Hierarchical porous carbon obtained from animal bone and evaluation in electric double-layer capacitors

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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

Hierarchical porous carbon obtained from animal bone and evaluation in electric double-layer capacitors Wentao Huang a, Hao Zhang

b,c

, Yaqin Huang

a,* ,

Weikun Wang b, Shaochen Wei

a

a

State Key Laboratory of Chemical Resource Engineering, The Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, 15 BeiSanhuan East Road, Beijing 100029, China b Research Institute of Chemical Defense, 35 Huayuan North Road, Beijing 100191, China c University of Science & Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Animal bone, an abundant biomass source and high volume food waste, had been converted

Received 3 June 2010

into a hierarchical porous carbon in a simple two-step sustainable manner to yield a highly

Accepted 16 October 2010

textured material. The structures were characterized by nitrogen sorption at 77 K, scanning

Available online 21 October 2010

electron microscopy and X-ray diffraction. The electrochemical measurement in 7 M KOH electrolyte showed that the porous carbon had excellent capacitive performances, which can be attributed to the unique hierarchical porous structure (abundant micropores with the size of 0.5–0.8 and 1–2 nm, mesopores and macropores with the size of 2–10 and 10– 100 nm), high surface area (SBET = 2157 m2/g) and high total pore volume (Vt = 2.26 cm3/g). Its specific capacitance was 185 F/g at a current density of 0.05 A/g. Of special interest was the fact that the porous carbon still maintained 130 F/g even at a high current density of 100 A/g. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Biological materials always possess elaborate structures and compositions, which grant them appropriate features to perform well in the nature system and can not be achieved through artificial synthesis absolutely. However, biological system, a vast material bank, can provide not only raw materials but also fantastic inspirations for human to develop new materials with refined mimic structures and outstanding functions like natural materials. Bioinspired design strategies are booming in the preparation of materials such as medical materials and other functional materials [1–4]. The development of novel materials exploited from natural preorganised systems, particularly those from inexpensive, abundant, and sustainable biomass would go in some way to achieving the goals of the future society. In this context the preparation of hierarchical porous carbon (HPC) materials from renewable natural resources is proposed.

HPC materials, owning interesting structures which include the combination of micropores, mesopores or macropores, display excellent potential for applications in gas storage, biosensor, catalysis, and energy storage [5–9]. Currently, HPCs are mainly synthesized using template methods [10–17]. With this approach, a carbon precursor/inorganic template composite is first formed, followed by carbonization, then chemical leaching of the template material. Such methodology is tedious, requiring multiple synthetic steps, especially the precursor infiltration into the template, caustic chemical treatments, and long curing times; scale-up has also proven difficult and is not cost-effective due to the destruction of expensive templates. The development of an inexpensive synthesis pathway for the generation of HPC materials through the recycling/utilization of natural biomaterials would be highly serviceable to overcome the weaknesses of the traditional template methods. Biominerals are mostly natural inorganic/organic composites and the inorganic and

* Corresponding author: Fax: +86 10 64438266. E-mail address: [email protected] (Y. Huang). 0008-6223/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.10.025

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organic components are often organised into nanometer sized domains. It is our hope that the biocomposites can act as useful precursor biomass for the preparation of HPCs, whereby the organic component can be carbonised in the presence of the (structure donating) inorganic component, which finally can be removed easily. Animal bone, a unique natural composite material, attracts our attention for the development of HPC as raw materials. First, bone is such a mineral and organic matrix [18,19], on a volumetric basis it consists of about 33–43% apatite minerals and 32–44% organics. Nano plate-like apatite crystals disperse within the discrete spaces within the collagen fibrils. The organic materials can act as carbon precursor, meanwhile the apatite crystals can act as natural template for the forming of abundant mesopores and macropores (30– 200 nm) [20]. Second, bone is a hierarchically structured material with concentric lamellae or plywood-like lamellae sub-microstructure, which is useful for forming hierarchical structure of the carbon materials. In addition, bone, one kind of food industry byproducts, is very cheap and environment friendly with abundant production, especially its application for preparation of porous carbon materials will be quite advantageous. Therefore we prepared HPC using animal bone natural template, in which the template preparation and tedious infiltration are unnecessary now. Furthermore, we tailored the pore structure and improved the surface area through chemical activation, and evaluated it in electric double-layer capacitor (EDLC). The results demonstrate that the tailored HPC exhibits attractive properties and is a promising electrode material for EDLC. The tailored HPC has the specific capacitance of 185 F/g at a current density of 0.05 A/g in 7 M KOH. Of special interest is the fact that it still maintains 130 F/g even at a high current density of 100 A/g.

2.

Experimental

2.1.

Preparation of HPC

The dried pig bone (purchased from market in Beijing) particles were pre-carbonized in a tuber furnace under N2 circumstance at 450 °C. Then the pre-carbonized bone particles were ground and carbonized again using the following heating program: (1) ramp at 2.5 °C/min to 850 °C, and hold for 1 h; (2) cool naturally to room temperature. The obtained product was washed in diluted HNO3, rinsed with distilled water, and dried at 110 °C for 12 h. The final product was named as HPC–0, some other ground pre-carbonized bone was mixed with KOH agent with a weight ratio of 1:1 and treated by using a similar procedure above, and the obtained product was labelled HPC-1.

2.2.

Structure and texture characterization

Micromeritics ASAP 2010 instrument was used to characterize the surface area and pore-structure of the carbon samples using N2 sorption at 77 K. The scanning electron microscopy (SEM) image was obtained on a HITACHI S-4700 electron microscope. Powder X-ray diffraction (XRD) pattern was

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recorded on a Rigaku D/max- 2500VB2+/pC diffractometer using Cu Ka radiation.

2.3. Electrode preparation and electrochemical measurements In order to evaluate the electrochemical performances of the as-prepared HPC-1 in EDLC, a mixture of 87 wt% of HPC-1, 10 wt% of acetylene black and 3 wt% of polytetrafluoroethylene (PTFE, as a binder, FR301B, 3F Corp., Shanghai, China) binder was pressed into pellets as electrodes. Then the electrodes were dried under vacuum at 120 °C for 12 h. A button-type capacitor was assembled with two carbon electrodes separated by polypropylene membrane using 7 M KOH aqueous solution as electrolyte. For comparison, a commercial activated carbon, YP17 from Kuraray Chemical Corporation, was used as a reference. A two-electrode system was used in cyclic voltammetry (CV), galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy. The galvanostatic charge/discharge test was carried out on a Land cell tester between 0 and 1.0 V. The cyclic voltammetry and impedance spectra were recorded on an electrochemistry workstation Solartron 1280B. The specific capacitance (Csp) of a single HPC-1 electrode was calculated from the discharge part of galvanostatic charge/discharge curves, by the formula Csp = 2It/ Vm, where I was the discharge current, t was the discharge time, V was the potential change in discharge between 0 and 1.0 V and m was the mass of the active electrode material in one electrode. The IR drop at the beginning of the discharge was omitted.

3.

Results and discussion

3.1.

Structure and texture characterization

The nitrogen adsorption–desorption isotherm and the corresponding DFT (density functional theory) pore size distribution curve of the HPC–0 are shown in Fig. 1. It can be seen from Fig. 1a that the isotherm of the HPC–0 taken up a shape of type IV (according to IUPAC classification). The steep increase of nitrogen uptake at low relative pressure close to 0.01 suggested the formation of micropores in large quantities, which could be produced by the dissociated leaving groups (e.g., CO2, H2O, H2S, NH3) caused by the pyrolysis of the organics. Obvious capillary condensation step (hysteresis loop) and the continuous accretion of nitrogen adsorption at the relative pressure from 0.4 to 1.0 indicated developed mesoporous structure and the appearance of macroporosity. Fig. 1b shows its pore size distribution, which was calculated from nitrogen desorption isotherms using DFT. The pores were mainly distributed in three regions: 0.6–0.9, 1.1–2.0 and 2–100 nm, while there were few macropores of pore size above 200 nm, representing two level micropores, mesopores and macropores respectively. This result indicated that the porous carbon prepared using bone possessed the hierarchical pore structure. The mesopores and macropores might be formed by the natural minerals, and the fact that the pore sizes were agreed with the sizes of apatite crystals in bone

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Fig. 1 – (a) Nitrogen adsorption–desorption isotherm and (b) the corresponding DFT pore size distribution curve of the HPC–0. The insert is a SEM image of the HPC–0.

demonstrates the role of apatite crystals as natural templates [18–20]. HPC-1 was developed by chemical activation using KOH. Fig. 2a and b displays the Nitrogen adsorption–desorption isotherm and the corresponding DFT pore size distribution curve. In Fig. 2a, the Nitrogen adsorption–desorption isotherm also showed a type IV shape with obvious hysteresis loop and the final increased tail at the relative pressure from 0.9 to 1.0. Compared with the HPC–0, the HPC-1 displayed more increment in nitrogen adsorption capacity at the entire relative pressure region, which suggested that additional pores were created and some small pores were widen by chemical activation [21,22]. The result can demonstrate that micropores, mesopores and macropores of the carbon were enhanced by KOH activation. It can be seen in Fig. 2b that the HPC-1 also has a hierarchical pore size distribution (mainly in four regions: 0.5–0.8, 1.0–2.0, 2–10 and 10–

100 nm), representing two level micropores, small mesopores, broad mesopores and macropores, respectively. Compared the DFT pore size distribution curves of the two samples, it can be seen that the volumes of two level micropores and small mesopores improved by KOH activation, meanwhile, the volume of broad mesopores and macropores decreased. The formation mechanisms of the hierarchical pore stuctrure need further investigation in detail. SEM image of the HPC-1 (inserted in Fig. 2b) also displays the microstructure of the porous carbon with interconnected pores. The textural characteristics of the HPC samples calculated from nitrogen adsorption–desorption isotherms are listed in Table 1. After KOH treatment, the specific surface area (SBET), total pore volume (Vt) and micropore volume (Vmi) of the hierarchical porous carbon increased remarkably from 861 to 2157 m2/g, 1.17 to 2.26 cm3/g, and 0.33 to 0.77 cm3/g,

Fig. 2 – (a) Nitrogen adsorption–desorption isotherm and (b) the corresponding DFT pore size distribution curve of the HPC-1. The insert is a SEM image of the HPC-1.

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Table 1 – Textural characteristics of the HPCs. Sample ID HPC–0 HPC-1 a b c d

SBETa (m2/g)

Vtb(cm3/g)

861 2157

1.17 2.26

Vmc (cm3/g) 0.33 0.77

Vmi/Vt

Waved (nm)

0.28 0.34

5.45 4.18

BET (Brunauer–Emmett–Teller) surface area, measured in relative pressure range of 0.04–0.20. Total pore volume, measured at P/P0 = 0.99. Micropore volume, calculated by HK (Horvath–Kawazoe) method from adsorption curve. Average pore width, estimated from the equation of 4 Vt/SBET.

respectively. The microporosity (Vmi/Vt) also rised from 0.28 to 0.34, but its amplification is lower than that of micropore volume, suggesting that additional mesopores were formed after activation. However, the average pore width (Wave) decreased from 5.45 to 4.18 nm. All the changes are probably due to the activation opening the closed pores, drilling new narrow micropores and widening the pre-existent pores. Furthermore, the weight ratio of KOH/pre-carbonized bone was only one, which is lower than that used in literatures [23,24], indicating that the porous and loose bone is in favor of improving the efficiency of activating agent and then cutting the cost. To examine the graphitic character of the HPC-1, wide angle powder XRD analysis was also performed (Fig. 3). The HPC-1 exhibited a broad reflection at a 2h value of about 24°, which may be attributed to the (002) reflection of a graphitic-type lattice. A weak reflection centered around 43° corresponds to a superposition of the (100) and (101) reflections of a graphitic-type carbon structure, indicating a limited degree of graphitization in this material.

3.2.

Electrochemical characterizations

Cyclic voltammetry measurements were conducted to test the EDLC performances of the HPC-1. Fig. 4a shows the cyclic voltammogram of HPC-1 based capacitor between 0 and 1.0 V scanned at 100, 200, and 500 mV/s. And the CV curves exhibit excellent rate performance without any redox peak in the chosen voltage range. It can be observed that the HPC-1 carbon presented a rectangular voltammogram shape at the 100 mV/s, indicative of excellent candidate as electrode material for electrochemical double-layer capacitor. Generally

Fig. 3 – XRD pattern of the HPC-1.

speaking, at higher scan rates, the CV profiles will usually deviate from the ideal rectangular shape due to polarization. But in this work, until the scan rate increased to 200 mV/s, the CV curve still remained its symmetrical rectangular shape, almost perfect for EDLCs. Even if the scan rate increased to as high as 500 mV/s, the CV curves almost kept rectangular, which reflected the capability of the HPC-1 based capacitor to cycle at high current densities. The rectangular shapes of all CV curves from HPC-1 are much better than those from the KOH-activated carbon nanotubes tested by the same way in [25]. According to the work of Wang et al. [11], ion-buffering reservoirs can be formed in the macropores to minimize the diffusion distances to the interior surfaces, the mesoporous walls provide low-resistant pathways for the ions through the porous particles, and the micropores strengthen the electric double layer capacitance. So this further confirms that the hierarchical porous structure helps to maintain good capacitive behavior of the HPC-1 at high sweep rates, which is due to fast ionic transportation within the mesopores and short diffusion distance from mesopores to micropores. A more detail investigation of the electrochemical properties of the sample HPC-1 was carried out by analysis of the electrochemical impedance spectrum (EIS). The Nyquist plots of HPC-1 electrode are shown in Fig. 4b. The curve presented a depressed semicircle in middle and high frequency region, and a nearly perpendicular line in low frequency region. The equivalent series resistance (ESR) of the HPC-1 was only 0.65 X, estimated from the value of the real axis at 1000 Hz, indicating the outstanding ionic conductivity which relates to the mobility of ions inside the pores and the electric conductivity of carbon materials. The previous XRD pattern in Fig. 3 also showed that the localized graphitic structure resulted in some ordered microcrystalline structure leading to high conductivity, which agreed with the quite low ESR of HPC-1. The nearly perpendicular line reflected the excellent properties of double layer capacitor of the HPC-1 electrode. Fig. 5 shows specific capacitance obtained at various discharge rates for the HPC-1. It is clear that with increasing discharge rate the specific capacitances decreased for the HPC-1, which was attributable to the decreased surface sites for electrochemical double layer formation. The specific capacitance of the HPC-1 reached 185 F/g at a current density of 0.05 A/g, which has a relatively high specific capacity [25,26]. Despite relatively high capacitance, the HPC-1 has slightly lower capacitance compared to some of the activated carbon with super high surface area. In future, it is maybe we can increase the surface area of HPC-1 by some methods which can be further improved the capacitance.

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Fig. 4 – (a) Cyclic voltammograms for the HPC-1 in 7 M KOH electrolyte corresponding EDLC systems at potential scan rates of 100, 200 and 500 mV/s and (b) Nyquist plots of HPC-1 electrode.

Fig. 5 – Specific capacitance at various discharging rates for the capacitors using HPC-1 and YP17 as electrode materials.

Fig. 6 – Charge–discharge curves of the HPC-1, current: (a) 100 A/g, (b) 50 A/g, (c) 40 A/g.

Moreover, the specific capacitance of the sample was dropped slowly as the current increases. When at 10 A/g, its specific capacitance only decreased to 143 F/g. However, its specific capacitance could be kept at 130 F/g even at a high current density of 100 A/g, which was 70.3% of the specific capacitance at 0.05 A/g, while the specific capacitance of commercial activated carbon YP17 were 158 F/g at 0.05 A/g and only 109 F/g at 10 A/g. Obviously, the HPC-1 carbon owned good capacitive performance at high currents and the efficiency of the ion reach micropores was really high because of the hierarchically porous structure. The galvanostatic charge/discharge curves of HPC-1 at different current densities are shown in Fig. 6. It showed that HPC-1 possessed typical triangular shapes with a little galvanostatic discharge decrease caused by the inner resistance throughout the current range of 40–100 A g 1, indicating good capacitive properties. The specific capacitances determined by Csp = 2It/Vm are 139, 139 and 130 F g 1 with the current of 40, 50, 100 A g 1 for HPC-1, respectively. The main reason

for the good performance at different charge/discharge rate may be that the mesopores/macropores in HPC-1 sample can improve the ion transfer and reduce the inner resistance of the electrodes.

4.

Conclusions

We report the fabrication and properties of novel hierarchical porous carbon materials using animal bone natural template and KOH activation, and its potential application as electrode materials for EDLC. Structure and texture analysis reveal that the porous carbon has high surface area (2157 m2/g), large pore volume (2.26 cm3/g), hierarchical pore structure of abundant interconnected micropores, mesopores and macropores and graphitic structure. Electrochemical studies revealed superior electrochemical performances of the porous carbon in KOH electrolyte. Its specific capacitance is 185 F/g at a current density of 0.05 A/g and maintains 130 F/g at 100 A/g, demonstrating superior capacitive properties. The investigation provides

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a simple, powerful, sustainable and cost-effective method to produce an advanced functional carbon materials with hierarchical porous structure, which could be an interesting candidate for not only energy storage but also other applications such as biosensor and catalysis.

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