Fuel cells, which directly convert the chemical energy from fuel into electricity, have been recognized as electrocatalysts for PEMFCs. Several approaches have been implemented to reduce the usage of Pt, such as dispersing the promising candidates for efficient and clean energy conversion. A problem is that the sluggish four-electron oxygen reduction reaction (ORR) at the cathode in fuel cell systems remains a major technical challenge and severely limits its widespread commercialization. Currently, expensive and chemically sensitive Pt-based alloys are the most effective ORR electrocatalyst in proton exchange membrane fuel cells (PEMFCs), which are commonly a reliable system for practical application. But due to the harsh acidic environment and high oxidation potential required for PEMFC cathode operation, few materials are stable enough to be considered as ORR electrocatalysts for PEMFCs.

… much effort has been devoted to replacing the Pt-based electrocatalysts for ORR, and especially for use in acid media which is a required condition of PEMFCs. As the limit in downsizing metal morphology, the concept of single-atom catalysts (SACs) has emerged since it maximizes the exposed atom efficiency. However, the preparation of SACs remains challenging because the high free energy of individual metal atoms leads to metal aggregation, affording nanoclusters or nanoparticles, particularly under harsh acidic reaction conditions.

… Here, we report the synthesis of atomically dispersed Ru species embedded on a N-doped graphene matrix (Ru-N/G) via a facile technique using GO containing trace amounts of Ru salts as the precursor. … This Ru and N-doped graphene-based catalyst exhibits high performance and stability toward ORR in an acidic media with extraordinarily low overpotential. Computational studies suggest that a Ru-oxo species with nitrogen coordination contributes favorably to the origin of oxygen reduction activity.

—Zhang et al.

Ruthenium is often a highly active catalyst when fixed between arrays of four nitrogen atoms, yet it is one-tenth the cost of traditional platinum. And since we are using single atomic sites rather than small particles, there are no buried atoms that cannot react. All the atoms are available for reaction.

—James Tour

Spreading single ruthenium atoms across a sheet of graphene involved dispersing graphene oxide in a solution, loading in a small amount of ruthenium and then freeze-drying the new solution and turning it into a foam.

Baking that at 750 ˚C Celsius (1,382 ˚F) in the presence of nitrogen and hydrogen gas reduced the graphene and locked nitrogen atoms to the surface, providing sites where ruthenium atoms could bind.

Schematic illustration of the synthetic process for the Ru-N/G catalyst annealed at 750 °C with NH3 (Ru-N/G-750). (I) GO is dispersed in deionized water. (II) Ru(NH3)6Cl3 is added into a GO solution followed by lyophilization; the dashed lines indicate the interaction between the Ru cation and GO. (III) Formation of the Ru-N/G-750 after annealing at 750 °C with NH3. The conversion of O2 to H2O is depicted. Credit: ACS, Zhang et al. Click to enlarge.

Materials made at higher and lower temperatures weren’t as good, and those made at the proper temperature but without either ruthenium or nitrogen proved the quality of the reaction depended on the presence of both.

The material showed excellent tolerance against methanol crossover and carbon monoxide poisoning in an acidic medium, both of which degrade the efficiency of fuel cells; such degradation is a persistent problem with traditional platinum fuel cells.

Lead authors of the paper are graduate students Chenhao Zhang of Rice and the Chinese Academy of Sciences, Shanghai; Junwei Sha of Rice, the Chinese Academy of Sciences and Tianjin University, China; and Juncai Dong and Dongliang Chen of the Chinese Academy of Sciences.

Co-authors are alumni Huilong Fei, Mingjie Liu and Qifeng Zhong, postdoctoral researchers Sadegh Yazdi and Xiaolong Zou and graduate student Jibo Zhang; Emilie Ringe, an assistant professor of materials science and nanoengineering, and Boris Yakobson, the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry, all of Rice; Naiqin Zhao of Tianjin University and the Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, China; and Haisheng Yu and Zheng Jiang of the Chinese Academy of Sciences.

Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.

The research was supported by the Air Force Office of Scientific Research and its Multidisciplinary University Research Initiative, the China Scholarship Council, the American Chemical Society Petroleum Research Fund, the Department of Energy, the Robert Welch Foundation, the National Natural Science Foundation of China and the Jianlin Xie Foundation of the Institute of High Energy Physics, Chinese Academy of Science.

Resources

• Chenhao Zhang, Junwei Sha, Huilong Fei, Mingjie Liu, Sadegh Yazdi, Jibo Zhang, Qifeng Zhong, Xiaolong Zou, Naiqin Zhao, Haisheng Yu, Zheng Jiang, Emilie Ringe, Boris I. Yakobson, Juncai Dong, Dongliang Chen, and James M. Tour (2017) “Single-Atomic Ruthenium Catalytic Sites on Nitrogen-Doped Graphene for Oxygen Reduction Reaction in Acidic Medium” ACS Nano doi: 10.1021/acsnano.7b02148