"Smart" electrolytes for supercapacitors
A group of researchers from RS2E (ICGM, Montpellier), the University of Bordeaux and the University of Graz has studied the impact of electrolyte design on the performance of supercapacitors. Their study, published in November 2016 in Nature Materials, differs from previous researches on improving the capacities of supercapacitors, which usually focuses on electrode materials. Here, by adding redox functions to both the cation and the anion of the electrolyte, the researchers have succeeded in forming ionic biredox liquids.
An electrolyte-centered approach
In the field of batteries (Li-air or Li-ion) and supercapacitors, there are already approaches to add redox molecules as additives in the solution with very different objectives, depending on the application. For Li-air batteries, the objective is to improve the kinetics of charge or discharge. With Li-ion, it is necessary to avoid the degradation of the electrolyte. Concerning supercapacitors, increasing capacities using redox molecules in the electrolyte had already been addressed. The originality of this study is to have functionalized the electrolyte by both reducing and oxidizing functions (so that each reacts to their respective electrode). This adds an additional function beyond conducting the ions. It can be said to be an "smart" electrolyte.The work was based on informal collaborations between electrochemistry enthusiasts, and in the continuity of previous long term collaborations between the universities of Graz and Bordeaux. In terms of exploratory work, the only support for the researchers was a ministerial thesis (Dr. Eléonore Mourad) and a thesis by the ANR Flexcap (Dr. Laura Coustan), both from the ICGM laboratory (Montpellier).
Fig 1 : Example of capacitance retention of supercapacitors with biredox electrolytes
(red) vs. Supercapacitors (blue) © Nature Materials / Researchers' work
The experiments were carried out thanks to the laboratory's own funds. An unprecedented role has been conferred to electrolytes based on ionic liquids. By controlled molecular assembly, the researchers have added storage functions, called redox, to the ions of the ionic liquid. These redox ionic liquids can store electrical energy in addition to that stored iin the electrode material. The scientists measured an increase of 100 to 300% in the specific capacity in the supercapacitors (Figure 1). This is a disruptive approach because performance improvement is no longer limited to the formulation of new electrode materials: the electrolyte is actively involved in the storage.
The “smart” electrolyte redox: the beginning of a large family?
In doing so, scientists have revealed a new concept of electrolyte, and re-evaluated its role in supercapacitor performance (Figure 2). A drastic increase of the specific capacity, usually considered as supercapacitors weakness compared to batteries, has been measured. The capacities of pseudocondensors have even been approached by using EDLC (electric double-layer capacitor) electrode materials: porous carbons!
Fig 2 : Comparison in a Ragon diagram of supercapacitors using various electrolytes, one sees in red the increase obtained in terms of specific energy © Nature Materials / Researchers' work
This concept shows great potential: the electrolyte can now be used to increase the amount of energy stored! But this concept of “smart” electrolyte is not limited to this single energy function: researchers are now interested in electrolytes that solidify when the device is charged, which limits the risk of leakage and maintains the device longer in the charged state when not used. This idea is an example among others because there is a large number of functions sensitive to various stimuli: temperature, mechanical stress, light ... These "smart" electrolytes will allow important scientific and technological openings for which only the imagination of researchers and engineers will be the limit.To find concrete applications, it is still necessary to obtain low-cost ionic bicarbon liquids and integrate them into 18650 prototypes (format used in industry) in order to conclude of their possible impact industrial.
Biredox ionic liquids with solid-like redox density in the liquid state for high-energy supercapacitors
Eléonore Mourad, Laura Coustan, Pierre Lannelongue, Dodzi Zigah, Ahmad Mehdi, André Vioux, Stefan A. Freunberger, Frédéric Favier and Olivier Fontaine
Nature Materials, 28/11/2016, DOI : 10.1038/NMAT4808