In Situ Growth of Ni-MOF Nanorods Array on Ti3C2Tx Nanosheets for Supercapacitive Electrodes

For the energy supply of smart and portable equipment, high performance supercapacitor electrode materials are drawing more and more concerns. Conductive Ni-MOF is a class of materials with higher conductivity compared with traditional MOFs, but it continues to lack stability. Specifically, MXene (Ti3C2Tx) has been employed as an electrochemical substrate for its high mechanical stability and abundant active sites, which can be combined with MOFs to improve its electrochemical performance. In this paper, a novel Ni-MOF nanorods array/Ti3C2Tx nanocomposite was prepared via a facile hydrothermal reaction, which makes good use of the advantages of conductive Ni-MOF and high strength Ti3C2Tx. The high density forest-like Ni-MOF array in situ grown on the surface of Ti3C2Tx can provide abundant active electrochemical sites and construct a pathway for effective ion transport. The formation of a “Ti-O···Ni” bond accomplished during an in situ growth reaction endows the strong interfacial interaction between Ni-MOF and Ti3C2Tx. As a result, the Ni-MOF/Ti3C2Tx nanocomposite can achieve a high specific capacitance of 497.6 F·g−1 at 0.5 A·g−1 and remain over 66% of the initial capacitance when the current density increases five times. In addition, the influence of the Ti3C2Tx concentration and reaction time on the morphology and performance of the resultant products were also investigated, leading to a good understanding of the formation process of the nanocomposite and the electrochemical mechanism for a supercapacitive reaction.


Introduction
Energy supplement devices with ultrathin volume, high energy density, and long lifespan are arousing amounts of interest along with the miniaturization and portability of smart equipment [1][2][3]. Among them, high performance supercapacitors have propelled rapidly because of their fast charge/discharge speed, convenient assembly, and high-level safety regardless of if the miniaturization process is performed [4][5][6]. However, further progress is severely constrained by their low energy density. Thus, it is necessary to investigate novel electrode materials with superior conductivity, high specific surface area, and easy preparation to overcome this restriction [7][8][9].
Metal-organic frameworks (MOFs) have been widely focused on in the past decades because of their considerable specific surface area, tunable structural architectures, and abundant active sites [10][11][12]. Formerly, the low conductivity limits traditional MOFs' appearance in a number of current advanced technologies with the requirement of high electrical conductivity. With the appearance of conductive MOFs such as M-HHTP (M is Co, Ni, Cu, etc. and HHTP is hexahydroxytriphenylene) [13,14], they gradually exhibit greater potential in supercapacitors [15,16]. For instance, Mircea et al. reported the first example of a supercapacitor electrode made entirely from neat conductive MOFs as Nanomaterials 2023, 13, 610 2 of 12 active materials [17]. However, the improved conductivity is still restricted by the shortages of low specific capacitances and structural stability in the long-term cycling process. Therefore, other conductive matrixes including conductive polymers (e.g., polypyrrole or polyaniline) and carbon-based materials (e.g., graphene or carbon nanotubes) have been brought into compositing with MOF to guarantee the effective control on electrochemical performances [18][19][20][21][22][23].
M n+1 X n T x (n = 1~4), in which M represents transition metals (e.g., Ti, V, etc.), X is C or/and N, and T x stands for terminal groups, such as -O, -OH, and -F, on the surface [24,25], and also named MXene, has been developed quickly since it has excellent conductivity [26,27], many surface groups for functional modification [28,29], and other fascinating tunable properties [30,31]. Recently, Ti 3 C 2 T x , the most widely studied MXene, has been used in supercapacitors by diversiform-processing techniques [32][33][34]. Nevertheless, there are also shortcomings in the easy stacking and oxidization of Ti 3 C 2 T x nanosheets during the electrochemical reaction process, which can be remitted by modifying them with other active materials [35,36].
Herein, combined with the above merits of MOFs, it can be clearly suggested that the nanocomposite synthesized from Ti 3 C 2 T x and conductive Ni-MOF will effectively make the utmost of their advantages since MXenes improve composite conductivity and provide sufficient surface groups to interact with MOFs while the Ni-MOF plays an important role in offering plentiful active sites and large specific surface area, reasonably obtaining incremental attention in electrochemical applications [37][38][39]. However, the direct combination out of in situ growth provides weak interfacial bonding, which can result in high transport impedance and low cyclic stability. In addition, the relationship between structural architectures and electrochemical performances is also unrevealed through regulating reaction conditions. Briefly, there is a significant attempt to develop MXene-MOF nanocomposites to solve the above issues on the account of the in situ growth strategy.
In this study, we propose a strategy for the in situ growing of forest-like hexagonalshaped Ni-MOF (Ni-HHTP) directly on the surface of Ti 3 C 2 T x rather than the simple mixture. The advantages of Ti 3 C 2 T x and Ni-MOF are effectively combined in this nanocomposite to address the aforementioned shortcomings, enabling reversible transport of electrons and ions at the interface as well as preventing the collapse of porous MOF structures during the insertion and de-intercalation of electrolyte ions. Additionally, the in situ growth mechanism of this nanocomposite was studied through analyzing the morphology and structure of serial products prepared from the controlled factors of reactant concentration and reaction time. Moreover, the electrochemical characterizations reveal that the Ni-MOF/Ti 3 C 2 T x nanocomposite exhibits a high specific capacitance value of 497.6 F·g −1 (0.5 A·g −1 ) with a high cyclic stability of 85% during 1000 cycles, which can be due to the synergistic effect of the underlying MXene substrate and porous MOF architectures. Furthermore, an all-solid-state asymmetric supercapacitor was also demonstrated by exploiting the Ni-MOF/Ti 3 C 2 T x nanocomposite as a positive-electrode material, successful in lighting a LED. The obtained results will possess great potential for electrochemical devices.

Synthesis of MXene Ti 3 C 2 T x
As the most reported method [40], Ti 3 C 2 T x nanosheet was synthesized as follows: 8 g of LiF was added to 100 mL of 9 M HCl in a PTFE bottle as the etchant. A total of 5 g Ti 3 AlC 2 was slowly added to the above etchant in an ice bath. The reaction was heated under 40 • C and magnetic stirring was maintained for 24 h. A dark green Ti 3 C 2 T x slurry was formed after washing by deionization water via centrifugation at 3500 rpm for 5 min until the pH reached 5~6. The Ti 3 C 2 T x powder was collected through centrifugating the slurry at 12,000 rpm for 30 min and drying the sediment at 80 • C in a vacuum oven for 24 h.

Synthesis of Ni-MOF/Ti 3 C 2 T x
The Ni-MOF/Ti 3 C 2 T x nanocomposite was synthesized as follows: 0.5 mg/mL of Ti 3 C 2 T x solution was prepared under ultrasonication for 30 min. An amount of 40 mg Ni(OAc) 2 ·4H 2 O and 28 mg HHTP was dispersed in 4 mL deionized water, respectively, before mixing the two homogeneous solutions above. Then, 8 mL mixture was added to 8 mL Ti 3 C 2 T x solution in a 20 mL vessel. The reaction was heated in an oven at 80 • C for 8 h and then cooled down to room temperature within 3 h. Black sediment was collected at the bottom of the vessel. The Ni-MOF/Ti 3 C 2 T x sample was obtained by washing the sediment using deionized water and ethanol for several times. The length of Ni-MOF nanorods in situ grown on the Ti 3 C 2 T x nanosheet was efficiently controlled by the hydrothermal reaction time. The pure Ni-MOF powder was synthesized through the same procedure without Ti 3 C 2 T x .

Activation of Ni-MOF/Ti 3 C 2 T x
The Ni-MOF/Ti 3 C 2 T x powder was activated after immersing it into deionized water at 80 • C overnight and exchanging the solvent with acetone that was replaced everyday for three days. The acetone-exchanged Ni-MOF/Ti 3 C 2 T x powder was collected after drying at 80 • C in a vacuum oven for 24 h to remove guest molecules.

Fabrication of the Electrodes in a Traditional Three-Electrode System
Electrochemical performance was conducted by a CHI 660E workstation in a threeelectrode system. A platinum wire electrode and a Hg/HgO electrode was used as the counter electrode and reference electrode, respectively. A concentration of 6.0 M KOH was chosen as the electrolyte. The working electrode was prepared by the mixing active materials, acetylene black and polytetrafluoroethylene (PTFE), with a weight ratio of 8:1:1, and then coating the mixture above on a 1 cm × 3 cm nickel foam. N-methyl-2-pyrrolidone was chosen as the additive when mixing and the coating area was about 1 cm 2 . Then, a thin foil was achieved by pressing the nickel foam under 10.0 MPa. The mass-loading of the active material was about 3-5 mg.

Fabrication of the Asymmetric Supercapacitor
The Ni-MOF/Ti 3 C 2 T x nanocomposite and the active carbon were used as the positive and negative electrode materials, respectively. The mass ratio of electrode materials is determined by C + m + = C − m − , where C + and C − are the specific capacitance of Ni-MOF/Ti 3 C 2 T x and active carbon. The KOH/PVA solution was used as the electrolyte and then sandwiched by the electrode. Lastly, the flexible supercapacitor was packeted with PET membrane. The resulting electrochemical measurement was conducted via a CHI 660E workstation.

Sample Preparation and Structure Characterization
The schematic diagram of the synthesis procedure of the Ni-MOF/Ti 3 C 2 T x nanocomposite is depicted in Figure 1a. Initially, the Ti 3 C 2 T x solution was prepared from its corresponding nanosheet powder that was obtained through etching the Al layers using LiF/HCl and dried in a vacuum; the two-dimensional Ti 3 C 2 T x nanosheets' morphology is shown in Figure S1. Next, a homogeneous precursor mixture was obtained by adding Ni(OAc) 2 ·4H 2 O and HHTP into the above Ti 3 C 2 T x solution, which was subsequently put in an 80 • C oven for a hydrothermal reaction. Lastly, the expected Ni-MOF/Ti 3 C 2 T x sample was received via washing the collected reaction sediment with DI water and ethyl alcohol repeatedly. Afterwards, the controlled products prepared from the various reactant concentrations and hydrothermal times were employed to investigate the in situ growth mechanism and electrochemical performance of the nanocomposite. concentrations and hydrothermal times were employed to investigate the in situ growth mechanism and electrochemical performance of the nanocomposite. The morphology of the as-prepared Ni-MOF/Ti3C2Tx nanocomposite (0.5 mg·mL −1 and 12 h) was revealed using the images from scanning electron microscopy (SEM), as shown in Figure 1b. It can be seen that the regular forest-like Ni-MOF nanorod arrays with hexagonal architectures and lengths of up to 550 nm were grown in situ on the surface of Ti3C2Tx, vertically, forming strong interfacial bonds, which can bring about low electrochemical impedance and high cyclic stability for this well-designed nanocomposite. In addition, there was no obvious stacking phenomenon of the 2D Ti3C2Tx nanosheets since the in situ growth of the Ni-MOF arrays can also effectively prevent the agglomeration of lamellae that occurred in the hydrothermal process. Furthermore, the elemental mapping of this Ni-MOF/Ti3C2Tx nanocomposite given by the energy dispersive spectrometer (EDS) also demonstrates the uniform in situ growth of the Ni-MOF arrays arranged on the surface of the Ti3C2Tx nanosheets, as consistent with the above SEM results (Figure 1e). This leads to abundant active sites for redox reactions and effective transport pathways for electron transfer and ion diffusion, which makes a great contribution to enhance the electrochemical properties of this novel Ni-MOF/Ti3C2Tx nanocomposite.
Furthermore, the nature of the crystal structure of the as-synthesized Ni-MOF/Ti3C2Tx products was characterized by transmission electron microscopy (TEM) (Figure 1c). It can be evidently seen in Figure 1c that the nanocomposite consists of numerous Ni-MOF nanorods in situ forming on the surface of the 2D Ti3C2Tx nanosheets, which is in agreement with the above SEM observations. Additionally, the apparent wrinkles of the 2D nanosheets and the whole forest-like constructions implicitly exhibit the Ni-MOF/Ti3C2Tx sample endowed with steady and strong interfacial bonding because there is no obvious crack or collapse occurring even after ultrasonication treatment. The high The morphology of the as-prepared Ni-MOF/Ti 3 C 2 T x nanocomposite (0.5 mg·mL −1 and 12 h) was revealed using the images from scanning electron microscopy (SEM), as shown in Figure 1b. It can be seen that the regular forest-like Ni-MOF nanorod arrays with hexagonal architectures and lengths of up to 550 nm were grown in situ on the surface of Ti 3 C 2 T x , vertically, forming strong interfacial bonds, which can bring about low electrochemical impedance and high cyclic stability for this well-designed nanocomposite. In addition, there was no obvious stacking phenomenon of the 2D Ti 3 C 2 T x nanosheets since the in situ growth of the Ni-MOF arrays can also effectively prevent the agglomeration of lamellae that occurred in the hydrothermal process. Furthermore, the elemental mapping of this Ni-MOF/Ti 3 C 2 T x nanocomposite given by the energy dispersive spectrometer (EDS) also demonstrates the uniform in situ growth of the Ni-MOF arrays arranged on the surface of the Ti 3 C 2 T x nanosheets, as consistent with the above SEM results (Figure 1e). This leads to abundant active sites for redox reactions and effective transport pathways for electron transfer and ion diffusion, which makes a great contribution to enhance the electrochemical properties of this novel Ni-MOF/Ti 3 C 2 T x nanocomposite.
Furthermore, the nature of the crystal structure of the as-synthesized Ni-MOF/Ti 3 C 2 T x products was characterized by transmission electron microscopy (TEM) (Figure 1c). It can be evidently seen in Figure 1c that the nanocomposite consists of numerous Ni-MOF nanorods in situ forming on the surface of the 2D Ti 3 C 2 T x nanosheets, which is in agreement with the above SEM observations. Additionally, the apparent wrinkles of the 2D nanosheets and the whole forest-like constructions implicitly exhibit the Ni-MOF/Ti 3 C 2 T x sample endowed with steady and strong interfacial bonding because there is no obvious crack or collapse occurring even after ultrasonication treatment. The high resolution TEM (HRTEM) (Figure 1d) reveals that the characteristic lattice fringe of 1.69 nm belongs to the (100) crystal face of the Ni-MOF structure.
As shown in Figure 2a, according to previous studies, the characteristic diffraction peaks of the Ni-MOF/Ti 3 C 2 T x nanocomposites at 2θ of 4.8 • , 9.7 • , and 14.2 • are attributed to the (100), (200), and (210) lattice planes of Ni-MOF [41], respectively, suggesting a high crystallinity of the products. Furthermore, compared with the peaks of the mixture of Ni-MOF and Ti 3 C 2 T ( Figure S1), the location of the (002) plane shifts from 6.6 • to approximately 6.2 • and the intensity decreases to near disappearance, demonstrating that the (100) face of Ni-MOF nanorods were grown in situ on the (002) face of the Ti 3 C 2 T x nanosheets rather than simply mixture [42]. It can also be inferred that there was no stacking of Ti 3 C 2 T x nanosheets from the pattern in which more electrolyte anions can be contained and serve as electroactive species in the Faradaic redox reaction. Nanomaterials 2023, 13, x FOR PEER REVIEW 5 of 12 resolution TEM (HRTEM) (Figure 1d) reveals that the characteristic lattice fringe of 1.69 nm belongs to the (100) crystal face of the Ni-MOF structure. As shown in Figure 2a, according to previous studies, the characteristic diffraction peaks of the Ni-MOF/Ti3C2Tx nanocomposites at 2θ of 4.8°, 9.7°, and 14.2° are attributed to the (100), (200), and (210) lattice planes of Ni-MOF [41], respectively, suggesting a high crystallinity of the products. Furthermore, compared with the peaks of the mixture of Ni-MOF and Ti3C2T ( Figure S1), the location of the (002) plane shifts from 6.6° to approximately 6.2° and the intensity decreases to near disappearance, demonstrating that the (100) face of Ni-MOF nanorods were grown in situ on the (002) face of the Ti3C2Tx nanosheets rather than simply mixture [42]. It can also be inferred that there was no stacking of Ti3C2Tx nanosheets from the pattern in which more electrolyte anions can be contained and serve as electroactive species in the Faradaic redox reaction. Fourier transform-infrared (FT-IR) measurements were performed to confirm the interactions between the components of Ti3C2Tx and Ni-MOF based on the location changes of the functional groups in the Ni-MOF/Ti3C2Tx nanocomposite. As shown in Figure 2b, the spectrum of nanocomposite exhibits the typical absorption features of its above components containing the stretching mode of the O-H group at 3422 cm −1 , the C=O group at 1643 cm −1 , graphene at 1462 cm −1 , and the C-O group at 1257 cm −1 , which can be observed from Ti3C2Tx and Ni-MOF in Figure S2 [41]. It is known that the O-H stretching vibration peak can evidently appear red-shift when the interactions take place from the molecular level. It is noted in Figure S2 that the O-H stretching vibration peak at 3422 cm −1 for the Ni-MOF/Ti3C2Tx sample shows a certain red-shift compared with that at 3441 cm −1 for Ti3C2Tx, indicating there exists strong interactions between Ni-MOF and Ti3C2Tx, as consistent with the XRD results.
As exhibited in Figure 2c, the Brunauer-Emment-Teller (BET) test of the Ni-MOF/Ti3C2Tx nanocomposite gains a specific surface area of 167.74 m 2 ·g −1 and an average pore width of 14.3 nm. A type I isotherm profile demonstrates its microporous nature. All Fourier transform-infrared (FT-IR) measurements were performed to confirm the interactions between the components of Ti 3 C 2 T x and Ni-MOF based on the location changes of the functional groups in the Ni-MOF/Ti 3 C 2 T x nanocomposite. As shown in Figure 2b, the spectrum of nanocomposite exhibits the typical absorption features of its above components containing the stretching mode of the O-H group at 3422 cm −1 , the C=O group at 1643 cm −1 , graphene at 1462 cm −1 , and the C-O group at 1257 cm −1 , which can be observed from Ti 3 C 2 T x and Ni-MOF in Figure S2 [41]. It is known that the O-H stretching vibration peak can evidently appear red-shift when the interactions take place from the molecular level. It is noted in Figure S2 that the O-H stretching vibration peak at 3422 cm −1 for the Ni-MOF/Ti 3 C 2 T x sample shows a certain red-shift compared with that at 3441 cm −1 for Ti 3 C 2 T x , indicating there exists strong interactions between Ni-MOF and Ti 3 C 2 T x , as consistent with the XRD results.
As exhibited in Figure 2c, the Brunauer-Emment-Teller (BET) test of the Ni-MOF/Ti 3 C 2 T x nanocomposite gains a specific surface area of 167.74 m 2 ·g −1 and an average pore width of 14.3 nm. A type I isotherm profile demonstrates its microporous nature. All of these confirm its relatively large specific surface area by virtue of the well-defined forest-like nanorod arrays with a hierarchically hexagonal configuration, which is beneficial for enhancing the electrochemical properties.
Furthermore, X-ray photoelectron spectroscopy (XPS) was performed to illustrate the specific interaction sites of the above FT-IR analysis via characterizing the changes of the composition and the valence state of the elements between the Ni-MOF, Ti 3 C 2 T x , and Ni-MOF/Ti 3 C 2 T x nanocomposites. The fitting spectra of Ti 2p, Ni 2p, and O 1s of the Ni-MOF/Ti 3 C 2 T x sample were detected, which demonstrate a high degree of interaction achieved between Ti 3 C 2 T x and Ni-MOF ( Figure S4). As shown in Figure 2e, the Ni 2p 3/2 and 2p 1/2 peaks of the nanocomposite (855.9 eV and 873.6 eV, respectively) show a definite change (0.2 eV shifting) compared with those of Ni-MOF (856.1 eV and 873.8 eV, respectively), revealing the formation of the "Ti-O···Ni" interaction between Ni-MOF and Ti 3 C 2 T x as similar to the formation mechanism of "V-O···Ni", as previously reported [39]. Furthermore, the Ti 2p spectra provide fairly powerful evidence to confirm the in situ growth mechanism of the nanocomposite as depicted in Figure 2d. It can be evidently found that the three doublets of the Ti 2p spectra of the Ni-MOF/Ti 3 C 2 T x products present a majority of Ti 4+ (458.8/464.4 eV) and few of Ti 2+ (456.1/461.1 eV) and Ti 3+ (457.6/463.2 eV), which are much different from the eight deconvoluted peaks of Ti (454.7 eV/460.7 eV), Ti 2+ (455.6 eV461.1 eV), Ti 3+ (456.7 eV/462.5 eV), and Ti 4+ (458.8 eV/464.4 eV) of the pure Ti 3 C 2 T x nanosheets. In addition, the Ti 4+ in the nanocomposite turns out not to be in the form of TiO 2 , which can be proved by no obvious TiO 2 characteristic peak in the XRD spectra. As a consequence, this is a sign that an abundance of active Ti 4+ sites can be formed during hydrothermal reactions, as previously reported [43,44], and they play an important role in the in situ anchoring of Ni-MOF via the "Ti-O···Ni" interaction. The spectra of O 1s are shown in Figure S5 and demonstrates further evidence that the strong "Ti-O···Ni" interaction occurs between Ti 3 C 2 T x and Ni-MOF during the in situ growth process. In other words, the XPS results correspond well with the conclusions of XRD and FT-IR, which ensures that the "Ti-O···Ni" interaction forms during the in situ growth of Ni-MOF via anchoring at active Ti sites on the surface of Ti 3 C 2 T x , as illustrated in Figure 2f.

Morphology Controlled by Concentration and Reaction Time
The concentration of Ti 3 C 2 T x and the reaction time have a direct effect on the micromorphology and structure of the Ni-MOF/Ti 3 C 2 T x products, which makes an important impact on the electrochemical performance. On the one hand, the concentration of the Ti 3 C 2 T x solution determines whether the forest-like Ni-MOF array vertically grows on the surface of Ti 3 C 2 T x successfully. When the concentration of the solution is 0.1 mg·mL −1 , a small quantity of Ti 3 C 2 T x is entirely covered by the relatively excessive Ni-MOF nanorods in no particular order (Figure 3a), resulting in the intensity of the corresponding (002) peaks of Ti 3 C 2 T x disappearing in its XRD patten as shown in the black line of Figure 3g. When the solution increases to 1.0 mg·mL −1 , compared with the morphology of the sample obtained at 0.5 mg·mL −1 (Figure 3c), Ni-MOF displays a nanosheet structure with a low density of distribution on the surface of Ti 3 C 2 T x . As a result, the XRD pattern shows the characteristic peaks of Ti 3 C 2 T x and Ni-MOF at 6.1 • and 4.8 • , respectively, indicating that Ni-MOF adhered to the surface prevents Ti 3 C 2 T x from accumulating and increases the lattice space of the Ti 3 C 2 T x nanosheets (blue line in Figure 2g). In addition, the intensity of Ti 3 C 2 T x 's and Ni-MOF's characteristic peaks all show strong signals because the low density of Ni-MOF cannot cover the surface and leads to exposure of the (002) face of the Ti 3 C 2 T x rather than disappearance or weakness of the intensity as for the above two samples. On the other hand, the reaction time decides the size of the final Ni-MOF products. It can be clearly found that the length of Ni-MOF increases from~250 nm of 8 h, 550 nm of 12 h, to~2.5 µm of 18 h by comparing the results of Figure 3d-f. To summarize, the concentration of Ti 3 C 2 T x and the reaction time play pivotal roles in controlling the architecture of the Ni-MOF/Ti 3 C 2 T x nanocomposite, which is due to providing different growth environments for Ni-MOF. A low Ti 3 C 2 T x concentration and a limited reaction time provide insufficient growth conditions for Ni-MOF anchoring on the surface of Ti 3 C 2 T x . By contrast, when up to 1.0 mg·mL −1 , the quantity of the Ni ions and the HHTP ligand cannot satisfy continuous growth after anchoring. Under 0.5 mg·mL −1 and for 12 h, the Ni-MOF nanorods can grow uniformly on the surface of the Ti 3 C 2 T x nanosheets. The different morphologies significantly impact the electrochemical performance of the products. provide insufficient growth conditions for Ni-MOF anchoring on the surface of Ti3C2Tx. By contrast, when up to 1.0 mg·mL −1 , the quantity of the Ni ions and the HHTP ligand cannot satisfy continuous growth after anchoring. Under 0.5 mg·mL −1 and for 12 h, the Ni-MOF nanorods can grow uniformly on the surface of the Ti3C2Tx nanosheets. The different morphologies significantly impact the electrochemical performance of the products.

Electrochemical Properties
The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were performed to measure the electrochemical capacitance via the traditional three-electrode system using 6 M KOH as the electrolyte and Hg/HgO as the reference electrode. The redox peak can be clearly seen from the CV curves rather than rectangular shapes, which demonstrates the typical pseudocapacitive behavior (Figure 4a). As shown in Figure 4c, the GCD curves clearly show that the specific capacitance of the Ni-MOF/Ti3C2Tx nanocomposite can reach as high as 497.6 F·g −1 (0.5 A·g −1 ), which can remain over 66% of the initial capacitance when the current density increases five times from 0.5 to 2.5 A·g −1 . Additionally, it shows a long discharge plateau period, which has great potential in practical applications. The good electrochemical performance of the Ni-MOF-Ti3C2Tx nanocomposite can be attributed to the synergetic combination of the excellent conductivity of the Ti3C2Tx substrate and the abundant redox active sites of MOF for electron transfer and ion transport. Because of the strong interaction formed during in situ growth, the Ni-MOF/Ti3C2Tx nanocomposite can maintain over 85% of the initial capacitance after 1000 cycles (Figure 4e). The large capacitance mainly displays a pseudocapacitance behavior due to the abundant active metal sites on the surface of the 2D Ti3C2Tx nanosheets, the porous skeleton (electric double layer capacitor), and the benzoquinone reaction that happened at the skeleton of the Ni-MOF, which can provide a good capacity for charge storage [45,46]. Additionally, the change of the impedance shows an obvious change ( Figure  S10), which might be caused by the less irreversible passivation behavior of the nanomaterials during the electrochemical cycles. (d-f) SEM images of the Ni-MOF/Ti 3 C 2 T x products at the same concentration 0.5 mg·mL −1 of Ti 3 C 2 T x within different reaction times of 8 h, 12 h, and 16 h, respectively; and (g) XRD pattern of the Ni-MOF/Ti 3 C 2 T x samples grown at different Ti 3 C 2 T x concentrations.

Electrochemical Properties
The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were performed to measure the electrochemical capacitance via the traditional three-electrode system using 6 M KOH as the electrolyte and Hg/HgO as the reference electrode. The redox peak can be clearly seen from the CV curves rather than rectangular shapes, which demonstrates the typical pseudocapacitive behavior (Figure 4a). As shown in Figure 4c, the GCD curves clearly show that the specific capacitance of the Ni-MOF/Ti 3 C 2 T x nanocomposite can reach as high as 497.6 F·g −1 (0.5 A·g −1 ), which can remain over 66% of the initial capacitance when the current density increases five times from 0.5 to 2.5 A·g −1 . Additionally, it shows a long discharge plateau period, which has great potential in practical applications. The good electrochemical performance of the Ni-MOF-Ti 3 C 2 T x nanocomposite can be attributed to the synergetic combination of the excellent conductivity of the Ti 3 C 2 T x substrate and the abundant redox active sites of MOF for electron transfer and ion transport. Because of the strong interaction formed during in situ growth, the Ni-MOF/Ti 3 C 2 T x nanocomposite can maintain over 85% of the initial capacitance after 1000 cycles (Figure 4e). The large capacitance mainly displays a pseudocapacitance behavior due to the abundant active metal sites on the surface of the 2D Ti 3 C 2 T x nanosheets, the porous skeleton (electric double layer capacitor), and the benzoquinone reaction that happened at the skeleton of the Ni-MOF, which can provide a good capacity for charge storage [45,46]. Additionally, the change of the impedance shows an obvious change ( Figure S10), which might be caused by the less irreversible passivation behavior of the nanomaterials during the electrochemical cycles. Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 12   (Figure S9), which surpasses the purely conducive MOF and Ti 3 C 2 T x and some of the MOF-Ti 3 C 2 T x composites.
Lastly, as depicted in Figure 5a, an all-solid-state asymmetric flexible supercapacitor was assembled by employing a Ni-MOF/Ti 3 C 2 T x nanocomposite as the cathode and activated carbon as the anode. Additionally, the voltage window was fixed to 1.5 V by the cycle voltammetry test on the anode and cathode, respectively ( Figure S11). The flexible device exhibits a specific capacitance of 128 F·g −1 , calculated through CV tests, and the devices have succeeded in lighting a LED as shown in Figure 5b. The GCD test of the device shows a good charge and discharge curve as depicted in Figure 5c. Additionally, the EIS test (Figure 5d) shows the small impedance of the flexible supercapacitor, which can reduce the energy loss effectively. Moreover, the device shows good electrochemical stability of more than 90 GCD cycles, as shown in Figure 5e. Additionally, it can be observed from Figure S12, with the bending angle change from 0 • to 30 • and 60 • , the CV curves show an inconspicuous change, which also demonstrates their good stability when applied to the bending scenario. Ni-MOF/Ti3C2Tx nanocomposite possesses an excellent energy density of 223.9 KWh·kg −1 at a power density of 1953.3 W·kg −1 (Figure S9), which surpasses the purely conducive MOF and Ti3C2Tx and some of the MOF-Ti3C2Tx composites. Lastly, as depicted in Figure 5a, an all-solid-state asymmetric flexible supercapacitor was assembled by employing a Ni-MOF/Ti3C2Tx nanocomposite as the cathode and activated carbon as the anode. Additionally, the voltage window was fixed to 1.5 V by the cycle voltammetry test on the anode and cathode, respectively ( Figure S11). The flexible device exhibits a specific capacitance of 128 F·g −1 , calculated through CV tests, and the devices have succeeded in lighting a LED as shown in Figure 5b. The GCD test of the device shows a good charge and discharge curve as depicted in Figure 5c. Additionally, the EIS test (Figure 5d) shows the small impedance of the flexible supercapacitor, which can reduce the energy loss effectively. Moreover, the device shows good electrochemical stability of more than 90 GCD cycles, as shown in Figure 5e. Additionally, it can be observed from Figure S12, with the bending angle change from 0° to 30° and 60°, the CV curves show an inconspicuous change, which also demonstrates their good stability when applied to the bending scenario.

Conclusions
In summary, we have developed a novel Ni-MOF/Ti3C2Tx electrode material with a high order nanorods forest in situ grown on 2D Ti3C2Tx nanosheets. By controlling the Ti3C2Tx concentration and the hydrothermal reaction time, the relationship between the microstructure and electrochemical performance is successfully revealed. Benefiting from Ni-MOF's abundant active redox sites and Ti3C2Tx's high conductivity, the in situ combination identifies an electrochemical performance of 497.6 F·g −1 (0.5 A·g −1 ) and a capacitive retention of over 66% of the initial capacity when the current density is increased five times, which can be attributed to the synergistic effect of Ni-MOF and Ti3C2Tx. The "Ti-O···Ni" bond formed during in situ growth offers a strong interfacial combination between Ni-MOF and Ti3C2Tx. Ni-MOF provides sufficient active sites, effective ion transportation

Conclusions
In summary, we have developed a novel Ni-MOF/Ti 3 C 2 T x electrode material with a high order nanorods forest in situ grown on 2D Ti 3 C 2 T x nanosheets. By controlling the Ti 3 C 2 T x concentration and the hydrothermal reaction time, the relationship between the microstructure and electrochemical performance is successfully revealed. Benefiting from Ni-MOF's abundant active redox sites and Ti 3 C 2 T x 's high conductivity, the in situ combination identifies an electrochemical performance of 497.6 F·g −1 (0.5 A·g −1 ) and a capacitive retention of over 66% of the initial capacity when the current density is increased five times, which can be attributed to the synergistic effect of Ni-MOF and Ti 3 C 2 T x . The "Ti-O···Ni" bond formed during in situ growth offers a strong interfacial combination between Ni-MOF and Ti 3 C 2 T x . Ni-MOF provides sufficient active sites, effective ion transportation pathways, and avoids stacking of Ti 3 C 2 T x , while Ti 3 C 2 T x acts as a high conductivity material. The device shows good performance and stability in tests, which shows its potential in practical applications. Thus, this work provides a new material for electrochemical applications.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13030610/s1, Figure S1. The SEM image of the Ti 3 C 2 T x nanosheets, Figure S2. The XRD pattern of the Ti 3 C 2 T x , Figure S3. FT-IR pattern of (a) Ti 3 C 2 T x and (b) Ni-MOF, respectively, Figure S4. The pore size distribution of Ni-MOF/Ti 3 C 2 T x , Figure  S5. XPS spectra of Ni-MOF/Ti 3 C 2 T x , Figure S6. High-resolution O 1s spectra of Ti 3 C 2 T x , Ni-MOF, Ni-MOF/Ti 3 C 2 T x , respectively, Figure S7. Comparation of rate capacitance of Ni-MOF/Ti 3 C 2 T x , with different growth conditions, Figure S8. Comparation of energy density of Ni-MOF/Ti 3 C 2 T x , with different growth conditions, Figure S9. Energy density at different Power density of Ni-MOF/Ti 3 C 2 T x , Figure S10. The EIS test of the Ni-MOF/Ti 3 C 2 T x before and after stability test, Figure S11. Energy density at different Power density of Ni-MOF/Ti 3 C 2 T x , Figure S12. The CV test under original or bending states.