Power/Performance Bits: Oct. 29

Supercapacitor for electricity storage
With the potential for solar cells that produce electricity 24/7 and mobile phones with built-in power cells that recharge in seconds and work for weeks between charges, researchers at Vanderbilt University have created a novel supercapacitor design with these and other applications in mind.

They believe it is the first supercapacitor made out of silicon so it can be built into a silicon chip along with the microelectronic circuitry that it powers and that it should be possible to construct these power cells out of the excess silicon that exists in the current generation of solar cells, sensors, mobile phones and a variety of other electromechanical devices, providing a considerable cost savings.

Instead of storing energy in chemical reactions the way batteries do, “supercaps” store electricity by assembling ions on the surface of a porous material. As a result, they tend to charge and discharge in minutes, instead of hours, and operate for a few million cycles, instead of a few thousand cycles like batteries.

It is these properties that have enabled the manufacture of commercial supercapacitors, made out of activated carbon, to capture a few niche markets such as storing energy captured by regenerative braking systems on buses and electric vehicles and to providing the bursts of power required to adjust of the blades of giant wind turbines to changing wind conditions. However, supercapacitors still lag behind the electrical energy storage capability of lithium-ion batteries and are too bulky to power most consumer devices but the researchers said they’ve been catching up rapidly.

Research to improve the energy density of supercapacitors has focused on carbon-based nanomaterials like graphene and nanotubes and since these devices store electrical charge on the surface of their electrodes, the way to increase their energy density is to increase the electrodes’ surface area, which means making surfaces filled with nanoscale ridges and pores.

The big challenge for this approach is assembling the materials, the researchers noted, and said that constructing high-performance, functional devices out of nanoscale building blocks with any level of control has proven to be quite challenging, and when it is achieved it is difficult to repeat.

Transmission electron microscope image of the surface of porous silicon coated with graphene. The coating consists of a thin layer of 5-10 layers of graphene which filled pores with diameters less than 2-3 nanometers and so did not alter the nanoscale architecture of the underlying silicon. (Source: Vanderbilt University)

As such, the team took a radically different approach and used porous silicon – a material with a controllable and well-defined nanostructure made by electrochemically etching the surface of a silicon wafer – which allowed them to create surfaces with optimal nanostructures for supercapacitor electrodes. This also left them with a major problem: silicon is generally considered unsuitable for use in supercapacitors because it reacts readily with some of chemicals in the electrolytes that provide the ions that store the electrical charge.

Nanopetals for sensors, batteries
Purdue University researchers are developing a method to mass-produce a new type of nanomaterial for advanced sensors and batteries as their research findings indicate the material shows promise as a sensor for detecting glucose in the saliva or tears and for “supercapacitors” that could make possible fast-charging, high-performance batteries.

However, for the material to be commercialized researchers must find a way to mass-produce it at low cost.

The researchers pointed out that it’s one thing to say you’ve got a new wonder material, but it’s another to prove that it can be made on a commercial scale. In many cases they find that fundamental research needs to be done for scaling up and want to be able to produce large quantities of the material at 50 cents per square meter.

As such, a team of Purdue researchers is aiming to do just that as part of a project that focuses on creating a nanomanufacturing method that is scalable.

The underlying technology consists of vertical nanostructures resembling tiny rose petals made of a material called graphene: a single-atom-thick film of carbon. Using these graphene nanopetals the researchers have realized exceptional performance in a wide range of devices at laboratory scales and now they hope to boost the production speed of nanopetal-coated surfaces to 10 square meters per hour, which would be a dramatic increase over the laboratory-scale production rate.

These color-enhanced scanning electron microscope images show nanosheets resembling tiny rose petals. The nanosheets are key components of a new type of biosensor that can detect minute concentrations of glucose in certain bodily fluids. The technology might eventually help to eliminate or reduce the frequency of using pinpricks for diabetes testing. (Source: Purdue University)

The graphene nanopetals also have shown promise as a thermal-interface material to keep computer chips from overheating with a slew of new device and material concepts based on graphene nanopetals emerging in applications as diverse as carbon fiber composites and new thermal-interface materials.