In the Near Future, Robots Will Eat Metal for Energy

Robots are becoming increasingly useful in search-and-rescue missions, from snake bots that can slither through tight spaces or cracks to reach a person in distress, to drones that can fly inside tight cave shafts to gather information about the tunnel system inside before deploying larger units. The problem? All of these robots are only as capable as their battery packs.

Once the energy inside the bots is depleted, they must stop whatever they're doing—no matter how important that mission is—and operators must either recharge the battery or load a new one. Not only that, but batteries give the bots additional undesirable weight.

But now, researchers at the University of Pennsylvania's School of Engineering and Applied Science have found a way to bypass those weaknesses. Their "metal-air scavenger" (MAS) makes use of an untapped power source that is quite literally all around us: metal.

"A lot of these robots are enabling some really remarkable capabilities," James Pikul, an assistant professor at Penn and the lead researcher on the project, tells Popular Mechanics. "They're being made down to the miniature scale, but batteries don’t scale that way."

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As robots become increasingly useful, they're quite literally tethered by their power sources. If researchers want to see a tripling in battery capability, it could take 20 years, Pikul says. "I refuse to wait that long."

In a paper published earlier this year in ACS Energy Letters, Pikul and his colleagues outline their metal-air scavenger process. The apparatus they've designed essentially "eats" metal in its surrounding environment, breaking down the chemical bonds like how our bodies break down bonds in the food we digest for energy.

"The biggest inspiration we drew from is kind of corny, but it's eating," Pikul says. "Animals eat and that's how they power themselves. But instead of eating glucose, [the robot] eats metal."

The current prototype includes a thin film of carbon cloth with a cathode embedded inside. The carbon cloth "breathes" in air through bits of platinum that act as reducing agents, pulling in oxygen from the environment.

Between the carbon film and the metal surface in question, a Jello-like hydrogel—loaded with saltwater—acts as an electrolyte, conducting electrons between the metal and the cathode. That way, when the hydrogel touches a metal surface, that area acts like the anode of a battery, letting electrons through to the cathode, which then powers the device.

Depending on a metal's potential for oxidation, different materials give the MAS system different energy densities, Pikul says. Iron is plentiful, but will yield less energy than aluminum due the nature of its chemical bonds. Aluminum and zinc, meanwhile, work really well. So just as our bodies metabolize apples and pizza differently, the MAS system breaks apart the bonds in metals differently.

Certain metals, like stainless steel, are specifically engineered to prevent corrosion. Oxidation is the culprit for corrosion, and therefore, rust. Because Pikul's system is essentially creating what he calls "controlled corrosion," your brand-new appliances wouldn't be a great energy source.

As robots have gotten smaller and smaller—from dragonfly agents to hummingbird robots–it's become more difficult to justify heavy batteries. Pikul's prototype is 10 times more power-dense than high-end energy harvesters (like solar cells) and 13 times more energy-dense than a lithium-ion battery, so it could eliminate the need for traditional batteries in robotics altogether.

Pikul's system makes the most sense in unstructured environments. Because the power grid provides a steady flow of electrons, and the stakes are low, there's no reason why your Roomba or a grocery store shelf-scanning robot would need a new energy source.

Pikul believes the metal-air scavenger could even prove useful in space one day. The oxygen the system requires is a clear constraint, but metal is plentiful.

"Rocket boosters are used once and then they get rid of them," Pikul says. They're released and burn up in space. What if, instead, the extra metal could be used to power spacecraft?

In the search-and-rescue example, meanwhile, the MAS technology has the potential to save lives. Just one return trip to switch out a robot's battery could be the difference between finding and pulling a person out of a burning building and never seeing them again.

That's reason enough to get robots away from wall chargers.