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. The findings show that freestanding graphene — a single layer of carbon atoms — ripples and buckles in a way that holds promise for energy harvesting.
Researchers found that at room temperature, the thermal motion of graphene induces an alternating current (AC) in a circuit, an achievement thought to be impossible. The team built the 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.
Additionally, the team increased the amount of power delivered. The on-off, switch-like behavior of the diodes amplifies the power delivered, rather than reducing it as previously thought. The rate of change in resistance provided by the diodes adds an extra factor to the power.
The team used a relatively new field of physics to prove the diodes increased the circuit’s power. In proving this power enhancement, they drew from the emergent field of stochastic thermodynamics. 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. A temperature difference between the graphene and circuit, in a circuit producing power, would contradict the second law of thermodynamics. The team also discovered that the relatively slow motion of graphene induces current in the circuit at low frequencies, which is important because electronics function more efficiently at lower frequencies.
The team’s next objective is 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 x 1mm chip, they could serve as a low-power battery replacement.