The Role of Graphene in Energy Storage Continues to Evolve
From supercapacitors to Li-ion batteries, graphene has something to offer
The Graphene Council Newsletter has been tracking the application of graphene to supercapacitors assiduously.
To fill in a little bit of the background, supercapacitors are a kind of cross between a battery and a capacitor. While batteries depend on a liquid electrolyte that changes the chemical states of ions in order to operate, a capacitor stores the ions on the surface of its electrodes in the form of static electricity. This translates into a capacitor being able to deliver energy very quickly in big bursts and to recharge almost as rapidly.
The speed at which an energy storage device can charge and discharge is known as “power density”. The power density of a capacitor is much higher than an electrolyte-based battery in which power is delivered slowly and it takes a long time for it to charge up. However, where batteries have capacitors beat is that they can store more energy than a capacitor and can then be used over an extended period of time. This ability to store energy is known as “energy density” and essentially means batteries can store more energy than a capacitor.
Supercapacitors, on the other hand, are a kind of hybrid between the electrolyte-based battery and the capacitor. Like a capacitor, the ions of a supercapacitor are stored on the surface of the electrodes in the form of static electricity. However, it differs from traditional capacitors in that an electrolyte is used to attract the ions to the electrodes during charging.
Supercapacitors are already used today, but typically in conjunction with traditional batteries so they can give a quick burst of energy in applications such as electric cranes that may need a little bit extra boost to lift a heavy load.
While that meets an industrial need, the hope has been that if the energy density of supercapacitors could be increased, they could offer an attractive alternative to traditional batteries for powering electric vehicles (EVs). The EVs of today, like a Tesla, have a range a little over 480 kilometers, but it can take 9.5 hours of charging from a 240-volt outlet to get from 0 back up to 480 kilometers. This has obviously limited their appeal as a viable alternative to fossil-fueled vehicles.
A supercapacitor could conceivably charge up much more quickly than that, but the problem is you couldn’t store enough energy in them to get very far on that charge.
Graphene has been looked at as an alternative to the current materials used in storing ions on the electrodes of supercapacitors. The reason for this is that you want a material that has a big surface area. The greater the surface area the more ions can be stored on it.
Graphene has a theoretical surface area of around 2600 square meters per gram. Unfortunately, that theoretical surface area can really be translated into a real device because that surface area is only achievable with a standalone sheet of graphene. In order to get it to work in a practical device that will provide a decent volumetric capacitance you need to stack a number of sheets on top of each other at which point the theoretical surface area is lost and you get about the same surface area you can get with the activated carbon used today.
Researchers Make Breakthrough in Storage Capacity for Graphene-based Supercapacitors
In research published in the Journal of Power Sciences, researchers in South Korea have developed a supercapacitor based on graphene that shatters the previous energy density records for these devices by reaching 131 watt-hours per kilogram (Wh/Kg), nearly four times the previous record for graphene-based supercapacitors of around 35Wh/Kg in lab prototypes.
Beyond these impressive storage figures, another attractive feature of this research is that the process for fabricating the graphene is non-toxic and can be carried out under low temperatures.
Despite the impressive figure of 131Wh/Kg for these supercapacitors, they still fall somewhat short of an average lithium-ion (Li-ion) battery that are used to power EVs of around 200Wh/Kg. Nonetheless the improvement is big enough to hold out hope that supercapacitors could still be an alternative to Li-ion batteries in EVs, offering a charge time of a few moments rather than nearly 10 hours.
The New Direction for Graphene in Supercapacitor Applications
While the South Korean research has rekindled notions that graphene could be the solution to increasing the storage capacity of supercapacitors to the point where they could offer an alternative to Li-ion batteries, the general research trend has moved away from this aim. Instead researchers have been trying to exploit the properties of graphene that competitive materials like activated carbon lack, namely high conductivity and the ability to be fabricated into different structures and sizes.
Along these lines, researchers at California NanoSystems Institute (CNSI) at UCLA are using graphene in a supercapacitor that could be small enough to be used as a wearable of implantable device.
In this application area, the supercapacitors actually have better storage capacity than thin-film Li-ion battery technology. The supercapacitor the CNSI researchers have developed is only one-fifth the thickness of a sheet of paper, and it can hold twice as much charge as a typical thin-film Li-ion battery, according to the researchers.
“Let’s say you wanted to put a small amount of electrical current into an adhesive bandage for drug release or healing assistance technology,” said Richard Kaner, a professor at UCLA, in a press release. “The microsupercapacitor is so thin you could put it inside the bandage to supply the current. You could also recharge it quickly and use it for a very long time.”
The key to the development was the type of graphne that the CNSI team used. Dubbed laser-scribed graphene (LSG), this form of graphene can hold an electrical charge for a long time, is highly conductive and charges very rapidly. The researchers combined the LSG with the two-dimensional material molybdneum disulfide and then combined that 2-D hybrid with manganese dioxide, which is often used in alkaline batteries. The manganese dioxide provides more charge to the supercapacitor and is inexpensive and abundant.
In their tests with the devices, the CNSI researchers were able to charge the supercapacitor with a solar cell and then have it power an LED light throughout the night on that one charge.
“The LSG–manganese-dioxide capacitors can store as much electrical charge as a lead acid battery, yet can be recharged in seconds, and they store about six times the capacity of state-of-the-art commercially available supercapacitors,” Kaner said. “This scalable approach for fabricating compact, reliable, energy-dense supercapacitors shows a great deal of promise in real-world applications, and we’re very excited about the possibilities for greatly improving personal electronics technology in the near future.”
“Holey” Graphene Continues its Crusade
Last year, we reported on previous work from the researchers again at CNSI, who developed a form of graphene dubbed as “holey” graphene that showed particular promise in supercapacitors.
Research has been continuing with this form of graphene for a number of research teams and recently a team at the University of California San Diego (UCSD) have developed a method for increasing the amount of electric charge that this form of graphene can store as an electrode material in supercapacitors.
The UCSD research team made its holes smaller than previous versions, with typical diameters around 1 nanometer. This is about 1000 times smaller than the holes that were reported in the CNSI. This translates into more holes in a given space and consequently greater charge density per unit area.
One of the key breakthroughs of the research was the process that the UCSD team developed for making the holey graphene. Instead of using chemical etching like the CNSI team did, the UCSD researchers used ions that made it possible to create atomic-scale holes.
This plasma processing (ions) technique is also used widely in the semiconductor industry that gives a certain familiarity and provides a high level of scalability.
While you may be wondering if this latest version of holey graphene can compete with the energy storage capacity of the recent devices produced out of South Korea, the short answer is that we don’t know. The UCSD team did not build a supercapacitor out of the material. The test will be to see how the material performs in actual device.
Two-Dimensional Materials Have a Role to Play in Li-ion Batteries Too
While the research we have covered here in graphene’s use in energy storage has just been in supercapacitors, the two-dimensional material molybdenum disulfide (MoS2) has been shown to improve the performance of Li-ion batteries. We have covered some of this work in the Graphene Council Newsletter. But in that work out of Rice University the potential application to batteries and supercapacitors was almost an afterthought.
Now researchers at Kansas State University (KSU) have focused on the potential of MoS2 on the electrodes of Li-ion batteries, where they say the material can dramatically boost its storage capacity.
In the research, which was published in the Nature’s Scientific Reports, the KSU researchers demonstrated that MoS2 sheets that had been wrapped in silicon carbonitride could store twice as many lithium ions as pure MoS2. The researchers believe that the reason the Li-ion batteries based on pure MoS2 perform comparatively poorly is that sulfur gets into the electrolyte, reducing its capacity.
"This kind of behavior is similar to a lithium-sulfur type of battery, which uses sulfur as one of its electrodes," Singh said in a press release. "Sulfur is notoriously famous for forming intermediate polysulfides that dissolve in the organic electrolyte of the battery, which leads to capacity fading. We believe that the capacity drop observed in molybdenum disulfide sheets is also due to loss of sulfur into the electrolyte."
By wrapping the MoS2 in silicon carbonitride, which is a ceramic material capable of withstanding high temperatures, the MoS2 is prevented from giving off its sulfur atoms and creating the polysulfides that would eventually dissolve in the electrolyte.
"The silicon carbonitride-wrapped molybdenum disulfide sheets show stable cycling of lithium-ions irrespective of whether the battery electrode is on copper foil-traditional method or as a self-supporting flexible paper as in bendable batteries," Singh said in the release.