In Situ Nitrogen Functionalization of 2D-Ti₃C₂T_x-MXenes for High-Performance Zn-Ion Supercapacitor

Abstract

Zinc (Zn) ion supercapacitors (ZISCs) have attracted considerable attention as a viable energy storage technology because they are cost-effective, safe, and environmentally friendly. However, cathode materials with suitable properties are rare and need to be explored. In this regard, metal carbides (MXenes) are a good choice for capacitive energy storage, but they exhibit low capacitance. The energy storage performance of MXenes can be bossed using functionalization with heteroatom doping, e.g., nitrogen (N), to simultaneously modify ZISCs’ fundamental characteristics and electrochemical properties. Herein, we present an in-situ N-functionalization of Ti₃C₂T_x-MXene via a hydrothermal reaction with urea (denoted as N-Ti₃C₂T_x-MXene). N-functionalization into Ti₃C₂T_x-MXene raised Ti₃C₂T_x-MXene’s interlayer spacing and boosted the Zn-ion storage in 1 M ZnSO₄ electrolyte. The N-Ti₃C₂T_x-MXene electrode delivered an excellent specific capacitance of 582.96 F/g at 1 A/g and retained an outstanding cycle stability of 94.62% after 5000 cycles at 10 A/g, which is 1.8 times higher than pristine Ti₃C₂T_x-MXene at identical conditions. Moreover, the N-Ti₃C₂T_x-MXene//Zn device demonstrated a maximum capacitance of 153.55 F/g at 1 A/g, retained 92% of its initial value after 5000 cycles, and its Coulombic efficiency was ~100%. This strategy considerably reduced Ti₃C₂T_x-MXene nanosheet restacking and aggregation and enhanced electrochemical performance. Further, this research elucidated N-Ti₃C₂T_x-MXene’s charge–storage process and offered a fresh approach to the rational design of novel electrode materials for ZISCs.

1. Introduction

Expanding energy crises and environmental degradation have provided a powerful impetus for the development of safe, environmentally sustainable, and inexpensive energy storage technologies [1,2]. Further, humans’ extensive energy demands require large-scale research into electrochemical energy storage and conversion technologies, which can store energy more efficiently [3,4]. Recently a significant increase in research efforts directed toward discovering energy storage devices that are safe and affordable have been connected with high-performing energy conversion devices [5,6]. Particular consideration has been given to designing rechargeable batteries [7] and supercapacitors (SCs) [8]. SCs are simple and risk-free technologies, but their low energy density is a primary limitation. Lithium-ion batteries (LIBs) have been widely commercialized for electronic devices because of their low weight, excellent energy density, and outstanding performance [9]. However, scaling up LIBs is restricted by significant safety and environmental issues [10]. Aqueous Zn-ion storage has drawn much interest as a potentially useful aqueous electrolyte-based device for widespread energy storage, because of the Zn anode’s exceptional stability in aqueous electrolyte and ease of fabrication [11,12,13]. The research and development of zinc ion supercapacitors (ZISCs) with excellent performance is therefore of utmost importance.

Electrode material is essential in determining aqueous ZISCs’ electrochemical performance. There have been numerous efforts to investigate novel electrode materials for high-performance ZISCs, including organic compounds [14,15], Prussian blue [16], and transition metal oxides [17,18]. Designing aqueous ZISCs with suitable capacitance, extended cycle life, and remarkable rate performance remains challenging. MXenes are a novel class of two-dimensional (2D) materials that have applications in energy conversion and storage [19]. MXenes are typically 2D transition metal nitrides, carbides and/or carbonitrides with the general formula M_n+1X_nT_x, where M denotes transition metal, n = 1–4, X denotes nitrogen and/or carbon, and T denotes different surface termination groups, such as –OH, –F or –O [20,21]. Owing to its metallic conductivity, layered structure, superior mechanical characteristics, and surface hydrophilicity [22], MXenes have already proven to be attractive options as electrode material in SCs, zinc ion batteries, ZISCs, and sensors [19,23,24]. Unfortunately, similar to many other 2D materials, MXenes have high interlayer restacking and aggregation, which reduces their electrochemical performance. In particular, MXenes with multi-layered accordion-like architectures exhibit exceptionally low water dispersibility [25]. Multi-layers can be separated into fewer further dispersible layers using ultrasonication. However, MXene layers quickly re-stack and agglomerate when sonication stops [26,27]. There have been several attempts to address these issues [7,28]; of these, nitrogen (N)-functionalization of MXene nanosheets is an excellent strategy to boost MXene’s performance. Additionally, to achieve excellent improvements in ZISCs’ performance, it is essential to incorporate pseudocapacitive properties into the electrode materials [29]. For instance, incorporating heteroatoms, such as N, into carbon-based materials has proven to be an efficient and successful strategy for improving their electrochemical characteristics [30]. N-functionalized carbon-based materials offer enhanced electrical conductivity and surface wettability and are employed as electrode materials for high-performance ZISCs [31].

This paper presents a straightforward approach for preparing a novel type of N-functionalized Ti₃C₂T_x-MXene (N-Ti₃C₂T_x-MXene) via a hydrothermal technique with urea; N-functionalization into Ti₃C₂T_x-MXene raised interlayer spacing. The N-Ti₃C₂T_x-MXene electrode demonstrated a maximum capacitance of 582.96 F/g at 1 A/g and retained 94.62% of its initial value for 5000 cycles at 10 A/g. In addition, the N-Ti₃C₂T_x-MXene//Zn device demonstrated a maximum specific capacitance of 153.55 F/g at 1 A/g, retention of 92% for 5000 cycles, and Coulombic efficiency equal to ~100%. This strategy considerably reduces Ti₃C₂T_x-MXene nanosheet restacking and aggregation and enhances electrochemical performance. Further, this research elucidated N-Ti₃C₂T_x-MXene’s charge–storage process and offered a fresh approach to the rational design of novel electrode materials for ZISCs.

2. Experimental Method

2.1. Preparation of Ti₃C₂T_x-MXene

To prepare layered Ti₃C₂T_x-MXene, Ti₃AlC₂ MAX powder (1 g) was mixed in 20 mL of HF (50 wt.%) containing solution at room temperature (RT) and constantly agitated for 90 h. The resultant Ti₃C₂T_x-MXene solution was rinsed with water and then centrifuged at 1500 rpm for 15 min to attain a pH of ~6. The Ti₃C₂T_x-MXene solution was filtered using a polyvinylidene fluoride (PVDF) membrane. The obtained Ti₃C₂T_x-MXene powder was vacuum-dried at RT.

2.2. Synthesis of N-Ti₃C₂T_x-MXene

First, 1 g of Ti₃C₂T_x-MXene was dissolved in 100 mL DI water. After that, urea was dropped into the Ti₃C₂T_x-MXene solution, which was continuously agitated. The proportion of urea to Ti₃C₂T_x-MXene by weight was 1:30. The mixture was placed in an autoclave. The solution of urea and Ti₃C₂T_x-MXene underwent a hydrothermal reaction for 6 h at 160 °C. Next, the nitrogen-functionalized Ti₃C₂T_x-MXene (N-Ti₃C₂T_x-MXene) was synthesized by letting the autoclave temperature naturally fall to RT. Finally, the N-Ti₃C₂T_x-MXene was cleaned using water before being dried at ambient temperature in a vacuum.

2.3. Physical Characterization

The sample was structurally analyzed using X-ray diffraction (XRD) (X’Pert Pro PANalytical), with a Cu Kα radiation of 0.15406 nm wavelength λ. The prepared sample’s morphology was analyzed using a transmission electron microscope (TEM JEM-2100F, JEOL). The material’s chemical states were analyzed using Raman spectroscopy (HJY Lab RAM Aramics 70 France). Elemental composition was investigated using X-ray photoelectron spectroscopy (XPS). Experiments involving the adsorption and desorption of nitrogen were carried out using a Brunauer–Emmett–Teller (BET) analyzer to determine surface area and pore structure.

2.4. Electrochemical Measurements

For three electrode configurations, the working electrode consisted of 80% N-Ti₃C₂T_x-MXene, 10% conductive carbon black, and 10% polyvinylidene fluoride (PVDF) as a binder. A homogenous slurry was prepared using N-Methyl-2-Pyrrolidone (NMP) as a solvent, which was pasted on a 1 × 1 cm² piece of pre-treated carbon cloth (CC) before being dried in a vacuum oven at 60 °C for 6 h. A platinum plate, with Ag/AgCl as a reference electrode using 1 M ZnSO₄ electrolyte, was employed as a counter. To fabricate aqueous ZISCs, the previous working electrode was served as positive, with Zn as negative, in 1 M ZnSO₄. An electrochemical workstation (CHI 660E, Wuhan, China) was used to perform cyclic voltammogram (CV) and galvanostatic charge–discharge (GCD) tests, and take electrochemical impedance spectroscopy (EIS) measurements.

2.5. Calculations

$$C_{sp}=\frac{I\Delta t}{m\times \Delta V}$$

(1)

$$C_d=\frac{I\Delta t_d}{M\times \left(V\right)}$$

(2)

$$E=\frac{1}{2}{C}_d{V}^2\times \frac{1000}{3600}$$

(3)

$$P=\frac{E}{\Delta t}$$

(4)

$$\eta (\%)=\frac{\Delta t_d}{\Delta t_c}\times 100$$

(5)

where C_d and C_sp denote capacitance for two- and three-electrode configuration, respectively; E and P denote energy and power densities, respectively; η denotes Coulombic efficiency; I(A) denotes current; M denotes the mass of two-electrode configurations; Δt (s) denotes discharge time; and V(V) denotes the potential window.

3. Results and Discussion

Figure 1 illustrates the preparation method for N-functionalized Ti₃C₂T_x-MXene (N-Ti₃C₂T_x-MXene). First, multilayer Ti₃C₂T_x-MXene with slightly increased layer spacing was formed via selective removal of the Al layer of Ti₃AlC₂ MAX using hydrofluoric acid (HF). During this procedure, several O-containing groups formed a negative charge on MXene nanosheets [32,33]. N-Ti₃C₂T_x-MXene was subsequently fabricated using a hydrothermal process with urea.

A transmission electron microscope (TEM) was employed to analyze the micromorphology and microstructural development of both Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene. Figure 2a shows a low-resolution TEM image of pristine Ti₃C₂T_x-MXene, which exhibits translucent and smooth nanosheets that ultimately overlapped to produce a wrinkled structure. Figure 2c is a TEM image of N-Ti₃C₂T_x-MXene, which reveals the porous structure of N-functionalized MXene nanosheets. Figure 2b shows a high-resolution TEM image of Ti₃C₂T_x-MXene, which exhibits a crystallite with an interplanar spacing of d₁₁₀ = 0.30 nm, corresponding to the (110) plane. After N-functionalization, the interlayer spacing in the N-Ti₃C₂T_x-MXene sample increased to d₁₁₀ = 0.32 nm, as shown in Figure 2d. The increased interlayer spacing facilitated the interfacial charge transfer and electrolytic ion’s accessibility to electroactive areas. Moreover, the presence of large pores on N-Ti₃C₂T_x-MXene nanosheets enhanced ion movement within the material, which enhanced its electrochemical performance. Figure 2e illustrates a uniform distribution of N, T, and C across the whole material via energy-dispersive X-ray spectroscopy (EDS) for N-Ti₃C₂T_x-MXene.

Figure 3a,b depict X-ray diffraction (XRD) images of Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene. Ti₃C₂T_x-MXene’s XRD pattern has a characteristic peak less than 10°, which confirms that Ti₃AlC₂ was successfully etched to fabricate Ti₃C₂T_x-MXene [34]. XRD images of N-Ti₃C₂T_x-MXene show that the N-functionalization approach had minimal effect on Ti₃C₂T_x-MXene’s phase structure; the only notable change was that the (002) peak moved toward a lower diffraction angle associated with increased spacing. After further examination, it was discovered that the (002) peak was situated at approximately 9.3°, which was associated with a 0.96 nm interlayer spacing. Following N-functionalization, the position of the (002) peak changed to 8.9°, and the interlayer spacing was modified to 0.99 nm (Figure 3b). Increased interlayer spacing might have efficiently exploited the potential area for Zn²⁺ accommodation, implying a high capacitance. Further, Figure 3c shows high resolution Ti-2p XPS spectra, which were deconvoluted into five components: Ti-C (454.65 eV), Ti²⁺-C (456.05 eV), Ti³⁺-C (458.35 eV), TiO₂ (460.74 eV), and Ti⁴⁺-C (464.21 eV). After N-functionalization, a new peak appeared at Ti⁴⁺-C (464.21 eV). Notably, each peak’s intensity was enhanced after N-functionalization, suggesting that Ti₃C₂T_x-MXene underwent partial oxidation during the N-functionalization process. N-Ti₃C₂T_x-MXene’s deconvoluted N-1s XPS spectra are shown in Figure 3d, which shows peaks at 399.30 eV corresponding to pyrrolic-N, whereas the peak at a 401.33 eV binding energy corresponds to graphitic-N. The contribution of pyrrolic-N and graphitic-N groups proved the improvement in the sample’s electrical conductivity and electrochemical activity. Figure 3e depicts the F-1s spectrum, which reveals peaks at 284.39 and 285.79 eV, related to Ti-F and Ti-F-Ti, respectively, indicating the presence of the -F group generated by HF etching. The deconvoluted C-1s spectrum is depicted (Figure 3f). This spectrum displays four peaks at binding energies 281.15, 284.61, 286.25, and 288.51 eV, related to the Ti–C, C–C, C–O–C, and COOH bonds, respectively.

Using a three-electrode configuration in 1 M ZnSO₄, the electrochemical performance of pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene were investigated. Figure 4a displays the CVs of pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene electrodes across the potential window range of −0.8–0.2 V. Both CV curves have a characteristic rectangular shape with anodic and cathodic peaks, suggesting that both electrodes retained their capacitive behavior. However, the individual capacitances displayed significant variation, as seen in the area under the CV curves. Compared to the pristine Ti₃C₂T_x-MXene electrode, an enhanced specific capacitance can be inferred from the larger CV area for N-Ti₃C₂T_x-MXene due to the outstanding conductivity and interconnectivity of N-Ti₃C₂T_x-MXene nanosheets. Figure 4b depicts CVs of N-Ti₃C₂T_x-MXene at various sweep rates (1–75 mV/s). Even when the sweep rate was very high (75 mV/s), N-Ti₃C₂T_x-MXene displayed extremely capacitive behavior, excellent ion responsiveness, and good rate capabilities, with slight shifts in cathodic and anodic peaks [35]. Additionally, the mechanism for charge storage in electrodes was analyzed using a power law study of electrochemical kinetics [36].

i(V) = a.v^b

(6)

log (i) = b log (v) + log (a)

(7)

where v denotes the sweep rate, i denotes the peak current density, and a and b denote arbitrary constants. A b-value = 0.5 implied that capacitance was regulated via ionic diffusion, whereas b-value = 1.0 showed that the capacitive mechanism dominated during charge–discharge. Figure 4c shows corresponding anodic and cathodic b-values of 0.81 and 0.88, respectively, demonstrating the synchronous diffusion and capacitive-controlled mechanisms in the electrochemical reaction of the N-Ti₃C₂T_x-MXene.

Figure 4d demonstrates that, at a sweep rate of 20 mV/s, N-Ti₃C₂T_x-MXene stored charge 28.8% through a diffusion-controlled mechanism and 71.2% through a capacitive-controlled mechanism. Furthermore, Figure 4e illustrates the capacitive- and diffusion-controlled mechanisms for pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene at various sweep rates (1 to 50 mV/s). The capacitive-controlled mechanism rose as the sweep rate increased, suggesting that the capacitive mechanism dominated the total capacitance, particularly at high sweep rates.

Figure 5a displays the GCDs of pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene at 5 A/g. N-Ti₃C₂T_x-MXene had a charge–discharge duration of 253.35 s, which was significantly longer than the charge–discharge duration of a pristine Ti₃C₂T_x-MXene electrode (152.69 s), and was consistent with CV observations. Additionally, Figure 5b displays the N-Ti₃C₂T_x-MXene electrode’s GCDs at 1 to 20 A/g; excellent capacitive responsiveness with highly reversible charge–discharge at the N-Ti₃C₂T_x-MXene electrode is indicated by the GCD curves’ symmetry across all current densities. A small IR drop in discharge curves indicates the low internal resistance of the N-Ti₃C₂T_x-MXene electrode [37]. A longer discharge duration at higher current density values and maintaining symmetry indicate good Coulombic efficiency and outstanding charge storage characteristics [38]. Using discharge times, capacitances were determined according to Equation (1). As seen in Figure 5c, the capacitances of pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene were 582.96 and 380.64 F/g at 1 A/g, respectively. Surprisingly, the N-Ti₃C₂T_x-MXene retained its capacitance of 400 F/g (68.6%) even at 20 A/g, and it was higher than that of pristine Ti₃C₂T_x-MXene (250.66 F/g, 65.8%). The higher capacitance of the N-Ti₃C₂T_x-MXene electrode was attributed to N-functionalization. Figure 5d depicts the Nyquist pattern, which helps explain the increased electrochemical performance of N-Ti₃C₂T_x-MXene, as determined using EIS tests. The semicircle’s diameter shows a charge transfer resistance in the high-frequency zone of R_ct ~ 10.71 Ω for the N-Ti₃C₂T_x-MXene electrode, which was lower than that of pristine Ti₃C₂T_x-MXene (R_ct ~ 13.3 Ω). The computed equivalent series resistance R_s from the x-intercept for N-Ti₃C₂T_x-MXene was R_s ~ 2.39 Ω, whereas it was R_s ~ 2.56 Ω for the Ti₃C₂T_x-MXene electrode. This demonstrates N-Ti₃C₂T_x-MXene’s higher electrical conductivity due to N-functionalization. The cyclic life of pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene were evaluated at 10 A/g. The cyclic stability of pristine Ti₃C₂T_x-MXene and N-Ti₃C₂T_x-MXene electrodes are depicted in Figure 5e. The N-Ti₃C₂T_x-MXene electrode exhibited 99.62% retention for 5000 cycles, whereas pristine Ti₃C₂T_x-MXene demonstrated 88.54% retention for 5000 cycles; the N-Ti₃C₂T_x-MXene electrode had exceptional cyclic performance.

In conjunction with their potential window, the improved energy storage performance of N-Ti₃C₂T_x-MXene electrodes in a three-electrode system suggested that a two-electrode energy storage device constructed from such material might exhibit excellent performance. Therefore, to investigate the viability of the N-Ti3C2Tx-MXene electrode for use in real-world applications, an aqueous N-Ti₃C₂T_x-MXene//Zn device was assembled. Figure 6a illustrates the assembly process and operation of an aqueous N-Ti₃C₂T_x-MXene//Zn device using a 1 M ZnSO₄. Figure 6b displays CV curves for the N-Ti₃C₂T_x-MXene//Zn device using a potential window of 0.0 to 1.2 V, where preservation of CVs’ shapes demonstrates excellent electrochemical stability. Further, the area under CV curves gradually increased with sweep rates, indicating that the N-Ti₃C₂T_x-MXene//Zn device had superior electrochemical performance. In addition, GCD measurements were performed from 1 to 7 A/g, as shown in Figure 6c. GCD curves for the N-Ti₃C₂T_x-MXene//Zn device demonstrate both highly reversible charge–discharge curves and high Coulombic efficiency. The maximum capacitance (C_d) of the N-Ti₃C₂T_x-MXene//Zn device was calculated according to Equation (2). The maximum computed specific capacitance was 153.55 F/g at 1 A/g, as illustrated in Figure 6d. At 10 A/g, 54% capacitance was retained, which indicated the N-Ti₃C₂T_x-MXene//Zn device’s exceptional rate performance. Figure 6e displays the Ragone plot, which demonstrates the E and P of the N-Ti₃C₂T_x-MXene//Zn device (calculated according to Equations (3) and (4), respectively). The N-Ti₃C₂T_x-MXene//Zn device delivered energy densities of 30.7, 24.3, 20.1, 19.1, 17.7, and 16.7 Wh/kg at power densities of 600.5, 1200.9, 1801.4, 3002.4, 4203.4, and 6004.8 W/kg, respectively. This indicates its superiority compared to most previously explored symmetric/asymmetric SCs devices [35,39,40,41,42,43]. Furthermore, Figure 6f displays the cyclic stability of the N-Ti₃C₂T_x-MXene//Zn device, which exhibited 92% retention for 5000 cycles, indicating exceptional cyclic stability with nearly 100% Coulombic efficiency. According to the findings of the single electrode and ZISCs device, N-Ti₃C₂T_x-MXene material demonstrates outstanding electrochemical performance as indicated by specific capacitance, cyclic performance, energy density, and power density. We believe that these exceptional properties are due to the favorable N-functionalization of the N-Ti₃C₂T_x-MXene electrode material. In particular, N-atoms’ presence enabled N-Ti₃C₂T_x-MXene to provide a relatively high active surface area and excellent electrochemical activity.

4. Conclusions

In conclusion, we presented a straightforward fabrication method for a newly designed N-Ti₃C₂T_x-MXene via a hydrothermal reaction with urea. N-functionalization into Ti₃C₂T_x-MXene raised the interlayer spacing of Ti₃C₂T_x-MXene. The fabricated N-Ti₃C₂T_x-MXene delivered a maximum capacitance of 582.96 F/g at 1 A/g and retained 94.62% capacitance for 5000 cycles at 10 A/g. Additionally, the N-Ti₃C₂T_x-MXene//Zn device demonstrated a maximum capacitance of 153.55 F/g at 1 A/g, with 92% retained capacitance for 5000 cycles and a Coulombic efficiency of ~100%. The N-Ti₃C₂T_x-MXene//Zn device revealed an energy density of 30.7 Wh/kg with a power density of 600.5 W/kg. This research provided insight into the rational assembling of innovative electrode materials for ZISCs.