Molybdenum-Based Electrode Materials

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This entry is adapted from the peer-reviewed paper 10.3390/batteries9090479

As a novel type of green energy storage device, supercapacitors exhibit several orders of magnitude higher capacities than the traditional dielectric capacitors and significantly higher power density than the traditional secondary batteries. Supercapacitors have been widely applied in energy storage fields. Electrode materials, as pivotal components of supercapacitors, play an important role in electrochemical performance. Molybdenum-based materials have attracted widespread attention for their high theoretical capacitance, abundant resources, and facile synthesis tactics. Therefore, it is necessary to systematically summarize the application of Mo-based electrode materials in high-performance supercapacitors and unveil their developmental direction and trends.

supercapacitors Mo-based electrode materials binary materials

1. Introduction

The energy crisis and environmental pollution are the two principal themes in the world. Therefore, renewable clean energy sources, such as wind, solar, and tidal energy, have attracted much attention in the energy fields ^[1]. However, the development of clean energy is limited by space and time factors ^[2]. Consequently, effective energy storage systems are needed to promote their commercial application. Among electrochemical energy storage devices, supercapacitors stand out for their high-power density and long cycle life ^[3]^[4].

The history of capacitors can be traced back to 1746, while the invention of the Leiden bottle is a pivotal milestone. A typical capacitor comprises two closely aligned conductors separated by an insulating medium. The stored energy of the capacitor is positively related to the voltage until the threshold of breakdown voltage ^[5]. In 1975, Conway et al. ^[6] claimed that the charge and discharge behaviors of RuO₂ closely resemble capacitors rather than batteries, and termed them “supercapacitors”.

Supercapacitors can be classified into two main types: electric double layer supercapacitors (EDLCs) and pseudocapacitors. EDLCs stored charge through ion adsorption and desorption on the surface of electrodes. Therefore, EDLCs often show long lifespan and high energy efficiency, because there are only physical reactions on the surface during the charge and discharge processes ^[7]. In contrast, pseudocapacitive supercapacitors undergo reversible redox reactions in the interior part of the electrode materials ^[8]. Therefore, pseudocapacitive electrodes often present higher specific capacitance and energy density than those of EDLCs.

Generally, a supercapacitor comprises four components: current collector, electrode, electrolyte, and separator ^[9]. The electrode material plays a pivotal role in determining the specific capacitance and energy density. As a result, the design and regulation of electrode materials have become the focus in the field of supercapacitors ^[10]^[11]^[12]^[13]^[14].

China is rich in molybdenum resources. The element of Mo exhibits variable valence states (+4, +5, and +6) ^[15]. In addition, Mo-based materials exhibit pronounced electrochemical activity, which have been widely investigated in recent years ^[16]^[17]^[18]. As shown in Figure 1, Mo-based electrode materials can be categorized as binary Mo-based materials (MoO₂, MoO₃), ternary Mo-based materials (NiMoO₄, CoMoO₄, and MnMoO₄, etc.), nanocomposites of Mo-based materials (Mo₂C@CNT and MoS₂/graphene), and Mo-based MOFs and Mo-based materials deriving from MOF materials (Mo-MOF/PANI and BiMo-MOF).

Figure 1. A series of Mo-based electrode materials for supercapacitors.

2. Mo-Based Electrode Materials for Supercapacitors

2.1. Binary Mo-Based Materials

Binary Mo-based materials are composed of two different types of elements, for example MoO₃, Mo₂C, MoN, and MoS₂, etc., which have attracted extensive research into supercapacitors.

2.1.1. Molybdenum Oxides

Molybdenum oxides mainly include MoO₂, MoO₃, and MoO_3−x, which are composed of MoO₆ octahedra through edge or corner sharing ^[19].

MoO₃ usually exists in three crystal types: α-MoO₃, β-MoO₃, and h-MoO₃. α-MoO₃ belongs to the orthorhombic crystal system, in which MoO₆ octahedra are connected with each other through weak van der Waals forces along the [010] direction and corner sharing along the [100] direction ^[20]. β-MoO₃ belongs to the monoclinic crystal system, in which MoO₆ octahedra are connected through corner sharing ^[21]. h-MoO₃ belongs to the hexagonal crystal system, in which MoO₆ octahedra share edges along the [001] direction and corners along the [100] direction ^[22]. Therefore, h-MoO₃ shows a zig-zag structure with a cavity of ~3 Å in diameter allowing for the insertion/deinsertion of ions ^[23]. Prakash et al. ^[24] employed the solution combustion method to synthesize a series of α-MoO₃ nanorods at different combustion temperatures. The SEM image in Figure 2a illustrates the morphology of α-MoO₃, which shows one-dimensional nanorods with a diameter of 50 nm and length of several micrometers. The α-MoO₃ nanorods show a specific capacitance of 176 F g⁻¹ at 1 mA g⁻¹. Even after 1000 cycles, the capacitance retention reaches 92%. Additionally, Niu et al. ^[25] synthesized the h-MoO₃ nanorods and nanoparticles using a hydrothermal method, as shown in Figure 2b. The nanorod structure facilitates adequate contact between the electrode materials and electrolyte, thereby improving the electrochemical performance. As a result, the MoO₃ nanorods achieved a specific capacitance of 229.0 F g⁻¹ at 0.2 A g⁻¹. Zhu et al. ^[26] fabricated MoO₃ nanoplates by the heat-treating of C₃N₄ and ammonium molybdate (Figure 2c). The extended layered structure enables the rapid insertion/deinsertion of ions. Notably, a high specific capacitance of 994.2 F g⁻¹ was achieved at 0.5 A g⁻¹. Moreover, the assembled YP50//MoO₃ device shows a capacitance retention of 84% after 1500 cycles at 3 A g⁻¹.

MoO₂ belongs to the monoclinic crystal system, in which the twisted MoO₆ octahedra units are connected to each other along the [001] direction by sharing edges. The distorted structure induces a change in the electronic state of Mo, leading to metallic conductivity ^[27]. Ma et al. ^[28] synthesized MoO₂ nanoparticles using a hydrothermal method (Figure 2d) and explored their electrochemical behavior in acid electrolytes. The charge storage behavior of MoO₂ under acidic conditions is ascribed to the surface redox reactions (Faradaic capacitance) and ion intercalation/deintercalation reactions (battery capacitance). However, the electrochemical mechanism of MoO₂ is ascribed to the electric double layer capacitors’ behavior under the neutral electrolyte. Notably, the MoO₂ electrode exhibited a specific capacitance of 509.8 F g⁻¹ at 0.5 A g⁻¹ in a 0.5 M H₂SO₄ solution. Additionally, the morphological structure of MoO₂ is also investigated to improve its capacitive performance. Wu et al. ^[29] synthesized hierarchical mesoporous MoO₂ spheres using a hydrothermal method (Figure 2e). The MoO₂ nanospheres exhibit a specific surface area of 29.5 m² g⁻¹, facilitating ion contact and improvement in charge transfer resistance. As a result, the specific capacitance reaches 381.0 F g⁻¹ at 0.3 A g⁻¹. Furthermore, the crystal structure of MoO₂ is further researched to reveal its influence on the performance of supercapacitors. Zhao et al. ^[30] introduced amorphous domains into MoO₂ nanosheets (Figure 2f), which enhances the ion diffusion and electron transport. Under a current density of 5 A g⁻¹, the capacitance retention remains 85% after 4000 cycles.

Non-stoichiometric MoO_3−x(0 < x < 1) enhances the intrinsic conductivity and increases the concentration of free carriers due to the introduced oxygen vacancies ^[31]. In comparison with MoO₃ (3.2 eV), the bandgap of MoO_3−x is reduced to 2.9 eV, which is expected to favor improved capacitance performance ^[32]. There have been many strategies to construct MoO_3−x. In addition, the size and shape adjustment of oxygen-deficient molybdenum oxides is relatively mature. At present, the reported methods mainly include the template method ^[33], surfactant method ^[34], solvothermal method ^[35], and sol–gel method ^[36], etc. Wu et al. ^[35] successfully synthesized MoO_3−x nanobelts with oxygen vacancy concentration of up to 20%. Figure 2g illustrates the MoO_3−x nanobelts, which show 30–40 nm in thickness and 100–200 nm in width. This structural design reduces the migration distance of electrolyte ions and enhances the availability of internal active sites. Notably, the MoO_3−x nanobelt exhibits a remarkable specific capacitance of 1,220 F g⁻¹ at 50 A g⁻¹ and shows an impressive capacitance retention of nearly 100% even after 38,000 cycles. Bai et al. ^[37] synthesized α-MoO_3−x nanobelts via a facile one-pot hydrothermal approach (Figure 2h). The enlarged interlayer spacing could weaken the interlamellar Van der Waals force, facilitating the rapid diffusion of ions. The α-MoO_3−x nanobelts exhibit a specific capacitance of 912.5 F g⁻¹ at 1 A g⁻¹. Salkar et al. ^[38] synthesized two-dimensional MoO_3−x microplates and microdisks. The SEM images illustrate the morphological structures in Figure 2i. According to the experimental data, the improved capacitor performance stems from the introduction of oxygen vacancies, the high specific surface area, and the enlarged layered structures. The MoO_3−x microplates and microdisks display specific capacitances of 410 F g⁻¹ and 226 F g⁻¹ at 20 A g⁻¹, respectively. Impressively, the capacitance retention is above 90% and the coulombic efficiency is above 98% after 10,000 cycles.

Figure 2. SEM images. (a) MoO₃ nanorods. (b) h-MoO₃ nanorods and nanoparticles. (c) MoO₃ nanoplates. (d) MoO₂ nanoparticles. (e) Mesoporous MoO₂ spheres. (f) MoO₂ nanosheets. (g) MoO_3−x nanobelts. (h) MoO_3−x nanobelts. (i) MoO_3−x microplates and microdisks.

2.1.2. Molybdenum Carbides

Transition metal carbides (TMCs) possess high electrical conductivity, and have been widely studied in energy storage, catalysis, and electromagnetic shielding fields. Now, the reported TMCs mainly include NbC ^[39], VC ^[40], TiC ^[41], and Mo₂C ^[42], etc. Notably, Mo₂C has been widely investigated in supercapacitors.

Yu et al. ^[43] conducted simulation calculations to analyze the electronic properties of molybdenum carbide. The energy band structure and projected DOS are presented in Figure 3a. For the molybdenum carbides (MoC, Mo₂C, MoC₂), an increase in the proportion of C means a reduction in free electrons on the surface. Mo₂C with the smallest proportion of C and numerous dangling bonds on the surface shows significant potential as an electrode material in supercapacitors. Illustrated in Figure 3b, Xu et al. ^[42] successfully synthesized lamellar Mo₂C through the calcination of amine–metal oxides, yielding ultrafine nanoparticles of ~10 nm. This microstructural design is beneficial for the adequate contact between the electrode and electrolyte, facilitating improved performance. Remarkably, a specific capacitance of 88 F g⁻¹ has been achieved at 0.5 A g⁻¹. In addition, the asymmetric Mo₂C//AC capacitor displays an impressive capacitance retention of 95% after 1200 cycles and an energy density of 44.1 W h kg⁻¹.

2.1.3. Molybdenum Nitrides

Transition metal nitrides (TMNs) have received extensive attention in the energy storage and catalysis fields due to their high electrical conductivity and robust chemical stability ^[44]. Xiao et al. ^[45] employed the salt template method to synthesize MoN nanosheets, as demonstrated in Figure 3c. The MoN nanosheets exhibit a negligible band gap and an exceptional electronic conductivity, thereby improving the electrochemical performance. Notably, the volume-specific capacitance reaches 928 F cm⁻³ at 2 mV s⁻¹, while maintains 200 F cm⁻³ even under 20 mV s⁻¹. Gao et al. ^[46] prepared zig-zag γ-Mo₂N thin film by magnetron sputtering. The SEM image is presented in Figure 3d. The zig-zag structured γ-Mo₂N presents an outstanding area capacitance of 248 mF cm⁻² at 50 mV s⁻¹. After 20,000 cycles at 200 mV s⁻¹, a remarkable capacitance retention of 95% is achieved. In addition, the symmetrical solid-state γ-Mo₂N//γ-Mo₂N device delivers an excellent power density of 107.1 W cm⁻³ at 33.8 mW h cm⁻³. Djire et al. ^[47] synthesized face-centered cubic γ-Mo₂N with high surface area through high-temperature treatment of molybdenum source and ammonia gas, as illustrated in Figure 3e. The pseudocapacitive charge storage mechanism was conducted through in situ experiments. As a result, the simultaneous insertion of hydrogen ions (H⁺) and electrons (e⁻) into the material leads to the reduction in Mo during the electrochemical processes. The γ-Mo₂N shows a high specific capacitance of 1500 F g⁻¹ with a potential window of 1.2 V in aqueous acidic electrolytes.

Figure 3. (a) Calculated band structures and projected DOS for Mo₂C, MoC, and MoC₂. (b) SEM image of Mo₂C nanosheets. (c) Schematic diagram of the synthesis of MoN nanosheets. (d) SEM image of zig-zag γ-Mo₂N films. (e) SEM image of γ-Mo₂N.

2.1.4. Molybdenum Sulfides

Molybdenum chalcogenides possess a layered structure, in which the large interlayer spaces are convenient for the storage and transport of ions ^[48]. This layered characteristic renders them an ideal candidate as an energy storage material. Molybdenum sulfide (MoS₂) and molybdenum selenide (MoSe₂) stand out in molybdenum chalcogenides. The high intrinsic ionic conductivity facilitates improved electrochemical performance. MoS₂ displays layered S-Mo-S stacks under van der Waals interactions ^[49]. There are two distinct phase states: the metallic 1T phase and the semiconducting 2H phase. The 1T phase easily transforms into the 2H phase due to its thermodynamic instability. MoS₂ predominantly exists in the form of the 2H phase in nature ^[50]. The active sites of the 2H phase are mainly distributed along the sulfur edge, while those of the metallic 1T phase are mainly distributed along both the edge and basal plane, inherently favoring the electrochemical process ^[51].

Liu et al. ^[52] used a simple one-step hydrothermal method to obtain rosette-like MoS₂ nanoflowers (Figure 4a). Figure 4b presents the GCD curves of MoS₂ nanoflowers at varying current densities; they exhibit a specific capacitance of 137 mF cm⁻² at 10 mA cm⁻². After 10,000 cycles, the capacitance retention reaches 81.6%. Teli et al. ^[53] reported amorphous MoS₂ nanoflakes (Figure 4c) using one-step electrodeposition, achieving a specific capacitance of 416.9 mF cm⁻² at 1 mA cm⁻². An asymmetric device was assembled using activated carbon and MoS₂ as negative and positive electrodes, respectively; this exhibited an area capacitance of 277.3 mF cm⁻² and energy density of 0.15 mW h cm⁻² at 5.33 mW cm⁻². Furthermore, 90.1% cyclability and excellent coulombic efficiency measured up to 3000 cycles were observed for an asymmetric device (Figure 4d). Additionally, Joseph et al. ^[54] prepared defect-rich 1T-MoS₂ nanosheets, which exhibited a specific capacitance of 379 F g⁻¹ at a current density of 1 A g⁻¹. The assembled supercapacitor device delivered an energy density of 21.3 W h kg⁻¹ and a power density of 750 W kg⁻¹. The capacitance retention remains 92% even after 3000 cycles (Figure 4e). Gupta et al. ^[55] utilized a hydrothermal method to synthesize MoS₂ nanoflowers. The surface features of the flakes and wrinkles facilitate the intercalation and deintercalation of cations. Notably, a specific capacitance of 255.65 F g⁻¹ is achieved at a current density of 0.25 A g⁻¹. Impressively, even after 1000 cycles, 70% of the initial specific capacitance is retained (Figure 4f).

Figure 4. (a) SEM image of MoS₂ nanoflowers; (b) GCD curves of MoS₂ nanoflowers at different current densities. (c) TEM image of MoS₂ nanoflakes; (d) cyclic performance of MoS₂ nanoflakes at 3 mA cm⁻². (e) Cyclic performance of 1T-MoS₂ nanosheets at 5 A g⁻¹. (f) Specific capacitance of MoS₂ nanoflowers at different current densities. (g) SEM image of mesoporous MoSe₂; (h) cyclic performance of mesoporous MoSe₂ at 1 A g⁻¹. (i) SEM image of 2H-MoSe₂; (j) cyclic performance of 2H-MoSe₂ at 5 A g⁻¹. (k) SEM image of MoSe₂ nanoflowers; (l) CV curves of MoSe₂ nanoflowers at different current densities.

The atomic structure of MoSe₂ closely resembles that of MoS₂, comprising three atomic layers held together by weak van der Waals interactions. Consequently, MoSe₂ has attracted considerable interest as a potential electrode material in supercapacitors ^[56].

MoSe₂ with a mesoporous structure shows a large specific surface area, providing significant benefits for ion transport. Vattikuti et al. ^[57] successfully synthesized a uniform dry leaf-like mesoporous MoSe₂ nanostructure using a microwave-assisted method, as illustrated in Figure 4g. The as-prepared leaf-like perforated MoSe₂ exhibited remarkable capacitance of 257.38 F g⁻¹ at 1 A g⁻¹ with a capacitance retention of almost 95% after 5000 cycles, see Figure 4h. In comparison to the mesoporous configuration, MoSe₂ with a nanosheet structure further increases the specific surface area, shortens the ion diffusion path, and improves the electrochemical performance. Upadhyay et al. ^[58] reported the synthesis of layered 2H-MoSe₂ nanosheets via an in situ selenization route. The SEM image is shown in Figure 4i. The MoSe₂ nanosheet exhibits a specific capacitance of 46.22 mA h g⁻¹ at 2 A g⁻¹. Remarkably, even after 2000 cycles at a current density of 5 A g⁻¹, a capacitance retention of 64% was observed (Figure 4j). Additionally, the nanoflower structure would offer ample channels for electrolyte diffusion during the electrochemical processes. Zhang et al. ^[59] synthesized smooth and irregular pleated flower-like MoSe₂ using a facile hydrothermal method. The SEM image is displayed in Figure 4k. Furthermore, Figure 4l illustrates the CV curves of MoSe₂ at different current densities. Notably, the specific capacitance reaches 641.5 mA h g⁻¹ at a current density of 0.1 A g⁻¹. The assembled hybrid MoSe₂//AC capacitors displayed a high energy density of 78.75 W h kg⁻¹ and a high power density of 3600 W kg⁻¹. In addition, the capacity retention rate is 70.28% after 5000 cycles with a potential window of 0.5–3.5 V.

2.2. Ternary Mo-Based Materials

Ternary Mo-based materials are composed of three elements, including metal molybdates and MXenes. The synthesis pathway of metal molybdate is simple and low cost while exhibiting remarkable physical and chemical properties ^[60]. MXenes exhibit a characteristic two-dimensional layered structure, offering a high specific surface area and exceptional electrical conductivity ^[61]. Notably, ternary Mo-based materials have been widely researched for supercapacitors in recent years.

2.2.1. Metal Molybdates

Metal molybdates, for example MMoO₄ (M = Cu ^[62], Zn ^[63], Bi ^[64], Ni ^[65], Mn ^[66], Sn ^[67], Co ^[68], etc.), constitute a significant category in inorganic materials.

Farahpour et al. ^[62] conducted a single-pot hydrothermal method to grow CuMoO₄ nanosheets on nickel foam. In Figure 5a, the prepared CuMoO₄ nanosheets are uniformly distributed with grass-like morphology. The specific capacitance of CuMoO₄ reached 2259.55 F g⁻¹ at 1 A g⁻¹. The cyclic stability analysis showed a capacitance retention of 90.08% at 16 A g⁻¹ after 5000 cycles. Moreover, the CuMoO₄//AC supercapacitor device displayed a high energy density of 52.51 W h kg⁻¹ at 600 W kg⁻¹. In addition, the device exhibited a capacitance retention of 78.6% after 5000 cycles at 4 A g⁻¹ (Figure 5b). Gurusamy et al. ^[63] produced a series of rod-shaped ZnMoO₄ using a template method by optimizing the concentration of CTAB. The schematic diagram of the synthesis process is presented in Figure 5c. The rod-like ZnMoO₄ material showed an impressive specific capacitance of 779 F g⁻¹ at 5 mV s⁻¹ and retained 90% of the initial capacitance even after 3000 cycles at 100 mV s⁻¹. Additionally, Yesuraj et al. ^[64] employed the hydrothermal method with a DNA template to synthesize Bi₂MoO₆ nanoplates, as depicted in Figure 5d. The Bi₂MoO₆ nanoplates with a large number of small nanoparticles (5–7 nm) on their surface result in an increased surface area, which facilitated charge transport and ion diffusion. The Bi₂MoO₆ exhibited a high specific capacitance of 698 F g⁻¹ at 5 mV s⁻¹, along with a capacitance retention of 86% even after 3000 cycles at a high scan rate of 100 mV s⁻¹ in 1 M NaOH electrolyte. Qu et al. ^[65] presented a rapid and zero-energy consumption method to obtain metal molybdate nanowires in supercapacitors (Figure 5e). The synthesized NiMoO₄ nanowires exhibited an impressive specific capacitance of 549 C g⁻¹ at 1 A g⁻¹. Furthermore, the assembled supercapacitor device demonstrated a specific capacitance of 156 F g⁻¹ at 0.8 A g⁻¹, along with an energy density of 55.6 W h kg⁻¹ at 640 W kg⁻¹. Additionally, Sheng et al. ^[66] employed a solid-state chemical synthesis approach to produce 1D MnMoO₄ 0.9H₂O and MnMoO₄ nanorods (Figure 5f), which exhibited a specific capacitance of 210.2 F g⁻¹ at 1 A g⁻¹. Notably, the MnMoO₄ nanorods displayed remarkable cycle stability, maintaining a cycle life of 112.6% even after 10,000 cycles. Furthermore, the electrochemical performance of MnMoO₄ underwent substantial enhancement upon the removal of crystal water from MnMoO₄·0.9H₂O, leading to a noteworthy 2.4-fold increase in specific capacitance. Remarkably, the asymmetric supercapacitor device achieved a high energy density of 23.5 W h kg⁻¹ at 187.4 W kg⁻¹. This remarkable electrochemical performance is attributed to the elevated electrical conductivity from the 1D nanostructure after the removal of crystallization water. Sakthikumar et al. ^[67] optimized the ratio of CTAB to metal salt and reaction conditions to synthesize sheet-like Sn(MoO₄)₂, as shown in the SEM image in Figure 5g. The specific capacitance of flake Sn(MoO₄)₂ is 109 F g⁻¹ at 5 mV s⁻¹ and the capacitance retention reaches 70% after 4000 cycles at 8 mV s⁻¹. Li et al. ^[68] synthesized CoMoO₄ material in situ on nickel foam using a hydrothermal method (Figure 5h). The prepared CoMoO₄ shows a cuboid rod-like structure with loose folds on the periphery, enhancing the contact between the electrode and electrolyte, and thereby facilitating ion diffusion and transmission. At a current density of 3 mA cm⁻², the discharge capacitance of CoMoO₄ reaches 11.112 F cm⁻².

Figure 5. (a) SEM image of CuMoO₄ nanosheets; (b) cyclic performance of CuMoO₄//AC at 4 A g⁻¹. (c) Schematic diagram of the synthesis of ZnMoO₄ nanorods. (d) Schematic diagram of the synthesis of Bi₂MoO₆ nanoplates. (e) Schematic diagram of the synthesis of NiMoO₄·xH₂O nanowires. (f) Schematic diagram of the synthesis of MnMoO₄ nanorods. (g) SEM image of Sn(MoO₄)₂ nanosheets. (h) SEM image of CoMoO₄ nanorods.

2.2.2. Mo-MXenes

In recent years, two-dimensional transition metal carbonitride (MXene) materials have attracted extensive attention in the energy storage field, owing to their unique physical and chemical characters ^[69]. The MAX phase is classified as a layered carbide or nitride and is characterized by the chemical formula M_n+1AX_n (n = 1~3). Here, M represents various transition metal elements, including Sc, Ti, Zr, Nb, Ta, Cr, Mo, etc. X stands for carbon and/or nitrogen, while A refers to a main group element. Through etching, A can be removed from the MAX phase, leading to the formation of a graphene-like MXene structure. The structural chemical formula of MXenes is M_n+1X_nT_x (n = 1~3), where T represents a functional group such as O, F, or OH ^[70]. The distinctive properties of MXene materials have sparked interest from researchers in supercapacitors.

Halim et al. ^[71] firstly put forward a large-scale synthesis strategy for 2D Mo₂CT_x flakes. LiF/HCl acts as an etchant to selectively etch Ga from Mo₂Ga₂C powder, leading to a delamination process, as depicted in Figure 6a. The obtained Mo₂CT_x flakes exhibit high conductivity and effective intersheet conduction due to their dense stacking. At a scan rate of 2 mV s⁻¹, the specific capacitance reaches 700 F cm⁻³, while the capacitance retention is nearly 100% even after 10,000 cycles at 10 A g⁻¹ (Figure 6b). Das et al. ^[72] conducted a theoretical analysis on the structure of Mo₂CO₂ to predict the electronic structure and investigate its capacitive behavior. As a result, the functionalized MXenes induce a change in charge transfer dynamics. Therefore, H inclines to form covalent bonds with O, leading to the sharing of electrons. Impressively, the theoretically calculated capacitance of Mo₂CO₂ is closely consistent with the experimental results. Zheng et al. ^[73] prepared Mo_1.33CT_z i-MXene films with a vacancy structure by etching (Mo_0.66Sc_0.33)₂AlC, as illustrated in Figure 6c. The inclusion of vacancies notably optimizes the ion transport. Notably, the Mo_1.33CT_z i-MXene attained an energy density of 25.4 mW h cm⁻³ at a power density of 152.4 mW cm⁻³ in a 15 M LiBr electrolyte, as depicted in Figure 6d. Even after 20,000 cycles at 100 mV s⁻¹, 99.4% of the initial specific capacitance is retained (Figure 6e).

Figure 6. (a) Schematic diagram of the synthesis of Mo₂CT_x; (b) cyclic performance of Mo₂CT_x at 10 A g⁻¹. (c) SEM image of Mo_1.33CT_z; (d) Ragone diagram of Mo_1.33CT_z in 1 M H₂SO₄ and 15 M LiBr compared to different Mxene; (e) cyclic performance of Mo_1.33CT_z at 100 mV s⁻¹.

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