2D metallic carbides, also called MXenes, have become the electrode materials of choice for the supercapacitor research community. Indeed, the 2D structure of these materials make them useful for capacitive storage because they offer the electrolyte ions a large accessible surface.
Although promising, MXenes still pose a problem: They are synthesized by reacting a precursor, called MAX, with an aqueous solution containing fluoride ions, such as HF. However, HF is a particularly dangerous solution to handle, which make the synthesis of MXenes quite hazardous. Therefore, for reasons of reaction, this choice limits the MAX precursors that we can use. It is also difficult to control the surface functions – and so their reactivity – of the materials produced. They usually contain high levels of –F and –OH.
An international team including members of the RS2E from CIRIMAT lab (University of Toulouse) and CEMHTI lab (University of Orleans) has developed a new synthesis route which makes it possible not to use aqueous solution containing fluoride ions and to broaden the range of MAX precursors that can be used. Their detailed study was published in the journal Nature Materials. You can find more general information about MXenes following this link.
Better performances in a non-aqueous electrolyte
Researchers have deepened existing research on a new synthesis route which replaces the use of aqueous solutions with Lewis acid molten salts.
[Ed: These salts are chemical entity of which one of its atoms has an electronic vacancy that make them susceptible to accept an electron doublet and thus to create a covalent bond with another chemical entity called Lewis base.]
Several consequences are due to this new reactivity. As previously mentioned, MXenes can be synthetized using a wider range of MAX precursor. Depending of the choice of the precursor/Lewis acid couple, the experimenters will be able to obtain different surface chemistry and more effectively control and anticipate the performances of the new electrode formed.
For example, the authors of this article have produced via the studied method a negative MXene electrode of formula Ti3C2 that has capacities of 200 mAh.g-1 in a commercial electrolyte for Li-ion batteries (LP30). Moreover, the unique electrochemical response of these materials makes it possible to achieve a high charge/discharge rate during the Li+ ion intercalation.
These capacity and power characteristics make these materials prepared in molten salts interesting electrodes for various energy storage systems such as high power batteries or Li-ion hybrid capacitors.
A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte
Youbing Li et al.
Nature Materials, 13/04/20, DOI : 10.1038/s41563-020-0657-0
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