Research Findings Uncover a More Sustainable Hybrid Energy Storage System

Researchers from TU Graz University in Austria, delve deeper into the electrochemical energy storage of a particular supercapacitor system. By doing so, the team addresses challenges facing energy storage which includes sustainability, ease of recyclability, and safety. 

The Graz researchers investigated the nature of a hybrid supercapacitor composed of carbon and saltwater. 

 

Researchers from Graz University, Harald Fitzek, Christian Prehal, and Qamar Abbas (from left) at the SAXS facility SAXSpoint 2.0 (Anton Paar GmbH). Image used courtesy of Graz University

 

The Hybrid Energy Storage System

The system that the research was centered upon was a hybrid system composed of a capacitor and battery. The potential energy of a capacitor is stored in an electric field, while a battery stores its potential energy in the chemical form before it is later converted into electric energy. 

Integrating a capacitor with a battery is beneficial as it brings together complementary features for enhancing energy storage and electrical power. Batteries are known for having a high energy density but slow charging time, while capacitors charge much more quickly and have low specific energy. Capacitors are also capable of high load currents and excellent temperature performance. On the other hand, batteries are more temperature-sensitive but tend to have a better leakage current than capacitors. A “hybrid supercapacitor” with the quick charge and discharge of a capacitor and high energy storage of conventional batteries provides a promising avenue for a new energy storage system. 

 

A Supercapacitor Based on Carbon and Saltwater

The Graz University researchers focused their research on a unique type of hybrid supercapacitor, one that is more sustainable. The system uses carbon and aqueous sodium iodide (NaI) electrolyte with a positive battery electrode and a negative supercapacitor electrode. Just how the supercapacitor works regarding electrochemical energy storage and what exactly goes on in the nanometer-sized pores of the carbon electrode was illuminated by the investigators at Graz. 

"The system we are looking at in detail consists of nanoporous carbon electrodes and an aqueous sodium iodide electrolyte, in other words, saltwater. This makes this system particularly environmentally friendly, cost-effective, incombustible, and easy to recycle," explains the first author of the study, Christian Prehal, in a recent news release.

 

Enhanced Energy Storage Capacity

During experimentation, researchers were able to show for the first time that solid iodine nanoparticles form within the carbon nanopores of the battery electrode during charging. 

These then dissolve once more during discharge. 

Prehal explained that this filling of the carbon nanopores is what determines the level of energy storage in the electrode. Storing the iodine nanoparticles in this way is what Prehal says allows for higher energy storage capacity for the iodine carbon electrodes. 

 

Christian Prehal, first author of the research recently relocated to ETH Zurich from Graz University. Image used courtesy of Graz University

 

In order to come about their findings, the Graz researchers utilized investigative methods known as small-angle X-ray scattering (SAXS) and Raman spectroscopy. The former method provides visibility for structural changes that may occur during electrochemical reactions. Raman spectroscopy uses the interaction between light and matter to establish the structure or properties of a material. The two methods were both used live during the charge and discharge of a hybrid supercapacitor. The SAXS method in particular was shown to be well suited for following structural changes in a supercapacitor or battery with nanometer precision. 

With further exploration and development, hybrid supercapacitor technology such as that seen here can pave the way toward cost-effective, safer, more efficient, and more sustainable stationary storage of electrical energy.