How fuel cells work

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 Polymer Exchange Membrane Fuel Cells uses one of the simplest reactions of any fuel cell.

What's in a PEM fuel cell:

The parts of a PEM fuel cell

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.

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 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.

Fuel Cell Efficiency
Pollution reduction is one of the primary goals of the fuel cell. By comparing a fuel-cell-powered car to a gasoline-engine-powered car and a battery-powered car, you can see how fuel cells might improve the efficiency of cars today.

Since all three types of cars have many of the same components (tires, transmissions, et cetera), 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.)

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.

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, USA researches have decided to concentrate on pure hydrogen fuel-cell vehicles, despite challenges associated with hydrogen production and storage.