Hydrogen
Hydrogen in combustion engines
There are two main ways of obtaining energy from hydrogen, the first being the combustion of hydrogen in a combustion engine. Hydrogen is burned with oxygen to release energy and water. The energy released is heat energy which causes the hot gases to expand, generating mechanical energy used in the engine to generate electricity.
2𝐻 2 (𝑔)+ 𝑂 2 (𝑔)→ 2𝐻 2 𝑂 + 𝑒𝑛𝑒𝑟𝑔𝑦
As seen in the equation, this method produces no carbon emissions and is seemingly faultless. However, a disadvantage of this method is that it could cause the formation of nitrogen oxides which are toxic and pollutant gases. The high temperatures can cause the NN triple bond in nitrogen to break and provide enough activation energy for a reaction between nitrogen and oxygen to form these harmful gases known as N 𝑂 𝑥 . It is for this reason that hydrogen combustion engines are not considered zero emission and have become less appealing.
Hydrogen in fuel cells
The other more attractive method of obtaining energy from hydrogen is the use of it in fuel cells. This makes use of an electrochemical process rather than a combustion process, meaning it completely avoids the chance of pollutant gases being formed. A fuel cell consists of a positive electrode and negative electrode. The hydrogen is pumped to the anode whilst air (containing oxygen) is supplied to the cathode. A catalyst (platinum) is used to split hydrogen molecules into hydrogen ions and electrons. The electrons and hydrogen ions now have two different pathways as they travel towards the oxygen. The electrons travel through an external wire allowing an electric current to flow in the circuit and
therefore a flow of electricity, while the hydrogen ions travel through the electrolyte 1 to the cathode and there is now a reaction between oxygen the electrons and hydrogen ions to form water and heat energy.
+ +4𝑒 −
𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑎𝑡 𝑎𝑛𝑜𝑑𝑒: 2𝐻 2 →4𝐻
+ + 4𝑒 − →2𝐻
Reaction at cathode: 𝑂 2 +4𝐻
2 𝑂
Overall reaction 2𝐻 2 + 𝑂 2 →2𝐻 2 𝑂
Figure 2: the processes in a fuel cell https://energyeducation.ca/encyclopedia/Fuel_cell; date accessed 19/08/23
The chemical energy of hydrogen is directly transferred to electrical energy making fuel cells very efficient in comparison to hydrogen combustion engines, as they require an intermediate store of thermal energy, meaning that there is some energy lost as heat. There are many different types of fuel cells that have function in a similar fashion but work best for different applications. An example is the
1 An electrolyte inhibits the movement of electrons.
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