What if your clothes could charge your mobile phone, fitness tracker and other devices using power harvested from your movements? Mundane tasks such as walking or sitting down could power your mobile devices.

At team from Vanderbilt University's Nanomaterials and Energy Devices Laboratory has developed an ultra-thin energy harvesting system that has the potential to do just that. The harvesting system is based on battery technology and made from layers of black phosphorus that are only a few atoms thick. It generates small amounts of electricity when it is bent or pressed even at the extremely low frequencies characteristic of human motion.

20170731_VU_NT1a Figure 1: Transmission electron microscope image showing the ultrathin layers of black phosphorus used in the energy harvesting device. An angstrom (Å) is about the width of a single atom and is one tenth of a nanometre (nm). (Source: Nanomaterials and Energy Devices Laboratory / Vanderbilt)

“In the future, I expect that we will all become charging depots for our personal devices by pulling energy directly from our motions and the environment,” said Cary Pint, Assistant Professor of Mechanical Engineering who directed the research.

The new energy harvesting system is described in a paper titled “Ultralow Frequency Electrochemical Mechanical Strain Energy Harvester using 2D Black Phosphorus Nanosheets” published online by ACS Energy Letters.

“This is timely and exciting research given the growth of wearable devices such as exoskeletons and smart clothing, which could potentially benefit from Dr. Pint’s advances in materials and energy harvesting,” said Karl Zelik, assistant professor of mechanical and biomedical engineering at Vanderbilt, an expert on the biomechanics of locomotion. Zelik is not part of the team who developed the harvesting device.

Currently, there is a tremendous amount of research aimed at discovering effective ways to tap ambient energy sources. These include mechanical devices designed to extract energy from vibrations and deformations; thermal devices aimed at pulling energy from temperature variations; radiant energy devices that capture energy from light, radio waves and other forms of radiation; and electrochemical devices that tap biochemical reactions.

“Compared to the other approaches designed to harvest energy from human motion, our method has two fundamental advantages,” said Pint. “The materials are atomically thin and small enough to be impregnated into textiles without affecting the fabric’s look or feel and it can extract energy from movements that are slower than 10Hz—10 cycles per second—over the whole low-frequency window of movements corresponding to human motion.”

Doctoral students Nitin Muralidharan and Mengya Li co-led the effort to make and test the devices. “When you look at Usain Bolt, you see the fastest man on Earth. When I look at him, I see a machine working at 5Hz,” said Muralidharan.

Extracting usable energy from such low frequency motion has proven to be extremely challenging. For example, a number of research groups are developing energy harvesters based on piezoelectric materials that convert mechanical strain into electricity. However, these materials often work best at frequencies of more than 100Hz. This means that they don’t work for more than a tiny fraction of any human movement so they achieve limited efficiencies of less than 5-10% even under optimal conditions.

“Our harvester is calculated to operate at over 25% efficiency in an ideal device configuration, and most importantly harvest energy through the whole duration of even slow human motions, such as sitting or standing,” Pint said.

The Vanderbilt lab’s ultrathin energy harvester is based on the group’s research on advanced battery systems. Over the past 3 years, the team has explored the fundamental response of battery materials to bending and stretching. They were the first to demonstrate experimentally that the operating voltage changes when battery materials are placed under stress. Under tension, the voltage rises and under compression, it drops.

20170731_VU_NT1b Figure 2: Mengya Li and Nitin Muralidharan adjust the energy harvesting device on the arm of Thomas Metke while Professor Cary Pint looks on. (Source: John Russell / Vanderbilt)

The team collaborated with Greg Walker, associate professor of mechanical engineering, who used computer models to validate these observations for lithium battery materials. These observations led Pint’s team to reconstruct the battery with both positive and negative electrodes made from the same material. Although this prevents the device from storing energy, it allows it to fully exploit the voltage changes caused by bending and twisting and so produce significant amounts of electrical current in response to human motions.

The lab’s initial studies were published in 2016. They were further inspired by a parallel breakthrough by a group at Massachusetts Institute of Technology who produced a postage-stamp-sized device out of silicon and lithium that harvested energy via the effect Pint and his team were investigating.

In response, the Vanderbilt researchers decided to go as thin as possible by using black phosphorus nanosheets, a material that has become the latest darling of the 2D materials research community because of its attractive electrical, optical and electrochemical properties.

20170731_VU_NT1c Figure 3: Graph showing the operating ranges of different types of energy harvesting devices. The red stars denote piezoelectric devices that use crystals that produce electricity when deformed. The blue circle represents another solid-state device called an ionic diode that generates electricity when compressed. The orange triangles depict triboelectric nanogenerators that produce electricity by sliding friction. The purple circles show the performance of the ultrathin strain harvester developed at Vanderbilt. (Source: Nanomaterials and Energy Devices Laboratory / Vanderbilt)

Because the basic building blocks of the harvester are about 1/5000th the thickness of a human hair, the engineers can make their devices as thin or as thick as needed for specific applications. They have found that bending their prototype devices produces as much as 40microwatts per square foot and can sustain current generation over the full duration of movements as slow as 0.01Hz, one cycle every 100 seconds.

The researchers acknowledge that one of the challenges they face is the relatively low voltage that their device produces. It’s in the millivolt range. However, they are applying their fundamental insights of the process to step up the voltage. They are also exploring the design of electrical components, like LCD displays, that operate at lower than normal voltages.

“One of the peer reviewers for our paper raised the question of safety,” Pint said. “That isn’t a problem here. Batteries usually catch on fire when the positive and negative electrodes are shorted, which ignites the electrolyte. Because our harvester has two identical electrodes, shorting it will do nothing more than inhibit the device from harvesting energy. It is true that our prototype will catch on fire if you put it under a blowtorch but we can eliminate even this concern by using a solid-state electrolyte.”

One of the more futuristic applications of this technology might be electrified clothing. It could power clothes impregnated with liquid crystal displays that allow wearers to change colours and patterns with a swipe on their smartphone. “We are already measuring performance within the ballpark for the power requirement for a medium-sized low-power LCD display when scaling the performance to thickness and areas of the clothes we wear.” Pint said.

Pint also believes there are potential applications for their device beyond power systems. “When incorporated into clothing, our device can translate human motion into an electrical signal with high sensitivity that could provide a historical record of our movements. Or clothes that track our motions in three dimensions could be integrated with virtual reality technology. There are many directions that this could go.”

Vanderbilt doctoral students Rachel Carter, currently a postdoctoral researcher at the Naval Research Laboratory, and Nicholas Galioto, an undergraduate mechanical engineering student also contributed to the research, which was supported by National Science Foundation grant CMMI 1400424 and Vanderbilt University’s discovery grant program.