In order to tackle climate change more and more countries (but not enough) are increasing the share or renewable energy in their energy mix. Solar and wind energies are renewable and have got cheaper than fossil energy (this point could worth a specific post) but are variable and really hard to forecast. As a consequence they have to be stored. What about Lithium batteries? Yes, they are efficient, reliable, ... But to store only day of electricity consumption of France, we would need 3 years of the world production of Lithium. It is not realistic, is it? Another solution could be to store this energy by producing Hydrogen. To make it short, electricity + water give hydrogen and oxygen. But currently most (95%) of the hydrogen produced for industrial purpose comes from fossil fuel cracking emitting green house gasses which is not compliant with a low carbon economy. That is why more and more electrolyzers (device producing hydrogen from electricty) have to be installed to produce carbon-free hydrogen. This hydrogen can be compressed and stored and then used in a car, a truck, a train or an industry for energy or chemical purposes. To produce electricity from this hydrogen we need a device named fuel cell. It is a kind of cell but that you have to fuel with something, here with hydrogen. This gas can be produced by many different ways from a primary energy like electricity, biomass, light, heat, ... and can converted to electricity or burned or used for chemical purposes. It is versatile and as such is a key enabling technology for the energy transition. Sorry if this post sound very technical. If you prefer a more poetic take-away keep in mind that hydrogen is sun in bottles.