A device made with sheets of black phosphorus, a two-dimensional semiconducting material, can generate electricity when it is bent or pressed at ultralow frequencies (ACS Energy Lett.2017, DOI: 10.1021/acsenergylett.7b00478). Incorporated into clothing, the paperthin device could produce electricity by harnessing relatively slow motions like walking, bending, or standing up.
Converting movement into electricity could reduce the need for batteries for low-power wearable or portable devices. Some groups have tried to harvest motion using piezoelectric materials, which turn mechanical force into an electric potential. These devices work best at frequencies around 100 Hz, typical of mechanical vibrations and much higher than human motion. Other groups have developed triboelectric generators that convert friction into electricity. Zhong Lin Wang’s group at Georgia Tech, for example, has demonstrated friction-based devices that generate electricity from walking or other body motions with frequencies as slow as 0.1 Hz.
In the new study, mechanical engineering professor Cary L. Pint and his colleagues at Vanderbilt University report a device that can capture movements with frequencies as low as 0.01 Hz—one-hundredth the rate of a beating heart. That opens up the possibility of generating power during slow motions, such as when someone walks, flexes their muscles, or fidgets in their chair. And unlike other approaches, the foil-like device should be easy to integrate into fabric without affecting its look or feel.
The researchers developed their novel energy-harvesting method based on what’s called strain engineering. The premise is that subtly squeezing or stretching semiconducting materials changes their conductivity. Pint and his colleagues reported this year that bending or pressing electrode materials changes their output voltage and the rate at which ions migrate into or out of the material (ACS Nano 2017, DOI: 10.1021/acsnano.7b02404).
They wanted to see if they could harness that ion movement with battery electrodes made of black phosphorus, a 2-D material that is excellent at storing ions for generating current and responds well to strain. The researchers coated two pieces of copper foil with black phosphorus flakes. They loaded the black phosphorus with sodium ions by pressing the foils against sodium metal and applying an electric current. Then they put the two foil electrodes together, black phosphorus sides facing each other, separated by a thin polypropylene film.
Bending or pressing the device compresses one electrode and stretches the other. Sodium ions travel from the compressed region to the stretched one across the separator, causing electrons to flow and creating current.
The researchers tested the device by bending or pressing it for 10 to 100 seconds and then letting go. It can produce 42 nW of power per cm2 and in the tens of millivolts right now, which is higher than what state-of-the-art piezoelectric materials produce at that frequency. But Pint says his team is exploring ways to increase that output for charging applications. They are also testing the energy harvesters on human volunteers. The device could also find uses such as tracking human motion for virtual reality applications, Pint adds.
This is a novel method with great potential, says Daniel Deng of Pacific Northwest National Laboratory, who works on piezoelectric energy harvesting systems. Energy-harvesting devices at low frequencies typically have power outputs in the nanowatt range, just like this one does, so the challenge will be increasing the low power output. Deng says the researchers still have a lot of work to do to optimize the performance of their laboratory prototype and turn it into a practical gadget.
CORRECTION: This story was updated on Aug. 23, 2017, to correct the output voltage of the device and the reference to the group’s previous work.