Freestanding MXene-hydrogels prepared via critical density-controlled self-assembly: high-performance energy storage with ultrahigh capacitive vs. diffusion-limited contribution

Abstract

Solvated network of two-dimensional materials in the form of hydrogel offers a unique platform for full utilization of surface dominated properties at the macroscopic scale. However, development of such hydrogels...

Electric Field Guided Fast and Oriented Assembly of MXene into Scalable Pristine Hydrogels for Customized Energy Storage and Water Evaporation Applications

“…Thus in order to achieve sufficient ordering and compactness, the process heavily relies on relative alignment of MXene sheets in the dispersion that requires certain liquid crystalline phases, obtained only above high concentrations depending on sheet size. [ 20 ] The proposed voltage‐controlled release of Zn ions was confirmed from energy‐dispersive X‐ray spectroscopy (EDS) analysis of the MXene hydrogel samples; developed at different potentials but before acid washing. As confirmed from EDS, the presence of Zn in hydrogels increase by five folds when under 1 V bias (7.51 at.%) compared to that of normal gelation (1.36 at.%) without any potential (0 V); and double the amount present in samples developed from 0.5 V applied bias (3.59 at.%).…”

“…Most importantly, application of potential enables the development of self‐standing hydrogel from MXene dispersions of <10 mg mL −1 overcoming the high sheet size dependent critical concentration ( C ct > 40–60 mg mL −1 ) requirement when no voltage was applied. [ 20 ] Figure 1g and the Movie S1 (Supporting Information) demonstrate the comparison between hydrogels developed with 10 mg mL −1 MXene dispersion with and without applied potential where a clear difference between their stability can be observed. Importantly, the stability of the field‐induced hydrogel developed in 30 s compared to the 20 min conventional 0 V hydrogel indicates the strong difference in the two gelation methods.…”

“…MXene hydrogels are highly attractive candidate for supercapacitive energy storage owing to several beneficial characteristics like the high metallic conductivity of MXene, its intrinsic pseudo‐capacitance, restacking controlled highly porous structure, and abundance of hydrated ion transport channels. [ 20,37 ] 2D hydrogels are highly suitable in this regard due to their ready‐to‐use structure for a large area electrode assembly and easy controllability of areal mass loading by tuning the time of gelation. The supercapacitive performance of field‐assisted MXene 2D hydrogels (FMH) of different areal mass loading (gelation time 30, 60, 180, and 300 s) was investigated in a three‐electrode Swagelok type cell in presence of 3 m H 2 SO 4 electrolyte.…”

“…To overcome these limitations, the self‐assembly of MXene over metal plates through interfacial assembly and simultaneous reduction was introduced. [ 20,21 ] However, the self‐limiting nature of this assembly process along with the requirement of a very high critical concentration of MXene dispersion, are the major concerns for their practical adoptability. More importantly, this self‐gelation cannot give any customized control over hydrogel thickness and it's impossible to extend beyond thin 2D hydrogel to a 3D monolithic structure.…”

“…The presence of robust, low-cost, facile assembly techniques for the development of macrostructures from nanoscale building blocks that incorporates intrinsic application-specific adapt-this self-gelation cannot give any customized control over hydrogel thickness and it's impossible to extend beyond thin 2D hydrogel to a 3D monolithic structure. [20] Nevertheless, the principal concern with this technique is that the usable freestanding hydrogel can only be obtained from MXene dispersion of very high critical concentration that is highly dependent on sheet sizes. Thus, the requirement of such high critical concentration and sheet size dependency increases the overall complexity of MXene hydrogel development with reduced scalability.…”