An international team led by researchers at UCLA and Caltech has demonstrated how altering the form of platinum nanoscale wires from a smooth surface to a jagged one could dramatically reduce the amount of precious metal used as catalysts in fuel cells and lower the cost.
Fuel cells use hydrogen combined with atmospheric oxygen to produce electricity and the only byproduct is water vapor. Consequently, they hold great promise as a clean and renewable technology for vehicles, considering how combustion engines use nonrenewable fossil fuels and emit substances responsible for global warming and atmospheric pollution. However, because fuel cells are reliant on platinum as a catalyst for the chemical reaction that powers them, the fuel cells and the cars powered by them have remained too expensive for widespread adoption.
But in a study published in Science, the research team showed how using jagged-shaped platinum nanowires increases their efficiency as a catalyst. 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. And, 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.
According to the findings, the newly developed catalyst is so active that the amount of platinum needed to make one fuel cell today could be used to make 50 of them with the jagged nanowire catalysts.
“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,” said Yu Huang, professor of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science. “This is fascinating world to explore for a material scientist.”
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.
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 the only viable catalyst to speed up the reaction and power generation. 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.
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 to selectively remove the nickel atoms from the alloy nanowires, leaving what looks like fuzzy wires composed of only platinum.
Following testing, the researchers found that the jagged nanowires made the reaction much more efficient, delivering 50 times more current (power) than that the same amount of commercial catalysts can. It could dramatically reduce the necessary platinum usage in fuel cells and lowering their cost.
“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.”