Facile Synthesis of Three-Dimensional Heteroatom-Doped and

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Facile Synthesis of Three-Dimensional Heteroatom-Doped and Hierarchical Egg-Box-Like Carbons Derived from Moringa Oleifera Branches for High-Performance Supercapacitors Yijin Cai, Ying Luo, Yong Xiao, Xiao Zhao, Yeru Liang, Hang Hu, Hanwu Dong, Luyi Sun, YingLiang Liu, and Mingtao Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10893 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 4, 2016

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ACS Applied Materials & Interfaces

Facile Synthesis of Three-Dimensional Heteroatom-Doped and Hierarchical Egg-Box-Like Carbons Derived from Moringa Oleifera Branches for High-Performance Supercapacitors Yijin Cai†, Ying Luo†, Yong Xiao†, Xiao Zhao†, Yeru Liang†, Hang Hu†, Hanwu Dong†, Luyi Sun**,‡, Yingliang Liu*,†, Mingtao Zheng*,†,‡ †

College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China.

E-mail: [email protected] (M. Zheng), [email protected] (Y. Liu). ‡

Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials

Science, University of Connecticut, Storrs 06269, USA. E-mail: [email protected] (L. Sun).

ABSTRACT: In this paper, we demonstrate that Moringa oleifera branch, a renewable biomass waste with abundant protein content, can be employed as novel precursor to synthesize three-dimensional heteroatom-doped and hierarchical egg-box-like carbons (HEBLCs) by a facile room temperature pre-treatment and direct pyrolysis process. The as-prepared HEBLCs possess unique egg-box-like frameworks, high surface area, interconnected porosity, as well as doping of hetero-atoms (oxygen and nitrogen), endowing its excellent electrochemical performances (superior capacity, high rate capability and outstanding cycling stability). Therefore, the resultant HEBLC manifests a maximum specific capacitance of 355 F g-1 at current density of 0.5 A g-1, and remarkable rate performance. Moreover, 95% of capacitance retention of HEBLCs can be also achieved after 20,000 charge/discharge cycles at an extremely high current density (20 A g-1), indicating a prominent cycling stability. Furthermore, the as-assembled HEBLC//HEBLC symmetric supercapacitor displays a superior energy density of 20 Whkg-1 in aqueous electrolyte and remarkable capacitance retention (95.6%) after 10,000 charge/discharge cycles. This work provides an environmentally friendly and reliable method to produce higher-valued carbon nanomaterials from renewable biomass wastes for energy storage application. KEYWORDS: hierarchical structured carbons, moringa oleifera, electrode materials, supercapacitors, cycling stability, heteroatom doping

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INTRODUCTION With environment and energy problems occurring more frequently, it is urgent to develop new

energy storage devices and further lower the consumption of fossil fuels, which produce greenhouse gases and have a limited supply in the world.1,2 Supercapacitors are emerging and potential energy storage devices, which have attracted high attention because of their rapider charge/discharge rate, more excellent cycling stability, and higher power density compared with Li-ion battery.3-7 According to the mechanism of energy storage, it contains two traditional types, pseudo-capacitors and electric double-layer capacitors (EDLCs).8 The capacitance of EDLCs can be attributed to the quick charge accumulating in the interface of electrode/electrolyte, resulting in a fast charge/discharge process.8 There are two key factors for enhancing the capacitance of EDLCs: (i) a higher specific surface area (SSA) of electrode material that will supply more active sites for charges to accumulate to form more electric double layers, (ii) hierarchical pore size distribution (PSD) which contains micro-, meso-, and macro-pores facilitating electrolyte ions diffusion and transportation into the internal pores of electrode materials especially at higher charge/discharge rates.9 In addition, it has been demonstrated that doping hetero-atoms (N, O, S, and F), into the matrix of carbon materials not only could effectively enhance the capacitance by providing the pseudo-capacitance, but also could improve the wettability as well as conductivity of electrode materials.10-13 In the last decade, various nanostructured carbon materials, including graphene,14-16 nanotubes,17-19 onion-like carbons,20-22 and templated carbons,23-25 have been explored for supercapacitor applications. However, the high cost and complicated preparation process severely limited their practical applications. Recently, as an outstanding candidate as electrode materials of EDLCs, hierarchical porous carbons (HPCs) have attracted great attention because of their cheapness, stable chemical and thermal capability, and good electronic conductivity.26-29 During the synthesis of HPCs, chemical and/or physical activation as traditional method were generally utilized to improve the surface area and porosity.27 However, this method of activation generates a high level of microporosity, leading to a low pore accessibility of electrolyte ions and poor rate performance, especially the fast decay at high current rates.15,30 Additionally, the excessive activation agents often lead to the decrease of yield and corrosion of the equipment. A reliable, effective, and low-cost method to synthesize and tune the pore structure of HPCs 2


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with a remarkable electrochemical performance is of great significance for the further research and applications. More recently, much efforts have focused on the HPCs synthesized from renewable biomass, which is abundant, sustainable, natural, readily available, and generally cheap,7,27,31 compared with carbohydrates32,33 and polymers34-36, which also have been widely used as carbon sources to synthesize HPCs for supercapacitors. Moreover, biomass-derived HPCs can satisfy the requirements, high SSA, suitable porosity, interconnect pore framework, and low cost, for high-performance EDLCs. Various types of biomass, such as pomelo peels,37 hemp fibers,38 soybeans,39 bamboos,40 dead leaves,41 fungus,42 baganese,43 have been successfully explored as the precursor to prepare HPCs. While a great progress has been achieved recently,28-31,37-43furtherresearch should focus on developing simple synthesis methods to fabricate HPCs without sacrificing the natural structure and morphology characteristics of biomass precursors. Moringa oleifera, a softwood tree, is a widespread biomass with large phytomass and fast growth rate. It is easy to collect and possesses a special natural structure and high protein content (up to 9.38 wt%) in the tropical and subtropical regions.44 Impressively, it is a multipurpose raw material and its roots, bark, leaves, flowers, fruits, and seeds have been widely used in various fields, such as sorption of heavy metals, water purification, as nutritious vegetables and medicines, and so on.45-47 However, there was no report on the utilization of the biomass waste of moringa oleifera as carbon precursor to fabricate higher-valued carbon nanomaterials with nitrogen doping and hierarchical framework for high-performance supercapacitors. Inspired by the unique structure and property of moringa oleifera, we herein report the synthesis of three-dimensional (3D) heteroatom-doped and hierarchical egg-box-like carbons (HEBLCs) using moringa oleifera branches (MOBs) as the precursor by a facile room temperature pre-treatment and direct pyrolysis process. Compared to conventional carbonization combined with post activation process that has the disadvantages of using excessive activation agents and violent reactions that easily irreversibly destroy the novel structure of the precursor, the present route is simple, efficient and benign. Significantly, the unique structure of MOBs can be well inherited and HEBLCs with interconnected framework can be achieved. Such micro-scale “egg-box” framework can provide more channels for 3



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electrolyte ions diffusing into internal carbons to improve the electrochemical performance, especially with a high active material loading. Moreover, the resultant HEBLCs have a high SBET, hierarchical porosity, and heteroatoms (O and N), endowing an excellent electrochemical performance. Besides, the assembled HEBLC symmetric supercapacitors manifest remarkable energy density and outstanding cycling stability.

EXPERIMENTAL SECTION Material Preparation. The employed raw MOBs were collected from South of China Agricultural

University (SCAU, China). In a typical process, MOBs were first treated by removing the bark and smashed into powders (ca. 0.1-0.5 mm). Then, 2.0 g of MOBs were dispersed in 50 mL KOH solutions with various concentrations (0.2-0.4 M) under stirring for 1 h, and then dried at 80 °C for 6 h. Subsequently, the pre-treated precursors were heated at 800 °C for 2 h with a heating rate of 5 °C min-1 in a tube furnace under N2 atmosphere. After cooled to room temperature naturally, the obtained products were washed with 2.0 M HCl aqueous solution, deionized water, and ethanol for several times. Finally, the resultant products were dried 6 h under 80 °C for 6 h. The as-prepared samples were named asHEBLC-2, HEBLC-3 and HEBLC-4, respectively, where 2, 3, and 4 refers to the concentration of KOH solution, 0.2, 0.3, and 0.4 M, respectively. For comparison, activated carbon derived from MOBs by the conventional two-step route, in which MOBs were first carbonized for at 800 °C 2 h in N2 atmosphere without pre-treatment, and then activated at 800 °C for 2 h by KOH (three times mass of carbon). The as-obtained sample was denoted as AC-800. Besides, MOBs were also carbonized directly under the same conditions as HEBLCs, named as C-800. The yield of HEBLC-2, HEBLC-3, HEBLC-4, AC-800, and C-800 is ca. 14.2, 12.5, 11.2, 8.0, and 16.2 wt% (compared to raw MOBs), respectively. Morphology and Microstructure Characterization. X-ray diffraction (XRD) data were gained using powder X-ray diffractometer (XD-2X/M4600, Cu Kα, λ=0.15405 nm) from 5 to 80°. Raman spectrum was recorded on Jobin-Yvon HR800 micro Raman spectrophotometer (λ= 457.9 nm). Porosity characteristics of all samples were determined from N2 adsorption-desorption isotherms (ASAP 2020 Micromeritics sorption analyzer). Before measuring, all the samples were degassed at 350 °C for 8 h. Brunauer-Emmett-Teller (BET) was performed to measure SSA, while the non-local density functional theory (NLDFT) was employed to calculate the PSD. The true density of carbon materials 4


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were determined by helium density measurement (AccuPyc II 1340, Micromeritics, America). The XPS characterization was conducted by VG ESCALAB Mark II (VG Scientific, UK). Microscopy observations of MOBs natural texture were conducted by an optical microscope (WSM500D, Micro optical instrument Co. Ltd.). The field emission electron microscope (FESEM, ZEISS Ultra 55) and high-resolution transmission electron microscope (HRTEM, JEM2100 HR) were used to observe the morphology of all samples. Electrochemical Performance Evaluation. The working electrode was prepared by mixing the as-prepared HEBLCs, carbon black, and PTFE binder with a mass ratio of 8:1:1 in ethanol to obtain slurry. Afterward, the slurry was spread on nickel foam used as current collector, and then dried overnight under the temperature of 105 °C, and then weighted and pressed into sheet (300±2 µm in thickness) under 10 MPa. The mass of active materials loaded on each electrode was ca. 4.0 mg. Three-electrode system was conducted in two aqueous solutions of 1.0 M Na2SO4 and 6.0 M KOH. In the process of test, Hg/HgO electrode was employed as reference electrode and Pt foil (1 cm2) as the counter electrode. Galvanostatic charge/discharge (GCD), cyclic voltammetry (CV), and cycle-life stability was performed on Chenhua electrochemical workstation (CHI660D, Beijin, China). The electrochemical impedance spectroscopy (EIS) was conducted on Im6ex electrochemical workstation (Zahnex Corp.). The frequency range of EIS is from 0.01 to 100 k Hz with 5 mV amplitude. The practical electrochemical performance of HEBLCs was assessed by assembling symmetric supercapacitor, which is comprised of a glassy fibrous separator and two electrodes of HEBLCs with same mass of active materials (ca. 4.0 mg) in 1.0 M Na2SO4 and 6.0 M KOH aqueous solutions. The gravimetric, areal and volumetric capacitance of electrodes were determined by the GCD curves, as following equations.42 CA =

I × ∆t A × ∆V

(1)

Cg =

I × ∆t m × ∆V

(2)

CV = C g × ρ ρ=

1 1 VT +

ρ carbon

(3)

(4)

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Where C A (F cm-2) and A (cm2) are the areal capacitance and geometrical area of the electrode, respectively. I (A), ∆t (s), and ∆ V (V) represents constant current, discharge time of GCD test, and voltage change excepting IR drop, respectively.

Cg

(F g-1), and CV (F cm-3) corresponds to gravimetric

specific and volumetric capacitance calculated from GCD test, respectively. ρ (g cm-3) represents the active material density. While

ρ carbon (g cm-3), and

VT (g cm-3) corresponds to the true density of

HEBLC-3 determined by helium density measurements (ca. 1.98 g cm-3), and total pore volume, respectively. The m (g) has various meaning in different systems, it refer to active material loading of single electrode in three-electrode system, while in two-electrode configuration, it represents total active materials loading in both electrodes. The power density ( P ) and energy density ( E ) of the supercapacitor were defined as following equations: E=

C × ( ∆V ) 2 2 × 3.6

(5)

P=

3600 × E ∆t

(6)

Where C (F g-1) represents specific capacitance of supercapacitor measured from Eq. 2, ∆ V (V) and

∆ t (s) represents the voltage change during the discharge process of GCD curves without including IR drop, discharge time, respectively.

RESULTS AND DISCUSSION 1. Morphology and Microstructures of the MOB precursor and HEBLCs. The overall

synthesis procedure of HEBLCs from MOBs is illustrated in Figure 1. At the beginning, MOBs was shattered into fine powders. Then, the obtained MOB powders were pre-treated by dipping into the low concentration KOH aqueous solution with magnetic stirring at room temperature (Figure 1a-c). It can be seen that the color of MOB powders turned from light yellow into brown yellow after this alkali pre-treatment (Figure 1b and d), implying that KOH was absorbed into the MOBs. Subsequently, the pre-treated MOB powders were simply heated at 800 °C. Since the pre-treatment was conducted in dilute KOH solutions, the unique structure of MOBs was well maintained, and thus HEBLCs with 6


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interconnected framework could be obtained (Figure 1e). Obviously, the employed MOBs act as in situ template and carbon precursor facilitating the formation of interconnected framework of HEBLCs. Unlike the traditional KOH activation during which the etching often occurs from outside to inside of the precursor,42 this activation in the present work takes place from inside to outside, because KOH was effectively absorbed into the networks of MOBs during the room temperature pre-treatment process. Therefore, the present process can significantly improve the activation efficiency, as well as reduce the amount of activation agent and the corrosion to equipment, and facilitate to prepare high performance electrode materials.

Figure 1. Schematic of the synthesis process of MOB-derived HEBLCs. (a) MOBs, (b) MOB powders, (c) KOH solution immersed MOB powders, (d) pre-treated MOB powders, and (e) as-resulted HEBLCs.

Figure 2a shows a photograph of one year old moringa oleifera trees, showing its very high growth rate. The inset in Figure 2a shows the photograph of the employed MOBs. The morphology and microstructure of the raw MOBs and pre-treated samples were characterized by optical microscope and FESEM. From Figure 2b and c, it can be observed that the xylem structure of MOBs consists of vessels which play an important role in transportation of nutrient/water transport from the root to leaves. Closer inspection toward their longitudinal section shows a certain amount of natural perforations, which facilitate the water and inorganic salts to longitudinally transport in the vessels. The natural structure of MOBs was also observed by FESEM images (Figure 2d-f). It can be seen that the cross section of the raw MOBs displays a honeycomb-like structure (Figure 2d). The longitudinal section image (Figure 2e), 7



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and an enlarged image (Figure 2f) of the raw MOBs reveal a large quantity of perforations on the vessel wall, in accordance with the optical microscopy image. It should be mentioned that the microstructure of MOBs is universal regardless of their geographical location. As shown in Figure S1a-c (Supplementary Information), the MOBs obtained from Kunming (Yunnan province, China), also present an interconnected honeycomb-like structure, similar to that of MOBs from SCAU (Figure 2d-f). After pre-treated by KOH solution, the vessels of MOBs shrunk, but still maintained their natural structure (Figure 2g-i). The shrink may be due to the evaporation of water during the drying process. Interestingly, there are a lot of holes or cavities with the range of several hundred nanometres in the longitudinal section (Figure 2h and i), which are highly favourable for the activation process, as KOH can be absorbed into the channels of MOBs.

Figure 2. (a) Digital photograph of moringa oleifera trees. Inset in (a) shows a photograph of MOBs. Optical photographs of MOBs in cross section (b) and longitudinal section (c). FESEM images of MOBs in cross section (d) and longitudinal section (e). (f) Magnified FESEM image of the red square in (e). FESEM images of pre-treated MOBs in cross section (g), longitudinal section (h), and (i) magnified FESEM image of the red square in (h).

The morphology and microstructure of the as-obtained HEBLCs derived from MOBs were

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characterized by FESEM and TEM (Figure 3). It is demonstrated that HEBLC-3 exhibits 3D hierarchically interconnected egg-box-like structure (Figure 3a-c). The HEBLC-3 has numerous macropores with a diameter from a few hundred nanometres to several micrometres, which can not only act as the ion-buffering reservoir for the electrolyte ions transportation, but also facilitate electrolyte ions to fast diffuse into the inner micropores of electrode materials, especially at high charging rates.9 Intriguingly, interconnected porous layered structure can also be observed clearly (Figure 3a and b), which can effectively shorten the electrolyte ions diffusion path and enhance the structure stableness of the sample during rapid charge/discharge process.42 The samples prepared from the MOBs treated with 0.2 and 0.4 M KOH solutions also show the egg-box-like structure (Figure S2a and b), indicating that our method is effective and reliable to produce HEBLCs. In contrast, the MOB-derived AC-800 synthesized through conventional two-step route of precarbonization and subsequent KOH activation displays a haphazard morphology (Figure S2c), revealing the complete destruction of the unique structure of MOB precursor. These results reveal that the present room temperature pre-treatment process is crucial to the structure retention and formation of HEBLCs.

Figure 3. FESEM (a-c), TEM (d, e), and HRTEM (f) images of the as-prepared HEBLC-3. Inset in (e) shows small mesopores in the carbon walls. TEM images further verify the porous layered structure with interconnected macropores with size of several hundred nanometres (Figure 3d and e), which is well consistent to the FESEM observations. Enlarged TEM image is shown in Figure 3e and its inset reveals the existence of plenty of mesopores in the carbon walls. The HRTEM image of HEBLC-3 displays an amorphous nature of HEBLC-3 (Figure 9



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3f). The pore wall with a thickness of ca. 25 nm can also be observed, in which a large amount of micro- and mesopores in the size of 1-4 nm co-exist. It is well known that micropores could provide numerous activated sites to form electronic double layers, while mesopores could offer fast ions diffusion/transport channels. Besides, plate edges can also be observed (Figure 3f), indicating a weakly ordered graphitic structure, which can effectively enhance the conductivity of materials. As expected, the as-resulted HEBLC-3 by using Kunming collected MOBs also exhibits the egg-box-like structure (Figure S1d-f), similar to HEBLC-3 from MOBs of SCAU. 2. Phase Structure and Composition of the Resultant HEBLCs. The wide-angle XRD patterns of the as-resulted HEBLCs were displayed in Figure 4a. The peak at ca. 24.6 ° is corresponding to the (002) reflection of the turbostratic carbon structure, suggesting an amorphous structure with low crystalline fraction.49 Another obvious peak located at ca. 44.4 ° can be assigned to the (100) diffraction of the graphitic carbon with an amorphous and disorder structure, revealing higher interlayer stacking extent in HEBLCs that can effectively enhance the electronic conductivity of the materials.50 Additionally, an obvious shift of the intensity from top to bottom towards low angle can be seen, indicating the existence of abundant micropores in HEBLCs.51 The specific nature of HEBLCs was further characterized by the Raman spectra. As depicted in Figure 4b, the peakcentered at 1352 cm-1 (D-band) is reflection of the defect and disorder of samples. Another peak located at 1585 cm-1 (G-band) can be assigned to the vibrating of all sp2 hybridized carbon atoms both in chains and rings.52,53 Moreover, the G-band peak is slightly weaker than the D-band peak, and the intensity proportion of D-band and G-band (IG/ID) values is determined to be 0.99, 0.98, and 0.88, for HEBLC-2, HEBLC-3, and HEBLC-4, respectively, suggesting that the defects and disorder sections in the as-resulted HEBLCs increase with an increasing concentration of KOH solution from 0.2 to 0.4 M. The chemical environment of elements of C and N, and the composition of the as-prepared HEBLCs were determined by XPS characterization. The survey spectra (Figure 4c) show two distinct and weak peaks at 284.6, 533, and 401 eV, corresponding to C 1s, O 1s, and N 1s peak, respectively. The C, N, and O content of all the samples were summarized in Table S1, according to the XPS analyses. The C, O, and N content in HEBLC-3 are 94.1%, 4.5%, and 1.3%, respectively. Intriguingly, the ratio of C/O is very high (20.8), suggesting a good electron conductivity of the carbon material.54 Moreover, the 10


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high-resolution C 1s shown in Figure 4d of HEBLC-3 can be fitted to four peaks centred at 288.5, 286.6, 285.7, and 284.8 eV, corresponding to C=O, C-O, C-C/N, and C=C, respectively.55 The O1s peaks (Figure S3g) centered at 534.7, 533.4, and 531.9 eV are attributed to COOH (carboxylic groups), C-O-C (oxygen ether or anhydride groups). And C=O (oxygen in carbonyl or anhydride groups), respectively.56 As shown in Figure 4e, the N 1s can be fitted to three peaks, oxidized N (N-O, at 402.6 eV), pyrrolic nitrogen (N-5, at 400.3 eV), and pyridinic nitrogen (N-6, at 398.4 eV).55,56 The high-resolution C 1s, O 1s, and N 1s spectra of HEBLC-2 and HEBLC-4 were shown in Figure S3a-f, displaying similar fitted results. These surface functional groups are important for supercapacitor applications since these surface functionalities could not only enhance the wettability of materials, facilitating to increase active surface areas accessibility to the electrolyte ions, but also take rapid redox reactions at the surface of electrode materials, resulting in a contribution of pseudo-capacitive to enhance the overall capacitance.55-57

Figure 4. (a) XRD profiles, (b) Raman spectra of the as-synthesized HEBLCs. (c) XPS spectra of the samples, high-resolution (d) C 1s and (e) N 1s spectra of HEBLC-3. (f) N2 adsorption-desorption, (g) pore size distribution (PSD) curves. The inset in (g) shows the PSD curves from 20 to 80 nm. 11



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3. Porosity of the as-Prepared HEBLCs. The porosity of the resultant HEBLCs were analyzed by the N2 adsorption-desorption measurements. The isotherms as shown in Figure 4f demonstrate that HEBLC-2, HEBLC-3 and HEBLC-4 are Ι-type adsorption-desorption isotherm with obvious N2 adsorption at low relative pressures from 0 to 0.05. Moreover, the obvious steep rise of N2 uptake at low relative pressures (lower than 0.1) reveals abundant micropores in the resulted HEBLCs. Impressively, compared with the other three isotherm curves, the curve of HEBLC-3 obviously increases from 0.1 to 0.5 (P/P0), indicating the existence of smaller mesopores. As shown in Figure 4a, with an increasing concentration of KOH solution, the isothermal platform of the samples initially rises, and then decreases, indicating that excessive KOH probably led to the structural collapse.48 As comparison, the as-prepared AC-800 displays Ι-type adsorption-desorption isotherm and quickly reaches the platform before the 0.2 (P/P0), indicating a microporosity characteristic of AC-800.Additionally, the as-resulted C-800 presents an I/IV-type isotherm and a clear hysteresis loop of H3-type (Figure S3i). The PSD curves of HEBLCs (Figure 4g) suggest a hierarchical porous structure including plenty of micropores, appreciable amount of mesopores, and some macropores. Although HEBLC-2, HEBLC-3, and HEBLC-4 exhibit similar PSD under 1 nm, they have distinctive difference from 1 to 4 nm and large mesopores (inset in Figure 4g). The as-resulted HEBLC-3 has more micropores and mesopores than other samples, consisting to the previous result of the isotherms. However, the as-synthesized AC-800 mainly presents microporosity in size of smaller than 2 nm and lacks of mesopores and macropores, resulting in a poor pore accessibility of electrolyte ions, low specific capacitance and poor rate capability.30 The characterization results show that AC-800 possesses very different microstructures from HEBLCs, indicating that the pre-treatment process plays a momentous role on the porosity of the as-resulted HEBLCs. The PSD curve of C-800 shows a hierarchical porosity distribution (inset of Figure S3i). More detailed pore parameters of HEBLCs, AC-800, and C-800 are shown in Table S2. As the concentration of KOH increased from 0.2 to 0.3 M, the SBET increased from 2033 to 2312 m2g-1, and the total pore volume (TPV) rose from 0.98 to 1.20 cm3 g-1. Nevertheless, both SBET and TPV of HEBLC-4 decreased dramatically to 1742 m2 g-1 and 0.79 cm3 g-1, respectively, probably owing to pores collapse. It has been demonstrated that the pores with a diameter of less than 1 nm could enhance capacitive effect through desolvation ions.58 However, the ultramicropores are difficult for electrolyte ions to penetrate into, 12


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especially at high charge/discharge rates, leading to a poor rate performance. Therefore, hierarchical porous framework will be helpful for improving the rate capability. According to the above results, the as-synthesized HEBLCs possess hierarchical porosity and interconnected framework, and HEBLC-3 has a larger SBET and more suitable PSD compared with other samples, indicating its potential for high rate discharge supercapacitors.

Figure 5. Electrochemical performance in 6.0 M KOH electrolyte solution of HEBLCs measured in a three-electrode system. (a) CV curves of the as-resulted HEBLCs at a scan rate of 50 mV s-1; (b) CV curves of HEBLC-3 at different scan rates from 5 to 200 mV s-1;(c) GCD curves of HEBLC-3, (d) IR (∆V) plots of HEBLCs, and (e) Specific capacitance of HEBLCs as a function of current density.at various current densities from 0.5 to 50 Ag-1, respectively. (f) Cyclic stability of HEBLC-3 over 20,000 cycles at a current density of 20 A g-1. The inset in (f) shows the GCD curves at 1st and 20, 000th cycles.

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4. Evolution of the Electrochemical Performance tested in a Three-Electrode System. The electrochemical properties of the as-synthesized HEBLCs were firstly analysed by cyclic voltammetry (CV) measurements in a three-electrode configuration using 6.0 M KOH aqueous solution as electrolyte. As shown in Figure 5a, the CV curves display nearly symmetrical rectangular shapes, indicating ideal EDLC behaviour. Additionally, the HEBLC-3 possesses the largest area of loop among these samples at the same scan rate, suggesting the highest capacitance. AC-800 exhibits a smaller specific capacitance than HEBLCs, owing to its high level of microporosity, which will hinder the outer electrolyte ions for diffusion into internal micropores. Figure 5b depicts the CV curves of HEBLC-3 electrode at various scan rates (5-200 mV s-1). A good rectangular-like shape and reversible behaviour of curves was remained, even the scan rate increased to 200 mV s-1, suggesting its remarkable rate performance. It has been demonstrated that the rectangular-like degree of CV curve is closely related to the charge transfer and ion diffusion rate in the pores of HEBLC-3 electrode.59 The CV curves ofHEBLC-2, HEBLC-4, AC-800, and C-800 electrodes are shown in Figure S4a, c, e, and g. One can see the similar supercapacitor performance, indicating the excellent capacitive behaviour of these MOB-derived carbon materials. Figure 5c presents the GCD curves of HEBLC-3 at various current densities ranging from 0.5 to 50 A g-1. The GCD curves present a quasi-symmetrical shape rather than completely symmetrical triangle. This may be due to the effect of heteroatom doping (O and N element). Especially, the N-doping ( N-5 and N-6) could provide the pseudo-capacity to the overall capacitance, and render the GCD curve to deform and deviate the symmetrical triangle at low current densities.11 The GCD curves of HEBLC-2, HEBLC-4, AC-800, and C-800 electrodes are depicted in Figure S4b, d, f, and i, respectively. Furthermore, the initial voltage loss (IR drop) at various current densities from 0.5 to 50 A g-1 for all the electrodes are manifested in Figure 5d. Among these samples, HEBLC-3 electrode exhibits the lowest IR drop regardless of current density, indicating the lowest internal series resistance. Figure 5e shows the gravimetric specific capacitance of HEBLCs electrodes calculated from the GCD curves at various current densities (0.5-50 A g-1), according to Eq. 2. As given in Figure 5e, S5, and Table S2, specific capacitances of 374 (1.26 F cm-2), 355 (1.10 F cm-2), 266 F g-1 (1.10 F cm-2), 203 F g-1(0.81 F cm-2), and 134 F g-1 (0.47 F cm-2) are obtained for HEBLC-2, HEBLC-3, HEBLC-4, AC-800, and C-800, 14


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respectively, at a current density of 0.5 A g-1. It is noteworthy that AC-800 with a high SBET of 2345 m2 g-1, however, displays a low capacitance (203 F g-1), which may be owing to its poor pore accessibility of electrolyte ions as discussed above. Intriguingly, HEBLC-2 shows a higher capacitance at lower current density (e.g. 0.5 A g-1), while HEBLC-3 exhibits superior capacitance at higher current density. This is probably because HEBLC-2 has more ultramicropores under 1 nm (as shown in Figure 4g) and a higher cumulative pore volume (Figure S3h), but HEBLC-3 possesses more suitable PSD for supercapacitors. It has been demonstrated that, the electrolyte ions on the electrode surface have no adequate time to diffuse into the inner pores if the charging current increases, because a large amount of micropores are difficult for electrolyte ions to transport especially at high current densities.41 Moreover, when the current density is up to 50 A g-1, HEBLC-3 electrode even can retain a capacitance of 230 F g-1, implying an excellent rate capability. Besides, HEBLC-3 manifests a higher specific capacitance than most of the HPCs prepared from other biomass precursors (Table S3). It is worthy of noting that the as-prepared HEBLC-3 from Kunming collected MOBs also exhibits high capacitance and excellent rate performance (Figure S6a-c). The effect of the active material loading on the electrode to electrochemical performance was also investigated. The capacitance slightly reduces with the raise of the active material loading for both HEBLC-3 and AC-800 when the current density is 1.0 A g-1 (Figure S7). Furthermore, when the loading of active material is increased from 4.0 to 12.0 mg, ca. 95% of the capacitance for HEBLC-3 can be achieved, which is superior to that of AC-800 (ca. 89%). These results indicate that the micro-scale “egg-box” structure of the as-resulted HEBLCs facilitates electrolyte ions to diffuse into inner active materials, especially with high active material loading. Thecyclic life stabilityof HEBLC-3 was analyzed by GCD of 20,000 cycles when the constant current density is 20 A g-1 (Figure 5f). It shows that, after 20,000 charge/discharge cycles, the resultant HEBLC-3 manifests high capacitance retention over 95% of the initial specific capacitance, suggesting its outstanding long-term cycling stability and excellent electrochemical reproducibility. Interestingly, there is a slight variation during the charge/discharge process, which may be due to the effect of the additional active surface areas exposed to the electrolyte, nitrogen-doping, and oxygen functional groups .12,51,60 The insert in Figure 5f displays the GCD curves of the 1st and 20,000th cycle, which 15



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present almost identical quasi-triangular and symmetric shape, indicating that the excellent electrochemical performance can be maintained even undergoing 20,000 charge/discharge cycles. To further assess the electrochemical property of the as-prepared HEBLCs, EIS measurements were performed in 6.0 M KOH electrolyte solution. In the Nyqusit plots of HEBLCs and AC-800 (Figure S8a, Supplementary Information), the intercepts on the real axis (Z′) in high frequency region represent equivalent series resistance (ESR), and steep linear curves at low frequency reveal nearly ideal capacitive behaviour. The diameter of semicircle is found to be the charge transform resistance (Rct) resulted from faradic reaction and the EDLCs, and the 45° sloped lines in the intermediate frequency indicate the Waburg impedance,61 as shown in the inset in Figure S8a. Notably, HEBLC-3 exhibits the lowest ESR (0.29 Ω), the smallest semicircle (0.10 Ω), the shortest length of 45° sloped region, and the steepest linear curve at low frequency, suggesting the rapid ion diffusion rate and transform of charge of HEBLC-3. In contrast, AC-800 with high ESR, Rct and Waburg impedance hinders the fast diffusion/transportation of electrolyte ions. The Bode plots of the three HEBLC and AC-800 electrodes, depicting that the phase angle varies with the frequency, as shown in Figure S8b. At high frequency ranges, the phase angle is near to zero, while at a frequency of 10 mHz, the phase angle nearly reaches to -90° (ideal capacitor). Correspondingly, HEBLC-3 exhibits the shortest time constant τ, which is defined as the inverse of the characteristic frequency at -45° in the Bode phase plots, of 0.8s, implying the fastest ion diffusion/transportation capability.62 The τ of AC-800 (3.3 s) is much longer than that of HEBLC-3, indicating a high ion diffusion/transportation impedance related to the poor pore structure. 5. Electrochemical Characterizations of Symmetric Capacitors in a Two-Electrode System. A symmetric supercapacitor based on the as-prepared HEBLC-3 was assembled in 6.0 M KOH solution to investigate the performance in the practical application for supercapacitors. Figure S9 displays the electrochemical performance of the as-assembled symmetrical supercapacitor. As depicted in Figure S9a and b, the CV and CGD curves reveal a good capacitive behaviour. The Ragone plot shows the relatively low specific energy density (6.3 Wh kg-1 at power density of 102.0 W kg-1), because of the low operation voltage of 6.0 M KOH solution (Figure S9c). It should be noted that over 90 % of capacitance retention after 3000 cycles can be achieved, indicating a good cycling stability in the alkaline solution (Figure S9d). 16


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In order to explore their practical application for supercapacitors with high energy density, neutral Na2SO4solution was also employed as electrolyte, due to the higher voltage window (0~1.8 V) than that of KOH aqueous solution (0~1.0 V), and its superior stability to organic electrolytes. Figure 6a depicts the CV curves at various voltage windows when the scan rate is 50 mV s-1 in 1.0 M Na2SO4 electrolyte. Even the voltage increased to 1.8 V, a good quasi-rectangular shape could be retained, suggesting that an ideal capacitive behaviour can be maintained when the voltage window is 0~1.8 V. Importantly, as shown in Figure 6b, good rectangular-like shape of CV curve scan be still kept even at a high scan rate of 300 mV s-1 without any deformation with an increase of the scan rate from 2 to 300 mV s-1, revealing an outstanding rate performance. GCD curves of the symmetric supercapacitor at various current densities exhibit triangular-like shape and low IR drop (Figure 6c), suggesting low internal series resistance and an excellent charge/discharge reversibility. The maximum capacitance of the cell calculated from GCD curves is ca. 45.2 F g-1 (26.7 F cm-3). Moreover, as displayed in Figure 6d, the maximum energy density is over 20 Wh kg-1 (11.8 Wh L-1) when the power density is 178.6 W kg-1 (105.4 W L-1), which is higher than most of the previously reported carbon-based symmetric supercapacitors in aqueous electrolytes.39,42,55,59,62-71 Figure 6e shows the cycle life over 10,000 cycles at a discharge current density of 5 A g-1. The high capacitance retention (ca.95.6%) of HEBLC-3 based symmetric supercapacitor explicitly indicates its outstanding long-term cycling stability. Besides, Figure S10a and b show the re-imaged FESEM and TEM images of HEBLC-3 electrode materials after 10,000 charge/discharge cycles. It can be observed that the hierarchical egg-box-like structure was well maintained, indicating the excellent structure stability of the as-resulted HEBLC-3 during the electrochemical process. For a better comparison, the electrochemical performance of the resultant HEBLC-3 in three-electrode system using 1.0 M Na2SO4 electrolyte was also studied in Figure S11. CV curves shown in Figure S11a present a good rectangular-like shape when the scan rate is 50 mV s-1. Figure S11b and c displays CV curves at various scan rates of 2-100 mV s-1 and CGD curves at different current densities ranging from 0.5 to 20 A g-1, respectively, revealing a good capacitive behaviour. The maximum capacitance of HEBLC-3 is ca. 255.5 F g-1 when the current density is 0.5 A g-1 and remains 70% of its primary capacitance when the current density reach to 20 A g-1 ( Figure S11d). 17



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Figure 6. Electrochemical performance of HEBLC-3 in a two-electrode system using 1.0 M Na2SO4 solutions. (a) CV curves of the supercapacitor conducted in various voltage windows when the scan rate is 50 mV s-1. (b) CV curves and (c) GCD of the supercapacitor at various scan rates from 2 to 300 mV s-1, and current densities from 0.2 to 10 A g-1. (d) Ragone plot of HEBLC-3//HEBLC-3 symmetric supercapacitors and comparison with other carbon based symmetric supercapacitors. (e) Cycle-life of symmetrical HEBLC-3 based supercapacitor.

Based on the above discussion, it can be concluded that the as-synthesized HEBLCs exhibit outstanding electrochemical performance, which may be ascribed to the features as follows: (i) the high surface area can furnish more active sites to accommodate more charges, forming more electric double layers to boost the overall capacitance, (ii) unique hierarchical egg-box-like interconnected framework structure will facilitate electrolyte ions to fast diffusing into inner pores of electrode materials during the rapid charge/discharge process, resulting in a low internal series resistance and excellent rate capability, 18


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(iii) doping of heteroatoms (O and N) not only offers the pseudo-capacitance for the over capacitance, but also improves the wettability of the electrode surface, resulting in reducing the diffusion impedance and an effective enhancement of active surface area for electrolyte ions to approach, (iv) stable carbon framework provides excellent structure stability and insures a long-term cycling stability at high current densities over 20,000 charge/discharge cycles, which is significant for practical applications.

CONCLUSIONS In summary, we have developed a facile, reliable, and effective method to prepare HEBLCs which

inherit the natural structure of the employed biomass of MOBs. The unique interconnected framework, high surface area, appropriate porosity, and doped heteroatoms (O and N), endow HEBLC-3 a high specific capacitance in a three-electrode system in 6.0 M KOH aqueous solution and remarkable long-term cycling stability (95% capacitance retention over 20,000 cycles charge/discharge process at a high current density of 20 A g-1). Remarkably, a high energy density of 20 Wh kg-1 in 1.0 M Na2SO4 electrolyte with the voltage window from 0 to 1.8 V and 95.6 % of capacitance retention were also achieved undergoing 10,000 charge/discharge cycles at current density of 5 A g-1 for the HEBLC-3 based symmetric supercapacitor. The outstanding performance clearly demonstrated that the interconnected HEBLCs derived from MOBs are promising electrode materials for high-performance supercapacitors. Furthermore, this work offers a novel and effective way to prepare higher-value hierarchical carbon nanomaterials from renewable biomass wastes, and the as-resulted HEBLCs may also find broad applications in fields of energy storage and conversion, including supercapacitors, lithium-sulphur batteries, catalysis, hydrogen storage, etc.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami. SEM images, element contents, high-resolution C1s, O1s and N1s of XPS spectra, Porosity parameters and capacitance performances, CV and GCD curves, areal capacitance, Bode and Nyquist plots of other resultant samples, the electrochemical measurements of HEBLC-3 in three-electrode system under 1.0 19



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M Na2SO4 solution, and electrochemical measurements of the as-assembled HEBLC-3//HEBLC-3 supercapacitor in 6.0 M KOH electrolyte.

AUTHOR INFORMATION

Corresponding Authors * E-mail: [email protected] (M. Zheng), [email protected] (Y. Liu), Tel/Fax: + 86 20 85280319. **E-mail: [email protected] (L. Sun). Author Contributions M.Z. and Y.L, conceived the experiments, Y.C., Y.L., and X.Z. designed and conducted the experiments, Y.X., H.H., H.D., and Y.L. analyzed the results, Y.C., M.Z., and L.S. wrote the manuscript. All the authors reviewed and approved the manuscript. Notes The authors declare no competing financial interest.

ACKNOLEDGEMENTS

The authors thank the financial supports from the National Natural Science Foundation of China (No. 21571066, U1501242, and 21371061), the Key Program of Science Technology Innovation Foundation of Universities of Guangdong Province (cxzd1113), Science and Technology Project of Guangdong Province (2014A010105038), and the key Laboratory of Functional Inorganic Materials Chemistry (Heilongjiang University), Ministry of Education, China. M. Z. also appreciates the support from China Scholarship Council.

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Table Of Contents (TOC)

A facile and low-cost strategy is developed to fabricate three-dimensional heteroatom-doped and hierarchical egg-box-like carbons (HEBLCs) with interconnected framework and excellent electrochemical properties for high-performance supercapacitors.

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Figure 1. Schematic of the synthesis process of MOB-derived HEBLCs. (a) MOBs, (b) MOB powders, (c) KOH solution immersed MOB powders, (d) pre-treated MOB powders, and (e) as-resulted HEBLCs. 170x93mm (300 x 300 DPI)



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Figure 2. (a) Digital photograph of moringa oleifera trees. Inset in (a) shows a photograph of MOBs. Optical photographs of MOBs in cross section (b) and longitudinal section (c). FESEM images of MOBs in cross section (d) and longitudinal section (e). (f) Magnified FESEM image of the red square in (e). FESEM images of pre-treated MOBs in cross section (g), longitudinal section (h), and (i) magnified FESEM image of the red square in (h). 120x90mm (300 x 300 DPI)


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Figure 3. FESEM (a-c), TEM (d, e), and HRTEM (f) images of the as-prepared HEBLC-3. Inset in (e) shows small mesopores in the carbon walls. 311x156mm (300 x 300 DPI)



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Figure 4. (a) XRD profiles, (b) Raman spectra of the as-synthesized HEBLCs. (c) XPS spectra of the samples, high-resolution (d) C 1s and (e) N 1s spectra of HEBLC-3. (f) N2 adsorption-desorption, (g) pore size distribution (PSD) curves. The inset in (g) shows the PSD curves from 20 to 80 nm. 160x147mm (300 x 300 DPI)


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Figure 5. Electrochemical performance in 6.0 M KOH electrolyte solution of HEBLCs measured in a threeelectrode system. (a) CV curves of the as-resulted HEBLCs at a scan rate of 50 mV s-1; (b) CV curves of HEBLC-3 at different scan rates from 5 to 200 mV s-1;(c) GCD curves of HEBLC-3, (d) IR (∆V) plots of HEBLCs, and (e) Specific capacitance of HEBLCs as a function of current density.at various current densities from 0.5 to 50 Ag-1, respectively. (f) Cyclic stability of HEBLC-3 over 20,000 cycles at a current density of 20 A g-1. The inset in (f) shows the GCD curves at 1st and 20, 000th cycles. 170x189mm (300 x 300 DPI)



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Figure 6. Electrochemical performance of HEBLC-3 in a two-electrode system using 1.0 M Na2SO4 solutions. (a) CV curves of the supercapacitor conducted in various voltage windows when the scan rate is 50 mV s-1. (b) CV curves and (c) GCD of the supercapacitor at various scan rates from 2 to 300 mV s-1, and current densities from 0.2 to 10 A g-1. (d) Ragone plot of HEBLC-3//HEBLC-3 symmetric supercapacitors and comparison with other carbon based symmetric supercapacitors. (e) Cycle-life of symmetrical HEBLC-3 based supercapacitor. 170x216mm (300 x 300 DPI)


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Facile Synthesis of Three-Dimensional Heteroatom-Doped and

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