Researchers develop metal eating robots

Mankind's use of minerals has evolved over the eons hand-in-hand with innovation. From the first crude fire starters and simple machines to the evolving wonders of the Information Age, human beings have looked to rocks and the minerals within them for the building blocks of technological advancement.

Now comes an innovation that could change everything, especially for electronics that require their own power sources.

University of Pennsylvania researchers have developed a system that enables robots to scavenge for the energy they need from metal. This is in lieu of batteries, which store energy internally, but are heavy and offer a limited supply of power, or harvesters such as solar panels, which collect energy from their environments but can operate only under limited conditions and can't deliver power quickly.

The new system, developed at the university's School of Engineering and Applied Science, has bridged the gap between the two technologies in the form of a "metal-air scavenger" that its inventors believe provide the best of both worlds.

The 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 the metal and air the unit can access as it operates.

Thus, it provides 10 times more power density than the best energy harvesters and 13 times more energy density than lithium-ion batteries.

The researchers – James Pikul, assistant professor in Penn's Department of Mechanical Engineering and Applied Mechanics, along with two members of his lab, Min Wang and Unnati Joshi – published a study recently demonstrating the metal-air scavenger's capabilities in the journal ACS Energy Letters.

The study demonstrates the new approach for powering robots and electronics by electrochemically scavenging energy from metal surfaces.

"This approach overcomes energy storage scaling laws by allowing robots and electronics to extract energy from large volumes of energy dense material without having to carry the material on-board," the researchers wrote.

In an article released by the university, they said the metal-air scavenger, or MAS, resulted from efforts to resolve the mismatch between the size of robots' brains and the power needed to operate them.

"We show that a range of hydrogel electrolyte compositions can be combined with air cathodes to extract 159, 87, and 179 mAh/cm2 capacities from aluminum, zinc, and steel surfaces at up to 130, 81, and 25 mW/cm2 power densities, which exceed the power density of the best energy harvesters by 10 times. (mAh stands for milliampere hour, which represents 1000th of an ampere-hour, and is the unit that measures electric power over time.)

"When moving across a metal surface, metal scavenging exceeds the energy densities of lithium-ion and metal-air batteries by 13 times and 2 times," the researchers wrote.

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.

Thus, metal scavenging is especially beneficial for small robots and electronics, whose size and performance are severely limited by the low energies provided by micro-energy storage technologies.

"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 explained in the statement. "There are robots the size of insects, but they can only operate for a minute before their battery runs out of energy."

Adding a bigger battery, however, won't allow a robot to last longer; instead, the added mass takes more energy to move, negating the extra energy it provides.

The researchers concluded that the only way to solve this frustrating dichotomy was to create a way for the robot to forage for chemical bonds, rather than pack them along.

"Harvesters, like those that collect solar, thermal or vibrational energy, are getting better," Pikul said. "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 10 times better than the best harvesters, to the point that we can compete against batteries," he said. "It's using battery chemistry, but doesn't have the associated weight, because it's taking those chemicals from the environment."

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, according to the researchers.

In the near term, the technology has already powered two spin-off companies. The winners of Penn's annual Y-Prize Competition plan to use metal-air scavengers to provide the energy for low-cost lights in off-grid homes in the developing world and long-lasting sensors in shipping containers that could alert to theft, damage or even human trafficking.

Like a traditional battery, the new 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, the researchers said there's little risk of it doing significant structural damage to the metal it's scavenging.

With so many possible uses, the new 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 US$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 US$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 said. "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 for it to work until its next meal."

Penn noted that the research was supported by a grant from the Office of Naval Research, and was carried out, in part, at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program.