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.

FUEL CELLSThe 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 H2 and O2 were being used up inside it. It involved a chemical reaction creating an electric current.

This presentation does not explain the production of the H2 and O2 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 (H2),

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 (O2), with a double bond,

and that each O atom has six

electrons very tightly held in its

valence level.

To simplify things, let’s look at just one O atom at a time.

This O atom has six valence electrons, but “wants” to have eight, so when an H2 molecule collides with it…

The O atom effectively pulls in these two extra electrons to complete its octet.

H

H

O

O

To simplify things, let’s 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 H2 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 (H2O) is produced.

If we go back to showing a diatomic O2 molecule,

then it would require two H2 molecules to react with it.

Here is a simplified representation of what that overall reaction would look like:

H

HO

OH

H2 H2 + O2 2 H2O

This is a very explosive, energy-releasing reaction, and it is important to realize that it is oxygen’s 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?That’s 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.

H2 molecules come in one side, and O2 molecules come in the other.

H2 intake

O2 intake

electric motor

wheel

electrodes

In case you’re wondering, these little guys are electrons and you’ll see in a moment why they are drawn in here…

These molecules come in contact with metal strips called electrodes, and the “electron-hungry” O2 molecule pulls the electrons toward it.

H

HO

OH

H

+ -H2 intake

O2 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

HO

OH

H

+ -H2 intake

O2 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

HO

OH

H

+ -H2 intake

O2 intake

electric motor

wheel

electrodes

Also notice that the electrons are being replenished by the H2 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

HO

OH

H

+ -H2 intake

O2 intake

electric motor

wheel

The oxygen, still “hungry” for more electrons, continues to pull electrons through the circuit.

H

HO

OH

H

+

+

-2-H2 intake

O2 intake

electric motor

wheel

This electric current continues to run the motor...

H

HO

OH

H

+

+

+

2-

-

H2 intake

O2 intake

electric motor

wheel

…which continues to spin the wheel.

H

HO

OH

H

+

+

+

+

-2-

2-H2 intake

O2 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.

O

O

H+

H+

H+

H+

2-

2-H2 intake

O2 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 that’s left is a bare proton. That’s why an H+ ion is the same thing as a proton.]

When the protons get to the other side, they bond to the O2- ions and cancel out their charges.

This creates two molecules of water (H2O).

O

H

H

OH

H

H2 intake

O2 intake

electric motor

wheel

2-

2-

+

+

+

+

These water molecules are then pushed out the exhaust tube by more incoming oxygen gas.

O

H

H

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O exhaust

And the entire process repeats itself…

H

HO

OH

H

+ -H2 intake

O2 intake

electric motor

wheel

H2O exhaust

H

HO

OH

H

+

+

-2-H2 intake

O2 intake

electric motor

wheel

H2O exhaust

And the entire process repeats itself…

H

HO

OH

H

+

+

+

2-

-

H2 intake

O2 intake

electric motor

wheel

H2O exhaust

And the entire process repeats itself…

H

HO

OH

H

+

+

+

+

-2-

2-H2 intake

O2 intake

electric motor

wheel

H2O exhaust

And the entire process repeats itself…

O

O

H+

H+

H+

H+

2-

2-H2 intake

O2 intake

electric motor

wheel

H2O exhaust

And the entire process repeats itself…

O

H

H

OH

H

H2 intake

O2 intake

electric motor

wheel

2-

2-

+

+

+

+

H2O exhaust

And the entire process repeats itself…

O

H

H

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O 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

HO

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O 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 H2 + O2 → 2 H2O + Energy

H

HO

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O exhaust

PEM

The important difference is that the PEM fuel cell is much more efficient when it comes to generating electricity. That’s because almost no energy is lost in the form of heat.

H

HO

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O exhaust

PEM

Now this presentation just showed one interaction at a time – as two H2 molecules transferred four electrons through the circuit to one O2 molecule.

H

HO

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O exhaust

PEM

In an actual fuel cell, trillions of these interactions are happening every second across every square millimeter of the cell membrane!

H

HO

OH

H

H2 intake

O2 intake

electric motor

wheel

H2O exhaust

PEM

It’s 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 sun’s 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 20.68 km (12.85 miles) on just one gram of hydrogen! That’s equivalent to driving 12,665 miles –more than half-way around the world – on just a single gallon of gasoline!

Here’s the fuel-cell car your teacher is hoping to buy some day!

It’s 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!

Here’s the one he’ll probably end up with!

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