Preparation Route of MXene

The preparation of MXene was initiated by etching the aluminum layer of Ti₃AlC₂ with a mixed solution of LiF and HCl. Firstly, 10 mL HCl (9 M) was added to 0.8 g LiF for a 10-min chemical reaction to prepare the etchant. Then, Ti₃AlC₂ was added into the etchant solution slowly to obtain the acidic suspension. Afterward, the suspension was magnetically stirred for 48 h at 40 °C and then rinsed with deionized water multiple attempts until almost neutral (pH~7). Finally, the obtained suspension was subjected to an ice bath for 6 h ultrasonic vibration followed by 30-min centrifugation in an Ar atmosphere to yield a stable supernatant of dark green MXene. Figure 1 displays the preparation route of MXene until its application as a supercapacitor.

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Fig. 1

Preparation route of the MXene/3D needled denim felts as electrodes and supercapacitors

Preparation of Carbonized MXene/3D Needled Denim Felts (CMDF)

3D needled waste denim felts (DF) was sourced from waste jeans via the cutting–opening–carding–needling process (Fig. 1), and more details about the preparation method can be found elsewhere [25]. The DF with an area density of 0.022 g cm⁻² was chosen as the substrate. After needling, DF samples were cleaned with acetone and deionized water to remove pollutants.

Various MXene suspension concentrations (2%, 4%, 6% and 8%) were adopted to obtain MXene/3D needled waste denim felts (MDF) with high MXene load. The loadings of MXene are shown in Fig. S1. Initially, the DF samples were immersed thoroughly in the prepared MXene suspension for 10 min. Then, DF samples were fished out and dried at room temperature for 120 min. The dried MDF was carbonized in a tube furnace at different temperatures (800, 1,000 and 1,200 °C) under a heating rate of 5 °C min⁻¹ for 60 min in an Ar environment to acquire CMDF. On the other hand, the DF (without MXene coating) was directly carbonized in a tube furnace to yield carbonized 3D needled denim felts (CDF) for comparison. The initial thickness of the DF was measured at 1.5 mm, which was subsequently carbonized to produce CMDF with a thickness of 0.7 mm. The MXene morphologies present on both the initial DF and the resulting CMDF are shown in Fig. S2.

Characterizations

The microstructure and surface morphology were inspected by transmission electron microscopy (TEM, JEM-2100F, Japan) and scanning electron microscopy (SEM, Quanta 450FEG, USA). The X-ray diffraction technology (XRD, D8 Advance, German) was utilized to investigate the sample phase compositions, while a inVia Reflex micro-Raman spectroscopy system was adopted to produce Raman spectra. X-ray photoelectron spectroscopy was utilized to examine the sample chemical components. The thermal decomposition process of samples (from 50 to 1200 °C) was evaluated by a thermal analysis infrared mass spectrometer under an N₂ environment at a thermal rate of 10 °C min⁻¹. The water contact angle of samples was investigated and calculated via the sessile drop method and SCA 20 software. Figures S3-S5 show the SEM images, electrical conductivity, fiber diameters and MXene load in MDF under different concentrations of MXene solution. It was determined that the optimal MXene solution concentration in this work was 6%.

Carbonization Mechanism of MXene and Architectural Feature of CMDF

The diameter of the fibers in MDF and CMDF that adhere to one another is wider than those of DF and CDF. This is likely due to the effectiveness of the MXene loading during the impregnation process (Fig. 3). There was a negligible effect of carbonization on the needled felt pore structure (Fig. S6). Furthermore, the Ti 2s spectrum of MDF is similar to that of MXene while no peak was detected for DF and CDF (Fig. 2d). It is observed that the CMDF presents two typical peaks at 459 eV (Ti–O) and 465 eV (Ti-C), suggesting Ti element was transformed into TiO₂ and further converted into TiC during the high carbonization temperature. No signal of the F 1s spectrum was detected for CMDF, implying the removal of -F groups in MXene during the carbonization process. Figure 3d1–d4 demonstrates the water droplets drop vertically on the surface of samples for 10 s. It was found that the coating of MXene had improved the hydrophilicity of the denim needled felt where the contact angle was reduced from 115° to 80°. At elevated temperatures, the hydrophilic –OH group of cellulose and MXene disappears, yet the carbonized denim fiber attains a fine texture that enhances water transfer [32]. This has led to the reduction in CMDF contact angle due to competitive effects between the two. The consequent increase in hydrophilicity is advantageous for electrolyte penetration, thereby elevating the electrochemical properties of CMDF. Figures S7 and S8 show the EDX mapping and electrochemical test of CMDF under various carbonization temperatures (800, 1000 and 1200 °C).

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Fig. 3

Comparison of SEM image, XPS spectra of F 1s and Ti 2s, water contact angle for different samples. a1, b1, c1, d1 DF, a2, b2, c2, d2 MDF, a3, b3, c3, d3 CDF and a4, b4, c4, d4 CMDF

Performance of CMDF Electrode

In order to ascertain the most effective MXene loading concentration, various MXene concentration solutions were utilized for verification. As evidenced in Fig. S9, when subjected to the same carbonization temperature, the specific capacitance of 1000CM_6%DF exceeded that of 1000CM_8%DF, which in turn surpassed that of 1000CM_4%DF, with 1000CM_2%DF being the least effective. In addition, when the current density is increased to 6 mA cm⁻², the capacitance of the CMDF electrode remains at 888 mF cm⁻², showing excellent rate performance. The findings obtained can potentially be attributed to the reduction in spacing between neighboring fibers and MXene sheets due to the coated MXene layer. This increases the overall bulk density of the electrochemically active material, thereby affecting its performance. However, it was observed that excessive MXene loading can lead to surface sealing of the DF, thereby hindering the complete contact of the material with the electrolyte, ultimately leading to compromised electrochemical performance. Therefore, it was concluded that a 6% MXene load is optimal under these circumstances.

During low scanning rates, CV curves of the CMDF electrode exhibit rectangular symmetric behavior, but at high scanning rates with significant capacitance, they become fusiform due to polarization (Fig. 4a). However, the analysis reveals that CDF exhibits negligible electrochemical properties (refer to Fig. S10), indicating that the electrochemical properties of CMDF are highly comparable to carbonized MXene. Notably, carbonized MXene exhibits a notably larger interlayer spacing than MXene. This is primarily attributed to the generation of TiC and TiO₂ at high temperatures, which facilitates the entry of the electrolyte into the interlayer, thereby enhancing the electrochemical performance of the material. The curves exhibit a triangle-like structure when the current density increases, indicating good reversibility, and also there are no obvious voltage drops, which further implies high electrical conductivity (Fig. 4b). The optimum specific capacitance of the CMDF (1748.5 mF cm⁻²) was detected at a current density of 0.5 mA cm⁻², suggesting that CMDF could demonstrate higher performance compared to other MXene supercapacitors. The fitting curves of CMDF show a higher slope at the low-frequency zone than the pristine Nyquist curve, indicating rapid ion diffusion occurs (Fig. 4c). Besides, the CMDF showed a 94% capacitance retention and a 99% coulombic efficiency after 15,000 GCD cycles (Fig. 4d), suggesting that the CMDF has excellent potential as an electrode for high-performance supercapacitors in the future.

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Fig. 4

a CV graphs under different scanning rates. b GCD graphs under different current densities. c EIS spectra. d Cycling stability (at 6 mA cm⁻²) of the CMDF. e MXene load and capacitance of the CMDF with other reported MXene relevant textiles. f TG-IR-MS 3D results, g TG curves and h FTIR spectra of MDF heated from 0 to 1,200 °C. i Decomposition gas analysis of DF and MDF during 0 to 1,200 °C

This study shows that the MXene load of CMDF is higher than most existing studies, thus demonstrating the structural superiority of 3D needled felt (Table S1, Fig. 4). At 0.5 mA cm⁻² current density, the CMDF specific capacitance detected was 1748.5 mF cm⁻² (Fig. 4e) which is significantly higher than the MXene-coated cotton fabric (362 mF cm⁻²) [33]. Furthermore, the specific capacitance of CMDF calculated from CV curves (1290 mF cm⁻²) is also higher than MXene/silk fabric (707 mF cm⁻²) at 2 mV s⁻¹ [34], MXene/cotton fabric (794.2 mF cm⁻²) [35] and MXene/carbon fabric (416 mF cm⁻²) at 10 mV s⁻¹ [26]. Again, the CMDF electrode with a remarkable specific capacitance offers great promise as a high-performance electrode.

The TG-IR-MS combined system was applied to acquire data of CMDF in real time during 0–1200 °C (Fig. 4f). During the first stage at 83 °C, obvious mass loss is caused by water evaporation while cellulose cracks lead to an increase of the H₂O, –OH and –O signals at 225 °C (Fig. 4g). The second stage occurs when surface groups of dyes or MXene sheets dissociate slowly at 383 °C into O₂ and H₂, with H₂ being released when the –OH terminals or molecular hydrogen trapped between MXene sheets combine. When the temperature reaches 1017 °C and above (the final stage), MXene begins to crack thermally and gradually releases carbon dioxide by eliminating –F groups. Figure 4h shows the corresponding FTIR spectra of the CMDF. This analysis indicates that the TiO₂ generation in CMDF begins gradually from 273 °C until it reaches equilibrium between 464 and 774 °C, which corresponds to the release of CO₂ at 2356 cm⁻¹ at 267 °C. Moreover, the F and -OH groups in MXene are continuously removed to generate CO₂. For the gas productions of DF and MDF during carbonization, the amount of CO₂ and H₂O shows significant differences despite their O₂ spectra almost overlapping (Fig. 4i), which could be ascribed to the existence of MXene.

The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 52073224, 32201491), the Textile Vision Basic Research Program of China (No. J202110), the Scientific Research Project of Shaanxi Provincial Education Department, China (No. 22JC035), the Advanced Manufacturing Technology Program of Xi'an Science and Technology Bureau, China (No. 21XJZZ0019), the Research Fund for the Doctoral Program of Xi'an Polytechnic University (No. BS202053) and the Youth Innovation Team of Shaanxi Universities and Institute of Flexible electronics and Intelligent Textile.