Supported data and manuscript "Efficient electrochemical performance of asymmetric supercapacitor based on nitrogen-doped Nb2CTx MXene in an alkaline electrolyte"
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Supported data and manuscript "Efficient electrochemical performance of asymmetric supercapacitor based on nitrogen-doped Nb2CTx MXene in an alkaline electrolyte" in Nano Research Energy 2025, 4: e9120164.
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Research Article Efficient electrochemical performance of asymmetric supercapacitor based on nitrogen-doped Nb2CT x MXene in an alkaline electrolyte AroomaSyed1,IrfanAli1,SanaMaqbool1,MuhammadYousaf2( ),IftikharHussain3,KailiZhang3( ), SaleemA.Khan4,andSyedRizwan1() 1Physics Characterization and Simulations Lab (PCSL), Department of Physics & Astronomy, School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan 2ZJU-Hangzhou Global Scientific and Technological Innovation Center, Zhejiang University, Hangzhou 311200, China 3Department of Mechanical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China 4New Technologies Research Center, University of West Bohemia, 30100, Pilsen, Czech Republic Received: 7 January 2025 / Revised: 9 March 2025 / Accepted: 13 March 2025 ABSTRACT The versatile, and tunable surface chemistry of two-dimensional (2D) MXenes coupled with their distinct properties includinghydrophilicnature,favorableiontransportandmetallicconductivitymakethemanidealcandidateforenergy storagedevices.Modifyingsurfaceterminationsbydopingheteroatomisanefficientapproachtoimprovelayerspacing andelectrochemicalactivesitesoftheMXenes.However,nitrogendopingin2Dmaterialshasbeenaneffectivewayto enhance their electrochemical characteristics. In this study, N-Nb2CTx MXene was synthesized by utilizing the hydrothermal method in which nitrogen doping in MXene was confirmed through several characterization techniques. TuningofMXenesurfacebyacost-effectivestrategyhasshownimprovedperformanceforenergystorage.Afterdoping nitrogeninNb2CTxMXene,ithasshownenhancedpseudocapacitanceperformancein1Mpotassiumhydroxide(KOH), elevatingtheelectrochemicalproperties.N-Nb2CTxMXenehasdisplayedabetterspecificcapacitanceofupto640F·g–1 while pristine Nb2CTx MXene has shown 276 F·g–1 from the cyclic voltammogram (CV) at a scan rate of 5 mV·s–1. In addition,anasymmetricdeviceofactivatedcarbon/N-Nb2CTxwasassembledforreal-worldapplications,ithasexhibited refined results. The asymmetric device has shown remarkable cyclic stability of 90% capacity retention at a current density of 5 A·g–1 for 5000 cycles. Additionally, the detailed density functional theory (DFT) calculations support the stabilityofNitrogenreplacingtheFluorinefunctionalgroup,complementingtheexperiment. KEYWORDS two-dimensional,MXene,nitrogendoping,supercapacitor,asymmetricdevice,DFTformationenergies 1Introduction The energy crisis has led to a decline in natural resources, so researchers are trying to produce certain ways to compensate or find other ways to eliminate these crises. Scientists are working on eco-friendly electrochemical energy storage devices to improve the lives of humans [1]. In energy storage systems like batteries, supercapacitors are well-known devices for storing energy due to their power and energy densities [2]. Electrochemical capacitors commonly referred to as supercapacitors exhibit significant promise owing to their rapid charge-discharge capabilities, excellent cycling stability, high power density, enhanced reliability, and safety features [3, 4]. Supercapacitors have obstacles to commercialization for storing high energy density, but the main benefit is they can store energy with improved power capacity [3, 5]. Similarly, electrode preparation is the crucial step to improve the energy density of electrode material that has a high surface area, excellent ion accessibility, good electrical conductivity, and higher electrochemical and thermal stability [1, 4]. Two-dimensional nanomaterials are well-known materials due to their distinct properties and their diverse potential for applications such as batteries, photocatalysis, supercapacitors and water-splitting [6–8]. In 2010, Yury Gogtosi and his colleagues discovered the potential material that’s known as MXene. Due to their unique characteristics such as rich chemistries, tunable properties, significant surface areas, and hydrophilic nature. MXenes have attracted a lot of attention for electrochemical ISSN 2791-0091 (print); 2790-8119 (online) https://doi.org/10.26599/NRE.2025.9120164 ©TheAuthor(s)2025.PublishedbyTsinghuaUniversityPress.The articlespublishedinthisopenaccessjournalaredistributedunderthetermsofthe CreativeCommonsAttribution4.0InternationalLicense(http://creativecommons.org/licenses/by/4.0/),whichpermitsuse,distributionandreproductionin anymedium,providedtheoriginalworkisproperlycited. Address correspondence to MuhammadYousaf, [email protected]; KailiZhang, [email protected]; SyedRizwan, [email protected]
investigations [9]. MXenes are derived from parent MAX phases in which M stands for the early transition metal, A represents the element of the 12‒16 group of the periodic table, where X belongs to Carbon/nitrogen or both. MAX phases are compact layered structures in which M-X bonding is stronger than M-A bonding, and by eliminating the A element, MXene is formed [10, 11]. The word ‘ene’ is a conventional name used for 2D nanomaterials like graphene, Silicene, phosphorene, etc. MXenes have a general formula Mn+1XnTx in which M represents the early transition metal (e.g Ti, Nb, Mo, V, etc.), X belongs to Carbon/nitrogen or both and Tx is the surface functional group (O, OH, F, Cl, etc.) attached to MXene depending on which method is used to eliminate A element from MAX phases where n is an integer varying from 1 to 4 [10–12]. The tunability of surface terminations enhances the features of MXenes, which is a desired property for a material, opening a wide range of applications [13–16]. MXene’s intrinsic electrochemical capabilities provide vital details on its charge storage mechanism, ion diffusion kinetics, and interactions with electrolytes [17–20]. MXene has favorable ion transport due to high electrical conductivity, which possesses rapid electron mobility with reduced charge resistance, improving overall electrochemical efficiency, additionally due to the layered structure of (2D) MXene, it contributes to successful ion diffusion which is effective for energy storage applications [21]. Several studies have been conducted on the first discovered Ti3C2Tx MXene whereas for Nb2CTx MXene there are not too many studies related to electrochemical studies for supercapacitor application [22]. Due to the lower Fermi level of Nb2CTx MXene, it is considered efficient for energy storage devices. The higher electroconductivity property of Nb2CTx MXene makes it promising for various applications such as batteries, photocatalysis, water splitting and photovoltaic, etc. [6, 23, 24]. To enhance the electrochemical features of MXene, different interlayer spacers have been introduced in its structures [16, 22, 25]. Moreover, doping of heteroatoms (N, P, S, etc.) also modify the electrochemical performance of different 2D nanomaterials by tuning their properties [26–28]. The incorporation of heteroatoms is an efficient approach to enhance the surface chemistry of MXenes. Nitrogen doping has been extensively investigated for the alteration of materials, with functional substitution, surface absorption and lattice substitution being proven to be three favourable forms of nitrogen doping. These approaches have shown improved specific capacitance of electrodes [29]. In previous reports, nitrogen doping in reduced graphene oxide (NrGO) has shown improved thermal stability and better capacitance and conductivity [30] whereas the nitrogen insertion in TiO2 exhibits a narrow bandgap showing improved photocatalysis. Yangyang Wen et al. have suggested that nitrogen doping in MXene and replacing carbon has significantly enhanced the interlayer spacing, similarly, by employing the annealing method, in Ti3C2Tx MXene has shown a 460% increment in gravimetric capacitance as compared to pristine Ti3C2Tx MXene [26]. In this report, we improved the gravimetric capacitance, energy and power density of Nb2CTx MXene by doping with hetero atoms (N) which have significantly decreased the -F surface terminations attached by selective etching using HF. Literature suggested that -F terminations attached to MXenes usually hinder the energy storage characteristics [7]. The most common way to prepare enhanced N-doped MXene is to use ammonia gas as the nitrogen source. However, due to its pungent smell, the risk of an explosion at high temperatures and difficulties in storage are significant challenges. Following an effective, safe, economical, and fast strategy to dope nitrogen by employing the hydrothermal method in Nb2CTx MXene enhances the active sites, which further leads to improved ion transport of electrolytes. Urea is typically used as a nitrogen source to prepare the nitrogen-doped MXene. Based on an increase in interlayer spacing of N-Nb2CTx MXene has shown tremendous electrochemical performance and stability. In this study, a structural, thermal, and morphological analysis is done by utilizing various characterizations. The findings related to the electrochemical properties of prepared electrode material were also studied in 1 M potassium hydroxide (KOH) basic electrolyte. Furthermore, DFT calculations were carried out to get insight into the structure stability for formation energies. 2Experimental Figure 1 demonstrates the synthesis route followed during the synthesis of pristine MXene and N-Nb2CTx MXene. 2.1SynthesisofNb2CT x MXene Nb2CTx MXene was synthesized by using a wet chemical etching approach. Initially, a Teflon-lined beaker was placed in an oil bath and 0.5 g of precursor Nb2AlC MAX was gradually added into 10 mL of hydrofluoric acid (HF). It was continuously stirred at 400 rpm to avoid aggregation for 72 h to get multilayered (ML) Nb2CTx MXene. The solution was transferred to a 50 mL Figure 1 Schematic for the fabrication of Nb2CTx MXene and N-Nb2CTx MXene. 2Nano Research Energy2025,4:e9120164
centrifuge tube and washed with deionized (DI) water and the supernatant was discarded after each cycle to neutralize the pH of the solution. The sediment obtained by centrifugation was further filtered by using a vacuum assembly. Finally, the obtained product ML-Nb2CTx MXene was dried for 24 h in a vacuum oven at 30 °C. 2.2SynthesisofN-Nb2CT x MXene N-Nb2CTx MXene was prepared using a hydrothermal route and the solution was prepared by adding Urea and ML-MXene in DI water. Then the solution was transferred into a Teflon beaker. Furthermore, an autoclave was utilized for hydrothermal reaction and kept at 120 °C for 8 h. The autoclave was cooled down at room temperature and then the solution was transferred to a centrifuge tube and washed multiple times with DI until pH became 7. The resulting solution was filtered and dried in a vacuum oven overnight at 60 °C. Figure 1 displays the sequential steps followed during the synthesis of MXene and N-Nb2CTx MXene. 3Material characterization X-ray diffractometry (XRD; Burker D8) was employed for the investigation of structural properties. Fourier transform infrared spectroscopy (FTIR; Spectrum 100) was utilized to examine the chemical bonding. To study the thermal characteristics, a thermal gravimetric analyzer (TGA; SDT650) was employed. The surface morphology was analyzed by scanning electron microscopy (SEM; JEOL JSM-6490A). In addition, the surface electronic states of the samples were investigated by employing X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha). 4Electrochemical measurement To examine the electrochemical characteristics of prepared materials a three-electrode setup was employed. Ag/AgCl was utilized as a reference electrode in a three-electrode setup to maintain the voltage while a platinum wire was taken as the counter electrode, and the active material electrode was carried out as a working electrode. The slurry of active materials was prepared by using polyvinylidene fluoride (PVDF), carbon black and active material with a ratio of 10:10:80 and two drops of N-methyl-2pyrrolidone (NMP) as a solvent were added to it. The slurry was sonicated till a homogeneous solution was formed and drop-cast in pre-washed Ni-foam. After the deposit of slurry in Ni-foam it was dried in a vacuum oven at 60 °C overnight. The prepared electrode was pressed at 1000 psi for 10 seconds. To access the electrochemical properties of prepared electrodes it was carried out in an alkaline solution of 1 M KOH. Moreover, the GAMRY 1011B workstation was utilized to record cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). All CV graphs were performed for different scan rates (5, 10, 20, 50, 100, 200 mV·s–1) whereas GCD curves were taken at various current densities (1, 2, 3, 4, 5 A·g–1), and EIS were performed at a frequency range of 0.1‒10 kHz with DC voltage of 0 V. 5Computational details For computational methodology, we have used the pseudopotential method, so-called projector-augmented wave method (PAW) [31] based on DFT calculations implemented in the Vienna Ab initio Simulation Package (VASP) [32, 33]. The pseudopotential method considers ion cores frozen, and valence electrons are specifically addressed. The Perdew–Burke–Ernzerhof formulism of Generalized gradient approximation (PBE-GGA) was utilized for exchange–correlation functional [34]. For internal geometry relaxation we used the conjugate gradient method with forces estimated using the Hellman ‐ Feynman theorem. The 520 eV kinetic energy cut-off was chosen for structure relaxation using tag ISIF=2 and selfconsistencies of the ground state energies. For k-point sampling, an automatic k-mesh was used with 20 k-points mapped on the irreducible Brillouin zone (IBZ), distributed according to a (6 × 6 × 1) k-grid. The force and energy convergence criteria were set at 10−3 eV·Å−1 and 10−6 eV, respectively. The results of the computational part are mentioned in section 8. 6Results and discussion Detailed crystal structure studies of prepared materials were carried out by employing X-ray diffraction (XRD). Structural analysis was conducted with an angle of 2θ from 5° to 80° for MAX phase Nb2AlC, Nb2CTx MXene and N-Nb2CTx MXene as illustrated in Fig. 2(a). Commercially purchased Nb2AlC MAX demonstrated the peaks at an angle of 2θ 12.7°, 25.7°, 33.3°, 33.9°, 38.7°, 42.7°,52.1°, 57.8°, 59.5°, 70.5° and 73.5° which were matched with the JCPDS card No: 00-030-0033 [23] corresponding to the planes (002), (004), (100), (101), (103), (104), (106), (107), (110), (109) and (203) respectively, which aligned with published data 10 20 30 40 50 60 70 80 Nb 2 C MXene N-doped Nb 2 C MXene (004) (100) (101) • Nb 2 AlC MAX ♣ ♣ ♣ (104) (106) (110) (103) ♦ • • 2θ ( ° ) Intensity (a.u.) (002) (002) (002) (a) 4000 3500 3000 2500 2000 1500 1000 500 Transmittance (%) Wave number (cm −1 ) N-doped Nb2C MXene Nb2C MXene O-H C-H O-H Nb-C C-O C-N Functional group region Fingerprint region (b) Figure 2 (a) XRD spectrum of Nb2AlC MAX phase, pristine Nb2CTx MXene, and N-Nb2CTx MXene (b) FTIR spectra of Nb2CTx and N-Nb2CTx MXene. Nano Research Energy2025,4:e9120164 3 https://www.sciopen.com|https://mc03.manuscriptcentral.com/nre|Nano Research Energy
[23]. The sharp intense peak at 38.7° (103) shows the presence of aluminum in MAX Nb2AlC which was reduced after the treatment with HF. Additionally, the intermetallic peaks in MAX also decrease after the etching process. The peak at 12.7° was shifted below to 9.81° indicating the expansion of the c-lattice parameter after exfoliation, which is the characteristic peak of Nb2CTx MXene (002) plane showing the successful preparation of MXene where the presence of a small amount of aluminum atom was seen in Nb2CTx MXene. By using Bragg’s law, the d-spacing of each material was calculated. An increase in the c-lattice parameter and d-spacing was observed from the transition from MAX to MXene. After the doping of heteroatom (nitrogen) by using a hydrothermal method in Nb2CTx MXene the peak at 9.81° shifted to a lower angle at 8.40° and became sharp exhibiting enhanced expansion of lattice parameter along the c-axis providing more active sites. Table S1 in the Electronic Supplementary Material (ESM) demonstrates the comparison of dspacing and c-lattice parameters of materials. Adding Nheteroatoms to the MXene structure results in an increased interlayer distance between MXene layers. The 9% increase in interlayer spacing was found in nitrogen-doped Nb2CTx MXene. Similarly, literature also suggested that Ti3C2Tx MXene annealed by NH3 (N-Ti3C2Tx shows similar behaviour in which the increased interlayer spacing was found [26]. Fourier transform infrared spectroscopy (FTIR) was conducted for the chemical composition of prepared materials. FTIR spectrum is divided into two regions, from the wavenumber 500 to 1500 cm–1 the region is referred to as a fingerprint in which the characteristic bonding of material will be present whereas the other region is named a functional group region ranging from 1500 to 4000 cm–1 in which the peaks primarily associated with functional groups attached to the material are observed [35, 36]. Figure 2(b) exhibits the FTIR spectrum of prepared material Nb2CTx and N-Nb2CTx MXene. The peaks around 1624 and 3426 cm–1 are attributed to O-H stretch bonding vibrations. Two small peaks at 2852 and 2919 cm–1 exhibit the C-H bending vibrations whereas the presence of a peak at 1071 cm–1 shows the bonding of C-O stretching [23]. The peak at 629 cm–1 on both spectra in the fingerprint region is associated with the characteristic bonding of Nb-C in Nb2CTx MXene whereas the peak intensity became prominent after the doping of nitrogen. Furthermore, the peak at 1377 cm–1 is attributed to the stretching of the C-N bond which also accomplished the sample preparation of N-Nb2CTx MXene [37]. Thermogravimetric analysis (TGA) was conducted for the thermal analysis of prepared material under a nitrogen atmosphere from 35 to 600 °C. Figures 3(c) and 3(d) shows the TGA curve of MAX Nb2AlC, Nb2CTx MXene and N-Nb2CTx MXene. The weight of the Nb2AlC MAX remains constant throughout the temperature range, with a small rise noted around 450 to 600 °C. This may be due to the selective oxidation of aluminum, which may have been impacted by oxygen that was still present while heating in the nitrogen atmosphere [36]. For Nb2CTx MXene the temperature range of 35 to 600 °C showed thermal stability with only a tiny, progressive weight drop of up to 6%. The presence of unstable surface functional groups like -F, -O, and -OH might be the cause of this drop or it can be due to the trapped water molecules between MXene sheets [38]. Furthermore, the N-doped MXene composite has shown thermal stability with 12% weight loss. In addition, the derivative curve of TGA was carried out by taking the differential of the weight loss. DTG (differential thermogravimetry) curve of MAX Nb2AlC, Nb2CTx MXene and N-Nb2CTx MXene as demonstrated in Fig. 3(d). The peak around 110 °C in all materials shows that it has absorbed heat indicating moisture release. The DTG results have an impact on the mass rate variations caused by heat flow and the subsequent endothermic processes. A second peak appears at around 490 °C, indicating significant mass rate changes in nanomaterial structures caused by the elimination of unstable surface-attached groups and their associated endothermic processes. Scanning electron microscope (SEM) was conducted to view the magnified image of materials including Nb2AlC MAX, N-Nb2CTx MXene and NNb2CTx MXene. Figures 4(a) and 4(b) shows SEM micrographs of Nb2AlC MAX at 10 and 5 µm displaying the compact and ceramic particle structure of the three-dimensional (3D) precursor. After exfoliation with HF, the irregular block-type structure of Nb2AlC MAX transformed into page-like morphology revealing the opening of the MXene layered structure that can be observed in Figs. 4(c) and 4(d) at 1 µm which also validates the successful synthesis of (2D) MXene. Figures 4(e) and 4(f) displays the SEM micrographs of N-Nb2CTx MXene at 5 and 1 µm indicating no change in the layered structural morphology which is also consistent with previously reported data [26, 28]. The weight% bar graph from EDS shown in Fig. 4(g) reveals the existence of a small amount of aluminum in layered Nb2CTx MXene nanosheet, exhibiting that there is untreated MAX in small amounts, which is also consistent with XRD data. The presence of fluorine and oxygen as termination indicated the treatment of MAX. After the doping of nitrogen in MXene, a 100 200 300 400 500 600 20 40 60 80 100 Weight (%) Temperature ( ° C) Temperature ( ° C) Nb 2 C MXene N-doped Nb 2 C MXene Nb 2 AlC MAX 12% wt. loss 6% wt. loss (a) 100 200 300 400 500 600 −10 −5 0 5 Nb2C MXene N-doped Nb2C MXene Nb2AlC MAX Heat flow (W·g−1) 110 °C 490 °C (b) Figure 3 (a) TGA spectra of Nb2AlC MAX phase, pristine Nb2CTx MXene, and N-Nb2CTx MXene (b) DTG plot of Nb2CTx and N-Nb2CTx MXene. 4Nano Research Energy2025,4:e9120164
significant decrease in -F termination has been observed from 3.0% to 0.1% whereas the -O termination increases from 14.9% to 18.7% which has enhanced the electrochemical properties of MXene. Usually, -F terminations hinder and -O improves the electrochemical characteristics [28]. Furthermore, the aluminum atoms have decreased as well from 2% to 1.2% due to hydrothermal synthesis, which also shows the effectiveness of our synthesis [38]. Elemental mapping was also conducted as shown in Figs. 4(h)–4(l) to confirm the presence of N hetero atoms in Nb2CTx MXene. X-ray photon spectroscopy (XPS) was carried out for detailed surface analysis and bond structure. The valence state of prepared materials was confirmed by using XPS. Figure 5(a) shows the XPS spectra of Nb2CTx MXene and NNb2CTx MXene whereas Figs. 5(b)–5(e) exhibits the deconvoluted spectra of N-Nb2CTx MXene. The presence of Nb 3d, C 1s and O 1s endorses the formation of niobium carbide MXene which is consistent with XRD and SEM. Figure 5(b) shows the HR XPS spectrum of the Nb 3d region in N-Nb2CTx MXene. The peaks around 203, 206, and 209 eV are associated with the energy level 3d5/2, 3d5/2 and 3d3/2 respectively. However, the peak at 203 eV represents the Nb-C bonding and 206 and 209 eV show the Nb-O bonding arises due to the oxygen surface termination in MXene. Figure 5(c) displays the spectrum of C 1s in which the peak at 281, 284 and 288 eV belongs to Nb-C, C-C and C-O bonding respectively. In O 1s spectra as shown in Fig. 5(d), the peaks at 529 and 531 eV represent the Nb-O and Nb-C-Tx bond respectively [6]. Moreover, the peak at 395 eV in N 1s spectra belongs to the Nb-N bond which confirms the successful doping of nitrogen in Nb2CTx MXene [39], whereas the peaks around 398 and 400 eV show the pyridine N and pyrrolic N as demonstrated in Fig. 5(e). Furthermore, XPS has verified the presence of oxygen termination on the Nb2CTx MXene surface, similarly the doping has been also confirmed which contributed towards enhanced electrochemical properties. Table S2 in the ESM exhibits the comparison of the atomic% concentration of each element presented in Nb2CTx and N-Nb2CTx MXene. After the hydrothermal process, the concentration of fluorine decreased from 17.26% to 1.39% while Hong Yu suggested that a decrease in -F termination enhanced the electrochemical properties [40]. The oxygen content increased from 33.08% to 42.68% and an aluminum reduction has been observed. The XPS results also revealed the presence of nitrogen doping in MXene which also affirms the EDS data. 7Electrochemical performance analysis Figures 6(a) and 6(b) displays the CV of Nb2CTx and N-Nb2CTx MXene at various scan sweeps (5‒200 mV·s–1) with the potential windows of 0.2 to 0.5 V and 0.1 to 0.52 V respectively. Nb2CTx MXene CV curves show typical pseudocapacitive behaviour as shown in Fig. 6(a), the curve simulation is almost the same as previously reported MXenes. The doping of nitrogen has improved the electrochemical characteristics of an electrode. The area under the curve significantly increased after the doping of nitrogen in MXene, which provided an increment in specific capacitance as both are proportional to each other indicating enhanced rate performance. The specific capacitance of N-Nb2CTx MXene is superior to Nb2CTx MXene. The increase in the interlayer spacing between the MXene layered structure provided by nitrogen doping has improved electrochemical behaviour by offering more active sites. Furthermore, the average oxidation and reduction peaks observed in CV scans for C=rI(V)dV mv∆V(1) In both cases of electrode materials, it has shown comparable capacitance at higher scan rates while at the lowest scan rate, doped MXene has shown higher capacitance due to the penetrations of ions into the layered structure of MXene. At the lowest scan rate of 2 mV·s–1 the Nb2CTx and N-Nb2CTx MXene based on CV plots show the capacitance of 276 and 640 F·g–1 respectively. N-Nb2CTx MXene has exhibited improved electronic conductivity and enhanced charge storage mechanisms due to Figure 4 SEM images of (a) and (b) Nb2AlC MAX phase at 10 and 5 µm (c) and (d) pristine Nb2CTx MXene at 1 µm. (e) and (f) N-Nb2CTx MXene at 5 and 1 µm. (g) Weight percent EDX plot of Nb2CTx and N-Nb2CTx MXene. (h)‒(l) Elemental mapping of each element in N-Nb2CTx MXene at 1 µm. Nano Research Energy2025,4:e9120164 5 https://www.sciopen.com|https://mc03.manuscriptcentral.com/nre|Nano Research Energy
rapid ion diffusion between the electrode and electrolyte. Table S3 in the ESM displays the comparison of capacitance with already published data. Moreover, GCD curves were recorded at different current densities (1‒5 A·g–1) to obtain information about the chargedischarge of prepared electrodes following the same CV plot's potential window. Figure 7(a) shows the GCD profile of Nb2CTx displaying the pseudocapacitive characteristics. N-Nb2CTx MXene has shown enhanced capacitance calculated by GCD plots which is attributed to the increase in the surface area as the lattice parameter along the c-axis increased [43]. The amazing reversibility observed between the charge and discharge processes further supports the pseudocapacitive nature of the charge storage mechanism, as illustrated by GCD data. Similarly, after the nitrogen incorporation in Nb2CTx MXene, an increase in the discharge time was observed in all current densities, as shown in Fig. 7(b), indicating an increase in capacitance. N-Nb2CTx MXene has the highest capacitance and longest discharging time at the smallest current density. A comparison of the GCD plot of Nb2CTx MXene and N-Nb2CTx MXene at the current density of 1 A·g–1 is shown in Fig. 7(c). The capacitance has been calculated by utilizing Eq. (2). C=Imtd ∆V(2)(2) In Eq. (2), I represent the current, td is the discharging time, ‘m’ belongs to active mass and ‘∆V’ stands for the difference in the potential voltage. From GCD profiles, 726 F·g–1 of capacitance has been recorded for N-Nb2CTx MXene at the current density of 1 A·g–1. Figure 7(d) demonstrates the GCD capacitance of N-Nb2CTx MXene at different current densities. The large d-spacing has provided more active sites to facilitate electrochemical reactions [44]. Figure S1 in the ESM shows the energy density and power 1000 900 800 700 600 500 400 300 200 100 Nb 2 CT x N-Nb 2 CT x Binding energy (eV) Intensity (Counts) Nb 3d C 1s Nb 3d Nb 3p O 1s F 1s F KLL O KLL N 1s (a) (c) (d) (e) (b) 220 215 210 205 200 195 Intensity (Counts) Binding energy (eV) Nb 3d N-Nb2CTx 295 290 285 280 Intensity (Counts) Binding energy (eV) C 1s N-Nb2CTx 540 535 530 525 Intensity (Counts) O 1s Binding energy (eV) N-Nb 2 CT x 394 396 398 400 402 404 Intensity (Counts) Binding energy (eV) N 1s N-Nb 2 CT x Figure 5 (a) XPS plot of pristine Nb2CTx MXene and N-Nb2CTx MXene. HR XPS deconvoluted graphs of (b) Nb 3d (c) C 1s (d) O 1s (e) N 1s. 6Nano Research Energy2025,4:e9120164
0.20 0.25 0.30 0.35 0.40 0.45 0.50 −40 −30 −20 −10 0 10 20 30 40 50 SR 5 SR 10 SR 20 SR 50 SR 100 SR 200 Current density (A·g −1 ) Potential (V vs. Ag/AgCl) Nb2C MXene (a) 0.1 0.2 0.3 0.4 0.5 0.6 −60 −40 −20 0 20 40 60 80 SR 5 SR 10 SR 20 SR 50 SR 100 SR 200 Potential (V vs. Ag/AgCl) Current density (A·g −1 ) N-doped Nb 2 C MXene (b) 0.1 0.2 0.3 0.4 0.5 0.6 -40 -20 0 20 40 60 N-doped Nb2C MXene Nb2C MXene Potential (V vs. Ag/AgCl) Current density (A·g −1 ) ( c) 12345 350 400 450 500 550 600 650 700 750 Current density (A·g −1 ) Specific capacitance (F·g −1 ) N-doped Nb 2 C MXene (d) Figure 6 Cyclic voltammograms of (a) Nb2CTx MXene, (b) N-Nb2CTx MXene. (c) Comparison plot of as-prepared materials at 20 scan rate. (d) gravimetric capacitance of as-prepared materials at each scan rate. 0 50 100 150 200 250 300 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Time (s) Potential (V vs. Ag/AgCl) Nb 2 C MXene 1 A·g −1 2 A·g −1 3 A·g −1 4 A·g −1 5 A·g −1 1 A·g −1 2 A·g −1 3 A·g −1 4 A·g −1 5 A·g −1 (a) 0 100 200 300 400 500 600 700 0.1 0.2 0.3 0.4 0.5 Time (s) Potential (V vs. Ag/AgCl) (b) N-doped Nb 2 C MXene 0 100 200 300 400 500 600 700 0.1 0.2 0.3 0.4 0.5 0.6 N-doped Nb2C MXene Nb 2 C MXene @ 1 A·g −1 Time (s) Potential (V vs. Ag/AgCl) (c) 0 50 100 150 200 0 100 200 300 400 500 600 700 gravimetric capacitance (F·g −1 ) Scan rate (mV·s −1 ) Nb 2 C MXene N-doped Nb 2 C MXene (d) Figure 7 GCD plots of (a) Nb2CTx MXene, (b) N-Nb2CTx MXene. (c) Comparison plot of as-prepared materials at 1 A·g–1. (d) Specific capacitance of as-prepared materials at each current density. Nano Research Energy2025,4:e9120164 7 https://www.sciopen.com|https://mc03.manuscriptcentral.com/nre|Nano Research Energy
density calculated by these GCD graphs at different current densities. Additionally, at a current density of 5 A·g–1, the prepared electrode was tested for long-term stability. The electrode material was charged and discharged for 5000 cycles to assess the feasibility and durability of the electrodes. Figure 8(a) shows the capacitance retention graph of Nb2CTx MXene and N-Nb2CTx MXene. Nitrogen-doped MXene has shown improved cyclic stability than Nb2CTx MXene. N-Nb2CTx MXene has exhibited 91% specific capacity retention while Nb2CTx MXene has displayed 68% retention. For further information about charge storage mechanisms, Dunn's plotting was utilized for different scan rates. Equation (3) was used to calculate the percentage contribution from CV curves. In Eq. (3), I(V) is the current response at a fixed voltage, which is measured by the sum of k1v (capacitive process) and k2v0.5 (diffusion-controlled process) whereas v stands for scan rate, k1 and k2 represent the slope and intercept value calculated from the CV profiles. Figures 8(b) and 8(c) show the diffusion and capacitive percentage contribution of Nb2CTx and N-Nb2CTx MXene respectively. From the graph, at a lower scan rate, the Nb2CTx MXene has shown a 95% diffusion-controlled process and 5% capacitive process whereas after doping in MXene with nitrogen, the capacitive process increased from 5% to 11%. The increase in capacitive contribution indicates a better supercapacitor behavior to store charge and deliver quickly through surface interactions. I(V) =k1v+k2v0.5(3) Electrochemical Impedance spectroscopy (EIS) was employed to locate the ion transport between the electrode and electrolyte. Figures 9(a) and 9(b) demonstrate the Nyquist plot of Nb2CTx MXene and N-Nb2CTx MXene, in which real impedance is along the x-axis whereas imaginary impedance is plotted along the yaxis. Resistance faced by ions in electrolytes while transfer from electrodes represents the solution resistance (Rs) whereas the opposition faced by electrons in charge transfer corresponds to charge transfer resistance (Rct). Furthermore, the impedance formed by diffusion is known as Warburg impedance (W). Constant phase element diffusion modified circuit is used for calculating the solution resistance, charge transfer resistance and Warburg impedance for prepared materials (Fig. S3 in the ESM). N-Nb2CTx MXene has shown an Rct value of 782.0e–3 Ω while Nb2CTx MXene has displayed 2.769 Ω, the smallest value of Rct for N-Nb2CTx MXene suggesting improved electron kinetics. Furthermore, N-Nb2CTx MXene has exhibited the lowest value of Rs which is 42.00e–3 Ω whereas Nb2CTx MXene has shown the Rs of 819.2e–3 Ω in KOH electrolyte. Similarly, N-Nb2CTx MXene has demonstrated the lowest Warburg impedance as compared to Nb2CTx MXene (14.36e–3 Ω for Nb2CTx MXene and 12.21e–3 Ω for N-Nb2CTx MXene) exhibiting rapid kinetics improving the energy storage. Table S4 in the ESM shows the parameters carried out by the fitted circuit in EIS plots (Fig. S3 in the ESM). In addition, the Warburg factor (σ) is also calculated for all prepared materials by plotting the low-frequency region along the 0 1000 2000 3000 4000 5000 0 20 40 60 80 100 N-doped Nb 2 C MXene Nb 2 C MXene Capacity retention (%) No. of Cycles 68% 91% (a) 5 5% 7% 9% 14% 19% 25% 75% 81% 86% 91% 93% 95% 10 20 50 100 200 0 20 40 60 80 100 Nb2CTx MXene Contribution (%) Scan rate (mV·s −1 ) Scan rate (mV·s −1 ) Diffusion control Capacitive control ( b) 5 10 20 50 100 200 0 20 40 60 80 100 N-doped Nb 2 CT x Contribution (%) Diffusion control Capacitive control (c) 11% 15% 20% 28% 35% 44% 56% 65% 72% 80% 85% 89% Figure 8 (a) Cyclic stability of Nb2CTx MXene and N-Nb2CTx MXene for 5000 cycles. Dunn’s plot of (b) Nb2CTx MXene and (c) N-Nb2CTx MXene. 8Nano Research Energy2025,4:e9120164
x-axis and real impedance along the y-axis (ω–0.5 vs. Zreal) to define the diffusion coefficient (D). Equation (4) is carried out to determine the value of D which is directly proportional to the universal gas constant (R) and absolute temperature (T) and inversely proportional to surface area (A), no of electrons (n), solution concentration (C), Faraday constant (F) and Warburg factor (σ). D=R2T2 2A2F4n4C2σ2(4) The prepared material Nb2CTx MXene has shown the “σ” value of 16 whereas after doping of nitrogen “σ” value becomes smaller (σ = 6.2) indicating higher diffusion as “σ” is inversely proportional to “D” showing fast ion diffusion. Additionally, for real-world applications, device testing was performed by employing two electrode system. Ni-foam was utilized as a current collector in which 1 mg slurry of active material and activated carbon (AC) was deposited. Further, it is assembled as an asymmetric supercapacitor device and tested in 1 M KOH electrolyte within the potential region of 0 to 1.1 V. The cyclic voltammograms of the AC/N-Nb2CTx device were taken at different scan rates (5, 10, 20, 50, 100, 200) displaying the quasirectangular shape as shown in Fig. 10(a). CV curves of the asymmetric device indicate the pseudocapacitive behavior. As the scan rate increases, the current density of the asymmetric device shows the improved charge storage mechanism and higher reversibility of the electrode material. Figure 10(b) illustrates the GCD profile of the AC/N-Nb2CTx device at various current densities (1, 2, 3, 4, 5 A·g–1). At the lowest current density of 1 A·g–1, the N-Nb2CTx electrode has shown improved chargedischarge capability. GCD curves show a non-linear triangle shape and results are persistent with the CV data. For more interpretation of the two-electrode system, the specific capacitance was calculated at each current density by employing Eq. (5). Figure 10(c) shows the specific capacitance at different current densities. At 1 A·g–1, the asymmetric AC/N-Nb2CTx device displayed a specific capacitance of 75 F·g–1. Figure 10(d) displays the Ragone comparison plot of energy and power density at each current density. In addition, energy and power density were calculated by employing Eqs. (6) and (7) respectively. The device has demonstrated improved energy and power density. Similarly, at 1 A·g–1 asymmetric AC/N-Nb2CTx device has shown an energy and power density of 45 Wh·kg–1 and 0.55 kW·kg–1 respectively. C=I×∆t m×∆V(5) E=C×∆V2 2(6) P=E ∆t(7) s1/2 In Eqs. (5)‒(7), C is the capacitance calculated by GCD plots, ∆V is the potential window, ∆t represents the discharging time, E is the energy density, P stands for power density, and m is the mass deposit in the electrode. Furthermore, the Nyquist plot of the device was carried out by employing EIS with a fitted circuit as shown in Fig. 10(e) within the frequency range of 0.1 Hz to 10 kHz. The charge transfer and solution resistance between electrode and electrolyte are 0.626 and 0.157 Ω respectively by employing a modified constant phase element (CPE) fitted circuit. In addition, Warburg impedance was also carried out in which the fitted circuit shows 0.057 S·s1/2 . Moreover, the stability was taken out for 5000 cycles at 5 A·g–1 as demonstrated in Fig. 10(f). The device has shown remarkable long-term stability with 90% retention showing an improved life cycle and lower degradation making it ideal for practical application. 8Crystal structure and calculated formation energies The crystal structure is a layered hexagonal structure with space group P63/mmc, whereas the preliminary lattice parameters (a=12.47 Å, c=18.0 Å) for the computational study of Nb2C MXene have been determined from experimental XRD data. In the present work, the Nb2CTx (Tx = O, F) and N-doped Nb2CTx 0 10 20 30 40 50 0 50 100 150 200 Z real (Ω) Z img (Ω) Z real (Ω) Z real (Ω) Z img (Ω) Nb 2 C MXene Circuit fitted (a) 0 5 10 15 20 0 10 20 30 40 50 N-doped Nb2C MXene Circuit fitted (b) 1.0 1.5 2.0 2.5 3.0 3.5 0 10 20 30 40 50 ω σ = 6.2 σ = 16 −0.5 (s −0.5 ) N-doped Nb 2 C MXene Nb 2 C MXene (c) Figure 9 EIS plots of (a) Nb2CTx MXene (b) N-Nb2CTx MXene (c) Plotted ω–0.5 vs. Zreal to calculate Warburg factor σ. Nano Research Energy2025,4:e9120164 9 https://www.sciopen.com|https://mc03.manuscriptcentral.com/nre|Nano Research Energy