Hydrogen may be a clean energy source, but one challenge is to produce it economically.

Researchers from North Carolina State University, Duke University, and Brookhaven National Laboratory published research that may hold promise for reaching that goal.

Researchers discover an inexpensive catalyst for hydrogen evolution.The key to creating hydrogen from water — hydrogen evolution — is an efficient catalyst. Current practice is to use platinum, which is too expensive for wide use. Another catalyst is molybdenum sulfide (MoS₂), which is both inexpensive and abundant.

It has been assumed however, that MoS₂ only has limited usefulness as a hydrogen evolution catalyst. The researchers have made a discovery that can change that. It had been thought that only the edges of MoS2 act as catalysts so that the bulk of the material is inactive.

The crystalline structure of MoS₂ is a grid of regularly spaced molybdenum and sulfur atoms. The condition in which one of the sulfur atoms is missing is called a sulfur vacancy.

“We found that these sulfur vacancies attract the hydrogen atoms in water at just the right strength: the attraction is strong enough to pull hydrogen out of the water molecule, but is then weak enough to let the hydrogen go,” says Linyou Cao, senior author of the paper describing the research.

Based on this discovery, the team realized that the best way to improve the catalytic activity is to engineer the sulfur vacancies. Although the edges of MoS₂ are twice as efficient at removing hydrogen atoms compared to sulfur vacancies, it is difficult to create a high density of edges.

However, a large number of sulfur vacancies can be engineered uniformly across the surface of the material. The researchers found that there is a sweet spot for maximizing the catalytic efficiency. The best results occur when between 7-10% of the sulfur sites are vacant.

They also found that high crystalline quality is important to optimize the catalytic activity.

“We now know that MoS₂ is a more promising catalyst than we anticipated, and are fine-tuning additional techniques to further improve its efficiency,” says Cao. “Hopefully, this moves us closer to making a low-cost catalyst that is at least as good as platinum.”