From flour to honeycomb-like carbon foam: Carbon makes room for high energy density supercapacitors

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From flour to honeycomb-like carbon foam: Carbon Makes room for High energy density supercapacitors Xiaoliang Wu, Lili Jiang, Conglai Long, Zhuangjun Fan

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S2211-2855(15)00109-3 http://dx.doi.org/10.1016/j.nanoen.2015.03.013 NANOEN762

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Received date: 16 December 2014 Revised date: 24 January 2015 Accepted date: 11 March 2015 Cite this article as: Xiaoliang Wu, Lili Jiang, Conglai Long, Zhuangjun Fan, From flour to honeycomb-like carbon foam: Carbon Makes room for High energy density supercapacitors, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.03.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

From Flour to Honeycomb-Like Carbon Foam: Carbon Makes Room for High Energy Density Supercapacitors Xiaoliang Wu, Lili Jiang, Conglai Long, and Zhuangjun Fan* Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China *

Address correspondence to [email protected]

Abstract An easy, one-step carbonization of alkali-treated wheat flour is proposed for the synthesis of three-dimensional (3-D) interconnected honeycomb-like porous carbon foam (HPC) with an excellent performance as supercapacitor electrodes. Due to its interconnected porous structure with narrow pore size distribution, high specific surface area (1313 m2 g-1), and heteroatom doping (N: 1.1 at.%, O:11.2 at.%), the HPC electrode exhibits high specific capacitance of 473 F g-1 at 0.5 A g-1 in 6 M KOH and outstanding electrochemical stability with capacitance retention up to 94.5% after 10,000 cycles. More interestingly, the assembled HPC//HPC symmetric supercapacitor delivers an ultrahigh energy density of 29.3 Wh kg−1 (based on the total mass of the active materials of the two electrodes), much higher than most of carbon-based supercapacitors. Additionally, the HPC//MnO2/ HPC asymmetric supercapacitor exhibits high energy density of 63.5 Wh kg-1 (based on the total mass of the active materials of the two electrodes) and excellent cycle stability (93.4% of initial capacitance retention after 5000 cycles).

Keywords: honeycomb-like porous carbon, high energy density, excellent cycle stability, supercapacitors 1

Introduction Supercapacitors have attracted considerable attention in recent years due to superior power density, fast charge/discharge rates and excellent cycling stability compared to other chemical energy storage devices [1-3]. However, supercapacitors suffer from limited energy density (less than 10 Wh kg−1) compared with other batteries such as lithium ion battery and lead-acid battery [4,5]. To meet the increasing energy demands for next-generation supercapacitors, there is an urgent need to increase the energy density without sacrificing their high power capability. According to the energy density equation, E = 1 CV 2 , C is the total capacitance of the cell and V is the cell voltage. Obviously, 2

the enhancement of energy density (E) can be obtained by increasing the specific capacitance (C) and/or the cell voltage (V). Recently, an effective method to enhance the cell voltage is to assemble asymmetric supercapacitors, such as graphene//Ni(OH)2/graphene [5], activated carbon (AC)//V2O5 [6], and AC//MnO2 [7]. Carbon materials have played a very important role in the design of high energy density supercapacitors due to their large surface area, high electrical conductivity and excellent thermal stability [8-15]. It is well-known that the specific surface area and pore size distribution are the main factor affecting the electrochemical performances of carbon materials, especially for rate performance at high current [10,11]. Commonly, large surface area is beneficial for the enhanced capacitance of carbon materials and the improvement of the energy density of supercapacitors. However, the conductive network is usually broken with increasing surface area, degrading its power performance. The discovery of graphene can effectively resolve the contradiction between surface area and electrical conductivity of carbon. Compared with traditional porous carbon materials, graphene has high electrical conductivity, and large surface area independence of its pore 2

structure but solid surface. However, graphene would spontaneously occur re-stacking and aggregation of nanosheets during both electrode manufacturing and cycling, thus resulting in the decreased practical surface available for charge storage [3,12]. Additionally, optimized pore architecture of porous carbons has shown to be a critical factor for achieving high performance supercapacitors. Recently, three-dimensionally (3D) interconnected porous templated carbons have well controlled narrow pore size distribution, ordered pore structure, large specific surface area and an interconnected pore network [16-20], resulting in high-rate performance supercapacitors. The interconnected pore wall provides 3D continuous electron pathway, and the interconnected ordered pores allow for the electrolyte to penetrate and evenly contact the electrode material. It is worth noting that the presence of micropores can enhance the charge storage [2,21]. However, the surface area of templated carbons by nanocasting is less than 1000 m2 g-1, in order to increase the surface area, it is essential for further activation techniques, such as oxidation in water vapor, KOH, or CO2 [10]. Therefore, it still remains a big challenge for the synthesis of 3D interconnected porous carbons with narrow pore size distribution, interconnected pore structure, and large surface area by using an effective, simple, and convenient process. Herein, we developed a novel strategy to synthesize interconnected honeycomb-like porous carbon foam (HPC) with narrow pore size distribution, high specific surface area, and heteroatom doping by one-step carbonization of alkali-treated wheat flour. The mechanism of formation of 3D porous structure may be the reason that KOH can act both as a template for the formation of macropores and as the activating agent for the formation of micropores and mesopores. As a result, the as-obtained carbon processes 3-D interconnected porous structure with high surface area, narrow pore size distribution and interconnected pore network, which are beneficial for the 3

ion/electron transport and charge storage in the electrode, resulting in high specific capacitance, excellent rate performance and cycling stability. Moreover, asymmetric supercapacitor based on HPC as a negative electrode material and HPC/MnO2 as a positive electrode material (as shown in Scheme 1) delivers high energy density of 63.5 Wh kg-1 (based on the total mass of the active materials of the two electrodes), and excellent cycle stability with 93.4% of initial capacitance retained after 5000 cycles. Additionally, the assembled symmetrical supercapacitor based on HPC delivers an ultrahigh energy density of 29.3 Wh kg−1 (based on the total mass of the active materials of the two electrodes), higher than those for previously reported carbon-based symmetrical supercapacitors in aqueous electrolyte [9,22,23].

Material and methods Synthesis of honeycomb-like porous carbon foam (HPC) HPC was prepared through a facile one-step pyrolysis of the mixture of wheat flour precursor and KOH. Typically, wheat flour and KOH were firstly mixed with a mass ratio of 1:1 in 75 mL stilled water under vigorous agitation, and then dried at 80 ºC for 24 h. The resulting product was heated in a tubular furnace under nitrogen atmosphere at 700 °C for 2 h with a heating rate of 3 °C min-1. Then, the product was washed with dilute HCl solution and distilled water, and dried at 80 °C for 12 h in a vacuum oven. For comparison, the different mass ratio of wheat flour and KOH (2:1; 2:3) were also investigated under the above process. In addition, wheat flour was heated at 700 ºC for 2 h under N2 flow with a heating rate of 3 °C min-1, and the as-obtained carbon was named as CF. Then CF and KOH with a mass ratio of 1:2 were thoroughly ground in an agate mortar, heated at 700 ºC for 2 h under N2 flow with a heating rate of 3 °C min-1, and the as-obtained carbon was denoted as ACF. All carbonized samples were thoroughly washed with dilute HCl and distilled water, and dried at 80 ºC for 12 h. 4

Synthesis of positive electrode material (HPC/MnO2) for Asymmetric Supercapacitor 100 mg of HPC was added into (1.35 mmol, 100 mL) KMnO4 solution and stirred for 2 h. Then the mixture was heated using a household microwave oven (Galanz, 2450 MHz, 700 W) for 8 min, and cooled to room temperature naturally. Then, the product was filtered, washed several times with distilled water, and dried at 80 °C for 12 h in a vacuum oven. Materials characterizations The morphology and microstructure of the samples were investigated by a SEM (Hitachi S-4800) and a TEM recorded on a Tecnai G2 operating at 200 kV. The crystallographic structure of the materials was determined by X-ray diffraction (XRD)equipped with Cu Kα radiation (λ= 0.15406 nm). X-ray photoelectron spectroscopy (XPS) was performed using a PHI5700ESCA spectrometer with Al Kα radiation (1486.6 eV); all of the data acquisition and processing were done on XPSPEAK software. Raman spectra were collected on a JY HR-800 Raman spectrometer (JobinYvon, France) with laser wavelength of 458 nm. N2 adsorption/desorption measurements were characterized by N2 adsorption at 77 K on an ASAP 2020 (Micrometritics, USA). The specific surface area was calculated by the modified Brunauer-Emmet-Teller (BET) method and the pore size distributions were analyzed from the adsorption branch isotherms by density functional theory (DFT) method.

Electrochemical measurements Electrodes were prepared by mixing electroactive material, carbon black and poly (tetrafluoroethylene) in a mass ratio of 75:20:5 to obtain slurry. Then the slurry was pressed onto the nickel foam current collector (1 cm × 1 cm) and dried at 80 °C for 12 h. The mass loading of the electrode materials was ~3 mg cm−2. The electrochemical tests of the individual electrode were 5

carried out using a three-electrode cell in 6 M KOH aqueous solution. Ni foam coated with electroactive materials was used as the working electrode; platinum foil and Hg/HgO electrode were used as the counter and reference electrodes, respectively. All of the above electrochemical measurements were carried out by a CHI 660C electrochemical workstation. The asymmetric supercapacitor was built with a glassy fibrous separator and performed in a two-electrode cell in 1.0 M Na2SO4 aqueous solution. The loading mass ratio of positive electrode material and negative electrode material were estimated from the equation as followed:

m+ C− ×V− = m− C+ ×V+

(1)

which C is the specific capacitance (F g−1), V is the potential range for the charge/discharge process (V), and m is the mass of the electrode (g). The gravimetric specific capacitance (C) was calculated using the following equations:

C=

C=

I∆t m∆V

(2)

∫ IdV

(3)

νmV

Where I is the current density, ∆V is the potential change within the discharge time ∆t, V is potential, ν is the potential scan rate, and m is the mass of the electroactive materials. The key parameters of the supercapacitors, power density (P) were calculated using equation：

P=

E t

(4)

where t is the discharge time (s) and E is energy density (Wh kg-1).

6

Results and discussion Approximately 700 million tons of wheat are now cultivated worldwide, making it the third most-produced grain after maize (Figure 1a). Wheat flour mainly consisting of starch (72-80%) and protein (8-10%), was dispersed in distilled water to form a suspension under vigorous stirring. When KOH was added into above suspension, alkali can promote the gelatinization process, resulting in granular swelling, loss of molecular orders and crystallinity, and volume expansion (Figure S1). During the carbonization, KOH acted as both template and activating agent for the formation of porous structure of carbon materials.

Gelatinization

Sol-gel

-

OH

Wheat flour

Pyrolyzation

KOH

Positive electrode Separator Negative electrode Collector

Swelling wheat flour

KMnO4 Microwave Irradiation HPC/MnO2

HPC

Scheme 1. Schematic illustration of the preparation of the asymmetric supercapacitor.

7

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2 µm

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(g)

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Figure 1. (a) Photograph of wheat field, the inset is wheat flour. (b) SEM image of HPC, the inset is the photograph of a honeycomb. (c,d) High resolution SEM images of HPC. (e) SEM image of HPC, and corresponding elemental mapping images of C (f), N (g) and O (h). Scanning electron microscopy (SEM) images show that HPC exhibits honeycomb-like structure (Figure 1b-c), which is favorable for the rapid ion diffusion and electron transfer as the electrode material for supercapacitor. High resolution SEM image further confirms its 3D highly interconnected porous structure (Figure 1d), and carbon wall has a thickness of <120 nm (inset in 8

Figure 1d), the corresponding element mapping images demonstrate the uniform distribution of C (Figure 1f), N (Figure 1g) and O (Figure 1h). Furthermore, transmission electron microscopy (TEM) image of HPC shows more micropores on the surface of carbon sheet (Figure S2a), which is beneficial to the energy storage for supercapacitors. With increasing ratio of KOH to wheat flour, the thickness of carbon wall for as-prepared samples becomes thinner until fragmentation (Figure S2d-f). By contrast, the carbonized wheat flour (CF) and KOH-activated CF (ACF, mass ratio of CF:KOH=1:2), display rather dense structure without honeycomb-like structure (Fig. S2b,c). It is worth mentioning that KOH can act as both template and activating agent for the formation of the honeycomb-like structure and micropores in carbon materials. Moreover, the macropores and mesopores in carbons also originate from the phase separation between hydrophobic carbon and water during the pyrolysis [17].

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Figure 2. (a) XRD patterns of CF, ACF and HPC. (b) Raman spectrum of HPC. (c) XPS survey spectrum of HPC. (d) N2 adsorption/desorption isotherms of CF, ACF and HPC, the inset shows the pore size distribution of HPC. X-ray diffraction (XRD) analysis of HPC exhibits broader and weaker intensity of the (002) peak compared with CF, ACF (Figure 2a), revealing a decreased degree of graphitization and crystallinity due to the existence in more pores and defects in carbon materials. Furthermore, the disordered structural feature of HPC is also investigated by Raman spectroscopy analysis (Figure 2b). The D peak at 1340 cm-1and G peak at 1589 cm-1 are corresponding to the disordered structures of carbon and graphite in-plane vibrations [9], respectively. High ID/IG ratio of HPC (1.16) means the existence of highly disordered carbon, which is in good agreement with XRD analysis. X-ray photoelectron spectroscopic (XPS) survey spectrum of HPC is shown in Figure 2c. The O and N content of HPC are calculated to be 11.2 and 1.1 at.%, respectively, which can contribute pseudocapacitance to the overall capacitance during the charge/discharge process. In addition, three different peaks centered at 284.5, 286.4 and 289.4 eV in the C1s XPS spectrum of HPC (Fig. S3a), are related with the C=C, C-O and C=O [24], respectively. To investigate the pore structure characteristics of as-obtained carbon materials, N2 adsorption/desorption isotherms of HPC are shown in Figure 2d. The isotherms of HPC exhibit the combined characteristics of type I and IV isotherms, in which an obvious steep increase in adsorption amount at very low relative pressure can be found due to the existence of micropores [14]. By contrast, ACF exhibits typical type I adsorption-desorption isotherms just like traditional activated carbon. Therefore, HPC exhibits the Brunauer-Emmett-Teller (BET) specific surface area of 1313 m2 g-1, comparable to ACF (1393 m2 g-1) but much higher than CF (less than 5 m2 g-1). Moreover, HPC with the mass ratio (precursor/KOH) of 1:1 exhibits the highest surface area among 10

other samples (2:1 for 1162 m2 g-1 and 2:3 for 1037 m2 g-1). Furthermore, it is worth noting that the size of micropores in the carbon wall of HPC based on the pore size distribution (inset in Figure 2d) is in agreement with TEM observation.

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Figure 3. (a) CV curves of CF, ACF, HPC at the scan rates of 20 mV s−1. (b) CV curves of HPC at different scan rates of 20, 50, 100 and 200 mV s−1 in 6.0M KOH aqueous solution. (c) specific capacitances of the CF, AF, and HPC at different current densities. (d) Cycling performance of HPC at a scan rate of 200 mV s-1. The inset shows the CV curves of the 1st and 10000th cycles. Due to its high surface area, 3D interconnected porous structure and heteroatom doping, HPC is expected to be as an excellent candidate electrode material for supercapacitors. Cyclic voltammetry (CV) was firstly used to test the electrochemical performance of as-obtained carbon materials using a three-electrode system in 6 M KOH aqueous solution. Notably, CV curve of the HPC electrode 11

exhibits a relatively rectangular shape compared with the ACF and CF electrodes (Figure 3a) even at high scan rate of 200 mV s-1 (Figure 3b), indicating excellent capacitive behaviors. Moreover, galvanostatic charge-discharge curves of HPC (Figure S4a) exhibit highly linear and symmetrical, suggesting excellent electrochemical reversibility and columbic efficiency. Additionally, there is no obvious IR drop even at a high current of 10 A g−1, meaning a low internal series resistance. The specific capacitance of HPC calculated from the galvanostatic discharge curves is 473 F g−1 at 0.5 A g−1 (Figure 3c), which is much higher than those of CF (134 F g−1) and ACF (374 F g−1). Even at 20 A g-1, the HPC electrode still delivers a specific capacitance of 275 F g-1. Addtionally, the HPC shows higher specific capacitance than other HPC samples with different ratios of precursor to KOH (Figure S4b), and other previously reported porous carbon materials (Table S2). The high capacitance of HPC is attributed to the reasons as follows: (1) The micro-pore sizes for HPC are mainly concentrated in lower than 1 nm, which is beneficial for the effective utilization of surface area [2,21]. (2) Based on the redox reactions as follows, highly oxygenated functionalities in carbon network (11.2 at.%) can provide high pseudocapacitance [10,17].

＞C − OH ⇔ ＞C = O + H

+

(5)

+ e−

− COOH ⇔ −COO + H + + e −

(6)

＞C = O + e ⇔ ＞C − O

(7)

−

−

(3) Nitrogen doping into carbon can improve the conductivity and surface wettability of the electrode [2]. Cycle stability of electrode materials is a critical requirement for practical applications. The cycle stability of the HPC electrode was investigated in 6 M KOH aqueous solution at 200 V s−1. After 10,000 cycles, it still retains 94.5% of the initial capacitance, and no obvious changes for CV curves are observed before and after 10000 cycles (inset in Figure 3d), demonstrating excellent 12

cycling stability. The excellent cycling stability of HPC can be ascribed to its 3D interconnected porous structure for fast ion diffusion, and high surface area and short ion diffusion length can

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Figure 4. (a) CV curves of the HPC//HPC symmetrical supercapacitor in different operation voltages at the scan rate of 50 mV s−1. (b) CV curves of the HPC//HPC symmetrical supercapacitor at different scan rates in the voltage window of 0-1.8 V. (c) Specific capacitance of the as-assembled HPC symmetrical supercapacitors based on based on the total mass of the active materials of the two electrodes at different scan rates. (d) Ragone plots of the HPC symmetrical supercapacitor and other previously reported carbon-based symmetric supercapacitors. To further demonstrate the excellent electrochemical performances of the HPC electrode, 13

two-electrode symmetric supercapacitor was fabricated and tested in 1 M Na2SO4 aqueous solution. Figure 4a displays the CV curves of HPC//HPC symmetric supercapacitor operated in different voltage windows at 50 mV s-1. It can be seen that there is no obvious increase of anodic current even at 1.8 V, indicating that such supercapacitor can be reversibly cycled within the voltage window of 0-1.8 V. CV curves of the HPC//HPC symmetric supercapacitor at different scan rates are shown in Fig. 4b. It is worth mentioning that CV curves still retain a relatively rectangular shape without obvious distortion with an increase in the scan rate, indicating good electrochemical capacitive behavior. The specific capacitances of symmetric supercapacitor was also calculated as shown in Figure 4c. The capacitance of HPC supercapacitor is 65 F g-1 at 2 mV s-1, higher than those of previously reported carbon-based symmetrical supercapacitors [9,22,23]. The energy and power densities of examples were calculated for the symmetrical supercapacitor is shown in Figure 4d. Benefitting from its high specific capacitance (65 F g−1) and a wide operating voltage (1.8 V), the HPC//HPC symmetric supercapacitor shows an energy density of 29.3 Wh kg-1, higher than most of previously reported carbon-based symmetric supercapacitors in aqueous electrolytes (Table S2), such as hierarchical porous carbon [9], seaweads drived carbon [22], mesoporous carbon [23], microporous carbon [24]. In addition, the HPC//HPC symmetric supercapacitor shows 90.6% of the initial capacitance retention after 10000 cycles, demonstrating excellent cycle stability. Recently, asymmetric supercapacitors (ASCs) have aroused extensive attention due to they can combination of different potential windows of the positive/negative electrodes to provide an optimized operation voltage in the cell system, resulting in a greatly enhanced energy density [5,7]. Meanwhile, rational design and match the positive/negative electrode materials is a key factor to maximize the electrochemical performance of the devices. 14

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Figure 5. (a) SEM image of HPC/MnO2. (b, c) High resolution SEM images of HPC/MnO2. (d) TEM image of HPC/MnO2. (e) XRD patterns of HPC/MnO2 composite. As the positive material, MnO2 has attracted a mass of attention due to its high capacitance, good rate performance, and low cost [7]. It is well known that carbon substrate can serve as the sacrificial reductant and thereby converts aqueous permanganate (MnO4-) to insoluble MnO2 deposits on the 15

carbon substrate, based on the redox reaction between KMnO4 and carbon in neutral solution [25]:

→

4MnO4- + 3C + H2O 4MnO2 + 3CO32-+ 2HCO32-

（ 8）

During the reaction, the reduced MnO2 can be closely anchored on the surface of carbon frameworks (Figure 5a), ensuring fast electron transport on the interfacial contact between MnO2 and carbon, which is beneficial for the enhanced electrochemical utilization of MnO2 [26]. High resolution SEM images confirms that the morphology of MnO2 shows nanowire (NW)-like structure, and the NWs are homogeneously distributed throughout the carbon wall (Figure 5b, c). Moreover, high resolution TEM image exhibits that MnO2 NWs have the lengths of 100-300 nm (Figure 5d) and the interlayer spacing of MnO2 crystals is about 0.7 nm (Figure S6a). The elemental mappings demonstrate the uniform distribution of C atom, Mn atom, and O atom (Figure S6c, d, e). Additionally, X-ray diffraction (XRD) patterns confirm the present of MnO2 with the characteristic peaks, which can be indexed to birnessite-type MnO2 (JCPDS 42-1317) as shown in Figure 5e. From XRD analysis, the d-spacing of MnO2 is about 7.0 Å, which is consistent with the high resolution TEM image. The electrochemical performances of HPC/MnO2 composite were tested in a three-electrode cell in 1.0 M Na2SO4 aqueous solution. The CV curves of the HPC/MnO2 composite at different scan rates from 20 to 200 mV s−1 are shown in Figure S6f. Notably, CV curve still maintains a nearly rectangular shape even at 200 mV s−1, indicating an excellent electrochemical response and reversibility. Furthermore, the HPC/MnO2 electrode exhibits a high capacitance of 259 F g-1 at 2 mV s-1 and excellent rate capability (68% retention at 200 mV s-1, Fig. S6g). Therefore, the excellent electrochemical performances of the HPC/MnO2 are attributed to the two reasons as follows: (1) MnO2 NWs grown on 3D interconnected porous carbon architecture can greatly shorten ion diffusion path, and reduce the ionic diffusion resistance and charge transfer resistance, resulting 16

in high electrochemical utilization of MnO2; (2) the excellent interfacial contact between MnO2 and carbon wall is great propitious to fast electron transfer throughout the whole electrode matrix.

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Power density (W kg )

Figure 6. (a) Comparative CV curves of HPC and HPC/MnO2 electrodes tested in a three-electrode cell in 1.0 M Na2SO4 aqueous solution at 10 mV s-1. (b) CV curves of an optimized HPC//MnO2/HPC ASC tested in different potential windows in 1.0 M Na2SO4 aqueous solution at 50 mV s-1. (c) Ragone plots of HPC//HPC/MnO2 (■) compared with HPC-based ASC (□) and other MnO2-based ASCs previously reported in the literatures, such as graphene//MnO2/graphene (◊) [31], V2O5/CNT//MnO2/mesoporous carbon (○) [38], and activated carbon//NaMnO2 (►) [39]. (d) Cycle performance of the HPC//MnO2/HPC ASC with a voltage of 2.0 V at 200 mV s-1 in 1.0 M Na2SO4 aqueous solution (the inset shows CV curves of the 1st and the 5000th cycle at 200 mV s-1). 17

An ASC was fabricated using HPC/MnO2 and HPC as positive and negative electrodes, respectively. It can be seen that the stable potential window is between -1.1 and -0.1 V for HPC and between -0.1 and 0.9 V for HPC/MnO2 (Figure 6a). In consequence, the cell voltage can be extended up to 2.0 V in 1 M Na2SO4 for the ASC of HPC//MnO2/HPC. Fig. 6b shows the CV curves of HPC//HPC/MnO2 ASC at a scan rate of 50 mV s-1 in various voltage ranges. Obviously, the CV curves still retain a rectangular-like shape even when the voltage is applied to 2.0 V, and no obvious distortion is observed even at higher scan rate of 200 mV s−1 (Fig. S7a). Therefore, the wide potential range of 2.0 V is chosen to investigate the electrochemical performance of the ASC. The specific capacitance of HPC//MnO2/HPC calculated from the CV curves is 114.3 F g-1 at 2 mV s-1, which is higher than HPC//HPC symmetric supercapacitor (Fig. S7b). Additionally, the electrochemical cyclability of HPC//MnO2/HPC was evaluated in the voltage window from 0 to 2V in 1.0 M Na2SO4 aqueous electrolyte at 200 mV s-1 for 5000 cycles (Figure 6c). Notably, the HPC//MnO2/HPC displays excellent electrochemical cyclability with 93.4% of the initial capacitance retention after 5000 cycles, which is comparable to those of other ASCs, such as, activated carbon (AC)//MnO2 (90% after 1500 cycles) [27], AC//MnO2 (93% after 100 cycles) [28], graphene hydrogel//MnO2 (83.4% after 5000 cycles) [29], FeOOH//MnO2 (85% after 2000 cycles) [30], graphene//MnO2/graphene (79% after 1000 cycles) [31], AC//Li2MnSiO4 (85% after 1000 cycles) [32], AC//RuO2/TiO2 (90% retention after 1000 cycles) [33], AC//Ni(OH)2 (82% after 1000 cycles) [34], reduced graphene oxide//Ni(OH)2/MnO2 (76% after 3000 cycles) [35] and carbon//hierarchical porous nickel oxide (50% after 1000 cycles) [36]. Ragone plots of the assembled ASCs were shown in Figure 6d. It is worth noting that the maximum energy density obtained for HPC//MnO2/HPC based on the total mass of the active materials of the two electrodes, is 63.5 Wh kg-1, much higher than those of MnO2-based ASCs in 18

aqueous electrolyte solution, such as FeOOH//MnO2 (12 Wh kg−1) [30], graphene//MnO2/graphene (30.4Wh kg−1) [31], AC//MnO2 (11.7Wh kg−1) [37], V2O5/CNT//MnO2/mesoporous carbon (16Wh kg−1) [38], AC//NaMnO2 (19.5 Wh kg−1) [39], Fe3O4//MnO2 (8.1 Wh kg−1) [40], and polyaniline// MnO2 (5.86 Wh kg-1) [41], The above excellent results exhibit that honeycomb-like carbon foam is a very promising electrode material for supercapacitor. The superior electrochemical performances of the HPC/MnO2 composite may be attributed to the reasons as follows: (1) Interconnected honeycomb-like pores can serve as continuous pathway or ion-buffering reservoirs for fast kinetic process of electrolyte ion transport; (2) A thin carbon wall with high surface area and plenty of micropores can provide abundant electroactive sites for the formation of electrical double layers; (3) Highly oxygenated functionalities can enhance the surface wettability and also provide high pseudocapacitance; (4) Carbon frameworks act as not only a highly conductive micro- current collector, but also support for the deposition of MnO2, and the excellent interfacial contact between MnO2 and carbon is beneficial for fast electron transport throughout the whole electrode.

Conclusions In summary, 3D interconnected honeycomb-like porous carbon foam were synthesized by one-step carbonization using wheat flour as carbon precursor. The obtained porous carbon has 3D highly interconnected pores and high specific surface, and in addition, carbon wall has a thickness of <120 nm, heteroatom doping and plenty of micropores. These unique structural properties allow short diffusion paths and consequently the rapid transport of ions throughout the carbonaceous matrix, as well as rapid electron transport, resulting in an excellent electrochemical performance. More importantly, the as-assembled symmetric and asymmetric supercapacitors based on such 19

interconnected honeycomb-like porous carbon deliver high energy densities of 29.3 and 63.5Wh kg−1, respectively, as well as excellent cycling performance. These promising results reveal the development of a one-step, facile, cost-effective, and scalable synthesis strategy for the production of carbon materials that are promising electrode materials for high-performance supercapacitors.

Acknowledgment The authors acknowledge financial support from Harbin Innovation Talents of Science and Technology Research Special Fund Project (2012RFXXG005), Fundamental Research Funds for the Central Universities, Natural Science Foundation of Heilongjiang Province (E201416), and Excellent Youth Foundation of Heilongjiang Province of China (JC201210).

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Highlights


three-dimensional interconnected honeycomb-like porous carbon foam (HPC) with narrow pore size

distribution, high specific surface area, and heteroatom doping were prepared .




The as-obtained carbon shows high gravimetric capacitances.




The as-assembled symmetric supercapacitor delivers high energy density.




asymmetric supercapacitor based on HPC as a negative electrode material and HPC/MnO2 as a positive electrode material delivers high energy density and excellent cycle stability,

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Graphical Abstract Keywords: honeycomb-like porous carbon, high energy density, excellent cycle stability supercapacitors X.L Wu, L.L Jiang, C.L Long, and Z.J Fan*

From Flour to Honeycomb-Like Carbon Foam: Carbon Makes Room for High

-1

Energy density (Wh kg )

Energy Density Supercapacitors

10

HPC Hierarchical porous carbon Seaweeds-drived carbon Mesoporous carbon Microporous carbon 1 100

2 µm

1000

-1

Power density (W kg )

10000

Interconnected honeycomb-like porous carbon foam (HPC) is prepared by a one-step carbonization of alkali-treated wheat flour. As a result, the as-obtained carbon processes 3-D interconnected porous structure with high specific surface area, narrow pore size distribution and interconnected pore network, resulting in high specific capacitance, excellent rate performance and cycling stability..

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