New findings pave the way to environmentally friendly supercapacitors


Date:
October 7, 2020
Source:
Graz University of Technology
Summary:
Similar to batteries, supercapacitors are suitable for the repeated
storage of electrical energy. Researchers have now presented a
particularly safe and sustainable variant of such a supercapacitor.



FULL STORY ========================================================================== Limited safety, sustainability and recyclability are key drawbacks
of today's lithium-ion battery technology, along with restricted
availability of starting materials (e.g. cobalt). In the search for
alternative electrochemical energy storage systems for use in e-mobility
and for storing energy from renewable sources, a combination of battery
and capacitor is very promising: the "hybrid supercapacitor." It can be
charged and discharged as quickly as a capacitor and can store almost
as much energy as conventional batteries. In comparison to the latter,
it can be charged and discharged much faster and much more frequently:
while a lithium-ion battery achieves a service life of a few thousand
cycles, a supercapacitor manages around one million charging cycles.


========================================================================== System made of carbon and salt water A particularly sustainable, but so
far quite unexplored variant of such a hybrid supercapacitor consists
of carbon and aqueous sodium iodide (NaI) electrolyte, with a positive
battery electrode and a negative supercapacitor electrode. Researchers
at Graz University of Technology have now investigated in more detail
how exactly the electrochemical energy storage in this supercapacitor
works and what happens in the nanometer-sized pores of the carbon
electrode, and have recently published their promising results in the scientific journal Nature Communications. "The system we are looking at
in detail consists of nanoporous carbon electrodes and an aqueous sodium
iodide electrolyte, in other words salt water. This makes this system particularly environmentally friendly, cost-effective, incombustible
and easy to recycle," explains Christian Prehal. He is the first author
of the study and has recently moved from the Institute of Chemistry and Technology of Materials at TU Graz to ETH Zurich.

Unexpectedly higher energy storage capacity With the aid of small-angle
X-ray scattering and Raman spectroscopy, the researchers were able to
show for the first time that solid iodine nanoparticles are formed in
the carbon nanopores of the battery electrode during charging, which
dissolve again during discharge. This corrects the previously suspected reaction mechanism and has far-reaching consequences, as Christian
Prehal explains: "The degree of filling of the nanopores with solid
iodine determines how much energy can be stored in the electrode. This
enables the energy storage capacity of the iodine carbon electrodes to
reach unexpectedly high values by storing all chemical energy in the
solid iodine particles." This new fundamental knowledge opens the way
to hybrid supercapacitors or battery electrodes with incomparably higher
energy density and extremely fast charging and discharging processes. Such hybrid capacitors have been very successfully investigated and further developed for several years by Qamar Abbas, currently a Lise Meitner
FWF scholarship holder at the Institute of Chemistry and Technology of Materials and another main author of the study.

With targeted improvements, hybrid supercapacitors can now be put to
use as a safe, non-flammable, cost-effective and sustainable alternative
for stationary storage of electrical energy. This can be an attractive
option especially for the storage of energy from photovoltaic cells in
private households, for example.

New investigation method for electrochemical energy storage systems The researchers achieved another breakthrough with regard to the investigation methods used. In Raman spectroscopy, the interaction of light with
matter is used to gain insight into the structure or properties of a
material. Small- angle X-ray scattering (SAXS) makes structural changes
during electrochemical reactions visible. Both methods took place in
operando, i.e. live during the charging and discharging of a specially developed electrochemical cell. "Both operando Raman spectroscopy and
operando SAXS were performed for the first time on a hybrid supercapacitor
with aqueous NaI electrolyte at the Institute of Electron Microscopy
and Nanoanalysis (FELMI) and in the soft matter application lab at Graz University of Technology. For the operando SAXS investigation, we have developed a special measuring cell for batteries and electrochemical
energy storage devices," explains Prehal. The results of the work show
that operando SAXS is ideally suited to follow structural changes in a supercapacitor or battery on the nanometer scale and directly "live"
during charging and discharging. This new investigation method could
therefore be widely used in future in the field of electrochemical
energy storage.


========================================================================== Story Source: Materials provided by
Graz_University_of_Technology. Original written by Susanne Eigner. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Christian Prehal, Harald Fitzek, Gerald Kothleitner, Volker Presser,
Bernhard Gollas, Stefan A. Freunberger, Qamar Abbas. Persistent
and reversible solid iodine electrodeposition in nanoporous
carbons. Nature Communications, 2020; 11 (1) DOI:
10.1038/s41467-020-18610-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201007093622.htm

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