Graphene supercapacitor - replacement for chemical batteries?

Graphene supercapacitor is one of the potential replacements of chemical batteries. While the thought of capacitors eventually replacing chemical batteries is not new, the up-and-coming super-material, graphene, may have been the necessary development theoretically allowing for the substitution to happen.

What is a capacitor? A capacitor comprises two metal sheets separated by an insulator. As it is charged, electrons accumulate on one of the sheets and an electrical field is gradually strengthened. Once a pathway for the electrons is opened, they are discharged.

Now what is a supercapacitor? A supercapacitor, or as it is more formally known, electric double-layer capacitor (ELDA) can store much higher charges than a regular capacitor. It exploits the fact that the charge it is capable of storing depend on the surface area of the sheet.

A common ELDA supercapacitor uses sheets coated by active carbon since its surface density is 3 000 m2 per gram (so a spoonful has the same area as a football pitch).

Difference between capacitors and batteries Unlike capacitors, batteries store energy in a chemical reaction. While capacitors deliver energy in rapid discharges (imagine tipping a bucket over), batteries deliver it in a continuous stream (imagine a bucket through a hole). This characteristic makes batteries more viable for important applications (like electric cars) than capacitors.

Supercapacitors may bridge the gap Supercapacitors may provide a solution halfway between batteries and capacitors. While they store much more energy than capacitors, it is released more slowly.

Furthermore, supercapacitors may solve one of the issues of battery-operated electric cars – it charges just as quickly as usual cars (the hours-long charging times seem to slow down their incorporation into the market).

The issue with current supercapacitors While supercapacitors may seem like a viable substitute, there is a challenge with energy density in comparison with chemical batteries. Current supercapacitors have only around 14% of Lithium ion’s 200Wh/kg energy density.

Possible solution may be found through the graphene supercapacitor Research indicates it may be viable to improve supercapacitors by substituting active carbon with graphene. At first, this substitution may seem counter-intuitive because graphene has lower surface area than active carbon (max. around 2 600 m2/g, while active carbon has 3 000 m2/g).

The benefit of graphene over active carbon is its very high conductivity. While active carbon’s conductivity is only 10-100 S/m, graphene can go well over 1700 S/m.

While it is promising in terms of conductivity, the energy density of max. 160 Wh/kg hints further development will be needed.

Benefits for the environment The benefit of active carbon/graphene-based supercapacitors is also an environmental one. While chemical batteries contain carcinogenic or toxic substances like lead and antimony, carbon is harmless for the environment. Hence the impact of production and products at the end of their lifecycle will be much more moderate.

Graphene supercapacitor - overview of the current situation

From the extensive research of graphene supercapacitors esp. in China and USA, it is apparent the scientific community is highly invested in the issue. Currently, a composite supercapacitor is being developed.

The supercapacitor uses a composite of graphene, nanotubes, and nano-scale ruthenium oxide; and excels in terms of conductivity. Again, the energy density will be a point in favor of common batteries; however carbon-based supercapacitors at the current technological level will be mostly selected for their low weight and excellent conductivity.

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