Vanderbilt University researchers developed an ultra-thin system that can harvest energy from the slightest of human motions — even sitting. Made from materials five thousand times thinner than a human hair, the technology may someday be woven into clothing to power personal devices.

Based on a Battery

The energy harvester is made from building blocks, said Professor Cary Pint, who directed the research.

Very small building blocks.

Pint and his team stitched together layers of two-dimensional black phosphorus nanosheets, each only about a few atoms thick. The result: a device that generates small amounts of electricity when bent or pressed.

Professor Cary Pint looks on as graduate students Mengya Li and Nitin Muralidharan adjust the energy harvesting device on the arm of undergraduate Thomas Metke. (Image Credit: John Russell/Vanderbilt)

A traditional battery contains a cathode and an anode, with a separator containing electrolyte. The Vanderbilt-developed unit puts a unique spin on the conventional battery configuration: The tiny black phosphorus particles of the super-thin materials serve as both electrodes, while retaining the spacer material and electrolyte familiar to a battery.

The components are all sandwiched together like they would be in a battery, but the configuration does not run the safety risk of catching fire like a typical electrochemical cell, according to the mechanical engineering professor.

“Here we're pulling insight from our experience with batteries to design an energy harvesting system that, instead of storing energy, harvests energy,” Pint told Tech Briefs.

Over the past three years, researchers at Vanderbilt University’s Nanomaterials and Energy Devices Laboratory have studied advanced battery systems, specifically the fundamental response of battery materials to bending and stretching. The engineers, including Pint, were the first to demonstrate that a voltage rating changes when mechanical stresses are applied to a cell material, such as the kind used in a lithium-ion battery.

"That was the 'lightbulb' idea that drove us to a strain energy harvester," said Pint.

Putting the Idea in Motion

To create the harvester, the Vanderbilt engineers reconstructed the battery with both positive and negative electrodes made from the same super-thin black phosphorus particles. Such a configuration prevents the device from storing energy, but allows the material to produce significant amounts of electrical current through the bending and twisting of human motion.

The team then validated the working principles of the energy harvesting device with computational models that assessed and predicted the voltage response from a given amount of strain.

The prototype devices produce as much as 40 microwatts per square foot, and sustain current generation over human movements as slow as 0.01 Hertz, one cycle every 100 seconds. The total power output of the prototype device, though low, is enough to power a small LCD screen.

Implementing strain engineering concepts into battery manufacturing will become more viable once batteries adopt new nanostructured materials, said Pint. For energy harvesting applications, however, the gain is more meaningful, given the lack of effective technologies that extract usable energy from low-frequency motion.

Graduate student Kathleen Moyer holds up the ultra-thin energy harvesting device, which can be embedded in fabric. (John Russell / Vanderbilt)

"The reason why this is game-changing for the harvesting systems is because it doesn't just improve existing harvesting methodologies, as strain engineering would do for batteries, and has done for semiconductor electronics,” said Pint. “It enables human motion harvesting, or more generally, wearable energy harvesting technologies to emerge from a technology space where these still remain elusive."

Moving Forward

One exciting possibility for the energy harvester is in the field of diagnostics. A FitBit, for example, gives a limited dimension of information, such as a user's steps. What about a shirt, made from the fiber-like material, that provides a local distribution of current for every movement, sending the precise nature of every motion?

"That's very useful for athletes who need to train to beat Michael Phelps at swimming, or someone who went through an injury and needs real-time 24/7 assistance and monitoring of a certain type of motion for rehabilitation," said Pint.

Pint’s team is currently working on using the system to harvest energy from different biomechanical motions. The next step is to integrate the technology into fabrics. The atomically thin materials can be woven into fiber textiles, extracting energy from a wearer’s movements that could then be used to power one’s personal devices.

"Within the next year or two, I expect that there's no reason why we can't have a multifunctional energy harvesting fabric that you'd make a shirt or jacket out of," said Pint.

What other possibilities do you envision with this kind of energy harvesting. Share your thoughts below.

The energy harvesting system is described in a study titled “Ultralow Frequency Electrochemical Mechanical Strain Energy Harvester using 2D Black Phosphorus Nanosheets,” published on Jul. 21 in the journal ACS Energy Letters.