Interfacial Engineering of Ti3C2Tx MXene Electrode Using g-C3N4 Nanosheets for High-Performance Supercapacitor in Neutral Electrolyte

The superior performance of the Ti3C2Tx (MXene)-based supercapacitor in acidic electrolytes has recently gained much interest in the energy storage community. Nevertheless, its performance in most neutral electrolytes is unfavorably low, plausibly due to limited ion diffusion between the MXene layers. Herein, protonated g-C3N4 (pg-C3N4) is incorporated into the Ti3C2Tx electrode by using a facile self-assembling process and annealing, which results in increased interlayer d-spacing and electrical conductivity of the composite electrode. As a result, the annealed Ti3C2Tx/pg-C3N4 film revealed an enhanced ion-accessibility and gravimetric capacitance of 140 F g–1 in 1 M aqueous MgSO4 electrolyte. The cyclic stability test also indicates excellent capacitance retention, with negligible loss of capacitance over 10000 cycles.


Synthesis of Delamination Ti3C2Tx
Ti3C2Tx was synthesized following the minimally intensive layer delamination (MILD) method. 1 Briefly, 1.6 g of lithium fluoride )LiF, >98% purity, Alfa Aesar( was dissolved in 20 mL of 9 M HCl (RCI Labscan, Ltd.) solution and stirred for 5 min in an HDPE bottle.This solution was then moved to an ice bath, and 1 g of Ti3AlC2 (MAX) powder (>99.5%,>500 mesh, Laizhou Kai Kai Ceramic Materials Co., Ltd.) was slowly added to the solution.Then, the resulting mixture was stirred at 35 °C for 24 h.The mixture was washed several times with deionized water via repeated centrifugation at 3500 rpm until the pH was neutral.After that, the mixture was shaken manually for 10 min for further exfoliation, followed by centrifugation for 1 h at 3500 rpm.The supernatant was extracted and stored in a refrigerator.The concentration of the colloidal solution was kept at 0.5 mg mL -1 .

Synthesis of Protonated Graphitic Carbon Nitride (pg-C3N4) Nanosheet
Protonated g-C3N4 was synthesized according to the literature. 2 Briefly, 1 g of dicyandiamide powder (99%, Sigma Aldrich) and 10 g of ammonium chloride were mixed and then heated in air at 550 °C for 4 h.The obtained pale-yellow powder was then sonicated in ethanol for 2 h.To ensure complete delamination of the nanosheets, 100 mL concentrated H2SO4 (98% RCI Labscan, Ltd) was added, and the mixture was vigorously stirred at room temperature for 12 h.Then, the obtained milk-like solution was washed with deionized water via vacuum filtration.The protonated g-C3N4 nanosheet (pg-C3N4) was dispersed in deionized water under sonication for 30 min, resulting in a stably dispersed pg-C3N4 solution.The concentration was kept at 0.02 mg mL -1 .

Fabrication of Ti3C2Tx/pg-C3N4 Freestanding Film
The composited freestanding films were fabricated via the self-assembling process.The prepared Ti3C2Tx colloidal solution was gradually added into pg-C3N4 solution and stirred for 30 min using varying pg-C3N4 contents from 1 wt.% to 10 wt.%.During this step, the negatively charged MXene sheets self-assembled with the positively-charged pg-C3N4 in solution.The mixture was then vacuum-filtrated through a hydrophilic PVDF membrane (47 mm, pore size 0.22 µm).After drying, the freestanding film of Ti3C2Tx/pg-C3N4 was peeled off from the membrane.The Ti3C2Tx/pg-C3N4 films prepared with 0, 1, 5, and 10 wt.% pg-C3N4 were denoted as MXene, MCN1, MCN5, and MCN10, respectively.The as-prepared freestanding electrode has a mass loading about 1.14 mg cm -2 .To improve the conductivity and interfacial contact of the composite, the prepared freestanding film was annealed at 200 °C for 2 h under an Ar atmosphere.The annealed samples were labeled as a-MXene, a-MCN1, a-MCN5, and a-MCN10.

Materials Characterization
X-ray diffractometer (XRD, PANalytical EMPYREAN, Cu Kα radiation 1.542 Å) was carried out to investigate the sample crystallography at 2θ = 5-70 at 2° min -1 of scan rate.A Cold field emission scanning electron microscope (FE-SEM, SU8010, Hitachi( and energydispersive X-ray analyzer (EDX) was utilized to obtain cross-sectional images and element mapping.A Zetasizer (Malvern instrument, Nanoseries ZS, Worcestershire, UK) was used for zeta potential and particle size distribution measurements.A Fourier-transform infrared spectrometer (FTIR, PerkinElmer 2000) was used for the characterization of the surface functional group.A UV-visible spectrophotometer (UV-VIS, Shimadzu UV-2600) was employed to measure the optical properties of the colloidal Ti3C2Tx solution, and the solution concentration was deduced from the absorbance at 760 nm.Atomic force microscope (AFM) (NX 10, Park Systems, Korea) was utilized for the thickness measurement of MXene single flake.Raman spectroscopy (Thermo Fisher Scientific, DXR smart Raman, 785 nm excitation laser) was used to study material interaction.The valence states of the elements were investigated using X-ray photoelectron spectroscopy (XPS) with a Kratos AMICUS instrument, employing an Mg Kα anode (1253.6 eV, 10 mA, 10 kV).The X-ray incidence angle on the sample was set to 50°, with an emission angle of 90°.The base pressure was maintained below 10 -8 Pa.Samples were cut into 6 mm circular films and fully loaded into the sample holder, with an analysis area of approximately 200 μm in beam diameter.Due to the highly conductive nature of our samples and the absence of sputtering during measurement, a charge neutralizer was not utilized.Prior to analysis, all samples were stored in vacuum-sealed polyethylene bags.The full width at half maximum (FWHM) of the Ag 3d5/2 peak was approximately 1 eV.Binding energies were calibrated using the C1s (C-C) peak of adventitious carbon at 284.88 eV.The reference binding energy (E B F ) was calculated as E B F = 289.98eV + φSA, where φSA represented the material's work function energy, approximately 4.7 eV. 3,4XPS spectra were analyzed using CasaXPS software, employing Shirley background subtraction and asymmetric line shape fitting.

Electrochemical Characterization
All electrochemical measurements were performed in a three-electrode Swagelok cell.
The prepared freestanding film of Ti3C2Tx/pg-C3N4, activated carbon freestanding film (Kuraray YP-50F), Ag/AgCl in 3 M KCl, and 1 M MgSO4 served as working electrode, counter electrode, reference electrode, and electrolyte, respectively.Cyclic voltammogram (CV) was measured from -1.0 V to 0.1 V with the scan rate from 2 to 200 mV s -1 .In three-electrode system, gravimetric (Cg) and areal capacitances (CA) were calculated from the CV discharging curves using eq 1 and eq 2, where i, V, m, ν, ∆V, A denote current (A), potential applied (V), mass of the freestanding film (g), scan rate (V s -1 ), potential window (V), and the area of electrode (cm 2 ), respectively.

Figure S2 .
Figure S2.(a) g-C3N4 powder obtained after sintering at 550 °C for 4 h without ammonium

Figure S3 .
Figure S3.Particle size distribution of the prepared precursors in water.

Figure S5 .
Figure S5.(a) The delaminated Ti3C2Tx suspension in water before and after being mixed with

Figure S7 .
Figure S7.Equivalent circuit model for EIS fitting, where Rs, RCT, CPE, and CDL corresponding

Figure S8 .
Figure S8.Physical appearance of pg-C3N4 in water (a) before and (b) after thermal treatment.

Figure S9 .
Figure S9.Raman spectra at 785 nm excitation wavelength of the freestanding films (a) before

Table S1 .
XPS peak fitting data for a-MCN1, consisting of peak position, FWHM, and the definition of each species.