Safe Etching Route of Nb2SnC for the Synthesis of Two-Dimensional Nb2CTx MXene: An Electrode Material with Improved Electrochemical Performance

In this study, low-temperature synthesis of a Nb2SnC non-MAX phase was carried out via solid-state reaction, and a novel approach was introduced to synthesize 2D Nb2CTx MXenes through selective etching of Sn from Nb2SnC using mild phosphoric acid. Our work provides valuable insights into the field of 2D MXenes and their potential for energy storage applications. Various techniques, including XRD, SEM, TEM, EDS, and XPS, were used to characterize the samples and determine their crystal structures and chemical compositions. SEM images revealed a two-dimensional layered structure of Nb2CTx, which is consistent with the expected morphology of MXenes. The synthesized Nb2CTx showed a high specific capacitance of 502.97 Fg−1 at 1 Ag−1, demonstrating its potential for high-performance energy storage applications. The approach used in this study is low-cost and could lead to the development of new energy storage materials. Our study contributes to the field by introducing a unique method to synthesize 2D Nb2CTx MXenes and highlights its potential for practical applications.


Introduction
Renewable energy resources are significant because they help us to diminish our dependence on fossil fuels. They are leading us to a sustainable future where we can live without the threat of climate change and pollution. Energy storage systems are combinations of procedures and techniques used to store energy that help to incorporate renewable energy sources into smart energy grids. There are many technologies used for energy storage, which can be classified based on the purpose for which energy is stored. Primarily, they are classified into two main methods: electrical energy storage and thermal energy storage, which is further divided into mechanical, chemical, and electrochemical energy. Among all energy storage technologies, electrochemical energy storage supercapacitors are better able to handle high power conversion rates than batteries. Another advantage of supercapacitors is that their charging times are nearly thousands of times faster than those of batteries with similar capacities [1]. In order to raise both the performance capability of batteries and the overall effectiveness of an energy storage system, supercapacitors have been used in conjunction with batteries [2]. In general, supercapacitors have been used in two major domains: high-power applications, where short-time power peaks are utilized by supercapacitors, to boost energy in hybrid vehicles, for instance, or to start heavy diesel engines; and low-power applications, where batteries can be more reliable, the most common examples of which are UPS and security installations [3]. strated the potential of various MXenes, including Ti 3 C 2 T x , V 2 CT x , and Nb 2 CT x , for use in supercapacitors, with excellent electrochemical performance reported in several cases. For instance, Dall'Agnese et al. [28] reported the use of a Ti 3 C 2 T x MXene as an electrode material for a symmetric supercapacitor, which exhibited high capacitance and excellent cycling stability. Similarly, Sandhya et al. [29] synthesized a V 2 CT x MXene via a facile wet-chemical method and demonstrated its use as an electrode material for an asymmetric supercapacitor, which showed high specific capacitance and energy density. Furthermore, by using in situ electrochemical Raman spectroscopy investigation, Hu et al. [30] studied the capacitance behavior of Ti 3 C 2 T x using aqueous electrolytes and three different types of sulfate ions (H 2 SO 4 , (NH 4 ) 2 SO 4 , and MgSO 4 ) and came to the conclusion that the Ti 3 C 2 T x electrode outperformed the other two electrolytes in terms of supercapacitor performance in an acidic medium. Ghidiu et al. [31] reported for the first time the clay-like Ti 3 C 2 T x materials as supercapacitor electrodes in acidic electrolyte, and the performance of these materials was found to be very promising, with volumetric capacitance up to 900 F cm −3 or 245 F g −1 . Lukatskaya et al. [32] concluded that the electrochemical behavior of Ti 3 C 2 T x in H 2 SO 4 is predominantly pseudo-capacitive, with specific capacitance near to 230 F g −1 . Apart from Ti 3 C 2 T x , other MXenes, such as V 2 CT x [33], Mo 2 CT x [34], Mo 1.33 TiC 2 T x [35], and Nb 2 CT x [36,37], have shown promising performance in supercapacitor and energy storage applications. Nb 2 CT x is not more studied as compared to the Ti 3 C 2 T x MXene, despite its having significant potential for many applications, such as biosensors [38] and energy storage [39]; most of the possible applications are still to be explored. Niobiumbased MXenes are theoretically proved to be more stable than titanium-based MXenes [15]. So far, various methods have been reported for the synthesis of Nb-based MXenes (Nb 2 CT x and Nb 4 C 3 T x ), in which different acids and reaction conditions have been used. HF (hydrofluoric acid) is the most common acid used for synthesis of Nb 2 CT x MXenes [40] under different etching times, such as 24 h, 48 h, and 96 h [41]. Apart from HF, a mixture of HCL (hydrochloric acid) and LiF (lithium fluoride) is also used to avoid the toxicity due to HF [42]. The Nb-based MXenes Nb 2 CT x and Nb 4 C 3 T x have proved potential in most applications, such as cancer nanomedicine [43], HER [16], EMI shielding [44], electrochemical sensors [40], and photocatalytic activities [45].
In this study, we investigated Nb 2 CT x , a supercapacitor electrode material, based on a two-dimensional nanostructure. While H 3 PO 4 etching was used for the synthesis of Nb 2 CT x MXenes, the solid-state reaction used to obtain the non-MAX phase Nb-Sn-C occurs at 1000 • C under the flow of nitrogen. XRD, SEM, FTIR, XPS, and TEM are some of the techniques that were used to describe the produced materials to examine their structural and morphological characteristics. By performing tests, such as galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) analyses, the electrochemical performance of the Nb 2 CT x -modified electrodes was examined. The Nb 2 CT x -modified electrodes displayed good capacitance performance, with a specific capacitance of 502.97 Fg −1 and a capacitance retention of 32.64% at a current density of 4.4 Ag −1 . The findings of this study show that Nb 2 CT x has promise as an electrode material for supercapacitors.

Materials
Nb (niobium powder, <45 µm, 99.7% metal basis), Sn (<125 µm, 99.8% metal), graphite powder (<30 µm), isopropyl alcohol (C 3 H 8 O), potassium hydroxide (KOH), acetylene black, and Nafion solution (binder) were purchased from Sigma Aldrich. The electrochemical characterizations were performed with three-electrode assembly, in which an Ag/AgCl electrode was used as the reference electrode, a platinum-wire electrode (purchased from Top Sky Technology China, Shenzhen, China) was used as an auxiliary electrode, and nickel foam on which the prepared sample was deposited was used as the working electrode. A mixture of distilled water (DI) and ethanol was used for the preparation of the solution and the cleaning of electrode materials throughout the experiment.

Synthesis of Nb 2 SnC and Nb 2 CT x MXenes
The Nb, Sn, and graphite powders were mixed at a molar ratio of 2:1.1:1 with a mortar and pestle and then ball-milled for 8 h using a Retsch PM 100 planetary ball mill with a 500 mL stainless steel jar and 10 mm-diameter stainless steel balls. The ball-to-powder weight ratio was 10:1, and the milling speed was set to 300 rpm. The resulting powder mixture was pressed into pellets with a size of 10 mm diameter and 1 mm thickness. The pressure exerted by the hydraulic press during compaction was 50 MPa, and each pellet weighed 1 g. The pellets were then heated in an atmospheric controlled tube furnace at 1000 • C for 8 h with nitrogen gas flowing through it. After cooling to ambient temperature, the pellets were manually ground into Nb 2 SnC powders and stored in a dry area.
As we know, the synthesis of graphene and black phosphorous [46] is performed by mechanical exfoliation, but this method is unfeasible for layers in the M n+1 AX n phase, due to the substantial metallic bonds among "M" and "A" elements. Among M-A and M-X bonds, the M-A bonds are chemically more active in comparison to the M-X bonds [47], and MXene can be synthesized by etching out the "A" element from the MAX phase with very strong acids, such as hydrofluoric acid (HF), lithium fluoride (LiF), or a mixture of both [48,49], though more commonly, fluoride-containing etchant [31,32] or heating is used [50,51].
The use of hydrofluoric acid (HF) in the synthesis of MXenes has been considered challenging, time-consuming, and hazardous due to its toxic nature. In this work, we focused on developing a new approach for acquiring MXenes without using HF. To achieve this, 500 mg of Nb 2 SnC non-MAX phase powder was combined with 50 mL of phosphoric acid, and the mixture was magnetically swirled for 24 h at 60 • C. Following the 24-h period, the solution was washed using the same procedure as before and then dried for an additional 24 h at 70 • C in an oven. Overall, this method provides a safer and more feasible way to synthesize MXenes and can be a promising alternative to the traditional pathway involving HF as shown in schematic diagram Figure 1.

Structural and Morphological Characterizations
The arrangement of crystalline structures and phases present in the synthesized materials was identified using XRD with the Phillips Pan-Analytical X'-pert XRD system. The structural morphology of the synthesized sample was determined using SEM

Structural and Morphological Characterizations
The arrangement of crystalline structures and phases present in the synthesized materials was identified using XRD with the Phillips Pan-Analytical X'-pert XRD system. The structural morphology of the synthesized sample was determined using SEM (scanning electron microscopy) with the Hitachi S-4800 at an applied potential of 2 kV. The elemental and atomic composition of the sample was calculated using EDS (energy-dispersive spectroscopy) with the Nova Nano 200 FEI Mark. XPS (X-ray photoelectron spectroscopy) was performed with the XPS Esca-lab 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) instrument, which was used employing an 800 µm monochromatic Al-Kα-X-ray to analyze the sample's surface chemistry as well as the electronic and chemical state of the element present in the prepared sample. The layered morphology and interlayer spacing were visible via HR-TEM using a JEM-2200FS microscope.

Preparation of Electrodes for Electrochemical Characterizations
A working electrode for three-electrode assembly was prepared by the drop-cast method. Homogeneous slurry was made by mixing 5 mg of etched Nb 2 CT x MXenes with 25 µL of Nafion and 25 µL of ethyl alcohol. The solution was ultrasonicated for 3 h to make it homogeneous. After the sonication, the homogeneous solution was dropped on the nickel foam, which was washed with 2 M HCL prior to deposition several times until a uniform layer of material was obtained as an electrode. After the deposition, the nickel foam was dried at 70 • C overnight in the oven.
The electrochemical characterizations were performed in a three-electrode assembly, and KOH was used as the electrolyte. The Ag/AgCl electrode and the platinum (Pt)-wire electrode were used as the reference and auxiliary electrode, respectively. Nickel foam surface modified with Nb 2 CT x nanomaterial was used as the working electrode. The VMP3 multi-channel potentiostat electrochemical workstation was used for all electrochemical characterizations. The integral area of CV was used to determine the value of specific capacitance (F g −1 ): where I is the current discharge, δ is the scan rate (mV s −1 ), V is the applied potential window, and m is the loading mass of the working electrode.
On the other hand, specific capacitance from the galvanostatic charge-discharge (GCD) curve was also calculated by finding out the integral area under the discharging curve using the following equation [52]: where j s is the current density, Vdt is the area under the discharge curve, V f is the final potential, and V i is the initial potential during the GCD measurement.

Structural and Morphological Analysis
EDS analysis was used to identify the elemental composition of the Nb 2 SnC non-MAX phase and Nb 2 CT x MXenes, as shown in Figure 2a. The reduction in the elemental composition of Sn (from 18.32% to 0.02%) and elevation in the elemental composition of C (from 7.72% to 54.37%) are evidence that Nb 2 CT x MXenes were successfully formed. Additionally, compared to Nb 2 SnC, the elemental composition of Nb and C in the Nb 2 C MXenes was elevated. The oxygen present in the EDS spectra of the Nb 2 CT x MXenes was associated with the intercalated water molecules and the surface terminations of OH ions. There were no impurities detected in the prepared sample. The SEM images of the Nb2C MXene and the NON-MAX phase were analyzed to investigate surface morphology. The pure Nb2SnC bulk structure can be seen in Error! Reference source not found.a,b. The morphology of the Nb2SnC non-MAX phase was altered to a sheet-like structure after being etched with phosphoric acid (H3PO4), as illustrated in Error! Reference source not found.c,d. The morphology of the Nb2CTx MXene is a structure that resembles two-dimensional sheets; the sizes of the layers' structures vary, but they are consistently arranged. The space between the internal layers is expanded, which is more suitable for ion circulation and more convenient for the junction between active ions and the active sites of the material [57]. In another study in the literature [9], it was reported that nanoparticles with comparable elevated active surface areas could exhibit prominent electrochemical performance, and we could observe good surface areas in the Nb2CTx materials, so these layered-structured nanomaterials are suitable for supercapacitor applications. The XRDs of the Nb 2 SnC non-MAX phase and Nb 2 CT x are displayed below in Figure 2b. As can be seen, the XRD analysis of Nb 2 SnC is consistent with the ICSD file (98-011-3800 hexagonal 63/mmc), with the planes (002), (013), and (016) found at the corresponding peaks 2θ = 38.76 • , 45.01 • , and 62.69 • , respectively, as in the literature [53][54][55][56]. Additionally, the other peaks of Sn, Nb, NbC, and Nb 2 C are consistent with the corresponding ICSD files, 01-086-2264, 01-077-0566, 00-038-1364, and 98-011-6716, respectively. After the selective etching with H 3 PO 4 , the obtained Nb 2 CT x MXene showed a similar pattern, with vanishing of the peaks at 2θ = 30.5 • , 32.0 • , 43.8 • , 55.3 • , 64.7 • , and 72.25 • , which belong to Sn. As can be seen in the XRD peaks, the Nb 2 SnC non-MAX phase has peaks with low intensity as compared with the MXene etched with Nb 2 CT x . The lattice parameters were calculated for the Nb 2 SnC NON-MAX phase and Nb 2 CT x . For hexagonal Nb 2 SnC, the lattice parameter was calculated as a = b = 2.90 Å and c = 12.9 Å, while for cubic Nb 2 CT x , the lattice parameter was calculated as a = 3.99 Å.
The SEM images of the Nb 2 C MXene and the NON-MAX phase were analyzed to investigate surface morphology. The pure Nb 2 SnC bulk structure can be seen in Figure 3a,b. The morphology of the Nb 2 SnC non-MAX phase was altered to a sheet-like structure after being etched with phosphoric acid (H 3 PO 4 ), as illustrated in Figure 3c,d. The morphology of the Nb 2 CT x MXene is a structure that resembles two-dimensional sheets; the sizes of the layers' structures vary, but they are consistently arranged. The space between the internal layers is expanded, which is more suitable for ion circulation and more convenient for the junction between active ions and the active sites of the material [57]. In another study in the literature [9], it was reported that nanoparticles with comparable elevated active surface areas could exhibit prominent electrochemical performance, and we could observe good surface areas in the Nb 2 CT x materials, so these layered-structured nanomaterials are suitable for supercapacitor applications. XPS (X-ray photoelectron spectroscopy) was used to investigate the surface chemistry of the prepared sample along with the chemical state of the present elements with binding energy levels. Error! Reference source not found.a show the XPS survey spectra of the Nb2SnC non-MAX phase and the Nb2CTx MXene. Error! Reference source not found.b shows the high-resolution spectrum of the Nb2CTx MXene in the Nb 3d region, which could be best fitted with the corresponding Nb2C MXene (Nb 3d 204.8eV and Nb 3d5/2 205.71 eV) and oxidized Nb (Nb 3d5/2 209.72 eV) [58,59]. In Error! Reference source not found.c, the peaks obtained at 496.3 eV and 487.65 eV are attributed to the binding energy of Sn4 + , while those at 493.5 eV and 487.65 eV belong to that of metallic Sn [55,60]. In Error! Reference source not found.d, the peaks of C 1s at 284.89 eV and 288.72 eV are ascribed to the binding energy of C-C and C=O bonds. XPS (X-ray photoelectron spectroscopy) was used to investigate the surface chemistry of the prepared sample along with the chemical state of the present elements with binding energy levels. Figure 4a show the XPS survey spectra of the Nb 2 SnC non-MAX phase and the Nb 2 CT x MXene. Figure 4b shows the high-resolution spectrum of the Nb 2 CT x MXene in the Nb 3d region, which could be best fitted with the corresponding Nb 2 C MXene (Nb 3d 204.8eV and Nb 3d 5/2 205.71 eV) and oxidized Nb (Nb 3d 5/2 209.72 eV) [58,59]. In Figure 4c, the peaks obtained at 496.3 eV and 487.65 eV are attributed to the binding energy of Sn4 + , while those at 493.5 eV and 487.65 eV belong to that of metallic Sn [55,60]. In Figure 4d, the peaks of C 1s at 284.89 eV and 288.72 eV are ascribed to the binding energy of C-C and C=O bonds.
To study the morphologies and structures of the prepared samples at atomic level, TEM analysis was performed. Figure 5a,b displays the TEM images of the Nb 2 SnC non-MAX phase at two different resolutions. The well layer structure of Nb 2 SnC NON-MAX can be seen in Figure 5a, which can also be confirmed from the SEM image of the Nb 2 SnC NON-MAX phase. For the same non-MAX phase, d-spacing calculated as shown in Figure 5b was found to be 6.4 Å, which corresponds to the (002) plane as compared to the XRD of the Nb 2 SnC non-MAX phase. Figure 5c,d display the TEM images of the Nb 2 CT x MXene at two different resolutions. The two-dimensional layer of the Nb 2 CT x MXene can be seen in Figure 5c at 50 nm resolution, which can also be confirmed from the SEM image of the same sample. The same sample d-spacing calculated as shown in Figure 5d was found to be 2.6 Å, which corresponds to the (010) plane as compared to the XRD of Nb 2 CT x .  To study the morphologies and structures of the prepared samples at atomic level, TEM analysis was performed. Error! Reference source not found.a,b displays the TEM images of the Nb2SnC non-MAX phase at two different resolutions. The well layer structure of Nb2SnC NON-MAX can be seen in Error! Reference source not found.a, which can also be confirmed from the SEM image of the Nb2SnC NON-MAX phase. For the same non-MAX phase, d-spacing calculated as shown in Error! Reference source not found.b was found to be 6.4 Å, which corresponds to the (002) plane as compared to the XRD of the Nb2SnC non-MAX phase. Error! Reference source not found.c, d display the TEM images of the Nb2CTx MXene at two different resolutions. The two-dimensional layer of the Nb2CTx MXene can be seen in Error! Reference source not found.c at 50 nm resolution, which can also be confirmed from the SEM image of the same sample. The same sample d-spacing calculated as shown in Error! Reference source not found.d was found to be 2.6 Å, which corresponds to the (010) plane as compared to the XRD of Nb2CTx.

Electrochemcial Analysis
Three-electrode assemblies were used for electrochemical determination for the Nb 2 CT x MXene. For the investigation of electrochemical characteristics, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) analyses were performed. In a three-electrode assembly, nickel foam was used as a working electrode, modified by drop-casting of the sample on the nickel foam.
Cyclic voltammetry (CV) is a significant approach used to analyze the capacitive behavior and electrochemical performance of modified electrodes for supercapacitors. CV was run for the Nb 2 CT x MXene, and the corresponding curves are shown in Figure 6a,b. All the CV curves were seen to have quasi-rectangular shapes, which suggest pseudocapacitive behaviors [61]. In addition, the Nb 2 CT x nanocomposite exhibits fragile and wide characteristics peaks, which is the outcome of oxidation-reduction reactions taking place at the surface of the electrode which demonstrate the pseudo-capacitive behavior of Nb 2 CT x . To further explicate the electrochemical performance of the Nb 2 CT x nanocomposite, CV was performed at various scan rates, starting from 10 mVs −1 up to 1000 mVs −1 in the applied potential range from −1 V to −0.2 V, as shown in Figure 6a. Additionally, the CV curve exhibited a similar rectangular pattern up to a high scanning rate of 1000 mVs −1 , which corresponds to adequate capacitance and rapid ion response. The specific capacitance at each scan rate was calculated from Equation (1), and these results are plotted in Figure 6b.

Electrochemcial Analysis
Three-electrode assemblies were used for electrochemical determination for the Nb2CTx MXene. For the investigation of electrochemical characteristics, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD) analyses were performed. In a three-electrode assembly, nickel foam was used as a working electrode, modified by drop-casting of the sample on the nickel foam.
Cyclic voltammetry (CV) is a significant approach used to analyze the capacitive behavior and electrochemical performance of modified electrodes for supercapacitors. CV was run for the Nb2CTx MXene, and the corresponding curves are shown in Error! Reference source not found.a, b. All the CV curves were seen to have quasi-rectangular shapes, which suggest pseudo-capacitive behaviors [61]. In addition, the Nb2CTx nanocomposite exhibits fragile and wide characteristics peaks, which is the outcome of oxidation-reduction reactions taking place at the surface of the electrode which demonstrate the pseudo-capacitive behavior of Nb2CTx. To further explicate the electrochemical performance of the Nb2CTx nanocomposite, CV was performed at various scan rates, starting from 10 mVs −1 up to 1000 mVs −1 in the applied potential range from −1 V to −0.2 V, as shown in Error! Reference source not found.a. Additionally, the CV curve exhibited a similar rectangular pattern up to a high scanning rate of 1000 mVs −1 , which corresponds to adequate capacitance and rapid ion response. The specific capacitance at each scan rate was calculated from Equation (1), and these results are plotted in Error! Reference source not found.b. The capacitance at 10 mVs −1 was found to be 260.38 Fg −1 and to exhibit a diminishing trend with stepwise increments in the scan rate, because, while increasing the scan rate, the diffusion of electrolyte ions into the internal electrode structure becomes challenging and there is no effective interaction between the electrode material and electrolyte, which leads to a decrease in specific capacitance. As the scan rate changed from 10 mVs −1 to 1000 mVs −1 , the Nb2CTx electrode retained the initial capacitance of 45.53% from its maximum value. The good rate capability may be elucidated by the high conductivity of the ions present in the electrolyte, which makes it appropriate for practical applications. This magnificent charge storage kinetic exhibits good electrochemical specifications, such as compact transfer resistance and smaller diffusion length [57].
The galvanostatic charge-discharge (GCD) technique is one of the electrochemical characterizations requisites for understanding the charging-discharging capability of a cell. For the Nb2CTx MXene, GCD was performed at current densities ranging from 1.0 Ag −1 to 4.4 Ag −1 in the applied potential range between −0.2 V and −1.2 V to analyze the capacitance. The GCD curves at various current densities for the Nb2CTx electrodes showed a symmetrical triangular pattern during the process of charging and discharging, which demonstrated EDLC behavior. The Nb2CTx nanocomposite exhibited elongated charging and discharging durations, which correspond to the typical pseudo-capacitive behaviors of metal carbides and nitrides [61,62]. The specific capacitance value calculated from the GCD curve was found to be 502.97 Fg −1 for Nb2CTx at the current density of 1.0 Ag −1 , and it exhibited a decreasing trend up to 165 Fg −1 at the current density of 4.4 Ag −1 , as shown in Error! Reference source not found.d,e. Additionally, as the current density varied from 1.0 Ag −1 to 4.4 Ag −1 , the Nb2CTx nanocomposite electrode material retained 32.64% of its initial specific capacitance.  The capacitance at 10 mVs −1 was found to be 260.38 Fg −1 and to exhibit a diminishing trend with stepwise increments in the scan rate, because, while increasing the scan rate, the diffusion of electrolyte ions into the internal electrode structure becomes challenging and there is no effective interaction between the electrode material and electrolyte, which leads to a decrease in specific capacitance. As the scan rate changed from 10 mVs −1 to 1000 mVs −1 , the Nb 2 CT x electrode retained the initial capacitance of 45.53% from its maximum value. The good rate capability may be elucidated by the high conductivity of the ions present in the electrolyte, which makes it appropriate for practical applications. This magnificent charge storage kinetic exhibits good electrochemical specifications, such as compact transfer resistance and smaller diffusion length [57].
The galvanostatic charge-discharge (GCD) technique is one of the electrochemical characterizations requisites for understanding the charging-discharging capability of a cell. For the Nb 2 CT x MXene, GCD was performed at current densities ranging from 1.0 Ag −1 to 4.4 Ag −1 in the applied potential range between −0.2 V and −1.2 V to analyze the capacitance. The GCD curves at various current densities for the Nb 2 CT x electrodes showed a symmetrical triangular pattern during the process of charging and discharging, which demonstrated EDLC behavior. The Nb 2 CT x nanocomposite exhibited elongated charging and discharging durations, which correspond to the typical pseudo-capacitive behaviors of metal carbides and nitrides [61,62]. The specific capacitance value calculated from the GCD curve was found to be 502.97 Fg −1 for Nb 2 CT x at the current density of 1.0 Ag −1 , and it exhibited a decreasing trend up to 165 Fg −1 at the current density of 4.4 Ag −1 , as shown in Figure 6d,e. Additionally, as the current density varied from 1.0 Ag −1 to 4.4 Ag −1 , the Nb 2 CT x nanocomposite electrode material retained 32.64% of its initial specific capacitance.
To further investigate the intrinsic resistance of the electrode and electrolyte, electrochemical impedance spectroscopy (EIS) was carried out at a frequency range of 100 MHz-100 KHz. Small electrode resistance was corroborated by EIS measurements, as shown in Figure 7, and the electrochemical performance of Nb 2 CT x is attributed to favorable electrochemical reaction kinetics. The equivalent circuit was plotted along with the graph, and values of resistance and capacitance were calculated as mentioned in the graph. The equivalent series resistance was found to be 1.37 Ω. In the EIS curve, the linear behavior in the medium-frequency range can be attributed to the traditional capacitive behavior leading to EDLC behavior [63]. The superior electronic conductivity and charge-transfer kinetics of Nb 2 CT x result in lesser charge transfer resistance, which helps in speeding up electrochemical reactions [64]. To further investigate the intrinsic resistance of the electrode and electrolyte, electrochemical impedance spectroscopy (EIS) was carried out at a frequency range of 100 MHz-100 KHz. Small electrode resistance was corroborated by EIS measurements, as shown in Error! Reference source not found., and the electrochemical performance of Nb2CTx is attributed to favorable electrochemical reaction kinetics. The equivalent circuit was plotted along with the graph, and values of resistance and capacitance were calculated as mentioned in the graph. The equivalent series resistance was found to be 1.37 Ω. In the EIS curve, the linear behavior in the medium-frequency range can be attributed to the traditional capacitive behavior leading to EDLC behavior. The superior electronic conductivity and charge-transfer kinetics of Nb2CTx result in lesser charge transfer resistance, which helps in speeding up electrochemical reactions [63].

Analysis of the Supercapacitive Behavior of the 2D Nb2CTx Nanomaterial
After analyzing all the electrochemical characterizations, the super capacitive behavior of the 2D Nb2CTx nanomaterial was ascribed to the following aspects: (a) The sheet and layered morphology of the Nb2CTx MXene, as shown in the SEM images,

Analysis of the Supercapacitive Behavior of the 2D Nb 2 CT x Nanomaterial
After analyzing all the electrochemical characterizations, the super capacitive behavior of the 2D Nb 2 CT x nanomaterial was ascribed to the following aspects: (a) The sheet and layered morphology of the Nb 2 CT x MXene, as shown in the SEM images, illustrates a prominent surface area and adequate conductivity, which reinforce the electrolytic diffusion and absorption of ions onto the electrode's surface. (b) The presence of functional group -O in Nb 2 CT x , which was confirmed by EDS analysis after etching with phosphoric acid (H 3 PO 4 ), helps in tuning the electrocatalytic properties, such as easy ion transfer, decreasing the internal resistance, and upgrading the electrical conductivity, which improves the electrochemical mechanism. The interlayer spacing in Nb 2 CT x eases the way for fast hydrated ion diffusion, which affords kinetics similar to the EDLC behavior and accessible active sites to an extent which ensures high capacity and rate performance. A comparison table (Table 1) is provided below, after the literature review of some MXenes prepared under different reaction conditions and via different etching methods which have been reported for supercapacitors, which shows that the prepared Nb 2 CT x MXene is a suitable candidate for supercapacitors.

Conclusions
• A novel synthesis method was developed for preparing Nb 2 SnC non-MAX phase powder at a lower temperature of 1000 • C, and two-dimensional nanostructures of Nb 2 CT x MXenes were synthesized by selective etching of Sn-layered Nb 2 SnC using mild phosphoric acid (H 3 PO 4 ).

•
The hexagonal crystal structure of Nb 2 SnC and the cubic structure of Nb 2 CT x were confirmed by analyzing the XRD patterns of the samples.

•
During the formation of Nb 2 CT x MXenes, the selective etching of Sn layers from Nb 2 SnC was evident in compositional analysis using EDX and XPS. • Two-dimensional layered nanostructures of Nb 2 CT x MXenes were observed in SEM images.

•
The specific capacitance of the synthesized materials was evaluated using CV and GCD techniques. The CV plot of Nb 2 CT x showed a specific capacitance of 260.38 Fg −1 , while the GCD curve exhibited a specific capacitance of 502.97 Fg −1 for Nb 2 CT x .

•
This study provides an eco-friendly and less hazardous method for synthesizing Nb 2 SnC and Nb 2 CT x . Nb 2 CT x has superior electrochemical performance, making it a potential candidate for high-performance supercapacitor applications. The presented synthesis and characterization techniques could be useful for developing other MXenes and two-dimensional materials for energy storage applications.