0 Introduction

Considering the ongoing advancements in science and technology, several problems are becoming increasingly prominent, such as the energy crises and environment pollution. The key to solving current problems is exploring and designing new friendly energy storage devices^[1-3]. Therefore, it is very crucial to create a large number of eminent energy storage and conversion devices. Electrochemical capacitors, also known as supercapacitors (SCs), have become people's choice due to their excellent electrochemical characteristics such as excellent power density, stable safety and ultralong cycle lifetime^[4-6].

MOF is considered a promising advanced material due to its high porosity and tunable crystal structure, which is usually used in catalysis, gas adsorption, supercapacitor and sensing^[7-8]. In particular, its porous structure is always modified to increase the specific capacity and electrical conductivity to achieve the desired high energy density. Compared with monometallic systems, bimetallic MOF exhibits superior electrochemical performance^[9-11]. Therefore, it has received more and more attention in supercapacitors. Particularly, LDH derived from MOFs is becoming prominent competitor. LDH is an application material which is suitable for supercapacitors. It is characterized by the rapid propagation of charged ions^[12-13]. Some reports indicate that the hydrophilicity of LDH facilitates the dispersion of ions in aqueous electrolytes. Faraday redox charge transfer occurs on the surface of LDH^[14-16]. Ran et al. revealed a carboxylated carbon nanotubes (C/CNTs) initiated the method of crystal growth to blend MOF polyhedrons associated by C-CNTs "bridge"^[17]. The results demonstrate that hollow nanostructures of NiCo-LDH/C-CNTs derived from MOF exhibit excellent electrochemical properties and extraordinary rate properties when used as battery-type electrodes for supercapacitor applications. However, due to the large size of MOFs, in situ growth of their conductive substrates has been a challenge^[18-19], and related studies are mostly performed on powders^[20]. However, the powder electrode is easy to agglomerate, resulting in poor conductivity, which seriously hinders the application of powder in the area of energy storage.

Herein, the self-sacrificial templates of Co-MOF anchored on CC are controllably transformed into NiCo-LDH three-dimensional porous nanostructure by the method of ion etching/exchange reaction. Among them, the best reason for the improvement of electrochemical properties is the layered double metal hydroxides with ultrathin nanoflakes. Its advantage is that it is a battery electrode with a higher specific capacity, a high energy density of 27.39 W·h/kg, and a capacity retention rate of 93.5% after 5000 cycles.

1 Experiment 1.1 Preparation of CC/Co-MOF

During this experiment, the chemicals were not used for further processing and were of analytical grade. The experimental water was deionized water. Co-MOFs were synthesized using a typical chemical solvent reaction. Firstly, Co(NO₃)₂·6H₂O(50 mL) and 2-methylimidazole (0.4 mol/L) were dissolved in 40 mL of deionized water with a clean CC immersed in the solution. It was magnetically stirred for 4 h at room temperature, and then dried at 80 ℃ for 12 h.

1.2 Preparation of CC/NiCo-LDH

Typically, 20 mg Ni(NO₃)₂·6H₂O was dispersed in 30 mL of ethanol under continuous magnetic stirring for 10 min. Next, the solution and the CC coated with the Co-MOF were poured into a 50 mL Teflon-lined autoclave and heated at 120 ℃ for 2 h at the rate of 10℃/min. Subsequent to drying at 60 ℃ for 12 h, the mass loading is ~1mg/cm².

1.3 Characterization

The crystalline and morphologies messages were characterized by X-ray diffraction (XRD, D/max TTR-III, Cu Kα), scanning electron microscope (SEM, SUPRA 40, German Zeiss) and transmission electron microscopy (TEM, Tecnai G² F20). The element content was determined using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Esxalab 250Xi) with a spectrometer with a dual Al Kα X-ray source (1486.6 eV).

1.4 Electrochemical Measurement

The electrochemical measurements of samples were conducted in 2 mol/L KOH with a three-electrode. Pt film and Ag/AgCl electrode were taken as counter electrode and reference electrode, respectively. The following Eq.(1) was used to calculate the specific capacity under different current densities^[21]:

$ C_s=(I \times \Delta t) /(3.6 \times m) $

(1)

where C_s, I and m are assigned to the specific capacity (mA·h/g), current (A) and the mass of the NiCo-LDH sample (g), respectively.

The following Eqs.(2)-(3) were employed to calculate the relevant energy and power density in the HSCs^[22]:

$ E=i \int V \mathrm{~d} t / M $

(2)

$ P=E / \Delta t $

(3)

where M and Δt were employed to be on behalf of the total mass of two electrodes (g) and the discharge time (h), respectively.

2 Results and Discussion

First, the Co-MOF nanosheets were anchored on CC through the coordination of 2-methylimidazole with metal ions in aqueous solution according to the chemical aqueous reaction.

Second, the Co-MOF was changed over into NiCo-LDH three-dimensional nanostructures by two steps. In the process of transformation, protons produced by Ni²⁺ hydrolysis destroy the coordination bond of Co-MOF and release Co²⁺ ions, which are then oxidized to Co³⁺ by ethanol^[23]. Then, the protons produced by Ni²⁺ hydrolysis continuously etch the Co-MOF template and the shaped Co²⁺/Co³⁺ co-precipitated with Ni²⁺ and OH^- to develop into NiCo-LDH nanosheets, which were then assembled into 3D structures^[24]. Finally, the three-dimensional hierarchical structure of NiCo-LDH was formed by assembling stacking.

The XRD images of NiCo-LDH converted from Co-MOF is depicted in Fig. 1(a). Clearly, the strong diffraction peak at 26.3° corresponds to the carbon cloth substrate. Otherwise, almost all the diffraction peaks of 11.6° and 46.9° correspond to NiCo-LDH (JCPDS No. 40-0216). In other words, the Co-MOF is successfully transformed into a porous NiCo-LDH nanostructure. In Fig. 1(b)-(c), it can be clearly seen that the dense triangular Co-MOF nanosheets are covered on the CC, and NiCo-LDH is obtained after the hydrothermal ion exchange reaction (Fig. 1(d) (e)). In addition, the enlarged view clearly reveals that NiCo-LDH is assembled with ultrathin nanosheets, as displayed in Fig. 1(f).

In Fig. 2(a)-(b), the porous structure of the CC/NiCo-LDH 2D nanosheet arrays can be clearly observed, which is due to the fact that the active nickel ions replace part of the cobalt ions on the CC/Co-MOF during the hydrothermal reaction ^[25]. Moreover, the thickness of the ultrathin nanosheet is ~20 nm. Images from the marginal region of HR-TEM distinctly show a plane spacing of 0.26 nm, which can be ascribed to the (012) plane of the CC/NiCo-LDH nanostructure arrays (Fig. 2(c)). In addition, the Co, Ni and O elements are uniformly present in the EDS mapping (Fig. 2(d)).

In Fig. 3(a), Ni, Co and O species are all present. As shown in Fig. 3(b), two convolution peaks are Ni 2p_1/2 and Ni 2p_3/2, which prove that both Ni²⁺ (856.43 and 873.91 eV) and Ni³⁺ (875.97 and 857.22 eV) co-exist in CC/Ni Co-LDH. Meanwhile, two peaks situated at 862.94 and 880.70 eV are attributed to the satellites (Sat.). Fig. 3(c) displays that the peaks at 782.68 and 798.22 eV for the Co 2p of CC/Ni Co-LDH are described to Co²⁺, in the meantime, the peaks situated at 782.63 and 796.94 eV are described to Co³⁺. In addition, the remaining two peaks are satellite peaks. In Fig. 3(d), three obvious diffraction peaks of O 1s can be observed, the lattice oxygen, oxygen vacancy and surface adsorbed oxygen correspond to 531.41, 532.19 and 533.60 eV, respectively.

In Fig. 4(a), the range of study is the scanning rate of 5 mV/s. From this figure, the curves exhibited distinct and paired redox peaks showing that the two electrodes are battery-type. Meanwhile, the CV curve of CC/NiCo-LDH has a larger specific current in the same potential range, indicating the largest specific capacity. Moreover, the GCD curves of CC/Co-MOF and CC/NiCo-LDH electrodes at 1 A/g are exhibited in Fig. 4(b). The specific capacity of CC/NiCo-LDH electrode can reach 226.3 mA·h/g, whereas only 18.6 mA·h/g is obtained for CC/Co-MOF electrode.

In Fig. 4(c), the CV curves represent a proliferation manipulated electrochemical process and rapid ionic and electronic transport. The probable faradaic reactions are shown below:

$ \mathrm{Ni}(\mathrm{OH})_2+\mathrm{OH}^{-} \leftrightarrow \mathrm{NiOOH}+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} $

(4)

$ \mathrm{Co}(\mathrm{OH})_2+\mathrm{OH}^{-} \leftrightarrow \mathrm{CoOOH}+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} $

(5)

In addition, as displayed in Fig. 4(d), the GCD curves of this electrode are observed at different current densities from 1 to 16 A/g. The symmetric GCD curves provide a signature of high-velocity function with highly reversible and high-speed reply dynamics. Remarkably, the Cs obtained from the CC/NiCo-LDH electrode is 226.3 mA·h/g and 172.4 mA·h/g at current densities of 1 A/g and 16 A/g, indicating the excellent rate properties based on the porous hierarchical structure of NiCo-LDH (in Fig. 4(e)). In addition, EIS tests also confirmed lower R_S impedance and faster kinetics on NiCo-LDH. As shown in Fig. 4(f), the equivalent series resistance (R_s) is the intersection with the real axis, and it includes electrolyte contact resistance, ion resistance and intrinsic resistance. Apparently, the R_s values are 1.41 and 1.36 Ω for CC/Co-MOF and CC/NiCo-LDH electrodes, respectively. It is worth noting that the CC/NiCo-LDH electrode shows high electronic conductivity, low resistance and rapid ion proliferation speed, revealing better charge transfer and ion pervasion dynamics behaviour.

The HSC device was assembled to prove the actual submission, where CC/ NiCo-LDH and AC are used as positive and negative electrodes, respectively (Fig. 5(a)). The mass ratio is calculated as follows:

$ m_{+} / m_{-}=\left(C_s^{-} \times \Delta V_{-}\right) /\left(C_s^{+} \times \Delta V_{+}\right) $

(6)

where m corresponds to the mass (g), C_s refers to the specific capacity (mA·h/g), and ΔV denotes the voltage window.

It is noted that the stabilizing working voltage of the HSC device is 1.4 V in Fig. 5(b). When the voltage is greater than 1.5 V, the oxygen is freed and palpable polarization happens. In a similar way, Fig. 5(c) shows GCD curves of the HSCs at various voltages, denoting that the stabilizing working voltage is 1.4 V at 5 mA/cm². Fig. 5(d) shows the CV curves of HSCs under various scan rates of 5-25 mV/s. The curves show a quasi-rectangular shape due to the reasonable matching of the positive and negative electrodes. Fig. 5(e) shows the GCD curves of the HSC, which depicts images at different current densities from 1 to 5 mV/s. The electrode had a high-capacity retention of 91.4%, which showed the excellent rate capability of our HSC device (Fig. 5(f)).

Fig. 6(a) summarizes the Nyquist diagrams, whose insets are the fitted equivalent circuit. It is worth nothing that relatively low intersection with the horizontal axis (R_s) and a slanted line in the low frequency (W) show the potential for energy storage. Certainly, the cycle function of HSC device is significant parameter for the actual submission. As shown in Fig. 6(b), the HSC device has excellent cycle stability after 5000 cycles, and the capacity retention rate reaches 93.5%, which reveals the outstanding stability. Ragone plot of HSC device is also detected for the functional application (inset in Fig. 6(b)). An energy density of CC/NiCo-LDH //AC device can be achieved to 27.39 W·h/kg (with a power density of 148.95 W/kg). It demonstrates that the HSC device composed of the layered structure CC/NiCo-LDH electrode has brilliant energy memory ability. Moreover, to verify the capacity for actual submission, two HSCs are connected in series to assemble a red light-emitting diode (LED) (inset in Fig. 6(b)).

3 Conclusions

In summary, we have successfully and controllably converted a self-sacrificial templated Co-MOF anchored on CC into a three-dimensional NiCo-LDH via an ion-etching/ exchange reaction method. Porous NiCo-LDH not only provides smooth channels for electrochemical reactions, but also offers high specific surface area and efficient kinetics. Therefore, CC/NiCo-LDH electrode exhibits excellent electrochemical performance. Moreover, the CC/NiCo-LDH//AC HSC device reaches a good cycle stability of 93.5%. This work comes up with a neoteric method for MOF-derived LDH electrodes with excellent electrochemical performance.