Materials engineers at Monash University in Australia have devised a method of producing graphene supercapacitors that have the same energy density as the lead-acid battery under your car’s hood. Not only are these supercapacitors about 10 times more energy-dense than commercial devices, but the method of producing the graphene inside the supercapacitors seems to be novel as well. The engineers say they used a process that is similar to traditional paper making – and that it could be easily and cost-effectively scaled up for the commercial production of graphene, and graphene-based supercaps.
Supercapacitors are essentially small batteries that can recharge and discharge almost instantly. While this results in a very high power density (lots of watts), their energy density is generally very low (watt-hours). For a conventional supercapacitor, we’re talking about a power density that’s 10-20 times higher than a conventional lithium-ion or lead-acid battery – but on the flip side, the energy density is 10-20 times worse.
In short, supercapacitors are fantastic for when you need a short burst of energy – such as a quick burst of acceleration from a car’s kinetic energy recovery system (KERS) – but useless for powering everyday consumer electronics like your smartphone.
Graphene, however, could change all that. The amount of energy stored by an electrochemical capacitor is closely tied to the amount of charge-carrying electrolyte that contacts the electrodes. The higher the surface area of the electrodes, the more charge-carrying ions that can be adsorbed (attached) to the electrodes, thus storing more energy.
You can probably see where this is going. Because graphene is the thinnest known substance, it is capable of providing an astonishingly large surface area; somewhere in the order of thousands of square meters (that’s multiple tennis courts) per gram. The surface area is so large that graphene could be used to create supercapacitors that bridge the massive energy density gap between supercaps and batteries, while still retaining huge power density.
That’s the theory, anyway. The problem, of course, as with all things graphene, is that it’s still very hard to mass-produce commercial-grade graphene. The Monash engineers claim to have solved this problem, though, using a solution-based process that’s “similar to that used in traditional paper making.”
Basically, they start with graphite (graphene) oxide, which is reduced to low-grade graphene flakes using a solution of hydrazine and ammonia. Then, the electrolyte and a solvent are added to the mix. As the mixture dries, the volatile solvent evaporates, causing capillary action to suck the graphene flakes together, with the electrolyte wedged between each of the flakes. Eventually the engineers are left with something that resembles a black sheet of paper – millions of layers of graphene, with oodles of charge-carrying electrolyte locked in (see the image below).
When fashioned into an electrochemical capacitor, this paper-like material has a volumetric energy density of almost 60 watt-hours per litre (Wh/l), which is just about comparable to a lead-acid battery. It retains about 90 per cent of its capacitance after 50,000 charge/discharge cycles, and it even holds 90 per cent of its charge after 300 hours.
Dan Li, the professor who led the work, says: “We have created a macroscopic graphene material that is a step beyond what has been achieved previously. It is almost at the stage of moving from the lab to commercial development.”
There is no word on when these graphene capacitors will come to market, but the solution-based chemical reduction of graphite oxide is one of the most likely routes for commercialisation of graphene.
Research paper: DOI: 10.1126/science.1239089 – “Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage”