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"Holey" Graphene Boosts Energy Density of Supercapacitors

Graphene still struggling to beat activated carbon in electrodes of supercapacitors

2 min read
"Holey" Graphene Boosts Energy Density of Supercapacitors
Image: California NanoSystems Institute/UCLA

Image: California NanoSystems Institute/UCLA

A research team at the California NanoSystems Institute (CNSI) at UCLA has developed what they call a “holey graphene framework” that they claim can significantly boost the energy density of supercapacitors.

Graphene has been held out by some as a silver bullet for enabling supercapacitors to combine the quick recharge and large bursts of energy of capacitors with the high storage capacity of electrochemical batteries. But so far using graphene as a replacement for activated carbon on the electrodes of supercapacitors has fallen short of expectations. It seems that graphene-based electrodes don’t enable energy storage much better than activated carbon, but because they can operate a higher frequencies than most supercapacitors they can be applied in novel ways, such as being used as part of filtering circuits in AC rectifiers.

Nonetheless, researchers working with graphene continue to try boost the energy density (the amount of energy stored per unit mass) of supercapacitors and still maintain the power density (the maximum amount of power that can be supplied per unit mass) of capacitors.

In research published in the journal Nature Communications, the CNSI team developed a three-dimensional graphene framework whose porous structure gives it a large ion-accessible surface area.

The researchers showed that the electrodes are capable of delivering a gravimetric capacitance of 298 Farads per gram (F/g) and a volumetric capacitance of 212 Farads per centimeter (F/cm). By way of comparison, researchers at George Washington University reported in April, that they were able to achieve 100 F/g with a combination of graphene and carbon nanotubes.

To give some more context, the specific energy density of the average lithium-ion (Li-ion) laptop battery is around 200 Watt-hour/kilogram (Wh/kg), whereas today’s upper average supercapacitor can get around 28 Wh/kg. In May, researchers at the University of California Riverside were able to achieve a full cell energy density of their graphene-and-nanotubes device of 39.28 Wh/kg.

The new CNSI researchers claim an energy density of a fully packaged device stack based on the holey-graphene framework is capable of 35 Wh/kg.

Because energy density is the brass ring that most of this research is reaching for, that’s what gets highlighted. However, the attractive bit about graphene-based electrodes for supercapacitors may be their superior electrical conductivity—making them useful for high frequency applications—and mechanical flexibility.

That said, the UCLA structure's unique hierarchical porosity may not be as attractive as it first seems once you consider that cigarette butts might have a better pore structure.

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Deep Learning Could Bring the Concert Experience Home

The century-old quest for truly realistic sound production is finally paying off

12 min read
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Image containing multiple aspects such as instruments and left and right open hands.
Stuart Bradford
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Now that recorded sound has become ubiquitous, we hardly think about it. From our smartphones, smart speakers, TVs, radios, disc players, and car sound systems, it’s an enduring and enjoyable presence in our lives. In 2017, a survey by the polling firm Nielsen suggested that some 90 percent of the U.S. population listens to music regularly and that, on average, they do so 32 hours per week.

Behind this free-flowing pleasure are enormous industries applying technology to the long-standing goal of reproducing sound with the greatest possible realism. From Edison’s phonograph and the horn speakers of the 1880s, successive generations of engineers in pursuit of this ideal invented and exploited countless technologies: triode vacuum tubes, dynamic loudspeakers, magnetic phonograph cartridges, solid-state amplifier circuits in scores of different topologies, electrostatic speakers, optical discs, stereo, and surround sound. And over the past five decades, digital technologies, like audio compression and streaming, have transformed the music industry.

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