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| Fig. 1: Water electrolysis process. (Image source: M. Verbaite) |
Green hydrogen is gaining much attention and support, and increasingly more projects and policies are being developed all around the world. Hydrogen has the potential to address a range of energy challenges and reduce emissions in hard-to-decarbonize industries such as aviation, iron, cement, steel, and chemicals. It has a variety of uses and applications, for example, in the production of fertilizers and the oil refining sector.
However, its commercial feasibility faces several significant hurdles, such as slow infrastructure growth and the high cost of production - preventing widespread adoption. Moreover, producing hydrogen from low-carbon energy is prohibitively expensive, and hydrogen is almost entirely supplied by natural gas and coal. Nonetheless, efforts have been made by industry leaders to implement green hydrogen in their products. Demand for hydrogen is increasing as hydrogen has a high heating value (HHV) of 120-142 MJ/kg. [1] It is suitable as a green energy source for fuel in hard-to-decarbonize industries and as electricity storage in the fuel cell. Moreover, hydrogen production from renewable power in a power-to-gas process may become more cost-effective due to the recent sharp decline in renewable energy costs. [2]
Hydrogen is a colorless gas, but it can be identified by thirteen different color codes, which refer to the manufacturing method or source used to produce it. These codes include green, blue, grey, brown, or black, turquoise, purple, pink, red and white. [2] The production of brown, grey, and black hydrogen is a significant source of GHG emissions as such hydrogen is produced using coal and natural gas, with the color representing the type of coal used, bituminous (black) or lignite (brown), respectively. Grey hydrogen is made from fossil fuels, often through steam methane reforming. This process releases carbon dioxide into the atmosphere. Green hydrogen, on the other hand, is a clean technology that aims to produce zero emissions. This technology uses renewable energy sources, such as solar panels, nuclear, wind turbines, to electrolyze water and to convert water into hydrogen gas and oxygen using electricity. A high-level overview of this process is shown in Fig. 1.
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| Fig. 2: Green hydrogen production methods. (Image source: M. Verbaite) |
The main production methods of green hydrogen include biomass processes (biological or thermochemical) and water splitting (electrolysis, thermolysis, and photolysis). The biological biomass pathway includes bio-photolysis, dark fermentation, and photo fermentation. The thermochemical biomass pathway includes pyrolysis, gasification, combustion, and liquefaction. Green hydrogen production is currently more expensive than blue hydrogen production, due to the cost of electrolyser materials. [3]
Electrolysis produces less than 2% of the world's hydrogen, although this figure is rapidly changing. [4] The demand for hydrogen is projected to increase to 500-680 million Mt by 2050 up from 87 million Mt in 2020. [2]
Green hydrogen is most often generated using renewable energy sources and through the process of water electrolysis, a zero-emission electrochemical process that uses electricity to split water into hydrogen and oxygen.
In recent years, four promising water electrolysis technologies have been developed based on their electrolyte, operating conditions, and ionic agents (OH-, H+, O2-): alkaline, anion exchange membrane (AEM), proton exchange membrane (PEM), and solid oxide electrolyser cells (SOECs). Fig. 2 illustrates the different methods for producing hydrogen from renewable sources.
All four technologies operate on the same principle of using electricity to split water molecules into hydrogen and oxygen. SOECs have several advantages over the other methodologies, which operate at lower temperatures. SOECs:
Can produce hydrogen at a higher electrochemical reaction rate,
Have lower electrical energy requirements,
Split steam and CO2 into H2 and CO, and
Operate in co-electrolysis mode, which converts a mixture of H2O and CO2 into syngas.
Amongst the four electrolysis methods, SOECs can generate H2 with the highest efficiency compared to other electrolysis cells, up to 90%. This is significantly higher than other technologies, such as alkaline electrolysis cells (50 - 60% efficiency) and PEM electrolysis cells (60 - 80% efficiency). [2] Despite having a longer start-up time, SOECs have lower material costs as they employ ceramic as the electrolyte instead of hydroxide exchange membrane electrolysers. The commercialization of SOECs is however hindered by their relatively short lifespan of less than 2-3 years, compared to 10 - 20 years for PEM and alkaline electrolysis technologies.
SOECs are the most efficient type of electrolyser, but they are also the most expensive. SOECs are still under development, but they have the potential to revolutionize H2 production. [3] The high-temperature operation of solid oxide electrolysis can be integrated with industrial processes, utilizing waste heat and improving overall efficiency.
Overall, these technologies have the potential to significantly reduce emissions and enhance energy efficiency in a variety of sectors. As these technologies continue to develop and become more cost-effective, they are expected to play a significant role in transitioning to a clean energy future.
In recent years, there have been several advancements in green hydrogen production technologies, which have the potential to make the production of green hydrogen more efficient, cost- effective, and scalable. Today, the cost of green hydrogen is around $2.50 - 6.80/kg. [2] However, due to the combined impacts of cheaper renewable energy and reduced electrolyser cost, the price of green hydrogen is falling so rapidly that it will soon be competitive with blue hydrogen. In the United States, the DOE has announced a goal of reducing the cost of hydrogen production to $1 per kilogram by 2030. [2]
Current hydrogen production is mainly involved in electrolysis and fossil fuel reforming. The former process is energy-intensive and expensive in terms of power consumption and requires renewable energy sources. Leading hydrogen production technologies, such as electrolysis, require around 9 kg of water is to produce 1 kg of hydrogen. This can be problematic in water-scarce regions, leading to competition for water between hydrogen production and other essential needs. Moreover, the quality of water used is crucial, as impurities can affect production efficiency and increase energy consumption.
Although hydrogen can be delivered in compressed form as a liquid at -253°C or at high pressures, the logistics of long-distance transport in this form remain a considerable barrier. [1] Liquid hydrogen storage requires specialized equipment and insulation to maintain extremely low temperatures. The development of technology that converts hydrogen generated via electrolysis into liquid fuels, such as diesel, is the subject of intense effort. This would facilitate transportation from areas with a high concentration of renewable energy to areas with limited renewable resources and fossil fuels. As the industry expands, it is expected that several new deployments will be made for these products. Moreover, substantial concerns have also been raised about hydrogen safety due to its high flammability, as it can ignite over a broad range of conditions. Developing safe and efficient hydrogen storage technologies remains a significant challenge. [5] Therefore, building a robust hydrogen infrastructure, including production facilities and distribution networks requires careful consideration of safety measures and regulations.
Green hydrogen remains relatively expensive compared to non-renewable alternatives as an input for businesses or an energy source. Green hydrogen, derived from renewable sources, offers a sustainable solution to decarbonize the energy sector, reducing reliance on fossil fuels and enhancing climate resilience. Its applications in various sectors enable significant reductions in GHG emissions, promoting environmental sustainability. Rapidly declining production costs position green hydrogen as a cost-effective alternative, pivotal for mitigating climate change and aligning with global sustainability goals such as UN Sustainable Development Goals and ESG principles.
© Maria Verbaite. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. A. Abdalla et al., "Hydrogen Production, Storage, Transportation and Key Challenges With Applications: A Review," Energy Convers. Manage. 165, 602 (2018).
[2] B. S. Zainal et al., "Recent Advancement and Assessment of Green Hydrogen Production Technologies," Renew. Sustain. Energy Rev. 189 A, 113941 (2024).
[3] G. Squadrito, G. Maggio, and A. Nicita, "The Green Hydrogen Revolution," Renew. Energy 216, 119041 (2023).
[4] D. Tonelli et al., "Global Land and Water Limits to Electrolytic Hydrogen Production Using Wind and Solar Resources," Nat. Commun. 14, 5532 (2023).
[5] P. Nikolaidis and A. Poullikkas, "A Comparative Overview of Hydrogen Production Processes," Renew. Sustain. Energy Rev. 67, 597 (2017).