Atomic cobalt on nitrogen-doped graphene catalyst shows promise to replace platinum for hydrogen production
The Rice lab of chemist James Tour and colleagues at the Chinese Academy of Sciences, the University of Texas at San Antonio and the University of Houston have developed a robust, solid-state catalyst that shows promise to replace expensive platinum for hydrogen generation.
The new electrocatalyst, based on very small amounts of cobalt dispersed as individual atoms on nitrogen-doped graphene (Co-NG), is robust and highly active in aqueous media with very low overpotentials (30 mV). In an open-access paper published in Nature Communications, the researchers suggested that the unusual atomic constitution of supported metals is suggestive of a new approach to preparing extremely efficient single-atom catalysts.
Electrochemical reduction of water through the hydrogen evolution reaction (HER) is a clean and sustainable approach to generate molecular hydrogen (H2), which has been proposed as a future energy carrier. Catalysts are needed to improve HER efficiency by minimizing reaction kinetic barriers, which manifest themselves as overpotentials (η). Although platinum (Pt) is the most active HER catalyst, its scarcity and high cost limit its widespread use. Thus, the transition to a hydrogen economy calls for alternative electrocatalysts based on earth-abundant elements, such as non-precious metal oxides, sulfides, phosphides, carbides and borides. In spite of their low η for HER, the active sites of these inorganic-solid catalysts, like other heterogeneous catalysts, are sparsely distributed at selective sites (that is, surface sites or edges sites). To expose more active sites, these catalysts are generally downsized into nanoparticulate form and stabilized onto certain substrates. Graphene is such a substrate that has a large specific surface area (high catalyst loading), good stability (tolerance to harsh operational conditions) as well as a high electrical conductivity (facilitated electron transfer) and therefore has been widely used to disperse nanoparticles for advanced electrocatalysis.
The dispersing ability of graphene is, however, far from being fulfilled unless single-atom catalysis (SAC) is achieved. SAC represents the lowest size limit to obtain full atom utility in a catalyst and has recently emerged as a new research frontier. … Here, we report an inexpensive, concise and scalable method to disperse the earth-abundant metal, cobalt, onto nitrogen-doped graphene (denoted as Co-NG) by simply heat-treating graphene oxide (GO) and small amounts of cobalt salts in a gaseous NH3 atmosphere. These small amounts of cobalt atoms, coordinated to nitrogen atoms on the graphene, can work as extraordinary catalysts towards HER in both acidic and basic water.—Fei et al.
In comparison tests, the new material nearly matched platinum’s efficiency to begin reacting at a low onset voltage—the amount of electricity it needs to begin separating water into hydrogen and oxygen.
This is an extremely high-performance material. No question, [platinum-carbon materials] are the best. But this is very close to it and much easier to produce and hundreds of times less expensive.—James Tour
The new catalyst is mixed as a solution and can be reduced to a paper-like material or used as a surface coating. Tour said single-atom catalysts have been realized in liquids, but rarely on a surface. This capability enables the building of electrodes, he noted. “It should be easy to integrate into devices.”
The researchers discovered that heat-treating graphene oxide and small amounts of cobalt salts in a gaseous environment forced individual cobalt atoms to bind to the material. Electron microscope images showed cobalt atoms widely dispersed throughout the samples.
They tested nitrogen-doped graphene on its own and found it lacked the ability to kick the catalytic process into gear. But adding cobalt in very small amounts significantly increased its ability to split acidic or basic water.
Atom-thick graphene is the ideal substrate, Tour said, because of its high surface area, stability in harsh operating conditions and high conductivity. Samples of the new catalyst showed a negligible decrease in activity after 10 hours of accelerated degradation studies in the lab.
Co-authors of the paper are Rice graduate students Huilong Fei and Gonglan Ye, postdoctoral researcher Nam Dong Kim, alumni Errol Samuel and Zhiwei Peng, 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 at Rice; Juncai Dong and Dongliang Chen of the Beijing Synchrotron Radiation Facility at the Chinese Academy of Sciences, Beijing; research associate M. Josefina Arellano-Jiménez and José Yacamán, chairman of the Department of Physics, at the University of Texas at San Antonio; and graduate students Zhuan Zhu and Fan Qin and Jiming Bao, an associate professor of electrical and computer engineering, at the University of Houston.
The research was funded by the Air Force Office of Scientific Research Multidisciplinary University Research Initiative, the National Institute on Minority Health and Health Disparities from the National Institutes of Health, the Welch Foundation and the National Natural Science Foundation of China.
Huilong Fei, Juncai Dong, M. Josefina Arellano-Jiménez, Gonglan Ye, Nam Dong Kim, Errol L.G. Samuel, Zhiwei Peng, Zhuan Zhu, Fan Qin, Jiming Bao, Miguel Jose Yacaman, Pulickel M. Ajayan, Dongliang Chen & James M. Tour (2015) “Atomic cobalt on nitrogen-doped graphene for hydrogen generation” Nature Communications 6, Article number: 8668 doi: 10.1038/ncomms9668