Exothermic

Just read an article in The Straits Times local newspapers on Hydrogen-Oxygen Fuel Cell few days back and it gave me an idea to blog about it. Huge potential to be a Chemistry question in your upcoming GCE O-Level Pure Chemistry or IP Chemistry examinations. I have always been reminding students in our Sec 4 O-Level and IP Chemistry Tuition Classes in Singapore on the importance of fuel cell and today i would like to share with you also.

Combustion of Fuels

First of all, let’s take a look at the primary source of energy which is fuels, which are substances that can burn easily in air to give out energy. The most common used fuels are fossil fuels (coal, petroleum and natural gas) which are formed from decayed plants and animals that lived millions of years ago. They are being covered more in the topic of Organic Chemistry. Other types of fuels being used are wood and hydrogen.

When fuels burn, combustion reaction between the fuel and oxygen in the air occurs. All combustion reactions are exothermic in nature and heat energy is given out to the surroundings.

Fuel Cells

Combustion reactions are only one way to extract useful energy from the fuels. Scientists have long ago explored ways to convert chemical energy of fuels directly to electrical energy (i.e. does not require combustion reaction).

This is known as a fuel cell which is a chemical cell in which reactants (usually a fuel and oxygen) are continuously supplied to produce electricity directly.

There are many types of fuel cell but the hydrogen-oxygen fuel cell is the best known example. It has been used as a source of electrical energy in space vehicles.

Hydrogen and oxygen gases are continuously pumped and supplied into the chemical cell.

At the positive electrode (known as the cathode), oxygen is reduced to form hydroxide ions.

O₂(g) + 2H₂O(l) + 4e^– → 4OH^–(aq)

At the negative electrode (known as the anode), hydrogen is oxidised to form water.

2H₂(g) + 4OH^–(aq) → 4H₂O(l) + 4e^–

Combining the two half equations above shows that hydrogen and oxygen are converted into water. The reaction equation is the same as that for the combustion of hydrogen.

2H₂(g) + O₂(g) → 2H₂O(l)

Fuel cells produce electricity indefinitely provided there is a continuous supply of fuel. This is unlike ordinary chemical cell (battery) which will be used up.

So what are the advantages of using hydrogen-oxygen fuel cell? Seems like hydrogen can be used as an alternative to fossil fuels which are rapidly declining because the demand for them has increases exponentially.

Advantages of using Hydrogen-Oxygen Fuel Cell:

1. It does not cause pollution since only water is produced.
2. Hydrogen is considered a renewable source of energy since it can be obtained from the electrolysis of water.
3. Fuel cells have a higher efficiency than diesel or gas engines. Less energy is lost to surrounding as compared to combustion.

But, how come fuel cells are not commonly used? Is there any limitations in using hydrogen-oxygen fuel cells as a source of energy?

Disadvantages of using Hydrogen-Oxygen Fuel Cell:

1. No cheap sources of hydrogen. Obtaining hydrogen from electrolysis is an expensive process. Currently, hydrogen is obtained from non-renewable energy sources such as cracking of petroleum.
2. Hydrogen is extremely flammable and explosive. As such, special precautions must be considered when it comes to storage and transportation of hydrogen.
3. Hydrogen is also very light (relative molecular mass of 2) and requires a larger storage volume as compared to other fuels. There is currently no effective way to store hydrogen.

Hydrogen-oxygen fuel cell has huge potential to be the main source of energy but at the moment, the technology is not well developed and the cost of using hydrogen-oxygen fuel cell is high. Main use is still in space vehicles. Developed countries are trying to implement hydrogen-oxygen fuel cells in vehicles. Singapore is one of them.

I hope you find the content easy for your understanding and if you have anything valuable to add, leave me a comment below.

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PS: For students based in Singapore taking the Pure Chemistry or IP Chemistry syllabus, do note that Fuel Cell is listed in the topic of Energy Changes and not the topic on Electrolysis. This applies to the Pure Chemistry textbooks titles by both Marshall Cavendish and Pearson publishers, which are approved by Ministry of Education (MOE).

PPS: Under related articles below, there are several blog post discussions and questions related to Energy Changes. You can also do a keyword search using the search box at the top right hand corner.