Excellent Supercapacitor Performance of Robust Nickel-Organic

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Excellent Supercapacitor Performance of Robust Nickel-Organic Framework Materials Achieved by Tunable Porosity, InnerCluster Redox and in-situ Fabrication with Graphene Oxide Xiang-Yang Hou, Xiao-Li Yan, Xiao Wang, Shuni Li, Yucheng Jiang, Mancheng Hu, and Quanguo Zhai Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00881 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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Crystal Growth & Design

Excellent Supercapacitor Performance of Robust Nickel-Organic Framework Materials Achieved by Tunable Porosity, InnerCluster Redox and in-situ Fabrication with Graphene Oxide Xiang-Yang Hou,†,‡ Xiao-Li Yan,† Xiao Wang*, †,‡ Shu’ni Li,† Yucheng Jiang,† Mancheng Hu,† Quan-Guo Zhai*† †

Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of

Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi, 710062, China ‡

Department of Chemistry and Chemical Engineering, Yan′an University Laboratory of New

Energy & New Function Materials, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan′an Key Laboratory of Analytical Technology and Detection, Yan’an University, Shaanxi 716000, China ABSTRACT: Metal-organic frameworks have showed promising applications as electrode materials for supercapacitors because of the high porosity and tunable structures, but their poor water stability and conductivity limit their capacitance and efficiency. To demonstrate how to overcome these drawbacks, three isostructural Ni-organic frameworks with [NiII2NiIII(µ3-OH)(COO)6] trinuclear building blocks are selected. Taking advantages of high-connected architectures, absence of open metal sites and effective inner-cluster redox process, three Ni-organic frameworks all are stable in KOH electrolyte and exhibit a pseudoapacitor behavior with high specific capacitances up to 394, 426 and 465 F g-1. The increasing porosity facilitates the diffusion of metal ions and electrons and thus increase their electrochemical performance. Furthermore, we demonstrate that the in-situ fabrication of metal-organic frameworks with graphene oxide can effectively promote their supercapacitor performance. With a given 3% graphene oxide doping amount, the pseudo-capacitance values of three compounds are improved to be 590, 576 and 504 F g-1, respectively. The values surpass most of the reported metal-organic framework supercapacitors up to now. The excellent supercapacitor performance of these Ni-MOFs provide a new route to explore potential electrode material by utilizing robust metal-organic frameworks on the base of [M3(µ3-OH)(COO)6] trinuclear building blocks. 1

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■ INTRODUCTION Supercapacitors (SCs) with the advantages of light weight, high power density, long cycling life, and small size, is emerging as a key enabling storage technology and electrical energy conversion, and which provides a low-cost alternative source of energy to fulfill future energy-storage needs1. There are two mechanisms for improving the SCs performance by increasing electrical capacity of the electrode materials or increasing the voltage window of the electrolyte2-3. But, the exploration of novel active materials is more important for the development of high-performance SCs 4. In this regard, the properties of a new SCs material has been improved by increasing specific surface area, tailored pore size, and conductivity. For electrochemical double layers (EDLs) and pseudo-capacitors, carbon-based active materials, metal oxides and conducting polymers have been investigated5-6. However, carbon materials can operate with a long-life cycle but have low capacitance, while metal oxides have high capacitance but low life cycle7. So, the development of new high porous and conductive materials is vital to improve the capacitance and operational lifetime of supercapacitors. Metal-organic frameworks (MOFs), a new family of versatile porous material with diverse metal ions and organic linkers, and wide variety structures, have been successfully utilized in many fields according to their structural features8-14. However, the MOF-based SCs are still on the beginning stage1,13-23. Up to now, most MOF-based SCs are fabricated by destroying MOFs to afford specific metal-oxides or porous carbon materials and thus improve the conductivity to further increase the capacitance. Obviously, such utilization cannot completely take advantages of MOF materials. In fact, the utilization of MOFs directly as

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electrode materials for SCs is an idea route. However, the poor water and pH stability and low conductivity usually limit their utilization efficiency and capacitive performance. To overcome these drawbacks, high stable MOFs are firstly desirable. High connectivity (> 8) can effectively improve the stability of MOFs. Moreover, MOFs with open metal sites (OMSs) maybe subject to be attacked by solvent and could lead to decreased framework rigidity and stability. Thus, high connected MOFs without OMSs should be idea candidates as electrode materials for SCs. Graphene oxide (GO) with ultrahigh conductivity and excellent stability significantly boosts the various applications such as ultrafast sensors, energy storage, electrodes, etc24. It is important that with the introduction of GO, the ions in electrolytes are easily charged and discharged through surface and pores of porous electrode material, which may efficiently generate large SCs capacitance. To our best knowledge, a lot of research works have been investigated on the introduction of GO to metal oxide or porous carbon SCs25-29, however, no such work has been carried out on the doping of GO in MOFs to improve their supercapacitor performance. It’s supposed that the introduction GO can effectively improve the conductivity of MOFs and thus promote their supercapacitor performance. On the base of above considerations, three isostructural Ni-organic frameworks with different pore sizes and porosity were selected in this work as electrode materials to comprehensively evaluate their supercapacitor performance. In these MOFs, three open metal sites of [NiII2NiIII(µ3-OH)(COO)6] trinuclear building blocks all are occupied by pyridine N atoms. Together with their 9-connected architectures, we speculate they should have high water and pH stability. On the other hand. the µ3-OH bridged Ni(II) and Ni(III) mixed valent

3



metal centers can provide a novel and effective inner-cluster electron transfer process (Scheme 1), which may generate large SCs capacitance. As a result, three Ni-MOFs all are stable in KOH electrolyte and exhibit a pseudo-apacitor behavior with specific capacitance up to 394, 426 and 465 F g-1 tuned by their increasing specific surface area and pore sizes. Also, we exhibited that the in-situ fabrication of MOFs with graphene oxide can effectively enhance the MOF conductivity, and thus led to improved pseudo-capacitance values. To the best of our knowledge, such MOF and GO-doping MOF materials are among the best MOF supercapacitors up to now. More importantly, our work demonstrates a new route for supercapacitor materials, which is promising because the [M3(µ3-OH/O)(COO)6] (M = Ni, Fe, Co, V, Cr, Mn, Ru, et al.) secondary building unit (SBU) has been extensively explored during the construction of new MOFs and thus provides vast MOF candidates for electrochemical applications.

Scheme 1. The trinuclear building blocks and proposed inner-cluster electron transfer process.

■ EXPERIMENTAL SECTION Materials. Benzene-1,4-dicarboxylic acid (L1, BDC), naphthalene-2,6-dicarboxylic acid (L2, NDC), isonicotinic acid (INA, L3), and 4-(pyridin-4-yl)-benzonic acid (PBA, L4) all were purchased from TCI Shanghai. Ni(NO3)2·6H2O, N,N’-dimethylacetylamide (DMA), acetylene

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black, graphene oxide (GO), and poly(vinyli dene fluoride) (PVDF) were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. The nickel foam was purchased from Changsha Li Yuan New Material Company with the pore size and volume density range from 0.1 to 10 mm and 0.1 to 0.8 g cm−3. All chemicals were of analytical grade and used directly. Preparation of MOFs-1-3. Three MOFs were prepared according to the slightly modified literature process30-32. MOF-1 was produced by heating the mixture of Ni(NO3)2.6H2O (0.174 g, 6.0 mmol), INA (0.738 g, 6.0 mmol), BDC (0.498 g, 3.0 mmol) and NaOH (0.560 g, 14.0 mmol) in DMA at 160 °C. MOF-2 was synthesized by heating the mixture of Ni(NO3)2.6H2O (0.870 mg, 3.0 mmol), PBA (0.60 g, 3 mmol), BDC (0.250 g, 1.5 mmol) and NaOH (0.280 g, 7.0 mmol) in DMA at 160 °C. MOF-3 was obtained by heating the mixture of Ni(NO3)2.6H2O (0.870 g, 3.0 mmol), PBA (0.600 g, 3 mmol), NDC (0.330 g, 1.5 mmol) and NaOH (0.280 g, 7.0 mmol) in DMA at 160 °C. Yields: ~ 65%, 48%, and 52% for MOFs-1-3, respectively. In-situ fabrication of MOFs-1-3 with GO. GO powder was firstly stirred in DMA for 8 h. Then Ni(NO3)2.6H2O and corresponding organic ligands were mixed with GO-DMA mixture by ultrasonication. The solvothermal synthesis conditions of MOF@GO materials were the same as those for pure MOF materials. The corresponding products were named as MOFn@GO-x% (n = 1, 2, 3; x = 1, 3, 5), where x is the amounts of GO. Characterizations. PXRD data were measured by using a Rigaku D/Max-2550V X-ray diffractometer (40 kV, 20 mA) with Cu Kα radiation. Thermal behaviors were done under N2 with a heating rate of 10 °C min-1 using a NETZSCH STA449C thermogravimetric analyzer. The FT-IR spectra were carried out using KBr pellets between 4000-400 cm–1 on a Bruker 5



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EQUINOX-55 Spectrophotometer. SEM (FEI Quanta 200) was used to observe morphologies. The porosity of MOFs was measured by N2 adsorption/desorption isotherms at 77 K using a Micromeritics ASAP 2020 HD88 Automatic Micropore Physisorption Analyzer. Electrochemical measurements. Pt counter electrode and Hg/Hg2Cl2 reference electrode were combined to form a three-electrode system to investigate capacitive properties of MOFn and MOF-n@GO-x% (n = 1, 2, 3; x = 1, 3, 5). According to the former reports 22, the working electrode with a geometric surface area of 1 cm2 was prepared by mixing 80 wt% MOF materials, 15 wt% acetylene black and 5 wt% PVDF with nickel foam. The CV (cyclic voltammetry), GCD (galvanostatic charge-discharge), and EIS (electrochemical impedance spectroscopy) characterizations for MOF-n and MOF-n@GO-x% (n = 1, 2, 3; x = 1, 3, 5) were done on a CHI660E Electrochemical Workstation. The specific capacitance (C), maximum storage energy (Emax) and maximum power density (Pmax) were obtained by the following equations:

C = ( I ∆t ) (m∆V )

(1)

E = (C×Vi2)/2

(2)

Pmax = Vi 2 4 MRs

(3)

where I is the charge or discharge current, ∆t is a full charge or discharge time, m indicates the mass of the active material, and ∆V is the voltage change after a full charge or discharge, Vi is the initial voltage of the discharge curve, M is the total mass of active material in two electrodes with a 1.0 V cell voltage.

■ RESULTS AND DISCUSSION 6


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MOF-1, MOF-2 and MOF-3 are isostructural with different pore sizes and surface areas. They all crystallize in the cubic space group I-43m with high symmetry and have 9-connected ncb topology, which is firstly reported by Chen and co-workers. In this structure, the planar trinuclear [NiII2NiIII(µ3-OH)] cluster is formed by six carboxylate and three pyridyl groups to generate a tricapped trigonal prismatic [NiII2NiIII(µ3-OH)(COO)6(PY)3] 9-connected SBUs. The trinuclear motifs are extended by dicarboxylate (L1 or L2) and pyridine-carboxylate (L3 or L4) ligands to form the 3D architectures of MOF-1, MOF-2 and MOF-3. As shown in Figure 1, each 3D MOF architecture contains three types of cages tuned by the length of ligands: triangular pyramid cages formed by dicarboxylate ligands (Cage A and Cage B); cubic cages formed by pyridine-carboxylate (Cage C and Cage D), and triangular pyramid cages formed by mixed dicarboxylate and pyridine-carboxylate ligands (Cage E and Cage F). The sizes of these cages are about 4.0, 5.3, 5.5, 6.0, 5.5, 6.3 and 7.7 Å for Cage A to Cage G, respectively. MOF-1 is generated by the interconnection of Cage A, Cage C and Cage E. MOF-2 is formed by Cage A, Cage D and Cage F. MOF-3 contains Cage B, Cage D and Cage G. As stated above, high stability of MOFs is important for their application as supercapacitor materials. The 9-connected structure and absence of OMSs for these three Niorganic frameworks effectively improve their water and pH stability. Clearly, these robust MOFs with same framework but different pore sizes provide an idea platform to evaluate the influence of porosity on their supercapacitor performance.

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Figure 1. The ligands, cage structures and 3D topological net of MOFs-1-3.

As shown in Figure 2a, experimental PXRD patterns of MOFs-1-3 are in good agreement with the simulated patterns from X-ray single-crystal diffraction data, which indicate the high purity of these MOF materials. Thermogravimetric analysis curves show that three MOF materials all can stabilize to above 350 °C (Figure S1), and their similar decomposition process also verify their similar frameworks. In order to further verify the porosity of MOFs1-3, the N2 uptakes at 77 K were done for three MOFs (Figures 2b-2d). The Langmuir surface areas and pore volume of MOF-1, MOF-2, MOF-3 are of 1250, 2280, 2660 m2g-1, and 0.41, 0.94, 0.96 cm3g-1, which are consistent with above structural analysis. Thanks to the large surface area and pore volume, these porous MOFs have much more active sites to contact with electrolyte ions, and thus may generate excellent supercapacitor performance.

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(a)

(b)

(c) (d) Figure 2. PXRD patterns and N2 adsorption and desorption isotherms for MOFs-1-3.

In order to find the best concentration of electrolyte for these MOF SCs, a preliminary experiment was performed. In different KOH electrolyte, SCs performance were investigated carefully for MOF-1, MOF-2, and MOF-3 films. The best KOH concentration of 6 M was determined by the synergy effect between KOH electrolyte and all of the as-prepared MOF samples. Lower KOH concentration will decrease the conductivity of the electrode material, while higher KOH concentration (> 6 M) will reduce the active sites of MOF electrode materials. In order to identify the applicability of MOFs as electrode materials, their CV performance was firstly investigated. Figures 3a and S2-S4 depicts the CV curves of MOF-1, MOF-2, and MOF-3 films in 6 M KOH electrolyte at different potential scanning rates (5 mv S-1, 10 mv S-1, 20 mv S-1 and 50 mv S-1) between the potential windows of 0.0-0.45 V. A pair of well-

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defined redox peaks are observed in all CV curves at different scan rates, confirming that the typical pseudo-capacitance features originates from the Ni3+ and Ni2+ ions’ redox reaction in alkaline electrolyte for MOF electrode materials1,33. MOFs-1-3 all exhibit similar CV curves due to their iso-structures on the base of the same trinuclear building blocks. Since three open metal sites of [NiII2NiIII(µ3-OH)(COO)6] cluster all are occupied by pyridine groups and OHgroups of KOH electrolyte cannot attack Ni ions, thus the usually ascribed redox process for Ni2+ is not suitable herein34. On the other hand, µ3-OH group trapped into the center of trinuclear cluster bridges two NiII and one NiIII ions, which provide an effective inner-cluster electron transfer. In our opinion, the following two inner-cluster reversible processes maybe used to explain above redox reaction mechanism. This inner-cluster redox does not need the dissociation of Ni ions from MOF electrode materials, which will effectively keep the MOF structure during the electrochemical process. [NiII2NiIII(µ3-OH)(COO)6] + OH- ←→ [NiIINiIII2(µ3-O)(COO)6] + H2O + e-

(4)

[NiIINiIII2(µ3-O)(COO)6] ←→ [NiIII3(µ3-O)(COO)6] + e-

(5)

(a)

(b)

10


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(c)

(d)

(e)

(f)

Figure 3. (a) CV curves of MOF-3 film in 6 M KOH at different scanning rates; (b) CV curves of three MOF films in 6 M KOH at 10 mv S-1 scanning rate; (c) GCD curves of MOF-3 in 6 M KOH at different current density; (d) GCD curves of MOF-3 film in different KOH concentration at 1 A g-1 current density; (e) GCD curves of three MOFs in 6 M KOH at 1 A g-1 current density; (f) Capacitance with cycle number of three MOF films in 6 M KOH at 1 A g-1 current density.

As show in the Figures 3c-3e and S2-S4, the GCD curves of the MOF-1, MOF-2, and MOF-3 electrodes show the potential windows from 0 to 0.37 V at 1, 2, 3 A g-1 current densities in KOH electrolyte. The GCD curves shape of all as-prepared MOFs samples reveal a distorted triangle. For all MOF electrode, the first linear stage of the charging step corresponds to the oxidation process, whereas the seconds segment represents the charging process itself. Due to the internal resistance and a slightly decaying, the discharging curve step has a sharp sloped part of potential drop. Such phenomenon reflects a pseudocapacitive 11



behavior of MOF electrode material, which is well consistent with the CV results. The specific capacitances are 391, 381, 340 F g-1 for MOF-1, 440, 393, 374 F g-1 for MOF-2 and 465, 426, 394 F g-1 for MOF-3 at a current density of 1, 2, 3 A g-1, respectively. To the best of our knowledge, these specific capacitance values surpass most of the reported MOF SCs. Concretely, the specific capacitance values follow the order: MOF-1 < MOF-2 < MOF-3. Since three MOF electrodes have the same inner-cluster redox reactions, the different electrochemical performance should mainly depend on their pore structure nature. Although MOF-1, MOF-2, and MOF-3 are isostructural, their different porous characters can affect the electron transport and the intercalation or deintercalation of electrolyte ions, and thus lead to different electrochemical performance. Clearly, large pore size can help to promote the specific capacitance of supercapacitor, which is in accordance with the reported porous carbon and metal oxide SCs. The long-term cycling stability under extreme load of MOF-1, MOF-2, and MOF-3 as electrodes is important for the practical applications. An endurance test is further done by a constant current charge/discharge experiments at 5 A·g-1 with the voltage ranging from 0 to 0.38 V in 6 M KOH electrolyte. As shown in Figure 3f, the capacitances of three as-prepared electrodes are decreased by only 21%, 16%, and 17% after 1000 cycles, indicating the good stability of these MOF crystal electrodes. In the meantime, the as-prepared electrodes were measured by FT-IR spectra after the 1000 cycles in 6 M KOH electrolytes (Figure S5). The characteristic signals in FT-IR spectra after 1000 cycles reveals that as-prepared electrodes maintained the structural characteristics of MOFs-1-3. Excellent stability, high specific capacitance values and remarkable long-term cycling

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stability of these MOF SCs encourage us to further overcome the other drawback of MOF electrodes, poor conductivity, which also limits their utilization efficiency and capacitive performance. Graphene oxide (GO) with ultrahigh electrical conductivity was selected in this work to be doped in MOFs-1-3 to give MOF-n@GO-x% (n = 1, 2, 3; x = 1, 3, 5) hybrid solids to improve their conductivity. As shown in Figure 4, the PXRD patterns of MOFn@GO-x% are well matched with the diffraction peaks for isolated MOF and GO. As clearly appears in the figures, the diffraction peaks of the MOF and GO are superimposable, which indicates the good combination of MOF and GO. FT-IR (Figure S5) and SEM analysis (Figure 4) also verify that the MOF and GO are well combined. The main characteristic peaks of pure MOF and GO have clearer presentation and do not change in the FT-IR spectra of MOF@GO. It's very clear, as shown in Figure 4, the surface morphologies of the as-doped GO in MOF-n@GO-x% are porous characteristics compared with three pure MOFs. The interactions between GO with strong electron transfer capability and MOF with high surface area can influence the morphology of MOF@GO and improve their electrochemical property. Obviously, such fabrication may help to better explore MOF application as supercapacitor electrode materials.

(a)

(b)

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(c)

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(d)

(e) (f) Figure 4. PXRD patterns (a, c, e) and SEM images (b, d, f) for MOF-n@GO-x% hybrid solids.

Firstly, the effect of GO amount on the electrochemical properties for MOF@GO was explored. The GO amount loading in MOF@GO was mediated to be 1, 3 and 5%, and the corresponding products were termed as MOF@GO-1%, MOF@GO-3%, MOF@GO-5%, respectively. Our results show that the best GO content is 3%, which may due to the synergy effect between GO and MOF in electrode materials. Lower content of GO will decrease the electron transfer rate, but higher content of GO will decrease the amount of active materials. Figures 5 and S6-S11 shows the CV curves of MOF-n@GO-x% films in KOH electrolyte at different potential scanning rates (5 mv S-1, 10 mv S-1, 20 mv S-1 and 50 mv S-1) between the potential windows of 0-0.45 V. Just like MOF electrodes at different scan rates, a pair of well-defined redox peaks are observed in all CV curves, confirming that the typical pseudo-

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capacitance features originate from the inner-cluster NiII and NiIII ions’ redox reaction in alkaline electrolyte for MOF electrode materials. Also, similar distorted triangle GCD curves were observed for MOF-n@GO-x% electrodes in KOH from 0 to 0.37 V at different current densities. It is noted that the discharging time of MOF@GO was much longer than that of corresponding MOFs at the same current density. As a result, the specific capacitances of the MOF@GO were higher than that of corresponding MOFs, which are 590, 529, 484 F g-1 for MOF-1@GO-3%, 576, 462, 446 F g-1 for MOF-2@GO-3%, and 504, 466, 434 F g-1 for MOF-3@GO-3% at a current density of 1, 2, 3 A g-1, respectively. Clearly, the introduction of GO remarkably improve the supercapacitor performance of MOFs. Under the same conditions, 151%, 131% and 108% enhancement of specific capacitances for MOF-n@GO-3% compared with that for corresponding MOF-n materials.

(a)

(b)

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(c)

(d)

(e)

(f)

Figure 5. (a) CV curve of MOF-1@GO-3% film in 6 M KOH at different scanning rates; (b) CV curves of three MOF@GO-3% films in 6 M KOH at 10 mv S-1 scanning rates; (c) GCD curve of MOF-1@GO3% in 6 M KOH at different current densities; (d) GCD curves of MOF-1@GO-x% in 6 M KOH at 1 A g-1 current density; (e) GCD curves of MOF-n@GO-3% in 6 M KOH and 1 A g-1 current density; (f) Capacitance with cycle number of three MOF-n@GO-3% film in 6 M KOH at 1 A g-1 current density.

In our opinion, the regular porosity and surface area of MOFs can provide effective diffusion or transportation paths for the ions/electrons, which improve the participation of electroactive sites during the electrochemical process. But the movement and accessibility of ions within the MOF framework will be hindered by their poor conductivity and thus generate insufficient faradaic redox reaction of the electrode materials35. The GO with excellent conductive properties were employed to improve the conductivity of MOF and 16


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accelerate the rate of NiIII and NiII ions redox reaction. The long-term cycling stability of MOF-1@GO-3%, MOF-2@GO-3%, and MOF-3@GO-3% electrodes is also evaluated at 5 A·g-1 within a voltage range between 0.V and 0.38 V in 6 M KOH electrolyte. The capacitances are decreased by 20%, 19% and 23% after 1000 cycles (Figure 5f) indicating the good cycle stabilities of MOF@GO electrodes. Figure 6 summarizes the specific capacitance values of all MOFs and MOF@GO electrodes under all investigated conditions. Thanks to the robust 9-connected architectures and novel inner-cluster redox process, all these Ni-MOF materials exhibit a pseudo-apacitor behavior with high specific capacitance. The electrochemical performances of MOFs are mainly related to their porous characters. Compared to MOFs, the enhanced specific capacitance values of MOF@GO solids maybe ascribed to the advantages of GO. The MOF crystalline particles were directly doped by GO through the coordination centers in the frameworks. The synergetic effects between MOF and GO effectively improve the electron transmission and lead to faster redox and energy storage process. As summarize in Table 1, the electrochemical performance of all Ni-MOF-based materials surpass most of reported MOF-based electrodes. Under the same conditions, only [Ni2(TATB)2(bid)2(H2O)]·2H2O37, [Cu2Br(OH)(L)2]·(CH3OH)438 and [Ni2(TATB)2(btd)2(H2O)]37 MOFs have higher specific capacitance values. Overall, our work provides a new opportunity for the exploration of novel electrode material by taking advantages of robust MOFs on the base of trinuclear building blocks. This method is promising since [M3(µ3-OH/O)(COO)6] trinuclear building blocks is a common SBU during the development of MOFs and thus provides vast MOF candidates for electrochemical applications.

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Figure 6. The specific capacitance values of all MOFs and MOF@GO electrodes at 1 A g-1 current density investigated in this work.

To evaluate the kinetic and mechanistic properties, the EIS of MOFs-1-3 and MOFn@GO-3% electrodes was further studied. Figure S12 shows a typical Nyquist of all the asprepared MOF and MOF@GO samples. The EIS spectra were fitted with the corresponding equivalent circuit1. A straight line is observed in the lower frequency region, showing a Warburg impedance (Zw) related to the diffusion of the electrolyte within the pores of the electrode39. The semicircle was found in the high frequency region at an operating voltage, which can be attributed to the bigger effect of interfacial impedance40. The lower Rct and Rs values of MOFs indicate that the synergistic effect of the structure with large surface area and pores41. The Rct and Rs values of MOF@GO are greater than the values of corresponding MOFs, which may be caused by the decrease of specific surface areas with the doping of GO. The introduction of GO in MOFs will reduce the pores, but the GO can improve the

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transmission of electrons. Hence, the specific capacitance of MOF@GO has been increased compared with corresponding MOF. Moreover, at lower frequencies, the curves of all the asprepared MOF samples have an angle larger than 45° showing that the diffusion process does not control electrochemical behavior and thus may improve ionic migration in the solid electrode.

Table 1 Summary of MOF SCs with top-high specific capacitance values Current density

MOFs

-1

(A g )

Electrolyte

0.5

Ni-MOF

C

Ref.

(F g-1) 1127

36

1

KOH/6.0 M

705

37

0.5

KOH /6.0 M

688

38

[Ni2(TATB)2(btd)2(H2O)]

1

KOH /6.0 M

666

37

MOF-1@GO-3%

1

KOH/6.0 M

590

This work

MOF-2@GO-3%

1

KOH/6.0 M

576

This work

Ni-DMOF-ADC

1

KOH /2.0 M

552

42

MOF-3@GO-3%

1

KOH/6.0 M

503

This work

[Co(HTATB)(m-bib)]·2H2O

2

KOH /6.0 M

502

43

Zn-MOF/PANI

1

H2SO4/1.0 M

477

44

MOF-3

1

KOH/6.0 M

465

This work

MOF-2

1

KOH/6.0 M

444

This work

Ni-DMOF-TM

1

KOH /2.0 M

440

42

Ni-DMOF-NDC

1

KOH /2.0 M

410

42

MOF-1

1

KOH/6.0 M

390

This work

Cu-MOF/GO

1

Na2SO4/0.5 M

385

45

Zr-MOF

1

KOH/6.0 M

207

46

Co-MOF

0.6

LiOH/1.0 M

207

1

1

KOH /6.0 M

166

47

Ni3(HITP)2

0.5

KOH /6.0 M

111

48

Ni3(HITP)2

0.5

TEABF4/CAN/1M

107

49

Cu-MOF

1.6

Na2SO4/1 M

85

49

Al-MOF

1

KOH /6.0 M

84

50

NaNO3/0.05 M

67

51

Li2SO4/1.0 M

22

52

[Ni2(TATB)2(bid)2(H2O)]·2H2O [Cu2Br(OH)(L)2]·(CH3OH)4

[Ni(HOC6H4COO)1.48(OH)0.5]·1.1H2O

[CoL(1,4-bdc)]·2DMF [Cd2(TDC)2(L)2]·4H2O

0.2 2.5 mAg

−1

19



Page 20 of 31

The energy deliverable efficiency (η%) is obtained according to the formula η = Td/Tc × 100 (Td and Tc are the discharging and charging time) from GCD curves at 1 A g-1 current densities. The η is determined to be 88.4%, 91.3%, 92.1%, 97.3%, 93.4%, and 91.6% for MOF-1, MOF-2, MOF-3, MOF-1@GO-3%, MOF-2@GO-3%, and MOF-3-@GO-3%, suggesting the excellent energy deliverable ability. Furthermore, the maximum storage energy (E) per unit mass were also calculated, and the values as 8.69, 9.78, 10.33, 13.11, 12.80 and 12.01 Wh kg−1 at a discharge current density of 1.0 A g-1. The maximum power density (Pmax) values are 3.03, 3.03, 2.47, 4.57, 3.78, and 2.68 kW kg-1. These values are superior to the capacitance and stability of Co8-MOF-517, SNNU-8022, Co-MOF18, and some metal oxide electrode materials53-55.

(a)

(b)

(c)

(d)

20


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(e)

(f)

Figure 7. Voltammetric current as a function of square root of scan rate of MOF-n and MOF-n@GO-3% electrodes.

The reaction mechanism of all MOF electrode materials is further speculated by the dependence between scanning rate (v) and voltammetric current (i). As shown in Figure 7, the straight lines of all MOF and MOF@GO electrode material corresponding to i∝v1/2 are observed, respectively. Such good linear relationship suggests that the redox reaction for all MOF and MOF@GO electrodes should be controlled by diffusion. Moreover, the areas of CV curves for all as-prepared samples become larger and the redox peaks shifted away with the increasing scanning rate. At the same scan rate, a better capacitive performance is usually indicated by the higher peak current. Herein, a fast electron transfer rate during diffusion process is indicated by the fact that the current enhances with the increasing scan rates. On the other hand, a circuitous diffusion of ions into the pores of MOFs together with their high resistance lead to the reduction of capacitance with the increasing scan rate.

■ CONCLUSIONS In conclusion, we demonstrate in this work how to overcome the drawbacks of MOF electrodes and develop a series of excellent MOF supercapacitors. The effects of specific surface area, pore diameter and doped-GO on electrochemical properties of MOF electrode 21



materials were discussed in detail for the first time. Thanks to our firstly supposed innercluster NiII and NiIII redox process, all MOF and MOF@GO solids behave as good pseudocapacitors. MOF-3 possesses the highest porosity as well as the largest specific capacitance of ca. 465 F g-1 (current density = 1 A g-1). Also, after 1000 cycles, it can retain 79% capacitance at a current density of 5 A g-1. Furthermore, the doping of GO greatly improve the specific capacitance of MOF-3 to ca. 590 F g-1 and its original capacitance can keep ca. 80 % after 1000 cycles. Such excellent electrochemical properties are mainly originated from its porous structures and fascinating synergetic effect between the unique architecture of MOF and the GO. These features also ensure that enough Faradaic reactions can come up for energy storage. What we presented in this work provide new ideas and channel for the preparation and utilizations of porous MOF materials in energy storage.

■ ASSOCIATED CONTENT Supporting Information TGA curves, FT-IR spectra, and additional electrochemical data. The Supporting Information is available free of charge on the ACS Publications website.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Q.-G. Zhai); [email protected] (X. Wang) Notes The authors declare no competing financial interest.

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■ ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21671126, 21663031 and 21503183), the Natural Science Foundation of Shaanxi Province (2018JC-019), and the Fundamental Research Funds for the Central Universities (GK201701003).

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For Table of Contents Use Only Excellent Supercapacitor Performance of Robust Nickel-Organic Framework Materials Achieved by Tunable Porosity, Inner-Cluster Redox and in-situ Fabrication with Graphene Oxide

Xiang-Yang Hou, Xiao-Li Yan, Xiao Wang, Shu’ni Li, Yucheng Jiang, Mancheng Hu, Quan-Guo Zhai

Taking advantages of high-connected Ni-organic frameworks on the base of [NiII2NiIII(µ3OH)(COO)6] trinuclear building blocks, we demonstrate herein how to overcome the drawbacks of MOF electrode, and develop a series of MOF supercapacitors with top-high specific capacitances achieved by tunable porosity, inner-cluster redox and in-situ fabrication with graphene oxide.

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Excellent Supercapacitor Performance of Robust Nickel-Organic

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