Scalable assembly of flexible ultrathin all-in-one MXene-based supercapacitors

Abstract

Supercapacitors are promising candidates for flexible energy storage devices as a result of their long cycle life and high power density. However, their scalable assembly is still challenging. Herein, a facile blade coating method is employed to realize the fabrication of flexible ultrathin supercapacitors with an integrated all-in-one configuration. The assembly of such supercapacitors can be easily scaled up by adjusting the area of substrates. The all-in-one structure effectively integrates two MXene electrodes, a gel electrolyte and a separator. As a result, they can not only enhance ion- and electronic-transfer capability, but also avoid the slippage or separation between adjacent components under frequent external strain. Moreover, owing to the unique physical and chemical properties of the MXene electrode as well as the structure and electrochemical stability of the integrated all-in-one configuration, the resulting supercapacitors exhibit excellent electrochemical properties, such as a high specific areal capacitance of 77.25 mF cm⁻² at 2 mV s⁻¹, a long cycle life of 16 000 cycles at 2 A g⁻¹, high rate ability and superior electrochemical stability even under varied bending states.

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1. Introduction

The recent boom in flexible electronic technology has promoted the development and production of flexible energy storage devices.^1–5 Among the various flexible energy storage devices, supercapacitors (SCs) are considered as promising candidates as a result of their long cycle life and high power density.^6–11 However, limited by electrode preparation and device design processes, the assembly of flexible SCs is still a challenge.^12–14 Recently, free-standing electrode materials have been widely used in flexible SCs because of the superior mechanical and electrochemical performance. Among various free-standing materials, MXenes have been widely utilized in flexible SCs as a result of their excellent physical properties including surface hydrophobicity, large specific surface area, and metallic conductivity.^15–18 Nevertheless, the reported flexible MXene-based SCs are still mainly assembled into conventional sandwich-stacked structures.^19–21 Since the components of these flexible sandwich-stacked SCs are independent, relative displacement between adjacent components may easily occur and some components or even the whole device may fracture when these devices are exposed to frequent external strain. Thus, serious electrochemical performance degradation may take place. In addition, the bulky architecture of sandwich-stacked SCs limits their further application in flexible electronic devices. Thus, flexible SCs with novel device structures should be further developed.

In comparison with conventional sandwich-stacked SCs, integrated SCs with an all-in-one configuration are favored due to their excellent structural stability.²² In all-in-one SCs, the electrode, separator, current collector, and other components of the device are integrated together. The seamless connection between adjacent components not only ensures effective ion- and electronic-transfer capability, but also avoids relative slippage and separation between adjacent components under frequent external deformation, thereby improving the mechanical and electrochemical stability of the device.^23–27 Moreover, all-in-one SCs can also reduce their thickness by simplifying the device structure, which helps improve device flexibility and electrochemical performance. The electrodes of flexible all-in-one SCs are generally prepared by depositing or coating active materials onto flexible current collectors.^28,29 Owing to the use of current collectors, ultrathin all-in-one SCs are hard to realize. Meanwhile, the corresponding assembly methods are also challenging to scale up. Consequently, methods to achieve the scalable assembly of flexible ultrathin all-in-one SCs should be developed. Porous cellulose paper is usually used as the separator in traditional SCs. Because of its flexibility, it could also serve as a flexible substrate for electrodes and buffer internal stress under external deformation. If all components of SCs could be integrated with the cellulose paper, flexible ultrathin SCs with an all-in-one configuration will be realized.

Inspired by this, a scalable assembly method was developed to prepare flexible ultrathin all-in-one MXene-based SCs with all components continuously and seamlessly integrated with a piece of cellulose paper by a facile blade coating method. The assembly of the resulting all-in-one SCs can be easily scaled up by adjusting the area of cellulose paper. Furthermore, with such an all-in-one configuration, the device can not only shorten the ion transfer distance and reduce interfacial contact resistance, but also avoid relative slip and separation between adjacent interfaces under external strain, thus ensuring effective electron- and ion-transfer. As a result, the all-in-one SCs demonstrate high flexibility and excellent electrochemical stability under different external strains.

2. Results and discussion

Fig. 1a schematically shows the assembly process of flexible ultrathin all-in-one SCs. In our case, owing to the superior physical properties such as large surface area, high conductivity, excellent mechanical properties and so on, Ti₃C₂T_x MXene (where T represents terminated groups such as –O, –OH, and –F generated from the etching processes) was selected to serve as the active material for flexible all-in-one SCs. A well-dispersed Ti₃C₂T_x MXene suspension was first prepared by selectively etching the Al layer in Ti₃AlC₂ ceramic material using LiF and HCl. The raw Ti₃AlC₂ consists of bulk particles with size ranging from several microns to tens of microns (Fig. S1†). After etching and sonication treatment, a uniformly dispersed Ti₃C₂T_x suspension was obtained (Fig. S2 and S3†). The structural evolution during this process is reflected in their X-ray diffraction (XRD) plots (Fig. 2a). The Ti₃AlC₂ XRD plot reveals typical Ti₃AlC₂ phase peaks according to the standard spectrum (JCPDS 52-0875). After etching, the strong XRD peaks corresponding to (101), (103), (104), (105) and (107) planes disappeared. Moreover, a distinct shift in the (002) peak from 9.5° to 7.2° is observed, suggesting an increase in the interlayer spacing.^30–32 All these results indicate the successful removal of the Al layer from Ti₃AlC₂. In order to inspect the morphology of MXene, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out. SEM images of freeze dried Ti₃C₂T_x exhibit a porous structure composed of Ti₃C₂T_x sheets (Fig. 2b and c). TEM shows their flat feature with wrinkling sheets (Fig. 2d). In addition, atomic force microscopy (AFM) characterization was further performed to verify that the obtained Ti₃C₂T_x MXene suspension consisted of MXene sheets with a single layer without restacking (Fig. 2e and f). As can be seen, the size of Ti₃C₂T_x MXene sheets lies within several microns with a thickness of about 1.65 nm. As the theoretical thickness of a Ti₃C₂T_x MXene monolayer is approximately 1 nm, and considering factors such as surface functional groups and the adsorption of water or other contaminants that may increase the thickness, it can be inferred that the Ti₃C₂T_x MXene suspension obtained in the experiment primarily comprised single-layer Ti₃C₂T_x MXene sheets, and the restacking of the Ti₃C₂T_x MXene sheets was prevented.^33–35

Fig. 1 (a) Schematic illustration of the assembly process of flexible ultrathin all-in-one SCs. (b and c) Photographs showing the blade coating processes. (d and e) Optical images of the flexible ultrathin all-in-one structure.

Fig. 2 (a) XRD plots of Ti₃AlC₂ and Ti₃C₂T_x samples. (b and c) SEM images of the freeze-dried Ti₃C₂T_x MXene. (d) TEM image of Ti₃C₂T_x flakes. (e) The AFM image of Ti₃C₂T_x sheets. (f) The thickness curve of Ti₃C₂T_x sheets received from the AFM image.

In a typical experiment, the Ti₃C₂T_x MXene slurry with a concentration of ∼26 mg mL⁻¹ was directly coated onto both sides of a piece of cellulose paper through a blade coating process, obtaining the ultrathin MXene film-separator-MXene film all-in-one structure (Fig. 1b–e and S4†). After that, a gel electrolyte was added to the structure, achieving the ultrathin all-in-one SCs. In order to validate the versatility of this method, the same MXene slurry was also blade coated onto various substrates, including Celgard 2400, filter paper, and glass fiber. Multimeter measurements revealed infinite bilateral resistance across all integrated structures, indicating that the MXene slurry did not permeate these substrates and this blade coating method works with both cellulose and noncellulose substrates (Fig. S5†). Besides, we also conducted repeated 1000 stretching and bending experiments (stretching to a flat state followed by bending until the distance between the two ends of the material reached zero) on a stretching platform to evaluate potential degradation over extended mechanical cycling. Initial SEM images of the electrode surface reveal flat and intact features (Fig. S6†). After 500 bending cycles, minor cracks began to appear on the surface. These cracks became more pronounced after 1000 bending cycles. Despite this, no macroscopic cracks that could compromise the structural integrity were observed, demonstrating excellent mechanical properties.

The surface of MXene films coated on the cellulose paper exhibits a densely packed structure composed of two-dimensional MXene sheets (Fig. 4a and b). The thickness of the all-in-one structure is less than 100 μm, approximating the thickness of a veil, as reflected by the SEM images and corresponding elemental mappings (Fig. 4c–f). In addition, the MXene layer with a thickness of approximately 4.5 μm adheres to both sides of the cellulose paper separator. In such an all-in-one structure, the Ti₃C₂T_x film electrodes can be firmly connected with the neighboring cellulose paper separator, displaying a continuous connection between the electrodes and separator. Impressively, as the blade coating method only involves coating MXene slurry on cellulose paper, there is no limit to the size of MXene films as well as the cellulose paper. Therefore, the preparation of MXene-based flexible ultrathin all-in-one SCs can be scaled up easily.

Since the electrochemical performance of Ti₃C₂T_x depends largely on the type and content of their terminated groups,³⁶ X-ray photoelectron spectroscopy (XPS) was conducted to analyze chemical bonding of their terminated groups (Fig. 3a–e). The XPS survey spectrum of Ti₃C₂T_x indicates the presence of Ti, C, O, and F elements (Fig. 3a). The C 1s spectrum of Ti₃C₂T_x is divided into three fitting peaks (Fig. 3b). The peaks centered at 282.5, 284.8 and 286.8 eV are ascribed to C–Ti, C–C/C=C, and C–O, respectively. Fig. 3c presents the Ti 2p spectrum of Ti₃C₂T_x MXene, showcasing peaks corresponding to Ti–C, Ti²⁺, and Ti³⁺. These findings align well with earlier studies.^37,38 Notably, there are no deconvoluted peaks indicative of TiO₂, suggesting that the Ti₃C₂T_x MXene remained unoxidized throughout the synthesis process. The XPS O 1s spectrum of Ti₃C₂T_x shows four fitting peaks of C–Ti–O_x, Ti–O, C–Ti–(OH)_x and H₂O at 532.6, 533.0, 534.3 and 536.0 eV, respectively (Fig. 3d). In the F 1s region of the Ti₃C₂T_x MXene sample, the primary component is C–Ti–F_x bond at 685.1 eV (Fig. 3e). The presence of Al–F_x is minimal, suggesting that Al has been effectively removed. These results confirm the successful synthesis of Ti₃C₂T_x MXene and indicate that the surface of Ti₃C₂T_x MXene is rich in oxygen-containing functional groups, which can provide significant pseudocapacitance to the Ti₃C₂T_x electrode, thus contributing to the improved electrochemical performance of SCs based on them.

Fig. 3 (a) XPS survey spectrum of Ti₃C₂T_x. High resolution (b) C 1s, (c) Ti 2p, (d) O 1s, and (e) F 1s spectra of Ti₃C₂T_x. (f) N₂ adsorption/desorption curves of Ti₃AlC₂ and Ti₃C₂T_x.

Fig. 4 (a and b) SEM images of the MXene film coating on a piece of cellulose paper. (c–f) Cross-sectional SEM image and the corresponding elemental mappings of the integrated structure.

Apart from the terminated groups, a large specific surface area is also required to meet the high energy density demand. As a result, a N₂ adsorption–desorption isotherm was further performed to evaluate the specific surface area of Ti₃C₂T_x (Fig. 3f). Significantly, a type-II isotherm is observed in Ti₃C₂T_x samples, corresponding to a single–multilayer reversible adsorption process occurring on the non-porous solid or macroporous solid surface. This indicates the existence of a macropore structure, which is consistent with the SEM results (Fig. 2b). The adsorption and desorption curve of Ti₃AlC₂ is a type III isotherm without an inflection point, corresponding to the multimolecular layer adsorption process on the hydrophobic surface, where the adsorption interaction between the solid and adsorbate is weaker than the interaction between adsorbate molecules. The specific surface area of Ti₃AlC₂ and Ti₃C₂T_x is 1.97 m² g⁻¹ and 28.12 m² g⁻¹, respectively. The Barrett–Joyner–Halenda (BJH) analysis (Fig. S7†) reveals that the pore volume of Ti₃C₂T_x MXene is 0.066 cm³ g⁻¹, almost 15 times larger than that (0.0045 cm³ g⁻¹) of Ti₃AlC₂. The pore size distribution of Ti₃C₂T_x primarily concentrates within the range of 1–16 nm, with a peak at 1.88 nm. However, due to the limitations of test conditions, the pore size distribution analysis of macropore structures is restricted. These results indicate that the surface area of Ti₃C₂T_x is greatly improved after etching of Ti₃AlC₂ with HCl and LiF, and the Ti₃C₂T_x material contains a significant number of micropores and mesopores.

Owing to its excellent electrical conductivity, abundant terminated groups, and large surface area, Ti₃C₂T_x is a good electrode candidate for SCs. For the purpose of understanding the electrochemical performance of Ti₃C₂T_x electrodes, SCs based on the all-in-one structure were fabricated with 1.0 mol per L H₂SO₄ electrolyte. Typical rectangle-like cyclic voltammetry (CV) curves were obtained even at a high scan rate of 200 mV s⁻¹ (Fig. 5a), suggesting the superior rate capability. It is also demonstrated by their isosceles triangle shaped galvanostatic charge/discharge (GCD) curves (Fig. 5c). Moreover, as the scan rate increases by 100 times (2–200 mV s⁻¹), the specific capacitance of the Ti₃C₂T_x MXene films retains 68% of its initial value (177.5–121.1 mF cm⁻², for a single electrode) as shown in Fig. 5b, indicating their excellent rate performance.

Fig. 5 (a–c) The electrochemical performance of the SCs with 1.0 mol per L H₂SO₄ electrolyte: (a) CV curves at different scan rates; (b) the corresponding areal capacitances at different scan rates; (c) GCD plots at varying current densities. (d) Schematic diagram of the fabricated flexible all-in-one SCs. (e–i) Electrochemical performance of SCs with all-in-one and stacked structures: (e) CV curves at 50 mV s⁻¹; (f) areal capacitances depend on scan rates; (g) GCD curves at 1 A g⁻¹; (h) Nyquist curves (100 mHz–100 kHz); (i) comparison of the electrochemical performance between all-in-one and stacked SCs.

Besides high electrical conductivity, abundant terminated groups, and large surface area, Ti₃C₂T_x also possesses excellent mechanical performance. Therefore, it can serve as flexible electrodes for all-in-one SCs. Based on Ti₃C₂T_x, flexible all-in-one SCs were fabricated with PVA/H₂SO₄ gel electrolyte, as depicted in Fig. 5d. Typical rectangle-like CV curves of all-in-one SCs indicate the low electrical resistance and excellent reversibility during the charging/discharging process of the flexible all-in-one SCs (Fig. 5e). The specific capacitance of the flexible all-in-one SCs is calculated to be 77.25 mF cm⁻² at 2 mV s⁻¹, which is comparable to those of stacked SCs (Fig. 5f) and the reported flexible supercapacitors prepared by spay/blade coating methods (Fig. S8†).^39–44 Moreover, they can still deliver a high specific capacitance of 51.35 mF cm⁻² even at a high scan rate of 50 mV s⁻¹. In contrast, the stacked SCs at the same scan rates suffer from more serious capacitance damping (Fig. 5f). These results are also supported by their GCD curves at 1 A g⁻¹ (Fig. S9†). In addition, the IR drop (58.8 mV) of the all-in-one SCs is also less than that of the stacked SCs (75.7 mV), demonstrating lower internal resistance of the flexible all-in-one SCs (Fig. 5g). Such an excellent rate performance is attributed to their shorter ion diffusion distance and faster electrochemical reaction kinetics, which is revealed by the electrochemical impedance spectroscopy (EIS) results (Fig. 5h). The measured Nyquist plots are fitted to an equivalent circuit (inset in Fig. 5h), and the fitting parameters are listed in Table S1† in the ESI.† In the high frequency region, all-in-one SCs show smaller internal resistance (R_s, 2.737 Ω) and charge-transfer impedance (R_ct, 3.864 Ω) than the stacked SCs. Furthermore, from the relationship between Z′ and ω^−1/2 (ω = 2πf) in the low frequency region (Fig. S10†), the all-in-one SCs show a lower slope, indicating their smaller charge transfer resistance and ion diffusion kinetics.⁴⁵ The excellent kinetics of the all-in-one device could be ascribed to the seamless connection between neighboring components, which provides a continuous electron-/ion-transfer pathway. Therefore, all-in-one SCs show superior electrochemical performance compared to the stacked SCs (Fig. 5i).

Fig. 6a shows the CV curves of flexible all-in-one SCs based on Ti₃C₂T_x at 2–50 mV s⁻¹. The rectangle-like CV curves even at high scan rates imply their excellent capacitive behavior. Besides, current responses of the all-in-one SCs exhibit a nearly linear connection with scan rates (Fig. 6b), evidencing the superior rate performance. With these excellent electrochemical properties, the all-in-one SCs can be charged and discharged over a wide current density ranging from 0.2 to 2 A g⁻¹ and nearly triangular shaped GCD plots can be achieved (Fig. 6c). In contrast, CV and GCD curves of the stacked SCs were shown in Fig. S11.† For the purpose of certifying stable electrochemical performance of all-in-one SCs, CV curves at different bending states (flat state as well as bent it to 40% (1.2 cm), 60% (1.8 cm), and 80% (2.4 cm) of its original length (3 cm)) were obtained (Fig. 6d). Little distinction is observed in the CV curves and their corresponding normalized capacitances under different bending states (Fig. 6e), suggesting the high electrochemical stability of the flexible all-in-one SCs. The stability of all-in-one SCs can be also suggested by their GCD plots at different bending states (Fig. 6f). In addition, cycle performance is another important factor for the applications of flexible devices. The capacitance retention of all-in-one SCs kept increasing during the initial 650 cycles. This can be attributed to the gradually improved electrolyte accessibility to the active sites resulting from the increased interlayer spacing during the cycling process.^46–48 After the initial 650 cycles, only a slight capacity fade is observed over the subsequent 15 350 cycles at 2 A g⁻¹ (Fig. 6g), indicating their superior cycling stability. Such excellent electrochemical performance makes our flexible all-in-one SCs promising for use as flexible power sources. To demonstrate the flexibility of our all-in-one SCs visually, three devices under flat and bending states in series were connected to light an LED (Fig. 6h and i). The nearly unchanged brightness of an LED indicates the applicability and compatibility of the devices as flexible energy storage devices for portable electronics.

Fig. 6 (a–g) Electrochemical properties of flexible all-in-one SCs: (a) CV curves, (b) current responses of CV curves, (c) GCD plots, (d) CV curves at 5 mV s⁻¹, (e) normalized specific capacitances based on CV curves and (f) GCD plots at 0.5 A g⁻¹ at varying L/L₀ values (C_s0 and L₀ represent the initial capacitance and the length of all-in-one SCs, C_s and L are capacitances and lengths under bending states), (g) cycling performance, (h and i) optical photographs of an LED lighted by all-in-one devices in series under (h) flat and (i) bending states.

The excellent electrochemical performance of flexible all-in-one SCs depends on their fast electrochemical reaction kinetics, which can be reflected in their CV curves (Fig. 6a). The current (i) in the CV curves can be usually considered as the sum of the surface capacitance control current (i_cap) and the diffusion control current (i_diff), and its empirical formula is as follows:⁴⁹

i = i_cap + i_diff = av^b (1)

or rewritten as:

log(i) = b log(v) + log(a) (2)

where v is the scan rate, and a and b are adjustable parameters. When b approaches 0.5, the electrochemical reaction becomes diffusion-controlled, and when b approaches 1.0, surface capacitance control with a quick kinetic behavior dominates the entire electrochemical reaction. According to eqn (2), the plots of lg v versus lg i were linearly fitted as shown in Fig. 7a and b. The b values of the all-in-one SCs range from 0.853 to 0.953, while those of the stacked SCs range from 0.790 to 0.900 under different potentials, revealing that the electrochemical reaction in both all-in-one and stacked SCs is mainly dominated by the surface capacitance control (Fig. 7c). However, the higher b values of all-in-one SCs indicate much faster electrochemical kinetics compared to stacked SCs.

Fig. 7 The electrochemical reaction kinetic analysis of the flexible all-in-one SCs during the charge/discharge process: the fitted plots of lg i versus lg v at different potentials: (a) stacked SCs and (b) all-in-one SCs. (c) The variation in b-values of all-in-one and stacked SCs under different potentials. CV curves of stacked SCs with a capacitance contribution at 2 and 50 mV s⁻¹, respectively: (d) stacked SCs, (e) all-in-one SCs. (f) Capacitance contribution comparison of all-in-one SCs with stacked SCs at various scan rates.

A quantitative analysis of the capacitance contribution percentage was further conducted by Dunn's method.⁵⁰ The current at specific potentials can be described as follows:⁵¹

i = k₁v + k₂v^1/2 (3)

where v is the scan rate, values k₁v and k₂v^1/2 represent the contributions of the surface capacitance control process and diffusion process to total current (i), respectively. The contributions of surface capacitance can be distributed in the form of the area in the CV curves (Fig. 7d and e). The capacitance contributions increase gradually with increasing scan rates, indicating that high scan rates are beneficial to the capacitance contribution, as the ion insertion/deintercalation process at high scan rates is confined to surface charging/discharging. Besides, the capacitance contributions of all-in-one SCs at 2, 5, 10, 20 and 50 mV s⁻¹ were calculated to be 56.42%, 61.33%, 65.28%, 70.63% and 84.53%, respectively, which are higher than those of the stacked SCs at the same scan rates (Fig. 7f). This result is consistent with the rate performance results (Fig. 5f), and it may be ascribed to the enhanced electron and/or load transfer capability of all-in-one SCs. Therefore, the all-in-one SCs exhibit faster charge transfer kinetics than the stacked SCs.

3. Conclusions

In summary, flexible ultrathin all-in-one MXene-based SCs were designed and successfully prepared through a scalable blade coating strategy, which effectively integrated the two MXene electrodes, a gel electrolyte and a separator in a single structure. The continuous interlayer connection can effectively avoid the sliding or relative separation of adjacent components under external mechanical deformation, thus ensuring stable ion- and electron-transfer ability. Therefore, the integrated all-in-one SCs show fast electrochemical reaction kinetics as well as excellent flexibility and structural stability even under varied bending states. The scalable strategy developed here provides a new idea of assembling all components of SCs into all-in-one structures with remarkable structural and electrochemical stability.

4. Experimental

4.1 Preparation of MXene

Ti₃C₂T_x MXene was prepared by exfoliating the Ti₃AlC₂ MAX phase through the LiF/HCl etching strategy. In a typical process, 2 g LiF powder (Alfa Aesar) was first added into 40 mL of 6 mol L⁻¹ HCl and the mixture was stirred for 10 minutes to dissolve the salt. After that, 2 g of Ti₃AlC₂ MAX phase was added to the solution which was kept at 45 °C for 3 days under continuous stirring. Subsequently, the obtained mixture was washed with deionized water until pH of the washing water reached approximately 6 and sediment was collected. Next, 300 mL deionized water was added to the above sediment and the mixture was stirred for 12 hours. After 30 minutes of sonication and 30 minutes of centrifugation treatment at 3500 r min⁻¹, the uniformly dispersed Ti₃C₂T_x suspension was obtained.

4.2 Assembly of flexible ultrathin all-in-one MXene-based SCs

The well-dispersed Ti₃C₂T_x MXene suspension was first subjected to high-speed centrifugation to obtain a concentrated MXene slurry. The concentration of the MXene slurry was adjusted to ∼26 mg mL⁻¹ with absolute ethanol serving as the dispersing agent. It was directly utilized as the electrode materials for flexible ultrathin all-in-one SCs. Then, 1 g concentrated H₂SO₄ and 10 mL deionized water were mixed uniformly. 1 g poly(vinyl alcohol) (PVA) was added to the above H₂SO₄ solution afterwards. The whole mixture was then heated to 85 °C under continuous stirring until it became clear, obtaining a PVA/H₂SO₄ gel electrolyte. After that, the MXene slurry was coated onto a piece of clean cellulose paper with the coating thickness precisely controlled at 200 μm using a doctor blade. In order to avoid stress concentration and cracking during the drying process, a staged drying method was employed, with room temperature drying for 2 hours to remove most of the solvent followed by vacuum drying at 60 °C for 12 hours to obtain the MXene–cellulose paper. The active material loading on one side was 0.75 mg cm⁻². Then, the same method was used to coat MXene slurry on the other side of the cellulose paper to obtain the MXene–cellulose paper-MXene integrated all-in-one structure. Finally, 100 μL of the hot PVA/H₂SO₄ electrolyte was added dropwise to the all-in-one structure (cut into a size of 3 × 1 cm²) and dried under vacuum, achieving the assembly of flexible ultrathin all-in-one MXene-based SCs.

4.3 Assembly of flexible stacked MXene-based SCs

The MXene electrodes of stacked MXene-based SCs were prepared by blade coating the same MXene slurry (∼26 mg mL⁻¹) on the commercial polypropylene membrane (Celgard 2400) with a coating thickness of 200 μm. After the drying process, free-standing MXene films were peeled off from the polypropylene membrane. A piece of cellulose paper and 100 μL of the hot PVA/H₂SO₄ solution were used as the separator and gel electrolyte, respectively. The flexible stacked SCs were fabricated by sandwiching the cellulose paper with two pieces of identical MXene films, each peeled off from the polypropylene membrane (cut into a size of 3 × 1 cm²).

4.4 Material characterization

The morphology of materials and all-in-one devices was characterized by scanning electron microscopy (SEM, Regulus8220) and transmission electron microscopy (TEM, FEI Talos F200X G2). Atomic force microscopy (AFM, Bruker Dimension ICON) was employed to characterize the morphology and thickness of MXene sheets. The crystal structure of materials was characterized by X-ray diffraction (XRD, DX-27008). Element type and valence information of the sample were characterized by X-ray photoelectron spectroscopy (XPS, PHI 1600 ESCA). The specific surface area of the material was measured using a nitrogen adsorption isotherm tester (ASAP 2020).

4.5 Electrochemical tests

The GCD and CV curves were measured on a CHI660E electrochemical workstation. The cyclic performance of SCs was carried out on an Arbin supercapacitor test system (BT 2000). The specific areal capacitance (C_s) of all-in-one devices versus current densities and scan rates are calculated with the formulae given below:where i (A) is the discharge current, Δt (s) is the discharge time, ΔV is the potential window, s (cm²) is the area of all-in-one devices, I (A) is the current during cyclic voltammetry tests and v (V s⁻¹) is the scan rate. EIS was carried out over a frequency range of 100 mHz–100 kHz on the same CHI660E electrochemical workstation with an amplitude of 5 mV.

Data availability

The data are available from the corresponding author on reasonable request.

Author contributions

Zifang Zhao: conceptualization, data curation, funding acquisition, writing – original draft, writing – review & editing; Zhilong Xu: project administration, writing – original draft; Yalei Wang: conceptualization, formal analysis, writing – review & editing; Weifeng Huang: formal analysis, investigation, methodology; Yinfeng Cheng: supervision, project administration; Wai-Yeung Wong: funding acquisition, resources, conceptualization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52102268 and 22309044), the RGC Senior Research Fellowship Scheme (SRFS2021-5S01), the Hong Kong Research Grants Council (PolyU 15307321), the Research Institute for Smart Energy (CDAQ), the Research Centre for Nanoscience and Nanotechnology (CE2H), and the Research Centre for Carbon-Strategic Catalysis (CE01 and CE2L), as well as by the Endowed Professorship in Energy (847S) funded by Miss Clarea Au, the Key Project of Science and Technology Program of Henan Province (No. 222102240001 and 252102231043), and the Key Scientific Research Project of Colleges and Universities in Henan Province (No. 24B150011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta00327j

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