From green to gold: colors and types of hydrogen

Hydrogen is the first element of the periodic table, has symbol H and atomic number one. This means that only one proton is in the nucleus and one electron gravitates around the nucleus.
It is colorless, tasteless, the lightest, and most abundant element in the universe. It is highly flammable, and it’s used as a fuel in stars, powering nuclear fusion and providing us with light and heat.

Whereas the most widespread form of hydrogen is molecular hydrogen (H2) in the cosmos, we found it bound to other atoms on Earth. For example, we have it in hydrocarbons such as methane (CH4), water (H2O) and ammonia (NH3). It doesn’t exist as a standalone molecule (H2). Therefore, it has to be produced synthetically using various methods labeled with different colors.

Green hydrogen or electrolytic hydrogen is the one produced by water electrolysis. The feedstock is water which, by means of an electric current, is converted into hydrogen (H2) and oxygen (O2) gasses (1).
To be defined as green hydrogen, the electric current must be supplied from renewable sources of energy such as wind or solar power.

This is the most desirable form of hydrogen as the only by-product is molecular oxygen (2), and the reagent of this chemical reaction is water, an abundant and clean substance. Additionally, if the energy supply comes from renewables, no greenhouse gasses, such as carbon dioxide (CO2) or methane (CH4) are involved in the process of hydrogen production.
Several technologies have been developed for the production of green hydrogen such as alkaline water electrolysis (AWE) or proton exchange membrane water electrolysis (PEM). They all work at relatively low temperatures (60-90 °C) and provide for the use of an electrochemical cell made of anode and cathode separated by a diaphragm or a membrane.

Hydrogen produced through water electrolysis is still expensive and represents an emerging clean tech market fueled by government funds and subsidies. It represents only 0.1% of the world’s hydrogen production, but it is expected to reach new highs in the coming decades due to more stringent climate laws on environmental topics such as greenhouse gas emissions.

Hydrogen can also be produced by coal gasification, and this is referred to as black hydrogen: starting from black coal, water, and oxygen, this process produces syngas, a mixture consisting of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2). If lignite is used instead of black coal, this type of hydrogen production is identified as brown hydrogen.

Grey hydrogen is produced from Steam Methane Reforming (SMR), a method for producing syngas (CO and H2) by reaction of hydrocarbons with water. Usually, natural gas, such as methane, is the feedstock.

This is the most common form of hydrogen production; in fact, nearly 95% of hydrogen produced worldwide nowadays comes from SMR. It is a mature technology in which high-temperature steam (700°C–1000°C) is used to produce hydrogen from a methane source. Using this technique, methane (CH4) reacts with steam (H2O) under 3–25 bar pressure in the presence of a catalyst to produce hydrogen (H2), carbon monoxide (CO), and a relatively small amount of carbon dioxide (2).

Subsequently, the carbon monoxide (CO) and steam (H2O) are reacted using a catalyst to produce carbon dioxide (CO2) and more hydrogen (H2) (3).

In the final process step, carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen.

Despite being the most widespread form of hydrogen production, steam methane reforming figures as a non-sustainable production technique as greenhouse gasses, methane and carbon dioxide, are both reagents and products of this chemical reaction. However, due to the maturity of the SMR technology, grey hydrogen stands out among the other forms of hydrogen production, especially compared to green hydrogen, for its low cost.

If the carbon dioxide produced in black or grey hydrogen is captured and either stored geologically or repurposed, the hydrogen is identified as blue hydrogen. This type of hydrogen is called low-carbon, as around 10%-20% of the generated carbon cannot be captured.
Hence, it cannot be considered fully carbon neutral. Despite being attractive due to its relatively “low” emissions, this form of hydrogen still produces and uses greenhouse gasses as an outcome of the chemical reaction and for its production, respectively.

Alternative forms of hydrogen exist, such as purple and gold hydrogen.
Purple hydrogen is hydrogen produced by water electrolysis, but the electric current is supplied by nuclear power instead of renewables. Finally, gold hydrogen, also referred to as white, natural, or geological hydrogen, is hydrogen produced naturally on Earth.
In fact, the Earth holds vast supplies of natural hydrogen that could be extracted from the ground through natural processes, such as serpentinization, water radiolysis, or biological activities, which yield this so-called gold hydrogen.

In conclusion, molecular hydrogen (H2) can be produced using various techniques. There is currently a lot of emphasis on the production of green hydrogen as it stands out among other forms for its sustainable outlook and ability to decarbonize energy and the hard-to-abate sectors.