A group at the University of California, Los Angeles, has created microsupercapacitors using a simple DVD burner to forge the one-atom- thick sheets known as graphene on which these devices are formed, in arrays. Together with a battery, such supercapacitors could run a cellphone for days. And because an array is less than 10 micrometers thick—far finer than a human hair—it is completely flexible. Build these arrays on flexible substrates and they could power a roll-up display.
All these things can be done at low cost. Our fabrication method can easily be scaled up, and our microsupercapacitors can be readily integrated onto silicon chips
New components combine laser-scribed graphene, or LSG — a material that can hold an electrical charge, is very conductive, and charges and recharges very quickly — with manganese dioxide, which is currently used in alkaline batteries because it holds a lot of charge and is cheap and plentiful.
Their hybrid device combines the best features of capacitors and batteries. The hybrid can be recharged in minutes, yet it has an energy density up to 10 times that of commercially available microbatteries. The hybrid device is only one-fifth the thickness of a sheet of paper; its footprint can vary from a few square micrometers up to the centimeter scale. The centimeter-scale devices would have capacitances in the range of 400 to 1,000 millifarads—easily enough to power an LED flashlight for an hour.
The design also sidesteps one of the main challenges in today’s power supplies: leaking electrolyte. Both batteries and conventional supercapacitors use highly corrosive liquid for this function, and as the devices age, this liquid sometimes escapes to eat away at circuits and surrounding components. The result is failure and sometimes even fire. Our microsupercapacitors employ an all-solid-state electrolyte, which we apply directly onto the interdigitated pattern.
For this solid electrolyte, we have plenty of choices. We can use gelled polymer electrolytes, made by swelling a polymer matrix with an electrolyte solution, or we can solidify ionic liquids by adding polymers or silica nanopowder. This nonleaking design, together with a virtually unlimited number of charge and discharge cycles, means that our supermicrocapacitors will likely outlast all other electronic devices on the chip. Such long life will be particularly useful whenever it is inconvenient or dangerous to open things up to replace a power source, as in pacemakers, defibrillators, and other medical implants.
Printing supercapacitor battery hybrids like burning a DVD
They can now produce 100 of the devices on a disc in less than 30 minutes, and there is plenty of room for improvement. Of course, a manufacturer could speed things up by simply running a roomful of DVD burners in parallel. It would be even better to optimize a burner for mass production with industrial-scale laser engravers, which are now widely used in industry to mark products so that they can be tracked later on. The laser engraving machines can be constructed on a conveyor-belt system using long rolls of graphite oxide.
A traditional capacitor is made of two metal plates separated by a thin insulating layer. It stores charge electrostatically, in an electric field created by the two oppositely charged plates. How much charge can be stored is determined by the capacitance of the device. It is a function of the area of one of the metal plates—typically less than a square meter—divided by the spacing between them, which is typically about a micrometer or less. Therefore, to increase the charge you must maximize the area and minimize the distance.
Supercapacitors minimize the distance by borrowing a bit of battery technology—the electrolyte. A supercapacitor is defined as an electric double-layer capacitor.