Advances in fuel-cell technology have been stymied by the inadequacy of
metals studied as catalysts. The drawback to platinum, other than cost, is that
it absorbs carbon monoxide in reactions involving fuel cells powered by organic
materials like formic acid. A more recently tested metal, palladium, breaks
down over time.
Now chemists at Brown
University have created a
triple-headed metallic nanoparticle that they say outperforms and outlasts all
others at the anode end in formic-acid fuel-cell reactions. In a paper
published in the Journal of
the American Chemical Society, the researchers report a 4-nm
iron-platinum-gold nanoparticle (FePtAu), with a tetragonal crystal structure,
generates higher current per unit of mass than any other nanoparticle catalyst
tested. Moreover, the trimetallic nanoparticle at Brown performs nearly as well
after 13 hrs as it did at the start. By contrast, another nanoparticle assembly
tested under identical conditions lost nearly 90% of its performance in just one-quarter
of the time.
“We’ve developed a formic acid fuel-cell catalyst that is the best to have
been created and tested so far,” said Shouheng Sun, chemistry professor at
Brown and corresponding author on the paper. “It has good durability as well as
good activity.”
Gold plays key roles in the reaction. First, it acts as a community
organizer of sorts, leading the iron and platinum atoms into neat, uniform
layers within the nanoparticle. The gold atoms then exit the stage, binding to
the outer surface of the nanoparticle assembly. Gold is effective at ordering
the iron and platinum atoms because the gold atoms create extra space within
the nanoparticle sphere at the outset. When the gold atoms diffuse from the
space upon heating, they create more room for the iron and platinum atoms to
assemble themselves. Gold creates the crystallization chemists want in the
nanoparticle assembly at lower temperature.
Gold also removes carbon monoxide from the reaction by catalyzing its
oxidation. Carbon monoxide, other than being dangerous to breathe, binds well
to iron and platinum atoms, gumming up the reaction. By essentially scrubbing
it from the reaction, gold improves the performance of the iron-platinum
catalyst. The team decided to try gold after reading in the literature that
gold nanoparticles were effective at oxidizing carbon monoxide—so effective, in
fact, that gold nanoparticles had been incorporated into the helmets of
Japanese firefighters. Indeed, the Brown team’s triple-headed metallic
nanoparticles worked just as well at removing carbon monixide in the oxidation
of formic acid, although it is unclear specifically why.
The authors also highlight the importance of creating an ordered crystal
structure for the nanoparticle catalyst. Gold helps researchers get a crystal
structure called “face-centered-tetragonal,” a four-sided shape in
which iron and platinum atoms essentially are forced to occupy specific
positions in the structure, creating more order. By imposing atomic order, the
iron and platinum layers bind more tightly in the structure, thus making the
assembly more stable and durable, essential to better-performing and
longer-lasting catalysts.
In experiments, the FePtAu catalyst reached 2,809.9 mA/mg Pt
(mass-activity, or current generated per milligram of platinum), “which is the
highest among all NP (nanoparticle) catalysts ever reported,” the Brown
researchers write. After 13 hrs, the FePtAu nanoparticle has a mass activity of
2,600 mA/mg Pt, or 93% of its original performance value. In comparison, the
scientists write, the well-received platinum-bismuth nanoparticle has a mass
activity of about 1,720 mA/mg Pt under identical experiments, and is four times
less active when measured for durability.
The researchers note that other metals may be substituted for gold in the
nanoparticle catalyst to improve the catalyst’s performance and durability.
“This communication presents a
new structure-control strategy to tune and optimize nanoparticle catalysis for
fuel oxidations,” the researchers write.