One method for generating hydrogen sustainably is via the hydrogen evolution reaction (HER), in which electrochemical reduction of protons is mediated by an appropriate catalyst—traditionally, an expensive platinum-group metal. Scalable production requires catalyst alternatives that can lower materials or processing costs while retaining the highest possible activity.

… Among available HER electrocatalyst candidates, layered transition-metal dichalcogenide (MX₂) catalysts based on molybdenum and tungsten have attracted substantial interest due to their low cost and high intrinsic per-site HER activity. However, these materials are significantly limited by the density of active sites, which are concentrated at the layer edges. Accordingly, significant research investment has been directed towards synthesis strategies that can expose additional active edge sites to enhance overall performance. An alternative strategy that obviates the need for complex nanostructuring involves the development of MX₂ catalysts that are not limited to edge activity but rather exhibit intrinsic basal-plane activity. In principle, such materials could enable far greater flexibility, materials compatibility and overall performance within existing electrode designs.

Here we address this critical need by employing first-principles calculations to reveal underlying electronic factors that control the surface activity of MX₂. A simple descriptor derived from this understanding leads to the discovery of group-5 MX₂ (H-TaS₂ and H-NbS₂) electrocatalysts whose performance derives from highly active basal-plane sites. The activity exceeds all reported MX₂ candidates…

—Liu et al.

Several catalysts were modeled by Rice theoretical physicist Boris Yakobson and lead author Yuanyue Liu, a former graduate student in his lab, and made and tested by Rice materials scientists led by Pulickel Ajayan and Jun Lou. They found the new dichalcogenide catalysts matched the efficiency of platinum—the most common hydrogen evolution reaction (HER) catalyst in water-splitting cells — and can be made at a fraction of the cost.

Schematic of the proposed mechanism for the morphology change, in which hydrogen evolution at basal-plane sites of stacked layers causes perforation and exfoliation. Plates represent MX2 layers; spheres represent H2 bubbles formed following HER. The top panel represents the state at an early stage of cycling, whereas the bottom panel corresponds to a late stage. Illustration by Yuanyue Liu. Click to enlarge.

Scientists who have been testing molybdenum and tungsten dichalcogenides as possible HER catalysts were frustrated to find the active sites tended to concentrate at the metal platelets’ edges, a small percentage of the material’s surface.

The Rice team turned to niobium and tantalum, two other transition metals (and dubbed Group-5 electrocatalysts for their middle position on the periodic table). They combined each with sulfur, expecting the new compounds would have active sites along their basal planes.

The hydrogen produced along the planes did something unexpected to make the materials even more effective.

The process generates hydrogen bubbles between the layers, which starts to break them apart. This makes the layers more accessible and increases the number of active sites.

—Boris Yakobson

The multilayer platelets that make up both catalysts became thinner, smaller and more dispersed as they self-optimized, the researchers observed. The thinning shortened the path electrons have to travel, which lowered charge-transfer resistance.

Liu said performance enhancements in both electrocatalysts were directly related to changes in the materials’ physical shape despite no observed changes in their chemical or crystal properties.

Yakobson said Liu’s method to model the material may be as important as the material itself.

Yuanyue in effect created a new shorthand way to evaluate the catalytic performance. The old-fashioned way was to directly compute the binding energy of the reactant, like hydrogen, to the surface. Instead, we chose the property of the catalyst itself to serve as the descriptor — without having to worry about what was absorbed.

This work is a rare example of the Materials Genome Initiative in action. The theory develops a descriptor to speed-search among numerous material possibilities and to accelerate discovery compared with trial-and-error experimentation.

—Boris Yakobson

The initiative is a federal program to speed the discovery and implementation of advanced materials.

The researchers expect the materials’ self-optimizing behavior will have practical advantages for scalable processing.

Finding surface-active catalysts in layered materials is a significant step forward for hydrogen production using non-noble metal catalysts. It is also very important that such surface activities could be directly verified experimentally, paving the way for future applications.

—co-author Jun Lou

Liu, who is now at the California Institute of Technology and will join the University of Texas-Austin as an assistant professor, is lead author of the paper. Co-authors from Rice are postdoctoral researchers Jingjie Wu, Jing Zhang and Yingchao Yang; alumnus Ken Hackenberg; graduate student Kunttal Keyshar; senior faculty fellow Robert Vajtai; and Pulickel Ajayan, chair of the Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry. Other co-authors are scientists Y. Morris Wang, Tadashi Ogitsu and Brandon Wood of the Livermore Lab and Jing Gu, an assistant professor of chemistry and biochemistry at San Diego State University. Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry.

The Livermore Lab, the US Department of Energy Fuel Cell Technologies Office, the Welch Foundation and a Department of Defense 2-D Multidisciplinary University Research Initiative grant supported the research.

Resources

• Yuanyue Liu, Jingjie Wu, Ken P. Hackenberg, Jing Zhang, Y. Morris Wang, Yingchao Yang, Kunttal Keyshar, Jing Gu, Tadashi Ogitsu, Robert Vajtai, Jun Lou, Pulickel M. Ajayan, Brandon C. Wood & Boris I. Yakobson (2017) “Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution” Nature Energy 6, Article number: 17127 doi: 10.1038/nenergy.2017.127