GRAPHENE SUPERCAPACITORS
What is a graphene supercapacitor?
The graphene supercapacitor is a high-capacitance capacitor made using graphene.
This kind of capacitor is also called a Double Layer Capacitor.
What makes it so unusual is its extreme capacitance;
while normal capacitors have typical capacitances less than 10mF,
a graphene supercapacitor can produce a capacitance of up to 12kF. If you are interested in hybrids between
supercapacitors and batteries using graphene, take a look at Graphene Batteries.
The extreme capacitance means that the Graphene Supercapacitor is able to store 10,000
times more electricity than an ordinary electrolytic capacitor of the same physical size.
Capacitors are interesting energy sources because they can recharge very quickly,
in a matter of seconds or minutes, while a battery usually takes hours to fully charge.
The problem with ordinary capacitors is they are unable to store large amounts of charge.
Your phone, running on ordinary capacitors, would recharge in a few seconds,
but you would have to charge it every few minutes. This is why we still use batteries.
The graphene supercapacitor seems to take the best of both worlds.
It combines the very quick charge time of an ordinary capacitor,
while its energy storage capacity is comparable to that of a battery.
If you are a potential investor, read up on How To Invest in Graphene.
Graphene supercapacitor discovery
The graphene supercapacitor was discovered by accident - sort of.
A UCLA graduate student, Maher El-Kady, used a LightScribe DVD burner,
commercially available and inexpensive, to create what is now known as a graphene supercapacitor.
Students at UCLA lab occasionally tried using LightScribe technology on different compounds with no results,
but graphite oxide turned out to be a different compound.
The results were amazing - by sandwiching a layer of electrolyte between two sheets of graphene peeled off a DVD
and connecting them to an LED, it was confirmed that this simple formation was in fact -
a graphene supercapacitor, as the LED kept shining for much longer than the scientists anticipated.
What is LightScribe?
LightScribe is a technology which uses a DVD drive’s laser to burn in a DVD cover image
into the top layer of a DVD. In order to do this,
the user loads the text or image to be burned into LightScribe software, and inserts the DVD upside down.
The laser then does the rest of the work, burning the artwork onto the DVD surface.
This technology was is aimed at the consumer market,
especially for people who can’t be bothered to write the contents of a DVD using a marker,
but still need a cover image or text marking the contents of the disc.
How exactly did they make a graphene supercap?
What El-Kady did was to coat a DVD-shaped piece of plastic with graphite oxide by
allowing an aqueous solution of graphite oxide to dry on the plastic surface.
He then inserted the DVD into the LightScribe drive and loaded the grahene capacitor shape into the computer.
The result was a, so to say, laser-printed graphene ultracapacitor with an unusually high capacitance value.
Graphene supercapacitor properties
Graphene is very porous.
The extreme capacitance value comes from graphene’s high internal surface area,
which is also known as Brunauer-Emmett-Teller surface area.
One gram of graphene has an equivalent surface area of approximately 1600 m2,
which is equivalent to 17200 square feet!
This means that 5 grams of graphene have an internal surface area equivalent to the area of an entire soccer field.
Graphene supercapacitor capacitance value
The capacitance value of a plate capacitor is in direct proportion to its plate area and
in inverse proportion to the distance between its plates.
A graphene supercapacitor uses graphene compounds as its plates and an electrolyte between them.
Graphene’s extreme Brunauer-Emmett-Teller surface area has an obvious effect on the capacitance,
and the fact that the distance between the two plates can also be nano-scaled
additionally contributes to the supercapacitor’s - super capacitance.
The maximum capacitance of a graphene supercapacitor is about 12 kilofarad as of 2013.
Graphene supercapacitor’s capacitance-to-weight ratio
A supercapacitor whose positive electrode is made of nickel-hydroxide-graphene
composite and negative electrode from porous graphene has a specific capacitance of approximately 220 F/g.
This allows scientists to make supercapacitors which are very lightweight.
It is easy to calculate that a 2 kilofarad capacitor would weigh under 10 grams.
It seems almost unfair to compare the capacitance of such supercapacitors to the capacitance of
the entire planet Earth: only 710 microfarads!
Maximum energy density of a supercapacitor
Besides the capacitance to weight ratio,
an important characteristic of any energy source is its maximum energy density.
Graphene supercapacitors have an energy density of up to 77.8 Wh/kg.
This means that a supercapacitor weighing 1 kg can provide power for a 1 W load for 77.8 hours.
Alternatively, it means that a 1 kg supercapacitor can provide power for a 77.8 W load for 1 hour.
This is a lot. It’s still worse than Li-Ion batteries, which boast an energy density of 100 to 125 Wh/kg,
but it takes hours to recharge a battery and only minutes to recharge a supercapacitor.
What is the cycle life of a graphene supercapacitor?
Graphene supercapacitors beat batteries in one more field: cycle life.
Cycle life basically defines how many times a battery or a supercapacitor can be fully discharged
and then fully charged again. Batteries can only last for about 500-1000 full charges.
You might have noticed this effect yourself. If you use your laptop or cell phone every day,
you’ll notice that its battery life reduces with age.
This is actually because the battery approaches its cycle life after many charge/discharge cycles.
As this happens, the maximum capacity of a battery degrades.
On the other hand,
graphene supercapactitors still retain 94% of their nominal charge after 3000 complete charge/discharge cycles.
The typical cycle life of a graphene supercapacitor can be up to 1 million cycles.
Graphene supercapacitor charge time
We’ve touched upon the extremely fast charge times required for supercapacitors.
Let us look at this fact more closely,
as it is one of the main advantages of graphene supercapacitors over batteries.
It takes between one hour and four hours to completely charge most Li-Ion batteries.
If you’re using an 850 mAh battery,
you might want to charge it with a current of 850 mA for one hour
(This is NOT the correct way to charge a Li-Ion battery.
We are only using this information to describe how much charge it takes to completely charge a battery,
since the total amount of electrical charge that can "fit into" a battery equals
charging current multiplied by the charging time.
Charging Li-Ion batteries in this manner could cause damage or even injury in case of overheating.
Only use certified chargers for your batteries!).
Charging the battery at higher currents will cause them to heat up more as
there is a certain power dissipation at the battery’s internal resistance equal to RI2.
The power dissipation limits the maximum charge current.
The lower the charging current, the more time it takes to reach 100% charge.
Capacitors have very low internal resistances, a few milliohms to several tens of milliohms,
so their maximum charging current can reach over a hundred amperes.
This directly translates to a charging time which is about a hundred times shorter
than the charging time for batteries.
Some calculations claim that you could charge your phone in under 10 seconds
if it were to use a graphene supercapacitor instead of a battery.
An electric car could be charged in a few minutes,
while batteries would take several hours to charge completely.
This page was last modified: April 26th, 2013.