Scientists find two ways to get more catalytic bang out of platinum

In a paper in Science, chemists at the US Department of Energy (DOE)'s Argonne National Laboratory report a new catalyst that can maximize the effectiveness of platinum.

Platinum is a precious metal rarer than silver or gold. Renowned in the fuel cell community for its effectiveness at converting hydrogen and oxygen into water and electricity, platinum offers unrivaled activity and stability for electrochemical reactions. But platinum is both scarce and expensive, which is encouraging scientists to create practical fuel cell catalysts that use far less of the costly precious metal.

In this new study, scientists at Argonne developed a new catalyst that uses only about a quarter as much platinum as current versions by maximizing the effectiveness of the available platinum.

In a fuel cell, platinum is used in two ways – to convert hydrogen into protons and electrons, and to break oxygen bonds and eventually form water. The latter reaction, known as the oxygen reduction reaction, requires an especially large quantity of platinum, and scientists have been looking for a way to reduce the platinum content of oxygen reduction catalysts.

Argonne scientists have now come up with a couple of ways for doing this. First, they tweaked the shape of the platinum to maximize its availability and reactivity in the catalyst. In this configuration, a few layers of platinum atoms cover a cobalt-platinum alloy nanoparticle to form a core-shell structure.

"If you're given only a very small amount of platinum in the first place, you have to make the best use of it," said Argonne chemist Di-Jia Liu, the corresponding author of the paper. "To use a platinum-cobalt core-shell alloy allows us to make larger number of catalytically active particles to spread over the catalyst surface, but this is only the first step."

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, Liu and his colleagues relied on another approach they knew well from past research. This involves producing a catalytically active, platinum group metal-free (PGM-free) substrate as the support for the cobalt-platinum alloy nanoparticles.

Using metal-organic frameworks as precursors, Liu and his colleagues were able to prepare a cobalt-nitrogen-carbon composite substrate with catalytically active centers that 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.

"You can think of it kind of like a molecular football team," Liu explained. "The core-shell nanoparticles act like defensive linemen thinly spread out all across the field, trying to tackle too many oxygen molecules at the same time. What we've done is to make the 'field' itself catalytically active, capable of assisting the tackling of oxygen."

As it turned out, the new combined catalyst possessed not only improved activity but also improved durability.

To produce this catalyst, Liu and his colleagues 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. Adding platinum followed by annealing causes platinum-cobalt core-shell particles to form, and these particles are surrounded by PGM-free active sites.

While the ultimate goal is to eliminate platinum from hydrogen fuel cell catalysts entirely, Liu said that the current research opens up a new direction for addressing both fuel cell catalyst activity and durability in a cost-effective way. "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," he said.

This story is adapted from material from Argonne National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.