Researchers have developed an advanced cobalt-based catalyst that extracts hydrogen from ammonia with remarkable efficiency, a breakthrough that could significantly lower the cost and accelerate the adoption of hydrogen-powered vehicles. The new material, designed by a team at The Hong Kong Polytechnic University, avoids the use of expensive precious metals like ruthenium, which have long been a barrier to the economic viability of clean energy transportation. This innovation addresses the dual challenges of hydrogen storage and catalytic cost, providing a practical solution for on-board hydrogen generation in fuel cell systems.
The global shift toward sustainable energy has put hydrogen vehicles at the forefront of clean transportation solutions, prized for their high efficiency and zero-emission output. However, the widespread use of these vehicles is constrained by the difficulty of storing and transporting hydrogen gas, which has a very low density. Using a chemical carrier like ammonia (NH3) is a promising strategy, as it can be stored and handled much more easily than pure hydrogen. The challenge then becomes releasing the hydrogen from the ammonia efficiently, a process called decomposition or cracking, which requires a highly active catalyst to perform effectively inside a vehicle.
The Precious Metal Problem
For decades, the most effective catalysts for chemical reactions in fuel cells and hydrogen production have relied on platinum-group metals. Ruthenium, in particular, has been a top performer for ammonia decomposition, but its high cost and scarcity present significant economic hurdles for mass-market applications. Scientists have explored more abundant and less expensive transition metals as alternatives, with cobalt emerging as a strong candidate. Cobalt is attractive due to its favorable electronic properties and lower susceptibility to poisoning compared to some other metals.
Despite its potential, conventional cobalt catalysts have a major drawback: they typically require very high temperatures, often exceeding 600°C, to achieve a satisfactory rate of hydrogen production. Such high operating temperatures are impractical for mobile applications like vehicles, where energy efficiency and compact, responsive systems are paramount. Overcoming this limitation required a fundamental redesign of the catalyst’s structure to enhance its activity at more moderate temperatures.
A Novel Core-Shell Architecture
The research team, led by Prof. Molly Mengjung Li, engineered a new class of catalyst with a unique core@shell structure to unlock cobalt’s low-temperature potential. The resulting material, designated Co@BaAl₂O₄₋ₓ, features a cobalt core encapsulated by a barium aluminate shell. This sophisticated design is central to its enhanced performance, leveraging advanced material science concepts to create a more effective and stable catalytic system. The work was published in the journal Advanced Materials.
Harnessing Interface Engineering
The catalyst’s success lies in the powerful synergy between the cobalt core and the specialized shell. The researchers utilized a strategy known as “lattice strain engineering,” where the atomic structure of the cobalt is intentionally stressed by the surrounding shell. This strain, combined with strong metal-support interactions at the interface between the two materials, fundamentally alters the electronic properties of the cobalt. This engineered interface makes the cobalt atoms more reactive, allowing them to break the strong chemical bonds in ammonia molecules at much lower temperatures than conventional cobalt catalysts.
Performance That Surpasses the Standard
Performance tests of the Co@BaAl₂O₄₋ₓ catalyst demonstrated its exceptional activity for ammonia decomposition under demanding conditions. Even at high space velocities, which simulate the rapid flow of fuel in a real-world system, the catalyst performed with impressive efficiency and durability. These results position the new material as a direct and viable competitor to costly ruthenium-based systems.
Key Performance Metrics
The PolyU team reported a hydrogen production rate of 64.6 millimoles of hydrogen per gram of catalyst per minute (mmol H₂ gcat⁻¹ min⁻¹). Furthermore, the catalyst maintained nearly complete conversion of ammonia into hydrogen across a broad and practical temperature range of 475°C to 575°C. This level of performance at moderate temperatures is a significant achievement, proving that an inexpensive cobalt-based material can match or even exceed the efficiency of established precious-metal catalysts.
Broader Impact on the Hydrogen Economy
The development of this catalyst extends beyond just making hydrogen vehicles more affordable. It contributes to the broader field of heterogeneous catalysis, where engineering interfaces between materials is a key strategy for designing next-generation systems for clean energy applications. Cobalt-based materials are also heavily researched for the hydrogen evolution reaction (HER), the process of producing hydrogen from water through electrolysis. Advances in tuning the properties of cobalt compounds through strategies like structural engineering, creating defects, and doping with other elements can improve their activity in both hydrogen production and its release from carriers.
The Path to Commercial Viability
This research represents a major step toward developing ruthenium-free catalysts for practical use in the hydrogen economy. The mechanistic insights gained from studying the Co@BaAl₂O₄₋ₓ system provide a roadmap for creating other advanced catalysts for a variety of clean energy technologies. While this breakthrough is promising, the next phase will involve addressing the challenges of scaling up production of the catalyst and integrating it into complete fuel system prototypes for long-term testing and validation.
The focus on cobalt, an element far more abundant and less costly than platinum-group metals, aligns with the global need for sustainable and economically sound technologies. As research continues to refine the stability and activity of these materials, the prospect of hydrogen-powered transportation becoming a mainstream reality moves closer than ever. This work underscores the transformative potential of materials science in solving critical energy problems and paving the way for a decarbonized future.
