A novel Ni Coordination Supramolecular Network hybrid monolith of ...

Materials Today Energy 6 (2017) 164e172

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Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/

A novel Ni Coordination Supramolecular Network hybrid monolith of 3D graphene as electrode materials for supercapacitors Hua Yao, Gaowei Zhang, Feng Zhang, Wei Li, Yangyi Yang*, Liuping Chen School of Materials Science and Engineering & School of Chemistry, Sun Yat-sen University, Guangzhou, 510275, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 June 2017 Received in revised form 23 August 2017 Accepted 17 September 2017

A novel Ni-based Coordination Supramolecular Network (CSN) combining with three-dimensional reduced graphene oxide hybrid monolith (Ni-pda@3DrGO, pda ¼ pyridine-2, 3-dicarboxylate) is fabricated by a facile one-pot surfactant-free method for the first time, and the electrochemical performances of the composite are investigated. The Ni-pda@3DrGO exhibits excellent electrochemical performance (952.85 F g1 at 1 A g1) which is much higher than that of the pure Ni-pda and the composite prepared by physically mixing. Furthermore, an asymmetric supercapacitor (ASC) is fabricated based on the Nipda@3DrGO and activated carbon (AC) with an energy density of 17.70 Wh kg1 at a high power density of 1500 W kg1. The results demonstrate the great potentials of Ni-pda@3DrGO as electrode materials for supercapacitors. As a proof of concept, two other hybrid monoliths (Ni(quin-2c)2(H2O)2@3DrGO, Co2(m2-H2O)(na)4$DMF@3DrGO) have also been fabricated via the same route, implying the proposed method can be further extended to fabricate other Coordination Supramolecular Networks and 3D graphene hybrid monoliths. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Coordination Supramolecular Networks (CSNs) Hybrid monolith Three-dimensional graphene Supercapacitor

1. Introduction The ever-growing renewable energy needs stimulate the exploration of low-cost and efficient energy storage systems (ESSs). Among numerous energy storage devices, electrochemical capacitors, also known as supercapacitors, as one of the most potential candidates, have attracted considerable interest because they can not only provide higher power density than batteries but also deliver higher energy density than conventional capacitors. Simultaneously, they also exhibit fast charge and discharge rate, long cycle lifetime and high reliability. Classified by the charge storage mechanism, there exist two kinds of supercapacitors, namely electrical double layer capacitors (EDLCs), which store charge by separating ions on the electrodeeelectrolyte interface and pseudocapacitors which utilize near-surface redox reactions of the electrode material [1e5]. In general, carbon-based materials, such as activated carbon (AC), carbon nanotubes (CNTs) and graphene are typically regarded as ideal electrode materials for EDLCs attributing to their good conductivity as well as excellent chemical stability [6e10], while transition metal oxides/hydroxides and conducting polymers are intensively researched as

* Corresponding author. E-mail address: [email protected] (Y. Yang). https://doi.org/10.1016/j.mtener.2017.09.012 2468-6069/© 2017 Elsevier Ltd. All rights reserved.

pseudocapacitive materials [11e17]. However, these conventional electrode materials cannot meet the requirement of supercapacitors due to their high cost, low capacitance or poor stability. Therefore, exploration of new electrode materials with high capacitance and long term cycling stability is desired. Metal organic frameworks (MOFs), a crystalline porous material assembled by metal ions (or clusters) and organic ligands through coordination bonds are drawing increasing attention in gas separation/storage [18,19], catalysis [20], drug delivery [21], sensing [22e25] and energy storage and conversion [26e28], due to their excellent characteristics, such as high porosity, tunable pore size and composition, and abundant pseudo-capacitive redox centers [29e31]. Recent years, MOFs have also been investigated as electrode materials for supercapacitors [32e34]. However, poor electrical conductivity in most MOFs frustrates their capacitance or rate performance [35,36]. Coordination Supramolecular Networks (CSNs) is a class of materials assembled by weak intermolecular interactions (e.g., hydrogen-bonding, pep stacking and electrostatic interactions) between discrete mononuclear/oligomeric coordination units. This kind of materials always contain abundant pores that can facilitate electrolyte ions transfer and many active metal sites that can be involved in the redox reactions. Compared with the relatively strong interaction of coordination bond in MOFs, CSNs constructed by weaker interactions such as pep stacking, hydrogen-bonding will be more efficient for

H. Yao et al. / Materials Today Energy 6 (2017) 164e172

electrolyte ion and charge transfer [37,38]. Besides, the selfhealing characteristics of these kinds of weak interactions may be beneficial to the cycling performance, and their flexible structures ascribed to the weak interactions make them more easily to be embedded into other conductive materials such as graphene, CNTs [39]. Furthermore, CSNs those assembled by p electron conjugated system such as nitrogen/sulfur heterocyclic ligands are likely to have relatively high conductivity [35]. The abovementioned merits imply the great potential of CSNs as electrode materials for supercapacitors. However, investigations on direct use of CSNs as supercapacitors materials are still rarely reported up to now [40]. To further enhance the stability, improve the electrical conductivity and make full use of the single-phase electrode-active materials, combination of capacitive materials with conducting materials has aroused increasing interests today [41]. Among many of the conducting materials, graphene has attracted great attention on account of its unusual properties such as superior electrical conductivity, high surface area and excellent chemical stability [42e45]. Especially, three-dimensional (3D) graphene-based hybrid materials are attracting more and more attention because of [46e49]: i) 3D structure, to some extent could prevent the selfaggregation behavior of graphene; ii) 3D-graphene has large active specific surface area, interconnected pore structure and outstanding mechanical strength; iii) Multi-component hybrid composites combined 3D-graphene with other materials may generate synergistic effects between them, and hence give rise to enhanced performances or even new properties that the single parts never achieves. Recently, considerable excellent 3D-graphene composites such as metal oxide/sulfide@3D-graphene [50,51], metal hydroxide@3D-graphene and conductive polymer@3Dgraphene have been successfully fabricated [52,53]. At the same time, many efforts have also been devoted to prepare metal coordinated polymer@3D-graphene composites [54,55]. However, their preparation processes are always complicated, resource- or timeconsuming. Therefore, a facile strategy for preparing MOFs@3DrGO hybrid monoliths composites is highly desired, and to the best of our knowledge, the fabrication of CSNs@3DrGO hybrid monoliths composites has not been reported up to now. Considering the above intrinsic characteristics of CSNs, and inspired by the preliminary works of our group [40], in this report, we choose Pyridine-2, 3-dicarboxylic acid (H2-pda) as ligand. After one-pot solvothermal reacting with Ni2þ and graphene oxide (GO) in H2O-DMF solution, a series of Ni-pda@Three-dimensional Reduced Graphene Oxide hybrid monoliths (Ni-pda@3DrGO) are successfully constructed. As a proof of practical application, the capacitive performances of the resulting composites are investigated. The optimal sample delivers a capacitance of 952.85 F g1 at the current density of 1 A g1, which is much higher than that of the composite fabricated by physically mixing (437.5 F g1), and the capacitance still remains at ~545 F g1 when the current density is increased up to 10 A g1. When used as positive electrode materials, the constructed Ni-pda@3DrGO//active carbon (AC) asymmetric supercapacitor (ASC) delivers high power density, high energy density as well as good cycling stability. Moreover, two other hybrid monoliths, Ni(quin-2-c)2(H2O)2@3DrGO (quin-2-c ¼ 2-quinolinecarboxylic) and Co2(m2-H2O)(na)4$DMF@3DrGO (na ¼ nicotinic acid) have also been fabricated successfully by the same method. 2. Experimental section 2.1. Synthesis of Ni-pda@3DrGO and Ni-pda/3DrGO composites All reagents used here were purchased from commercial sources and used directly as received otherwise stated.

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Graphene oxide (GO) was prepared from natural graphite powder by a modified Hummer's method as reported previous [56]. The Ni-pda@3DrGO composite was synthesized by a one-step solvothermal method as follows: GO was dispersed in 15 mL DMF-H2O solvent by sonicating for 30 min, then 0.1 g Ni(NO3)2$6H2O was dissolved in the GO dispersion solution, after sonicating for another 1 h, 0.06 g H2-pda was added and sonicated for 30 min to form a stable suspension. The suspension was then transferred to a 20 mL Teflon-lined stainless steel autoclave, sealed tightly, and heated at 180 C for 18 h. After cooling to room temperature, the as-prepared Ni-pda@3DrGO gel was immersed in alcohol and deionized water for 24 h in sequence to wash the unreacted precursors and DMF, then the gel was dried at room temperature to get Ni-pda@3DrGO composites, noted as Ni-pda@3DrGO-x, where x ¼ 10, 20 and 30 are the amounts of GO added in the preparation of the composite (being 10%, 20% and 30% based on the total mass of nickel salts of the initial material weight, respectively). The amount of reduced graphene in Ni-pda@3DrGO monoliths was measured by dissolving Ni-pda@3DrGO in hydrochloric acid solution (36 wt%) and weighing the remains (Table S1) [57]. The one-step synthesis of Nipda@3DrGO composite is illustrated in Fig. 1. For comparison, Pure Ni-pda and 3DrGO were also prepared under the same experiment conditions respectively. Ni-pda/3DrGO was prepared by physically mixing pure Ni-pda with pure 3DrGO in agate mortar. Besides, two other CSNs@3DrGO hybrids, Ni(quin-2c)2(H2O)2@3DrGO (quin-2-c ¼ 2-quinolinecarboxylic) and Co2(m2H2O)(na)4$DMF@3DrGO (na ¼ nicotinic acid) were also fabricated by the same method (Supporting information Section 1). 2.2. Characterization Powder X-ray diffraction (PXRD) patterns, which used to investigate the crystallographic characteristics of the resultants were recorded on Rigaku X-ray diffractometer with Cu-Ka radiation (1.5418 Å). Field emission scanning electron microscopy (Hitachi SU8010) and transmission electron microscopy (Tecnai G2 20) images of the samples were collected to investigate the morphologies. Fourier transform infrared spectroscopy (FTIR) analysis was carried out on FTIR instrument (Bruker) from 600 to 4000 cm1 with a resolution of 2 cm1. 2.3. Electrochemical measurements The electrochemical performances of the as-synthesized samples were investigated by using a typical three-electrode system on an electrochemical workstation (CHI 760E, Shanghai Chenhua instrument, Inc., China). The three-electrode setup comprise active materials as working electrode, a platinum foil as counter electrode and a saturated Ag/AgCl as the reference electrode, respectively. To make the working electrode, acetylene black and polyvinylidene fluoride (PVDF) were used as conducting additive and binder respectively. After being mixed with the electrode-active materials in a ratio of 1:1:8 in the 1-Methyl-2-pyrrolidinone (NMP), the homogeneous mixture were coated on the current collector (nickel foam, 1.0 1.0 cm2) [58]. Then the electrode was dried at 60 C for a period of 24 h. Typical mass loadings of active material (it means the total mass of Ni-pda and 3DrGO if the material is a composite) on a piece of electrode were about 1.5 mg cm2. During the electrochemical testing, an aqueous solution of LiOH (1.0 M) was used as the electrolyte. All electrochemical measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) (0.01 Hz e 100 kHz) and galvanostatic charge/discharge (GCD) were conducted at room temperature. The specific capacitances were calculated based on the branches of discharge curves used the followed equation:

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Fig. 1. Fabrication processes for Ni-pda@3DrGO composites.

C ¼ I t=ðDV mÞ

(1)

herein, I (A), t (s), DV (V) and m (g) are the discharge current, discharge time, potential window and mass of active materials, respectively. 2.4. Asymmetric supercapacitors (ASCs) Asymmetric supercapacitors were fabricated by sandwiching positive electrodes (Ni-pda@3DrGO) and negative electrodes (active carbon) with the cellulose paper separator. Here, the ACbased negative electrodes were prepared via the same procedure used to prepare the positive electrode mentioned above except for substitution of AC for Ni-pda@3DrGO. Prior to the fabrication of the asymmetric supercapacitor, the mass ratio of positive and negative electrodes was calculated based on charge balance principle as follows [52]:

mþ =m ¼ ðC DV Þ=ðCþ DVþ Þ

(2)

herein, C and DV are the specific capacitance and working potential window of AC, respectively (the capacitive performance of AC can be seen in Fig. S7). Cþ and DVþ are the specific capacitance and potential range of Ni-pda@3DrGO, respectively. The electrochemical performances of the constructed ASCs were tested in a two-electrode system using a 1.0 M LiOH aqueous solution as electrolyte. The followed equations were used to calculate the energy density (E) and power density (P) [59]:

. E ¼ 1=2 C V 2 3:6

(3)

P ¼ 3600 E=t

(4)

In these equations, C represents the specific capacitance of assembled ASCs (based on the total mass of active carbon and Nipda@3DrGO-20). V and t represent the potential window and discharge time, respectively. 3. Results and discussion 3.1. Ni-pda@3DrGO electrodes A surfactant-free one-pot method is applied to prepare Nipda@3DrGO composites. The scheme of the synthesis is illustrated in Fig. 1. During the process, the nickel ions Ni2þ tend to diffuse toward to GO sheets by electrostatic interactions with the oxygen-containing functional groups on the GO surface [60,61]. As the reaction occurred, the ligands can coordinate with the metal ions and form the Ni-pda nuclei on the GO. At the same time, GO sheets are gradually reduced and form three-dimensional framework by pep interaction. Then the Ni-pda nuclei continue to grow and lead to the formation of nano/micron particles anchored on the rGO nanosheets. It is noteworthy that the ratio of H2O to DMF has great influence on the formation of Ni-pda and 3D graphene composites. As shown in the XRD patterns (Fig. S1), the nucleation and growth of Ni-pda


only occur under appropriate volume ratio of water (H2O: DMF ¼ 1:1 or 1:2). The mechanism may be proposed as follows, when polar-protic solvents, H2O takes up a great portion of the total reaction mixture, they surround possible negative anions by hydrogen bonding which raises activation energy, hampering the reaction between the Ni cation and the ligands [62]. While the proportion of H2O in the reaction mixture decreases dramatically, it may hinder the deprotonation process of the ligands and the process of coordination. On the other hand, the hydrophobic property of rGO makes it more easily be dispersed in DMF. Thus, only disperse rGO could be obtained in pure DMF solution at the present of Ni cation and the ligands (photo S-5 in Fig. S1). Thus, the ratio for H2O to DMF was fixed to 1:2 thereinafter unless otherwise stated. The XRD spectrums were first recorded to identify the crystal phase of the samples. Fig. 2a shows the XRD patterns of pure Ni-pda and Ni-pda@3DrGO-x (x ¼ 10, 20 and 30). As shown, all the patterns are well consistent with the simulated patterns (CCDC 276662) [63], confirming the formation of Ni-pda during the solvothermal process and the presence of graphene does not change the peak position of Ni-pda, indicating that graphene does not hinder the formation of linkages between metal centers and organic ligands. The crystal structure of Ni-pda CSNs is displayed in Fig. 2b. In the complex, the basic unit, [Ni(pda)(H2O)3]n, extends infinitely to form an 1D chain. One nitrogen atom and two oxygen atoms from pda ligand coordinate with the center ions (Ni(II)). At the same time, another three oxygen atoms from the water molecules link with the center ions from different directions to form a slightly distorted octahedral structure. These water molecules give rise to strong hydrogen bonds between the one dimensional chains to form a two dimensional layered supramolecular networks [63]. This special layered structure is likely to provide enough space for the

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electrolyte storage and be more favorable to OH intercalation and deintercalation during charge and discharge. Moreover, the nitrogen heterocyclic ligand may benefit for electron transporting [35,64]. The samples were further characterized by FT-IR spectra. As shown in Fig. S2, for pure reduced graphene oxide (curve I), no absorption peaks related to oxygen-containing groups (such as C]O, CeOeC and eOH) appear, indicating successful reduction of GO after the solvothermal treatment [65]. As seen from the curve II, an obvious absorption peak located around 3408 cm1 appears, indicating the existence of water molecules and hydrogen bonds. The band related with C]O and CeO stretching of the ligand is distinguished at 1633 cm1 and 1379 cm1, respectively. The three peaks around 676, 721 and 777 cm1 are due to the wagging vibrations of pyridine ring. Besides, the characteristic peaks of C]N group locating at 1583 and 1570 cm1 also appear [63]. Comparing the FTIR spectra of pure Ni-pda and Ni-pda@3DrGO (curve III) composite, similar peaks indicating different vibrational and stretching modes in Ni-pda showed up except for an emergence of new peak near 1793 cm1, a C]N stretch peak shift from 1570 cm1 to 1580 cm1 and peak changes near 710 cm1 and 825 cm1 (denoted with the dotted box in Fig. S2) that indicate the interaction between Ni-pda and graphene [65e67]. Furthermore, the intensities of the characteristic peaks decreased apparently after incorporating with 3DrGO. The microstructures of the samples were examined by SEM technique. SEM images (Fig. 2c and d) reveal an interconnected, porous three-dimensional graphene framework with continuous macropores in the micrometer size range, which is similar to that of the reported work previously [52,68]. Compared with the pure Ni-pda, after adding graphene oxide, the Ni-pda particles are attached on the graphene surface tightly and the size of the particles are reduced dramatically, which demonstrates that the

Fig. 2. Structure and morphology of resulting composites, (a) XRD patterns of pure Ni-pda and Ni-pda@3DrGO-10/20/30, (b) Structure of Ni-pda CSNs, SEM images (c, d) pure graphene, (e, f) pure Ni-pda, (g, h) Ni-pda@3DrGO-10, (i, j) Ni-pda@3DrGO-20, (k, l) Ni-pda@3DrGO-30.

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presence of graphene can effectively hinder the growth of crystals. The above results demonstrate that graphene play an important role in regulating material morphology and structure during the solvothermal process. As shown in Fig. 2gel, when the content of graphene oxide is only 10%, there exist some redundant large crystals on the surface (shown by arrow in Fig. 2g), suggesting the excessive generation of Ni-pda particles. When the proportion of graphene oxide is increased up to 20%, the SEM images shown in Fig. 2i and j indicate the particles of Ni-pda are uniformly attached on the graphene surface, which is further confirmed by the EDS mapping of Ni-pda@3DrGO-20 in Fig. S3. But with further increasing of graphene oxide to 30%, the surfaces of some graphene nanosheets are bare (shown by arrow in Fig. 2k and l). The interior structures of Ni-pda@3DrGO-10/20/30 are further investigated by TEM. As shown in Fig. S4, the Ni-pda particles attach on the graphene nanosheets to form conductive networks. The distribution density of Ni-pda particles on the surfaces of graphene decreases with the increasing of GO content (from 10% to 30%). Most of the particles show uniform octahedron shapes except for some large blocks in the Ni-pda@3DrGO10, coinciding with the morphology in the SEM images. Besides, the XRD patterns and SEM images of Ni(quin-2-c)2(H2O)2@ 3DrGO and Co2(m2-H2O)(na)4$DMF@3DrGO are also displayed in Figs. S5 and S6, respectively. As shown, both of the XRD patterns are well consistent with their simulated profiles, indicating that

the Ni(quin-2-c)2(H2O)2@3DrGO and Co2(m2-H2O)(na)4$DMF@ 3DrGO could also be prepared by one-pot solvothermal method successfully. The SEM images clearly show that the crystals in both composites are coated with interconnected graphene nanosheet. To further optimize the ratio of Ni-pda and 3DrGO, typical techniques, such as cyclic voltammetry (CV), galvanostatic charge/ discharge (GCD) and electrochemical impedance spectroscopy (EIS) were used to evaluate the electrochemical behaviors of Nipda@3DrGO-x (x ¼ 10, 20 and 30). The CV curves of the electrode materials recorded at a scan rate of 10 mV s1 are shown in Fig. 3a. It clearly shows that there are a couple of redox peaks located near 0.42 V and 0.2 V respectively, corresponding to the redox pair of Ni2þ/Ni3þ in the present of OH during charge/discharge process, which indicates that the capacitance of Ni-pda may be mainly attributed to the combination of ion exchange and redox mechanism (Fig. 3f) [61,69]. The GCD curves of the electrode composites were recorded at a current density of 1 A g1 (Fig. 3b). As seen, the distinct plateau region near 0.33 V at the charge/discharge curves is in good agreed with the result of the CV test and shows a dominant pseudo-capacitive behavior. The capacitances of Ni-pda@3DrGO-10/20/30 calculated from discharge branches of the GCD curves are 729.45 F g1, 952.85 F g1 and 748.75 F g1, respectively. Obviously, with the increase of GO amount from 10% to 30%, the specific capacitances of the

Fig. 3. Electrochemical performance of Ni-pda@3DrGO-x (x ¼ 10, 20 and 30), (a) CV curves, (b) GCD profiles, (c) EIS plots, (d) CV cures of Ni-pda@3DrGO-20 at different scan rate, (e) GCD profiles of Ni-pda@3DrGO-20 at different current density and (f) The proposed electron transport process between the Ni-pda and graphene networks.


composites increase firstly and then decrease, indicating that only a moderate amount ratio of rGO to Ni-pda can obtain the desirable capacitance. The content of rGO rises with the increase in GO concentration from 10% to 30% in the precursor. In the Nipda@3DrGO-10, the content of rGO is relatively low and the conductive network could not form effectively, resulting in poor capacitive performance. With the amount of GO increasing to 20%, a maximum capacitance is obtained. In contrast, the capacitance decreases slightly when the amounts of GO are further increased to 30%, which can be ascribed to the low relative percentage of pseudocapacitive active material (Ni-pda) in Ni-pda@3DrGO-30. Fig. 3c shows the Nyquist plots of Ni-pda@3DrGO-10/20/30. It is obvious that the impedance spectra are all composed of a semicircle in the high frequency region and a straight line at in the range of low frequency range. Intercepts of the EIS curves with the real impedance axis represent the internal resistance of the composites electrode which consists of three parts: electrolyte resistance, contact resistance between the electrode materials and current collector and the intrinsic resistance of the active material. The diameter of the semicircle associate with charge transfer resistance of the electrode materials, while the straight line is related to the Warburg resistance [52,70]. Fig. 3c shows that Ni-pda@3DrGO-20 has the smallest internal resistance and moderate charge transfer resistance. These electrochemical performances of the samples in agreement with the previous SEM results, further confirming that the optimum content of GO could be 20%. As known, rate capacity is a vital characteristic for electrodes of supercapacitors. Hence, the rate performance of Ni-pda@3DrGO-20 is further evaluated by CV and GCD tests. Fig. 3d shows the CV curves recorded at different scan rates (10e50 mV s1). As shown, with the speed of scan rate increasing, both the intensity of responding current and the separation between oxidation peak and reduction peak increase. However, the shape of the CV curve almost retains even the scan rate increased to 50 mV s1, indicating the good reversibility of this hybrid. The GCD profiles recorded at

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different current density are shown in Fig. 3e, they clearly shows that the separation of the redox platforms on the charge and discharge profiles increase along with the increasing current densities, which are well consistent with the CV results. The specific capacitances of the electrode calculated from discharge branches of the GCD curves are to be 952.85 F g1, 803.45 F g1, 644.38 F g1 and 545.50 F g1 corresponding to the current densities of 1 A g1, 2 A g1, 5 A g1 and 10 A g1, respectively. The excellent capacitive performances are comparable to or even better than many of the previously reported Ni-base electrode materials (Table S2). Moreover, 57% of the specific capacitance could be retained when the current density increased from 1 A g1 to 10 A g1, indicating its excellent rate capacity. In order to exploit the influence of three dimensional graphene networks on the electrochemical process, we also test the electrochemical performances of Ni-pda@3DrGO, Ni-pda/3DrGO and pristine Ni-pda. Fig. 4a clear shows that the enclosed area of the CV curve from the Ni-pda@3DrGO composite is much larger than that of Ni-pda/3DrGO and pure Ni-pda, indicating a enhanced specific capacitance. The specific capacitance of Ni-pda@3DrGO, Ni-pda/ 3DrGO and pure Ni-pda calculated from the GCD (Fig. 4b) profiles are to be 952.85 F g1, 437.5 F g1 and 363.50 F g1 at the current density of 1 A g1, respectively, which are well consistent with the results of CV test. EIS plots (Fig. 4c) show that both the internal resistance and charge-transfer resistance of Ni-pda@3DrGO are much lower than that of Ni-pda/3DrGO and Ni-pda. Moreover, Nipda@3DrGO also exhibits better cycling performance than Ni-pda/ 3DrGO (Fig. 4d). Thus, the above-mentioned results further confirmed that the in-situ incorporation of 3DrGO could effectively improve the electrochemical performance of the composites, which can be ascribed to the following three merits [49,58]: i) The interconnected networks of the 3DrGO monoliths not only act as an excellent charge transfer network to shorten the path length of the electron collection/transport but also

Fig. 4. Electrochemical performance of Ni-pda@3DrGO-20, Ni-pda/3DrGO and Ni-pda, (a) CV curves, (b) GCD, (c) EIS plot and (d) Cycling performance at a constant current density of 5 A g1.

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Fig. 5. (a) Schematic illustration of the assembled structure of the Ni-pda@3DrGO//AC ASC device, (b) CV curves of the ASC device at different scan rates, (c, d) GCD profiles of the ASC at different current densities, (e) Specific capacitance of ASC at various current densities, (f) Ragone plot of the ASC.

provide facile pathways for the electrolyte ion diffusing to the active materials surface. ii) The presence of the GO during the synthetic process can effectively regulate the material morphology and structure, as well as reduce the particles size of Ni-pda, which can promise sufficient contact between the active material and the electrolyte and greatly shorten the distance for electrolyte ion diffusion during the charge/discharge process. iii) The tight adherence of Ni-pda on the 3DrGO nanosheets benefit the electron transfer through the pep stacking and synergistic cooperation between the Ni-pda and rGO (as shown in Fig. 3f), resulting high electrochemical activity and excellent stability.

3.2. Asymmetric supercapacitors To understand the possibility of Ni-pda@3DrGO for practical use, asymmetric supercapacitors used Ni-pda@3DrGO as positive electrode and commercial AC as negative electrode were assembled (Fig. 5a) and tested. The CV curves of the ASC recorded at different scan rates shown in Fig. 5b demonstrate that the device can reach a high operating voltage up to 1.5 V. As shown, a couple of redox peaks is observed, which could be attributed to the Faradaic redox

reactions of the Ni-pda@3DrGO electrode. The shapes of all curves recorded at different scan rates almost remain the same, indicating good reversibility and rate performance of the device. The GCD profiles recorded at different current densities from 1 A g1 to 25 A g1 are shown in Fig. 5c and d. As the current density increases from 1 to 25 A g1, the “iR” drop increases obviously, ascribing to the internal resistance of the ASC. In addition, the symmetric configurations of the charge/discharge curves even at a high current density of up to 10 A g1 suggest the good electrochemical reversibility. The specific capacitances obtained at different applied specific current density based on the total mass of active materials on both electrodes are displayed in Fig. 5e and Table S3. As shown, the device delivers a specific capacitance of 66 F g1 at 1 A g1. When the current density is increased to 25 A g1, the specific capacitance still stays around 16.33 F g1. As clearly shown in Fig. 5e, with the increase of charge/discharge current density, the specific capacitance gradually decreases. As known, with the decrease of the charge/discharge time, the diffusion distance of electrolyte ion decrease, it means that the utilization efficiency of active materials decrease at the same time, resulting diminution of specific capacitance [71]. Generally, energy density and power density are another two important parameters for the practical application of ASC. The Ragone plot calculated according to the different current densities


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Fig. 6. (a) Cycling performance of the Ni-pda@3DrGO//AC ASC at current density of 20 A g1, (b, c) Photographs of Ni-pda@3DrGO//AC ASCs power a small fen and a red LED indicator. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

is illustrated in Fig. 5f. As shown, the ASC delivers an energy density of ~20 Wh kg1 at a power density of 750 W kg1 and the energy density is found to decrease with increasing power energy. However it still retains as high as 14.80 Wh kg1 at a very high power density of 3.75 kW kg1 (Table S3), which is superior to that of many reported ASCs, such as CoMoO4//AC (21.1 Wh kg1 at 300 W kg1) [72], Ni(OH)2//AC (12.6 Wh kg1 at 1670 W kg1) [73], meso-NiO/Ni-3//CNCc (19.1 Wh kg1 at 700 W kg1) [74], and V2O5-rGO//MnO2-rGO (15.4 Wh kg1 at 436.5 W kg1) [75] and GNCC//AC (7.6 Wh kg1 at 5600 W kg1) [76]. Furthermore, the repeated charge/discharge cycling and practical application of the ASCs are also shown in Fig. 6a. Here, no capacitance loss is observed after 6000 charging/discharging cycles, indicating its excellent cycling stability. For a demonstration of the practical application of the ASC device, a small fan can be powered by a single ASC after charging to 1.5 V (Video S1) and two ASCs connected in series are able to light up a 2.5 V red LED indicator for several minutes (Video S2) after charging to 3 V (Fig. 6b and c). These exhibitions indicate the great potential of the Ni-pda@3DrGO//AC ASCs in practical application. Supplementary video related to this article can be found at https://doi.org/10.1016/j.mtener.2017.09.012. 4. Conclusion In summary, a novel Ni-pda CSN@3DrGO monolith is prepared by a facile one-step solvothermal method and investigated as electrode materials for supercapacitors. The self-assembly of graphene sheets into three dimensional networks and crystallization of Ni-CSN occur simultaneously, resulting in Ni-CSN@3DrGO frameworks hybrid monoliths. The porous 3DrGO not only

facilitates electrolyte ion transport, but also acts as a framework to support the CSNs and conductive networks to enhance the charge transfer. Furthermore, the synergistic effect of the Ni-pda with specific structure and 3DrGO with high conductivity can generate large benefits for taking full use of the pseudocapacitive active materials. On account of the above-mentioned merits, the NiCSN@3DrGO-20 sample delivers a high capacitance of 952.85 F g1 at the current density of 1 A g1. When used as positive electrode materials, the constructed Ni-pda@3DrGO//AC ASC delivers high power density, high energy density as well as good cycling stability, demonstrating the great potential of CSNs in energy storage field. In addition, the one-pot surfactant-free solvothermal strategy can also be applied to fabricate Ni(quin-2c)2(H2O)2@3DrGO and Co2(m2-H2O)(na)4$DMF@3DrGO hybrid monoliths, demonstrating it can offer an important guideline for further integration of other metal coordinated polymer into 3D graphene networks, which are promising for energy storage, adsorbent and electrochemical catalytic. Acknowledgements This work was financially supported by the National Natural Science Foundation of China [grant numbers 51472275, 91022012 and 20973203] and Guangdong Natural Science Foundation [grant numbers 2014A030313207]. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.mtener.2017.09.012.

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