An Overview of Recent Advances in the Synthesis and Applications of the Transition Metal Carbide Nanomaterials

Good stability and reproducibility are important factors in determining the place of any material in their respective field and these two factors also enable them to use in various applications. At present, transition metal carbides (TMCs) have high demand either in the two-dimensional (2D) form (MXene) or as nanocomposites, nanoparticles, carbide films, carbide nano-powder, and carbide nanofibers. They have shown good stability at high temperatures in different environments and also have the ability to show adequate reproducibility. Metal carbides have shown a broad spectrum of properties enabling them to engage the modern approach of multifacet material. Several ways have been routed to synthesize metal carbides in their various forms but few of those gain more attention due to their easy approach and better properties. TMCs find applications in various fields, such as catalysts, absorbents, bio-sensors, pesticides, electrogenerated chemiluminescence (ECL), anti-pollution and anti-bacterial agents, and in tumor detection. This article highlights some recent developments in the synthesis methods and applications of TMCs in various fields.


Introduction
Metal carbides, particularly transition metal carbides (TMCs), are among the good additions in the world of materials. TMCs can be of different size and morphology. Among these TMCs, nano-particles, 2D carbides, and composites have gained much attention due to their various applications [1].
Carbon is one of the most abundant material on Earth and has the ability to show various hybridization states. Therefore, synthesis of carbon nanomaterials showing versatile electrical, thermal, mechanical and chemical properties are much in focus. Carbon-based materials, like carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, and MXenes are synthesized by various methods and their applications are examined in different fields [2,3].
In 2011, the first MXene was reported and the most popular ones are Ti 2 CT x , Ti 3 C 2 T x , and Nb 4 C 3 T x [1]. Until now, 70 different MXenes have been synthesized, among them only a few are well developed [4]. MXenes belongs to the 2D class of carbides, which can have different transition metals. MXenes having two or more transition metals that can exist in two different forms, i.e., ordered phases or solid solutions. In solid solutions, the transition metals are arranged randomly, while in the ordered phase one transition metal layer lies between the layers of another transition metal [1]. Transition metal-containing carbides, particularly tungsten carbide in comparison with Pt/Ru, are low cost with high durability as anode electrocatalysts [5,6]. The position of the transition metal in the periodic table and its ability to bear defects help to form the carbides with various 2.1. Synthesis of 2D Carbides 2.1. 1

. Acid Etching Method
The etching of aluminum (Al) from the Ti 3 AlC 2 phase was done by using HF solution [8,[37][38][39] or a solution mixture of HCl and LiF [1,40], or ammonium fluoride or ammonium hydrogen bifluoride [1,41] and fluoride containing etchant or heating [39]. The purposed reaction mechanism was demonstrated as below [39]: Ti 3 C 2 + 2H 2 O → Ti 3 C 2 (OH) 2 + H 2 (2) Ti 3 C 2 + 2HF → Ti 3 C 2 F 2 + H 2 After the selective etching process, the resulting mixture was washed with ultrapure distilled water until the pH of the solution became neutral or near neutral. Then it was separated and dried. The time period, reaction conditions, and concentrations of solutions for the etching were dependent on the size of MAX phase and the temperature of the reaction. The M-Al bond energies also played an important role in determining the reaction parameters [1,8]. From the 10 different A elements of MAX phase, which belongs to groups 13 and 14, only Al was successfully etched to form MXenes. The process of etching was kinetically controlled and a different time period was required for each kind of MAX phase to achieve MXene [1,37]. Vanadium carbide MXenes were also synthesized by HF etching [42]. A schematic view of acid etching is depicted in Figure 1.
The samples obtained by these methods are characterized by different techniques. Clear exfoliation of particles was observed in SEM results. The results also showed that these layers were almost parallel to each other, which clarified that the etching was done in a certain orientation [8]. V 4 C 3 MXene also showed increased d-spacing than the parental MAX phase. The results showed that the synthesized samples were electronically transparent and the thickness of the sample and transparency were inversely proportional to each other [42]. MAX phase. The results showed that the synthesized samples were electronically transparent and the thickness of the sample and transparency were inversely proportional to each other [42].

Etching with Molten Salts
'A' elements can be replaced in the MAX phase at high temperatures with the help of molten salts. With the help of the replacement reaction between the Al-based MAX phase and ZnCl2 at 550 °C, Al is replaced by Zn, which gives a Zn-based MAX phase. When excess ZnCl2 is used, a Cl-terminated MXene is obtained, like Ti2CCl2 and Ti3C2Cl2. Thus, these Lewis acidic melts could etch Al from MAX phases providing a variety of MXene based on redox-controlled mechanisms [43]. It is found that the lattice parameters of MXene etched by salt are greater than the one etched by acid and the layer spacing is also increased in salt-etched MXene [44].

Acid Etching and Intercalation
Ti3AlC2 powder was acid etched by HF solution. The grams (10 g) of the sample was immersed in 100 mL of 40-49% HF solution for 1 min at different temperatures. Samples were then centrifuged, washed, dried, and intercalated with dimethylsulfoxide (DMSO), urea, NH4OH 0.5 g MXene in 10 mL DMSO, 3 g urea, and 6 mL (25-28%) NH4OH was used accordingly. The mixture was then magnetically stirred for 18 h, 24 h, and 2 h, respectively. Samples were washed and oven-dried and then they were characterized by various techniques which showed that high temperature and high concentration of HF were favorable for MXene synthesis and that intercalation of small molecules increased the d-spacing [45] (Figure 2).  24 h, very high magnification to show thin layers without balls and the layer thickness is ~30 ± 5 nm [45] (reproduced from Li et al. [45] with permission from Elsevier).

Etching with Molten Salts
'A' elements can be replaced in the MAX phase at high temperatures with the help of molten salts. With the help of the replacement reaction between the Al-based MAX phase and ZnCl 2 at 550 • C, Al is replaced by Zn, which gives a Zn-based MAX phase. When excess ZnCl 2 is used, a Cl-terminated MXene is obtained, like Ti 2 CCl 2 and Ti 3 C 2 Cl 2 . Thus, these Lewis acidic melts could etch Al from MAX phases providing a variety of MXene based on redox-controlled mechanisms [43]. It is found that the lattice parameters of MXene etched by salt are greater than the one etched by acid and the layer spacing is also increased in salt-etched MXene [44].

Acid Etching and Intercalation
Ti 3 AlC 2 powder was acid etched by HF solution. The grams (10 g) of the sample was immersed in 100 mL of 40-49% HF solution for 1 min at different temperatures. Samples were then centrifuged, washed, dried, and intercalated with dimethylsulfoxide (DMSO), urea, NH 4 OH 0.5 g MXene in 10 mL DMSO, 3 g urea, and 6 mL (25-28%) NH 4 OH was used accordingly. The mixture was then magnetically stirred for 18 h, 24 h, and 2 h, respectively. Samples were washed and oven-dried and then they were characterized by various techniques which showed that high temperature and high concentration of HF were favorable for MXene synthesis and that intercalation of small molecules increased the d-spacing [45] (Figure 2). MAX phase. The results showed that the synthesized samples were electronically transparent and the thickness of the sample and transparency were inversely proportional to each other [42].

Etching with Molten Salts
'A' elements can be replaced in the MAX phase at high temperatures with the help of molten salts. With the help of the replacement reaction between the Al-based MAX phase and ZnCl2 at 550 °C, Al is replaced by Zn, which gives a Zn-based MAX phase. When excess ZnCl2 is used, a Cl-terminated MXene is obtained, like Ti2CCl2 and Ti3C2Cl2. Thus, these Lewis acidic melts could etch Al from MAX phases providing a variety of MXene based on redox-controlled mechanisms [43]. It is found that the lattice parameters of MXene etched by salt are greater than the one etched by acid and the layer spacing is also increased in salt-etched MXene [44].

Acid Etching and Intercalation
Ti3AlC2 powder was acid etched by HF solution. The grams (10 g) of the sample was immersed in 100 mL of 40-49% HF solution for 1 min at different temperatures. Samples were then centrifuged, washed, dried, and intercalated with dimethylsulfoxide (DMSO), urea, NH4OH 0.5 g MXene in 10 mL DMSO, 3 g urea, and 6 mL (25-28%) NH4OH was used accordingly. The mixture was then magnetically stirred for 18 h, 24 h, and 2 h, respectively. Samples were washed and oven-dried and then they were characterized by various techniques which showed that high temperature and high concentration of HF were favorable for MXene synthesis and that intercalation of small molecules increased the d-spacing [45] (Figure 2).  24 h, very high magnification to show thin layers without balls and the layer thickness is ~30 ± 5 nm [45] (reproduced from Li et al. [45] with permission from Elsevier). (a) Twenty-four hours, high magnification to show the fully exfoliated grain; (b) twenty-four hours, high magnification to show thin layers with small balls with the diameter of 40 ± 10 nm; (c) 24 h, very high magnification to show thin layers without balls and the layer thickness is~30 ± 5 nm [45] (adapted with permission from Li et al. [45]. Copyright Elsevier).
Among all the listed methods for the synthesis of 2D carbides, the acid etching method is the simplest one, and by adjusting the conc. of acid, better MXene layers will be obtained. Among different acids that can be used in this method, HF gave good results. This method is found successful for different sizes of MAX phase with different "A" elements. Another Nanomaterials 2021, 11, 776 5 of 34 advantage associated with this method is obtaining a parallel layer MXene structure. In the high-temperature etching method, one should require a sophisticated apparatus to attain high temperature and vacuum, also a drawback associated with this method is that usually, the shape of the MXene may be damaged. An increase in d-spacing changes the crystal structure of MXenes and also affects their physical properties. To obtain MXenes with different properties, intercalation with different molecules can be done by acid etching and intercalation technique.

Electrochemical Etching
As the fluoride-based etchants are highly toxic, other methods which are greener in approach are highly in demand. Electrochemical etching is fluoride-free and used to etch Ti 2 AlC into Ti 2 CT x . For this MAX phase is kept in HCl (2 M) at 0.6 V for five days. The drawback associated with it is that etching occur only at the surface of MAX phase. This problem is addressed by another group suggesting the use of binary aqueous electrolyte of ammonium chloride (1 M) and tetramethylammonium hydroxide (0.2 M) with pH above 9. MAX phase can be used as both the anode and cathode but etching occurs only at the anode. In this process Clrapidly etch Al because of its strong binding ability. The cations intercalate and increase the d-spacing [43].

One-Pot Solvothermal Method
The nano-composite of Ni/Ni 3 C/Ni 3 N was synthesized by the one-pot solvothermal method. Under magnetic stirring, the precursors, Ni(NO 3 ) 2 .6H 2 O and triphenylphosphine oxide (TPPO) were mixed in ethylene glycol and hydrazine hydrate was added to the solution dropwise. After 15 min stirring, the solution was then transferred into an autoclave at 200 • C for 24 h. After that, the product was obtained, washed, and vacuum oven-dried. Characterization results showed that the size of the nano-composite was increased from 14 to 21 nm with an increase in hydrazine amount, the elemental analysis showed the presence of carbon and nitrogen and it was found that the nitrogen content decreased with an increase in hydrazine hydrate. SEM results showed that the nano-composite has spherical morphologies [46].
Nanoparticles of TMCs on rGO were synthesized by adding metal precursor and rGO, in 25 wt% of composite in the solvent, i.e., 2-methyl-1-propanol. The components of the reaction were sonicated and transferred into a cylindrical reactor of stainless steel. The reactor was sealed and heated in a sand bath for 1 h at 400 • C. After that, the mixture was cooled, filtered, washed, and dried in a vacuum oven overnight. The carburization of the material was conducted in the tube furnace. The formation of metal carbide from metal oxide was strongly dependent on the conditions and gas composition. The XRD patterns and SEM images showed that the Mo 2 C had a smaller crystallite size, i.e., 13.5 nm, synthesized at 650 • C in the H 2 environment, and it (Mo 2 C) was dispersed on rGO. CH 4 was used as a carbon source for the synthesis of Fe 3 C nanoparticles of size 23.3 nm which were dispersed on rGO. For the synthesis of WC or W 2 C, a 5% H 2 /Ar gas mixture was used with a slow ramp rate, i.e., 1 • C/min [19].

Electrospinning Technique
In this technique, fine fibers with very thin diameters were produced using electrostatic approaches from the polymer solution. The surface area obtained was larger than that obtained by conventional spinning methods [47].
MoC/CNF (carbon nanofiber) composites were prepared using this approach. Precursors were electrospun at a specific ratio with a constant flow rate and a specific voltage. This sample was then heated under the air atmosphere and then nitrogen atmosphere to obtain the required product which is further analyzed by different techniques. The synthesized product had a 3D porous structure, with a diameter of approximately 300 nm and lengths of various µm [48] (Figure 3).
MoC/CNF (carbon nanofiber) composites were prepared using this approach. Precursors were electrospun at a specific ratio with a constant flow rate and a specific voltage. This sample was then heated under the air atmosphere and then nitrogen atmosphere to obtain the required product which is further analyzed by different techniques. The synthesized product had a 3D porous structure, with a diameter of approximately 300 nm and lengths of various μm [48] (Figure 3).  [48] (reproduced from Lee et al. [48] with permission from Elsevier).

Simple Stirring to Obtain Ag@Ti3C2Tx
Ti3C2Tx nanosheets were prepared by acid etching of MAX phase and then the homogeneous suspension was prepared in de-ionized (DI) water by ultrasonic treatment. AgNO3 solution was added slowly under continuous stirring. After 10 min of stirring, the resulting suspension was centrifuged and washed to obtain Ag@Ti3C2Tx. The surface morphology studied by SEM and XRD was used to confirm the synthesis of nanocomposites. The chemical bonding between Ag and nanosheets was confirmed by XPS [49].
For the synthesis of nano-composites of MXenes different approaches can be used. The easiest one and green approach is the simple stirring of MXene and the desired element salt solution in DI water. But the one-pot solvothermal approach has an advantage over it, i.e., by changing conditions, different morphologies, and size of metal carbides will obtain. With the help of the electrospinning technique, large surface area thin fibers are obtained. In the carbothermal synthesis of metal carbides, direct carburization of metal was done with some carbon source at elevated temperature [50]. Generally, the insertion of carbon in metal structure from any carbon source was done at elevated temperature, i.e., more than 1200 °C [28]. The general reaction for the diffusion of carbon from any carbon source into the metal structure is given as follows [28].

Synthesis of TMCs
Carburization, when done with the gaseous source of carbon had the main advantage of low temperature, i.e., 750 °C or above and another advantage it had over solid phase carbon source carburization was that it gave well-defined composites, which provided good stability and reproducibility [28]. When a well-defined carbon source like graphene is used then metal carbides had customized dimensions and specific properties. Graphene also served as a growth substrate [50].  [48] (adapted with permission from Lee et al. [48]. Copyright Elsevier).

Simple Stirring to
Obtain Ag@Ti 3 C 2 T x Ti 3 C 2 T x nanosheets were prepared by acid etching of MAX phase and then the homogeneous suspension was prepared in de-ionized (DI) water by ultrasonic treatment. AgNO 3 solution was added slowly under continuous stirring. After 10 min of stirring, the resulting suspension was centrifuged and washed to obtain Ag@Ti 3 C 2 T x . The surface morphology studied by SEM and XRD was used to confirm the synthesis of nanocomposites. The chemical bonding between Ag and nanosheets was confirmed by XPS [49].
For the synthesis of nano-composites of MXenes different approaches can be used. The easiest one and green approach is the simple stirring of MXene and the desired element salt solution in DI water. But the one-pot solvothermal approach has an advantage over it, i.e., by changing conditions, different morphologies, and size of metal carbides will obtain. With the help of the electrospinning technique, large surface area thin fibers are obtained.

Carbothermal Method Direct Carburization of Metal
In the carbothermal synthesis of metal carbides, direct carburization of metal was done with some carbon source at elevated temperature [50]. Generally, the insertion of carbon in metal structure from any carbon source was done at elevated temperature, i.e., more than 1200 • C [28]. The general reaction for the diffusion of carbon from any carbon source into the metal structure is given as follows [28].
Carburization, when done with the gaseous source of carbon had the main advantage of low temperature, i.e., 750 • C or above and another advantage it had over solid phase carbon source carburization was that it gave well-defined composites, which provided good stability and reproducibility [28]. When a well-defined carbon source like graphene is used then metal carbides had customized dimensions and specific properties. Graphene also served as a growth substrate [50].
α-Molybdenum carbide was also synthesized by a direct carbothermal method using carbon black as a carbon source and MoO 3 as a metal precursor [51]. Face centered cubic η-MoC 1-x and hexagonal close-packed based β-Mo 2 C had been synthesized using MoO 3 and propane gas as carburization source. η-MoC 1-x was produced via direct carburization when excess propane (carbon source) was used and it changed into β-Mo 2 C after treatment with hydrogen, XRD pattern showed the transformation from fcc carbide to hcp carbide [52].

Carburization of Metal Plates
The carburization of plates of Cr, W, Mo, Nb, and Ta was done by using CH 4 as a carbon source in H 2 (20% vol) with a flow rate of 300 mL/min and temperature range 800-1000 • C [28]. Whereas Ta, V, W, Mo, and Nb carbide synthesis was done by using melamine as a starting material. The temperature varied from 1100 to 1200 • C and the molar ratio of reactants was from 4:1 to 8:1 [53].
Characterization of these carburized plates was done by XRD. The results showed that the metal was converted into their respective carbides while chromium showed two carbide phases, i.e., Cr 3 C 2 and Cr 7 C 3 [28]. Characterization results confirmed the conversion of oxides into respective carbides with an average size starting from 5 nm (V 2 O 5 ) to 180 nm (MoO 3 ) [53].

Carburization of Metal Wires
The coating of carbide on transition metal wires was done in a two-step process. The metal wires were treated initially with O 2 (20%) in the Ar atmosphere at a suitable temperature and then the carburization was done by using methane (25%) in H 2 . This was also done in the temperature range of 700 to 1000 • C. During the process of carburization, transition metal wires changed their surface color which is the visual indication for a successful process [29]. A scheme for Carburization of metal wires is depicted in Figure 4.
α-Molybdenum carbide was also synthesized by a direct carbothermal method using carbon black as a carbon source and MoO3 as a metal precursor [51]. Face centered cubic η-MoC1-x and hexagonal close-packed based β-Mo2C had been synthesized using MoO3 and propane gas as carburization source. η-MoC1-x was produced via direct carburization when excess propane (carbon source) was used and it changed into β-Mo2C after treatment with hydrogen, XRD pattern showed the transformation from fcc carbide to hcp carbide [52].

Carburization of Metal Plates
The carburization of plates of Cr, W, Mo, Nb, and Ta was done by using CH4 as a carbon source in H2 (20% vol) with a flow rate of 300 mL/min and temperature range 800-1000 °C [28]. Whereas Ta, V, W, Mo, and Nb carbide synthesis was done by using melamine as a starting material. The temperature varied from 1100 to 1200 °C and the molar ratio of reactants was from 4:1 to 8:1 [53].
Characterization of these carburized plates was done by XRD. The results showed that the metal was converted into their respective carbides while chromium showed two carbide phases, i.e., Cr3C2 and Cr7C3 [28]. Characterization results confirmed the conversion of oxides into respective carbides with an average size starting from 5 nm (V2O5) to 180 nm (MoO3) [53].

Carburization of Metal Wires
The coating of carbide on transition metal wires was done in a two-step process. The metal wires were treated initially with O2 (20%) in the Ar atmosphere at a suitable temperature and then the carburization was done by using methane (25%) in H2. This was also done in the temperature range of 700 to 1000 °C. During the process of carburization, transition metal wires changed their surface color which is the visual indication for a successful process [29]. A scheme for Carburization of metal wires is depicted in Figure 4. XRD was performed by grinding the outermost layers of carbide coated wires. It was found that a good coating with no crystalline impurity was done. SEM was carried out to determine the thickness of the carbide coating. In Mo2C the thickness of carbide coating was 6 μm whereas; the other three, i.e., WC, TaC, and NbC showed coating thickness in the range of 42-52 μm [29]. XRD was performed by grinding the outermost layers of carbide coated wires. It was found that a good coating with no crystalline impurity was done. SEM was carried out to determine the thickness of the carbide coating. In Mo 2 C the thickness of carbide coating was 6 µm whereas; the other three, i.e., WC, TaC, and NbC showed coating thickness in the range of 42-52 µm [29].

Solid-State Reaction Method
The Scheelite ore (CaWO 4 ), which was the starting material for this reaction, was ground in the ball mill for about 40 h to reduce its particle size. It was then milled with charcoal (which is almost double the amount of scheelite) for 50 h and 100 h separately. The powder was then recovered, in the form of pellets, and calcined at various temperatures under the Ar atmosphere. The samples were then characterized by XRD which showed both the peaks of WC and CaWO 4 (high intensity). The 100-h milled sample, which was calcined at elevated temperature and then leached with HCl (1:1), followed by NaOH showed the enhanced peaks of WC [54].
Another green approach used to synthesize tungsten carbide was grinding the ammonium carbonate and ammonium metatungstate, and then dissolved in deionized water in a 10% mass percentage. After 72 h, needle-like crystals appeared which were calcined at 600 • C in the air for 2 h. CO/H 2 mixture in a temperature-programmed gas-solid reaction was then used to synthesize the mesoporous hollow crystalline needle-like tungsten carbide particles. Initially, the temperature was kept at 400 • C and then it was raised to 800 • C for 2 h and then decreased to room temperature. The prepared sample was then characterized by XRD and its morphology was studied by SEM and thermogravimetric analysis was also performed. These nanocrystalline structures of WC material showed to have needles like crystals of 3 mm length and also showed the weight loss of about 14% due to NH 4 + and H 2 O species [55]. The XRD of various forms is given in Figure 5. charcoal (which is almost double the amount of scheelite) for 50 h and 100 h separately. The powder was then recovered, in the form of pellets, and calcined at various temperatures under the Ar atmosphere. The samples were then characterized by XRD which showed both the peaks of WC and CaWO4 (high intensity). The 100-h milled sample, which was calcined at elevated temperature and then leached with HCl (1:1), followed by NaOH showed the enhanced peaks of WC [54].
Another green approach used to synthesize tungsten carbide was grinding the ammonium carbonate and ammonium metatungstate, and then dissolved in deionized water in a 10% mass percentage. After 72 h, needle-like crystals appeared which were calcined at 600 °C in the air for 2 h. CO/H2 mixture in a temperature-programmed gas-solid reaction was then used to synthesize the mesoporous hollow crystalline needle-like tungsten carbide particles. Initially, the temperature was kept at 400 °C and then it was raised to 800 °C for 2 h and then decreased to room temperature. The prepared sample was then characterized by XRD and its morphology was studied by SEM and thermogravimetric analysis was also performed. These nanocrystalline structures of WC material showed to have needles like crystals of 3 mm length and also showed the weight loss of about 14% due to NH4 + and H2O species [55]. The XRD of various forms is given in Figure 5. Tantalum and carbon black powders were mixed in a dry mill for 10 h in stoichiometric ratios of 1:1 and 2:1 for TaC and Ta2C, respectively. Samples were cold-pressed in a cylindrical sample, in an Ar environment, using a stainless-steel combustion chamber. The SHS experiment was conducted and the initial temperature of the sample was raised before the ignition started. Combustion behavior affected the ratio of reactants taken. Activation energies were 187.4 and 299.3 KJ/mol for the combustion synthesis of TaC and Ta2C, respectively [56].
Transition metal oxide, twice in amount than calcium carbide and metallic magnesium, were taken and mixed in agate mortar after that mixture was transferred to stainless steel autoclave and kept at 600 °C for 10 h. Dark precipitates were collected and washed with dilute HCl and vacuum dried for 5 h. Pure cubic TiC and hexagonal structures of V2C and Mo2C were synthesized. The TiC samples showed a flower-like microstructure of 0.5-1 μm in diameter while V2C and Mo2C showed an average size of 50 nm [57].
Zirconium carbide nanoparticles were prepared by grinding the zirconium powder with toluene in a planetary ball mill having tungsten carbide vials and ball. The product was removed, dried, and characterized. XRD data showed that with an increase in milling Tantalum and carbon black powders were mixed in a dry mill for 10 h in stoichiometric ratios of 1:1 and 2:1 for TaC and Ta 2 C, respectively. Samples were cold-pressed in a cylindrical sample, in an Ar environment, using a stainless-steel combustion chamber. The SHS experiment was conducted and the initial temperature of the sample was raised before the ignition started. Combustion behavior affected the ratio of reactants taken. Activation energies were 187.4 and 299.3 KJ/mol for the combustion synthesis of TaC and Ta 2 C, respectively [56].
Transition metal oxide, twice in amount than calcium carbide and metallic magnesium, were taken and mixed in agate mortar after that mixture was transferred to stainless steel autoclave and kept at 600 • C for 10 h. Dark precipitates were collected and washed with dilute HCl and vacuum dried for 5 h. Pure cubic TiC and hexagonal structures of V 2 C and Mo 2 C were synthesized. The TiC samples showed a flower-like microstructure of 0.5-1 µm in diameter while V 2 C and Mo 2 C showed an average size of 50 nm [57].
Zirconium carbide nanoparticles were prepared by grinding the zirconium powder with toluene in a planetary ball mill having tungsten carbide vials and ball. The product was removed, dried, and characterized. XRD data showed that with an increase in milling time, ZrC x peaks dominate. The average size determined was less than 10 nm and TEM images showed the presence of single crystalline nanoparticles [58].
The 8-12:1 molar ratio of dicyandiamide and a given metal oxide were mixed together and pressed into the pellet then kept these into the silica ampoule which was evacuated and heated for about half an hour at 750 • C. Powder of the desired product was obtained when the ampoule was opened after attaining room temperature. The results showed that WC, VC, NbC, TaC, and MoC had particle size 4 nm, 5 nm, 15 nm, 25 nm, and 200 nm, respectively [53].

Liquid-Phase Synthesis Method
Nickel acetyl acenoate was decomposed in hot oleylamine and oleic acid. Nickel oleate was formed from Ni precursor which was then further reacted with the carbon of Nanomaterials 2021, 11, 776 9 of 34 oleylamine and oleic acid. In this method, oleic acid not only acted as a source of carbon but also as a surfactant for the controlled synthesis of Ni 3 C [59,60]. Liquid phase synthesis of NiC was done in three-neck round bottom flask using Ni-acetylacetonate hydrate and oleylamine as a precursor. 1-octadecene or octadecane was added to the reaction mixture. The reaction mixture was heated for 5 to 8 h at 250-270 • C temperature. After that the product was obtained and washed out. XRD results showed that rhombohedral Ni 3 C with loose particles were present and the size was around 18 nm according to the SEM results [60].

Spin Coating of Tungsten Nanoparticles
Tungsten carbide was prepared by the spin coating of tungsten nanoparticles onto the substrate i.e., graphite and then it was calcined at various temperatures ranging from 900-1450 • C in the Ar environment for 150 min. The characterization of different samples at different temperatures was done and it was observed that phase changes started from 1000 • C and at 1450 • C WC pure was obtained [61].

Sol-Gel Method
The formation of a solid product from the solution phase precursor when it was passed through the gel intermediate and the reactants were taken at the molecular level was known as the sol-gel process. The main advantage of this process was the low working temperature, control of product size, morphology, and porosity. MO x was synthesized in the first step, and in the second step elevated temperature was required for the synthesis of non-oxidic material [62].

Urea Glass Route
The metal salt precursor was dissolved in alcohol (ethanol) and then a specific amount of urea was added and stirred to get a completely clear solution. The time for dissolution may vary depending on the urea ratio/amount. The solution was then dried on glass and kept in the oven under an N 2 atmosphere for heat treatment at 700-800 • C. Here temperature and time played an important role in determining the final product. Additionally, it was observed that the ratio of precursor salt and the urea played a twisting role in determining the chemical composition from pure nitride to carbide. XRD results also confirmed the synthesis of the desired product [62].
In addition or in place of urea, macromolecules can be used in the derivative of the urea glass route [62].

Biopolymer Route
By dispersing the molten salt in the biopolymer, nanoparticles have been prepared. The gel here provided the source of carbon or nitrogen and it also restricted the size of the particles. The carbothermal reduction process of the intermediate iron oxide nanoparticles was done by a suitable thermal process and iron carbide nanoparticles were obtained [62].

Inorganic Precursors
Using hydroxyl propyl cellulose (HPC), carbon-coated tungsten or molybdenum carbides were synthesized. K 2 WO 4 or K 2 MoO 4 were dissolved in an HPC solution. The solutions were cooled at room temperature to get gel and then dried at 100 • C. After that, it was further heat treated and washed with H 2 SO 4 (1 M) and distilled water. The XRD results ensured the formation of WC, W 2 C, and Mo 2 C. At elevated temperature, only WC was formed. From TEM results, the observed particle size for WC was less than 100 nm while Mo 2 C showed a wide range from 5 nm to 200 nm. Thermogravimeter was used to determine carbon content by ignition loss at 470 • C, at this temperature metal carbides were not oxidized [63].

Organic Precursors
Zirconium carbide was prepared by dissolving saccharide in acetic acid at 80 • C, the solution was then cooled to room temperature and zirconium n-propoxide was then dissolved and gelling began. It was dried at 120 • C, then ground in a vibrating grinder to get fine powder. Then for carbothermal reduction, kept this mixture in Pyrex furnace in the Ar environment for 3 h at 1400-1800 • C, after that it was placed in a graphite boat. The XRD pattern confirmed the synthesis of ZrC (particle size~15 nm) and backscattering of SEM images ensured the homogeneity of the sample [64].

Solvothermal Synthesis of TMCs
Historically, this technique was defined as the synthesis in a non-aqueous solvent under the action of high temperature and pressure. However, now in the case of nanostructure synthesis due to the use of an autoclave, it was defined as the synthesis in heated solvent whereas the requirement of high pressure diminished [65].
Tantalum chloride and carbon were mixed with a mortar and pestle to get maximum homogeneity, lithium was added and the mixture was then transferred into a fused quartz test tube with a rubber stopper. All the mixing was done in the glove box. After loosening the rubber stopper, the sample was ignited in a tube furnace and after ignition, it was removed from the furnace, and water was added and allowed for air quenching. The product was separated and washed to get TaC which was characterized by different techniques to confirm the formation of TaC. Near-spherical TaC had a particle size of 25 nm. For the small-scale and large-scale synthesis of TaC, the specific surface area was 39.9 and 19.5 m 2 /g, respectively [65].

TMCs by Thermal Reduction
The precursors; polytetrafluoroethylene (PTFE), transition metal oxides (Nb 2 O 5 , V 2 O 5 , TiO 2 , and MoO 3 ), and sodium metal were kept in a stainless-steel autoclave at 600 • C in an electronic furnace for 12 h. After cooling the autoclave, the collected sample was washed with dilute HCl, water, and alcohol and was then dried in an oven at 60 • C for 5 h. Crystal structure and phase purity were verified by XRD results and FESEM, SAED, TEM, and HRTEM were used to study morphologies and microstructures. NbC showed a good crystalline form while the other three carbides showed different dimensions [66].

TMCs by Direct Element Combination
A direct combination of metal and non-metal at elevated temperature was carried out by this method. The drawbacks associated with this method were high temperature, irregular size and morphology, and a lack of mesoporosity which did not make it a popular approach. High energy consumption and significant time demand made this method non-popular [67].

TMCs by the Laser Ablation Method
Nanoparticles of iron carbide was synthesized by laser ablation method in organic liquids. For this purpose an iron plate was placed at the bottom of the cell containing solvent (tetrahydrofuran (THF, inhibitor-free), acetonitrile (AN), dimethylformamide (DMF), dimethylsulfoxide (DMSO), toluene (TOL), and ethanol (EtOH), and a pulse of about at 5 J/cm 2 at a 10 Hz repetition rate was used. In all the solvent's iron gives a yellow-brownish colloidal suspension. TEM and XRD were performed to determine the structure. Large Fe 3 C and smaller iron oxide nanoparticles were obtained in EtOH [68].
Silicon carbide nanowires were prepared by the laser ablation method. SiC target (25 mm × 25 mm × 5 mm) was placed in the alumina tube at the center of furnace and a graphitic substrate placed inside the alumina tube was kept at the one end of the furnace. Temperature was set at 900 • C. The laser beam focused on the target for 2 h and product on substrate was then examined by SEM and TEM. Results showed the large quantities of nanowires present on the substrate with diameter ranging from 59 to 110 nm [69].
Onion-like carbon-encapsulated cobalt carbide core/shell nanoparticles (Co 3 C/OLC NPs) were synthesized by laser ablation of metallic cobalt in acetone. Characterization techniques, i.e., XRD showed that Co 3 C has orthorhombic structure and TEM showed the spherical particles with size ranging from 5 nm to 45 nm [70].
For the synthesis of carbides nanoparticles, various approaches have been used. Among these, few have advantages over the other. The solid-state reaction method is the greener approach to synthesize carbides nanoparticles with reduced size. The advantages attained while using the sol-gel approach are low working temperature, control particle size, morphology, and porosity. The direct carburization method gives good results when a gaseous carbon source is used but it requires high temperature. The liquid phase synthesis method and solvothermal approaches give slightly larger-sized particles and direct element combination is unpopular due to high energy consumption, time consumption, and irregular size.

Bimetallic Carbides 2.4.1. Thermal Decomposition Method
Co(Ni)-Mo carbides were obtained when the thermal decomposition of complexes was performed under flowing nitrogen. The complexes were prepared by mixing the aqueous solution of ammonium heptamolybdate with cobalt nitrate, cobalt acetylacetonate hydrate, or hexamethylenetetramine with constant stirring. Then, the sample was heated from room temperature to 1173 K for 2 h and cooled suddenly by removing the quartz tube from the furnace, and kept for at least 2 h to prevent oxidation. The XRD results of the synthesized samples showed the formation of Co 3 Mo 3 C and Ni 2 Mo 3 C. The pure Co 3 Mo 3 C was obtained when the sample was heated at 1073 K [20].

Carbothermal Hydrogen Reduction Method
Bimetallic carbides can also be prepared with the help of activated carbon (obtained by the chemical activation of the coconut shell) and an aqueous solution of ammoniumheptamolybdate and cobalt nitrate in a rotary evaporator. After evaporation, the sample was calcined at 500 • C in the Ar atmosphere for 2 h. Then, the sample was quenched to room temperature and pure hydrogen was passed through this at a rate of 200 cm 3 /min. The XRD pattern of Co 3 Mo 3 C showed the η-carbide structure type with an average particle size of 16 nm [71].
An easy and relatively simple approach for the synthesis of bimetallic carbide is the carbothermal hydrogen reduction approach.

Synthesis of Carbide Films Epitaxial Growth at Low Temperature
For the film deposition of metal carbide, two different methods were used: 1.
Co-evaporation of metal and C 60 ; and 2.
Evaporation of C 60 and magnetron sputtering of metal under Ar pressure.
Using a Knudsen effusion cell, C 60 was introduced into the chamber and the vapor pressure was kept at 0.04-0.25 Pa. The metal was evaporated from a standard metal rod with the help of an e-beam evaporator. In the magnetron sputtering method, a DC magnetron was used. The type of metal and Me/C 60 flux ratio affects the interaction between them. At 100 • C, the films deposited had a cubic structure and were polycrystalline by the co-evaporation method and the grain size calculated was 60-100 Å. The quality of carbide films obtained by the sputtering process was better at a lower temperature in comparison with those obtained by the evaporation process. The drawback associated with this technique is that the grain size of particles obtained was almost double than those obtained by other technique. WC cubic epitaxial films were obtained by this process [72].
Zr-Si-C films were also deposited by the magnetron sputtering method. Rotatory deposition holder was used for the homogeneous composition at 350 • C under Ar plasma, while the deposition was 40-70 Å. TEM images of Zr 0 . 60 Si 0 . 33 C 0 . 07 film showed the absence of crystallinity, the films deposited were completely amorphous. XRD also confirmed the amorphous nature of the films [73].
2.6. Synthesis of Carbide Nano-Powders 2.6.1. Aqueous Synthesis (NH 4 ) 2 Cr 2 O 7 and C 6 H 12 O 6 (65 and 35 wt%, respectively), were dissolved in 100 mL deionized water at 80 • C to obtain a homogeneous solution. The solution was dried at 200 • C for 1 h to obtain the final precursor and then heated at different temperatures (800 • C, 900 • C, 1000 • C, 1100 • C) in a vacuum to obtain Cr 3 C 2 nano-powder. XRD results showed that at 800 • C no carbide formation takes place but at elevated temperatures, Cr 3 C 2 formation was confirmed. The average particle sizes obtained according to the Scherrer equation were 32 nm, 45 nm, and 93 nm, respectively, at 900 • C, 1000 • C, and 1100 • C. These powders showed good dispersion and spherical or nearly-spherical shape. The advantages of using this method were low reaction temperature and less time consumption [22]. The XRD and TEM images of nanopowders synthesized at various temperatures are shown in Figure 6.
films obtained by the sputtering process was better at a lower temperature in comparison with those obtained by the evaporation process. The drawback associated with this technique is that the grain size of particles obtained was almost double than those obtained by other technique. WC cubic epitaxial films were obtained by this process [72].
Zr-Si-C films were also deposited by the magnetron sputtering method. Rotatory deposition holder was used for the homogeneous composition at 350 °C under Ar plasma, while the deposition was 40-70 Å. TEM images of Zr0.60Si0.33C0.07 film showed the absence of crystallinity, the films deposited were completely amorphous. XRD also confirmed the amorphous nature of the films [73].

Synthesis of Carbide Nano-Powders
2.6.1. Aqueous Synthesis (NH4)2Cr2O7 and C6H12O6 (65 and 35 wt%, respectively), were dissolved in 100 mL deionized water at 80 °C to obtain a homogeneous solution. The solution was dried at 200 °C for 1 h to obtain the final precursor and then heated at different temperatures (800 °C, 900 °C, 1000 °C, 1100 °C) in a vacuum to obtain Cr3C2 nano-powder. XRD results showed that at 800 °C no carbide formation takes place but at elevated temperatures, Cr3C2 formation was confirmed. The average particle sizes obtained according to the Scherrer equation were 32 nm, 45 nm, and 93 nm, respectively, at 900 °C, 1000 °C, and 1100 °C. These powders showed good dispersion and spherical or nearly-spherical shape. The advantages of using this method were low reaction temperature and less time consumption [22]. The XRD and TEM images of nanopowders synthesized at various temperatures are shown in Figure 6.

In-Situ Synthesis
For the synthesis of chromium carbide nano-powder, Cr 2 O 3 and magnesium turnings were mixed with acetone. The reactants were kept in a stainless-steel autoclave which was then put in the electric furnace at 700 • C for various time intervals. The autoclave was then cooled and the solid product was removed and washed with 50% HCl and then distilled water. The product was then dried at 100 • C for 24 h. With continuous stirring, the dried Cr 3 C 2 was treated with 0.5 M NaOH solution for 1 h. After leaching, the sample was characterized and the XRD pattern clearly showed the presence of chromium carbide. Results showed that the reduction process of the chromium oxide was not affected by the reaction time although it was affected by increasing the carbon content. The conversion followed the following path when the carbon content was increased.
The average diameter determined was 35-50 nm. HRTEM showed the presence of an outer coating of carbon which protects the synthesized powder from the oxidative environment. This also increased the stability of the product up to 625 • C as determined by TGA [21].

Sol-Gel Using Inorganic Precursors
Zirconium carbide was synthesized by using the sol-gel method. Inorganic precursors were dissolved in the solution of ethanol and water (4:1) and pH was adjusted to 4, after the gelation process, it was attrition milled and then the aging process took 24 h. The gel was dried and ground to get ZrC which was further heated in a graphite resistance furnace at a temperature of 1100-1400 • C. Samples heated at 1100 • C showed a weak peak of ZrC but samples that were prepared at 1400 • C showed the only crystalline phase which is ZrC [74].
For the synthesis of metal carbide nano-powders both approaches have their own advantages. The aqueous synthesis approach required low temperature and less time, thus, it is more useful than the other one. While the sol-gel approach, although time-consuming, its precursors are inorganic in nature, which makes it attractive.
2.7. Synthesis of Carbide Nano-Fibers 2.7.1. Thermal Decomposition Method Thermal decomposition at 800 • C in the Ar flow of anilinium trimolybdate fibers produced Mo 2 C nano-fibers. β-form of Mo 2 C crystals with hexagonal ABAB packing were formed. The surface area of 50 m 2 /g was obtained from nitrogen sorption isotherm which indicated the presence of meso/macroscale pores. SEM results revealed that the material had nanofiber morphology with bundled and stacked fibers with an average diameter of 100 nm. TGA results showed that the material was thermally stable beyond 750 • C [75].
This method of synthesis for nano-fibers requires high temperature but the product obtained has good mesoporosity with high-temperature stability. SEM image of Mo 2 C nanofiber is shown in Figure 7.

Hydrolysis in Colloidal Solutions
MXene synthesized by Huang et al. [76] was examined for its stability in water and iso-propanol. It had been found that Ti2CTx and Ti3C2Tx were stable in iso-propanol/O2 and iso-propanol/Ar solutions, while they were unstable in water/O2 and water/Ar solutions. Within two weeks, Ti2CTx in water/O2 solution were completely degraded and Ti3C2Tx in water/Ar solution took 4 weeks, and still a very little persistence in water/Ar solution. These results showed that instead of air or oxygen, water played a key role in the degradation of MXene [76]. The most possible reaction which occurs during the degradation of MXene in water is: or 2Ti C (OH) + 11H O → 6TiO + CO + CO + 2CH + 9H [70].

Hydrolysis in Colloidal Solutions
MXene synthesized by Huang et al. [76] was examined for its stability in water and iso-propanol. It had been found that Ti 2 CT x and Ti 3 C 2 T x were stable in iso-propanol/O 2 and iso-propanol/Ar solutions, while they were unstable in water/O 2 and water/Ar solutions. Within two weeks, Ti 2 CT x in water/O 2 solution were completely degraded and Ti 3 C 2 T x in water/Ar solution took 4 weeks, and still a very little persistence in water/Ar solution. These results showed that instead of air or oxygen, water played a key role in the degradation of MXene [76]. The most possible reaction which occurs during the degradation of MXene in water is: or [70].

Volumetric Changes in Ionic Liquids
2D MXenes showed volumetric changes in liquids. Studies in two different ionic liquids (IL); one with similar ion sizes and the other with different ion sizes were carried out. EMIM-TFSI (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) having comparable ionic sizes and BMIM-BF 4 (1-butyl-3-methylimidazolium tetrafluroborate) with cations much larger than anions were used. It had been found that, in dry MXene, the d-spacing was 11.6 Å, which increased quickly when it came in contact with ionic liquid without any applied voltage, and became 13.5 Å with the first IL and 13.7 Å with the second IL. The contraction was observed when the positive potential (0.5 V) was applied and expansion was observed when negative potentials (0.5 and 1 V) were applied. Intercalation was not observed in the positive potential, whereas it was observed in the negative potential. In BMIM-BF 4 , with the increase in negative potential, the expansion of MXenes did not increase further. This might be due to the large size differences between the cations and anions. This change in volume of MXene sheets played an important role in the electrochemical behavior [77].

Tunable Electronic Properties
The electronic properties of 2D metal carbide can be altered from metallic to semiconducting by replacing the outer layer titanium with molybdenum in M 3 C 2 and M 4 C 3 MXenes. M' 2 M"C 2 and M 2 M" 2 C 3 are the 2D double transition layer carbides with M occupy the outer layer and M" fill the inner layer. Molybdenum atomic layers, which sandwich the Ti-C layers in Mo 2 TiC 2 T x and Mo 2 Ti 2 C 3 T x , changed the conductive behavior of these MXenes. Ti 3 C 2 T X was metallic in nature and its resistivity decreased with a decrease in temperature from 250 to 130 K. However, the resistivity of both the mentioned MXenes, which were semiconductor in nature, was increased with the decrease in temperature from 250 to 10 K [78].

Diffusion between Binary Carbides
The IV-B and V-B group metal carbides hafnium, tantalum, and zirconium were considered as refractory TMCs. The solid solution and their diffusion behavior of TaC-HfC, TaC-ZrC, and HfC-ZrC were studied. The composition of these solutions with different molar ratios was formed. It was found that the rate of diffusion was higher for the HfC-ZrC system and lowest for the TaC-HfC system. Zr had a higher dissolution rate into TaC than Hf at constant temperature [79].

Mechanical Properties
Mechanical properties of MXenes depend on the surface termination. It has been found that the MXene with O-terminated group have shown higher stiffness than the one with other terminated groups like F and OH. This may be because that the O-terminated MXene have smaller lattice parameters than the other ones. The number of layers in MXenes also played an important role in determining its elastic stiffness [80].

Magnetic Properties
Magnetic properties of MXenes varies with the functional groups attached to them. The electronegativity of the functional group decides the magnetic properties of MXenes [81,82]. They can be varied from ferromagnetic (FM) half metal to antiferromagnetic (AFM) metals or AFM semiconductors [81].

Superconductivity
DFT studies showed that MXenes having group 6 transition metals have superconductive behavior. Band structures of these carbides showed changes due to spin orbit coupling (SOC) and it was observed that these changes are negligible in Mo but strong in W [83]. Superconductivity of MXenes is also affected by the chemical functionalization. Below 30 K, the resistivity of Nb 2 CCl 2 MXene was increased. In the case of chalcogenides superconductivity was observed in low temperature regions, Nb 2 CS 2 (Tc~6.4 K), Nb 2 CSe (Tc~4.5 K), and Nb 2 C(NH) (Tc~7.1 K) showed superconductive behavior, whereas Nb 2 COx did not show the superconducting state [84].

Electromagnetic Interference Shielding Property
An electronic device which transmits, distributes, or uses electrical energy has the ability to create electromagnetic interference (EMI), which determines the device performance as well as the surrounding environment. Undesirable emissions are reduced and components are protected from the stray external signals by an effective EMI shielding material. MXene polymer composite showed good tensile strength and good conductivity, which is also maintained by less polymer loading. Materials that show good conductivity have large EMI shielding effectiveness (SE) values. There is a direct relation between EMI SE and electrical conductivity. In MXene, EMI shielding originates from good electrical conductivity and partially due to the layered structures of the films. Thus, it can be said that the 2D structure which has high electrical conductivity and also good electronic coupling between layers is responsible for the very high EMI shielding efficiency of the MXenes [85].

Applications
Transition metal carbides in their various forms showed a wide range of applications. A variety of their applications have been determined and improvements are continuously in demand. TMC applications were found in water splitting; a green fuel for today's energy production, HER, OER; important reactions towards energy production from water, energy storage using batteries and supercapacitors (SCs), as a catalyst in different fields, like in hydrogen production, carbon nano coil synthesis, carbon monoxide hydrogenation, hydrodeoxygenation of anisole, photo-degradation of toxic material, CO 2 capture, biosensors for pesticides, antimicrobial activity, and chemiluminescence biosensors.

Water Splitting
Plenty of water is available in Earth's crust and is found to be an important source of hydrogen, the energy source of the modern era. HER and OER are involved in the complete catalytic splitting of water. Different catalysts are involved in the reaction, i.e., HER and OER of water splitting. The time demands the development of such a catalyst or combination of catalysts that can perform both reactions in a particular environment. Pt is known for its good activity in HER and Ru/Ir is known for OER [25].
Splitting of water can be done using electricity that can be produced by renewable sources, e.g., wind or tidal energy sources. The effectiveness of the method is enhanced by using a catalyst/photocatalyst for water splitting [24].
TMCs showed their ability to split water into its components more efficiently than Pt (for HER) and Ru/Ir (for OER). TMCs also showed them capable of both HER and OER at the same time, like Ni/Mo 2 C, nano-arrays of tungsten carbide, bi-TMCs (bi-transition metal carbides). Pt is a good catalyst for HER but is expensive and in a limited amount. TMCs of various metals showed good catalytic behavior towards HER and were also costeffective, thus trying to compete with Pt. OER is an important reaction to compete for green energy production demand. The problem facing the development of an OER catalyst is toxicity tolerance and carbides showed good toxicity tolerance with good catalytic activity, e.g., coupling of MXene and graphitic carbon nitride, iron carbide, tungsten carbide, etc., showed good catalytic behavior in OER.
Ni/Mo 2 C supported on porous carbon acts as a dual catalyst for water splitting. It had been found that HER activity was being sensitive towards the valence state of the metal. In this case, Ni was found to be an important part of the catalyst towards HER. For water dissociation, it was believed that Ni +2 played an active role. This catalyst was found active for OER in alkaline media. More redox peaks of Ni, i.e., Ni +2 and Ni +3 , were observed during OER operation. From this, it was concluded that the higher OER was due to the enhanced transfer of electrons in nickel valance shell. Porous carbon supported Ni/Mo 2 C showed excellent activity for water splitting and visible bubbling of hydrogen and oxygen at respective electrodes was observed. η 10 (overpotential at 10 mA/cm 2 ) in acidic, alkaline, and neutral electrolytes observed is 210 mV, 179 mV, and 250 mV, respectively [25].
Another efficient electrocatalyst for water splitting was nano-arrays of tungsten carbide doped with nitrogen. Doping of nitrogen increased the catalytic activity of the WC. The reaction kinetics for HER was improved much with N-WC nano-arrays and it also showed very small charge transfer resistance, i.e., 0.5 ohms in comparison with hydrogen electrode. The commercial Ir/C and IrO 2 catalyst showed OER activity at 1.5 V. While OER started at 1.35 V by N-WC nano-arrays but the stability in acidic conditions for OER was needed to be improved. Splitting of water began at 1.4 V by N-doped WC-nano arrays. The electrolyte used for the set up was 0.5 M H 2 SO 4 [24].
Mo 2 C was embedded in nitrogen-doped porous carbon nanosheet and it was used for the splitting of water in alkaline media as well. It showed excellent HER activity in alkaline media. This high activity was because of the 2D hybrid structure which had high nitrogen content with the abundance of active sites and strong interaction with carbon matrix and Mo 2 C. Mo 2 C@2D-NPC (Mo 2 C-embedded nitrogen-doped porous carbon nanosheets) showed three times and 75 times higher HER activity than the Mo 2 C@NPC and com-Mo 2 C (commercial Mo 2 C), respectively. The Tafel slope calculated for these had the value 67 mV/dec, 52 mV/dec, and 46 mV/dec, respectively, for com-Mo 2 C, Mo 2 C@NPC, and Mo 2 C@2D-NPC [86].
In the field of catalysis, bi-TMC (BTMCs) had their own significance but the reduction in their size was quite difficult. Small-sized (less than 20 nm) Mo x Co x C (BTMC) was synthesized by trapping method. It was confined in the uniform carbon polyhedron (Mo x Co x C@C) and the synthesis was based on trapping [PMo 12 O 40 ] 3-(PMo 12 ) clusters into uniformed ZIF-67, which was pre-synthesized. The HER and OER activities were observed and it was observed that Mo and Co played an important role in both these activities. The calcination temperature varied and it was found that the crystallinity increased with the increase in temperature. PMo/ZIF-67-6-7N and PMo/ZIF-67-6-6N calcined under nitrogen for 4 h at 700 • C and 600 • C and they showed better activity towards OER and HER, respectively [87].
Using the green approach, corn stalks were used as a source for carbon, and carbon nanosheets were prepared on which Mo 2 C nanoparticles were strongly attached. Different concentrations of ammonium molybdate had been used and the samples were thermally treated at 900 • C. This gave an efficient bifunctional catalyst for water splitting. Nano-sized Mo 2 C and the carbon matrix had a good cooperative effect and enhanced the activity of catalyst as well as the stability. The sample which was synthesized at 0.01 M concentration of ammonium molybdate showed the Tafel slope of 70 mV/dec and 74.2 mV/dec for OER and HER, respectively [88].
MXene sheets that were wrapped on octahedral crystals of nickel selenide were used for the splitting of water. An increase in the conductive current was observed by this hybrid while pure Ni, NiSe 2 , or MXene did not show such high activity. The Tafel slope obtained by MXene sheets and NiSe 2 were 222.2 mV/dec and 46.9 mV/dec while the hybrid (NiSe 2 /Ti 3 C 2 T x ) showed 37.7 mV/dec. It had been found that the activity increased because of the charge transfer from MXene towards nickel selenide which enabled the system for efficient utilization of the active sites and also gave faster adsorption kinetics to the system [27]. The Tafel plots of various materials are shown in Figure 8 for comparison. MXene nanoparticles (Ti3C2) were synthesized by using the solvent exfoliation method and were used to synthesize heterostructures with TiO2 nanoparticles. These structures were used as photocatalysts for water splitting. The amount of hydrogen and oxygen evolved was determined, being 0.15 mL h −1 for hydrogen and 0.07 mL h −1 for oxygen, which is in the ratio of 2:1. This ensured the splitting of water. This photocatalyst was found stable in the presence of light, showing no catalyst degradation and producing a stable photocurrent density. The current density at 1.6 V was determined using TiO2, TiO2/100 μL Ti3C2, TiO2/50 μL Ti3C2, and TiO2/500 μL Ti3C2, and 0.2 mA/cm 2 , 1.96 mA/cm 2 , 1.3 mA/cm 2 , and 1 mA/cm 2 current densities were observed, respectively. The obtained current densities showed that the sample of TiO2/100 μL Ti3C2 had the best performance [89]. Various catalysts and their activities towards water splitting are shown in Table 1. MXene nanoparticles (Ti 3 C 2 ) were synthesized by using the solvent exfoliation method and were used to synthesize heterostructures with TiO 2 nanoparticles. These structures were used as photocatalysts for water splitting. The amount of hydrogen and oxygen evolved was determined, being 0.15 mL h −1 for hydrogen and 0.07 mL h −1 for oxygen, which is in the ratio of 2:1. This ensured the splitting of water. This photocatalyst was found stable in the presence of light, showing no catalyst degradation and producing a stable photocurrent density. The current density at 1.6 V was determined using TiO 2 , TiO 2 /100 µL Ti 3 C 2 , TiO 2 /50 µL Ti 3 C 2 , and TiO 2 /500 µL Ti 3 C 2 , and 0.2 mA/cm 2 , 1.96 mA/cm 2 , 1.3 mA/cm 2 , and 1 mA/cm 2 current densities were observed, respectively. The obtained current densities showed that the sample of TiO 2 /100 µL Ti 3 C 2 had the best performance [89]. Various catalysts and their activities towards water splitting are shown in Table 1.

Hydrogen Evolution Reaction
Hydrogen is a renewable energy source and an important energy carrier for technologies based on electrochemical energy conversion. Production of hydrogen from water electrolysis is a major challenge because of the involvement of high cost and less abundant catalyst. Currently, the most efficient catalyst is Pt, which shows good catalytic activity in HER. It has a long lifetime and small overpotential, the drawbacks associated with it are; it's high cost and least abundance [28][29][30][31]. To overcome issues regarding the catalyst for the electrocatalytic activity in HER, developing a new class of catalysts is in progress. The most focused class is carbides in which TMCs show good catalytic activity and more catalytic cycles like Pt, as they both have similarities in electronic properties [28][29][30][31]. TMCs, e.g., W, Nb, Ta, Cr, and Mo carbides were prepared by carburization with methane at elevated temperature. These carbide electrodes, in phosphoric acid solution (100%) electrolyte which have a highly saturated hydrogen environment, showed good catalytic activity. Group 6 elements carbides were found more efficient than group 5 elements' carbides. Among these carbides, the highest activity was shown by WC having a Tafel slope of 56 mV/dec [28]. Carbide-coated transition metal wires were prepared when carburization of pre-oxidized transition metal wires was done at elevated temperature. The activity in HER was determined in comparison with platinum and it was found that WC showed even better activity than platinum in molten KH 2 PO 4 . At −0.85 V, the current density shown by WC was −30 mA/cm 2 which was the highest current obtained at minimum voltage [29]. VC nanosheets and particles were prepared and their activity in HER was determined and compared with Pt. VC nanosheets showed more activity than VC particles because the specific surface area was large with a small pore size distribution. As a result, they had more active sites which enhanced the HER activity many times. While the VC particles had a small specific surface area whereas they bear large contact resistance, due to this they showed less activity. The VC nanosheets showed much higher activity with long life, thus, making it an excellent catalyst having outstanding HER performance with a Tafel slope of 56 mV/dec [30]. The electrocatalytic activity of tungsten carbide synthesized at different temperatures was determined in a 0.5 M H 2 SO 4 solution. The best HER activity was observed by the mixture of W, WC, and W 2 C with good stability. A current of 310 mV was observed at an overpotential of 10 mA/cm 2 [61]. It had been found that the application of monolayer Pt on carbide showed much-enhanced activity in HER and reduced the amount of Pt usage. Pd was used in the replacement of Pt because it was cost-effective and also showed its activity when methanol (alkaline conditions) was used instead of water. WC and Mo 2 C foils were used to provide support to the Pd catalyst. Both these foils provided good support for the solid catalyst and enhanced the magnitude of activity by the order of two. Both the supporting layers, i.e., WC and Mo 2 C, were similar but had a slight difference in their lattice structure and electronic properties. Due to this, the Pd on the WC sample showed more catalytic activity than the other carbide [31]. α-Mo 2 C, with a crystallite size 35 nm, at different electrode loads, was immersed in 0.1 M HClO 4 electrolyte to determine its HER activity. HER measurements were conducted, and the minimum loaded amount of catalyst on carbon support was 8.5 µg/cm 2 showed a Tafel slope of 73.8 mV, with other loaded amounts of 12.7, 16.9, 25.4, 50.8, 100.8 µg/cm 2 showing Tafel slope values of 79.6, 75.5, 75.5, 77.8, 79.0 mV, respectively. It has also been determined that along with catalyst load, catalyst dispersion also played an important role in determining the efficiency of the catalyst [51].
2D MXene sheets, which were terminated with different ratios of the functional group 'F', were examined for HER activity. It was found that HER activity was greatly varied with the percentage of 'F'. A decrease in the percentage of 'F', results in enhanced HER activity. 2D MXene terminated with LiF-HCl, 10% HF and 50% HF in 0.5 M H 2 SO 4 (electrolyte) showed the Tafel slope 128 mV/dec, 138 mV/dec and 190 mV/dec respectively. MXene sheets with different amounts of Ti also studied for HER activity. It was found that Mo 2 CT x showed a Tafel slope value of 75 mV/dec, which is less than Ti 2 CT x (88 mV/dec) and Mo 2 Ti 2 C 3 T x (99 mV/dec), and made it a more efficient electrode for HER [90].
HER activities shown by various catalysts are listed in Table 2.

Oxygen Evolution Reaction
OER is a key reaction while producing renewable green energy, e.g., hydrogen. It is coupled with a system like water splitting or metal-air batteries. In this reaction, the catalyst, which has non-precious metal and long stability, is under development. The combination of metal, nitrogen, and carbon material to develop a non-precious and effective catalyst is found to be attractive for research. In this regard, MXenes have great importance as they have hydrophilic surfaces, good electrical conductivity, and structural and chemical stability. Coupling of MXenes (Ti 3 C 2 ) and graphitic carbon nitride (g-C 3 N 4 ) because of similar 2D geometry and metal nitrogen interaction is thought to improve the electrocatalytic properties and a hybrid film (TCCN) of overlapped g-C 3 N 4 and Ti 3 C 2 was formed. TCCN showed great catalytic activity and showed good OER with excellent durability [91].
To evolve oxygen from the energy carrier reactions in fuel cells and batteries, various catalysts have received much attention. Among these, TMCs are of great importance. Lowcost catalysts with high stability are under development phases. Iron carbide, tungsten carbide, and vanadium carbide are found to have good catalytic activity. Nitrogen-doped graphene was found good support as well as a catalyst for oxygen reduction reactions. FeMo carbide supported on graphene was synthesized by nanoparticle growth on graphene oxide and then pyrolysis by urea was conducted. The electrocatalytic activity was evaluated by cyclic voltammetry (CV) and rotating disk electrode measurements. It showed good stability and toxicity tolerance along with high catalytic activity [92].

Energy Storage
In the field of energy storage, batteries and SCs play an important role. In both these devices, recharging ability is an important factor. The more the number of cycles a device can work, the better it will be. Despite this, the scaling and toxicity of batteries and SCs are also important factors to determine the life of a battery. For the improvement of energy storage devices, TMCs are being used as electrodes, either anodes or cathodes. MXene/silver composites showed recharging capacity up to 5000 cycles in batteries while polymeric composites of MXene showed 92% retention of capacitance in KOH electrolyte. Energy and charge storage mechanism in MXenes is based on intercalation and de-intercalation of ions [16,93] (the interlayer spacing is strongly affected by the nature of the ions being intercalated) [93], redox reactions at transition metal sites [94], and it depends on the electrolyte used [16,93] (good results were obtained with 1M H 2 SO 4 ) [93], either aqueous or non-aqueous [16], and the termination groups of MXenes [16].

Batteries
In the area of energy storage and its conversion, two devices are dominating, i.e., batteries and supercapacitors. Research in the field of EES (electrochemical energy storage) devices is particularly in progress. The dominating fields in this system are sodium-ion batteries (SIB), lithium-ion batteries (LIBs), and supercapacitors (SCs). The combination of good ionic and electronic conductivities in a system is desirable in EES [95].
Ti 3 C 2 T x is considered a good anode material in comparison with Ti 2 CT x . Diffusion and adsorption of lithium-ion on bare Ti 3 C 2 and Ti 3 C 2 T x (-OH, -F) was studied and it was found that lithium ions migrate easily on bare Ti 3 C 2 due to the lower energy barrier and shorter diffusion pathway in comparison with other derivatives [96]. However, for LIBs, transition metal-based MXenes having an oxidized surface were found to be the most dominating material for the anode. These materials had large surface areas, which is an important requirement for the LIBs' anode. Exfoliated MXene (Ti 2 C) had an open structure and high surface area which showed five times more reversible capacity than its precursor. The stable capacity of exfoliated material was determined to be 225 mA/g at the rate of C/25. These results encouraged the use of MXene as an intercalation electrode in LIBs [97]. TiC/NiO core/shell nano-architecture was used as an electrode in LIBs and the results showed that it has good rate capability and high overall capacity. A total of 568.1 mAh/g was the reversible capacity observed at 200 mA/g with 90% retention of capacity, observed even after 60 cycles. TiC nanowire, which had double-layer capacitive behavior with high electrical conductivity, showed many contributions towards electrical properties [98]. Further work for developing the anode structure for LIBs was the development of the spheres of MoO 2 /Mo 2 C/C. The material was synthesized by using the hydrothermal and calcination approach. The structure had porous carbon spheres and showed good rate capabilities. The electrode resistance was reduced due to good contact between MoO 2 and Mo 2 C nanoparticles and the carbon sphere. All three components worked together and provided short pathways for electron transportation, which resulted in good rate capabilities and improved cyclic performance [99].
Among the studied electrode materials for LIBs, the most studied one is titanium carbide, along this NbC, MoC, WC, and TaC in different forms are also under study [95]. In another research for determining an anode material for LIBs, MoC/C nanowires were found as a stable material with good cyclic stability and high capacity. The stacking of carboncoated MoC nanostructure resulted in the mesoporous structure which was responsible for the shortening of the path for Li + ions diffusion and also made electron transfer easy. The reversible capacity, they had is 650 mAh/g even after 350 cycles [100]. Another promising step in this field was the use of multilayer MXene as a current collector, i.e., an anode, and it was found that its performance-matched to the conventional metals, i.e., Cu or Al, but its lightweight decreased the weight and occupied space by the device [32]. MXene and its composites were also studied as a candidate for LIBs. The groups -OH or -F present on the surface of MXene limit the capacity. MXene with an -OH group (Ti 3 C 2 (OH) 2 ) had shown the capacity of 67 mAh/g and with -F group (Ti 3 C 2 F 2 ) the capacity was 130 mAh/g. The MXenes which were treated with hydrogen peroxide showed better rate capabilities and charge specific capacity was also enhanced. In composite material of MXene, multi-stacked Ti 3 C 2 /carbon nanofiber particles showed a capacity of 320 mAh/g at 1 C after 295 cycles and 2D Ti 3 C 2 T x /CNT showed enhanced storage capability and cycling stability after 100 cycles, at 500 mAh/g. A composite of MXene with metals i.e., MXene silver composite showed the specific capacity of~500 mAh/g and reversible capacities at 1 C, 310 mAh/g, and at 50 C, 150 mAh/g and it showed stability up to 5000 cycles [8]. As the Li-O 2 cell's performance and rechargeability greatly depend on the positive electrode material where ORR and OER takes place. Mo 2 C nanofibers synthesized [75] mentioned in Section 2.7.1 were used as the electrode material in the above-mentioned cells. The properties of Mo towards electrical conductivity, catalytic dehydrogenation, and electrochemical hydrogen evolution are well known. This material Mo 2 C nano-fibers was used with two different electrolytes, i.e., 0.5 M lithium bis(trifluoromethanesulfonyl)imide in tetra-ethylene glycol dimethyl ether (TEGDME) and 0.5 M LiClO 4 in DMSO. Compatibility of the current electrode with these electrolytes was studied and it was found that TEGDME showed good stability below 4 V while DMSO showed parasitic behavior when it was charged above 3.25 V, although, in DMSO, lithium superoxide was more soluble. At 100 µA/cm 2 current density, the cells were discharged to 2.4 V. It was also found that there was a close relationship between oxygen consumption and electrochemically cell discharge. Using 0.5 M of both the electrolytes, the nano-fibers showed a high potential of 4 V with a current capacity 0.5 mAh in LiTFSI-TEGDME and less potential, approximately 3.5 V, with high current capacity, greater than 1.6 mAh in DMSO [75].
Ti 3 C 2 T x hybrid films with Co 3 O 4 (hybrid sandwich) and NiCo 2 O 4 (by in situ growth and spray coating) had been prepared. These films were found to be flexible and have excellent conducive and were used directly as anodes in LIBs. No binder or current collector was required. The spray-coated Ti 3 C 2 T x /NiCo 2 O 4 was found best among these samples and showed reversible capacities of 1330, 650, and 330 mAh/g at 0.1, 5, and 10 C, respectively. In-situ grew Ti 3 C 2 T x /NiCo 2 O 4 showed the reversible capacities of 1200 and 390 mAh/g at 0.1 and 5 C, respectively. Ti 3 C 2 T x /Co 3 O 4 showed the maximum reversible capacity at 0.1 C which was~1200 mAh/g. This capacity decreased very fast and at 5 and 10 C it was 150 and 100 mAh/g, respectively. Comparative studies also showed that MXene/TMO had the advantage of superior rate performances and high volumetric capacities. This showed that smaller devices can store the same amount of energy if MXene/TMO sheets were used [101].
As sodium is abundant in the Earth's crust so it has been considered that in the future the cheapest rechargeable energy device will be the SIBs. For these batteries, the development of an electrode is a challenge. In this regard, MXene was studied and it was found that the multistage storage mechanism of Na ions was involved in OH terminated MXenes. MXene nanosheets although found unstable due to a large number of active sites but found to have a high capacity to store metal ions. Vanadium carbide-based MXenes were also studied as a positive electrode for SIBs [95]. MXene showed good rate capabilities due to the absorption of sodium onto the surface and intercalation of ions between the layers. Good rate capabilities with a specific capacity of 100 mAh/g for 100 cycles were observed. When Sb 2 O 3 nanoparticles were dispersed uniformly on the surface of MXene, the discharge capacity observed at 100 mAh/g after 100 cycles were 472 mAh/g. When the composites of MXenes with MoS 2 was formed, the observed discharge capacities were 250.9 mAh/g at 100 mAh/g after 100 cycles [8]. MXene@SnS composite showed good electrochemical properties and have good rate performance at different current densities. At 500 mA/g, the specific capacity is 320 mA/g with excellent cyclic properties [44]. The cyclic stability of various materials is given in Figure 9. excellent conducive and were used directly as anodes in LIBs. No binder or current collector was required. The spray-coated Ti3C2Tx/NiCo2O4 was found best among these samples and showed reversible capacities of 1330, 650, and 330 mAh/g at 0.1, 5, and 10 C, respectively. In-situ grew Ti3C2Tx/NiCo2O4 showed the reversible capacities of 1200 and 390 mAh/g at 0.1 and 5 C, respectively. Ti3C2Tx/Co3O4 showed the maximum reversible capacity at 0.1 C which was ~1200 mAh/g. This capacity decreased very fast and at 5 and 10 C it was 150 and 100 mAh/g, respectively. Comparative studies also showed that MXene/TMO had the advantage of superior rate performances and high volumetric capacities. This showed that smaller devices can store the same amount of energy if MXene/TMO sheets were used [101].
As sodium is abundant in the Earth's crust so it has been considered that in the future the cheapest rechargeable energy device will be the SIBs. For these batteries, the development of an electrode is a challenge. In this regard, MXene was studied and it was found that the multistage storage mechanism of Na ions was involved in OH terminated MXenes. MXene nanosheets although found unstable due to a large number of active sites but found to have a high capacity to store metal ions. Vanadium carbide-based MXenes were also studied as a positive electrode for SIBs [95]. MXene showed good rate capabilities due to the absorption of sodium onto the surface and intercalation of ions between the layers. Good rate capabilities with a specific capacity of 100 mAh/g for 100 cycles were observed. When Sb2O3 nanoparticles were dispersed uniformly on the surface of MXene, the discharge capacity observed at 100 mAh/g after 100 cycles were 472 mAh/g. When the composites of MXenes with MoS2 was formed, the observed discharge capacities were 250.9 mAh/g at 100 mAh/g after 100 cycles [8]. MXene@SnS composite showed good electrochemical properties and have good rate performance at different current densities. At 500 mA/g, the specific capacity is 320 mA/g with excellent cyclic properties [44]. The cyclic stability of various materials is given in Figure 9.  [98]. (b) Cyclic performance of MoC at a current density of 0.2 A/g [100]. Li-ion batteries (c) long-term cycling performance and coulombic efficiency of MoS2/Ti3C2-MXene @ C electrode at high current densities (Na-ion battery) [8] (reproduced from Huang et al. [98] with permission from ACS Publications and Deng et al. [100], Tang et al. [8] with permission from Elsevier).

Supercapacitors
Electrochemical SCs have attained much attention nowadays due to the high power density and cyclic stability. SCs are divided into electrochemical double-layer capacitors and pseudocapacitors, depending on the energy storage mechanisms. The former have rapid charging and discharging mechanisms and the latter has a high capacity [102].
The properties of MXenes and its composites had been determined regarding supercapacitors. Simple MXene; 2D-Ti3C2 with thickness ~40 nm showed the specific capacitance of (i) MXene (95%) and PTFE and (ii) MXene and 10% carbon black was 53 F/g and 65 F/g in 3 M KOH electrolyte, respectively. This showed that carbon black increased the Figure 9. (a) Cyclic stability and coulombic efficiency of the NiO and TiC/NiO core/shell nanoarchitecture at 200 mA/g [98]. (b) Cyclic performance of MoC at a current density of 0.2 A/g [100]. Li-ion batteries (c) long-term cycling performance and coulombic efficiency of MoS 2 /Ti 3 C 2 -MXene @ C electrode at high current densities (Na-ion battery) [8] (adapted with permission from Huang et al. [98]. Copyright ACS Publications and adapted with permission from Deng et al. [100], Tang et al. [8]. Copyright Elsevier).

Supercapacitors
Electrochemical SCs have attained much attention nowadays due to the high power density and cyclic stability. SCs are divided into electrochemical double-layer capacitors and pseudocapacitors, depending on the energy storage mechanisms. The former have rapid charging and discharging mechanisms and the latter has a high capacity [102].
The properties of MXenes and its composites had been determined regarding supercapacitors. Simple MXene; 2D-Ti 3 C 2 with thickness~40 nm showed the specific capacitance of (i) MXene (95%) and PTFE and (ii) MXene and 10% carbon black was 53 F/g and 65 F/g in 3 M KOH electrolyte, respectively. This showed that carbon black increased the specific capacitance and decreases the resistance of the electrode. The resistances were 16.7 Ω and 3.5 Ω [103]. Ti 2 CT x 75%, 20% carbon black, and 5% PTFE were used to design an electrode that had shown good ion response and highly capacitive behavior. The specific capacitance shown at the current density of 2 mV/s was 517 F/cm 3 and at the current density of 100 mV/s was 307 F/cm 3 in 1 M KOH electrolyte. At a current density 20 A/g, capacitance retention was observed up to 3000 cycles [104]. It has been found that the simple MXene showed capacitance by volume up to 350 F/cm 3 at 20 mV/s in the KOH solution, used as an electrolyte. Capacitance properties of MXene with rGO, CNT, and PVA were observed. MXene, when combined with rGO at a weight ratio of 7:1, showed the maximum capacitance of 154 F/g at 2 A/g. Between two different layers of MXene, the graphene acted as the conducting bridge with amazing retention capacity after 6000 cycles. When the composite of MXene was formed with a carbon nano-tube (single-walled), the volumetric capacitance of 345 F/cm 3 was attained with no or negligible degradation up to 10,000 cycles. When polymer composites of MXenes were formed, different properties were observed with a different polymer. With PVA, 528 F/cm 3 at 2 mV/s and 306 F/cm 3 at 100 mV/s volumetric capacitance were observed in KOH electrolyte and, with PPy, 1000 F/cm 3 volumetric capacitance with 92% retention after 25,000 cycles was observed [8].
To improve the energy storage capacity of titanium carbide, the surface of its nanosheets was decorated with pseudocapacitive materials. In this study, it was decorated with MnO 2 due to its low toxicity and low cost. Using a direct chemical synthesis approach, MnO 2 was deposited on the surface of MXene sheets. This approach increased the specific capacitance of MnO 2 . It had been found that the specific capacitance was increased by three times when the MXene sheets were loaded with MnO 2 . The obtained results strongly suggest that there was great potential in the hybrid-MXene electrode [105]. MXene nanosheets wrapped in NiSe 2 showed a higher ability as a supercapacitor compared to unmodified NiSe 2 , which was confirmed by the large discharge time and CV area. The stability of both these samples was also determined and it was found that the wrapped MXene sheets had much higher stability and its capacitance retention was 47.3% higher, even after 1000 cycles than unmodified NiSe 2 [27]. MXene nanosheets/Ni/Al layered double hydroxide (LDH) was prepared and its electrochemical properties were determined in comparison with LDH. It was found that with an increasing amount of MXene (up until an optimized amount), the dispersion of LDH became more homogeneous, forming an open and porous structure. MXene/LDH showed more specific capacities and capacitance retention at various current densities than LDH. This confirmed the increase in the electrochemical behavior of MXene/LDH [102].
2D titanium carbide sheets (MXenes) were prepared by the etching method from the MAX phase and its electrochemical measurements were determined. 447 F/g by Ti 3 C 2 and 248 F/g by Ti 2 C at a scan rate of 1 mV/s was the specific capacitance observed in 6 M KOH. These properties made MXene a suitable candidate for SCs [106]. Tantalum carbide was synthesized by etching the MAX phase and the solid, 2D-layered structure of Ta 4 C 3 was obtained, which was fabricated into the electrode to determine its properties as an electrochemical material. For this purpose, CV of the prepared sample was performed. An 89% retention rate after 2000 cycles was observed with a specific capacitance of 481 F/g at 5 mV/s in 0.1 M H 2 SO 4 . This study showed tantalum carbide as a suitable material for supercapacitors [107]. V 2 C synthesized by the acid-etched method of V 2 AlC and its films were directly used as a binder-free electrode. Over capacitive activated carbon the counter electrode and the gravimetric capacitance was determined using H 2 SO 4 , KOH, and MgSO 4 (1 M each). The specific capacitance of V 2 C obtained in a three-electrode system showed that the electrode (V 2 C) had the best performance in 1 M H 2 SO 4 with a capacitance 487 F/g at 2 mV/s, and in KOH and MgSO 4 the capacitance were 184 F/g and 225 F/g, respectively [108]. V 4 C 3 MXenes (synthesis discussed in Section 2.1.1) showed a specific capacitance of 330 F/g in 1 M H 2 SO 4 electrolyte at 5 mV/s. It also showed as high as 90% of the retention even after 3000 cycles [42].
2D-MXene nanosheets having phenyl sulfonic groups attached to their surface were synthesized and their electrochemical properties were evaluated in a three-electrode cell. The capacitance of functionalized MXene was found to be almost double than the original one and it also showed good cycling ability and 91% capacity retention after 10,000 cycles [109]. Nitrogen-modified surfaces of Ti 3 C 2 T x nanosheets greatly enhance the electrochemical performance. All nitrogen-containing functional groups played their role in the electrochemical properties, but the role of O-Ti-N (oxygen-titanium-nitrogen) was found more pronounced than Ti-O-N (titanium-oxygen-nitrogen), particularly for application in a capacitor. With enhanced cyclic performance, 415 F/g gravimetric capacitance at 2 mV/s, and 75.9% rate capacity at 200 mV/s in H 2 SO 4 (3 M) the aqueous electrolyte was determined [110]. Similar results had been noted by Wen et al., in 2017 when they used nitrogen-doped Ti 3 C 2 T x . Their results also concluded that the doping of the hetero atom played a major role in enhancing the electrochemical behavior and made it a suitable candidate for supercapacitor [111].
Bimetallic carbide was a new addition in this field. Molybdenum iron carbide was prepared by a simple hydrothermal and carbothermal method and its properties were evaluated. It was found that these methods were good in fabricating this bimetallic carbide. The improved electrochemical performance was observed due to its small size, less resistance, and large surface area. Despite great electrochemical performance, it also offered high energy density, i.e., 6.74 Wh/Kg in a system of 21 kW/kg power density [112].
For the electrochemical capacitor, carbon-coated tungsten and molybdenum carbide (synthesis discussed in Section 2.3.5.3) were used. The electrolyte was 1 mol/L H 2 SO 4 . Carbon-coated WC showed a capacitance of 350 F/cm 3 and carbon-coated Mo 2 C showed a capacitance of 550-650 F/cm 3 . As the size of particles was small, due to the coating of carbon, they were converted into respective hydroxides after the first cycle of charging/discharging [63]. W, Mo, and V carbides were synthesized by the temperatureprogrammed carburization method required activation as they were passivated to avoid oxidation on exposure to air. Before activation, W 2 C and VC were electrochemically unstable, but activation in 0.3 mol/dm 3 NaOH solution was conducted to remove the thick passivated oxygen layer. Activation not only enhances the stability in the electrochemical environment but also increases the surface area. With a total of 0.1 mol/dm 3 H 2 SO 4 , a three-electrode system was used to determine the electrochemical behavior of both passivated and activated material. VC and W 2 C were unstable before activation and after activation the CV of these materials showed no prominent redox peaks, which means charge and discharge occur at a pseudo-constant rate, while α + β Mo 2 C showed enhanced capacitance after activation, which persists even after the 2000 cycle, which means more than 24 h. The maximum capacitance of 150 F/g was observed, which was kept the same after almost 2100 cycles, i.e., 135 F/g [113]. A green and catalyst-free approach to synthesize metal carbides was laser ablation and it was used in this study to synthesize the transition metal carbide/carbon core/nanosphere shell. All the carbides in the core had a cubic phase structure. Among the synthesized samples, TaC/C/NSs (metal carbide/carbon core/nanosphere shell) had the highest energy storage capacity and their CV indicated high charge propagation within the electrode. It also showed good cycling ability and rate capability [114].
The materials used in energy storage devices are summarized in Table 3.

As a Catalyst
MXene/TMCs showed diversity in their behavior and showed activity in different fields. Some of the applications shown by them in various fields are discussed here.

Photocatalyst for Hydrogen Production
MXenes Ti 3 C 2 T x provided a 2D platform and served as a co-catalyst to assist TiO 2 nanoparticles for solar hydrogen production. Pure TiO 2 , Ti 2 CT x , Ti 3 C 2 T x , Nb 2 CT x suspensions, and TiO 2 /Ti 2 CT x , TiO 2 /Ti 3 C 2 T x , and TiO 2 /Nb 2 CT x suspensions were made and their catalytic behavior was determined. TiO 2 /Ti 3 C 2 T x , 5 wt% composites showed a good increase in hydrogen production via photocatalytic water splitting. It showed a 400% increase in photocatalytic hydrogen production as compared to pure TiO 2 . The hydrogen production rate shown by it was 18 µmol/h/g (catalyst) [115].

Carbon Monoxide Hydrogenation
The hydrogenation of carbon monoxide was carried over face-centered molybdenum carbide and face-centered tungsten carbide. When molybdenum carbide catalyst was used, initially the conversion of synthesis gas was less but increased up to 25% when the temperature was raised by 20 K. i.e., from 523 K to 543 K. When space velocity was increased at elevated temperature, a decrease in the conversion was observed. An equal amount of methane and carbon dioxide were prepared initially, but with the increase in temperature and space velocity, the percentage of carbon dioxide produced was raised. This showed that, initially, CO was converted into CH 4 and H 2 O and H 2 O further reacted with CO to give CO 2 and H 2 . This also confirmed that carbidic carbon was not used in this reaction because if it was used no water formation could happen. As a result no CO 2 formation could occur. In the case of tungsten carbide, the catalyst was not as active as molybdenum carbide, but the major product was methane and no water gas shift reaction occurred [17].

Carbon Nano-Coils Synthesis
The effect of various metal carbides on the growth of carbon nano-coils was observed. Carbon nano-coils (CNC) were twisted with a diameter of 100-600 nm and the length was 1-3 mm. TiC was found as the most effective catalyst among TaC, Cr 3 C 2 , Mo 2 C, and WC under various temperature and gas conditions. The effect of simple Ti, Ta, Mo, Cr, and W metals were also observed on the growth of CNC and it was found that they did not affect its growth [116].

Catalytic Enhanced Hydro-Deoxygenation Selectivity
Enhanced hydrodeoxygenation (HDO) selectivity of mesoporous metal carbides has been observed. Tungsten and molybdenum carbides synthesized by the nano-casting method have been used for the HDO of anisole. It was found that mesoporous W 2 C was more active with more than 96% benzene selectivity at low temperatures and Mo 2 C showed almost 80% benzene selectivity. The active catalytic centers found per gram of W 2 C and Mo 2 C were about 5 µmol, which made them a good catalyst for HDO of anisole [117]. Due to various anthropogenic sources, the level of CO 2 is increasing in the environment. TMCs were found to be good in absorbing CO 2 . It has been studied that CO 2 can be captured and activated on various stable surfaces of TMCs, including Ti, Zr, Nb, Hf, Mo, and Ta carbides [118]. Another method to decrease the level of CO 2 is its conversion into CH 4 , a hydrocarbon fuel, using MXene sheets. Group IV to group VI MXene sheets have been investigated for this application and Cr 3 C 2 and Mo 3 C 2 were found better among these candidates. Thecarbons in the inner layers are octahedral-coordinated with two kinds of transition metals; those constituting the central inner layer (also octahedral-coordinated) and the three-coordinated terminal metals that are especially reactive due to their empty d-like orbitals and are therefore where the catalytic activity will take place. Chemisorption of CO 2 to MXene was good due to spontaneous binding energy (B.E.), which decreased in a group. This was the reason why group VI elements had shown a stronger ability to capture material. Theoretical studies also showed that the hydrogenation of captured C of CO 2 was more preferable than the hydrogenation of its oxygen. Thus, MXenes were found to be capable of CO 2 conversion to CH 4 . Bare MXenes required more energy input, 1.05 and 1.35 eV for Cr 3 C 2 and Mo 3 C 2 , respectively, than surface-terminated ones, i.e., 0.35 and 0.54 eV for -O and -OH-terminated MXenes [119].

Biosensors for Pesticides
AChE/CS/Ti 3 C 2 T x /GCE (acetylcholinesterase/chitosan/Ti 3 C 2 T x /glassy carbon electrode) were fabricated and used as bio-sensors for the detection of organophosphate pesticides. The MXene sheets here enhanced the electron transfer and chitosan provided good biocompatibility to AChE. This made a good biosensor with a low detection limit, good reproducibility, and a wide linear range. It showed good detection of malathion, having a linear range of 1 × 10 −14 to 1 × 10 −8 M and the detection limit was found to be 0.3 × 10 −14 M [18].
The synthesis of Ag@Ti 3 C 2 T x discussed in Section 2.2.3, was used as a biosensor for the detection of pesticides. AChE biosensors were fabricated using Ag@Ti 3 C 2 T x nanocomposites. The current of Ag@Ti 3 C 2 T x /GCE was higher than Ti 3 C 2 T x /GCE, which showed the improved effect of Ag nanoparticles and Ti 3 C 2 T x sheets. When AChE was introduced, the current was decreased which showed that AChE blocked the electrode transfer due to the in-conductivity of protein. This biosensor was based to detect malathion with good linearity, a low limit of detection, excellent stability, fast response rate, good reproducibility, and acceptable stability [49].

Antipollution
Metal carbides also played their role in the betterment of the environment. Their activity towards the reduction of toxic chemicals was also observed. The accordion-like structure of MXene (Ti 3 C 2 ) was found to be active in absorbing Cr(VI) from the aqueous solution. From the experiment, it was observed that the 80 mg of potassium dichromate was completely removed from its 100 mg/L solution at room temperature in 8 h when 1 g of synthesized product was added to it. SEM results showed the immobilization of Cr-containing compounds on the surface of Ti 3 C 2 layers [8]. Ti 3 C 2 T x nanosheets reduced Cr(VI) to Cr(III) and adsorbed Cr(III) simultaneously, also showing the reductive removal of KMnO 4 , NaAuCl 4 , and K 3 [Fe(CN) 6 ]. Delaminated (DL) and multilayer (ML) MXene showed the removal of Cu 2+ ions from water [120]. For the degradation of rhodamine blue (RhB), the composite of Ti 3 C 2 with Fe 2 O 3 was used. About 90% of the degradation of dye was observed under visible light. The catalyst showed recyclability and was found to be active for up to four cycles. The activity and reuse stability of the catalyst was due to its heterostructure interfaces [121]. Fe 2 O 3 /Ti 3 C 2 T x showed the removal of phosphate ions from water by the sorption selectivity method. TiO 2 /Ti 3 C 2 T x showed photocatalytic degradation (UV radiation) of methyl orange (MO) [120]. The nanocomposites of MXene-Co 3 O 4 were found to be active against methylene blue and rhodium blue. Ten milligrams (10 mg) of the nanocomposite sample, in the presence of 15 mL H 2 O 2 , degraded the dye in 100 mL solution (12.5 mg/L of MB or 5 mg/L of RhB) at room temperature. The catalyst also showed good repeatability and was found to be active for up to eight consecutive cycles [40]. The degradation of methylene blue (MB) and acid blue 80 (AB80) was also performed by using Ti 3 C 2 T x , a representative multilayered MXene. It has been found that, in the dark, MXene (anionic terminating groups) developed electrostatic interactions with MB (cationic dye). It was found that no AB80 was degraded in the dark (20 h exposure) but under UV light about 62% of dye was degraded in 5 h, and about 18% MB was degraded in the dark and 81% when exposed to UV light for 5 h [122]. Phytic acid-MXene composite was also used to adsorb RhB and MB. Ten milligrams (10 mg) of the catalyst was added in 40 mL of MB (12 mg/L) and RhB (6 mg/L) solution and it showed considerable adsorption. The composite was recovered and repeatedly used for the same experiment [123]. For the adsorption of MB and AB 80 Ti 3 C 2 T X -based MXene and Al-based A100-MOF were used. It was observed that the isoelectric point of these two adsorbents at pH values were 3 and 9, respectively. MB and AB (having positive and negative charges, respectively) removed by MXene and MOF, respectively, by the adsorption method. The adsorption rate of both decreases after four cycles [124]. Efforts are continuous in the process to develop catalysts that can completely degrade the toxic material from the environment. Using the co-precipitation approach, nano-structures of Gd 3+ -and Sn 4+ -doped bismuth ferrite with MXene sheet were synthesized and the degradation of Congo dye in the presence of sunlight was observed. The amount of iron and tin was varied in these nano-hybrid structures Bi 1-x Gd x Fe 1-y Sn y (BGFSO). The photocatalytic activity was determined by adding 100 mg of hybrid catalyst into 100 mL of Congo red dye solution and kept under a xenon lamp source with a wavelength 400-700 nm. The catalyst Bi 0 . 90 Gd 0 . 10 Fe 0 . 80 Sn 0 . 20 showed maximum activity, i.e., 100% degradation of dye in 120 min [125]. Similar degradation was observed when La and Mn co-doped bismuth ferrite/Ti 3 C 2 MXene composite was prepared. Bi 0 . 9 La 0 . 1 FeO 3 /Ti 3 C 2 and Bi 1−x La x Fe 1−y Mn y O 3 /Ti 3 C 2 nanohybrids showed the degradation of Congo red dye in the dark by~92% and~93%, respectively, and they showed 100% degradation of dye when irradiated for 30 min [126]. The degradation of Congo dye and acetophenone was also observed using non-doped bismuth ferrite/Ti 3 C 2 MXene and 100% photocatalytic degradation of Congo red dye was achieved in 42 min and acetophenone in 150 min. [127].

Ecotoxicity
The toxic effects by the MXene on the environment were determined and it was found that Ti 3 C 2 stimulated the growth of green algae at lower concentrations. This effect reversed when the concentration was increased. However, the ecotoxicity depends upon the type of components along with the amount [35].

Phytotoxicity
Phytotoxicity of MXene and its derivatives with Ag and Pd were determined using a phytotoxic set. Plant seeds of two species were used for this purpose, i.e., sorghum and charlock, and it was found that nanoparticles can affect the Golgi apparatus and endoplasmic reticulum. They could also produce reactive oxygen [35].

Anti-Bacterial Activity
MXene sheets (Ti 3 C 2 ) had negatively-charged surfaces and high hydrophilicity. Thus, they have more ability to come in contact with bacterial membranes, and this resulted in the inactivation of micro-organisms when they comes in direct contact, according to the direct contact-based mechanism [128]. Antibacterial activity against Gram-positive and Gram-negative (Escherichia coli) strains were determined, and except for the reference MXene Ti 3 C 2 , the derivatives with Ag and Pd showed considerable activity against bacterial strains [35]. The antibacterial activity of MXene is enhanced considerably when it is conjugated with gold nanoclusters (AuNCs). The activity was observed against a Gram-positive bacteria model and a Gram-negative bacteria model. For E. coli, the bacteria killing efficiency of the conjugation is increased by >96%, which was much higher than the efficiency shown by them individually. MXene-AuNCs have the ability to damage bacterial DNA, which results in the enhanced anti-bacterial activity. This conjugation has both physical (through MXene) and chemical (via MXene-AuNCs) mechanisms which showed enhanced antimicrobial activity towards Gram-positive and Gram-negative bacterial strains [129].

Chemiluminescence Biosensor
Exosomes are considered biomarkers for early cancer diagnosis. Ti 3 C 2 MXene nanosheets, as the ECL biosensor, were developed to detect exosomes. Due to their large surface area, excellent conductivity, and catalytic properties, they became highly-sensitive ECL biosensors with a detection limit of 125 µ particles/L. This became a sensitive, feasible, and reliable tool in exosome clinical diagnostics [130].

Cancer Therapeutics and Diagnostics
MXene has been used in the field of biomedical applications in the past three years and it has been found that the most suitable and compatible MXenes for biomedical application are titanium-based MXenes, e.g., Ti 3 C 2 and Ti 2 C. Among the number of tested MXenes, Mo 2 C, Ta 4 C 3 , and Nb 2 C have also shown themselves to be good candidates for cancer treatment. MXenes have hydrophilic surfaces, a number of active sites for drug incorporation, good biocompatibility, strong light absorption in the near-infrared region (NIR), and ferromagnetism suitable for MRI imaging. The unique architecture and characteristics make it a suitable tool for (i) cancer therapeutics, e.g., photodynamic therapy, photothermal therapy, and chemo and radio therapy; (ii) cancer diagnostics, e.g., sensing electrode material, as a contrast agent in MRI, CT bioimaging, and photo-acoustic imaging; and (iii) cancer theranostics as a cargo agent for anticancer drug delivery and as a contrast agent for real-time imaging [36,131].

MXene in Memory Devices
MXene shows versatile properties and have found their application as a storage medium in memory devices. MXene-TiO 2 core shell nanosheets have shown themselves as an excellent storage medium for data in memory devices. The oxidation of multilayered MXene was controlled and it act as a floating gate and TiO 2 as a tunneling layer simultaneously by a facile water-based process in nano-floating-gate transistor memories (NFGTMs). The as-prepared NFGTMs showed good memory characteristics, including a large memory window, high programming, long retention time, and cyclic endurance [132].

Conclusions
TMCs, either in the form of MXenes or nano-composites or nano-powders/-particles, opened a wide spectrum of research. Until now, a great deal of work has been conducted on these materials, but there is still much more to do. Several new inventions and proceedings are on the way. The compounds/materials synthesized until now have brought revolutions to various fields. For example, by using these materials lightweight batteries and energy storage devices become possible. However, their charging, discharging, and life span are significant issues now. TMCs have a number of such properties that make them versatile and suitable for their applications in multiple fields. They have the ability to split water for the production of energy, but good stability and maximum output in both acidic and alkaline environment of these dual catalysts is a significant challenge. Ni/Mo 2 C acts as a dual catalyst for water splitting, but its stability is a challenge. MXene sheets with two different transition metals are thought to be good for water splitting. MXenes synthesized with group 6 elements are better than one with group 5 elements for HER, but the stability of the electrode with an enhanced number of cycles is a challenge. MXene sheets other than Ti are also found to be more effective than ones with it, for HER. For OER, Fe, W, and V carbides are found to be good. MXenes with terminated groups and in the composite form have shown good efficiency for their use in batteries and hybrid MXenes for supercapacitors. In LIBs, instead of bare MXene, multilayer hybrid films or composites of MXene showed good performance. Intercalation of CNTs in the films also enhanced the capacity of LIBs.
TMCs, due to their versatile nature, will show their capabilities in different fields. Future studies should focus on the development of synthesis methods that yield precise and uniform shape, size, and morphologies. The sol-gel method for the synthesis of carbide nano-particles is more suitable for future synthesis because of its greener approach. Synthesis of nano-rods and nano-wires of carbides with enhanced optical activity and electronic transmission and 2D double-layered MXenes will be in much focus with group 4 and 6 elements, and also with group 5 and 7 elements. This will change the conductive behavior of carbides. For the application towards the environment or medicinal group 10 to group 14 elements composites with MXene will be investigated against different aqueous and non-aqueous pollutants and bacterial strains. MXene sheets with different d-spacing and intercalating ions will also play an important role in detecting cancer cells in different parts of the body.