Technologies of the energy transition: Low and zero-carbon hydrogen

Hydrogen, a versatile energy carrier, can be produced through three main technological routes, which are often identified through a colour code: (1) direct generation from fossil fuels such as natural gas and coal (‘grey’ hydrogen); (2) fossil fuel-based generation with the addition of carbon capture technology to mitigate the resulting CO₂ emissions (‘blue’ hydrogen); (3) water electrolysis using renewable electricity (‘green’ hydrogen). Hydrogen produced through the third route is zero-carbon, whereas ‘blue’ hydrogen is low-carbon.