Aluminium:

There is increasing interest in energy and transport technologies based around hydrogen. It must be emphasised that hydrogen cannot replace oil as a source of energy - there are no significant reserves of natural hydrogen. Instead, other energy sources are used to make hydrogen, which then becomes a transportable store of energy.

Diagram: hydrogen molecule

Hydrogen Fuel Cells

Although hydrogen releases a lot of energy when it burns, much of the interest is in the use of fuel cells. A fuel cell uses hydrogen and oxygen to generate electricity in a reaction that is the reverse of the electrolysis of water.

Photo and Equation: Fuel cell

Fuel Cell Vehicles

In a vehicle the electricity produced by the cell is used to power an electric motor, lights and so on. Some advantages of using a fuel cell are:

· The only emission from a working cell is water, so they have the potential to improve air quality in congested cities
· Fuel cells and electric motors are much quieter than engines
· Unlike electric vehicles using batteries, which need lengthy recharging, refuelling is quick

Making hydrogen

The hydrogen has to be made somehow, and the usual methods are:

· Electrolysis of water
· Steam reforming of methane gas
· Hydrogen is also produced as a by-product in other processes
· Some green algae produce hydrogen under certain conditions, and research into this is in progress

Photo: Research into hydrogen from algae

Diagram: Chemical equations for making hydrogen

Energy

Electrolysis and reforming both require a considerable amount of energy, and the hydrogen generated is therefore acting as an energy store. The electrical energy eventually produced by the fuel cell cannot, of course, be more than the energy needed to make the hydrogen in the first place! A hydrogen fuel cell is not necessarily a source of "clean electricity" - it really depends on how you produce the hydrogen.

Diagram: Hydrogen stores energy

Storing Hydrogen

In order to use hydrogen in a fuel cell vehicle, a sufficient quantity has to be stored in a safe, easily transportable form. There are currently several different approaches.

Compressed gas	Hydrogen can be kept in a tank compressed to 200 atmospheres
Liquefied gas	Combining refrigeration and compression will liquefy hydrogen
Metal hydride tanks	Some metal hydrides have a sponge-like structure that can absorb hydrogen molecules
Nano technology	Hydrogen molecules will occupy the spaces inside and between carbon nanotubes
Chemical storage as metal hydrides	Alkali metal hydrides react with water and liberate hydrogen from both the hydride and water
Chemical storage as methanol	Direct methanol fuel cells do not make use of molecular hydrogen, but the reaction involves the oxidation of hydrogen

Carbon Nanotubes

When hydrogen is taken into the structure of carbon nanotube bundles, it takes up much less room than it would do as a compressed gas. Research has shown the H₂ molecules are mostly between the nanotubes, but inside too if the tubes are short enough.

Diagram: Hydrogen storage in nanotubes

Alkali Metal Hydrides

Another way of producing hydrogen is using reactive metals and water. Sodium hydride (rather than sodium metal) is encapsulated inside polythene spheres, about the size of a table tennis ball, referred to as powerballs.

The power balls are stored in water inside a hydrogen generator, and are cut in half automatically as required to release the sodium hydride and produce hydrogen. During refuelling sodium hydroxide, also produced by the reaction, is removed along with empty cut spheres, and fresh power balls are put in.

The sodium hydroxide can be used to make more sodium hydride, and the plastic shells are recycled into new powerballs.

Diagram: "Powerball" cycle