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 — they can only operate in certain conditions and can’t turn that energy into useful power very quickly.
Researchers are bridging the gap between these two fundamental technologies for the first time in the form of a metal-air scavenger (MAS) that gets the best of both worlds. The MAS 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 MAS. 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.
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 shrinks, 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; for example, there are robots the size of insects that can only operate for a minute before their battery runs out of energy.
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 often used to power sensors and electronics that are off the grid or where batteries cannot be changed easily. They have low power density, meaning they can’t take energy out of the environment as fast as a battery can deliver it.
Like a traditional battery, the MAS starts with a cathode 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.
To demonstrate the technology, 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. The MAS vehicle drove 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. The MAS vehicles also were tested 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.