Tech Briefs Magazine - April 2022 - 28

Power & Energy
tric material converts waste heat from
the reaction to usable power.
Vapor of a high-energy fuel - in this
case, methanol - is combusted on a thin,
440-nanometer film on platinized silicon
wafers. The device converts the heat from
this reaction into pyroelectric power.
Nanostructured iridium oxide is the
top electrode and combustion catalyst.
Iridium is a dense, corrosion- and
heat-resistant metal, making it an excellent
candidate for this application.
Iridium oxide is first activated at temperatures
as low as 105 °C and fully catalyzes
methanol to carbon dioxide at
120 °C. This is an advantage compared
to platinum-based catalysts, which do
not achieve full conversion until 150 °C.
This means less heat must be applied to
the device for it to be fully effective. The
on-chip combustion technology has a 90
percent combustion efficiency rate.
This technology would be significantly
more powerful than lithium-ion batteries,
the common rechargeable batteries
used in electronics. The energy density
of methanol is 22 times greater than a
lithium-ion battery.
Pyroelectric power is a clean alternative
to fossil fuels and nuclear energy,
which still constitute more than 80 percent
of the United States' power. Thus,
this technology has broad energy applications
on large and small scales.
For more information, contact Kim Krieger
at kim.krieger@uconn.edu; 860-486-0361.
Stable, Efficient, Anode-Free Sodium Battery
The battery is smaller than a traditional lithium-ion battery due to the elimination of dendrites.
Washington University, St. Louis, MO
A
traditional lithium-ion battery consists
of a cathode and anode, both of which
store lithium ions; a separator to keep the
electrodes separated on either side; and
an electrolyte - the liquid through which
the ions move. When lithium flows from
the anode to the cathode, free electrons
leave through the current collector to the
device being powered while the lithium
passes the separator to the cathode. To
charge, the process is reversed and the
lithium passes from the cathode, through
the separator, to the anode.
The concept of replacing lithium with
sodium and doing away with the anode
isn't new. The problem has been that the
anode-free battery does not have a reasonable
lifetime. They always fail very quickly,
have a very low capacity, or require special
processing of the current collector.
Anode-free batteries tend to be unstable,
growing dendrites - finger-like
growths that can cause a battery to short
or simply to degrade quickly. This conventionally
has been attributed to the
reactivity of the alkali metals involved; in
this case, sodium. In the new battery, only
a thin layer of copper foil was used on the
anode side as the current collector, so the
battery has no active anode material. Instead
of flowing to an anode where they
sit until time to move back to the cathode,
in the anode-free battery, the ions
are transformed into a metal. First, they
plate themselves onto copper foil, then
they dissolve away when it's time to return
to the cathode.
Traditionally, when a battery fails, in
order to determine what went wrong, a
researcher can open it up and take a look.
But that after-the-fact observation has limited
usefulness.
28
The researchers saw smooth deposits of sodium in the new battery. (Photo: Washington University)
All of the battery's instabilities accumulate
during the working process. What
matters is instability during the dynamic
process and there's no method to characterize
that. So, the team developed a
unique, transparent capillary cell that offers
a new way to look at batteries. Watching
the anode-free capillary cell, one
could clearly see that if there is not good
quality control of the electrolyte, various
instabilities will be seen including the formation
of dendrites.
Alkali metals react with water, so the
research team brought the water content
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down. Watching the battery in action,
they saw shiny, smooth deposits of sodium.
It's the smoothness of the material
that eliminates morphological irregularities
that can lead to the growth of dendrites.
Water content must be lower than
10 parts-per-million. With that realization,
the team was able to build not just a
capillary cell but a working battery that is
similar in performance to a standard lithium-ion
battery but takes up much less
space because of the lack of an anode.
For more information, contact Chuck Find er
at chuck.finder@wustl.edu; 314-935-4333.
Tech Briefs, April 2022
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Tech Briefs Magazine - April 2022

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