Facile Surface Properties Engineering of High-Quality Graphene

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Facile Surface Properties Engineering of High-Quality Graphene: Towards Advanced Ni-MOF Heterostructures for High-Performance Supercapacitor Electrode Yao Xiao, Wei Wei, Meijiao Zhang, Sa Jiao, Yuchuan Shi, and Shujiang Ding ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02201 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Energy Materials

Facile Surface Properties Engineering of High-Quality Graphene: Towards Advanced Ni-MOF Heterostructures for High-Performance Supercapacitor Electrode Yao Xiao, Wei Wei,* Meijiao Zhang, Sa Jiao, Yuchuan Shi, Shujiang Ding School of Science; MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Materials; Xi’an Key Laboratory of Sustainable Energy Material Chemistry, Xi’an Jiaotong University, Xi’an 710049, P. R. China.

*Email: [email protected] Abstract: Metal-organic frameworks (MOFs) have emerged as one class of the most promising electrode materials for supercapacitors because of their high surface area and tunable pore structures. The development of MOF heterostructures based on high-quality graphene (HQG) which aims at superior capacitive properties while overcoming poor solution processability and reactivity of HQG is of great concern. In this work, a hetero-layered MOF composite was prepared by a facile glucose modification of HQG and controlled nucleation and growth of lamellar Ni-MOF on the HQG surface. The introduction of oxygen-containing functionalities in our approach not only improves the interfacial interactions to form 2D heterostructures but also enhanced the synergistic effects between Ni-MOF and HQG substrates. The constructed GM-LEG@Ni-MOF exhibits excellent capacitance of 987.6 F g-1 at the current density of 0.5 A g-1 and a high capacity retention (85.6 %) after 3000 cycles, owing to its favorable electrochemical kinetics and highly structural stability. Keywords: graphene, surface engineering, Ni-MOF, supercapacitor, glucose

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1. INTRODUCTION With the rapid development of the electronic technologies and the growth of world population, the increasing requirement of energy supply and the urgent environmental problems have to be addressed by developing more efficient, cleaning and sustainable energy storage and conversion devices.1-5 Supercapacitors (SCs) as one of the energy storage devices have attracted wide attention,6-10 not only because of its outstanding capability to store charges compared with conventional capacitors, but also its higher power density (10 times more) compared to the secondary batteries. Consequently, SCs have become one of the efficient electrochemical energy storage devices for use in hybrid vehicles and mobile communications field, owing to the advantages of fast charge and discharge rate, long and stable cycle life and eco-friendly features.11-16 Metal-organic frameworks (MOFs) are a class of porous organic materials that were first reported by the Yaghi and co-works in 199517 and have received immerse attention for its potential applications in gas storage and separation,18,19 sensors,20 catalysts,21 and the field of electrochemical energy storage.22,23 MOFs possess three-dimensional network-like crystal structures formed by inorganic metal center and organic ligand (mainly aromatic acid, base nitrogen or oxygen polydentate organic ligand),24 giving rise to controllable microporous structure, large specific surface area and pore volume, thereby exhibiting great potential as electrode for SCs applications. To date, utilization of MOFs as SCs electrode has been mainly achieved by the following three routes. First of all, thanks to the intrinsic microporous


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structure, MOFs can be used as the electrode materials for SCs directly. Typically, Ni-,25,26 Zr-,27 ,Co-,28,29, Fe-,30 and Zn-derived MOF31 materials have been widely investigated. Furthermore, MOFs can be used as versatile precursors for preparing various hybrids comprised of porous carbon and the metallic compound by a simple calcination method.32-35 The organic component of MOF converted into carbon matrix, while the metal ions transformed to metallic active materials with pseudo-capacitive properties. This method provides a simple and efficient approach for preparing SCs electrodes in high capacity. However, there are two fundamental limitations on utilizing MOFs as SCs electrode: the poor conductivity and low structural stability. Recently, combining MOF materials with various carbon matrixes (e.g. graphene,36,37 CNTs38,39etc.) to form a hybrid structure provides an effective way to overcome these disadvantages for developing high-performance SCs electrodes. Among these, graphene is the most popular building block for the construction of MOF hybrids electrodes, owing to its fascinating properties including high electrical conductivity, exceptional mechanical strength, and chemical stability. The highly conductive and unique layered structure of graphene enables fast transport and diffusion for both electrons and ions, thus ensures favorable electrochemical kinetics towards charge storage and delivery.40,41 Importantly, construction of hetero structured architectures and synergistic effects between graphene and MOFs effectively improve the interfacial interactions and overall electrochemical performance.42 Integration of MOF with graphene into two-dimensional (2D) heterostructures



can increase the contact area with electrolyte and improve kinetics of ion transfer.43 Taking advantage of abundant surface oxygen-containing functional groups, most of the reported 2D MOF hybrids used graphene oxide (GO) as the building block, achieving facile processing and efficient growth of various MOF components.44-46 However, the intrinsic crystal structure of graphene has been inevitably destroyed during the aggressive oxidation preparation of GO. The resulted large amount of structural defects of GO and poor interfacial adhesion between GO and MOFs consequently restrict an optimal capacitive storage performance.47 In contrast, liquid-phase exfoliation of graphite is one of the most desirable approaches to prepare high-quality graphene. Very recently, our group reported a facile and efficient liquid-exfoliation method towards industrial-scale production of graphene (denoted as LEG) for numerous applications.48,49 Notably, LEG with low defects density and excellent electrical conductivity can significantly improve the interface bonding properties and electrochemical performance of the coated MOFs. Nevertheless, owing to the hydrophobic nature and inert surface properties, LEG also exhibits associated shortcomings such as poor dispersibility and low reactivity, which prevent it from material synthesis and practical applications. Therefore, searching for an efficient processing protocol to engineer LEG surface and prepare the LEG-derived MOFs electrode material remains a great challenge. In this work, we present a facile surface functionalization and construction of 2D heterostructured MOF composites based on high-quality LEG. The glucose derivatives were readily grafted onto LEG surface by a simple solvothermal process,


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which afforded numerous surface oxygen-containing functionalities. Owing to the enhanced interfacial interactions and geometrical compatibility, a lamellar nickel-based Ni-MOF homogeneously nucleated around and closely adhered to the LEG surface, giving rise to LEG-derived Ni-MOF composites with 2D hierarchical structures. The resulted GM-LEG@Ni-MOF ensures favorable transport kinetics for both electrons and ions, which demonstrated profound capacitive performance compared to that of pure Ni-MOF and LEG-based composites without surface modification.

2. EXPERIMENTAL SECTION 2.1 Materials: Natural graphite powder was purchased from Aladdin Reagent Company (Shanghai). All reagents were used without further purification. 2.2 Preparation of LEG LEG was prepared by a direct liquid-phase exfoliation of graphite in solvent. 3 g of graphite powder was first dispersed into 500 mL of N-methyl-2-pyrrolidinone (NMP) to form a homogeneous solution. The dispersion was sonicated for 3 h in an ice water bath (prevent graphene from aggregation due to overheating) then allowed to stand for 72 h. Finally, the LEG was obtained by filtration and dried under vacuum at 80℃ over 12 h. 2.3 Preparation of GM-LEG Firstly, 10 mg of LEG was dispersed into mixed solutions of 15 mL of NMP and



5 mL of deionized water. 50 mg of glucose was then added into the above solutions and sonicated for 30 minutes to form a homogeneous solution. The as-prepared solution was transferred into a Teflon-lined stainless steel autoclave (50 mL volume) and treated at 180 ° C for 6 h. After the autoclave naturally cooled to room temperature, the precipitate was washed with water/ethanol several times and dried under vacuum at 80℃ over 12 h to obtain the GM-LEG. 2.4 Synthesis of Ni-MOF 0.096 g of NiCl2▪6H2O and 0.166 g of terephthalic acid (PTA) were dissolved in 20 ml of N,N-dimethylformamide (DMF). Then, 2 ml of NaOH (0.4 M) was added dropwise under stirring for 30 min. The above mixtures were then transferred into Teflon-lined stainless steel autoclave and treated at 100 ℃ for 8h. After the autoclave naturally cooled to room temperature, the Ni-MOF was obtained by washing with ethanol and DI water three times and dried under vacuum at 80℃ over 12 h. 2.5 Fabrication of 2D GM-LEG@Ni-MOF heterostructures 10 mg of LEG and 0.096 g of NiCl2▪6H2O were dispersed in 20 ml DMF under stirring for 30 min. Then, 2 ml of NaOH (0.4 M) was added dropwise and stirred vigorously at room temperature for 3 h. After that, 0.166 g of PTA was added and stirred for 30 min. The as-obtained mixtures were transferred into Teflon-lined stainless steel autoclave and treated at 100 ℃ for 8 h. After the autoclave naturally cooled to room temperature, the GM-LEG@Ni-MOF composites was obtained by washing with DMF, ethanol several times and dried under vacuum at 80℃ over 12 h.


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2.6 Fabrication of LEG@Ni-MOF composites The processes were same to that of GM-LEG@Ni-MOF except the GM-LEG substrates were replaced by pristine LEG. 2.7 Characterizations The morphology features were characterized by transmission electron microscopy (TEM, JEOL Jem-2100, JEOL) equipped with an energy-dispersive spectrometer (EDS). X-ray photoelectron diffraction (XRD, D8 ADVANCE, Bruker) was employed to investigate the crystal structure and phase purity. Raman spectroscopy was examined on a Raman spectrometer with a 532 nm laser (HR800, Horiba JOBIN YVON, Japan). X-ray photoelectron spectrum (XPS) measurements were carried out on XPS system (ESCALAB Xi+, Thermo Scientific, America). Fourier transformed infrared (FTIR) spectrum were obtained on FTIR spectrograph (VERTEX70, China). Specific surface area was determined according to the Brunauer-Emmett-Teller (BET) analysis. 2.8 Electrochemical measurements For investigating the electrochemical performance, the as-synthesized samples were tested in a three-electrode configuration in 3M KOH aqueous solution using a CHI660E electrochemical workstation. Different active materials, Super P carbon black and polymer binder of polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1 were mixed in NMP and stirred to obtain black homogeneous slurry. Then the slurry was coated on the Ni foam and transferred into a vacuum oven for drying at 80℃ over 12 h. The dried and loaded Ni foams were pressed into thin foils at a pressure of



5.0 MPa to obtain the working electrodes. Platinum electrode and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Cyclic voltammetry (CV) and galvanostatic discharge-charge were tested with the potential range of 0-0.45V and 0-0.4V, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on an AC voltage of 5 mV amplitude over the frequency range 10-1-105 Hz. The specific capacitance measured by chronopotentiometry was calculated from the current density per mass of active material according to the equation (1) as follows: 𝐶𝑠𝑝 =

𝐼 × ∆𝑡 ∆𝐸 × 𝑚

(1)

where Csp (F g-1) is the specific capacitance of the electrode, I (A) is the constant discharging current, ∆t (s) is the discharging time, ∆E (V) is the potential range during the discharge process and m is the mass of active materials deposited on the electrode. The current density was calculated according to the mass of active materials iron oxide. The asymmetric supercapacitor was consisting of GM-LEG@Ni-MOF as a positive electrode, commercial active carbon (AC) as a negative electrode and one piece of cellulose paper as a separator, respectively. The optimal mass ratio between the positive electrode and negative electrode can be obtained according to equation as follows: 𝑚+ 𝑚―

=

𝐶 ― × ∆𝐸 ― 𝐶 + × ∆𝐸 +

(2)

Where 𝑚 + and 𝑚 ― are the masses of positive electrode and negative electrode,


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respectively; 𝐶 + and 𝐶 ― are the specific capacity of positive electrode and negative electrode, respectively. The power density (P) and energy density (E) of the asymmertric supercapacitor is calculated by equation (3) and (4): 𝐶 × ∆𝑉2 𝐸= 2 × 3.6

(3)

3.6 × 𝐸 ∆𝑡

(4)

𝑃=

Where C (F g-1) is the specific capacity, ∆V (V) is voltage range, ∆𝑡 is the discharge time. 3. RESULTS AND DISCUSSION

Scheme 1. Schematic synthesis procedure of the GM-LEG@Ni-MOF composites

Scheme 1 shows the schematic synthesis procedure of the 2D LEG-derived MOF composites. First, a high concentration of LEG dispersion was prepared by liquid-phase exfoliation of graphite in mixed NMP/H2O solvents. The as-exfoliated graphene was then modified with glucose (GM-LEG) via a simple solvothemal



process to afford numerous surface oxygen-containing functionalities. Consequently, The Ni2+ precursor was mixed with GM-LEG and anchored on the LEG surface by strong electrostatic interactions. By adding the PTA ligands, the suspensions were placed in an autoclave for solvothemal treatment to give rise to LEG-based Ni-MOF (GM-LEG@Ni-MOF).

Figure 1. TEM images of LEG (a), GM-LEG (b) and GM-LEG/Ni-MOF (c, d) and the SAED pattern (inset) of the GM-LEG/Ni-MOF; EDS (e) and elemental mapping images (f-h) of the GM-LEG@Ni-MOF composites.

The morphology and microstructure of the as-prepared LEG, GM-LEG and


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GM-LEG/Ni-MOF samples were first investigated by transmission electron microscopy (TEM). It can be clearly seen that the exfoliated LEG has nanosheet-like structures with lateral sizes up to micrometer range (Figure 1a). The presence of crinkled and rough textures along LEG edges is attributed to the thin and flexible nature of graphene.50 After modification with glucose, the GM-LEG exhibits similar morphology to the pristine LEG sheets, with no obvious glucose-derived residuals detected on the graphene surface (Figure 1b). Figure1c shows that the GM-LEG/Ni-MOF is composed of lamellar Ni-MOF nanosheets closely adhered to the GM-LEG surface. The high-resolution TEM (Figure 1d) and selected area electron diffraction (inset) pattern of GM-LEG/Ni-MOF reveals the amorphous form or weakly crystalline structure of the coated Ni-MOF. Under a highly powerful electron beam, the crystalline structure of Ni-MOF is easily decomposed, therefore it is difficult to detect the diffraction spots in the SAED pattern.51 For the purpose of comparison, TEM observation of pure Ni-MOF and LEG@Ni-MOF are also presented in Figure S1 and S2 (see Experimental section on preparation). The pure Ni-MOF is composed of a large number of primary MOF nanosheets that stack layer-by-layer to form an aggregated structure. For LEG@Ni-MOF, it is found that most of the Ni-MOF aggregation particles are randomly distributed around the LEG instead of uniform coating (Figure S2). These comparative results validate that the surface engineered oxygen functionalities from glucose serve as crucial binding sites for homogeneous growth of Ni-MOF. Moreover, the mapping analysis of energy-dispersive X-ray spectrometry (EDS) for C, O, and Ni elements clearly



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demonstrates that nickel and oxygen element homogeneously distributed on the sheets,

while

C

exists

throughout

the

whole

structure

(Figure

1e-h).

Thermogravimetric analysis of GM-LEG@Ni-MOF and Ni-MOF (Figure S3) reveals that the weight ratio of Ni-MOF in the composite was 72.7 %.

Figure 2. (a) FTIR spectrum of as-prepared LEG, GM-LEG and GM-LEG@Ni-MOF. (b) XRD pattterns of the Ni-MOF and GM-LEG@Ni-MOF. (c) Raman spectra of the Ni-MOF and GM-LEG@Ni-MOF. (d) XPS high-resolution C1s spectra of GM-LEG. (e) XPS spectrum and (f)


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high-resolution Ni 2p of GM-LEG@Ni-MOF.

The FTIR spectrum of the as-prepared samples is shown in Figure 2a. For GM-LEG/Ni-MOF, the band at 3431 cm-1 is attributed to H2O molecules, confirming that coordinated H2O exists within the Ni-MOF structures. The strong bands at 1570 and 1385 cm-1 are ascribed to the asymmetric and symmetric stretching modes of the coordinated －COO- groups, respectively, which indicates that －COO- of H2BDC is coordinated to Ni2+ via a bidentate ligand mode.52 For the LEG and GM-LEG samples, the band at 1576 cm-1 of LEG is attributed to the para-aromatic CH groups of graphene sheets. The bands at 3447 and 1637cm-1 of GM-LEG is associated to the stretching vibrations of － OH and C=O groups,53 suggesting that the oxygen functional groups were successfully grafted on the LEG surface. The influence of different reaction time on surface functionalization of LEG was also investigated. FTIR spectra indicate that the surface of all GM-LEG samples is obviously functionalized with same oxygen-containing groups (Figure S4). XRD was employed to investigate the structure and phase purity of the as-prepared samples. As shown in Figure 2b, the diffraction peaks of both Ni-MOF and GM-LEG@Ni-MOF are in good agreement with [Ni3(OH)2(C8H4O4)2·H2O)4]·2H2O (CCDC no.638866), in which the diffraction peaks at 9.2°, 11.8°, 15.6 ° and 23.7 ° are corresponded to the (100), (010), (101) and (020) planes of Ni-MOF, respectively. For GM-LEG@Ni-MOF, the peak appeared at 26.5 ° is indexed to (002) plane of graphite, implying the multilayered structure of the LEG sheets (Figure S5). Raman spectrum further proves the combination of GM-LEG substrate and Ni-MOF. The weak D band (at 1341 cm-1)



and low ID/IG ratio (0.15) is comparable to pristine graphite, indicating the high quality of exfoliated LEG sheets (Figure S6). It is noteworthy that the Ni-MOF exhibits the D band and G band at 1439 and 1610 cm-1 respectively, which partially overlap with that of in-between LEG sheets of the composites (Figure 2c).54 To examine the chemical structure of the GM-LEG@Ni-MOF, X-ray photoelectron spectrum was performed and the results are presented in Figure 2d-e. The XPS survey (Figure S7) and high-resolution C1s spectra of GM-LEG (Figure 2d) disclose the presence of substantial oxygen-containing functional groups (C–OH, C=O and C–O– C) after the glucose modification. Particularly, the peak located at a binding energy (BE) of 284.6 eV is attributed to the C=C/C-C species, while the peaks at 285.6, 286.5 and 287.3 eV are assigned to the C-OH, C (epoxy/alkoxy) and C=O, respectively.55 The XPS survey of GM-LEG@Ni-MOF reveals the presence of carbon, oxygen, and nickel elements derived from Ni-MOF and LEG (Figure 2e). The presence of N1 signal might result from the coordination of solvent DMF and Ni2+.56,57 The high-resolution Ni 2p spectrum (Figure 2f) at 856.3 eV and 873.8 eV correspond to Ni 2p3/2 and Ni 2p1/2, respectively, implying a divalent state of the Ni element.

58,59

The N2 adsorption/desorption measurement discloses that the

as-prepared GM-LEG@Ni-MOF possesses a Brunauer-Emmett-Teller (BET) surface area of 24.1 m2 g-1 and a bimodal pore distribution at 2.3 nm and 9 nm calculated using the Barrett-Joyner-Halenda (BJH) method (Figure S8). The formation of 2D graphene-based heterojunctions and bimodal porous structure of GM-LEG@Ni-MOF are beneficial to enhance the diffusion of the ions and electrolyte, thereby improving


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the electrochemical performance.

Figure 3. (a) Electrochemical performances of different Ni-MOF samples. (a) CV curves at 10 mV s-1 scan rates and (b) galvanostatic charge-discharge curves at 1 A g-1 of Ni-MOF, GM-LEG and GM-LEG@Ni-MOF. (c) Specific capacities obtained from different charge-discharge current densities. (d) CV curves of GM-LEG@Ni-MOF at different scan rates from 5 mV s-1 to 50 mV s-1. (e) Galvanostatic charge-discharge curves of GM-LEG@Ni-MOF at different current densities from 0.5 A g-1 to 10 A g-1. (f) Nyquist plots (inset showing the date of the high frequency range)



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of Ni-MOF, GM-LEG and GM-LEG@Ni-MOF.

To investigate the electrochemical performance for capacitive energy storage, the as-prepared Ni-MOF samples were tested in a three-electrode configuration in 3M KOH aqueous electrolyte. The cyclic voltammetry curves of the Ni-MOF, LEG@Ni-MOF and GM-LEG@Ni-MOF composites at a constant scan rate of 10 mV s-1 are exhibited in Figure 3a. All the electrodes exhibit non-standard rectangular shape

with

obvious

redox

peaks,

suggesting

typical

pseudo-capacitance

performance.60 The strong redox peaks in the curves correspond to the conversion between

Ni2+

and

Ni3+

[Ni3(OH)2(C8H4O4)2·H2O)4]·2H2O+OH﹣

with

the －

presence e﹣↔

of

OH﹣:

[Ni3O(OH)

(C8H4O4)2·(H2O)4]·2H2O+H2O.61 The valence change of Ni during the charging and discharging process is verified by the XPS analysis (Figure S9). Notably, the GM-LEG@Ni-MOF composites exhibit stronger redox peaks than LEG@Ni-MOF and pure Ni-MOF, which indicates much improved electrochemical activity of Ni-MOF based on the GM-LEG substrates. It can be clearly seen that the GM-LEG@Ni-MOF exhibits an excellent specific capacitance up to 913.9 F g-1 at 1 A g-1, which is 1.5 times higher than that of the pure Ni-MOF (606.8 F g-1, Figure 3b). The specific capacitance of all electrodes at different charge-discharge current densities is summarized in Figure 3c. Obviously, the GM-LEG@Ni-MOF yields substantially higher specific capacitance and rate capability than those of the counterpart samples. These comparative results indicate that the introduction and surface modification of LEG greatly improved the capacitive performance of the


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anchored Ni-MOF, which could be explained by following characters: compared to pristine LEG, GM-LEG possess abundant oxygen functionalities that directed the growth of Ni-MOF around LEG into sandwich-like structures, thereby providing more active sites for the adsorption and transport of ions. In addition, the in-between LEG effectively improved the electrical conductivity of the composites and accelerates the transmission rate of electrons. The formation of hetero-layered architectures enhanced synergistic effect between the two components, leading to superior electrochemical performance. Further, Figure 3d shows cyclic voltammetry curves of the GM-LEG@Ni-MOF composites at different scan rates. Upon increasing the scan rates from 5 to 50 mV s-1, the corresponding currents show a rising trend accordingly and the curve shape remains intact, indicating the good rate capability of the electrode material. Figure 3e shows the galvanostatic charge-discharge curves of GM-LEG@Ni-MOF composites at current densities of 0.5 A g-1, 1 A g-1, 2 A g-1, 5 A g-1 and 10 A g-1. The specific capacitances are 987.6 F g-1, 913.9 F g-1, 847.5 F g-1, 764.4 F g-1 and 685.7 F g-1 respectively, much higher than those of Ni-MOF and LEG@Ni-MOF electrodes under the same conditions (Figure S10). The specific capacitances of Ni-MOF and LEG@Ni-MOF at different current density are compared and listed in Table. S1. The influence of MOF weight ratio on the electrochemical property of GM-LEG@Ni-MOF was also investigated. As shown in Figure S11, increasing mass ratio of MOF precursors/GM-LEG from 0.5 to 1.0 lead to enhanced storage capacity. Further increase the loading amount (MOF precursors /GM-LEG up to 1.5) does not contribute to the capacitive performance. To verify the



improved electrochemical performance of the GM-LEG@Ni-MOF electrode, EIS of the three electrodes were tested under the open circuit voltage as shown in Figure 3f. It is noteworthy that GM-LEG@Ni-MOF reveals a lower internal resistance (0.77 Ω) than that of LEG@Ni-MOF (0.86 Ω) and Ni-MOF (0.92 Ω) obtained from the intercept of the Nyquist plots with the real axis, manifesting an optimized electrode structures and interfacial connections within GM-LEG@Ni-MOF electrode.62 The GM-LEG@Ni-MOF also has good cycle stability with high capacity retention of 85.6 % after 3000 cycles (Figure S12), which is mainly attributed to the LEG supported stable microstructures towards efficient ion adsorption/desorption and electron transport. To evaluate the practical application of GM-LEG@Ni-MOF, we fabricated a GM-LEG@Ni-MOF//activated carbon (AC) asymmetric supercapacitor (ASC) and measured its capacitance properties. The electrochemical properties of AC electrode were also tested in 3 M KOH solution using a three electrode system (Figure S13). The specific capacity of AC electrode is calculated to be 188.3 F g-1 at the current density of 1 A g-1, leading to an optimal mass ratio of 1:2 between GM-LEG@Ni-MOF and AC. Figure 4a shows corresponding potential windows of the GM-LEG@Ni-MOF and AC electrodes at the scan rate of 10 mV s-1, respectively. Therefore, it is supposed that the maximum operation potential of the proposed ASC could reach 1.6 V. Noticeably, the CV curves (Figure 4b) exhibit no obvious deformation even at a scan rate as high as 100 mV s-1, which indicates a good electrochemical reversibility of the GM-LEG@Ni-MOF//AC ASC. The galvanostatic


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charge-discharge tests of the ASC were carried out at different current densities (Figure 4c). The constructed ASC exhibits specific capacity of 104.5 F g-1 at the current density of 1 A g-1 and retains 77.8 % of the initial capacity at the current density of 10 A g-1, manifesting a high rate performance. Further, Ragone plot was used for comparative performance evaluation of the GM-LEG@Ni-MOF//AC ASC (Figure 4d). The ASC delivers an energy density of 32.7 W h kg-1 at a power density of 750 W kg-1, even remaining 25.4 W h kg-1 at a power density of 7500 W kg-1, which benefits from the high specific capacity and wide electrochemical voltage window. The detailed energy density and power density performance at different current densities are summarized in Tables S2, and a comparison of the energy density and power density obtained from GM-LEG@Ni-MOF//AC ASC with those in

previous reports is shown in Table S3.



Figure 4. (a) CV curves of GM-LEG@Ni-MOF and AC electrodes performed in three-electrode cell at the scan rate of 10 mV s-1. (b) CV curves of the GM-LEG@Ni-MOF//AC ASC at different scan rates ranging from 10 mV s-1 to 100 mV s-1. (c) Galvanostatic charge-discharge curves of the GM-LEG@Ni-MOF//AC ASC at different current densities. (d) Ragone plots relating power density versus energy density of the ASC device.

4. CONCLUSION In summary, 2D LEG-based Ni-MOF composite has been successfully prepared by a facile solvothermal process and exploited as electrode material for SCs applications. The as-prepared GM-LEG@Ni-MOF hybrid electrode displayed high specific capacitance, good rate capability and cycling stability. A capacitance of 987.6 F g-1 was achieved at the current density of 0.5 A g-1 and 85.6 % capacity was retained after 3000 cycles, better than pure Ni-MOF and the LEG@Ni-MOF counterparts. The unique glucose modification protocol provides abundant oxygen functionalities of LEG, which significantly improve the solution processibility of LEG and facilitated the controlled growth of Ni-MOF nanosheets around LEG. Such a lamellar heterostructure provides more and stable active sites for adsorption and transmission of electrons and ions for SCs applications. This work opens up a new pathway to design heterostructured MOF hybrids based on high-quality graphene for applications in supercapacitors, batteries, electrocatalysis and other high-performance energy and conversion devices.

▪ ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on


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the ACS Publications website. ▪ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Wei Wei: 0000-0002-8357-8427 Notes: The authors declare no competing financial interest.

▪ ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51602245), the Fundamental Research Funds for the Central Universities (2016qngz07), and the “Young Talent Support Plan of Xi’an Jiaotong University (HX1J002). ▪ REFERENCES (1) Wang, H. L.; Casalongue, H. S.; Liang, Y. Y.; Dai, H. J. Ni(OH)2 Nanoplates Grown On Graphene as Advanced Electrochemical Pseudocapacitor Materials. J. Am. Chem. Soc. 2010, 132, 7472-7477. (2) Wang, G. P.; Zhang, L.; Zhang, J. J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (3) Long, J. Y.; Yan, Z. S.; Gong, Y.; Lin, J. H. MOF-derived Cl/O-doped C/CoO and C Nanoparticles for High Performance Supercapacitor. Appl. Surf. Sci. 2018, 448, 50-63. (4) Wu, Q. N.; Wen, J. H.; Wen, M.; Wu, Q. S.; Fu, Y. Q. Bioinspired Sea-Sponge Nanostructure Design of Ni/Ni(HCO3)2-on-C for a Supercapacitor with a Superior



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