“An energy-harvesting circuit based on graphene could be incorporated into a chip to provide clean, limitless, low-voltage power for small devices or sensors,” said Paul Thibado, professor of physics and lead researcher in the discovery.
The findings, published in the journal Physical Review E, are evidence of a concept that the physicists developed in the U of A 3 years ago that freestanding graphene — a single layer of electrons — ripples and buckles in a way that holds promise for energy harvesting.
The thought of harvesting energy from graphene is controversial since it refutes physicist Richard Feynman’s well-known assertion that the thermal motion of atoms, known as Brownian motion, can’t do work. Thibado’s team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an accomplishment thought to be impossible.
In the 1950s, physicist Léon Brillouin released a landmark paper refuting the idea that adding one diode, a one-way electric gate, to a circuit would be the solution to harvesting energy from Brownian motion. Recognizing that, Thibado’s group assembled their circuit using two diodes for converting AC into a direct current (DC). With the diodes in resistance allowing the current to flow either way, they provide separate paths through the circuit, so making a pulsing DC present that performs work on a load resistor.
Additionally, they discovered their design increased the amount of power delivered. “We also found that the on-off, switch-like behavior of the diodes actually amplifies the power delivered, rather than reducing it, as previously thought,” said Thibado. “The rate of change in resistance provided by the diodes adds an extra factor to the power.”
The team used a relatively new discipline of physics to prove that the graphene diodes improved the circuit’s energy. “In proving this power enhancement, we drew from the emergent field of stochastic thermodynamics and extended the nearly century-old, celebrated theory of Nyquist,” said coauthor Pradeep Kumar, associate professor of physics and coauthor.
According to Kumar, the graphene and circuit share a symbiotic relationship. Though the thermal environment is doing work on the loading resistor, the graphene and circuit are in precisely the same temperature and heat does not flow between the two.
That’s an important distinction, said Thibado, because a temperature difference between the graphene and circuit, in a circuit producing power, would contradict the second law of thermodynamics. “This means that the second law of thermodynamics is not violated, nor is there any need to argue that ‘Maxwell’s Demon’ is separating hot and cold electrons,” Thibado said.
The team also found that the relatively slow movement of graphene causes current in the circuit at low frequencies, which can be significant from a technological standpoint because electronics work more efficiently at lower frequencies.
“People may think that current flowing in a resistor causes it to heat up, but the Brownian current does not. In fact, if no current was flowing, the resistor would cool down,” Thibado explained. “What we did was reroute the current in the circuit and transform it into something useful.”
The team’s next objective is to determine if the DC current could be stored in a capacitor for later use, a target that requires miniaturizing the circuit and patterning it onto a silicon wafer, or chip. If countless very small circuits could be constructed onto a 1-millimeter from 1-millimeter chip, they could function as a low-power battery replacement.
The University of Arkansas holds many patents pending at the U.S. And global markets on the technology and has licensed it for industrial applications throughout the university’s Technology Ventures branch. Researchers Surendra Singh, University Professor of physics; Hugh Churchill, associate professor of mathematics and Jeff Dix, assistant professor of engineering, worked on the graphene project, which was funded by the Chancellor’s Commercialization Fund supported by the Walton Family Charitable Support Foundation.
Related Journal Article: https://journals.aps.org/pre/abstract/10.1103/PhysRevE.102.042101