Hydrogen production

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 process produces greenhouse gas carbon dioxide. However, in the newer methane pyrolysis process no greenhouse gas 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. 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.

Current production methods

Steam reforming – gray or blue

Hydrogen is industrially produced from steam reforming, 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 produces hydrogen from natural gas, mostly methane, 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.
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, 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 reacts with methane in an endothermic reaction to yield syngas.
In a second stage, additional hydrogen is generated through the lower-temperature, exothermic, water-gas shift reaction, performed at about :
Essentially, the oxygen atom is stripped from the additional water 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 by a third, while the improved heating increases the overall efficiency. SMR needs 4,2 kWh/Nm3 H2, eSMR 3,6 kWh/Nm3 H2.

From water

Methods to produce hydrogen without the use of fossil fuels involve the process of water splitting, or splitting the water molecule 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 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 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 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 or natural gas. 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, polymer electrolyte membrane cells and alkaline electrolysis cells. Traditionally, alkaline electrolysers are cheaper in terms of investment, but less-efficient; PEM electrolysers, conversely, are more expensive 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, 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.
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 and at high temperatures, often near.

Efficiency and economics

Efficiency of modern hydrogen generators is measured by energy consumed per standard volume of hydrogen, assuming standard temperature and pressure of the H₂. The lower the energy used by a generator, the higher would be its efficiency; a 100%-efficient electrolyser would consume of hydrogen,. Practical electrolysis typically uses a rotating electrolyser, where centrifugal force helps separate gas bubbles from water. Such an electrolyser at 15 bar pressure may consume, and a further if the hydrogen is compressed for use in hydrogen cars.
Conventional alkaline electrolysis has an efficiency of about 70%, however advanced alkaline water electrolysers with efficiency of up to 82% are available. Industrial PEM efficiency is expected to increase to approximately 86% before 2030.
In 2022, a Nature publication described a capillary-fed electrolysis cell, which reached 98% energy efficiency due to various design optimizations minimizing overpotentials.
As of 2020, the cost of hydrogen by electrolysis is around $3–8/kg. Considering the industrial production of hydrogen, and using current best processes for water electrolysis which have an effective electrical efficiency of 70–82%, producing 1 kg of hydrogen requires 50–55 kWh of electricity. At an electricity cost of $0.06/kWh, as set out in the Department of Energy hydrogen production targets for 2015, the hydrogen cost is $3/kg.
The US DOE target price for hydrogen in 2020 was $2.30/kg, requiring an electricity cost of $0.037/kWh, which is achievable given recent PPA tenders for wind and solar in many regions. In 2021, the US DOE established the Hydrogen Energy Earthshot with a target of $1 for 1 kg of hydrogen in 1 decade, i.e., $1/kg by 2031. This low price was selected to be competitive with the price of hydrogen from natural gas in the United States which is approximately $1.50/kg. In comparison, the cost baseline for hydrogen from electrolysis in 2020 was approximately $5/kg, requiring an 80% cost reduction to meet the Hydrogen Shot goal.
The report by IRENA.ORG is an extensive factual report of present-day industrial hydrogen production consuming about 53 to 70 kWh per kg could go down to about 45 kWh/kg. The thermodynamic energy required for hydrogen by electrolysis translates to 33 kWh/kg, which is higher than steam reforming with carbon capture and higher than methane pyrolysis.
One of the advantages of electrolysis over hydrogen from steam methane reforming is that the hydrogen can be produced on-site, meaning that the costly process of delivery via truck or pipeline is avoided.