Hydrogen is a naturally occurring gas and is the most abundant substance in the universe, estimated to contribute 75% of the mass of the universe. It is a chemical element ‘H’ and atomic number ‘1’. It is an invisible gas and is the lightest element.

Hydrogen can be produced from a variety of resources, such as natural gas, nuclear power, biogas and renewable power like solar and wind.

• Green: hydrogen is made by using clean electricity from surplus renewable energy sources, such as solar or wind power, to electrolyse water. Electrolysers use an electrochemical reaction to split water into its components of hydrogen and oxygen, emitting zero-carbon dioxide in the process. Yellow hydrogen may also be referred to when using electrolysis with solar power.
• White/Gold: hydrogen is a naturally occurring, geological hydrogen found in underground deposits and created through fracking. White hydrogen may be renewable. It is non-polluting and offers lower costs than industrial hydrogen.
• Blue: hydrogen is produced mainly from natural gas (through steam reforming), which brings together natural gas and heated water in the form of steam. Carbon dioxide is also produced as a by-product creating greenhouse gases. Blue hydrogen includes the use of carbon capture and storage (CCS) to trap and store this carbon.
• Grey: this is the most common form of hydrogen production. Grey hydrogen is created from natural gas, or methane, using steam methane reformation but without capturing the greenhouse gases made in the process. Grey hydrogen is essentially the same as blue hydrogen, but without the use of carbon capture and storage.
• Black/Brown: the opposite to green hydrogen, made from black coal or lignite (brown coal) and the most environmentally damaging.
• Pink/Purple/Red: hydrogen is generated through electrolysis powered by nuclear energy.
• Turquoise: hydrogen is made using a process called methane pyrolysis to produce hydrogen and solid carbon.

• Fuel cells use a chemical process to convert hydrogen into electricity. A hydrogen fuel cell works by passing hydrogen through the anode, where it is split into electrons and protons. The electrons flow through an external circuit to produce electricity, while the protons move through a membrane to the cathode. At the cathode, electrons, protons, and oxygen from the air combine to form water.
• Hydrogen combustion engines burn hydrogen, producing power through combustion, like conventional engines.

• Hydrogen fuel cells produce no emissions except water vapor and warm air.
• Hydrogen engines release near zero, trace amounts of CO2 (from ambient air and lubrication oil), but do produce very high levels of nitrogen oxides (NOx). Hydrogen combustion, like any combustion reaction that heats air to high temperatures, creates harmful pollutants called NOx. These are linked to smog, acid rain, and damaging health impacts such as asthma and respiratory infections. As a result, they are not suitable for indoor use and require exhaust after treatments to reduce NOx emissions, which then causes further reductions in efficiency and increases cost.

• Hydrogen has 0% GHG emissions and can therefore contribute to a rapid decrease of the average GHG emissions for shipping. The maritime industry faces increasingly stricter air emissions and climate legislation, whereby hydrogen that is produced with renewable energy (green hydrogen) has been identified as a fuel that could offer a ‘near-zero’ carbon solution on a well-to-wake basis.
• Due to hydrogen’s low volumetric energy density (which would increase the storage needs onboard a ship) hydrogen-fuelled vessels may prove to be a more appropriate solution for short-sea shipping rather than deep-sea.