New advances in Green Hydrogen Production

Introduction:

Fossil fuels contribute significantly to greenhouse gas emissions and climate change, accounting for 75% of global energy consumption with demand projected to increase1. This underscores the urgent need for renewable, carbon-free energy sources. Green hydrogen, produced through electrolysis using renewable electricity, offers a reduced carbon footprint compared to conventional hydrogen production methods. According to the International Renewable Energy Agency (IRENA), green hydrogen has a carbon footprint of less than 1 kg CO2-eq/kg H2, versus 9-12 kg for natural gas-derived hydrogen2. Electrolyzer technologies, such as Solid Oxide Electrolysis Cells (SOECs) and Solid Polymer Electrolyte Electrolysis (SPE), enable this green hydrogen production. SOECs operate at high temperatures for increased efficiency, while SPEs function at lower temperatures suitable for distributed applications. These advanced technologies represent promising approaches for sustainable hydrogen production through electrolysis.

Solid Oxide Electrolysis Cells

Solid Oxide Electrolysis Cells (SOECs) have emerged as a pivotal technology in electrochemical energy conversion. They use conductive ceramics and operate at high temperatures and pressures, achieving superior efficiency compared to traditional alkaline electrolysis. SOECs avoid issues like electrolyte evaporation and corrosion and can reach up to 95% power-to-hydrogen efficiency when using external heat for steam generation. Two main SOEC architectures are electrolyte-supported cells (ESCs) and cathode-supported cells (CSCs), each with distinct structural and performance characteristics.
While current research on CO2 electrolysis by solid oxide electrolysis cells (SOECs) primarily centers on electrode material development and performance evaluation, researchers widely acknowledge the value of a validated mechanistic model in elucidating the intricate chemical, electrochemical, and mass transport processes governing SOEC operation4,5. Several models have been proposed for high-temperature H2O electrolysis in SOECs, aimed at studying reactions and transport phenomena within the electrode structure6,7,8. However, few studies have focused specifically on modeling CO2 electrolysis. Chan et al.9 developed a model for CO2 electrolysis, but it did not incorporate gas transport processes within the porous electrodes. Omitting gas transport processes within porous electrodes in CO2 electrolysis modeling is problematic because it neglects critical mass transfer limitations, concentration gradients, and two-phase flow effects, leading to inaccurate system performance predictions and potentially suboptimal electrode designs.
Yixiang Shi’s study from Tsinghua University develops a validated one-dimensional elementary reaction model for a solid oxide electrolysis cell with CO/CO2 gas mixtures. The model couples cathodic reactions, mass transport, and charge transfer, predicting cell performance, species distributions, and carbon deposition. The experimental data was used for calibration and validation at various conditions10.
Figure 1 shows that increasing CO concentration at the SOEC cathode leads to higher carbon deposition coverage at 700°C9. The CO/CO2 ratio significantly affects this process, with higher ratios intensifying carbon deposition. Low CO content is crucial to reducing carbon deposition risk during SOEC operation. Green hydrogen production seems promising but also faces challenges because of high operating temperatures and long transient periods10. This makes frequent on/off cycling impractical. Intermittent renewable energy sources can be integrated with programmable renewables or upstream energy storage. SOEs require a minimum operational load (60-100%) to maintain temperature through exothermic reactions, avoiding external heat input11. Comprehensive model coupling various cell processes can lead to more efficient SOEC designs and tailored solutions for specific applications. Understanding the operational constraints, such as minimum load requirements, informs better integration with renewable energy sources and the design of hybrid systems. These advancements could result in enhanced overall efficiency in hydrogen production, reduced operational risks, and increased investor confidence, ultimately accelerating the adoption of SOEC technology in green hydrogen production and contributing to its economic viability.

Solid Polymer Electrolyte Electrolysis

The Solid Polymer Electrolyte (SPE) water electrolyzer employs a cost-effective membrane electrode assembly (MEA). This assembly contains a thin perfluorosulfonic acid ionomer membrane, typically Nafion, which acts as the solid electrolyte. The membrane, about 10-12 mil thick, transports hydrogen protons between sulfonic acid sites. The anode and cathode are formed by hot-pressing platinum black or noble metal alloy catalyst layers onto opposite sides of the membrane.
The electrochemical reactions take place at the anode, where water molecules are oxidized to produce oxygen gas, protons, and electrons (2H2 O → O2  + 4H + 4e-), and at the cathode, where travelling protons combine with electrons from the external circuit to form hydrogen gas (4H+ 4e-  → 2H2). To enhance the kinetics of these reactions, SPE electrolyzers typically operate at elevated temperatures around 80-150°C and elevated pressures up to 40 atm, increasing green hydrogen solubility and decreasing gas bubble formation12,13. SPE and SOEC systems show promise for green hydrogen production, with SOECs particularly suited for high-temperature applications. Both technologies face unique challenges and benefit from ongoing research to improve efficiency, durability, and performance. This research focuses on enhancing components, materials, and operating conditions to address specific technical hurdles and optimize overall system performance.
Non-precious metal catalysts, such as transition metal compounds or metal-organic frameworks, have seen use as alternatives to precious metal catalysts like platinum or iridium. For instance, Park et al. reported a novel nickel-based catalyst with excellent stability and activity for the hydrogen evolution reaction (HER) in acidic media, offering a potential non-precious metal alternative to platinum18. These materials provide comparable catalytic activity for HER and oxygen evolution reaction (OER) while being more abundant and cost-effective14. Using solid polymer electrolytes (SPEs) with non-precious metal catalysts enables the design of efficient      electrolyzers for green hydrogen production. Popczun et al. (2013) demonstrated that nanostructured nickel phosphide (Ni2P) exhibited excellent HER activity in acidic solutions, achieving a current density of 20 mA/cm² at an overpotential of -130 mV, comparable to platinum-based catalysts19. This study highlighted the efficiency of SPEs combined with non-precious metal catalysts. This study’s findings demonstrate that non-precious metal catalysts like nanostructured nickel phosphide can achieve HER performance comparable to platinum-based catalysts, a significant breakthrough for cost-effective hydrogen production. The high efficiency of Ni2P in acidic solutions opens up new possibilities for developing more affordable and scalable electrolysis systems, potentially accelerating the adoption of hydrogen as a clean energy carrier and reducing the dependence on rare and expensive platinum-group metals in electrocatalysis. Overcoming these challenges through ongoing research is crucial for making SPE a commercially viable technology for scalable green hydrogen production15.

Solid Oxide Electrolysis vs. Solid Polymer Electrolyte

Solid Oxide Electrolysis Cells (SOECs) and Solid Polymer Electrolyte (SPE) Electrolyzers are both promising technologies for green hydrogen production through water electrolysis, they differ in their operating conditions, materials, and underlying mechanisms, leading to contrasting strengths in efficiency, cost, and application suitability as highlighted in the table. The table compares these key aspects between the two leading electrolyzer technologies.
SOECs and SPE systems have complementary roles in hydrogen production, with SOECs suited for high-temperature, integrated processes and SPEs for flexible, lower-temperature applications.

Conclusion

In conclusion, both solid oxide electrolysis cells (SOECs) and solid polymer electrolyte (SPE) electrolyzers offer promising pathways for hydrogen production but represent distinct technological approaches. SOECs achieve high efficiency by operating at extreme temperatures, requiring costly ceramic materials. SPE cells operate under milder conditions using cheaper polymers but at the trade-off of lower thermal efficiency16. SOECs suit high-temperature processes, while SPEs fit distributed renewable applications. Continued research and development efforts in materials, system design, and process integration will be crucial to fully realizing the potential of these complementary electrolysis technologies in enabling a sustainable hydrogen economy.

References:

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2.    Wang, Y., Chen, K.S., Mishler, J., Cho, S.C. and Adroher, X.C., 2011. A review of polymer electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research. Applied Energy, 88(4), pp.981-1007.
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14.    Wang, Y., Chen, K.S., Mishler, J., Cho, S.C. and Adroher, X.C., 2011. A review of polymer electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research. Applied Energy, 88(4), pp.981-1007.
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About the Authors

Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 30 years. He is an elected Fellow by his peers at IChemE, AOCS, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at “ASTM’s Long Awaited Fuels and Lubricants Handbook 2nd Edition Now Available”, https://bit.ly/3u2e6GY
He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honourific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology), The Pennsylvania State University, State College, PA ( School of Engineering Design and innovation ),  Auburn Univ (Tribology), SUNY Farmingdale, (Engineering Management) and State university of NY, Stony Brook ( Chemical engineering/ Material Science and engineering). An Adjunct Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical engineering, Raj also has over 650 publications and has been active in the energy industry for over 3 decades.
More information on Raj can be found at https://bit.ly/3QvfaLX
Contact: rshah@koehlerinstrument.com

Udithi Kothapalli is a chemical engineering student at Carnegie Mellon University, set to graduate in May 2025. She is also pursuing a minor in Biomedical Engineering. Udithi is actively involved in campus organizations, serving as the current president of the Indian Organization at her university. Additionally, she holds the position of industrial liaison for the American Institute of Chemical Engineers Chapter at Carnegie Mellon, demonstrating her commitment to both cultural and professional development within her field of study.