Associate Professor Yang Shao-Horn, left, and graduate student Jin Suntivich, co-authors of Nature Chemistry paper. Photo: Melanie Gonick |
MIT
researchers have found a new way to predict which materials will perform best
as catalysts for oxygen reduction, a core process in metal air batteries and
fuel cells, opening up the possibility of faster and more effective development
of new high-efficiency, low-cost energy-storage technologies.
Such
catalysts are the crucial materials that govern the performance of fuel cells,
as well as air-breathing batteries and other energy storage systems that are
becoming increasingly important for everything from portable electronic devices
to cars to the electric grid—where inexpensive storage is seen as key to
increasing use of renewable but intermittent energy sources, such as solar or
wind. But so far, selecting and testing such materials has essentially been a
matter of trial and error, and most of the high-performing materials found have
been rare and expensive, such as palladium and platinum.
The
new principle, by contrast, should allow rapid assessment of a range of
alternative catalysts made of metal-oxide materials, many of which are made of
inexpensive and abundant elements.
The
MIT researchers’ analysis found that the effectiveness of different materials
could be determined by the arrangement of electrons in the outer shells of
their atoms, and the way surface metal ions bond to oxygen. The research—led by
Yang Shao-Horn, an associate professor of mechanical engineering and materials
science and engineering at MIT, and Hubert A. Gasteiger, a visiting professor
at MIT and a chemistry professor at the Technische Universität München in Garching, Germany—was
published in Nature Chemistry. Graduate student Jin Suntivich of MIT’s
Department of Materials Science and Engineering is the lead author, and John B.
Goodenough of the Univ. of Texas at Austin
is a co-author.
This diagram illustrates the way the electronic configuration of metal ions can control the activity of metal oxides for oxygen reduction, varying it by a factor of at least 10,000 times. This can serve as a design principle (symbolized as a “volcano plot”) to screen metal oxide candidates and accelerate the development of efficient fuel cells, metal-air batteries and other energy storage technologies. Image: Jin Suntivich, Eva Mutoro, and Yang Shao-Horn |
Shao-Horn
explains that until now, there has not been any systematic approach to looking
for new, inexpensive and high-performance oxides to use as electrodes in fuel
cells or metal-air batteries. Typically, good catalysts for oxygen reduction
will bind neither too strongly nor too weakly with oxygen. “For some time, we
knew that platinum was good” as a catalyst, she says, but it was unclear
whether the explanation could be applied to other materials such as metal oxides.
Research
pioneered by Jens Nørskov and colleagues at Stanford Univ.
and Denmark Technical Univ. established a simple parameter: average energy of
the outermost electron, which correlates with the binding energy of oxygen to
metal surfaces. This principle explains why certain metals perform better than
others, and it turns out that platinum just has the right electronic structure
to provide optimum binding of oxygen—and thus high catalytic activity.
Now, “we have a theoretical framework and experimental evidence that explains why”
certain metal oxides perform better than others, Shao-Horn says. When plotted
on a chart comparing the electronic configuration of oxides’ surface metal ions
and their catalytic activity, the result is a “volcano” shape: a sharp peak at
the center, with steeply sloping sides indicating poorer performance. By simply
changing the electronic configuration, materials can vary in their activity by
a factor of at least 10,000 from the base of the volcano to the peak, Shao-Horn
says.
The
new work now makes it possible to screen thousands of candidate metal-oxide
materials without the time-consuming tests needed to prove their exact
performance. A material’s behavior can now be predicted from a single
parameter: how its electrons are distributed in the orbitals responsible for the
bonding of metal to oxygen.
Robert
Savinell, the George S. Dively Professor of Chemical Engineering at Case
Western Reserve Univ., says this work is “of very high quality in rigor and innovation.”
He adds that it “allows for optimizing chemical compositions for other
important catalyst characteristics such as ease of synthesis, durability,” and
other qualities. He says the research results will have an impact on searches
for catalyst materials “for alkaline fuel cells, metal-air batteries, and even
for electrolysis cells [that] … will be important for converting renewable
energy sources such as wind and solar to hydrogen for energy storage or for use
in fuel cells for transportation.”
