Hierarchical Two-Dimensional Conductive Metal–Organic Framework

Communication pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Hierarchical Two-Dimensional Conductive Metal−Organic Framework/Layered Double Hydroxide Nanoarray for a HighPerformance Supercapacitor Yan-Li Li, Jiao-Jiao Zhou, Meng-Ke Wu, Chen Chen, Kai Tao, Fei-Yan Yi, and Lei Han* State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China S Supporting Information *

regard, Ni-CAT (CAT = catecholate) is a classical 2D conductive MOF,27,28 in which the 2D hexagonal lattice nets are further packed along the crystallographic c axis to form a honeycomb porous structure (Figure 1a). Effective orbital overlaps between

ABSTRACT: A novel hierarchical nanoarray material based on a two-dimensional metal−organic framework (Ni-CAT) and a layered double hydroxide (NiCo-LDH) was fabricated on a nickel foam substrate. By taking advantage of the regular nanostructure and making full use of the high porosity and excellent conductivity, the hybrid material exhibits a high areal capacitance for a supercapacitor (3200 mF cm−2 at 1 mA cm−2).

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etal−organic frameworks (MOFs), as one class of crystalline porous materials self-assembled through coordination bonds between inorganic metal ions or clusters and organic ligands, have attracted increasing attention because of their controllable structure and functionality, higher surface area, ultrahigh porosity, and inherently physicochemical properties.1,2 Besides applications in gas storage,3 separation,4,5 catalysis,6,7 sensing,8,9 and drug delivery,10,11 MOFs are receiving increasing research interest in energy storage,12−15 such as batteries and supercapacitors. Although these material-based traditional MOFs have been applied in supercapacitors,16,17 their poor electrical conductivity and large steric hindrance hamper their direct utilization as electrode materials. Hence, it is essential to explore new strategies to enhance the inherent electrochemical performances of MOFs.18 Recently, intrinsically two-dimensional (2D) conductive MOFs demonstrated high electrochemical performances for supercapacitors.19−21 2D conductive MOFs possess many special properties: atomic thicknesses with short pathways and large lateral sizes allow rapid mass transport and superior electron transfer; extremely high accessible active sites exposed on the surface of 2D MOFs instead of enclosed inside their pores or channels, which accelerate interactions between active sites and the electrolyte so that their performance is greatly improved in electrochemistry.22−25 To improve the supercapacitive properties of these 2D conductive MOFs, further studies are extremely desired by engineering MOFs at nanoscale or by incorporating pseudocapacitive components. Furthermore, hierarchical and regular nanoarray composites, integrating the unique properties of each building block, have proven to be effective in the advancement of supercapacitors.26 Herein, we present, for the first time, the fabrication of hierarchical-hybrid-nanoarray-combined 2D conductive MOFs (Ni-CAT nanorods) and layered double hydroxides (NiCoLDH nanosheets) on a nickel foam (NF) substrate. In this © XXXX American Chemical Society

Figure 1. (a) Morphology and crystal structure of pure Ni-CAT nanorods. (b) NiCo-LDH nanosheets coated on NF. (c) SEM image of a Ni-CAT/NiCo-LDH/NF array. (d) Powder XRD patterns of the samples.

Ni ions and organic ligands endow Ni-CAT with good properties of charge transport. On the other hand, a 2D LDH with the general formula [M2+1−xM3+x(OH)2]x+[An−x/n·yH2O]x−, possessing a positively charged layer,29−31 has been reported as an ideal electrode material for pseudocapacitors because its nanochip-interleaved structure assists in the shortening of the transport distance of the ions and its large surface area and longtime stability in a strong alkaline solution can markedly increase the exchange rate and cycling stability during the electrochemical processes.32 The key for this design strategy is to combine the Faradaicactive NiCo-LDH with the 2D conductive Ni-CAT on a conductive substrate, forming a Ni-CAT/NiCo-LDH/NF hierarchical integrated nanoarray without any conductive additives or other binders. This regular nanostructure displays an excellent areal capacitance of 3200 mF cm−2 at 1 mA cm−2 Received: February 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b00493 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry because of the following advantages: (I) the synergistic effect between Ni-CAT nanorods and NiCo-LDH nanosheets contributes to improving the capacitance; (II) 2D nanomaterials with multiple metallic ions provide more active sites; (III) the excellent electrical conductivity of the 2D MOF accelerates the electronic transmission; (IV) a hierarchical integrated nanoarray avoids aggregation of the nanoparticles, which greatly improves the performance. The facile two-step hydrothermal synthesis of the hierarchical Ni-CAT/NiCo-LDH/NF nanoarray is demonstrated in Scheme S1. First, the NiCo-LDH nanosheet was in situ grown on NF. Second, the Ni-CAT nanorods were coated onto the surface of NiCo-LDH with a staggered architecture under hydrothermal conditions at 85 °C for 16 h (Figure S1). The possible procedure for the deposition of Ni-CAT on NiCo-LDH may be attributed to the fact that outside hydroxides on the surface of LDH could trap the Ni ions, and then the Ni ions coordinated with HHTP ligands, gradually forming Ni-CAT nanorods. The nanosized morphology and crystal structure of Ni-CAT were observed in Figure 1a, showing the nanorod-shaped aggregation. As shown in Figure 1b, the NiCo-LDH nanosheets are intersected and aligned vertically on NF with a thickness about 550 nm. The final Ni-CAT/NiCo-LDH/NF nanoarray is exhibited in Figure 1c, numerous Ni-CAT nanorods have intensively grown on NiCo-LDH nanosheets with a thickness around 40 nm. The high-resolution transmission electron microscopy (HRTEM) image (Figure S2) indicates their mutual cross-arrangement. The visible lattice fringes with interplanar spacings of 0.67 and 0.15 nm correspond to a d210 spacing of NiCAT27 and a d015 spacing of NiCo-LDH,32 respectively. Powder X-ray diffraction (XRD) analyses further confirm the successful formation of the final products. As shown in Figure 1d, the well-defined diffraction peaks at around 11.1°, 22.2°, 34.4°, and 38.5° (blue dots) can be indexed to (003), (006), (009), and (015) plane reflections, respectively, of the hydrotalcite-like LDH phase.32 The weak but distinctive peaks (red dots) indicate the successful growth of Ni-CAT on NiCo-LDH. In addition, energy-dispersive spectroscopy (EDS) and mapping test (Figure S3) demonstrate the coexistence of the two substances and clearly manifest the uniform distribution of each element. The electrochemical performances of Ni-CAT/NiCo-LDH/ NF nanoarrays were evaluated by cyclic voltammetry (CV) in a 1 M KOH aqueous electrolyte. As depicted in Figure 2a, typical CV curves at scan rates from 5 to 100 mV s−1 clearly exhibit welldefined redox peaks, manifesting that the pseudocapacitive characteristics mainly arise from Faradaic redox reactions. It is obvious that the current intensity is increasing with increasing scan rate. Simultaneously, the position of the redox peak is slightly shifted because of the increase in the internal diffusion resistance of the active materials. Galvanostatic charge− discharge (GCD) curves of the Ni-CAT/NiCo-LDH/NF electrode were further examined within a potential range of 0− 0.45 V (Figure 2b). For comparison, the electrochemical performances of NiCo-LDH/NF and Ni-CAT are shown in Figures S4 and S5. It can be seen that the NiCo-LDH/NF electrode also has a significant pair of redox peaks in the CV curves, and while the scanning rate increases, the summit appears to slightly shift. However, the peak area of Ni-CAT/NiCo-LDH/ NF is much higher than that of the NiCo-LDH/NF electrode in the case of the same speed, which corresponds to the GCD curves in Figure S4b. Obviously, the CV of Ni-CAT has a good pair of redox peaks at different scan rates, also indicating the pseudocapacitive characteristic (Figure S5a). The GCD curves

Figure 2. Electrochemical performances of the Ni-CAT/NiCo-LDH/ NF electrode: (a) CV curves; (b) GCD curves; (c) specific capacitance; (d) cycling stability of Ni-CAT/NiCo-LDH/NF.

show a significant symmetry, indicating its excellent electrochemical performance and reversible redox process. Capacitances were obtained from their charge−discharge curves at different current densities (Figures 2c and S4c and S5c). The specific capacitance of Ni-CAT is 253 mF cm−2 (320 F g−1) at 1 mA cm−2, which decreases to 141 mF cm−2 at 8 mA cm−2, while the specific capacitance of NiCo-LDH/NF is 1770 mF cm−2 (890 F g−1) at 1 mA cm−2, which decreases to 1272 mF cm−2 (635 F g−1) at 8 mA cm−2. For Ni-CAT/NiCo-LDH/NF, the specific capacitance of 3200 mF cm−2 (2133 F g−1) observed at 1 mA cm−2 is decreased to 2073 mF cm−2 (1373 F g−1) at 8 mA cm−2. This result is significantly better than those of some previously reported electrode materials, as compared in Table S1. The cycling stabilities of NiCo-LDH/NF, Ni-CAT, and NiCAT/NiCo-LDH/NF are shown in Figures S4d and S5d and 2d, respectively. The capacitance of Ni-CAT/NiCo-LDH/NF remains at 80% of the initial value after 1000 cycles, which shows that it can provide a highly reliable capacitance performance at high charge−discharge rates for power depletion applications. Similarly, the specific capacitance of the Ni-CAT electrode only maintains at 67% of the initial value. Therefore, NiCo-LDH/NF has a great contribution to the specific capacitance and stability of hybrid Ni-CAT/NiCo-LDH/NF by the synergistic effect. Although the areal performance of NiCAT/NiCo-LDH/NF is higher than that of NiCo-LDH/NF, the cycling stability of Ni-CAT/NiCo-LDH/NF is slightly lower than that of NiCo-LDH/NF (87%), which may be due to the poor stability of Ni-CAT in an alkaline environment. The electrochemically active surface area (ECSA) is important to the redox reactions on the surface of the electrode. The ECSA of electrodes is proportional to the double layer capacitance (Cdl) estimated from non-Faradaic region CV curves. The potential window of the CV curves was in the range of 0.21−0.37 V (vs Ag/AgCl). The scan rates were 2, 5, 10, and 20 mV s−1. The linear slope of the plot of the difference of the charging current density Δj at 0.26 V versus scan rate, equivalent to twice Cdl, was used as an indicator for the ECSA. As shown in Figure S6, Cdl of Ni-CAT/NiCo-LDH/NF is 203.85 mF cm−2, which is much higher than those of NiCo-LDH (97 mF cm−2) and Ni-CAT (31.7 mF cm−2). This means that Ni-CAT/NiCo-LDH/NF has enriched active sites at the solid−liquid interface, which contributes to the higher electrochemical performance. B

DOI: 10.1021/acs.inorgchem.8b00493 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry To further evaluate the hierarchical Ni-CAT/NiCo-LDH/NF for practical applications, an asymmetric supercapacitor (ASC) was assembled by using Ni-CAT/NiCo-LDH/NF as a positive electrode and activated carbon (AC) as a negative electrode in a 1 M KOH electrolyte. Obviously, the CV curves of the AC electrode exhibits typical electric double layer capacitance behavior within the range of −1.0 to 0 V, as shown in Figure S7a, while the working potential of the Ni-CAT/NiCo-LDH/NF electrode varies from 0 to 0.6 V, indicating a working window of 1.7 V for the asymmetric cell. Conventionally, a high potential window means a high energy density for ASCs.33 GCD curves of the AC electrode at different current densities remain almost symmetric, indicating high reversibility (Figure S7b). Figure S8b shows some typical CV curves for the asymmetric cell in various voltage windows (1.5−1.8 V) at a scan rate of 50 mV s−1. The shapes of the CV curves obtained in different voltage ranges indicated that not only the electric double layer but also the Faraday reaction make contributions to the charge storage process. Besides, the polarization occurs in the CV curves with the potential increasing to 1.7 V. Figure 3a shows CV curves at

power density. This result outperforms many previously reported ASCs (Table S2). As shown in Figure 3d, the ASCs also exhibited excellent cycle stability, with over 74% capacity after 1000 cycles, indicating that Ni-CAT/NiCo-LDH/NF has potential applications. In addition, relevant tests on the stability of the material were performed after testing the performance. There was a slight change in the morphology after the cycle (Figure S9a). The morphology of Ni-CAT changed, which was tightly attached to the surface of the distorted NiCo-LDH nanosheet. From the XRD image (Figure S9b), it can be seen that the weak peaks of NiCo-LDH and Ni-CAT were also observed. The CV curves (Figure S9c) were also tested at the same scan rate to further characterize the stability of the material after the cycle. There are no significant changes, which indicates that Ni-CAT/NiCoLDH/NF has good cycling stability. In summary, a hierarchical Ni-CAT/NiCo-LDH/NF electrode material has been fabricated. Because of the excellent features of each component, the hybrid Ni-CAT/NiCo-LDH/ NF shows a higher area capacitance of 3200 mF cm−2 at a current density of 1 mA cm−2. In view of the 2D structure of NiCo-LDH itself, with good stability and high capacitance performance, the growth of a uniform array of Ni-CAT nanorods on the NiCoLDH/NF surface avoids the powder Ni-CAT agglomeration. It can be seen that the electrochemical performance of the 3D multilayer Ni-CAT/NiCo-LDH/NF complex with respect to NiCAT has been greatly improved. At present, the 2D conductive MOFs seem to have just begun to be used in the field of energy storage, and more work should focus on improving the stability of conductive MOFs in the future. This work will open research on the construction of hybrid materials based on 2D conductive MOFs and provide a great prospect for electrochemical energy storage materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00493. Experimental preparation, EDS analysis, HRTEM image, electrochemical experiments, CV and ASC curves, Scheme S1, and Tables S1 and S2 (PDF)

Figure 3. Electrochemical performances of an ASC with Ni-CAT/ NiCo-LDH/NF as a positive electrode and AC as a negative electrode: (a) CV curves; (b) GCD curves; (c) specific capacitances; (d) cycling stability of the ASC at 25 mA cm−2.

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different scan rates at potentials of 0−1.7 V. In Figure 3b, GCD curves show the symmetric behavior at different current densities in a stable operating voltage window. The ASC exhibited excellent rate capability with capacitance retention values of 435, 406, 368, and 330 mF cm−2 at current densities of 1, 3, 5, and 10 mA cm−2, respectively (Figure 3c). The electrochemical impedance spectroscopy tests were performed at open circuit in the frequency range from 100 kHz to 0.01 Hz (Figure S8c). Before and after the contrast cycle, the lines essentially overlap in the high-frequency region, indicating almost equal amounts of solution bulk resistance (Rs).34 However, the semicircle at the intermediate frequency range corresponds to the charge-transfer process. After cycling, Rct is larger than that before cycling, mainly because the hydroxide ions are repeatedly intercalated and deintercalated between the electrode and electrolyte, resulting in the collapse of the Ni-CAT porous structure, thereby hindering the electrolyte wetting electrode. Figure S8d shows the energy densities and power densities of ASCs at different current densities, which deliver a high energy density of 93 μWh cm−2 at 18.3 mW cm−2

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.H.). ORCID

Fei-Yan Yi: 0000-0003-0733-9712 Lei Han: 0000-0002-2433-9290 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21471086 and 51572272), the Science and Technology Department of Zhejiang Province (Grant 2017C33007), the Natural Science Foundation of Ningbo (Grants 2017A610062 and 2017A610065), and the K.C. Wong Magna Fund at Ningbo University. C

DOI: 10.1021/acs.inorgchem.8b00493 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00493 Inorg. Chem. XXXX, XXX, XXX−XXX