Researchers Demonstrate Viability of Eco-Friendly Supercapacitors

The main disadvantages of present-day lithium-ion battery technology include limited safety, recyclability, and sustainability, coupled with limited availability of starting materials (for example, cobalt).

Harald Fitzek, Christian Prehal, and Qamar Abbas (from left) at the SAXS facility SAXSpoint 2.0 (Anton Paar GmbH): With their work at Graz University of Technology, the researchers are providing new insights into hybrid supercapacitors. Image Credit: © Lunghammer—TU Graz.

In the hunt for alternative electrochemical energy storage systems for e-mobility and renewable energy storage, a specific blend of battery and capacitor shows robust potential: It has been tentatively called thhe “hybrid supercapacitor.”

The hybrid supercapacitor can be charged and discharged as fast as a capacitor and can store nearly as much energy as standard batteries. In contrast to the latter, it can be charged and discharged a lot faster and for more number of times: while the service life of a lithium-ion battery is a few thousand cycles, a supercapacitor can handle about one million charging cycles.

System Made of Carbon and Salt Water

A specifically sustainable, but hitherto largely uninvestigated variant of such a hybrid supercapacitor comprises carbon and aqueous sodium iodide (NaI) electrolyte, with a negative supercapacitor electrode and a positive battery electrode.

Recently, scientists at the Graz University of Technology closely explored how precisely the electrochemical energy storage in this supercapacitor takes place and what occurs in the nanometer-sized pores of the carbon electrode. They have published their promising findings in Nature Communications, a scientific journal.

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.

Christian Prehal, Study First Author, ETH Zurich

Prehal recently moved from the Institute of Chemistry and Technology of Materials at TU Graz to ETH Zurich.

Unexpectedly Higher Energy Storage Capacity

The team used small-angle X-ray scattering and Raman spectroscopy to demonstrate for the first time that solid iodine nanoparticles are formed in the carbon nanopores of the battery electrode while charging, and dissolve again while discharging.

This is in contrast to the formerly suspected reaction mechanism and is highly significant.

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.

Christian Prehal, Study First Author, ETH Zurich

This new basic know-how paves the way to hybrid supercapacitors or battery electrodes with higher energy density like never before and very fast charging and discharging processes. Hybrid capacitors such as these have been very effectively analyzed and advanced further for a number of years by Qamar Abbas, presently a Lise Meitner FWF scholarship holder at the Institute of Chemistry and Technology of Materials and another author of the study.

Targeted developments would enable hybrid supercapacitors to be used as a safe, economical, non-flammable, and sustainable substitute for stationary storage of electrical energy. This can be a lucrative option particularly to store the energy obtained from photovoltaic cells in residential houses, for instance.

New Investigation Method for Electrochemical Energy Storage Systems

The team accomplished another innovation pertaining to the investigation approaches used. In the case of Raman spectroscopy, researchers use the interaction of light with matter to understand the properties or structure of a material.

Structural variations that occur during electrochemical reactions are made visible by small-angle X-ray scattering (SAXS). Both approaches were performed in operando—live during the charging and discharging of a uniquely designed 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.

Christian Prehal, Study First Author, ETH Zurich

The research findings reveal that operando SAXS is the best choice to detect structural variations in a supercapacitor or battery on the nanoscale, directly “live” during charging and discharging. This new exploration method could thus be extensively used in the future, in the area of electrochemical energy storage.

Journal Reference:

Prehal, C., et al. (2020) Persistent and reversible solid iodine electrodeposition in nanoporous carbons. Nature Communications.

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