A. Choudhari, Tata Consulting Engineers Ltd., Mumbai, India
The widespread adoption of hydrogen (H2) faces technical barriers, particularly due to its physical properties. One key challenge is H2’s low energy density, which requires large volumes for storage. To store H2 in liquid form, it must be maintained at cryogenic temperatures (−253°C). Achieving such extremely low temperatures demands highly complex and expensive storage solutions that are difficult to scale for widespread use. Another major concern is H2’s flammability. Due to its small molecular size, H2 is prone to leaks, which can easily go unnoticed and create significant safety risks. These factors—cryogenic storage demands, leakage risks and flammability—have limited H2's commercial viability for widespread applications, especially when compared to other energy carriers.
Ammonia (NH3) is emerging as one of the most promising H2 carriers, offering solutions to several critical challenges associated with H2 storage and transportation. As H2 is increasingly recognized as a pivotal component in the transition to a carbon-free energy future, efficient storage and transportation systems have become crucial for its widespread adoption.
Despite its potential, H2 presents significant logistical challenges due to its low volumetric energy density and the complexities involved in storing and transporting it. In its gaseous form, H2 is notoriously difficult to compress and maintain at pressures that are practical for large-scale transport. Although liquefied H2 provides a higher energy density, it must be stored at extremely low temperatures (around –253°C ), which significantly increases the complexity, cost and safety concerns associated with its use.
NH3 offers a more practical and efficient alternative for H2 storage and transport. As depicted in FIG. 1, NH3 (comprising 17.6% H2 by weight) has a high H2 content compared to other potential carriers. Its volumetric H2 density is approximately 123 kg-H2/m3, a substantial improvement over many current H2 storage methods. What sets NH3 apart is its ability to be liquefied at ambient temperature around –33°C and stored under relatively low pressures, typically around 10 atm. This makes NH3 far easier and less energy-intensive to handle compared to liquefied H2, offering a practical solution for long-distance transportation and large-scale storage of H2.
In addition to its favorable physical properties, NH3 also presents significant environmental advantages. Upon decomposition, NH3 yields nitrogen and H2 without emitting carbon dioxide (CO2), positioning it as a zero-carbon fuel. This characteristic aligns with the global push for decarbonization and achieving carbon neutrality. NH3’s ability to serve as a carbon-free energy source—either directly or as a H2 carrier—makes it a strong contender in future energy systems.
Moreover, NH3 can be synthesized using renewable energy sources like solar and wind power. Through processes such as electrolysis and the Haber-Bosch method, NH3 can be produced in a sustainable manner, further bolstering its potential as a key component in a renewable energy economy. This ability to integrate with green energy technologies enhances NH3’s appeal as an eco-friendly and scalable solution for H2 storage and transportation, making it a cornerstone of the transition to a clean energy future.
Therefore, NH3’s high H2 content, ease of liquefaction, stable storage properties and carbon-neutral decomposition make it a compelling candidate for addressing the challenges of H2 storage and transport. As renewable energy adoption accelerates, NH3 could play a crucial role in overcoming the logistical barriers associated with H2, enabling a more sustainable and efficient energy ecosystem.
NH3 as a H2 carrier. NH3 offers a promising alternative as a H2 carrier, addressing many of the challenges associated with H2 (TABLE 1). NH3 is easier to liquefy and store at moderate conditions (–33°C under atmospheric pressure), unlike H2, which requires extremely low temperatures. These more manageable storage conditions make NH3 a more practical option for large-scale energy storage and transportation. Additionally, NH3 is already widely produced and used in various industrial processes, primarily in the manufacture of fertilizers.
A well-established global infrastructure for NH3 production, storage and transportation already exists, significantly reducing the logistical and financial barriers to using NH3 as a H2 carrier. As a result, the conversion of NH3 back into H2 at the point of use could offer a more cost-effective and scalable solution for H2 transport and storage, especially for energy-intensive industries or regions with limited access to renewable resources.
As seen in TABLE 1, NH3 presents several distinct advantages over other H2 storage methods. First, its storage and transportation infrastructure are mature and extensive. As one of the most widely traded chemicals globally, NH3 benefits from well-developed supply chains that can be adapted for H2 transport. Second, NH3's high energy density and liquid state at ambient temperatures make it more energy-efficient to store and transport than gaseous H2. Third, NH3's decomposition process is well-understood and new technologies are emerging to make it more efficient, including the use of catalysts to lower reaction temperatures and increase H2 yield.
NH3 decomposition for H2 production. In the H2-to-NH3 energy flow process, the journey begins with the production of H2, which can be sourced from low-carbon or renewable methods. For renewable H2, electrolysis is commonly used, where electricity generated from renewable sources like wind or solar is applied to split water into H2 and oxygen. This green H2 is then stored for further use. Alternatively, H2 can also be produced through methane reforming or other processes paired with carbon capture and storage (CCS) to minimize emissions.
Once H2 is produced, it undergoes conversion into NH3. This is achieved using the Haber-Bosch process, a widely used industrial method that synthesizes NH3 by reacting H2 with nitrogen sourced from the atmosphere. The process operates under high pressure (150 bar–250 bar) and high temperature (400°C–500°C) with the help of a catalyst, typically iron-based. This conversion is vital because NH3 is much more manageable to store and transport than H2 due to its higher energy density and ease of liquefaction.
After production, NH3 is stored and transported to regions where H2 demand exists, often over long distances. NH3 shipping infrastructure closely mirrors that of liquid natural gas (LNG), with specialized tankers designed to carry large volumes of liquid NH3. Shipping NH3 in liquid form over long distances is cost-effective, taking advantage of the infrastructure and logistics already in place for handling chemicals like NH3. Once NH3 reaches its destination at the user’s site, it must be decomposed back into H2. NH3 decomposition, also called "cracking," is the process that breaks down NH3 molecules into H2 and nitrogen using various technologies. The goal is to extract H2 from NH3 with minimal energy input, ensuring the process remains efficient and sustainable. The entire process is depicted in FIG. 2.
This flow from H2 production to NH3 synthesis, to global shipment and finally decomposition forms a critical part of the future H2 economy. The key challenge in using NH3 as a H2 carrier lies in efficiently decomposing it back into H2 and nitrogen. The decomposition of NH3 is an endothermic process, represented by the chemical reaction in Eq. 1:
2NH₃(g) → N₂(g) + 3H₂(g), ΔH = +46 kJ/mol (1)
This reaction requires energy input, as NH3 is relatively stable due to the strong N-H bonds. To break these bonds, temperatures of around 400°C–600°C are typically required, especially when using thermocatalytic processes. However, the reaction can be catalyzed using various metal catalysts to lower the energy input and increase the reaction efficiency.
An overview of some of the technologies that are under development for NH3 decomposition is detailed below. These processes allow the H2 stored in NH3 to be liberated at the point of use, enabling a clean and efficient energy supply chain. By utilizing NH3 as a carrier, regions with abundant renewable energy can export H2 in a form that is practical and scalable for global energy markets, while minimizing the challenges associated with direct H2 transport. The entire system offers a pathway to decarbonize industries, transportation and power generation while harnessing the benefits of renewable H2.
Thermocatalytic decomposition of NH3. Thermal catalytic decomposition is the most widely established method for NH3 decomposition, particularly when converting NH3 into H2 for energy applications. In this process, NH3 is passed over a solid metal catalyst—commonly ruthenium (Ru), nickel (Ni) or iron (Fe)—at elevated temperatures ranging from 500°C–800°C. The catalyst facilitates the breaking of the strong N-H bonds in NH3, resulting in the production of H2 and nitrogen. The H2 generated through this method can then be purified and utilized in a variety of applications, including fuel cells, industrial processes or directly as a clean energy source.
The role of the catalyst is crucial. It provides active sites on its surface where NH3 molecules can adsorb and undergo dehydrogenation, a step-by-step removal of H2 atoms, ultimately yielding nitrogen and H2. Without a catalyst, NH3’s stable N-H bonds would require temperatures exceeding 900°C to decompose, making the process highly energy-intensive. By employing a catalyst, the decomposition can occur at a significantly lower temperature, typically in the range of 400°C–600°C, which helps reduce overall energy consumption.
Among the available catalysts, Ru is considered one of the most efficient for thermocatalytic NH3 decomposition. Ru-based catalysts, such as Ru/Al2O3, Ru/CeO2 and Ru/La2O3, have exhibited high catalytic activity and stability, achieving nearly complete NH3 decomposition at temperatures as low as 450°C. However, the high cost and limited availability of Ru pose challenges for its widespread use in large-scale industrial applications.
As a result, alternative catalysts like Ni and Fe are extensively studied due to their lower cost and greater availability. Ni-based catalysts, including Ni/CeO2 and Ni/Al2O3, demonstrate reasonable performance at higher temperatures (500°C–600°C), although their catalytic efficiency is generally lower compared to Ru-based systems. Current research focuses on improving the performance of Ni-based catalysts by alloying them with other metals or modifying their surface properties to increase the density of active sites for NH3 adsorption, thereby enhancing their catalytic activity. The ongoing development of more cost-effective and efficient catalysts is key to making NH3 decomposition a more viable and scalable process for H2 production, especially as NH3 continues to gain traction as a H2 carrier for future energy systems.
Photocatalytic decomposition of NH3. Photocatalytic decomposition of NH3 is another promising approach that uses light energy (typically from sunlight) to drive the reaction at ambient temperatures. Photocatalysts, such as titanium dioxide (TiO2), zinc oxide (ZnO) and molybdenum nitride (Mo2N), can absorb photons and generate electron-hole pairs. These charge carriers then participate in the redox reactions required to split NH3 into nitrogen and H2.
In photocatalytic NH3 decomposition, the reaction is initiated when the energy of incident light exceeds the bandgap of the semiconductor catalyst. For example, TiO2, one of the most studied photocatalysts, has a bandgap of about 3.2 eV, which is sufficient to drive the NH3 decomposition process under UV light. The reaction can be described using Eqs. 2–4:
Photocatalyst + hv → e– + h⁺ (2)
2NH₃ + 6h⁺ → N₂ + 6H⁺ (3)
2H⁺ + 2e– → H₂ (4)
While photocatalytic processes have the advantage of operating at room temperature and utilizing renewable solar energy, their efficiency remains relatively low compared to thermocatalytic methods. Research is ongoing to develop more efficient photocatalysts that can operate under visible light, thereby increasing the practical applicability of this technology. For example, doping TiO2 with noble metals, such as platinum (Pt) or palladium (Pd), has been shown to improve its photocatalytic activity.
Plasma-catalytic decomposition of NH3. Plasma-catalytic NH3 decomposition is a novel approach that combines non-thermal plasma technology with catalysis to enhance the decomposition reaction. Plasma is a highly ionized gas that contains a mixture of energetic electrons, ions and neutral species. These energetic electrons can initiate chemical reactions at lower temperatures than those required for traditional thermal processes.
In plasma-catalytic systems, NH3 is exposed to a plasma field, which generates reactive nitrogen and H2 species. These species then interact with a catalyst, such as Fe or cobalt (Co), to drive the decomposition reaction. The main advantage of plasma-catalytic decomposition is its ability to operate at temperatures as low as 100°C–300°C, significantly reducing energy consumption.
However, the stability of plasma during the reaction and the energy efficiency of the process remain challenges. In plasma systems, the energy required to sustain the plasma field can sometimes outweigh the benefits of lower reaction temperatures. As such, plasma-catalytic NH3 decomposition is still in the experimental stage, with more research needed to optimize the technology for large-scale use.
Electrocatalytic decomposition of NH3. Electrocatalytic decomposition of NH3 involves the use of electrical energy to split NH3 into nitrogen and H2. This process is similar to water electrolysis, but it requires significantly lower voltages. The theoretical decomposition potential for NH3 electrolysis is just 0.077 V, compared to 1.23 V for water electrolysis. In an alkaline electrolyte, NH3 is oxidized at the anode, releasing nitrogen and electrons, while water is reduced at the cathode to form H2.
The reaction mechanisms for electrocatalytic NH3 decomposition can be summarized in Eqs. 5–7:
Anode: 2NH₃ + 6OH⁻ → N₂ + 6H₂O + 6e– (5)
Cathode: 2H₂O + 2e– → H₂ + 2OH– (6)
Overall: 2NH₃ → 3H₂ + N₂ (7)
While electrocatalytic processes offer low energy requirements and the potential for integration with renewable electricity sources, they face challenges related to slow reaction kinetics and the need for efficient electrocatalysts. Pt and Ru are highly effective catalysts for NH3 electrolysis, but their high cost and limited availability hinder their widespread use. Efforts are underway to develop alternative electrocatalysts based on transition metals, such as Ni and Fe, which can offer comparable performance at a fraction of the cost.
Technology readiness and commercial potential. The technology readiness level (TRL) of NH3 decomposition technologies is a key factor in determining their viability for large-scale process plants consisting of H2 production, H2 storage and transport, followed by NH3 decomposition at the user end. NH3 decomposition technologies are progressing through different stages of development, ranging from basic research and proof-of-concept designs to pilot-scale demonstrations and early commercial prototypes. These developments fall within a TRL framework, which spans from TRL 1 (basic principles observed) to TRL 9 (actual system proven in operational environment).
Most NH3 decomposition technologies are in the TRL 4–6 range, meaning they have moved beyond laboratory-scale experiments but are still being tested in relevant industrial or pilot-scale environments. A few technologies are nearing commercialization (TRL 7–8), but large-scale, economically viable deployment remains a challenge. The main technological hurdles include achieving high conversion efficiency, lowering operational temperatures and reducing energy consumption.
The TRL of these NH3 decomposition methods varies, with thermocatalytic decomposition being the most mature, reaching TRL levels of 7–8. Ru-based catalysts are already in use in industrial processes, particularly in locations where waste heat can be utilized to drive the reaction. However, the high cost of Ru limits its application to specialized industries.
Photocatalytic and plasma-catalytic methods are still in the research and pilot phases, with TRL levels between 4 and 6. While these technologies hold significant promise, especially in terms of lowering operational temperatures and integrating with renewable energy sources, their commercial readiness is still several years away.
Electrocatalytic NH3 decomposition is similarly in the pilot stage (TRL 5–6), with ongoing research focusing on improving catalyst performance and reducing costs. Once fully developed, electrocatalytic systems could offer a highly efficient and scalable solution for H2 production, particularly when combined with renewable energy sources.
One of the most promising approaches under investigation is the use of catalysts to lower the decomposition temperature, which typically ranges between 500°C and 700°C in conventional systems. Researchers are exploring advanced catalysts such as Ru-based or Ni-based systems that can operate at lower temperatures (around 300°C–450°C) with high efficiency. The ability to reduce the temperature not only enhances the system’s efficiency but also opens up possibilities to use waste heat from industrial processes, further improving the economics of NH3 cracking.
In terms of ongoing R&D efforts, there are notable advancements in both the academic and industrial sectors. For example, a research team at Japan's Kyushu University is working on a novel catalyst-based reactor designed to decompose NH3 at lower temperatures using a manganese oxide (MnO2)-based catalyst. This system has demonstrated NH3 decomposition efficiencies of > 90% at 450°C, a significant reduction from conventional methods. The project is currently at a TRL 5–6 level, with pilot-scale testing planned in collaboration with industry partners.
Moreover, the European Union-funded project Hydrogen Processing by Ammonia Cracking (HYPER) is aiming to develop and demonstrate a scalable, cost-effective solution for NH3 decomposition. This project brings together several European research institutes and industrial partners to design a modular cracking system that can be integrated into existing H2 supply chains. Currently, the project is in the TRL 6–7 range, with full-scale demonstration plants scheduled for testing by 2025. This initiative highlights the growing focus on NH3 as a H2 carrier, with decomposition technology playing a critical role in enabling the transition to a H2-based economy.
Another prominent example is the partnership between the Australian company Fortescue Future Industries and Monash University that focuses on improving NH3 decomposition catalysts. The project, currently at TRL 5, aims to develop a Ni-based catalyst that operates at lower temperatures while maintaining high H2 yields. Early results from laboratory-scale experiments have shown promising performance, and the project plans to scale up the technology to a pilot plant by 2026. This collaboration represents the ongoing global push to make NH3 decomposition more efficient and cost-competitive, particularly in regions like Australia, where green H2 production is poised to grow significantly in the coming years.
On the industrial front, companies such as Topsoe are developing commercial-scale NH3 cracking systems that target integration into green H2 production facilities. Topsoe’s process involves an advanced catalyst system that improves both efficiency and scalability. The technology is currently at TRL 7, with pilot plants operating in Europe and Asia. The company aims to fully commercialize the technology by 2027, with potential applications in NH3-to-H2 fueling stations and as part of the broader H2 export strategy.
In terms of ongoing challenges and future directions, researchers are increasingly focusing on integrating NH3 decomposition technology with renewable energy sources. For example, coupling NH3 cracking with solar or wind power to provide the energy needed for decomposition is being explored in several pilot projects. These efforts are crucial in reducing the overall carbon footprint of H2 production from NH3. However, technical challenges, such as the need for continuous operation under variable renewable energy inputs, remain unsolved and are the subject of ongoing R&D.
NH3 decomposition technologies are advancing rapidly but are still in the intermediate stages of development. Current R&D is focused on improving catalytic efficiency, reducing operational temperatures and scaling up from pilot to commercial levels. As exemplified by projects like Kyushu University’s catalytic reactor and the European HYPER project, the next few years are likely to see significant progress, potentially pushing NH3 decomposition technologies to higher TRLs. These advancements are critical to realizing NH3's potential as a viable H2 carrier, which will play a pivotal role in the global transition to a H2-based energy system.
Future potential and economic impact. The future potential of NH3 as a H2 carrier is immense. According to estimates from the International Energy Agency (IEA), NH3 could account for up to 25% of global H2 production by 2050. This is largely due to NH3's advantages in terms of storage, transportation and scalability. Countries with limited H2 production capacity, such as Japan and South Korea, are already investing heavily in NH3-based H2 strategies, including NH3 imports from renewable-rich countries like Australia.
NH3’s use as a H2 carrier could also have a significant economic impact, particularly in sectors that are hard to decarbonize, such as shipping and heavy industry. For example, NH3 could be used directly as a fuel in maritime transport, replacing bunker fuel and reducing carbon emissions in one of the most polluting sectors. Similarly, NH3-based H2 could be used in steel production and other industrial processes that require high-temperature heat.
Moreover, the cost of producing and transporting NH3 is expected to decrease as renewable energy technologies become more widespread and NH3 synthesis processes, such as green NH3 production, become more efficient. The global NH3 market is projected to grow significantly, with demand for green NH3 expected to rise in parallel with the growth of the H2 economy.
Takeaways. NH3’s unique properties make it an ideal candidate for H2 storage and transportation, offering practical solutions to many of the challenges associated with H2 logistics. The ability to store large quantities of H2 in a stable, easily transportable form, coupled with the existing infrastructure for NH3 production and transport, positions NH3 as a critical component of the future H2 economy.
Technological advancements in NH3 decomposition—particularly in thermocatalytic, photocatalytic, plasma-catalytic and electrocatalytic methods—will play a key role in determining how quickly and efficiently NH3 can be adopted as a H2 carrier. Each method has its advantages and challenges, with ongoing research aimed at improving efficiency, reducing costs and scaling up for commercial use.
As the world transitions to a low-carbon future, NH3 is poised to play a significant role in facilitating the widespread adoption of H2 as a clean energy carrier. With the potential to drastically reduce carbon emissions and provide a reliable, scalable solution for H2 storage and transport, NH3 offers a path toward a sustainable energy future. HP
Atul Choudhari is the Chief Technology Officer (CTO) of Tata Consulting Engineers (TCE). He joined the company in 2017, and presently leads a team of technology experts and is responsible for knowledge management, innovation, technology development and selection, process optimization and process safety management.
With 31 yr of experience in core process engineering, including flowsheet simulations, process synthesis, and basic and detailed engineering for petrochemicals, petroleum refineries and hydrocarbon processes, Choudhari is a trusted advisor and expert in process engineering. He has contributed several technical papers addressing a wide range of issues related to process design engineering at both national and international journals and conferences, and is actively involved with academia through the development of curriculum and mentoring undergraduate chemical engineers in various colleges. His areas of interest include emerging new energy markets and alternate fuels, including LNG.
Before joining TCE, Choudhari worked at Aker Solutions for more than two decades as Senior Engineering Manager. He is a gold medalist (University Rank First in chemical engineering) and holds a B.E. degree and a Ch.E. degree from Marathwada University, Aurangabad. The author can be reached at achoudhari@tce.co.in.