Hydrogen: energy of the future?

Imagine a car whose exhaust pipe drips nothing but clean water. No exhaust cloud, no CO₂. That is exactly what hydrogen buses promise, and they already run in some cities today. Many see hydrogen as the energy carrier of the future, for steel plants, ships and aircraft. Whether it can keep that promise is decided by chemistry, and a little by honest arithmetic.

Burning without soot and CO₂

When hydrogen burns with pure oxygen, nothing but water is produced: 2 H₂ + O₂ → 2 H₂O. Count for yourself: on the left there are 4 hydrogen atoms and 2 oxygen atoms, and exactly the same on the right. The equation is balanced, no atom gets lost. Because the fuel contains no carbon, no CO₂ and no soot can form. When hydrogen burns in ordinary air, however, the high flame temperature can additionally form nitrogen oxides. Water as the only exhaust is delivered only by the fuel cell. Burning it releases a lot of energy: per kilogram, hydrogen delivers about three times as much energy as petrol. Per litre, though, it is far less than for petrol, because the gas is extremely light.

The catch: where does the hydrogen come from?

There is hardly any free hydrogen on Earth, so it has to be produced first. The cleanest route is electrolysis: electricity splits water into hydrogen and oxygen, which is combustion run backwards. But this hydrogen is only green if the electricity comes from wind, sun or hydropower. If the electricity comes from coal or gas, you merely shift the CO₂ from the exhaust pipe to the power plant. Today most hydrogen is actually made directly from natural gas, releasing CO₂ in the process. So the colour of the hydrogen decides whether it truly helps the climate.

Storage, fuel cells and open questions

Hydrogen can store energy when wind and sun deliver more than we currently need. In a fuel cell it later reacts with oxygen in a controlled way and delivers electricity directly, again with water as the only exhaust. But honest questions remain: the gas takes up a lot of space and must be strongly compressed or cooled to about −253 °C, a large share of the energy is lost during production, transport and conversion back, and green electricity is still scarce. Safety matters too: hydrogen is extremely flammable and forms an explosive mixture with oxygen, including the oxygen in air, called oxyhydrogen. That is why it is stored in especially sturdy high-pressure tanks, at around 700 bar in a car, and installations must be checked carefully for leaks, because the tiny molecule escapes easily through fine gaps. Much suggests using hydrogen first where batteries fall short: in steel plants, on ships and in aircraft. Whether it will also win over the ordinary car is an open question.