Ultrafine jagged Pt nanowires extremely efficient ORR catalysts; 50x more power than current commercial catalyst
An international team led by researchers at UCLA and Caltech has demonstrated that altering the form of platinum nanoscale wires from a smooth surface to a jagged one can significantly reduce the amount of precious metal required as a catalyst for the oxygen reduction reaction (ORR) in fuel cells and thus lower the cost. According to the findings, the newly developed catalyst is so active that the amount of platinum required for a fuel cell could be 1/50 of what is needed today.
In a paper published in Science, the team reports that the jagged Pt nanowires exhibit an ECSA (electrochemical active surface area) of 118 m2 per gram Pt and a specific activity of 11.5 mA per square centimeter for ORR for a mass activity of 13.6 ampere per milligram Pt, nearly doubling previously reported best values. Reactive molecular dynamics simulations suggested that the highly stressed, under-coordinated rhombohedral-rich surface configurations of the jagged nanowire enhanced ORR activity versus more relaxed surfaces.
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—
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.
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