by Grubbs in 1959 to function as an ion-
conductive gas barrier [10]. Initially aqueous acids
