TENGs exploit the triboelectric effect, a phenomenon in which a charge Electricity builds up in a material after it has been separated from another material it was in contact with. It is what for example is responsible for the static charge that occurs when combing your hair.
credits: Dalian Maritime University In this particular device, a thin layer of porous spongy material is sandwiched between a couple of 38 by 76 mm made of two different polymers. These strips are both coated with a conductive ink, with the sponge creating a link between them. The whole is sealed with waterproof tape.
When the resulting TENG bends back and forth even in a relatively weak underwater current, the two polymer strips squeeze across the sponge to intermittently enter and exit contact the with each other, generating an electric current in the process. In wave tank tests, it was shown that multiple TENGs could be used to continuously power devices such as marine environmental sensors, eliminating the need for battery changes.