Will graphene supercapacitors be the best?
Jun 14, 2013 Dr Peter Harrop
Black magic has a new meaning. It is the sub-optimal way most supercapacitors rely on soot from burnt coconut shells and some homespun trickery. The soot is deposited on something akin to cooking foil and that often has the 40 micron thickness of lithium-ion battery electrodes ten years ago, before they were optimised.
Contrary to teaching based on yesterday's understanding, supercapacitors are competitors of lithium-ion batteries. Increasingly, they either go across them to protect them, enhance their performance and release more of their power - meaning less battery is needed - or they completely replace them as with over 600,000 modules that let conventional cars automatically switch off even when briefly stopped, firing them on again when the accelerator is next depressed. That saves several percent of fuel consumption and pollution. At the other extreme supercapacitors have started to replace lithium-ion batteries in hybrid vehicles from Formula One cars to trash trucks despite their up-front cost being over ten times those batteries per unit of energy stored. Ironically, this is always because those roasted coconut shells perform much better and last much longer than lithium-ion batteries despite being based on Flintstones technology. They even cost less over life, in many cases.
The question therefore arises as to how much supercapacitors can improve if organisations more like NASA took over. After all, if supercapacitors can replace many batteries when, as today, they store one tenth of their energy, what will happen in the marketplace if they approach the theoretical ten times the lithium-ion battery figure while acting as a near-perfect "battery"? That perfection is something real batteries can never achieve. The calculations are based on utilising the huge area of graphene "soot" though aerogels, nanotubes and other allotropes are impressive as well. If exposed and made useful for this particular electrostatic process without collapsing or degradation, graphene may get up to 1000 W/kg, say the professors, giving pure electric vehicles, from e-planes to e-cars, more range than conventional ones today. A killer blow indeed.
Today, many companies make graphene supercapacitors in small quantities and some have increased the energy density ten times in the laboratory. Why not more? Well, it turns out that their graphene is very impure, it does not reveal enough useful area, and it is on thick foil with sub-optimal electrolytes, all of which is good news because the potential of graphene sheets gripped vertically on, say 0.5 micron aluminium foil has yet to be demonstrated. There is plenty of development space ahead as we move from such things as carbon form carbide, which has a bit of graphene - and most other things - in it to properly tailored and mounted sheets of one atom thickness. Progress is rapid with non-flammable and non-toxic electrolytes and other useful advances in supercapacitor structure and the half-way house of supercabatteries is showing promise. In most respects, supercapacitors continue to improve faster than lithium-ion batteries.
Indeed, making the supercapacitor as more-useful smart skin or load-bearing structure in vehicle, as researched at Imperial College London and in the USA has little parallel in the lithium-ion battery industry. Supercapacitors are the ones to watch whether or not there are painted naked people dancing around them during manufacture.
For more attend Supercapacitors USA, Santa Clara, California on November 20-21 2013.