Chemistry Reference
In-Depth Information
Table 19.2
Types of fuel cells and characteristics
Fuel cell type and mobile ion
Applications
Alkaline (AFC) OH
-
ion
Space vehicles
Proton exchange membrane (PEM)
Vehicle and mobile low-
H
+
ion
power CHP systems
Phosphoric acid (PAFC) H
+
ion
200-kW CHP systems
Molten carbonate (MAFC)
Medium- and large-
CO
3
2
-
ion
scale CHP systems
Solid oxide (SOFC) O
2
-
ion
All sizes of CHP systems
Fig. 19.7
Generation and use of hydrogen as part of a water
energy economy.
Thus, at 80°C the potential for a hydrogen oxygen
fuel cell is given by
E
= 226 100/2 ¥ 96 485 = 1.15 V.
Since the first demonstration of a fuel cell by
William Grove in 1839, five principal types of fuel
cells have been developed, according to the elec-
trolyte in the cell (see Table 19.2). The characteris-
tics of a fuel cell are:
(hydropower, solar, wind and wave) or on photo-
chemical or photobiological methods.
Hence, overall, electrochemistry is set to play a
major role in energy production and generation in
the future. This role covers the areas of storage bat-
teries, super-capacitors, redox batteries, solar cells
and fuel cells, the latter being a key technological
factor. The function of the battery for energy store in
portable devices is well established in many markets
for medium- and small-scale power generation.
Exciting new battery couples continue to be devel-
oped, including metal hydride, e.g. nickel/lithium
polymer, sodium/nickel chloride, etc. [9]. For sus-
tainability of battery technology, methods are
required to recycle and reuse, particularly the active
battery components in primary cells. Recycling of
lead acid batteries is well established whereas other
processes for nickel cells, etc. are developing, in some
cases using electrochemical technology.
• Electrochemical device to convert directly a fuel
(hydrogen) plus oxygen to electricity (heat +H
2
O)
• Potential to operate with other hydrogen-rich
fuels: alcohols, natural gas, petrol, NH
3
?
• Solid state and silent
• Compact electrical power generation
• Low emissions
Until recent years the technological success of the
fuel cell has been limited. Polymer electrolyte mem-
brane (PEM) cells were used in the first manned
spacecraft, alkaline fuel cells were used in the Apollo
mission and then PEM cells returned to the space
shuttle Orbitor vehicles. Many 200-kW combined
heat and power (CHP) phosphoric acid fuel cells
have been installed in the USA, Europe and Japan.
In the last decade the interest in the PEM cell has
gained momentum rapidly for use in transportation
and portable electronic equipment. The attraction of
the fuel cell (Table 19.2), particularly in the latter
applications, lies in its good efficiency, simplicity in
construction, low emissions and low noise.
The issue of efficiency is of particular interest
because the fuel cell is not Carnot cycle limited. The
maximum fuel cell efficiency usually is defined as
the electrical energy produced per mole divided by
the enthalpy of formation, i.e. -D
G
f
/-D
H
f
. At temper-
atures of <750°C the fuel cell efficiency can be greater
than that of a heat engine. For example, at 25°C for
liquid water as the product (D
G
f
=-327.2 kJ mol
-1
)
the efficiency maximum is 0.83 or 83%. Although
4.1 Fuel cells
The principal type of fuel cell uses hydrogen as the
fuel and in conjunction with oxygen produces elec-
trical energy by virtue of the favourable thermody-
namics of the overall cell reaction:
Hg
()+
05
.
Og HOg
2
()=
()
2
2
which at 80°C has a Gibbs free energy D
G
of
-226.1 kJ mol
-1
. If the system is reversible, the
overall electrical work done (in the absence of losses)
is equal to the Gibb's free energy of formation, i.e.
D
G
f
=-
nFE
, where
E
is the electromotive force (EMF)
or reversible, open-circuit cell potential and
F
is the
Faraday constant or charge on 1 mol of electrons.
