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Improving supercapacitors with hybrid graphene

Graphene hybrid made from metal organic frameworks (MOF) and graphenic acid make an excellent positive electrode for supercapacitors, which thus achieve an energy density similar to that of nickel-metal hydride batteries. Image credit: Prof. Dr. J. Kolleboyina/IITJ

Supercapacitors are … well capacitor with a super capacity in terms of energy storage. Like capacitors they can accumulate energy in a blink of an eye and likewise discharge it. Batteries take more time to be charged (in a capacitor you measure time in seconds, 1 to 10 seconds to fully charge it, in batteries you measure time in hours) but they have a much higher energy density (meaning that for a certain mass, you can store more energy (a lithium-ion battery can store some 200Wh/kg, 300Wh/l). Today’s supercapacitors have an energy density that is two order of magnitude lower than batteries.

Wouldn’t it be nice to have a supercapacitor with the same energy density of batteries? Think about it: recharging an electric car could be done in a matter of seconds! Now that will make electric cars really convenient.

Researchers at the Technical University of Munich (TUM) have been working with graphene looking for ways to increase its capacity. Graphene is a very thin, one atom thin!, layer of carbon and we are now starting to have industrial manufacturing capacity to create it in volumes at an industrial price. If you think about it, a capacitor is nothing but two sheets of material that can contain charges (electron). The amount of charges that con be contained per volume unit or per weight unit is what gives us the energy density, the more the better.  If you can store charges using very thin sheets of material than you can use larger sheets with lower weight and if can bend and roll these sheets you can fold them into a very little volume.

By using graphene TUM researchers have been able to do just that: they created a structure that packs a surface of 900 square meters in 1 gram of material!

Actually, to develop a supercapacitors (as well as a battery) you need several materials. They used chemically modified graphene and combine it with a nano structured metallic-organic framework (MOF) to create one side of the capacitor (one electrode) and a graphene acid for the other side (the other electrode). You can get all details reading their paper (you need to like technical stuff).

So is our next electric car going to come equipped with this kind of supercapacitor? Well, as with any lab research result it will take a few more year to move from a prototype to an industrial product. It is not just about having an affordable and effective manufacturing process, it is about controlling these supercapacitors.

It is great that you can charge them in a few seconds but that requires a huge amount of energy (power) that is way beyond what we currently have. A Tesla supercharger can fill up the car battery can deliver 150kW and takes 75 minutes for a full charge (20′ to charge the battery at 50%, that is the fastest part of the charging, the remaining 50% takes 55′). Now, 150kWh is no peanuts (at home you may have between 3 and 20 kWh). Recharging a supercapacitor with an energy storage equivalent to the one of a Tesla battery in a few seconds would require some 10 MW, the output of a small power plant!

It is worse than that. A supercapacitor is not just extremely fast in charging, it can be extremely fast in discharging! You will have electronic circuit to use the power at the rate you need, so over the period of a few hours driving, but what if there is an accident and the supercharger gets damaged and discharges at once! That is like having a bomb exploding under your seat. Not nice.

Hence, you see why it will take some time to move into real application. It is however impressive to see how much researchers are progressing thanks to material science, one of the key enablers for the progress we are going to see in this decade.

About Roberto Saracco

Roberto Saracco fell in love with technology and its implications long time ago. His background is in math and computer science. Until April 2017 he led the EIT Digital Italian Node and then was head of the Industrial Doctoral School of EIT Digital up to September 2018. Previously, up to December 2011 he was the Director of the Telecom Italia Future Centre in Venice, looking at the interplay of technology evolution, economics and society. At the turn of the century he led a World Bank-Infodev project to stimulate entrepreneurship in Latin America. He is a senior member of IEEE where he leads the Industry Advisory Board within the Future Directions Committee and co-chairs the Digital Reality Initiative. He teaches a Master course on Technology Forecasting and Market impact at the University of Trento. He has published over 100 papers in journals and magazines and 14 books.