Graphene may be key to leap in supercapacitor performance
Aug 26, 2014 Dr Peter Harrop
Graphene electrodes are one of the best prospects for enabling supercapacitors and superbatteries to take up to half of the lithium-ion battery market in 15 years - amounting to tens of billions of dollars yearly. They may also be key to supercapacitors taking much of the multibillion dollar aluminium electrolytic capacitor business. That would make supercapacitors and supercabatteries (notably in the form of lithium-ion capacitors) one of the largest applications for graphene.
Heirarchical to exohedral?
Today's supercapacitor electrodes usually have hierarchical electrode structures with large pores progressing to small pores letting appropriate electrolyte ions into monolithic masses of carbon. In research, this is often giving way to better results from exohedral structures - where the large functional area is created by allotropes of carbon often only one atom thick. Examples are graphene, carbon nanotubes and nano-onions (spheres within spheres). Add to that the newer aerogels with uniform particles a few nanometers across.
It is not simply an area game. The exohedral structure must also be optimally matched to the electrolyte, then the pair assessed not just for specific capacitance (capacitance density) but voltage increase, because that also increases the commercially-important energy density when competing with batteries.
It is not a done deal. Graphene is expensive when good purity and structural integrity are required. Exohedral structures like graphene, with the greatest theoretical area, tend to improve gravimetric but not volumetric energy density. Poor volumetric energy density will cut off many applications unless structural supercapacitors prove feasible. Here the supercapacitor would replace dumb structures like car bodies, taking effectively no volume, regardless of measured volumetric energy density. Some of these formulations increase the already superb power density but that is not very exciting commercially.
Other parameters matter
Of course cost, stability, temperature performance and many other parameters must also be appropriate in all potential applications of graphene in supercapacitors and supercabatteries. Indeed for replacing electrolytic capacitors, working at 120Hz is key. In other applications, increased power density may be valuable when combined with other improvements. Nevertheless, energy density improvement is the big one for sharply increasing the addressable market - probably around 2025 or later.
Highest energy density by leveraging new generation electrolytes
Graphene gives some of the highest energy densities in the laboratory and it is particularly effective in exhibiting high specific capacitance with the new electrolytes. That means aqueous electrolytes with desirably low cost and non-flammability, and ionic electrolytes with desirably simplified manufacturing, high voltage, non-flammability, low toxicity and now exceptional temperature range.
With ionic electrolytes, graphene works despite the high viscosity that makes them ineffective in hierarchical electrode structures. On the other hand, graphene does not exhibit good specific capacitance with the old acetonitrile and propylene carbonate organic solvent electrolytes. It is advantageous that there is no solvent or solute with ionic electrolytes, though sometimes they are added to tailor the ionic supercapacitor to obtain certain performance in experiments.
With aqueous electrolytes, graphene's accessible area is large and this offsets the low voltage to give good energy density in some experiments. Curved graphene is often used. Under a microscope it looks like crushed paper so further optimisation is possible. In the laboratory, the energy density of lead-acid and nickel cadmium batteries and even lithium-ion batteries has been achieved with various formulations involving graphene so it is likely that one of them will prove commercial in due course.
Recent developments by industrial companies demonstrate that graphene lithium-ion capacitor supercabattery systems can operate up to 3.7 V. They have a very good cycle life and excellent power performance.
AC graphene supercapacitors
Potentially, inverters in electric vehicles can be made smaller, lighter and have lower installed cost thanks to planned graphene supercapacitors replacing their large aluminium electrolytic capacitors. So far, it is only with vertically stacked graphene that the necessary time constant of 200 microseconds has been demonstrated suitable for such 120Hz filtering.
For more see the brand new IDTechEx report Functional Materials for Supercapacitors / Ultracapacitors / EDLC 2015-2025 and also Graphene Markets, Technologies and Opportunities 2014-2024.
In addition, attend IDTechEx's events Supercapacitors LIVE! USA 2014 and Graphene & 2D Materials LIVE! USA 2014 taking place in November.
Top image: Carbon nano-onions