Power/Performance Bits: Nov. 23

Graphene energy
Researchers from the University of Arkansas, University of Pennsylvania, and Universidad Carlos III de Madrid built a circuit capable of capturing graphene’s thermal motion and converting it into an electrical current.

“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 at University of Arkansas.

In previous research, it was found that freestanding graphene ripples and buckles in a way that holds promise for energy harvesting. However, it came up against Richard Feynman’s assertion that the thermal motion of atoms, Brownian motion, cannot do work. The team found that at room temperature the thermal motion of graphene does in fact induce an alternating current in a circuit, an achievement thought to be impossible.

The group 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, 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,” 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 field of physics to prove the diodes increased the circuit’s power. “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 Pradeep Kumar, associate professor of physics at University of Arkansas.

According to Kumar, the graphene and circuit 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, 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 discovered that the relatively slow motion of graphene induces current in the circuit at low 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 can be stored in a capacitor for later use, a goal that requires miniaturizing the circuit and patterning it on a silicon wafer. If millions of these tiny circuits could be built on a 1-millimeter by 1-millimeter chip, they could serve as a low-power battery replacement, according to the researchers.

Vanillin grid batteries
Researchers at the Graz University of Technology and Montanuniversitaet Leoben developed a way to make grid-scale batteries using vanillin, the common flavor compound in vanilla.

The liquid electrolyte of redox-flow batteries is typically comprised of heavy metals or rare earth elements. But using vanillin, which can be refined from paper-making waste product lignin, “is ground-breaking in the field of sustainable energy storage technology,” said Stefan Spirk from the Institute of Bioproducts and Paper Technology at Graz University of Technology.

The process is highly scalable and suitable for continuous production, said Spirk. “The plan is to hook up our plant to a pulp mill and isolate the vanillin from the lignin that is left over as waste. Whatever is not needed can subsequently flow back into the regular cycle and be used energetically as usual. We are in concrete talks with Mondi AG, a leading global manufacturer of paper-based products, which is showing great interest in the technology.”

The company is now looking for energy supply companies that can integrate the start-up’s redox flow technology into its infrastructure. Spirk added, “We can keep the value chain ranging from the procurement of raw materials and components to the generation of electricity on a regional basis, enable storage capacities of up to hundreds of MWh, relieve the strain on the electricity grid and make an important contribution to the green energy storage.”