University of Connecticut (UConn) chemists have developed a new material that could make hydrogen capture more commercially viable and provide a key element for a new generation of cheaper, lightweight hydrogen fuel cells. The new metal-free catalyst uses carbon graphene nanotubes infused with sulfur.

Hydrogen is the most abundant element in the universe and a promising source for clean energy. But producing high-grade hydrogen is an expensive and energy-consuming process. Current hydrogen production uses intense heat to separate hydrogen from the hydrocarbons found in crude oil. But the resulting hydrogen isn’t very pure, and byproducts must be scrubbed out.

Hydrogen is the most abundant element in the universe and a promising source for clean energy. Image credit: ORNL.An alternative process, capturing hydrogen in water, is cleaner and more sustainable, but it too has limitations. Electrocatalysts involved in this process are usually made of rare earth metals such as platinum and iridium that are very expensive, making the commercialization of pure hydrogen fuels difficult.

Steven Suib, director of UConn’s Institute of Materials Science, and fellow UConn researcher and electrochemistry expert James Rusling knew that sulfur-infused carbon graphene nanotubes were a potentially efficient non-metal catalyst for an oxygen reduction reaction (ORR)—a key component of hydrogen-based fuel cells. In an ORR, hydrogen gas used to power the cells passes through a catalyst, currently a corrosive-resistant metal like platinum, causing an oxygen reduction electrochemical reaction that creates energy and, as a byproduct, water.

But reversing that process—starting with water and extracting pure hydrogen from it—is a much greater challenge electrochemically. The key, Suib says, was manipulating the sulfur and carbon atoms to create stable bonds and structures within the nanotubes, while maintaining or improving the tubes’ electrochemical potential so that it mirrored those found in the rare metals.

The process developed in Suib’s and Rusling’s labs uses a dual doping procedure involving sulfur and benzyl disulfide treated at high heat. The researchers had to carefully add heteroatoms of sulfur at extremely low levels to strike the delicate balance needed to maintain usability and stability. Add too much sulfur and the sample would be unstable—not enough and it would be ineffective.

Suib says the procedure for isolating hydrogen in water, in a very general way, is similar to trying to separate flour and sand after they’ve been mixed together thoroughly. In the end, he says, the sulfur-doped nanotubes used much less energy in the chemical reaction process than other known processes and were much more active and efficient catalysts than other known products.

Most importantly, he points out, the sulfur-infused nanotubes are efficient for both separating hydrogen from water and reducing oxygen into water. Materials with those dual properties are rare, he notes.