Natural Biomass-Derived Hierarchical Porous Carbon Synthesized by

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Natural biomass-derived hierarchical porous carbon synthesized by in-situ hard template coupled with NaOH activation for ultra-high rate supercapacitors Longfeng Hu, Qizhen Zhu, Qi Wu, Dongsheng Li, Zhongxun An, and Bin Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02299 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Sustainable Chemistry & Engineering

Natural biomass-derived hierarchical porous carbon synthesized by in-situ hard template coupled with NaOH activation for ultra-high rate supercapacitors Longfeng Hu a, Qizhen Zhu a, Qi Wu a, Dongsheng Li b, Zhongxun An c, Bin Xu a, ∗ a

State Key Laboratory of Organic-Inorganic Composites, Beijing Key Laboratory

of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beisanhuan East Road, Beijing, China 100029 b

Key laboratory of inorganic nonmetallic crystalline and energy conversion

materials (China Three Gorges University), University Road, Yichang 443000, China c

National Engineering Research Center of Ultracapacitor System for Vehicles,

Shanghai Aowei Technology Development Co., Ltd, Guoshoujing Road, Shanghai 201203, China *Corresponding author. E-mail addresses: [email protected] (B. Xu).

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ABSTRACT An in-situ hard template strategy coupled with NaOH activation is proposed to prepare hierarchical porous carbons with high surface area from biomass for high-performance supercapacitors. The preparation of the carbon includes the sol-gel process of lotus seed shell and sodium phytate, followed by carbonization and NaOH activation. The soluble sodium phytate is pyrolyzed to nano-Na5P3O10 during carbonization and then reacts with NaOH to convert to nano-Na2CO3 and nano-Na3PO4 particles, which are encapsulated in the carbon matrix as the in-situ hard templates and leave large mesopores/macropores after being removed in the subsequent washing treatment. Combined with the micropores created by NaOH activation, the as-prepared carbons possess developed hierarchical pores with a large surface area of 3188 m2 g1

and a pore volume of 3.2 cm3 g-1. Furthermore, the carbons are rich in the heteroatoms of O, N,

and P originated from the biomass precursors and sodium phytate. As a result, the biomassderived hierarchical porous carbon exhibits high capacitance (286 F g-1 at 0.5 A g-1), great rate performance (241 F g-1 at 200 A g-1), and excellent cycle stability in 6 M KOH electrolyte. Outstanding electrochemical performances are also achieved in 3 M H2SO4 electrolyte. Thus, the biomass-derived carbon with hierarchical porous structure and outstanding performance is considered as a promising electrode material for high-rate and high-energy density supercapacitors, and the strategy based on in-situ template method opens a new door for the preparation of hierarchical porous carbons with high surface area from biomass precursors.

KEYWORDS: natural biomass, hierarchical porous carbon, in-situ hard template, ultra-high rate performance, supercapacitors


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INTRODUCTION Nowadays, the concern in the overexploitation of fossil fuels and the resulting environmental pollution attracts considerable attention for clean and renewable energy sources as well as the effective energy storage systems. As one type of energy storage devices characterized with high power density, short charging/discharging time and ultra-long cycle life, supercapacitors have recently gained considerable attention owing to their wide applications in uninterruptible power supply, consumer electronics, electric vehicle and renewable energy sources utilization.1-3 Among various electrode materials, porous carbons possess large specific surface area, high conductivity, good chemical stability and low cost,4,5 which can not only offer electric doublelayer capacitances via physical adsorption of ions, but also produce pseudo-capacitances through the superficial redox reactions between the doped heteroatoms and the electrolyte.6-9 Therefore, porous carbons are the most widely used electrode materials in supercapacitors. The pore structure of the carbons has great influence on their electrochemical performances.10-14 As the energy stored in porous carbons mainly depends on the accumulation of electrical charge on the electrode/electrolyte interface, substantial micropores (50 nm) are helpful to shorten the transmission distance by providing ion-buffering reservoirs, guaranteeing high rate capability and high power density of supercapacitors.15 Therefore, it is proposed that hierarchical porous structure with interconnected micro-, meso-, and macropores is recognized as an ideal design to ensure the porous carbons present both high capacitance and excellent rate performance in supercapacitors.15-18


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Some strategies have been proposed to synthesize hierarchical porous carbons, such as double template method,19,20 combination of template carbonization and activation,21,22 and one-step activation of low carbon content polymers (PVDC, PVDF).23,24 Among these strategies, the combination of template carbonization and chemical activation has been the most promising way to prepare the hierarchical porous carbons. Firstly, the carbon precursors incorporated with hard templates, such as silica,25,26 nano-CaCO3,27,28 nano-MgO,29,30 nano-Fe2O3,31,32 and nanoZnO33,34 are carbonized at elevated temperature in an inert atmosphere to create large meso/macropores. And then, chemical activation is employed to introduce micropores with KOH or NaOH as activation agent. For template carbonization, the nanoparticle templates should be evenly dispersed in the carbon precursors to obtain template/precursor composites, which can be easily realized for polymer resin and carbohydrates by mechanical mixing or solution dispersion. Besides, the most widely used precursors include resin, gelatin, sucrose, starch and glucose, etc. 21,22,25-34

Recently, from the view of sustainable, green, and abundant resource, more and more attentions have been focused on the utilization of agriculture residues and forestry by-products to prepare porous carbons, such as shells, leaves, straws, seeds, etc.35-38 Moreover, as these natural substances are usually rich in heteroatoms, such as nitrogen and phosphorus, heteroatom-doped porous carbons can be easily obtained, which is helpful for increasing the performance of the carbons in supercapacitors. Chemical activation is a commonly used method to synthesize porous carbon from various natural biomasses. Although the surface area of the as-prepared carbons can reach over 2000 m2 g-1, the carbons are usually microporous carbons as the pore size is predominately below 2 nm. Therefore, these biomass-derived carbons can present high capacitance but poor rate performance due to the absence of mesopores and macropores. As the


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above mentioned nanoparticles such as nano-CaCO3 and nano-MgO can not be easily inserted into the biomass precursors, the widely used template carbonization-activation method is not suitable to natural biomass resources. Hence, it is a challenge to prepare carbon materials with well-developed hierarchical porous structure from natural biomass. Under these circumstances, in this work, we propose a novel and effective strategy for fabricating a natural biomass-derived porous carbon with a hierarchical porous architecture based on an in-situ template coupled with NaOH activation. Lotus seed shell, a nitrogencontaining agriculture waste in lotus seed processing, is used as the carbon precursor, and sodium phytate is used as the hard template precursor. The lotus seed shell and sodium phytate aqueous solution are thoroughly mixed at 60 °C to form a mixed gel. The soluble sodium phytate is pyrolyzed to nano-Na5P3O10 during carbonization and then reacts with NaOH to convert to nano-Na2CO3 and nano-Na3PO4 particles, which are homogeneously dispersed in the carbon substrate and leave large mesopores/macropores after washing treatment. Combined with the micropores created by NaOH activation, a well-developed hierarchical porous carbon with hollow nest-like structure is obtained. This biomass-derived hierarchical porous carbon exhibits a high specific surface area of 3188 m2 g-1 and numerous interconnected pores with a large pore volume up to 3.20 cm3 g-1 composed of micro-, meso-, and macropores. Benefiting from the well-developed hierarchical porous structure, the obtained carbon shows ultrahigh rate capacitances of 241 F g-1 and 217 F g-1 at 200 A g-1 in 6 M KOH and 3 M H2SO4 electrolyte, respectively. The biomass-derived porous carbon with the advanced hierarchical architecture and excellent performance is a promising electrode material for high-rate and high-energy density supercapacitors. Furthermore, a green and sustainable strategy is introduced to synthesize


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hierarchical porous carbon from biomass, which can be easily industrialized for turning the agricultural residues into electrode materials.

EXPERIMENTAL SECTION Synthesis of biomass-derived porous carbon materials. The preparation of the porous carbon from lotus seed shell is schematically illustrated in Scheme 1, including sol-gel reaction, and the following carbonization and NaOH activation. Lotus seed shell produced from Hubei Province, China, was used as carbon precursor, and sodium phytate was used as template precursor and phosphorus source. Firstly, the dried lotus seed shell was grinded into powders. Sodium phytate was dissolved in deionized water under stirring at 60 °C for several minutes to form a uniform solution in concentration of 0.45 g mL-1, then the lotus seed shell powders were added. After stirring at 60 °C for 5 h, sol-gel reaction occurred and the resultant reddish brown and sticky gel was kept in an oven at 100 °C for 6 h to obtain the biomass/sodium phytate mixed gel. Secondly, the biomass/sodium phytate gel was calcinated at 450 °C for 1 h in a tube furnace under flowing N2 (200 mL min-1) with a heating rate of 5 °C min-1. Then the pyrolysized products (named as PP450) was mixed with NaOH in a PP450/NaOH mass ratio of 1:2. Subsequently, the mixture was heated at 650 °C for 2 h with a heating rate of 5 °C min-1 under N2 atmosphere. Finally, after washed with HCl to remove the inorganic templates, and then washed with deionized water for several times to neutral, the final hierarchical porous carbons were obtained. The porous carbons prepared with sodium phytate/biomass ratio of 0:1, 1:1, 2:1 and 3:1 were denoted as LAC, HBC1, HBC2 and HBC3, respectively. In addition, the control


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sample was synthesized by heating the PP450 at 650 °C for 2 h with a heating rate of 5 °C min-1 under N2 atmosphere without the addition of NaOH, which was signed as PP650. Materials characterizations. To investigate the morphologies of the carbons, scanning electron microscope (SEM, Hitachi S-4800) and high-resolution transmission electron microscope (HR-TEM, JEOL 2100) were carried out. N2 (77 K) adsorption and desorption isotherms were performed on Micrometrics (ASAP 2460) to measure the porosity of the carbons. The specific surface area and the pore size distribution of the samples were characterized using the Brunauer-Eumett-Teller (BET) and the density function theory (DFT) methods, respectively. The total pore volume was taken from the amount adsorbed at a relative pressure of p/p0=0.99. The micropore volume (Vmic) was calculated with the Dubinin–Radushkevich analysis in the relative pressure range from 10-4 to 10-2, while the mesopore/macropore volume (Vmeso-macro) was obtained by subtracting the micropore volume from the total pore volume. The X-ray diffraction (XRD) measurements were employed on a X-ray D8 Advance Instrument (Cu Kα) in the Bragg's angle (2θ) range of 5-90°. The ray photoelectron spectroscopy (XPS) with the excitation source of Al Kα X-rays was used to study the electronic and structural properties of the porous carbons. Electrochemical measurements. The electrochemical performances of the biomass-derived hierarchical porous carbons were measured using both two-electrode and three-electrode test systems. The working electrodes were prepared by mixing the carbons (85 wt%) as active materials, acetylene black (10 wt%) as conductive agents and polytetrafluoroethylene (PTFE, 5 wt%) as binder. The slurries were pressed onto nickel foam and then cut into pellets with a diameter of 10 mm. After dried at 120 °C in a vacuum oven for 4 h, the electrodes were obtained. The active material loading of the electrodes is about 8~10 mg cm-2. The electrochemical


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measurements of the carbons were detected in both two-electrode setups and three-electrode systems, including cyclic voltammetry (CV) carried out on CHI1100C electrochemical workstation, galvanostatic charge / discharge (GCD) test conducted on an Arbin BT2000 cell tester, and the electrochemical impedance spectroscopy (EIS) recorded on a CS350 electrochemical workstation. For two-electrode setups, button type capacitors were assembled with two as-formed electrodes separated by polypropylene membrane using 6 M KOH aqueous solution as electrolyte. The specific capacitance (Cs, F g-1), energy density (E, Wh kg-1) and the power density (P, W kg-1) are calculated from galvanostatic charge/discharge curves according to the equations as follows:

Cs=

4I∆t

…………………………(1)

m∆V

‫=ܧ‬

଴.ହ஼௏ మ ଷ.଺

ܲ=

…………………(2)

ா ௧

……………………..………(3)

where I, ∆t, ∆V and m is the discharge current density (A), time (s), potential change in discharge (V) and mass of the active material (g), respectively. Three-electrode test systems were also employed to evaluate the electrochemical performances in 3 M H2SO4 aqueous solution, using active carbon (the mass is over ten times than that of working electrode) as the counter electrode and Ag/AgCl electrode as the reference electrode. The operation potential range was set from -0.2 to 0.7 V. The discharge specific capacitance is calculated from the GCD curves according to the formula: Cm =

ூ∆௧ ௠∆௏

, where Cm (F g-1) is the


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specific capacitance based on the mass of the active material in working electrode, I (A) is the discharge current, and ∆t (s) is the time consumed in potential window of ∆V (V) during the discharge process.

Scheme 1 Schematic illustration of the preparation of natural biomass-derived hierarchical porous carbons

RESULTS AND DISSUCTION As sodium phytate acts as template resource, the effect of the amount of sodium phytate on the porous structure of the carbons was investigated. The morphologies of the HBCs samples prepared with different sodium phytate/lotus seed shell mass ratios, and the carbon obtained by direct carbonization of lotus seed shell at 650 °C (LSS650) for comparison, were characterized by SEM and HR-TEM images as shown in Fig. 1. Fig. 1a indicates the lotus seed shell-derived carbon (LSS650) has typical fibrous fractures with a diameter of ~400 nm, close packing of


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blocks and smooth surface, implying a non-porous carbon. Fig. 1b shows the SEM image of LAC prepared by conventional carbonization of lotus seed shell and subsequent NaOH activation without sodium phytate addition. Due to the etching effect of NaOH at high temperature, the fibrous fractures of the char were cut into fragment parts with rough surface. As NaOH activation can only create small micropores,21,37 no large pores can be observed. In contrast, with sodium phytate added into lotus seed shell as a template resource, the as-prepared carbons show honeycomb-like morphology with abundant large mesopores/macropores (Fig. 1c-e). With the increase of sodium phytate/lotus seed shell mass ratio, the porous structure becomes more developed and the pore size becomes larger. As shown in Figure. S1, the pore size transforms from 3-8 nm of HBC1 to 5-15 nm of HBC2. When the mass ratio of sodium phytate/lotus seed shell increases to 3:1, parts of small pores turn into the larger ones with the size of over ~30 nm. HR-TEM image (Fig. 3f) confirms the developed interconnected porous structure of HBC2 with pore size ranging in 2-60 nm. The disordered graphite ribbons indicate amorphous carbon, and the defects on the wall of the mesopores confirm the existence of micropores created by NaOH activation. However, for the HBC3 (Fig. 1e), some ultra large pores with size of several hundred nanometers are observed accompanied by the collapsing structure, which can be ascribed to the agglomeration of the template nanoparticles. These results indicate the crucial role of sodium phytate in the porous structure creation of the biomass-based carbons.


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Fig. 1. The SEM images of (a) LSS650; (b) LAC; (c) HBC1; (d) HBC2 and (e) HBC3; (f) the HR-TEM image of HBC2.

In order to further characterize the porous structure of the HBCs, nitrogen (77K) adsorptiondesorption isotherms were performed as shown in Fig. 2a. LAC prepared by carbonization of lotus seed shell and the following NaOH activation without sodium phytate addition shows typical type I adsorption-desorption isotherms,39 characteristic of microporous structure. With sodium phytate introduced, the HBCs show combined type I and type IV isotherms with much enhanced nitrogen adsorption volume, indicating more developed porous structure.32 The sharp N2 adsorption increase at low relative pressure (p/p0 < 0.01), the continuous increase at p/p0=0.10.5 and the large hysteresis loop in the p/p0 range of 0.5-0.9 indicate the presence of a large amount of mesopores as well as some micropores. The DFT pore size distribution curves plotted in Fig. 2b indicate the LAC has an overwhelming majority of micropores (

Natural Biomass-Derived Hierarchical Porous Carbon Synthesized by

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