Power/Performance Bits: August 13

Graphene-based device enables next-gen energy storage
Monash University researchers have brought next generation energy storage closer with an engineering first: a graphene-based device that is compact, yet lasts as long as a conventional battery.

A research team in the Department of Materials Engineering has developed a completely new strategy to engineer graphene-based supercapacitors (SC), making them viable for widespread use in renewable energy storage, portable electronics and electric vehicles.

SCs are generally made of highly porous carbon impregnated with a liquid electrolyte to transport the electrical charge. Known for their almost indefinite lifespan and the ability to re-charge in seconds, the drawback of existing SCs is their low energy-storage-to-volume ratio – known as energy density. Low energy density of five to eight Watt-hours per litre, means SCs are unfeasibly large or must be re-charged frequently.

The Monash researchers have created an SC with energy density of 60 Watt-hours per litre – comparable to lead-acid batteries and around 12 times higher than commercially available SCs.

Graphene, which is formed when graphite is broken down into layers one atom thick, is very strong, chemically stable and an excellent conductor of electricity. To make their uniquely compact electrode, the team exploited an adaptive graphene gel film they had developed previously. They used liquid electrolytes – generally the conductor in traditional SCs – to control the spacing between graphene sheets on the sub-nanometre scale. In this way the liquid electrolyte played a dual role: maintaining the minute space between the graphene sheets and conducting electricity.

To create their material, the research team used a method similar to that used in traditional paper making, meaning the process could be easily and cost-effectively scaled up for industrial use.

Squeezed light allows precise measurements at lower power levels
One of the many counterintuitive and bizarre insights of quantum mechanics is that even in a vacuum, all is not completely still. Low levels of noise, known as quantum fluctuations, are always present. Always, that is, unless you can pull off a quantum trick. And that’s just what a team led by researchers at the California Institute of Technology (Caltech) has done. The group has engineered a miniature silicon system that produces a type of light that is quieter at certain frequencies—meaning it has fewer quantum fluctuations—than what is usually present in a vacuum.

This special type of light with fewer fluctuations is known as squeezed light and is useful for making precise measurements at lower power levels than are required when using normal light. Although other research groups previously have produced squeezed light, the Caltech team’s new system, which is miniaturized on a silicon microchip, generates the ultraquiet light in a way that can be more easily adapted to a variety of sensor applications.

This system should enable a new set of precision microsensors capable of beating standard limits set by quantum mechanics, the researchers noted, as the experiment brings together, in a tiny microchip package, many aspects of work that has been done in quantum optics and precision measurement over the last 40 years.

In the past, squeezed light has been made using so-called nonlinear materials, which have unusual optical properties. This latest Caltech work marks the first time that squeezed light has been produced using silicon, a standard material.

(a) SEM image of the silicon micromechanical resonator used to generate squeezed light. Light is coupled into the device using a narrow waveguide and reflects off a back mirror formed by a linear array of etched holes. Upon reflection, the light interacts with a pair of double-nanobeams (micromechanical resonator/optical cavity), which are deflected in a way that tends to cancel fluctuations in the light. (b) Numerical model of the differential in-plane motion of the nanobeams.
(Source: Caltech)

They believe this new way of ‘squeezing light’ in a silicon micro-device may provide new, significant applications in sensor technology.

~Ann Steffora Mutschler