Structural supercapacitors take a load on

Stress and squeeze: the new structural supercapacitor design

A solid-state supercapacitor that works under great stresses and vibrations has been developed by researchers from the US. Unlike traditional supercapacitors, the new design does not delaminate under stress and could lead to a variety of practical applications, from more-efficient devices to renewable-energy storage.

Unlike batteries – which work through chemical reactions – supercapacitors store energy in the form of electrically charged ions, which are assembled on the surfaces of porous electrodes. Supercapacitors also have numerous benefits: they can charge and discharge in minutes – unlike batteries, which take hours – and have a much longer lifespan, lasting for millions of cycles rather than thousands. Their drawback, however, comes in their reduced storage capacity – to hold a given charge, most supercapacitors need to be much larger and heavier than an equivalent lithium-ion battery.

Heavy-weight storage

One idea to overcome this limitation lies in creating supercapacitors that act as both energy storage and structural support. By doubling up the otherwise "dead weight" of structural materials, the rapid-charging, long-lasting nature of supercapacitors could be utilized without needing an internally distinct power source. This concept of a "structural supercapacitor" could have many potential applications – for example, a laptop where the case acts as a battery, renewable energy stored within the walls of a house, or even a rapidly charging electric car that stores power in its own chassis. To be used as such a structural device, the supercapacitor would need to work under considerable stresses and vibrations. Traditional supercapacitors are ill-suited to this task – being layered, their electrodes and electrolytes are prone to separate when such forces are applied.

Strong bonds

Load bearing

To overcome this problem, a team of researchers, led by Cary Pintfrom Vanderbilt University in Tennessee, has engineered a supercapacitor with better-integrated layers. The design features electrodes made from silicon wafers, the inner sides of which are electrochemically etched to create a surface covered in nano-sized pores. These are then coated with a protective, ultra-thin layer of carbon, before being vacuum-squeezed together around an ion-conducting polymer. This electrolyte seeps into the silicon's nanopores, setting into a strong mechanical bond, which does not come apart easily.

When tested, the researchers found that the design offers an energy density of up to 10 W h/kg and operates perfectly, even under stresses of 44 psi and vibrational accelerations over 80 g – the latter of which is greater than the forces acting within working jet engines. While designs for structural supercapacitors have been reported previously (mostly based around the use of carbon fibres) the team's design is capable of 3 to 4 orders of magnitude more charge storage – comparable, in fact, with (regular) commercial supercapacitors. Pint points out that the design showed that its "performance is not compromised while we operate the device under compression, shearing, tensile stretching, high-amplitude vibrations, and impact forces". He adds that the manufacturing process is simpler than in traditional supercapacitors and involves minimal cost, with both the component materials and the production process being relatively cheap. Furthermore, the constituent parts are both bio-friendly and non-flammable, thereby removing some of the safety concerns traditionally associated with lithium-ion batteries.

"The study is also a nice contribution to the larger area of research on mechanically robust electrodes for electrochemical energy-storage systems," says Vadym Mochalin, a nanomaterial expert at Drexel University in Philadelphia, who was not involved in the research. "[This] will likely inspire similar designs for lithium-ion batteries, micro-supercapacitors on chip, sensors, and other devices."

The research is described in Nano Letters.

About the author

Ian Randall is a science writer based in New Zealand

http://physicsworld.com/

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105-bit optical memory built on a chip

Two fully functional optical memories on single chips have been fabricated by researchers in Japan. The devices use bistable optical cavities to store the bits, and allow multiple bits to be controlled simultaneously by the same waveguide. The researchers hope that, in future, such a memory could be used for optical logic operations to increase the speed of computation.

Today, optical fibres are the material of choice for transmitting data, thanks to their lower signal attenuation compared with copper wires and their much higher bandwidth. Currently, however, optical signals have to be converted into electronic ones for processing, and then once more, to convert the output back to an optical signal. Such conversions consume energy and time, and fail to utilize the biggest advantage of optical transmission – that photons do not interfere with each other, meaning that several signals with different frequencies can travel down one fibre simultaneously in a process known as "multiplexing". Photonic signals have to be "demultiplexed" before an electronic processor can deal with them, and so optical processors are of interest to many researchers.

Random memories

A key element in any processing unit is the random access memory (RAM), in which data are stored temporarily while the computer runs a program. A modern electronic RAM usually stores each bit of memory as the charge on a capacitor, and various optical equivalents have been proposed. In 2012 Masaya Notomi and colleagues at NTT Laboratories in Kanagawa, Japan, designed a four-bit RAM made from a photonic crystal – a periodic optical nanostructure comprising a network of holes that allows some wavelengths of light to propagate while blocking others. Inside the photonic crystal were four identical cavities that had two possible refractive indices – a pulse of light at the cavity's resonant frequency would allow a switch between the two indices, while light at a different frequency would reveal the cavity's state without disturbing it. By designating the two states as 0 and 1, the researchers created a readable and rewritable memory. However, each of the cavities had to be controlled by a separate waveguide.

Now, the same researchers have made the cavities much smaller and non-identical, allowing them to introduce multiplexing. They created two different types of optical RAM – one made from silicon and the other from indium phosphide and indium gallium arsenide phosphide. In each RAM, multiple cavities were arranged lengthways, with a single waveguide passing all of them. The researchers used computer modelling to work out exactly how to move specific holes in the photonic crystal such that each cavity had a slightly different resonant frequency. They were then able to send a "write" pulse down the waveguide containing the frequencies of whichever bits they wanted to switch and only those cavities would respond.

Stable lifetimes?

The silicon RAM contained 105 working cavities, with all the resonant wavelengths falling between 1540 nm and 1570 nm, at an average spacing of just 0.23 nm, all of which was fabricated on a silicon crystal just 1 mm long. Unfortunately, the cavity states were stable for less than 10 ns – too short for a viable optical memory. However, the lifetime of the bits in the indium-phosphide-based RAM was, in principle, infinite. Because indium phosphides are less well established in industry than silicon, the technology for manufacturing indium-phosphide components is less precise, and so Notomi and colleagues could only produce a 28-bit memory. However, they believe this provides a better blueprint for future research. "Our final goal is to produce better indium-phosphide systems by improving the fabrication accuracy," says Notomi.

Martin Hill of the University of Western Australia in Crawley describes the paper as "a nice piece of work on a difficult area of photonics". But he also points out that, at present, the switching speed of the optical cavities is lower than the switching speed of electrical transistors, and says that before the device becomes useful as a product, the researchers need a way of making the switching frequencies more predictable and reproducible.

The research is published in Nature Photonics.

About the author

Tim Wogan is a science writer based in the UK

http://physicsworld.com/

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