A fuel cell is an
electrochemical energy conversion device that converts hydrogen and oxygen
into water, producing electricity and heat in the process. 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. The fuel cell will compete with many other types of
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 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 your house.
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Proton Exchange
Membrane
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 Proton
Exchange Membrane 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.
- 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.
Chemistry of a Fuel Cell
Anode side:
2H2 => 4H+ + 4e-
Cathode side:
O2 + 4H+ + 4e- => 2H2O
Net reaction:
2H2 + O2 => 2H2O
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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.
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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.
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