Selenized Binary Transition Metals‐MXene Composite for High‐Performance Asymmetric Hybrid Capacitors

Abstract

The exploration of innovative and high‐efficiency energy storage materials is crucial for advancing high‐performance supercapacitors. In this study, a novel composite material is synthesized, comprising multilayered MXene (Ti₃C₂T_x) nanoparticles integrated with porous NiCo₂Se₄ nanosheets. The accordion‐like nanostructure of MXene and its strong interfacial interactions enhance the surface area and cycling stability of the nanocomposite. Additionally, substituting selenium (Se) for Ni‐Co‐based hydroxides modulates orbital hybridization with the corresponding metal cations, significantly improving electrochemical activity and reducing the adsorption/desorption energy barrier for electrolyte ions. The synergistic interaction between these two materials enabled the composite electrode to achieve a high specific capacity of 796.25 C g⁻¹ at 1 A g⁻¹ while maintaining over 90% of its initial capacity after 8000 cycles. Furthermore, the as‐fabricated asymmetric hybrid capacitor, employing activated carbon as the negative electrode, delivered an energy density of 64.36 Wh kg⁻¹ at a power density of 0.8 kW kg⁻¹, surpassing the performance of most previously reported hybrid capacitors. The developed composite structure holds significant potential for integration into various electrochemical devices, such as batteries, sensors, and electrolyzers.

A 3D MXene–NiCo₂Se₄ composite is synthesized via a facile strategy. The introduction of selenium modulates orbital hybridization with transition metal cations, thereby enhancing electron transport and redox kinetics. The hierarchical porous architecture and strengthened interfacial coupling collectively facilitate ion diffusion and charge transfer, demonstrating great potential for advanced asymmetric hybrid capacitor applications.

1. Introduction

Renewable energy sources, such as wind and solar power, have gained prominence in recent years, becoming primary drivers in meeting the growing global energy demand. Consequently, the development of efficient and stable electrochemical energy storage systems has become essential.^{[ 1} , ^{2 ]} Supercapacitors (SCs) have emerged as promising candidates for rapid energy harvesting and power delivery.^{[ 3} , ^{4 ]} For traditional SCs, electric double‐layer capacitors (EDLCs) store charge via electrostatic adsorption, while pseudocapacitors (PCs) utilize fast surface or near‐surface Faradaic redox reactions.^{[ 5} , ⁶ , ^{7 ]} Due to limitations inherent to their charge storage mechanisms, their energy densities remain insufficient and restrict widespread practical applications. Therefore, researchers are focusing on developing SCs with higher energy and power densities.^{[ 8 ]} Based on the equation E = 0.5 CV², materials that facilitate a wider operating voltage (V) and greater specific capacitance (C) are expected to provide a higher energy density (E). To achieve this, asymmetric hybrid capacitors (AHCs) that increase the output voltage by utilizing positive and negative electrodes with different working voltages have been proposed as an effective strategy for enhancing energy density.^{[ 9} , ¹⁰ , ^{11 ]} Additionally, various combinations of active materials with distinct structural properties have been explored to achieve higher properties.^{[ 12} , ^{13 ]}

Transition metal compounds (TMCs)—including hydroxides, oxides, and sulfides—are considered Faradaic electrode materials. They have recently attracted significant attention as active materials for AHCs due to their natural abundance, cost‐effectiveness, multiple oxidation states, and high theoretical capacitance.^{[ 14} , ^{15 ]} Among these, nickel‐based monometallic materials have been extensively investigated due to the high electrochemical activity of Ni. However, their practical application is limited by the low electrical conductivity of redox intermediate products.^{[ 16} , ¹⁷ , ¹⁸ , ^{19 ]} Integrating cobalt into nickel‐based monometallic compounds has significantly improved the energy storage performance of AHCs due to the synergistic interaction between Ni and Co.^{[ 20} , ^{21 ]} The improved electrical conductivity of Co facilitates charge transport within the compound, while lattice distortions in the crystal structure of Ni‐Co TMCs create abundant active sites that lead to the formation of multiple bimetallic oxidation states (Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺/Co⁴⁺).^{[ 22} , ²³ , ^{24 ]} However, the intrinsic low conductivity of TMCs remains a significant limitation in achieving high‐performance capacitors. Therefore, enhancing the conductivity of Ni‐Co‐based TMCs is essential, which can be achieved by modifying the type of coordinating anions. In this study, selenium (Se), a group 16 element, was selected as the counter anion due to its superior conductivity compared to that of oxides and sulfides.^{[ 25} , ^{26 ]} The electronic properties of TMCs are primarily determined by the orbital hybridization of chalcogen p orbitals and metal d orbitals, which significantly affect electron mobility and the activation energy barrier during redox reactions.^{[ 27 ]} In specific hybridization states, electron redistribution occurs between the metal and chalcogen atoms, a process governed by the outermost electron orbitals of these atoms. Figure  1a illustrates that the energy level of Se 4p orbitals is consistent with that of transition metal (Ni/Co) 3d orbitals, in contrast to the orbital states of oxygen (O: 2s ²2p ⁴) and sulfur (S: 3s ²3p ⁴). This alignment facilitates strong orbital hybridization, leading to a smaller energy difference between bonding and antibonding orbitals. Consequently, the valence bands become more dispersed and delocalized, resulting in a smaller energy barrier for the Faradaic reaction.^{[ 28} , ²⁹ , ³⁰ , ^{31 ]} Binary Ni‐Co transition metal selenides (TMSe) are regarded as promising active materials for AHCs.

Figure 1.

a) Atomic orbital energies of O, S, Se, Ni, and Co. b) Schematic illustration of morphological changes in the NiCo₂Se₄@NF electrode before and after introducing MXene. c) Fabrication process of the multilayered Ti₃C₂T_x and NiCo₂Se₄ nanocomposite electrode on NF.

Cycling stability is a critical factor for optimizing the performance of AHCs. During the charge–discharge process of quasi‐reversible Faradaic reactions, the fragile nanostructure of TMSe is susceptible to collapse and aggregation due to phase transitions and structural distortions.^{[ 32} , ^{33 ]} Therefore, maintaining the integrity of the original nanostructure by mitigating the brittleness of TMSe is highly desirable. One effective approach is to incorporate conductive nanoscale support, such as graphene or carbon nanotubes (CNTs).^{[ 34} , ^{35 ]} Another promising alternative is the use of MXenes, a broad class of 2D transition metal carbides and/or nitrides.^{[ 36} , ³⁷ , ^{38 ]} For instance, recent works have demonstrated MXene‐based composites with metal chalcogenides for SCs.^{[ 39} , ^{40 ]} However, these studies mainly focus on single‐metal systems, or multi‐metal systems combined with oxides or sulfides. In contrast, our strategy leverages binary metallic selenides to further enhance redox activity and employs a facile synthesis method that ensures intimate contact and compatibility with MXene. Moreover, MXene possesses surface functional groups with tunable hydrophilicity, enabling strong electrostatic interactions with TMSe and facilitating robust interfacial adhesion. A previous study has demonstrated that integrating TMC with MXene on a nickel foam (NF) current collector significantly enhances the mechanical stability of the electrode compared to pristine TMC on NF. This enhancement is attributed to the clay‐like nature of MXene, preventing delamination from the NF collector.^{[ 41} , ^{42 ]} Consequently, the capacity degradation of the electrode is highly mitigated.^{[ 43 ]} Moreover, Figure 1b illustrates the accordion‐like 3D nanostructure of MXene particles, providing an expansive surface area that facilitates the intercalated growth of TMSe. The unique layer‐by‐layer architecture of MXene inhibits the collapse of the TMSe nanostructure, thereby enabling the development of high‐performance AHCs.^{[ 44 ]}

Therefore, this study aims to develop a hierarchically porous and highly conductive TMSe (NiCo₂Se₄)‐MXene (Ti₃C₂T_x) composite on an NF electrode without the use of binders, designated as MXene‐NiCo₂Se₄@NF. To achieve a high active surface area free from oxide impurities, the composite is synthesized by combining MXene with Ni‐Co layered double hydroxides (NiCo‐LDH) and subjecting the composite to thermal selenization under an inert atmosphere using the chemical vapor deposition method for the first time. The structural, morphological, and electrochemical properties of MXene‐NiCo₂Se₄@NF are thoroughly investigated. Furthermore, the charge‐storage performance, cycling stability, and underlying charge‐storage mechanism of the electrode, along with its associated limitations, are examined. The findings could lead to the development of an AHC device incorporating activated carbon (AC), demonstrating its potential in powering real‐world appliances such as digital thermometers and LED bulbs. Furthermore, the developed material could potentially attract researchers and industries focused on energy, environmental sustainability, and healthcare applications.

2. Results and Discussion

The fabrication process of the MXene‐NiCo₂Se₄@NF electrode is graphically described in Figure 1c. Initially, MXene particles were synthesized by etching Ti₃AlC₂ (MAX phase) in hydrofluoric acid (HF). Subsequently, these particles were deposited onto an NF substrate via the dip‐coating method, forming a binder‐free electrode (Ti₃C₂T_x@NF).^{[ 37} , ⁴⁵ , ^{46 ]} Subsequently, the NiCo‐LDHs were electrodeposited onto the Ti₃C₂T_x@NF electrode using the cyclic voltammetry method, and the curves are shown in Figure S1 (Supporting Information). A general equation for the electrodeposition of metal hydroxides is expressed as follows: M²⁺ _(aq) + 2OH^– _(aq) → M(OH)₂₍s₎ + 2e^–. The electrodeposition method is more effective in suppressing MXene oxidation than the conventional hydrothermal synthesis conducted at high temperatures. The unique nanostructure of LDHs provides a high specific surface area and promotes the formation of mesoporous architectures, significantly facilitating electrolyte transport.^{[ 47 ]} During electrodeposition, metal cations such as Ni²⁺ and Co²⁺ exhibit a strong affinity toward the electronegative surface functional groups of MXene, such as ─O, ─OH, and ─F, via a strong electrostatic adsorption.^{[ 48 ]} This interaction promotes homogeneous deposition of the metal precursors and enhances both interfacial compatibility and structural integrity of the composite. Simultaneously, the hydrogen evolution reaction (2H₂O(l) + 2e⁻ → H₂(g) + 2OH^–(aq), E₀ = −0.83 V versus NHE) occurs on the surface of the MXene‐LDH nanocomposite. The generated hydrogen bubbles at the electrode surface hinder the further adsorption of metal ions, thereby inhibiting metal hydroxide formation in localized regions. However, the formation of a mesoporous NiCo‐LDH around the Ti₃C₂T_x promotes the mass transfer of electrolyte ions, accelerating redox reactions.^{[ 49} , ^{50 ]} Finally, the MXene‐NiCo‐LDH was subjected to thermal selenization at 350 °C under a nitrogen gas flow, forming the MXene‐NiCo₂Se₄ composite via chemical vapor deposition. The thermal treatment resulted in the selenization of the MXene‐NiCo‐LDH and the simultaneous removal of hydroxyl groups from the LDH layers, along with fluorine terminal groups from the MXene via pyrolysis.^{[ 51 ]}

The crystal structures of the composites were comprehensively analyzed using X‐ray diffraction (XRD) (Figure  2a). The analysis revealed a distinct peak at 6.27° in the MXene‐NiCo₂Se₄ composite, corresponding to the (002) crystal plane of the MXene phase. Additionally, diffraction peaks at 33.53°, 45.10°, and 51.12° were assigned to the characteristic monoclinic phase of NiCo₂Se₄ (JCPDS: 04–5241), correlating with the (002), (311), and (313) crystal planes, respectively. Notably, a significant shift toward a lower angle in the (002) graphitic plane was observed compared to pristine MXene (Figure 2a). The supporting information provides the interlayer spacing (d‐value) of MXene in the nanocomposite (1.41 nm) determined using Bragg's law, significantly larger than that of the original MXene sample (0.97 nm). This expansion can be attributed to the increased interlayer spacing between adjacent MXene sheets due to the intercalation of NiCo₂Se₄, further confirmed in the subsequent sections.^{[ 52} , ^{53 ]} This expanded interlayer spacing facilitates electrolyte diffusion and increases the available surface area for the redox reaction. Furthermore, the diffraction peaks corresponding to the (311) and (313) planes in the composite demonstrated a minor shift toward lower angles compared to the reference PDF card of NiCo₂Se₄. This peak shift suggests a lattice distortion potentially induced by the Jahn–Teller effect. Despite their similar atomic radii—Ni (0.124 nm) and Co (0.126 nm)—their distinct electronic configurations significantly affect the atomic structure of the compound. Specifically, Ni²⁺, with an electronic configuration of t_2g ⁶e_g ², evenly occupied t_2g and e_g orbitals, leading to a minimal or negligible Jahn–Teller effect. In contrast, Co²⁺ (t_2g ⁶e_g ¹) contained a single vacancy in the e_g orbital, specifically in the d_x2‐y2 or d_z2 orbital, inducing a significant Jahn–Teller effect. This effect, a fundamental chemical phenomenon, induced a slight atomic displacement that stabilized electron occupancy by minimizing asymmetric energy levels of the orbitals. Consequently, the Jahn–Teller effect effectively eliminated orbital degeneracy.^{[ 54} , ⁵⁵ , ^{56 ]} Variations in Jahn–Teller distortion may introduce additional active sites for redox reactions by generating uneven local Coulomb forces. Additionally, the elongation of the Co‐Se bond due to this distortion alters the electrostatic repulsion within the electron clouds, influencing overall electronic interactions. This process facilitates electron transfer by enabling effective overlap between the Se 4p orbital and the M^n+/(n+1)+ redox states.^{[ 57} , ^{58 ]} These slight structural mismatches are crucial in enhancing the electrochemical performance of the MXene‐NiCo₂Se₄ composite by optimizing conductivity and facilitating efficient ion transport.

Figure 2.

a) XRD patterns of MXene, NiCo₂Se_4, and the MXene‐NiCo₂Se₄ nanocomposite. High‐resolution XPS spectra of b) Ni 2p, c) Co 2p, and d) Se 3d in the MXene‐NiCo₂Se₄ nanocomposite.

The chemical composition of the electrode was analyzed using X‐ray photoelectron spectroscopy (XPS) to gain detailed insights into its surface chemistry. Figure S2a (Supporting Information) illustrates that the XPS survey spectrum reveals distinct peaks corresponding to Ti, C, and O from MXene (Figure S2b–d, Supporting Information) and Ni, Co, and Se from TMSe (Figure 2b–d). A detailed analysis of the high‐resolution Ti 2p spectrum reveals three doublets corresponding to Ti─C, Ti(II), and Ti─O bonds (Figure S2b, Supporting Information). The absence of a detectable TiO₂ peak confirms that MXene remained unoxidized during thermal treatment. Figure S2b,c (Supporting Information) illustrates that the deconvoluted peaks corresponding to C─Ti─O, C═C, C─C, O─C═O, O─Ti, O─Ti─C, OH─Ti─C, and Ti─O─Ni appeared at their respective binding energies. The presence of the Ti─O─Ni signal indicates the formation of a covalent bond between Ti₃C₂T_x and NiCo₂Se₄, effectively inhibiting the delamination of nanoporous NiCo₂Se₄ from MXene@NF, thereby maintaining structural stability.^{[ 59} , ⁶⁰ , ⁶¹ , ^{62 ]} The peakfitting analysis of the Ni 2p spectrum (Figure 2b) revealed distinct chemical states, including Ni³⁺ 2p_3/2 (853.4 eV), Ni²⁺ 2p_3/2 (855.7 eV), Ni³⁺ 2p_1/2 (872.9 eV), and Ni²⁺ 2p_1/2 (875.3 eV). Additionally, two peaks at 861.3 eV and 880.2 eV are assigned to shake‐up satellites (Sat). Similarly, the Co 2p spectrum (Figure 2c) is deconvoluted into two spin‐orbit doublets, revealing four fitted peaks at 778.5, 780.9, 793.3, and 797.0 eV, corresponding to Co³⁺ 2p_3/2, Co²⁺ 2p_3/2, Co³⁺ 2p_1/2, and Co²⁺ 2p_1/2, respectively.^{[ 63 ]} The Se 3d spectrum (Figure 2d) displays distinct peaks at 54.1 eV and 54.9 eV, corresponding to Se 3d_5/2 and Se 3d_3/2, respectively. These peaks indicate the presence of Se²⁻ bonds in binary TMSe. Furthermore, a secondary peak at 58.78 eV suggests surface oxidation of Se due to air exposure.^{[ 64 ]} The Se 3d_5/2 binding energy at 54.1 eV in the composite underwent a negative shift compared to CoSe₂ (Se 3d_5/2  =  54.4 eV), indicating an increased electron density due to composite formation. Furthermore, the composite material exhibited a positive shift in the Ni 2p_3/2 (853.4 eV) and Co 2p_3/2 (778.5 eV) peaks compared to NiSe₂ (Ni 2p_3/2 = 853.1 eV) and CoSe₂ (Co 2p_3/2 = 778.3 eV), indicating modified electronic interactions.^{[ 65} , ⁶⁶ , ^{67 ]} This finding indicates electronic modulation within the composite, attributed to charge transfer between these elements due to differences in electronegativity and electron cloud density around the bonds. The resulting increase in binding energy differences enhances local electric dipole moments, thereby promoting the adsorption and desorption processes in surface redox reactions while reducing the kinetic barrier.^{[ 68 ]}

A comprehensive analysis was conducted to investigate the morphologies of pristine MXene, pristine NiCo₂Se_4, and the MXene‐NiCo₂Se₄ composite using field‐emission scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM). Figure  3a b, depict the morphological characteristics of pristine MXene and NiCo₂Se₄, respectively. Pristine MXene exhibited a multilayered, accordion‐like structure with a smooth surface, while pristine NiCo₂Se₄ formed a porous network. After synthesizing the MXene‐NiCo₂Se₄ composite, NiCo₂Se₄ adhered uniformly to the multilayered MXene, forming an interconnected nanostructure (Figure 3c,d). This surface integration was further confirmed using TEM (Figure 3e), revealing a single nanoparticle of the MXene‐NiCo₂Se₄ composite. The porous NiCo₂Se₄ particles encapsulated the edges of the MXene nanosheets. Furthermore, the high‐resolution TEM image (Figure 3f) revealed well‐defined d‐spacing values of 0.27 and 0.17 nm, corresponding to the (002) and (313) crystal planes of NiCo₂Se₄, respectively.^{[ 69 ]} The d‐spacing of the (002) plane in MXene (Figure 3g) was measured at 0.97 nm and 1.41 nm, aligning precisely with the XRD results and consistent with interlayer spacing (d‐value) calculations based on Bragg's law. Additionally, elemental mapping images of Ti, Ni, Co, and Se elements (Figure S3, Supporting Information) confirmed the uniform distribution of NiCo₂Se₄ on the MXene nanosheets. The elemental maps demonstrated an even dispersion of Ni, Co, Se, and Ti, indicating the successful integration of NiCo₂Se₄ with MXene.

Figure 3.

SEM images of a) pristine Ti₃C₂T_x MXene, b) pristine NiCo₂Se_4, and MXene‐NiCo₂Se₄ nanocomposite on c) plane‐ and d) cross‐sections. e–g) TEM images of the MXene‐NiCo₂Se₄ nanocomposite at various magnifications.

The electrochemical performance of the engineered MXene‐NiCo₂Se₄@NF electrode was evaluated in a 6 M potassium hydroxide (KOH) electrolyte using a three‐electrode cell configuration. First, the composition of MXene and NiCo₂Se₄ was optimized for charge storage efficiency through cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) experiments. The supporting information (Figures S4–S7, Supporting Information) provides the detailed results. The MXene‐NiCo₂Se₄@NF electrode exhibited higher specific capacity (786.25, 745.40, 660.79, 617.34, and 589.22 C g⁻¹ at 1, 2, 5, 10, and 20 A g⁻¹) and better rate capability (74.9% of its initial capacity at 20 A g⁻¹) when the MXene‐to‐NiCo₂Se₄ mass ratio was 2:3 (denoted as MXene‐NiCo₂Se₄‐2 in the supporting information. This enhanced performance may be attributed to the increased nanoporous structure and the abundant active sites due to the optimal proportion between MXene and NiCo₂Se₄.

A comparative analysis was conducted to evaluate the performance of various electrodes fabricated using the same loading ratio but varying compositions, such as NiCo‐LDH, NiCo₂O₄, and NiCo₂S₄. CV measurements were performed at multiple scan rates to analyze the electrochemical behavior of these electrodes (Figure S8, Supporting Information). Figure  4a presents the CV curves of various electrodes recorded at a scan rate of 5 mV s⁻¹ for comparison. Among these electrodes, the MXene‐NiCo₂Se₄@NF electrode exhibited the highest peak current, indicating accelerated Faradaic processes due to the improved conductivity of selenium. Additionally, the anodic (E_pa) and cathodic (E_pc) peak potentials, along with the peak‐to‐peak separation (ΔE_pp) obtained from the CV curves, are summarized in Table S1 (Supporting Information). MXene‐NiCo₂Se₄ exhibited lower E_pa and E_pc values than others, indicating that lower potentials are needed for deprotonation and protonation processes in the Faradaic reaction (Mⁿ⁺ ↔ M⁽ⁿ⁺¹⁾⁺ + e⁻, M = Ni or Co).^{[ 70 ]} This behavior is attributed to selenium substitution for sulfur or oxygen anions, which weakens cation interactions due to its lower electronegativity, larger ionic radius, and higher‐lying orbital hybridization, enhancing covalent bonding. As the anions within the same family move across the periodic table, increasing ionic radius decreases electron binding energy and electrode redox potentials. This phenomenon occurs because the Se (4s4p) nuclei attract outer‐shell electrons less strongly than S (3s3p) or O (2s2p) anions. In addition, selenium exhibits the same coordination and valence as S and O with transition metal cations. Weaker atomic or electronic interactions reduce internal energy, decreasing electrochemical redox potential and the energy barrier governing electron transfer. Consequently, Mⁿ⁺/M⁽ⁿ⁺¹⁾⁺ redox reactions are easily initiated.^{[ 71} , ^{72 ]} Furthermore, the reduced electron transfer barrier accelerates the Nernstian equilibrium. Consequently, the smallest ΔE_pp of the selenide electrode indicates a faster, more reversible electrochemical redox process than that of the other electrodes.

Figure 4.

a) CV curves at a scan rate of 5 mV/s for various electrodes. b) A linear relationship exists between peak current and the square root of the scan rate. c) Nyquist plot. d) Cycling stability of MXene‐NiCoLDH@NF, MXene‐NiCo₂O₄@NF, MXene‐NiCo₂S₄@NF, MXene‐NiCo₂Se₄@NF, and pristine NiCo₂Se₄@NF electrodes with the same mass ratio over 8000 cycles at a current density of 10 A/g in 6 M KOH electrolyte.

To determine reversibility and the electrode charge storage mechanism, the relationship between the oxidation peak current density (I_p ) and scan rate (v) was investigated. Figure 4b shows a linear relationship between I_p  and the square root of v (v ^1/2), suggesting a diffusion‐controlled redox process. The diffusion coefficient (D) of the ions was calculated using the Randles–Sevcik equation as follows^{[ 73 ]}:

$$i_p=0.4463nFAC{\left(\frac{nFvD}{RT}\right)}^{1/2}$$ (1)

At an electrolyte temperature of 25 °C, the equation can be simplified to:

$$i_p=2.69\times {10}^5{n}^{3/2}A{D}^{1/2}C{v}^{1/2}$$ (2)

where i_p , n, F, A, D, C, R, T, and v represent the peak current in ampere, number of electrons involved in the redox reaction, Faraday constant in C mol⁻¹, specific surface area of the electrode in cm², diffusion coefficient in cm² s⁻¹, concentration of electrolyte in mol cm⁻³, gas constant, temperature, and scan rate in V s⁻¹, respectively. In this study, the electrochemical activity of different electrodes was evaluated and compared. MXene‐NiCo₂Se₄ exhibited a higher slope than other electrodes, which indicates a fast and reversible electron transport owing to an improved diffusion coefficient caused by MXene and the higher conductivity of selenium. Table S2 (Supporting Information) lists the calculated diffusion coefficients for all electrodes. Notably, the MXene‐NiCo₂Se₄ electrode showed a significantly higher diffusion coefficient (≈3, 7, and 8 times higher) than that of sulfide, oxide, and hydroxide electrodes, respectively (Table S3, Supporting Information). Moreover, the pristine NiCo₂Se₄ system exhibited nonlinearity at high scan rates due to reduced electrolyte diffusion and increased charge transfer resistance. Similarly, the MXene‐NiCo₂O₄ electrode demonstrated nonlinearity, attributed to the oxidation‐induced increase in charge transfer resistance.

Electrochemical impedance spectroscopy (EIS) was utilized to investigate the interfacial resistance of the prepared electrodes over a frequency range of 100 kHz–0.01 Hz. The corresponding results are shown in Figure 4c and Table S4 (Supporting Information). In the high‐frequency region, the Nyquist plot intercept at the real axis (Z^′) represents the equivalent series resistance (ESR), which comprises electrolyte resistance, ohmic resistance of active materials, and contact resistance at the active material‐substrate interface. The inset of Figure 4c indicates that the ESR values for MXene‐containing samples are significantly lower than those of pure NiCo₂Se₄, except for the MXene‐NiCo₂O₄ electrode. This can be attributed to the improved internal conductivity caused by the enhanced active sites of multilayered Ti₃C₂T_x. Notably, the MXene‐NiCo₂O₄@NF electrode exhibited the highest ESR due to the complete oxidation of Ti₃C₂T_x to TiO₂, indirectly confirming the positive role of MXene in improving conductivity. At mid‐frequency, the height and diameter of the semicircular arc represent the double‐layer capacitance (C_dl) and charge‐transfer resistance (R_ct), respectively. R_ct originates from the Faradaic reaction impedance at the electrode‐electrolyte interface. Both the MXene‐NiCo₂Se₄ and pure NiCo₂Se₄ electrodes exhibited essentially lower R_ct values (0.27 and 0.33 Ω) than other samples. It can be ascribed to the surface‐decorated TMSe, which lowers the charge transfer barrier compared to sulfides or hydroxides, thereby facilitating charge transfer, consistent with the aforementioned. In the low‐frequency region, the length of the 45° sloped line represents the Warburg resistance, which relates to ion diffusion in the electrolyte. The MXene‐NiCo₂Se₄ electrode exhibited a shorter projected length on the real axis than the pure NiCo₂Se₄ electrode, indicating that layered MXene particles effectively shorten ion transport length within the electrolyte. Furthermore, the plot becomes more vertical at lower frequencies, reflecting a transition from diffusion‐limited behavior to an ideal capacitive response. This increased verticality indicates improved ion accessibility to the electrode surface, facilitating electric double‐layer formation. Such behavior is characteristic of high‐performance capacitive materials, where charge storage primarily occurs through surface redox reactions and ion adsorption. The enhanced verticality observed in MXene‐containing electrodes suggests that MXene integration improves capacitive performance by promoting faster ion diffusion and better active site utilization. These advantages underscore the potential of MXene‐based composites for advanced SC applications.

Cycling stability, a key factor in determining the lifespan of a hybrid capacitor, was assessed through a comparative analysis of pristine NiCo₂Se₄ and MXene‐based composite electrodes (Figure 4d). Notably, the MXene‐NiCo₂Se₄@NF electrode retained over 90% of its capacity, with Coulombic efficiency stabilizing at ≈98% after 8000 cycles at 10 A g⁻¹. To understand the enhanced stability, SEM images of the pristine NiCo₂Se₄@NF and MXene‐NiCo₂Se₄@NF electrodes were analyzed before and after the stability test (Figure S9, Supporting Information). The analysis revealed a significant decrease in pristine NiCo₂Se₄ porosity after the stability test due to phase transitions and volume changes during the redox process. As previously discussed, the charge–discharge process induces Ni valence fluctuations between +2 (t_2g ⁶e_g ²) and +3 (t_2g ⁶e_g ¹), repeatedly triggering geometric Jahn–Teller distortion, causing nanostructure and hindering electrolyte diffusion.^{[ 74} , ^{75 ]} Consequently, the specific capacity decreased. In contrast, the nanocomposite retained its multilayer structure (Figure S9c,d, Supporting Information), with MXene enhancing stability and preserving a high electrochemically active surface area. Additionally, Ti₃C₂T_x terminal groups mitigate NiCo₂Se₄ structural distortions through electroadsorption, thereby preventing significant charge storage degradation.

To quantitatively evaluate the charge–discharge performance of MXene‐NiCo₂Se₄ electrodes in a full‐cell system, an AHC device was fabricated using an alkaline electrolyte with MXene‐NiCo₂Se₄ electrode and AC serving as the positive and negative electrodes, respectively (Figure  5a). Initially, the operational potential range was determined by evaluating the CV curves of the MXene‐NiCo₂Se₄ and AC electrodes in a three‐electrode system at a scan rate of 10 mV s⁻¹ (Figure S10a, Supporting Information). The AC and MXene‐NiCo₂Se₄ electrodes exhibited potential windows of −1.0 to 0 V and 0–0.6 V, respectively, making them suitable for an asymmetric device. The AHC device uses a PVA‐KOH gel as both a polymer electrolyte and membrane separator to prevent short circuits between the anode and cathode. CV curves were recorded for voltage ranges of 1.0–1.8 V at a scan rate of 50 mV s⁻¹ to determine the optimal operating voltage window (Figure S10b, Supporting Information). The integrated CV curve area increases with voltage expansion up to 1.6 V, beyond which a significant asymmetric current increment occurs due to oxygen evolution from water‐splitting reactions. Therefore, the optimal operating voltage for the device is 1.6 V. Figure 5b,c depict the CV and GCD curves of the AHCs, which remained stable even at higher scan rates and current densities, indicating a high‐performance rate and stable reversibility. At the nanoscale level, the measured current consists of diffusion‐controlled (i_diff ) and surface‐controlled ( i cap ) components. To determine the charge storage mechanism of the system, a widely accepted equation (Equation 3) that correlates measured current (i) with scan rate (v) was employed.^{[ 76} , ^{77 ]}

$$i=i_{cap}+i_{diff}=a{v}^b$$ (3)

where a and b are the fitting parameters. The b value is derived from the slope of a log(i) versus log(v) plot using the equation log(i) = log(a) + blog(v). The electrochemical response is influenced by the b value, where b = 0.5 indicates a diffusion‐controlled process, whereas a b value close to 1 indicates a surface‐controlled capacitive process. Figure 5d and its inset show b values ranging from 0.77 to 0.89, indicating a combined charge storage mechanism involving surface‐controlled (k₁v) and diffusion‐controlled (k₂v ^1/2) contributions. The capacitive contribution can be estimated by rewriting Equations (3) as (4).^{[ 78} , ^{79 ]} To facilitate analysis, Equation (4) is rearranged as Equation (5):

$$i\left(V\right)=k_{1}v+k_2{v}^{1/2}$$ (4)

$$i\left(V\right)/v^{1/2}=k_1{v}^{1/2}+k_2$$ (5)

where i(V) and v represent the current density (A/g) at different voltages (V) and scan rate (V s⁻¹), respectively. The parameters k₁ and k₂ are determined by plotting i/v ^1/2 versus v ^1/2. This enables the separation of surface‐controlled capacitive and diffusion‐controlled redox process contributions. Figure 5e and Figure S11 (Supporting Information) show the CV curves of MXene‐NiCo₂Se₄ in the ASC device, with capacitive current contributions calculated at scan rates ranging from 5 to 100 mV s⁻¹. At a low scan rate (5 mV s⁻¹), charge storage is evenly split between capacitive (50%) and diffusion‐controlled (50%) contributions, facilitated by the interlinked MXene‐NiCo₂Se₄ composite, which enables OHˉ ions to penetrate freely. As the scan rate increases, the capacitive process contributes significantly to the charge storage, as shown in the histogram (Figure 5f). These results show that diffusion control is dominant at low scan rates, while surface‐controlled capacitance dominates at high scan rates because of finite diffusion time and fast reaction kinetics.^{[ 80 ]} Therefore, the MXene‐NiCo₂Se₄@NF||AC electrode performs well at high charge/discharge currents due to its high capacitive contribution.

Figure 5.

Electrochemical performance of the MXene‐NiCo₂Se₄||AC ASC device. a) Schematic diagram of the ASC device. b) CV curves at various scan rates and c) charge–discharge curves at various current densities of MXene‐NiCo₂Se₄@NF in the ASC device. d) Plot of b‐values (calculated from inset, log i versus log v) versus various potentials. e) Capacitive contribution in the CV curve at a scan rate of 5 mV s⁻¹. f) Ratio of capacitive and diffusion contributions at various scan rates (5–100 mV s⁻¹). g) Ragone plot of the developed ASC device along with previously reported devices. h,i) Photographic images of the ASC device, lighting a green LED bulb, and a digital thermometer, respectively.

To assess the feasibility of the hybrid device, energy density and power density were calculated using Equations (6) and (7), respectively:^{[ 81 ]}

$$E=\frac{1}{2}{C}_s\mathrm{\Delta }{V}^2$$ (6)

$$P=\frac{E}{\mathrm{\Delta }t}$$ (7)

where P, E, Δt, Cs, and ΔV represent power density (W/kg), energy density (Wh kg⁻¹), discharge time (s), specific capacitance (F g⁻¹), and voltage window for discharge (V), respectively. The E and P values derived from the GCD curves (Figure 5c) are shown in the Ragone plot (Figure 5g). The assembled ASC achieves a high energy density of 64.36 Wh kg⁻¹ at a power density of 0.8 kW/kg while maintaining an energy density of 23.8 Wh kg⁻¹ at a high power density of 18 kW kg⁻¹. This performance surpasses that of most reported MXene‐based capacitor devices or similar works, including MXene‐NiCoAl LDH||AC (45.8 Wh kg⁻¹ at 0.35 kW kg⁻¹),^{[ 82 ]} MXene‐MnO₂||AC (29.8 Wh kg⁻¹ at 0.75 kW kg⁻¹),^{[ 83 ]} MXene‐CuS||AC (15.4 Wh kg⁻¹ at 0.75 kW kg⁻¹),^{[ 84 ]} graphene‐NiCo₂Se₄||AC (37.83 Wh kg⁻¹ at 1.44 kW kg⁻¹),^{[ 85 ]} CNT‐CoZnSe||PCNF (61.4 Wh kg⁻¹ at 0.75 kW kg⁻¹),^{[ 86 ]} Ni_4.5Co_4.5Se||AC (47.4 Wh kg⁻¹ at 1.5 kW kg⁻¹),^{[ 87 ]} and NiCoSe₂||AC (35 Wh kg⁻¹ at 0.19 kW kg⁻¹).^{[ 88 ]} Additionally, the results indicate that asymmetric devices offer better energy storage ability compared to symmetric devices, such as doped C‐MXene (10.8 Wh kg⁻¹ at 0.6 kW kg⁻¹),^{[ 89 ]} MXene film (9.2 Wh kg⁻¹ at 0.1 kW kg⁻¹),^{[ 90 ]} PANI‐MXene (25.6 Wh kg⁻¹ at 0.15 kW kg⁻¹),^{[ 91 ]} and MXene‐graphene (11.5 Wh kg⁻¹ at 0.06 kW kg⁻¹).^{[ 92 ]} Finally, the fabricated ASC device demonstrated its practicality by powering a green LED and a digital thermometer (Figure 5h,i). This highlights its potential for smart devices and sensor applications.

3. Conclusion

In summary, we successfully fabricated an advanced MXene (Ti₃C₂T_x)‐TMSe (NiCo₂Se₄) material for SCs using binder‐free electrodeposition and thermal selenization under an inert atmosphere on a nickel foam electrode. Oxygen‐free thermal selenization of NiCo‐LDH enhanced conductivity, while combining multilayered Ti₃C₂T_x with porous NiCo₂Se₄ (derived from double‐layered hydroxides of Ni and Co) increased the active surface area, which facilitated ion diffusion and improved charge storage performance. The synergy between Ti₃C₂T_x and NiCo₂Se₄, along with the hierarchical nanostructure, enables a high specific capacity of 786.25 C g⁻¹ at a current density of 1 A g⁻¹ while maintaining a 91.8% retention rate over 8000 cycles at a current density of 10 A g⁻¹. Furthermore, an AHC device assembled using the MXene‐NiCo₂Se₄@NF and AC@NF as positive and negative electrodes, respectively, achieved an energy density of 64.36 Wh kg⁻¹ at a power density of 0.8 kW kg⁻¹. This study emphasizes the benefits of selenization as an effective strategy for improving the electrochemical performance of transition metal compounds for asymmetric hybrid capacitors. Additionally, multilayered MXene‐based heterostructure composites provide a promising avenue for advancing pristine MXene applications in SCs, batteries, sensors, and electrolyzers.

4. Experimental Section

Synthesis of Ti₃C₂T_x MXene

Ti₃C₂T_x was synthesized from Ti₃AlC₂ via HF etching at room temperature. Initially, 3.0 g of 98 wt.% Ti₃AlC₂ powder (100 mesh) was added to 60 mL of 49% HF solution and stirred continuously at room temperature for 24 h. The resulting MXene suspension was treated with deionized (DI) water and centrifuged at 4000 rpm to separate the powder. This process was repeated until the supernatant reached a pH of ≈6. The final powder was vacuum‐dried at 80 °C for 12 h.

Fabrication of MXene@NF Electrode

The 0.1 g of dried Ti₃C₂T_x powder was dispersed in 50 mL of DI water and sonicated for 10 min. Pre‐cleaned NF was cut into 1 × 4 cm² tiles and placed on a hot plate. Repeated dipping and drying cycles of the Ti₃C₂T_x suspension were conducted to ensure optimal concentration of MXene loading on the NF.^{[ 93} , ^{94 ]}

Synthesis of NiCo₂Se₄ Nanosheets on MXene@NF Electrodes

The synthesis process involved co‐electrodeposition of a mixed nickel‐cobalt hydroxide, followed by annealing in a Se vapor atmosphere. As previously reported, the initial electrochemical deposition of bimetallic nickel‐cobalt hydroxide onto the MXene@NF electrode was conducted using a CV electrodeposition method.^{[ 95 ]} A 4 mm Co(NO₃)₂•6H₂O and 2 mM Ni(NO₃)₂•6H₂O aqueous solution was used at room temperature, with the MXene@NF electrode serving as the working electrode, a saturated Ag/AgCl electrode as the reference, and a Pt sheet as the counter electrode. The potential was cycled from −1.2 to 0.2 V at a scan rate of 5 mV s⁻¹ for 5 cycles. The resulting NiCo LDH‐MXene@NF electrode was then dried in an oven for 12 h. The NiCo LDH‐MXene@NF electrode was positioned at the center of a tube furnace, while Se powder was positioned upstream. The selenylation process was conducted under an N₂ atmosphere at 350 °C with a heating rate of 2 °C min⁻¹ for 3 h. Finally, the selenized electrode, MXene‐NiCo₂Se₄@NF, was repeatedly washed with DI and ethanol and then dried under vacuum at 60 °C for 12 h.

Fabrication of Asymmetric Hybrid Capacitor Devices

The ASC device was fabricated with a two‐electrode configuration, employing MXene‐NiCo₂Se₄@NF as the positive electrode and AC@NF as the negative electrode. The two electrodes were separated by a PVA‐KOH polymer gel electrolyte membrane. The AC@NF electrode was fabricated using the drop‐casting method by blending 80 wt.% active material, 10 wt.% acetylene black, and 10 wt.% polytetrafluoroethylene in NMP solvent.

Characterization

The crystalline structure and phase of the materials were examined using an automated XRD system (Bruker D8 Advance, Germany) equipped with Cu‐K_α radiation (λ = 1.5418 Å) at 40 kV and 40 mA. The analysis was conducted in the 2θ range of 5° to 90° at room temperature. The structural and morphological features of the samples were characterized using XPS (ESCA2000, Al K_α excitation laser), field‐emission SEM, and TEM (JEM‐2100F).

Electrochemical Measurements

Electrochemical experiments were performed in a 6 m KOH aqueous electrolyte using an RST 5100F electrochemical workstation. The prepared electrode served as the working electrode, while a Pt plate and Hg/HgO were used as the counter and reference electrodes, respectively. GCD measurements were conducted within the appropriate potential range at varying current densities of 1, 2, 5, 10, and 20 A g⁻¹. EIS was conducted at open‐circuit potential with a 5 mV AC perturbation over a frequency range of 100 kHz–0.01 Hz. The cycling stability of the electrodes was evaluated over 8000 charge–discharge cycles at a current density of 10 A g⁻¹. An AHC device was assembled using MXene‐NiCo₂Se₄@NF and AC@NF as the positive and negative electrodes, respectively. The mass loading of the negative electrode was determined by balancing the charges stored in the two electrodes.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Li H., Kalaiyarasan G., Cao X., et al. “Selenized Binary Transition Metals‐MXene Composite for High‐Performance Asymmetric Hybrid Capacitors.” Small 21, no. 36 (2025): 21, e04350. 10.1002/smll.202504350

Data Availability Statement

All data generated or analyzed during this study are included in this published article and its supplementary information file.