Types of Fuel Cells

The fuel cell will compete with many other energy conversion devices, including the gas
turbine in your city's power plant, the gasoline engine in your car and the battery in your
laptop. Combustion engines like the turbine and the gasoline engine burn fuels and use
the pressure created by the expansion of the gases to do mechanical work. Batteries
convert chemical energy back into electrical energy when needed. Fuel cells should do
both tasks more efficiently.
A fuel cell provides a DC (direct current) voltage that can be used to power motors, lights
or any number of electrical appliances.
There are several different types of fuel cells, each using a different chemistry. Fuel cells
are usually classified by their operating temperature and the type of electrolyte they use.
Some types of fuel cells work well for use in stationary power generation plants. Others
may be useful for small portable applications or for powering cars. The main types of fuel
cells include:
Polymer exchange membrane fuel cell (PEMFC)
The Department of Energy (DOE) is focusing on the PEMFC as the most likely candidate
for transportation applications. The PEMFC has a high power density and a relatively low
operating temperature (ranging from 60 to 80 degrees Celsius, or 140 to 176 degrees
Fahrenheit). The low operating temperature means that it doesn't take very long for the
fuel cell to warm up and begin generating electricity. We’ll take a closer look at the PEMFC
in the next section.
Solid oxide fuel cell (SOFC)
These fuel cells are best suited for large-scale stationary power generators that could
provide electricity for factories or towns. This type of fuel cell operates at very high
temperatures (between 700 and 1,000 degrees Celsius). This high temperature makes
reliability a problem, because parts of the fuel cell can break down after cycling on and off
repeatedly. However, solid oxide fuel cells are very stable when in continuous use. In fact,
the SOFC has demonstrated the longest operating life of any fuel cell under certain
operating conditions. The high temperature also has an advantage: the steam produced by
the fuel cell can be channeled into turbines to generate more electricity. This process is
called co-generation of heat and power (CHP) and it improves the overall efficiency of the
system.
Alkaline fuel cell (AFC)
This is one of the oldest designs for fuel cells; the United States space program has used
them since the 1960s. The AFC is very susceptible to contamination, so it requires pure
hydrogen and oxygen. It is also very expensive, so this type of fuel cell is unlikely to be
commercialized.
Molten-carbonate fuel cell (MCFC)
Like the SOFC, these fuel cells are also best suited for large stationary power generators.
They operate at 600 degrees Celsius, so they can generate steam that can be used to
generate more power. They have a lower operating temperature than solid oxide fuel cells,
which means they don't need such exotic materials. This makes the design a little less
expensive.
Phosphoric-acid fuel cell (PAFC)
The phosphoric-acid fuel cell has potential for use in small stationary power-generation
systems. It operates at a higher temperature than polymer exchange membrane fuel cells,
so it has a longer warm-up time. This makes it unsuitable for use in cars.
Direct-methanol fuel cell (DMFC)
Methanol fuel cells are comparable to a PEMFC in regards to operating temperature, but
are not as efficient. Also, the DMFC requires a relatively large amount of platinum to act
as a catalyst, which makes these fuel cells expensive.
In the following section, we will take a closer look at the kind of fuel cell the DOE plans to
use to power future vehicles -- the PEMFC.

Polymer Exchange Membrane Fuel Cells

The polymer exchange membrane fuel cell (PEMFC) is one of the most promising fuel
cell technologies. This type of fuel cell will probably end up powering cars, buses and
maybe even your house. The PEMFC uses one of the simplest reactions of any fuel cell.
First, let's take a look at what's in a PEM fuel cell:

Figure 1. The parts of a PEM fuel cell

In Figure 1 you can see there are four basic elements of a PEMFC:
• The anode, the negative post of the fuel cell, has several jobs. It conducts the
electrons that are freed from the hydrogen molecules so that they can be used
in an external circuit. It has channels etched into it that disperse the
hydrogen gas equally over the surface of the catalyst.
• The cathode, the positive post of the fuel cell, has channels etched into it
that distribute the oxygen to the surface of the catalyst. It also conducts the
electrons back from the external circuit to the catalyst, where they can
recombine with the hydrogen ions and oxygen to form water.
• The electrolyte is the proton exchange membrane. This specially treated
material, which looks something like ordinary kitchen plastic wrap, only
conducts positively charged ions. The membrane blocks electrons. For a
PEMFC, the membrane must be hydrated in order to function and remain
stable.
• The catalyst is a special material that facilitates the reaction of oxygen and
hydrogen. It is usually made of platinum nanoparticles very thinly coated
 onto carbon paper or cloth. The catalyst is rough and porous so that the
maximum surface area of the platinum can be exposed to the hydrogen or
oxygen. The platinum-coated side of the catalyst faces the PEM.

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Figure 2. Animation of a working fuel cell Chemistry
of a Fuel Cell
Figure 2 shows the pressurized hydrogen gas (H2) entering Anode side:
the fuel cell on the anode side. This gas is forced through 2H2 => 4H+ + 4e-
the catalyst by the pressure. When an H2 molecule comes in
contact with the platinum on the catalyst, it splits into two Cathode side:
H+ ions and two electrons (e-). The electrons are conducted O2 + 4H+ + 4e- => 2H2O
through the anode, where they make their way through the Net reaction:
external circuit (doing useful work such as turning a motor) 2H2 + O2 => 2H2O
and return to the cathode side of the fuel cell.
Meanwhile, on the cathode side of the fuel cell, oxygen gas (O2) is being forced through the
catalyst, where it forms two oxygen atoms. Each of these atoms has a strong negative
charge. This negative charge attracts the two H+ ions through the membrane, where they
combine with an oxygen atom and two of the electrons from the external circuit to form a
water molecule (H2O).
This reaction in a single fuel cell produces only about 0.7 volts. To get this voltage up to a
reasonable level, many separate fuel cells must be combined to form a fuel-cell stack.
Bipolar plates are used to connect one fuel cell to another and are subjected to both
oxidizing and reducing conditions and potentials. A big issue with bipolar plates is
stability. Metallic bipolar plates can corrode, and the byproducts of corrosion (iron and
chromium ions) can decrease the effectiveness of fuel cell membranes and electrodes. Low-
temperature fuel cells use lightweight metals, graphite and carbon/thermoset
composites (thermoset is a kind of plastic that remains rigid even when subjected to high
temperatures) as bipolar plate material.
In the next section, we'll see how efficient fuel-cell vehicles can be.
 Hydrogen
Hydrogen is the most
common element in the
universe. However, hydrogen
does not naturally exist on
Fuel Cell Efficiency Earth in its elemental form.
Pollution reduction is one of the primary goals of the fuel Engineers and scientists must
cell. By comparing a fuel-cell-powered car to a gasoline- produce pure hydrogen from
engine-powered car and a battery-powered car, you can see hydrogen compounds,
including fossil fuels or water.
how fuel cells might improve the efficiency of cars today.
In order to extract hydrogen
Since all three types of cars have many of the same from these compounds, you
components (tires, transmissions, et cetera), we'll ignore have to exert energy. The
that part of the car and compare efficiencies up to the point required energy may come in
where mechanical power is generated. Let's start with the the form of heat, electricity or
fuel-cell car. (All of these efficiencies are approximations, even light.
but they should be close enough to make a rough
comparison.)
If the fuel cell is powered with pure hydrogen, it has the potential to be up to 80-percent
efficient. That is, it converts 80 percent of the energy content of the hydrogen into
electrical energy. However, we still need to convert the electrical energy into mechanical
work. This is accomplished by the electric motor and inverter. A reasonable number for the
efficiency of the motor/inverter is about 80 percent. So we have 80-percent efficiency in
generating electricity, and 80-percent efficiency converting it to mechanical power. That
gives an overall efficiency of about 64 percent. Honda’s FCX concept vehicle reportedly
has 60-percent energy efficiency.

Photo copyright 2007, courtesy AutoMotoPortal.com

Honda's FCX Concept Vehicle

If the fuel source isn’t pure hydrogen, then the vehicle will also need a reformer. A
reformer turns hydrocarbon or alcohol fuels into hydrogen. They generate heat and
produce other gases besides hydrogen. They use various devices to try to clean up the
hydrogen, but even so, the hydrogen that comes out of them is not pure, and this lowers
the efficiency of the fuel cell. Because reformers impact fuel cell efficiency, DOE researches
have decided to concentrate on pure hydrogen fuel-cell vehicles, despite challenges
associated with hydrogen production and storage.
Gasoline and Battery Power Efficiency
The efficiency of a gasoline-powered car is surprisingly low. All of the heat that comes out
as exhaust or goes into the radiator is wasted energy. The engine also uses a lot of energy
turning the various pumps, fans and generators that keep it going. So the overall efficiency
of an automotive gas engine is about 20 percent. That is, only about 20 percent of the
thermal-energy content of the gasoline is converted into mechanical work.
A battery-powered electric car has a fairly high efficiency. The battery is about 90-percent
efficient (most batteries generate some heat, or require heating), and the electric
motor/inverter is about 80-percent efficient. This gives an overall efficiency of about 72
percent.
But that is not the whole story. The electricity used to power the car had to be generated
somewhere. If it was generated at a power plant that used a combustion process (rather
than nuclear, hydroelectric, solar or wind), then only about 40 percent of the fuel required
by the power plant was converted into electricity. The process of charging the car requires
the conversion of alternating current (AC) power to direct current (DC) power. This process
has an efficiency of about 90 percent.
So, if we look at the whole cycle, the efficiency of an electric car is 72 percent for the car,
40 percent for the power plant and 90 percent for charging the car. That gives an overall
efficiency of 26 percent. The overall efficiency varies considerably depending on what sort
of power plant is used. If the electricity for the car is generated by a hydroelectric plant for
instance, then it is basically free (we didn't burn any fuel to generate it), and the efficiency
of the electric car is about 65 percent.
Scientists are researching and refining designs to continue to boost fuel cell efficiency. One
approach is to combine fuel cell and battery-powered vehicles. Ford Motors and Airstream
are developing a concept vehicle powered by a hybrid fuel cell drivetrain named the
HySeries Drive. Ford claims the vehicle has a fuel economy comparable to 41 miles per
gallon. The vehicle uses a lithium battery to power the car, while the fuel cell recharges the
battery.
 Photo © 2007, courtesy Airstream
Ford's Airstream Concept

Fuel-cell vehicles are potentially as efficient as a battery-powered car that relies on a non-
fuel-burning power plant. But reaching that potential in a practical and affordable way
might be difficult. In the next section, we will examine some of the challenges of making a
fuel-cell energy system a reality

Fuel Cell Problems

Fuel cells might be the answer to our power problems, but first scientists will have to sort
out a few major issues:
Cost Golden Catalysts
Chief among the problems associated with fuel cells is how Nanoscale science may
expensive they are. Many of the component pieces of a fuel provide fuel cell developers
cell are costly. For PEMFC systems, proton exchange with some much sought after
membranes, precious metal catalysts (usually platinum), answers. For example, gold is
gas diffusion layers, and bipolar plates make up 70 percent usually an unreactive metal.
of a system's cost [Source: Basic Research Needs for a However, when reduced to
Hydrogen Economy]. In order to be competitively priced nanometer size, gold particles
(compared to gasoline-powered vehicles), fuel cell systems can be as effective a catalyst
must cost $35 per kilowatt. Currently, the projected high- as platinum.
volume production price is $110 per kilowatt [Source:
Testimony of David Garman]. In particular, researchers must either decrease the amount
of platinum needed to act as a catalyst or find an alternative.
Durability
Researchers must develop PEMFC membranes that are
durable and can operate at temperatures greater than 100 Aromatic-based
degrees Celsius and still function at sub-zero ambient
temperatures. A 100 degrees Celsius temperature target is membranes
An alternative to current
required in order for a fuel cell to have a higher tolerance to
impurities in fuel. Because you start and stop a car perfluorosulfonic acid
relatively frequently, it is important for the membrane to membranes are aromatic-
based membranes. Aromatic
remain stable under cycling conditions. Currently
in this case does not refer to
membranes tend to degrade while fuel cells cycle on and off,
the pleasing scent of the
particularly as operating temperatures rise.
membrane -- it actually refers
Hydration to aromatic rings like
Because PEMFC membranes must by hydrated in order to benzene, pyridine or indole.
transfer hydrogen protons, researches must find a way to These membranes are more
develop fuel cell systems that can continue to operate in stable at higher temperatures,
sub-zero temperatures, low humidity environments and high but still require hydration.
What’s more, aromatic-based
operating temperatures. At around 80 degrees Celsius,
membranes swell when they
hydration is lost without a high-pressure hydration system.
lose hydration, which can
The SOFC has a related problem with durability. Solid oxide affect the fuel cell's efficiency.
systems have issues with material corrosion. Seal integrity
is also a major concern. The cost goal for SOFC’s is less restrictive than for PEMFC
systems at $400 per kilowatt, but there are no obvious means of achieving that goal due to
high material costs. SOFC durability suffers after the cell repeatedly heats up to operating
temperature and then cools down to room temperature.
Delivery
The Department of Energy’s Technical Plan for Fuel Cells states that the air compressor
technologies currently available are not suitable for vehicle use, which makes designing a
hydrogen fuel delivery system problematic.
Infrastructure
In order for PEMFC vehicles to become a viable alternative for consumers, there must be a
hydrogen generation and delivery infrastructure. This infrastructure might include
pipelines, truck transport, fueling stations and hydrogen generation plants. The DOE
hopes that development of a marketable vehicle model will drive the development of an
infrastructure to support it.
Storage and Other Considerations
Three hundred miles is a conventional driving range (the distance you can drive in a car
with a full tank of gas). In order to create a comparable result with a fuel cell vehicle,
researchers must overcome hydrogen storage considerations, vehicle weight and volume,
cost, and safety.
While PEMFC systems have become lighter and smaller as improvements are made, they
still are too large and heavy for use in standard vehicles.
There are also safety concerns related to fuel cell use. Legislators will have to create new
processes for first responders to follow when they must handle an incident involving a fuel
cell vehicle or generator. Engineers will have to design safe, reliable hydrogen delivery
systems.
Researchers face considerable challenges. In the next section, we will explore why the
United States and other nations are investing in research to overcome these obstacles.