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Breakthrough in energy storage Supercapacitors with high energy and power density

From Kristin Rinortner 2 min Reading Time

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Supercapacitors have a high power, but not a high energy density - meaning the storage capacity is limited. Chinese scientists have developed a novel approach for a supercapacitor with a "breathing" electrode made of carbon nanotubes with a significantly better energy density.

Supercapacitors: Scientists have achieved a breakthrough in the storage capacity of supercapacitors with an innovative "breathing" electrode made of chlorine gas.(Image: (c) Wiley-VCH)
Supercapacitors: Scientists have achieved a breakthrough in the storage capacity of supercapacitors with an innovative "breathing" electrode made of chlorine gas.
(Image: (c) Wiley-VCH)

Supercapacitors are energy storage devices that can complement or partially replace batteries. They provide very high currents in very short periods (high power density). However, their runtime is very short (low energy density). Modern electrical energy storage, however, should combine both. Unfortunately, approaches to increasing energy density have so far always come at the expense of power density – a stumbling block for the use of supercapacitors.

A team from the East China University of Science and Technology (Shanghai, China) and the University of Oxford (UK), led by Long Chen, Cheng Lian, Xiangwen Gao, and Chunzhong Li, is now on their way to master this challenge. They have published their results in the journal Angewandte Chemie.

Supercapacitors with "breathing" electrode

The inspiration came from a small lizard that breathes in an air bubble while diving underwater: Anoles live on land, but dive to forage for food. To do this, they carry an air bubble that sticks firmly to a scale layer on their head. Underwater, they repeatedly breathe in and out into this bubble.

The newly developed electrode made of porous carbon materials (multi-walled carbon nanotubes with approximately 3 nm pores were found to be most favorable) can also hold a layer of gas when dipped in a saline solution as an electrolyte. However, air is not used, but instead, chlorine gas.

During charging and discharging, in addition to the charge shifts typical for supercapacitors, a redox reaction occurs at this electrode: When charging, the electrode transfers electrons to the chlorine gas and reduces chlorine to chloride ions, which dissolve – the electrode "exhales". During discharging, chloride ions are oxidized back to chlorine, the gas reforms in the pores of the electrode – the electrode "inhales".

Drastically increased energy density, extremely high power density

Using various analytical methods, the team showed that no chlorine gas escapes from the electrode. The very rapid reduction/oxidation and the fast mass transfer in the thin gas layer drastically increase the energy density of the supercapacitor while maintaining the extremely high power density. According to the scientists, the capacity remains almost unchanged high even after thousands of cycles.(kr)

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