Green hydrogen production technologies include hydrogen generation via water electrolysis using renewable energy sources (wind, hydro, solar), solar photolysis of water, and biomass-based hydrogen production. Among these, water electrolysis powered by renewable energy is the most widely applied and technologically mature method.

I. Definition of Green Hydrogen

Green hydrogen refers to hydrogen produced using renewable energy sources such as solar, wind, or nuclear power. With minimal carbon emissions during production, green hydrogen is also termed "zero-carbon hydrogen."

The electricity used in green hydrogen production must originate from renewable sources such as solar, wind, or hydropower.This distinguishes green hydrogen from conventional "gray hydrogen," which is derived from fossil fuels (e.g., coal, oil, natural gas) and associated with significant carbon emissions.

II. Green Hydrogen Production Technologies

Green hydrogen production technologies include renewable energy-powered water electrolysis (using wind, hydro, or solar power), solar water splitting, and biomass-derived hydrogen.Renewable energy-powered water electrolysis remains the most mature and widely utilized approach. Additional technologies include thermochemical water splitting, biomass reforming, and microbial electrolysis cells. Below are key green hydrogen production processes:

1. Water Electrolysis for Hydrogen Production

The most common method involves splitting water into hydrogen and oxygen through electrolysis. The electricity used in this process is sourced from renewable energy, such as solar or wind power. This method is currently the most widely adopted and commercially mature technology.

(Water Electrolysis Hydrogen Production | Image source: Aibang Hydrogen Technology Network)

2. Biomass Gasification for Hydrogen Production

Biomass materials (e.g., wood, crop residues) undergo gasification to produce combustible gas (primarily hydrogen, carbon monoxide, and carbon dioxide). Hydrogen is extracted through gas treatment and purification steps. This method leverages renewable organic resources but requires careful consideration of carbon dioxide emissions generated during gasification.

(Biomass Gasification for Hydrogen Production | Image source: Biomass Chemical Engineering)

3. Photocatalytic Hydrogen Production

Semiconductor-based photocatalysts drive water splitting into hydrogen and oxygen under visible or ultraviolet light. Though demonstrated in laboratories, this method remains under development due to challenges in efficiency and cost-effectiveness.

(Photocatalytic Water Splitting | Image source: Hua Shuo Hydrogen Energy)

4. Biological Hydrogen Production

Specific microorganisms or enzymatic reactions convert organic waste or substrates into hydrogen. This method offers potential for waste resource utilization but requires further research and development.

(Biomass Catalytic Gasification | Image source: New Energy Technologies & Equipment)

5. Thermochemical Hydrogen Production

Thermochemical hydrogen production utilizes high temperatures and chemical reactions to convert compounds into hydrogen. For example, reacting high-temperature steam with natural gas or methane produces hydrogen and solid carbon. This method reduces carbon dioxide emissions and enables the application of the resulting solid carbon in areas such as carbon capture and storage (CCS).

(Thermochemical Sulfur-Iodine Cycle | Image source: National Green Technology Exchange)

6. High-Temperature Electrolysis for Hydrogen Production

High-Temperature electrolysis for hydrogen production employs high-temperature electrolysis technology to split water into hydrogen and oxygen, enhancing overall efficiency. Compared to conventional water electrolysis, high-temperature electrolysis reduces energy consumption and increases hydrogen production rates. While requiring higher operating temperatures and specialized electrolytic materials, it achieves significantly improved hydrogen generation efficiency.

(High-Temperature Electrolysis | Image source: Yan Zhi Cheng Li)

7. Microbial Electrolysis for Hydrogen Production

Microbial electrolysis for hydrogen production leverages the metabolic activity under controlled conditions to generate hydrogen through electrochemical reactions. This approach integrates microbial and electrochemical processes, utilizing bio-generated electrons from microorganisms to facilitate water-splitting reactions, thereby enabling green hydrogen production. Microbial Electrolysis for hydrogen production represents an emerging technological field that remains under active research and development.

(Microbial Hydrogen Conversion | Image source: Hydrogen Vision)

8. Solar Thermochemical Hydrogen Production

Solar thermochemical hydrogen production employs solar thermal systems to convert sunlight into thermal energy, which drives chemical reactions to split water into hydrogen and oxygen. This method integrates solar energy with chemical processes, enabling sustainable green hydrogen production under controlled conditions.

(Solar Thermochemical Hydrogen Production | Image source: CRES Solar Thermal Power Committee)

III. Green Hydrogen Applications

Green hydrogen is both environmentally friendly and sustainable, with versatile applications driving energy system transformation and modernization in the following sectors:  

1. Chemical Industry:Production of ammonia, methanol, and hydrides.

2. Transportation: Fuel for road/rail vehicles, aviation, and maritime transport.

3. Power Generation: Fuel cell power plants, backup power for fuel cells, gas turbine electricity generation, and hybrid systems with biogas.

4. Building Sector: Heating/hot water systems and hydrogen fuel cell backup power.