This schematic shows the process of cutting 2D sheets into nanoribbons. Image: KAIST.
This schematic shows the process of cutting 2D sheets into nanoribbons. Image: KAIST.

One of the biggest challenges to making hydrogen production clean and cheap has been finding an efficient catalyst for splitting water that is less expensive and made from more abundant materials than the platinum catalysts currently used. Researchers in Korea may now have come up with a way to produce just such a catalyst, by 'snipping' a 2D material into tiny nanoribbons that possess a catalytic efficiency at least that of platinum.

The researchers, who are led by Sang Ouk Kim in the Department of Materials Science and Engineering at the Korea Advanced Institute of Science and Technology (KAIST), report their work in a paper in Nature Communications.

Hydrogen is likely to play a key role in the transition away from fossil fuels and other processes that produce greenhouse gas emissions. There are a raft of transportation sectors, such as long-haul shipping and aviation, that are difficult to electrify and so will require cleanly produced hydrogen as a fuel or as a feedstock for other carbon-neutral synthetic fuels. Likewise, fertilizer production and the steel sector are unlikely to be 'de-carbonized' without cheap and clean hydrogen.

The problem is that the cheapest current way to produce hydrogen gas is from natural gas, resulting in the production of carbon dioxide and thus defeating the whole purpose of transitioning to hydrogen.

Alternative techniques for producing hydrogen, such as passing an electric current between two electrodes to split water into hydrogen and oxygen, are very well established. But one of the factors contributing to the high cost of splitting water, beyond being extremely energy-intensive, is the need to use platinum, which is relatively rare and expensive, as a catalyst on the electrodes.

Researchers have long been on the hunt for a substitute for platinum that is more abundant and less expensive. One potential candidate is the family of 2D materials known as transition metal dichalcogenides (TMDs). These 2D materials are composed of one atom of a transition metal (the metallic elements in the middle part of the periodic table) and two atoms of a chalcogen element (specifically sulfur, selenium or tellurium).

TMDs offer several advantages as a replacement for platinum. For a start, they are made from more abundant elements, while their electrons are structured in a way that gives the electrodes a boost. In addition, their ultrathin nature allows a great many more TMD molecules to be exposed during the catalytic process than would be possible with a bulk material, thus promoting the water-splitting reaction.

But TMD molecules are only reactive at the four edges of a nanosheet; in their flat interior, not much is going on. In order to increase the hydrogen-production rate, the nanosheet would need to be cut into very thin – almost one-dimensional – strips, thereby creating many edges. To achieve this, Kim and his team have now developed what are, in essence, a pair of chemical scissors that can snip TMD into tiny strips.

"Up to now, the only substances that anyone has been able to turn into these 'nanoribbons' are graphene and phosphorene," said Kim. "But they're both made up of just one element, so it's pretty straightforward. Figuring out how to do it for TMD, which is made of two elements was going to be much harder."

The chemical scissors utilize a two-step process. First, the researchers insert lithium ions into the layered structure of the TMD sheets. Next, they use ultrasound to cause a spontaneous 'unzipping' in straight lines.

"It works sort of like how when you split a plank of plywood: it breaks easily in one direction along the grain," Kim explained. "It's actually really simple."

The researchers then tested this process with various types of TMDs, including those made of molybdenum, selenium, sulfur, tellurium and tungsten. They found that it worked with all them, producing nanoribbons with a catalytic efficiency similar to platinum.

Because of its simplicity, this method could be used not just for the large-scale production of TMD nanoribbons, but also to make similar nanoribbons from other multi-elemental 2D materials for purposes beyond just hydrogen production.

This story is adapted from material from KAIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.