How to maximize platinum's efficiency in fuel cells

Scientists with the U.S. Department of Energy's Argonne National Laboratory have identified a new catalyst that uses only about a quarter as much platinum as current technology does to convert hydrogen and oxygen into water and electricity.

As it is widely known, when it comes to fuel cell, platinum offers both activity and stability for electrochemical reactions. However, given the rarity and price of the precious metal, the Argonne researchers have been trying to use less of it.

Up until now, large amounts of the element were used in oxygen reduction reactions but the US scientists were able to do things more efficiently. First, they tweaked the shape of the platinum to maximize its availability and reactivity in the catalyst. In this configuration, a few layers of pure platinum atoms cover a cobalt-platinum alloy nanoparticle core to form a core-shell structure.

Chemist Lina Chong (foreground) holds a sample catalyst while chemist Di-Jia Liu looks on. Photo by Argonne National Laboratory.

But the core-shell nanoparticles on their own still could not handle a large influx of oxygen when the fuel cell needs to crank up the electric current. To increase the efficiency of the catalyst, the scientists produced a catalytically active, platinum group metal-free or PGM substrate as the support for the cobalt-platinum alloy nanoparticles.

By using metal-organic frameworks as precursors, they were able to prepare a cobalt-nitrogen-carbon composite substrate in which the catalytically active centers are uniformly distributed near to the platinum-cobalt particles. Such active centers are capable of breaking the oxygen bonds by themselves and work synergistically with platinum.

In a media statement, the researchers explained that the new catalyst not only improved activity but also the durability as compared to either component alone.

Previously, the experts created a patented process that involves first heating up cobalt-containing metal-organic frameworks. As the temperature increases, some of the cobalt atoms interact with organics to form a PGM-free substrate while others are reduced to well-dispersed small metal clusters throughout the substrate. After the addition of platinum followed by annealing, platinum-cobalt core-shell particles are formed and surrounded by PGM-free active sites.

"Since the new catalysts require only an ultralow amount of platinum, similar to that used in existing automobile catalytic converters, it could help to ease the transition from conventional internal combustion engines to fuel cell vehicles without disrupting the platinum supply chain and market," said Di-Jia Liu, the corresponding author of the study in the press release.