Yucheng Luan at start-up firm East Eight Energy in China, Li and their colleagues created an energy-harvesting device just 1-centimetre square, which consists of a top and bottom electrode with several 25-nanometre-wide strands of zinc oxide attached to each. The material was chosen for its ability to generate electrical charge under mechanical deformation.
“We thought it would be interesting and meaningful to see if this motion can be harvested and converted into electricity,” says Wei Li at Nankai University in China.
A miniature generator can convert the movements of molecules in room temperature liquid into electricity. The device could one day be used to power devices like medical implants or even small household appliances.
The device looks a little like two toothbrushes with bristles facing each other, says Luan.
The team submerged the device into a container of a liquid called n-octane – a hydrocarbon like propane or butane only with a longer chain of carbons and hydrogens – at room temperature. The molecules in the liquid bumped into the miniature strands of zinc oxide, says Luan, and the device generated a small voltage of 2.28 millivolts and a 2.47 nanoampere current.
The researchers hope the device could be used to supply energy for nanotechnologies, such as tiny implants for drug delivery. They also want to scale up the technology to be able to power larger things.
“We hope people can use it at home. For example, to charge our phones or TV,” says Luan.
“[This demonstrates] that the mechanical energy arising from the thermal motion of the n-octane liquid molecules could be converted into constant electrical power at room temperature without the need of any other energy source,” says João Ventura at the University of Porto in Portugal. Although it generates only low voltages for now, it could lead to a clean and ubiquitous energy source for low-power devices, he says.
Molecular thermal motion harvester for electricity conversion
Molecular thermal motion is a special kind of dynamic motion14 that is essentially different from ordinary mechanical motion. It is a component of the internal energy of the physical system, which means that the molecules of all substances are in constant and random movement above absolute zero temperature. Brownian motion of particles is one example that is caused by the molecule thermal motion of the surrounding liquid or gaseous molecules. Molecule thermal motion contains an enormous amount of energy, taking an ideal gas as an example, the average kinetic energy of thermal motion per mole of gas molecules at room temperature (27 °C) is 3.7 kJ.14 If this form of energy could be utilized from the huge amounts of liquids and gases on the planet effectively, this would provide a new source of energy on an enormous scale. However, to our knowledge, the energy of the liquid molecular thermal motion has never been utilized as an energy source to date, perhaps due to the challenge of creating a viable device that can convert this energy to form electricity. In this work, we demonstrated that it was feasible to convert the energy of molecular thermal motion into electrical energy by using a molecular thermal motion harvester (MTMH). Because of the random collision of liquid molecules, small and slim objects15,16 such as nano-tubes, nano-wires, nano-rods, and nano-sheets with one-ends attached to a wall or surface will undergo worm-like Brownian motion such as bending, flexing, or wriggling, when suspended in the liquid instead of moving around as a whole due to the geometry constraint.17 The factors influencing Brownian motion are the friction coefficient (interfacial tension), the viscous drag force from the surrounding fluid flow and temperature, etc.17 Recently, the Brownian motion of tethered nanowires in a liquid has been reported.17,18 If these nanowires are made of a material that possesses piezoelectric properties, bending or deforming these nanowires will lead to piezoelectric potential. In an array of piezoelectric nanowires, each wire will develop a piezoelectric potential individually.9 Therefore, by designing a device that can regulate the direction of the output electric current, the thermal motion energy of the surrounding liquid molecules can be collected as electrical energy. Here we developed nanoarray electrodes made of a piezoelectric material that can be bent/deformed/flexed readily by surrounding liquid molecules, demonstrating that the energy of liquid molecule thermal motion can be converted into electricity for the first time. The design of the novel MTMH is based on ZnO and gold-coated ZnO (Au@ZnO) nanoarray electrodes and n-octane liquid, etc. n-octane was chosen to drive MTMH because of its low dielectric constant, low toxicity, suitable boiling point, and low viscosity. The purity of n-octane is electronic grade (≥99.999%) to ensure there are no free-moving ions, which would leak the electricity from the nano-wires. ZnO was chosen as the piezoelectric material because it can form nano-whiskers of different structures such as tubes, sheets, wires, and rods at mild conditions,19 which could be deformed by the surrounding liquid.
Tiny generator uses the motion of molecules to produce electricity
Yucheng Luan at start-up firm East Eight Energy in China, Li and their colleagues created an energy-harvesting device just 1-centimetre square, which consists of a top and bottom electrode with several 25-nanometre-wide strands of zinc oxide attached to each. The material was chosen for its ability to generate electrical charge under mechanical deformation.