Facile Synthesis of NiCo2O4 Nanowire Arrays/Few-Layered Ti3C2-MXene Composite as Binder-Free Electrode for High-Performance Supercapacitors

Herein, a 3D hierarchical structure is constructed by growing NiCo2O4 nanowires on few-layer Ti3C2 nanosheets using Ni foam (NF) as substrate via simple vacuum filtration and solvothermal treatment. Ti3C2 nanosheets are directly anchored on NF surface without binders or surfactants, and NiCo2O4 nanowires composed of about 15 nm nanoparticles uniformly grow on Ti3C2/NF skeleton, which can provide abundant active sites and ion diffusion pathways for enhancing electrochemical performance. Benefiting from the unique structure feature and the synergistic effects of active materials, NiCo2O4/Ti3C2 exhibits a high specific capacitance of 2468 F g−1 at a current density of 0.5 A g−1 and a good rate performance. Based on this, an asymmetric supercapacitor (ASC) based on NiCo2O4/Ti3C2 as positive electrode and activated carbon (AC)/NF as negative electrode is assembled. The ASC achieves a high specific capacitance of 253 F g−1 at 1 A g−1 along with 91.5% retention over 10,000 cycles at 15 A g−1. Furthermore, the ACS presents an outstanding energy density of 90 Wh kg−1 at the power density of 2880 W kg−1. This work provides promising guidance for the fabrication of binder-free, free-standing and hierarchical composites for energy storage application.


Introduction
With climate change and fossil energy depletion of the world, the development of high-performance energy conversion and storage devices is urgently demanded [1,2]. Supercapacitors (SCs) are considered as a new type of promising candidate devices and have been extensively developed due to their high power density, fast charging/discharging rate and long cycling life compared with batteries [3,4]. However, the relatively lower energy density of SCs restricts their development and actual application. It is well known that the energy density (E) is related to working potential range (∆V) and specific capacitance (C) according to the formula of E = C (∆V) 2 /2 and the electrochemical behavior of electrode materials plays an important role in performance of SCs [5,6]. Thus, exploring suitable electrode material is promising for improving the energy density and performance of SCs.
MXene (Ti 3 C 2 ), a recently novel emerging 2D material, has been widely studied in SCs by virtue of its unique properties, such as high conductivity, unique layered structure, excellent theoretical capacitance energy (1500 F cm −3 ), good mechanical stability and environmentally friendly characteristics [7][8][9]. Moreover, recent researches have proven that MXene shows much promise over other supercapacitor electrode materials and is one of the prospective candidates [10]. Particularly, few-layer Ti 3 C 2 nanosheets can facilitate the full exposure of O-functionalized surface, which will expedite the accessibility of ions and promote the sufficiently utilization of their functional surfaces [11,12]. However, the MXene materials usually exist in powdery form and must be mixed with conductive carbon and a polymeric binder, followed by painting and pressing on a current collector, when they are Figure 1 shows the synthesis process of the NiCo 2 O 4 /Ti 3 C 2 composite. (1) The construction of Ti 3 C 2 /NF 3D structure via vacuum filtration. Firstly, 1 g Ti 3 AlC 2 (Figure S1a, Supplementary Materials) was etched by using 30 mL HF (40 wt%) under magnetic stirring at 50 • C for 36 h. The resultant sediments were washed with deionized water and ethanol several times via centrifugation until pH reached neutrality and dried at 60 • C in an oven. The obtained accordion-like Ti 3 C 2 powders ( Figure S1b) were dispersed in 20 mL deionized water followed by sonicating for 20 min at ice-bath ambient and centrifuged at 3500 rpm for 1 h to gain the supernatant of few-layer Ti 3 C 2 nanosheets. Then, Ni foams (NFs, 1 × 1.5 cm 2 ) were cleaned in acetone, 6 M HCl, deionized water and ethanol for ultrasonic cleaning for 15 min, respectively, to remove the stains and oxides on their surface and dried for further use. Afterwards, few-layer Ti 3 C 2 nanosheet supernatant was dropped on the pretreated NF with the assist of vacuum filtration to gain Ti 3 C 2 /NF 3D network skeleton structure. (2) The growth of NiCo 2 O 4 on the surface of Ti 3 C 2 /NF was realized via a solvothermal process. 0.05 M Ni(NO 3 ) 2 ·6H 2 O, 0.1 M Co(NO 3 ) 2 ·6H 2 O and 0.42 M CH 4 N 2 O were dissolved in the solution consisted of 20 mL deionized water and 20 mL ethanol followed by magnetic stirring for 20 min to form a clear pink solution. Subsequently, the above obtained Ti 3 C 2 /NF was immersed into the solution, which was put in a Teflon-lined stainless-steel autoclave. The autoclave was heated at 150 • C for 4 h to grow Ni−Co precursor nanowires on Ti 3 C 2 /NF surface. Finally, the NiCo 2 O 4 /Ti 3 C 2 composite was obtained after annealing at 350 • C for 2 h in N 2 at a heating rate of 1 • C min −1 with the transform from precursor to NiCo 2 O 4 . The active material on NF in NiCo 2 O 4 /Ti 3 C 2 is about 1.5 mg. Noteworthily, the abundant functional terminal groups (-F, -OH, -O, etc.) on the few-layer Ti 3 C 2 surface will provide a large number of nucleation sites for in situ growth of NiCo 2 O 4 nanowires as a result of electrostatic interaction between Co and Ni cations and negatively charged MXene [12]. For comparison, the NiCo 2 O 4 /NF and pure NiCo 2 O 4 powders were synthesized by the same method.

Instruments and Characterization
The structures and morphologies of electrodes were characterized by field emission scanning electron microscope (FE-SEM, FEI, Quanta 650 FEG, New York, America), and the detailed microstructure was further observed by transmission electron microscopy (TEM, JEOL, JEM-2100Plus, Osaka, Japan) at 200 KV. The elemental compositions were tested by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD, Rigaku, Ultima IV, Osaka, Japan) was performed to investigate phase compositions using Cu Kα radiation (λ = 0.15418 nm). Surface valences were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha, New, York, NY, USA) with Al Kα radiation. Specific surface and pore information were measured by nitrogen adsorption-desorption isotherms (Micromeritics, ASAP 2460, Norcross, GA, USA) at 77 K. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) formula, and the pore size distribution was obtained by the Barret-Joyner-Halenda (BJH) method.

Fabrication of the NiCo2O4/Ti3C2//AC Asymmetrical Supercapacitor (ASC)
The AC/NF negative electrode was prepared by painting the uniform mixtures of activated carbon (AC), polyvinylidene fluoride (PVDF) binder and acetylene black conductive agent with the mass ratio of 8:1:1 on the pretreated NF. The ASC was assembled in a coin cell (CR2032) with the order of positive case, NiCo2O4/Ti3C2, electrolyte (6 M KOH), separator (Celgard 3501), electrolyte, AC/NF, spacer, spring, negative case, respectively.

Instruments and Characterization
The structures and morphologies of electrodes were characterized by field emission scanning electron microscope (FE-SEM, FEI, Quanta 650 FEG, New York, America), and the detailed microstructure was further observed by transmission electron microscopy (TEM, JEOL, JEM-2100Plus, Osaka, Japan) at 200 KV. The elemental compositions were tested by energy dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD, Rigaku, Ultima IV, Osaka, Japan) was performed to investigate phase compositions using Cu Kα radiation (λ = 0.15418 nm). Surface valences were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, K-Alpha, New, York, NY, USA) with Al Kα radiation. Specific surface and pore information were measured by nitrogen adsorption-desorption isotherms (Micromeritics, ASAP 2460, Norcross, GA, USA) at 77 K. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) formula, and the pore size distribution was obtained by the Barret-Joyner-Halenda (BJH) method.

Fabrication of the NiCo 2 O 4 /Ti 3 C 2 //AC Asymmetrical Supercapacitor (ASC)
The AC/NF negative electrode was prepared by painting the uniform mixtures of activated carbon (AC), polyvinylidene fluoride (PVDF) binder and acetylene black conductive agent with the mass ratio of 8:1:1 on the pretreated NF. The ASC was assembled in a coin cell (CR2032) with the order of positive case, NiCo 2 O 4 /Ti 3 C 2 , electrolyte (6 M KOH), separator (Celgard 3501), electrolyte, AC/NF, spacer, spring, negative case, respectively.

Electrochemical Measurements
The electrochemical performance of the NiCo 2 O 4 /Ti 3 C 2 electrode and the asymmetrical supercapacitor (ASC) was tested on an electrochemical workstation (CHI660E). For NiCo 2 O 4 /Ti 3 C 2 electrode, the cyclic voltammetry (CV, 0-0.6 V), galvanostatic chargedischarge (GCD, 0-0.5 V) and electrochemical impedance spectroscopy (EIS, 5 mV, 100 KHz-0.01 Hz) curves were investigated by a three-electrode system consisting of platinum foil counter electrode, double salt bridge Hg/HgO reference electrode and NiCo 2 O 4 /Ti 3 C 2 working electrode. For ACS, the electrochemical performance was mea-sured by the electrode clamp connected to the electrochemical workstation. The CV (0-1.6 V) and GCD (0-1.6 V) curves were recorded at various scan rates of 10-100 mV s −1 and current densities of 1-10 A g −1 , respectively. In addition, the specific capacitance of NiCo 2 O 4 /Ti 3 C 2 in a three-electrode system and ACS was calculated from the GCD curves by Equations (1)-(3) [25][26][27]: where C m (F g −1 ), I (A), ∆t (s), m (g), ∆V (V) are the gravimetric specific capacitance, load current, discharge time, the mass of active material on NF and working potential range of NiCo 2 O 4 /Ti 3 C 2 electrode, respectively. The volumetric specific capacitance was calculated using the Equation (2): where C A (F g −1 ), I (A), ∆t (s), m (g), ∆V (V) represent for the specific capacitance, load current, discharge time, total mass of active materials in NiCo 2 O 4 /Ti 3 C 2 positive and AC/NF negative electrodes and working potential range of the ACS, respectively. The specific capacitance of NiCo 2 O 4 /Ti 3 C 2 and AC/NF based on CV curves at 10 mV s −1 was gained according to Equation (4) [26]. The mass ratio between NiCo 2 O 4 /Ti 3 C 2 positive and AC/NF negative electrodes was determined using the Equation (5) [26] for optimizing the ACS: where C p (F g −1 ), A, m (g), s (mV s −1 ), ∆V (V) represent for specific capacitance, the integrated area in CV curves, the mass of active material, scan rate and working potential range of the positive and negative electrodes, respectively. The mass of active material on NiCo 2 O 4 /Ti 3 C 2 and AC/NF was 1.5 mg and 0.85 mg initially and was well adjusted to be about 1:1 (1.5 mg:1.5 mg) ultimately in this work. The energy density (E, Wh kg −1 ) and power density (P, W kg −1 ) of ACS were obtained according to the Equations (6) and (7) [25,26]: where C A (F g −1 ), ∆V (V), ∆t (s) represent for specific capacitance, working potential range, discharge time of ACS.

Structural and Morphological Characterization
XRD patterns were performed to analyze the phase compositions of the samples. Figure S2 (Supplementary Materials) shows the XRD patterns of Ti 3 AlC 2 and the etched accordion-like Ti 3 C 2 powders. The disappearance of characteristic peak of (104) plane in Ti 3 AlC 2 indicates the removal of Al layer after HF etching. Meanwhile, the (002) plane located at 8.6 • in Ti 3 C 2 is obviously broadened and shifts to a low angle compared with Ti 3 AlC 2 , implying a larger interlayer distance [28]. In Figure 2, besides the three strong peaks of NF, there are no diffraction peaks of Ti 3 C 2 that can be observed in Ti 3 C 2 /NF, which is different from Ti 3 C 2 powders due to the poor crystallinity of Ti 3 C 2 [29]. The diffraction peaks located at 31 Ti3AlC2, implying a larger interlayer distance [28]. In Figure 2, besides the three strong peaks of NF, there are no diffraction peaks of Ti3C2 that can be observed in Ti3C2/NF, which is different from Ti3C2 powders due to the poor crystallinity of Ti3C2 [29]. The diffraction peaks located at 31.16°, 36.7°, 59.06° and 64.94° in NiCo2O4/NF are well assigned to the (220), (311), (511) and (440) planes of NiCo2O4 (JCPDS No. 20-0781) [30], respectively. As for NiCo2O4/Ti3C2, only the diffraction peaks of NiCo2O4 can be observed and no obvious peaks of Ti3C2 can be found due to the poor crystallinity of Ti3C2 or maybe the blocking effect of NiCo2O4 resulted from the full decoration of Ti3C2/NF by NiCo2O4. The NiCo2O4/Ti3C2 and Ti3C2/NF samples were characterized by SEM in Figure 3. As observed in Figure 3a, graphene-like Ti3C2 nanosheets are successfully attached on NF surface, due to the strong physical adsorption between Ti3C2 and NF caused by vacuum filtration. The powerful vacuum filtration also contributes for the close connection between NF and Ti3C2, which is conducive to the electron transport and lower resistance in electrode [31]. From Figure 3b,c, it can be seen that the surface of Ti3C2/NF becomes rough when Ti3C2/NF is used as substrate to grow NiCo2O4, and Ti3C2 can hardly be found owing to the dense needle-like NiCo2O4 nanowires covering on its surface. However, it can still be deduced from the uneven surface that Ti3C2 nanosheets exist in the composite. The cross-section of NiCo2O4/Ti3C2 in Figure S3 further shows NiCo2O4 nanowires with a length of up to several micrometers vertically grow on Ti3C2/NF, while the pure NiCo2O4 powders possess an urchin-like microsphere structure as shown in Figure S4. The phenomenon indicates that the separately and uniformly dispersed NiCo2O4 nanowires on Ti3C2/NF will provide large specific surface area and abundant active sites for improving electrochemical performance. The element mapping (Figure 3d) clearly shows the existence and uniform distribution of Ti, C, O, Ni and Co elements in as-prepared composite electrode, which further reveals the successful synthesis of NiCo2O4/Ti3C2 nanostructure.
The active material of NiCo2O4/Ti3C2 was peeled from NF under ultrasound in a vacuum to further characterize the microstructure, as shown in Figure 3e-h. The peeling took a long time, indicating the strong contact of active material components, which is beneficial to the electron transport and structural stability of NiCo2O4/Ti3C2 and synergistic effects among components. As observed in Figure 3e, the broken NiCo2O4 nanowires are distributed around the few-layer Ti3C2 nanosheet. In Figure 3f, needle-like NiCo2O4 nanowires are composed of numerous nanoparticles, similar to the previous work [32], with a diameter of about 15 nm. Massive channels existing in nanoparticles will provide large surface area, which is beneficial to the sufficient contact between electrolyte and electrode The NiCo 2 O 4 /Ti 3 C 2 and Ti 3 C 2 /NF samples were characterized by SEM in Figure 3. As observed in Figure 3a, graphene-like Ti 3 C 2 nanosheets are successfully attached on NF surface, due to the strong physical adsorption between Ti 3 C 2 and NF caused by vacuum filtration. The powerful vacuum filtration also contributes for the close connection between NF and Ti 3 C 2 , which is conducive to the electron transport and lower resistance in electrode [31]. From Figure 3b,c, it can be seen that the surface of Ti 3 C 2 /NF becomes rough when Ti 3 C 2 /NF is used as substrate to grow NiCo 2 O 4 , and Ti 3 C 2 can hardly be found owing to the dense needle-like NiCo 2 O 4 nanowires covering on its surface. However, it can still be deduced from the uneven surface that Ti 3 C 2 nanosheets exist in the composite. The cross-section of NiCo 2 O 4 /Ti 3 C 2 in Figure S3 further shows NiCo 2 O 4 nanowires with a length of up to several micrometers vertically grow on Ti 3 C 2 /NF, while the pure NiCo 2 O 4 powders possess an urchin-like microsphere structure as shown in Figure S4. The phenomenon indicates that the separately and uniformly dispersed NiCo 2 O 4 nanowires on Ti 3 C 2 /NF will provide large specific surface area and abundant active sites for improving electrochemical performance. The element mapping (Figure 3d) clearly shows the existence and uniform distribution of Ti, C, O, Ni and Co elements in as-prepared composite electrode, which further reveals the successful synthesis of NiCo 2 O 4 /Ti 3 C 2 nanostructure.
The active material of NiCo 2 O 4 /Ti 3 C 2 was peeled from NF under ultrasound in a vacuum to further characterize the microstructure, as shown in Figure 3e-h. The peeling took a long time, indicating the strong contact of active material components, which is beneficial to the electron transport and structural stability of NiCo 2 O 4 /Ti 3 C 2 and synergistic effects among components. As observed in Figure 3e, the broken NiCo 2 O 4 nanowires are distributed around the few-layer Ti 3 C 2 nanosheet. In Figure 3f, needle-like NiCo 2 O 4 nanowires are composed of numerous nanoparticles, similar to the previous work [32], with a diameter of about 15 nm. Massive channels existing in nanoparticles will provide large surface area, which is beneficial to the sufficient contact between electrolyte and electrode material, and promote the rapid ions transport and pseudocapacitance reactions. As observed in Figure 3g material, and promote the rapid ions transport and pseudocapacitance reactions. As observed in Figure 3g, the interplanar spacings of 0.27 nm and 0.10 nm can be indexed to the (311) and (731) planes of NiCo2O4, respectively. In addition, SADE pattern in Figure 3h displays many diffraction rings with different diameters, which can be ascribed to the (400), (511), (531), (711) planes of NiCo2O4 showing the polycrystalline structure. The surface chemical states and elemental compositions of prepared materials were examined by XPS, as shown in Figure 4. In Figure 4a, the XPS survey spectra indicate Ni, Ti, C and O elements are presented in Ti3C2/NF, and a new peak with the binding energy of 782.1 eV corresponding to Co element appears in NiCo2O4/Ti3C2 except for the same elements as Ti3C2/NF, confirming that the NiCo2O4 has been loaded on the Ti3C2/NF surface. Moreover, Ti element peak in NiCo2O4/Ti3C2 is weakened, resulting from the complete coverage of NiCo2O4 on Ti3C2/NF, which is in agreement with the XRD. Fine XPS spectra of C 1s, Ti 2p, Co 2p, Ni 2p, and O 1s for NiCo2O4/Ti3C2 are presented in Figure  4b-f. The typical fitted C 1s spectrum in Figure 4b shows that the peaks located at 282.9 eV, 284.8 eV, 285.7 eV and 287.7 eV are assigned to C−Ti, C−C, C−O and O−C=O, respectively, which prove the presence of functional groups on Ti3C2 [33]. Ti 2p spectrum (  [34,35]. In Co 2p spectrum (Figure 4d), the peaks at 779.6/795.8 eV and 783.2/799.8 eV can be indexed to Co 2+ and Co 3+ [36]. Similar to Co 2p, the Ni 2p spectrum (Figure 4e) possesses a typical doublet The surface chemical states and elemental compositions of prepared materials were examined by XPS, as shown in Figure 4. In Figure 4a, the XPS survey spectra indicate Ni, Ti, C and O elements are presented in Ti 3 C 2 /NF, and a new peak with the binding energy of 782.1 eV corresponding to Co element appears in NiCo 2 O 4 /Ti 3 C 2 except for the same elements as Ti 3 C 2 /NF, confirming that the NiCo 2 O 4 has been loaded on the Ti 3 C 2 /NF surface. Moreover, Ti element peak in NiCo 2 O 4 /Ti 3 C 2 is weakened, resulting from the complete coverage of NiCo 2 O 4 on Ti 3 C 2 /NF, which is in agreement with the XRD. Fine XPS spectra of C 1s, Ti 2p, Co 2p, Ni 2p, and O 1s for NiCo 2 O 4 /Ti 3 C 2 are presented in Figure 4b-f. The typical fitted C 1s spectrum in Figure 4b shows that the peaks located at 282.9 eV, 284.8 eV, 285.7 eV and 287.7 eV are assigned to C−Ti, C−C, C−O and O−C=O, respectively, which prove the presence of functional groups on Ti 3 C 2 [33]. Ti 2p spectrum (Figure 4c) [34,35]. In Co 2p spectrum (Figure 4d), the peaks at 779.6/795.8 eV and 783.2/799.8 eV can be indexed to Co 2+ and Co 3+ [36]. Similar to Co 2p, the Ni 2p spectrum (Figure 4e) possesses a typical doublet of Ni 2p 3/2 (854.48 eV) and Ni 2p 1/2 (872.58 eV) and broad shakeup satellites [36]. The multivalent valences of Ni and Co will make contributions to high specific capacitance in SCs. The spectrum of O 1s is shown in Figure 4f. The peaks at 529.7 eV, 530.7 eV and 533.4 eV correspond well to the Co−O or Ni−O, C=O and OH − , respectively [21,33].
of Ni 2p3/2 (854.48 eV) and Ni 2p1/2 (872.58 eV) and broad shakeup satellites [36]. The multivalent valences of Ni and Co will make contributions to high specific capacitance in SCs. The spectrum of O 1s is shown in Figure 4f. The peaks at 529.7 eV, 530.7 eV and 533.4 eV correspond well to the Co−O or Ni−O, C=O and OH -, respectively [21,33]. N2 adsorption-desorption isotherms were conducted to demonstrate the specific surface area and pore diameter distributions of the NiCo2O4/Ti3C2 and Ti3C2/NF. NiCo2O4/Ti3C2 displays a distinct type IV hysteresis loop in Figure 5a, indicating the presence of mesoporous structure and a higher calculated specific surface area (35.2 m 2 g −1 ) compared to Ti3C2/NF (20.7 m 2 g −1 ). Moreover, the pores size values in Figure 5b analyzed by the BJH method further show that the pore volume of NiCo2O4/Ti3C2 is higher than Ti3C2/NF, which can be mainly attributed to the massive mesoporous structure of NiCo2O4. Porous structure and large active surface in NiCo2O4/Ti3C2 both promote ions or electron transport and penetration of electrolyte, optimizing the electrochemical performance [37].

Electrochemical Performances of Samples
The electrochemical performance of the as-prepared samples was investigated using a three-electrode system in 6 M KOH as illustrated in Figure 6. Firstly, the performance N 2 adsorption-desorption isotherms were conducted to demonstrate the specific surface area and pore diameter distributions of the NiCo 2 O 4 /Ti 3 C 2 and Ti 3 C 2 /NF. NiCo 2 O 4 /Ti 3 C 2 displays a distinct type IV hysteresis loop in Figure 5a, indicating the presence of mesoporous structure and a higher calculated specific surface area (35.2 m 2 g −1 ) compared to Ti 3 C 2 /NF (20.7 m 2 g −1 ). Moreover, the pores size values in Figure 5b analyzed by the BJH method further show that the pore volume of NiCo 2 O 4 /Ti 3 C 2 is higher than Ti 3 C 2 /NF, which can be mainly attributed to the massive mesoporous structure of NiCo 2 O 4 . Porous structure and large active surface in NiCo 2 O 4 /Ti 3 C 2 both promote ions or electron transport and penetration of electrolyte, optimizing the electrochemical performance [37].
of Ni 2p3/2 (854.48 eV) and Ni 2p1/2 (872.58 eV) and broad shakeup satellites [36]. The multivalent valences of Ni and Co will make contributions to high specific capacitance in SCs. The spectrum of O 1s is shown in Figure 4f. The peaks at 529.7 eV, 530.7 eV and 533.4 eV correspond well to the Co−O or Ni−O, C=O and OH -, respectively [21,33]. N2 adsorption-desorption isotherms were conducted to demonstrate the specific surface area and pore diameter distributions of the NiCo2O4/Ti3C2 and Ti3C2/NF. NiCo2O4/Ti3C2 displays a distinct type IV hysteresis loop in Figure 5a, indicating the presence of mesoporous structure and a higher calculated specific surface area (35.2 m 2 g −1 ) compared to Ti3C2/NF (20.7 m 2 g −1 ). Moreover, the pores size values in Figure 5b analyzed by the BJH method further show that the pore volume of NiCo2O4/Ti3C2 is higher than Ti3C2/NF, which can be mainly attributed to the massive mesoporous structure of NiCo2O4. Porous structure and large active surface in NiCo2O4/Ti3C2 both promote ions or electron transport and penetration of electrolyte, optimizing the electrochemical performance [37].

Electrochemical Performances of Samples
The electrochemical performance of the as-prepared samples was investigated using a three-electrode system in 6 M KOH as illustrated in Figure 6. Firstly, the performance

Electrochemical Performances of Samples
The electrochemical performance of the as-prepared samples was investigated using a three-electrode system in 6 M KOH as illustrated in Figure 6. Firstly, the performance comparison of NF, Ti 3 C 2 /NF, NiCo 2 O 4 /NF and NiCo 2 O 4 /Ti 3 C 2 is carried out to exclude the effect of NF as substrate on capacitance, as shown in Figure 6a,b. It is well known that the larger integrated area of CV curves and longer charge-discharge time of GCD reflect higher specific capacitance. Thus, it can be deduced from Figure 6a,b that the contribution of NF to capacitance of electrodes is very little. Furthermore, it can be seen that the introductions of Ti 3 C 2 and NiCo 2 O 4 into NF result in higher current signal and specific capacitance mainly due to their mutually synergistic effects, compared with Ti 3 C 2 /NF and NiCo 2 O 4 /NF. Nyquist plots of EIS spectrum will provide the information about the electrochemical reaction kinetics of electrode. A semicircle in high frequency region and a straight line in low frequency region are comprised in a Nyquist plot. The diameter of semicircle is related to the charge transfer resistance value (Rct), the straight line is interpreted as the Warburg resistance (Zw), reflecting the ions diffusion resistance in the electrode, and the intercept with the real axis represents the equivalent series resistance (Rs) [41,42]. As shown in Figure 6f, the Rs of NiCo2O4/Ti3C2 (0.69 Ω) is lower than that of Ti3C2/NF (0.81 Ω) on the basis of equivalent circuit [43], which mainly results from quick electrolyte penetration into massive channels of NiCo2O4. Simultaneously, these channels and the top sharp ends of nanowires can easily capture ions, decreasing the ions' diffusion resistance, as explained by a more vertical line.
The storage mechanism of NiCo2O4/Ti3C2 electrode is explored based on the CV curves. The contributions of surface capacitive process and diffusion-controlled process are analyzed according to the b value in formula i = av b [44,45], where i represents peak current, v signifies scan rate. "b = 1" declares a surface capacitive process and "b = 0.5" describes a diffusion-controlled process. As shown in Figure 7a,b, values in charge and  [38]. With the increase of scan rate, the approximately same CV shape indicates the good rate performance of NiCo 2 O 4 /Ti 3 C 2 . The oxidation peak and reduction peak shift to the positive range and negative range of values, respectively, which may result from the polarization effect of electrode material [39]. The GCD curves at various current densities of NiCo 2 O 4 /Ti 3 C 2 ranging from 0.5 A g −1 to 10 A g −1 are shown in Figure 6d. It can be seen that the discharge time at 0.5 A g −1 can even up to 2500 s, which is almost longest among previous reports. The approximately triangular and symmetrical charge-discharge curves can still be kept even at 10 A g −1 , indicating the rapid I−V response and high charge-discharge reversibility of NiCo 2 O 4 /Ti 3 C 2 [40]. Moreover, the corresponding gravimetric specific capacitances (C m ) and volumetric specific capacitances (C p ) are calculated by Equations (1) and (2) in Figure 6e. The gravimetric specific capacitance of 2468 F g −1 and volumetric specific capacitances of 24.7 F cm −3 at 0.5 A g −1 are gained, and 1782 F g −1 (C m ) and 17.8 F cm −3 (C p ) are obtained at 10 A g −1 , both with a high capacitance retention of 72%, demonstrating the good rate performance of composite.
Nyquist plots of EIS spectrum will provide the information about the electrochemical reaction kinetics of electrode. A semicircle in high frequency region and a straight line in low frequency region are comprised in a Nyquist plot. The diameter of semicircle is related to the charge transfer resistance value (R ct ), the straight line is interpreted as the Warburg resistance (Z w ), reflecting the ions diffusion resistance in the electrode, and the intercept with the real axis represents the equivalent series resistance (R s ) [41,42]. As shown in Figure 6f, the R s of NiCo 2 O 4 /Ti 3 C 2 (0.69 Ω) is lower than that of Ti 3 C 2 /NF (0.81 Ω) on the basis of equivalent circuit [43], which mainly results from quick electrolyte penetration into massive channels of NiCo 2 O 4 . Simultaneously, these channels and the top sharp ends of nanowires can easily capture ions, decreasing the ions' diffusion resistance, as explained by a more vertical line.
The storage mechanism of NiCo 2 O 4 /Ti 3 C 2 electrode is explored based on the CV curves. The contributions of surface capacitive process and diffusion-controlled process are analyzed according to the b value in formula i = av b [44,45], where i represents peak current, v signifies scan rate. "b = 1" declares a surface capacitive process and "b = 0.5" describes a diffusion-controlled process. As shown in Figure 7a,b, values in charge and discharge process of NiCo 2 O 4 /Ti 3 C 2 are 0.629 and 0.568, respectively, indicating that the energy storage is determined by both capacitive process and diffusion-controlled process. The quantitative contributions of surface capacitive process and diffusion-controlled process at different scan rates are analyzed based on formula i(V) = k 1 v + k 2 v 1/2 [46], where k 1 v and k 2 v 1/2 represent the currents from surface capacitive process and diffusion-controlled process, respectively. Figure 7b shows that the contribution of surface capacitive process at 10 mV s −1 is approximately 22%. Moreover, the contribution proportions of surface capacitive process (Figure 7c) show a constant rising tendency with the increase of scan rate because of the diffusion delay of electrolyte ions to internal electrode, leading to the decrease of the contribution of the diffusion-controlled process [47]. NiCo 2 O 4 nanowires composed with massive nanoparticles vertically grow on 3D Ti 3 C 2 /NF skeleton in NiCo 2 O 4 /Ti 3 C 2 . The excellent electrochemical performance of NiCo 2 O 4 /Ti 3 C 2 can be attributed to its unique structure feature. The excellent capacitance performance of NiCo 2 O 4 /Ti 3 C 2 in a three-electrode system is compared with other carbonbased electrodes and metal oxides/sulfides-based electrodes in previous works [48][49][50][51][52][53][54][55] and electrodes with similar morphology (Gd/CeOx nanoflowers/porous carbon [56], Co 3 O 4 nanosheets/carbon foam [57]). The comparison results are listed in Table S1 [48][49][50][51][52][53][54][55][56][57]. It is worth emphasizing that the results in our work are in a high level overall, especially the specific capacitance (2468 F g −1 at 0.5 A g −1 , 2226 F g −1 at 1 A g −1 ) is even superior to other works. Firstly, the direct contact without binders among components and amounts of micropores of NiCo 2 O 4 effectively reduce the interface resistance, and promote ions and electron transport. Moreover, Ti 3 C 2 nanosheets can make up for the conductivity of NiCo 2 O 4 by a close contact leading to a continuous electron pathway. Electron transport in the internal electrode is accelerated. Thirdly, NiCo 2 O 4 nanowires vertically grow on the Ti 3 C 2 /NF substrate rather than the agglomerated urchin behavior, which will reduce the "dead volume" and further improve electrochemical performance. What is more, the situ-growth of NiCo 2 O 4 on Ti 3 C 2 /NF incites a robust heterostructure, improving the cycle performance of NiCo 2 O 4 /NF.
cess at different scan rates are analyzed based on formula i(V) = k1v + k2v 1/2 [46], where k1v and k2v 1/2 represent the currents from surface capacitive process and diffusion-controlled process, respectively. Figure 7b shows that the contribution of surface capacitive process at 10 mV s −1 is approximately 22%. Moreover, the contribution proportions of surface capacitive process (Figure 7c) show a constant rising tendency with the increase of scan rate because of the diffusion delay of electrolyte ions to internal electrode, leading to the decrease of the contribution of the diffusion-controlled process [47]. NiCo2O4 nanowires composed with massive nanoparticles vertically grow on 3D Ti3C2/NF skeleton in NiCo2O4/Ti3C2. The excellent electrochemical performance of NiCo2O4/Ti3C2 can be attributed to its unique structure feature. The excellent capacitance performance of NiCo2O4/Ti3C2 in a three-electrode system is compared with other carbonbased electrodes and metal oxides/sulfides-based electrodes in previous works [48][49][50][51][52][53][54][55] and electrodes with similar morphology (Gd/CeOx nanoflowers/porous carbon [56], Co3O4 nanosheets/carbon foam [57]). The comparison results are listed in Table S1 [48][49][50][51][52][53][54][55][56][57]. It is worth emphasizing that the results in our work are in a high level overall, especially the specific capacitance (2468 F g −1 at 0.5 A g −1 , 2226 F g −1 at 1 A g −1 ) is even superior to other works. Firstly, the direct contact without binders among components and amounts of micropores of NiCo2O4 effectively reduce the interface resistance, and promote ions and electron transport. Moreover, Ti3C2 nanosheets can make up for the conductivity of NiCo2O4 by a close contact leading to a continuous electron pathway. Electron transport in the internal electrode is accelerated. Thirdly, NiCo2O4 nanowires vertically grow on the Ti3C2/NF substrate rather than the agglomerated urchin behavior, which will reduce the "dead volume" and further improve electrochemical performance. What is more, the situ-

Electrochemical Performances of the ASC
With the destination of investigating the electrochemical performance of NiCo 2 O 4 /Ti 3 C 2 electrode applied in supercapacitors, an NiCo 2 O 4 /Ti 3 C 2 //AC ASC was established. Initially, the CV curves of AC/NF negative electrode ( Figure S5a) were conducted in a threeelectrode system to detect the appropriate potential window. It can be seen that AC/NF shows polarization in CV curves of −1.1-0 V and −1.2-0 V. Moreover, an approximate rectangle without polarization can be observed at −1-0 V, indicating the double-electriclayer energy storage of AC, which also can be proved by the triangle shapes in GCD curves ( Figure S5b). The high rate performance and brilliant kinetics behavior of AC/NF were evaluated by CV curves at various scan rates (10-100 mV s −1 ) and GCD curves at various current densities (0.5-40 A g −1 ) at −1-0 V ( Figure S5c,d) to ensure its superiority and eliminate its negative effects to ASC.
The CV curves of NiCo 2 O 4 /Ti 3 C 2 (0-0.6 V vs. Hg/HgO) and AC/NF (−1-0 V vs. Ag/AgCl) at 10 mV s −1 in a three-electrode system are displayed in Figure 8a. The pseudocapacitance characteristics of NiCo 2 O 4 /Ti 3 C 2 with redox peaks and the electrical double-layer characteristics of AC/NF with quasi-rectangle are coupled with each other and displays an irregular shape, as shown in Figure 8b, suggesting the mutual contribution of EDLC-and battery-type electrodes to the total capacitance [41]. To our knowledge, extensive potential window will not only improve the energy density but also increase the output potential of supercapacitors, which will push them towards a more practical use. For further determining and optimizing the potential window of ACS, the CV curves (0-1 V to 0-1.9 V) and the GCD curves (0-1 V, 0-1.2 V, 0-1.4 V, 0-1.6 V with 0.2 V steps increasing) were tested ( Figure S6a,b). It can be observed that the phenomenon of polarization becomes obvious with the increase in potential window from Figure S6a, and the optimal potential window is considered to be 0-1.6 V. Moreover, the GCD curves further confirm the accessibility of 0-1.6 V ( Figure S6b), which is ultimately certified as the working potential window of ACS for further study. The cyclic life is a critical factor for the device and was evaluated at 15 A g −1 for 10,000 cycles, as depicted in Figure 8e. The capacitance increases slightly at first due to the activation progress of the electrode [3], and tends to be steady over 700 cycles, and ultimately retains as high as 91.5% of its initial value, revealing an excellent cyclic stability. The excellent cyclic performance is on account of the close contact among active materials in NiCo2O4/Ti3C2 and the robust Ti3C2/NF substrate, holding an interlaced network structure with outstanding structural stability, which can support the NiCo2O4 nanowires and avoid their collapse and agglomeration in the cycling process. SEM images of NiCo2O4/Ti3C2 before and after cycles are displayed in Figure S7. By comparison, no significant change is found over 10,000 cycles, and the nanowires structure can be maintained well but only some white knots generating on the top of NiCo2O4 nanowires, leading to the decrease in the active surface and little loss of capacitance (about 8.5%). Energy density and power density are further calculated and plotted to evaluate the device performance for practical applications. From the inset of Figure 8f, Figure 8b presents the CV curves of the ACS device at various scan rates from 10 mV s −1 to 100 mV s −1 . The similar shape and the increased area of the CV curves can be found with the scan rate increasing, suggesting the fast transport of ions and electron at the electrode/electrolyte interfaces and the good high-current charge/discharge performance within the device [58]. The GCD curves ranging from 1 A g −1 to 10 A g −1 at a high potential of 0-1.6 V show approximately triangular-like and symmetrical shape and the negligible IR drop (Figure 8c), manifesting the ideal capacitive behavior [59], the high coulombic efficiency [60] and the low IR of the device [61]. The corresponding specific capacitances as a function of current densities are calculated by Equation (3), as shown in Figure 8d. The specific capacitance can reach up to 253 F g −1 at 1 A g −1 , and it decreases with current density increasing due to the delay of ions diffusion. The specific capacitance is 135 F g −1 with a retention of 53% at a high current density of 10 A g −1 .
The cyclic life is a critical factor for the device and was evaluated at 15 A g −1 for 10,000 cycles, as depicted in Figure 8e. The capacitance increases slightly at first due to the activation progress of the electrode [3], and tends to be steady over 700 cycles, and ultimately retains as high as 91.5% of its initial value, revealing an excellent cyclic stability. The excellent cyclic performance is on account of the close contact among active materials in NiCo 2 O 4 /Ti 3 C 2 and the robust Ti 3 C 2 /NF substrate, holding an interlaced network structure with outstanding structural stability, which can support the NiCo 2 O 4 nanowires and avoid their collapse and agglomeration in the cycling process. SEM images of NiCo 2 O 4 /Ti 3 C 2 before and after cycles are displayed in Figure S7. By comparison, no significant change is found over 10,000 cycles, and the nanowires structure can be maintained well but only some white knots generating on the top of NiCo 2 O 4 nanowires, leading to the decrease in the active surface and little loss of capacitance (about 8.5%). Energy density and power density are further calculated and plotted to evaluate the device performance for practical applications. From the inset of Figure 8f, the ultra-high energy densities up to 90, 74.84, 67.67, 59, 51.38, 48 Wh kg −1 can be achieved, when the power densities are 2880, 5760, 8640, 14,400, 23,119.2, 28,800 W kg −1 at 1, 2, 3, 5, 8, 10 A g −1 , respectively. The presented energy density is compared with other reported ASCs based on Ti 3 C 2 and Ni−Co materials in previous works (Figure 8f), such as Ti 3 C 2 T x /NiS//C/AC (17. [68]. By comparison, the ACS in our work exhibits relatively high and comparable energy density and power density simultaneously, which manifests the NiCo 2 O 4 /Ti 3 C 2 great potential in the construction of ASCs.

Conclusions
The NiCo 2 O 4 /Ti 3 C 2 composite was constructed by the two-step method of vacuum filtration and solvothermal treatment using NF as substrate. The close and direct contact among vertical hierarchical layers is in favor of the electron transport, and the porous structure of the obtained NiCo 2 O 4 /Ti 3 C 2 provides abundant conductive channels and active areas, which can reduce ions diffusion resistance and improve kinetic condition. Due to the synergistic effects of hierarchical components, the NiCo 2 O 4 /Ti 3 C 2 possesses a high specific capability (2468 F g −1 at 0.5 A g −1 and 1782 F g −1 at 10 A g −1 ) and a good rate capability. Based on this, a high-performance ACS device based on NiCo 2 O 4 /Ti 3 C 2 positive and AC/NF negative electrodes was assembled. The ACS device exhibits a competitive capacitance of 253 F g −1 at 1 A g −1 and 135 F g −1 at 10 A g −1 with a relatively high retention of 53%. Moreover, only 8.5% capacitance loss is discovered after 10,000 cycles at 15 A g −1 .
More importantly, the brilliant energy density of 90, 74.84, 67.67, 59, 51.38, 48 Wh kg −1 are achieved at the power density of 2880, 5760, 8640, 14,400, 23,119.2, 28,800 W kg −1 , respectively. This work provides an approach for the design and development of binderfree electrode in high performance energy storage application.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27196452/s1, Figure S1: SEM images of (a) Ti 3 AlC 2 , (b) accordion-like Ti 3 C 2 ; Figure S2: XRD patterns of Ti 3 AlC 2 and Ti 3 C 2 ; Figure S3: (a,b) SEM images of NiCo 2 O 4 /Ti 3 C 2 ; Figure S4: SEM image of pure NiCo 2 O 4 ; Figure S5: (a,b) CV and GCD curves at various potential windows, (c) CV curves at various scan rates, (d) GCD curves at various current densities of AC/NF; Figure S6: (a,b) CV and GCD curves of ACS at various potential windows; Figure S7: (a,b) Morphology before and after 10,000 cycles of NiCo 2 O 4 /Ti 3 C 2 ; Table S1: Electrochemical performances of carbon-based electrodes and metal oxides/sulfides based electrodes and electrodes with similar morphology for supercapacitor application in a three-electrode system.