Batteries and solar panels are the usual sources of power for mobile devices. Batteries, of course, need recharging and solar panels only work when there's sufficient light. What if a device could harvest energy in a different way, perhaps by absorbing metal ions from an external source. Now, researchers, James Pikul and colleagues, at the University of Pennsylvania have developed a technology that hybridizes the pros of batteries and energy harvesting systems to give them a metal-air scavenger" that works like a battery making and breaking bonds to generate electricity but replenishing itself by tapping into the inherent energy of the chemical bonds between metal ions and the air around the device.

The device has a PTFE-coated platinum on carbon electrode as cathode; PTFE is the synthetic "non-stick" polymer, polytetrafluoroethylene. Underlying this is a hydrogel electrolyte. The device is then place on a metal surface as the anode and it harvests metal ions, oxidizing the surface of the metal, and generating electricity. A robotic device can be self-powered by harvesting the metal ions from an aluminum, zinc, or steel surface on which it sits, leaving a thin oxidized layer in its wake just a few micrometers deep.

The prototype device has a power density ten times higher than any current energy-harvesting technology and thirteen times the energy density of a conventional rechargeable lithium-ion battery. The researchers muse that their device might ultimately be used to power robotic systems that essentially scavenge and "eat" metal from their environment to keep themselves powered up. The inverted relationship between computing performance and energy storage makes it difficult to keep tiny devices such as robots the size of insects powered up. Shrink the battery the power tails off too much so that a device might only function for a minute before running out of power, explains Pikul. A bigger battery does not solve the problem as it simply adds weight and bulk to what was meant to be a tiny device.

"Metal scavenging is especially beneficial for small robots and electronics, whose size and performance are severely limited by the low energies provided by microenergy storage technologies," the team writes. For the time being, the system is being developed to power lights for buildings in the developing world that are not on any electricity and to power long-lasting sensors in shipping containers. [Wang, M. et al. ACS Energy Lett, 2020; 5 (3): 758 DOI: 10.1021/acsenergylett.9b02661]