Electrode with chlorine gas for high power and energy density
Supercapacitors are energy-storage devices that complement
rechargeable batteries, and could even partially replace them. Current
supercapacitors do not have sufficient energy density, so they don’t
last long enough. A novel approach for making a supercapacitor with a
“breathing” electrode is far superior. As the team that developed it
explain in the journal Angewandte Chemie, they drew inspiration from a lizard that brings along an air bubble to breathe from when it dives under water.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Today, the applications for supercapacitors include compensation for
short-term power outages in facilities like hospitals or data processing
centers, and buffering for consumption spikes by electronic devices.
Supercapacitors that are charged by braking energy help modern
streetcars and buses to save electricity. They are also increasingly of
interest to the solar energy industry for stabilizing voltage
fluctuations.
In contrast to rechargeable batteries, supercapacitors are the
“sprinters” of energy storage: they can produce very high currents in a
very short time (high power density). However, they are poor
“long-distance runners” because, even with low electricity usage, they
don’t last very long (low energy density). Modern electric energy
storage needs to combine both traits, and have a low weight.
Unfortunately, methods for increasing energy density have so far always
come at the cost of power density—the stumbling block for the
advancement of supercapacitors.
A team led by Long Chen, Cheng Lian, Xiangwen Gao, and Chunzhong Li
at East China University of Science and Technology (Shanghai, China) and
the University of Oxford (UK) has now begun to overcome this challenge.
Their inspiration came from a little lizard. Anolis lizards live
on land but can also breathe underwater when they dive in search of
food. To do this, they bring along an air bubble that is attached to a
layer of scales on their head. Under water, they repeatedly breathe this
bubble in and out. The newly developed electrode made of porous carbon
materials (most favorable were multi-wall carbon nanotubes with pores of
about 3 nm in diameter) can also hold onto a layer of gas when it is
submerged in a solution of table salt as electrolyte. The gas used isn’t
air, however, it is chlorine.
During charging and discharging, this electrode undergoes a redox
reaction in addition to the charge separation usual for supercapacitors.
Upon charging, the electrode transfers electrons to the chlorine gas,
reducing chlorine to chloride ions, which go into solution—the electrode
“exhales”. Upon discharging, the chloride ions are oxidized back to
chlorine, which returns the gas to the pores of the electrode—the
electrode “inhales”. By using a variety of analytical methods, the team
demonstrated that no chlorine gas escapes the electrode. The very rapid
reduction/oxidation and rapid mass transfer in the thin layer of gas
drastically increases the energy density of the supercapacitor while
maintaining an extremely high power density. The capacity remains at the
same high level even after thousands of cycles.
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About the Author
Dr. Chunzhong Li is a
Professor at the department of Chemical Engineering, East China
University of Science and Technology. His main specialty is synthesis
technology of new energy materials by confined reaction, fabrication;
and processing of polymer-based nanocomposites, and the engineering
properties and process scale-up for nanomaterials.
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