Rusty film generates salty shock

Salty water flowing over metal nanolayers can generate tiny pulses of electricity, an effect that could be exploited by energy-harvesting devices (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1906601116). In a study, researchers used vapor deposition to lay 10 nm thick films of iron, nickel, or vanadium on glass slides and flexible plastics. Exposed to air, the metals developed an oxide coating just a few nanometers deep. The team then passed water containing varying amounts of sodium chloride over the surface, alternating between low and high salinity. Each change in salt concentration generated a spike of electricity, measuring tens of millivolts and several microamperes per square centimeter. Faster flows or larger changes in salinity generated more current. “I expected nanoamps or picoamps . . . then boom, microamps. Unbelievable!” says Franz M. Geiger of Northwestern University, who was part of the team. The researchers think that at high salinity, sodium ions bind to exposed metal oxide groups in the film. When lower-salinity water flows over the sodium ions, they pop off and change the strength of the electric field. This triggers charge transfer between metal ions in the oxide layer, which mobilizes electrons in the underlying metal. Similar effects have been seen in graphene, but the metal films are much cheaper and easier to fabricate at large scale. Geiger suggests that they could be used to harvest energy from wastewater flows or rainwater runoff.