Michigan Tech researchers are using carbon foam to develop greener, ultra-long-life batteries. Image: Michigan Technological University |
A lighter, greener, cheaper, longer-lasting battery. Who wouldn’t want
that?
Researchers at Michigan
Technological University
are working on it. Their design is a twist on an asymmetric capacitor, a new
type of electrical storage device that’s half capacitor, half battery. It may
be a marriage made in heaven.
Capacitors store an electrical charge physically and have important
advantages: they are lightweight and can be recharged (and discharged) rapidly
and almost indefinitely. Plus, they generate very little heat, an important
issue for electronic devices. However, they can only make use of about half of
their stored charge.
Batteries, on the other hand, store electrical energy chemically and can
release it over longer periods at a steady voltage. And they can usually store
more energy than a capacitor. But batteries are heavy and take time to charge
up, and even the best can’t be recharged forever.
Enter asymmetric capacitors, which bring together the best of both worlds.
On the capacitor side, energy is stored by electrolyte ions that are physically
attracted to the charged surface of a carbon anode. Combined with a
battery-style cathode, this design delivers nearly double the energy of a
standard capacitor.
Now, Michigan Tech researchers have incorporated a novel material on the
battery side to make an even better asymmetric capacitor.
Their cathode relies on nickel oxyhydroxide, the same material used in
rechargeable nickel-cadmium or nickel-metal hydride batteries. “In most batteries
that contain nickel oxyhydroxide, metallic nickel serves as a mechanical support
and a current collector,” says chemistry professor Bahne Cornilsen, who had
been studying nickel electrodes for a number of years, initially with NASA
support. A few years ago, the Michigan Tech team had a chance to experiment
with something different: carbon foam. He suggested replacing the nickel with
carbon foam.
Carbon foam has advantages over nickel. “It’s lighter and cheaper, so we
thought maybe we could use it as a scaffold, filling its holes with nickel
oxyhydroxide,” says Tony Rogers, associate professor of chemical engineering.
Carbon foam has a lot of holes to fill. “The carbon foam we are using has
72% porosity,” Rogers
says. “That means 72% of its volume is empty space, so there’s plenty of room
for the nickel oxyhydroxide. The carbon foam could also be made of renewable
biomass, and that’s attractive.”
But how many times can you recharge their novel asymmetric capacitor?
Nobody knows; so far, they haven’t been able to wear it out. “We’ve achieved
over 127,000 cycles,” Rogers
says.
Other asymmetric capacitors have similar numbers, but none have the
carbon-foam edge that could make them even more desirable to consumers.
“Being lighter would give it a real advantage in handheld power tools and
consumer electronics,” says Rogers.
Hybrid electric vehicles are another potential market, since an asymmetric
capacitor can charge and discharge more rapidly than a normal battery, making
it useful for regenerative braking.
The group has applied for a patent on their new technology.
