Dense Charge Accumulation in MXene with a Hydrate-Melt Electrolyte

Abstract

Electrochemical double-layer (EDL) capacitors operating at high charge/ discharge rates are an important class of electrochemical energy storage devices. Aqueous EDL capacitors show great potential for use as inexpensive devices with much higher power; however, their energy density is severely limited by the narrow electrochemical window of water (1.23 V) and the small specific capacity of the electrodes. Here, we develop a highvoltage aqueous supercapacitor based on a highly concentrated Li + aqueous electrolyte (hydrate melt) and a two-dimensional titanium carbide MXene electrode. Experimental and theoretical analyses reveal the existence of dense hydrated Li + in the interlayer space of the deeply charged MXene, which is realized by the wide electrochemical window of a hydrate-melt electrolyte. The hydrate-melt electrolyte together with the largecapacitance MXene Ti 2 CT x improves the performance of an aqueous lithium-ion supercapacitor, offering a promising strategy for advanced aqueous capacitors.

Capacitive versus Pseudocapacitive Storage in MXene

“…[6,7] For example, we observed confinement effects where hydrated Li + and Na + ions in a nanoslit can deliver enhanced capacitance because of overscreening of the hydration shells. [9,10] Another type of EC is a pseudocapacitor (or a redox capacitor) that stores charge via fast surface Faradaic reactions. [11][12][13][14] There are several pseudocapacitive charge-storage mechanisms, such as underpotential deposition, quantum capacitance, redox capacitance, and intercalation pseudocapacitance.…”

“…[11] MXenes, an emerging class of 2D materials with the chemical formula M n+1 X n T x (where M is a transition metal, X is carbon or nitrogen, and T is the surface termination groups), behave either as a capacitor or pseudocapacitor electrode. [9,10,[15][16][17][18][19][20][21] With aqueous electrolytes, MXenes exhibit cation intercalation and have rectangular-shaped cyclic voltammetry (CV) curves, [9,10,22] which is typical of a capacitor electrode. [9,10] However, with nonaqueous electrolytes, MXenes show pseudocapacitive behaviors with highly distorted rectangular CV curves.…”

“…[9,10,[15][16][17][18][19][20][21] With aqueous electrolytes, MXenes exhibit cation intercalation and have rectangular-shaped cyclic voltammetry (CV) curves, [9,10,22] which is typical of a capacitor electrode. [9,10] However, with nonaqueous electrolytes, MXenes show pseudocapacitive behaviors with highly distorted rectangular CV curves. [16,17] For the rational development of high-performance MXene capacitors, clarifying the conditions under which MXene electrodes behave capacitively or pseudocapacitively is important, particularly from the viewpoint of their electronic states during charge/discharge.…”

Binder-free 2D MXene/activated carbon for High-performance Supercapacitors and Methylene Blue Adsorption

“…2D nanomaterials, for instance graphene, transition metal dichalcogenides, transition metal oxides/hydroxides and MXene have opened new possibilities in terms of energy storage applications because they could significantly improve the electrochemical properties as binder-free electrodes materials 15,16 . MXenes, as an emerging family of 2D transition metal carbides and nitrides has opened new possibilities in terms of energy harvesting and storage because they exhibit impressive mechanical performance, high electronic conductivity and exceptional rate capability [15][16][17] .…”

“…2D nanomaterials, for instance graphene, transition metal dichalcogenides, transition metal oxides/hydroxides and MXene have opened new possibilities in terms of energy storage applications because they could significantly improve the electrochemical properties as binder-free electrodes materials 15,16 . MXenes, as an emerging family of 2D transition metal carbides and nitrides has opened new possibilities in terms of energy harvesting and storage because they exhibit impressive mechanical performance, high electronic conductivity and exceptional rate capability [15][16][17] . 2D transition metal carbides MXene are generally produced by selective etching of the A-group (generally, group III A or IV A) element layers from MAX phase precursors with a general formula of M n+1 X n T x , where M is an early transition metal, X is carbon and/or nitrogen, T x denotes surface functional groups such as -OH, -F and/or -O and n is1-3 [18][19][20] .…”