Mench M.M. Fuel Cell Engines
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20 Introduction to Fuel Cells
Figure 1.18 Three generations of Ballard Mark 1030 (1320 W rated power) technology. The
Mark 1030 V3 (on left) is 40% lighter and 26% smaller than the previous generation of residential
cogeneration fuel cell. (Image courtesy of Ballard Power Systems, Inc.)
available online at www.fuelcells.org, and some manufacturers do not advertise prototype
demonstrations, so that the exact numbers in Fig. 1.19 are not precise, but the qualitative
trend of explosive growth is clear.
Distributed power plants are even larger than stationary systems and are designed
for megawatt-level capacity. Several have been demonstrated to date, including, a 2-MW
MCFC demonstrated by FuelCell Energy in California [25]. However, as the power is
y
u
s
Figure 1.19 Estimated number of projects initiated to install stationary power sources for 1985–
2002 based on data from [26].
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1.5 Potential Fuel Cell Applications and Markets 21
scaled up to megawatt levels, the efficiency advantages of fuel cells become less favorable
compared to gas turbine based cogeneration units, which has limited development of fuel
cells for megawatt distributed applications.
Transportation Applications Perhaps the most exciting potential application for fuel cell
power is in transportation. The potential for high efficiency with low pollution, and severing
the umbilical cord between the oil industry and the world economy is a strong driver toward
development of fuel cell vehicles. Even early fuel cell pioneers also dreamed of fuel cell
vehicles, and some early prototypes were built, such as the alkaline fuel cell built by
Kordesh shown in Figure 1.20. The bottles on the roof of the car are actually laboratory
cylinders of hydrogen, an approach that is obviously not recommended for safety but was
nevertheless successful. Hundreds of fuel-cell-powered prototype vehicles are now in the
testing stage. In 2003, Toyota leased around 20 fuel cell Rav-4 vehicles for ∼ US$10,000
per month. Figure 1.21 shows a prototype fuel cell vehicle built by Hyundai and Kia Motor
Company.
Although the potential benefits to society are tremendous, fuel cells for transportation
applications suffer the most daunting technological hurdles. The existing combustion engine
technology market dominance will be difficult to usurp, considering its low comparative
cost (∼$30/kW), high durability, high power density, suitability for rapid cold start, and
high existing degree of optimization. Additionally, the success of high-efficiency hybrid
electric/combustion engine technology adds another rapidly evolving target fuel cells must
match. The lack of an existing hydrogen fuel infrastructure and other issues make it highly
likely that initial introduction would be in the form of fleet vehicles such as buses, taxi
cabs, or postal vehicles. In this case, a single refueling station would be needed, and driving
cycles and maintenance could be controlled and measured. In fact, many such demonstration
Figure 1.20 Early 5-kW alkaline fuel cell car built and driven on public roads by K. Kordesch
[27]. Tanks on the top are for hydrogen storage. (Reproduced with permission from Ref. [27].)
c01 JWPR067-Mench December 19, 2007 19:42 Char Count=
22 Introduction to Fuel Cells
Figure 1.21 UTC Power develops proton exchange membrane fuel cell technology for next gen-
eration automobiles and works with major automobile manufacturers, including Hyundai. (Image
courtesy of UTC Power Corporation.)
projects exist. Fuel cell buses have been in operation in cities such as Vancouver, Canada,
and Chicago, Illinois, for several years.
Other Niche Applications Any application requiring electrical power could potentially
operate on fuel cells, although not all make practical sense. The military has a need for fuel
cells for battery replacement and transportation applications. The heavy weight and cost of
primary batteries make fuel cells attractive, even just for training purposes. Replacing some
of the nonrechargeable batteries presently used with reusable fuel cells would represent
a major reduction in waste. Additionally, the potential for stealth and long life wireless
operation is attractive for military reconnaissance and remote-sensing applications. For
military transportation, higher efficiency and longer ranges would be a great benefit, as
some estimates put the cost of a gallon of fuel delivered to the battlefield at over $100.
Regenerative fuel cells are especially suited for space applications, where cargo can
cost up to $10,000 per pound to take to space. In a regenerative fuel cell powered by hydro-
gen and oxygen, the closed fuel cell vessel actually operates as a battery, producing current
from stored oxygen and hydrogen when in the power mode. In the power mode, product
water is generated from the electrochemical reactions. In the regeneration mode, the water
is electrolyzed back into oxygen and hydrogen by reversal of the hydrogen oxidation and
oxidizer reduction reactions. Through this cycle, the fuel and oxidizer are continually recy-
cled and reused, reducing the weight of reactants required to be put into orbit for a chosen
duty cycle. Of course, the second law of thermodynamics requires more electrical power
is input into the electrolysis than is produced by power generating hydrogen oxidation
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1.6 History of Fuel Cell Development 23
reaction.
5
In regenerative fuel cells for space application, however, the additional input
power can come from solar panels, which take advantage of the radiation from the sun.
1.6 HISTORY OF FUEL CELL DEVELOPMENT
In 1839, Sir William Grove conducted the first known demonstration of the fuel cell [28].
It operated with separate platinum electrodes in oxygen and hydrogen gas, submerged in a
dilute sulfuric acid electrolyte solution, essentially reversing a water electrolysis reaction.
Early development of fuel cells focused on use of plentiful coal for fuel, but poisons formed
by the coal gasification limited the fuel cell usefulness and lifetime [29]. High-temperature
SOFCs began with Nernst’s 1899 demonstration of the still-used yttria-stabilized zirconia
solid-state ionic conductor, but significant practical application was not realized [30]. The
MCFC was first studied for application as a direct coal fuel cell in the 1930s [31]. In
1933, Sir Francis Bacon began development of an AFC that achieved a short-term power
density of 0.66 W/cm
2
—high even for today’s standards. However, little additional practical
development of fuel cells occurred until the late 1950s, when the space race between the
United States and the Soviet Union catalyzed development of fuel cells for auxiliary power
applications. Low-temperature PEFCs were first invented by William Grubb at General
Electric in 1955 and generated power for NASA’s Gemini space program. However, short
operational lifetime and high catalyst loading contributed to a shift to AFCs during the
NASA Apollo program, and AFCs still serve as APUs for the Space Shuttle orbiter.
After the early space-related applications, development of fuel cells went into relative
abeyance until the 1980s, when rising energy costs fueled development. The PAFC built by
United Technologies Company (UTC) became the first fuel cell system to reach commer-
cialization in 1991. Although only produced in small quantities of twenty to forty 200-kW
units per year, UTC has installed and operated over two hundred forty-five 200-kW units
similar to that shown in Figure 1.13 in 19 countries worldwide. As of 2002, these units have
successfully logged over five million hours of operation with 95% fleet availability [29].
Led by researchers at Los Alamos National Laboratory in the mid 1980s, resurgent
interest in PEFCs was spawned through the development of an electrode assembly technique
that enabled an-order-of-magnitude reduction in noble metal catalyst loading [29]. This
major breakthrough and ongoing environmental concerns, combined with availability of a
non-hydrocarbon-based electrolyte with substantially greater longevity than those used in
the Gemini program, instigated a resurrection of research and development of PEFCs that
continues today.
Research and development toward commercialization of high-temperature fuel cells
including MCFC and SOFC systems have also grown considerably since 1980, with a
bevy of demonstration units in operation and commercial sales of MCFC systems. Figure
1.22 shows the estimated number of fuel cell systems built for all applications (excluding
educational kits) per year. In 2004, the cumulative number of independent power-generating
systems topped 11,000, with 10,000 of those units built in the preceding decade.
Currently, the PEFC and SOFC are the most promising candidates for conventional
power system replacement, with the MCFC also under continued development. The PAFC
and AFC have all but ceased development efforts, with the exception of niche applications.
Based on the continued market drivers of devinding petroleum resources, environmental
5
If this were not so, we could make a perpetual-motion device, which is impossible.
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24 Introduction to Fuel Cells
Portable Fuel Cell Systems
Figure 1.22 Estimated cumulative number of portable fuel cell systems built (excluding educational
kits) by year. (Based on data adapted from Ref. [32].)
concerns, and wireless technology needs, it is evident that, despite the lingering technical
challenges, continued development of a variety of fuel cell systems will evolve toward
implementation in many, but certainly not all, potential applications. In some cases, im-
provement of existing or other alternative power sources or the exiting technical barriers
will ultimately doom ubiquitous application of fuel cells, while some applications are likely
to enjoy commercial success.
1.7 SUMMARY
The goal of this chapter was to introduce the reader to the wide variety of fuel cell
engine technologies available and begin to dissect them in terms of operating parameters,
strengths and limitations, and potential applications. The basic nomenclature of fuel cells
was introduced, along with the various methods for classification of the various systems
under development. Fuel cell implementation in portable, stationary, and transportation
applications, is highly likely to occur during this century, although the pace is limited by
some technological, safety, and infrastructural hurdles. Some fuel cell systems, such as the
PAFC and MCFC, have already made it to the commercialization stage, although not yet on
a major scale. Other fuel cell systems were heavily developed in the past but development
has been nearly ceased due to certain technical or cost limitations. In the end, fuel cells
are not a panacea that will solve every power generation need, but they do have significant
potential advantages that engender their long-term implementation in many applications.
APPLICATION STUDY: SOCIOECONOMIC IMPACT
OF FUEL CELL IMPLEMENTATION
Imagine that fuel cells are implemented on a worldwide basis in consumer automotive
vehicles in the next 20 years. On the one hand, the possible elimination of petroleum
c01 JWPR067-Mench December 19, 2007 19:42 Char Count=
Problems 25
as a primary fuel source seems incredibly attractive: Middle Eastern instability and the
growing Chinese and Indian economies threaten to increase oil prices and stifle world-
wide economic growth and development. On the other hand, automotive fuel cells would
require hydrogen, which is not a readily available resource and must be produced in an
energy-intensive process that is still many times more expensive per unit of energy than
petroleum. Additionally, the most likely automotive power plant replacement is the PEFC,
which requires platinum catalyst. Platinum is a precious metal and ironically less readily
available than the petroleum it replaces. The question posed is this: Are the worldwide
platinum resources readily available or not? That is, if fuel cells do replace a significant
number of automotive combustion engines over the coming years, will the market for plat-
inum be as precarious as the future market for petroleum? The answer to this question is
quite complex and varies depending on the source of information. For this assignment, you
must investigate the available resources and make some engineering estimates to come to
your own conclusion. Answer the following question in a short written report and include
the websites and resources you consulted. Do you think replacing 50% of the cars on the road
with fuel cells would affect world platinum markets? You will need to estimate the
automotive power plants required worldwide, the amount of platinum per fuel cell,
the average power of the fuel cell used, and several other things. The point of this ex-
ercise is to look beyond the text to find reliable Internet and other resources and use
engineering logic to come to a reasonable scientific conclusion. It is likely that not
every person will come to the same conclusion, but your conclusion should be justi-
fied with reasonable assumptions. Hint: You can use the numbers in Problem 18 to get
started.
PROBLEMS
Calculation/Short Answer Problems
1.1 Go online and try to find 10 reliable fuel cell infor-
mation websites. There are good general sites as well as
industrial sites with reliable information available. There
are also dozens in not hundreds of sites with questionable
accuracy.
1.2 Go online and identify companies that currently are de-
veloping each fuel cell technology listed in Table 1.1. Which
technologies have the most/least current developers?
1.3 Go online to the U.S. Patent and Trade Office and de-
termine how many fuel-cell-related patents were granted
for the latest year available. How does it compare to
Figure 1.1?
1.4 Describe the differences between a battery and a fuel
cell.
1.5 List the relative advantages and disadvantages of the
PAFC, SOFC, MCFC, AFC, H
2
PEFC, and DMFC. List
one potential power application well suited for each type of
fuel cell.
1.6 List some potential niche applications for fuel cells.
1.7 Why do you think AFCs have been successfully imple-
mented in space applications?
1.8 Why don’t fuel cell manufacturers simply use one large
fuel cell plate to obtain the required power output instead
of stacking many fuel cell plates?
1.9 Why would a stack be aligned in series or parallel and
what specific advantages can you think of where a combi-
nation of series and parallel would be useful?
1.10 Find a market price for hydrogen gas and normalize
the energy content compared to the current local price of
gasoline. Using octane as an approximation for gasoline
energy content, compare the cost per kilojoule of energy of
hydrogen to gasoline. What cost per liter of gasoline do you
think must be reached before the hydrogen fuel source is
economically competitive?
1.11 Estimate the current approximate cost per kilowatt
of power from a standard automotive combustion engine.
c01 JWPR067-Mench December 19, 2007 19:42 Char Count=
26 Introduction to Fuel Cells
(Hint: You can find this information if you do an internet
search of “crate engines.”) This is the cost target for fuel
cell systems.
1.12 Estimate the current approximate cost per kilowatt
of power from a standard laptop computer and cell phone
battery. This is the cost target for portable cell systems.
1.13 Estimate the current approximate cost per kilowatt of
power from a standard stationary power generator system.
This is the cost target for stationary cell systems.
1.14 Estimate the power density of a combustion engine
on a volume basis, that is, kilowatts per liter. Compare this
to a power density for PEFCs of 1.3 kW/L.
1.15 Make a plot of the volume of gas-phase hydrogen
required to contain the equivalent energy in a gallon of
gasoline as a function of pressure of 0–10,000 psig. (Hint:
Assume the ideal gas law is valid.)
1.16 List the three products of reaction from an operating
fuel cell and discuss how each can be utilized in a fuel cell
system.
1.17 Consider an automotive application where nominally
300 V is required to operate the electric motors powering
the wheels. If the fuel cells operate at an average cell voltage
of 0.6 V, 1.1 A/cm
2
, how many cells in series will the stack
have? If the cells are 300 cm
2
active area each, what is the
total current out of the stack? If the stack is now arranged
with the individual cells in parallel, rather than in series,
what is the current and voltage output of the stack? Is there
a difference in power output (P
e
= IV) of the two designs
(parallel vs. series)? Why would we choose one design over
the other in a practical application?
Open-Ended Problems
1.18 Consider that, at present, about 0.8 mg/cm
2
total of
platinum is used to make a hydrogen fuel cell that can
operate at around 0.65 W/cm
2
. Calculate how much plat-
inum would be needed to replace 50% of the cars on the
road in the United States alone with fuel cells. Do you
think this would affect world platinum markets? Note:You
will have to go outside the textbook to answer this ques-
tion as well as make some reasonable estimates to enable
calculation. Although it is clear that a great reduction in
the precious metal loading of a hydrogen fuel cell would
be desirable to achieve ubiquitous implementation, there
are differing views about the effect that such a conversion
would have. Do you think platinum recycling would be
neccessary?
1.19 List some potential power source applications where
a fuel cell could potentially replace a battery and discuss
the relative advantages and disadvantages of this. Use Ta-
ble 1.1 to help you decide which fuel cell would be most
appropriate for each application.
1.20 List some potential power source applications where
a fuel cell could potentially replace a combustion-based
power source and discuss the relative advantages and
disadvantages of this. Use Table 1.1 to help you de-
cide which fuel cell would be most appropriate for each
application.
1.21 For a hydrogen-based economy, a large supply of hy-
drogen will be needed. There are many ways in which hy-
drogen generation can be achieved. Do some research and
discuss the various potential sources of hydrogen and com-
ment on the advantages and drawbacks of each source. You
can start in Chapter 8 of this text, but there are many more
resources you can find.
1.22 Consider development of a fuel cell for weight loss.
That is, a fuel cell would be attached to the human body
and be fueled from the glucose in the bloodstream, effec-
tively burning calories from the user’s blood effortlessly.
Consider a current biological glucose-burning fuel cell cur-
rent density of 5 mA/cm
2
at 0.4 V and a fuel cell stack
mounted next to the person with 100 cm
2
per plate and 3
mm total per plate. Calculate the approximate volume of a
weight loss fuel cell stack to burn 500 food calories (e.g.,
500,000 thermodynamic calories) in an hour of use. Is this
practicals? What current density would make this approach
feasible?
REFERENCES
1. U.S. Department of Energy 2003 Multi-Year Research, Development and Demonstration Plan
for Fuel Cells, http://www.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/3.4
fuelcells.pdf.
2. D. Sperling, and J. S. Cannon, The Hydrogen Energy Transition: Moving Toward the Post
Petroleum Age in Transportation, Academic, 2004.
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References 27
3. J. M
¨
uller, G. Frank, K. Colbow, and D. Wilkinson, “Transport/Kinetic Limitations and Effi-
ciency Losses,” in Handbook of Fuel Cells—Fundamentals, Technology and Applications,Vol.4,
W. Vielstich, A. Lamm, and H. A. Gasteiger, Eds., Wiley, 2003, pp. 847–855.
4. R. Doshi, V. L. Richards, J. D. Carter, X. Wang, and M. Krumpelt, “Development of Solid-Oxide
Fuel Cells that Operate at 500
◦
C,” J. Electrochem. Soc., Vol. 146, No. 4, pp. 1273–1278, 1999.
5. R. F. Service, “New Tigers in the Fuel Cell Tank,” Science, Vol. 288, pp. 1955–1957, 2000.
6. http://www.siemenswestinghouse.com/en/fuelcells/hybrids/index.cfm.
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8. B. S. Baker, Ed., Hydrocarbon Fuel Cell Technology, Academic, New York, 1965.
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Cells—Fundamentals, Technology and Applications, Vol. 1, W. Vielstich, A. Lamm, and H. A.
Gasteiger, Eds., Wiley, 2003, pp. 287–300.
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poration, San Diego, CA, 2000.
11. M. Zhao, C. Rice, R. I. Masel, P. Waszczuk, and A. Wieckowski, “Kinetic Study of Electro-
Oxidation of Formic Acid on Spontaneously-Deposited Pt/Pd Nanoparticles—CO Tolerant Fuel
Cell Chemistry,” J. Electrochem. Soc., Vol. 151, No. 1, pp. A131–A136, 2004.
12. M. M. Mench, H. M. Chance, and C. Y. Wang “Dimethyl Ether Polymer Electrolyte Fuel Cells
for Portable Applications” J. Electrochem. Soc., Vol. 151, pp. A149–A150, 2004.
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Direct Oxidation Fuel Cells,” Electrochem. Solid-State Lett., Vol. 4, No. 4, pp. A38–A41, 2001.
14. C. Lamy and E. M. Belgsir, “Other Direct-Alcohol Fuel Cells,” in Handbook of Fuel
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H. A. Gasteiger, Eds., Wiley, 2003, pp. 323–334.
15. H. Liu, R. Narayanan, and B. Logan, “Production of Electricity during Wastewater Treatment
Using a Single Chamber Microbial Fuel Cell,” Environ. Sci. Technol., Vol. 38, pp. 2281–2285,
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Miniature Biofuel Cell,” J. Am. Chem. Soc. Vol. 123, pp. 8630–8631, 2001.
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portation, Elsevier, 2004.
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Energy and Environmental Systems, Division on Engineering and Physical Sciences, The Hydro-
gen Economy: Opportunities, Costs, Barriers, and R&D Needs, National Research Council and
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Washington, DC, 2003.
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28 Introduction to Fuel Cells
24. Toshiba Press Release, June 24, 2004, http://www.toshiba.com/taec/press/dmfc
04 222.shtml.
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2
Basic Electrochemical Principles
The most urgent, long-term security requirement for the United
States is to reduce our dependence on imported oil by developing
clean, safe, renewable energy systems, and energy conservation
programs.
—Rear Admiral Eugene Carroll, U.S. Navy, Retired, Deputy
Director, Center for Defense Information
2.1 ELECTROCHEMICAL VERSUS CHEMICAL
REACTIONS
In chemical or power production, we often have a choice of obtaining a desired result by
either an electrochemical reaction or a purely chemical reaction. An example is electricity.
To generate electrical power, purely chemical combustion of a fuel and oxidizer can be
used to generate heat, which is converted into motion (e.g., in a piston), then converted
into electrical power through a generator. There is also an option to obtain electrical power
directly through an electrochemical reaction of the fuel and oxidizer, which produces
current. There are many reasons why one option is chosen over another in practice, including
convenience, quality, safety, and cost. Whether chemical or electrochemical, the overall
reaction of the same fuel and oxidizer begins with the same chemical energy stored in
the bonds of the reactants and releases the same chemical energy per mole of reactant.
The difference lies in the form of the chemical energy released. Electrochemical reactions,
such as in a battery or fuel cell, provide a direct conversion of chemical energy stored in
bonds between atoms into electrical energy (i.e., current and voltage), whereas a chemical
reaction converts this chemical energy into heat.
Thus, a heat engine is limited in thermal efficiency by the Carnot cycle,
1
and an
electrochemical reaction engine is not. The thermal efficiency of a Carnot cycle, which
is the measure of the maximum thermal efficiency of a chemical reaction heat engine,
is
η
th,carnot
= 1 −
T
L
T
H
(2.1)
1
It is suggested that the reader consult with an undergraduate thermodynamics textbook if unfamiliar with the
Carnot cycle.
29
Fuel Cell Engines
Copyright © 2008 by John Wiley & Sons, Inc.
Matthew M. Mench