The design of a nature-inspired material that can make
energy-storing hydrogen gas has gone holistic. Usually, tweaking the design of
this particular catalyst—a work in progress for cheaper, better fuel cells—results
in either faster or more
energy-efficient production but not both. Now, researchers have found a
condition that creates hydrogen faster without a loss in efficiency.
And, holistically, it requires the entire system—the
hydrogen-producing catalyst and the liquid environment in which it works—to
overcome the speed-efficiency tradeoff. The results, published online in the Proceedings of the National Academy of
Sciences, provide insights into making better materials for energy
“Our work shows that the liquid medium can improve
the catalyst’s performance,” said chemist John Roberts of the Center for
Molecular Electrocatalysis at the Department of Energy’s Pacific Northwest
National Laboratory. “It’s an important step in the transformation of
laboratory results into useable technology.”
The results also provide molecular details into how the
catalytic material converts electrical energy into the chemical bonds between
hydrogen atoms. This information will help the researchers build better
catalysts, ones that are both fast and efficient, and made with the common
metal nickel instead of expensive platinum.
A solution solution
The work explores a type of dissolvable nickel-based catalyst, which is a
material that eggs on chemical reactions. Catalysts that dissolve are easier to
study than fixed catalysts, but fixed catalysts are needed for most real-world
applications, such as a car’s pollution-busting catalytic converter. Studying
the catalyst comes first, affixing to a surface comes later.
In their search for a better catalyst to produce hydrogen
to feed into fuel cells, the team of PNNL chemists modeled this dissolvable catalyst
after a protein called a hydrogenase. Such a protein helps tie two hydrogen
atoms together with electrons, storing energy in their chemical bond in the
process. They modeled the catalytic center after the protein’s important parts
and built a chemical scaffold around it.
In previous versions, the catalyst was either efficient
but slow, making about a thousand hydrogen molecules per second; or inefficient
yet fast—clocking in at 100,000 molecules per second. (Efficiency is based on
how much electricity the catalyst requires.) The previous work didn’t get
around this pesky relation between speed and efficiency in the catalysts—it
seemed they could have one but not the other.
Hoping to uncouple the two, Roberts and colleagues put
the slow catalyst in a medium called an acidic ionic liquid. Ionic liquids are
liquid salts and contain molecules or atoms with negative or positive charges
mixed together. They are sometimes used in batteries to allow for electrical
current between the positive and negative electrodes.
The researchers mixed the catalyst, the ionic liquid, and
a drop of water. The catalyst, with the help of the ionic liquid and an
electrical current, produced hydrogen molecules, stuffing some of the electrons
coming in from the current into the hydrogen’s chemical bonds, as expected.
As they continued to add more water, they expected the
catalyst to speed up briefly then slow down, as the slow catalyst in their
previous solvent did. But that’s not what they saw.
“The catalyst lights up like a rocket when you start
adding water,” said Roberts.
The rate continued to increase as they added more and
more water. With the largest amount of water they tested, the catalyst produced
up to 53,000 hydrogen molecules per second, almost as fast as their fast and
Importantly, the speedy catalyst stayed just as efficient
when it was cranking out hydrogen as when it produced the gas more slowly.
Being able to separate the speed from the efficiency means the team might be
able to improve both aspects of the catalyst.
The team also wanted to understand how the catalyst worked in its liquid salt
environment. The speed of hydrogen production suggested that the catalyst moved
electrons around fast. But something also had to be moving protons around fast,
because protons are the positively charged hydrogen ions that electrons follow
around. Just like on an assembly line, protons move through the catalyst or a
protein such as hydrogenase, pick up electrons, form bonds between pairs to
make hydrogen, then fall off the catalyst.
Additional tests hinted how this catalyst-ionic liquid
set-up works. Roberts suspects the water and the ionic liquid collaborated to
mimic parts of the natural hydrogenase protein that shuffled protons through.
In these proteins, the chemical scaffold holding the catalytic center also
contributes to fast proton movement. The ionic liquid-water mixture may be
doing the same thing.
Next, the team will explore the hints they gathered about
why the catalyst works so fast in this mixture. They will also need to attach
it to a surface. Lastly, this catalyst produces hydrogen gas. To create a fuel
technology that converts electrical energy to chemical bonds and back again,
they also plan to examine ionic liquids that will help a catalyst take the
hydrogen molecule apart.