Platinum is considered the standard metal for fuel cells.
While scarce and extremely expensive, platinum is considerably more effective than silver and gold at converting hydrogen and oxygen into water and electricity.
Now, researchers from the U.S. Department of Energy’s Argonne National Laboratory have found a new catalyst that uses about 25 percent of the platinum that is used in current technology, while still maintaining the activity and stability of a full supply of the metal for electrochemical reactions.
Fuel cells use platinum to convert hydrogen into both protons and electrons, while also breaking oxygen bonds apart to eventually form water in a process that requires a substantial amount of the catalyst.
To reduce the amount of platinum needed for this process, the researchers first tweaked the shape of the metal so that a few layers of pure platinum atoms cover a cobalt platinum alloy nanoparticle core, which maximizes platinum’s availability and reactivity in the catalyst.
“If you’re given only a very small amount of platinum in the first place, you have to make the best use of it,” Argonne chemist Di-Jia Liu, the corresponding author of the study, said in a statement. “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.”
On its own, the core-shell nanoparticles cannot handle a large influx of oxygen when the fuel cell needs to ratchet up the electric current. The researchers increased the efficiency by producing a catalytically active, platinum group metal-free substrate to support the cobalt-platinum alloy nanoparticles by serving as precursors that enabled the team to prepare a cobalt-nitrogen-carbon composite substrate.
In this substrate, the catalytically active centers, which are capable of breaking oxygen bonds by themselves and work synergistically with the platinum, are uniformly distributed near to the platinum-cobalt particles.
“You can think of it kind of like a molecular football team,” Liu said. “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,” Liu added.
The researchers first heated up the cobalt-containing metal-organic frameworks, which allowed some of the cobalt to interact with organics and form a PGM-free substrate. At the same time, the remaining cobalt atoms were reduced to well-dispersed small metal clusters throughout the substrate.
They then added platinum and annealed the mixture to form the platinum-cobalt core-shell particles that are surrounded by PGM-free active sites.
Along with improved activity, the new catalyst featured better durability than either component that was used along.
“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,” Liu said.
The study was published in Science.
