The record-breaking catalyst stuffs electrons – the backbone of electricity, seen here as yellow balls or yellow halos – into chemical bonds between hydrogen atoms (H) stolen from water. It uses inexpensive nickel (Ni) to do so, instead of the more common and expensive platinum. |
Looking
to nature for their muse, researchers have used a common protein to
guide the design of a material that can make energy-storing hydrogen
gas. The synthetic material works 10 times faster than the original
protein found in water-dwelling microbes, the researchers report in the
August 12 issue of the journal Science, clocking in at 100,000 molecules
of hydrogen gas every second.
This
step is just one part of a series of reactions to split water and make
hydrogen gas, but the researchers say the result shows they can learn
from nature how to control those reactions to make durable synthetic
catalysts for energy storage, such as in fuel cells.
In
addition, the natural protein, an enzyme, uses inexpensive, abundant
metals in its design, which the team copied. Currently, these
materials—called catalysts, because they spur reactions along—rely on
expensive metals such as platinum.
“This
nickel-based catalyst is really very fast,” said coauthor Morris
Bullock of the Department of Energy’s Pacific Northwest National
Laboratory. “It’s about a hundred times faster than the previous
catalyst record holder. And from nature, we knew it could be done with
abundant and inexpensive nickel or iron.”
Stuffing bonds
Electrical
energy is nothing more than electrons. These same electrons are what
tie atoms together when they are chemically bound to each other in
molecules such as hydrogen gas. Stuffing electrons into chemical bonds
is one way to store electrical energy, which is particularly important
for renewable, sustainable energy sources like solar or wind power.
Converting the chemical bonds back into flowing electricity when the sun
isn’t shining or the wind isn’t blowing allows the use of the stored
energy, such as in a fuel cell that runs on hydrogen.
Electrons
are often stored in batteries, but Bullock and his colleagues want to
take advantage of the closer packing available in chemicals.
“We
want to store energy as densely as possible. Chemical bonds can store a
huge amount of energy in a small amount of physical space,” said
Bullock, director of the Center for Molecular Electrocatalysis at PNNL,
one of DOE’s Energy Frontier Research Centers. The team also included
visiting researcher Monte Helm from Fort Lewis College in Durango, Colo.
Biology
stores energy densely all the time. Plants use photosynthesis to store
the sun’s energy in chemical bonds, which people use when they eat food.
And a common microbe stores energy in the bonds of hydrogen gas with
the help of a protein called a hydrogenase.
Because
the hydrogenases found in nature don’t last as long as ones that are
built out of tougher chemicals (think paper versus plastic), the
researchers wanted to pull out the active portion of the biological
hydrogenase and redesign it with a stable chemical backbone.
Two plus two equals one
In
this study, the researchers looked at only one small part of splitting
water into hydrogen gas, like fast-forwarding to the end of a movie. Of
the many steps, there’s a part at the end when the catalyst has a hold
of two hydrogen atoms that it has stolen from water and snaps the two
together.
The
catalyst does this by completely dismantling some hydrogen atoms from a
source such as water and moving the pieces around. Due to the
simplicity of hydrogen atoms, those pieces are positively charged
protons and negatively charged electrons. The catalyst arranges those
pieces into just the right position so they can be put together
correctly. “Two protons plus two electrons equals one molecule of
hydrogen gas,” says Bullock.
In
real life, the protons would come from water, but since the team only
examined a portion of the reaction, the researchers used water stand-ins
such as acids to test their catalyst.
“We
looked at the hydrogenase and asked what is the important part of
this?” said Bullock. “The hydrogenase moves the protons around in what
we call a proton relay. Where the protons go, the electrons will
follow.”
A bauble for energy
Based
on the hydrogenase’s proton relay, the experimental catalyst contained
regions that dangled off the main structure and attracted protons,
called “pendant amines.” A pendant amine moves a proton into position on
the edge of the catalyst, while a nickel atom in the middle of the
catalyst offers a hydrogen atom with an extra electron (that’s a proton
and two electrons for those counting).
The
pendant amine’s proton is positive, while the nickel atom is holding on
to a negatively charged hydrogen. Positioned close to each other, the
opposites attract and the conglomerate solidifies into a molecule,
forming hydrogen gas.
With
that plan in mind, the team built potential catalysts and tested them.
On their first try, they put a bunch of pendant amines around the nickel
center, thinking more would be better. Testing their catalyst, they
found it didn’t work very fast. An analysis of how the catalyst was
moving protons and electrons around suggested too many pendant amines
got in the way of the perfect reaction. An overabundance of protons made
for a sticky catalyst, which pinched it and slowed the
hydrogen-gas-forming reaction down.
Like
good gardeners, the team trimmed a few pendant amines off their
catalyst, leaving only enough to make the protons stand out, ready to
accept a negatively charged hydrogen atom.
Fastest cat in the West
Testing
the trimmed catalyst, the team found it performed much better than
anticipated. At first they used conditions in which no water was present
(remember, they used water stand-ins), and the catalyst could create
hydrogen gas at a rate of about 33,000 molecules per second. That’s much
faster than their natural inspiration, which clocks in at around 10,000
per second.
However,
most real-life applications will have water around, so they added water
to the reaction to see how it would perform. The catalyst ran three
times as fast, creating more than 100,000 hydrogen molecules every
second. The researchers think the water might help by moving protons to a
more advantageous spot on the pendant amine, but they are still
studying the details.
Their
catalyst has a drawback, however. It’s fast, but it’s not efficient.
The catalyst runs on electricity—after all, it needs those electrons to
stuff into the chemical bonds—but it requires more electricity than
practical, a characteristic called the overpotential.
Bullock
says the team has some ideas on how to reduce the inefficiency. Also,
future work will require assembling a catalyst that splits water in
addition to making hydrogen gas. Even with a high overpotential, the
researchers see high potential for this catalyst.
A Synthetic Nickel Electrocatalyst With a Turnover Frequency Above 100,000 s-1 for H2 Production
