Knowing the position of
missing oxygen atoms could be the key to cheaper solid oxide fuel cells with
longer lifetimes. New microscopy research from the U.S. Department of Energy’s
Oak Ridge National Laboratory (ORNL) is enabling scientists to map these
vacancies at an atomic scale.
Although fuel cells hold
promise as an efficient energy conversion technology, they have yet to reach
mainstream markets because of their high price tag and limited lifespans.
Overcoming these barriers requires a fundamental understanding of fuel cells,
which produce electricity through a chemical reaction between oxygen and a
fuel. As conducting oxygen ions move through the fuel cell, they travel through
vacancies where oxygen atoms used to be. The distribution, arrangement, and
geometry of such oxygen vacancies in fuel cell materials are thought to affect
the efficiency of the overall device.
“A big part of making a
better fuel cell is to understand what the oxygen vacancies do inside the
material: how fast they move, how they order, how they interact with interfaces
and defects,” says ORNL’s Albina Borisevich. “The question is how to
study them. It’s one thing to see an atom of one type on the background of
atoms of a different type. But in this case, you want to see if there are a few
atoms missing. Seeing a void is much more difficult.”
In research published in Nature Materials, ORNL scientists used
scanning transmission electron microscopy to determine the distribution of
oxygen vacancies in a fuel cell cathode material below the level of a single
unit cell. The team verified its findings with theoretical calculations and
neutron experiments at the laboratory’s Spallation Neutron Source.
“Even though the vacancy
doesn’t generate any signal in the electron micrograph, it’s still a big
disturbance in the structure,” Borisevich says. “You can see that the
lattice expands where vacancies are present. So we tracked the lattice
expansion around vacancies and compared it with theoretical models, and we were
able to develop a calibration for this type of material.”
By providing a means to study
vacancies at an atomic scale, the ORNL technique will help inform the
development of improved fuel cell technologies in a systematic and deliberate
fashion, in contrast to trial and error approaches. Beyond its relevance to
applications in fuel cells and information storage and logic devices, ORNL
coauthor Sergei Kalinin explains that the team’s research is also building a
bridge between two scientific communities that traditionally have had little in
common.
“From my perspective, it
is physics marrying electrochemistry,” Kalinin says. “The idea is
that vacancies are important for energy, and vacancies are important for
physics. The materials that physicists like to study are exactly the same as
the materials used for fuel cells, and unless we understand how vacancies
behave at interfaces, ferroic domain walls, and in thin films, we will not be
able to fully appreciate the physics of these systems.”
The team’s research is also
reinforced by a parallel study published in Physical
Review Letters, with Borisevich and Kalinin as coauthors, that explains how
to obtain parameters that describe vacancy-ordered systems from electron
microscopy data.
Source: Oak Ridge National Laboratory