When electronics need their own
power sources, there are two basic options: batteries and harvesters. Batteries
store energy internally, but are therefore heavy and have a limited supply.
Harvesters, such as solar panels, collect energy from their environments. This
gets around some of the downsides of batteries but introduces new ones, in that
they can only operate in certain conditions and can’t turn that energy into
useful power very quickly.
New research from the University of
Pennsylvania’s School of Engineering and Applied Science is bridging the gap
between these two fundamental technologies for the first time in the form of a
“metal-air scavenger” that gets the best of both worlds.
This metal-air scavenger works like
a battery, in that it provides power by repeatedly breaking and forming a
series of chemical bonds. But it also works like a harvester, in that power is
supplied by energy in its environment: specifically, the chemical bonds in
metal and air surrounding the metal-air scavenger.
The result is a power source that
has 10 times more power density than the best energy harvesters and 13 times
more energy density than lithium-ion batteries.
In the long term, this type of
energy source could be the basis for a new paradigm in robotics, where machines
keep themselves powered by seeking out and “eating” metal, breaking down its
chemical bonds for energy like humans do with food.
In the near term, this technology is
already powering a pair of spin-off companies. The winners of Penn’s annual
Y-Prize Competition are planning to use metal-air scavengers to power low-cost
lights for off-grid homes in the developing world and long-lasting sensors for
shipping containers that could alert to theft, damage or even human
trafficking.
The
researchers, James Pikul, assistant professor in the Department of Mechanical
Engineering and Applied Mechanics, along with Min Wang and Unnati Joshi,
members of his lab, published a study demonstrating their scavenger’s
capabilities in the journal ACS Energy Letters.
The motivation for developing their
metal-air scavenger, or MAS, stemmed from the fact that the technologies that
make up robots’ brains and the technologies that power them are fundamentally
mismatched when it comes to miniaturization.
As the size of individual
transistors shrink, chips provide more computing power in smaller and lighter
packages. But batteries don’t benefit the same way when getting smaller; the
density of chemical bonds in a material are fixed, so smaller batteries
necessarily mean fewer bonds to break.
“This inverted relationship between
computing performance and energy storage makes it very difficult for
small-scale devices and robots to operate for long periods of time,” Pikul
says. “There are robots the size of insects, but they can only operate for a
minute before their battery runs out of energy.”
Worse still, adding a bigger battery
won’t allow a robot to last longer; the added mass takes more energy to move,
negating the extra energy provided by the bigger battery. The only way to break
this frustrating inverted relationship is to forage for chemical bonds, rather
than to pack them along.
“Harvesters, like those that collect
solar, thermal or vibrational energy, are getting better,” Pikul says. “They’re
often used to power sensors and electronics that are off the grid and where you
might not have anyone around to swap out batteries. The problem is that they
have low power density, meaning they can’t take energy out of the environment
as fast as a battery can deliver it.”
“Our MAS has a power density that’s
ten times better than the best harvesters, to the point that we can compete
against batteries,” he says, “It’s using battery chemistry, but doesn’t have
the associated weight, because it’s taking those chemicals from the
environment.”
Like a traditional battery, the
researchers’ MAS starts with a cathode that’s wired to the device it’s
powering. Underneath the cathode is a slab of hydrogel, a spongy network of
polymer chains that conducts electrons between the metal surface and the
cathode via the water molecules it carries. With the hydrogel acting as an
electrolyte, any metal surface it touches functions as the anode of a battery,
allowing electrons to flow to the cathode and power the connected device.
For the purposes of their study, the
researchers connected a small motorized vehicle to the MAS. Dragging the
hydrogel behind it, the MAS vehicle oxidized metallic surfaces it traveled
over, leaving a microscopic layer of rust in its wake.
To demonstrate the efficiency of
this approach, the researchers had their MAS vehicle drive in circles on an
aluminum surface. The vehicle was outfitted with a small reservoir that
continuously wicked water into the hydrogel to prevent it from drying out.
“Energy density is the ratio of
available energy to the weight that has to be carried,” Pikul says. “Even
factoring in the weight of the extra water, the MAS had 13 times the energy
density of a lithium ion battery because the vehicle only has to carry the
hydrogel and cathode, and not the metal or oxygen which provide the energy.”
The researchers also tested the MAS
vehicles on zinc and stainless steel. Different metals give the MAS different
energy densities, depending on their potential for oxidation.
This oxidation reaction takes place
only within 100 microns of the surface, so while the MAS may use up all the
readily available bonds with repeated trips, there’s little risk of it doing
significant structural damage to the metal it’s scavenging.
With so many possible uses, the
researchers’ MAS system was a natural fit for Penn’s annual Y-Prize, a business
plan competition that challenges teams to build companies around nascent
technologies developed at Penn Engineering. This year’s first-place team, Metal
Light, earned $10,000 for their proposal to use MAS technology in low-cost
lighting for off-grid homes in the developing world. M-Squared, which earned
$4,000 in second place, intends to use MAS-powered sensors in shipping
containers.
“In the near term, we see our MAS
powering internet-of-things technologies, like what Metal Light and M-Squared
propose,” Pikul says. “But what was really compelling to us, and the motivation
behind this work, is how it changes the way we think about designing robots.”
Much of Pikul’s other research
involves improving technology by taking cues from the natural world. For
example, his lab’s high-strength, low-density “metallic wood” was inspired by
the cellular structure of trees, and his work on a robotic lionfish involved
giving it a liquid battery circulatory system that also pneumatically actuated
its fins.
The researchers see their MAS as
drawing on an even more fundamental biological concept: food.
“As we get robots that are more intelligent
and more capable, we no longer have to restrict ourselves to plugging them into
a wall. They can now find energy sources for themselves, just like humans do,”
Pikul says. “One day, a robot that needs to recharge its batteries will just
need to find some aluminum to ‘eat’ with a MAS, which would give it enough
power to for it work until its next meal.”
Journal
article: https://pubs.acs.org/doi/10.1021/acsenergylett.9b02661
Source: https://myfusimotors.com/2020/04/26/new-scavenger-technology-allows-robots-to-eat-metal-for-energy/
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