Today, however, nearly all synthetic hydrogen is produced with fossil fuels. Three main methods of sourcing hydrogen can be distinguished: grey, blue, and green hydrogen.

• Grey hydrogen is obtained by steam reforming, i.e., converting natural gas or coal under heat into H2 and CO2, with about 10t of CO2 being released into the atmosphere as GHG for each tonne of hydrogen produced. H2 sourced from methane creates about 285 grams of CO2 emis­sions per kWh, while coal gasification has an emission factor of around 675 grams of CO2 per kWh of hydrogen power.

• Blue hydrogen is produced in the same way, but with the addition of carbon capture and storage (CCS) to prevent the CO2 from being released. It is seen as a potential bridging tech­nology for a transitional period until the cost of synthesizing hydrogen from renewables goes down.

• Green hydrogen is produced by electrolysis of water using only renewable, CO2-free elec­tricity sources. While the process is already viable, the speed of its adoption will depend on the cost of electrolyzers, supply chain logis­tics, and regulatory policies in support of H2-based sector coupling.

Electrolysis, or the process of splitting water (H2O) into hydrogen and oxygen using direct current, was discovered in 1800 by the English scientists William Nicholson and Anthony Carlisle, who went on to lay the groundwork for the new discipline of electrochemistry. Until the arrival of steam reforming from coal and gas, electrolysis from water was the standard method for producing hydrogen, prompting French author Jules Verne to write in his 1874 novel The Mysterious Island: “Water will be the coal of the future.” More than two centuries later, that time may finally have come, as the need to store excess renewable power and convert it into a chemical energy source has kicked off a renaissance of H2 electrolysis.

The basic principle of this technology is that two electrodes, connected to a power source, are placed in the electrolyte – in this case, water – which facilitates the charge exchange. Hydrogen will begin to form in bubbles at the cathode, and oxygen will evolve at the anode, at a rate of production that is proportional to the electric charge. This chemical reaction will yield 1 kg of hydrogen, with an energy content (HHV) of 39.4 kWh, per 9 liters of water (stoichiometric), based on a power input of 52 kWh – an efficiency rate that compares favorably to other industrial processes.

Safety and environmental impact

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