Hierarchically Porous 3D Freestanding Holey-MXene Framework via Mild Oxidation of Self-Assembled MXene Hydrogel for Ultrafast Pseudocapacitive Energy Storage

The true promise of MXene as a practical supercapacitor electrode hinges on the simultaneous advancement of its three-dimensional (3D) assembly and the engineering of its nanoscopic architecture, two critical factors for facilitating mass transport and enhancing an electrode’s charge-storage performance. Herein, we present a straightforward strategy to engineer robust 3D freestanding MXene (Ti3C2Tx) hydrogels with hierarchically porous structures. The tetraamminezinc(II) complex cation ([Zn(NH3)4]2+) is selected to electrostatically assemble colloidal MXene nanosheets into a 3D interconnected hydrogel framework, followed by a mild oxidative acid-etching process to create nanoholes on the MXene surface. These hierarchically porous, conductive holey-MXene frameworks facilitate 3D transport of both electrons and electrolyte ions to deliver an excellent specific capacitance of 359.2 F g–1 at 10 mV s–1 and superb capacitance retention of 79% at 5000 mV s–1, representing a 42.2% and 15.3% improvement over pristine MXene hydrogel, respectively. Even at a commercial-standard mass loading of 10.1 mg cm–2, it maintains an impressive capacitance retention of 52% at 1000 mV s–1. This rational design of an electrode by engineering nanoholes on MXene nanosheets within a 3D porous framework dictates a significant step forward toward the practical use of MXene and other 2D materials in electrochemical energy storage systems.


Figure S2 .
Figure S2.Photographs of MXene hydrogels.Digital images of a) acid washed MH and b) HMH3.

Figure S3 .
Figure S3.Characterization of MXene hydrogels.Thermogravimetric (TG) analysis of MH and HMH3, displaying the water content in the hydrogels.

Figure S4 .
Figure S4.Morphological feature of MXene hydrogel.Low magnification TEM image of MH showing the clean surface feature and absence of any TiO 2 particles.

Figure S5 .
Figure S5.Morphological characterization of MXenes hydrogels.High magnification SEM images of a) HMH1 and b) HMH6 showing porous morphology of 3D hydrogel structure after freeze-drying, which confirms the mild etching process does not alter the microstructure of the MXene hydrogels.

Figure S6 .
Figure S6.Determination of surface area and pore size distribution of MXene hydrogels.a) N 2 sorption isotherms, and b) pore size distribution plots of MH and HMH3.

Figure S7 .
Figure S7.Phase characterization of MXene and MAX.XRD patterns of Ti 3 C 2 T x MXene and Ti 3 AlC 2 MAX.The XRD peaks of the used MAX match well with the standard XRD pattern of Ti 3 AlC 2 MAX.

Figure S8 .
Figure S8.XPS analysis of MXene and MXene hydrogels.Comparison of the XPS survey spectra of pure MXene, MH and HMH3.The arrows in the spectra represent the possible location of Zn 2p peak, if Zn was present in the sample.

Figure S10 .
Figure S10.Morphological characterization of MXene hydrogels.(a) High and (b) low magnification SEM images of MXene hydrogel prepared by ZnCl 2 before washing with 3 M H 2 SO 4 , indicating agglomerated MXene sheets in the hydrogel structure.

Figure S12 .
Figure S12.Rheological characterization of MH.Comparison of the rheological characteristics of MH, (a) before and (b) after acid wash, displaying the acid washing strengthens the hydrogel structure.

Figure S13 .
Figure S13.Morphology analysis of MXene electrode.SEM images of MXene hydrogel electrodes (a) before and (b) after Swagelok assembly.

Figure S15 .
Figure S15.Resistive drop characterization of different MXene hydrogels.Comparison of the drop values of MH and HMH3 at different current densities calculated from their  respective GCD curves.

Figure S17 .
Figure S17.Phase characterization of MXene and various MXene hydrogels.Comparison of the XRD patterns of different holey-MXene hydrogels (HMH1, HMH3 and HMH6) and pristine MXene.Increase in etching time (from 1h in HMH1 to 6h in HMH6) causes minor restacking of MXene sheets in the hydrogel.

Figure S19 .
Figure S19.Electrochemical characterization of various MXene hydrogels.Comparison of the EIS spectra of holey-MXene hydrogels at different electrode mass loadings.

Figure S21 .
Figure S21.Electrochemical characterization of various MXene hydrogels.CV profiles of (a) MH and (b) HMH3 at different scan rates in symmetric two-electrode configuration.

Figure S22 .
Figure S22.Electrochemical characterization of various MXene hydrogels.EIS spectra of (a) MH and (b) HMH3 in symmetric two-electrode configuration.The inset shows a zoomed view in EIS spectra at the high frequency regime.The absence of charge-transfer resistance in HMH3 indicates a significantly increased number of active-sites due to the introduction of nanoholes on the surface of MXene.

Figure S23 .
Figure S23.Rate performance characteristics.Variation of specific capacitance of MH and HMH3 at different scan rates in symmetric two-electrode configuration.

Figure S24 .
Figure S24.Long term stability test.Variation of capacitance retention and Coulombic efficiency with number of charge-discharge cycles for HMH3 in symmetric two-electrode configuration.The inset shows the comparison of CV curves before and after the stability test.

Figure S25 .
Figure S25.Supercapacitor performance test for various MXene hydrogels.Ragone plot of MH and HMH3 displaying the variation of energy densities and power densities of the devices at different scan rates.

Figure S26 .
Figure S26.Three-electrode test.Schematic of the Swagelok cell for three-electrode test showing the cell configuration and its components.

Table S1 .
Assignments of Ti 2p XPS spectra of pure MXene, MH and HMH3

Table S2 .
Assignments of C 1s XPS spectra of pure MXene, MH and HMH3

Table S3 .
Comparison of the electrochemical performance of HMH3 with other reported

Table S4 .
Comparison of areal capacitances and rate performances of various high mass

Table S5 .
Comparison of the gravimetric energy and power densities of various MXene-based