Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Most hydrogen is gray hydrogen made through steam methane reforming. In this process, hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. When carbon capture and storage is used to remove a large fraction of these emissions, the product is known as blue hydrogen.
Green hydrogen is usually understood to be produced from renewable electricity via electrolysis of water. Less frequently, definitions of green hydrogen include hydrogen produced from other low-emission sources such as biomass. Producing green hydrogen is currently more expensive than producing gray hydrogen, and the efficiency of energy conversion is inherently low. Other methods of hydrogen production include biomass gasification, methane pyrolysis, extraction of underground natural hydrogen, and in situ hydrogen synthesis.
As of 2023, less than 1% of dedicated hydrogen production is low-carbon, i.e. blue hydrogen, green hydrogen, and hydrogen produced from biomass.
In 2020, roughly 87 million tons of hydrogen was produced worldwide for various uses, such as oil refining, in the production of ammonia through the Haber process, and in the production of methanol through reduction of carbon monoxide. The global hydrogen generation market was fairly valued at US$155 billion in 2022, and expected to grow at a compound annual growth rate of 9.3% from 2023 to 2030.
Overview
Molecular hydrogen was discovered in the Kola Superdeep Borehole. It is unclear how much molecular hydrogen is available in natural reservoirs, but at least one company specializes in drilling wells to extract hydrogen. Most hydrogen in the lithosphere is bonded to oxygen in water.
Manufacturing elemental hydrogen requires the consumption of a hydrogen carrier such as a fossil fuel or water. The former carrier consumes the fossil resource and in the steam methane reforming (SMR) process produces greenhouse gas carbon dioxide. However, in the newer methane pyrolysis process no carbon dioxide is produced. These processes typically require no further energy input beyond the fossil fuel.
Decomposing water, the latter carrier, requires electrical or heat input, generated from some primary energy source (fossil fuel, nuclear power or a renewable energy). Hydrogen produced by electrolysis of water using renewable energy sources such as wind and solar power, referred to as green hydrogen. When derived from natural gas by zero greenhouse emission methane pyrolysis, it is referred to as turquoise hydrogen.
When fossil fuel derived with greenhouse gas emissions, is generally referred to as grey hydrogen. If most of the carbon dioxide emission is captured, it is referred to as blue hydrogen. Hydrogen produced from coal may be referred to as brown or black hydrogen.
Classification based on production method Hydrogen is often referred to by various colors to indicate its origin (perhaps because gray symbolizes "dirty hydrogen"). +Colors that refer to method of productionColorProduction sourceProcess / method / chemistryGreenhouse gas footprintNotes ReferencesGreen Renewable electricity: wind, solar, hydro, tidal, geothermal. May also include electricity from low-emission sources such as biomass.Electrolysis of water 2 H2O → 2 H2 + O2Minimal Turquoise Fossil hydrocarbons: natural gas, a.k.a. Methane Methane pyrolysis (thermal splitting) CH4 → C + 2 H2MinimalSolid carbon byproduct Blue Fossil hydrocarbons: natural gas Gas reforming with carbon capture and storage 1st stage: CH4 + H2O → CO + 3 H2
2nd stage: CO + H2O → CO2 + H2LowCCS networks required Gray Fossil hydrocarbons: natural gasSteam reforming of natural gas 1st stage: CH4 + H2O → CO + 3 H2
2nd stage: CO + H2O → CO2 + H2HighCO2 produced Black Fossil hydrocarbons: Coal (anthracite)Coal carbonisation or gasification 1st stage: 3 C (i.e., coal) + O2 + H2O → H2 + 3 CO
2nd stage: CO + H2O → CO2 + H2
C24H12 + 12 O2 → 24 CO + 6 H2Very highCO2 produced Brown Fossil hydrocarbons: brown coal (lignite) Coal carbonisation or gasification as black hydrogenVery highCO2 produced Red Nuclear powerNuclear heat: thermolysis Thermochemical water splitting H2O(l) ⇌ H2(g) + 1/2 O2(g) MinimalMauve Decay heat from radioactive isotopesRadiogenic heat: thermolysis Thermochemical water splitting H2O(l) ⇌ H2(g) + 1/2 O2(g) MinimalPink Nuclear powerNuclear electricity plus water: electrolysis 2 H2O → 2 H2 + O2Minimal Purple Nuclear power Nuclear heat plus water: Electrolysis and thermolysis 2 H2O → 2 H2 + O2MinimalAlso contributing steam to natural gas reforming Yellow Solar photovoltaics Electrolysis 2 H2O → 2 H2 + O2Minimal Gold Hydrogen Microbial activity in depleted oil wells, drillingLowCCS networks required White Hydrogen occurring naturally in underground depositsDrilling, miningMinimal
Current production methods
Steam reforming – gray or blue
Hydrogen is industrially produced from steam reforming (SMR), which uses natural gas. The energy content of the produced hydrogen is around 74% of the energy content of the original fuel, as some energy is lost as excess heat during production. In general, steam reforming emits carbon dioxide, a greenhouse gas, and is known as gray hydrogen. If the carbon dioxide is captured and stored, the hydrogen produced is known as blue hydrogen.
Steam methane reforming (SMR) produces hydrogen from natural gas, mostly methane (CH4), and water. It is the cheapest source of industrial hydrogen, being the source of nearly 50% of the world's hydrogen. The process consists of heating the gas to in the presence of steam over a nickel catalyst. The resulting endothermic reaction forms carbon monoxide and molecular hydrogen (H2).
In the water-gas shift reaction, the carbon monoxide reacts with steam to obtain further quantities of H2. The WGSR also requires a catalyst, typically over iron oxide or other oxides. The byproduct is CO2. Depending on the quality of the feedstock (natural gas, naphtha, etc.), one ton of hydrogen produced will also produce 9 to 12 tons of CO2, a greenhouse gas that may be captured.
For this process, high temperature steam (H2O) reacts with methane (CH4) in an endothermic reaction to yield syngas.
CH4 + H2O → CO + 3 H2
In a second stage, additional hydrogen is generated through the lower-temperature, exothermic, water-gas shift reaction, performed at about :
CO + H2O → CO2 + H2
Essentially, the oxygen (O) atom is stripped from the additional water (steam) to oxidize CO to CO2. This oxidation also provides energy to maintain the reaction. Additional heat required to drive the process is generally supplied by burning some portion of the methane.
Electrified Steam Methane Reforming In May 2019, Science published the results of a Danish study in which the tin catalyst is heated electrically. This reduces natural gas consumption (and CO2-emission) by a third, while the improved heating increases the overall efficiency. SMR needs 4,2 kWh/Nm3 H2, eSMR 3,6 kWh/Nm3 H2 (2,6 kWh natural gas and 1.0 kWh electricity).
From water
Methods to produce hydrogen without the use of fossil fuels involve the process of water splitting, or splitting the water molecule (H2O) into its components oxygen and hydrogen. When the source of energy for water splitting is renewable or low-carbon, the hydrogen produced is sometimes referred to as green hydrogen. The conversion can be accomplished in several ways, but all methods are currently considered more expensive than fossil-fuel based production methods.
Electrolysis of water – green, pink or yellow
Hydrogen can be made via high pressure electrolysis, low pressure electrolysis of water, or a range of other emerging electrochemical processes such as high temperature electrolysis or carbon assisted electrolysis. However, current best processes for water electrolysis have an effective electrical efficiency of 70-80%, so that producing 1 kg of hydrogen (which has a specific energy of 143 MJ/kg or about 40 kWh/kg) requires 50–55 kWh of electricity.
In parts of the world, steam methane reforming is between $1–3/kg on average excluding hydrogen gas pressurization cost. This makes production of hydrogen via electrolysis cost competitive in many regions already, as outlined by Nel Hydrogen and others, including an article by the IEA examining the conditions which could lead to a competitive advantage for electrolysis.
A small part (2% in 2019) is produced by electrolysis using electricity and water, consuming approximately 50 to 55 kilowatt-hours of electricity per kilogram of hydrogen produced.
Water electrolysis is using electricity to split water into hydrogen and oxygen. As of 2020, less than 0.1% of hydrogen production comes from water electrolysis. Electrolysis of water is 70–80% efficient (a 20–30% conversion loss) while steam reforming of natural gas has a thermal efficiency between 70 and 85%. The electrical efficiency of electrolysis is expected to reach 82–86% before 2030, while also maintaining durability as progress in this area continues apace.
Water electrolysis can operate at , while steam methane reforming requires temperatures at . The difference between the two methods is the primary energy used; either electricity (for electrolysis) or natural gas (for steam methane reforming). Due to their use of water, a readily available resource, electrolysis and similar water-splitting methods have attracted the interest of the scientific community. With the objective of reducing the cost of hydrogen production, renewable sources of energy have been targeted to allow electrolysis.
There are three main types of electrolytic cells, solid oxide electrolyser cells (SOECs), polymer electrolyte membrane cells (PEM) and alkaline electrolysis cells (AECs). Traditionally, alkaline electrolysers are cheaper in terms of investment (they generally use nickel catalysts), but less-efficient; PEM electrolysers, conversely, are more expensive (they generally use expensive platinum group metal catalysts) but are more efficient and can operate at higher current densities, and can therefore be possibly cheaper if the hydrogen production is large enough.
SOECs operate at high temperatures, typically around . At these high temperatures, a significant amount of the energy required can be provided as thermal energy (heat), and as such is termed high-temperature electrolysis. The heat energy can be provided from a number of different sources, including waste industrial heat, nuclear power stations or concentrated solar thermal plants. This has the potential to reduce the overall cost of the hydrogen produced by reducing the amount of electrical energy required for electrolysis.In the laboratory, water electrolysis can be done with a simple apparatus like a Hofmann voltameter:
PEM electrolysis cells typically operate below . These cells have the advantage of being comparatively simple and can be designed to accept widely varying voltage inputs, which makes them ideal for use with renewable sources of energy such as photovoltaic solar panels. AECs optimally operate at high concentrations of electrolyte (KOH or potassium carbonate) and at high temperatures, often near .
Efficiency and economics