have heard a lot recently about fuel cells. According to many
news reports, we may soon be using the new energy-saving
technology to generate electrical power for our homes and
cars. The technology is extremely interesting to people in all
walks of life because it offers a means of making power more
efficiently and with less pollution. But how does it do this?
In this edition of HowStuffWorks,
we'll take a quick look at each of the existing or emerging
fuel-cell technologies. We'll detail how one of the most
promising technologies works, and we'll discuss a few
potential applications of fuel cells.
What is a Fuel Cell? A fuel cell is an
electrochemical energy conversion device that converts
hydrogen and oxygen into electricity and heat. It is very much
like a battery
that can be recharged while you are drawing power from it.
Instead of recharging using electricity, however, a fuel cell
uses hydrogen and oxygen.
fuel cell will compete with many other types of energy
conversion devices, including the gas
turbine in your city's power plant,
engine in your car
and the battery in
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 store electrical
energy by converting it into chemical energy, which can be
converted back into electrical energy when needed.
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 the type of
electrolyte they use. Some types of fuel cells show
promise for use in power generation plants. Others may be
useful for small portable applications or for powering cars.
The proton exchange membrane fuel cell (PEMFC) is
one of the most promising technologies. This is the type of
fuel cell that will end up powering cars, buses and maybe even
Proton Exchange Membrane The proton
exchange membrane fuel cell (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
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
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
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.
The catalyst is a special material that
facilitates the reaction of oxygen and hydrogen. It is
usually made of platinum powder 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.
Figure 2. Animation of a fuel cell
Chemistry of a Fuel
Anode side: 2H2 => 4H+ + 4e-
Cathode side: O2 + 4H+ + 4e- => 2H2O
Net reaction: 2H2 + O2 => 2H2O
2 shows the pressurized hydrogen gas (H2) entering the fuel cell on the anode
side. This gas is forced through the catalyst by the pressure.
When an H2 molecule comes in
contact with the platinum on the catalyst, it splits into two
H+ ions and two electrons
(e-). The electrons are
conducted through the anode, where they make their way through
the external circuit (doing useful work such as turning a
motor) 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
PEMFCs operate at a fairly low temperature (about 176
degrees Fahrenheit, 80 degrees Celsius), which means they warm
up quickly and don't require expensive containment structures.
Constant improvements in the engineering and materials used in
these cells have increased the power density to a level
where a device about the size of a small piece of luggage can
power a car.
Problems with Fuel Cells We learned in the
last section that a fuel cell uses oxygen and hydrogen to
produce electricity. The oxygen required for a fuel cell comes
from the air. In fact, in the PEM fuel cell, ordinary air is
pumped into the cathode. The hydrogen is not so readily
available, however. Hydrogen has some limitations that make it
impractical for use in most applications. For instance, you
don't have a hydrogen pipeline coming to your house, and you
can't pull up to a hydrogen pump at your local gas station.
Hydrogen is difficult to store and distribute, so it would
be much more convenient if fuel cells could use fuels that are
more readily available. This problem is addressed by a device
called a reformer.
A reformer turns hydrocarbon or alcohol fuels into hydrogen,
which is then fed to the fuel cell. Unfortunately, reformers
are not perfect. 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.
Some of the more promising fuels are natural gas, propane
and methanol. Many people have natural-gas lines or propane
tanks at their house already, so these fuels are the most
likely to be used for home fuel cells. Methanol is a
liquid fuel that has similar properties to gasoline. It is
just as easy to transport and distribute, so methanol may be a
likely candidate to power fuel-cell cars.
Efficiency of Fuel Cells In this section, we
will take a look at how fuel cells might improve the
efficiency of cars today. Remember that pollution
reduction is one of the primary goals of the fuel cell.
We will compare a fuel-cell-powered car to a gasoline-engine-powered
car and a battery-powered
car. Since all three types of cars have many of the same
components (tires, transmissions,
etc.), we'll ignore that part of the car and compare
efficiencies up to the point where mechanical power is
generated. Let's start with the fuel-cell car. (All of these
efficiencies are approximations, but they should be close
enough to make a rough comparison.)
Car 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. But, as we learned in the
previous section, hydrogen is difficult to store in a car.
When we add a reformer
to convert methanol to hydrogen, the overall efficiency drops
to about 30 to 40 percent.
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 30- to
40-percent efficiency at converting methanol to electricity,
and 80-percent efficiency converting electricity to mechanical
power. That gives an overall efficiency of about 24 to 32
Gasoline-Powered Car 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
Battery-Powered Electric Car
This type of 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,
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.
Surprised? Maybe you are
surprised by how close these three technologies are. This
exercise points out the importance of considering the whole
system, not just the car. We could even go a step further and
ask what the efficiency of producing gasoline, methanol or
Efficiency is not the only consideration, however. People
will not drive a car just because it is the most efficient if
it makes them change their behavior. They are concerned about
many other issues as well. They want to know:
Is the car quick and easy to refuel?
Can it travel a good distance before refueling?
Is it as fast as the other cars on the road?
How much pollution does it produce?
of course, goes on and on. In the end, the technology that
dominates will be a compromise between efficiency and
Other Types of Fuel Cells There are several
other types of fuel-cell technologies being developed for
possible commercial uses:
Alkaline fuel cell (AFC): This is one of the
oldest designs. It has been used in the U.S. space
program 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.
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 PEM fuel cells, so it has a longer warm-up
time. This makes it unsuitable for use in cars.
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 (around
1,832 F, 1,000 C). This high temperature makes reliability a
problem, but it also has an advantage: The steam produced by
the fuel cell can be channeled into turbines to generate
more electricity. This improves the overall efficiency of
Molten carbonate fuel cell (MCFC): These fuel
cells are also best suited for large stationary power
generators. They operate at 1,112 F (600 C), so they also
generate steam that can be used to generate more power. They
have a lower operating temperature than the SOFC, which
means they don't need such exotic materials. This makes the
design a little less expensive.
Applications of Fuel Cells As we've
discussed, fuel cells could be used in a number of
applications. Each proposed use raises its own issues and
Automobiles Fuel-cell-powered cars will
start to replace gas- and diesel-engine cars in about 2005. A
fuel-cell car will be very similar to an electric car but with
a fuel cell and reformer instead of batteries. Most likely,
you will fill your fuel-cell car up with methanol, but some
companies are working on gasoline reformers. Other companies
hope to do away with the reformer completely by designing
advanced storage devices for hydrogen.
Check out these links for more information about
Portable Power Fuel cells
also make sense for portable electronics like laptop
phones or even hearing aids. In these applications, the
fuel cell will provide much longer life than a battery would,
and you should be able to"recharge" it quickly with a liquid
or gaseous fuel.
Check out these links for more information on
portable-power fuel cells:
buses are already running in several cities. The bus was one
of the first applications of the fuel cell because initially,
fuel cells needed to be quite large to produce enough power to
drive a vehicle. In the first fuel-cell bus, about one-third
of the vehicle was filled with fuel cells and fuel-cell
equipment. Now the power density has increased to the point
that a bus can run on a much smaller fuel cell.
Check out these links for more information on fuel-cell
Generation This is a promising application that you
may be able to order as soon as 2002. General
Electric is going to offer a fuel-cell generator system
made by Plug
Power. This system will use a natural gas or propane
reformer and produce up to seven kilowatts of power (which is
enough for most houses). A system like this produces
electricity and significant amounts of heat, so it is possible
that the system could heat your water and help to heat your
house without using any additional energy.
Check out these links for more information on fuel-cell
home power generation:
Generation Some fuel-cell technologies have the
potential to replace conventional combustion power plants.
Large fuel cells will be able to generate electricity more
efficiently than today's power plants. The fuel-cell
technologies being developed for these power plants will
generate electricity directly from hydrogen in the fuel cell,
but will also use the heat and water produced in the cell to
power steam turbines and generate even more electricity. There
are already large portable fuel-cell systems available for
providing backup power to hospitals and factories.
Check out these links for more information on large
fuel-cell power generation: