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January 2013

Electrocatalysts Show Their Rougher Face

Molybdenum sulfide nanostructures with highly stepped surfaces are effective catalysts for hydrogen production from water

Gonzalo Prieto

Novel nanostructured MoS2 electrocatalysts show unprecedented activity in the production of hydrogen from water. The image shows how the reaction proceeds on the atoms located at the step edges on the surface of the MoS2 catalyst. Yellow=sulfur, blue=molybdenum, white=hydrogen. 

By applying a nanometer-scale casting process, scientists have successfully synthesized thin films of a molybdenum sulfide, MoS2, catalyst with unprecedented activity for the production of hydrogen from water. Electrocatalysts are materials that drive chemical reactions with the aid of electricity. Researchers at the Center on Nanostructuring for Efficient Energy Conversion, or CNEEC, demonstrated this innovative approach to synthesize a MoS structure, which contains up to four-fold more catalytically active atoms on its surface than previous thin-film MoS2 catalysts. By maximizing the number of active centers, the nanostructured MoS2 catalyst exhibits greatly enhanced activity.

Turning water to hydrogen. The use of hydrogen in fuel cells and other applications is considered the Holy Grail for a future sustainable society. Its use does not result in environmentally harmful emissions. Its production can be fully renewable if sunlight or electricity derived from wind power is employed. Hydrogen can be produced from water by the combination of electricity and a catalyst, a process called electrocatalytic evolution of hydrogen.

The action occurs at the edge. MoS2 is an inexpensive and efficient catalyst for the electrocatalytic evolution of hydrogen. However, only a minor fraction of the atoms on the surface of MoS2 is indeed capable of driving the reaction. "Those efficient catalytic sites are located at step edges on the MoS2 surface," explains Thomas Jaramillo, who led the study at Stanford University. "The majority of the surface atoms, located at flat terraces, are mere spectators."

A rough surface, with a high proportion of steps, would be therefore more efficient than a more regular, flat surface. To design superior electrocatalysts, one would ideally want to maximize the proportion of step edges on the MoS2 surface.

Getting the nano-catalysts out of the mold. The scientists at CNEEC developed a synthesis strategy that imparts a high degree of curvature to the porous MoS2 catalyst at the nanometer scale. As a frame of reference, if the walls of the pores of this material would be as thick as our apartment's walls, a virus would be larger than a person. To achieve such curvature, MoS2 was prepared by first synthesizing a silica material with a highly twisted, three-dimensional porous structure as a "mold." The precursors for MoS2 were deposited inside the pores of the silica. The silica cast was chemically removed leaving behind a porous MoS2 replica. This can be seen as the nanoscale version of taking a cake out of the mold. The highly tortuous MoS2 structure exhibits an unusually high proportion of the desired step edges on its surface.

As an essential step towards integration in electrochemical devices, the novel "nano-casted" catalyst was assembled in the form of thin films supported on a conducting material or substrate. "Discovering a route to fabricate thin films of the porous MoS2 that were electrically connected and contiguous over large areas proved to be the greatest challenge," said graduate student Zhebo Chen. "We were able to leverage knowledge that we previously gained working with metallic films to also achieve this unique nanoscale architecture with MoS2."

Jakob Kibsgaard, who participated in the project as a visiting scholar, envisages improvements: "Further research on the addition of other metal atoms to the structure is expected to enhance the activity and potentially enable our porous MoS2 to compete with the best precious metal catalysts available today."

The new nanostructuring approach opens horizons to controllably prepare thin films of MoS2 and other materials with superior properties for applications in catalytic reactions driven by temperature, electricity or sunlight for the production of green fuels.

More Information

Kibsgaard J, Z Chen, BN Reinecke and TF Jaramillo. 2012. "Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis." Nature Materials 11:963-969. DOI: 10.1038/nmat3439

Acknowledgments

All physical and electrochemical characterization was supported by the Center on Nanostructuring for Efficient Energy Conversion, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences. Early stage development of the synthetic procedure to obtain the nanostructured MoS2 catalyst was supported by DOE's Office of Energy Efficiency and Renewable Energy. Jakob Kibsgaard gratefully acknowledges the Villum Kann Rasmussen Foundation for a postdoctoral fellowship.

About the author(s):

  • Gonzalo Prieto is a postdoctoral researcher at the Debye Institute for Nanomaterials Science, Utrecht University (The Netherlands), and member of the Center for Atomic Level Catalysts Design based in Louisiana State University. His research interests are in the design and assembly of nanoparticulate materials in porous carriers as solid catalysts for the sustainable synthesis of fuels and chemicals.

Casting Your Catalyst

Tiny molds create rough material that produces hydrogen quickly

Dissociation of water to produce hydrogen driven on the surface of a porous MoS2 electrocatalyst obtained by nanoscale casting.

A catalyst reduces the energy needed to make a reaction happen. A catalyst's efficiency influences the energy needed and the waste produced by many chemical reactions. Creating the hydrogen necessary to produce biofuels or run fuel cells requires an efficient catalyst. Scientists discovered that they could speed up the production of hydrogen from water by creating a molybdenum sulfide catalyst, MoS2, with many step edges, instead of a smooth surface. They built this rougher surface by first making a porous silica-based mold. They added the catalyst's precursors inside the pores, then chemically removed the mold, creating porous MoS2. The catalyst has an unusually high number of the desired step edges on its surface and, therefore, drives the production of hydrogen from water more efficiently. Molding at this tiny scale could lead to controllably building thin catalytic films with highly desirable properties for hydrogen production or other energy-related reactions. Scientists at the Center on Nanostructuring for Efficient Energy Conversion, led by Stanford University, did this work.

More Information

Kibsgaard J, Z Chen, BN Reinecke and TF Jaramillo. 2012. "Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis." Nature Materials 11:963-969. DOI: 10.1038/nmat3439

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.