Redox-active organic molecules functionalized nitrogen-doped porous carbon derived from metal-organic framework as electrode materials for supercapacitor

Electrochimica Acta 223 (2017) 74–84

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Redox-active organic molecules functionalized nitrogen-doped porous carbon derived from metal-organic framework as electrode materials for supercapacitor Bingshu Guo, Yuying Yang, Zhongai Hu* , Yufeng An, Quancai Zhang, Xia Yang, Xiaotong Wang, Hongying Wu Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China

A R T I C L E I N F O

Article history: Received 14 September 2016 Received in revised form 15 November 2016 Accepted 3 December 2016 Available online 5 December 2016 Keywords: Asymmetrical supercapacitor Organic molecules Metal-organic frameworks Non-covalent functionalization Rate capability

A B S T R A C T

Metal-organic frameworks (MOFs) have been turned out to be an excellently self-sacrificing template for preparing porous carbon. Herein, we synthesized a nitrogen-doped porous carbon materials (NPCs) by direct thermolysis of zinc-based MOFs (ZIF-8). Fortunately, the NPCs with high specific surface area and abundant pore structure was suitable for using as conductive substrate to anchor organic molecules. Anthraquinone (AQ), 1, 4-naphthoquinone (NQ) and tetrachlorobenzoquinone (TCBQ) were selected to functionalize NPCs via noncovalent interactions, respectively. As a consequence, the multielectron redox centers possessed by AQ, NQ and TCBQ were implanted in the NPCs. More interestingly, the electrochemical rate-determining step for the functionalized NPCs was surface process rather than diffusion, which is similar to capacitive behavior of the electrical double layer. The functionalized NPCs revealed an enhanced overall capacitance (about 1.4 times higher than NPCs) because the electrochemical capacitance was superposed on the electrical double layer capacitance. Furthermore, the as-assembled asymmetrical supercapacitor (ASC) exhibited excellent energy storage performance. The topological structure of MOFs skeleton and the potential self-matching behavior between the positive and negative electrodes were responsible for high energy density (23.5 Wh kg
1 at 0.7 kW kg
1, which is 1.54 times higher than that of NPCs symmetrical supercapacitor) of the device. © 2016 Published by Elsevier Ltd.

1. Introduction The fossil energy such as coal, oil and gas, which is once used to support the rapid development of human civilization in the 20th century, is undergoing an unprecedented crisis in recent years. Except for reserves dwindles, the emissions coming from burning those fossil fuels lead to global warming, hazy weather and a series of environmental problems. Therefore, developing environmentally friendly, inexpensive and trustworthy energy storage systems is a foundation of global sustainable development [1–3]. Supercapactors, which have also been called as electrochemical capacitors or ultracapacitors, are emerging energy storage devices. In general, supercapacitors possess higher power density (ca. 5– 15 kW kg
1), superior cycle stability (over 106 cycles) and rapid charge-discharge rate. But their drawback is limited energy density

* Corresponding author. E-mail address: [email protected] (Z. Hu). http://dx.doi.org/10.1016/j.electacta.2016.12.012 0013-4686/© 2016 Published by Elsevier Ltd.

(ca. 5–10 Wh kg
1) comparing with lithium-ion batteries (ca. 120– 170 Wh kg
1) and lead acid batteries (ca. 20–35 Wh kg
1) [4,5]. Thus, the researchers have to face an unavoidable problem: how to improve the energy density of the supercapacitor to meet the growing power supply demand of a variety of applications such as portable devices, electric/hybrid vehicles, stand-by power systems, and so on. As is known to all, the energy density (E) of a supercapacitor is expressed as E = 0.5 CDV2, where C is the total specific capacitance and DV is the cell voltage of a device [6]. Apparently, increasing C or broadening DV of the supercapacitor is an effective and feasible method to enhance the energy density. According to the calculation formula of the capacitance (Cc, Cc = eA/d), Cc is a function that depends on the specific surface area (A) of electrode [7]. Thus, nanoporous materials with optimal pore size and pore distribution naturally become a class of promising electrode to improve the energy density of the supercapacitor [8]. Recently, metal-organic frameworks (MOFs) or porous coordination polymers have been investigated in the field of supercapacitors because of tremendous specific surface area, abundant

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topological structure, tunable pore distribution and high ordered crystalline state [9–11]. They are usually applied in following ways: firstly, pristine MOFs crystals can be directly used as electrode materials due to their essential porous structures and exposed metal centers, which store charges by absorbing electrolyte ions or corresponding reversible redox reactions, respectively; secondly, converting MOFs into porous metal oxides in an air atmosphere to store charges by taking place fast Faradaic reactions; thirdly, pyrolyzing MOFs to achieve porous carbons that store charges through electrostatic accumulation of charges [12]. Amongst these ways, each approach has their own merits and shortcomings in supercapacitor applications. Pristine MOFs always show poor conductivity, while the metal oxides are non-renewable resources. However, MOFs derived nanoporous carbons are not only able to inherit the structural characteristics of the precursors but also environmentally friendly. Comparing with other methods to prepare nanoporous carbons, this method not need to remove template from the resultant and the experimental procedures are simple and suitable for wholesale production. To a certain extent, the MOFs derived porous carbon has embodied its superiority in the construct of supercapacitors with high performances. For example, Yusuke Yamauchi et al. obtained nanoporous carbon and cobalt oxide (Co3O4) electrode materials from cobalt based MOFs (ZIF-67) by controlling the annealing conditions. The asymmetric supercapacitor (ASC), which is constructed by nanoporous carbon and Co3O4, exhibited high specific energy (36 W h kg
1, 1600 W kg
1) and excellent rate capability [13]. Gao Qiuming et al. also fabricated an ASC by using MOF-5 derived porous carbon as negative electrode and Ni-Zn-Co oxide/hydroxides as positive electrode. The device delivered a high energy density of 41.65 W h kg
1 and displayed a good cycle stability [14]. Salunkhe R. et al. prepared a high specific surface area (1523 m2 g
1) nanoporous carbon materials via direct carbonization of ZIF-8 for assembling symmetric supercapacitors (SSC). The SSC could deliver a high energy density of 10.86 W h kg
1 with a power density of 225 W kg
1 in 1 M H2SO4 electrolyte [15]. Besides, Torad N.L. et al. were also synthesis another nanoporous carbon materials by using cobalt based MOFs as sacrificial templates. When the as-prepared nanoporous carbon materials were used as electrode materials for a symmetric supercapacitor, the energy density could reach to 19.6 W h kg
1 at a power density of 700 W kg
1 in 0.5 M H2SO4 electrolyte [16]. But no matter how to adjust and control the pore structure and size, the electrical double-layer energy storage mechanism of the MOFs-derived porous carbon, as a kind of carbon-based materials, is always not involved in Faradaic charge reactions and results in limited specific capacitance of 100–300 F g
1 in an aqueous electrolyte [12]. Against this background, the functionalization of the MOFs-derived porous carbon becomes particularly important. Namely, redox-active organic molecules can be used to improve the surface properties of them. As a result, the electrochemical capacitance originated from the anchored redox-active organic molecules is superposed on the electrical double-layer capacitance of the MOFs-derived porous carbon [17]. As well-known, there are a great number and various sorts of redox-active organic molecules in the available chemical resources, which can meet the selection needs for researchers [18,19]. More importantly, the electrochemical properties of the organic molecules can be selectively tuned by adjusting the organic functional groups or molecular scaffolds [20]. However, the solubility of the selected organic molecules should be not be too high in the applied electrolyte, or else the active species will migrate between the positive and negative electrode and generate shuttle effect, which always leads to capacitive fading. Based on above analysis, it is believed that the functionalized MOFs-derived porous carbon can collect many merits at once. On the one hand, it

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has the feature of tunable pore size and plentiful topological structures. On the other hand, it can acquire the desired surface properties which are regulated and controlled by organic molecule functionalization. Based on our previous reports [21], the electrochemical kinetics of the organic molecules modified carbon-based materials have following characteristics: firstly, the peak currents (ip) are linear to the applied sweep rates (v); besides, the cyclic voltammetry (CV) curves have small peak separations between oxidation and reduction peaks. This implies that surface redox reactions of anchored organic molecules will not be controlled by diffusion, which is similar to the behaviors of electrical double layer capacitance [22]. In this work, we attempted to provide a new strategy for preparing electrode materials of supercapacitors, in which the MOFs were employed as a self-sacrificing template to derive porous carbon with high specific surface area, tunable pore distribution and controllable pore size while the redox-active organic molecules were used to realize the surface functionalization. The monodispersed ZIF-8 crystals with well-defined and regular polyhedral morphologies were synthesized by means of typical coordination chemistry method. Then, the as-obtained ZIF-8 crystals as precursors were converted into nitrogen-doped porous carbon, which could retain their original morphologies and possess large specific surface area (1140 m2 g
1) and appropriate porosity after the pyrolysis treatment. Subsequently, the AQ was selected to functionalize NPCs to gain negative electrode material (AQ-NPCs) while NQ and TCBQ collectively decorated NPCs to obtain positive electrode material (TN-NPCs). The resultant positive and negative electrodes generated a potential self-matching behaviour because the electrochemical response of AQ, NQ and TCBQ molecules were located at about
0.09, +0.28 and +0.53 V. More importantly, the asymmetric supercapacitor fabricated by AQ-NPCs and TN-NPCs with homologous pore structures can deliver an energy density of 23.5 Wh kg
1 (along with the power density of 0.7 kW kg
1) in 1 M H2SO4 aqueous electrolyte. It is pointed out that the abovementioned strategy is promising for constructing a supercapacitor in term of green, all-carbon and excellent supercapacitive performances. 2. Experimental 2.1. Synthesis of ZIF-8 derived nitrogen-doped porous carbon Firstly, according to the common method [23], the ZIF-8 crystals were prepared as following: 0.988 g of zinc acetate dehydrate and 2.956 g of 2-methylimidazole (MeIm) were dissolved in the 270 mL anhydrous ethanol, respectively. The above two solutions were mixed and vigorously stirring for 5 min, and then the resulting solution was aged at room temperature for 24 h. After that, the white precipitates were collected by filtration and washed carefully with ethanol for several times. Secondly, the as-obtained ZIF-8 crystals were directly carbonized at 800 C for 8 h under a N2 atmosphere with the heating rate of 3 C min
1. After cooling down to room temperature, the black powders were washed by 1 M HCl to remove ZnO nanoparticles and washed with distilled water to neutral, then dried at 70 C and achieved NPCs. 2.2. Synthesis of organic molecules-functionalized NPCs Based on our previous report [24], 0.04 g of pure organic compound (AQ) and mixed organic compound (0.024 g NQ and 0.016 g TCBQ) were dissolved in 40 mL acetone and then 0.05 g NPCs was added, respectively. The mixtures were sonicated for 30 min. By evaporating the acetone at 70 C, the organic compounds were anchored on the NPCs. After that, the samples

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were washed scrupulously with distilled water. For convenience, the products were labelled as AQ-NPCs and TN-NPCs, respectively. 2.3. Physical characterizations The morphologies of the samples were characterized by field emission scanning electron microscopy (FESEM; ULTRA plus, Germany) and transmission electron microscope (TEM; JEOL, JEM-2010, Japan). Fourier-transform infrared (FT-IR) spectra were recorded by Nicolet Nexus 670 FT-IR instrument. The crystalline structures of the samples were determined by using powder X-ray diffraction (XRD, D/Max-2400, Cu Ks, l = 1.5418 Å). The Raman spectra were collected using inVia Raman spectrometer (Renishaw) with an argon ion laser (l = 514 nm). X-ray photoelectron spectroscopy (XPS) analysis was measured using monochromatic Al Ka radiation source (ThermoVG Scientific). Nitrogen adsorption-desorption isotherm data was achieved using Micromeritics ASAP 2020 nitrogen adsorption apparatus. 2.4. Electrochemical measurements The electrochemical performances were evaluated by using three- and two-electrode configurations with 1 M H2SO4 as electrolyte. In the three-electrode system, the working electrode was prepared by dispersing 4 mg of active electrode materials (85%) and 0.7 mg of acetylene black (15%) in 0.4 mL of Nafion (0.25 wt%). Then 6 mL of the resultant homogeneous slurry was dropped onto the glassy carbon electrode using a pipet gun and dried at room temperature. The cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were tested on a CHI760E electrochemical working station (Chenghua, Shanghai, China), in which platinum foil and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. For the ASC test, the charge (Q) on both negative and positive electrodes was balanced based on the following equation: Q = C  V  m, where C, V and m are specific capacitance, operating potential window and the mass of the active materials, respectively. The total mass loading on both positive and negative electrodes was over 1 mg cm
2. In the two-electrode system, TN-NPCs, AQ-NPCs and glass fiber were served as positive electrode, negative electrode and separator, respectively. The distance between positive and negative electrodes is not more than 5 mm. All electrochemical measurements were carried out on a CHI760E electrochemical working station. The cycle stability was performed on LANHE test system (Wuhan electronic co., LTD, China).

3. Results and discussion The schematic illustration of the experimental process is presented in Fig. 1. In the first step, Zn2+ and MeIm take place coordination reaction and produce ZIF-8 crystals. In the ZIF-8, each metal center is solely coordinated by the N atoms in the 1, 3postions of the MeIm rings. The MeIm rings act as a bridging unit between the metal centers and build an interpenetrating network structure, which generally allows electrolyte unrestricted access but goes against the transmission of electrons [25]. In the second step, the as-prepared ZIF-8 as sacrificial templates and precursors are directly pyrolyzed to obtain NPCs. Finally, the NPCs as an appropriate substrate is functionalized by different organic molecules (AQ, NQ and TCBQ). 3.1. Substrate material Herein, the NPCs is used as substrate materials to further prepare high-performance electrode material. Fig. 2a and b show the FESEM images of NPCs with different magnification, it can be found that the MOF-derived carbon triumphantly inherit the original morphology of precursors (Fig. S1a and b). Each NPCs particle possesses uniform and well-defined polyhedron shape with a sharp edge as well as the smooth surface. The average diameter of NPCs particles is about 500 nm. This near perfect morphology mainly benefits from the slow nucleation rate of the ZIF-8 crystals. Namely, low dissolvability of Zn(CH3COO)2 in anhydrous ethanol slows down the coordination reaction rate and accordingly prevents the rapid nucleation of ZIF-8 crystals [26]. The low-magnification TEM images of NPCs particles are presented in Fig. 2c and Fig. S2 (a, b and c), which are well consistent with that of FESEM. In addition, Fig. 2d displays a highmagnification TEM image on the edge of NPCs particles, which clearly reveals the presence of abundant porous structures. The porosity of the NPCs was also investigated by nitrogen adsorption/ desorption measurement. As shown in Fig. 3a, the sorption isotherm displays mixed type I and type IV isotherm. The adsorptions show an obvious rising trend at the low relative pressure (type I, P/P0 < 0.4), suggesting the presence of micropores [27]. Furthermore, a hysteresis hoop at the high relative pressure (type IV, 0.4 < P/P0 < 0.9) indicates the mesoporous characteristics. The Brunauer Emmett Teller (BET) surface area of the NPCs is around 1140 m2 g
1 and the total pore volume is 0.57 cm3 g
1. Pore size distribution was observed by using the Barrette-JoynereHalenda (BJH) method (inset in Fig. 3a), suggesting the mesoporous diameter is 3.9 nm [28]. Besides, a clear upward trend peak

Fig. 1. Schematic illustration of the preparation procedure for samples.

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Fig. 2. (a and b) FESEM images of NPCs at different magnifications, (c) Low-magnification TEM images of NPCs and (d) High-magnification TEM image is collected from the ringed area.

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of the pore size distribution curve (< 2 nm) further confirms the existence of micropores in NPCs [27]. The average pore size distribution is 2.1 nm. The XRD pattern of the as-prepared ZIF-8 (inset in Fig. 3b) matches well with the published pattern [29]. Compared with ZIF-8, the XRD pattern of the NPCs (Fig. 3b) emerges a sharp diffraction peak at around 23 and a broad diffraction peak near 43 , corresponding to a typical (002) and (101) lattice planes of graphite-type carbon nanosheets, respectively [30,31]. Fig. 3c presents the Raman spectrum for as-prepared NPCs particles, where the D and G bands are located at 1347 cm
1 and 1574 cm
1 originating from the defective or disordered graphitic and E2g mode first-order scattering of the sp2 carbon domains, respectively [32]. The intensity ratio of the D band to the G band (ID/IG) is about 0.98 indicating that NPCs includes a portion of disordered carbons, which may be attribute to the edge doped N atoms [33]. More importantly, a broad and weak 2D band is detected at near 2836 cm
1, which further confirms a high graphitizing degree of NPCs [34,35]. The XPS spectrum of the NPCs (Fig. S1c) clearly shows the existence of C 1s (284.8 eV), N 1s (398.7 eV) and O 1s (532.5 eV) signal peaks. Among them, the N content is about 7.6% through element analysis and enlarged N 1s spectrum (Fig. 3d) can be mainly deconvoluted into three peaks located at around 398.4, 399.9 and 401.1 eV, which are correspond to pyridinic-N (N-6) and pyrrolic-N (N-5) and quaternary N (N-Q), respectively [36]. For the N-6 and N-5, the N atoms are bond with two sp2 carbon atoms and contribute electrons to the p system, resulting in electron donor properties to the carbon layers [33]. Whereas, for N-Q, the N atoms are embed in the graphitic carbon plane and produce positive charge property, which is conducive to improve the electrical conductivity of the NPCs [37]. To sum up, the as-obtained NPCs not only possesses porous feature to provide interconnected paths for the electrolyte transport, but also has high-level nitrogen content to improve wettability of the electrode material and synchronously introduce faradic pseudocapacitance

into aqueous electrolyte during electrochemical process [33,38]. Moreover, the NPCs is here employed as substrate to adsorb different quinones organic molecules to achieve high-performance electrode material for supercapacitors. 3.2. Negative electrode material Anthraquinone (AQ) with multi-electron reaction center can transfer two electrons during electrochemical redox process [21]. Its electrochemical response is located in the negative region (about
0.09 V). Therefore, we selected AQ to functionalize NPCs and obtained AQ-NPCs composite as a negative electrode material. Fig. 4a and b present the SEM and TEM images of AQ-NPCs. After absorbing the organic compounds, the morphologies of AQ-NPCs remain unchanged comparing to the bare NPCs. Besides, the XRD pattern of AQ-NPCs (Fig. 4c) reveals that there is no visible diffraction peak for pure AQ excepting a characteristic diffraction peak for graphite-type carbon. Based on the above analysis, the free phase of organic molecules is in absence, indicating that the organic compounds are anchored in the form of molecules, which are further proved by FT-IR analysis (Fig. 4d) [24]. According to the FT-IR spectrum of the bare NPCs, the broad absorption band at around 3438 cm
1 derives from N

H and O

H stretching vibration [39], and some other characteristic peaks located at around 1621 and 1290 cm
1 are attributed to the C¼N and C

N stretching deformation mode, respectively [37]. For AQ-NPCs, a strong absorption peak at 1674 cm
1, corresponding to characteristic band of AQ, is derived from the C¼O stretching vibrations. A weak absorption peak at 1588 cm
1 is corresponding to aromatic C¼C vibrations of AQ [40]. Besides, another obvious characteristic absorption peak of AQ is located at 1280 cm
1. Furthermore, the C

H out-of-plane bending vibrations bond of the AQ appears at 692 cm
1 [40]. Compared with bare NPCs, the C¼N and C

N stretching vibrations are covered by characteristic peaks of the

Fig. 4. (a) FESEM images, (b) TEM images and (c) XRD patterns of AQ and AQ-NPCs, (d) FT-IR spectra of bare NPCs and AQ-NPCs.

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organic compound. The FT-IR analysis confirms that the AQ molecules are successfully implanted into NPCs materials. The nitrogen adsorption/desorption isotherm of the AQ-NPCs was shown in Fig. S3a. The isotherm is not closed completely, which may arise from the organic molecules are absorbed in the micropores and mesoporous. The BET surface area of the AQNPCs is about 268 m2 g
1 and the average pore size distribution is 2.96 nm (Fig. S3b). Prompted by the unique porous polyhedron structure and redox groups (C¼O), it is expected that the AQ-NPCs exhibits good supercapacitive behaviour as a negative electrode for supercapacitor. Fig. 5a compares the cyclic voltammetry (CV) of the pure AQ, NPCs and AQ-NPCs within a potential window of
0.4–0.2 V at the scan rate of 10 mV s
1. As expected, the bare NPCs material reveals an

electric double layer (EDL) energy storage feature which is expressed by the rectangular shape of the CV curve. By comparison, the CV curve of AQ-NPCs arises a pair of additional peak and situates at
0.09 V, which corresponds to the reversible redox of AQ (inset in Fig. 5a) [21]. Obviously, we combined EDL charge storage with faradaic charge storage together and enhanced the overall specific capacitance successfully because the specific capacitance is proportional to the integral area of the CV curve at the same scanning rate. However, the background current of AQNPCs is a little smaller than that of the bare NPCs. It’s caused by the two main factors: firstly, the specific surface area of AQ-NPCs becomes much lower than that of NPCs; secondly, the capacitances are calculated through the overall quality including organic molecules and substrate materials. Under such circumstances,

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Fig. 5. (a) CV curves of AQ, bare NPCs and AQ-NPCs at the scan rate of 10 mV s
1, (b) CV curves of AQ-NPCs at different scan rates, (c) GCD curves of AQ, bare NPCs and AQ-NPCs at 1 A g
1, (d) Specific capacitances of AQ-NPCs at various current density, (e) The relationships between the peak specific current (ip) and scan rate (n) of AQ-NPCs and (f) Capacity versus square root of discharge time.

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the decreased mass fraction of NPCs in composite could cause the background currents reduced. As shown in Fig. 5b, the CV curve area and the peak current gradually increase with the increase of the scan rate from 5 to 100 mV s
1, but the reversible redox peak still be clearly seen even at a high scan rate. This objective fact suggests that the AQ-NPCs electrode possesses good rate ability in 1 M H2SO4 electrolyte [41]. Owning to the uncompensated resistance, the peak potential separations also increase slightly as the scan rate increased, but still smaller than theoretical value (0.058/n, n is the charge transfer number) even at the scan rate of 20 mV s
1. Moreover, peak current ratio (ipa/ipc) is approaching to 1.0, indicating that the redox process is reversible and faster (details are displayed in Table S1) [21]. The result is profit from the well-defined polyhedral substrate materials with appropriate pore size and high graphitizing degree. Furthermore, the GCD curve of AQ-NPCs (Fig. 5c) is also different from that of bare NPCs and exhibits two overtly plateaus resulting from the redox of AQ. The specific capacitance as high as 373 F g
1 at a current density of 1 A g
1 is obtained for the composite, which is larger than that of pure AQ and bare NPCs (42 and 254 F g
1). More significantly, the specific capacitance for AQ-NPCs can remain 83.2% of the initial value when the current density increases from 1 to 50 A g
1 (Fig. 5d). However, the terminal specific capacitance for the bare NPCs only remains the initial value of 62.3% (presented in Fig. S3c). For such an improved rate capability is inevitably associated with the fast redox reaction of AQ because it is only involved in adsorption and desorption of H+ which does not need to diffuse into the material's bulk. Besides, the conductive substrates (NPCs) with abundant porous structure are accessible to free and quick transport of electrolyte ions and the N-doped can further improve the hydrophilic of carbon materials [42,33]. The mass loading of AQ in composite can be determined by using electrochemical technique and the calculations results indicated that the average content of AQ is about 15.5% of the total mass (the details are presented in Table S2). In order to research the kinetic feature of electrode material, we analysed the relationship between the peak current (ip) and applied scan rate (v) based on the Eq. (1): i = avb,

(1)

where a and b are adjustable parameters. Generally, b = 1 means surface-controlled reaction, while if b = 1/2 indicates ideal diffusion-controlled faradaic process [43,44]. As shown in Fig. 5e, a linear dependence of ip on v confirms the redox reaction of AQ as a surface-controlled so that the overall behaviour is similar to capacitive process. Furthermore, it is considered by the some researchers that the specific capacitance (C) commonly contains electric double layer capacitor component (k1) and a diffusion limited component (k2 t1/2), which determined by the Eq. (2): C = k1 + k2 t1/2.

(2)

Among Eq. (2), the k1 can be concluded from the y-axis intercept point if we plot by using t1/2 and C [45]. Thus, through this method, we can further determine the contribution fraction of capacitive effects to the whole specific capacitance (Fig. 5f). Apparently, k1 accounts for 80.1% in AQ-NPCs, while accounts for 67.2% in bare NPCs. The result adequately explains that capacitive effects play a primary role in the composite.

3.3. Positive electrode material To obtain a positive electrode material well match with resultant AQ-NPCs, we chose other organic compounds, NQ (1, 4-naphthoquinone) and TCBQ (tetrachlorobenzoquinone), to collectively modify NPCs and achieved TN-NPCs composite. Namely, both NQ

and TCBQ are anchored on the surface of NPCs in the form of molecules. The SEM, TEM and XRD characterizations indicate that the morphology and structure of the TN-NPCs are heritage to NPCs (Fig. S5a and b and Fig. 6a). FT-IR spectra of the bare NPCs and TNNPCs were also performed and displayed in Fig. 6b. The characteristic peaks at 1689 cm
1 and 1566 cm
1 can be assigned to C¼O and aromatic C¼C stretching vibrations of NQ and TCBQ in the TN-NPCs. Compared with negative electrode material, the C¼O absorption peak shows a slight blue-shift, while the C¼C absorption peak displays a red-shift. It might be because the TCBQ has electronwithdrawing groups at a position and the NQ and TCBQ molecules have a smaller conjugate system than AQ. Besides, the other characteristic peaks of TCBQ can be observed at near 1108 and 746 cm
1. Among them, the latter is corresponding to C

Cl stretching vibrations. Furthermore, a weak peak located at 695 cm
1 can be indexed to C

H out-of-plane bending vibrations, indicating the existence of NQ molecules in TN-NPCs [24]. The analysis result of nitrogen adsorption/desorption isotherm for TNNPCs suggested that the BET specific surface area is 356 m2 g
1 and the average pore size distribution is 2.40 nm (Fig. S5a). It is also noted that the AQ-NPCs possess the biggest average pore size of the three samples (NPCs and AQ/TN-NPCs). Therefore, it could be speculated that the organic molecules fill in and even cover micropores partially, which leads to the average pore size increased. The electrochemical measurements for the TN-NPCs are carried out in 1 M H2SO4 electrolyte. As shown in Fig. 6c, the composite decorated with NQ and TCBQ emerges two pairs of distinct and nearly symmetrical redox peaks, O1/R1 couple and O2/R2 couple located at near +0.28 and +0.53 V, respectively. The former corresponds to the reversible redox reaction of NQ, while the latter is attributed to the reversible redox reaction of TCBQ (inset in Fig. 6c) [46,20]. With the addition of the peak position of AQ, this result demonstrates that the electrochemical properties of the organic molecules can be selectively tuned by adjusting organic functional groups or changing the molecular scaffolds. Besides, the variation of peak potential separations with the scan rate increase can be observed through Fig. 6d and the details are also presented in Table S3. The specific capacitances of TN-NPCs, NPCs, NQ and TCBQ are respectively 392, 279, 30 and 17 F g
1, which are calculated through discharging time at a current density of 1 A g
1 (Fig. 6e). When the current density increases from 1 to 50 A g
1, the specific capacitance of TN-NPCs is still able to maintain 76.9% of the initial value, while the NPCs material can remain 73.2% (Fig. 6f and Fig. S6b). The rate capability with such a slightly increase may be related to the positive synergistic effect between organic molecules and substrate materials [47]. Similarly, the TN-NPCs electrode process is characterized by capacitive behaviors rather than semi-infinite diffusion because the peak current ip varies with v as shown in Fig. S6c and d. Furthermore, according to the equation (1 and 2), the contribution fraction of capacitive effects to the whole specific capacitance is 73.3% for TN-NPCs, while 71.6% for bare NPCs (Fig. S6e). In addition, the mass loadings of NQ and TCBQ in the TN-NPCs composite are about 12.3 and 1.2%, respectively (the details are shown in Table S2). 3.4. Asymmetric supercapacitor It has been demonstrated that the microporous materials are conducive to improve the energy density, while the mesoporous materials are favor to enhance the power density [48]. The above analysis shows that both AQ-NPCs and TN-NPCs have a micropore and mesopore feature with homologous porous substrates. They are used as negative and positive electrode for assembling asymmetric supercapacitor, respectively. At the same time, we used bare NPCs to fabricate a symmetric supercapacitor for comparing and analysing. Fig. 7a presents typical CV curves of the

B. Guo et al. / Electrochimica Acta 223 (2017) 74–84

(
0.09 V, shown in Fig. 7b). The two half-reactions lead to the peak of ca.0.34 V, which is approximately equal to the difference between redox peak positions of NQ and AQ in the three-electrode configuration. As the ASC charging process continues, the cell voltage is up to 0.65 V. At this moment, the positive electrode (TNNPCs) takes place oxidation reaction of the TCBQ (+0.53 V, shown in Fig. 7b) and the negative electrode occurs reduction reaction of the remaining AQ, which are responsible for the second peak in two-electrode configuration (AQ-NPCs//TN-NPCs ASC) [17,24]. On the contrary, the corresponding reversible processes take place in the discharging segment. This prominent self-matching and selfsynergy effect may originates from the following reasons: firstly, the positive and negative electrodes possess homologous porous structure, which stores enough electrolyte ions (H+) to meet the requirement for the electrical double layer or the redox reactions of organic molecules; secondly, the redox reactions of organic molecules in the electrodes are not controlled by diffusion, which

30

40

50

60

70

3438

-1

Potential (V vs. SCE)

-1

OH

OH

R2 Cl

Cl

Cl

0.2

Cl

OH NPCs NQ

R1

OH

0.4

0.6

0.8

Potential (V vs. SCE)

TN-NPCs TCBQ

1.0

0.6 0.4 0.2 0.0 0

200

400

Time (s)

600

25 0 -25

800

-1

10 mVs-1

5 mVs

-50

-1

-1

20mVs

30 mVs

-1

-75

-1

50 mVs

70 mVs

-1

100 mVs

0.0

1.0

NPCs TN-NPCs TCBQ NQ

0.8

50

-100

-1

0.0

e

2e-+2H+

O

-10 -15

Cl O

75

0.2

0.4

0.6

0.8

1.0

Potential (V vs. SCE)

1.2

f 400 350

76.9 %

300

Potential (V vs. SCE)

-5

Cl

Cl

2e-+2H+

0

Cl

Specific current (A g )

O2

Specific capacitances (F g )

-1

Specific current (A g )

O

10 5

749 695

Wavenumber (cm )

d 100

O1

O

1108

4000 3500 3000 2500 2000 1500 1000 500

80

2 Theta (degree)

c 15

1290

NQ TCBQ

3438

TN-NPCs

20

TN-NPCs NPCs

Transmittance (a.u.)

Intensity (a.u.)

b

1689 1566

a

1621

NPCs//NPCs SSC and AQ-NPCs//TN-NPCs ASC with a high operating cell voltage of 1.4 V without any electrolyte decomposition even over the water splitting voltage at about 1.23 V [49]. For the NPCs SSC, the CV curve exhibits a quasi-rectangular shape and not observes any redox peaks, while the CV curve for AQ-NPCs//TNNPCs ASC appears two pairs of distinguished peaks (P1 and P2 in Fig. 7a) and reveals a capacitive feature superimposing redox capacitance on electrical double layer capacitance. The first couple of peak centers at ca. 0.34 V and the second situates at ca. 0.65 V. Herein, we referred the CV curves of AQ-NPCs and TN-NPCs in the three-electrode system (Fig. 7b) to explain the electrochemical behaviour of the AQ-NPCs//TN-NPCs ASC (two-electrode system). As the ASC starts charging, its cell voltage begins to increase from 0 V. When the cell voltage of the ASC increases to 0.34 V at the charging segment, the positive electrode (TN-NPCs) takes place oxidation reaction of the NQ (+0.28 V, shown in Fig. 7b) while the negative electrode takes place reduction reaction of partial AQ

10

81

50 A g-1

1.0

250

1 A g-1

0.8

200

0.6

150

0.4

100

0.2 0.0

50 0 0

10

0

100

200

Time (s) 20 30

300

400

-1

40

Current density (A g )

50

Fig. 6. (a) XRD patterns of NQ, TCBQ and TN-NPCs, (b) FT-IR spectra of bare NPCs and TN-NPCs, (c) CV curves of NQ, TCBQ, bare NPCs and TN-NPCs at the scan rate of 10 mV s
1, (d) CV curves of TN-NPCs at different scan rates, (e) GCD curves of NQ, TCBQ, bare NPCs and TN-NPCs at 1 A g
1 and (f) Specific capacitances of TN-NPCs at various current density.

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calculated from the discharging curve of the AQ-NPCs//TN-NPCs ASC, which can reach to 86 F g
1 at a current density of 1 A g
1 and still remain 60.1% of the initial value at 30 A g
1 (Fig. 7d). The decay of the specific capacitances derives from the low utilization efficiency of active materials under high charge/discharge current [23]. The cycling stability test of the ASC reveals that there is 75.3% retention of the initial specific capacitance after 5000 cycles at a current density of 5 A g
1. Furthermore, in order to compare with

provide a kinetic condition for self-matching negative and positive half reactions. It is without doubt that these characteristics or behaviors are foundations for obtaining high-performance supercapacitors. As displayed in Fig. S7a, ranging of scan rates from 5 to 100 mV s
1, the shapes of the CV curves for ASC are unchangeable. In addition, all of discharge curves are almost symmetric to their corresponding charging segments, suggesting an excellent electrochemical reversibility (Fig. 7c). The specific capacitance is

a

3

b

P 2 0.65 V

0.34 V P 1

Specific current (A g-1)

-1

Specific current (A g )

4

2 1 0

-1 -2

NPCs//NPCs AQ-NPCs//TN-NPCs

-3 0.2

0.4

0.6

0.8

1.0

1.2

8 0 -8

0.53 V TN-NPCs -0.09 V 0.28 V AQ-NPCs

-16

-4 0.0

0.62 V 0.37 V

16

-0.4 -0.2

1.4

Voltage (V)

1A

-1

30 A

1.4

g-1

g-1

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

50

100

Specific capacitances (F g )

c 1.6

0.0

0.2

0.4

0.6

0.8

1.0

Potential (V vs. SCE)

Voltage (V)

150

200

250

Time (s)

d 96 80 64

60.1 %

48 32 16 0 0

5

10

15

20

25

30

-1

Current density (A g )

-1

Energy density (Wh kg )

e 100 36 s

10

R.53 R.15

R.52

R.13

3.6 s

R.54

R.51

0.36 s ASC AC//AC NPCs//NPCs

1 0.036 s 0.0036 s

0.1 0.1

1

10

-1

Power density (kW kg )

100

Fig. 7. (a) CV curves of ASC and SSC at a scan rate of 10 mV s
1, (b) Schematic diagram of imaginative three-electrode system, (c) GCD curves of ASC at various current density, (d) Specific capacitances of ASC at various current density, (e) Ragone plots of SSC and ASC as compared to AC//AC and data in other literatures and (f) Radar plots of the ASC, the red curve is generated from the AQ-NPCs//TN-NPCs ASC using 1 M H2SO4 electrolyte.

B. Guo et al. / Electrochimica Acta 223 (2017) 74–84

NPCs//NPCs, we also tested cycle life for the SSC with the same current density. As a result, the retention rate for SSC is 88.9% after 5000 cycles. Therefore, it is deduced that the decay of 10.6% for the ASC originates from the loss of electrochemically active materials (Fig. S7b). In addition, in the high frequency region of EIS (Fig. S7c), the inconspicuous semicircle indicates a negligible charge-transfer resistance and fast faradic response within electrode materials. And also, the almost vertical plot in the low frequency region suggests fast ion diffusion and ideal capacitive behaviors [50]. Fig. 7e presents the Ragone plot (energy density vs power density), which is often used to evaluate the performance of energy storage devices. It can be observed that the TN-NPCs//AQ-NPCs ASC delivers an energy density of 23.5 Wh kg
1 along with the power density of 0.7 kW kg
1, while the SSC based on NPCs//NPCs reveals only energy density of 15.6 Wh kg
1 at the same condition. The energy density of the ASC reveals a gain about 66.3% compared with the SSC due to the redox activity implanted into NPCs. Moreover, in order to evaluate the energy density, we assembled a symmetric supercapacitor (AC//AC) by using commercial activated carbon as electrode material to compare with NPCs//NPCs and AQNPCs//TN-NPCs. The electrochemical measurement results were also presented as Fig. 7e. The result shows that the energy density of AC//AC is 4.3 Wh kg
1 at a power density of 0.7 kW kg
1 when the same test conditions are remained. Thus it can be seen that although the cell construction in the present work is not the same as the one reported in the referenced literatures, the energy density for NPCs//NPCs and AQ-NPCs//TN-NPCs is relatively high compared with some reported symmetric or asymmetric supercapacitors, such as nanoporous carbon SSC (10.86 Wh kg
1 at 0.225 kW kg
1) [15], carbon//carbon (7.1 Wh kg
1 at 0.8 kW kg
1) [13], HPGM//HPGM (9.1 Wh kg
1 at 5.9 kW kg
1) [51], C-AQ//CDHB (10.0 Wh kg
1 at 0.18 kW kg
1) [52], Co0.85Se//N-PCNs (21.1 Wh kg
1 at 0.4 kW kg
1) [53] and FMCNTs//UPMNFs (10.4 kg
1 at 2 kW kg
1) [54]. Obviously, implanting organic molecules with multi-electron redox center into the carbon substrates can effectively enhance the energy density of supercapacitor. Finally, a radar plot (Fig. 7f) was employed to comprehensively assessing the electrochemical performances of the as-assembled AQ-NPCs//TN-NPCs ASC. 4. Conclusions Metal-organic frameworks can be used as self-sacrificial template to prepare nitrogen-doped porous carbon with high specific surface area (1140 m2 g
1) and appropriate pore size. The as-obtained NPCs can be easily functionalized by using electrochemically active organic molecules (AQ, NQ and TCBQ) via noncovalent interactions. Interestingly, the electrochemical redox reactions of the functionalized NPCs are not controlled by diffusion and similar to capacitive behaviour of the electrical double-layer. More importantly, the AQ-NPCs have a current response in the negative potential region, while the TN-NPCs show a current response in the positive potential region. They generate a potential self-matching behaviour in the two-electrode configuration, which leads to a high-performance asymmetrical supercapacitor with a high energy density (23.5 Wh kg
1, 0.7 kW kg
1). Besides, it is worth noting that the ASC is 1.54 times in energy density higher than that of the SSC due to the redox active groups implanted into NPCs. Acknowledgements We gratefully acknowledge the financial support offered by the National Natural Science Foundation of China (Nos. 20963009, 21163017 and 21563027), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20126203110001).

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.12.012.

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