Chemistry only

Fuel cells

Cells in a charger being charged. One of the problems with cells and batteries is that the chemicals that react to produce useful electrical energy eventually run out and the cell/battery has to be disposed of; that is unless the cell is of a type which can be recharged. However even rechargeable cells can only be recharged so many times before they need replacing. A potential solution to this problem is to use a fuel cell. A fuel cell is similar to an ordinary cell/battery but the chemicals which react to produce electrical energy are not contained inside the battery but are supplied continually from an external source; this means that unlike a battery or a cell as long as the reacting chemicals are continually supplied the fuel cell will never stop working.

There are several different types of fuel cells but they have many similarities in common. A fuel commonly hydrogen, alcohol or methane is supplied continually to the fuel cell. All fuel cells contain an anode, a cathode and an electrolyte. In cells and batteries the anode has a negative charge and the cathode is positively charged; this is the other round from what you will have met before in electrolysis where the anode has a positive charge and the cathode has a negative charge so be careful not to get them mixed up!

The electrolyte varies in different types of fuel cells but common electrolytes are phosphoric acid or a potassium hydroxide solution. The anodes and cathodes are made from porous carbon and they have a coating of small catalyst particles on their surface. The anode catalyst is normally platinum and the cathode catalyst is commonly nickel. The anode, cathode and electrolyte are kept apart in a fuel cell by a semi-permeable membrane. This membrane allows ions to move across it but not electrons. The electrons are forced through an external circuit where they are used to power whatever is attached to the fuel cells; this could for example be a car, a motorcycle or some appliance. The diagram below shows an outline of the main parts of a fuel cell.

fuel cell schematic. Simple diagram to show how a fuel cell works.

The diagram above shows an outline of a typical fuel cell. This one is a hydrogen-oxygen fuel cell. In this cell hydrogen gas; the fuel enters the porous carbon anode. Here the hydrogen is oxidised by the catalysts on the anode surface to form hydrogen ions (H⁺). This can be shown as:

Anode half-equation: H_2(g) → 2H⁺_(aq) + 2e This is oxidation- loss of electrons

The electrons produced at the anode then flow through the wire connected to the bulb, but it can be any electrical item. The hydrogen ions (H⁺) now diffuse across the semi-permeable membrane and enter the electrolyte solution and head towards the cathode. At the cathode oxygen gas (from the air) is fed in and here it undergoes a catalysed reaction on the cathode surface to form water. The oxygen gas reacts with the hydrogen ions (H⁺) from the electrolyte and the electrons which flow from the anode to the cathode via the external circuit. The oxygen gas is reduced on the surface of the cathode to form water.

Cathode half-equation: O_2(g) + 4H⁺_(aq) + 4e → 2H₂O_(g) This is reduction- gain of electrons

The overall equation is simply obtained by combining the anode and cathode half-equations; although we need to multiply the anode half-equation by two to balance off the electrons: Overall cell equation: oxidation at the anode and reduction at cathode- redox reaction

cathode half-equation O_2(g) + 4H⁺_(aq) + 4e → 2H₂0_(l)

anode half-equation 2H_2(g) → 4H⁺_(aq) + 4e

Overall equation: 2H_2(g) + O_2(g) + → 2H₂0_(g)

The fuel cell operates at temperatures between 150-200⁰C; this means that the water which is produced can be converted into steam which can be used for other purposes e.g. generating electricity or supplying hot water to homes, this can increases the efficiency of the fuel cell from around 50% to as much as 90% efficient.

How fuel cells work

The image below shows how a fuel cell works.

Model showing how a fuel cell works with description of the anode and cathode reactions.

Starting from the left hand-side of the fuel cell in the diagram we can say that:

At the anode:

The hydrogen fuel is fed into the fuel cell where it is oxidised on the surface of the anode. The catalyst which is on surface of the anode will help speed up the oxidation of the hydrogen gas. As we saw above once the hydrogen gas has been oxidised it forms hydrogen ions (H⁺) and releases 2 electrons.
The hydrogen ions flow through the semi-permeable membrane and head into the electrolyte and then towards the cathode. The electrons flow through the wire (the external circuit) and light up the bulb; as shown in the diagram.

At the cathode:

Oxygen gas is fed into the fuel cell at the cathode. On the surface of the cathode the oxygen gas reacts with the hydrogen ions from the electrolyte. The oxygen atoms also gain electrons from the external circuit flowing through the bulb.
The oxygen atoms on the cathode are reduced to form water as the only product of this reaction.
The overall reaction that happens inside the fuel cell is simply hydrogen reacting with oxygen to form water. The hydrogen is oxidised to form water.

Uses of fuel cells

Fuel cells can be made in a large range of sizes and power outputs from units which can be used to generate electricity for factories, homes and offices to small scale devices which can charge your phone and other electronic devices. Car manufacturers including Honda, Toyota, and Hyundai have models of cars running on hydrogen fuel cells.

Advantages and disadvantages of fuel cells

The main drawback with fuel cells is their fuel, hydrogen. Though hydrogen is an excellent fuel which burns to release large amounts of clean energy hydrogen is not widely available. Large amounts of energy are needed to split water using electrolysis as the main source of hydrogen gas. Unless the energy used for this electrolysis comes from renewable sources then the carbon footprint is likely to be large. Hydrogen can also be obtained from methane but as this is a fossil fuel the long term sustainability of this is also questionable. The extraction of hydrogen from methane also requires large amounts of energy, this energy would need to be obtained from renewable sources. It is unlikely that there are sufficient renewable energy resources available to supply enough hydrogen to convert all cars, bikes, trains etc away from traditional fossil fuels.

Hydrogen is also a highly flammable and explosive gas which means that there will be additional safety concerns with transporting and storing it safely.

Car with a hydrogen oxygen fuel cell produce only water and no greenhouse gases or other pollutants

The costs to the country of converting all garages from selling petrol and diesel to hydrogen would be massive. There would also be considerable costs in storing and transporting hydrogen safely around the country in sufficiently large quantities. The other main problem with hydrogen is that it is a gas and so would need to be cooled and stored under pressure as a liquid. This again costs money and also you have the additional problem that hydrogen is a very explosive gas. Would there be a high risk of death and serious injury in fitting hydrogen tanks to all motor vehicles? However provided that the hydrogen needed for the fuel cells is produced in an environmentally friendly way then fuel cells with their high efficiencies and high energy outputs and low emissions are viable alternative to providing clean energy for the future.

Fuel cells have no moving parts so are reliable, efficient and useful for providing power for remote and isolated communities and businesses. Fuel cells are cheaper to produce than batteries and they are also not as polluting or harmful to the environment to produce or to dispose of. Fuel cells also have longer working life than batteries and so could be considered more environmentally friendly and sustainable. Hydrogen-oxygen fuel cells also produce water as the only waste product and so are much more environmentally friendly than fossil fuels; no carbon dioxide, sulfur dioxide or nitrogen oxides are produced and are more environmentally friendly than batteries which contain some corrosive and toxic chemicals.

Key Points

Fuel cells convert the stored chemical energy in fuels such as hydrogen and methane into useful electrical energy.
Unlike batteries and cells a fuel cell will produce electricity for as long as they are supplied with a suitable fuel and oxygen.
In a fuel cell the anode is negatively charged and the cathode is positively charged. The anode is the site of oxidation and the cathode is where reduction occurs.
In a hydrogen oxygen fuel cell hydrogen gas is oxidised and oxygen gas is reduced to form water.
Hydrogen oxygen fuel cells are efficient, reliable and produce only water as a waste product.

Fuel cells

Anode half-equation: H_2(g) → 2H⁺_(aq) + 2e This is oxidation- loss of electrons

Cathode half-equation: O_2(g) + 4H⁺_(aq) + 4e → 2H₂O_(g) This is reduction- gain of electrons

cathode half-equation O_2(g) + 4H⁺_(aq) + 4e → 2H₂0_(l)

anode half-equation 2H_2(g) → 4H⁺_(aq) + 4e

Overall equation: 2H_2(g) + O_2(g) + → 2H₂0_(g)

How fuel cells work

Uses of fuel cells

Advantages and disadvantages of fuel cells

Key Points

Practice questions

Check your understanding - Questions on fuel cells

Check your understanding - Quick Quiz on Fuel cells

Next

Fuel cells

Anode half-equation: H2(g) → 2H+(aq) + 2e This is oxidation- loss of electrons

Cathode half-equation: O2(g) + 4H+(aq) + 4e → 2H2O(g) This is reduction- gain of electrons

cathode half-equation O2(g) + 4H+(aq) + 4e → 2H20(l)

anode half-equation 2H2(g) → 4H+(aq) + 4e

Overall equation: 2H2(g) + O2(g) + → 2H20(g)

How fuel cells work

Uses of fuel cells

Advantages and disadvantages of fuel cells

Key Points

Practice questions

Check your understanding - Questions on fuel cells

Check your understanding - Quick Quiz on Fuel cells

Next

Anode half-equation: H_2(g) → 2H⁺_(aq) + 2e This is oxidation- loss of electrons

Cathode half-equation: O_2(g) + 4H⁺_(aq) + 4e → 2H₂O_(g) This is reduction- gain of electrons

cathode half-equation O_2(g) + 4H⁺_(aq) + 4e → 2H₂0_(l)

anode half-equation 2H_2(g) → 4H⁺_(aq) + 4e

Overall equation: 2H_2(g) + O_2(g) + → 2H₂0_(g)