Introduction

During the last decades, wearable and portable electronic textiles have attracted a considerable attention for medical, military and communication applications because of their low volume, lightweight nature, high electrical properties, and flexibility [1, 2]. The development of wearable, flexible and high-performance supercapacitors is of critical importance for meeting the power requirements of wearable electronics [3,4,5,6]. For textile supercapacitor electrode fabrication, the loading of different electroactive materials onto the textile surface is carried out using different techniques. Drop casting has significant advantages, since it is a fast, low-cost and effective method to prepare high-performance electrodes and supercapacitors [7,8,9,10].

Textiles provide many advantages for wearable energy storage applications due to their fibrous surface and hierarchical morphologies [11]. Cotton textile products, such as fibers and yarns are among the most promising substrates for the fabrication of flexible and wearable energy storage devices [12, 13]. Especially, fiber network of cotton fabric with abundant oxygen-containing groups provides a suitable substrate to form conductive and electrochemically active paths [14]. Recently, various studies have been carried out investigating cotton fabric-based supercapacitors and micro-supercapacitors [15,16,17,18,19]. Ye et al. [20] fabricated flexible cotton-based MXene/polyaniline (PANI) electrodes and supercapacitor by dip-coating to benefit from synergistic effect of MXenes and PANI. They reported that cotton/MXene/PANI electrode in 1 M H2SO4 electrolyte exhibited an excellent stability and an areal specific capacitance up to 1027.5 mF cm−2 at 1 mA cm−2 which is higher compared to cotton/MXene and cotton/PANI electrodes. Li et al. [16] have reported that MXene dip-coated cotton fabric electrodes have shown a specific capacitance of 48 mF cm−2 at 2 mV s−1 in 1 M H2SO4 electrolyte. Li et al. [17] have prepared rGO/cotton supercapacitors by dry-coating method. They stated that flexible and sandwich supercapacitor had a specific capacitance of 464 F g−1 at 0.25 A g−1.

MXenes are novel two-dimensional transition metal carbides, nitrides and carbonitrides. Ti3C2Tx is the first synthesized MXene material from Ti3AlC2 MAX precursor via the selective etching of Al layers from the structure [21]. Since then, Ti3C2Tx MXenes were extensively used for the fabrication of wearable and flexible supercapacitor electrodes thanks to their excellent electrical, electrochemical and hydrophilic properties [22]. Reduced graphene oxide has also been widely used in supercapacitor applications due to its outstanding electrical properties and large specific surface area [23]. The combination of MXene and rGO materials could provide several advantages and improvements for energy storage applications. The pseudocapacitive properties of MXene and electrical double layer properties of rGO could be obtained in a single electrode for a better supercapacitor performance [23,24,25]. The presence of rGO sheets could provide a better intercalation between MXene nanosheets. In addition, MXene nanosheets could also act as dispersing agent by incorporating rGO sheets. Thereby, the agglomeration problem could be avoided for both materials [25]. Zhou et al. [25] developed Ti3C2Tx/rGO stretchable supercapacitor electrodes with a high electrochemical performance. Xu et al. [26] reported that flexible rGO/Ti3C2Tx films exhibited a specific capacitance of 405 F g−1 at 1 A g−1 in 6 M KOH electrolyte. In another study, Wen et al. [27] developed Ti3C2Tx/rGO hybrid inks for the fabrication of flexible inkjet-printed supercapacitor electrodes. They reported that the printed composite electrode displayed a high volumetric capacitance of 183.5 F cm−3 at 5 mVs−1. Zheng et al. [23] developed high-performance MXene/rGO cotton fabric electrodes by chemically reducing graphene oxide coated cotton fabrics. On the other hand, the most important problem in the use of rGO materials is their agglomeration and poor dispersion capability in water [28]. Recently, it was proposed that the use of CNC as a dispersing agent for rGO dispersion in water is advantageous due to the abundant hydroxyl groups of the CNCs [29]. Ding et al. [30] claimed that CNCs connect to the residual oxygen functional groups of rGO by hydrogen bonds and increase hydrophilicity. Ye et al. [29] investigated rGO dispersibility in water with different CNC mass ratios and found that increasing CNC led to a stable rGO dispersions in water. In addition, the presence of CNC could also improve the interaction between cellulosic cotton fabric surface and electroactive materials because of the formation of strong hydrogen bonds [31]. Although there are several studies concerning Ti3C2Tx/rGO hybrid structures, there is a lack in the literature regarding the synergistic effects and electrochemical performance of Ti3C2Tx, rGO and CNCs for textile-based supercapacitor applications. Hence, it is critical to elucidate the efficiency of hybrid dispersions loaded into fabric substrates in terms of electrochemical characteristics.

In the present paper, fabric supercapacitor electrodes were prepared by coating the plain-woven cotton fabrics via the simple drop-casting method using Ti3C2Tx and Ti3C2Tx/rGO/CNC dispersions. The main goal of the study is to take advantage of the synergistic effect between Ti3C2Tx, rGO and CNC in a single fabric electrode for a high specific capacitance and cyclic stability. CNC was used as a dispersing agent for rGO in the water-based hybrid dispersions. In addition, it was aimed to prevent possible agglomeration problem of rGO and MXene components in hybrid dispersions and to obtain a better coating formation on the cotton fabrics by benefiting the abundant hydroxyl groups of CNC. Thereby, it was aimed to facilitate the access of electrolyte ions to the active sites on MXene layers and rGO sheets by avoiding the restacking phenomena to increase the electrochemical performance of fabric electrodes. The effect of rGO/CNC on the electrochemical performance of the Ti3C2Tx/rGO/CNC coated fabric electrodes was investigated depending on the rGO/CNC weight percentage of 5%, 15%, 30% and 50%, respectively. To the best of the authors' knowledge, Ti3C2Tx/rGO/CNC coated cotton fabric electrodes were not investigated in detailed previously in terms of electrochemical performance and synergistic effect between Ti3C2Tx and rGO/CNC. In this regard, it was shown that Ti3C2Tx/rGO/CNC hybrid dispersions have displayed a good structural stability and significant improvement in electrochemical performance and specific capacitance of cotton fabrics after coating.

Experimental section

Materials

Ti3AlC2 MAX phase (90%, 325 mesh) and LiF (98.5%) were obtained from Sigma-Aldrich for synthesis of Ti3C2Tx MXene and preparation of delaminated Ti3C2Tx dispersions. Cellulose nanocrystal (diameter of 10–20 nm and length of 300–900 nm) was purchased from Nanografi Nano Technology company. Graphene oxide was synthesized by improved Hummers method using graphite powder from Fischer Scientific. Hydrochloric acid (HCl, 37%, purity 99%) was purchased from Sigma-Aldrich. The commercially available raw plain woven 100% cotton fabrics (115 g/m2) were used to obtain fabric electrodes.

Preparation of delaminated Ti3C2Tx MXene

Delaminated Ti3C2Tx nanosheets were synthesized using LiF and HCl for HF-free MXene synthesis [32]. 1 g of LiF was dissolved in 20 ml, 9 M HCl solution. After dissolving of LiF, 1 g of Ti3AlC2 MAX powder was slowly added to the solution in an ice bath to avoid overheating. Then, the solution was stirred for MXene synthesis at 50 °C for 24 h. After 24 h, the obtained mixture was washed using deionized water through centrifugation at 3500 rpm for 5 min until the pH ~ 6 was achieved. The washed MXene sediment was collected and ultrasonicated for 90 min using ultrasonic homogenizer to obtain homogenous MXene dispersion. Delaminated Ti3C2Tx phase was then filtrated using vacuum filtration and used for Ti3C2Tx and hybrid dispersions.

Synthesis of water-dispersible rGO/CNC

GO synthesis was carried out following improved Hummers method reported in the literature [33]. In order to develop water-based rGO dispersion, CNC structure was combined with the rGO structure during chemical reduction. Therefore, the dispersions of GO and CNC were prepared separately by stirring at room temperature. The mass ratio is determined according to electrical conductivity and dispersibility of rGO/CNC samples after synthesis. Then, the two dispersions were mixed in 80:20 mass ratio and ultrasonicated for 1 h to obtain homogenous GO/CNC hybrid dispersion [30, 34]. The hybrid mixture was heated to 95 °C with continuous stirring, and NaOH was added dropwise to the mixture to ensure colloidal stability. The chemical reduction was carried out by addition of ascorbic acid (1 mg/mL) to the mixture. The obtained composite structure was washed with deionized water for neutral pH and dried overnight. The sample was named rGO/CNC and was used for following preparation of hybrid dispersions.

Preparation of cotton fabric

For the preparation of fabric electrodes, firstly, the cotton fabric was treated using 1 M NaOH solution at 95 °C in order to increase hydrophilicity of fabric surfaces. The treated fabrics were then washed with deionized water and dried overnight at room temperature. The fabrics were then cut into rectangular strips with a size of 2 × 3 cm, washed with deionized water and ethanol and dried in oven at 60 °C for coating process.

Preparation of Ti3C2Tx and Ti3C2Tx/rGO/CNC coated fabric electrodes

MXene and MXene/rGO/CNC hybrid dispersions were used for the coating of cotton strips to fabricate conductive fabric electrodes. The schematic illustration of hybrid dispersion preparation and fabric electrode fabrication by drop-casting technique is given in Fig. 1. Ti3C2Tx MXene dispersion with concentration of 5 mg/mL was prepared using ultrasonication method for 1 h. All the dispersions were prepared using only deionized water, and no additives or other solvents were used while preparing the dispersions. MXene/rGO/CNC hybrid dispersions in the same concentration were also obtained after sonication for 1 h with rGO/CNC weight percentages of 5%, 15%, 30% and 50%. For the fabrication of electrodes, the prepared cotton fabric strips were placed on polytetrafluoroethylene film on a hot plate at 60 °C to provide a hydrophobic surface for better coating of the water-based hybrid dispersion on the cotton surface. After each dropping step, the fabrics were dried for 30 min at 60 °C to ensure complete evaporation of water. In this work, MXC shows Ti3C2Tx MXene coated cotton fabric electrode, while MXRC-1, MXRC-2, MXRC-3 and MXRC-4 denote Ti3C2Tx /rGO/CNC coated electrodes with rGO/CNC weight percentage of 5%, 15%, 30% and 50%, respectively.

Figure 1
figure 1

Schematic illustration of Ti3C2Tx and Ti3C2Tx/rGO/CNC dispersion preparation and drop-casting of cotton fabric.

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Characterization

X-Ray Diffraction analysis (XRD) of synthesized materials and coated fabric electrodes was conducted using X-Ray Diffractometer (Bruker AXS D8 Advance) with Cu-Kα radiation (λ = 1.54 Å). The chemical structure of fabric electrodes was investigated by Fourier transform infrared spectroscopy (FT-IR, NICOLET—IS50) in the range of 4000–400 cm−1. Morphological analysis of nanoparticles and fabric electrode surfaces was carried out by scanning electron microscopy (SEM) (Carl Zeiss / Gemini 300). The surface resistance of fabric electrodes was measured by two-point probe method by Keithley 2000 multimeter at room temperature and the relative humidity of 50%.

Electrochemical measurements

Electrochemical characterization of fabric electrodes was carried out using CHI608E Electrochemical analyzer/workstation. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charge/discharge tests were conducted to evaluate electrochemical performance of the fabric electrodes. Electrochemical measurements were carried out in 1 M H2SO4 electrolyte solution in a conventional three-electrode system, containing fabric electrode as working electrode, Ag/AgCl (3 M KCl) reference electrode and platinum counter electrode. The CV tests were performed by sweeping the potential from −0.4 V to 0.2 V at scan rates of 2, 5, 10, 20, 30 and 50 mVs−1. GCD tests were also carried out in the potential range of 0.2 and −0.4 V at a current density of 0.3, 0.6, 1 and 1.5 A g−1, and the fabric electrode showing the best performance was investigated in terms of cycling stability using GCD test for 500 charge/discharge cycles. EIS tests were conducted from 100 to 0.01 Hz frequency range with AC voltage amplitude of 5 mV vs. open circuit potential. The gravimetric specific capacitance values of single fabric electrodes were calculated from GCD tests using following equation [35].

$$C = \frac{I\Delta t}{{m\Delta V}}$$
(1)

In equation, I (A), t (s), m (g) and ΔV (V) represent the discharge current, the discharge time, the total mass of the loaded active material and the voltage window, respectively.

The supercapacitor device was also prepared for evaluation of the performance of two-electrode system. The coated cotton strips showing the highest specific capacitance in three-electrode system, were used in the assembly of the sandwich type supercapacitor. The cotton fabric which was coated by 1 M of PVA/H2SO4 gel electrolyte was used as separator in the device. For the calculation of energy and power density of the symmetric supercapacitor device following equations were used.

$$E = \frac{{C_{{{\text{sp}}}} \times \Delta V^{2} }}{2 \times 3.6}$$
(2)
$$P = \frac{E \times 3600}{{\Delta t}}$$
(3)

In Eq. (2) and Eq. (3), E (Wh kg−1), Csp (F g−1), ΔV (V), P (W kg−1) and Δt (s) represent energy density, specific capacitance and voltage window, power density and discharge time, respectively.

Results and discussion

Structural characterization

The SEM images of nanoparticles and hybrid structures were given in Fig. 2. The single layer Ti3C2Tx nanoflakes is clearly seen in Fig. 2a. According to the SEM images, the average flake length of Ti3C2Tx phase was determined as 1.4 µm ± 0.2. In Fig. 2b–e, the SEM images of hybrid structures consisting of Ti3C2Tx/rGO/CNC in different weight percentages were given. According to the SEM images, it is evident that the formation of a homogenous structure occurs, especially in Ti3C2Tx/rGO/CNC hybrid structure with rGO/CNC weight percentage of 5, 15, 30%. It is also observed that the rGO sheets become more visible with the increasing rGO/CNC weight percentage. On the other hand, the rough and coarse flakes and sheets shown in Fig. 2e, indicate the possible agglomeration of rGO and MXene nanoparticles because of the increasing concentration of rGO/CNC in the dispersions. In Fig. 2f, the SEM image of the synthesized rGO/CNC exhibits the wrinkled rGO sheets covered by CNC nano-rods. The rod-like shape of CNC structure can also be seen in Fig. 2g. In order to determine the C:O ratio of rGO/CNC nanohybrid, EDS analysis was carried out, as can be seen in Fig. 2h. The C:O ratio of the synthesized rGO/CNC was determined as 2.26 by EDS technique. The SEM images and EDS results also confirm that a homogenous hybrid nanostructure was obtained in rGO/CNC synthesis.

Figure 2
figure 2

The SEM images of Ti3C2Tx (a), hybrid Ti3C2Tx/rGO/CNC structures with rGO/CNC weight percentage 5% (b), 15% (c), 30% (d), 50% (e), rGO/CNC (f), CNC (g). EDS analysis results of rGO/CNC (h).

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Figure 3 shows the structural characterization of synthesis products, pristine cotton fabric and coated cotton fabric electrodes which was carried out by XRD and FT-IR spectroscopy. According to Fig. 3a, the characteristic peaks of CNC corresponding to (1–10), (110), (200) and (004) lattice planes were observed at 2θ = 13.8°, 14.9°, 21.5° and 33.1°. These peaks prove the presence of characteristic crystal structure of cellulose Iβ in cellulose nanocrystals [36]. Figure 2a shows the XRD pattern of synthesized Ti3C2Tx MXene. It is clear that the peak of (104) crystallographic plane was eliminated and (002) plane was observed at 2θ = 6.87° indicating successful delamination and increasing interlayer spacing [37]. In addition, XRD pattern of rGO/CNC shows that (002) and (001) crystallographic planes are observed at 2θ = 22.3° and 42.6°, respectively. The relatively sharp (002) peak indicates the presence of CNC in the structure. Figure 3b shows, XRD patterns of pristine cotton fabric, Ti3C2Tx MXene-coated and Ti3C2Tx /rGO/CNC-coated cotton fabrics in different weight percentages. The characteristic diffraction peaks of cotton fabric were seen at 13.1°, 14.8°, 21.7° and 32.8° corresponding to (101), (10–1), (002) and (040) which shows lattice planes of cellulose Iβ [38]. According to Fig. 3b, c, it can be interpreted that the coating process was successfully carried out due to the presence of (002) peaks in MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4 samples. Moreover, it is evident that (002) peak shifts to lower degrees and peak intensities decrease as a result of increasing amount of rGO/CNC in the structure. In addition, it was observed that (002) plane shifts to 2θ = 5.6° in MXRC-2 sample which could be favorable for electrolyte ion diffusion[39]. On the other hand, after rGO/CNC weight percentage reaches 30% in MXRC-3, the shifting was not observed possibly because of increasing agglomeration between MXene and rGO phases. Moreover, it is also obvious that the intensity of (200) crystallographic peak of cotton fabric decreases significantly possibly because of a good dispersion capability of components and successful deposition of 2D nanoflakes onto the cotton fibers.

Figure 3
figure 3

XRD patterns (a–c) and FT-IR spectra (d, e) of CNC, CF, Ti3AlC2, Ti3C2Tx, MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4 samples. The sheet resistance of MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4 samples (f).

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The FT-IR analysis results were given in Fig. 3d, e. Figure 3d shows the FT-IR spectra of CNC, Ti3C2Tx and rGO. FT-IR results of CNC show that the characteristic peak of O–H stretching is observed at 3329 cm−1 [40]. The peak at 2898 cm−1 corresponds to the stretching vibration of C-H. The peak observed near 1622 cm−1 is ascribed to the C = O stretching, while the peaks at 1428 cm−1 and 1365 cm−1 is indication of C-H bending. The peak at 1028 cm−1 show C-O stretching vibrations, while the peak at 900 cm−1 is attributed to C-H vibration of the β-glycosidic linkages in CNC structures [41]. On the other hand, FT-IR spectra of MXene exhibit a broad peak of O–H stretching at 3000 cm−1 with low intensity. The peak at 1600 cm−1 is attributed to C = O stretching [42]. Furthermore, FT-IR spectra of rGO/CNC sample, a weak and broad peak near 3333 cm−1 reveal the presence of CNC in the structure. In addition, the peak at 1570 cm−1 is ascribed to C = C bonds [43]. It is also evident for rGO/CNC that the chemical reduction was clearly carried out using ascorbic acid as reductant. In Fig. 3e, it was seen that characteristic peaks of cotton at 3333 cm−1, 2890 cm−1, 1620 cm−1, 1428 cm−1 and 1032 cm−1 were disappeared with coating process, and this indicates that drop-casting was applied effectively for all the fabrics. The sheet resistance values of coated fabric electrodes were measured by two-probe method. The sheet resistance of MXC, MXRC-1 and MXRC-2 samples are 4.8, 5.0 and 8.8 Ω sq−1, respectively, as shown in Fig. 3f. In addition, the relatively higher sheet resistance of MXRC-3 (42.9 Ω sq−1) and MXRC-4 (116.7 Ω sq−1) is a result of increasing rGO/CNC amount in hybrid dispersion.

Morphological characterization

To evaluate the efficiency of the coatings comprehensively, the uncoated and coated fabrics were investigated by SEM analysis in terms of morphology and coating/fiber interaction, as shown in Fig. 4. The smooth and clean surface of uncoated cotton fibers is shown in Fig. 4a. In Fig. 4b, SEM image of MXC fabric demonstrates that cotton fibers were completely wrapped by MXene sheets and the gaps between fibers were uniformly coated. A similar case is obvious in MXRC-1 and MXRC-2 in Fig. 4c, d, respectively. It can be clearly seen that warp and weft yarns of cotton fabric were bridged because of the uniform coating of the surface. This case also contributes to form continuous conductive paths for the access of electrolyte ions [23]. Furthermore, MXRC-2 fabric shows a more uniform coating between fiber bundles compared to MXC and MXRC-1 fabrics which resulted from positive effect of increasing rGO/CNC amount in hybrid dispersion. In addition, the compact conductive network of fibers and yarns is favorable for capacitive properties of MXRC-2 fabric electrode [20, 44]. SEM image of MXRC-2 also confirms that rough surface of MXene coated fabrics turns into a smoother coating surface which may be attributed to a good interaction between Ti3C2Tx and rGO nanosheets. Contrary to MXC, MXRC-1 and MXRC-2 fabrics, it is easily noticed that coating surface of MXRC-3 starts to become slightly rougher possibly caused by the increasing rGO/CNC amount in the coating dispersions, as given in Fig. 4e. However, the fabric surface of MXRC-3 still maintains the continuous coating structure resulted from MXene and rGO phases. On the contrary, Fig. 4f shows that MXRC-4 fabric exhibits a crinkled and coarse coating surface. In this case, it can be interpreted that the agglomeration of rGO starts to appear, and the coating adhesion could not be achieved completely. The unfilled gaps between fibers and yarns also affect negatively the electrical and electrochemical properties of cotton fabric because of inhibiting of electrolyte ion transport [45]. SEM images prove that an efficient interaction between cotton surface and Ti3C2Tx/rGO/CNC nanoflakes can be achieved up to 30% wt. rGO/CNC in Ti3C2Tx/rGO/CNC hybrid dispersions.

Figure 4
figure 4

SEM images of cotton fabric (a), MXC (b), MXRC-1 (c), MXRC-2 (d) MXRC-3 (e), MXRC-4 (f).

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Electrochemical characterization

Electrochemical characterization of the fabric electrodes was carried out in three-electrode system using 1 M of H2SO4 aqueous electrolyte. CV curves of fabric electrodes were given in Fig. 5a, b. According to the results, MXRC-2 electrode exhibits the largest CV area indicating a high specific capacitance compared to the other fabric electrodes. This improvement can be explained by the expansion of Ti3C2Tx layers by rGO/CNC sheets, as shown by XRD results and the formation of more electrochemically active surface for fast H+ ion transport. In addition, the partial increase in CV area in MXRC-1 electrode demonstrates the positive effect of rGO/CNC presence on the electrochemical performance of MXene structure. MXRC-1 also displays a larger CV area than MXC, MXRC-3 and MXRC-4 electrodes. It is obvious that a better interaction between MXene and rGO/CNC structures could be obtained in MXRC-1 and MXRC-2 electrodes which results in a high electrical conductivity and shorter ion transport channels. On the contrary, the smallest CV area was observed in MXRC-4 electrode possibly because of low electrical conductivity resulting from rGO agglomeration in MXene nanosheets [46]. To evaluate the rate performance of MXRC-2 electrode, CV tests were performed under different scan rates, as shown in Fig. 5b. It is clear that increasing scan rates lead to a higher current value. However, CV area starts to decrease especially after 20 mV s−1 indicating the decreasing rate performance of the electrode. This situation may result from the low accessibility of electrolyte ions to the active sites of coated fabric at high scanning rates because of the local agglomerations and blocked active sites across the fabric electrode surface [47]. MXRC-2 shows gravimetric specific capacitance of 324.7, 135.2, 66.1, 28.2, 16.2 and 7.8 F g−1 at scan rates of 2, 5, 10, 20, 30 and 50 mV s−1, respectively.

Figure 5
figure 5

a CV curves of MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4 at scan rate of 2 mV s−1 and b CV curves of MXRC-2 at different scan rates.

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GCD tests were performed to assess electrochemical performance of fabric electrodes at current densities of 0.3, 0.6, 1 and 1.5 A g−1 and GCD curves were obtained as shown in Fig. 6. GCD curves reveal that electrical double-layer and pseudocapacitive behavior take place together in the fabric electrodes. In Fig. 6a, it could be observed that the increased rGO/CNC amount up to 15% wt. in MXRC-2 electrode leads to a better charge–discharge performance and the longest discharge time at current density of 0.3 A g−1 suggesting the highest specific capacitance. The high specific capacitance of MXRC-2 also shows that the interconnected fiber structure coated by Ti3C2Tx and rGO/CNC nanostructures facilitates the ion transport phenomenon to the active sites through the fabric network [48]. This case also confirms that Ti3C2Tx nanosheets and rGO/CNC could provide a synergistic effect by preventing the self-restacking and creating more ion accessible channels for an improved charge storage capability. On the other hand, this advantageous structure for ion transportation begins to disappear in MXRC-3 electrode due to the inefficient fiber coating and restacking phenomenon of Ti3C2Tx and rGO components of the coating. Additionally, MXRC-4 electrode shows the shortest discharge time indicating the lowest specific capacitance compared to the other fabric electrodes because of inaccessible electroactive surface area and nonuniform coating of fabric surface which was also confirmed by SEM analysis. The calculated gravimetric specific capacitances were 438.3, 446.6, 501.1, 208.9 and 86.6 F g−1 at current density of 0.3 A g−1 for MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4, respectively. The GCD curves of MXRC-2 at different scanning rates were given in Fig. 6b. The linear and almost symmetrical GCD curves of MXRC-2 at various current densities confirm that an enhanced electrochemical performance and resulting high specific capacitance. Figure 6c, d also shows that the lowest IR drop was observed in MXRC-2 at different current densities which is ascribed to the presence of more conductive pathways and less electrical resistance of the electrode [49]. In addition, MXC and MXRC-4 electrodes display higher IR drop values at different current densities compared with other fabric electrodes. The specific capacitance of fabric electrodes at different current densities was given in Fig. 6d. It can be noted that MXRC-1 and MXRC-2 electrodes exhibit high specific capacitance values indicating a better rate performance. On the contrary, the decrease in specific capacitance values of MXRC-3 and MXRC-4 is more apparent with increasing current density. The possible reason of this situation is the low interaction time between electrolyte ions and electrode active sites caused by limited conductive pathways of the fabric electrodes [44]. In this case, the impregnation of the electrolyte into the fabric is restricted and electrochemical interactions occurring only on the electrode surface cause relatively lower specific capacitance values [32, 44].

Figure 6
figure 6

a GCD curves of MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4 at current density of 0.3 A g−1 and b GCD curves of MXRC-2 at different current densities. c IR drop of fabric electrodes at different current densities. d Specific capacitance of fabric electrodes at different current densities.

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Electrochemical impedance spectroscopy (EIS) was carried out to investigate internal resistance and charge transfer properties of fabric electrodes. The Nyquist plot, electrolyte resistance (Rs) and charge transfer resistance (Rct) of different electrodes was given in Fig. 7. The intersection of Nyquist plot with the real part (X-axis) at high frequency region shows the Rs value which resulted from electrolyte resistance [50]. Furthermore, the diameter of semicircle of Nyquist plot corresponds to Rct of the electrodes [51]. Nyquist plots in Fig. 7a display that the lowest Rs value was observed in MXRC-2 (2.9 Ω) electrode approving electrochemical efficiency of the electrode, while the highest Rs value is obtained in MXRC-4 (5.4 Ω) electrode at high frequency region. In addition, Rs decreases gradually starting from MXC up to 15% wt. of rGO/CNC (MXRC-2) indicating high electrical conductivity stemming from synergistic effect of 2D flakes. Moreover, MXRC-2 shows the lowest Rct (0.9 Ω) which compatible with Rs. The larger semicircle and smaller slope at low frequency region for MXRC-3 and MXRC-4 demonstrate high ion diffusion resistance and lower capacitive performance. This may be explained by the presence of high amount rGO/CNC resulting in a high electrical resistance in MXRC-3 and MXRC-4. Additionally, the decreasing semicircle diameter of the electrodes and increasing slope of Nyquist plot at low frequency region up to 15% wt. affirms the improvement of ion transport kinetics of the MXRC-2 electrode. The near vertical line in the low frequency region demonstrates Warburg region representing the electrolyte ion diffusion into the electrode [50]. It is evident that MXRC-2 exhibits the shortest Warburg line indicating the efficient and fast ion diffusion to the fabric electrode which is favorable for electrode performance [52]. Conversely, the longest Warburg line with a lower slope of MXRC-4 is the result of poor ion diffusion and lower capacitance.

Figure 7
figure 7

Nyquist plots (a), Rs and Rct values (b) of MXC, MXRC-1, MXRC-2, MXRC-3 and MXRC-4.

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To further investigate rate performance of MXRC-2 fabric electrode, GCD test was performed applying 500 cycles and EIS test was carried out after cycling. Cycling stability test shows that charge/discharge curves do not display a remarkable change over 500 charge discharge cycles in −0.4 to 0.2 V at current density of 1.5 A g−1, as shown in Fig. 8a. The capacitance retention of MXRC-2 electrode is calculated as 95.6% after 500 charge discharge cycles. In addition, Fig. 8b shows the Nyquist plots of MXRC-2 fabric electrode and equivalent circuit model before and after cycling test. The solid lines in Nyquist plots depict the fitting curves. It seems that GCD cycling tests does not significantly affect the internal resistance and charge transfer resistance properties of MXRC-2. According to the EIS results, Rs increased from 2.9 to 3.2 Ω, while Rct increased from 0.9 to 1.3 Ω after cycling tests. Furthermore, it is noteworthy that a small increase in slope occurs at low frequency region after charge discharge tests which may be attributed to the fast and continuous electrolyte ion movement during charge discharge cycles.

Figure 8
figure 8

Cycling stability test of MXRC-2 electrode (a) and Nyquist plots of MXRC-2 before and after cycling test (b).

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The prepared flexible MXRC-2 electrodes were assembled as a symmetric supercapacitor using cotton fabric separator which was coated by 1 M of PVA/H2SO4 gel electrolyte. The two-electrode performance of the prepared symmetric device and the change in electrochemical performance after 500 bending cycles at the angle of 180° were investigated. The CV curves, specific capacitance values at different scan rates, EIS and GCD test results before and after bending process were given in Fig. 9. CV curves and calculated gravimetric specific capacitance in Fig. 9a, b show that the supercapacitor device consisting of MXRC-2 electrodes exhibited gravimetric specific capacitance of 84.8, 74.9, 52.5, 33.7, 26.4 and 19.7 F g−1 at scan rates of 2, 5, 10, 20, 30 and 50 mV s−1, respectively. According to Nyquist plots in Fig. 9c, Rs and Rct values of the device were determined as 22.3 Ω and 10.1 Ω, respectively. On the other hand, the Rs and Rct values displayed an increase to 24.1 Ω and 12.8 Ω after 500 bending cycles, respectively. The change in the device performance can be seen in GCD test results in Fig. 9d. The supercapacitor device exhibited a specific capacitance of 103.8 F g−1 and 60.7 F g−1 at a current density of 0.3 A g−1. It was shown that the device delivers a maximum energy density of 5.19 Wh kg−1 at a power density of 68.7 W kg−1 in normal state. On the other hand, a relative decrease occurred in energy and power densities of the device after 500 bending cycles. The energy density of the device was determined as 2.98 Wh kg−1 at a power density of 63.2 W kg−1 after bending. It was also shown that the prepared symmetric supercapacitor has a flexible structure. The specific capacitance, energy density and power density from different studies were also given in Table 1 for comparison. The specific capacitance, energy density and power density results of MXRC-2 single electrode and flexible symmetric device show that MXene/rGO/CNC hybrid electrodes can reach higher specific capacitance and energy densities without complex processes compared to only MXene or rGO containing textile electrodes.

Figure 9
figure 9

CV curves (a), specific capacitance at different scan rates of MXRC-2 supercapacitor (b). Nyquist plots (c) and GCD curves of MXRC-2 supercapacitor before and after 500 bending cycles.

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Table 1 Comparison of specific capacitance, energy density and power density of different supercapacitors from the literature with this work
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Conclusions

In this study, it was shown that Ti3C2Tx and rGO 2D materials can form a homogenous dispersion in water with the presence of CNC and uniform coatings can be applied to the plain-woven cotton fabrics by fast and simple drop-casting method to fabricate wearable supercapacitor electrodes. The good interaction between cotton fibers, Ti3C2Tx and Ti3C2Tx/rGO/CNC nanostructures could be obtained as shown by XRD, FT-IR and SEM analysis. In addition, the formation of uniform and homogenous coating of fabrics resulted in a low electrical resistance especially for MXC, MXRC-1 and MXRC-2 fabric electrodes. Electrochemical characterization techniques indicated that Ti3C2Tx and rGO/CNC components resulted in a synergistic effect on fabric electrode structure by preventing restacking of 2D materials and creating conductive ion transportation pathways through the coated fibers. Ti3C2Tx coated fabric electrode (MXC) displayed a specific capacitance of 438 F g−1 at a current density of 0.3 A g−1. The presence of rGO/CNC up to 15 wt. % in MXRC-2 led to a continuous conductive fiber network and increased specific capacitance to 501.1 F g−1 at a current density of 0.3 A g−1. It was also shown that MXRC-2 electrode possesses a good electrochemical stability. The symmetric supercapacitor consisting of MXRC-2 fabric electrodes exhibited a specific capacitance 103.8 F g−1 at a current density of 0.3 A g−1. The results also offer a promising approach for simple and fast fabrication of textile supercapacitor electrodes.