The first to form in the moments after the big bang, the most abundant in the galaxy, the first on the list in the periodic table because it is the lightest of all the chemical elements…

A long and turbulent cosmic evolution has also brought a bit of it to Earth, combining with other elements and distributing itself in different ways in its “spheres”. It is present as water in the atmosphere, hydrosphere and, in part, the lithosphere which also contains it in the form of organic compounds, as in the biosphere. As H₂, elemental hydrogen is present in negligible quantities, being such a light gas that it escapes the force of gravity and exits the atmosphere. That’s why it was not easy to find! Water generator After Paracelsus and Robert Boyle, in 1766, Henry Cavendish noted that the reaction of a metal with an acid released a strange colourless and odourless gas which he called “flammable air”. A few years later, Lavoisier verified the discovery of this gas; its combustion produced water, so he decided to call it “Hydrogène”, in Greek “water generator”. After the discovery, all that remained was to study its properties, No mean feat given its high flammability. Under normal environmental conditions, hydrogen spontaneously and explosively combines with oxygen, producing invisible flames that move upwards very fast and reach temperatures as high as 2700 °C.

So, a gas that releases energy by reacting naturally with the oxygen in the air, emitting harmless water… Interesting! So far it seems rather simple, until we think think back to its presence in the various “spheres”. Water and hydrocarbons are the main sites of hydrogen, which can be extracted in different ways. Today, the most widespread involves treating methane gas (CH₄) or other hydrocarbons with high-temperature steam. Processes of this type involve the formation of polluting compounds such as CO and CO₂ mixed with hydrogen. Various studies are also evaluating the efficiency of obtaining hydrogen from the activity of red bacteria, cyanobacteria and microalgae, as well as from the gasification of municipal solid waste and non-recyclable plastics. This last process involves the decomposition of organic matter with a high-temperature heat treatment, in an almost oxygen-free atmosphere.

The extraction from water molecules is cleaner as these are most efficiently broken down by the process of electrolysis. In electrolytic cells, an electric current passes through the water that dissociates it into H⁺ and OH^– ions. These are drawn respectively towards the cathode and the anode where hydrogen is formed on one hand and gaseous oxygen on the other.  To keep it clean, the energy for electrolysis must come from a renewable source, such as the Sun. Hydrogen produced in this way can be considered an ideal resource for powering electric cars. To make them work, we have to rewind the tape and we do this inside fuel cells. Here, the inverse of what we saw in the electrolytic cells occurs. The hydrogen, in the molecular form H₂ which we painstakingly obtained, is broken up into two positively charged H⁺ atoms and electrons. These pass through an external circuit, supplying a current that feeds the electric motor. Once they return to the cell’s cathode along with the positive hydrogen ions, they combine with oxygen from the air and produce… water again! This fuel cell system is associated with the hydrogen tank, which must be maintained at a pressure that can reach up to 700 bar (more or less the same as a depth of 2.5 km below us). This high value is necessary to counter its low energy density in terms of volume, as much as is possible. In fact, despite having an energy density in terms of mass that exceeds that of all the most common fuels (142 MJ/kg compared to 54 of methane and to 46 of gasoline), the molecules don’t tend to want be very close to each other, let alone to switch to the liquid state (unless you force them to 259 °C below zero!). So we need to oblige them to get closer, whatever their feelings on the matter, to make them stay inside the tank as long as possible and extend the range of travel. Long, winding paths, studded with observations, experiments and inspirations from nature are leading our cars towards mobility that is increasingly compatible with the environment and with people.

Geologist, from atoms to stars through the Earth.