Overview of the relative greenness of the main hydrogen production processes

•The hydrogen production processes can be considered as being more or less green.•The processes involving fossil fuels are the least green.•The processes involving photolytic and photobiological processes are the greenest.

Hydrogen, H2, is one of the most promising (bio-)fuels for our near-future as it is considered as a green energy carrier. However, its natural occurrence is extremely low. In fact, it has to be produced through various processes, from various sources, given that atomic hydrogen, H, exists combined with other light atoms, e.g. oxygen in water, carbon in methane, or both atoms in cellulose. As molecular hydrogen has to be produced, the relative greenness of the production processes merits analysis; we do this here qualitatively. The analysis is notably based on the 12 principles of green chemistry. After an overview of the main hydrogen processes, we show that the combination of renewable raw materials with biological or electrochemical methods and with renewable energies, makes hydrogen production “green”.