Fuel Cells

• This article is about fuel cells.
• We will first go over what a hydrogen fuel cell is.
• Then, we will explore a hydrogen fuel system.
• After, we will go through how to write a half equation for a hydrogen fuel cell.
• We will then explore the applications of fuel cells.
• Finally, we will explore the advantages and disadvantages of fuel cells.

Hydrogen fuel cell

Usually, the first thing that may come to your head when you think of hydrogen is water because water is made up of two hydrogen molecules and one oxygen. But using hydrogen and oxygen, we can actually find an alternative form of energy that does not harm our planet. This is because no fossil fuels are produced, reducing carbon emissions and mitigating climate change. So, this is sounding like a good solution so far, but later on in the article you will be able to see why this may not be the most perfect solution.

Fuel cell system

If you look above, you will see a diagram of a hydrogen fuel cell. There are two graphite electrodes, one positive and one negative, and two inputs and one output. You can also see on the top there is an external circuit, where electrons are mobile and therefore, allowing the flow of energy to power an object.

The way this works is, hydrogen in a gas state is inputted into the system through the input of the negative electrode. The gas diffuses through the graphite electrode and reacts with hydroxide ions (OH^-). This results in the formation of water and delivers a source of electrons for the external circuit.

Oxygen in its gas form, on the other hand, is inputted into the system through the input of the positive electrode. However, instead of reacting with hydroxide ions, it reacts with mobile electrons to form the hydroxide ions, which as mentioned previously reacts with hydrogen and produces water.

So now, if we can picture this we can identify that using the reaction of hydrogen gas and the reaction of the oxygen gas, the electrons that were produced and then taken up cancel each other out and the only product is water, which although is a water product is not harmful like greenhouse gases.

Fig. 2: Coal.

Half-equations for hydrogen fuel cells

We now have explored what hydrogen fuel cells are and how they work, so as chemists, let's explore this through a chemical equation. Fuel cells in general usually have two separate elements reacting together to generate a flow of electrons, so this also means we have two separate equations, which one of the equations show the elements being oxidised and the other showing reduction and what we need to know is, how to join them both together to showcase the total reaction.

For hydrogen fuel cells we know that we use hydrogen and oxygen, therefore were going to be using the half equation for oxygen and the half equation for hydrogen.

The half equation for hydrogen is:

$$2H_2(g)+4OH^-(aq)\rightarrow4H_2O(l)+4e^-$$

The half equation for oxygen is:

$$O_2(g)+2H_2O(l)+ 4e^-\rightarrow4OH^-(aq)$$

If we first go through hydroxide ions, we can see that there are 4 of them in the half equation for hydrogen on the left and on 4 of them in the half equation for oxygen on the right. Because they are on two different sides of the equation and there are 4 on each side, they cancel each other out.

We now have these equations:

$$2H_2(g)\rightarrow4H_2O(l)+4e^-$$

$$O_2(g)+2H_2O(l)+ 4e^-$$

You will also notice another part of the equation that is similar, and that is the number of electrons, with 4 being in each. In the hydrogen half equation it is on the right side and in the oxygen half equation it is on the left side, so we can now cancel these out as well and leads to the following equations:

$$2H_2(g)\rightarrow4H_2O(l)$$

$$O_2(g)+2H_2O(l)$$

The final part that we can simplify is the water. We have 4 in the hydrogen half equation and 2 in the oxygen half-cell, so we can take 2 away from each equation, but unlike the electron and hydroxide ions, we will still have 2 waters in the hydrogen half equation and the equations will not look like this:

$$2H_2(g)\rightarrow2H_2O(l)$$

$$O_2(g)$$

We can now no longer simplify or cancel any part of the equation out, so we can join them both together, and it will look like this:

$$2H_2(g)+O_2(g)\rightarrow2H_2O(l)$$

This is our complete equation for a hydrogen fuel cell.

Application of fuel cell

Fuel cells in general can be used in different objects to power, and they do this by converting chemical energy into electrical energy. Fuel cells are different to batteries as they need a constant supply of fuel, whereas batteries do not need a constant supply, but this does mean batteries eventually end up not being able to supply constant energy unlike fuel cells.

Fig 3. Toyota Sora fuel cell bus 2017

However, there have been some inventions that use fuel cells, like cars and buses. If you look above, you will see a Toyota bus that runs on a fuel cell. There has been wider research and development into this field, especially as the threat from climate change becomes larger.

Advantages of fuel cell

At the beginning of this article, we mentioned how fuel do not produce any greenhouse gasses or have carbon emission. This is heavily advantageous, especially at a time when we are seeing the adverse effects of pollutants and carbon. Another advantage is that they do not need to be electrically recharged, this means we are able to use them constantly instead of having to recharge like rechargeable batteries, or replace them and produce waster. Finally, like batteries, fuel cells can be produced in different sizes for different objects, which is very useful.

Disadvantages of fuel cell

With anything, there are advantages and disadvantages. We have gone through the advantages, so let us go through the disadvantages. The main disadvantages can be seen in hydrogen fuel cells. This is because hydrogen is highly flammable and hard to store, and as for fuel cells we need a constant supply it can make the situation tricky. In addition, sometimes to produce hydrogen, we may need to use non-renewable sources, so although the fuel cell itself will not harm the environment, the raw materials for it to function will.