FUEL CELLS The presentation that follows is meant to explain what was going on inside the fuel cell when the model car was running along the floor and the H 2 and O 2 were being used up inside it. It involved a chemical reaction creating an electric current. This presentation does not explain the production of the H 2 and O 2 that you saw earlier, in which electrical energy from the solar panels was used to break up water. To get a better understanding of that process, just imagine the following presentation shown in reverse!

Before we discuss fuel cells and how they produce an electric current we need to have some understanding about what electricity is. The wires and electrodes used in a fuel cell are of course made of metal, and metal atoms… have what is known as a sea of electrons flowing among them at all times. (in reality, these electrons move much faster than this!)

If some of the electrons are removed from one end of the wire, the rest of the electrons, just by random movement, tend to head toward that end of the wire.

If some of the electrons are removed from one end of the wire, the rest of the electrons, just by random movement, tend to head toward that end of the wire. Electrons flowing like this in a specific direction is what is commonly known as an electric current. Direction of electron flow

So how exactly does a hydrogen fuel cell work? It all begins with the reaction between hydrogen and oxygen: H H Recall that hydrogen is diatomic (H 2 ), and that each H atom has just one electron held very weakly in its valence level. O O And recall that oxygen is also diatomic (O 2 ), with a double bond, and that each O atom has six electrons very tightly held in its valence level.

To simplify things, lets look at just one O atom at a time. This O atom has six valence electrons, but wants to have eight, so when an H 2 molecule collides with it… The O atom effectively pulls in these two extra electrons to complete its octet. H H OO

To simplify things, lets look at just one O atom at a time. This O atom has just six valence electrons, but wants to have eight, so when an H 2 molecule collides with it… The O atom effectively pulls in these two extra electrons to complete its octet. H H O The H atoms form bonds and a molecule of water (H 2 O) is produced.

If we go back to showing a diatomic O 2 molecule, then it would require two H 2 molecules to react with it. Here is a simplified representation of what that overall reaction would look like: H H O O H H 2 H 2 + O 2 2 H 2 O

This is a very explosive, energy-releasing reaction, and it is important to realize that it is oxygens strong pull on the electrons that provides the driving force for this reaction. This strong pull on electrons is what makes oxygen such a reactive gas in the first place. But is it possible for oxygen molecules to pull the electrons off the hydrogen molecules before they even come in contact with each other? Thats what a hydrogen fuel cell is all about. The oxygen molecules essentially pull the electrons through a wire which has the hydrogen molecules on the other end.

This is just like how a battery works: a chemical reaction is set up so that an exchange of electrons takes place through a wire rather than a direct hand-off between the atoms. Thus the chemical reaction creates an electric current. So how is a fuel cell different than a regular battery? In a regular battery, the chemicals are locked inside, and when they run out, the battery is dead – although of course some batteries can be recharged. In a fuel cell, the chemicals are run through it and continuously being replenished. A fuel cell could theoretically run forever, provided it had a limitless supply of fuel.

Here is a diagram of a hydrogen fuel cell hooked up to an electric motor, which in turn is hooked up to a wheel. H 2 molecules come in one side, and O 2 molecules come in the other. H H OO H H H 2 intake O 2 intake electric motor wheel electrodes In case youre wondering, these little guys are electrons and youll see in a moment why they are drawn in here…

These molecules come in contact with metal strips called electrodes, and the electron-hungry O 2 molecule pulls the electrons toward it. H H OO H H + - H 2 intake O 2 intake electric motor wheel electrodes

Notice how this removal of electrons from one side of the wire causes electrons to flow all the way through the circuit, just as we described in the beginning of this presentation. H H OO H H + - H 2 intake O 2 intake electric motor wheel electrodes

This movement of electrons through the wire creates an electric current, and this electric current runs the motor which in turn spins the wheel. H H OO H H + - H 2 intake O 2 intake electric motor wheel electrodes

Also notice that the electrons are being replenished by the H 2 molecules on the other side of the cell. As they lose electrons, they turn into H + ions. Meanwhile the O atoms are obtaining a negative charge. Now watch how this continues… H H OO H H + - H 2 intake O 2 intake electric motor wheel

The oxygen, still hungry for more electrons, continues to pull electrons through the circuit. H H OO H H H 2 intake O 2 intake electric motor wheel

This electric current continues to run the motor... H H OO H H H 2 intake O 2 intake electric motor wheel

…which continues to spin the wheel. H H OO H H H 2 intake O 2 intake electric motor wheel

Now while this is going on, the H + ions (also known as protons) are migrating across the membrane that separates the two sides. OO H + H + H + H + 2- H 2 intake O 2 intake electric motor wheel [Side note: Recall that a typical H atom has just one proton and one electron – no neutrons. When it loses the electron, all thats left is a bare proton. Thats why an H + ion is the same thing as a proton.]

When the protons get to the other side, they bond to the O 2- ions and cancel out their charges. This creates two molecules of water (H 2 O). O H H O H H H 2 intake O 2 intake electric motor wheel

These water molecules are then pushed out the exhaust tube by more incoming oxygen gas. O H H O H H H 2 intake O 2 intake electric motor wheel H H OO H H H 2 O exhaust

And the entire process repeats itself… H H OO H H + - H 2 intake O 2 intake electric motor wheel H 2 O exhaust

H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust And the entire process repeats itself…

H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust And the entire process repeats itself…

H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust And the entire process repeats itself…

OO H + H + H + H + 2- H 2 intake O 2 intake electric motor wheel H 2 O exhaust And the entire process repeats itself…

O H H O H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust And the entire process repeats itself…

O H H O H H H 2 intake O 2 intake electric motor wheel H H OO H H H 2 O exhaust And the entire process repeats itself…

Because it allows protons to pass across it, this membrane is known as a proton exchange membrane or PEM. And so this type of fuel cell is called a PEM fuel cell. H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust PEM

Whether the hydrogen and oxygen are reacting directly in a combustion type reaction or indirectly through a PEM fuel cell, the overall reaction is still the same: 2 H 2 + O 2 2 H 2 O + Energy H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust PEM

The important difference is that the PEM fuel cell is much more efficient when it comes to generating electricity. Thats because almost no energy is lost in the form of heat. H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust PEM

Now this presentation just showed one interaction at a time – as two H 2 molecules transferred four electrons through the circuit to one O 2 molecule. H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust PEM

In an actual fuel cell, trillions of these interactions are happening every second across every square millimeter of the cell membrane! H H OO H H H 2 intake O 2 intake electric motor wheel H 2 O exhaust PEM

Its exciting to imagine a future where all the cars and trucks and buses are powered by fuel cells…

Drivers would refill their tanks at hydrogen filling stations….

And they could drive silently for hundreds of kilometers…. …with only water vapor coming out of their exhaust pipes!

And where would all that hydrogen come from?

Ideally it would be produced the same way we produced it in the lab, only on a larger scale…

Like this huge solar farm which has thousands of solar panels harnessing the suns energy.

Or perhaps the hydrogen would be generated by the electricity produced by wind farms like this one.

Unlike gasoline, hydrogen is an infinitely renewable resource.

PAC-Car II, the fuel cell car shown at right, holds the world record for most fuel efficient automobile. It drove km (12.85 miles) on just one gram of hydrogen! Thats equivalent to driving 12,665 miles – more than half-way around the world – on just a single gallon of gasoline !

Heres the fuel-cell car your teacher is hoping to buy some day!

Its exciting to imagine a future where all the cars and trucks are powered by fuel cells. People would refill their tanks with hydrogen gas, and drive silently for hundreds of kilometers with only water vapor coming out of their exhaust pipes! Heres the one hell probably end up with!

Is there a fuel cell car in your future? (the end)