One-step fragmentation of a 2D MXene across the fine 1D MnO₂ surface and its supercapacitance

Abstract

Functional materials are being studied for their promising applications. Here, for the first time, a novel approach is highlighted to bring down the morphologies of MXene into small fragments with the aid of finer one-dimensional (1D)/nanorods of MnO₂. This unique grown morphology was characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS). The BET surface area showed an enhancement in surface area from 39 to 201 m² g⁻¹ on incorporating 1D MnO₂ with MXene. Morphological tension as developed between MnO₂ and the MXene surface helped in the considerable improvement of the supercapacitive behaviour of MnO₂. An increase of 92.4% in the capacitive behaviour of MnO₂ was observed with 818.5 F g⁻¹ at 3 A g⁻¹. Electrochemical device characterization was undertaken to achieve a promising energy density of 77.2 W h kg⁻¹ at 1725 W kg⁻¹. Favourable stability retention of 192.3% in a 3-electrode system and stable performance with 80% retention in a 2 electrode system were achieved after 5000 cycles of galvanostatic charge–discharge. The hydrothermal growth process of (1D) MnO₂ is quite effective in bringing MXenes down to fragments, thereby enhancing their overall activity for showcasing one of the best supercapacitive behaviours.

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1. Introduction

Novel functional materials have attracted considerable attention for their considerable changes in physical and chemical behaviours. These nanomaterials have bright prospects for developing efficient renewable energy systems. Future renewable energy systems will benefit greatly from energy storage devices as they can compensate for the intermittent nature of renewable energy.^1–2 Owing to their high-power density, long cycle life, and fast charge/discharge characteristics and long cycle life, supercapacitors are considered to be an important class of energy storage devices.^3–4 Despite this, their practical applications are limited due to their lower energy density values than batteries.^5–6 Thus, to meet the high power and energy density requirements of next-generation energy storage devices, significant research efforts have been devoted to the design and development of high-energy-density supercapacitors.^7–9 To achieve this, new electrode materials with higher energy densities must be developed without compromising cycling stability or power density. Among the various transition metal oxides investigated for supercapacitors, MnO₂ has attracted great interest due to its low cost and toxicity, friendly interfacial properties with carbon materials, high theoretical super capacitance and natural abundance.^10–12 Further studies have revealed that the usage of hybrid electrode materials with high specific capacitances could result in higher energy densities.^{4–5,13–15} Hybrid composite electrodes can be prepared by dispersing/inserting a supercapacitor material like a transition metal oxide/conducting polymer over carbon.^16–19

MXenes, a family of two-dimensional early transition metal carbides, are emerging as innovative electrode materials due to their superior properties, such as high in-plane electrical conductivity, large surface areas, and metallic conductivity on their hydrophilic surfaces.^20–24 Gogotsi et al. have demonstrated that MXenes can act as an intercalation electrode for a wide variety of cations, such as Na⁺, Li⁺, K⁺, Mg²⁺, Al³⁺ and NH⁴⁺.^25–27 By adapting post-etching annealing conditions or by coating MXene sheets with metal ions, the electrochemical energy storage properties of MXenes can be altered.²⁸ Electrodes made of MXenes by conventional routes possess lower gravimetric specific capacitances than those made of graphene.^29–30 This technical constraint can be overcome by surface coating the MXene with supercapacitive materials, which will significantly improve its electrochemical energy storage performance. To the best of our knowledge, MnO₂/MXene hybrid materials have not been studied extensively for their electrochemical energy storage properties.

In the present work, we have tried to increase the supercapacitive behaviour of MnO₂ by incorporating it with MXenes. We have done this by modifying our previously reported hydrothermal synthesis technique to grow fine 1D MnO₂.^31–40 Herein, we report for the first time that heavy clustered sheets of MXenes can be dispersed into small fragments by force generated during the growth process of fine 1D MnO₂ to develop a superior functional material. The resulting hybrid material is one of the best electrochemical super capacitances due to the novel surface enhancement of 1D MnO₂. Standard characterization techniques have been employed to study its morphology, structure and growth mechanism.

2. Experimental

2.1. Synthesis

First, 1 g of Ti₃AlC₂ MAX phase powder was mixed with a solution of 40 wt% hydrofluoric acid (HF) with continuous stirring under normal conditions. The stirring was continued for 5 days, followed by ultrasonication. The supernatant was removed, and the resulting precipitate was washed several times with ethanol and distilled water. The process was repeated and finally, Ti₃C₂ MXene powder was obtained after drying. Then, a known weight of KMnO₄ (potassium permanganate) was mixed into 37 ml of distilled water under continuous magnetic stirring and 10 wt% of the as-prepared MXene powder was added to the solution. NaNO₂ (sodium nitrite) was added to make the molar ratio between KMnO₄ and NaNO₂ 2 : 3. Finally, under continuous stirring, 3 ml of a 0.3 M H₂SO₄ solution was slowly added dropwise and the solution was then treated hydrothermally at 170 °C for 12 h to obtain the MnO₂/MXene nanocomposite after properly washing and drying the resulting precipitate. For a comparative analysis, the same procedure was repeated in the absence of MXene to obtain fine one-dimensional MnO₂ structures.

2.2. Fabrication of the working electrode

In the preparation of the working electrode, super p carbon, polyvinylidene fluoride (PVDF) and the as-prepared material were taken at a weight ratio of 1 : 1 : 8, respectively. N-Methyl-2-pyrrolidone (NMP) was added to make a slurry out of these materials after grinding. The slurry was easily coated over a 1 cm² surface area of 0.05 mm thick stainless steel plate to be used as a working electrode after drying. The test was followed in 1 M KOH electrolyte with a mass of the active material of 1 mg. In contrast to a 3-electrode system, the electrode size was cut down to half of its length for a 2-electrode system.

3. Results and discussions

3.1. XRD

Fig. 1 shows the XRD patterns of all the prepared samples. The MXene sample exhibited peaks at 2θ values of about 9.0, 18.3, 27.5, 38.7, 41.6 and 60.4, attributed to the Ti₃C₂ phase with miller indices of (002), (004), (006), (008) and (110).^41–45 In addition, peaks at 2θ values of 35.9 and 41.6 were attributed to TiC with miller indices of (111) and (200). XRD measurements of MnO₂ exhibited peaks at 2θ values of 12.3, 17.9, 24.8, 28.5, 36.6, 37.5, 41.0, 41.82, 49.6, 56.1, 60.0, 65.2, 69.4 and 72.7 degrees. According to JCPDS 44-0141, these peaks are related to α-MnO₂ with a tetragonal phase for miller indices (110), (200), (220), (310), (400), (211), (420), (301), (411), (600), (521), (002), (541) and (312), respectively. The composite sample, MnO₂/MXene, exhibited all the above peaks plus an additional peak at 9.0 degrees corresponding to the (002) plane of MXene. No other impurity peaks in the XRD patterns were observed due to the high purity of the as-prepared samples. The presence of only a minute peak for MXene is related to its morphologically reduced shape and may be in strictly two dimensions, which results in unnoticeable lattices by passing X-rays. Therefore, the structural composition could be further verified by XPS analysis.

Fig. 1 XRD measurements of MXene, MnO₂ and MnO₂/MXene.

3.2. XPS

The chemical valence states and surface composition of MnO₂/MXene were analyzed using XPS (Fig. 2). The obtained survey spectrum (Fig. 2a) indicated the presence of Mn, Ti, and C elements, which corroborate the EDS analysis. The Mn2p signal (Fig. 2b) consists of four peaks at 642.4, 643.9, 654.1, and 656.3 eV. Two of the peaks at 642.4 and 654.1 eV correspond to Mn (2p_3/2 and 2p_1/2, respectively) of the trivalent manganese ion (Mn³⁺), while the other two peaks at 643.9 and 656.3 eV correspond to the tetravalent manganese ion (Mn⁴⁺).⁴⁶ The presence of Mn³⁺ species indicates the oxygen vacancies on the surface of the synthesized manganese dioxide. The Ti 2P signal (Fig. 2c) consists of two peaks at 458.3 and 459.3 corresponding to Ti–O (2p_3/2), whilst the third peak at 464.1 eV represents O (2p_1/2) bonds.^46,47 The O1s peaks (Fig. 2d) observed at 529.7, and 531.9 eV arise from the Mn–O and Ti–C–O_X, respectively and confer fruitful bonding with carbon.⁴⁸ The C1s peaks (Fig. 2e) consist of two divisions at 284.8 and 286.1 eV, which represent C–C and C–O bonding, respectively.⁴⁷

Fig. 2 (a) Survey scan, (b) Mn2p, (c) Ti2p, (d) O1s and (e) C1s XPS spectra of MnO₂/MXene.

3.3. SEM & TEM

The SEM image shown in Fig. 3a corresponds to a fine one-dimensional (1D) structure of MnO₂ with a diameter in the range of 10–80 nm. Energy Dispersive X-ray Spectroscopy (EDS) analysis of the MnO₂ sample shown in the inset in Fig. 3a has characteristic peaks of only Mn and O elements. Two-dimensional MXene sheets can be visualized in SEM images shown in Fig. 3b. The MnO₂/MXene sample images at different time intervals of the hydrothermal reaction are shown in Fig. 3(c and d). After approximately 6 h of the hydrothermal reaction (Fig. 3c), fine two-dimensional sheets of MXene were seen to be stacked together layer by layer, surrounded by minute particles considered to be the mixed reagents of KMnO₄ and NaNO₂. The reagents were distributed well over the MXene surface like a uniform coating (Fig. 3c). These reagents act as precursors for the growth of 1D MnO₂ following redox kinetics. It was also predicted that these precursors were intercalated inside the thin layers of the MXene sheet. Herein, nitrite ions function as a perfect reducing agent for KMnO₄ with the help of an adequate amount of acid content, as depicted below.

2MnO₄⁻ + 3NO₂⁻ + 2H⁺ → 2MnO₂ + 3NO₃⁻ + H₂O (a)

After 12 h of hydrothermal reaction, the above kinetics exerts pressure to form fine 1D structures of MnO₂. It is interesting to note here that during the growth process of 1D structures, the sheets of MXene were considered to be fragmented as if 1D structures were emanating from inside the surfaces of the MXene. This led to the fragmentation of MXene sheets across the surface of 1D MnO₂ as seen in Fig. 3d. EDS of sample MnO₂/MXene shown in the inset in Fig. 3d revealed extra peaks for C and Ti elements, which account for the compound Ti₃C₂. The absence of any impurity peaks relates to the pure nature of the prepared MnO₂ and MnO₂/MXene samples.

Fig. 3 SEM images of (a) 1D MnO₂ and its EDS as the inset, (b) MXene and MnO₂/MXene after a hydrothermal reaction time of (c) 6, and (d) 12 h, and the EDS in the inset.

In the TEM images shown in Fig. 4(a–c), fragmented MXene sheets, as highlighted, are seen to be settled around 1D structures. The precise attachment between MXene and MnO₂ could be due to the weak van der Waals forces, which need to be studied in the near future. The HRTEM image shown in Fig. 4d provides the approximate interlayer spacing of MXene sheets as 0.95 nm.^49,50 Also seen in the image is a d-spacing of 0.48 nm in the (200) plane for MnO₂. This shows close interaction between MXene and MnO₂.

Fig. 4 (a–c) TEM images of MnO₂/MXene, (d) HRTEM image of MnO₂/MXene with an interlayer spacing of 0.95 nm for MXene and d-spacing of 0.48 nm in the (200) plane for MnO₂.

The phenomenon of fragmentation of MXene across 1D structures can be observed from the schematic shown in Fig. 5. As portrayed, after 6 h of hydrothermal reaction, a uniform coating of MnO₂ precursors over MXene layered structures was formed. This coated material was then transformed into fine 1D structures after 12 h of hydrothermal reaction. Interestingly, the phenomenon of the growth of 1D structures deeply affects the layered structures of MXenes. As a fruitful effect, the layered structures of MXene are broken down into small fragments. Even after this breakdown, these fragmented parts of MXenes are seen to be morphologically attached to the 1D structures therein, giving rise to a novel blend of nanocomposites in the form of MnO₂/MXene.

Fig. 5 Schematic of MnO₂/MXene nanocomposite formation.

3.4. BET

Surface analysis by the Brunauer–Emmett–Teller (BET) method is highlighted with isotherms of nitrogen (N₂) absorption–desorption and Barrett–Joyner–Halenda (BJH) pore size distribution (PSD) as an inset of Fig. 6(a and b) for MnO₂ and MnO₂/MXene, respectively. The 1D MnO₂ structure exhibits a type H4 loop under a type IV isotherm, indicating a mesoporous structure.⁵¹ The specific surface area of 1D MnO₂ was found to be 39 m² g⁻¹. The PSD histogram of MnO₂ showed the greatest distribution around 5–10 nm giving rise to an average pore diameter of 7.4 nm. For the MnO₂/MXene sample, the isotherm was observed to be a type H1 loop within a type IV isotherm, which again showcases the mesoporous structure. However, the specific surface area considerably increased to 201 m² g⁻¹, and the PSD histogram showed the greatest distribution around 4–9 nm, predicting an average pore diameter of 6.3 nm. This increase in specific surface area and reduction in average pore diameter can be attributed to the fragmented sheets of MXene being precisely attached to the fine 1D structures of MnO₂.

Fig. 6 Isotherms of nitrogen absorption–desorption and BJH pore size distribution as an inset of the samples: (a) MnO₂ and (b) MnO₂/MXene.

3.5. Supercapacitive behaviour

3.5.1. Three-electrode system. The standard electrochemical capacitive behaviour of the as-prepared material was tested in a 3-electrode cell configuration, and its formulation in the device was tested in 2-electrode cells. For the 3-electrode cell set-up, the first electrode was taken as platinum wire to serve the counter purpose, while the reference purpose was satisfied by the Ag/AgCl electrode. The final electrode was composed of prepared material to serve as the working electrode.

Cyclic voltammetry measurements in a potential window of −0.3 to 0.8 V for the MXene sample and −0.3 to 1.0 V for MnO₂ and MnO₂/MXene at 20 mV s⁻¹ are shown in Fig. 7a. It was observed that the pristine MXene sample exhibited a considerably smaller area under the CV curve with undiscernible redox peaks. This indicates its poor electrochemical activity in contrast to MnO₂ and MnO₂/MXene samples. For MnO₂ and MnO₂/MXene samples, the larger curve area with two similar anodic oxidation peaks at around 0.15 and 0.65 V and two cathodic reduction peaks at around 0.0 and 0.2 V revealed the two-stage oxidation and reduction reaction of electrode surfaces. This suggests a promising charge flow in and out of the working electrodes, which is favourable for supercapacitive behaviour. In both the MnO₂-based electrodes, the transition between Mn⁴⁺ and Mn³⁺ is reversible. Faradaic activity following the intercalation and de-intercalation of K⁺ ions as well as charges into the electrode surface can be interpreted as follows:

MnO₂ + K⁺ + e⁻ ↔ MnO·OK (b)

The [2 × 2] and [1 × 1] tunnel structures found in the tetragonal α phase of MnO₂ are suitable for providing increased interstitial sites to support the above faradaic activity. An extra-large CV curve observed for MnO₂/MXene could account for the increased surface area from the attached fragmented sheets of Ti₃C₂ across MnO₂. This increased surface area could accommodate extra ions and charges in a reversible adsorption and desorption process leading to non-faradaic activity as follows:

Ti₃C₂ + K⁺ + e⁻ ↔ (Ti₃C₂)⁻K⁺ (c)

Hence, a synergistic effect could have arisen, leading to an enhancement in the supercapacitive performance of 1D MnO₂. CV curves at different scan rates of 10, 20, 30, 50, 75 and 100 mV s⁻¹ for samples MXene, MnO₂ and MnO₂/MXene are shown in Fig. 7(b–d), respectively. The mathematical equation for calculating specific capacitance C_s in F g⁻¹, using the CV curve is as follows:Here, A denotes the area enclosed by the CV curve and m, k, and ΔV denote mass in grams, scan rate in mV s⁻¹ and the potential window in volts, respectively. C_s were calculated as 141.0, 113.8, 95.2, 72.8, 58.0 and 49.3 F g⁻¹ for MXene, 299.5, 263.2, 242.0, 205.8, 169.2 and 140.0 F g⁻¹ for MnO₂, and 573.7, 352.4, 278.6, 213.1, 180.5 and 159.0 F g⁻¹ for MnO₂/MXene samples at scan rates of 10, 20, 30, 50, 75, and 100 mV s⁻¹, respectively. Fig. 7e gives the comparative specific capacitances at various scan rates for MXene, MnO₂ and MnO₂/MXene samples. Out of the three samples, MnO₂/MXene performed the best with 573.7 F g⁻¹ at 10 mV s⁻¹. Surprisingly, MXene, in its purest state, cannot match the 1D structure of MnO₂ for capacitive performance. However, when it is blended with 1D MnO₂, it leaves an impact by abruptly increasing the capacitive performance of 1D MnO₂. When compared to pure MXene, the presence of the one-dimensional (1D) structure is crucial for good capacitive performance. Short diffusion paths provided by 1D MnO₂ make it simple to accommodate charged particles. The combined impact of MnO₂ and MXene in composite MXene/MnO₂ corresponds to a reversible conversion in faradaic and non-faradaic activities as described above for its maximum capacity.

Fig. 7 CV curves for (a) MXene, MnO₂, and MnO₂/MXene at 20 mV s⁻¹, (b) MXene, (c) MnO₂, (d) MnO₂/MXene; (e) specific capacitances of MXene, MnO₂ and MnO₂/MXene at different scan rates.

Galvanostatic charge–discharge measurements in a potential window of −0.3 to 0.8 V for the MXene sample and −0.3 to 1.0 V for MnO₂ and MnO₂/MXene at 3 A g⁻¹ are shown in Fig. 8a. Fig. 8(b–d) show the measurements for MXene, MnO₂ and MnO₂/MXene, respectively, at current densities of 3, 5, 7, 10, 20 and 30 A g⁻¹. The mathematical equation for calculating specific capacitance C_s in F g⁻¹ using the GCD curve is as follows:

C_s = IΔt/mΔV (2)

Here, IΔt denotes the product of current and discharge time in mA and seconds, respectively. C_s were calculated as 136.0, 103.2, 75.3, 62.72, 31.3, and 17.7 F g⁻¹ for MXene, 425.3, 325.4, 285.6, 249.2, 182.6, and 129 F g⁻¹ for MnO₂, and 584.6, 496.4, 448.7, 391.7, 249.4, and 185.7 F g⁻¹ for MnO₂/MXene sample at current densities of 3, 5, 7, 10, 20, and 30 A g⁻¹, respectively. Fig. 8e gives the comparative specific capacitances at current densities for MXene, MnO₂ and MnO₂/MXene samples.

Fig. 8 GCD curves for (a) MXene, MnO₂ and MnO₂/MXene at 3 A g⁻¹, and (b) MXene (c) MnO₂ (d) MnO₂/MXene at 3, 5, 7, 10, 20, and 30 A g⁻¹. (e) Specific capacitances of MXene, MnO₂ and MnO₂/MXene at various current densities.

In line with the findings from CV waveforms, composite MnO₂/MXene demonstrated superior capacitive behaviour as compared to MnO₂ and MXene, respectively. Table 1 compares the new work to previously reported efforts.

Table 1 Comparison of specific capacitance values

S. no.	Electrode	Specific capacitance (F g^−1)	Current density (A g^−1)	Ref.
1	MnO2/MXene	340	1	47
2	MnO2/Ti3C2Tx	130.5	0.2	52
3	MnO2/Ti3C2Tx	452	1	53
4	MnO2@MXene/CNT	181.8	1	48
5	MnO2/MXene	611.5	1	54
6	MnO2/MXene	457	1	55
7	MnO2	365	0.25	56
8	MnO2/Fe3O4	243.7	0.1	57
9	MnO2/Fe3O4	590	2.9	39
10	MnO2	425.3	3	This work
11	MnO2/MXene	584.6	3	This work

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The attained morphology may be used to explain why the developed material (MnO₂/MXene) performs better capacitively than the previously reported works. When fine 1D materials are covered with MXene fragmented structures, the physical complexity of their surface is increased. It is clear from the TEM investigation that MXene is present in a variety of fragmented structures along with 1D morphologies. As a result of the interaction between 1D MnO₂ and 2D MXene, the surface tension is supposed to be enhanced. The fragments of MXene are predicted to remain attached on the 1D MnO₂ surface due to the weak van der Waals forces. The BET study, which shows a dramatic rise in the specific surface area of 1D MnO₂ when mixed with MXene from 39 to 201 m² g⁻¹, provides insight into this interaction. However, this interaction does not support any chemical bonding between MnO₂ and MXene, as XRD analysis showcased α-MnO₂ with a tetragonal phase according to ICDD 44-0141. Therefore, this weak interaction could have resulted in a synergistic effect between the two interacting surfaces for a promising charge storage capability. In addition, as compared to pure MXene, the composite MXene was shown to have reduced morphologies. The reduction in morphology must be from the surface effects of 1D MnO₂. This reveals that nanoscale materials try to interact closely with a substance having comparable dimensions for stability, thus giving rise to weak van der Waals forces. The surface tension is further increased by this change in form, creating an enhanced synergistic effect. Therefore, even though the pure MXene sample employed comparable crystallographic structures, the modification in morphology, as given, leads to greater capacitive behaviour. Surface tension that results from this significant morphological change may increase surface activity, creating more active spots for the convenient accommodation of charged particles. The material must react more quickly for both the non-faradaic and faradaic activities mentioned above due to the intricacy of the surface shape. However, the precise relationship between morphology and electrochemical kinetics may be examined thoroughly with an established synthesis and characterization approach, a topic that is debatable and requires more technological advancement.

GCD cycles were performed for 5000 continuous cycles at 20 mA for all the samples to study their stability in capacity retention as presented in Fig. 9a. The MXene sample was quite stable with a good retention value of 89.54% after continuous running over 5000 GCD cycles at 20 mA. However, under similar conditions, MnO₂ and MnO₂/MXene electrodes showed variations. MnO₂ and MnO₂/MXene electrodes showed a gradual increase with prolonged cycling. These increases could account for the breaking of interfacial layers between electrode and electrolyte surfaces, which is a common phenomenon in a 3-electrode system. However, the MnO₂ electrode was more stable and showed a steady response after 1000 cycles with a capacity retention of 123.6%, whilst the MnO₂/MXene electrode took a longer cycling duration of 3500 cycles to get stabilized with a capacity retention of 137.3%. This stabilization could be accounted for by the creation of a continuous pathway for charged particles across the interfacial layers. In the case of the MnO₂/MXene electrode, the longer cycling duration for stabilization directly reflects the breaking of a much large number of interfacial layers. The larger surface area of MnO₂/MXene (201 m² g⁻¹ against 39 m² g⁻¹ of MnO₂) sample observed through BET analysis and the complexity in morphology observed through TEM analysis suggest the existence of a large number of interfacial layers. For a steady reference, the first and last 10 GCD cycles of the MnO₂/MXene sample are presented in Fig. 9(b and c), respectively.

Fig. 9 (a) Specific capacity vs. cycle number for up to 5000 GCD cycles for MXene, MnO₂, and MnO₂/MXene. (b) The first 10 GCD cycles and (c) the last 10 GCD cycles of MnO₂/MXene at 20 A g⁻¹.

3.5.2. Two-electrode system. Electrochemical device characterization was done by fabricating a 2-electrode symmetric supercapacitor cell using 1 M KOH as an electrolyte. The active material MnO₂/MXene was used at the same weight for both the positive and negative electrodes. The same processes as for the 3-electrode system were used to prepare the working electrodes for the cell. The amount of active substance coated on each electrode, however, was decreased to 0.5 mg. CV curves at different scan rates of 10, 20, 30, 50, 75 and 100 mV s⁻¹ for symmetric supercapacitors composed of MnO₂/MXene electrodes are shown in Fig. 10a. Specific capacitances were calculated as 260.6, 168.6, 149.5, 125.7, 106.3, and 93.3 F g⁻¹ at scan rates of 10, 20, 30, 50, 75, and 100 mV s⁻¹, respectively. GCD curves presented in Fig. 10b, revealed the specific capacitances as 247.0, 215.5, 180.2, 155.6, 124.9 and 77.0 F g⁻¹ and current densities of 2, 3, 5, 7, 10 and 20 A g⁻¹, respectively. The variations in specific capacity upon prolonged GCD cycling for up to 5000 cycles at 10 A g⁻¹ are presented in Fig. 10c with GCD curves of the last 10 cycles as the inset. A stable capacity retention value of 80% was observed. Interestingly, unlike the case of the 3-electrode system, a gradual increase in capacitive behaviour upon prolonged cycling was not observed. This suggests that under symmetrical conditions or with the use of an identical counter and the working electrode, the interfacial layers between electrode surfaces were comparatively less accountable.

Fig. 10 Symmetric supercapacitor (a) CV curves at scan rates of 10, 20, 30, 50, 75 and 100 mV s⁻¹. (b) GCD curves at 2, 3, 5, 7, 10 and 20 A g⁻¹. (c) Specific capacity vs. cycle number for up to 5000 cycles at 10 A g⁻¹ with GCD curves for the last 10 cycles as an inset in the potential range of 0–1.5 V. (d) Ragone plot.

Using the following equations and discharge duration as t in seconds, the energy density, E (Wh kg⁻¹) and power density, P (W kg⁻¹), were computed as 77.2, 67.3, 57.5, 48.6, 39.1, and 24.1 Wh kg⁻¹, and 1725.0, 2587.5, 4404.2, 6035.7, 8635.3, and 17388.0 W kg⁻¹, respectively (Fig. 10d, Ragone plot).

A comparison of energy density as achieved in the 2-electrode configuration with the available reports is presented in below Table 2:

Table 2 Comparison of energy density values

S. no.	Electrode	Energy density (Wh kg^−1)	Power density (W kg^−1)	Ref.
1	MnO2/CNT//Ti3C2Tx	36	3200	58
2	MnO2/Ti3C2Tx	12.25	20 000	52
3	MnO2/Ti3C2Tx	14.3	790	53
4	rGO//Ti3C2Tx	23.7	22 500	59
5	MnO2@Ti3C2Tx//activated carbon	31.4	180	60
6	MnO2/Ti3C2	8.3	221.33	61
7	MnO2/Ti3C2Tx	14.42	700	62
8	Ti3C2Tx/MWCNT//rGO	20	65	63
9	Ti3C2/CuS//Ti3C2	15.4	750.2	64
10	Ti3C2/nickel foam//Ti3C2	18.1	397.8	65
11	MnO2/CNTs//activated carbon	13.3	600	66
12	MnO2/MXene	77.2	1725	This work

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The results predict that the as-prepared material could be one of the best in class for utilization as supercapacitor electrodes. It is noteworthy that the same elemental composition of MXene with MnO₂ can give different results based on their morphologies. The fine morphological tuning of MXene sheets as fragments over 1D MnO₂ has gained a considerable advantage for adding surface activity to MnO₂, resulting in enormously high capacitive performance. The presented study is an addition to the exploration of fascinating material properties.^67–74

4. Conclusions

We have successfully presented a simple one-step synthesis method for morphologically fragmenting the 2D sheets of MXene across the hydrothermally grown 1D MnO₂ structure. TEM analysis presented weak bonds between fragmented parts of MXene and the 1D MnO₂ surface. BET analysis further supported the promising interactions between MXene and MnO₂ with an increment in surface area from 39 to 201 m² g⁻¹. As a consequence, promising supercapacitive behaviour was achieved to the extent of 818.5 F g⁻¹ at 3 A g⁻¹ in a three-electrode system. Also, the device performance was successfully tested in a two-electrode symmetric supercapacitor system with 77.2 Wh kg⁻¹ at 1725 W kg⁻¹. A stable electrode material was thus developed with a capacity retention of 80% as tested over 5000 GCD cycles in the two-electrode configurations. The improved surface activity could be attributed to the novel morphological structure of MnO₂/MXene, following considerable enhancement in the overall charge distribution.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to acknowledge the financial support provided by Uttaranchal University under the seed money project scheme for accomplishing the work reported in this research article. We further acknowledge the DST-FIST, Government of India (via Project No. SR/FST/PS-II/2021/190) along with SRM-SCIF and Nanotechnology Research Center (NRC) for providing some of the important characterization facilities at SRMIST. Also, the authors are thankful to the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program) for funding.

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