Molecule of the Month: It ran dirigibles, but could it run your car?: John Emsley looks at the problems and possibilities of hydrogen as a potential answer to our energy needs
Monday 21 September 1992
Sir James Dewar was born on 20 September 1842 at Kincardine-on-Forth, in Scotland, and died in 1923. The invention was the vacuum flask, which we associate with keeping hot drinks hot but which Dewar used to keep liquids cold.
It was thanks to his silvered glass vessel, with its vacuum layer, that Dewar was able to liquefy hydrogen gas for the first time in 1898, and produce solid hydrogen the following year.
Hydrogen gas, which has the formula H2 and is the lightest of all gases, was first isolated by Henry Cavendish in London in 1766 by reacting metals with acids. Hydrogen itself melts at minus 259 C and boils at minus 253 C.
The energy given out by burning hydrogen exceeds that of conventional fuels on a weight-for-weight basis. Hydrogen releases 25 kilocalories per gram, against 10kcal/g for natural methane gas. And there are environmental benefits: when hydrogen burns it produces only steam. However, since hydrogen is less dense than methane, it requires three times the volume of gas to provide the same amount of heat.
Throughout its history, hydrogen has had links with transport, and the same is true today.
It was used by Henri Giffard to raise an airship in Paris 140 years ago. Hydrogen-filled dirigibles had a brief but spectacular career early this century. They were used to bomb London and Paris in the First World War, and ran passenger services in the Twenties and Thirties - until the spectacular Hindenburg disaster occurred in New Jersey in 1937.
The US space programme uses large amounts of liquid hydrogen and transports it in road and rail tankers carrying 75,000 litres at a time. One storage tank at Cape Canaveral holds three million litres of liquid hydrogen.
Hydrogen has also been advocated as a fuel for cars; the problem here, however, is storage. A ton of hydrogen gas occupies a volume of 11 million litres, whereas the same amount as a liquid takes up only 14,000 litres, and this is how it is transported and stored in bulk.
Hydrogen cars have recently been demonstrated in Japan. A team of scientists led by Shoichi Furuhama at the Musashi Institute of Technology has been working on a hydrogen car for more than 20 years, and this June its car completed trials, running 200 miles on a 100-litre tankful of liquid hydrogen, held in a stainless steel version of a Dewar vacuum flask. The car is a Nissan Fairlady Z sports model in which the diesel engine has been modified to ignite hydrogen gas at a pressure of 100 atmospheres.
But hydrogen need not be carried round as a liquid. Alloys of titanium and iron, or magnesium and nickel, can absorb 1,000 times their own volume of hydrogen and release it as required. Inside the alloy, the hydrogen molecule splits into atoms which pack into the spaces between the atoms of metal.
At the Tokyo motor show last autumn, Mazda exhibited a hydrogen car that stores its fuel in this way. The hydrogen is not burnt, but used in a fuel cell to generate electricity for the motor. In this cell, hydrogen releases its electrons to produce an electric current, and then combines with oxygen to form water.
Storage of hydrogen in alloys is beset with difficulties, says Professor Alfred Tseung, director of the chemical energy research centre at the University of Essex. Pumping hydrogen in and out of the alloy makes the metal brittle, and after a time reduces it to dust. If any moisture gets into the storage tank, its capacity is much reduced.
Professor Tseung, the UK representative on the International Committee for Hydrogen Utilisation, believes that these problems, and the high cost of hydrogen, make it impractical as a fuel for cars.
If there ever were a 'hydrogen economy', in which the gas was piped into homes for heating and cooking, as well as fueling the family car, one thing is certain - we would need a lot of gas.
Hydrogen is already manufactured on a large scale for the chemicals industry and is piped hundreds of miles across Europe and America. It is used in many ways, but most goes into making ammonia for fertilisers, and plastics. Some is used to turn vegetable oils into margarine - a deficiency of hydrogen is what differentiates an unsaturated oil from a saturated one.
World production of hydrogen is around 350 billion cubic metres per year, or 30 million tons. According to Ron Chappell, technical services manager at ICI's chloralkali works in Cheshire, the gas is usually produced as a co-product. In ICI's case, hydrogen is formed during the manufacture of sodium hydroxide (caustic soda). The company uses the gas to generate electricity; pipes it to other companies, for example, to make hydrogen peroxide; and sells it in high-pressure cylinders.
A truck stacked with cylinders demonstrates the economic difficulties of hydrogen: a fully loaded 40-ton lorry is actually transporting less than half a ton of the gas.
There are two natural sources of hydrogen: water (H2 0) and hydrocarbons such as methane (CH4 ). The estimated 130 million tons in the Earth's atmosphere is too diluted to be reclaimed.
Hydrogen can be released by passing an electric current through water - but this is not economical, despite improvements in efficiency such as the electrolysis of steam inside porous electrodes of zirconium oxide. One way to make the gas cheaply would be to use the surplus electricity of nuclear power stations at night to generate hydrogen by the electrolysis of water.
Another way of generating hydrogen is 'coal gasification' - blowing steam through white hot coal. But again, it is not economical, and in any case produces a mixture of hydrogen and carbon monoxide gas, which is better converted into the alternative fuel, methanol.
A third method, still only a research novelty, is to use sunlight to split water into its component gases, oxygen and hydrogen. Powdered titanium dioxide doped with platinum metal was found to do this 20 years ago, but the amount of hydrogen given off was tiny. Earlier this year, the National Chemical Laboratory for Industry in Japan boosted the yield of hydrogen by adding washing soda (sodium carbonate) to the water. However, we are still a long way from a commercially viable way of generating hydrogen from sunlight.
Currently, the best way to make hydrogen is from alkanes such as methane and propane. React these with steam at 1,000 C and you obtain a mixture of hydrogen with a small amount of carbon monoxide. Obtaining pure hydrogen gas from such a mixture is no problem, thanks to a curious property of the element palladium: a thin sheet of this metal allows hydrogen to filter rapidly through, while stopping everything else.
It seems like a magic solution, but not magical enough yet to make hydrogen the source of energy that will solve the world's problems.
The author is science writer in residence in the department of chemistry, Imperial College, London.
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