Graphene Tackles the Supercapacitor With Mixed Results
Graphene’s contribution to the advancement of supercapacitors may not be what people expected, but it still may be pretty significant
Not too long after research labs around the world first started experimenting with graphene, one of the early ideas for an application was to see if it could replace the activated carbon used in the electrodes of supercapacitors.
Supercapacitors, or ultracapacitors, or for the more technically inclined, electrochemical double layer capacitors (EDLCs), inhabit a world between electrochemical batteries (like lithium-ion (Li-ion) batteries) and capacitors. Capacitors are capable of delivering a lot of power in quick bursts; this ability is called power density. Electrochemical batteries are unable to deliver a lot of power like that, but they can store a lot of electrical energy and release it slowly over time. This ability to store energy is called energy density. Another key difference in the performance characteristics of capacitors and batteries is that capacitors can be charged up in seconds while batteries can take hours to be fully charged.
Supercapacitors lie between these two energy storage methods. They can store electricity at levels approaching batteries and they can deliver big bursts of power like a capacitor. They can also charge up very quickly like a capacitor.
Sounds great, right? Why don’t we just get rid of batteries and replace them with supercapacitors? Not so fast. The energy density (the amount of energy stored per unit mass) of supercapacitors currently on the market is capable on average of around 28 Watt-hour per kilogram (Wh/kg) whereas a Li-ion battery has about 200Wh/kg. Supercapacitors are good, but not that good…yet.
Graphene Offers an Under-appreciated Solution in Supercapacitors
That’s where many believe graphene would come in and make it possible for supercapacitors to compete with batteries in energy storage, plus be able to get fully charged in seconds. The idea of all-electric vehicles (EVs) that could be topped up at an electrical station just as fast as gas-powered cars are filled up with gasoline started to spread through the popular imagination.
While graphene-based supercapacitors in the lab have been able to achieve 90 to 160Wh/kg figures, it wasn’t clear that graphene was going to replace activated carbon on the merits of its energy density alone. The key to the energy storage capacity of a supercapacitor depends on the surface area of its electrodes. The greater the surface area the more ions it can store and the greater its storage capacity.
The theoretical limit for the surface area of graphene is 2630m2/g. That’s the theoretical limit and no one has achieved anything higher than 1520m2/g for a supercapacitor electrode. Meanwhile there already exist activated carbon-based electrodes that have surface areas of 3000m2/g.
Graphene’s superiority over activated carbon for the electrodes of supercapacitors is not in surface area and the resulting higher storage capacity, but in its conductivity. Graphene’s high conductivity (>1700 Siemens per meter (S/m)) compared to activated carbon (10 to 100 S/m) means that it could address the market for high-frequency applications that current supercapacitors cannot. Also, graphene’s ability to be structured and scaled down, unlike other supercapacitor materials, means that it could be used in computer processing units (CPUs) and integrated circuits (ICs).
The Most Recent Developments in Graphene for Supercapacitors
In the second quarter of this year, a trend in graphene research for supercapacitors is that it’s taking on a decidedly hybrid approach. Instead of graphene on its own to improve supercapacitors, it is being teamed up with a variety of nanomaterials, most notably carbon nanotubes.
In research published in the Journal of Applied Physics, a team at George Washington University has combined carbon nanotubes and graphene to create a supercapacitor they claim is low cost and high performance.
The combination of the two carbon nanomaterials takes advantage of graphene’s high conductivity and high surface area while the carbon nanotubes connect the structures to make a unified network. The device’s specific capacitance—its ability to store a charge—was reported as 100 Farads per gram (F/g), three times higher than the specific capacitance of a supercapacitor made by carbon nanotubes alone.
While that capacitance may be good compared to carbon nanotubes, it’s not quite as impressive when compared to activated carbon supercapacitors that can achieve a capacitance of 250 F/g—and activated carbon is made from crushed coconut shells.
Research at Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University in the United States has also developed a supercapacitor based on the combination of graphene and carbon nanotubes.
In research published in the journal Nature Nanotechnology, the international team developed a supercapacitor that can take the shape of a fiber and be woven directly into clothing. The research exploits graphene’s and carbon nanotubes’ advantage over activated carbon for supercapacitors: they can be built into unique geometric structures.
Because of its unique geometry, the researchers have given the energy density of the supercapacitor in volume rather than mass, which is usually presented in watts per kilogram. They claim that the volumetric energy density is the highest yet reported for carbon-based microscale supercapacitors: 6.3 microwatt-hours per cubic millimeter, which is comparable to a 4-volt-500-microampere-hour thin-film lithium ion battery that can be used to power smart cards and RFID tags.
The researchers have been able to produce these fiber-like supercapacitors as long as 50 meters and have a charge capacity 300 Farad per cubic centimeter (again, a measurement given in volume rather than mass).
The researchers are planning early applications for the supercapacitor fibers to be woven into clothing that could power biomedical-monitoring devices a patient wears at home.
Researchers at the University of California Riverside have added a third nanomaterial for the ingredients to making a supercapacitor. In the journal Nature Scientific Reports, graphene-and-nanotube hybrid foam was dipped into a slurry of hydrous ruthenium oxide (RuO2) nanoparticles that resulted in a few-layer-thick graphene foam architecture covered with hybrid networks of RuO2 nanoparticles and anchored nanotubes.
While the device walks the tightrope between being a supercapacitor and a battery—a confusion that some argue is holding back developments in the field -- the hybrid device is reported to have high conductivity and high specific capacitance. However, the reported numbers for its energy density for this latest device 39.28 Wh/kg, far short of the 200 Wh/kg we see in Li-ion batteries. Once again, geometry and conductivity may be the factors that get this supercapacitor adopted, not its energy density.