A new catalyst structure can facilitate the use of electricity to produce hydrogen gas from water.
This new catalyst avoids the rare, expensive metal platinum normally required to garner this reaction. (Catalysts speed up chemical reactions without themselves being consumed.)
The material under study, molybdenum disulfide, contains two common elements, said Mark Lukowski, a Ph.D. student working with associate professor Song Jin in the University of Wisconsin-Madison chemistry department.
“Most people have tried to reduce the cost of the catalyst by making small particles that use less platinum, but here we got rid of the platinum altogether and still got reasonably high performance.”
The research group produced milligram quantities of the catalyst, “but in principle you could scale this up,” Lukowski said. “Molybdenum disulfide is a commercially available product. To control purity and structure, we go through the trouble of synthesizing it from the bottom up, but you could buy it today.”
To make the new material, Lukowski and Jin deposit nanostructures of molybdenum disulfide on a disk of graphite and then apply a lithium treatment to create a different structure with different properties.
Just as carbon can form a diamond for jewelry and graphite for writing, molybdenum disulfide can be a semiconductor or a metallic phase, depending on structure. When the compound grows on the graphite, it is a semiconductor, but it becomes metallic after the lithium treatment. Lukowski and Jin discovered the metallic phase has far greater catalytic properties.
“Like graphite, which is made up of a stack of sheets that easily separate, molybdenum disulfide is made up of individual sheets that can come apart, and previous studies have shown that the catalytically active sites are located along the edges of the sheets,” Lukowski said.
“The lithium treatment both causes the semiconducting-to-metallic phase change and separates the sheets, creating more edges. We have taken away the limitation from molybdenum disulfide and made the active sites both more pervasive and more reactive.”
The experiment is a proof of concept for a new approach for improving these catalysts, Jin said.
“Even though the efficiency of producing hydrogen has been greatly improved, it is still not as good as what platinum can achieve,” Jin said. “The next steps include finding ways to further improve the performance by optimizing all aspects of the process and exploring related compounds. There are many hurdles to achieving a hydrogen economy, but the advantages in efficiency and pollution reduction are so significant that we must push ahead.”
As technological advances put further strain on the supply of platinum and other rare elements, using common elements is a major advantage, Jin said.
“The elements we use are cheap and abundant in earth’s crust, and the raw material is already commercially available at low cost,” he said. “Building on this discovery and new understanding, we would like to further improve these materials to achieve the efficient production of hydrogen without using precious metals.”