With the rise of renewable energy and electric vehicles, hydrogen-powered vehicles have attracted growing interest. Prof. Molly Mengjung Li, Assistant Professor of the Department of Applied Physics at The Hong Kong Polytechnic University is dedicated to researching ammonia as a hydrogen carrier and has recently developed a highly efficient, low-cost catalyst, helping to advance the practical adoption of hydrogen vehicles.
The global transition towards sustainable energy has placed hydrogen-powered vehicles at the forefront of clean transportation solutions. As governments and industries strive to decarbonise mobility, the acceptance of hydrogen fuel cell vehicles is gaining momentum due to their high energy efficiency and zero-emission credentials. However, the widespread adoption of hydrogen energy vehicles hinges not only on the development of fuel cell technology but also on the safe, efficient, and cost-effective storage and release of hydrogen itself.
Prof. Li, and her research team are investigating the possibility of using ammonia as a hydrogen fuel carrier and studying the stability of hydrogen energy storage in order to promote the popularisation of hydrogen-powered vehicles. Their study, published in Advanced Materials, introduces an efficient and cheap catalyst to facilitate the hydrogen energy generation reaction.
Hydrogen (H2), when used in fuel cells, reacts with oxygen (O2) to generate electricity, emitting only water (H2O) as a by-product. This reaction offers a compelling alternative to fossil fuel combustion, promising both environmental and operational advantages. However, hydrogen’s low volumetric density and the challenges associated with its storage and transport have long been recognised as significant barriers to its practical deployment. Among the various strategies proposed, chemical carriers such as ammonia (NH3) have emerged as promising solutions. NH3 boasts a well-established production and distribution infrastructure, a high hydrogen density and the ability to release hydrogen without generating carbon oxides. The decomposition of NH3 into N2 and H2 is thus a critical reaction for on-board hydrogen generation in fuel cell vehicles.
Despite its promise, the practical implementation of NH3 cracking technology faces a major hurdle—the reliance on ruthenium (Ru)-based catalysts. Ru catalysts are highly effective for low-temperature NH3 decomposition but their scarcity and high cost impede large-scale adoption. This has spurred a global research effort to identify alternative catalysts based on earth-abundant, non-noble metals.
Cobalt (Co) has emerged as a particularly attractive candidate, given its favourable nitrogen binding energy and lower susceptibility to catalyst poisoning compared to other transition metals. However, conventional Co-based catalysts typically require high temperatures (>600°C) to achieve satisfactory hydrogen yields, limiting their utility for mobile applications where energy efficiency and compact reactor design are paramount considerations.
To address these challenges, recent research has focused on innovative catalyst design strategies that can enhance the low-temperature activity of Co-based systems. One such approach is the engineering of lattice strain at the catalyst-support interface, which can modulate the electronic structure of active sites and thereby optimise their interaction with reactants. Drawing inspiration from advances in strain engineering in other catalytic systems, Prof. Li’s research team has developed a new class of core@shell catalysts, exemplified by the Co@BaAl₂O₄₋ₓ heterostructure.
Performance testing of the Co@BaAl₂O₄₋ₓ catalyst reveals remarkable activity for NH3 decomposition at moderate temperatures. Under high space velocity conditions, the catalyst achieves a hydrogen production rate of 64.6 mmol H₂ gcat-1 min-1 and maintains nearly complete NH3 conversion between 475°C and 575°C. These results are on par with, or even surpass, those of many Ru-based catalysts, but without the associated cost and supply constraints. Advanced characterisation techniques, including synchrotron X-ray absorption spectroscopy and electron microscopy, confirm the formation of a well-defined core@shell structure and the presence of nitrogen species at the interface after reaction, highlighting the critical role of the heterostructure in facilitating the catalytic process.
To further elucidate the advantages of the core@shell design, a comparative study was conducted with a conventional supported catalyst, Co/BaAl₂O₄₋ₓ, which lacks the encapsulating shell. Both catalysts were prepared with similar cobalt nanoparticle sizes to ensure a fair comparison. The results are striking: while both systems exhibit increasing NH3 conversion with temperature, the core@shell Co@BaAl₂O₄₋ₓ catalyst demonstrates a significantly lower onset temperature for activity (200°C versus 250°C) and achieves near-complete conversion at 500°C, compared to even higher temperature for the supported analogue. Moreover, the core@shell structure exhibits superior stability under high flow rates, whereas the supported catalyst suffers from a sharp decline in performance.
The development of the Co@BaAl2O4-x core@shell catalyst represents a significant advance in the quest for efficient, Ru-free catalysts for ammonia cracking in hydrogen energy vehicles. By leveraging lattice strain engineering and strong metal-support interactions, this system achieves low-temperature activity and stability previously attainable only with precious metals. The mechanistic insights gained from this work not only inform the design of next-generation catalysts for clean energy applications but also underscore the transformative potential of interface engineering in heterogeneous catalysis. As the hydrogen economy continues to evolve, such innovations will be pivotal in realising the full potential of hydrogen as a sustainable fuel for the future of mobility.
Source: Innovation Digest
