A team from the US Department of Energy’s Ames Laboratory and Zhengzhou University in China has demonstrated a metal-free carbocatalyst—nitrogen-assembly carbons (NCs)—for the release of hydrogen from liquid organic hydrogen carriers (LOHCs) even at ambient temperature, showing greater activity than transition metal–based catalysts. An open-access paper on their work is published in the journal Science Advances.

Hydrogen offers one potential solution in the effort to decrease reliance on fossil fuels. According to the DOE, improving hydrogen storage is key to advancing hydrogen fuel cell technologies. One of the ways to store hydrogen is chemically.

Chemical storage relies on materials that react with hydrogen molecules and store them as hydrogen atoms, such as in LOHCs. This type of storage allows large amounts of hydrogen to be stored in small volumes at ambient temperatures. However, for the hydrogen to be used, catalysts are needed to activate LOHCs and release the hydrogen. This process is called dehydrogenation.

Schematic representation of the other LOHC dehydrogenation strategies and the new approach for catalytic dehydrogenation to release dihydrogen. Hu et al.

Ames scientist Long Qi said that currently there are other dehydrogenation methods, but they raise some challenges. Some methods rely on metal-based catalysts, which involve critical platinum group metals. Supplies of these metals are limited and expensive. Other methods require additives to release the hydrogen. The additives are not reusable and result in a higher overall cost because they need to be added in each cycle.

The new catalyst requires neither metals nor additives.

It’s fairly simple. Basically, just add the metal-free catalyst into the LOHC, and then the hydrogen gas is just popping out, even at room temperature.

—Long Qi

The catalyst is composed of nitrogen and carbon. The key to its efficiency is the structure of the nitrogen. Catalytic activity can take place at room temperature because of the unique closely spaced graphitic nitrogens as nitrogen assembly which were formed during the carbonization process.

The nitrogen assembly catalyzes the cleavage of carbon-hydrogen (C‒H) bonds in LOHCs and facilitates the desorption of hydrogen molecules. This process is what makes the catalyst more efficient than other catalysts in use.

DOE goals for vehicle hydrogen storage capacity needs to be close to 6.5% by weight. The researchers are optimistic about the future of their work to meet the goal with molecules that have a larger capacity.

Resources

• Haitao Hu, Yunqing Nie, Yuewen Tao, Wenyu Huang, Long Qi And Renfeng Nie (2022) “Metal-free carbocatalyst for room temperature acceptorless dehydrogenation of N-heterocycles” Science Advances doi: 10.1126/sciadv.abl9478