Shriram Ramanathan’s laboratory setup for testing solid-oxide fuel cells. The fuel cell is hidden under the circular component at the top, which pins it down to create a tight seal with the hydrogen fuel entering from below. Two needles connect with the electrodes to measure the electricity produced. Photos by Caroline Perry/SEAS

Imagine
a kerosene lamp that continued to shine after the fuel was spent, or an
electric stove that could remain hot during a power outage.

Materials
scientists at Harvard have demonstrated an equivalent feat in clean
energy generation with a solid-oxide fuel cell (SOFC) that converts
hydrogen into electricity but can also store electrochemical energy like
a battery. This fuel cell can continue to produce power for a short
time after its fuel has run out.

“This
thin-film SOFC takes advantage of recent advances in low-temperature
operation to incorporate a new and more versatile material,” explains
principal investigator Shriram Ramanathan, associate professor of
materials science at the Harvard School of Engineering and Applied
Sciences (SEAS). “Vanadium oxide (VOx) at the anode behaves as a
multifunctional material, allowing the fuel cell to both generate and
store energy.”

The finding, which appeared online in the journal Nano Letters
in June, will be most important for small-scale, portable energy
applications, where a very compact and lightweight power supply is
essential and the fuel supply may be interrupted.

“Unmanned
aerial vehicles, for instance, would really benefit from this,” says
lead author Quentin Van Overmeere, a postdoctoral fellow at SEAS. “When
it’s impossible to refuel in the field, an extra boost of stored energy
could extend the device’s life span significantly.”

Ramanathan,
Van Overmeere, and their co-author Kian Kerman (a graduate student at
SEAS) typically work on thin-film SOFCs that use platinum for the
electrodes (the two “poles” known as the anode and the cathode). But
when a platinum-anode SOFC runs out of fuel, it can continue to generate
power for only about 15 seconds before the electrochemical reaction
peters out.

Purple plasma is visible through the window of this vacuum deposition chamber. The equipment is used for creating the extremely thin-layered electrodes and electrolyte on a wafer of silicon.

 

The
new SOFC uses a bilayer of platinum and VOx for the anode, which allows
the cell to continue operating without fuel for up to 14 times as long
(three minutes, 30 seconds, at a current density of 0.2 mA/cm2). This
early result is only a “proof of concept,” according to Ramanathan, and
his team predicts that future improvements to the composition of the
VOx-platinum anode will further extend the cell’s life span.

During
normal operation, the amount of power produced by the new device is
comparable to that produced by a platinum-anode SOFC. Meanwhile, the
special nanostructured VOx layer sets up various chemical reactions that
continue after the hydrogen fuel has run out.

“There
are three reactions that potentially take place within the cell due to
this vanadium oxide anode,” says Ramanathan. “The first is the oxidation
of vanadium ions, which we verified through XPS [X-ray photoelectron
spectroscopy]. The second is the storage of hydrogen within the VOx
crystal lattice, which is gradually released and oxidized at the anode.
And the third phenomenon we might see is that the concentration of
oxygen ions differs from the anode to the cathode, so we may also have
oxygen anions being oxidized, as in a concentration cell.”

All
three of those reactions are capable of feeding electrons into a
circuit, but it is currently unclear exactly what allows the new fuel
cell to keep running. Ramanathan’s team has so far determined
experimentally and quantitatively that at least two of three possible
mechanisms are simultaneously at work.

Each dark speck within the nine white circles at left is a tiny fuel cell. The AA battery is shown for size comparison.

Ramanathan
and his colleagues estimate that a more advanced fuel cell of this
type, capable of producing power without fuel for a longer period of
time, will be available for applications testing (e.g., in micro-air
vehicles) within two years.

This
work was supported by the U.S. National Science Foundation (NSF), a
postdoctoral scholarship from Le Fonds de la Recherche
Scientifique-FNRS, and the U.S. Department of Defense’s National Defense
Science and Engineering Graduate Fellowship Program. The researchers
also benefited from the resources of the Harvard University Center for
Nanoscale Systems (a member of the NSF-funded National Nanotechnology
Infrastructure Network) and the NSF-funded MRSEC Shared Experimental
Facilities at the Massachusetts Institute of Technology.

Source: Harvard University