Hydrogen can be produced using four broad process groups: thermal, electrolytic, biological, and solar-driven. Within each group, multiple forms of technology have been developed. For the map, we have divided the dataset into these groups. The database and map are intended to be evergreen. As the hydrogen sector develops, the PE Media Network data and maps team will be adding more projects. Most of the current projects indicated on the map fall into the thermal and electrolytic process groups.

Thermochemical processes
Thermal processes for hydrogen production typically involve steam reforming, a high-temperature process in which steam reacts with a hydrocarbon fuel to produce hydrogen. Many hydrocarbon fuels can be reformed to produce hydrogen, including natural gas, diesel, renewable liquid fuels, gasified coal, or gasified biomass. According to the IEA, hydrogen is almost entirely supplied from natural gas and coal today.

Electrolytic processes
Water can be separated into oxygen and hydrogen through a process called electrolysis. Electrolytic processes take place in an electrolyser, which functions much like a fuel cell in reverse. Instead of using the energy of a hydrogen molecule like a fuel cell, an electrolyser creates hydrogen from water molecules.

Biological processes
Biological processes use microbes, such as bacteria and microalgae, to produce hydrogen. In microbial biomass conversion, the microbes break down organic matter like biomass or wastewater to produce hydrogen, while in photobiological processes the microbes use sunlight as their energy source.

Direct solar water splitting processes
Solar-driven processes use light as the agent for hydrogen production. Solar-driven processes include photobiology, photoelectrochemical reactions and solar thermochemical reactions. Photobiological processes use the natural photosynthetic activity of bacteria and green algae to produce hydrogen. Photoelectrochemical processes use specialised semiconductors to separate water into hydrogen and oxygen. Solar thermochemical hydrogen production uses concentrated solar power to drive water splitting reactions, often along with other species such as metal oxides.

Electrolytic processes

Low temperature electrolysis (LTE)
The electrolysis process uses electricity to split water into hydrogen and oxygen molecules. A typical electrolyser cell consists of two electrodes (cathode and anode) and a conducting electrolyte. The functioning of different types of electrolysers differs depending on the electrolyte material, as explained below.

The type of cell used determines the operating temperature for electrolysers. LTE refers to electrolysis at operating temperatures below approximately 300°C and typically uses alkaline water electrolysis (AWE) cells or polymer electrolyte membrane (PEM) cells.

Alkaline water electrolysis (AWE)
Alkaline water electrolysers employ alkaline liquid (usually aqueous KOH or NaOH) electrolytes which readily conduct electricity. In AWE cells, water dissociation and hydrogen generation occurs at cathode and hydroxyl ions (OH¬-) transfer through the porous diaphragm producing water and oxygen at the anode. These cells use inexpensive metals as electrode material and produce hydrogen of purity up to 99.9% at approximately 70-80% efficiency but operate best at low current densities. Alkaline electrolysis is a mature technology where significant advances with respect to electrode and separating diaphragm materials have improved the cost, efficiency, and durability of the cells. Alkaline electrolyzers however, do have higher start-up and cycling times, and may require higher maintenance than PEM cells.

Polymer electrolyte membrane electrolysis (PEM)
Polymer electrolyte membrane (PEM) electrolysers use a solid ion-conducting membrane as the electrolyte instead of a liquid. In PEM cells, water dissociation occurs at the anode and protons migrate selectively through the membrane generating hydrogen at the cathode. This type of electrolysis has advantages such as smaller footprint, high current density, higher efficiency, low (20-80°C) temperature operation, higher dynamic operation, and higher hydrogen purity. PEM cells, however, use high activity noble metal electrocatalysts which makes them more expensive than AWE cells.

High temperature steam electrolysis (HTSE)
High-temperature steam electrolysis employs high temperature (750-1000°C) for electrolysis of water. It utilizes inexpensive thermal energy from nuclear reactors to convert water to steam and the dissociation reaction proceeds in vapor phase producing hydrogen. The energy demand for electrolysis in vapor phase is lower thus consuming nearly 35% less electricity compared with LTE. The overall efficiency for HTSE is therefore higher than LTE.

In HTSE, high temperature electrolysis of steam at the cathode produces hydrogen molecules and oxygen ions that migrate through the solid oxide electrolyte to form oxygen molecules at the anode. HTSE cells are also called solid oxide electrolyser cells (SOEC).

Power to X (PtX)
Besides being stored in batteries, electricity may be stored by converting it to another form of energy, including hydrogen. Power-to-X describes the process of converting power generated from various renewable sources to other forms of energy carriers for use in other sectors or for storage and then re-conversion to power when needed. The X in the terminology can refer to one of the following: power-to-ammonia, power-to-chemicals, power-to-fuel, power-to-gas, power-to-hydrogen, power-to-liquid, power-to-methane, power-to-food, power-to-power, and power-to-syngas. For example – power-to-hydrogen refers to the process of using electricity to split water and produce hydrogen through electrolysis. Similarly, power-to-gas refers to converting renewable grid energy to gaseous carriers such as hydrogen or methane.

Thermal processes

Autothermal reforming (ATR)
ATR is a process for generating hydrogen and carbon dioxide by partial combustion of a gaseous hydrocarbon feed followed by catalytic reforming in the presence of steam. The process supports a variety of feeds such as natural gas, refinery off-gas, tail gas, light ends or naphtha. ATR often combines exothermic partial oxidation and endothermic steam reforming in a single reactor for a thermoneutral combined reforming process.

A key advantage of ATR is that the H₂/CO ratio can be varied to maximize hydrogen production (or optimized for next generation biofuel production). This technology is popular foroffers the advantage of a compact design, low-cost reactor system with higher hydrogen production, lower operating temperature, less coking, easier start-up, and fast response times. These advantages are critical requirements upon which the deployment and commercialization of several hydrogen applications such as fuel cells hinges, making ATR a popular technology.

Steam methane reforming (SMR)
Steam reforming or steam methane reforming is a mature and widely used technology that uses high temperature (700-1000°C) steam to produce hydrogen from a methane source such as natural gas. In SMR, methane reacts with high pressure steam in presence of a catalyst to yield hydrogen (H₂), carbon monoxide (CO), and carbon dioxide (CO₂). This is followed by the water-gas shift reaction where CO and steam react in presence of a catalyst to produce more hydrogen and CO₂. The product H₂ is separated and further purified using pressure swing adsorption (PSA). A variety of feedstock such as gasoline, LPG, naphtha, refinery off-gas, ethanol or propane may be used for SMR. Feed flexibility, low operating cost and existing infrastructure make steam reforming the most common method for producing hydrogen today.

Gasification processes

Gasification
Gasification is a non-combustion process that converts solid organic biomass or fossil fuel based carbonaceous material such as coal to hydrogen, carbon dioxide and carbon monoxide using controlled amount of steam and oxygen at high temperatures. The carbon monoxide then reacts with water via the water gas shift reaction producing more hydrogen and carbon dioxide. Hydrogen is usually separated and concentrated from the effluent gas stream through adsorption or membrane separation.

Direct air capture (DAC)
Direct air capture, as the name suggests, is a technology to capture CO₂ directly from the atmosphere. It involves contacting atmospheric air with a chemical solution which reacts with the CO2 in the air to trap it in the form of a salt. The CO₂ depleted air is recycled back to the atmosphere while the trapped CO₂ is separated, concentrated, stripped via calcination, and compressed to be utilized or stored as pure CO₂. The concentration of CO2 in the atmosphere is about 390 ppm which is nearly 300 times less than a typical flue gas stream. The very low mass transfer potential results in prohibitively high cost of the current direct air capture technologies making them a less attractive option compared to the others available.

Carbon capture & Storage (CCS)

Carbon capture & Storage (CCS)
Carbon capture and storage (CCS) is the process of capturing the carbon dioxide (CO₂) formed from burning of fossil fuels in industrial processes, power plants etc. and storing it so that it is not emitted to the atmosphere. Employing CCS involves three main steps – capture, transportation, and storage. Various technologies may be used for CO₂ capture at source and may be categorized as 1) post-combustion carbon capture – capturing CO₂ from exhaust/flue gases, 2) pre-combustion carbon capture – separating CO₂ from gasified fuel, and 3) oxy-fuel combustion systems –burning fuel in an oxygen rich environment producing concentrated stream of easily separable CO₂. The captured CO₂ is usually compressed into a liquid for transportation before being stored in underground geological reservoirs. Current CCS technologies can capture 90% and above of the CO₂, thus avoiding emission to the atmosphere. The term sequestration is often used to indicate long term storage of the captured carbon in underground reservoirs.

Carbon capture, Utilisation & Storage (CCUS)
Carbon capture, utilisation, and storage (CCUS), refers to the CCS process explained above and adds the concept of ‘utilization’ of the CO₂ for making useful products or for applications other than underground storage. One primary use of captured CO2 is to inject CO₂ and water in oilfields to improve yields, known as Enhanced Oil recovery (EOR), while sequestering the CO₂ underground. Other CO₂-utilization technologies of interest include: CO₂ conversion to fuels via methanol, methane, alkanes and syngas; CO₂ as feedstock for chemicals such as organic polycarbonates, inorganic resins, or agrochemicals such as urea; CO₂ mineralization for non-geologic storage; and the use of carbon to fabricate materials for construction or commercial products.

PE Media Network has adopted the hydrogen colour coding system, as used by the Energy Industries Council (EIC). We have categorised our data into these broad colour bands (see below). The colour coding is based principally on carbon intensity. It runs from green hydrogen, which has zero carbon emissions, through to black and brown hydrogen, which have high carbon emissions.

Hydrogen production via fossil fuels

Blue hydrogen
Blue hydrogen is produced when natural gas is split into hydrogen and CO₂ by steam methane reforming (SMR) or auto thermal reforming (ATR), for example, and the CO₂ is captured and then stored. The ‘capturing’ is done through a process called Carbon capture and storage (CCS) or carbon capture, utilisation, and storage (CCS).

Turquoise hydrogen
Hydrogen produced from natural gas using pyrolysis technology In which Natural gas is passed through, for example, a reactor containing molten metal to facilitate a reaction that releases hydrogen gas as well as solid carbon.

Grey hydrogen
Grey hydrogen has been produced for many years. It is a similar process to blue hydrogen using SMR or ATR to split natural gas into Hydrogen and CO₂, although the CO₂ is not captured and is released into the atmosphere.

Brown hydrogen
Brown hydrogen is created through brown coal (lignite) gasification. Hydrogen is produced by first reacting coal with oxygen and steam under high pressures and temperatures to form synthesis gas, a mixture consisting primarily of carbon monoxide and hydrogen. If brown hydrogen is combined with CCS it is then considered to be ‘blue’ hydrogen.

Black hydrogen
Black hydrogen is created through black (bituminous) coal gasification. Hydrogen is produced by first reacting coal with oxygen and steam under high pressures and temperatures to form synthesis gas, a mixture consisting primarily of carbon monoxide and hydrogen. If black hydrogen is combined with CCS it is then considered to be ‘blue’ hydrogen.

Hydrogen production via electricity

Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyser. Electrolysers can range in size from small, appliance-size pieces of equipment that are well-suited for small-scale distributed hydrogen production to large-scale, central production facilities that could be tied directly to renewable or other non-greenhouse-gas-emitting forms of electricity production.

Green hydrogen
Green hydrogen is made by using 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.

Purple/pink hydrogen
Pink hydrogen is generated through electrolysis powered by nuclear energy. Nuclear-produced hydrogen can also be referred to as purple hydrogen or red hydrogen. In addition, the very high temperatures from nuclear reactors could be used in other hydrogen productions by producing steam for more efficient electrolysis or fossil gas-based steam methane reforming.

Yellow hydrogen
Yellow hydrogen is produced from mixed-origin grid energy.