Flexible, highly conductive, and microscopically disordered MXene film electrode constructed via in-situ carbon dots intercalation toward high-performance supercapacitors

INTRODUCTION

Supercapacitors are efficient energy storage systems owing to their extended cycle lifespan, rapid charge-discharge rate, and high power output^[1,2]. They are categorized as either electric double-layer capacitors (EDLC) or pseudocapacitors according to their charge-storage mechanism^[3]. EDLC stores charge via the electrosorption of ions at the electrode/electrolyte interface. This mechanism typically restricts the capacitance of the electrode materials within 300 F g^-1, owing to the limited surface area of involved materials^[4,5]. Contrary to EDLC, pseudocapacitors involving materials such as transition metal compounds^[6,7] and conducting polymers^[8] can achieve significantly higher capacitance owing to their reliance on surface Faradic redox reaction. However, pseudocapacitive materials often experience poor electrical conductivity and structural instability during continuous cycling, resulting in inferior rate capability and cycle performances. Thus, the advancement of supercapacitor technology relies heavily on developing high-performance pseudocapacitive materials that demonstrate higher rate capability and extended lifespan.

Among the emerging materials, MXenes, a class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, produced typically from ternary M_n+1AX_n precursors via selectively etching the A atom layers (mainly IIIA and IVA group elements) are getting immense recognition as advanced energy storage materials. MXenes have a representative formula of M_n+1X_nT_x, where M is early transition metals, X is carbon and/or nitrogen with n between 1~4, and T is the associated terminations (-O, -OH, and -F)^[9-12]. Their unique structural configuration, inherited from MAX phases, gives them metallic conductivity, rich surface chemistry, high density, hydrophilicity, and excellent flexibility, recognizing them as a promising electrode material^[13,14]. Specifically, the 2D lamellar-like structure of MXene makes it versatile for the construction of flexible film electrodes, which are ideal for application in portable and wearable microelectronic devices^[15,16]. For example, a MXene film prepared through blade-assisted coating can exhibit high conductivity (15,100 S cm^-1 at 214 nm thickness), high tensile strength (~570 MPa at 940 nm thickness), and excellent flexibility, allowing the film to withstand numerous deformations^[13]. Although such a configuration is promising, MXene flakes arranged face-to-face might cause significant layer-restacking due to high interlayer van der Waals forces, undermining the surface-active sites and extended ion diffusion pathways and thus causing low capacitance and inferior kinetics.

To address the problem of MXene-layer restacking, the structural modification of densely packed MXene films into open designs, for example, 3D configurations^[17,18] and vertical arrays^[19], has become a favorable approach. This can be achieved through various methods, including template methods^[20,21], self-assembly^[22], mechanical shearing^[19], and 3D printing^[23]. For example, poly(methyl methacrylate) spheres have been reported as a template to facilitate the fabrication of hollow MXene spheres, which, when accumulated, form macroporous MXene films with exposed active surfaces and developed ion diffusion channels^[24]. However, the macroporous structure significantly decreases the film density (0.25~0.4 mg cm^-3), thus deteriorating the electrode's volumetric performance. Additionally, modulating the structure of the precursor has proven effective in enlarging the interlayer spacing of the MXene films^[25]. A hexagon Ti₃AlC₂ with a spiral chiral symmetry structure may be pre-stratified using a Lewis acid molten salt technique and etched using a microwave-assisted procedure to form an MXene film with a large interlayer spacing of 1.39 nm and excellent structural stability. Among other strategies, the use of “interlayer spacers” such as graphene^[26], carbon nanotubes (CNTs)^[27], and celluloses^[28] has also shown promising results in effective modulation of the interlayer structure of MXene films.

Our previous work demonstrated the improved ion transportability of CNT-intercalated MXene films, achieving a high capacitance (200 F g^-1 at 500 A g^-1) compared to its pristine counterpart (153 F g^-1). Nevertheless, similar capacitances observed at lower rates suggest inadequate exposure to surface active sites, limiting capacitance improvement^[29]. At the same time, the spacer-intercalated MXene films may also suffer from decreased film density due to the increased interlayer spacing. Therefore, developing a simple yet efficient strategy to fabricate flexible, open MXene architectures without compromising the integrity and film density remains a significant challenge.

Herein, we propose a rationally designed MXene film electrode from the perspective of inducing microscopical disorder. This MXene film was constructed via the integration of gelatin into MXene flakes followed by carbonization for the in-situ formation of CDs that act as intercalators, enabling increased interlayer spacing and disorder while preventing layer restacking and exposing MXene active sites, ensuring rapid ion diffusivity and superior electrode kinetics. Meanwhile, the carbonization removes ineffective surface groups, resulting in high-density carbon dots-intercalated MXene films (CDs-MF). The CDs-MF, when tested as an electrode, demonstrated improved capacitive performances compared to its pristine counterpart with a high gravimetric capacitance of 396.4 F g^-1 at 1 A g^-1 and a volumetric capacitance of 1,153.2 F cm^-3 at 1 A cm^-3. Moreover, the assembled flexible symmetric supercapacitor (SSC) realized a volumetric energy density of 21.2 Wh L^-1 while maintaining excellent capacitive performance under various operating conditions.

EXPERIMENTAL

RESULTS AND DISCUSSION

Figure 1A depicts the intercalation of the gelatin molecules within the MXene interlayers, followed by the carbonization-enabled fabrication of CDs-MF. Initially, accordion-like Ti₃AlC₂ [Supplementary Figure 1A] was etched by LiF/HCl to produce multilayer Ti₃C₂T_x, followed by ultrasonication to exfoliate multilayers into single or few-layer MXene flakes with excellent aqueous dispersity [Supplementary Figure 2 and 3]. The corresponding X-ray diffraction (XRD) pattern exhibits a characteristic (002) peak of Ti₃C₂T_x at 7° [Supplementary Figure 3B], which was lower than that of the Ti₃AlC₂ (9.5°) [Supplementary Figure 1B], confirming the successful etching process. The MXene dispersion was mixed with gelatin solution and subjected to vacuum filtration, allowing the gelatin to intercalate within MXene flakes. The resulting G-MF exhibited a flake-stacked structure, slightly less ordered than p-MF [Supplementary Figure 4]. In G-MF-3, the MXene flakes were closely packed due to the high gelatin content. Finally, the G-MFs were annealed to in-situ carbonize the gelatin into CDs, forming CDs-MF architectures.

Figure 1. (A) Schematic diagram illustrating the fabrication process of CDs-MF, cross-sectional SEM images of the (B) p-MF, (C) CDs-MF-1, (D) CDs-MF-2, and (E) CDs-MF-3 and corresponding (F) film thickness variation, (G and H) HRTEM image of the specific regions of CDs-MF-2 with corresponding lattice fringe and FFT pattern, (I) SEM image (top-view) of the CDs-MF-2 and corresponding energy-dispersive X-ray spectroscopy (EDS) mapping showing Ti, C, and N elements, (J) comparison of surface macroscopic morphology of the CDs-MF-2 and p-MF, (K) flexibility of CDs-MF-2 film, withstanding bending and rolling without structural damage.

Cross-sectional scanning electron microscope (SEM) images of the CDs-MF in reference to their p-MF and a-MF counterparts are presented in Figure 1B-E and Supplementary Figure 5. Despite exhibiting similar microscopic morphologies, the film thickness varied significantly among CDs-MF, p-MF, and a-MF, as summarized in Figure 1F. The thicknesses of the p-MF and a-MF were measured at 4.47 ± 0.29 and 3.17 ± 0.16 μm, respectively. The decrease in the thickness of a-MF compared to p-MF is attributed to MXene surface terminations removal and increased layer restacking post-annealing. In contrast, in-situ intercalated CDs substantially expanded the MXene film. As a result, the CDs-MF-1, CDs-MF-2, and CDs-MF-3 exhibited film thicknesses of 3.47 ± 0.13, 4.65 ± 0.04, and 4.66 ± 0.11 μm, respectively, signifying the enlarged interlayer spacings in CDs-MF that contribute to exposing active surface regions and facilitating ion diffusion in the electrode.

Figure 1G and H depict the high-resolution transmission electron microscope (HRTEM) image of the CDs-MF-2 and the fast Fourier transform (FFT) pattern, revealing numerous nano-size particles (< 10 nm) on the MXene flakes post-annealing. The lattice fringe of these nanoparticles was determined to be ~0.358 nm, corresponding to the (002) crystal plane of carbon, indicating the CDs-intercalated nature of the CDs-MF-2^[29,30]. Given that the amino groups in the gelatin chain led to nitrogen doping during carbonization, the distribution of CDs within CDs-MF was confirmed through elemental mapping [Figure 1I]. Ti, C, and N element mappings corroborated the homogeneous CD intercalation throughout the MXene film. Figure 1J shows the surface macroscopic morphologies of the CDs-MF-2 and p-MF, where CDs-MF-2 appear tarnished and coarsened due to the in-situ intercalation of CDs during annealing. Despite these changes, CDs-MF-2 maintained excellent flexibility, as demonstrated by its ability to withstand various deformations such as bending and rolling [Figure 1K]. This confirms its potential as a flexible supercapacitor electrode for wearable electronic devices.

The electrical conductivities of CDs-MF, p-MF, and a-MF were investigated and are presented in Figure 2A. The p-MF exhibited an electrical conductivity of 2,764 ± 57 S cm^-1, which significantly decreased after the intercalation of insulating gelatin due to reduced electrical contact between the MXene flakes [Supplementary Figure 6]. The conductivity of a-MF reached 4,687 ± 303 S cm^-1 after annealing due to the partial elimination of MXene surface terminations and improved flake orientation^[13]. Upon intercalation of CDs within the MXene, the electrical conductivity of CDs-MF decreases with increasing CD content due to the successively enlarged interlayer spacing and microscopic disorder. Specifically, the conductivities of the CDs-MF-1, CDs-MF-2, and CDs-MF-3 were measured to be 4,554 ± 344, 3,493 ± 454, and 2,195 ± 321 S cm^-1, respectively. Despite this reduction, the electrical conductivities of CDs-MF remained competitive compared to p-MF, ensuring excellent electron transportation within the electrode. The film densities of the electrodes are summarized in Figure 2B. The p-MF exhibited a film density of 2.74 ± 0.12 g cm^-3, which increased to 3.86 ± 0.17 g cm^-3 for the a-MF due to the significantly decreased film thickness. In contrast, the intercalated CDs expanded the film thickness of a-MF, resulting in reduced film densities for the CDs-MF. Specifically, CDs-MF-1 achieved a film density of 3.52 ± 0.15 g cm^-3, which decreased to 2.85 ± 0.13 and 2.73 ± 0.11 g cm^-3 with increasing CD content within MXene film. Nevertheless, the CDs-MF films demonstrate superior film density compared to the p-MF, which is beneficial for achieving excellent volumetric performance for supercapacitive applications.

Figure 2. (A) Electrical conductivities, (B) film densities, (C) XRD patterns of CDs-MF and their corresponding (D) interlayer spacing variation, (E) Raman spectra of the CDs-MF, (F) water contact angles of the CDs-MF-2, a-MF, and p-MF, (G and H) SAXS scattering intensities of the CDs-MF-2 and p-MF, WAXS patterns of the (I) p-MF, (J) CDs-MF-1, (K) CDs-MF-2, and (L) CDs-MF-3 showing their FWHM.

XRD analysis was employed to investigate the variation of interlayer spacings of the CDs-MF, p-MF, and a-MF [Figure 2C and D]. The variations in interlayer spacings of these MXene-based electrodes positively correlate with their respective thickness changes. Specifically, the a-MF exhibited a right-shifted (002) peak at 7.5° compared to p-MF, indicating reduced interlayer spacing from 12.6 to 11.8 Å, subsequent to the partial surface terminations elimination and the deteriorated layer-restacked structure. In contrast, the intercalation of gelatin molecules resulted in lower-angle (002) peaks for G-MF compared to p-MF [Supplementary Figure 7]. The (002) peak of CDs-MF-1, following CD intercalation, exhibited a similar feature to that of the a-MF, implying no significant interlayer structure change occurred due to the inadequate CD intercalation. However, a notable shift was observed in the CDs-MF-2 with increased CD content. The (002) peak of CDs-MF-2 was positioned at 6.6°, indicating a 13.4 Å interlayer spacing. As the CD content continued to increase, the CDs-MF-3 displayed a marginally downward shift in the (002) peak location to 6.4°, aligning with the observations made regarding film thickness. Figure 2E presents the Raman spectra of CDs-MF compared to p-MF counterpart, showing similar peak profiles in the range of 100~1,000 cm^-1, including the A_1g symmetric out-of-plane vibrational peaks near 188 and 720 cm^-1, and E_g shear vibrational peaks at 246, 369, and 611 cm^-1[20,31]. The absence of TiO₂ vibrational states confirms the well-preserved structure of MXene with no oxidation post-in situ CD intercalation. Notably, distinct D and G carbon bonds were observed for the CDs-MF near 1363 and 1607 cm^-1, confirming their successful intercalation with CDs^[32,33].

Water contact angle measurement confirmed the variation in wettability for the CDs-MF, a-MF, and p-MF. In Figure 2F, p-MF had a water contact angle of 75.3°, which decreased to 71° and 60.3° for a-MF and CDs-MF-2, respectively, confirming improved hydrophilicity with the rise of CD intercalation. This increased hydrophilicity is beneficial for permeating aqueous electrolytes and ion diffusion within the electrode. X-ray photoelectron spectroscopy (XPS) confirmed the surface chemical variation of CDs-MF-2 compared to the p-MXene counterpart. The C 1s binding energy profile for CDs-MF-2 was deconvoluted into major binding energies for C-Ti, C-Ti-O, C-C, C-O, and O-C=O bonds at 281.6, 282.9, 284.6, 285.8, and 288.6 eV, respectively [Supplementary Figure 8]^[20,34]. As seen, the intercalation of CDs has strengthened the C-C bond in CDs-MF-2 compared to p-MXene. Supplementary Figure 9 shows the O 1s binding energy profiles of the CDs-MF-2 and p-MF, with the latter exhibiting four peaks associated with Ti-O, C-Ti-O_x, C-Ti-(OH)_x, and H₂O_ads at 530.1, 531.1, 532.2, and 533.8 eV, respectively^[35-37]. In contrast, the CDs-MF-2 exhibited a similar profile with an additional peak of C=O at 531.7 eV, owing to intercalated CDs and the annealing treatment^[38-40].

Given the significant differences in the interlayer structures, film density, electrical conductivity, and surface chemistry between CDs-MF and p-MF owing to in-situ CD intercalation, small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) measurements were conducted to elucidate the porosity and orientation degree of the MXene flakes. The SAXS pattern of CDs-MF-2 exhibited lower intensity than p-MF, confirming developed porosity [Figure 2G and H]^[41]. The (002) peak azimuth diagram and full width at half maximum (FWHM) were analyzed through WAXS patterns [Figure 2I-L]. The p-MF showed a FWHM value of 20.3°, which increased to 25.3, 27.9, and 29.0° for the CDs-MF-1, CDs-MF-2, and CDs-MF-3, respectively. Correspondingly, the orientation degree factor (Π), which signifies the flake orientation degree relative to the film plane, was calculated [Π = (180° - FWHM)/180° × 100%]^[42]. The high Π value of 88.7% indicates an ordered p-MF structure aligned with the horizontal axis of the 2D assembly. In contrast, the CDs-MF showed decreased Π values of 85.9%, 84.5%, and 83.9% with increasing CD intercalation, demonstrating progressively developed microscopic disorder^[43,44]. The highly ordered orientation of MXene flakes in p-MF imparts characteristics such as high electrical conductivity and low interlayer spacing, whereas the developed porosity in CDs-MF allows for exposed active surfaces and convenient ion transport channels, resulting in improved capacitive performance.

The capacitive characteristics of CDs-MF were assessed in a three-electrode setup with activated carbon as a counter electrode and a 3 M H₂SO₄ electrolyte. Figure 3A compares the cyclic voltammetry (CV) curves of CDs-MF, p-MF and a-MF at a scan rate of 5 mV s^-1 in the potential range of -0.6~0.2 V (vs. Ag/AgCl). All samples exhibited similar peak profiles with distinct redox peaks, confirming their pseudocapacitive nature. The redox chemistry stems from MXene’s reversible proton intercalation/de-intercalation and Ti atom oxidation variation^[24,45]. The increased integral CV areas of CDs-MF, compared to p-MF, suggest greater capacitances resulting from the expanded interlayer spacing and exposed surface-active sites of CDs intercalated MXene after annealing. The a-MF, characterized by its reduced interlayer spacing, had a CV curve encompassing its p-MF counterpart due to the exposed redox active sites on the MXene surface after annealing^[46]. Figure 3B shows the galvanostatic charge-discharge (GCD) curves of the CDs-MF, a-MF, and p-MF at 1 A g^-1, where all samples realized approximately symmetric nonlinear profiles differing from the typical triangle-like profile of EDLC, further corroborating the electrode’s inherent pseudocapacitive mechanism. Among all, the CDs-MF-2 had the longest duration for the charge-discharge cycle, indicating its superior capacitance performance.

Figure 3. Electrochemical performances of the CDs-MF in reference to p-MF and a-MF counterparts, (A) CV curves at 5 mV s^-1, (B) GCD curves at 1 A g^-1, (D) gravimetric capacitances, and (F) volumetric capacitances of the CDs-MF, p-MF, and a-MF at various current densities, (C) CV curves at different scan rates and (E) GCD curves at different current densities of the CDs-MF-2, (G) long-term cycle performance of the CDs-MF-2 at 10 A g^-1 with inset depicting its initial, middle, and last 5 GCD curves.

Figure 3C and Supplementary Figure 10 show the CV profiles of the electrodes at different scan rates. The p-MF and a-MF show distorted profiles in comparison to CDs-MF, which retains its peak patterns even at scan rates exceeding 200 mV s^-1, confirming the superiority of intercalated CDs in promoting ion diffusion and accessibility, thereby improving its overall electrochemical performance at high current rates. Correspondingly, the gravimetric capacitances of CDs-MF calculated from the CV curves at different scan rates are provided in Supplementary Figure 11 and Supplementary Table 1. As seen, CDs-MF offers significantly improved capacitance compared with the a-MF and p-MF, highlighting the advantages of CD intercalation within MXenes film.

The gravimetric capacitances of CDs-MF, a-MF, and p-MF at current densities ranging from 1 to 1,000 A g^-1 are shown in Figure 3D. As seen, the p-MF realized a capacitance of 258.8 F g^-1 at 1 A g^-1, which then increased to 325.7 F g^-1 post-annealing due to exposure of additional active surface sites. In contrast, the intercalation of CDs endowed the CDs-MF with enlarged interlayer spacings and developed microscopic disorder, resulting in high capacitances of 381.5, 396.4, and 360.3 F g^-1 for the CDs-MF-1, CDs-MF-2, and CDs-MF-3, respectively. Remarkably, even at 1,000 A g^-1, the CDs-MF-2 maintained a high capacitance of 123.2 F g^-1, outperforming the p-MF (11.9 F g^-1) and a-MF (36.6 F g^-1) counterparts. This improved capacitive behavior validates the superiority of CD intercalation in enhancing ion transportation and accessibility within MXene film, thus achieving excellent rate capability. In addition, even at current densities as high as 200 A g^-1, the GCD curves of CDs-MF-2 showed symmetrical nonlinear profiles [Figure 3E and Supplementary Figure 12], confirming the electrode’s excellent reversibility. Additionally, the capacitances of CDs-MF-2 were compared to previously reported MXene-based electrodes at varying current densities [Supplementary Figure 13]. The CDs-MF-2 exhibits competitively superior performances in capacitance and rate capability, highlighting its potential as a high-performance supercapacitive material.

The high density and excellent gravimetric performance also contribute to outstanding volumetric performance of CDs-MF. As shown in Figure 3F, the p-MF achieved a volumetric capacitance of 847.6 F cm^-3 at 1 A cm^-3, which rapidly declined to 274.8 F cm^-3 when the current density reached 1,000 A cm^-3. In contrast, the exposed active surface and improved ion transportability enabled the CDs-MF to have much enhanced volumetric capacitances and rate capability. Specifically, CDs-MF-2 exhibited a volumetric capacitance of 1,153.2 F cm^-3 at 1 A cm^-3, with a capacitance retention of 53.3% at 1,000 A cm^-3, much higher than p-MF (32.4%). Due to the high density of 3.5 g cm^-3, CDs-MF-1 outperformed CDs-MF-2 in volumetric capacitance, obtaining 1,347.0 and 826.8 F cm^-3 at 1 and 1,000 A cm^-3, respectively. The exceptional gravimetric and volumetric performances of CDs-MF-2 showcase its potential application in wearable and portable electronics. Furthermore, the extended cycling of the CDs-MF-2 was tested at 5 and 10 A g^-1, as depicted in Figure 3G and Supplementary Figure 14. The CDs-MF-2 achieved excellent cycling stability at 5 A g^-1, exhibiting no capacitance decay over 8,000 cycles. More importantly, it can withstand an ultralong charge/discharge cycling up to 100,000 times at 10 A g^-1. The GCD curves of CDs-MF-2 remained consistent throughout the first, middle, and final five cycles, signifying its extraordinary cycling stability and reversibility.

Electrochemical impedance spectroscopy (EIS) analysis was conducted to investigate the electrode kinetics of the CDs-MF-2 and p-MF. The Nyquist plots show that the straight line of CDs-MF-2 in the low-frequency region is almost perpendicular to the longitudinal axis and larger than that of the p-MF [Figure 4A]. The relationship between Z’ and ω (ω = 2πf) in the low-frequency region demonstrated a much lower slope for the CDs-MF-2 compared to p-MF [Supplementary Figure 15]. These results confirm the significantly improved ion diffusion capability within CDs-MF-2 due to the in-situ intercalation of CDs. Moreover, the evolution of impedance characteristics for CDs-MF-2 was assessed through daily EIS curves during a 21-day immersion in 3 M H₂SO₄ electrolyte. As shown in Supplementary Figure 16, the CDs-MF-2 exhibited a pronounced decrease in charge transfer impedances over time, attributable to the gradual infiltration of electrolyte ions into the MXene interlayer, thereby activating the electrode. In contrast, the equivalent series resistance (the x-intercept in the high-frequency region) remains constant across the entire 21-day immersion period, signifying the remarkable structural stability of the CDs-MF-2 in H₂SO₄ electrolyte. The impedance behaviors of the CDs-MF-2 and p-MF were further analyzed using the complex model of capacitance C(ω), which includes a real part of C′(ω) and an imaginary part C″(ω) [Figure 4B and C]^[47,48]. CDs-MF-2 exhibited a higher normalized C′(ω) that decreases much slower than the p-MF with increasing frequency. The relaxation time constant (τ₀, minimal time needed to discharge energy with above 50% efficiency) was determined to be 2.49 s for CDs-MF-2, significantly lower than p-MF (4.87 s), corroborating the former faster ion diffusion ability.

Figure 4. (A) Nyquist plots, (B) normalized real and (C) imaginary capacitances as a function of the frequency of the CDs-MF-2 and p-MF, (D and E) relationship between the peak current and scan rates, (F) capacitive distribution for the CDs-MF-2 and p-MF at different scan rates.

The charge storage kinetics of the CDs-MF-2 and p-MF were evaluated from the power law equation describing the peak current (i) and scan rate (v) relationship [Supplementary Equation 3]^[49-51]. The b values of p-MF in the cathodic/anodic scan were 0.88 and 0.89, respectively [Figure 4D and E]. In contrast, the b values of CDs-MF-2 increased to 0.95 and 0.93, reflecting the predominance of capacitive-controlled contribution to the total capacitance. The specific contributions at different scan rates calculated via representative Supplementary Equation 4 are shown in Figure 4F. As seen, CDs-MF-2 achieved a capacitive-controlled contribution of 84% at 5 mV s^-1, which increased to 97% as the scan rate approached 200 mV s^-1. For comparison, only 64% of the capacitance originated from the capacitive-controlled behavior at 5 mV s^-1 for the p-MF due to its densely stacked structure. The predominated capacitive behavior of CDs-MF-2 ensures fast charge transfer on the MXene surface even at high rates, resulting in high capacitance and superior rate capability.

A flexible SSC was configured to assess the practicality of the CDs-MF-2. Figure 5A shows two identical CDs-MF-2 electrodes configured within polyethylene terephthalate (PET) packages with a 3 M polyacrylic acid (PAA)/H₂SO₄ gel electrolyte. Figure 5B shows the CV curves of the CDs-MF-2 SSC at various scan rates (5 to 200 mVs^-1), depicting non-significant distortion with the increasing scan rates, further corroborating its superior high-rate endurance and reversibility. The extended cycling of the CDs-MF-2 SSC was evaluated at 10 A g^-1 [Figure 5C]. The electrode maintained 94% of its original capacitance after 10,000 consecutive cycles, demonstrating its exceptional cycling stability. Furthermore, the Ragone plot of CDs-MF-2 SSC [Figure 5D] indicated a maximum energy density of 21.2 Wh L^-1 at a power density of 112 W L^-1 (based on the total mass of CDs-MF-2 electrode). Even when the power density was raised to 12 kW L^-1, the constructed SSC maintained an energy density of 12.3 Wh L^-1, highlighting the competitive superiority of CDs-MF-2 for supercapacitive applications.

Figure 5. (A) Digital photograph and a structure illustration of the assembled CDs-MF-2 SSC, and corresponding (B) CV curves at various scan rates, (C) cycle performance at 10 A g^-1 and (D) Ragone plots of the CDs-MF-2 SSC, (E) CV curves and (F) digital photographs of the CDs-MF-2 SSC at 0°, 45°, 90°, and 180° degree bends, (G) CV curves at 20 mV s^-1 and (H) GCD curves at 2 A g^-1 of two SSC connected in series and parallel.

Figure 5E and F shows that the CV curves of the SSC at different bending angles of 0°, 45°, 90°, and 180° were almost identical, indicating excellent mechanical robustness of the device, capable of withstanding large deformation without compromising the electrochemical performance. The practical application potential of SSC was further assessed by operating two devices in series or parallel to fulfill high voltage and capacitance demands. The corresponding CV and GCD curves [Figure 5G and H] show that series-assembled SSC had a working voltage of 1.6 V, double that of a single SSC. In contrast, the parallel-assembled device achieved doubled current output and discharge time, confirming the promising application of CDs-MF-2 in flexible and wearable devices.

CONCLUSIONS

A flexible, highly conductive, and microscopically disordered MXene film electrode was successfully designed using a simple in-situ CD intercalation process. Gelatin, pre-intercalated within MXene, transformed into CDs during annealing, which acted as interlayer spacers to expand the interlayer spacing and induce microscopic disorder between the MXene flakes, resulting in exposed active sites and rapid ion diffusivity, thereby achieving excellent kinetics. Consequently, the optimal CDs-MF-2 electrode delivered a high gravimetric capacitance of 396.4 F g^-1 at 1 A g^-1, a superior rate capability of 123.2 F g^-1 at 1,000 A g^-1, and an impressive cycle stability with no capacitance decay over 100,000 cycles. Moreover, the high-density characteristics of CDs-MF-2 and CDs-MF-3 resulted in high volumetric capacitances of 1,153.2 and 1,347.0 F cm^-3. When assembled in an SSC device, CDs-MF-2 realized a maximum energy density of 21.2 Wh L^-1 and a power density of 12 kW L^-1. The SSC maintained its capacitive performance under various operating conditions, including bending at multiple angles and configurations in series or parallel assemblies. These results demonstrate the superiority of CD intercalation in MXene film electrodes for developing high-performance supercapacitors, making them suitable for use in flexible and wearable electronics.