Chemistry only

Fuel cells

cells in a charger 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.

There are several different types of fuel cell but they have many similarities in common. A fuel, commonly hydrogen, alcohol or methane is supplied to the fuel cell. All fuel cells contain an anode, a cathode and an electrolyte. In cells the anode has a negative charge and the cathode is positively charged, this is the other round round in electrolysis, 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 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 by a semi-permeable membrane. This membrane allows ions to move across it but not electrons. The diagram below shows the main parts of a fuel cell

fuel cell schematic 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. This can be shown as:
Anode half-equation: H2(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+) diffuse across the semi-permeable membrane and enter the electrolyte solution. At the cathode oxygen gas (from the air) is fed in and here it undergoes a catalysed reaction on the cathode to form water. The oxygen gas reacts with the hydrogen ions from the electrolyte and the electrons which flow from the anode to the cathode via the external circuit.
Cathode half-equation: O2(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 O2(g) + 4H+(aq) + 4e → 2H20(l)
anode half-equation 2H2(g)4H+(aq) + 4e
Overall equation: 2H2(g) + O2(g) + → 2H20(g)
The 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.

fuel cell description

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

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. 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 is sufficient renewable energy resources available to supply enough hydrogen to convert all cars, bikes, trains etc away from traditional fossil fuels.

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, 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 and useful for providing power for remote and isolated communities and businesses. Fuel cells can provide more energy and are cheaper to produce than batteries, they are also not as polluting or harmful to the environment to produce. Fuel cells 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, or batteries which contain some corrosive and toxic chemicals.

Practice questions

Check your understanding - Questions on fuel cells