By Jeremy Cook
The transition to renewable power sources like solar and wind requires new methods of energy storage. Clouds can obscure the sun for days at a time, and solar is completely unavailable at night; wind can be even more fickle. Storage gets us through unavoidable generation gaps and covers localized surges in power use.
Today’s lithium-ion batteries are incredible, but they can only charge and discharge so fast and have a limited lifespan. Additionally, extracting lithium from the earth is complicated with its own environmental impacts. Today the U.S. produces less than 1% of the world’s lithium, making it a potential bottleneck for production.
Supercapacitors, which can charge/discharge at a much faster rate and at a greater frequency than lithium-ion batteries are now used to augment current battery storage for quick energy inputs and output. Graphene battery technology—or graphene-based supercapacitors—may be an alternative to lithium batteries in some applications.
Instantaneous power and long-term energy supply
The big advantage of supercapacitors is their high-power capability. The disadvantage is a low total energy density. These properties may seem at odds, but consider the definition of both terms:
Power = work/time, expressed in SI units as joules/second
Energy Density = energy stored/volume, expressed in SI units as joules/m^3
While the unit in the numerator is the same they are two distinct quantities. Power is the ability to release a quantity of energy over a specific time period, while energy density is the capability to store a specific quantity of energy, regardless of the time period.
High values for both would be ideal, but supercapacitors typically have a high-power discharge capacity and a low energy density. Therefore, while a supercapacitor can produce a massive burst of energy for an instant, it can’t maintain this (or a lower rate) of energy output for nearly as long as a comparable lithium-ion battery. Today’s applications often use a supercapacitor to even out massive energy inputs or expenditures (e.g., regenerative braking and quick acceleration), while longer-term energy needs are handled by batteries.
The challenge is to increase supercapacitor energy density while still maintaining excellent instantaneous power capacity. The answer comes in the form of the wonder material, graphene.
Is graphene energy storage the future?
Supercapacitors commonly use anode and cathode layers made from metal foils coated with activated carbon and separated by a semi-permeable membrane containing an electrolyte solution. Layers of this carbon/membrane sandwich are rolled or stacked into a capacitor housing, allowing them to store charges via ion movement in the electrolyte.
Attribution: By Tosaka - Own work, CC BY 3.0 |Schematic construction of a supercapacitor with stacked electrodes 1. positive electrode, 2. negative electrode, 3. separator
Activated carbon can be made very thin in this role, on the order of 1/10 mm thick but has a high surface area: several square centimeters for each .1mm particle. Graphene, however, comes in sheets of 2D molecules that are 1 atom thick, with a similar specific surface area to activated carbon. It can be spread out in an extremely thin layer for an ultra-dense conductor arrangement. Graphene is an excellent conductor, meaning minimal heat loss and hypothetically better power delivery than even activated carbon supercapacitors.
The problem is manufacturing graphene capacitors at scale. Given graphene’s promise however, researchers are working on this sort of implementation behind closed doors. While graphene might not eliminate lithium-ion batteries completely, supercapacitor improvements using graphene could help this power storage device become more energy-dense and efficient.
Other advanced storage options... still carbon?
Graphene isn’t the only advanced storage option being developed. The use of carbon nanotubes — another arrangement of carbon in long tubular molecules, as opposed to graphene’s sheets —has also been put forth for the role of energy storage. Graphene balls and curved/crumpled graphene are other carbon-based possibilities for energy storage.
Handling the power
While the ability to deliver a massive amount of power is a good thing, it must be controlled for proper usage. SiC transistors can be used in this role. Along with this, current measurement technologies must be implemented to ensure proper power and energy application.
Interestingly, the advanced production of carbon in its various forms along with silicon (and the combination of the two), appears to be the wave of the future for energy handling. The difficulty today is taking ideas that will work in theory and/or at small scales and ramping them up to everyday products that enhance our lives. Once these techniques are perfected and implemented, we can foresee a more energy-efficient future with even more capable devices.
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