When it comes to driving hydrogen production, a new catalyst
built at Pacific Northwest National Laboratory (PNNL) can do what was
previously shown to happen only in nature: Store energy in hydrogen and release
that energy on demand. This nickel-based complex drives the reaction, but is
not consumed by it. While slow, the catalyst wastes little energy. It turns
electrons and protons into hydrogen. The hydrogen molecule holds the energy in
a very small space until it is needed. The same catalyst then breaks the single
bond in the hydrogen molecule, releasing electrons to do work.
Reducing our reliance on fossil fuels benefits the economy,
national security, and the environment. However, solar and wind power cannot be
major players on the energy stage until the intermittent power they generate
can be stored and used when needed. One option is to transform the electrical
energy from solar and wind into hydrogen, which can be used in fuel cells. To
create the hydrogen, scientists want a single, efficient catalyst, which had
eluded them. This research proves that such a catalyst can be synthesized.
“We are trying to build metal catalysts that will
convert between electrical and chemical energy to make it possible to use renewable
sources,” said Morris Bullock, PhD, who worked on the research at PNNL and
is the Director of the Center for Molecular Electrocatalysis.
Often learned in high school chemistry classes, the reaction
for working with hydrogen looks pretty simple.
“However, the mechanism is remarkably
complicated,” said Bullock. “There is a lot of detail in this
process: taking the hydrogen apart, moving protons and electrons, and putting
it back together.”
The team began with the type of catalyst they’ve worked with
for more than two years at the Center for Molecular Electrocatalysis. The
catalyst relies on a nickel center or active site to do the work. This metal
was chosen for its low cost and abundance.
“Replacing fossil fuels with devices that require
precious metals is simply not reasonable,” said Bullock.
Wrapped around and attached to the nickel active site are
several molecular strands or ligands. These ligands function as arms,
transporting molecules, protons, and electrons to and from the active site. The
team systematically explored how changing the size, structure, and behavior of
the ligands affected the reaction. They characterized each version of the
catalyst using nuclear magnetic resonance spectroscopy and electrochemical
With the catalyst characterized, they tested its ability to
drive the reaction forward and back. The tests involved measuring the electric
current produced by adding hydrogen to the catalyst. Using complex mathematical
formulas, they determined the speed and efficacy of the reactions.
The catalyst proved very efficient, wasting little energy.
Energy waste is measured by determining the overpotential, a ratio of energy
used under real world conditions versus the energy needed under perfect
conditions. “This [catalyst] has a lower overpotential than we usually
find,” said Bullock. “Sadly, it is also slow.”
Speed. The team is working to speed up the catalyst by
tweaking the molecular structure of the ligands to transport protons to and
from the active site more quickly.
“We’ll figure out what the slow step is and then figure
out how to speed it up. Then, we’ll take on the next slowest step, and so on,
until we get the speed we need,” said Bullock.