C A R B O N 4 9 ( 2 0 1 1 ) 8 3 8 – 8 4 3

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Hierarchical porous carbon obtained from animal boneand 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, Chinab Research Institute of Chemical Defense, 35 Huayuan North Road, Beijing 100191, Chinac 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:

Received 3 June 2010

Accepted 16 October 2010

Available online 21 October 2010

0008-6223/$ - see front matter Crown Copyrdoi:10.1016/j.carbon.2010.10.025

* Corresponding author: Fax: +86 10 64438266E-mail address: huangyaqin9@gmail.com

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

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

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

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 � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Biological materials always possess elaborate structures and

compositions, which grant them appropriate features to per-

form 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 mate-

rials 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 develop-

ment 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.

ight � 2010 Published by

.(Y. Huang).

HPC materials, owning interesting structures which

include the combination of micropores, mesopores or mac-

ropores, display excellent potential for applications in gas

storage, biosensor, catalysis, and energy storage [5–9]. Cur-

rently, HPCs are mainly synthesized using template methods

[10–17]. With this approach, a carbon precursor/inorganic

template composite is first formed, followed by carboniza-

tion, 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 destruc-

tion of expensive templates. The development of an inexpen-

sive 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

Elsevier Ltd. All rights reserved.

C A R B O N 4 9 ( 2 0 1 1 ) 8 3 8 – 8 4 3 839

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, at-

tracts our attention for the development of HPC as raw mate-

rials. First, bone is such a mineral and organic matrix [18,19],

on a volumetric basis it consists of about 33–43% apatite min-

erals 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, mean-

while 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 infil-

tration are unnecessary now. Furthermore, we tailored the

pore structure and improved the surface area through chem-

ical activation, and evaluated it in electric double-layer capac-

itor (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 capaci-

tance 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) parti-

cles were pre-carbonized in a tuber furnace under N2 circum-

stance at 450 �C. Then the pre-carbonized bone particles were

ground and carbonized again using the following heating pro-

gram: (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 la-

belled 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

recorded on a Rigaku D/max- 2500VB2+/pC diffractometer

using Cu Ka radiation.

2.3. Electrode preparation and electrochemical measure-ments

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 polytetrafluoroethyl-

ene (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 but-

ton-type capacitor was assembled with two carbon electrodes

separated by polypropylene membrane using 7 M KOH aque-

ous 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 elec-

trochemical 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 gal-

vanostatic 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 corre-

sponding DFT (density functional theory) pore size distribu-

tion 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 in-

crease of nitrogen uptake at low relative pressure close to

0.01 suggested the formation of micropores in large quanti-

ties, 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 mes-

oporous 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 hierarchi-

cal 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

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.

840 C A R B O N 4 9 ( 2 0 1 1 ) 8 3 8 – 8 4 3

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 iso-

therm and the corresponding DFT pore size distribution

curve. In Fig. 2a, the Nitrogen adsorption–desorption iso-

therm 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–

Fig. 2 – (a) Nitrogen adsorption–desorption isotherm and (b) the

The insert is a SEM image of the HPC-1.

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,

corresponding DFT pore size distribution curve of the HPC-1.

Table 1 – Textural characteristics of the HPCs.

Sample ID SBETa (m2/g) Vt

b(cm3/g) Vmc (cm3/g) Vmi/Vt Wave

d (nm)

HPC–0 861 1.17 0.33 0.28 5.45HPC-1 2157 2.26 0.77 0.34 4.18

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

C A R B O N 4 9 ( 2 0 1 1 ) 8 3 8 – 8 4 3 841

respectively. The microporosity (Vmi/Vt) also rised from 0.28 to

0.34, but its amplification is lower than that of micropore vol-

ume, 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. Further-

more, the weight ratio of KOH/pre-carbonized bone was only

one, which is lower than that used in literatures [23,24], indi-

cating 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 an-

gle 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 gra-

phitic-type lattice. A weak reflection centered around 43� cor-

responds to a superposition of the (100) and (101) reflections

of a graphitic-type carbon structure, indicating a limited de-

gree 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 car-

bon presented a rectangular voltammogram shape at the

100 mV/s, indicative of excellent candidate as electrode mate-

rial 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, al-

most 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-buf-

fering reservoirs can be formed in the macropores to mini-

mize 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 fur-

ther 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 proper-

ties 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 con-

ductivity of carbon materials. The previous XRD pattern in

Fig. 3 also showed that the localized graphitic structure re-

sulted 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 dis-

charge rates for the HPC-1. It is clear that with increasing dis-

charge rate the specific capacitances decreased for the HPC-1,

which was attributable to the decreased surface sites for elec-

trochemical 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 fur-

ther improved the capacitance.

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.

842 C A R B O N 4 9 ( 2 0 1 1 ) 8 3 8 – 8 4 3

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 com-

mercial 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 effi-

ciency of the ion reach micropores was really high because

of the hierarchically porous structure.

The galvanostatic charge/discharge curves of HPC-1 at dif-

ferent current densities are shown in Fig. 6. It showed that

HPC-1 possessed typical triangular shapes with a little galva-

nostatic 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 den-

sity of 0.05 A/g and maintains 130 F/g at 100 A/g, demonstrat-

ing superior capacitive properties. The investigation provides

C A R B O N 4 9 ( 2 0 1 1 ) 8 3 8 – 8 4 3 843

a simple, powerful, sustainable and cost-effective method to

produce an advanced functional carbon materials with hierar-

chical porous structure, which could be an interesting candi-

date for not only energy storage but also other applications

such as biosensor and catalysis.

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