Qualitative Electrochemical Study of Titanium Carbide (Mxene)/Graphene Nanocomposites in Organic Electrolytes
Authors: Zhang Qing 
Published: May 2020

Titanium carbide (Ti3C2Tx, also called Mxene), uniquely combining metallic conductivity with hydrophilicity, has attracted burgeoning attention in batteries and supercapacitors application since reported in 2013. To achieve promising electrochemical performance in supercapacitors, aqueous electrolytes are favored for Mxene so far while using organic electrolytes commonly fails to deliver high power/energy densities due to the limited ionic diffusion between the hydrophilic Mxene intralayers in organic solvents such as acetonitrile or propylene carbonate, despite the virtues of high potential window and wide temperature-tolerance range of these organic electrolytes.
The energy densities of Mxene in various aqueous electrolytes (such as H2SO4(aq), K2SO4(aq), Na2SO4(aq), and KOH(aq))1-4 reported so far are as high as (400-500) F/g, which is quite impressive given that in organic electrolytes the values of which is only (70-100) F/g at slow scan rates5,6. Raising the energy storage capacity of Mxene in organic electrolytes will be of significant impact not only to improve the energy density through utilizing the high working voltage window, but also to extending the possible application in extreme conditions such as low temperature environment.
Herein, to explore the possibility of improving Mxene’s energy density in organic electrolytes, an unique structure of Mxene/graphene nano-sponge, comprised of “quasi-nanotube” like structures, has been fabricated, where the graphene skeleton serves as ionic conducting pathway as well as mechanical robust support, the synergistic effects of Mxene/graphene nanocomposites as electrode material has been observed, together with quite peculiar and interesting increase of specific capacitance (nearly 60%-70%) during cycling (as shown in Fig.1, in which Mxene only takes up 40 wt%. Higher the Mxene ratio is, higher the specific capacitance is; for examples, the specific capacitances before cycling of 90 wt% Mxene increase by ~3.87 times compared to that of 40 wt% Mxene).
Furthermore, together with investigation on structural transformation, extensive qualitative analysis are also performed on the ionic behavior and charge storage of Mxene/graphene nanocomposites in Et4NBF4 in Acetonitrile (AN) and in Propylene Carbonate (PC) solvents, to study the influences of graphene skeleton and organic solvents on ionic diffusion and specific capacitance, and to provide guidance in designing Mxene-based composite electrode for more advanced performance.
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Figure 1