Hydrogen occurs naturally in water, so resources are vast

Hydrogen fuel cells are silent

The only by-product is water vapour, in contrast to the burning of fossil fuels, which emits greenhouse gases and other pollutants

The biggest problem is getting at the hydrogen. This is done either by electrolysis - using electricity to split water molecules into oxygen and hydrogen - or by reforming fossil fuels

Electrolysis creates no harmful by-products directly, but is only as clean as the process used to generate the electricity in the first place

It is also expensive – costing about $2.40 for a kilogram of hydrogen

The more widely used method is to split the hydrocarbons in fossil fuels into hydrogen and carbon. This is much cheaper – about $0.65 a kilogram. But it defeats the point somewhat as it still uses fossil fuels and creates carbon dioxide as a by-product. Even using hydrocarbons, however, hydrogen fuel cells would still reduce air pollution

Hydrogen is a flammable gas, so there are safety concerns

Hydrogen is bulky to transport