Hydrogen is classified according to the chemical process that produced it, each of which is associated with a color. “Brown” hydrogen is obtained through coal gasification, and it is the one that pollutes the atmosphere and the environment the most because it emits 25 kg of CO2 for every kg of hydrogen it produces.
“Grey” hydrogen is obtained from natural gas through the process of steam methane reforming and it has an environmental impact of 11 kg of CO2 for every kg of H2. If it’s then associated with another process to capture and store carbon dioxide, preventing it from being released into the atmosphere, it’s called “blue” hydrogen. However, due to the current technology’s limitations or due to CO2 leaks from the storage systems, this process still emits an estimated 3-6 kg CO2 / kg H2.
Hydrogen can also be obtained with electricity through the process of electrolysis, which separates the atoms of hydrogen and oxygen contained in water molecules. If the electricity used comes from a nuclear power plant, it’s called “pink” hydrogen; it doesn’t release carbon dioxide into the atmosphere, but it produces radioactive waste. If, instead, the electricity used to produce it comes from renewable sources, it’s called “green” or “renewable” hydrogen, and doesn’t produce the CO2 emissions.