Green hydrogen is essential for decarbonizing hard to abate sectors.
Recognizing this, the US Department of Energy announced $15M just this week in funding to support R&D projects that use raw materials to produce green hydrogen at a low cost.
What is green hydrogen?
Hydrogen is the most abundant element in the universe. It is also the lightest element, composed of one proton and one electron. It has high energy density by mass of 120 MJ/kg, which is about three times higher than that of gas and diesel. However, its energy density by volume is low, at 8 MJ/L, so it requires high-pressure tanks or very low-temperature (cryogenic) storage in many applications.
Hydrogen is not readily available on the planet; it is chemically combined in water, fossil fuels, and biomass, so the main challenge is separating hydrogen from these compounds. There are several methods for producing hydrogen, and each has a different environmental impact, categorized by color. The following are the four main methods:
Gray
Gray hydrogen is produced by splitting methane (CH4) and steam into hydrogen and CO2. It is also often produced from coal. It accounts for 95% of hydrogen production today.
Blue
Blue hydrogen is produced in the same process as gray hydrogen, but blue hydrogen captures and stores the resulting CO2. However, not all the CO2 can be captured and stored effectively.
Turquoise
Turquoise hydrogen is produced using feedstock through methane pyrolysis, a process that splits methane into hydrogen and solid carbon at high temperatures, without using oxygen. The process does not release CO₂, and the solid carbon can be discarded or reused. The technology is still in early development stages.
Green
Green hydrogen is produced through water electrolysis and renewable electricity is used to power the process.
Electrolysis
The process of electrolysis occurs in devices called electrolyzers. Electrolyzers consist of a negative electrode (anode) and a positive electrode (cathode) separated by an electrolyte. The electrolyte is a substance that conducts ions between the cathode and anode. In water electrolyzers, the electrolyte prevents oxygen and hydrogen gasses from mixing. The electrode is an electrical conductor used to make contact with non-metallic parts of a circuit.
Water electrolyzers use electricity to split water (H₂O) into hydrogen gas and oxygen gas. At the anode, water molecules lose electrons, which then flow through an external circuit, producing oxygen gas and releasing hydrogen ions (H⁺). At the cathode, these ions gain the electrons from the external circuit, producing hydrogen gas (H₂). The only byproduct is oxygen gas.
Electrolyzers and fuel cells use similar technologies but serve opposite purposes. Instead of using electricity to split water into hydrogen and oxygen, fuel cells combine oxygen (from the air) and hydrogen to create electricity, with water as the only byproduct.
There are four types of water electrolysis:
1. Proton exchange membrane (PEM) electrolysis
In PEM electrolysis, the electrolyte is a solid plastic material that conducts hydrogen ions from the anode to the cathode, where the ions meet the electrons to form hydrogen gas.
PEM electrolysis operates at 50°C to 80°C. Its low temperature, compact design, and rapid response make PEM electrolysis the fastest type of electrolysis, allowing it to adjust its output to match supply from intermittent renewable energy sources.
Due to the cost of the membrane material, PEM electrolyzers are expensive, but the PEM electrolysis market is strong and expected to grow, especially for the production of high-purity hydrogen.
Technology Readiness Level (TRL): 9
2. Solid oxide electrolysis cell (SOEC)
In solid oxide electrolysis cells, the electrolyte is a solid ceramic material that conducts oxygen ions (O²⁻) from the cathode to the anode, where the ions combine to form oxygen gas (O₂).
SOEC systems operate at high temperatures, 675°C to 825°C, which makes it easier to set off the electrolysis reaction.
SOEC systems have high efficiency and are best fit for large-scale green hydrogen industry applications.
TRL: 7
3. Alkaline electrolysis
In alkaline electrolysis, the electrolyte is a liquid sodium solution or a potassium hydroxide alkaline solution that conducts hydroxide ions (OH-) from the cathode to the anode, where the ions lose electrons to produce water and oxygen gas.
Alkaline electrolysis operates at moderate temperatures, 80°C to 160°C. Although alkaline electrolyzers are less efficient and are slower to respond, they are cost-effective and have been commercial for decades.
TRL: 9
4. Anion exchange membrane (AEM) electrolysis
In AEM electrolyzers, the electrolyte is a polymer material that conducts hydroxide ions from the cathode to the anode, same as in alkaline electrolyzers.
AEM electrolysis operates at low temperatures, 60°C to 80°C. AEM electrolyzers are cost-effective because they can be made of non-precious metals, and they are efficient because they have a higher pH, which reduces corrosion of electrodes.
TRL: 7
Key Sectors For Green Hydrogen
The following three sectors cannot be decarbonized through electrification but can be decarbonized through green hydrogen:
1. Transportation
For heavy-duty transportation, including buses, trucks, planes, and ships, the weight of batteries required for long distances would be prohibitive. Therefore, fuel cells and hydrogen-based fuels provide alternatives.
Fuel Cells
Hydrogen fuel cells in vehicles convert hydrogen, stored in a tank, and oxygen from the air into electricity, which then powers the vehicle’s electric engine.
Green Ammonia and E-Methanol
Green ammonia and e-methanol are derived from green hydrogen, and they can be used to decarbonize planes and industrial shipping ships.
Two challenges are that green ammonia and e-methanol have higher costs than fuel oil and that new hydrogen or ammonia-powered planes and ships have to be designed, built, and sold to airlines and shipping companies to replace existing jet-fuel-powered planes and ships before these fuels can be used.
Drop-in Fuels
Drop-in fuels are synthetic fuels derived from green hydrogen. They are similar to jet fuel and methanol, so they can be used in existing planes and ships with minimal adjustments. However, drop-in fuels do not eliminate CO2.
2. Industry
Steel Production
Green hydrogen is used to reduce iron ore to produce steel and eliminates CO2 emissions from the process.
Cement Production
Green hydrogen is used to fuel cement kilns and reach the high temperatures needed for cement clinker formation, replacing coal and natural gas.
Chemical Production
Green hydrogen is used as feedstock to produce ammonia, which is used as a fertilizer, refrigerator, purificator, and chemical stabilizer, as well as in household cleaning products.
Green hydrogen is also used to produce methanol, which is used to produce other chemical compounds, fuels, and additives, including in the conversion of crude oil.
3. Energy Storage
Excess renewable energy from wind and solar is used to power electrolysis. The resulting green hydrogen can be stored and later converted back into electricity through a fuel cell, which supports intermittent renewable sources and stabilizes the grid.
Green Hydrogen Market
China is the leading electrolyzer hydrogen producer, accounting for over 40% of global production. The EU, Germany, Saudi Arabia, Sweden, the UK, and the US are also key producers. Their policies focus on investing in green hydrogen R&D and bridging green hydrogen price gaps between supply and demand.
The biggest green hydrogen companies include Air Products and Chemicals, Plug Power, Linde, Bloom Energy, Shell, and Reliance Industries.
In addition to these, the following three startups are innovating in the green hydrogen space:
Supercritical Solutions
Supercritical Solutions builds electrolyzers that do not have membranes and that output industry-specific temperature and pressure hydrogen, which makes its process more efficient and cost-effective.
Electric Hydrogen
Electric Hydrogen is building a factory to produce 100-megawatt PEM electrolyzers by 2024. These will be sold to hydrogen producers to support green large-scale transportation, industry, and energy storage applications.
Enapter
Enapter mass-produces modular AEM electrolyzers that are flexible and scalable, with a range of 2.4 kilowatts to 100 megawatts.
What’s next?
The high production cost due to renewable electricity, lack of infrastructure for storage and transportation, and safety risks due to hydrogen’s flammability are the three main challenges that the green hydrogen market is working to solve. The US Department of Energy’s Hydrogen Shot is investing in these areas to reduce the cost of green hydrogen by 80% to $1/kg by 2031. Globally, subsidies for green hydrogen are projected to reach $360B in 2024.
More demand-side long-term green hydrogen purchasing commitments are needed to scale up the industry. Infrastructure and shipping as well as public acceptance and regulation also need to be further developed. By making it competitive with fossil fuels, green hydrogen will play an essential role in decarbonizing the transportation, industry, and energy storage sectors.