"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 lead researchers Paul Thibado, a professor of physics and lead researcher in the discovery at UArk.
The findings published in the journal Physical Review E drew on previous works from the same UArk lab. The research showed that freestanding graphene rippled and buckled in a way that could be harvested for energy.
The researchers harnessed both the nanometer-sized rippling and the Brownian motion – the thermal motion of atoms – found in graphene to produce an electric current that could be put to a variety of uses.
"The origin of these nanometer-sized ripples is still an open question," the team wrote in their study. They noted that the graphene ripple seems to stem from the interaction of subatomic particles in the material.
The idea itself of harvesting energy from graphene is controversial as it refutes physicist Richard Feynman's well-known assertion that Brownian motion couldn't do work. But Thibado's team found that the thermal motion of graphene does in fact induce an alternating current (AC) at room temperature.
To capture the energy, the team used two diodes in the circuit to convert the AC current from the graphene into direct current (DC). This allowed the current to flow both ways along separate paths through the circuit.
This resulted in a pulsing DC current that could perform work on a load resistor – a current that could potentially power small electronic devices. In addition, the two-diode design also boosts the power delivered by the system.
"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," Thibado said. "The rate of change in resistance provided by the diodes adds an extra factor to the power."
The research also shows a symbiotic set-up between the graphene and the circuit which avoids conflict with the second law of thermodynamics, maintaining one uniform temperature so that heat isn't transferred. In other words, the current flowing through the resistor does not heat it up. (Related: Scientists generate novel form of magnetism from graphene.)
In addition, the slow motion of the graphene only induced a low-frequency current in the circuit, which is important for efficiency – electronics function much more efficiently at low frequencies.
This efficiency and low heat generation could make graphene an ideal method for powering small electronic devices in lieu of batteries in the future, but more research is needed to realize it.
For their next step, Thibado's team is looking to find out if the direct current from the circuit could be stored on a capacitor for later use. For this to happen, the circuit should be miniaturized so that it could be printed on a silicon wafer or chip.
If the team succeeds in doing this, millions of these tiny graphene circuits could be built on a 1-millimeter square chip that could then replace batteries in certain, low-power applications and devices.
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