[Author: Girum Tiruye-Predoctoral Researcher, Electrochemical Processes Unit]
Currently, climate change and decreasing availability of fossil fuels require the scientific community to search for alternative renewable energy resources and high power & energy storage devices. Some of renewable energy resources (e.g. wind and solar energy) are intermittent, meaning sun does not shine continuously and wind does not blow on demand, but we all expect to get sufficient energy for our daily activities. Therefore, electrochemical energy storage devices combine with renewable energy resources are very crucial to store energy when there is excess energy production and to use it later whenever it is necessary. Electrochemical energy storage devices include batteries and electric double layer capacitors (EDLC).
Electric double-layer capacitors (EDLC) also known as supercapacitors (SCs) are one of the promising electrochemical energy storage devices that meet requirements for applications where high power values and fast charge-discharge cycles are needed. SCs utilize electrodes with high surface area and electrolytes to store energy by means of electrostatic interactions. It is necessary to keep in mind the differences between batteries and Supercapacitors both in their structure or material of construction and the way how they store energy either in physical or chemical processes. Batteries store energy chemically and as a result, they deliver higher energy than the one in SCs- that means they can keep running the device for long time, but they can also take long time to recharge when they run down and results in slow power delivery. Despite lower energy density, SCs have higher power density than batteries, meaning power delivery and charging process is very fast (fig 1).
Figure 1. Ragone plot of energy vs. power for batteries and Supercapacitors
Supercapacitors are constructed in such a way that a separator is sandwiched between two carbon electrodes sealed in electrolytes (fig 2). During charging, the ions of electrolyte move towards to opposite sign of electrode and during discharging, the ions move back to the electrolyte. Since energy density of SCs is the product of half of capacitance and squared operating voltage of electrolyte, the strategy to increase energy is mainly focus on increasing both capacitance and voltage. This can be achieved by two approaches: using an electrolyte with higher electrochemical stability window (ESW) such as Ionic Liquids (ILs) and developing active materials (mainly activated carbons) with high intrinsic capacitance and higher conductivity.
Figure 2. Representative of SCs during its charged state
The interest in ILs is growing worldwide due to their particular nature such as low vapour pressure, non-flammability, and high chemical, thermal & electrochemical stabilities. These unique properties of ILs make them promising electrolytes for the development of safer SCs compared to other electrolytes (water and organic based electrolytes).
Related to the second approach, an electrode material that has high conductivity, high surface area (>2000 m2/g), controlled pore structure, good corrosion resistance, high intrinsic capacitance or even pseudo-capacitance, and relatively low cost is more important to have best performing SCs.
By developing ionic liquid based electrolytes that able to withstand voltage up to 3.5 V and high capacitance electrode materials, the energy density of SCs can be increased . This would create the chance that in the near future, SCs will bridge the gap between high energy batteries and high power capacitors.
This is one of the challenges of the “RENAISSANCE ITN” European Project (http://www.renaissance-itn.eu). RENAISSANCE ITN is a multidisciplinary and intersectorial European research and training network in innovative polymers for a sustainable society. The main goal of RENAISSANCE ITN is to improve the career perspective of researchers by training at the forefront of research in the field of innovative polyelectrolytes for applications in Energy and Environment technologies. The role of IMDEA Energy within this project is to develop high performance supercapacitors through the use of innovative polyelectrolytes based on ionic liquids.
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