It doesn't sound like a huge number, but 6.3 milliwatt-hours per cubic mm is a breakthrough: it's the highest volumetric energy density so far achieved in a microscale carbon-based supercapacitor.
Such devices are keenly sought in electronics research to drive the growing wearables market, since battery life is a big issue among glassholes and fitness-tracker owners alike. The right supercaps would be a boon, offering decent battery life and faster charging than the ubiquitous Li-ion battery.
In a paper at Nature Nanotechnology (abstract here), researchers from Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University in the United States describe their supercapacitor as: “a hierarchically structured carbon microfibre made of an interconnected network of aligned single-walled carbon nanotubes with interposed nitrogen-doped reduced graphene oxide sheets.”
Apart from energy density, what's really excited the group is that they've created a scalable process to produce their materials. They say they've produced their carbon microfibres in 50 metre lengths, and see no serious limit to scalability.
The challenge for supercapacitors is that you need to offer a large surface area for carrying charges. A conventional capacitor is a simple creature indeed: charge-carrying foil plates, separated by a dielectric. That kind of cap' can deliver high current, but only for a short time – to store more charge, a supercapacitor has to squeeze a much higher surface area in a similar volume, while still keeping the positive and negative plates separated.
The group, led by NTU professor Yuan Chen, created a setup in which a solution containing single-wall nanotubes, graphene oxide, and ethylenediamine is pumped through a capillary column and heated in an oven for six hours.
The process causes sheets of graphene and carbon nanotubes to self-assemble into a network that runs along the length of the fibre. The resulting material, essentially a long fibrous capacitor, presents 396m2 of surface area per gram of fibre, giving the material its high capacity.
It can also be woven, which opens up applications like “smart clothing” or, more prosaically, to power medical devices, and the group claims good performance over 10,000 charge cycles.
The group says the 6.3 milliwatt-hour per cubic mm result is comparable to a 4V, 500 micro-amp-hour lithium thin film battery. ®