Room-Temperature Fabrication of a Nickel-Functionalized Copper Metal–Organic Framework (Ni@Cu-MOF) as a New Pseudocapacitive Material for Asymmetric Supercapacitors

A nickel-functionalized copper metal–organic framework (Ni@Cu-MOF) was prepared by a facile volatilization method and a post-modification synthesis method at room temperature. The obtained Ni@Cu-MOF electrode delivered a high capacitance of 526 F/g at 1 A/g and had a long-term cycling stability (80% retention after 1200 cycles at 1 A/g) in a 6 M KOH aqueous solution. Furthermore, an asymmetric supercapacitor device was assembled from this Ni@Cu-MOF and activated carbon electrodes. The fabricated supercapacitor delivered a high capacitance of 48.7 F/g at 1 A/g and a high energy density of 17.3 Wh/kg at a power density of 798.5 kW/kg. This study indicates that the Ni@Cu-MOF has great potential for supercapacitor applications.


Introduction
With global energy demands and the high consumption of fossil fuels, accompanied by increasing environmental pollution, sustainable and renewable energy supplies are in high demand [1][2][3]. Supercapacitors (SCs) are energy storage devices (similar to lithium-ion batteries and fuel cells) that provide efficient renewable systems [4][5][6][7]. SCs have attracted widespread attention due to their large power density, fast charging and discharging time, and long life span [8][9][10].
According to the charge storage mechanism, SCs are divided into two types: electrical double-layer capacitors (EDLCs) and faradaic pseudocapacitors. In an EDLC, the ions are stored at the Helmholtz double-layer interface found on materials such as carbon nanotubes and carbon fibers [11][12][13]. In pseudocapacitors, Faradic redox reactions occur at the surface or near-surface of electroactive sites on, for example, transition metal oxides and hydroxides [14][15][16]. To achieve a higher capacitance, it is important to search for and design porous materials with better energy storage properties for SCs.
Metal-organic frameworks (MOFs) are composed of metal ions and organic ligands. They are porous materials with a remarkable surface area, permanent porosity, and metal cations that participate in various redox reaction. MOFs are used as promising electrode materials in SCs [17][18][19]. According to the energy storage required for the application, there are two types of SCs formed from MOFs: (1) MOFs used as controlled templates or precursors for synthesizing mesoporous carbon, metal oxides, 660E potentiostat electrochemical workstation (Thermo-Fisher, Berlin, Germany). X-ray diffraction (XRD) patterns were recorded with a Rigaku Miniflex 600 X-ray diffractometer (Rigaku, Tokyo, Japan) from 5° to 50°. An infrared spectrum was recorded using an IR Affinity-1 FT-IR spectrometer (Shimadzu, Kyoto, Japan) in the range of 400-4000 cm −1 . Cu(NO3)2, Ni(NO3)2, 2,6pyridinedicarboxylic acid were bought commercially(Jinan Henghua Technology Co., Ltd., Jinan, China), acetylene carbon black and PolyVinylidene Fluoride were bought commercially(TIMCAL, Changzhou, China) Figure 1 shows the X-ray diffraction (XRD) patterns of the as-prepared Cu-MOF, Ni@Cu-MOF, and simulated Cu-MOF. The measured XRD patterns of the as-prepared Cu-MOF were in good agreement with the XRD pattern of the simulated Cu-MOF [44]. After introducing Ni 2+ into the Cu-MOF, the PXRD pattern of the Ni@Cu-MOF remained unchanged. The results suggested that the Cu-MOF and the Ni@Cu-MOF were synthesized successfully. The SEM ( Figure 1b) and TEM ( Figure S1) images of the Cu-MOF revealed that the size of the Cu-MOF nanorod was about 230 nm. On the basis of the SEM (Figure 1c) and TEM ( Figure S2)images of the Ni@ Cu-MOF after the introduction of Ni 2+ into the Cu-MOF, we speculated that the Cu-MOF nanorod dissolved in the nickel nitrate aqueous solution; meanwhile, the phase structures of the Cu-MOF particles changed from nanorods to smaller nanoparticles.

Results and Discussion
The surface areas of the Cu-MOF (67.908 m 2 ·g −1 ) and the Ni@Cu-MOF (95.365 m 2 ·g −1 ) were calculated by a multipoint BET model. The increase in surface area of the Ni@Cu-MOF was attributed to the smaller size of the Ni@Cu-MOF. In the range of P/P0 = 0.8-1.0, the hysteresis loop in the isotherms showed that mesopores and macropores existed in the MOF. Obviously, the pore distribution curve suggested that the Ni@Cu-MOF also possessed mesopores and macropores, and the pore diameters were about 60-90 nm. In addition, these mesopores and macropores were helpful to the rapid diffusion of ions and promoted a rapid faradaic reaction and high rate performance.  Figure 2 shows the FT-IR spectra. Several bands between 3200 and 3500 cm −1 were characteristic of the n(O-H) mode of carboxyl groups. The absorption band at 1620 cm −1 belonged to the n(C=O) mode of carboxyl groups. In addition, the bands at 1300 and 920 cm −1 were assigned to the d(O-H) mode of carboxyl groups. The band at 1420 cm −1 belonged to the n(C=N) mode of carboxyl groups. After the introduction of Ni 2+ into the Cu-MOF, the IR bands of the Ni@Cu-MOF were consistent with those of the Cu-MOF. The results showed that the introduction of Ni 2+ did not influence the crystalline structure, as shown by the PXRD patterns. The energy storage capacitance of the as-prepared Ni@Cu-MOF was assessed by cyclic voltammetry (CV) measurements. Figure 3 shows the CV responses of the Ni@Cu-MOF at different scan rates (2-100 mV s −1 ). The CV curves of the Ni@Cu-MOF showed an integrated pair of redox peaks due to the redox reaction (Ni@Cu-MOF) in the 6 M KOH electrolyte solution (reversible valence-state changes between Ni 2+ and Ni 3+ ), which occurred due to the adsorption/desorption of OH − in the layer during the electrochemical reaction. Furthermore, the peak current of the CV   Figure 2 shows the FT-IR spectra. Several bands between 3200 and 3500 cm −1 were characteristic of the n(O-H) mode of carboxyl groups. The absorption band at 1620 cm −1 belonged to the n(C=O) mode of carboxyl groups. In addition, the bands at 1300 and 920 cm −1 were assigned to the d(O-H) mode of carboxyl groups. The band at 1420 cm −1 belonged to the n(C=N) mode of carboxyl groups. After the introduction of Ni 2+ into the Cu-MOF, the IR bands of the Ni@Cu-MOF were consistent with those of the Cu-MOF. The results showed that the introduction of Ni 2+ did not influence the crystalline structure, as shown by the PXRD patterns. The energy storage capacitance of the as-prepared Ni@Cu-MOF was assessed by cyclic voltammetry (CV) measurements. Figure 3 shows the CV responses of the Ni@Cu-MOF at different scan rates (2-100 mV s −1 ). The CV curves of the Ni@Cu-MOF showed an integrated pair of redox peaks due to the redox reaction (Ni@Cu-MOF) in the 6 M KOH electrolyte solution (reversible valence-state changes between Ni 2+ and Ni 3+ ), which occurred due to the adsorption/desorption of OH − in the layer during the electrochemical reaction. Furthermore, the peak current of the CV The energy storage capacitance of the as-prepared Ni@Cu-MOF was assessed by cyclic voltammetry (CV) measurements. Figure 3 shows the CV responses of the Ni@Cu-MOF at different scan rates (2-100 mV s −1 ). The CV curves of the Ni@Cu-MOF showed an integrated pair of redox peaks due to the redox reaction (Ni@Cu-MOF) in the 6 M KOH electrolyte solution (reversible valence-state changes between Ni 2+ and Ni 3+ ), which occurred due to the adsorption/desorption of OH − in the layer during the electrochemical reaction. Furthermore, the peak current of the CV increased (scan rate from Polymers 2019, 11, 821 5 of 10 2 to 100 mV s −1 ); accordingly, the current response increased. The results showed a good rate and reversibility of the fast charge-discharge response [17,45].
Polymers 2018, 10, x FOR PEER REVIEW 5 of 10 increased (scan rate from 2 to 100 mV s −1 ); accordingly, the current response increased. The results showed a good rate and reversibility of the fast charge-discharge response [17,45].  Figure 4 shows the galvanostatic charge-discharge (GCD) curves of Ni@Cu-MOF and Cu-MOF at different current densities (ranging from 0 to 0.5 V). Specific capacitances of 526,484, 390, 320, and 280 F g −1 were observed at 1.0, 2.0, 5.0, 8.0, and 10 A g −1 , respectively. The discharge curves of the Ni@Cu-MOF and the Cu-MOF at 1 A g −1 were different from straight and flat curves, which indicated that the process of GCD can be ascribed to the Faradaic process. The GCD curves of Ni@Cu-MOF showed a better performance than the GCD curves of the Cu-MOF, reducing the polarization. As calculated, the specific capacitance of the Ni@Cu-MOF (526 F/g) was more than four times higher than the specific capacitance of the Cu-MOF (126 F/g) at 1 A/g (Figures S5 and S6). The cycling tests of the Ni@Cu-MOF were also recorded by GCD measurements. Figure 5 shows the GCD curves of the Ni@Cu-MOF electrode at 1 A/g in a 6 mol/L KOH solution for 1200 cycles. The results showed that the Ni@Cu-MOF electrode maintained 80% of its specific capacitance after 1200 cycles, which demonstrated good long-term cycling stability and capacitance.  Figure 4 shows the galvanostatic charge-discharge (GCD) curves of Ni@Cu-MOF and Cu-MOF at different current densities (ranging from 0 to 0.5 V). Specific capacitances of 526, 484, 390, 320, and 280 F g −1 were observed at 1.0, 2.0, 5.0, 8.0, and 10 A g −1 , respectively. The discharge curves of the Ni@Cu-MOF and the Cu-MOF at 1 A g −1 were different from straight and flat curves, which indicated that the process of GCD can be ascribed to the Faradaic process. The GCD curves of Ni@Cu-MOF showed a better performance than the GCD curves of the Cu-MOF, reducing the polarization. As calculated, the specific capacitance of the Ni@Cu-MOF (526 F/g) was more than four times higher than the specific capacitance of the Cu-MOF (126 F/g) at 1 A/g (Figures S5 and S6).
Polymers 2018, 10, x FOR PEER REVIEW 5 of 10 increased (scan rate from 2 to 100 mV s −1 ); accordingly, the current response increased. The results showed a good rate and reversibility of the fast charge-discharge response [17,45].  Figure 4 shows the galvanostatic charge-discharge (GCD) curves of Ni@Cu-MOF and Cu-MOF at different current densities (ranging from 0 to 0.5 V). Specific capacitances of 526,484, 390, 320, and 280 F g −1 were observed at 1.0, 2.0, 5.0, 8.0, and 10 A g −1 , respectively. The discharge curves of the Ni@Cu-MOF and the Cu-MOF at 1 A g −1 were different from straight and flat curves, which indicated that the process of GCD can be ascribed to the Faradaic process. The GCD curves of Ni@Cu-MOF showed a better performance than the GCD curves of the Cu-MOF, reducing the polarization. As calculated, the specific capacitance of the Ni@Cu-MOF (526 F/g) was more than four times higher than the specific capacitance of the Cu-MOF (126 F/g) at 1 A/g ( Figures S5 and S6). The cycling tests of the Ni@Cu-MOF were also recorded by GCD measurements. Figure 5 shows the GCD curves of the Ni@Cu-MOF electrode at 1 A/g in a 6 mol/L KOH solution for 1200 cycles. The results showed that the Ni@Cu-MOF electrode maintained 80% of its specific capacitance after 1200 cycles, which demonstrated good long-term cycling stability and capacitance. The cycling tests of the Ni@Cu-MOF were also recorded by GCD measurements. Figure 5 shows the GCD curves of the Ni@Cu-MOF electrode at 1 A/g in a 6 mol/L KOH solution for 1200 cycles. The results showed that the Ni@Cu-MOF electrode maintained 80% of its specific capacitance after 1200 cycles, which demonstrated good long-term cycling stability and capacitance. Electrochemical impedance spectroscopy (EIS) (potential of 0.3046 V in a 6 M KOH solution) was studied in the frequency range of 0.01 Hz to 100 kHz. In Figure 6, the Nyquist plots of the Ni@Cu-MOF show a semicircle and linear curve in the high-and low-frequency regions, respectively. The semicircle represents Faradaic charge transfer processes (Rct). The internal resistance (Rs) was deduced from the real axis intercept (Z') and included the intrinsic resistance (Ni@Cu-MOF), the bulk resistance of the solution, and the resistance of the Ni@Cu-MOF and Ni foam (the current collector). The Rct of the Ni@Cu-MOF was 0.75 Ω. After 1200 cycles, the value of the Rct (Ni@Cu-MOF) showed a slight increase from 0.75 to 0.77. The phase angle of the line (Ni@Cu-MOF after 1200 cycles) was similar to 45 °C in the low-frequency region, which resulted from the synergistic effect of the Ni@Cu-MOF. To further explore the energy storage application of the Ni@Cu-MOF electrode, an ASC device was built with a positive electrode (Ni@Cu-MOF) and a negative electrode (activated carbon, AC). Electrochemical impedance spectroscopy (EIS) (potential of 0.3046 V in a 6 M KOH solution) was studied in the frequency range of 0.01 Hz to 100 kHz. In Figure 6, the Nyquist plots of the Ni@Cu-MOF show a semicircle and linear curve in the high-and low-frequency regions, respectively. The semicircle represents Faradaic charge transfer processes (Rct). The internal resistance (Rs) was deduced from the real axis intercept (Z') and included the intrinsic resistance (Ni@Cu-MOF), the bulk resistance of the solution, and the resistance of the Ni@Cu-MOF and Ni foam (the current collector). The Rct of the Ni@Cu-MOF was 0.75 Ω. After 1200 cycles, the value of the Rct (Ni@Cu-MOF) showed a slight increase from 0.75 to 0.77. The phase angle of the line (Ni@Cu-MOF after 1200 cycles) was similar to 45 • C in the low-frequency region, which resulted from the synergistic effect of the Ni@Cu-MOF. Electrochemical impedance spectroscopy (EIS) (potential of 0.3046 V in a 6 M KOH solution) was studied in the frequency range of 0.01 Hz to 100 kHz. In Figure 6, the Nyquist plots of the Ni@Cu-MOF show a semicircle and linear curve in the high-and low-frequency regions, respectively. The semicircle represents Faradaic charge transfer processes (Rct). The internal resistance (Rs) was deduced from the real axis intercept (Z') and included the intrinsic resistance (Ni@Cu-MOF), the bulk resistance of the solution, and the resistance of the Ni@Cu-MOF and Ni foam (the current collector). The Rct of the Ni@Cu-MOF was 0.75 Ω. After 1200 cycles, the value of the Rct (Ni@Cu-MOF) showed a slight increase from 0.75 to 0.77. The phase angle of the line (Ni@Cu-MOF after 1200 cycles) was similar to 45 °C in the low-frequency region, which resulted from the synergistic effect of the Ni@Cu-MOF. To further explore the energy storage application of the Ni@Cu-MOF electrode, an ASC device was built with a positive electrode (Ni@Cu-MOF) and a negative electrode (activated carbon, AC). To further explore the energy storage application of the Ni@Cu-MOF electrode, an ASC device was built with a positive electrode (Ni@Cu-MOF) and a negative electrode (activated carbon, AC). According to the CV results, in the three-electrode system in the 6 M KOH solution, the potential  (Figures S7 and  S8). As shown in Figure 7a,b, the CVs at various scan rates from 2 to 100 mV/s and the GCD curves at various current densities from 1 to 1 0 A/g displayed a potential window from 0 to 1.6 V. The CV curve did not change at a high scan rate (100 mV s −1 ). The results showed that the ASC device has the potential for a high power delivery [24]. The specific capacitance (C), power density (P), and energy density (E) of the ASCs were calculated from the GCD curves at 1 A/g. As shown in Figure 7b, the ASCs achieved a potential as high as 1.6 V. Moreover, the value of C s was calculated from the GCD curves, and a specific capacitance of 48.7 F/g at a 1 A g −1 was observed. The ASCs delivered a high energy density of 17.3 Wh/kg at a power density of 798.5 kW/kg. According to the CV results, in the three-electrode system in the 6 M KOH solution, the potential window of the Ni@Cu-MOF and AC was in the ranges of 0-0.6 V and −1.0 to 0 V ( Figures S7 and S8). As shown in Figure 7a,b, the CVs at various scan rates from 2 to 100 mV/s and the GCD curves at various current densities from 1 to 1 0 A/g displayed a potential window from 0 to 1.6 V. The CV curve did not change at a high scan rate (100 mV s −1 ). The results showed that the ASC device has the potential for a high power delivery [24]. The specific capacitance (C), power density (P), and energy density (E) of the ASCs were calculated from the GCD curves at 1 A/g. As shown in Figure 7b, the ASCs achieved a potential as high as 1.6 V. Moreover, the value of Cs was calculated from the GCD curves, and a specific capacitance of 48.7 F/g at a 1 A g −1 was observed. The ASCs delivered a high energy density of 17.3 Wh/kg at a power density of 798.5 kW/kg. Furthermore, cycling stability tests of the ASCs were evaluated at 1 A/g, as shown in Figure 8. After 1000 cycles, the capacitance of the ASCs was maintained at ∼63%. Therefore, the Ni@Cu-MOF has potential for practical applications.  Furthermore, cycling stability tests of the ASCs were evaluated at 1 A/g, as shown in Figure 8. After 1000 cycles, the capacitance of the ASCs was maintained at~63%. Therefore, the Ni@Cu-MOF has potential for practical applications. According to the CV results, in the three-electrode system in the 6 M KOH solution, the potential window of the Ni@Cu-MOF and AC was in the ranges of 0-0.6 V and −1.0 to 0 V ( Figures S7 and S8). As shown in Figure 7a,b, the CVs at various scan rates from 2 to 100 mV/s and the GCD curves at various current densities from 1 to 1 0 A/g displayed a potential window from 0 to 1.6 V. The CV curve did not change at a high scan rate (100 mV s −1 ). The results showed that the ASC device has the potential for a high power delivery [24]. The specific capacitance (C), power density (P), and energy density (E) of the ASCs were calculated from the GCD curves at 1 A/g. As shown in Figure 7b, the ASCs achieved a potential as high as 1.6 V. Moreover, the value of Cs was calculated from the GCD curves, and a specific capacitance of 48.7 F/g at a 1 A g −1 was observed. The ASCs delivered a high energy density of 17.3 Wh/kg at a power density of 798.5 kW/kg. Furthermore, cycling stability tests of the ASCs were evaluated at 1 A/g, as shown in Figure 8. After 1000 cycles, the capacitance of the ASCs was maintained at ∼63%. Therefore, the Ni@Cu-MOF has potential for practical applications.

Conclusions
In summary, a hybrid bimetallic Ni@Cu-MOF was prepared by a facile volatilization method and a post-modification synthesis method at room temperature. The bimetallic Ni@Cu-MOF delivered a higher specific capacitance of 526 F g −1 at 1 A g −1 than the monometallic Cu-MOF (126 F g −1 ). The results were attributed to the coexistence of Ni and Cu elements with good cycling stability (80% retention over 1200 cycles). An ASC based on Ni@Cu-MOF and activated carbon (as positive and negative electrodes) delivered a specific capacitance of 48.7 F g −1 at 1 A g −1 and a high energy density of 17.3 W h kg −1 at a power density of 798.5 W kg −1 . These results show that bimetallic MOFs are promising materials for SCs. This work encourages us to design and fabricate multicomponent MOF electrode materials for SCs.