0620 · 4.2Hydrogen–oxygen fuel cells

How hydrogen–oxygen fuel cells work

0:00A car can use a chemical reaction to produce electricity, with water as its only chemical product. Instead of burning petrol in an engine, it can supply hydrogen to a fuel cell. The electricity then powers an electric motor. Understanding this reaction helps us judge both the benefits and the difficulties of hydrogen vehicles. A hydrogen–oxygen fuel cell uses hydrogen and oxygen to produce electricity with water as the only chemical product.

0:25Hydrogen is the fuel, while oxygen is the other reactant. In a vehicle, the oxygen usually comes from the surrounding air. Both substances are used up as the cell operates, so they must keep being supplied. The useful starting idea is that chemical reactions can release energy. A fuel cell transfers some of that energy directly into electrical energy. An electric current involves the movement of charged particles. Inside a metal wire, those moving particles are electrons, and their flow can operate an electric motor.

0:54The fuel cell controls how hydrogen reacts with oxygen. Reactions take place at two electrodes, which are conducting surfaces connected through an external circuit. Electrons released at the hydrogen electrode travel through that circuit towards the oxygen electrode. This flow provides the electric current, rather than simply releasing the reaction energy as heat. For this topic, the important idea is the overall conversion, not the detailed electrode chemistry. Chemical energy from the reactants becomes electrical energy that can power a vehicle.

1:23The cell also releases some energy as heat. It continues producing electricity while hydrogen and oxygen are supplied, unlike a battery with a limited store of reactants inside. The word equation is hydrogen plus oxygen gives water. The balanced symbol equation is 2H₂ plus O₂ gives 2H₂O. Hydrogen and oxygen atoms are rearranged into water molecules, with no carbon atoms involved. Electricity is an energy output, not another substance, so water remains the only chemical product.

1:54Do not confuse a fuel cell with electrolysis. Electrolysis uses an electrical supply to drive a chemical change. A fuel cell does the opposite kind of energy conversion, producing electricity from a chemical reaction. Here, hydrogen and oxygen are consumed rather than being made from water.

Benefits of hydrogen fuel cells

2:12This next part is for the Extended paper. Comparing fuel cells with petrol engines means considering pollution, efficiency, cost and practical use. An advantage should explain what becomes better for the vehicle or its surroundings. A disadvantage identifies a difficulty that still exists, even though the fuel cell produces only water. The first advantage is the lack of harmful exhaust products from the fuel cell itself. Petrol contains carbon, so burning it produces carbon dioxide.

2:39Petrol engines can also release other pollutants. A hydrogen–oxygen fuel cell forms only water, so it releases no carbon dioxide or other pollutants at the vehicle. This benefit matters especially where many vehicles operate close to people. Replacing petrol engines with fuel cells can reduce local air pollution from exhaust emissions. However, cleaner operation does not automatically mean that obtaining the fuel caused no pollution. We must distinguish what happens in the vehicle from what happened before the hydrogen reached it.

3:10Another advantage is that fuel cells are more efficient than petrol engines. Efficiency describes how much of the energy supplied becomes a useful output. A petrol engine first releases energy by combustion, then uses that energy to produce movement. The fuel cell produces electricity directly, so a larger proportion can become useful energy rather than wasted heat.

Hydrogen fuel cell limitations

3:31Hydrogen storage is a major disadvantage. As a gas, hydrogen occupies a large volume unless it is compressed. Vehicle storage therefore commonly requires large tanks containing hydrogen under high pressure. These tanks need strong walls, take up space and make storage more difficult than keeping liquid petrol in a tank. Hydrogen is also flammable, meaning that it can catch fire. A leak can create a fire risk if the escaping gas encounters an ignition source.

3:57Petrol is flammable too, so this does not mean petrol vehicles have no safety risks. The relevant point is that hydrogen requires careful storage and handling. Refuelling presents another practical difficulty because there are relatively few hydrogen refuelling stations. A driver needs access to a suitable station before the fuel runs out. Petrol stations are much more widely available. Even an efficient vehicle becomes inconvenient if its driver cannot easily obtain the fuel.

4:24Hydrogen production can also reduce the environmental advantage. Hydrogen is often made using fossil fuels, and this production can release carbon dioxide. The vehicle still produces only water from its fuel cell. However, emissions earlier in the fuel supply mean the complete journey is not necessarily free from carbon dioxide emissions. Fuel cells are also expensive to manufacture. Their materials and construction contribute to this cost. That can make a fuel-cell vehicle more expensive to buy than a comparable petrol vehicle.

4:54Reduced pollution and greater efficiency are advantages, but they do not remove the financial and practical disadvantages.

Evaluating fuel cells in exams

5:01Consider a delivery company choosing vehicles for busy residential streets. Its hydrogen would be made using fossil fuels, and only one nearby station could supply it. We should first separate the vehicle’s operation from the production of its fuel. At the vehicle, replacing petrol with a fuel cell reduces harmful exhaust emissions because the cell produces only water. Next, consider the supply information rather than ignoring it. Making the hydrogen using fossil fuels can release carbon dioxide, so the company cannot claim that the whole process is emission-free.

5:33The limited refuelling network could also disrupt deliveries. A sensible judgement recognises cleaner local operation while acknowledging fuel production and access difficulties. An examination may ask you to state the chemical product, then describe advantages and disadvantages compared with petrol engines. For the state part, give the product directly, which is water. For describe, identify specific benefits or difficulties rather than writing that hydrogen is better or worse. Select separate points so that each sentence adds something new.

6:04In a model answer, the first advantage is that the fuel cell forms only water, with no carbon dioxide or other pollutants at the vehicle. It is also more efficient than a petrol engine. On the other side, hydrogen is difficult to store because it needs large pressurised tanks. There are also relatively few refuelling stations for drivers to use. Avoid three common mistakes in your answers. Water is the only chemical product, but heat is also released, so do not confuse substances with energy transfers.

6:32Saying hydrogen is always pollution-free ignores how it was produced. Finally, a vague phrase such as better for the environment earns no credit, so name carbon dioxide as the gas that is not released. The central idea is a controlled reaction that produces electricity from hydrogen and oxygen. Water is the only chemical product, and an electric motor can use the electrical output. Compared with petrol engines, fuel cells offer cleaner operation at the vehicle and greater efficiency.

7:00Difficult storage, flammability, limited refuelling, production emissions and cost explain why those benefits come with important trade-offs.