A Review of Nb₂CT_x MXene: Synthesis, Properties and Applications

Abstract

Nb₂CT_x is an important member of MXene family. It has attracted widespread attention because of its abundant functional groups, high hydrophilicity, high electrical conductivity as well as low ion transport barrier, showing great potential in various applications. In order to utilize the advantages of Nb₂CT_x MXene, the progress of preparation, properties and applications are reviewed in this work. This work focuses on different methods of Nb₂CT_x preparation and applications in electrochemical energy storage (supercapacitors and secondary batteries), electrocatalytic hydrogen evolution, photocatalytic hydrogen evolution, sensors, etc. Additionally, the main problems of self-stacking and prospect of Nb₂CT_x MXene are discussed.

1. Introduction

MXene, a 2D layer-structured material, has high conductivity, excellent mechanical property and large surface area [1,2]. MXene consists of transition metal nitrides, carbides or carbonitrides. Its molecular formula is M_n+1X_nT_x, where M is the transition metals, including Ti, Sc, Cr, V, Zr, Nb, and Mo, X is carbon or nitrogen and T_x stands for surface terminals, including -F, -OH as well as -O groups [3,4]. Usually, MXene can be obtained by selectively etching the A atom (Si or Al atom) layers of MAX phase (precursor of MXene, M represents transition metal element; A represents main group element; X represents carbon or nitrogen.) [5,6]. Because of its chemical composition and unique structure, MXene and its composites have been applicated in electrochemical energy storage (supercapacitors and secondary batteries) [7], catalysis [8], sensors [9], electromagnetic interference shielding [10], etc.

So far, most of the research on MXene has focused on Ti₃C₂T_x, for the preparation of Ti₃C₂T_x is quite mature. In MXene materials, as the number of atomic layers increases, the molecular mass increases and the theoretical capacity decreases. Therefore, compared with Ti₃C₂T_x, MXene with M₂X structure has a higher specific capacity. Nb₂CT_x exhibits outstanding conductivity, near zero bandgap, and has special metallic properties because of the flexible combination of surface functional groups [11,12]. Nb₂CT_x can be obtained by acid etching of the Nb₂AlC MAX phase. Theoretical studies have shown that Nb₂CT_x and its modified materials will show excellent performance in many fields because of the improved properties.

Figure 1 shows that Nb₂CT_x with different properties have been applied in various fields in recent years. Cheng et al. [13] have studied the charge storage mechanism of Nb₂CT_x in lithium-ion batteries, by combining density functional theory (DFT) calculations and complementary experiments. It is found that the outstanding electrochemical performance of Nb₂CT_x is mainly related to the unique structure of -O functional groups on Nb₂CT_x surface. As the oxidation states of O and Nb changes, Nb₂CT_x will store charge, while Li-ion can intercalate or deintercalation between the interlayers of Nb₂CT_x for charge compensation. The theoretical capacity value of Nb₂CT_x can reach 542 mAh/g [14]. Xin et al. [15] have shown that Nb₂CT_x nanosheet is an outstanding material as supercapacitor electrode by evaluating the theoretical quantum capacitances. It can be calculated using the equation, C_Q = e²D(-Ve), where D(E) is the electronic densities of state (DOS), E is the energy with respect to the Fermi level, V is the voltage of the electrode, and e is the elementary charge. The theoretical quantum capacitances of Nb₂CT_x electrode are 1091.1 and 1828.4 F/g at negative and positive electrodes, respectively. Nb₂CT_x is an excellent candidate for catalysis. In photocatalytic application, the structure of 2D Nb₂CT_x can promote photogenerated carrier to transfer from interior to Nb₂CT_x surface. This leads to improved separation efficiency and photodegradation performance [16]. Similarly, Nb₂CT_x has also been applied in electrocatalytic hydrogen evolution (HER) due to its abundant hydrophilic functional groups, especially the terminated O atoms. The Gibbs free energy (ΔG^a_H*) for the adsorption of atomic hydrogen is close to the ideal value (0 eV) [17]. In addition, Nb₂CT_x has also been applied in sensors due to its highly hydrophilic surface and high charge-carrier mobility [18].

The outstanding properties of Nb₂CT_x have ensured its applications in different areas, such as energy storage devices [19,20,21,22], electrocatalysis [23,24], photocatalysts [25,26], and sensors [27,28,29]. In addition, applications of Nb₂CT_x on other areas have also been reported, such as on perovskite solar cells (PVSCs) [30], photodetector [31], biomedicine [32], electromagnetic shielding [33], adsorbents [34], etc. In this paper, the progress of Nb₂CT_x preparation, its properties and different applications are reviewed, while also focusing on the exfoliating methods as well as applications in energy storage (supercapacitors and secondary batteries), photocatalytic hydrogen evolution, electrocatalytic hydrogen evolution, sensors, etc. Additionally, the main problems of self-stacking of Nb₂CT_x are discussed and prospects for the future are explored.

2. Synthesis

In Nb₂AlC MAX, the Nb-C bonds are strong, while the metal bonds of Nb-Al are relatively weak, therefore, Nb₂CT_x is usually obtained by selectively removing Al atom layers of Nb₂AlC. Even though the metal bond energy between the Nb-Al atomic layers is relatively weak, the bonding strength is still greater than that of the Van der Waals force. Hence, it seems difficult to peel the precursor into few-layer or monolayer using ultrasound-assisted liquid-phase exfoliation or other mechanical stripping method. Chemical exfoliation method is the most effective and widely used approach, where Nb₂CT_x can be prepared by etching away the Al atomic layer without destroying the Nb-C atomic layers. Figure 2 shows the structures of various types of MAX phases and their corresponding MXene structures obtained using chemical exfoliation method.

Studies have also shown that Nb₂CT_x can be intercalated by a variety of metal cations [36] or organic molecules [20] to increase the interlayer spacing, and to facilitate the etching of Al atoms and exfoliation of Nb₂CT_x nanosheets. After exfoliation, the single-layer or few-layer Nb₂CT_x nanosheets can be obtained.

2.1. Synthesis of Nb₂AlC

Most of Nb₂CT_x is obtained from Nb₂AlC MAX. The quality of Nb₂AlC MAX affects the quality of Nb₂CT_x nanosheets directly. In order to get Nb₂CT_x nanosheets with high quality, Nb₂AlC MAX with high quality should be prepared first. Many methods are developed to prepare Nb₂AlC MAX, such as high-temperature solid-state reaction method [37,38,39,40], self-propagating high-temperature synthesis (SHS) process [41,42,43,44] and spark plasma sintering technique (SPS) [45].

Stumpf et al. [38] have synthesized Nb₂AlC powders using the high-temperature solid-state reaction method. Firstly, high-purity Nb, NbC and Al powders are uniaxially compacted at 250 MPa to obtain powder particles, then Nb₂AlC powder is obtained by grounding particles at a high temperature of 1600 °C. Yeh et al. [41] have synthesized Nb₂AlC using a productive self-propagating SHS process. To obtain Nb₂AlC, Al₄C_3, Nb₂O₅ and Al undergo chemical reactions under different stoichiometric parameters and high temperatures (1200–1800 °C). Although these methods can prepare high-purity Nb₂AlC, the equipment and high temperature required for the synthesis lead to huge investment and production costs.

SPS is considered as an effective method for the synthesis of many ceramics. Compared with the thermocompression method, SPS shows obvious advantage in heating as it can be heated by pulsed direct current, and requires lower temperature and shorter cycle time. Zhou et al. [45] have synthesized Nb₂AlC using spark SPS, whereinhigh-purity Nb, Al and NbC powders are uniaxially compacted at 20 MPa and then vacuum sintered in a spark plasma at a heating temperature of 1450 °C.

Among all these preparation methods mentioned above, the preparation of Nb₂AlC using the SPS method has advantages in temperature, reaction time and cost, thus promises to be an efficient and commonly used method for the synthesis of Nb₂AlC.

2.2. Synthesis of Nb₂CT_x

Nb₂CT_x is usually obtained from Nb₂AlC MAX using the chemical exfoliation methods. Commonly used chemical etching methods include hydrofluoric acid (HF) etching [46,47,48,49,50], hydrofluoric acid/lithium fluoride (HCl/LiF) in situ generation of hydrofluoric acid etching [51,52], hydrothermal etching [53], Lewis acid molten salt etching [54,55] and electrochemical etching [56]. Etching with a high-concentration HF solution is currently the most common method for preparing Nb₂CT_x. Using the HF etching method, Zhao et al. [46] have obtained few-layer Nb₂CT_x nanosheets by changing the reaction temperature, time and other parameters, shown in Figure 3a. After HF etching, the densely packed layers of Nb₂AlC become unconsolidated, and the angle of (002) diffraction peak is smaller due to action of different surface functional groups and interlayer water molecules. In the meantime, Nb₂CT_x, exposed to the solution will combine with different functional groups (-OH, -F and -O). However, some Nb₂AlC usually remains in the Nb₂AlC prepared using the etching method. In

Table 1. Different parameters of HF etching method for the preparation of Nb₂CT_x.

Etching Solution	Temperature/°C	Time/h	References
50% HF	25	240	[14]
40% HF	60	90	[46]
48% HF	55	48	[57]
50% HF	55	72	[58]
48% HF	80	24	[59]
50% HF	30	24	[60]
40% HF	50	72	[61]

, different experimental parameters of HF etching method is compared, since etching conditions will affect the purity and layer spacing of Nb₂CT_x [14,46,57,58,59,60,61]. As shown in Table 1, proper acid concentration, reaction temperature and reaction time will promote the etching reaction and benefit in obtaining pure Nb₂CT_x nanosheets.

Since the high concentration of hydrofluoric acid is harmful to humans and the environment, developing environmentally friendly and milder methods to prepare Nb₂CT_x is urgent and important. Hydrofluoric acid/lithium fluoride (HCl/LiF) in situ generation of hydrofluoric acid etching method is an effective alternative to the HF etching method. As Figure 3b shows, Xiao et al. [51] have obtained few-layer Nb₂CT_x using HCl/LiF in situ generation of hydrofluoric acid etching method, where Li⁺ can be embedded in the Nb₂CT_x layers to facilitate the delamination of Nb₂CT_xnanosheets. However, HF is still generated and released during the HCl/LiF in situ generation of hydrofluoric acid etching process, and thus this method is harmful to humans and environment in some ways. Therefore, it is imperative to propose green preparation methods which can avoid the direct use and release of hydrofluoric acid. Peng et al. [53] have obtained Nb₂CT_x using solvothermal treatment by mixing hydrochloric acid (HCl) and sodium tetrafluoroborate (NaBF₄) together, followed by ultrasonication and intercalation. Specifically, the mixture of NaBF₄, HCl and Nb₂AlC is hydrothermally reacted at 180 °C for 15–35 h. The etching process takes place in a confined hydrothermal system, without exposure to toxic hydrofluoric acid vapors. The hydrothermal method can remove the Al layer more completely compared to traditional HF etching method.

Since -F functional groups are electronegative, too much -F functional groups will greatly reduce the conductivity of MXene [36], so proposing fluorine-free etching methods is also necessary. In recent years, some fluorine-free etching methods have been reported, such as to obtain Ti₂CT_x using electrochemical etching method [62] and to obtain Ti₃C₂T_x by etching Ti₃AlC₂ using the molten salt method [63]. Similar to these methods of obtaining Ti₃C₂T_x and Ti₂CT_x, fluorine-free methods to obtain Nb₂CT_x have also been reported in recent years.

Song et al. [56] have synthesized fluoride-free Nb₂CT_x flakes using an electrochemical etching method (E-etching method), as shown in Figure 3c. This method is carried out in a three-electrode system where Nb₂AlC phase powder and carbon black are mixed and evenly drop-cast onto the CFC substrate. This electrode is oxidized in HCl electrolyte for 4 h at 50 °C with a voltage of 1 V to etch away the Al atomic layers. Li et al. [55] have used a fluoride-free method using direct redox coupling of the cation from the Lewis acid molten salt and the Al atom of Nb₂AlC. Specifically, AgCl is used as salt, while Nb₂AlC is mixed with AgCl, KCl and NaCl in a molar ratio of 1:5:2:2, and reacted at 700 °C for about 24 h. During this reaction, Al atoms will be oxidized by AgCl and eventually removed from Nb₂AlC to obtain Nb₂CT_x.

In general, hydrofluoric acid etching is the most commonly used method for synthesizing Nb₂CT_x. However, considering the danger of hydrofluoric acid to humans and the fact that fluorine functional groups will reduce the electrochemical performance of Nb₂CT_x, many fluorine-free etching methods have been suggested. However, these methods still have problems, such as harsh reaction conditions or low yields. Therefore, it is still important to explore more efficient fluorine-free etching methods.

2.3. Delamination of Nb₂CT_x

After the Nb₂AlC etching reaction, the Al atoms are selectively removed, and replaced with functional groups, such as -F, -OH as well as -O. The obtained Nb₂CT_x layers are held together by hydrogen bonds and weak Van der Waals forces. However, the Nb₂CT_x sheets are multi-layered, and further delamination/exfoliation processing is required to form single-layer/few-layer Nb₂CT_x sheets. The specific surface area of two-dimensional materials will be greatly increased after further delamination.

Generally, the materials for delamination involve organic, ionic molecules or inorganic cation. Non-alkaline organic solvent intercalation is an effective way to obtain few-layer Nb₂CT_x. Such organic solvent intercalants include DMSO [26,64], isopropylamine (i-PrA) [20,48], etc. For example, Mashtalir et al. [20] have obtained few-layer Nb₂CT_x sheets using isopropylamine (i-PrA), as shown in Figure 3d. When mixed with water, i-PrA can form ammonium cation R-NH₃⁺, which can be intercalated between Nb₂CT_x layers using electrostatic force. In addition, the i-PrA molecule has a three-carbon-atom alkyl tail, which can overcome the intercalation steric hindrance and push the Nb₂CT_x nanosheets apart, resulting in weaker interlayer interactions and leading to the delamination of Nb₂CT_x.

Cation intercalation, treating Nb₂CT_x with alkalization, is another widely used method. The intercalants include LiOH [14], KOH [36], TMAOH [16,65], and TPAOH [28,66,67]. As shown in Figure 3e, Cui et al. [16] have obtained few-layer Nb₂CT_x sheets by intercalating with tetramethylammonium hydroxide (TMAOH) for about 3 days at room temperature. Thus, the numerous functional groups on the surface of Nb₂CT_x nanosheet make it negatively charged. In order to achieve charge balance, H⁺ will enter the interlayers. When treated with alkali, the cations of the alkali will replace H⁺ and insert between the layers. After cation intercalation, the layer spacing of Nb₂CT_x will be larger and the -F functional groups will be replaced by hydroxyl functional groups. The intercalated alkali cations will weaken the interaction between the Nb₂CT_x sheets, so single-layer/few-layer Nb₂CT_x flakes can be obtained after simple exfoliation, such as shaking or water-bath sonication.

3. Structure and Properties of Nb₂CT_x

3.1. Structure of Nb₂CT_x

The Scanning Electron Microscope (SEM) image (Figure 4a) and Transmission Electron Microscope (TEM) image (Figure 4b) show that the few-layer Nb₂CT_x sheets are ultrathin and transparent. The high-resolution transmission electron microscopy (HRTEM) image (Figure 4c) and the selected area electron diffraction (SAED) pattern image (inset of Figure 4c) of the Nb₂CT_x nanosheets show good crystallinity. A clear crystal lattice patten of (0001) crystal plane can be seen, showing good single crystal characteristics [46]. The X-ray diffraction (XRD) image (Figure 4d) indicates that the (002) peak of few-layer Nb₂CT_x sheets shifts toward a lower angle, suggesting a larger interspacing due to the elimination of Al atoms [57].

The crystal structure of computationally modeled Nb₂C MXene is similar to that of 1T-MoS₂, including three layers with Nb-C-Nb sequence in a 3 × 3 × 1 supercell. The C layer is sandwiched by two Nb layers; C layers and Nb layers are stacked along the hexagonal c-axis [57]. Because of the highly unsaturated surface in Nb₂C, functionalized elements (F, O, OH and N) can be easily trapped in the hollow sites. Figure 4e [68] exhibits the modeling structures, including Nb₂C and Nb₂CX₂, where X groups can be located at top of a Nb (1) atom (top layer), a C atom and a Nb (2) atom (bottom layer), and both Nb₂C sides, therefore, there are a total of six configurations of adsorption sites.

3.2. Properties of Nb₂CT_x

Seredych et al. [69] have explored the thermal stability of Nb₂CT_x using mass spectrometry analyses and combined thermogravimetric. Nb₂CT_x etched by HF tends to release water at 246 °C and the main phase transition occurs at 807 °C, with a weight loss of 10–15 wt%. The weight loss coincides with the massive release of CO₂ as well as CO, indicating a possible transformation of Nb₂CT_x into the stable corresponding massive carbides. The Nb₂CT_x, further delaminated by TMAOH, has a weight loss of about 8.5 wt%, in a temperature range of 50 °C to 600 °C. NH₃, NH₂, and NH₄⁺ species release because of the decomposition process and desorption process of TMA⁺ ions. The results indicate that the intercalant is strongly bound to the surface of Nb₂CT_x nanosheets to hinder water adsorption. The transformation temperature slightly increases to 820 °C with less mass loss of about 4 wt%, indicating a higher stability of Nb₂CT_x. Therefore, intercalants, such as TMAOH, can replace water and affect the decomposition products at low-temperature, making Nb₂CT_x with better thermal stability.

Babar et al. [70] have explored the magnetic properties of Nb₂CT_x. The peculiar magnetic properties of Nb₂CT_x shows the existence of Meissner effect, and Nb₂CT_x is supposed to have superconductivity (SC) with the transition temperature of 12.5 K. Evaluated with the DFT first-principles calculation, the Nb₂CT_x shows a negative magnetic moment, which corresponds well to the experimental diamagnetic nature. Both the experimental analysis and calculation indicate Nb₂CT_x showing a superconductor-like diamagnetic behavior. The superconductivity-like magnetic property in Nb₂CT_x can be attributed to the higher density-of-states at the Fermi level and strong electron–phonon interaction [71].

Xu et al. [72] have studied the electronic properties and structural stability of Nb₂CT_x based on DFT calculations, paying special attention to the influence of surface groups. The study suggests that Te and O groups are more inclined to locate at the central C hollow sites on both Nb₂CT_x sides. The interaction strength between different functional groups and Nb atoms will gradually decrease from O to S, Se and to Te. This study has further analyzed bonding interactions using projected crystal orbital Hamiltonian populations. As the surface terminal gains less electrons from O to Te group, the integrated projected crystal orbital Hamilton population (pCOHP) value decreases gradually from O to Te. The altered interactions can also affect the electronic performance of Nb₂CT_x. This study has found that it is the different types of surface groups that determine the electronic properties and conductivity of Nb₂CT_x, with adsorption sites as a secondary factor. That is, Nb₂CO₂ and Nb₂CTe₂ are metallic, while Nb₂CS₂ and Nb₂CSe₂ can be metallic or semiconducting, depending on the adsorption sites of different surface groups on Nb₂CT_x.

Halim et al. [73] have obtained Nb₂CT_z and Ti₂CT_z film via physical vapor deposition on sapphire and LiF/HCl etching method, and then measured the optical and electronic properties of these films. Ti₂CT_z film exhibits metallic behavior while Nb₂CT_z film is not metallic, which can be supported by optical properties via spectroscopic ellipsometry measurements. The difference in optical response between Nb₂CT_z film and Ti₂CT_z film can be attributed to the absorption of Nb₂CT_z film near infrared. Additionally, the resistivity of Nb₂CT_z film increases when temperature decrease, because of the percolation between Nb₂CT_z flakes. The different optical and electronic properties of these films are influenced by the type of transition metal.

Palisaitis et al. [74] have investigated the stability of Nb₂CT_x in plan-view geometry using electron energy loss spectroscopy (EELS), atomically resolved scanning transmission electron microscopy (STEM) as well as STEM simulations. This research has found that the Nb adatoms on Nb₂CT_x surface can attract and be linked to ambient O and then form clusters which will ripen over time, leading to the destruction of Nb₂CT_x structure stability. Considering the structure stability at a long time, the adatoms on the Nb₂CT_x surfaces are detrimental for Nb₂CT_x structure. Echols et al. [75] have explored ways to enhance the oxidation stability of Nb₂CT_x. It is found that the shelf life of Nb₂CT_x can be significantly improved with low temperature storage, using antioxidants, increasing the concentration of Nb₂CT_x solution, etc.

4. Applications

4.1. Secondary Batteries

MXene has rich chemical and structural diversity, and is a promising material for secondary batteries electrode. When the layer of MXene materials increases, the molecular mass increases, so the theoretical capacity will decrease [76]. Hence, compared to M₃X₂ and M₄X₃, the M₂X structured MXene tends to have a higher specific capacity [77]. Nb₂CT_x MXene, showing low diffusion barriers, fast ion mobility and low average intercalation potentials, has generated great interests in recent years. Currently, the research on Nb₂CT_x application on secondary batteries is still at the exploratory stage. Taking lithium-ion batteries as an example, Nb₂CT_x has a low lithium-ion diffusion barrier (<0.1 eV), and thus Li⁺ ions can easily intercalate into the Nb₂CT_x nanosheets. The surface functional groups of Nb₂CT_x also affect the lithium storage performance of Nb₂CT_x. The specific capacity of Nb₂CT_x with -O functional groups are higher than that of Nb₂CT_x with -F and -OH functional groups. The -F and -OH functional groups can inhibit the adsorption of lithium ions, which is detrimental to the electrochemical properties. The reaction of Nb₂CT_x with -O functional groups to adsorb lithium ions is as follows:

Nb₂CT_xO₂ + 2Li ↔ Nb₂CT_xO₂Li₂

(1)

4.1.1. Lithium Ion Battery

As an important type of energy storage device, Lithium-ion batteries (LIBs) are playing an increasingly important role in our society owing to their ability to offer power for both our daily life and industrial manufacturing [78,79,80]. Numerous studies on Li-ion battery materials have been reported in recent years. For example, the first DFT calculations of a high-energy, 5 Volt rechargeable Li-ion battery cathode material Li₂CoMn₃O₈ were performed by Eglitis and Borstel [81]. It was found that the reversible capacity of Nb₂CT_x is higher than that of Ti₃C₂T_x, and the capacity value of Nb₂CT_x can reach 542 mAh/g [14], which is much larger than that of Ti₃C₂T_x (320 mAh/g). However, considering the strong Van der Waals force between Nb₂CT_x layers, Nb₂CT_x nanosheets are easy to aggregate or stack together. The self-restacking will lead to densely packed structure, reducing the specific surface area and increasing difficulty in the transportation of ions, thus to decreasing the rate performance and cycling performance of Nb₂CT_x electrodes. Qi et al. [77] have reported that the specific capacity of Nb₂CT_x after 200 cycles at 500mA is less than 200 mAh/g, which is far from the theoretical value. Therefore, strategies should be carried out to improve the electrochemical properties from Nb₂CT_x.

Reports have been published to solve the above problems in order to fully utilize the electrochemical properties of Nb₂CT_x [82,83,84,85,86] Li et al. [82] have mixed Nb₂CT_x with 1,3,5-benzenetricarboxylic acid (BA) to obtain BA-Nb₂C, as shown in Figure 5a. BA molecules can be anchored into Nb₂CT_x nanosheets through hydrogen bonding interaction between Nb₂CT_x sheets and BA molecules, enlarging the interlayer spacing and enhancing the stability of Nb₂CT_x. Figure 5b shows that BA-Nb₂C exhibits a larger specific surface area which can facilitate the Li⁺ diffusion, provide more active sites, and improve the capability rate of Nb₂CT_x. The BA-Nb₂C electrode exhibits a high specific capacitance of 328.2 mAh/g at 0.1A/g, as shown in Figure 5c.

The surface functional group of MXene plays an important role in the electrochemical reaction [84,85]. The -F functional and hydroxyl groups on Nb₂CT_x surface can hinder the diffusion of Li-ions in the interlayer, thereby greatly reduce the lithium storage specific capacity [86]. However, the -O groups on Nb₂CT_x surface will promote the diffusion and adsorption of Li-ions and effectively inhibit the formation of lithium dendrites [13]. Zhao et al. [14] have mixed LiOH with Nb₂CT_x to introduce more oxygen-containing functional groups and analyzed the corresponding electrochemical properties and Li storage dynamics. The surface functional groups (-O, -OH and -F) of Nb₂CT_x nanosheets changes when annealed at different temperatures. After annealing at 400 °C, -O surface functional groups of Li⁺ intercalated Nb₂CT_x increase while the -OH and -F groups decrease. The Li-Nb₂CT_x-400 electrode shows a high specific capacity of 448 mAh/g at 0.05 A/g owing to the large number of -O surface functional groups.

4.1.2. Sodium Ion Battery

Sodium ion batteries (SIBs) have become a promising alternative to LIBs because of certain advantages, such as low cost, abundant sodium resources and similar energy storage mechanism. However, the radius of Na-ion is larger than that of Li-ion, and the development of SIBs is constrained by the short of suitable anode materials [87,88]. MXene, with its unique properties and structure, is promising to serve as a new type of anode material for SIBs [89,90]. As shown in Figure 6a, Yuan et al. [77] have constructed a robust 3D Nb₂CT_x@MoS₂@C composites, by hydrothermal synthesis of MoS₂ and carbon coating on Nb₂CT_x framework. The Nb₂CT_x@MoS₂@C composites show a reinforced cross-linked three-dimensional structure (Figure 6b). The high specific surface area and stable structure can better utilize the electrode material. The Nb₂CT_x@MoS₂@C electrode shows an improved charge transfer efficiency, and avoid structural collapse or volume expansion caused by Na-ion intercalation. The Nb₂CT_x@MoS₂@C exhibit a high capacity of 530 mAh/g after 200 cycles test in 0.1 A/g (Figure 6c). Zhang et al. [91] have prepared a sandwich-like composite including N-doped carbon nanotubes (N-CNTs) and Nb₂CT_x nanosheets using the electrostatic self-assembly approach. The N-CNTs are trapped between two MXene layers as conductive agents and spacers. In addition, the N-doped sites on the carbon nanotube walls can act as defects, promoting the diffusion and storage of Na-ion. The N-CNT@Nb₂C is supposed to have good electrochemical performance as Na-ion batteries anode. Xiao et al. [92] have synthesized Ag-Nb₂CT_x composite by physically mixing Ag nanoparticles and Nb₂CT_x nanosheets. The Ag-Nb₂CT_x can provide larger specific surface area and higher conductivity. The Ag-Nb₂CT_x shows a capacity of 183 mAh/g at 0.05 A/g after 50 cycles in Na-ion battery.

4.1.3. Potassium Ion Battery

Potassium-ion batteries (KIBs) is also a potential alternative to LIBs because of the low cost and abundance of potassium-related resources. However, the ion migration of electrodes is difficult because of the much larger radius of K-ion compared to Li-ion [93,94]. Therefore, developing suitable electrode materials for KIBs is essential. The unique structure and excellent conductivity of Nb₂CT_x make it an ideal electrode candidate for KIBs [95]. As shown in Figure 7a, Liu et al. [58] have prepared in situ carbon-intercalated and surface-modified Nb₂CT_x as electrode for Potassium-ion batteries. The carbon-intercalated Nb₂C nanocomposite (Nb₂C/C) can be obtained by intercalating dodecylamine (DDA) into the Nb₂CT_x layers, followed by annealing (Figure 7b). The Nb₂C/C electrode shows less surface functional groups, enlarged interlayer spacing as well as increased specific surface area. The Nb₂C/C electrode, with high electrical conductivity and good K⁺ diffusion coefficient, performs a specific capacity of 199.8 mAh/g at 0.02 A/g (Figure 7c). Mao et al. [20] have prepared self-standing composites of Nb₂CT_x and reduced graphene (rGO) (3D-rGO/Nb₂C) using the electrostatic self-assembly approach. The paper electrode with 3D porous network can provide more active sites and promote the transfer of K⁺. The 3D-rGO/Nb₂C shows a good capacity of 139 mAh/g after 1000 cycles at 500 mA/g, owing to the diffusion and pseudocapacitive mechanism in potassium ion storage.

4.1.4. Lithium Sulfur Battery

Lithium-sulfur (Li-S) batteries have been considered as the best alternative to LIBs because of advantages such as long cycle life, low cost of active materials, as well as high energy density. However, the commercialization of this technology has been plagued by some key challenges, such as the insulating shuttling effect of sulfur cathodes, volume changes during charging/discharging, and poor electronic/ionic conductivity [96,97]. As previously reported, MXene with unique two-dimensional structure, negatively charged surface, and high conductivity can immobilize soluble polysulfides by adsorption to suppress the shuttling effect [98,99]. As shown in Figure 8a, Song et al. [100] have designed Nb₂C/Nb₂O₅ heterostructures using water vapor etching. The Nb₂C has high adsorption capacity, while Nb₂O₅ can catalyze the activity of lithium polysulfides. Meanwhile, the accordion-like and porous structure (Figure 8b,c) formed during water vapor etching can suppresses the aggregation of Nb₂C sheets and Nb₂O₅ nanoparticles, promote Li⁺ transport and facilitate diffusion of the anchored lithium polysulfides. The S-Nb₂C/Nb₂O₅ electrode exhibits a capacity of 621 mAh/g after 500 cycles test at 1.0 C (Figure 8d). By freeze-drying, Zhong et al. [101] have prepared petal-like Nb₂CT_x and MoS₂ is grown on the surface of Nb₂CT_x via hydrothermal method. The Nb₂CT_x and MoS₂ is deemed to form a heterojunction owing to their high lattice adaptability. The MoS₂/Nb₂C heterojunction structure is favorable for the conductivity and adsorption capacity of lithium polysulfides. The specific capacity of MoS₂/Nb₂C cathode is 919.2 mAh/g at the current density of 0.2 C.

In addition to LIBs, SIBs, KIBs, and LSBs, Nb₂CT_x has also been applied to other batteries. The calcined Nb₂CT_x cathode used for aluminum batteries (Al batteries) shows a specific capacity of 08 mAh/g at 0.2 A/g after 500 cycle tests [57]. Li et al. [102] have proposed a new method of achieving high-energy-density MXene electrodes with a distinct discharge voltage plateau for aqueous zinc ion batteries. The authors use a high-voltage-scanning approach to trigger the behavior of Nb₂CT_x MXene cathode in aqueous Zn-ion batteries, which shows a stable Zn²⁺ storage with a 1.55 V plateau. The energy density of the MXene electrode reaches 146.7 Wh/kg, which is much higher than that of the capacitive Nb₂CT_x electrode and other conventional cathodes.

Table 2. The specific capacities of Nb₂CT_x for secondary batteries.

Battery	Materials	Current Density	Cycles	Capacity (mAh/g)	References
LIB	Li-Nb2CTx-400	2 A/g	2000	342	[14]
LIB	Nb2CTx-CNT	2.5 C	300	430	[20]
LIB	Chl@Nb2C	0.5 A/g	200	163	[77]
LIB	BA-Nb2C	1 A/g	500	261.7	[82]
LIB	Nb4N5@Nb2C	2 A/g	800	109.2	[83]
KIB	3D-rGO/Nb2C	0.5 A/g	1000	139	[21]
KIB	Nb2C/ rGO	2 A/g	500	301.7	[94]
SIB	Nb2CTx@MoS2@C	1 A/g	2000	403	[76]
SIB	Ag-Nb2CTx	0.05 A/g	50	183	[93]
Li-S	Nb2C/Nb2O5	1.0 C	500	621	[100]
Li-S	MoS2/Nb2C	0.2 C	200	919.2	[101]
AIB	Nb2CTx	0.5 A/g	500	80	[57]

lists the performance of Nb₂CT_x with its derivative materials for secondary batteries by category.

The restacking of Nb₂CT_x will impair the electrochemical properties, by hindering the transport of electrolyte ions inside the electrode and reducing the utilization of active sites in Nb₂CT_x electrode materials. the functional groups also greatly affect the electrochemical properties. In this section, different applications of Nb₂CT_x in secondary batteries have been introduced, to provide solutions or strategies in improving the electrochemical performance of Nb₂CT_x. Methods of intercalation in Nb₂CT_x nanosheets can effectively inhibit the restacking, with the intercalation materials of organic compounds [82,96], carbon materials [91], and metals [93]. Regulation the functional groups of Nb₂CT_x (for example, by substituting -O functional groups for -F, and -OH [14]), can obviously improve the ability of Nb₂CT_x to store charges. Studies have also shown that, increasing the ion transport channels of Nb₂CT_x by manufacturing porous or three-dimensional structures can also effectively improve the electrochemical performance [20,76,100]. All these methods have obvious effects on the specific capacity, cycle life or rate performance of Nb₂CT_x.

4.2. Supercapacitors

Supercapacitors have been an important and widely applicated electrochemical energy storage device because of their high power density, fast charge–discharge capability and long-cycle stability [59,103]. For electrode materials of supercapacitors, the charge storage mechanism is an electrical double-layer capacitance mechanism and pseudo-capacitance mechanism [104,105]. In electric double-layer capacitive mechanism, ions store charges through electrostatic adsorption at the interface between supercapacitor electrodes and electrolytes, while in pseudocapacitive mechanism, ions store charges via fast reversible redox reactions on the surface of active materials. MXene with abundant surface functional groups, high electrical conductivity and rapid redox reactions on the surface is deemed to be an ideal pseudocapacitive material [106,107].

Nb₂CT_x is theoretically predicted to be an excellent electrode material with higher specific areal capacitance and gravimetric capacitance than Ti₃C₂T_x. Nb₂CT_x can provide redox-active Nb sites for pseudocapacitive-intercalation and the Nb element in Nb₂CT_x has a wider chemical valence or better electrochemical activity compared with that of Ti element [108]. However, the strong Van der Waals force between Nb₂CT_x sheets easily results in the aggregation of Nb₂CT_x nanosheets, leading to the degradation of electrochemical performance of Nb₂CT_x electrode. Recently, many works on Nb₂CT_x in supercapacitors have been devoted to improve the electrochemical performance. s. The electrochemical performance of Nb₂CT_x and its composites are listed in

Table 3. The specific capacities of Nb₂CT_x for supercapacitors.

Materials	Electrolyte	Capacitance	References
Nb2C/Ti3C2	1 M H2SO4	584 F/g at 2 A/g	[59]
Nb2CTx/CNT	1 M H2SO4	202 F/g at 2 mV/s	[109]
Ti3C2Tx/Nb2CTx	2 M H2SO4	370 F/g at 2 mV/s	[60]
Co3O4-Nb2CTx	6 M KOH	1016 F/g at 2 mV/s	[110]
Nb2CTx	1 M LiSO4	154 F/g at 5 mV/s	[111]
S-P-NNO/f-Nb2CTx	1 M LiPF6	157 mAh/g at 2.0 A/g	[112]

.

Nasrin et al. [59] have prepared Nb₂C/Ti₃C₂ (NCTC) heterointerface nanostructures via one-pot synergistic in situ etching of Nb₂C/Ti₃C₂. The NCTC is synchronized to enhance surface dynamics of charge storage by exposing more active sites and ion diffusion channels. With the individual and synergistic charge storage at the heterointerface, the NCTC shows a pseudocapacitive property with a specific capacitance of 584 F/g at 2 A/g. Xiao et al. [109] have mixed carbon nanotubes with Nb₂CT_x to enlarge the interlayers spacing of Nb₂CT_x sheets. The conductive CNT can improve the rate ability of Nb₂CT_x/CNT. The Nb₂CT_x /CNT//AC asymmetric supercapacitor, with Nb₂CT_x/CNT and activated carbon as electrodes, exhibits an areal capacitance of 462.0 mF/cm². Li et al. [60] have prepared flexible all-MXene hybrid films by mixing Nb₂CT_x and Ti₃C₂T_x together. With the introduction of Ti₃C₂T_x nanosheets, the interlayer spacing of Nb₂CT_x is increased and the stacking of MXene sheets is hindered. The flexible all-MXene film supercapacitor demonstrates a specific capacitance of 370 F/g at 2 mV/s and a good performance rate. As shown in Figure 9a, Shen et al. [110] have introduced Co₃O₄ into Nb₂CT_x layers using the electrostatic self-assembly approach to form a robust 2D cross-linked structure. Figure 9b shows that Co₃O₄ nanoparticles (marked in yellow circles) are uniformly distributed on Nb₂CT_x surface to suppress the self-restacking of Nb₂CT_x sheets and provide more active sites. The 2D cross-linked Co-MXene electrode shows a specific capacitance of 730 F/g at 20 mV/s (Figure 9c). Li et al. [22] have constructed Nb₂CT_x hydrogels via a 4D printing using a heat-stimulated self-assembly process, with the conducting polymer PEDOT:PSS working as the crosslinker. The 3D porous structured Nb₂CT_x hydrogels, shows with enhanced mechanical strengths, high specific surface area, good electrical conductivities and good electrochemical performance.

Liao et al. [111] have studied the capacitance behavior of Nb₂CT_x under cation intercalation by intercalating different cations. Compared to Na⁺ and K⁺, the insertion of Li⁺ tends to achieve better electrochemical performance and capacitance behavior. Nb₂CT_x electrode in the Li₂SO₄ electrolyte shows a higher specific capacitance of 120 F/g at 20 mV/s, due to the fast transport and easy intercalation of the small-sized Li⁺ between Nb₂CT_x layers.

Hybrid capacitors, with battery-type and capacitive-type materials as electrodes, can take into consideration high power density of supercapacitors as well as high energy density of batteries. Recently, the application of Nb₂CT_x in hybrid capacitors has attracted extensive attention. As shown in Figure 10a, Qin et al. [112] have prepared single-crystal perovskite NaNbO₃ nanocubes (S-P-NNO NCs) using a hydrothermal alkalization method with few-layer Nb₂CT_x (f-Nb₂CT_x). The f-Nb₂CT_x and S-P-NNO NCs are further assembled via freeze-drying to obtain S-P-NNO/f-Nb₂CT_x composites. Benefiting from the 3D-connected porous conductive network (Figure 10b) and synergistic effect between f-Nb₂CT_x and S-P-NNO NCs, the composites in Lithium-ion capacitors (LICs), shows a capacity of 157 mAh/g at 2.0 A/g (Figure 10c) and an energy density of 56 Wh/kg at the power density of 13 kW/kg in a full hybrid cell (Figure 10d).

At present, the experimental conditions of research on Nb₂CT_x are relatively mild and are mainly focused on aqueous electrolyte. Since the electrochemical stability of Nb₂CT_x MXene is relatively poor, designing a stable structure and optimizing application conditions to inhibit the oxidation and dissolution of Nb₂CT_x are also necessary.

4.3. Electrocatalytic Hydrogen Evolution

Electrocatalytic hydrogen evolution reaction (HER) is an important clean and efficient hydrogen production method, as hydrogen is considered to be an effective substitute for traditional fossil fuels [113,114]. Recent studies have proven that MXene-based catalysts have potential application in HER [115], or the -O functional groups on MXene surface are the active sites for HER [116]. Figure 11a provides a volcano map of the theoretical exchange current and the average Gibbs free energy of hydrogen adsorption. The predictions have shown that, the high binding strength of Nb₂CO₂ and H leads to a slightly poorer HER catalytic performance than that of Ti₃C₂O₂ [17]. Currently, some studies are devoted to weakening the hydrogen adsorption of Nb₂CT_x and improving the electrocatalytic hydrogen evolution activity.

Anchoring single transition metal atom into 2D materials is an important strategy to develop electrocatalysts with high HER performance. As shown in Figure 11b, Fan et al. [117] have immobilized platinum nanoclusters on Nb₂CT_x via mechanochemical ball milling method to get electrocatalysts for hydrogen evolution. Ultrafine Pt₃Nb nanoclusters are formed on the Pt/Nb₂CT_x composite after annealing at 600 °C (Figure 11c) and the average diameter of Pt₃Nb nanoclusters is 1.5 nm. The Pt/Nb₂CT_x-600 demonstrates a low overpotential of 46 mV and 5 mV at the current density of 100 mA/cm² and 10 mA/cm², respectively (Figure 11d). After 20 h test, the Pt/Nb₂CT_x-600 also shows 5% degradation (Figure 11e). The good HER property can be attributed to more active sites and homogenous Pt dispersion after the thermal treatment and mechanochemical process. Zhang et al. [118] have studied the effect of surface functionalization of Nb₂CT_x-supported Pd catalysts for HER performance. Compared with oxygen-rich materials, the F-rich Nb₂CT_x-based Pd nanocatalyst shows better HER property and long-term stability. The Pd/Nb₂C-HF shows an overpotential of 34 mV at 10 mA/cm², with the Tafel slope of 34 mV/dec. The improved HER property of Pd/Nb₂C with F-rich terminated catalyst can be attributed to the weakened adsorption of hydrogen, which make the evolution of hydrogen easier. Zhao et al. [23] have anchored Co single atoms on Nb₂CT_x to obtain Co@ Nb₂CT_x composites. Because of the stronger hybridization between surface-terminated O_2p orbitals of Nb₂CT_x and Co_3d orbitals, the Co atoms in the Co@ Nb₂CT_x have an optimized electronic structure, leading a lower HER energy barriers and accelerated catalytic kinetics.

Except for anchoring transition metals, other strategies to prepare effective catalysts are also adopted. Jiang et al. [24] have tailored 2D Nb₂CT_x nanosheets into quantum dots (QDs), with active sites at the Nb₂CT_x QDs edges enriched. The intermediate H* (absorbed hydrogen) species can be more moderately bound with metal atoms at the edge of Nb₂CT_x QDs, delivering a lower energy barrier. Nb₂CT_x QDs exhibits 6.8 times higher activity than pure Nb₂CT_x nanosheets and has good stability.

4.4. Photocatalytic H₂ Evolution

Nb₂CT_x is an ideal candidate for photocatalyst because of its rich functional groups, hierarchical lamellar structure and good photocatalytic properties.

As shown in Figure 12a, Huang et al. [64] have prepared 2D CdS/2D Nb₂CT_x composite using the electrostatic self-assembly and solvothermal reaction. After the hydrothermal reaction of 2D CdS with Nb₂CT_x, the 2D CdS nanosheets will cover Nb₂CT_x, and form a flower-like morphology (Figure 12b). The photocatalytic H₂ evolution rate of CdS/Nb₂CT_x composite is about 5040 μmol/g/h, which is 4.3 times higher than that of pure 2D CdS (Figure 12c). In CdS/Nb₂CT_x composite, Schottky heterojunction formed can the redistribution of charges on this 2D CdS/2D Nb₂CT_x composite and accelerate the photo-generated charge separation and transfer of 2D CdS nanosheets, thus to promote the water splitting reaction with more photo-generated electrons. Cui et al. [16] have synthesized ultrathin Bi₂WO₆/Nb₂CT_x composite photocatalyst using the hydrothermal method. The Nb₂CT_x flakes can greatly improve the separation efficiency of photogenerated carriers and boost the photocatalytic activity of Bi₂WO₆. Through in situ calcination, Makola et al. [119] have prepared a metal-free Nb₂CT_x@g-C₃N₄ Schottky junction photocatalyst. The heterostructure exhibits a narrow energy bandgap and can significantly suppress electron recombination rate, indicating potential application in the photocatalytic. Peng et al. [25] have synthesized Nb₂O₅ nanorod arrays on Nb₂CT_x nanosheets via hydrothermal method, followed by photodepositing Ag nanoparticles to form hierarchical composites. The Ag/Nb₂O₅@Nb₂CT_x nanohybrid exhibits 3.6 times higher photocatalytic H₂ evolution rate than Nb₂O₅ nanoparticles. The performance can be explained using the photogeneration efficient of electron-hole pairs provided by Nb₂O₅ nanorods and abundant active sites provided by Ag nanoparticles. Tayyab et al. [120] have synthesized In₂S₃/Nb₂O₅/Nb₂C (INNC) via a one-pot in situ hydrothermal method. During the hydrothermal process, Nb₂CT_x flakes are partial oxidized into Nb₂O₅ nanorods, and In₂S₃ nanoparticles are chemically anchored on Nb₂CT_x flakes and Nb₂O₅ nanorods. Due to the heterojunction of In₂S₃/ Nb₂O₅, INNC composite has an improved photoexcited charge carrier transfer with a higher photocatalytic H₂ evolution rate (68.8 μmol/g/h) than Nb₂O₅/Nb₂C or In₂S₃. Xu et al. [121] have synthesized Ru/Nb₂O₅@Nb₂C using one-pot method. During the hydrothermal reaction, Nb₂C is partially oxidized into semiconductor Nb₂O₅ nanowires and Ru³⁺ is reduced to Ru on Nb₂O₅/Nb₂C in the deposition process. The Ru/Nb₂O₅@Nb₂C with metal/semiconductor/Nb₂CT_x MXene heterojunction has a reduced photo-generated carrier transfer resistance, showing improved photocatalytic HER performance of 10.11 mmol/g/h.

The photocatalytic H₂ evolution performance of Nb₂CT_x and its composites are listed in

Table 4. The specific capacities of Nb₂CT_x for Photocatalytic H₂ evolution.

Materials	H2 Production (μmol/g/h)	Sacrificial Reagent	References
Ag/Nb2O5@Nb2CTx	824.2	Methanol	[25]
1D CdS/2D Nb2CTx	5300	Lactic acid (10 vol%)	[26]
2D CdS/2D Nb2CTx	5040	Lactic acid (10 vol%)	[64]
In2S3/Nb2O5/Nb2C	68.8	Methanol	[120]
Ru/Nb2O5@Nb2C	10,110	Methanol	[121]

.

4.5. Sensors

MXene is also widely applied in sensing because of high conductivity, rich functional groups and high hydrophilicity. Nb₂CT_x, as another typical member of MXene family, has also generated wide interests for its potential use in sensors in recent years.

Zhao et al. [61] have designed an electrospinning neuron-like Nb₂CT_x/SA composite film, used as self-powered humidity sensors. The composite film can provide many nanochannels and adsorption sites for the directional ionization and transfer of water molecules. The moist-electric humidity sensor (MEHS) of Nb₂CT_x/SA composite shows fast response/recovery time, good long-term cycle stability, linearity, repeatability as well as low detection limit. Bi et al. [18] have studied the Nb₂CT_x in relative humidity (RH) measurement (Figure 13a). Microfibers and Nb₂CT_x are integrated using optical deposition for fiber-optic-based RH sensing. The RH sensing experiments show that as the RH levels increase from 18.5% to 72.4%, the transmission spectra (Figure 13b) initially blue shift with the sensitivity of −86 pm/% RH because of the change in effective refractive index of Nb₂CT_x. The transmission spectra (Figure 13c) exhibit a red shift in the RH from 72.4% to 95.4% RH, and the sensitivity is 585 pm/% RH due to the variations in Nb₂CT_x structure.

Li et al. [122] have synthesized a new heterogeneous convex fiber-tapered seven core fiber-convex fiber (CTC) probe via Nb₂CT_x, gold nanoparticles (AuNPs) and creatinase (CA) enzyme. The probe, combined with localized surface plasmon resonance (LSPR) technology, is used to test the concentration of creatinine. Since strong evanescent fields can induce the LSPR effect and stimulate AuNPs, the sensitivity of this probe increases. This sensor has a sensitivity of 3.1 pm/µM as well as limit of detection, which is 86.12 µM at the linear from 0 to 2000 µM. Wang et al. [28] have prepared a novel Nb₂CT_x/PANI sensor to efficiently detect NH₃ (Figure 14a). The gas-sensing response of the Nb₂CT_x/PANI-2 sensor (301.31%) is about 2.34 times higher than PANI sensor (128.81%), upto 100 ppm NH₃ under 87.1% RH (Figure 14b). The p-n junction is favorable to the enhancement of the gas-sensing properties of the Nb₂CT_x/PANI sensor.

In addition to the above applications, Nb₂CT_x sensors are also used for water quality monitoring [29], NO₂ gas detection [123], fluorescence detection of heavy metal ions [27], biosensing application [56], and detection of human hemoglobin concentration [124], etc.

4.6. Other Applications

4.6.1. Electromagnetic Interference (EMI) Shielding and Microwave Absorption (MA)

Nb₂CT_x is used in EMI shielding because of its unique structure, special metallic properties and abundant natural defects [33,125,126]. Rajavel et al. [126] have prepared a mixed composite of lamellar-structured Nb₂CT_x and one-step-extracted low-stratified Nb₂CT_x for EMI shielding. The composite material shows good EMI shielding efficiency of 44.09 ± 1.99 dB at 12 GHz. Similarly, Nb₂CT_x has also been used for microwave absorption (MA) [127,128]. Song et al. [119] have synthesized Nb₂O₅/ Nb₂CT_x hybrids using microwave-assisted hydrothermal method. The hybrids shows good microwave absorption performance and a minimum reflection loss (RL) vof −44.1 dB at 2.8 GHz with the thickness of 5 mm.

4.6.2. Photodetector

Zhang et al. [67] have grown Bi NPs on the Nb₂CT_x surface to prepare Nb₂CT_x@Bi Schottky heterojunction. The Nb₂CT_x@Bi flexible photodetector has unique self-powered photodetection capability, and the response and recovery speed is both 0.08 s. In addition, Nb₂CT_x@Bi shows a specific detectivity of 4.63 × 10¹² Jones and a responsivity of 585.25 mA/W. Because of the reduced charge transmission time as well as the Schottky barrier and built-in electric field at the interface of Bi and Nb₂CT_x, the uniflow spreading process of electrons is accelerated, the separation of photo-induced electron/hole pairs is facilitated and carrier recombination dynamics is hindered. Therefore, Nb₂CT_x@Bi shows improved photodetection capability.

4.6.3. Perovskite Solar Cells (PVSCs)

Due to adjustable optoelectrical properties and numerous surface functional groups, Nb₂CT_x has also been used in Perovskite solar cells [30,129]. Zhang et al. [129] have reduced the work function (WF) by replacing -F groups with -NH₂ groups via hydrazine treatment and obtained T-Nb₂CT_x. The T-Nb₂CT_x electron transport layer (ETL) shows outstanding optoelectrical properties, highly matched energy level with perovskite layer, high transmittance and conductivity. After incorporating T-Nb₂CT_x into the perovskite precursor, hydrogen bond with iodine ions is formed to retard the crystallization rate. The T-Nb₂CT_x as ETL achieves a high power conversion efficiency (PCE) of 21.79% with good stability.

In addition to the above applications, Nb₂CT_x is also used in nitrogen reduction reaction (NRR) catalysts [130], dye adsorption [34], solar-driven water evaporation [131], H₂O₂ synthesis catalysts and organic dyes degradation [132].

5. Summary and Outlook

This review has summarized the synthesis and properties of Nb₂CT_x as well as its different applications in secondary batteries, supercapacitors, electrocatalysis and sensing. The wide application of Nb₂CT_x is attributed to its high conductivity, high theoretical specific capacity, abundant functional groups and high hydrophilicity. At present, the study of Nb₂CT_x is at the preliminary stage, as there are many challenges related to synthesis, properties and application which should be fixed before it can be studied further. This review also deals with some potential application such as EMI, MA and photodetector, that needs to be focused on.

In terms of synthesis, Nb₂CT_x is etched using mainly acid etching methods, especially the high concentration HF etching method, which is effective but harmful to the humans and the environment. Therefore, exploring fluorine-free etching methods is essential. Both molten salt etching and electrochemical etching have been reported as fluorine-free etching methods, but the experimental conditions and parameter details of these methods still need to be fully discussed. In addition, the delamination of Nb₂CT_x is a necessary step to get single-layer or few-layer Nb₂CT_x nanosheets. Most of the intercalation currently used are organic bases, such as TMAOH and TPAOH. Nonetheless, further exploration is needed to develop low-cost and harmless intercalants.

Because of the Van der Waals force between Nb₂CT_x sheets, Nb₂CT_x nanosheets tend to be aggregated or restacked to form a dense structure, sacrificing the specific surface area and active sites. When using as an electrode in energy storage, it will lead to poorer cycling stability, lower specific capacitance value and rate capability. Therefore, more work should be carried out to suppress the self-stacking of Nb₂CT_x in order to increase the specific surface area, provide more active sites, and optimize the electrochemical utilization of Nb₂CT_x.

Treatment of surface functional groups (alkali treatment or heat treatment) is an effective way to tune the property of Nb₂CT_x. Specially, increasing -O functional groups and reducing -F functional groups on Nb₂CT_x can greatly improve the electrochemical performance. Studies have shown that after treating with alkali (KOH, LiOH) or transition metal salts (CuSO₄, FeSO₄), the Nb₂CT_x or Nb₂CT_x composites tends to form a porous structure, with increasing specific surface area and more active sites. It is found that introducing carbon materials (CNTs and CQDs), conductive polymers, metal oxides or other metal cations into the Nb₂CT_x layers can affect the interlayer spacing of Nb₂CT_x, inhibiting the self-stacking of Nb₂CT_x. In addition, single-atom doping, designing heterostructures, and constructing 3D porous structures are also effective methods to improve the electrochemical reaction active sites and enhance the electrochemical performance.

In terms of application, Nb₂CT_x is widely used in secondary batteries and supercapacitors. However, the mechanism of charge storage needs to be studied more in order to lay a solid foundation to guide future research or commercial application. Moreover, at present, there is no research on some areas of Nb₂CT_x applications. For example, aqueous zinc metal (Zn⁰) batteries (LAMBs) are among the most promising beyond-lithium technologies [133]. However, not many studies have been conducted to assess the application of Nb₂CT_x in LAMBs and as such it needs to be further carried out and developed. Additionally, application of Nb₂CT_x in the fields of biology, medicine, photoelectric detection and electromagnetic shielding also need more attention and effort.