AN
INTRODUCTION TO FUEL CELLS AND HYDROGEN TECHNOLOGY
(YUNI SALMA D.)
Brian Cook
Heliocentris
3652 West 5th Avenue
Vancouver, BC V6R-1S2
Canada
December 2001
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
ii
An Introduction to Fuel Cells and Hydrogen Technology
1. What is a fuel
cell?..................................................................................................................
1
2. History of fuel
cells.................................................................................................................
2
2.1 The “Gas Battery”
..........................................................................................................
2
2.2 The “Bacon fuel cell”
.....................................................................................................
4
2.3 Fuel cells for
NASA.......................................................................................................
5
2.4 Alkaline fuel cells for terrestrial applications
................................................................ 6
2.5 The PEM fuel
cell...........................................................................................................
7
2.5.1 Ballard Power
........................................................................................................7
2.5.2 Los Alamos National Laboratory
.......................................................................... 8
3. Fuel cell applications
..............................................................................................................
8
3.1 Transportation
................................................................................................................
8
3.2 Distributed power generation
.......................................................................................
10
3.2.1 Grid-connect
applications....................................................................................11
3.2.2 Non-grid connect
applications.............................................................................
11
3.3 Residential
Power.........................................................................................................
12
3.4 Portable
Power.....................................................................................................14
3.4.1 Direct methanol fuel cells for portable power
..................................................... 16
4. The science of the PEM fuel cell
..........................................................................................
17
4.1 The Chemistry of a Single Cell
....................................................................................
17
4.2 The Polymer Electrolyte Membrane
(PEM)................................................................. 19
4.3 Cell Voltage and Efficiency
.........................................................................................
20
5. Direct methanol fuel
cell.......................................................................................................
22
6. Where will the hydrogen come
from?...................................................................................
22
6.1 Reformation of hydrocarbon
fuels................................................................................
23
6.2 Renewable Energy Systems .........................................................................................
24
6.3 Biological Methods
......................................................................................................25
7. Benefits and obstacles to the success of fuel cells and the
development of a hydrogen-based
economy........................................................................................................................................
25
7.1 Benefits........................................................................................................................
25
7.2 Obstacles
......................................................................................................................
26
8. Conclusion ............................................................................................................................
27
9.
References............................................................................................................................
27
9.1 Internet
sources.............................................................................................................
28
10. Aknowledgements
............................................................................................................
28
11. About the Author
..............................................................................................................
28
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
iii
FIGURES
FIGURE 1. Trends in the use of fuels. As fuel use has developed
through time, the
percentage of hydrogen content in the fuel has increased.
.................................... 2
FIGURE 2. The principle of an
electrolyzer.............................................................................
3
FIGURE 3. Grove’s ‘gas battery’ (1839) produced a voltage of about
1 volt, and Grove’s ‘gas
chain’ powering an electrolyzer
(1842)................................................................. 3
FIGURE 4. Bacon’s laboratory, at the Department of Chemical
Engineering, Cambridge
University
(1955)...................................................................................................
4
FIGURE 5. NASA Space Shuttle Orbiter fuel cell. .................................................................
5
FIGURE 6. Two prototype automobiles powered by fuel cells, the
NECAR 5 and JEEP
Commander, from
DaimlerChrysler......................................................................
9
FIGURE 7. A fuel-cell distributed power plant.
.................................................................... 12
FIGURE 8. A fuel-cell power plant for residential applications
provides 7 kilowatts heat and
electricity, enough power for a modern energy efficient home. ..........................
13
FIGURE 9. A prototype portable fuel cell provides 50 watts
electrical power. .................... 14
FIGURE 10. Graphs comparing the energy density of compressed
hydrogen (3000 psi) versus
lithium-ion and lead acid batteries.......................................................................
15
FIGURE 11. A prototype direct methanol fuel cell used as a lithium
battery charger provides
up to 20 watts electrical power.
.......................................................................... 17
FIGURE 12. Diagram of a single PEM fuel
cell....................................................................... 18
FIGURE 13. Chemical structure of a PEM fuel cell membrane. Long
chains of PTFE
(Teflon®) with side chain ending with sulphonic acid (HSO3)...........................
20
FIGURE 14. Close-up of a PEM fuel cell membrane.
............................................................. 20
FIGURE 15. Graph comparing carbon dioxide emissions of cars, using
different types of fuel
sources.
................................................................................................................
23
FIGURE 16. Electrical power from renewable energy sources.
............................................... 24
AN
INTRODUCTION TO FUEL CELLS AND HYDROGEN TECHNOLOGY
by
Brian Cook, Heliocentris (Vancouver, Canada)
Whereas the 19th Century was the century of
the steam engine and the 20th
Century was
the century of the internal combustion engine,
it is likely that the 21st Century will be the
century of the fuel cell.
Full cells are now on the verge of being
introduced commercially, revolutionizing the
way we presently produce power. Fuel cells can
use hydrogen as a fuel, offering the
prospect of supplying the world with clean,
sustainable electrical power.
1. WHAT
IS A FUEL CELL?
A fuel cell by definition is an electrical cell,
which unlike storage cells can be
continuously fed with a fuel so that the
electrical power output is sustained indefinitely
(Connihan, 1981). They convert hydrogen, or
hydrogen-containing fuels, directly into
electrical energy plus heat through the
electrochemical reaction of hydrogen and oxygen
into water. The process is that of electrolysis
in reverse.
Overall reaction: 2 H2(gas) + O2(gas) → 2 H2O + energy (Eq.1)
Because hydrogen and oxygen gases are
electrochemically converted into water, fuel
cells have many advantages over heat engines.
These include: high efficiency, virtually
silent operation and, if hydrogen is the fuel,
there are no pollutant emissions. If the
hydrogen is produced from renewable energy
sources, then the electrical power produced
can be truly sustainable.
The two principle reactions in the burning of
any hydrocarbon fuel are the formation of
water and carbon dioxide. As the hydrogen
content in a fuel increases, the formation of
water becomes more significant, resulting in
proportionally lower emissions of carbon
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
2
dioxide (Fig. 1). As fuel use has developed
through time, the percentage of hydrogen
content in the fuels has increased. It seems a
natural progression that the fuel of the future
will be 100% hydrogen.
W o o d
C o a l O i l N a t u r a l
G a s
H y d r
o g e n
2. HISTORY
OF FUEL CELLS
2.1
The “Gas Battery”
Sir William Grove (1811-96), a British lawyer
and amateur scientist developed the first
fuel cell in 1839. The principle was discovered
by accident during an electrolysis
experiment. When Sir William disconnected the
battery from the electrolyzer and
connected the two electrodes together, he
observed a current flowing in the opposite
direction, consuming the gases of hydrogen and
oxygen (Fig. 2). He called this device a
‘gas battery’. His gas battery consisted of
platinum electrodes placed in test tubes of
hydrogen and oxygen, immersed in a bath of
dilute sulphuric acid. It generated voltages
Figure 1. Trends in the use of fuels. As fuel
use has developed through time, the percentage of hydrogen content
in the fuel has increased.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
3
Dilute
acid
electrolyte
Platinum
electrodes
O2 H2 O2 H2
A
of about one volt. In 1842 Grove connected a
number of gas batteries together in series to
form a ‘gas chain’. He used the electricity
produced from the gas chain to power an
electrolyzer, splitting water into hydrogen and
oxygen (Fig. 3). However, due to
problems of corrosion of the electrodes and
instability of the materials, Grove’s fuel cell
was not practical. As a result, there was little
research and further development of fuel
cells for many years to follow.
Figure 2. The principle of an electrolyzer,
shown left; of a fuel cell, shown right. (Larminie, 2000).
Figure 3. Grove’s ‘gas battery’ (1839) produced
a voltage of about 1 volt, shown left. Grove’s ‘gas
chain’ powering an electrolyzer (1842), shown
right. (Photo courtesy of Berry, 2000)
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
4
2.2
The “Bacon fuel cell”
Significant work on fuel cells began again in
the 1930s, by Francis Bacon, a chemical
engineer at Cambridge University, England. In
the 1950s Bacon successfully produced
the first practical fuel cell, which was an
alkaline version (Fig. 4). It used an alkaline
electrolyte (molten KOH) instead of dilute
sulphuric acid. The electrodes were
constructed of porous sintered nickel powder so
that the gases could diffuse through the
electrodes to be in contact with the aqueous
electrolyte on the other side of the electrode.
This greatly increased the contact area contact
between the electrodes, the gases and the
electrolyte, thus increasing the power density
of the fuel cell. In addition, the use of
nickel was much less expensive than that of
platinum.
Alkaline fuel cell:
Anode reaction: 2 H2 + 4 OH- → 4 H2O+ 4 e- (Eq. 2.1)
Cathode reaction: O2 + 4 e- + 2 H2O → 4 OH- (Eq. 2.2)
Overall reaction: 2 H2 + O2 → 2 H2O (Eq. 2.3)
Figure 4. Bacon’s laboratory, at the Department
of Chemical Engineering, Cambridge University
(1955). A fuel cell stack can be seen being
assembled on the left of the photograph (Photo courtesy of
Department of Chemical Engineering, University
of Cambridge).
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
5
2.3
Fuel cells for NASA
For space applications, fuel cells have the
advantage over conventional batteries, in that
they produce several times as much energy per
equivalent unit of weight.
In the1960s, International Fuel Cells in Windsor
Connecticut developed a fuel cell power
plant for the Apollo spacecraft. The plant,
located in the service module of the
spacecraft, provided both electricity as well as
drinking water for the astronauts on their
journey to the moon. It could supply 1.5
kilowatts of continuous electrical power. Fuel
cell performance during the Apollo missions was
exemplary. Over 10,000 hours of
operation were accumulated in 18 missions,
without a single in-flight incident (IFC).
In the 1970s, International Fuel Cells developed
a more powerful alkaline fuel cell for
NASA’s Space Shuttle Orbiter (Fig. 5). The
Orbiter uses three fuel cell power plants to
supply all of the electrical needs during
flight. There are no backup batteries on the space
Figure 5. NASA Space Shuttle Orbiter fuel cell.
One of three fuel cells aboard the Space Shuttle.
These fuel cells provide all of the electricity
as well as drinking water when Space Shuttle is in flight. It
produces 12 kilowatts electricity, and occupies
154 litres. (Photo courtesy of NASA).
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN TECHNOLOGY
6
shuttle, and as such, the fuel cell power plants
must be highly reliable. The power plants
are fuelled by hydrogen and oxygen from
cryogenic tanks and provide both electrical
power and drinking water. Each fuel cell is
capable of supplying 12 kilowatts
continuously, and up to 16 kilowatts for short
periods. The Orbiter units represent a
significant technology advance over Apollo,
producing about ten times the power from a
similar sized package. In the Shuttle program,
the fuel cells have demonstrated
outstanding reliability (over 99% availability).
To date, they have flown on 106 missions
and clocked up over 82,000 hours of operation
(NASA).
2.4
Alkaline fuel cells for terrestrial applications
Compared with other types of fuel cells, the
alkaline variety offered the advantage of a
high power to weight ratio. This was primarily
due to intrinsically faster kinetics for
oxygen reduction to the hydroxyl anion in an
alkaline environment. Therefore alkaline
fuel cells were ideal for space applications.
However, for terrestrial use, the primary
disadvantage of these cells is that of carbon
dioxide poisoning of the electrolyte. Carbon
dioxide is not only present in the air but also
present in reformate gas, the hydrogen rich
gas produced from the reformation of hydrocarbon
fuels.
In the poisoning of an alkaline fuel cell, the
carbon dioxide reacts with the hydroxide ion
in the electrolyte to form a carbonate, thereby
reducing the hydroxide ion concentration
in the electrolyte. This reduces the overall
efficiency of the fuel cell. The equation of
carbon dioxide reacting with a potassium
hydroxide electrolyte is shown below:
2 KOH + CO2 → K2CO3 + H2O (Eq. 2.4)
Because of the complexity of isolating carbon
dioxide from the alkaline electrolyte in
fuel cells for terrestrial applications, most
fuel cell developers have focused their
attention on developing new types using
electrolytes which are non-alkaline. These fuel
cells include: solid oxide fuel cells (SOFC), phosphoric
acid fuel cells (PAFC), molten
carbonate fuel cells (MCFC) and polymer
electrolyte membrane (PEM) fuel cells.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
7
2.5
The PEM fuel cell
In the early 1960s, General Electric (GE) also
made a significant breakthrough in fuel
cell technology. Through the work of Thomas
Grubb and Leonard Niedrach, they
invented and developed the first polymer
electrolyte membrane (PEM) fuel cell. It was
initially developed under a program with the US
Navy’s Bureau of Ships and U.S. Army
Signal Corps to supply portable power for
personnel in the field.
Attracted by the possibility of using GE´s PEM
fuel cell on the Apollo missions, NASA
tested its potential to provide auxiliary power
onboard its Gemini spacecraft. The Gemini
space program consisted of 12 flights in
preparation for the Apollo missions to the moon.
For lunar flights, a longer power source was
required than could be provided by batteries,
which had been used on previous space flights.
Unfortunately, the GE fuel cell, model
PB2, encountered technical difficulties prior to
launch, including the leakage of oxygen
through the membrane. As a result the Gemini
missions 1 to 4 flew on batteries instead.
The GE fuel cell was redesigned and a new model,
the P3, successfully operated on the
Gemini flights 6 to 12. The Gemini fuel cell
power plant consisted of two fuel cell battery
sections, each capable of producing a maximum
power of about 1000 watts (NASA).
2.5.1
Ballard Power
A further limitation of the GE PEM fuel cell at
that time was the large quantity of
platinum required as a catalyst on the
electrodes. The cost of PEM fuel cells was
prohibitively high, restricting its use to space
applications. In 1983, Geoffrey Ballard a
Canadian geophysicist, chemist Keith Prater and
engineer Paul Howard established the
company, Ballard Power. Ballard took the
abandoned GE fuel cell, whose patents were
running out and searched for ways to improve its
power and build it out of cheaper
materials (Koppel, 1999).
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
8
Working on a contract from the Canadian
Department of National Defence, Ballard
developed fuel cells with a significant increase
in power density while reducing the
amount of platinum required. From these developments
it was recognized that fuel cells
could be made smaller, more powerful and cheaply
enough to eventually replace
conventional power technologies.
Ballard Power has since grown to become
recognized as a world leader in PEM fuel cell
technology, developing fuel cells with power
outputs ranging from 1 kilowatt, for
portable and residential applications, through
to 250 kilowatts for distributed power.
Ballard has formed alliances with a wide range
of companies, including DaimlerChrysler,
Ford, Coleman and Ebara Power in Japan.
2.5.2
Los Alamos National Laboratory
In the late 1980s and early 1990s Los Alamos
National Laboratory and Texas A&M
University also made significant developments to
the PEM fuel cell. They also found
ways to significantly reduce the amount of
platinum required and developed a method to
limit catalyst poisoning due to the presence of
trace impurities in the hydrogen fuel (Los
Alamos National Laboratory).
3. FUEL
CELL APPLICATIONS
3.1
Transportation
The California Low Emission Vehicle Program,
administered by the California Air
Resources Board (CARB), has been a large
incentive for automobile manufacturers to
actively pursue fuel cell development. This
program requires that beginning in 2003, ten
percent of passenger cars delivered for sale in
California from medium or large sized
manufacturers must be Zero Emission Vehicles,
called ZEVs. Automobiles powered by
fuel cells meet these requirements, as the only
output of a hydrogen fuel cell is pure
water.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
9
The NECAR 5 (Fig. 6) is the latest prototype
fuel cell automobile by DaimlerChrysler.
This automobile is fuelled with liquid methanol
which is converted into hydrogen and
carbon dioxide through use of an onboard fuel
processor. The vehicle has virtually no
pollutant emissions of sulphur dioxide, oxides
of nitrogen, carbon monoxide or
particulates, the primary pollutants of the
internal combustion engine. The efficiency of a
fuel cell engine is about a factor of two higher
than that of an internal combustion engine
and the output of carbon dioxide is considerably
lower.
Figure 6. Two prototype automobiles powered by
fuel cells, the NECAR 5 and JEEP Commander, from
DaimlerChrysler (Photo courtesy of
DaimlerChrysler).
The NECAR 5 drives and feels like a “normal”
car. It has a top speed of over 150 km/hr
(90 mph), with a power output of 75 kW (100
horsepower). It is also believed that this
vehicle will require less maintenance. It
combines the low emission levels, the quietness
and the smoothness associated with electric
vehicles, while delivering a performance
similar to that of an automobile with an
internal combustion engine.
In April 1999 the California Fuel Cell
Partnership was developed. Founding members
included DaimlerChrysler, the California Air
Resources Board, the California Energy
Commission, Ballard Power, Ford, Shell and
Texaco. The primary objective was to help
commercialize fuel cell technology for vehicles
through joint demonstration programs by
the partners. Since then new participants have
included General Motors, Honda, Hyundai,
Nissan, Toyota, Volkswagen, British Petroleum,
Exxon Mobil, Xcellsis, US Department
of Energy and US Department of Transportation.
To date seven of the world’s ten leading
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
10
auto manufacturers have announced that they plan
to introduce fuel cell automobiles
beginning in the 2003 to 2005 timeframe.
There are also plans for buses, trucks and
trains all powered with fuel cell engines. In
2000, Ballard completed a two-year program
testing six fuel cell buses, three in
Vancouver, British Columbia and three in
Chicago. The design and maintenance
requirements of fuel-cell vehicles as well as
public acceptance were included in the study.
The results of the tests were exemplary. Thirty
new buses powered by Ballard’s fuel cell
will be introduced to 10 European cities
beginning in 2002 for additional field testing.
The resulting data will be used to further
develop a commercial fuel cell bus.
3.2
Distributed power generation
Electrical energy demands throughout the world
are continuing to increase. In Canada the
demand is growing at an annual rate of
approximately 2.6%. In America the rate is about
2.4% (IEA., 1997), and in developing countries
it is approximately 6% (Khatib., 1998).
How can these energy demands be met responsibly
and safely? Distributed power plants
using fuel cells can provide part of the
solution.
Distributed or “decentralized” power plants,
contrasted with centralized power plants, are
plants located close to the consumer, with the
capability of providing both heat and
electrical power ( a combination known as
“cogeneration”). Heat, the by-product of
electrical power generation, is transferred from
the fuel cell to a heat exchanger. The
exchanger transfers the heat to a water supply,
providing hot water to local customers.
The overall efficiency of a cogeneration system
can be in excess of 80 percent,
comparatively high compared to a system
producing electricity alone. An increase in
efficiency naturally corresponds to a decrease
in fuel consumption.
Distributed power plants have many additional
advantages. For example, they can
provide power to a remote location without the
need of transporting electricity through
transmission lines from a central plant. There
is also an efficiency benefit in that the cost
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
11
of transporting fuel is more than offset by the
elimination of the electrical losses of
transmission. The ability to quickly build up a
power infrastructure in developing nations
is often cited. Using fuel cell power plants
obviates the need for an electrical grid.
3.2.1
Grid-connect applications
Distributed power plants can provide either
primary or back-up power. In primary
applications they can provide base-load power,
operating virtually continuously from the
consumption of natural gas, reducing the demand
from the electrical grid. This not only
decreases the cost of displaced power, but can
also result in a reduction of demand
charges imposed by the utility. Should the power
plant provide an excess of electricity,
the excess can be fed back into the electrical
grid, resulting in additional savings.
In case of a power outage on the grid, a
distributed power plant can continue to provide
power to essential services; eliminating the
need for both an uninterruptible power supply
(UPS), presently handled by lead-acid battery
banks, and a stand-by generator, for
extended periods of power outage. An additional
quality of a fuel cell power plant for
UPS applications is that the average “down time”
is anticipated to be low, 3.2 to 32
seconds per year versus typically nine hours for
a conventional battery-bank UPS (HDR
Engineering). For industries where UPS systems
are critical, such as banking, minimizing
down time is of up most importance.
3.2.2
Non-grid connect applications
Other applications for fuel cell distributed
power plants are also possible e.g. stand-alone
back-up power generators. The fuel cell plant
can be started in seconds, supplying power
for as long as required from stored hydrogen,
producing electrical power cleanly and
virtually silently.
Shown in Figure 7 is a prototype fuel cell
distributed power plant, by Ballard Power. This
unit provides 250 kilowatts of electricity and
an equivalent amount of heat. This is
enough power for a community of about 50 homes,
or a small hospital or a remote
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
12
school. This particular unit incorporates a fuel
processor so that natural gas can be used
as a fuel. The fuel processor converts the
natural gas, through the process of reformation,
into a hydrogen-rich gas composed primarily of
hydrogen and carbon dioxide. The
hydrogen is used by the fuel cell and the carbon
dioxide is released into the atmosphere.
Figure 7. A fuel-cell distributed power plant.
This unit, produced by Ballard Power provides 250 kilowatts
heat and electricity which is enough power for
an industry, a school or a community of up to 50 homes.
(Photo courtesy of Ballard Power)
Eventually as an infrastructure for hydrogen
develops, these units could be powered with
hydrogen directly without the need of a fuel
processor. Ballard Power is presently fieldtesting
five of these units in the United States,
Germany, Japan and Switzerland, with
four more units planned for 2002. Testing is
expected to continue until 2004 after which
commercial introduction is planned (Ballard
Power).
3.3
Residential Power
Fuel cell power plants are also being developed
by several manufacturers to provide
electricity and heat to single-family homes.
Fuelled by either natural gas or propane,
these plants will be able to supply base-load
power or all of the electricity required by a
modern-day home.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
13
Ballard Power has developed a one-kilowatt fuel
cell designed to supply both base-load
electrical power as well as heat to a dwelling.
This unit can also be fuelled by natural gas.
It does not provide enough power to supply the
total electrical demands of a residence,
but it does shift a portion of the demand from
the electrical grid to natural gas. The
electrical efficiency of this fuel cell system
is rated at 42% and the heat efficiency is rated
at 43%. Therefore the combined cogeneration
efficiency of the system can be as high as
85%. This particular generator is targeted at
the Japanese residential market. Ballard’s
goal is to commence sales of these units in
2004.
Plug Power, based in Latham, New York has
developed a new fuel cell power plant that
supplies seven kilowatts of electrical power to
the home plus heat, using either natural
gas or propane as the fuel (Fig. 8). This is
enough power to supply the electrical needs of
a modern energy efficient house. At present,
these units are designed to be used in
parallel with the grid. This means the fuel cell
will supply base-load power and the utility
grid will handle momentary power surges. Should
the electric grid fail, the fuel cell
operates as a back-up generator providing power
for the home’s critical requirements.
Figure 8. A fuel-cell power plant for
residential applications provides 7 kilowatts heat and electricity,
enough power for a modern energy efficient home.
(Photo courtesy of Plug Power)
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
14
Second generation products will be designed to
run independent of the grid. During 2000,
Plug Power installed and tested 52 systems in
the field and accumulated over 133,000
hours of system run-time. General Electric which
is marketing Plug Power’s unit, has
announced that commercial introduction of this
home fuel cell power plant is expected in
2002 (Plug Power).
3.4
Portable Power
Several manufacturers are also developing fuel
cell power supplies for portable
applications, providing a few watts up to
several kilowatts of electricity (Fig. 9). Fuelled
by stored natural gas, propane, methanol or
hydrogen gas, portable fuel cells may one day
replace both gasoline and diesel-engine
generators for portable applications as well as
conventional batteries for uses such as remote
lighting, laptop computers and mobile
phones.
Figure 9. A prototype portable fuel cell
provides 50 watts electrical power. The supply of hydrogen for
this unit, by Heliocentris, is stored in the
attached metal hydride canister.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
15
Compared with engine-driven mobile electrical
generators, fuel cells have the significant
advantage of being quiet and having low
emissions. As they have few moving parts
(only external pumps and fans) they are operate
virtually silently. If stored hydrogen is
the fuel, again the only emission is pure water.
A significant advantage of the fuel cell over
its battery counterpart is that of its energy
density (Fig. 10). Portable power packs using
fuel cells can be lighter and smaller in
volume for an equivalent amount of energy,
particularly the direct methanol fuel cell.
Note that the comparison here is the fuel tank.
0
50
100
150
200
250
300
Compressed
hydrogen Lithium-ion battery Lead-acid battery
Storage
Density by Volume
Energy
Density
(Whr/l)
Storage
Density by Weight
Compressed
hydrogen Lithium-ion battery Lead-acid battery
240
100
70
240
100
Energy
Density
(Whr/l)
Compressed
hydrogen (3000 psi) vs. Lithium-ion and lead-acid batteries
Comparing
Energy Density:
Figure 10. Graphs comparing the energy density
of compressed hydrogen (3000 psi) versus lithiumion
and lead acid batteries.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
16
“The fuel cell makes sense when the energy
storage required by an application represents
many hours of operation at full power. The
durability of batteries in this sort of
application is at best a few hours. The size, weight,
and cost of energy storage for a fuel
cell powerplant easily out competes batteries.
You do have the fixed cost (and size and
weight) of the plant, which is a function of
power. This is why it is important to note that
the advantage of fuel cells is for low power,
high energy applications.” (Ric Pow of Pow
Consulting, 2001)
Rechargeable batteries will discharge over time;
the colder the ambient temperature the
quicker they will discharge. Also the charge
capacity of a rechargeable battery decreases
with the number of times of charge and
discharge. Conversely, providing the hydrogen
supply is sealed correctly, a fuel cell will not
discharge over time, maintaining its full
charge capacity almost indefinitely.
3.4.1
Direct methanol fuel cells for portable power
Direct methanol fuel cells were invented and
initially developed at the Jet Propulsion
Laboratory in Pasadena, California. They were
designed to supply electricity for field
troops in the Armed Forces and for applications
with NASA (Fig. 11). The direct
methanol fuel cell has the advantage over the
hydrogen fuel cell in that they can use a
liquid fuel i.e. methanol without the need for
external reforming. Liquid fuel is easy to
store and has a high energy density compared to
compressed hydrogen. At present, the
direct methanol fuel cell suffers from
relatively low efficiency and high cost, owing to
required platinum loading compared to that of
the hydrogen fuel cell. However, as this
improves, it is expected that the direct
methanol fuel cell will play a leading role in
providing power for portable and possibly
transportation applications.
Ballard Power, Motorola, the Los Alamos National
Laboratory and Manhattan Scientific
are all actively pursuing the development of the
direct methanol fuel cell. Motorola
claims that a portable cell phone will be able
to remain fully charged on standby for a
month rather than days. The company has also
announced that it plans to have its version
commercially available in three to five years.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
17
4. THE
SCIENCE OF THE PEM FUEL CELL
4.1
The Chemistry of a Single Cell
In a PEM fuel cell, two half-cell reactions take
place simultaneously, an oxidation
reaction (loss of electrons) at the anode and a
reduction reaction (gain of electrons) at the
cathode. These two reactions make up the total
oxidation-reduction (redox) reaction of
the fuel cell, the formation of water from
hydrogen and oxygen gases.
As in an electrolyzer, the anode and cathode are
separated by an electrolyte, which allows
ions to be transferred from one side to the
other (Fig. 12). The electrolyte in a PEM fuel
cell is a solid acid supported within the
membrane. The solid acid electrolyte is saturated
with water so that the transport of ions can
proceed.
Figure 11. A prototype direct methanol fuel cell
used as a lithium battery charger provides up to 20
watts electrical power. (Photo courtesy of Jet
Propulsion Laboratory/NASA)
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
18
PEM Fuel Cell:
Anode reaction: H2 → 2H+ + 2e- (Eq. 3.1)
Cathode reaction: ½O2 + 2e- + 2H+ → H2O (l) (Eq. 3.2)
Overall reaction: H2 + 1/2 O2 → H2O (l) (Eq. 3.3)
At the anode, the hydrogen molecules first come
into contact with a platinum catalyst on
the electrode surface. The hydrogen molecules
break apart, bonding to the platinum
surface forming weak H-Pt bonds. As the hydrogen
molecule is now broken the oxidation
reaction can proceed. Each hydrogen atom
releases its electron, which travels around the
external circuit to the cathode (it is this flow
of electrons that is refered to as electrical
current). The remaining hydrogen proton bonds with
a water molecule on the membrane
surface, forming a hydronium ion (H3O+). The hydronium ion travels through the
membrane material to the cathode, leaving the
platinum catalyst site free for the next
hydrogen molecule.
Figure 12. Diagram of a single PEM fuel cell.
When an electrical load is attached across the anode
and the cathode of the fuel cell a redox
reaction occurs. The working voltage produced by one cell
in this process is between 0.5 and 0.8 volts,
depending on the load. To create practical working
voltages, individual fuel cells are stacked
together in series to form a fuel cell stack.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
19
At the cathode, oxygen molecules come into
contact with a platinum catalyst on the
electrode surface. The oxygen molecules break
apart bonding to the platinum surface
forming weak O-Pt bonds, enabling the reduction
reaction to proceed. Each oxygen atom
then leaves the platinum catalyst site,
combining with two electrons (which have
travelled through the external circuit) and two
protons (which have travelled through the
membrane) to form one molecule of water. The
redox reaction has now been completed.
The platinum catalyst on the cathode electrode
is again free for the next oxygen molecule
to arrive.
This exothermic reaction, the formation of water
from hydrogen and oxygen gases, has
an enthalpy of -286 kilojoules of energy per
mole of water formed. The free energy
available to perform work decreases as a
function of temperature. At 25º C, 1 atmosphere
the free energy available to perform work is
about -237 kilojoules per mole. This energy
is observed as electricity and heat.
4.2
The Polymer Electrolyte Membrane (PEM)
The membrane material used in a PEM cell is a
polymer. PEMs are generally produced in
large sheets. The electrode catalyst layer is
applied to both sides, and is cut to the
appropriate size. A single PEM sheet is
typically between 50 to 175 microns thick, or
around the thickness of 2 to 7 sheets of paper.
A common PEM material used today is Nafion®. Developed in the 1970s by Dupont,
Nafion consists of Polytetrafluoroethylene
(PTFE) chains, commonly known as Teflon®
forming the backbone of the membrane. Attached
to the Teflon chains, are side chains
ending with sulphonic acid (HSO3) groups (Fig. 13). A close-up view of the membrane
material shows long, spaghetti-like chain
molecules with clusters of sulphonate side
chains (Fig. 14). An interesting feature of this
material is that whereas the long chain
molecules are hydrophobic (repel water), the
sulphonate side chains are highly
hydrophylic (attract water).
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
20
For the membrane to conduct ions efficiently the
sulphonate side chains must absorb
large quantities of water. Within these hydrated
regions, the hydrogen ions of the
sulphonic acid groups can then move freely,
enabling the membrane to transfer hydrogen
ions, in the form of hydronium ions from one
side of the membrane to the other.
Polytetrafluoroethylene
(PTFE) chains
F F F F
F F F F F F F F F F F
- C-C-
C- C- C- C-C-C-C-C-C-C-C-C-C-
F F F F
F F F O F F F F F F F
F -C- F
F -C- F
O
F -C- F
F -C- F
O=S =O
OH
+
Figure 13. Chemical structure of a PEM fuel cell
membrane. Long chains of PTFE (Teflon®) with
side chain ending with sulphonic acid (HSO3). (Source: Larminie & Dicks, February 2000)
Figure 14. Close-up of a PEM fuel cell membrane.
Diagram shows long spaghetti-like chain molecules
of Teflon surrounding clusters of hydrated
regions around the sulphonate side chains. The Teflon chains
form the backbone of the membrane. The hydrated
regions around the sulphonate side chains become
the electrolyte. (Source: Larminie & Dicks,
February 2000)
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
21
4.3
Cell Voltage and Efficiency
If the fuel cell was perfect at transferring
chemical energy into electrical energy, the ideal
cell voltage (thermodynamic reversible cell
potential) of the hydrogen fuel cell would be
at 25º C, 1 atmosphere, 1.23 volts. As the fuel
cell heats up to operating temperature,
around 80º C the ideal cell voltage drops to
about 1.18 volts. However there are many
limiting factors that reduce the fuel cell
voltage further. The voltage out of the cell is a
good measure of electrical efficiency; the lower
the voltage, the lower the electrical
efficiency and the more chemical energy is
released in the formation of water and
transferred into heat.
The primary losses that contribute to a
reduction in cell voltage are:
Activation losses. Activation losses are a result of the energy required to initiate
the reaction. This is a result of the catalyst.
The better the catalyst the less
activation energy is required. Platinum forms an
excellent catalyst however there
is much research underway for better materials.
A limiting factor to power density
available from a fuel cell is the speed at which
the reactions can take place. The
cathode reaction, (the reduction of oxygen) is
about 100 times slower than that of
the reaction at the anode, thus it is the
cathode reaction that limits power density.
Fuel crossover and internal currents. Fuel crossover and internal currents are a
result of fuel that crosses directly through the
electrolyte, from the anode to the
cathode without releasing electrons through the
external circuit, thereby
decreasing the efficiency of the fuel cell.
Ohmic losses. Ohmic
losses are a result of the combined resistances of the
various components of the fuel cell. This
includes the resistance of the electrode
materials, the resistance of the electrolyte
membrane and the resistance of the
various interconnections.
Concentration losses (also referred to as “mass transport”). These losses result
from the reduction of the concentration of
hydrogen and oxygen gases at the
electrode. For example, following the reaction
new gases must be made
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
22
immediately available at the catalyst sites.
With the build up of water at the
cathode, particularly at high currents, catalyst
sites can become clogged,
restricting oxygen access. It is therefore
important to remove this excess water,
hence the term mass transport.
5. DIRECT
METHANOL FUEL CELL
A direct methanol fuel cell also uses a PEM
membrane. However, other catalysts in
addition to platinum are required on the anode
side of the membrane to break the
methanol bond in the reaction forming carbon
dioxide, hydrogen ions and free electrons.
As with the hydrogen fuel cell, the free
electrons flow from the anode of the cell through
an external circuit to the cathode and the
hydrogen protons are transferred through the
electrolyte membrane. At the cathode the free
electrons and the hydrogen protons react
with oxygen to form water.
Direct Methanol Fuel Cell:
Anode reaction: CH3OH + H2O → CO2 + 6H+ + 6e- (Eq. 4.1)
Cathode reaction: 3/2O2 + 6H+ + 6e- → 3H2O (Eq. 4.2)
Overall reaction: CH3OH + 3/2O2 → CO2 + 2 H2O (Eq. 4.3)
6. WHERE
WILL THE HYDROGEN COME FROM?
One of the most important questions to be asked
is: where the hydrogen will come from?
A very interesting study published by the
Pembina Institute, based in Calgary, Alberta,
compared total carbon dioxide emissions of fuel
cell vehicles using hydrogen produced
from a variety of methods (Fig 15). The results
clearly show that the choice as to which
method will be used to produce the hydrogen will
be a critical environmental decision.
For example, if hydrogen is produced from the
electrolysis of water and the electrolysers
are powered from the electrical grid, whereby
the electricity is produced from a coal
burning power station, then there will be no
reduction in carbon dioxide emissions
compared with the levels of the present day
internal combustion engine. In fact, there will
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
23
be an increase in metals & pollutants into
the environment. If on the other hand the
electrolyzer is powered from a renewable energy
source, through use of a solar panel, a
wind turbine or a hydroelectric turbine, there
will be no emissions of carbon dioxide. The
choice has yet to be made as to which method of
hydrogen production will dominate as
the fuel cell industry grows.
Carbon
dioxide emissions (kilograms) per 1000 km
0
50
100
150
200
250
300
1 2 3 4
5 6
1 Car
with internal combustion engine
2 Fuel
cell car with hydrogen produced from Alberta electric grid (coal generation)
3 Fuel
cell car with onboard gasoline reformer
4 Fuel
cell car with onboard methanol reformer
5 Fuel
cell car using hydrogen from natural gas (distributed from urban retail
outlets)
6 Fuel
cell car using hydrogen from natural gas (made at large refineries)
6.1
Reformation of hydrocarbon fuels
For the short term, because of the abundance of
natural gas, the availability of methanol
and propane, and the lack of a hydrogen
infrastructure, it is expected that hydrocarbon
fuels will be the dominant fuels for stationary
fuel cell applications. For as long as these
fuels are cheaply available, reformation of a
hydrocarbon fuel is the most cost efficient
method of producing hydrogen. In the reformation
of a hydrocarbon fuel however, there
is an emission of carbon dioxide. Although
carbon dioxide is not considered a pollutant,
controversy exists that man-made emissions may
contribute to global warming.
Figure 15. Graph comparing carbon dioxide
emissions of cars, using different types of fuel sources.
(Reprinted by permission from Pembina Institute)
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
24
6.2
Renewable Energy Systems
Hydrogen can be produced sustainably with no
emission of carbon dioxide from
renewable energy systems. An example of such a
system is the use of a solar panel, a
wind turbine or a micro-hydro generator to
convert the radiant energy of sunlight into
electrical power, which drives an electrolyzer.
The electrolyzer breaks apart water
producing hydrogen and oxygen gases. The
hydrogen is stored for use by the fuel cell and
the oxygen is released into the atmosphere. Thus
when the sun shines, the wind blows or
the water flows, the electrolyser can produce
hydrogen.
A power system incorporating hydrogen from
renewable sources and a fuel cell is a
closed system, as none of the products or
reactants, water, hydrogen and oxygen are lost
to the outside environment. The water consumed
by the electrolyzer is converted to
gases. The gases are converted back to water.
The electrical energy produced by the solar
panel is transferred to chemical energy in the
form of gases. The gases can be stored and
transported, to be reconverted back to
electricity (Fig. 16).
Micro hydro
Storage
H2
Oxygen
Oxygen
Water Water
Fuel
Electrolyzer Cell
Solar Cell
Wind
Figure 16. Electrical power from renewable
energy sources. In the past, the limiting factors of
renewable energy have been the storage and
transport of that energy. With the use of an electrolyzer, a
method of storing and transporting hydrogen gas,
and a fuel cell, electrical power from renewable
energy sources can be delivered where and when
required, cleanly, efficiently and sustainably.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
25
These systems are truly sustainable, for as long
as there is sunlight there can be electrical
power, available where and when required.
6.3
Biological Methods
Research and development is taking place on the
production of hydrogen from biological
methods (BioHydrogen). For example, Dr. A. Melis
at the University of California,
Berkeley has discovered a metabolic switch in
common green algae
(Chlamydomonas reinhardtii) that causes the
algae to oxidize water and to produce pure
hydrogen gas when sulphur nutrients are depleted
from the growth medium. This and
other BioHydrogen mechanisms are presently in
the R & D
stage but may one day provide the world with an
additional source of hydrogen.
7. BENEFITS
AND OBSTACLES TO THE SUCCESS OF FUEL CELLS AND THE
DEVELOPMENT
OF A HYDROGEN-BASED ECONOMY
7.1
Benefits
•
Fuel cells are efficient.
They convert hydrogen and oxygen directly into electricity
and water, with no combustion in the process.
The resulting efficiency is between 50
and 60%, about double that of an internal
combustion engine.
•
Fuel cells are clean. If
hydrogen is the fuel, there are no pollutant emissions from a
fuel cell itself, only the production of pure
water. In contrast to an internal
combustion engine, a fuel cell produces no
emissions of sulphur dioxide, which can
lead to acid rain, nor nitrogen oxides which
produce smog nor dust particulates.
•
Fuel cells are quiet. A
fuel cell itself has no moving parts, although a fuel cell
system may have pumps and fans. As a result,
electrical power is produced relatively
silently. Many hotels and resorts in quiet
locations, for example, could replace diesel
engine generators with fuel cells for both main
power supply or for backup power in
the event of power outages.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
26
•
Fuel cells are modular.
That is, fuel cells of varying sizes can be stacked together to
meet a required power demand. As mentioned
earlier, fuel cell systems can provide
power over a large range, from a few watts to
megawatts.
•
Fuel cells are environmentally safe. They produce no hazardous waste products,
and their only by-product is water (or water and
carbon dioxide in the case of
methanol cells).
• Fuel cells may give us the opportunity to
provide the world with sustainable electrical
power.
7.2
Obstacles
At present there are many uncertainties to the
success of fuel cells and the development
of a hydrogen economy:
•
Fuel cells must obtain mass-market acceptance to succeed. This acceptance
depends largely on price, reliability, longevity
of fuel cells and the accessibility and
cost of fuel. Compared to the price of present
day alternatives e.g. diesel-engine
generators and batteries, fuel cells are
comparatively expensive. In order to be
competitive, fuel cells need to be mass produced
less expensive materials developed.
• An
infrastructure for the mass-market availability of hydrogen, or methanol fuel
initially,
must also develop. At present there is no
infrastructure in place for either of
these fuels. As it is we must rely on the
activities of the oil and gas companies to
introduce them. Unless motorists are able to
obtain fuel conveniently and affordably,
a mass market for motive applications will not
develop.
• At
present a large portion of the investment in fuel cells and hydrogen
technology
has come from auto manufacturers. However, if fuel cells prove
unsuitable for automobiles, new sources of
investment for fuel cells and the
hydrogen industry will be needed.
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
27
•
Changes in government policy could also derail fuel cell and hydrogen
technology
development. At present stringent
environmental laws and regulations,
such as the California Low Emission Vehicle
Program have been a great
encouragement to these fields. Deregulation laws
in the utility industry have been a
large impetus for the development of distributed
stationary power generators. Should
these laws change it could create adverse
effects on further development.
• At
present platinum is a key component to fuel cells. Platinum is a scarce natural
resource; the largest supplies to the world
platinum market are from South Africa,
Russia and Canada. Shortages of platinum are not
anticipated, however changes in
government policies could affect the supply.
8. CONCLUSION
As our demand for electrical power grows, it
becomes increasingly urgent to find new
ways of meeting it both responsibly and safely.
In the past, the limiting factors of renewable
energy have been the storage and transport
of that energy. With the use of fuel cells and
hydrogen technology, electrical power from
renewable energy sources can be delivered where
and when required, cleanly, efficiently
and sustainably.
9. REFERENCES
BERRY M. & MACDONALD A., (2000) Energy
through Hydrogen, Heliocentris
COLELL H., (1998) Solar
Hydrogen Technology, Heliocentris
CONNIHAN M.A., (1981) Dictionary
of Energy, Routledge and Kegan Paul
INTERNATIONAL ENERGY AGENCY, (1997) Energy
Policies of IEA countries,
OECD Publications
KHATIB H., (1998) "Electrical power in
developing countries" Power Engineering Journal, Vol.12, no.10
(October)
KOPPEL, T., (1999) Powering
the Future, John Wiley and Sons
LARMINIE J. & DICKS A., (2000) Fuel
Cell Systems Explained,
John Wiley & Sons
AN INTRODUCTION TO FUEL CELLS AND HYDROGEN
TECHNOLOGY
28
MELIS, A AND HAPPE , T., (2001) Hydrogen
Production: Green Algae as a Source of Energy. Plant
Physiology, Volume 127: pp.740-748
THOMAS, S & ZALBOWITZ, M., (1999) booklet: Fuel
Cells, Green Power, Los Alamos National
Laboratory
9.1
Internet sources
Ballard Power: http://www.ballard.com/250k_stationary.asp
HDR Engineering and Architecture: http://www.hdrinc.com/information/search.asp?PageID=476
IFC: http://www.internationalfuelcells.com/spacedefense/heritage.shtml
Jet Propulsion Laboratory: http://www2.jpl.nasa.gov/files/images/captions/p48600.txt
Los Alamos National Laboratory: http://www.lanl.gov/worldview/science/features/fuelcell.html
NASA: Gemini: http://science.ksc.nasa.gov/history/gemini/gemini-v/gemini-v.html
Space Shuttle Orbiter: http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-eps.html
Pembina Institute: http://www.pembina.org/pubs/pdf/fuelcell.pdf
Plug Power: http://www.plugpower.com/technology/
Smithsonian Institution: http://americanhistory.si.edu/csr/fuelcells/pem/pemmain.htm
10.
AKNOWLEDGEMENTS
The author would like to thank Rachel Browne for
supplying the graphs and drawings
and editing the text, and Ric Pow, from Pow
Consulting, Vancouver for reviewing the
paper and providing technical advice.
11.
ABOUT THE AUtHOR
Brian Cook, from Vancouver, Canada is Director
of Heliocentris North America. He has
a background in electrical technology and a
special interest in environmental
sustainability. Heliocentris develops and
manufacturers products for the classroom
demonstrating fuel cell and hydrogen technology.
Brian Cook can be contacted at:
b.cook@heliocentris.com.
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