Abstract
Carbons are the main electrode materials used in electrochemical capacitors, which are electrochemical energy storage devices with high power densities and long cycling lifetimes. However, increasing their energy density will improve their potential for commercial implementation. In this regard, the use of high surface area carbons and high voltage electrolytes are well known strategies to increase the attainable energy density, and lately ionic liquids have been explored as promising alternatives to current state of the art acetonitrile-based electrolytes. Also, in terms of safety and sustainability ionic liquids are attractive electrolyte materials for electrochemical capacitors. In addition, it has been shown that the matching of the carbon pore size with the electrolyte ion size further increases the attainable electric double layer (EDL) capacitance and energy density. The use of pseudocapacitive reactions can significantly increase the attainable energy density, and quinonic-based materials offer a potentially sustainable and cost effective research avenue for both the electrode and the electrolyte. This perspective will provide an overview of the current state of the art research on electrochemical capacitors based on combinations of carbons, ionic liquids and quinonic compounds, highlighting performances and challenges and discussing possible future research avenues. In this regard, current interest is mainly focused on strategies which may ultimately lead to commercially competitive sustainable high performance electrochemical capacitors for different applications including those requiring mechanical flexibility and biocompatibility.
Highlights
Nowadays, there is a great interest in improving our use of energy to make it more affordable and sustainable, driving research in grid management and renewable energy implementation and integration
While there are many different energy storage technologies, each one characteristic of a certain range of energy and power density range and appropriate for certain applications, electrochemical energy storage devices are attractive since they can achieve energy and power ranges not accessible by other technologies and which better suit most of the expected near future demands
More advanced carbon materials such as carbon nanotubes (CNTs) and graphene exhibit promising performance due to their outstanding properties arising from their peculiar structures
Summary
There is a great interest in improving our use of energy to make it more affordable and sustainable, driving research in grid management and renewable energy implementation and integration. Double layer mechanisms are based on electrostatic forces between electrical charges in the electrode and the electrolyte, while pseudocapacitive mechanisms are based on Faradaic redox reactions taking place at the surface of electrodes The increase of both quantities will result in an increased capacitance and energy stored, which can be further improved by increasing the operational voltage of the devices. Quinonic compounds, and ionic liquids (ILs) are considered to be the most promising materials capable of increasing double layer capacitances, pseudocapacitances, and operational voltages, respectively. These topics will be the focus of this Perspective
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