Why hydrogen?

There is a great deal of talk about hydrogen and its role in tackling climate change, yet it was hardly on anyone’s radar a year ago, so why all the great interest?

Most of the world’s population now accept that we have to stop the rise in the average world air temperature, which is caused by the release of carbon dioxide into the air by burning fossil fuels.

When these are burnt, it is the carbon and hydrogen that they contain that produce water and the greenhouse gas carbon dioxide; water is OK but carbon dioxide is not, so if we burn just hydrogen then we should be fine.

Hydrogen is not a source of energy, it’s an energy carrier and abundant on earth, mainly strongly bound to oxygen as water (H2O).

It takes a lot of energy to split water back into hydrogen and oxygen, which can be done with electricity in an electrolyser. When hydrogen is combined again with oxygen to make water, this releases the same large amount of energy as was needed to split it.

The high strength of the hydrogen-oxygen bond means that hydrogen has a high energy potential and good for carrying energy. Hydrogen can be burnt with oxygen in the air or by combining it with the oxygen in a fuel cell to produce electricity; both processes just produce water.

A hydrogen fuel cell works like a battery, but instead of burning the hydrogen, it is forced under pressure through a membrane to meet oxygen from the air on the other side to form water, while causing an electric current to flow from one side of the membrane to the other .

But why do we need hydrogen when we can just use clean or “green” electricity? it’s fine for fixed machines in factories and buildings, but for vehicles that move around, we need to store it on board.

Batteries are fine for small vehicles, but to power heavy transport that need a lot more energy, the batteries become too heavy as the vehicles become larger and need a longer range.

The long charging time of batteries is a problem for some users, such as car owners that don’t have a drive with their own charging point, and fork-lift trucks in a distribution warehouse that works 24/7.

Hydrogen is a solution as an energy store for larger moving applications such as freight vehicles, buses, trains, ships, aircraft and remote off-grid applications; it can be quickly dispensed, just as fast as petrol or diesel.

[1] Most fuel cells now have a proton exchange membrane (PEM) that separates the anode + side where the hydrogen comes in, from the cathode̵  side where the oxygen from the air comes in. A molecule of hydrogen gas is too big to pass through the PEM, but when it splits into 2 protons and 2 electrons, the protons are small enough to pass through the PEM to meet oxygen on the other side. The two electrons flow around to the other side of the cell to meet with oxygen and the protons coming through the PEM, to make water. The flow of electrons powers the device that the fuel cell is connected to.

Lunar Society Fellow David Dundas, who has held several senior positions in energy companies, has a strong interest in climate change and alternative sources including fusion nuclear energy – and hydrogen. In this article, he examines the role of hydrogen that is not an energy source but an energy carrier.