While most of today’s electric vehicles rely on batteries to store energy, supercapacitors have enjoyed significant improvements that have made them serious competitors to batteries. Batteries traditionally have the upper hand in terms of capacity, since supercapacitors’ low capacities mean very short driving ranges for electric vehicles. Supercapacitors’ biggest advantage lies in their much higher power density compared to batteries, enabling a quicker charge time and the ability to quickly discharge for fast acceleration.
A new study by a group of researchers at the University of Alberta and the National Research Council of Canada, both in Alberta, Canada, has shown that supercapacitors have great potential for continuing improvements. The researchers have synthesized a new material that they call sponge-like graphene due to its 3D macroporous structure and demonstrated that it can be used to make supercapacitor electrodes. Supercapacitors with these new electrodes have fair energy density when operating at low power densities, but their biggest attraction is when operating at ultrahigh power densities of around 48,000 W/kg, where they’re able to deliver an attractive energy density of 7.1 Wh/kg.
As the researchers explain in their study, they synthesized the sponge-like graphene out of multiwalled carbon nanotubes and cobalt phthalocyanine (PC) molecules that attach to nucleation sites in the nanotube “skeleton.” These materials were microwave-heated for 20 minutes to yield graphite, and then immediately quenched with ice water to transform the graphite into graphene flakes. Scanning electron microscope images revealed a very uniform sponge-like morphology in the carbon structure. In experiments, the researchers demonstrated that electrodes made of the sponge-like graphene are stable in two common electrolytes (ionic liquid and aqueous) used in supercapacitors. While many supercapacitor electrodes perform well only at temperatures of 60 °C (140 °F) or higher, the sponge-like graphene electrodes work very well at room temperature. The researchers attribute both the good room-temperature operation and the ability for fast electrolyte transfer (and resulting high power density) to the electrode’s sponge-like macroporous structure. The sponge-like graphene electrodes also exhibit an excellent cycle life. After running through 10,000 charge-discharge cycles, the electrodes retained 90% of their capacity in the ionic liquid electrolyte and 98% in the aqueous electrolyte.