An
illustration of the 2D boron nitride substrate with imperfections that host
tiny nickel clusters. The catalyst aids the chemical reaction that removes
hydrogen from liquid chemical carriers, making it available for use as a fuel.
Credit: Jeff Urban/Berkeley Lab.
Hydrogen
is a sustainable source of clean energy that avoids toxic emissions and can add
value to multiple sectors in the economy including transportation, power
generation, metals manufacturing, among others. Technologies for storing and
transporting hydrogen bridge the gap between sustainable energy production and
fuel use, and therefore are an essential component of a viable hydrogen
economy. But traditional means of storage and transportation are expensive and
susceptible to contamination. As a result, researchers are searching for
alternative techniques that are reliable, low-cost and simple. More-efficient
hydrogen delivery systems would benefit many applications such as stationary
power, portable power, and mobile vehicle industries.
Now, as
reported in the journal Proceedings of the National Academy of Sciences,
researchers have designed and synthesized an effective material for speeding up
one of the limiting steps in extracting hydrogen from alcohols. The material, a
catalyst, is made from tiny clusters of nickel metal anchored on a 2-D
substrate. The team led by researchers at Lawrence Berkeley National
Laboratory's (Berkeley Lab) Molecular Foundry found that the catalyst could
cleanly and efficiently accelerate the reaction that removes hydrogen atoms
from a liquid chemical carrier. The material is robust and made from
earth-abundant metals rather than existing options made from precious metals,
and will help make hydrogen a viable energy source for a wide range of
applications.
"We
present here not merely a catalyst with higher activity than other nickel
catalysts that we tested, for an important renewable energy fuel, but also a
broader strategy toward using affordable metals in a broad range of
reactions," said Jeff Urban, the Inorganic Nanostructures Facility
director at the Molecular Foundry who led the work. The research is part of the
Hydrogen Materials Advanced Research Consortium (HyMARC), a consortium funded
by the U.S. Department of Energy's Office of Energy Efficiency and Renewable
Energy Hydrogen and Fuel Cell Technologies Office (EERE). Through this effort,
five national laboratories work towards the goal to address the scientific gaps
blocking the advancement of solid hydrogen storage materials. Outputs from this
work will directly feed into EERE's H2@Scale vision for affordable hydrogen
production, storage, distribution and utilization across multiple sectors in
the economy.
Chemical
compounds that act as catalysts like the one developed by Urban and his team
are commonly used to increase the rate of a chemical reaction without the
compound itself being consumed—they might hold a particular molecule in a
stable position, or serve as an intermediary that allows an important step to
be reliably to completed. For the chemical reaction that produces hydrogen from
liquid carriers, the most effective catalysts are made from precious metals. However,
those catalysts are associated with high costs and low abundance, and are
susceptible to contamination. Other less expensive catalysts, made from more
common metals, tend to be less effective and less stable, which limits their
activity and their practical deployment into hydrogen production industries.
To improve
the performance and stability of these earth-abundant metal-based catalysts,
Urban and his colleagues modified a strategy that focuses on tiny, uniform
clusters of nickel metal. Tiny clusters are important because they maximize the
exposure of reactive surface in a given amount of material. But they also tend
to clump together, which inhibits their reactivity.
Postdoctoral
research assistant Zhuolei Zhang and project scientist Ji Su, both at the
Molecular Foundry and co-lead authors on the paper, designed and performed an
experiment that combatted clumping by depositing 1.5-nanometer-diameter nickel
clusters onto a 2-D substrate made of boron and nitrogen engineered to host a
grid of atomic-scale dimples. The nickel clusters became evenly dispersed and
securely anchored in the dimples. Not only did this design prevent clumping, but
its thermal and chemical properties greatly improved the catalyst's overall
performance by directly interacting with the nickel clusters.
"The
role of the underlying surface during the cluster formation and deposition
stage has been found to be critical, and may provide clues to understanding
their role in other processes" said Urban.
Detailed
X-ray and spectroscopy measurements, combined with theoretical calculations,
revealed much about the underlying surfaces and their role in catalysis. Using
tools at the Advanced Light Source, a DOE user facility at Berkeley Lab, and
computational modeling methods, the researchers identified changes in the
physical and chemical properties of the 2-D sheets while tiny nickel clusters
formed and deposited on them. The team proposed that the material forms while
metal clusters occupy pristine regions of the sheets and interact with nearby
edges, thus preserving the tiny size of the clusters. The tiny, stable clusters
facilitated the action in the processes through which hydrogen is separated
from its liquid carrier, endowing the catalyst with excellent selectivity,
productivity, and stable performance.
Calculations
showed that the catalyst's size was the reason its activity was among the best
relative to others that have recently been reported. David Prendergast,
director of the Theory of Nanostructured Materials Facility at the Molecular
Foundry, along with postdoctoral research assistant and co-lead author Ana
Sanz-Matias, used models and computational methods to uncover the unique
geometric and electronic structure of the tiny metal clusters. Bare metal
atoms, abundant on these tiny clusters, more readily attracted the liquid
carrier than did larger metal particles. These exposed atoms also eased the
steps of the chemical reaction that strips hydrogen from the carrier, while
preventing the formation of contaminants that may clog the surface of the
cluster. Hence, the material remained free of pollution during key steps in the
hydrogen production reaction. These catalytic and anti-contamination properties
emerged from the imperfections that had been deliberately introduced to the 2-D
sheets and ultimately helped keep the cluster size small.
"Contamination
can render possible non-precious metal catalysts unviable. Our platform here
opens a new door to engineering those systems," said Urban.
In their
catalyst, the researchers achieved the goal of creating a relatively
inexpensive, readily available, and stable material that helps to strip
hydrogen from liquid carriers for use as a fuel. This work came out of a DOE
effort to develop hydrogen storage materials to meet the targets of EERE's
Hydrogen and Fuel Cell Technologies Office and to optimize the materials for
future use in vehicles.
Future
work by the Berkeley Lab team will further hone the strategy of modifying 2-D
substrates in ways that support tiny metal clusters, to develop even more
efficient catalysts. The technique could help to optimize the process of
extracting hydrogen from liquid chemical carriers.