This magnified image from a transmission electron microscope reveals details of the unexpected nanosheet structure of the nickel-molybdenum-nitride catalyst, seen here as dark, straight lines. |
Hydrogen gas offers one of the most promising sustainable energy
alternatives to limited fossil fuels. But traditional methods of producing pure
hydrogen face significant challenges in unlocking its full potential, either by
releasing harmful carbon dioxide into the atmosphere or requiring rare and
expensive chemical elements such as platinum.
Now, scientists at the U.S. Department of Energy’s (DOE)
Brookhaven National Laboratory have developed a new electrocatalyst that
addresses one of these problems by generating hydrogen gas from water cleanly
and with much more affordable materials. The novel form of catalytic
nickel-molybdenum-nitride—described in a paper published online in Angewandte Chemie
International Edition—surprised scientists with its high-performing
nanosheet structure, introducing a new model for effective hydrogen catalysis.
“We wanted to design an optimal catalyst with high activity and
low costs that could generate hydrogen as a high-density, clean energy source,”
said Brookhaven Lab chemist Kotaro Sasaki, who first conceived the idea for
this research. “We discovered this exciting compound that actually outperformed
our expectations.”
Goldilocks chemistry
Water provides an ideal source of pure hydrogen—abundant and
free of harmful greenhouse gas byproducts. The electrolysis of water, or
splitting water into oxygen and hydrogen, requires external electricity and an
efficient catalyst to break chemical bonds while shifting around protons and electrons.
To justify the effort, the amount of energy put into the reaction must be as
small as possible while still exceeding the minimum required by thermodynamics,
a figure associated with what is called overpotential.
For a catalyst to facilitate an efficient reaction, it
must combine high durability, high catalytic activity, and high surface area.
The strength of an element’s bond to hydrogen determines its reaction level—too
weak, and there’s no activity; too strong, and the initial activity poisons the
catalyst.
“We needed to create high, stable activity by combining
one non-noble element that binds hydrogen too weakly with another that binds
too strongly,” said James Muckerman, the senior chemist who led the project. “The result becomes this well-balanced Goldilocks compound—just right.”
Unfortunately, the strongest traditional candidate for an
electrocatlytic Goldilocks comes with a prohibitive price tag.
Problems with
platinum
Platinum is the gold standard for electrocatalysis, combining low
overpotential with high activity for the chemical reactions in water splitting.
But with rapidly rising costs—already hovering around $50,000/kg—platinum and
other noble metals discourage widespread investment.
“People love platinum, but the limited global supply not
only drives up price, but casts doubts on its long-term viability,” Muckerman
said. “There may not be enough of it to support a global hydrogen economy.”
In contrast, the principal metals in the new compound
developed by the Brookhaven team are both abundant and cheap: $20/kg for nickel
and $32/kg for molybdenum. Combined, that’s 1000 times less expensive than
platinum. But with energy sources, performance is often a more important
consideration than price.
Turning nickel into
platinum
In this new catalyst, nickel takes the reactive place of
platinum, but it lacks a comparable electron density. The scientists needed to
identify complementary elements to make nickel a viable substitute, and they
introduced metallic molybdenum to enhance its reactivity. While effective, it
still couldn’t match the performance levels of platinum.
“We needed to introduce another element to alter the
electronic states of the nickel-molybdenum, and we knew that nitrogen had been
used for bulk materials, or objects larger than one micrometer,” said research
associate Wei-Fu Chen, the paper’s lead author. “But this was difficult for
nanoscale materials, with dimensions measuring billionths of a meter.”
The scientists expected the applied nitrogen to modify the
structure of the nickel-molybdenum, producing discrete, sphere-like
nanoparticles. But they discovered something else.
Subjecting the compound to a high-temperature ammonia
environment infused the nickel-molybdenum with nitrogen, but it also
transformed the particles into unexpected 2D nanosheets. The nanosheet
structures offer highly accessible reactive sites—consider the surface area
difference between bed sheets laid out flat and those crumpled up into balls—and
therefore more reaction potential.
Using a high-resolution transmission microscope in
Brookhaven Lab’s Condensed Matter Physics and Materials Science Department, as
well as X-ray probes at the National Synchrotron Light Source, the scientists
determined the material’s 2D structure and probed its local electronic
configurations.
“Despite the fact that metal nitrides have been
extensively used, this is the first example of one forming a nanosheet,” Chen
said. “Nitrogen made a huge difference—it expanded the lattice of
nickel-molybdenum, increased its electron density, made an electronic structure
approaching that of noble metals, and prevented corrosion.”
Hydrogen future
The new catalyst performs nearly as well as platinum, achieving
electrocatalytic activity and stability unmatched by any other non-noble metal
compounds. “The production process is both simple and scalable,” Muckerman
said, “making nickel-molybdenum-nitride appropriate for wide industrial
applications.”
While this catalyst does not represent a complete solution
to the challenge of creating affordable hydrogen gas, it does offer a major
reduction in the cost of essential equipment. The team emphasized that the
breakthrough emerged through fundamental exploration, which allowed for the
surprising discovery of the nanosheet structure.
“Brookhaven Lab has a very active fuel cell and
electrocatalysis group,” Muckerman said. “We needed to figure out fundamental
approaches that could potentially be game-changing, and that’s the spirit in
which we’re doing this work. It’s about coming up with a new paradigm that will
guide future research.”
