Energy harvesting circuit captures graphene’s thermal motion

Technology News | October 8, 2020

By Rich Pell

Power Management Analog Sensing / Conditioning Batteries / Power Supplies Energy Harvesting Materials & processes

The discovery, say the researchers, is proof of a theory they developed three years ago that freestanding graphene – a single layer of carbon atoms – ripples and buckles in a way that holds promise for energy harvesting.

“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,” says Paul Thibado, professor of physics and lead researcher in the discovery.

The idea of harvesting energy from graphene is controversial, say the researchers, because it refutes physicist Richard Feynman’s well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. Instead, the researchers found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit – an achievement thought to be impossible.

In the 1950s, physicist Léon Brillouin published a landmark paper refuting the idea that adding a single diode – a one-way electrical gate – to a circuit is the solution to harvesting energy from Brownian motion. Knowing this, the researchers built their circuit with two diodes for converting AC into a direct current (DC). With the diodes in opposition allowing the current to flow both ways, they provide separate paths through the circuit, producing a pulsing DC current that performs work on a load resistor.

In addition, they say, they discovered that 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,” says Thibado. “The rate of change in resistance provided by the diodes adds an extra factor to the power.”

To prove the diodes increased the circuit’s power, the researchers used a relatively new field of physics.

“In proving this power enhancement,” says coauthor of a paper of the work Pradeep Kumar, associate professor of physics and coauthor, “we drew from the emergent field of stochastic thermodynamics and extended the nearly century-old, celebrated theory of Nyquist.”

The graphene and circuit, says Kumar, share a symbiotic relationship. Though the thermal environment is performing work on the load resistor, the graphene and circuit are at the same temperature and heat does not flow between the two.

That’s an important distinction, say the researchers, 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,” says Thibado.

The researchers also discovered that the relatively slow motion of graphene induces current in the circuit at low frequencies, which is important from a technological perspective because electronics function 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,” says Thibado. “In fact, if no current was flowing, the resistor would cool down. What we did was reroute the current in the circuit and transform it into something useful.”

The researchers next plan to determine if the DC current can be stored in a capacitor for later use – a goal that requires miniaturizing the circuit and patterning it on a silicon wafer, or chip. If millions of these tiny circuits could be built on a 1-millimeter by 1-millimeter chip, say the researchers, they could serve as a low-power battery replacement.

For more, see “Fluctuation-induced current from freestanding graphene.”