Overview of Hydrogen Energy and Fuel Cells
Motivation: Sustainable Energy
Devising sustainable energy resources that are readily accessible to all nations,
environmentally friendly, and cost competitive with fossil fuels (coal, natural gas, and
petroleum oil) is one of the greatest challenges of the 21st century. As of January 1, 2006,
proven world oil reserves were estimated at 1,293 billion barrels with about 71 percent
located in the Middle East or Canada [United States Energy Information Administration,
http://tonto.eia.doe.gov/country/index.cfm?view=reserves]. In 2007, the world
consumption of petroleum was 86 million barrels per day. During the same time period,
the consumption in the United States of America was 21 million barrels per day. [United
States Energy Information Administration, http://www.eia.doe.gov/emeu/ipsr/t21.xls,
accessed January 2009].
Further, as more nations become industrialized, the consumption of these fixed oil
reserves increases. The last few decades of the 20th century alone saw several new
nations become significantly industrialized. The depleting supply of fossil fuel alone is a
reason to seek an alternative fuel for the world. As a final note, energy generation from
fossil fuel has led to significantly increased atmospheric greenhouse gas (e.g., CO, CO2)
levels as well as those of byproducts (NOx, SOx, particulate matter, hydrocarbons etc.).
The Hydrogen Economy and Fuel Cells
A futuristic view of energy supply is based on hydrogen rather than fossil fuels. The
coined term ‘hydrogen economy’ is a hypothetical economy in which energy for
vehicular transportation and electricity is derived from reacting hydrogen (H2) with
oxygen (O2). Depending upon the hydrogen source, a shift to a hydrogen economy may
diminish fossil fuel usage and greenhouse gas emissions. A key technology to enabling a
hydrogen economy is the fuel cell.
Fuel cells are electrochemical energy conversion devices that yield electrical energy from
chemical reactions. These devices vary depending on the chemical reactions generating
the electricity. Furthermore, fuel cells are scalable, as they can be used for a wide range
of uses from small as portable electronics up to power plants. In addition, fuel cells
operate quietly and efficiently.
Engineers calculate the efficiency of any energy converting technology such as fuel cells
as a means to discuss the merits of its design to do work. One method to calculate the
efficiency of a fuel cell at converting chemical energy into electrical energy is to
calculate the ratio of actual electrical energy obtained to the amount that would be
obtained if all the heat energy released from the chemical reactions taking place were
completely converted into electrical energy.
Fuel cells are under consideration to replace traditional technologies such as the internal
combustion engines for transportation applications because they are more efficient at
converting chemical energy to electrical energy. In the internal combustion engine, the
chemical energy in the fuel is converted into heat which is used to provide work to move
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the vehicle. The engine efficiency is highly dependent upon the load. However, in the
fuel cell the chemical energy of the fuel is converted into electricity. The electrical
energy is then converted into work to move the vehicle using an efficient electric motor.
Though conversion efficiency varies with the type of fuel cell, the fuel-to-electricity
efficiencies are typically 40% – 50%. In stationary applications, the waste heat of the fuel
cell can also be used, resulting in higher overall efficiencies.
Fuel cells may also have applications in modern electrical power plants due to their
modular design, scalability, and rapid response to load changes. There may also be
advantages to more localized electricity generation.
Fuel Cell Types
The first “fuel cell” noted was discovered by British scientist Sir William Grove in 1839.
He reacted hydrogen with oxygen to form water and produced direct current (DC)
electricity. The half-cell reactions are simple and can provide clean, drinkable water to
many parts of the world. Since the pioneering fuel cell, several other electrochemical
reactions have been employed to generate electricity thereby forming the basis for a large
variety of fuel cells. Several fuel cell types are presented in Table 1 along with their
respective operating temperature and range of power output.
Table 1. Fuel Cell Types and Operating Ranges*
Type
Mobile Ion
OH-
Approximate
Power Range
500 W – 10 kW
Operating
Temperature
50 – 200 oC
Alkaline Fuel
Cell (AFC)
Proton
Exchange
Membrane Fuel
Cell (PEMFC)
Direct
Methanol Fuel
Cell (DMFC)
Phosphoric
Acid Fuel Cell
(PAFC)
Molten
Carbonate Fuel
Cell (MCFC)
Application
H+
1 W – 100 kW
30 -100 oC
Vehicles and
mobile
applications
H+
1 W – 100 W
20 – 90 oC
Portable
electronics
H+
10 kW – 1 MW
220 oC
Combined heat
and power
CO3-2
100 kW – 100
MW
650 oC
Space program
Medium to
large scale
power
generation
Solid Oxide
O-2
2 kW – 100
500 -1000 oC
Large scale
Fuel Cell
MW
power
(SOFC)
generation
*Reference: Larminie and Dicks, Fuel Cell Systems Explained, 2nd Edition, Wiley
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Electrochemistry of A Proton Electrolyte Membrane Fuel Cell (PEMFC)
The electrochemical events occurring within a PEMFC are shown schematically in
Figure 1. A PEMFC consists of three main components: the anode, the electrolyte, and
the cathode. Different types of fuel cells are distinguished by the nature of the electrolyte.
The electrolyte of a PEMFC is a proton conducting membrane that does not conduct
electrons.
e-
eH2
H2
Anode Reaction:
H2 → 2H+ + 2e-
H2O
O2
H+
H2O
H2
O2
H+
H2
O2
H2
H2
H2
H2O
H2
H2O
H+
H2
H+
Cathode Reaction:
½ O2 + 2H+ +2e- → H2O
O2
Anode
Cathode
Electrolyte
Overall Reaction
H2 + ½ O2 → H2O
Figure 1. Reactions within a PEMFC
At the anode of a PEMFC, hydrogen molecules are dissociated into protons and
electrons, hydrogen oxidation as seen in Equation 1. The released electrons flow through
an external circuit, whereas the protons travel through the electrolyte to the cathode.
H2 → 2H+ + 2e-
(1)
The overall performance of a PEMFC is governed by four parameters:
1. porosity allowing for diffusion of reactants (hydrogen) and products (protons,
electrons)
2. catalytic activity for the reaction of interest
3. electronic conductivity for transport of electrons
4. ionic conductivity for the transport of H+
In particular, the electrolytic membrane of a PEMFC should have high ion conductivity
to allow for the transport of protons from the anode to the cathode, but have negligible
electronic conductivity. The electrolyte must also be an effective barrier to crosselectrode diffusion of fuel and oxidant (air, oxygen). The most common electrolyte
material used in PEMFCs is Nafion®, a sulfonated tetrafluorethylene copolymer, i.e. a
Teflon polymer backbone with perfluorovinyl ether groups terminated with sulfonate
(SO3-) groups. Nafion was the first synthetic polymer developed with ionic properties, a
class of materials now referred to as ionomers.
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At the cathode of a PEMFC, oxygen reacts with protons which have come across the
electrolytic membrane and electrons there by the action of the external circuit to produce
the reaction products water and heat according to Equation 2.
½ O2 + 2H+ + 2e- → H2O + heat (Q)
(2)
Ideally, the water generated is removed in the flowing oxidant exit stream. However, an
active area of research is water management in PEMFCs so as to prevent electrode
‘flooding’ when the latter does not happen efficiently.
The four parameters affecting the performance of a PEMFC cathode are essentially the
same as those for the anode. In particular, porosity and catalytic activity should be high
for the reactants and products specific to the cathode half-cell reaction. For a hydrogenoxygen PEMFC, catalytic activity should be high for oxygen reduction at the cathode.
Both the anode and cathode of a PEMFC are prepared in a similar manner. An electrode
“ink”, having a viscous liquid consistency, is applied to a carbon based gas diffusion
layer (cloth, paper). The “ink” is typically a mixture of supported catalyst (platinum),
electrically conductive particles such as porous carbon black (particles, fibers or
nanotubes), and Nafion ionomer in solution.
The electrochemical reactions that take place in a PEMFC occur in a relatively small
region of space found near the electrode/electrolyte interface referred to as the active
region. Within this active region, the parameters of porosity, catalytic activity, electronic
conductivity, and ionic conductivity together govern the overall performance of a
PEMFC. The importance of the electrochemical events occurring in the active region on
the overall fuel cell performance is true in general for all types of fuel cells.
Basic Fuel Cell Calculations
A fuel cell, as with all electrochemical cells, operates according to Faraday’s Law which
states that the amount of a substance consumed or produced at one of the electrodes is
directly proportional to the amount of electricity (electrons) that passes through the cell.
Thus, the hydrogen consumption rate is proportional to the current in the fuel cell. One
must make use of Faraday’s constant which is equal 96,485 coulombs (C) per mole of
electrons as well as the half cell stoichiometry (2 moles of electrons per mole of
hydrogen, as seen in Equation 1).
For example, in a single cell fuel cell in which hydrogen is being consumed at a rate of
0.00005 mol/s, the electrical current generated by the fuel cell can be calculated by
Equation 3 as:
I
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5 105 mol H 2 2 mol e- 96485C
C
 9.6  9.6 A
s
mol H 2 mol e
s
(3)
The performance of a fuel cell is typically characterized by a polarization plot, which is a
graph of the cell voltage Vc as a function of the electrical current density, i. The current
density is calculated by dividing the current by the cross-sectional area of the fuel cell. A
typical polarization plot is shown in Figure 2 below.
Figure 2. Polarization Plot
The polarization curve gives insight into both chemical and electrical events occurring
within the active region of the fuel cell. The voltage drop, also called overvoltage, can be
attributed to three different physical mechanisms: kinetic losses (for current densities less
than 50 mA/cm2), ohmic losses (for current densities in the range of 50 – 900 mA/cm2),
and mass transport losses (for current densities above 900 mA/cm2). It is noted that all of
the above losses result in fuel being converted into heat instead of electricity.
The open cell voltage (OCV), when no current is drawn (no external voltage is applied),
is the maximum attainable voltage for a given fuel cell. At low current densities, the noninstantaneous reaction kinetics result in a rapid voltage drop from OCV. At intermediate
current densities, there is a linear drop in voltage. This is due to Ohm’s law, V = IR, as
there is some resistance to electrical flow in the fuel cell components. At the higher
current densities, the hydrogen consumption rate begins to approach the mass transfer
rate (hydrogen must be transported through the diffusion layer to the catalyst). Fuel cells
are typically operated in the ohmic loss region.
When judging fuel cell performance, the amount of electrical power produced is of the
greatest interest. Electrical power is calculated from:
P = Vc·I
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(4)
where Vc is the cell voltage and I is the current.
Scaling Up Fuel Cells
A fuel cell based energy generation system capable of supplying enough electrical energy
to meet the needs of a large community such as an urban city can not be achieved by a
single fuel cell. One way to scale up a fuel cell based energy generation system is to
make a very large “sandwich” of individual fuel cells referred to as a fuel cell stack, as
seen in Figure 3. The plates found at the very top and very bottom of the fuel cell stack
are called end plates between which are found a series of complete fuel cells (anode side
bipolar plate, electrolytic membrane, and cathode bipolar plate).
Fuel cell stacks are used when the cell voltage needed is greater than 0.8-1.0 volts. This
typically correlates with 1 Watt of power. As with all electrical systems, voltages placed
in series are additive. Thus, a fuel cell stack with n cells has an effective stack voltage
Vstack equal to the sum of the cell voltages, or Vstack = nVc. When operating at the same
current, more electrical power is produced.
External Circuit
e-
Anode (-)
Cathode (+)
x
Hydrogen out
Air in
(N2, O2,)
Bipolar plate
Thermocouple
insertion cavity
Gas flow
distribution field
(commonly a serpentine pattern)
Hydrogen in
Air out
(N2, O2, H2O)
x
Key
x
Gas flow in
Gas flow out
Membrane Electrode
Assemblies (MEAs)
Figure 3. A schematic of a polymer electrolyte membrane fuel cell stack
This introductory fuel cell module provides the conceptual foundation on which the other
modules build. Chemical engineering concepts and principles will be studied in more
detail in each of the course specific modules.
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Example Problem
Consider a 12 cell fuel cell with a cell area of 100 cm2.
a) If the fuel cell operates at a current of 80 A and uses the polarization plot shown
in Figure 2, determine the stack power in W and hydrogen consumption rate in g
H2 / s.
b) Verify your answers with the animation at http://tinyurl.com/FCabacus which is
also linked to the fuel cell curriculum project website.
Example Problem Solution
a) We can use the polarization plot for this fuel cell to solve for voltage.
Step 1) Determine the current density:
i
80 A 1000 mA
mA
 800 2
2
A
100 cm
cm
Step 2) Use the polarization plot to determine the cell voltage Vc = 0.62 V.
Step 3) Determine stack voltage: Vstack = nVc = 12 (0.62 V) = 7.44 V
Step 4) Determine stack power P = Vstack·I = 7.44 V x 80 A = 595 W
Step 5) Determine hydrogen consumption rate by rearranging Equation 3, noting
that you need to divide by the number of cells:
MH2 
g H2
12cells 80A C/s mol H 2 mol e- 2g H 2
 0.01
A 2 mol e 96485C mol H 2
s
b) The results from using the animation site are similar to those calculated in part a)
above as shown in the image below.
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Fuel Cell Curriculum Project
The fuel cell curriculum website project was initiated to describe fuel cell operating
principles in a chemical engineering context. Fuel cell technology can simultaneously
address energy and environmental challenges facing the world in the 21st century.
Therefore, future chemical engineering students must be directly exposed to fuel cell
science and technology in order to become the forerunners in achieving ground-breaking
advancements in these areas. With a strong academic foundation, upcoming engineers
can make timely innovations in the development and/or optimization of fuel cells.
The chemical engineering modules developed by a CACHE Corporation task force
illustrate the use of core chemical engineering principles to design and engineer better
performing fuel cells and are course specific. These modules are easily integrated into
established chemical engineering curricula.
Fuel Cell References
Fuel Cell Systems Explained, James Larminie and Andrew Dicks, 2nd Edition, Wiley,
New York, 2003, ISBN 0-470-84857-X
Fuel Cells and Their Applications, Karl Kordesch and Gunter Simader, VCH, Federal
Republic of Germany, 1996, ISBN 3-527-28579-2
Websites
Fuel Cells 2000 (www.fuelcells.org)
U.S. Fuel Cell Council (www.usfcc.com)
Department of Defense Fuel Cell Demonstration Project (dodfuelcell.cecer.army.mil)
Los Alamos National Lab (http://www.lanl.gov/orgs/mpa/mpa11/animation.htm)
Ballard (www.ballard.com)
American Institute of Chemical Engineers position statement,
(http://www.aiche.org/uploadedFiles/About/WhoWeAre/Structure/Committees/Departme
ntUploads/PDF/fuelcells.1.pdf)
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