One Step Construction of NitrogenâCarbon Derived from

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One Step Construction of Nitrogen-Carbon Derived from Bradyrhizobium Japonicum for Supercapacitor Applications with Soybean Leaf as Separator Qiufang Yao, Hanwei Wang, Chao Wang, Chunde Jin, and Qingfeng Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03777 • Publication Date (Web): 17 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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ACS Sustainable Chemistry & Engineering

A green Supercapacitor with Soybean Leaf as Separator and Nitrogen-Carbon Derived from Bradyrhizobium Japonicum as Electrode via One Step Construction 101x44mm (300 x 300 DPI)

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One Step Construction of Nitrogen-Carbon Derived from Bradyrhizobium Japonicum for Supercapacitor Applications with Soybean Leaf as Separator Qiufang Yao,† Hanwei Wang,† Chao Wang,† Chunde Jin,† Qingfeng Sun †* †

School of Engineering, Zhejiang A&F University, Hangzhou, 311300P. China.

KEYWORDS: hierarchical porous carbon, supercapacitor, separator, bradyrhizobium japonicum, soybean leaf

ABSTRACT Bradyrhizobium japonicum (BJ), which was a symbiotic relationship with soybean roots, possessed an abundant three-dimensional (3D) structure with high N content. Soybean leaf (SL) with a hierarchically ordered macro-porous network and numerous polar hydroxyl groups was proposed as a separator for a supercapacitor. 3D hierarchical porous carbon was prepared by the facile carbonization with chemical activation of BJ. The as-prepared material, possessing large specific surface area (1275 m2·g−1) and unique 3D hierarchical porosity, and good electrical conductivity. The electrochemical performance of the bradyrhizobium japonicum-derived porous carbons at a mass ratio (ZnCl2/BJ = 1.5) (BJPC-1.5) for supercapacitors with a SL separator showed a high capacitance (358 F/g at 1 A·g−1) and superior cycle stability of 91% over 8000


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cycles, and superior rate capability in a symmetric two-electrode supercapacitor in 6 M KOH. Furthermore, electrochemical performance of the BJPC-1.5 with a SL separator was comparable to that with commercialized cellulose and polypropylene (PP) separators. More attractively, the SL separator with preferable water uptake showed a much better performance in the BJPC-1.5 cell than PP separator. These results provide an insight into the full-usage of natural and biodegradable biomass for separator and electrode materials for supercapacitor.

INTRODUCTION Based on electrical double layer (EDL), supercapacitors charge storage is being gradually prevalent on account of their advantages of rapid charge-discharge rate, high-power density, and long cycle life

1-7

. With diverse pore structures, relatively good conductivity, and superior

thermal and chemical stability, carbon materials are widely applied in EDL capacitors

8-14

.

Carbon sources and their synthesis procedures, which determine the chemistry property, pore characteristic, and electrical conductivity of the carbon product, heavily influenced the electrochemical performances 15-17. The carbon was obtained by pyrolysis derived from materials such as kiwifruit 9, coal

18

, polymers

19, 20

, carbon nanotubes

21-23

, and graphene

24,25

. But the

carbon from coal might be full with too many impurities, such as CaO and Al2O3 26. The carbon from polymers such as polyvinyl chloride (PVC) has an extraordinary low specific surface area value for absence of rich pores

27

. With good conductivity, carbon nanotubes and graphene are

often used as supercapacitor electrodes, but it is hardly commercialization for their high cost. Nowadays, more and more researchers are focused on biomass-derived porous carbon as a potential electrode material, such as loofah sponge

28

, kiwifruit9, and cattle bone

29

. These

biomass materials are abundant, clean, and renewable. Furthermore, there are rich in N, O, and S in the biomass materials, which could convert to self-doped heteroatoms after the pyrolysis


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process. These heteroatoms especially O and N can contribute to electrical conductivity of carbon 30. Importantly, more and more attentions have been focused on utilizing biomass waste to synthesize a three dimensional (3D) hierarchical porous carbons (PCs), due to the abundance, low cost, and unique porous structure of biomass precursors

31,32

. These PCs show excellent

capacitances and rate performances. Soybeans are widely 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 33-36. However, tons of bradyrhizobium japonicum (BJ), which was a symbiosis relationship with soybean roots, that is, the byproduct of soybeans, are directly incinerated or abandoned, which leads to a large amount of waste and results in an enormous loss of resources. Therefore, transforming BJ into 3D hierarchical PCs is an effective way to utilize this biomass waste. Importantly, there is a large amount of proteins (around 50%) in BJ. That is to say, it contains an abundance of nitrogen (around 8-10 wt. %). Due to its high nitrogen content, BJ could be regarded as a promising precursor for producing N-doped porous carbons. In addition, a separator plays an important role in a high-performance supercapacitor. Separators have been made up by resorcinol formaldehyde polymers, aqua-gel, plastic, polyolefin films, and rubber and so on. However, there was few research on soybean leaf (SL) as a separator. In this work, N-doped microporous carbons had been performed by one step carbonization process. In detail, activated carbon electrodes was prepared by ZnCl2 activation of BJ for a supercapacitor. The electrochemical performance of these specimens with SL as a separator was tested in basic (6 M KOH) aqueous electrolyte. Furthermore, SL with ordered natural channels provided suitable places for storage of the electrolytes. The resulting BJPC showed a high


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specific surface area (SBET) (1275 m2·g-1) and multi-scale porous structure with abundant micropores, appropriate meso-pores, and macro-pores. With large SBET and unique structure, the as-prepared BJPC with SL separator present high specific capacitances, good rate capabilities, and long-term stabilities in 6 M KOH. Importantly, the electrochemical performance of the BJPC with a SL separator was comparable to that with commercialized cellulose and PP separators. More attractively, the SL separator showed a much better performance in the BJPC cell at 25 °C than PP separator. The SL separator by using natural raw materials would play an important contribution to the sustainable development of renewable energy storage systems. MATERIAL AND METHODS Materials Bradyrhizobium japonicum and soybean leaves were collected from Hangzhou Province of China. Acetylene black (Super-P), polyvinylidene fluoride (PVDF), Zinc chloride (ZnCl2), Hydrochloric acid (37 wt. %, HCl), and ethanol were purchased from Aladdin Chemistry Co., Ltd. (China). Nickel foam, steel coin cell, commercialized cellulose and polypropylene (PP) separator, and acetylene black were used as purchased. All chemicals were analytical grade. Preparetion of Bradyrhizobium Japonicum-Derived Porous Carbons (BJPC) First, BJ were washed with ethanol and water several times and then was endured a hydrothermal treatment at 200 °C for 3h to remove inorganic impurities in BJ. Next, the BJ was washed with distilled water and then dried in a vacuum oven at 60 °C for 12h. The cleaned BJ was carbonized by one step process through a two-stage heating process under the protection of Ar gas. In detail, they were first heated at 300 ºC for 1 h with a ramping rate of 2 ºC·min-1, followed by a further thermal annealing at 800 ºC (3h, 1ºC·min-1) in a horizontal tube furnace to form bradyrhizobium japonicum carbons (BJC). In other hand, some cleaned BJs were mixed with ZnCl2 at different


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mass ratio (ZnCl2/BJ = 1, 1.5, and 3) and then carbonized by the process of BJC to form the bradyrhizobium japonicum derived porous carbons (BJPC) and remarked as BJPC-1, BJPC-1.5 and BJPC-3. Afterwards, all the samples were soaked into a 6 M hydrochloric acid solution for 10 h to remove soluble salts and then thoroughly washed with ultrapure water until the filtrate became neutrality. The obtained samples were dried in a vacuum oven at 60 ºC. In addition, SL was washed with water several times and then immersed in ethanol for 6 h ethanol to remove chlorophyll for using as separator. And then the SL was flattened through the glass plate and then freeze-dried using a Scientz-10N freeze-dryer. Physical Characterization Scanning Electron Microscopy (SEM, FEI, Quanta 200, USA) and Transmission Electron Microscope and High Resolution Transmission Electron Microscopic (TEM and HRTEM, FEI, Tecnai G20, USA) were appiled to characterize microstructures of the specimens. X-ray Photoelectron Spectroscopy (XPS) spectra (Thermo Fisher Scientific-K-Alpha 1063, UK) with monochromatic Al Kα (1486.71 eV) X-ray radiation (15 kV and 10 mA) was used to investigate the chemical compositions and valence states. Crystalline structures were identified by the X-ray diffraction technique (XRD, Rigaku, D/MAX 2200, Japan) operating with Cu Kα radiation (λ = 1.5418 Å) at a scan rate (2θ) of 2°·min-1. The Raman Spectra were carried out by using the Renishaw inVia Raman microscope with the excitation length of 532 nm. The pore structures of all the samples were characterized by nitrogen adsorption-desorption isotherms obtained on a surface area and porosity analyzer (Quantachrome Instruments v4.01) at -196 °C. Specific surface area (SBET) of all the specimens was calculated by the Brunauer-Emmett-Teller (BET) model, and the pore size distribution (PSD) plots and pore volumes were by the density functional theory (DFT) method. The porosity of the separator was measured using the method


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of n-butyl alcohol immersion by immersing in n-butanol 1 h, and then calculating the porosity using the equation: porosity = (ma/ρa)/(ma/ρa + mb/ρb) × 100%, where ma and mb are the mass of n-butanol and the separator and ρa and ρb were the density of n-butanol and the separator, respectively. The contact angle (CA) of the spencimens was measured by a contact angle analyzer (SDC-200, Shengding precision instrument, China) at 25 ºC. The tensile strengthelongation test of the separators were carried out on an electromechanical uni-versal testing machine (CMT-6104, MTS systems Co. ltd, China) equipped with two flat-surface compression stages and a 10-N loadcell. Electrochemical Characterization The specimens were investigated as electrode materials for supercapacitor via a symmetric twoelectrode system using 2032 stainless steel coin-type test cells. A slurry, consisted of active material (80 wt. %), PVDF (10 wt. %), and Super-P (10 wt. %), was distributed in ethanol solvent and was rolled to be sheet with a density of 1.8 mg·cm-2 by a glass rod, and then dried in a vacuum oven at 60 ºC for 12 h. The sheet was cut into small round pieces with a size (1.0 cm × 1.0 cm) as the working electrode. The cells were assembled using the SL with a size (1.1 cm × 1.1 cm) as separator. 6 M KOH as the electrolyte. For comparison, cells using the commercial cellulose and PP as the membranes to separate two working electrodes and tested under the same conditions. All electrochemical measurements of the specimens were performed by a twoelectrode system with a CHI 660E electrochemical workstation (CHI 660E, Shanghai Chenhua Instrument Co.Ltd., China) at 25°C without specification. Obviously, the tap density of the supercapacitor electrode was about 0.18 g·cm-3 estimated by direct mass and physical dimension measurements. Galvanostatic charge/discharge (GCD) measurements and Cyclic voltammetry


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(CV) curves were recorded with a potential window ranging from 0 to -1 V in 6 M KOH solution. The gravimetric specific capacitance of a single electrode was deduced from the chargedischarge curves according to the Equation (1) 36: C = 2I × ∆t/(m × ∆V)

(1)

where I, ∆t m, and ∆V represent the discharge current (A), the discharge time (s), and the mass of active material loaded on a single working electrode (g), and the voltage change excluding the ohmic drop within ∆t (V), respectively. RESULTS AND DISCUSSION Morphology and Structure The schematic in Figure 1a illustrated the entire process to prepare the BJC and BJPCs for electrode materials and SL for separator. Figure 1b showed SEM images of SL. The total thickness of SL was about 129 µm, with the upper surface and the back surface 6 µm and 3 µm thick, respectively. The inner side of SL was highly porous with many the well-aligned tube arrays. In addition, it was manifestly shown a dense upper surface, which was consisted of some grinning-mouth like open macro pores (stomata) and a large number of folded the platelets. These unique porous structures facilitated the transport and storage of the electrolyte. Nitrogen adsorption-desorption isotherms of BJ, BJC, and BJPCs were performed to investigate the development of porosity by ZnCl2 activation, as showed in Figure 1c. BJ exhibited a type II isotherm, indicating a little porous characteristic, whereas all BJC and BJPCs showed typical type I isotherms with higher N2 quantity absorbed ability, suggesting microporous characteristics. The prominent nitrogen uptake at a relative pressure (P/P0) below 0.01 was attributed to micropores, whereas the uptake at P/P0 > 0.05-0.3 is due to meso-pores. The N2 adsorption with


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a plateau at higher relative pressures followed by an increase at P/P0 > 0.9 illustrated the existence of large and open meso-pores and macropores

37

. Such microporosity greatly benefits

charge accumulation, thus improving the specific capacitance of the electrodes. The SBET and different pore volumes of BJ, BJC, and BJPCs were listed in Table 1. BJ showed undeveloped pore structure with a very low pore volume (0.02 cm3·g-1) and SBET (39 m2·g-1) and BJC exhibited a pore structure with a pore volume (0.36 cm3·g-1) and SBET (684 m2·g-1). The porosity of the resultant BJPCs was evidently developed during the chemical activation and significantly influenced by the ZnCl2/BJ weight ratio. The SBET of BJPC-1 and BJPC-1.5 were 907 and 1275 m2·g-1, respectively, whereas the pore volumes calculated by the DFT are 0.47 and 0.63 cm3·g-1, respectively. The pore volume and SBET of BJPC-3 were dramatically lower, that were, 0.56 cm3·g-1 and 1029 m2·g-1, because of the collapse of pores caused by the excessive activation. The product yields are 69.7, 59.8, 55.4, and 49.3 wt. % for BJC, BJPC-1.5, BJPC-1.5, and BJPC-1.5, respectively. The decrease in the yield is mainly attributed to more ZnCl2 consuming more carbon during the activation process. As shown in Figure 1d, pore diameters ranging from 0.5 to 3 nm dominated the porous structures of all SRPCs. The meso-pore volume of BJPC-1.5 is 0.41 cm3·g-1, which was much higher than those of BJPC-1 (0.29 cm3·g-1) and BJPC-3 (0.38 cm3·g-1). The hierarchical structure with the co-occurrence of micropores and meso-pores in the specimens was perfect for supercapacitors because the charge accumulation happens primarily in the micropores, whereas the meso-pores can transfer abundant electrolytes to and from the micropores, improving the utilization of the micropores. According to the previous results

38,39

,

the energy storage property of carbon materials with macroporous/mesoporous structures could be increased because these structures were more beneficial for electrolyte storage and electron transfer. The micropores acted as electrolyte ion storage sites and the mesorpores acted as


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electrolyte transportation channels. Thus, BJPC-1.5, possessing the hierarchical structure with the highest micropore and meso-pore volume, was expected to display outstanding electrochemical performance as a supercapacitor electrode material. Although containing abundant pores, the specimens exhibited high electrical conductivities, which were 172, 183, 194, and 179 S·m-1 for BJC, BJPC-1, BJPC-1.5, and BJPC-3, respectively. Figure 2 indicated that the microstructure of the specimens. For BJ, shown in Figure 2a, there were a lot of particles with an average diameter of 3.6 µm surrounded by graphene-like sheets. After the carbonization in Figure 2b, BJC showed there were intertwined silk-like slice derived from the pyrolysis of the particles and sheets of BJ. By ZnCl2 activation, BJPC-1.5 exhibited a 3D porous structure in Figure 2c. There were consisted of the hollow, micrometer- or submicrometer- sized holes and were interconnected through the smaller pores that interpenetrate through the thin shells of holes. The high magnification SEM image shown in Figure 2c3 seemed to be plenty of meso-pores in the matrix of BJPC-1.5. This was attributed to ZnCl2induced etching which tended to disrupt the structure by forming a 3D porous structure. These results illustrated a hierarchical porous structure was created. The structure of BJPC-1.5 could provide as much contact area with the electrolyte as possible. The TEM images of the specimens were displayed in Figure 3. For BJ in Figure 3a, it showed long shuttle type of BJ with thicker inner wall than the shell. For BJC in Figure 3b, many micro and meso pores were created on nanosheets during the carburization process. For BJPC-1.5 in Figure 3c, a similar hierarchical structure was also formed at the nanoscale, as revealed by the TEM images at different magnifications in Figure 3c and d. There existed a lot of nanometers pores with a different diameter distribution. The pores were interconnected in a 3D structure with the smaller pores nested inside the larger pores. The HRTEM image of BJPC-1.5 in Figure 3c


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suggested that there were micropores with a diameter below 2 nm on the carbon surface. Intriguingly, the HRTEM images of BJPC-1 and BJPC-3 in Figure 3e and Figure 3f were similar to that of BJPC-1.5. Chemical Composition XRD patterns of the specimens were shown in Figure 4a. For BJ, the peak at 21.8º was attributed to cellulose in BJ. After carburization or activization process, the peak located at 24.6º was referred to the (002) reflection of the turbostratic carbon structure, suggesting an amorphous structure with low crystalline fraction. Another obvious peak centered at 44.5°could be assigned to the (100) diffraction of the graphitic carbon with an amorphous and disorder structure, revealing higher interlayer stacking extent in BJC and BJPCs that could effectively enhance the electronic conductivity of the materials 40. Additionally, compared with BJ, the peak positions of BJC and BJPCs had a slight shift toward low angle after the carbonization process, attributing to that the graphitization degree of BJC and BJPCs samples were enhanced at the carbonization temperature of 800 °C. The results were consistent with the observations from HRTEM, which indicated that the graphitic stacking and micro and meso pores form after annealing 41.” As shown in Figure 4b, the specific characters of the specimens were further characterized by the Raman spectra. The peak located at 1352 cm-1 (D-band) was reflection of the defect and disorder of samples. Another peak located at 1590 cm-1 (G-band) could be referred to the vibrating of all sp2 hybridized carbon atoms both in chains and rings. Moreover, the G-band peak was slightly weaker than the D-band peak. The intensity ratio of D and G bands (ID/IG) indicated the disorderness and the degree of graphitization in the material.

42

The smaller the ID/IG ratio

meant the higher the degree of graphitization. The results showed that the ID/IG peak ratio is 0.98, 0.92, 0.89 and 0.95 for BJC, BJPC-1, BJPC-1.5, and BJPC-3, respectively. The lowest ID/IG of


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BJPC-1.5 suggested BJPC-1.5 was rich in graphitic phase with a low degree of amorphous, which was consistent with the XRD. The graphitic phase offered good electric conductivity, which was consistent with the results of electrical conductivities. In Figure 4c, the survey spectra showed four peaks at 284.6, 533, 401, and 162 eV corresponding to C 1s, O 1s, N 1s and S 2p peak, respectively. The C, O, N, and S content of all the specimens were BJPC-1.5 were 90.37%, 5.21%, 4.08%, and 0.24%, respectively. In detail, the ratio of C/O was high (17.3), suggesting a superior electron conductivity of the material. For BJ in Figure 4e, the high-resolution C1s of BJ could be fitted to four peaks located at 284.8, 286.2, 287.8, and 288.9 eV, belonging to C-C, C-O, C-N, and C=O, respectively. For C1s spectrum of BJPC-1.5 in Figure 4f, there was the main peak of C=C (284.4 eV) and other weak peaks at 284.8, 285.8, 286.3, 287.6 and 288.8 eV belong to C-C, C=N, C-O, C-N, and C=O, respectively. 43 For BJ in Figure 4g, the N 1s could be fitted to three peaks at 398.3 eV (pyridinic N), 400.1 eV (pyrrolic N), and 402.6 eV (oxidized N). As shown in Figure 4h and i, the N 1s of BJC and BJPC-1.5 could be fitted to four peaks at 398.3 eV (pyridinic N), 400.1 eV (pyrrolic N), 400.9 eV (graphitic N), and 402.6 eV (oxidized N). The difference between BJC and BJPC-1.5 was the intensity of graphitic N and that of BJPC-1.5 was more than that of BJC. Supercapacitor Performance The electrochemical capacitive performances of the specimens were evaluated by CV and GCD measurement. Figure 5a showed the CV curves of the specimens at scan rates of 10 mV·s-1. The curves of all specimens were similar with the typical rectangular shape of supercapacitors. Especially, BJPC-1.5 showed the largest current response and the longest discharge time owing to the highest specific surface area (1275 m2·g-1). Figure 5b summarized the initial ohmic drop


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(IR drop) under various current densities of the specimens. The ohmic drops linearly increased with the increasing current density, and the slope of those lines represented the internal resistance value of the overall cells. Among these specimens, BJPC-1.5 exhibited a lower equivalent internal resistance (0.011 Ω g) than BJC (0.032 Ω g), BJPC-1 (0.018 Ω g), and BJPC3 (0.016 Ω g), which exhibited the lowest IR drop regardless of current density, indicating the lowest internal series resistance. The pyridinic N (N-6), pyrrolic N could provide abundant defects and active sites within the carbon, which was beneficial for ion insertion and diffusion.44 Graphitic N was formed on the undamaged carbon skeleton by the replacement of C atoms by N atoms in favor of the promotion of electronic conductivity. Moreover, pyridinic N, pyrrolic N played major roles in providing pseudocapacitance in the aqueous electrolyte solution for supercapacitors.43 The content of N in BJPC-1.5 was 5.21%, which was higher than that of BJPC-1 and BJPC-3 while the content of C in BJPC-1.5 was similar with other samples. Therefore, the N and C element in the BJPC-1.5 were beneficial for improving the electrochemical performance by enhancing the conductivity and providing a pseudocapacitance. Furthermore, Figure 5c described the specific capacitance versus current density of the specimens. The specific capacitances of all specimens decreased as the current density increased, when increased the current density to 20 A·g-1, the capacitance retention of BJC was 79.2%, while the retention of BJPC-1, BJPC-1.5, and BJPC-3 under the same current density was 79.9%, 83.8%, and 80.7%. BJPC-1.5 also displayed the largest specific capacitance (358 F/g) at 1 A·g-1, which was better than BJC (251 F/g), BJPC-1 (304 F/g), and BJPC-3 (329 F/g). The high specific capacitance of BJPC-1.5 resulted from not only the hybridization from surface oxygen and nitrogen functional group but also the hierarchical pore structure with high SBET and large meso-


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pore volume matching with the aqueous electrolyte ions. Moreover, BJPC-1.5 exhibited high capacitance retention (83.8%) at 20 A·g-1, which was much higher than that of BJC, BJPC-1, and BJPC-3, because its large meso-pore volume (61%) allowed electrolyte ions to easily move in and out of the inner micropores despite a lower electrical conductivity. The enhancement of the rate capability of BJPC-1.5 was mainly attributed to the improved conductivity which facilitating the electron transportation, resulting in a faster electronic response. Electrochemical impedance spectroscopy (EIS) analysis of the specimens were as shown in Figure 5d. The Nyquist plot exhibited two distinct parts (a linear part at low frequency and a semicircle part at high frequency). The solution resistance (Rs) and the charge transfer resistance (Rct) could be founded from the intercept at real axis (Z0) and the semicircle intercept in the Nyquist plot, respectively. Rct indicated the migration rate of ions at the interface between the solution and the electrode surface. The smaller diameter of semicircle in the spectrum of BJPC1.5 indicated lower Rct (1.68 Ω). Meanwhile, the slightly larger resistances of BJC possibly resulted from the high doping contents of nitrogen and oxygen groups. In the low frequency region, this line was a finite slope representing the diffusive resistivity of the electrolyte within the pore structure of the specimen. The slope of the BJPC-1.5 approached to an ideally straight line implying the enhanced accessibility of the ions. The CV curves of the BJPC-1.5 at scan rates ranging from 10 to 500 mV·s-1 were presented in Figure 5e. The nearly rectangular shapes, at low scan rates, illustrated their classical capacitive behavior as the electrodes of EDL supercapacitors. At 200 mV·s-1, the CV profiles of BJPC-1.5 maintained the rectangular shape of the voltammograms with little distortion, attributing to the excellent electrical conductivity and the low mass transport resistance of BJPC-1.5. However, at


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the high scan rates of 500 mV·s-1, the CV profiles of BJPC-1.5 transformed from rectangular shape to shuttle shape. Figure 5f presented the GCD curves of BJPC-1.5 at various current densities (0.5-30 A·g-1). The excellent capacitive behavior of BJPC-1.5 for supercapacitors was also reflected in their long GCD times and a quasi-symmetrical shape. Here, there were quasi-symmetrical shapes rather than completely symmetrical isosceles triangle-like profiles. This might be contributed to the effect of heteroatom doping (O and N element). The specific capacitance of BJPC-1.5 was up to 357 F/g at a current density of 1 A·g-1, where the GCD curve exhibited nearly triangular shape without obvious IR drop, suggesting a high reversibility of a typical capacitor with a rapid I-V response owing to its improved graphitization degree which could enhance the electrical conductivity. Although ZnCl2 activation produced carbons with much lower BET surface areas than the KOH activated carbons (SBET = 2967 cm3/g) investigated by G. Yushin et al.

50

, the ZnCl2 activated

BJPC-1.5 in the present study showed significantly higher specific capacitances (358 F/g) than the KOH activated carbons derived from wood saw dust (236 F/g). Further evidence that ZnCl2 activation produced carbons with superior double-layer capacitance to alkali metal activated carbons was provided by the electrochemical performance of ZnCl2 activated carbons from biomass listed in Table 3. Importantly, BJPC-1.5 showed superior electrocapacitive performance with high specific capacitances (358 F/g), which was just little lower than that of the coffee beans researched by Rufford et al. 50. The morphology, thickness, and the CA of SL, cellulose, and PP separator were shown in Figure 6a, Figure 6b, and Figure 6c, respectively. Obviously, in Figure 6a, it was observed that SL had a dense surface consisted of some grinning-mouth like open macro pores and the inside was


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highly porous well-aligned tube arrays. Cellulose separator possessed tortuous pores in Figure 6b while PP separator shown in Figure 6c possessed elliptic pores. The thickness of the SL, cellulose, and PP separator was 129, 100, 25 µm, respectively. The CA of the electrolyte on SL, cellulose, and PP separator was 11°, 29°, and 74°, respectively, which was implied that the SL separator exhibited better wettability to the electrolyte than PP separator. The SL and cellulose separator separators were quickly wetted by the liquid electrolyte, where the electrolyte droplets easily spread over whole surface of the separator. This good wettability of SL separator to the liquid electrolyte was due to the unique structure of SL. The water uptake properties of the separators were evaluated by the mass changes of the specimens after different storage times. Typically, the separators immersed in 6 M KOH solution for different days at 25 ºC to reach certain dilation, respectively. And then the separators were taken out of the solution and carefully wiped with an absorbent paper before they were weighed (Wwet). The degree of water uptake (Dw) was determined by the weight of wet separators (Wwet) and corresponding dry ones (Wdry), according to Equation (2) 52: ‫= ݓܦ‬

୛௪௘௧ି୛ௗ௥௬ ௐௗ௥௬

× 100%

(2)

The thickness, porosity, tensile strength, CA, and Dw of the separators were listed in Table 2. The improvement in the microporous structure of the SL separator was further confirmed by the porosity data of the separators. It was worth noticing that the porosity of SL separator (85%), similar with that of cellulose separator (75%), was higher than that of PP separator (50%). It was well known that highly porous structure was beneficial to increase permeability. The tensile strength of the SL separator was 16 MPa, lower than that of cellulose (24 MPa), while that of PP separator was only 13 MPa. In terms of porosity, tensile strength, and Dw, the SL separator can be regarded as a suitable candidate for separator applied in supercapacitors.


15

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Meanwhile, the changes in weight was monitored and the Dw was calculated according to Equation (1), which was presented in Figure 6d. To some extent, the SL and cellulose separator were swelled in the solution due to the hydrophilic groups such as -OH and -NH on the backbone of separators. It was shown that the Dw of SL and cellulose was kept around 52% and 41%, respectively. Obviously, the Dw of PP separator was about 4%, which was ascribed to the low wettability of the separator. Therefore, these excellent properties of SL were favorable to holding sufficient liquid electrolyte in facilitating rapid ionic transportation for superior electrochemical properties. As a contrast, shown in Figure 6e, the CV tests of supercapacitor based on conventional cellulose and PP separator were recorded. The CV shapes of supercapacitor with SL and cellulose separators were nearly a rectangular at 100 mV·s-1, but the CV curves based on PP separator exhibited obvious distortion at the same sweep rate. These results indicated the SL separator illustrated better properties than the conventional PP separator while was similar with that of cellulose separator. The inset in Figure 6e was digital photograph of LEDs powered by the device assembled with symmetric two-electrode capacitors of BJPC-1.5 with SL as separator. Moreover, the whole supercapacitor device was wrapped with Tin foil papers for fixation. Figure 6f showed the GCD measurements of supercapacitor based on SL, conventional cellulose and PP separator. Obviously, the iR drop of supercapacitor based on PP separator was about 0.28 V, which was much larger than that of supercapacitor employing SL (0.13 V) and cellulose separator (0.19 V). This was due to the low wettability of PP separator, a large part of capacity was exhausted by the inner resistance. Hence, the output capacity was smaller than the ones with SL and cellulose separator. From the discussion above, The SL could be as a potential separator


16


Page 18 of 34

for supercapacitor. The inset of Figure 6f exhibited illustration fo an EDL capacitor with simplified electric circuit with SL as separator and BJPC-1.5 as electrodes. Figure 6g exhibited the cycling performance of BJPC-1.5 based on a two-electrode cell by the GCD test at 1 A·g-1. These results demonstrated that the specific capacitance of BJPC-1.5 retained about 91% after 8000 consecutive cycles, suggesting the highly stable cyclic ability of the BJPC-1.5-based electrode. The nearly identical GCD curves of the 1st and 8000th cycle also indicated the high cycling stability (inset in Figure 6g). All these results revealed that this type of porous nanostructure and high graphitization degree of the BJPC-1.5 could facilitate the electrolyte ion diffusion and electron transportation, leading to a reduced internal resistance and an improved electrochemical performance. CONCLUSIONS In summary, 3D hierarchical porous carbon was prepared by the one step carbonization with ZnCl2 activation of BJ. The N-doped carbon prepared at 800 ºC with a ZnCl2 to BJ of 1.5 (BJPC1.5) had the highest SBET of 1275 m2·g-1 and the SBET of BJPCs was exhibited to first increase and then decrease with the growth of mass ratio of ZnCl2/BJ. The volume of meso-pores was found to increase with the ZnCl2 to BJ mass ratio. Moreover, the electrochemical performance of the specimens with SL as a separator for a supercapacitor had been evaluated. The BJPCs displayed the specific capacitances as high as 358 F/g. The BJPC-1.5 delivered the highest specific capacitance and the most stable electrochemical performance. These results demonstrated the benefit of multiscale pores to double-layer capacitance at fast charge-discharge rates. Furthermore, compared with commercialized cellulose and PP separators, the electrochemical performance of the BJPC-1.5 with a SL separator showed a much better performance in the BJPC-1.5 cell. Hence, the activated carbon electrodes prepared by activation


17

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of biomass with high N content and SL as a separator for a supercapacitor might be the potential materials for supercapacitor with excellent electrochemical performances such as outstanding cycling stability (91% over 8000 cycles). These outstanding results might pave the way for successfully exploring a high-efficient and stable biologic activated carbon electrodes and separator for supercapacitors. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was financially supported by Zhejiang Provincial Natural Science Foundation of China under Grant No. LZ15C160002 and Scientific Research Foundation of Zhejiang A&F University (Grant No. 2014FR077). REFERENCES (1) Gan, S.; Zhong, L.; Gao, L.; Han, D.; Niu, L. Electrochemically Driven Surface Confined Acid/Base Reaction for Ultrafast H+ Supercapacitor. J. Am. Chem. Soc. 2016, 138, 1490–1493. (2) Zeng, Y.; Yu, M.; Meng, Y.; Fang, P.; Lu, X.; Tong, Y. Iron-Based Supercapacitor Electrodes:

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(49) Rufford, T. E.; Hulicova-Jurcakova, D.; Zhu, Z.; Lu, G. Q., Nanoporous carbon electrode from waste coffee beans for high performance supercapacitors. Electrochem. Communicat. 2008, 10 (10), 1594–1597. (50) Wei, L.; Sevilla, M.; Fuertes, A. B.; Mokaya, R.; Yushin, G., Hydrothermal Carbonization of Abundant Renewable Natural Organic Chemicals for High‐Performance Supercapacitor Electrodes. Adv. Energy Mater. 2011, 1 (3), 356–361. (51) Yun, Y. S.; Cho, S. Y.; Shim, J.; Kim, B. H.; Chang, S. J.; Baek, S. J.; Huh, Y. S.; Tak, Y.; Park, Y. W.; Park, S., Microporous carbon nanoplates from regenerated silk proteins for supercapacitors. Adv. Mater. 2013, 25 (14), 1993–1998. (52) Yu, H.; Tang, Q.; Wu, J.; Lin, Y.; Fan, L.; Huang, M.; Lin, J.; Li, Y.; Yu, F. Using Eggshell Membrane as a Separator in Supercapacitor. J. Power Sources 2012, 206, 463-468.


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Figure 1. (a) Scheme of the preparation of BJPCs; (b) SEM image of the leave of soybean; N2 adsorption–desorption isotherms (c) and pore size distribution analysis (d) of the samples.


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Figure 2. SEM images and macrographs of BJ (a), BJC (b) and BJPC-1.5 (c), respectively


27

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Figure 3. TEM images and HRTEM images of BJ (a), BJC (b), and BJPC-1.5 (c, d); HRTEM images of BJPC-1 (e) and BJPC-3 (f), respectively


28


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Figure 4. XRD patterns (a) and Raman spectra (b) of the samples; XPS analysis of the samples: Survey scan (c), component contents (d), C1s curve-fitted peaks of BJ (e) and BJPC-1.5(f), N1s curve-fitted peaks of BJ (g), BJC (h) and BJPC-1.5 (i).


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Figure 5. (a) CV curves at 10 mV·s-1 of the specimens in a symmetric two-electrode supercapacitor in 6 M KOH aqueous solution; (b) IR (∆V) plots of the specimens; (c) Gravimetric capacitance as a function of current density; (d) Nyquist plots of the specimens electrodes measured with amplitude of 10 mV over the frequency range from 100000 to 0.1 Hz, inset: the magnified high frequency region; (e) CV curves of BJPC-1.5 at different scan rates; (f) GCD curves of the BJPC-1.5 at different current densities (0.5-30 A·g-1).


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Figure 6. Morphology and CA photos of the different separators: SL (a), cellulose (b) and PP (c); (d) The Dw for different separators in 6 M KOH aqueous solution; (e) CV curves at 100 mV·s-1 of supercapacitors with different separators and BJPC-1.5 as electrodes (inset: digital photograph of LEDs powered by the device); (f) GCD curves of supercapacitors with different separators and BJPC-1.5 as electrodes at 0.5 A·g-1; the inset: Illustration fo an EDL capacitor with simplified electric circuit with SL as separator and BJPC-1.5 as electrodes; (g) cyclic stability at 1 A·g-1 for 8000 cycles of BJPC-1.5 with SL as a separator (inset: GCD curves of the 1st and 8000th cycle).


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Table 1 Yield and Textual Parameters of the samples Sample

Yield (wt. %)

SBET (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

Vmeso(cm3/g)

BJ

/

39

0.02

0.01

0.01

BJC

69.7

684

0.36

0.09

0.26

BJPC-1

59.8

907

0.47

0.18

0.29

BJPC-1.5

55.4

1275

0.63

0.27

0.41

BJPC-3

49.3

1029

0.56

0.21

0.38

Table 2 Physical parameters of separators Separators

Thickness (µm)

Porosity (%)

Tensile trength (MPa)

CA (°)

Dw (%)

SL

129

85

16

11

52

Cellulose

100

75

24

29

41

PP

25

50

13

74

4


32


Page 34 of 34

Table 3 Comparation of biomass-derived carbon electrodes through activation Materials

Activation agent

SBET

Vtotal (cm3/g)

Electrolyte

C (F/g)

BJC

/

684

0.36

6M KOH

251

This work

BJPC-1

ZnCl2

907

0.47

6M KOH

304

This work

BJPC-1.5

ZnCl2

1275

0.63

6M KOH

358

This work

BJPC-3

ZnCl2

1029

0.56

6M KOH

329

This work

Filter paper

ZnCl2

2232

1.49

6M KOH

302

(45)

Coffee beans

ZnCl2

1021

1.3

TEABF4/AN

134

(46)

Sugar cane bagasse

ZnCl2

1788

1.74

11M H2SO4

300

(47)

Banana fiber

ZnCl2

1097

/

1M Na2SO4

296

(48)

Wood saw dust

KOH

2967

1.35

TEABF4/AN

236

(49)

Coffee beans

ZnCl2

1019

0.48

1M H2SO4

368

(50)

Silk fibroin

KOH

2557

/

1M H2SO4

264

(51)

Ref.


33

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