Bioresource Technology 189 (2015) 285–291

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Bioresource Technology

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Converting biowaste corncob residue into high value added porouscarbon for supercapacitor electrodes

http://dx.doi.org/10.1016/j.biortech.2015.04.0050960-8524/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: wencuili@dlut.edu.cn (W.-C. Li).

Wen-Hui Qu, Yuan-Yuan Xu, An-Hui Lu, Xiang-Qian Zhang, Wen-Cui Li ⇑State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P.R. China

h i g h l i g h t s

� Corncob residue was used to prepareporous carbon for supercapacitorelectrodes.� Green and low-cost steam activation

without pre-carbonization was used.� The obtained porous carbon achieves

high surface area and high yield afterash-removal.� The porous carbon exhibits a high

capacitance of 314 F g�1 in 6 M KOHelectrolyte.� No capacitance decay was observed

after 100,000 cycles for a symmetricalcell.

g r a p h i c a l a b s t r a c t

A high value added porous carbon was obtained from a corncob residue, which shows a superior capaci-tive performance compared to those polymer-based synthetic carbons as electrode material for asupercapacitor. The corresponding symmetrical cell shows a superb cycling stability. Almost no capaci-tance decay was observed after 100,000 cycles.

%

120120 F g-1 for a Symmetrical Cell

Cycle number / 1040 2 4 8 10

Capa

cita

nce

rete

ntio

n /

0

20

40

60

80

100

Biowaste Corncob

Porous CarbonElectrode Material

6

a r t i c l e i n f o

Article history:Received 6 March 2015Received in revised form 31 March 2015Accepted 1 April 2015Available online 4 April 2015

Keywords:BiowasteCorncobPorous carbonSupercapacitor

a b s t r a c t

In this report, corncob residue, the main by-product in the furfural industry, is used as a precursor toprepare porous carbon by a simple and direct thermal treatment: one-step activation withoutpre-carbonization. As a consequence, the corncob residue derived porous carbon achieves a high surfacearea of 1210 m2 g�1 after ash-removal. The carbon material has the advantages of low cost and lowenvironmental impact, with a superior electrochemical performance compared to those polymer-basedsynthetic carbons as electrode material for a supercapacitor. The carbon electrode exhibits a highcapacitance of 314 F g�1 in 6 M KOH electrolyte. The corresponding sample also shows a superb cyclingstability. Almost no capacitance decay was observed after 100,000 cycles. The excellent electrochemicalperformance is due to the combination of a high specific surface area with a fraction of mesopores andhighly stable structure.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Electrical double layer capacitors (EDLC), also calledsupercapacitors, as a class of state-of-the-art energy storage

devices, have been attracting significant research interest latelydue to their wide range of applications in electrical vehicles, digitaldevices, pulsing techniques, etc (Choi et al., 2012; Dyatkin et al.,2013). The storage of electrical energy in supercapacitors is basedon the electrostatic attraction of the opposite charges occurred ona double-layer at the electrode/electrolyte interface. Thus,supercapacitor is effective for instantaneous charge–discharge of

286 W.-H. Qu et al. / Bioresource Technology 189 (2015) 285–291

large current supplies, compared to redox reaction based batteries.It exhibits a higher specific power (�10 kW kg�1) and longer cyclelife (�100,000 cycles) compared with other rechargeable electro-chemical energy storage devices, such batteries and fuel cells.

Carbon-based materials are currently pursued as supercapacitorelectrodes due to their large surface area, high electrical conductiv-ity and good capacitive performance (Jiang et al., 2013; Zhang andZhao, 2009). Considering the demand for sustainable eco-friendlyresources and process, more and more attention has been focusedon transforming biomass into valuable carbon materials, due toabundance and low cost of biomass precursors, such as beer waste(Hao et al., 2014), human hair (Saravanan and Kalaiselvi, 2015), nutshells (Xu et al., 2014), plant leaves (Biswal et al., 2013), pollens(Zhang et al., 2013), cotton stalk (Chen et al., 2012) and sugar canebagasse (Rufford et al., 2010).

According to the recent report from United States Departmentof Agriculture (USDA), global corn production for 2013/14 reached989.2 million metric tons. Assuming that all current corncobs arecollected, 41.1–54.7 million metric tons of corncobs are annuallyavailable. As a widely available and environmentally friendly bio-mass, agricultural residue, corncob has been employed for thelarge-scale production of biofuels and varieties of high value addedchemicals (Li et al., 2010; Mao et al., 2013). For example, usingcorncobs as the raw feedstock can product furfural, one of the mostuniversal industrial chemicals (Li et al., 2014; Oh et al., 2013;Zhang et al., 2014). A million tons of corncob residues (by-productfrom the process of furfural production) are generated. It has beenestimated that around 12–15 metric tons of residue is obtainedafter 1 metric ton of furfural is produced. An average of 23 millionmetric tons of corncob residues is available annually for alternativeuses in China (Sun et al., 2011). Despite their abundance, theinvestigations on how to transform corncob residue into valuableproducts are still very limited. It is necessary to find green and sim-ple solutions to turn such a significant amount of biomass residueinto high value added materials. It has been reported that corncobresidue can be used to prepare porous carbons for water pollutantsremoval and gas separation after chemical activation or a combina-tion with microwave irradiation (Hou et al., 2013; Sun et al., 2012).

Corncob residue mainly composes of lignin and cellulose(Mamman et al., 2008; Zeitsch, 2000). Lignin is a three-dimen-sional, highly cross-linked poly-phenolic polymer without anyordered repeating units. Cellulose is a polysaccharide consistingof a linear chain of several hundred to many thousands of linked

D-glucose units, which is organized into micro-fibrils surroundedby hemicellulose and encased inside a lignin matrix. The highlyheterogeneous of corncob residue can cause preferential etch oneor several of the phases in the process of pyrolysis and activation.Therefore, such multi-phase tissue is an ideal precursor to achieveporous carbon with a high porosity.

The conventional physical activation process is a two-step pro-cess: (i) pyrolysis in an inert atmosphere (usually nitrogen) of theprecursor normally at 400–900 �C to eliminate the bulk of volatilematter; (ii) partial gasification using an oxidizing gas at350–1000 �C to develop the porosity and surface area. The porousstructure of obtained porous carbons depends on precursor, thetemperature and degree of activation. Although this process hasbeen sufficiently studied (Sevilla and Mokaya, 2014), the regimesof activation for different raw materials still requires optimization.

Considering the environmental concern and energy consump-tion, it is still a challenge to develop an easy and benign methodto prepare porous carbons from biowaste corncob residue.Meanwhile, corncob residue derived porous carbon for supercapaci-tor electrodes was rarely reported. In this work, a one-stepactivation process is demonstrated for the preparation of porouscarbons from corncob residue. The porous carbons can be made

with high surface area up to 1210 m2 g�1, and exhibit a high capaci-tance of 314 F g�1 at a scan rate of 5 mV s�1, when they were used asan electrode material for supercapacitors. The capacitance is still259 F g�1, even at a high scan rate of 100 mV s�1. The capacitanceretention is as high as 82%. Almost no capacitance decay wasobserved after 100,000 cycles. The maximum energy density inaqueous and organic electrolyte is 6.8 Wh kg�1 and 17 Wh kg�1,respectively. Such a remarkable performance shows that corncobresidue derived porous carbon is a promising material for commer-cial supercapacitors. This study also opens a new approach for thepreparation of high value added products from biowaste.

2. Methods

2.1. Materials

Corncob residues were obtained from a continuous acidhydrolysis process of corncob-to-furfural. Before processing, thecorncob residue was water-rinsed to remove the residual acid,dried at 90 �C overnight, and then sieved for irregular granuleswith the size of ca. 150–250 lm. KOH (Sinopharm ChemicalReagent Co., Ltd., China, AR) was used as received without any fur-ther purification. Distilled water was used to prepare solutions andwash samples.

2.2. Proximate analysis of corncob residue

The proximate analysis of the corncob residue was carried outto estimate the characteristics. The moisture content was deter-mined as the weight loss in an air oven at 110 �C until a constantweight. A silica crucible and a muffle furnace were used to estimateash (heating up to 815 �C for 1 h) and volatile matter (heating up to900�C for 7 min) contents. Fixed carbon content was calculated bythe difference. All analyses were performed in triplicate.

2.3. Sample preparation

In a typical one-step activation process, i.e., steam activationwithout pre-carbonization, was conducted by directly activatingcorncob residues (CR) at 750, 800 or 850�C. The obtained carbonswere accordingly denoted as CR-x, where x represented the activa-tion temperature. CR-750, CR-800 and CR-850 were prepared byheating the raw material to a set activation temperature (750,800 or 850�C) in a N2 atmosphere, and then immediately switchedto steam/N2 at this temperature and maintained for 45 min. Thesteam used was generated by a steam generator at a constant200 �C and carried into the reactor with nitrogen carrier gas.

CR-850 was further treated by ash removal with acid soaking,subsequent alkali immersing, deionized water washing and drying.The obtained sample was denoted CR-850-RA. For comparison, acontrol simple was prepared firstly by a pre-carbonization, whichwas heated up to 350 �C for 1 h. Subsequently, the sample washeated to 850 �C and activated by steam for 45 min (denoted CR-CA-850). All heating rates in the experiments are 5 �C min�1.

2.4. Characterization

Thermo gravimetric analysis was performed in a temperaturerange of 25 �C to 800 �C under N2 with a heating rate of 10 �C min�1

using a STA 449 F3 Jupiter thermo gravimetric analyzer (NETZSCH).Scanning electron microscopy (SEM) was carried out with a HitachiS-4800 instrument. Nitrogen adsorption/desorption isothermswere measured with a Micromeritics TriStar 3000 physisorptionanalyzer. Prior to the test, the samples were degassed under vac-uum at 200 �C for at least 4 h. The Brunauer–Emmett–Teller

W.-H. Qu et al. / Bioresource Technology 189 (2015) 285–291 287

(BET) equation and t-plot method were utilized to calculate thespecific surface area (SBET) and micropore surface area (Smic),respectively. Pore size distributions were determined by theadsorption branch of the isotherms using density functional theory(DFT) for micropores and the Barrett–Joyner–Halenda (BJH) modelfor mesopores. The total pore volume (Vtotal) was calculated fromthe amount adsorbed at a relative pressure P/P0 of 0.97. Themicropore volume (Vmicro) was calculated using the t-plot method.Fourier transform infrared spectroscopy (FTIR) was performedwith a Nicolet 6700 (Thermo Scientific Co., Ltd., USA) by averaging64 scans in the 670–4000 cm�1 spectra range at 4 cm�1 resolution.Elemental analysis was carried out on a CHNO elemental analyzer(Vario EL III, Elementar).

2.5. Electrochemical evaluation

To evaluate the electrochemical performance of porous carbons,the active material (90 wt.%) was ground and then mixed withpoly(tetrafluoroethylene) (PTFE) binder (10 wt.%) in 7 mL of etha-nol followed by an ultrasonication treatment for 25 min. Thehomogenized mixture was then dried in 50 �C, rolled into a thinfilm accompanied by adding several drops of N-methyl-2-pyrroli-done (NMP). Then the electrode film was dried at 150 �C for 4 h.After NMP was completely evaporated, the electrode film wouldbe tailored to a proper size, placed on a foam nickel current collec-tor and pressed under 10 MPa for 5 min to fabricate an electrode.The mass loading of the active material is ca. 4–6 mg cm�2.

The electrochemical performance was evaluated by cyclic vol-tammetry (CV) and galvanostatic charge/discharge cycling (GC)test technology in 6 M KOH aqueous electrolyte. The test was per-formed on a CHI660D electrochemical workstation (CHInstruments Inc., Shanghai, China) with a conventional three-elec-trode setup, in which the active material electrode served as theworking electrode and a platinum filament coil, and Hg/HgO wereused as the counter electrode and reference electrode, respectively.A symmetric capacitor was assembled in a coin cell with two iden-tical electrodes. A membrane filter (MPF50AC purchased fromNippon Kodoshi plant, Japan) was used as a separator. The perfor-mance overview of the symmetric capacitor was obtained.Electrochemical impedance spectra (EIS) and cycle life were mea-sured using CHI660D and an Arbin SCTS-165,699-T multichannelelectrochemical workstation, respectively. The specific gravimetriccapacitance of the single electrode tested in a three-electrode sys-tem derived from the CV tests was calculated by Eq. (1):

CðF g�1Þ ¼R Vb

VaIdV

mmðVb � VaÞð1Þ

where I (A) is the instant current based on CV curves, m (g) the massof active material on the electrode, m (V s�1) is the potential scanrate, Va (V), and Vb (V) were low and high potential limits, respec-tively. The specific gravimetric capacitance of a single electrodewas also calculated from GC curves by Eq. (2):

CðF g�1Þ ¼ I � Dtm � DV

ð2Þ

where I (A) is the discharge current, Dt (s) is the discharge time, m(g) is the mass of active material on the electrode and DV (V) is thevoltage range from the end of the voltage drop to the end of the dis-charge process. For the supercapacitor cell, the specific capacitancewas calculated from the galvanostatic discharge process, accordingto Eq. (3):

CðF g�1Þ ¼ 4I � Dtm � DV

ð3Þ

where I (A) is the discharge current, Dt (s) is the discharge time, DV(V) is the voltage range from the end of the voltage drop to the end

of the discharge process and m (g) is the total mass of active mate-rial in two electrodes.

The capacitance of a capacitor cell (Ccell) was calculated fromthe discharge process after 20 cycles’ activation, according to theequation below:

CcellðF g�1Þ ¼ I � DtDV

ð4Þ

where I (A g�1) is the discharge current based on the total mass ofactive material, the definitions of Dt (s) and DV (V) are as same asthose in Eq. (3). The energy density (E) and power density (P) wereobtained from the capacitance of a capacitor cell (Ccell):

EðWh kg�1Þ ¼ Ccell � V2

2� 3:6ð5Þ

PðW kg�1Þ ¼ 3600� EDt

ð6Þ

where Dt (s) is the discharge time.

3. Results and discussion

Chemical compositions of corncobs before and after furfuralproduction are listed in Table S1 according to the correspondingresearches. It can be seen that content of hemicelluloses decreasefrom 39.8% for corncobs to 3.6% for corncob residues, suggestingthat hemicelluloses were significantly removed during furfuralproduction. Prior to activation, the corncob residue was character-ized using proximate analysis method. The residue comprised fixedcarbon (48.23 wt.%), volatiles (42.11 wt.%), moistures (4.63 wt.%)and ash (5.03 wt.%). Due to high fixed carbon and low ash content,the corncob residue was considered to be a good precursor forpreparing porous carbons.

Mass loss steps in the pyrolysis process can be seen in thethermo gravimetric (TG) curve of the corncob residue (Fig. S1).The mass loss during the pyrolysis of corncob residue can bedivided into two steps according to the differential thermal gravi-metric (DTG) curve (Fig. S1). The first step occurs around 100 �C,which is related to desorption of physically adsorbed moisture.As the temperature increases, the decomposition of corncob resi-due occurs and an evident dTGA/dt peak around 390 �C is ascribedto elimination of volatiles from cellulose. When the temperature ishigher than 390 �C, the TG curve tends to flatten due to a slowdecomposition of lignin (Kruse et al., 2013; Wettstein et al.,2012). When the pyrolysis temperature reaches 800 �C, 58 wt.%of the initial mass is left. The mass loss is associated with elim-ination of the heteroatoms (N, O, H) from the carbonaceous matter,which is consistent with the element analysis results (see below).

The structural parameters of the porous carbons prepared areshown in Table 1 using a nitrogen sorption technique at 77 K. Itcan be seen that the SBET and Vtotal of the samples increase withthe increase of activation temperature from 750 to 850 �C, whichresults from faster activating reactions at the higher temperature.On the other hand, the yield decreases gradually with the increaseof activated temperature. CR-850 has a lower yield than that of CR-750 and CR-800 because more volatile matter burns at a high tem-perature. CR-850, directly activated at 850 �C without pre-car-bonization, has a SBET of 1043 m2 g�1, as high as CR-CA-850(1018 m2 g�1). Because the yield and SBET of products are reason-ably high, one-step activation method is suitable to prepare CRderived porous carbon.

A lower yield of CR-850 can result in a higher ash level. Toinvestigate the effect of ash level on the structural properties ofporous carbons, the sample CR-850 with high ash content was cho-sen to undergo an ash removal treatment. The obtained sample,CR-850-RA, has the highest SBET of 1210 m2 g�1, with good yield

Table 1Structural parameters of corncob residue derived porous carbons.

Samples Yield/wt.%

SBET/m2 g�1

Smic/m2 g�1

Vtotal/cm3 g�1

Vmic/cm3 g�1

CR (rawmaterial)

– 6 – – –

CR-750 36.6 872 645 0.479 0.299CR-800 28.6 916 677 0.489 0.312CR-850 24.5 1043 757 0.572 0.349CR-850-RA 23.2 1210 900 0.671 0.416CR-CA-850 27.2 1018 757 0.565 0.348

Table 2Elemental analysis of corncob residue derived porous carbons.

Samples C H N Oa

CR 65.30 4.30 1.06 29.35CR-750 85.61 1.22 0.71 12.46CR-800 90.50 0.89 0.76 7.85CR-850 93.32 0.95 0.45 5.28CR-850-RA 87.78 1.17 0.68 10.37

a O is estimated by difference.

288 W.-H. Qu et al. / Bioresource Technology 189 (2015) 285–291

of 23.2 wt.%, as shown in Table 1. Its microporous surface area(Smic) increases from 757 to 900 m2 g�1 after ash removal. Thisdemonstrates that the process of ash removal is efficient forpore-forming, especially so for well-developed micropores. Thus,the high SBET combined with a fraction of mesopores renders theporous carbons hopeful as effective electrode materials forsupercapacitors.

Fig. 1a shows the nitrogen adsorption and desorption isothermsof the samples obtained under different conditions. All samplesgive Type IV isotherms, which confirm the presence of mesopores.The total specific surface area of these samples herein is found tobe in the range of 872–1210 m2 g�1 with an approximate averagepore size of 2.2 nm (Fig. 1b).

The surface morphologies of the raw material and some typicalporous carbon samples are presented (Fig. S2). It can be seen thatthe micrograph of the raw material shows almost no pore struc-ture. After activation, the unordered porous structure forms onthe surface of the sample. This can be attributed to the release ofvolatile matter and meanwhile the steam activation results in sur-face etching on the raw material. A certain amount of porosity wasgenerated after ash removal (Fig. S2d). This is ascribed to the elim-ination of inorganic impurities generated during a high tempera-ture process, which leads to the opening of some closed pores.The surface morphologies coincide with the results on the struc-tural parameter properties. Porosity with abundant microporesand well-developed mesopores was fabricated by removing ash.The framework of CR-850-RA is suitable for good electrical conduc-tivity. Hence, this sample was expected to perform as an excellentelectrode material.

In order to study the surface chemistry of the carbons, qual-itative identification of the functional groups was examined byFTIR over the range of 4000–400 cm�1 (Fig. S3). All the carbon sam-ples present a broad band at ca. 3400 cm�1 which is assigned to thestretching vibration of the hydroxyl groups. The absorptionobserved at ca. 2900 cm�1 corresponds to asymmetric C–H

P / P0

V ads

/ cm

3g-1

, STP

(a)

0.0 0.2 0.4 0.6 0.8 1.0

200

250

300

350

400

450

CR-750CR-800

CR-850CR-CA-850CR-850-RA

Fig. 1. The structure characterizations of porous carbons, (a) N2 adsorption and desorpti�10 and 10 cm3 g�1, respectively; (b) Pore size distributions according to adsorption br

vibration of the alkyl groups. With the increase of activation tem-perature, the C–H vibration becomes weak, indicating –CH3 groupsare progressively removed from the substituted aromatic rings athigh temperature. Absorption peaks around 1600–1450 cm�1

originate from the skeletal and stretching vibrations of the benzenering. It is obvious that the raw material displays different peakpositions with those of the other samples. For the raw material,the band at 1710 cm�1 is due to the asymmetric C@O stretching,and once treated at high temperature, this band disappears. Theappearance of the band at 1260 cm�1 is ascribed to guaiacyl ringbreathing with C–O stretching, which is characteristic of lignin.In addition, aromatic ring C–H in plane bending vibration isdetected at 1026 cm�1 but only for the raw material (Yan et al.,2012).

Elemental analysis was performed to determine the chemicalcompositions of the raw material, one-step activated samplesand the ash removal sample. The results in Table 2 show that thecarbon content increases with the increase of activation tempera-ture, but the opposite trend is true for the heteroatoms. This isdue to the heteroatoms derived from the raw material becomingpart of the chemical composition as a result of incomplete car-bonization. As for the ash removal sample, the increase of heteroa-tom content can be attributed to the heteroatom incorporation tothe carbon surface during subsequent treatment or exposure toair (Pandolfo and Hollenkamp, 2006). All the as-prepared productspresent the carbon content over 85%, much higher than that of theraw material. A high carbon content results in good cycle stability,and the existence of heteroatoms can improve the wettability ofthe carbon surface to electrolyte and thus increase the electro-chemical activity (Qu, 2002).

To explore any advantages of the porous carbons and theirpotential as electrode materials for supercapacitors, the capacitiveperformance was investigated in 6 M KOH electrolyte by a three-electrode system. Fig. 2a exhibits CV curves of porous materialsCR-750, CR-800, CR-850 and CR-850-RA at a scan rate of10 mV s�1. The CV curves of all the electrodes present a unique

dV/d

log(

D)

D / nm1 10 100

CR-750CR-800

CR-850CR-CA-850CR-850-RA

(b)

on isotherms at 77 K. The isotherms of CR-750 and CR-850 were offset vertically byanches.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-20

-10

0

10

E / V vs Hg/HgO

I / A

g-1

5-100 mV s-1

(c)

Time / s0 100 200 300 400 500 600

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

E /V

vs

Hg/

HgO

0.5-10 A g-1

(d)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-3

-2

-1

0

1

2

E / V vs Hg/HgO

I / A

g-1 CR-750

CR-800

CR-850

CR-850-RA

(a)

10 mV s-1

0 20 40 60 80 1000

100

200

300

Cap

acita

nce

/ F g

-1

Scan rates / mV s-1

CR-750CR-800CR-850CR-850-RA

(b)

Fig. 2. (a) CV curves of the products at 10 mV s�1 and (b) rate performance of the products in the range of 5–100 mV s�1; (c) CV and (d) GC curves of CR-850-RA tested with athree-electrode setup, in the range of 5–100 mV s�1 and 0.5–10 A g�1, respectively.

W.-H. Qu et al. / Bioresource Technology 189 (2015) 285–291 289

quasi-rectangular shape, indicating typical electrical double layercapacitive behavior and all the electrodes are stable in aqueouselectrolyte (6 M KOH). For samples of CR-750, CR-800, CR-850,and CR-850-RA, their specific capacitances calculated from theCV curves tested at the scan rate of 10 mV s�1 are respectively291, 295, 287, and 298 F g�1. Rate capability is another key factorfor the application of carbon-based electrode materials. Fig. 2bshows the specific capacitance of the as-prepared samples at awide range of scan rates from 5 to 100 mV s�1. It is well knownthat specific capacitance of most current material decreases withthe increase of scan rates, which is ascribed to diffusion limitation.But the capacitance retention of CR-850-RA exhibits as high as 82%at a high scan rate of 100 mV s�1, implying a good rate perfor-mance. Notably, the capacitance is maintained at 259 F g�1, whichis much higher than that of other samples. The results also indicatethat the sample with ash removal treatment (CR-850-RA) has awell-developed pore structure, as shown in the SEM images.

Fig. 2c displays CV curves for the sample CR-850-RA at a rangeof scan rates from 5 to 100 mV s�1. It is well known that CV curvesfor an ideal double-layer capacitor are characterized by a perfectlyrectangular-shaped profile. A high capacitance of 314 F g�1 wasobtained at a scan rate of 5 mV s�1. The CV curve still retains asatisfactory rectangular shape and no dramatic distortion isobserved at 100 mV s�1. The capacitance retention is as high as82%, indicating a quick charge propagation capability and easyion transport within the CR-850-RA electrode material. Thecharge–discharge profiles retain the linearity and symmetry verywell as shown in Fig. 2d. The high capacitance and good rate per-formance of CR-850-RA are attributed to the well-developedporosity and good conductivity of this sample.

In order to further determine the electrochemical performanceof corncob residue derived carbon, a symmetrical cell was assem-bled with two equal sized CR-850-RA electrodes in 6 M KOH aque-ous electrolyte. The facilitated ion and electron transport behaviorof the porous materials was confirmed by the EIS test in the open-circuit voltage. Fig. 3a presents the EIS of a two-electrodesupercapacitor in a range of 10 mHz–100 kHz. In all cases, a

straight line approaching 90� is obtained in the low frequency,representing an ideal capacitive behavior of the electrodes.Almost no semicircle is observed in the high-frequency region,indicating fast charge transfer in the porous carbons. The lengthof the line with a slope of �45�, caused by diffusion impedanceand known as the Warburg impedance, increases in the order ofCR-850-RA, CR-850, CR-800 and CR-750. The CR-850-RA has thelowest impedance benefiting from its enhanced porosity derivingfrom the elimination of inorganic impurities and the resultantadded pores. This coincides with the aforementioned capacitiveperformance. The intercept in the high frequency region at the realaxis represents the equivalent series resistance (ESR). For all thesamples, ESR is as low as 0.7 X (Taberna et al., 2003).

The capacitive performance of this symmetrical cell was alsoevaluated with a two-electrode cell by CV curves at various scanrates from 5 to 200 mV s�1. As shown in Fig. 3b, a rectangularshape is retained well when the scan rate increases, indicating afast charge–discharge process. This fast ionic transfer propertycan be ascribed to the well-developed micro-mesoporous structureand surface chemistry.

As it is well known, long cycle life of a supercapacitor isrequired for its practical application. Fig. 3c displays the specificcapacitance retention of CR-850-RA as a function of charge–dis-charge cycle numbers at a constant current density of 1 A g�1.The specific capacitance gradually increases in the first 2000cycles, reaching 104% of the initial value and is then maintained.The initial increase is probably attributed to an activation processof the electrode material, i.e. gradual wetting of the electrolytedeep inside the electrode material (Lee et al., 2013; Yoon et al.,2013). A similar phenomenon was also observed in other carbonelectrode (Yan et al., 2012). Remarkably, no capacitance decay isobserved after consecutive 100,000 cycles. GC curves of the firstfive and the last five cycles are almost identical isosceles triangles(Fig. 3c inset), demonstrating outstanding long-term cycle stabil-ity. The calculated capacitance based GC curve is 120 F g�1 at aconstant current density of 1 A g�1 in 6 M KOH solution. InFig. 3d, the CV curve after 100,000 cycles is more rectangular than

(d)

before 100 000 cycles

after 100 000 cycles

E / V I /

A g

-110 mV s-1

Cap

acita

nce

rete

ntio

n / %

Cycle number / 104

(c)

0 2 4 6 8 100

20

40

60

80

100

120

First 5 cycles Last 5 cycles

E /V

0 100 200 3000.0

0.5

1.0

0 100 200 3000.0

0.5

1.0

Time / s

0.0 0.4 0.8 1.2

-0.4

-0.2

0.0

0.2

0.4

0.0 0.4 0.8 1.2-8

-4

0

4

8

5-200 mV s-1

E / V

I / A

g-1

z' / Ohm500 10 20 30 40

0

40

80

120

-z"

/ Ohm

CR-750CR-800CR-850CR-850-RA

(a)160 (b)

Fig. 3. (a) Electrochemical impedance spectra of the samples with the symmetrical cell, represented as Nyquist plots; (b) CV curves of as-assembled cell of CR-850-RA at ascan rate range of 5–200 mV s�1; (c) cycle life performance of CR-850-RA tested with an as-assembled cell at a constant current density of 1 A g�1 as a function of cyclenumber and the inset is GC curves of the first five and the last five cycles, respectively; (d) the CV curves recorded before and after 100,000 cycles.

290 W.-H. Qu et al. / Bioresource Technology 189 (2015) 285–291

before, which illustrates the enhanced ideal electric double layercharacteristic during the cycling process. These results denote thathighly reversible electrostatic adsorption and desorption of elec-trolyte ions occur on the electrode surface, indicating an infinitelife time for the repetitive charge–discharge cycling. This illus-trates that the corncob residue derived porous carbon possessessuperb electrochemical stability with long cycle life, making it apromising candidate for long-term energy storage devices.

In order to further determine the electrochemical performanceof corncob residue derived carbon, a symmetrical cell was assem-bled with two equal sized CR-850-RA electrodes in 1 M TEABF4-ANorganic and 6 M KOH aqueous electrolyte. The energy density andpower density of the electrochemical CR-850-RA capacitor wereestimated in Fig. 4. The energy density can be improved by maxi-mizing the specific capacitance and increasing the usable operatingvoltage. The cell voltage can reach 1.2 V. The CR-850-RA basedsupercapacitor still retains an energy density of 5.3 Wh kg�1 at a

10 100 1000 10000 1000000.1

1

10

100 1 M TEABF4

6 M KOH

Power density / W kg-1

Ener

gy d

ensi

ty /

Wh

kg-1 3600 s

3.6 s

36 s

0.36 s

360 s

18.6 s

Fig. 4. Ragone plots of CR-850-RA tested by using a two-electrode symmetricsupercapacitor with both 6 M KOH aqueous and 1M TEABF4-AN organic electrolyte.

high power density of 8276 W kg�1 in 6 M KOH. At a current draintime of 18.6 s, the energy and power density values are found to be15 Wh kg�1 and 2827 W kg�1, respectively, in organic electrolyte.The maximum energy density in aqueous and organic electrolyteis 6.8 Wh kg�1 and 17 Wh kg�1, respectively. A comparison indi-cates that the CR-850-RA is superior to commercial carbon andcomparable to many reported biomass-based carbon electrode athigh rates (Table S2). Hence, the corncob residue derived porouscarbons are appropriate electrode materials for supercapacitors.

4. Conclusions

High value added porous carbon materials derived from bio-waste corncob residue were prepared by green and low-cost steamactivation without pre-carbonization. As corncob residue is acheap, abundant, widely available and sustainable biologicalresource, the resultant porous carbons can be industrially pro-duced at a high efficiency/cost ratio. With a high specific surfacearea and a moderate mesoporosity, the as-prepared porous carbondemonstrates excellent electrochemical performance in both aque-ous and organic electrolyte, making it a promising material forcommercial supercapacitors. A feasible approach for large-scaleutilization and high value added conversion of biowaste has beendemonstrated.

Acknowledgements

We would like to sincerely thank the financial support byNational Program on Key Basic Research Project (No.2013CB934104) and National Natural Science Foundation ofChina (No. U1303192).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2015.04.005.

W.-H. Qu et al. / Bioresource Technology 189 (2015) 285–291 291

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