Soybean Root-Derived Hierarchical Porous Carbon as Electrode

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Soybean Root-Derived Hierarchical Porous Carbon as Electrode Material for High-Performance Supercapacitors in Ionic Liquids Nannan Guo,† Min Li,† Yong Wang,†,‡ Xingkai Sun,† Feng Wang,*,† and Ru Yang*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ North Institute for Scientific & Technical Information, Beijing 100089, People’s Republic of China S Supporting Information *

ABSTRACT: Soybeans are extensively cultivated worldwide as human food. However, large quantities of soybean roots (SRs), which possess an abundant three-dimensional (3D) structure, remain unused and produce enormous pressure on the environment. Here, 3D hierarchical porous carbon was prepared by the facile carbonization of SRs followed by chemical activation. The as-prepared material, possessing large specific surface area (2143 m 2 g −1 ), good electrical conductivity, and unique 3D hierarchical porosity, shows outstanding electrochemical performance as an electrode material for supercapacitors, such as a high capacitance (276 F g−1 at 0.5 A g−1), superior cycle stability (98% capacitance retention after 10,000 cycles at 5 A g−1), and good rate capability in a symmetric two-electrode supercapacitor in 6 M KOH. Furthermore, the maximum energy density of as-assembled symmetric supercapacitor can reach 100.5 Wh kg−1 in neat EMIM BF4. Moreover, a value of 40.7 Wh kg−1 is maintained at ultrahigh power density (63000 W kg−1). These results show that the as-assembled supercapacitor can simultaneously deliver superior energy and power density. KEYWORDS: soybean root, hierarchical porous carbon, energy storage, supercapacitor, ionic liquid ties.8−10 Thus, some ionic liquids with imidazolium cations (EMIM+) have been recently applied in high-energy supercapacitors. Wang et al. reported that the operation voltages in EMIM BF4 and BMIM PF6 reached 4 V.11 Zhang et al. assembled a two-electrode supercapacitor by incorporating porous carbon electrodes and an EMIM BF4 electrolyte, in which the operating voltage reached 3.5 V.12 Tian et al. prepared bioinspired beehive-like hierarchical porous carbon and found that the operating voltage in EMIM TFSI electrolyte also reached 3.5 V.13 The high operating voltage resulted in a remarkable improvement in the power density and energy density. As we all know, for a given electrolyte, the capacitance of a supercapacitor closely depends on the intrinsic properties of the electrode materials, such as good electronic conductivity, high specific surface area, hierarchical porous structure, and chemical functionality.14,15 Therefore, many researchers are dedicated to developing new electrode material for high-performance supercapacitors. Carbon materials, such as porous carbons (PCs), carbon spheres, graphene, and carbon nanotubes, are regarded as ideal materials for practical supercapacitors because

1. INTRODUCTION Supercapacitors, possessing superior power density, long lifetime, and short charging time, have attracted tremendous attention as the best potential device for electrical energy storage.1,2 However, the intrinsic low energy density compared with batteries has greatly limited the practical applications of supercapacitors in energy-efficient industrial equipment.3,4 Therefore, enhancing their energy densities without sacrificing their excellent rate capabilities is a pressing need.5 Considerable effort has been directed toward either or both the capacitance and operating voltage according to E = CV2/2. The operating voltage of a supercapacitor mainly depends on the electrochemically stable voltage of the electrolytes. Conventional aqueous electrolytes significantly limit the widespread application of supercapacitors because of their low operating voltages (1−1.8 V), which are attributed to the low decomposition potential of water. Although the operating voltage of organic electrolytes can reach the range of 2.5−2.8 V, they also have many critical and inevitable problems, such as higher costs, lower conductivities, flammability, toxicity, and environmental impact.6,7 Recently, ionic liquids have been recognized as a promising candidate to replace conventional electrolytes of supercapacitors owing to their higher operation voltage (>3 V), negligible volatility, nonflammability, excellent electrochemical/ thermal stability, and tunable physical and chemical proper© XXXX American Chemical Society

Received: September 5, 2016 Accepted: November 15, 2016 Published: November 15, 2016 A

DOI: 10.1021/acsami.6b11162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

stabilities in 6 M KOH. To improve the operating voltage, we also assemble symmetric two-electrode supercapacitors working in a neat ionic liquid (EMIM BF4). This device simultaneously shows excellent energy and power density in ionic liquid electrolyte.

of their excellent thermal stabilities, large surface areas, high electrical conductivities, and good cycle stabilities.16−18 Although many recent papers have shown that graphene and carbon nanotubes have great potential for future applications, PCs are still the most promising candidates for supercapacitor electrode materials due to their easy preparation and low cost.19,20 The energy storage of a supercapacitor generally relies on the accumulation and release of electrolyte ions at the interface between the electrolyte and electrode. Thus, a large surface area is necessary to obtain high specific capacitance. However, PCs possessing large surface areas usually display microporous characteristics with narrow and tortuous micropore distributions ranging from 0.5 to 1.1 nm. This distribution leads to poor rate performance because high ion transport resistance results from the inadequate ionic diffusion within the narrow micropores, which greatly hinders their practical applications in high power density supercapacitors.21,22 Thus, considerable effort has been devoted to fabricating three-dimensional (3D) hierarchical PCs possessing abundant micropores, interconnected mesopores, and appropriate macropores. Macropores can play the role of ion-buffering reservoirs to store more electrolyte ions, and mesopores serve as channels that decrease the ion diffusion distance to the inner micropores, whereas micropores can furnish rich sites as the locations for charge accumulation.23 Hence, a wide range of synthetic routes, such as using silica oxides and metallic compounds as templates, have been developed for obtaining novel 3D hierarchical PCs.24,25 Due to the consumption of expensive templates, energy, and time, it is still a primary challenge to prepare 3D hierarchical PCs by a simple and low-cost method. In addition, considering the excessive consumption of fossil fuels and the demand for sustainable eco-friendly resources and processes, increasing attention has been focused on utilizing biomass waste to synthesize 3D hierarchical PCs due to the abundance, low cost, and unique porous structure of biomass precursors.13,23,26 Although these PCs show excellent capacitances and rate performances, their low energy densities limit their practical applications. Thus, developing a more suitable precursor for high-performance 3D hierarchical PCs is still an impending need for high-performance supercapcitors. Soybeans are extensively cultivated all over the world as human food because of their abundant nutritional value and low cost. Recently, soybean- and soybean shell-derived carbon exhibited promising performance for supercapacitors.27,28 However, tons of soybean roots (SRs), that is, the byproduct of soybeans, are directly incinerated or abandoned, which leads to a large amount of waste and results in enormous pressure on the environment. In addition, SRs are rich in honeycomb-like epidermal, vascular, and ground tissues. Therefore, transforming SRs into 3D hierarchical PCs is an effective way to utilize this biomass waste. In this paper, 3D hierarchical PCs are synthesized from SRs through a KOH activation method and employed as the electrode materials for high-performance supercapacitors. The ordered natural channels of the precursor provide suitable places for the activating reaction to get high surface area PCs. The resulting soybean root-derived PC shows a high specific surface area (2143 m2 g−1) and hierarchical porous structure with abundant micropores, appropriate mesopores, and limited macropores. Benefiting from the large specific surface area and unique structure, all of the as-prepared samples present high specific capacitances, good rate capabilities, and long-term

2. EXPERIMENTAL SECTION 2.1. Materials. Soybean roots were collected from Henan province of China. Hydrochloric acid (37 wt %, HCl) and potassium hydroxide (85 wt %, KOH) were purchased from Shanghai Chemical Reagents Co., Ltd. (China). Neat EMIM BF4 was purchased from Lanzhou Institute of Chemical Physics. (China). Polytetrafluoroethylene solution was obtained from Aladdin Chemistry Co., Ltd. (China). Nickel foam, steel coin cell, separator, and acetylene black were used as purchased. 2.2. Synthesis and Characterization of Soybean RootDerived Porous Carbons. First, soybean roots were thoroughly washed with ethanol and water several times and dried in an oven at 80 °C. The cleaned soybean roots were first carbonized for 2 h at 500 °C under a nitrogen atmosphere. Then, the obtained char was mixed with KOH in different proportions (ratio KOH/char = 3, 4, and 4.5) in a minimum volume of water, and the mixed solution was stirred until forming a homogeneous slurry. Then, the slurry was activated for 2 h at 800 °C (heating rate, 5 °C/min) under a nitrogen atmosphere in a horizontal tube furnace. The residue was washed with 6 M HCl solution to remove the soluble salts and then thoroughly washed with ultrapure water until the filtrate became neutrality. The obtained sample was dried in a vacuum oven. The resulting char was denoted SRC, and the activated PCs were named SRPC-nK, where n represents the KOH/char weight ratio. To eliminate surface oxygen functional groups, the sample of SRPC-4K was heated for 2 h at 900 °C under a nitrogen atmosphere, and the annealed sample was denoted SRPC-4K900. 2.3. Physical Characterization. The obtained samples were characterized with Raman scattering spectroscopy (Renishaw, InViaReflex, using laser excitation at 514 nm), X-ray diffraction (XRD Siemens D5000), and scanning electron microscopy (SEM, Hitachi S4700). Element analysis was characterized by a Vario EL CUBE in a stream of pure O2, and surface elements were examined by X-ray photoelectron spectroscopy (XPS, Thermo VG ESCALAB 250 spectrometer). The pore structure of all the samples was characterized by nitrogen adsorption−desorption isotherms obtained on a surface area and porosity analyzer (Micromeritics ASAP 2020) at −196 °C. Prior to measurement, the SRPCs and SRC were degassed under vacuum at 300 °C for 12 h. Specific surface area of all the samples was calculated by the Brunauer−Emmett−Teller (BET) model, and the pore size distribution (PSD) was calculated by the density functional theory (DFT) method. 2.4. Electrochemical Characterization. The electrochemical experiments were investigated under a symmetric two-electrode system. Working electrodes were constructed by mixing 80 wt % SRPCs, 10 wt % polytetrafluoroethylene, and 10 wt % acetylene black on nickel foam. The mass of the SRPC loading on the working electrode was approximately 2.0 mg cm−2. The samples were dried for 24 h at 80 °C and pressed for 5 s at 15 MPa.11 The symmetric twoelectrode supercapacitors were constructed in a 2032 stainless steel coin cell using a nonwoven polypropylene mat (MPF 30AC) as the membrane to separate two working electrodes, 6 M KOH, and neat EMIM BF4 as the electrolyte. Before use, EMIM BF4 was dried at 120 °C in a vacuum oven. The gravimetric specific capacitance of a single electrode was deduced from the charge−discharge curves using the formula

C = 2I × Δt /(m × ΔV )

(1)

where I represents the discharge current (A), Δt refers to the discharge time (s), and m is the mass of active material loaded on a single working electrode (g). ΔV corresponds to the voltage change excluding the ohmic drop within Δt (V). The energy density E (Wh B


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Figure 1. (a) Scheme of the scalable preparation of SRPCs; (b) SEM image of the SRC carbonized at 500 °C; (c, d, e) SEM images of SRPC-4K.

Figure 2. (a) N2 physisorption isotherms; (b) PSD curves of SRC and SRPCs; (c) Raman spectra; (d) high-resolution O 1s XPS spectra of SRPCs. kg−1) and power density P (W kg−1) based on the electrode were obtained from the following formulas:

E = C × ΔV 2/(2 × 4 × 3.6)

(2)

P = 3600E /Δt

(3)

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. Approximately 110 million tons of soybeans have been cultivated in China for human food.28 However, a large amount of SRs are directly incinerated or abandoned, which results in a huge pressure on C


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ACS Applied Materials & Interfaces Table 1. Yield and Textual Parameters of the SRC and SRPCs sample

yield (wt %)

SBETa (m2/g)

Vtotalb (cm3/g)

Smicroc (m2/g)

Smesod (m2/g)

Vmicroe (cm3/g)

Vmesof (cm3/g)

SRC SRPC-3K SRPC-4K SRPC-4.5K

27.4 62.5 55.7 54

36.5 1708 2143 1937

0.01 0.62 0.94 0.72

10.9 1151 1772 1261

0 26 125 77

0.01 0.6 0.81 0.64

0 0.01 0.13 0.07

a

Specific surface area. bTotal pore volume obtained by DFT. cMicropore (micro < 2 nm) surface area calculated by DFT. dMesopore (2 nm < meso < 50 nm) surface area calculated by DFT. eMicropore volume calculated by DFT. fMesopore volume calculated by DFT.

activation process. As seen in Figure 2b, pore diameters ranging from 0.5 to 3 nm dominate the porous structures of all SRPCs. The mesopore volume of SRPC-4K is 0.13 cm3 g−1, which is much higher than those of SRPC-3K (0.01 cm3 g−1) and SRPC4.5K (0.07 cm3 g−1). The hierarchical structure with the cooccurrence of micropores and mesopores in PCs is perfect for supercapacitors because the charge accumulation happens primarily in the micropores, whereas the mesopores can transfer abundant electrolytes to and from the micropores, improving the utilization of the micropores.32 Thus, SRPC-4K, possessing the hierarchical structure with the highest micropore and mesopore volume, is expected to display outstanding electrochemical performance as a supercapacitor electrode material. The XRD patterns of SRPCs are shown in Figure S2a. All of the samples display two typical peaks at approximately 2θ = 25 and 42°, corresponding to diffractions of (002) and (100), respectively, suggesting that all of the SRPCs are amorphous. The Raman spectra are shown in Figure 2c, also reflecting the degree of structural disorder of SRPCs. All samples display two broad peaks around at 1330 cm−1 (D band, disorder and defects) and 1590 cm−1 (G band, graphitic).33 The intensity of the D band is obviously lower than that of the G band, suggesting that all of the SRPC are partially graphitized. Furthermore, the ID/IG ratios of SRPC-3K, SRPC-4K, and SRPC-4.5K are determined to be 0.84, 0.87, and 0.91, respectively, which are significantly lower than that for a commercial activated carbon (Norit, ID/IG = 1.92).34 The KOH activation process causes considerable cavity construction and destroys the graphite crystallite, leading to a developed surface area but disordered structure.34,35 However, the SRPCs exhibit a relatively high degree of graphitization, which is attributed to the inherent structure of the soybean roots. This results in the good conductivities of SRPC-3K (3.1 S cm−1), SRPC-4K (2.8 S cm−1), and SRPC-4.5K (3 S cm−1). 3.2. Chemical Composition. Doping heteroatoms can also improve the electrochemical performance of carbon materials. The carbon, nitrogen, and oxygen contents of SRs evaluated from the elemental analysis are 49.2, 1.8, and 42.5 wt %, respectively. However, the carbon content obviously increases, whereas mainly nitrogen and oxygen are removed after carbonization and activation (Table S1). We next conducted XPS to confirm the surface elemental composition (Figure S2b). As listed in Table S1, the carbon contents are 86.1, 90.5, and 91 at. %, and the oxygen contents are 11.5, 8.4, and 8.3 at. % in SRPC-3K, SRPC-4K, SRPC-4.5K, respectively. The existence of oxygen in the surface of the PC materials is a universal phenomenon. The oxygen functional groups usually come from the activation agents and precursors.36 The O 1s XPS spectra of SRPCs possess three peaks at 531.5, 532.6, and 533.7 eV, seen in Figure 2d, which represent CO quinone groups and carbonyl oxygen of keto (O-I), COC ether-

the environment. Moreover, soybean roots are rich in honeycomb-like epidermal, vascular, and ground tissues, which are beneficial for preparing PCs by thermochemical conversion. Figure 1a illustrates the detailed preparation procedure for SRPCs. In a typical process, the soybean roots were precarbonized under a nitrogen atmosphere, and then the carbonized product was mixed with KOH and activated in a ceramic crucible under nitrogen atmosphere. As seen in Figure 1b, SRC possesses two different type channels. Round or elliptical channels exhibit large pore diameters of 30−50 μm and many macroscopic well-organized pores arranged on the carbon wall, which is a result of the vascular bundles. The polygonal channels possess small pore diameters of 5−10 μm, which are derived from the ground tissues. The polygonal channels continuously distribute around the elliptical channels, forming a honeycomb-like structure, which provides suitable places for the activating reaction to get high surface area PCs during the activation process. After activation and grinding, the polygonal channels are broken along the length of the root and generate elongated blocks that possess interconnected porous networks on the channel walls (Figure 1c). Moreover, the network structure of the round channels is effectively preserved, and sheet-like carbon flakes are obtained after the activation process (Figure 1d). In addition, the cross-linked 3D structure is clearly observed by magnifying the carbon flakes (Figure 1e). The 3D structure has excellent permeability, delivers more usable active sites for the more interfacial accumulation of ions, and shortens the diffusion distances of electrolyte ions into the pores.29 Nitrogen adsorption−desorption isotherms of SRPCs and SRC were performed to investigate the development of porosity by KOH activation, as shown in Figure 2a and Figure S1. SRC exhibits a type II isotherm, indicating an imporous characteristic, whereas all SRPCs show typical type I isotherms with higher N2 quantity absorbed ability, suggesting microporous characteristics. Such microporosity greatly benefits charge accumulation, thus improving the specific capacitance of the electrodes.30 The surface area and different pore volumes of SRC and SRPCs are listed in Table 1. SRC shows undeveloped pore structure with a very low pore volume (0.01 cm3 g−1) and BET surface area (36.5 m2 g−1). The porosity of the resultant SRPCs is evidently developed during the chemical activation and significantly influenced by the KOH/char weight ratio. The specific surface areas of SRPC-3K and SRPC-4K are determined to be 1708 and 2143 m2 g−1, respectively, whereas the pore volumes calculated by the DFT are 0.62 and 0.94 cm3 g−1, respectively. The pore volume and specific surface area of SRPC-4.5K are dramatically lower, that is, 0.72 cm3 g−1 and 1937 m2 g−1, because of the collapse of pores caused by the excessive activation.31 The product yields are 62.5, 55.7, and 54 wt % for SRPC-3K, SRPC-4K, and SRPC-4.5K, respectively. The decrease in the yield is mainly attributed to more KOH consuming more carbon during the D


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Figure 3. (a) CV curves at 5 mV s−1 of SRPC-3K, SRPC-4K, and SRPC-4.5K in a symmetric two-electrode supercapacitor in 6 M KOH aqueous solution; (b) CV curves of SRPC-4K and SRPC-4K-900 at 5 mV s−1; (c) GCD curves at 1 A g−1; (d) GCD rate performance SRPC-3K, SRPC-4K, and SRPC-4.5K; (e) GCD curves at various current densities; (f) cyclic stability at 5 A g−1 for 10,000 cycles of SRPC-4K (inset: GCD curves of the 1st and 10,000th cycles).

These curves also show a two-part deviation from the rectangular shape in Figure 3b, suggesting the pseudocapacitance induced by the abundant surface oxygen functional groups.13,42 The excellent capacitive behavior of SRPC-based supercapacitors is also reflected in their long galvanostatic charge−discharge (GCD) times and symmetrical isosceles triangle-like profiles (Figure 3c). Figure 3d exhibits the specific capacitance of all the SRPCs at various current densities. The specific capacitance of SRPC-4K is 276 F g−1 at 0.5 A g−1, which is better than SRPC-3K (239 F g−1), SRPC-4.5K (245 F g−1), and some carbon-based electrodes in an aqueous electrolyte system (Table S2).13,23,43−46 Compared with these previously reported PCs, the high specific capacitance of SRPC4K results from not only the pseudocapacitance from surface oxygen functional group but also the hierarchical pore structure with high specific surface area and large mesopore volume matching with the aqueous electrolyte ions. Moreover, SRPC4K exhibits high capacitance retention (81.5%) at 20 A g−1, which is much higher than that of SRPC-3K and SRPC-4.5K, because its large mesopore volume (14%) allows electrolyte ions to easily move in and out of the inner micropores despite a lower electrical conductivity. This result can be further demonstrated by GCD curves from 0.5 to 20 A g−1 and the CV curves from 5 to 200 mV s−1 for SRPC-4K (Figure 3e and Figure S4). The cycling performance of SRPC-4K based on a two-electrode cell was also measured by the GCD tests in Figure 3f. The specific capacitance shows 98% retention after 10,000 consecutive cycles, suggesting the highly stable cyclic ability of the SRPC-4K-based electrode. The nearly identical GCD curves of the 1st and 10,000th cycles also indicate the high cycling stability (inset in Figure 3f). After removal of the surface oxygen functional groups by heating at 900 °C, the specific surface area of SRPC-4K-900 shows no obvious change (Figure S5a). However, the specific capacitances are lower than that of SRPC-4K at various current densities due to the evaporation of surface oxygen, which

type oxygen in ester and/or anhydrides (O-II), and the oxygen of carboxylic groups and/or water (O-III), respectively.37 >C−OH ⇔ >C = O + H+ + e−

(A)

−COOH ⇔ COO + H+ + e−

(B)

−

−

>C = O + e ⇔ >C−O

(C)

Faradaic pseudocapacitance of SRPCs may be induced by electrochemical reaction C at the surface of the electrode in aqueous electrolytes.38−40 The surface nitrogen contents of the SRPC are 0.5, 0.3, and 0.2 at. %, decreasing with increasing KOH/char ratio. Nitrogen located at the surface of the carbon materials can also trigger pseudocapacitance, which comes from the reactions in Figure S2d.39 Moreover, the abundant surface oxygen functional groups and small amount of nitrogen functional groups can not only improve the hydrophilia of the carbon material but also increase the electrochemical performance. Moreover, the doping nitrogen can also improve the conductivity of SRPCs.41 3.3. Supercapacitor Performance. The electrochemical properties of SRPCs and SRC are first measured in 6 M KOH aqueous solution in a three-electrode system. Figure S3a shows the cyclic voltammetry (CV) curves of all samples obtained at 1 mV s−1. SRC shows the smallest special shape that is obviously different from the typical rectangular shape of supercapacitors, indicating a low specific capacitance owing to the lowest specific surface area (36.5 m2 g−1). For all SRPCs, the current response is significantly greater than that of SRC due to higher surface area, and SRPC-4K shows the largest current response and the longest discharge time (Figure S3). To fully measure the practical applications of the as-prepared SRPCs, we tested the SRPC-based symmetric two-electrode supercapacitor in 6 M KOH. Figure 3a displays the corresponding CV curves at 5 mV s−1. All of the curves show quasi-rectangular shapes, suggesting that the double-layer capacitance is the main contribution to the total capacitance. E


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Figure 4. (a) CV profiles at 50 mV s−1 and (b) GCD profiles at 1 A g−1 of SRPC-4K symmetrical supercapacitor with different operation voltages in neat EMIM BF4 at room temperature; (c) GCD profiles at 10 A g−1 of SRPC-4K-based supercapacitors tested at room temperature and 60 °C; (d) GCD curves at 10 A g−1; (e) ohmic drop associated with the equivalent internal resistance at various current densities; (f) GCD rate performance for SRPC-based symmetrical supercapacitors in neat EMIM BF4 at 60 °C.

Figure 5. (a) CV profiles at various sweep rates; (b) GCD profiles at various current densities; (c) cycling performance at 10 A g−1 for SRPC-4Kbased devices at 60 °C (inset: digital photograph of LEDs powered by the cell assembled with symmetric two-electrode capacitors of SRPC-4K); (d) Ragone plots of symmetrical supercapacitors with different electrolytes; (e) packaged Ragone plots of SRPC-based devices compared with commercial capacitors and batteries.

according to eq 2. Therefore, we assembled the symmetrical two-electrode supercapacitors in neat EMIM BF4 (an ionic liquid electrolyte). To confirm the operating voltage range of this device, the SRPC-4K based supercapacitor in neat EMIM BF4 was first tested at various voltage ranges from 2.0 to 3.6 V at room temperature, as shown in Figure 4a. Surprisingly, all CV profiles exhibit perfect rectangular shapes even when the operating voltage increases to 3.6 V, suggesting ideal doublelayer capacitive behavior and outstanding reversibility. Figure 4b exhibits the GCD profiles of SRPC-4K in a symmetrical

weakens the redox reactions (Figure S5d and Table S1). The CV curve of SRPC-4K-900 shows a more rectangular shape after the heating treatment (Figure 3b), further suggesting that the capacitance mainly results from the electric double-layer capacitance. Furthermore, SRPC-4K-900 shows better rate performance than SRPC-4K as seen in Figure S5d, due to lower oxygen content (2.9 at. %) and higher conductivity (4 S cm−1). However, the practical applications of supercapacitors in aqueous electrolytes are critically limited by their low operating voltage of 1−1.8 V, which leads to relatively low energy density F


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liquid and organic electrolytes. This result is comparable to that of electrodes based on graphene, carbon nanotubes, or their composite in ionic liquid (Table S2).12,13,34,48−58 More importantly, the energy density of 40.7 Wh kg−1 also remains at 63000 W kg−1. These results suggest that the SRPC-4Kbased symmetric two-electrode supercapacitor can simultaneously deliver ultrahigh power density and high energy density. High energy density and fast charge−discharge performance lay the groundwork for the fabrication of practical energy conversion devices. Figure 5c shows a digital photograph of red LEDs (3 V) lit by a cell assembled with symmetric two-electrode (SRPC-4K) supercapacitors in EMIM BF4 solutions. The SRPC-based supercapacitors have much higher packaged energy density than traditional electrochemical capacitors and come up with commercial lithium-ion and nickel−metal hydride batteries. Furthermore, the power density of these devices can remain at an ultrahigh value, which implies a faster charge or discharge rate than that of commercial lithium-ion nickel−metal hydride batteries (Figure 5e).59 For example, the specific energy based on the device weight can reach 25 Wh kg−1 at 1088 W kg−1 (SRPC-4K), as the active material is approximately 25% of the overall device mass.5 These results show that SRPC-based two-electrode supercapacitors can serve as an energy bridge between the traditional capacitors and batteries. These electrochemical results suggest that SRPC-4K possesses an excellent performance for supercapacitor applications, which is ascribed to the 3D structure with high surface area and hierarchical PSD. To confirm the important role of the 3D structure on the SRs for high surface area and superior electrochemical property of SRPCs, a contrast sample with irregular blocks as its structure, no obvious porous structure, and lower specific surface area was prepared. The corresponding data are shown in Figure S8. The unique 3D structure of the SRs can enhance KOH activation, leading to high specific surface area and excellent supercapacitor performance. In addition, the control sample displays a lower capacitance response and a weaker rate performance compared to SRPC-4K in both ionic liquid and aqueous systems, which can be ascribed to the lower specific surface area without the 3D hierarchical PSD to buffer large amounts of electrolyte ions.

two-electrode system at different operating voltages, and all GCD profiles display an isosceles triangle shape, which further confirms the consequence of CV profiles. Nevertheless, the resistance of these devices is considerably higher than that of supercapacitors in aqueous and organic electrolytes due to the relatively high viscosity and low ionic conductivity of EMIM BF4 at ambient temperature.34 Fortunately, supercapacitors with ionic liquid electrolytes can exhibit better performance at high temperatures (≥60 °C).10 Figure 4c shows the GCD data of SRPC-4K tested in EMIM BF4 at room temperature and 60 °C. The supercapacitor tested at 60 °C exhibits a longer discharge time than that tested at room temperature, which is attributed to the lower viscosity and higher ionic conductivity of EMIM BF4 at 60 °C.34 The result shown in Figure S6 further demonstrates this conclusion. Therefore, the operating voltage of 3.6 V and temperature of 60 °C are chosen for the subsequent electrochemical performance tests. The internal resistance voltage drop plays a very important role in energy delivery due to the high internal resistance, leading to a large energy loss according to eq 2. Figure 4e summarizes the ohmic drop under various current densities of SRPCs. The ohmic drops linearly increase with the increasing current density, and the slope of those lines represents the internal resistance value of the overall cells.47 SRPC-4K exhibits a lower equivalent internal resistance (0.01 Ω g) than SRPC-3K (0.018 Ω g) and SRPC-4.5K (0.0185 Ω g). The low equivalent internal resistance provides a longer discharge time of the GCD curve for SRPC-4K compared to those of SRPC-3K and SRPC4.5K (Figure 4d and Figure S7). Figure 4f shows the specific capacitance versus current density of SRPCs. SRPC-4K also displays the largest specific capacitance (239 F g−1) and the most excellent rate performance in all SRPCs. Surprisingly, even at 100 A g−1, the SRPC-4K-based supercapacitor retains >76% of its capacitance at 5 A g−1. As seen in Figure 5a, the CV profiles of the as-assembled SRPC-4K supercapacitor also show rectangular shapes even at 500 mV s−1, and the GCD profiles (Figure 5b) display perfect linear and symmetrical shapes even at an ultrahigh current density of 100 A g−1, further confirming the superior electrochemical reversibility. The peak at approximately 1.5 V in the CV profiles can also be found in previous works,11,48,49 which may be attributed to the redox hump induced by the surface oxygen functional groups.49 To demonstrate this result, SRPC-4K-900 was also used in a symmetric supercapacitor in neat EMIM BF4 and tested at 60 °C. The CV curve of SRPC4K-900 shows a weak peak at approximately 1.5 V due to the evaporation of the oxygen content (Figure S5). Figure 5c displays an excellent cycling stability of the SRPC-4K-based supercapacitor. After 4000 cycles at 10 A g−1, the supercapacitor also shows 84% retention of its initial capacitance. Compared with the other previously reported PCs in ionic liquid or organic systems, the excellent electrochemical property of SRPC-4K is derived from (1) the large specific surface area, which leads to a significant enhancement of specific capacitance because many electrolyte ions can accumulate at the micropore surface; and (2) the ideal 3D hierarchical PSD in the range from 0.5 to 3 nm, which allows electrolyte ions, that is, EMIM+ and BF4−, to easily diffuse through the pores even at high discharge rates.13,34 Ragone plots based on the active material weight are exhibited in Figure 5d. The maximum energy density of the SRPC-4K-based device is 100.5 Wh kg−1 at 4353 W kg−1, which is higher than that of previously reported PC materials in ionic

4. CONCLUSION In summary, our work demonstrated that soybean root was an excellent precursor to obtain 3D hierarchical PCs by chemical activation for supercapacitors. The high surface area, 3D network-like structure with abundant micropores and some mesopores, and high content of surface oxygen endowed SRPC-4K with outstanding electrochemical properties, such as a high capacitance (276 F g−1 at 0.5 A g−1), excellent cycling stability (only 2% capacitance decay over 10,000 cycles at 5 A g−1), and superior rate capability (224 F g−1 at 20 A g−1) in symmetric two-electrode supercapcitors in 6 M KOH. A superior energy density of 100.5 Wh kg−1 was obtained at a power density of 4353 W kg−1 in neat EMIM BF4. A value of 40.7 Wh kg−1 was maintained even at an ultrahigh power density of 63000 W kg−1. Therefore, the cost-effective synthesis method of SRPCs gives them great potential for applications in supercapacitors. G


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11162. XPS spectra, electrochemical performances of SRPC-3K, SRPC-4K, and SRPC-4K in three- and two-electrode systems, nitrogen sorption isotherms, and electrochemical performances of SRPC-4K-900 and control sample (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(F.W.) E-mail: 64451996. Phone: *(R.Y.) E-mail: 64436736. Phone:

[email protected]. Fax: +86 10 +86 10 64451996. [email protected]. Fax: +86 10 +86 10 64436736.

ORCID

Ru Yang: 0000-0002-2017-2323 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (51372012 and 51432003). REFERENCES

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