Ammonia (NH₃) can be broken down to generate hydrogen gas without emitting CO₂. Its high density and ease of transport make it a significant asset for the green energy sector. However, a major limitation of NH₃ is the extremely high temperatures required for its decomposition reactions. Recently, a collaborative effort by Japanese researchers introduced a surface protonics-assisted technique to produce green hydrogen from ammonia on demand, using an electric field alongside a Ru/CeO₂ catalyst.
Hydrogen gas is receiving increased attention as a clean energy source for a sustainable future due to its high energy density and the fact that it does not emit carbon. Although hydrogen is the most plentiful element in the universe, it rarely exists independently; it is usually found in compounds like ammonia, metal hydrides, and other hydrogen-rich substances.
Among available hydrogen carriers, ammonia is particularly promising due to its wide availability, high hydrogen content (approximately 17.6% of its mass), and the ease with which it can be liquefied and transported. However, a significant drawback to using ammonia for on-demand green hydrogen production is the requirement for very high temperatures (over 773K) for effective decomposition. Efficient hydrogen production for use in fuel cells and combustion engines demands high conversion rates of ammonia at lower temperatures.
To address this challenge, Professor Yasushi Sekine from Waseda University, along with his colleagues Yukino Ofuchi, Sae Doi from Waseda University, and Kenta Mitarai from Yanmar Holdings, proposed a new compact process that could function at reduced temperatures. They showcased an experimental setup that converts ammonia to hydrogen at significantly lower temperatures by applying an electric field and utilizing a highly active and readily available Ru/CeO2 catalyst. This research was published in Chemical Science on August 27, 2024.
“This is a collaborative project between our lab at Waseda University and Yanmar Holdings, a leader in ammonia utilization. Our goal was to create a process that effectively uses ammonia to produce hydrogen on demand,” explains Sekine. He further elaborates, “We began by exploring traditional thermal catalytic systems in which the reaction occurs via the formation of N and H adsorbates from N-H bond dissociation, which then recombine to form N2 and H2 gases.”
The team discovered that, at lower temperatures, the bottleneck was nitrogen desorption on the active metal Ru, while at higher temperatures, it was the dissociation of N-H bonds. To overcome these challenges, they turned to electric field-assisted catalytic reactions, which enhanced proton conduction at the catalyst’s surface and lowered the activation energy needed for the reactions, enabling more efficient ammonia conversion even at reduced temperatures.
Using these insights, the researchers created a new thermal catalytic system that facilitates the low-temperature decomposition of ammonia into hydrogen with the assistance of the easily producible Ru/CeO2 catalyst and a DC electric field. They discovered that their innovative approach successfully decomposed ammonia at temperatures below 473 K, achieving a full conversion rate of 100% at 398 K, which exceeds the expected equilibrium conversion rate. This success was attributed to the electric field’s ability to enhance surface protonics, which involves proton movement on the catalyst’s surface, ultimately lowering the apparent activation energies of the ammonia conversion process.
In contrast, without the electric field, the nitrogen desorption process was significantly slowed down, inhibiting the ammonia decomposition reaction from continuing after a period. The importance of surface protonics in enhancing ammonia conversion rates was further validated through various experimental and density functional theory calculations performed by the research team.
This innovative strategy shows that it is possible to produce green hydrogen from ammonia at low temperatures through an irreversible pathway, which allows for nearly 100% conversion rates and high reaction speeds. “We believe that our proposed method can expedite the widespread adoption of clean alternative fuels, making the on-demand production of CO2-free hydrogen more accessible than ever,” concludes Sekine.
