How it works: scavenger robots that consume metal for energy

For a long time now, factories and warehouses have employed autonomous robots to perform heavy-duty and repetitive tasks, from raw material handling to finished goods packaging. Yet when it comes to recharging, manual intervention remains inevitable.

Refuelling happens most commonly in one of two ways: energy harvesters and batteries. In the energy harvesting method, energy is mined from the environment. It is then converted into an accessible and usable form. However, this methodology has its drawbacks as energy harvesters, such as solar panels, only function under specific and often unique conditions. With batteries, large quantities of energy can be stored in a relatively small space. Then again, this arrangement makes batteries heavy and difficult to accommodate.

There is now a third way, substantially more promising than the previous two. Remedying the shortcomings in traditional charging methods, an engineering laboratory at the University of Pennsylvania has developed a metal-air-scavenger (MAS) machine that makes its own electrical power from metal and air. Essentially, the machine can ‘eat’.

The motivation for developing a self-reliant, cannibalistic robot flows from the idea that batteries don’t necessarily get better with miniaturisation, but computing chips do. As transistors get smaller, computing power is delivered in more powerful yet lighter packages. The metal-air-scavenger (MAS) machine compensates this lack of symmetry by using the same battery chemistry minus the weight.

“Harvesters, like those that collect solar, thermal or vibrational energy, are getting better,” explains James Pikul, assistant professor in the Department of Mechanical Engineering and Applied Mechanics. “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. It’s using battery chemistry, but doesn’t have the associated weight, because it’s taking those chemicals from the environment.”

Furthering the study, the researchers attached a motorised or self-propelled vehicle to the MAS system. Small and light, the vehicle tows a hydrogel slab behind it as it travels over the metal surface (aluminium). This in turn oxidises the metal surface and a microscopic layer of rust starts to develop. The vehicle continues to move around until the hydrogel dries out or the surface is thoroughly corroded. That said, it’s entirely possible for a free-moving robot to pick up new sources of water and metal.

The highlight in this process is that the metal-air scavenger vehicle procures energy by breaking chemical bonds in the aluminium surface it travels over, totally eliminating the need for a battery. When tested with metals such as zinc and stainless steel, it was found that MAS acquired different energy densities, depending on the metal’s potential for oxidation. The chances of structural damage to the metal remain low as the oxidation reaction happens within 100 microns of the metal surface.

A small storage reservoir is hooked onto the vehicle to continuously infuse water into the hydrogel. This prevents the gel from drying out completely. “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.”

Casting light on the future of scavenger robots, Pikul explains: “In the near term, we see our MAS powering internet-of-things technologies, like what Metal Light and M-Squared propose. But what was really compelling to us, and the motivation behind this work, is how it changes the way we think about designing robots.

“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. One day, a robot that needs to recharge its batteries will just need to find some aluminium to ‘eat’ with a MAS, which would give it enough power to for it work until its next meal.”

Similar to a regular battery, the metal-air scavenger (MAS) consists of a cathode that is wired to a device that needs to be powered – in the case shown in the diagram above, a little toy vehicle. Right below the cathode, which in this case is a carbon electrode, a layer of hydrogel transmits electrons that travel to and fro between the cathode and the metal surface. Practically any metal surface that comes in contact with the hydrogel electrolyte will function as the anode of a battery, causing electrons to flow to the cathode which subsequently powers the connected device.