A research team at the California NanoSystems Institute (CNSI) at UCLA has developed what they call a “holey graphene framework” that they claim can significantly boost the energy density of supercapacitors.
Graphene has been held out by some as a silver bullet for enabling supercapacitors to combine the quick recharge and large bursts of energy of capacitors with the high storage capacity of electrochemical batteries. But so far using graphene as a replacement for activated carbon on the electrodes of supercapacitors has fallen short of expectations. It seems that graphene-based electrodes don’t enable energy storage much better than activated carbon, but because they can operate a higher frequencies than most supercapacitors they can be applied in novel ways, such as being used as part of filtering circuits in AC rectifiers.
Nonetheless, researchers working with graphene continue to try boost the energy density (the amount of energy stored per unit mass) of supercapacitors and still maintain the power density (the maximum amount of power that can be supplied per unit mass) of capacitors.
In research published in the journal Nature Communications, the CNSI team developed a three-dimensional graphene framework whose porous structure gives it a large ion-accessible surface area.
The researchers showed that the electrodes are capable of delivering a gravimetric capacitance of 298 Farads per gram (F/g) and a volumetric capacitance of 212 Farads per centimeter (F/cm). By way of comparison, researchers at George Washington University reported in April, that they were able to achieve 100 F/g with a combination of graphene and carbon nanotubes.
To give some more context, the specific energy density of the average lithium-ion (Li-ion) laptop battery is around 200 Watt-hour/kilogram (Wh/kg), whereas today’s upper average supercapacitor can get around 28 Wh/kg. In May, researchers at the University of California Riverside were able to achieve a full cell energy density of their graphene-and-nanotubes device of 39.28 Wh/kg.
The new CNSI researchers claim an energy density of a fully packaged device stack based on the holey-graphene framework is capable of 35 Wh/kg.
Because energy density is the brass ring that most of this research is reaching for, that’s what gets highlighted. However, the attractive bit about graphene-based electrodes for supercapacitors may be their superior electrical conductivity—making them useful for high frequency applications—and mechanical flexibility.
That said, the UCLA structure's unique hierarchical porosity may not be as attractive as it first seems once you consider that cigarette butts might have a better pore structure.