A fuel cell is a device that converts chemical energy directly into electrical energy through electrochemical reactions. There are several types of fuel cells classified by their electrolyte, including alkaline fuel cells (AFC), proton exchange membrane fuel cells (PEMFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), and solid oxide fuel cells (SOFC). Fuel cells have advantages over heat engines like higher efficiency, fewer moving parts requiring less maintenance, and modularity to increase capacity. However, fuel cells also have challenges to overcome like fuel processing requirements, catalyst costs, startup times, and high temperature durability for some types.

1. Fuel Cells
NIGEL BRANDON
Imperial College
London, United Kingdom
1. What Is a Fuel Cell?
2. Fuel Cell Types
3. Fuel Selection
4. Fuel Cell Applications
5. Fuel Cell Costs
6. Environmental Benefits
Glossary
anode The fuel electrode, where electrons are donated as
the fuel is oxidised. The negative electrode in a fuel cell.
balance of plant All the components that make up a power
system, other than the fuel cell stack, for example, the
control system, reformer, power conditioner, or com-pressor.
bipolar plate A dense, electronic (but not ionic) conductor
that electrically connects the anode of one cell to the
cathode of another. It also distributes fuel or air to the
electrodes.
cathode The air electrode, where electrons are accepted and
oxygen is reduced. The positive electrode in a fuel cell.
electrocatalyst A catalyst incorporated into both the anode
and the cathode to promote the electrode reactions.
electrolyte A dense ionic (but not electronic) conductor.
Each fuel cell type is distinguished by the nature of the
electrolyte used within it.
interconnect Another term for the bipolar plate (see
above).
membrane electrode assembly (MEA) An assembled an-ode,
cathode, and electrolyte.
open circuit voltage The voltage from the fuel cell when no
current is being drawn from it.
overpotential The voltage loss within an operating fuel cell
due to electrode kinetics, mass transport limitations,
and component resistance.
positive electrolyte negative (PEN) The assembled cath-ode,
electrolyte, and anode of a solid oxide fuel cell.
reformer The fuel processor that converts the fuel to a
hydrogen-rich gas suitable for the fuel cell.
stack An assembly of many individual fuel cells, complete
with gas manifolds and electrical outputs.
A fuel cell is a device for directly converting the
chemical energy of a fuel into electrical energy in a
constant temperature process. Fuel cells operate on a
wide range of fuels, including hydrogen, and are seen
as a clean, high-efficiency power source and an
enabling technology for the hydrogen economy.
1. WHAT IS A FUEL CELL?
The fuel cell can trace its roots back to the 1800s
when a Welsh-born, Oxford-educated barrister, Sir
William Robert Grove, realized that if electrolysis,
using electricity, could split water into hydrogen and
oxygen, then the opposite would also be true. Grove
subsequently built a device that would combine
hydrogen and oxygen to produce electricity—the
world’s first gas battery, later renamed the fuel cell.
It was another British scientist, Francis Thomas
Bacon, who was the first to develop a technologically
useful fuel cell device. Bacon began experimenting
with alkali electrolytes in the late 1930s, using
potassium hydroxide instead of the acid electrolytes
used by Grove. Bacon’s cell also used porous ‘‘gas-diffusion
electrodes’’ rather than Grove’s solid
electrodes. These increased the surface area over
which the electrochemical reactions occurred, im-proving
power output. In the early 1960s, Pratt and
Whitney licensed the Bacon patents and won the
National Aeronautics and Space Administration
contract for the Apollo spacecraft utilizing onboard
liquid hydrogen and oxygen to provide both power
and water. Innovation and development has con-tinued
since that time with pioneering work by, for
example, Westinghouse, Union Carbide, and General
Electric, among others.
Today, potential applications for fuel cells range
from battery replacement in consumer goods and
portable computers, through residential scale com-bined
heat and power (CHP), to distributed energy
generation. However, the key problem limiting the
Encyclopedia of Energy, Volume 2. r 2004 Elsevier Inc. All rights reserved. 749

2. significant commercial uptake of fuel cells is cost,
and it is cost reduction, together with the need to
demonstrate high levels of reliability and durability,
that are the primary concerns of fuel cell developers
today.
As demonstrated by Grove’s gas battery, a fuel cell
is analogous to a battery, but one that is constantly
being recharged with fresh reactants. In a similar
manner to a battery, each fuel cell comprises an
electrolyte, which is an ionic conductor, and two
electrodes (the negative anode and positive cathode),
which are essentially electronic conductors.
The nature of the ion transfer varies between the
different types of cell, but the principle shown in
Fig. 1 for a polymer electrolyte membrane fuel cell
(PEMFC) is representative. In this case, hydrogen is
fed to the anode of the cell where it splits into a
proton and electron, the former passing through the
electrolyte and the latter forced around an external
circuit where it drives a load. The proton and
electron combine with oxygen from the air at the
cathode, producing pure water and a small amount
of heat. The overall reaction is thus
H2 þ 0:5O23H2O: ð1Þ
The anode is then the negative electrode of the
device, and the cathode the positive.
The fuel cell differs from a conventional heat
engine (such as the internal combustion engine or the
gas turbine), in that it does not rely on raising the
temperature of a working fluid such as air in a
combustion process. The maximum efficiency of a
heat engine is subject to the Carnot efficiency
limitation, which defines the maximum efficiency
that any heat engine can have if its temperature
extremes are known:
Carnot efficiency ¼ðTH TLÞ=TH; ð2Þ
where TH is the absolute high temperature and TL is
the absolute low temperature. In contrast, the
theoretical efficiency of a fuel cell is related to the
ratio of two thermodynamic properties, namely the
chemical energy or Gibbs free energy (DG0) and the
enthalpy (DH0) of the fuel oxidation reaction:
Maximum fuel cell efficiency ¼ DG0=DH0: ð3Þ
Figure 2 provides an illustration of the theoretical
efficiency possible from a fuel cell running on
hydrogen and air as a function of temperature and
compares this to the Carnot efficiency of a heat
engine at the same temperature, assuming a
low temperature of 251C. As the Gibbs free energy
for reaction (1) falls with increasing temperature,
while the enthalpy remains largely unchanged, the
Electrolyte
Anode Cathode
H+
H+
H+
H+
Ion migration through
the electrolyte
Bipolar plate
Bipolar plate
H2
O2
H2
O2
H2O,
Heat
e−
e−
FIGURE 1 Schematic illustration of a polymer electrolyte fuel cell.
750 Fuel Cells

3. 100%
90%
80%
70%
60%
H2 fuel cell
theoretical efficiency of the fuel cell falls with
increasing temperature. Indeed, at high temperatures
the theoretical efficiency of a heat engine is higher
than that of a hydrogen driven fuel cell. However,
because of the need for motion in a heat engine,
either rotary or linear, significant materials issues are
associated with operating them at high temperatures,
from the perspective of both durability and cost. Fuel
cells do not have moving parts operating at high
temperatures and thus are less susceptible to this
problem.
However, other factors play a role in determining
the actual efficiency of an operating fuel cell, in
particular operating temperature, fuel type, and
materials selection. For example, losses associated
with the kinetics of the fuel cell reactions fall with
increasing temperature, and it is often possible to use
a wider range of fuels at higher temperatures.
Equally, if a fuel cell is to be combined with a heat
engine, for example, in a fuel cell/gas turbine
combined cycle, then high fuel cell operating tem-peratures
are required to maximize system efficiency.
All these factors mean that there is considerable
interest in both low-temperature and high-tempera-ture
fuel cells, depending on the application.
Figure 3 illustrates the shape of the current-voltage
characteristics that would be expected from
a typical fuel cell. When no current is being drawn
from the fuel cell, the cell voltage is at a maximum,
termed the open circuit voltage (E), which increases
with the partial pressures of the fuel and air gases
and decreases with increasing temperature, accord-ing
to the Nernst equation:
E ¼ E0 RT=nF ln pH2O=pH2 pO0:5
2 : ð4Þ
where E0 is related to the Gibbs energy for the
reaction via:
DG0
298 ¼ nE0F; ð5Þ
where n is the number of electrons involved, 2 for
reaction (1), and F is the Faraday constant
(96495Cmol1). As the value of DG0
298 for the
reaction of hydrogen with oxygen to form water is
229 kJ mol1, then an open circuit voltage of
around 1.2V would be expected from a hydrogen/
air fuel cell operating at near ambient temperatures
under standard conditions.
As current is drawn from the cell, additional
irreversible losses result in a decrease in the cell
voltage (Ecell), according to:
Ecell¼ E iR Za Zc; ð6Þ
where iR refers to ohmic losses within electrodes,
interconnects, and current take-off’s due to the finite
resistance of the materials used, Za refers to the
overpotential at the anode, reflecting losses due to
both electrode kinetics and mass transport limita-tions,
and Zc refers to the equivalent overpotential at
the cathode. Different loss terms are reflected in
different regions of the current-voltage curve, with
the initial fall in voltage reflecting electrode kinetics,
the central linear region being dominated by iR
losses, before mass transport limitations dominate at
high current densities.
Hence, under load a single cell produces a
reduced cell voltage due to the losses highlighted
here. It is the task of the fuel cell designer to
minimize these losses by judicious selection of
materials and cell geometry. Electrodes, for example,
50%
100 200 300 400 500 600 700 800 900 1000
Temperature (oC)
Percentage LHV efficiency
Carnot heat engine
FIGURE 2 Maximum efficiency (on a lower heating value
basis) of a hydrogen/air fuel cell, and a heat engine limited by the
Carnot cycle, as a function of temperature.
1.0
0.8
0.6
0.4
0.2
0.2 0.4 0.6 0.8 1.0 1.2
Voltage (V)
Current (A cm− 2 )
FIGURE 3 Schematic illustration of a typical fuel cell current-voltage
curve.
Fuel Cells 751

4. are required to be porous to enable gas transport to
and from the active catalyst region adjacent to the
electrolyte, yet also to be good electronic conduc-tors.
As such, there is considerable research into
electrode structures and materials, so as to minimize
overpotentials. Another choice facing the fuel cell
system designer is the operating cell voltage.
At any point other than very close to the open
circuit voltage (where parasitic loads become sig-nificant),
then, the higher the cell voltage, the higher
the cell efficiency—but the lower the power density.
In general, a cell voltage of about 0.6 to 0.7V is
taken as the operating point. This is a compromise
between efficiency (a high-efficiency reducing fuel
consumption) and power density (a low-power
density increasing stack size and weight, and hence
capital cost). To achieve a useful output power,
individual cells are connected together in a ‘‘stack’’.
This is achieved using an interconnect or bipolar
plate, which joins the anode of one cell to the
cathode of the next cell. The interconnect also
separates and often distributes the fuel and oxidant.
An example of a PEMFC stack is shown in Fig. 4. In
this case, the interconnect is known as a flow field
plate, and the combination of electrolyte and
electrodes as the membrane electrode assembly
(MEA). Other fuel cell types use different terminol-ogy
(positive electrolyte negative or PEN for an
SOFC, for example) for these components, but their
function remains the same.
The fuel cell stack is then incorporated into a
system, which meets the demand of a specific
application. Fuel cell systems can include a range of
balance of plant (BoP), depending on the fuel used,
the application, and the fuel cell type. BoP can
include a fuel processor, compressor, power condi-tioner,
and control system. Together, these can add
significant cost to the overall system. As well as high
efficiency and low emissions levels (discussed further
in Section 6), a number of further advantages are
cited for fuel cells:
1. They are quiet and involve few moving parts,
other than some fans or a compressor to blow air
into the device, and hence do not require much
maintenance.
2. They are modular, such that several can be
coupled together to increase the capacity of a
system, but can be mass-manufactured to reduce
cost.
3. They exhibit an increase in efficiency at low
loads, unlike a heat engine, which normally only
exhibits maximum efficiency around the design
point for the device.
2. FUEL CELL TYPES
There are five main classes of fuel cell, each with
differing characteristics, and differing advantages
and disadvantages. The five types are summarized in
Table I, with each taking the name of the electrolyte
used in its fabrication. These five classes of fuel cell
can essentially be further grouped into one of two
classes, distinguished as either low-temperature fuel
cells (AFC, PEMFC, PAFC), or high-temperature fuel
Fuel Flow field
plates
Membrane
electrode
assembly
Air
Electricity
+
FIGURE 4 Example of a polymer electrolyte fuel cell (PEMFC) stack.
752 Fuel Cells

5. cells (MCFC, SOFC). The low-temperature fuel cells
can be distinguished by the following common
characteristics:
* They require a relatively pure supply of hydrogen
as a fuel (e.g., AFCs are sensitive to carbon
dioxide, PEMFCs to carbon monoxide). This
usually means that a fuel processor and some
form of gas cleanup is required, adding cost and
reducing system efficiency.
* They incorporate precious metal electrocatalysts
to improve performance.
* They exhibit fast startup times.
* They are available commercially (AFC, PAFC) or
are approaching commercialization (PEMFC).
In contrast, high-temperature fuel cells can be classed
as having the following general features:
* They have fuel flexibility; they can be operated on
a range of hydrocarbon fuels at high efficiency.
* Their increased operating temperature reduces the
need for expensive electrocatalysts.
* They can generate useful ‘‘waste’’ heat and are
therefore well suited to cogeneration applications.
* They exhibit slow startup times.
* They can require expensive construction materials
to withstand the operating temperature,
particularly in the balance of plant.
* Reliability and durability are concerns, again due
to the operating temperature.
* They are suitable for integration with a gas
turbine, offering very high efficiency combined
cycles.
* They are further from commercialization, though
a significant number of demonstrators are in
operation.
The previous summary is, of course, a general-ization.
For example, a significant amount of work is
ongoing to develop intermediate temperature SOFCs
(IT-SOFCs), which operate at temperatures toward
6001C and which aim to overcome a number of the
disadvantages of high-temperature fuel cells cited
here. Similarly, work also is being carried out to
develop low-temperature fuel cells that operate
directly on methanol rather than clean hydrogen,
Fuel Cells 753
eliminating the need for a fuel processor. None-theless,
it is reasonable to distinguish between the
low-temperature and high-temperature variants as
being broadly best suited to transportation and
stationary power applications, respectively, applica-tions
that place differing requirements on the fuel
cell stack and system. Each of the main fuel cell
types is discussed in more detail in the following
sections.
2.1 Alkaline Fuel Cells (AFCs)
The alkaline fuel cell has a long history in the space
program. It is still used in the space shuttle in an
expensive guise, producing power for the onboard
systems by combining the pure hydrogen and oxygen
stored in the rocket-fuelling system.
The electrolyte is concentrated (85 wt%) potas-sium
hydroxide (KOH) in AFCs operated at 2501C,
or less concentrated (35 to 50 wt%)KOH for
operation below 1201C. The electrolyte is retained
in a porous matrix (typically asbestos), and electro-catalysts
include nickel and noble metals. The main
difficulties with this fuel cell type are (1) carbon
monoxide, always found in hydrogen produced by
reforming hydrocarbon or alcohol fuels (see Section
III), is a poison to the precious metal electrocatalysts,
and (2) carbon dioxide (in either fuel or air) will
react with the KOH electrolyte to form potassium
carbonate. As such, applications are essentially
limited to those where either pure oxygen and
hydrogen can be used.
2.2 Polymer Electrolyte Fuel
Cells (PEMFCs)
PEMFCs have high-power density, rapid startup,
and low-temperature operation (around 80 to
1201C), and so are ideal for use in applications such
as transport and battery replacement. The electro-lyte
used is a proton conducting polymer. This is
typically a perfluorinated polymer, though other
hydrocarbon-based membranes are under develop-ment
in an attempt to reduce cost or to enable
TABLE I
Summary of the Five Main Fuel Cell Types
Fuel cell type Alkaline Polymer Phosphoric acid Molten carbonate Solid oxide
Acronym AFC PEMFC PAFC MCFC SOFC
Operating temperature 60–2501C 80–1201C 150–2201C 600–7001C 600–10001C

6. operation at temperatures approaching 2001C. The
catalytically active layer sits adjacent to the mem-brane,
supported on a PTFE treated carbon paper,
which acts as current collector and gas diffusion
layer. For operation on pure hydrogen, platinum is
the most active catalyst, but alloys of platinum and
ruthenium are used when higher levels of carbon
monoxide are present (CO is a poison in all low-temperature
fuel cells). Water management in the
membrane and electrodes is critical for efficient
performance, because the membrane must be hy-drated,
while avoiding flooding of the electrode
pores with water.
The present cost of PEMFC systems is high. While
mass-manufacturing techniques will help bring down
those costs, further technical innovation is also
needed. The potential for replacing diesel standby
generators that are often noisy, expensive, and
unreliable is engendering great interest. As many
countries move toward liberalization of their energy
systems, the opportunities for decentralized genera-tion
are also growing. Demonstrator plants of
250kWe are also being produced and operated.
These run on natural gas using a reformer, for the
main part, and offer very low emissions but high
efficiency as their benefit.
2.3 Phosphoric Acid Fuel Cells (PAFCs)
Phosphoric acid fuel cells have been very successful
in fuel cell terms over the past 5 years as stationary
cogeneration plants, with more than 220 commercial
power plants delivered. The PAFC fleet has demon-strated
over 95%þavailability, and several units
have passed 40,000 hours operation. PAFCs operate
at 150 to 2201C, using a 100% phosphoric acid
electrolyte retained in a silicon carbide matrix.
Generally platinum electrocatalysts are used in both
anode and cathode.
Typical applications lie in hospitals, where the
waste heat can be used in laundry and other areas
and where consistent and reliable power is required;
in computer equipment power provision, where the
absence of power surges and spikes from the fuel cell
enables systems to be kept running; and in army
facilities and leisure centers that have a suitable heat
and power requirement. While the PAFC is by far the
most commercial of the fuel cells to date, it may well
be superseded in the longer term by PEMFC plants
that can potentially be produced more cheaply and
by SOFC or MCFC plants that have more useful heat
output and that operate at higher efficiencies on
hydrocarbon fuels such as natural gas.
2.4 Molten Carbonate Fuel
Cells (MCFCs)
The electrolyte in the MCFC is usually a combina-tion
of alkali carbonates, retained a ceramic LiAlO2
matrix. The temperature of operation is 600 to
7001C, where the alkali carbonates form a highly
conducting molten salt, with carbonate ions provid-ing
the means for ionic conduction. The increased
temperature of operation means that precious metal
electrocatalysts are not needed, and generally nickel
anodes and nickel oxide cathodes are used. A design
constraint with the MCFC is the need for CO2
recirculation, meaning that it is difficult to operate
below the 100kWe’s scale, removing the market in
micro combined heat and power (micro-CHP).
2.5 Solid Oxide Fuel Cells (SOFCs)
Solid oxide fuel cells operate at elevated tempera-tures,
generally above 8001C for the all ceramic high-temperature
variant and in the range 600 to 8001C
for metal-ceramic intermediate-temperature solid
oxide fuel cells. The electrolyte is a dense ceramic,
usually yttria stabilized zirconia (YSZ), which is an
oxide ion conductor at elevated temperatures. The
cathode is typically a perovskite material such as
strontium doped lanthanum manganite, often mixed
with the YSZ in the form of a composite. The anode
is a cermet of nickel and YSZ. The main difference
between the high temperature SOFC and the IT-SOFC
lies in (1) the thickness of the electrolyte,
which tends toward 20 mm thick films for IT-SOFCs
to reduce ionic resistance, and (2) the interconnect
material, with stainless steel being used at the lower
temperatures of the IT-SOFC, whereas more expen-sive
high chrome alloys, or oxides such as lanthanum
chromite, are needed at higher temperatures.
SOFCs lend themselves to applications in which
their high-temperature heat can be used. This heat
can be used in two basic ways: for heating processes
such as those in industry or in homes or for
integration with turbines in hybrid cycles for very
high efficiency electricity production. Recent ad-vances
in microturbines have led to the concept of a
combined power plant of 250kWe with an efficiency
approaching 70%. Specific applications in which
SOFCs may be used are in decentralized electricity
generation of 250kWe to 30MWe, industrial cogen-eration
of 1–30MWe, or domestic applications of 1–
5 kWe. Intermediate temperature SOFCs are also of
interest for vehicle auxiliary power unit (APU)
applications, operating on diesel or gasoline. Carbon
754 Fuel Cells

7. monoxide is not a poison for SOFCs, meaning that a
wide range of fuels can be used, together with a
simpler, and therefore cheaper, fuel processor. It is
also possible to recuperate heat from the fuel cell
within the fuel reformer, improving system efficiency
when compared to low temperature fuel cells when
operating on hydrocarbon fuels.
3. FUEL SELECTION
All fuel cells can run on hydrogen as a fuel. This is
combined with oxygen, normally fed into the fuel
cell as air, to form water, as shown in reaction (1).
However, high-temperature fuel cells can also run on
other fuels, especially hydrogen-rich gases, and some
low-temperature systems are able to run on specific
liquid fuels. The application of the fuel cell system
often determines on which fuel it will run. Although
fuel cells require comparatively clean fuels, they are
also flexible in which ones they will accept. High-temperature
fuel cells will operate directly on
hydrogen-rich gases such as methane and are being
successfully tested on forms of biogas. They, and
their lower temperature cousins, will also accept any
form of gas produced from liquid or solid fuels,
provided that it contains a high percentage of
hydrogen. This allows for great flexibility according
to location and enables the efficient conversion of
many indigenous or waste resources. This is dis-cussed
in more detail in the following for two
applications, stationary power generation and trans-portation.
3.1 Stationary Power Generation
The common fuel for those commercial stationary
systems that exist (mainly PAFC based) is natural
gas, which is reformed by a separate steam reformer
before the hydrogen is fed into the fuel cell stack.
Demonstration PEMFC systems also use this meth-od.
In contrast, high-temperature fuel cells are able
to operate directly on some gas streams, though they
may need to be cleaned up by the removal of sulfur
and higher hydrocarbons. The advantage is that
high-temperature cells are able to reform the fuel
(methane, for example) directly on the anode of the
fuel cell because they have sufficient thermal energy
and catalyst present to do so. Both steam reforming
and partial oxidation reforming can be used. The
former is preferred on the grounds of efficiency,
though partial oxidation may be needed in an
external reformer or for startup:
Steam reforming:
Fuel Cells 755
CnHmþnH2O3nCOþðn þ m=2ÞH2 ð7Þ
Partial oxidation:
CnHm þ ðn=2ÞO23nCOþðm=2ÞH2: ð8Þ
Good thermal integration by incorporating the
reformer within a high-temperature fuel cell stack
offers significant efficiency gains over an external
reformer. However, the internal reforming process
can cause severe temperature gradients across the
stack, and many designers prefer to use a separate
catalyst bed, distributed within the fuel cell stack.
These processes are called direct and indirect internal
reforming, respectively. As well as the reformer,
low temperature fuel cells also require a high- and
low temperature shift reactor to carry out the water
gas shift reaction of CO to CO2:
CO þ H2O3CO2þH2; ð9Þ
together with selective oxidation, to reduce the CO
content to parts per million levels.
Fuel cells will also run on reformate from other
fuel sources, and there are successful demonstration
PAFC systems using gas from landfill sites or sewage
farms.
3.2 Transportation
The fuelling problem in transport is more complex
than in stationary systems, because there are far
more severe size, weight, cost, and performance
constraints. Vehicles could be fuelled using pure
hydrogen, stored onboard the vehicle. Indeed, this is
how most demonstration buses are supplied, making
them true zero-emission vehicles. However, there is
rarely provision for hydrogen fuelling, so if fuel cells
are to be used in cars there may have to be another
solution.
Like stationary systems, transport fuel cells
(usually PEMFC) can operate on a hydrogen-rich
reformate. This is less efficient than using pure
hydrogen but may be a pragmatic approach to the
fuelling issue. Methanol has been suggested as a good
compromise fuel. Although it also requires a supply
infrastructure to be developed, it is a liquid and can
be handled in a similar way to gasoline, though
concerns have been raised regarding its toxicity level.
It is also comparatively easy to process into hydro-gen.
Gasoline can also be reformed using a sequence
of autothermal, partial oxidation, and water gas shift
reformers, though the resulting reformate stream
contains less hydrogen than from methanol and
reduces the system performance somewhat more.

8. The fuelling question remains significant. If
methanol turns out to be the fuel of choice, then
not only will methanol reformers with sufficiently
high performance and low cost have to be integrated
into all fuel cell cars, but the problem of supplying
methanol to the consumer will also have to be
addressed. However, the consensus if opinion in this
sector seems to be moving toward hydrogen as the
fuel of choice. For this to become a reality, hydrogen
storage systems need to be improved, though it is
possible to use compressed hydrogen in cylinders
designed for compressed natural gas. Equally im-portant
are the standards and regulatory decisions
that have to be made to allow hydrogen vehicles on
the road.
In some cases, fuels other than hydrogen can be
used within the fuel cell without reforming into
hydrogen. Significant effort, for example, is being
applied to the development of a direct methanol fuel
cell that can use methanol as a fuel directly without
any preprocessing. While this decreases system
complexity, it makes the fuel cell more difficult to
design and could result in lower efficiency. However,
this route is particularly attractive for microfuel cells,
which aim to replace battery technology.
4. FUEL CELL APPLICATIONS
There are many areas in which fuel cells could
potentially be used to replace conventional power
equipment, discussed under the broad headings of
stationary power generation, transport, and battery
replacement.
4.1 Stationary Power Generation
Even within stationary applications there are a
number of distinct divisions, though the most
important ones have to do with temperature and
the amount of waste heat that can be used. In
general, plant sizes of several hundred kWe’s (enough
for a leisure center or small office block) down to 1
to 5 kWe (a single house or a portable power unit)
are under development.
High-temperature systems can be used in more
demanding applications where larger systems are
required or additional heat is useful. Such systems
are also more efficient when operating on fuels such
as natural gas, as they enable internal reforming.
While heat of 801C or more can be used for space
heating, hot water and possibly absorption chillers to
allow for cooling, industrial steam raising, or gas
turbine bottoming cycles require temperatures of at
least 5001C. This not only allows for the potential of
generating extra electrical power and thus improving
the overall system efficiency to nearly 70%, but also
the possibility of using cogenerated heat and
increasing total energy efficiency to 90%. Either of
these options brings down the cost per unit of energy
even if the capital cost of the system is high, but can
only be effectively based around a high-temperature
fuel cell variant.
4.2 Transport
Transport applications tend to demand rapid startup
and instant dynamic response from fuel cell systems,
so a high-temperature fuel cell is unlikely to be
competitive as the main engine in applications such
as cars and buses. The prime candidate for these
vehicle propulsion systems is the PEMFC, which
exhibits both of the above characteristics while also
having very high power density. This is important as
it must also occupy a similar amount of space to an
internal combustion engine. Of recent interest has
been the development of auxiliary power units for
vehicles, in which the fuel cell meets the onboard
electric load of the vehicle. Both PEMFCs and
ITSOFCs are under development for this application.
AFC systems have traditionally been used in space
applications by NASA––in the Gemini, Apollo, and
space shuttle programs—but are also being investi-gated
for certain transport applications, mainly in
vehicles with limited duty cycles such as delivery
vehicles and fork lifts.
Of course, transport is not confined to the car and
aerospace markets—locomotives, ships, scooters,
and a whole host of other applications offer potential
for a variety of fuel cell systems. For ships and trains,
where the application is almost akin to having a
stationary power plant running all the time, startup
and system dynamics are less important than noise,
emissions, fuel consumption, and vibration. Serious
investigations are under way as to the benefits of
installing SOFCs, for example, on ships to remove
the marine diesel—traditionally a source of heavy
pollution, very high noise levels, and damaging
vibration. However, in these cases the preferred fuel
would be heavy fuel oil, one of the most difficult to
process for a fuel cell application.
PEMFC systems have also been commissioned for
a number of the world’s navies––Ballard and Siemens
have each been active in their respective countries
putting PEMFCs into submarines.
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9. 4.3 Battery Replacement
An area in which it is possible that fuel cells may
break through to commercialization in the very near
future is in the replacement of conventional batteries.
Battery power for laptop computers, mobile phones,
and many other devices is expensive and often
inconvenient if recharging is required every few
hours. A small fuel cell with an equally small fuel
source could potentially operate for longer than a
battery, but with refueling only taking a few minutes
instead of many hours. Batteries cost much more for
the amount of power they can supply than almost
any other application, and this gives the fuel cell a
good chance of entering the market, although it is
still expensive. The ideal fuel cell for these applica-tions
is the PEMFC, which not only has the high
power density required for miniaturization, but also
has the least challenging electrolyte management
issues of the low-temperature fuel cells.
4.4 The Fuel Cell Industry
The recent upsurge of interest in fuel cell systems has
led to rapid growth in industries developing fuel cell
technology around the world. As well as fuel cell
technology developers, there are an even larger
number of component suppliers, which are not
dedicated to the fuel cell industry but have a
significant role to play, for example, in the supply
of compressors, pumps, valves, and so forth. Indeed,
it is estimated that typically only around one-third of
the cost of a fuel cell system is the actual fuel cell
stack, with around one-third being balance of plant,
and one-third integration, installation, and commis-sioning
costs. Cost issues are discussed in more detail
in the following section.
5. FUEL CELL COSTS
The true costs associated with fuel cells are not yet
clear—either from a capital or operating perspective.
Present costs are well above conventional technolo-gies
in most areas, though this depends slightly on
the type of fuel cell and the market area in which it
may play a part.
The economics of fuel cell systems are also
different in different market niches. The fuel cell
has the potential to usurp many traditional technol-ogies
in a variety of markets, from very small
batteries and sensors to multimegawatt power
plants. Each system has very different characteristics
Fuel Cells 757
and will accept very different prices. For example, a
laptop battery substitute that could run for 20 h
instead of 2 h could command a high price, especially
if it could be refueled in seconds from a canister
rather than recharged over several hours. At the
other end of the scale, the potential for building
modular power plants in which maintenance can be
carried out on each module without shutting down
the system is worth a significant amount of money to
the owner.
Traditional economic calculations have suggested
that the fuel cell system for large-scale power
generation needs to be less then $1500/kWe before
it will be competitive, while the fuel cell system for
automobiles and mass production must be competi-tive
with the internal combustion engine at $50/kWe
or below. That said, it must be remembered other
drivers exist for the technology, including environ-mental
benefits, and an issue of increasing strategic
importance for many counties, namely a reduced
reliance on oil. Some fuel cell systems will sell
themselves at $10,000/kWe, however, if they can be
installed where there is currently no available
technology capable of meeting requirements.
However, it is clear that all fuel cell costs at
present—and these are estimated at anything be-tween
500 and 10,000 dollars per kilowatt (a mature
technology such as a gas turbine costs about $400/
kWe)—are high because they are representative of an
emerging technology. Once in mass production,
recent estimates predict costs of $40 to $300/kWe
for PEMFCs for transport applications, depending
on assumptions regarding technology development.
It is clear that both further technical innovation as
well as mass manufacture will be needed to compete
solely on a cost basis with the internal combustion
engine.
High-temperature systems tend to be more ex-pensive
as they require significant investment in
associated balance of plant, but should still be able to
be manufactured for close to $600 per kilowatt, not
far from the current price for a gas turbine or gas
engine.
6. ENVIRONMENTAL BENEFITS
One of the areas in which fuel cells should
demonstrate significant advantages is in their poten-tial
for minimal environmental impact. As well as
offering a high theoretical efficiency, all fuel cells
emit low levels of pollutants such as the oxides of
sulfur (SOx) and nitrogen (NOx). SOx emissions are

10. low because low sulfur fuels such as methanol or
desulfurized natural gas are used. NOx emissions are
low because even the high-temperature fuel cells such
as the molten carbonate fuel cell and the solid oxide
fuel cell operate at temperatures well below those
needed to form NOx by the thermal combination of
nitrogen and oxygen. The formation of NOx at high
temperatures is a problem for those trying to push up
the efficiency of heat engines by increasing their
maximum operating temperature. Fuel cell systems
are already exempt from permitting requirements in
some U.S. states, including New York, meaning that
they can bypass one of the planning stages (in itself a
useful bonus in reduced cost and time).
The high efficiency of the electrochemical process
is an added advantage, since less fuel is required to
produce a given amount of power than would
otherwise be needed. This means that less fuel is
used and, of increasing importance, that less CO2 is
released. Estimates suggest that a fuel cell power
plant running on a traditional cycle could produce 20
to 30% less CO2 than traditional power plants, and
that vehicles powered by fuel cells would provide
similar benefits.
In the case of vehicles, it is particularly important
to examine the fuel on which they run. While fuel cell
vehicles powered on gasoline reformer technology
will have little or no benefit in reduced greenhouse
gas emissions, methanol-powered cars may produce
25% less CO2. Using hydrogen produced from
natural gas will result in cuts of up to 40%, while
hydrogen from renewably generated electricity, such
as wind or solar, emits no greenhouse gas at any stage
of the process.
In the ultimate renewable/zero-emission scenario,
fuel cells can be run on pure hydrogen produced
from renewable energy, using electrolysis of water.
This ensures that there are no polluting emissions at
any stage of the fuelling process (though there will
inevitably be some from the manufacture and
construction of the plant). This scenario is applicable
both to stationary energy and to transport, and
would enable countries to reduce their dependence
on imported energy in various forms, in addition to
being very clean. Hydrogen can also be produced
from a whole variety of primary resources, including
biomass and fossil fuels.
SEE ALSO THE
FOLLOWING ARTICLES
Alternative Transportation Fuels: Contemporary
Case Studies Batteries, Overview Batteries,
Transportation Applications Cogeneration Fuel
Cell Vehicles Fuel Economy Initiatives: Interna-tional
Comparisons Hydrogen, End Uses and
Economics Hydrogen, History of Hydrogen
Production Transportation Fuel Alternatives for
Highway Vehicles
Further Reading
Fuel Cell Today. www.fuelcelltoday.com. Accessed on November,
2003.
Kordesch, K., and Simader, G. (1996). ‘‘Fuel Cells and Their
Applications.’’ VCH, Weinheim, Germany.
Larminie, J., and Dicks, A. (2000). ‘‘Fuel Cell Systems Explained.’’
Wiley, Chichester, United Kingdom.
Minh, N. Q., and Takahashi, T. (1995). ‘‘Science and Technology
of Ceramic Fuel Cells.’’ Elsevier, Amsterdam.
Steele, B. C. H., and Heinzel, A. (2001). Materials for fuel cell
technologies. Nature 414, 345–352.
U.S. Department of Energy (2000). ‘‘Fuel Cell Handbook.’’ 5th Ed.
Produced under contract DE-AM26–99FT40575.
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