1. INTRODUCTION
Petroleum, the worlds most prolific
fuel, is becoming more scarce and its burning produces emissions which
shoulder much of the responsibility for air pollution (Fig.
1). Contributions also come from deforestation, carbon dioxide from
the burning of coal, and methane release. In order to reverse the trend
of destroying the environment, a change to a more ecologically mundane
resource, or method of producing energy such as hydrodynamic, wind, geothermal,
solar and tidal is desirable. These methods are presently employed in a
somewhat small scale, but require specific environments in order to work
effectively. Fuel cells need no particular environment to work well (other
than a heat sink) and is highly efficient both electrically and physically
(without sound and with far fewer harmful air pollutants).
A fuel cell is an electrochemical
device which brings together hydrogen and oxygen, or air in the midst of
a catalyst to produce electricity, heat and water. (Fig.
2) or (Fig.
2a) The single cell fixture consists a single electrolyte sandwiched
between electrodes. This inner sandwich is then placed in-between current
collectors which usually serve as the poles of the cell. A fuel cell generates
current by transforming (usually by using the catalyst in the electrodes)
hydrogen gas into a mixture of hydrogen ions and electrons on the anode
side of the cell. Because of the insulating nature of the electrolyte,
the anions transfer through the electrolyte to the cathode side of the
cell while the electrons are conducted to the current collectors and through
a load to do work. The electrons then travel to the cathode side current
collector where they disperse onto the electrodes to combine with incoming
hydrogen anions, oxygen, or air in the presence of a catalyst to form water
completing the circuit.
This process occurs in all types
of fuel cells (alkaline, solid polymer, phosphoric acid and solid oxide)
except for molten carbonate. The molten carbonate cell transfers the carbonate
ions formed by the reaction of oxygen and carbon monoxide in the presence
of electrons from the cathode side to the anode side to react with hydrogen
and form water and two electrons for current. Thus the net flow of ions
in the electrolyte is opposite of that in all other fuel cells, but since
the current flows in the same direction as the other fuel cell types, the
anode and cathode keep their polarity.
The fuel cell was first invented
in 1839 by Sir William Grove, a professor of experimental philosophy at
the Royal Institution in London. He tested what turned out to be the precursor
to the phosphoric acid fuel cell by enclosing platinum in tubes of hydrogen
and oxygen gas while submerging the tubes in sulfuric acid. (Fig.
3) Unfortunately, he was hampered by the inconsistency of cell performance
(a common feature of cells today), but realized the importance of the three
phase contact (gas, electrolyte and platinum) to energy generation. He
spent most of his time searching for an electrolyte that would produce
a more constant current. He found several electrolytes which produced current,
but still struggled with consistent results. He also noted the potential
of the energy production method commercially if hydrogen could replace
coal and wood as energy sources (1).
Since that time, researchers world
wide have attempted to increase cell performance electrically, chemically
as well as physically. Their experiments ranged from an improved three
phase contact to smart materials and the adoption of off gases from other
power sources. After over 150 years of research, fuel cells can be divided
into five major categories named after the electrolyte used in each; alkaline,
solid polymer, phosphoric acid, molten carbonate and solid oxide. The five
types resulted from the knowledge that heat accelerates chemical reaction
rates and thus the electrical current. The materials used for electrolytes
have their best conductance only within certain temperature ranges and
thus other materials must be used in order to take advantage of the temperature
increase (2).
The Solid Oxide Fuel Cell
Solid oxide fuel cells (SOFC) which
operate at the highest temperature (1000 - 1100 degrees Celsius) are not
the most reactive because of the low conductivity of its ionic conducting
electrolyte (yttria-stabilized zirconia). (Fig.
4) Many advances have been made in solid oxide fuel cell (SOFC) research
to increase the chemical to electrical efficiency to 50%, but because of
the conductivity and the heat, it has been used mainly in large power plants
which can use the cogeneration of steam for additional power. Because of
the high temperature, the cell requires no expensive catalysts, or additional
humidification and fuel treatment equipment which excludes the cost of
these items. The primary drawback to this type of fuel cell is the cost
of the containment which requires exotic ceramics which must have similar
expansion rates. SOFCs are now being considered for large power plants
and for industrial applications because of its electrolytic resistance
to poisoning which allows internal reforming of many carbon compounds into
hydrogen to create power. (1, 2)
The Molten Carbonate Fuel Cell
The molten carbonate, which operates
at 600 degrees Celsius can use CO as a fuel input on the cathode side but
needs hydrogen on the anode. Although the high temperature allows carbon
in the cell, sulfur can poison the cell in small quantities (~1ppm). Carbonate
ions are produced at the cathode and flow across the membrane to react
with hydrogen and form two electrons, water and carbon dioxide. (Fig.
5) In an actual system, because of the internal heat, the cell can
reform methanol into hydrogen for the anode reaction and use the carbon
dioxide and extra hydrogen (burned in the presence of air) as fuel for
the cathode reaction. The temperature is high enough for additional power
production through cogeneration of steam and low enough eliminate the need
of expensive catalysts and containment required in the SOFC. A MCFC operates
nominally at 0.16 A/cm^2 and 0.75 volts per cell with better performance
under pressurized conditions. Nickel compounds are used for the electrodes
while the electrolyte contains a mixture of 68% lithium carbonate and 32%
potassium carbonate in a porous gamma-lithium-aluminum oxide matrix. The
efficiency using this system has risen to 50% in a combined (electrical
and steam) cycle. MCFC, like the SOFC, is also used for mega-watt size
power plants because of its heat (1, 2).
The Phosphoric Acid Fuel Cell
Phosphoric acid fuel cells (PAFC)
are the oldest type whose origins extend back to the creation of the fuel
cell concept. Many different acids have been used in order to boost performance
such as sulfuric and perchloric acids, but when the temperature increases
above 150 degrees Celsius, high rates of oxygen reduction are possible
which enable phosphoric acid to perform best. The temperature allows the
cell to tolerate 1-2% CO and a few PPM of sulfur in the reactant stream
which benefits the steam reforming process by reducing the requirement
of pure hydrogen input to the anode. The heat generated is not enough for
cogeneration of steam, but is able to warm water and act as a heater for
an increased overall efficiency. The structure of the cell is similar to
figure 2. The electrolyte is flanked by porous graphite carbon coated with
Teflon to allow gases to the reaction sites, but not allow the liquid electrolyte
out. The efficiency of this system is much lower than that of other systems
at 40%, but because of its history, it can be controlled better. PAFCs
are now in production and sell for $2875.00 per kilowatt by International
Fuel Cells, but are also sold be Fugi Electric (1, 2).
The Alkaline Fuel Cell
Although alkaline fuel cells (AFC)
are the most temperamental of all fuel cells, it can produce the maximum
amount of energy (80% efficiency (4) when used as a water heating device).
They use KOH (potassium hydroxide) electrolytes because it is the most
conducting of all alkaline hydroxides, but this requires extremely pure
hydrogen and oxygen input to avoid poisoning. The cell cannot internally
reform any fuel because of the 80 degree Celsius cell operating temperature.
Hydrogen at the anode reacts with the electrolyte creating water and two
electrons which both meet at the cathode with oxygen to complete the circuit.
(Fig.
6) The electrolyte constantly flows through the cell which provides
cooling by convection the porous (and catalyzed) graphite electrodes from
which it picks up hydroxyl ions and a small amount of water in the process.
In an actual cell, one third of the water produced drains on the cathode
side while two thirds resides on the anode side. Again, because of liquid
nature of the electrolyte, semi-permeable, Teflon coated carbon material
is used as electrodes which are heavily catalyzed as compared with other
types of fuel cells because of the low operation temperature (1).
The Solid Polymer Membrane Fuel Cell
Solid polymer fuel cells operate
at around 80 degrees Celsius like AFCs, but do not have the strict requirements
on reactant input, or the efficiency of the alkaline, but at 60% efficiency,
it comes in second only to alkaline. The perfluorinated sulfuric acid membrane
is sandwiched between two platinum catalyzed porous electrodes and compressed
using bolts. (Fig.
7) The electrolyte is very sensitive to CO contamination, so the inlet
streams must be purified. Air can be used instead of oxygen as a reducing
agent, but the efficiency of the system falls drastically as the CO content
in the cathode rises above 0.17 %. The temperature and the solid type of
membrane make mobile power generation possible (1, 5, 6).
Many automotive companies have
decided to use methanol as a fuel for fuel cells by reforming it into hydrogen
because of the capacity of safe hydrogen storage and transportation that
methanol provides. (Fig.
8) A direct methanol fuel cell (DMFC) works on the same premise as
the SPFC except that the temperature is increased (to around 120 degrees
Celsius) for internal reformation of methanol into hydrogen for use in
the cell. This type of fuel cell is still in the design stages because
of the problems of finding a good electrocatalyst to both, transform methanol
into carbon dioxide and hydrogen efficiently and to reduce oxygen in the
presence of methanol should any transfer through the electrolyte which
is in itself a problem (1, 7, 8).
The drive towards new advancements
in fuel cells occurred in lieu of major spending from such groups as the
NASA Apollo (F. T. Bacons' alkaline fuel cells in 1961) and Gemini programs
(General Electrics' solid polymer fuel cell, 1962 - 1966: (Fig.
9)), the oil crisis of 1973 (Westinghouse and others in solid oxide
fuel cells) and todays automotive industry (Ballards' solid polymer fuel
cell and others: (Fig.
10)). The automotive interest occurred mostly because of the political
pressure to lower emissions to cut the increasing degradation of air quality
caused by cars. In terms of environmental damage, it has been proven that
the methane released in the atmosphere is more destructive than pollutants
emitted by automotive engines, but the sanctions are against auto emissions
because they are controllable. Other than public concerns, fuel cell research
has remained largely academic, but the devices needed to handle and store
hydrogen as well as the reforming process from other fuels have been well
employed by industry because of the use of hydrogen in producing ammonia
and other products. Besides the automotive industry, major power companies
world wide have also ascribed to the fuel cell power generation method
in response to emissions problems similar to those posed by automotive
engines. Thus companies such as ONSI, Ballard, Westinghouse, Fugi and others
(see: (Fig.
11)) have begun to fabricate power plants in order to support the plea
for fuel cells for large scale power generation using all five types of
fuel cells. Telecommunication companies are now buying fuel cells for backup
power as well as for cooling equipment.
2. FUEL CELL THEORY
Several processes are involved in
the operation of a fuel cell. The processes can be summarized as: gas transfer
to the reaction sites, the electrochemical reaction at those sites, the
transfer of ions and electrons and their combination at the cathode. Gases
must diffuse through the electrode leaving behind any impurities which
may disrupt the reaction while liquid produced at the surface of the electrolyte,
or added through humidification must be either added to the electrolyte
for hydration, or drawn away from the reaction sites so as not to block
reaction sites. Gases move towards the reaction sites based on the concentration
gradient between the gas channel (high concentration) and the reaction
sites (low concentration). Platinum, which is used as an electrocatalyst
(in AFC, SPFC and PAFC) serves as the actual electrode, but since it must
be physically supported to allow minimum concentrations (and thus less
cost), the support and the platinum become the electrode. (Fig.
12) The concentration gradient refers to the difference between the
concentration of free flowing gas in the channels and the concentration
at the platinum reaction sites. This gradient varies depending on pressure
and temperature of the gases and the diffusion coefficient of the electrode
material. When the gas comes nears the reaction sites, the flow is dominated
by a capillary action based on the reaction rates at the sites (2).
Two main electrochemical reactions
occur in a fuel cell. One at the anode and the other at the cathode.
At the anode;
and at the cathode;
which together give the result:
At the anode, the reaction releases
hydrogen ions and electrons whose transport is crucial to energy production.
The ions build up on the anode creating a positive potential which pushes
the outer ions away from the anode. The ions transfer through the electrolyte
either by remaining connected through an attraction to a water molecule
which travels through the electrolyte, or by transferring between water
molecules. The hydrogen side of the water molecule contains a slight negative
charge which attracts the ions and may become attached to it, but the attraction
is weak so any forces made are easily broken. The actual method of transfer
varies, but is based on the thickness of the membrane, the amount of water
in the membrane and the number of ions transported. Thus, the anode contains
a net positive charge while the cathode, towards which the ions drift,
contains a negative potential.
The acid in the electrolyte serves
to provide structure for the electrolyte as well as a barrier to electrons.
It is conducive for electrons to flow through materials whose electrons
are held loosely (conductive materials) because of the process of electron
transport. Thus, electrons move from the reactions sites on the anode through
the gas diffusion section of the electrode, through the anode current collector,
through a load to do work, across the cathode current collector, through
the gas diffusion section of the electrode on the cathode and then to the
catalyzed reaction sites on the cathode. The electrons do not move through
the electrolyte because the acid chains hold their electrons tightly and
thus constitute an electric insulator. Other criteria for selecting an
electrolyte are its structural stability, low resistance to ionic movement
and low porosity.
If the fuel cell were perfect in
transferring the energy resulting from the formation of water, it would
create 1.23 Volts with little heat. Since the theoretical voltage is a
function of the number of electrons, the free energy of the fuel involved
and the temperature of the reaction, the voltage may be increased using
a higher temperature. Unfortunately there are some inefficiencies which
tend to reduce the actual voltage. These are separated into three categories
ohmic, activation and concentration polarizations (Fig.
13).
Activation polarization is produced
because of the energy intensive activity of the making and breaking of
chemical bonds at the cathode and anode. At the anode, hydrogen fuel enters
a reaction site and is broken into ions and electrons through the use of
the catalyst. The resulting ions form bonds with the catalyst surface while
electrons remain near the ions until another fuel molecule begins to react
with the catalyst, thus breaking the bond with the ion. The energy input
to break the bond with the ion determines whether the electron will bond
again with the ion, or will remain separate. The electrons migrate through
the bipolar plate while the ions diffuse through the electrolyte.
The same procedure occurs at the
cathode. Incoming oxygen is broken up into its components by the catalyst
where it draws electrons, ions and oxygen atoms together to form water
that is taken from the electrode, ejected into the gas channel and out
of the cell. The amount of energy needed for the forming and destroying
of these bonds comes from the fuel, and thus reduces the overall energy
the cell can produce. The reduction is controlled by the reaction rate
of the cell. If the reaction rate increases, the flow rate for fuel must
also increase which increases the kinetic energy and thus lowers the energy
required to break bonds. Increasing temperature, active area of the electrode
and the utilization of the catalyst also lower the effect of activation
polarization.
Ohmic polarization is caused by
electrical losses in the cell. The resistances of the current collecting
plates, electrodes and the electrolyte are all factors which add to the
energy loss. Resistance is added by the electrodes because of the contact
resistance with the current collectors, with the electrolyte and through
the electrode material itself. The electrolyte can add to ohmic polarization
through resistance to ionic flow. Resistances can also be observed from
the material in the plates and when using different methods of energy retrieval
from the cell.
Concentration polarization results
from restrictions to the transport of the fuel gases to the reaction sites.
This usually occurs at high current because the forming of product water
and excess humidification blocks the reaction sites. This polarization
is also affected by the physical restriction of the transfer of a large
atom ,oxygen, to the reaction sites on the cathode side of the fuel cell.
Concentration polarization can be reduced by increasing the gas pressure
which drives water out of the cell and increases fuel concentration, using
high surface area electrodes, or using thinner electrodes which shortens
the path of the gas to the sites (2).
The combination of all three polarizations
have distinguishable effects on the performance of the cell because each
is dominant at different current stages. (Fig.
14) When no current flows (open circuit voltage), the difference between
the theoretical and the actual voltage is the activation polarization.
The activation polarization continues to decrease the voltage at low currents,
but decreases its rate of voltage declination as the current increases
in a parabolic form (usually). The low current inflection in the performance
curve is due to the characteristic of ohmic polarization which adds electrical
resistance. The line of ohmic polarization is almost always straight indicating
a constant resistance (V=IR). The high current inflection point, where
the voltage again begins to plummet, is when concentration polarization
is added to the ohmic and activation resistances. In this high current
range, fuel is used faster than it can be replenished. The volume of water
produced within this current range may also prove to be a determining factor
in the voltage loss. The graph varies in lengths and slopes due to construction
materials, temperatures and pressures of the cell.
The mathematical expression:
(2)
where
= Elemental (usually by area)
Power (kW)
= Temperature (C)
= Total Pressure (kPa)
= Partial Pressure of Oxygen
(kPa)
= Relative air humidity (%)
= Elemental Current (A)
= Open circuit Voltage (mV)
b = Empirical equation constant (mV/dec)
m = Empirical equation constant (mV)
n = Empirical equation constant (cm/mA)
R = Empirical equation constant (Ohm/cm^2)
is used iteratively to solve for the current with power and physical
conditions given. Because of the quadratic nature of the equation with
respect to , there are two possible
solutions of which the lowest is always taken because that will be the
one reached when starting from zero current to the elemental power used
in the equation which is generally per cm^2 active area. The voltage can
then be calculated using:
(2)
which incorporates the inefficiencies of the activation polarization
(first two terms), ohmic polarization (second term) and the concentration
polarization (last two terms). Vo, b, R, m, n, are all constants based
on the type of MEA, the type of oxidant, and the local pressure, temperature,
humidity and oxygen concentration for the elemental unit. Thus the MEA
plane is broken into small area segments which are used to calculate the
performance of the cell. Because the factors which determine the constants
varies from element to element on an MEA and from MEA to MEA, the voltage
and current produced by each cell will not be the same. The data described
in practice is the average of experimental performance rather than the
actual per unit performance is given because of the difficulty in modeling
the flow and the chemical composition of the reactants at every elemental
step of energy production.
The possibility of calculating cell
performance enables researchers to measure the influence of their design.
There are several other methods to calculate the efficiency such as one
which is based on more experimental data taking into account the inefficiencies
of the voltage, current and total stack design. Using this alternate method,
only the efficiency of the total stack, or single cell can be modeled although
this method uses more realistic data and is easier to calculate.
It is important to note that in
order to increase the power of a fuel cell, the voltage, current, or both
must increase. Because of the electrical dynamics of the system, cells
generate current per unit of electrolyte surface area while voltage increases
by stacking the cells (electrical series). Present day fuel cells design
their plates to take advantage of this concept by using large planar cells
with small thicknesses (9).
3. BASIC DESIGNING
In designing the fuel cell assembly,
many functional characteristics must be observed. Hydrogen is extremely
difficult to contain, so the tolerances between materials, the density
of materials and their thickness in the hydrogen circuit must be controlled
so as not to allow the fuel to escape. Spaces in-between materials have
traditionally been sealed through gasketing while the thickness of the
material depends on the type and preparation of the material. Hydrogen
is also flammable and unstable in the presence of other elements. It will
react with most metals on contact thus only gold, silver, aluminum, stainless
steel, nickel and copper have been used. Copper will form a film resulting
in lower electrical efficiency while aluminum in the presence of air automatically
forms a film of aluminum oxide which also reduces its electrical conductance.
In time, all materials except for gold show signs of degradation when placed
in the hydrogen path, but with the materials just described (except for
copper), this occurs at slow pace. Finally, the hydrogen circuit must not
be near any ignition source to reduce the risk of explosion.
The oxygen (reduction) circuit
contains more lenient stipulations. The cross section of flow needs to
be large enough for the small fraction of oxygen in air (and carbon monoxide
in the case of the MCFC) to react at a rate which would support current
demand. If oxygen is used, then the circuit must be free of any ignition
source. The gasketing required for oxygen, or air, is less important than
on the hydrogen side because of the size of the molecule, but it is still
needed to keep the reducing agent from expanding out of crevices under
pressure. The circuit must also be free of those elements which corrode
in the presence of oxygen. Bipolar plates, which are commonly used in stacks,
allow hydrogen on one side and oxygen on the other, so it must conform
to both environments.
When dealing with SPFCs (as with
any other type of fuel cell), electrolytes are not all the same. (Fig.
15) Currently, Dow, DuPont, Etek and others produce their own electrolytes
using different mixtures in order to develop a membrane which resists electrons,
but encourages the travel of hydrogen ions. DuPont (which produces nafion)
was the first to produce the membrane, but recently, Dow and others have
improved on the design which provides a more powerful cell. When designing,
these electrolytes must never come in contact with copper as they will
easily be poisoned and loose their conductivity. If copper could be used
as current collectors, or for electrode substrate material, an appreciable
increase in current would result because of the lower ohmic resistance.
Besides these factors, the presence
of water in the system must be controlled. Water is needed for the electrochemical
processes of the electrolyte, but proves a barrier to reaction gases. Thus,
those reaction sites covered by the liquid reduce the performance of the
cell. Water forms at the anode through humidification of hydrogen and at
the cathode by the product of the chemical reaction as well as the humidification
of the reducing gas. Water is mostly a problem on the cathode side of the
fuel cell at high currents (reaction rates), but an increase in flow velocity
usually clears any blockage based on current collector design.
A fuel cell generates power though
a voltage difference and current, therefore electrical separation of the
poles is required in order not to short the cell thus subverting all current
through the short and not through the load. Any conducting material which
touches both poles will suffice for this action to proceed. This is an
important criteria when selecting a method of compressing the plates together
as most high tensile strength materials have conductive properties. The
cooling water used also may partially short the cell by charging the liquid
in the water thus deionized water is used. The compression of the plates
by these members must be uniform so as to not fracture the plates, but
to improve conductance between the MEA and the current collector.
The gas pressures and gas and cell
temperatures must also be monitored for their influence on performance
and for safety. When increased, these variables have a positive effect
on performance, but if the difference in the pressures of the gases is
too large, structural failure of the MEA, or ejection of the electrolyte
into the gas stream may result. Also, if the temperature varies beyond
material limits, the cell may experience a decline in performance, or failure.
Temperature control is important
during all fuel cell operation, but is especially important when a stack
of cells is placed together. Because thinner cell design has always been
an objective of designers, the cooling of these cells has also required
designing. Many designs use cooling plates with holes drilled through for
water passage, or use the heat of vaporization of water by injecting the
liquid into the inlet stream of the hydrogen. Each provides adequate cooling,
but the water injection requires less actual stack volume. (Fig.
16)
4. THE CURRENT DESIGN
PEMFCs (Proton Exchange Membrane
Fuel Cells) are traditionally designed such that a planar MEA fits between
two parallel plates with serpentine, parallel, series, or series-parallel
channel construction. Gas can then be input and distributed on either side
of the MEA in order to utilize the maximum electrode area (active area).
In actuality, the only portion of MEA surface area that produces power
is that where the reactive gases touch the electrode on either side. In
the traditional design, plates are made to be exact mirrors of each other
such that the high portions of the channels fit over the ribs of the adjacent
current collector thus creating an electrically nonproductive area at the
plateau of the rib. During construction, these plates must be aligned to
limit overlap which may cause flow restriction, or gas release, thus most
MEAs must have an additional hole where an aligning tool may be inserted.
5. THE FUTURE
The future plan for the Kettering
fuel cell laboratory is to produce, or test a five kilowatt fuel cell which
would be connected to an electric motor and a dynamometer for power testing.
Knowing the efficiency of the electrical motor and the horsepower, the
efficiency of the fuel cell will be known. Other plans involve creating
excitement about fuel cells by allowing an automotive company to come on
campus with a fuel cell electric vehicle and give a talk about the system
and how it functions. Any testing of outside fuel cells would also be included
in the labs schedule as well.
Other fuel
cell related pictures.
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