Platinum is used to catalyze the key oxygen reduction reaction in the fuel cell’s cathode, directly converting the chemical energy in hydrogen fuel into electricity, with water vapor as the byproduct. Oxygen reduction represents the key reaction step that limits the rate of power generation, and platinum is currently still the most viable catalyst to speed up the reaction and power generation, despite a great deal of research exploring alternatives. However, current state-of-art platinum catalysts are not active enough, which necessitates the use of a relatively large amount of platinum, contributing to the high price of fuels cells.

Manufacturing nanowires with jagged surfaces, rather than smooth, creates new types of highly active sites that can significantly reduce the reaction barrier and speed up the oxygen reduction reaction. Further, the thin body of the nanowire ensures most of the platinum atoms are exposed on the surface to actively participate in the reaction instead of being embedded inside the body and making little contribution to the reaction. All of that results in a reduction in the amount of platinum used, and therefore the cost, while at the same ramping up the reaction efficiency and power generation rate.

This work is a perfect example of what one can achieve by the atomic scale control of nanoscale materials, and how structural modifications at such small dimension can lead to big gain in functions for real applications. This is fascinating world to explore for a material scientist.

—Yu Huang, professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science

Huang was a co-principal investigator on the research along with Xiangfeng Duan, UCLA professor of chemistry and biochemistry. Both are members of the California NanoSystems Institute at UCLA. The other principal investigator was William Goddard, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at Caltech. The lead author of the research was Mufan Li, a UCLA doctoral student advised by Duan and Huang.

The researchers created the wires in a two-step process. First, they used a heating process to create nanowires of a platinum-nickel alloy. Then they used an electrochemical process selectively to remove the nickel atoms from the alloy nanowires.

Jagged platinum nanowires are made by removing nickel from a platinum-nickel alloy. Source: UCLA. Click to enlarge.

Following testing, the researchers found that the jagged nanowires made the reaction much more efficient, delivering 50 times more current (power) than the same amount of commercial catalysts can.

The one-dimensional geometry and the jagged surfaces offers abundant and highly active sites that can greatly accelerate the reaction rate and last for repeated reaction cycles,” Duan said “So, this has enabled a breakthrough performance that was not previously possible.

—Xiangfeng Duan

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Other authors hailed from the National Research Council of Italy; Tsinghua University in China; The Chinese Academy of Sciences; California State University, Long Beach; Northeastern University; and Lawrence Berkeley National Laboratory.

The research was supported by the Department of Energy and the National Science Foundation.

Resources

• Mufan Li, Zipeng Zhao, Tao Cheng, Alessandro Fortunelli, Chih-Yen Chen, Rong Yu, Qinghua Zhang, Lin Gu, Boris Merinov, Zhaoyang Lin, Enbo Zhu, Ted Yu, Qingying Jia, Jinghua Guo, Liang Zhang, William A. Goddard III, Yu Huang, Xiangfeng Duan (2016) “Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction” Science doi: 10.1126/science.aaf9050