全 文 ：文章编号: 1007-8827(2016)05-0475-10
葱叶一步法裂解制备多孔炭及其电容性能研究
于 晶1， 高利珍1， 李雪莲1， 吴 超2， 高丽丽1，3， 李长明2
(1．太原理工大学 环境科学与工程学院，山西 太原 030024;
2．西南大学 清洁能源与先进材料研究所，重庆 400715;
3．太原理工大学 绿色能源材料与储能系统实验室，山西 太原 030024)
摘 要: 以葱叶为炭前驱体，在不添加任何活化剂的条件下，炭化活化同时进行，制备了孔径分布主要集中于 0． 6 ～ 1． 2 nm
和 3 ～ 5nm 之间的葱基多孔炭材料，并对其电容性能进行研究。分别采用扫描电子显微镜(SEM)、场发射扫描电子显微镜
(FE-SEM)、能量弥散 X 射线光谱(EDX)、火焰原子吸收光谱(FAAS)、X 射线衍射(XＲD)、热重分析(TGA)和氮气吸脱附曲
线等方法表征了葱基炭的形貌、成分、比表面积及孔径分布等性能;通过循环伏安(CV)、交流阻抗(EIS)、恒流充放电(GCD)
等电化学方法考察了材料的比电容和循环寿命等电化学性能。结果表明，葱叶中本身含有的微量矿物质如钙、钾等在其炭化
的过程中同时起到了活化的作用。研究了不同温度下(600 ～ 800 ℃)制备的多孔炭的性能，发现 800 ℃条件下制得的样品性
能最佳，以微孔为主，介孔辅之，孔径为 0． 6 ～ 1． 2 nm 的微分孔隙体积达 2． 608 cm －3 /g /nm，3 ～ 5 nm 的微分孔隙体积有
0． 144 cm －3 g /nm，BET 比表面积为 551． 7 m2 /g，质量比电容为 158． 6 F /g，有效面积电容可高达 28． 8 μF /cm2。这表明孔径分
布情况对多孔炭的电荷存储能力有很重要的影响，此法也为提高“有效面积电容”提供了思路。
关键词: 多孔炭;葱叶;一步炭化活化法;有效面积电容
中图分类号: X712 文献标识码: A
基金项目:山西省青年科技研究基金资助项目(2013021011-3);山西省留学人员科研基金资助项目(2013-041);太原理工大学
人才引进资助项目(tyut-rc201110a)．
通讯作者:高丽丽，博士后，讲师． E-mail:gaolili@ tyut． edu． cn
Porous carbons produced by the pyrolysis of
green onion leaves and their capacitive behavior
YU Jing1， GAO Li-zhen1， LI Xue-lian1， WU Chao2， GAO Li-li1，3， LI Chang-ming2
(1． School of Environmental Science and Engineering，Taiyuan University of Technology，Taiyuan 030024，China;
2． Institute for Clean Energy ＆ Advanced Materials，Southwest University，Chongqing 400715，China;
3． Lab of green energy materials and storage systems，Taiyuan University of Technology，Taiyuan 030024，China)
Abstract: Porous carbons were prepared by the simple carbonization of green onion leaves at temperatures from 600 to 800 ℃ and
used as the electrode materials of supercapacitors． SEM，FESEM，EDX，AAS，XＲD，TGA and nitrogen adsorption were used to
characterize their morphology，pore structure and surface elemental composition． Cyclic voltammetry，electrochemical impedance
spectroscopy and galvanostatic charge /discharge were carried out to evaluate their specific capacitance，resistance and cycling life．
Ｒesults showed that the initial mineral elements present in the leaves such as calcium (Ca)and potassium (K)play an activating
role during the carbonization． All samples have a bimodal pore distribution of micropores (mainly 0． 6-1． 2 nm)and mesopores
(mainly 3-5 nm)． The carbon prepared at 800 ℃ had the highest surface area of 551． 7 m2 /g，a specific capacitance of 158． 6 F /g
at 0． 2 A /g and an effective areal capacitance of 28． 8 μF /cm2 ． The effective areal capacitance of the carbon prepared at 800 ℃ is
higher than of most porous carbons reported in the literature，which is ascribed to its pore size distribution that favors ion access to its
pores．
Keywords: Porous carbon;Green onion leaves;One-step carbonization and activation;Effective areal capacitance
Ｒeceived date:2016-06-10; Ｒevised date:2016-07-28
Foundation:Shanxi Province Science Foundation for Youths (2013021011-3) ;Shanxi Scholarship Council of China (2013-041) ;
Project for Importing Talent of Taiyuan University of Technology(tyut-rc201110a)．
Corresponding author:GAO Li-li，Post-doctor，Lecturer． E-mail:gaolili@ tyut． edu． cn
English edition available online ScienceDirect (http:www ． sciencedirect． comsciencejournal18725805 )．
DOI:10． 1016 /S1872-5805(16)60026-4
第 31 卷 第 5 期
2016 年 10 月
新 型 炭 材 料
NEW CAＲBON MATEＲIALS
Vol． 31 No． 5
Oct． 2016
1 Introduction
With the increase of the environmental pollution
and the scarcity of fossil fuels，the demand for clean
energy sources is growing rapidly all around the
world． Supercapacitor，as a kind of clean energy con-
version and storage device，has attracted much atten-
tion owing to its high power density，long cycle life
and high dynamic of charge propagation， which
bridges the power /energy gap between traditional die-
lectric capacitor and battery［1-6］． Especially，electrical
double-layer supercapacitors (EDLSs) ，draw much
more attention owing to their simple charging mecha-
nism，long cycling life and short charging time． Since
pure physical charge accumulation occurs at the elec-
trochemical interface between electrode and electrolyte
during the charge /discharge process，EDLS is able to
store and deliver energy at a relatively high rate［7-10］．
Compared to batteries，supercapacitors have the ad-
vantages of high power density，long life expectancy，
long shelf life，high efficiency，wide range of operat-
ing temperatures，environmental friendliness and safe-
ty． However，they also face challenges at the current
stage of technology，such as low energy density，high
cost and high self-discharging rate． Among the com-
ponents of a supercapacitor，electrode materials domi-
nate the performance of supercapacitors［11］． There-
fore，developing new materials with improved per-
formance is important to improve the property of su-
percapacitors［12］． In general，electrode materials of
supercapacitors include three types［13，14］:carbon ma-
terials，conducting polymers，and metal oxides． Por-
ous carbons have large surface areas，relatively good
electrical conducting properties and the 3D porous net-
work structure that ensures fast electronic and ionic
conduction through charge /discharge process． Fur-
thermore，porous carbons are considered as the most
promising candidate materials for supercapacitors in
industry owing to their moderate cost［3，7，15］． Gener-
ally，the synthesis of porous carbons includes two
steps:carbonization and activation． Among various
precursors，cheap and renewable biomass such as ag-
ricultural byproducts have attracted much attention
owing to their low cost and environmental friendly
properties［16-18］． Activation is a crucial procedure，
which include physical and chemical activation． For
these two methods，either high temperature or large
amount of chemical agent is used，which require ex-
pensive equipments or bring about difficulty in post-
treatment［19-25］． Though various porous carbons have
been tried as electrode materials in supercapacitors，
their applications are still limited owing to their com-
plicated production processes［26］． As reported，natural
constituents such as mineral substances in some kinds
of leaves may replace the additional pore generators to
create micropores， thereby simplifying the
process［27，28］． Green onions are widely planted in
China and could be stored in winter． However，dur-
ing the storage，the leaves of green onions are usually
withered and need to be discarded． Therefore，we re-
ported a facile，cost-effective approach to synthesize
porous carbon via one-step pyrolysis of the discarded
green onion leaves without any additive． The reason
might be that green onion leaves contain Ca and K
that act as pore generators［27，28］． The pore sizes are
mainly centered around 0． 6-1． 2 and 3-5 nm． Al-
though the specific surface area and the mass specific
capacitance for the green onion leave-derived carbons
(GOLCs)are not so high，their“effective areal ca-
pacitance”is high，indicating that the proportion of
their effective pores in GOLCs is high．
2 Experimental
2． 1 Chemicals
The green onions used in this study were directly
obtained from the local farm． Nafion solution was
purchased from Sigma． All other chemical reagents，
such as hydrochloric acid (HCl，36%) ，nitric acid
(HNO3，65%) ，perchloric acid (HClO4，70%) ，
hydrogen peroxide (H2O2，30%)and potassium hy-
droxide (KOH，98%) ，were purchased from Sinop-
harm Chemical Ｒeagent Co． Ltd and used as received
without any further purification． All the aqueous solu-
tions were prepared with Millipore water having a re-
sistivity of 18． 2 MΩ (Purelab Classic Corp．，USA)．
2． 2 Synthesis of porous carbons
The synthesis process of green onion leave-de-
rived carbons (GOLCs)is shown in Fig． 1．
Fig． 1 Schematic diagram for the synthesis of
porous carbons from green onion leaves．
The leaves of green onion were separated from
the white stem，washed thoroughly with deionized
·674· 新 型 炭 材 料 第 31 卷
water and dried at 60 ℃ in an oven over night． The
dried leaves were crushed into powder． The carboni-
zation and activation processes were carried out at one
step． The dried leave powder was heated at 600-
800 ℃ under the protection of argon for 2 h in a tubu-
lar furnace． The heating rate was 10 ℃ /min． After
cooled down to room temperature under argon，the
green powder was totally turned into black color． The
obtained products were washed thoroughly by deion-
ized water and then dried in an oven over night． For
comparison，some products were rinsed by a diluted
hydrochloric solution (0． 1 M)．
2． 3 Electrochemical measurements
Electrochemical characterizations were carried
out in a three-electrode electrochemical system using
Hg /HgO electrode and platinum foil as the reference
and counter electrode，respectively． The GOLC pow-
der was dispersed in water by sonication． Then the
suspension was dripped on a glassy carbon electrode
and coated by Nafion solution．
All the electrochemical measurements were car-
ried out on a CHI 660D electrochemical workstation
(Shanghai Chenhua Co． Ltd，China)in 3 M KOH
aqueous electrolyte solution at room temperature． Cy-
clic voltammetry (CV)curves were obtained between
a potential range of － 1． 0-0． 1 V at different scanning
rates． The electrochemical impedance spectroscopy
(EIS)was performed in a three-electrode system at
5 mV-alternating current-disturbance around the open
circuit potential vs Hg /HgO． The scanning frequency
was from 0． 01 to 100 kHZ． The galvanostatic
charge /discharges (GCD)were carried out under dif-
ferent current densities．
The mass specific capacitance is calculated from
GCD curves through equation (1) :
C s =
2I dtdv
m (1)
where“C s”is the specific capacitance，“I” is the
current，“m”is the active mass and“dv /dt”is the
slope obtained from the discharge curve．
Effective areal capacitance (C ea， μF /cm
2 )
means the ratio of“mass specific capacitance (Cms，
F /g)”and“BET surface area (A，m2 /g)”，which
is calculated by the equation (2)．
C ea =
Cms
A (2)
2． 4 Characterizations
The morphology of GOLCs was observed by a
JSM-6510LV (Japan)scanning electron microscope
(SEM) and a JSM-7800F field-emission scanning
electron microscope (FE-SEM，Japan)． Elemental
composition analysis was qualitatively measured by
JSM-6510LV (Japan)energy dispersive X-ray spec-
troscopy (EDX) and quantitatively determined by
WFX-110 flame atomic absorption spectrometry
(FAAS)． The samples were pretreated before FAAS
measurement． Firstly，they were ground into powder
and poured into an acid mixture of HNO3 and HClO4，
followed by heating and dissolving at a hot plate until
most of water evaporated． Then H2 O2 was added to
get rid of the residual acid． Through the treatment，
minerals such as K and Ca could be totally dissolved
from the samples，which could be used for FAAS
measurements． The nitrogen adsorption and desorp-
tion isotherms at 77 K were measured using a Quanta-
chrome Instruments (USA)Inc． Nova 1200e surface
area and pore size analysis system． The specific sur-
face area was calculated from the N2 adsorption iso-
therm by applying the Brunauer-Emmett-Teller
(BET)equation． In order to reflect the pore size dis-
tribution exactly，both Barrett-Joyner-Halenda (BJH)
and Density functional theory (DFT)models were
applied． BJH model is more suitable to mesopore
analysis while DFT for micropore analysis． XＲD pat-
terns were obtained by a XＲD-7000 (Japan)． Ther-
mogravimetric analysis (TGA)and differential ther-
mogravimetric (DTG)analysis were carried out using
with a Thermogravimetric Analyzer Q50 (USA)．
3 Ｒesults and discussion
The morphology of all GOLCs prepared at differ-
ent temperatures is shown in Fig． 2． From the SEM
images (from 2a to 2f) ，all the samples prepared at
different temperatures show similar fiber structure as
the original leaves，implying that the macroscopical
structure haven’t been changed during carbonization．
However，mesopores and micropores could not be
clearly observed under SEM，which might be caused
by the low magnification and resolution of SEM． The
GOLC prepared at 800 ℃ (GOLC-800)under FE-
SEM is shown in Fig． 2g-h，which reveals that more
tiny pores can be observed，but still not quite clear．
This might be because some of the pores may be hid-
den by the original mineral substances that are uni-
formly distributed in green onion leaves．
The pore structure could be further verified by
nitrogen adsorption-desorption isotherms as shown in
Fig． 3． An obvious hysteresis loop can be observed in
the isotherms in Fig． 3a at the relative pressure from
0． 4 to 0． 9． The hysteresis loop can be categorized as
H4 type，revealing that mesopores exist in the sam-
ples［28，29］． The specific surface areas for different
GOLCs prepared at 600，700 and 800 ℃，abbreviated
·774·第 5 期 YU Jing et al:Porous carbons produced by the pyrolysis of green onion leaves……
as GOLC-600，GOLC-700 and GOLC-700，are cal-
culated with standard BET method to be and respec-
tively 230． 5，348． 4 and 551． 7 m2 /g，respectively．
Fig． 3b-d depict the pore size distributions of GOLCs
with the two models，which show bimodal distribu-
tion of micropores and mesopores． Through calcula-
tion， the differential pore volumes of micropores
(0． 6-1． 2 nm ) are 1． 432， 1． 449 and
2． 608 cm －3 /g /nm for GOLC-600，GOLC-700 and
GOLC-800，respectively． Furthermore，most of the
micropores are centered around 0． 6-0． 8 nm． Micro-
pores have a high surface area to volume ratio and
contribute more to surface area when present in signif-
icant amounts． Some studies have reported that pore
sizes around 0． 7 nm may be a suitable dimension for
aqueous electrolyte，which could match the dimension
of the aqueous ion［2，32，33］． And the corresponding dif-
ferential pore volumes of mesopores (3 to 5 nm)are
0． 016，0． 071 and 0． 144 cm －3 /g /nm for GOLC-
600，GOLC-700 and GOLC-800，respectively． As
reported［30］，mesopores play a significantly important
role to obtain an ideal capacitor behavior，because
they can not only contribute to the surface area but al-
so provide wide transport channels for adsorbate ac-
cessibility［31］． Both the differential micropore volume
and differential mesopore volume for GOLC-800 are
the highest among the three samples，implying that
high activation temperature is favorable for the gener-
ation of pores． Therefore，GOLC-800 is the most ex-
cellent material among the three，followed by GOLC-
700 and then GOLC-600，if it is judged merely from
the pore size distributions and BET surface areas．
Fig． 2 (a-f)SEM and (g-h)FESEM images of green onion leave-derived
carbons prepared at different temperatures: (a-b)600 ℃，(c-d)700 ℃ and (e-h)800 ℃ ．
The elements and their relative contents in the
GOLC-800 were also determined by EDX as shown in
Fig． 4a． It is seen that carbon (C)is the most promi-
nent ingredient，implying that the green onion has
been well carbonized． Trace of inorganic elements
such as oxygen (O) ，sulphur (S) ，chlorine (Cl)
and phosphorus (P)can be observed as shown in
Fig． 4a． The existing of oxygen (O) implies that
there are lots of oxygen-groups on the surface of the
carbon． Furthermore，some mineral substances can be
as well detected，such as calcium (Ca)and potassi-
um (K)． Since no element addition was involved
·874· 新 型 炭 材 料 第 31 卷
during the carbonization of GOLC-800，it can be in-
ferred that all the mineral substances originate directly
from the green onion leaves．
Fig． 3 (a)Nitrogen adsorption-desorption isotherms for green onion leave-derived carbons
prepared at different temperatures; (b-d)pore size distributions with the BJH and DFT models．
Fig． 4 (a)Images of EDX analysis and (b)XＲD patterns for green onion leave-derived carbon at 800 ℃ ．
To further verify the content of these mineral
substances，TGA measurement of original green onion
leaves was also carried out as shown in Fig． 5．
Stage I from 25 to approx． 200 ℃ might corre-
spond to the elimination water including free and
bonded water，and the total content of water in green
onion is 15 wt% ． The main pyrolysis of green onion
occurs at Stage II (200-300 ℃)and Stage III (300-
500 ℃) ，which show highest weight loss． Stage II
may be correlated to the decomposition of carbohy-
drates and proteins［27］ while stage III to cellulose and
hemicellulose［34］． The weight loss for stage II and III
is approximately 55% in general． When the tempera-
ture is higher than 500 ℃ (stage IV) ，only a 5% -
8% weight reduction happens until 800 ℃，which
might be caused by the decomposition of the small
amount of lignin contained in green onion［34］． The re-
sidual content after Stage IV is above 20%，part of
which may be due to the large amount of minerals
such as Ca，K originally present in green onion
leaves．
The XＲD patterns of GOLCs in Fig． 4b could
·974·第 5 期 YU Jing et al:Porous carbons produced by the pyrolysis of green onion leaves……
further confirm the existence of mineral substances．
The upper line in Fig． 4b represents the GOLC-800
that was washed only with pure water，from which，
two sharp peaks near 28° and 33° could be seen obvi-
ously;however，after the GOLC-800 was rinsed by
diluted HCl solution，these two peaks disappeared as
shown in the lower line． Through comparison to the
standard spectrum diagrams，the sharp peaks might be
attributed to CaC2 ． After rinsing with HCl，CaC2
might reacts with in water． Furthermore，a broad peak
near 2θ = 25°can be seen in both lines，corresponding
to the crystalline graphite． As reported［27，28］，Ca and K
salts can be acted as pore generators to create pores
during the synthesis． Nakagawa［35］ reported that more
mesopores and micropores could be obtained in the
porous carbons by adding some calcium compound into
the raw material before activation． Ｒaymundo also il-
lustrated that the presence of K derivatives in carbon
precursor played the same role as additives of chemical
pore generators during the activation［27］．
Fig． 5 TGA and (DTG)analysis of green onion leaves
under a nitrogen atmosphere (heating rate:10 ℃ /min)．
To quantitatively analyze the contents of mineral
substances (K，Ca) ，FAAS was applied． Three dif-
ferent samples were measured， dried green onion
leaves prepared by drying green onion leaves under
60 ℃ at vacuum oven for 12 h，GOLC-800 and
GOLC-800 rinsed by HCl solution． The results are
listed in Table 1，which reveal that the original con-
tents of K and Ca in dried green onion leaves are 20． 5
and 3． 5 mg /g，respectively，which are similar to the
reported results［28］． After the carbonization at 800 ℃，
the contents of K and Ca increase to 42． 7 and
7． 3 mg /g，respectively． The increase of their relative
contents in the samples might be attributed to pyroly-
sis of carbohydrates and proteins，namely，the loss of
H，O and other elements． These results agree well
with the TGA conclusions as shown in Fig． 5． Com-
pared with the amount of the activating agents added
in chemical activation，the contents of K and Ca are
very low ． However，as reported by Biswal［28］，natu-
ral constituents such as mineral substances in biomass
are distributed uniformly． So despite the very few
amounts，they are very effective to create pores in ac-
tivation． In this work，the total content of K and Ca
in GOLC-800 is 50 mg /g，so they could play an im-
portant role to generate pores in carbonization as acti-
vating agents． This is why no more external activating
agents are needed． After the GOLC samples were
thoroughly rinsed in HCl solution，the K and Ca were
removed to an extent too little to be detected．
Table 1 Contents of K and Ca in dried green onion
leaves，GOLC-800 and GOLC-800 rinsed by HCl．
Dried green
onion leaves
GOLC-800
GOLC-800
rinsed by HCl
K 20． 5 42． 7 －
Ca 3． 5 7． 3 －
Electrochemical behaviors of GOLCs prepared
under different temperatures were measured in 3 M
KOH aqueous electrolyte，as shown in Fig． 6 and
Fig． 7． To measure whether the residual K and Ca in
GOLC-800 have great effect on capacitance， the
GOLC-800 samples were thoroughly rinsed by HCl，
as shown in Fig 6a． The XＲD results in Fig 4b have
shown that materials such as Ca could be gotten rid of
through rinsing with HCl． However，it could be obvi-
ously seen that CV curves of GOLC-800 and GOLC-
800 rinsed by HCl are similar，implying that the min-
eral substances as K and Ca in GOLC have little
effects． Thus，GOLCs were just washed by deionized
water and measured in the following samples．
Fig． 6b is the galvanostatic charge /discharge
curve at 0． 2 A /g of GOLCs，linear and nearly sym-
metrical curves could be seen in all samples，confir-
ming that the product has excellent electrochemical re-
versibility and charge /discharge properties． Compari-
son of the three samples at the same charge /discharge
current density of 0． 2 A /g，discharge time of GOLC-
800 is nearly 870 s，and GOLC-700 and GOLC-600
is 570 and 520 s，respectively，implying that GOLC-
800 has better electrochemical performance than
GOLC-600 and GOLC-700． The mass specific capac-
itances for GOLC-800，GOLC-700 and GOLC-600 at
a current density of 0． 2 A /g calculated from equation
(1)are 158． 6，104． 2 and 94． 8 F /g，respectively．
The higher mass specific capacitance for GOLC-800
may be ascribed to its larger specific surface area and
higher differential pore volume［36］． Actually，this ca-
pacitance value is relatively higher than those of other
electrode materials for supercapacitor application from
biomass precursor［8，37］． Fig 6c is the galvanostatic
charge /discharge curves of GOLC-800 at different
·084· 新 型 炭 材 料 第 31 卷
current densities． It can be seen that the capacitances
drastically change for GOLC-600，GOLC-700 and
GOLC-800 when the current density increases from
0． 2 to 5． 0 A /g as shown in Fig． 6d． This can be ex-
plained as follows［38］． At lower current densities，
ions can be transported and diffused into the pores
easily，which results in higher capacitance． However，
when the current density increases，ions cannot be
easily diffused into the pores so that the effective
double layers are formed at the surface of the elec-
trode． Hence，the capacitance at high current densi-
ties are low ．
Fig． 6 Measurements of GOLCs’electrochemical behavior．
(a)CV curves of GOLC-800 and GOLC-800 rinsed by HCl at a scanning rate of 2 mV /s;
(b)Galvanostatic charge /discharge curves of GOLCs prepared at different temperatures under 0． 2 A /g;
(c)Galvanostatic charge /discharge curves of GOLC-800 at different current densities
and (d)Discharge-specific capacitances of supercapacitors at different current densities for GOLCs．
Fig． 7a and Fig． 7b depict the cyclic voltamme-
try curves of GOLC-800 at different scanning rates．
At lower scanning rate such as 2 mV /s，a redox
hump could be observed betwwen － 0． 15-0． 25 V，
which might be casued by oxygen-groups reaction at
the carbon surface［39］． This Faradaic redox reaction
also contributes to the capacitance． However，in the
whole scaning rang from － 1． 0 to 0． 1 V，the CV
curves represent nearly rectangular shape，revealing
an ideal capacitance behavior and the charge /dis-
charge process is nearly reversible［23，40］． With the in-
creasing of scanning rate，there is almost no deviation
from rectangular shape in CV curves，implying the
low ohmic polarization and high electrolyte ion trans-
fer rate． At the same time，when the direction of the
scanning rate changes，current responses quickly，im-
plying the fast kinetics of the double layer formation．
Electrochemical impedance spectrometry (EIS)
is a steady state technique with small potential varia-
tion，which is more reliable for measuring the capaci-
tance． The sloping line in the range of low frequency
corresponds to the diffusive resistance． In Fig． 7c，
the Nyquist plots for all the samples are dominated by
nearly vertical trend capacitive lines in the range of
low frequency which indicate capacitive behavior ac-
cording to the equivalent circuit theory and could be
attributed to the capacitive properties． However，the
sloping line for GOLC-800 is more vertical than that
for GOLC-700 and GOLC-600，revealing that GOLC-
800 represents low diffusive resistance and high ca-
pacitance． In the range of high frequency，no obvious
semicircle could be observed，implying that the intrin-
sic resistance of the active material is relatively small，
which agree well with the results in Fig． 7a，b．
·184·第 5 期 YU Jing et al:Porous carbons produced by the pyrolysis of green onion leaves……
Furthermore，the GOLC-800 shows an excellent
cycling stability as shown in Fig． 7d． The mass spe-
cific capacitance still remains 96% of the initial after
5 000 galvanostatic charge /discharge cycles at a cur-
rent density of 10 A /g．
Fig． 7 Measurements of GOLCs’electrochemical behavior．
(a，b)CV curves of GOLC-800 at different scanning rates; (c)Nyquist plots for different GOLCs;
(d)Charge /discharge stability for GOLC-800 at a current density of 10 A / g．
Some other carbons synthesized from biomass
materials are compared with ours as shown in Table
2． Ｒice husk［41］， firewood［25］， bamboo［42］， bean
dregs［43］ and many other biomass materials were ap-
plied as precursor． Mass specific capacitance is an im-
portant factor that should be considered in practical
application． However，for some small electronic de-
vices，effective areal capacitance is very important in
supercapacitor applications［44，45］． Compared with oth-
er biomass derived carbons，BET surface area and
mass specific capacitance of GOLC prepared in this
work might not be that high，but the effective areal
capacitance is much high，reaching 28． 8 μF /cm2 at
0． 2 A /g．
Table 2 Comparison of carbon synthesized from biomass materials．
Original materials
Activating
agent
BET surface
area (m2 /g)
Mass capacitance
(F /g)
Effective areal
capacitance
(μF /cm2)
Measurements
done at
Electrolyte Ｒeference
Ｒice husk NaOH 1886 210 11． 1 0． 2 mA /g 3 M KCl ［41］
Firewood KOH 1064 180 16． 9 10 mV /s 0． 5 M H2SO4 ［25］
Bamboo KOH 1251 260 20． 8 1 mA /g 30 wt% H2SO4 ［42］
Banana fibers ZnCl2 1097 74 6． 7 500 mA /g 1 M Na2 SO4 ［24］
Corn grains KOH 3199 257 8． 0 1 mA /g 6 M KOH ［19］
Sugar-cane bagasse ZnCl2 1788 300 16． 8 250 mA /g 1 M H2SO4 ［22］
Sunflower seed sell KOH 2509 311 12． 4 250 mA /g 30 wt% KOH ［17］
Argan sed shell KOH /melamine 2062 355 17． 2 125 mA /g 1 M H2SO4 ［16］
cattail KOH 1951 336 17． 2 2 mV /s 6 M KOH ［29］
bean dregs KOH 1680 482 28． 7 1 A /g 1 M H2SO4 ［43］
Spring onion No activation 552 159 28． 8 0． 2 A /g 3 M KOH This work
·284· 新 型 炭 材 料 第 31 卷
4 Conclusions
Green onion leaves derived carbons (GOLCs)
were prepared by a simple carbonization without any
external additives． Three kinds of GOLCs were pre-
pared at different carbonization temperatures:GOLC-
600，GOLC-700 and GOLC-800． All the carbons
have a bimodal pore distribution of micropores and
mesopores，and GOLC-800 has highest differential
pore volume in both micropore and mesopore range．
GOLC-800 shows the highest mass specific capaci-
tance and specific surface area among the three． More
importantly，the effective areal capacitance of GOLC-
800 could reach 28． 8 μF /cm2 at 0． 2 A /g，which is
the highest among the samples reported． This is main-
ly due to the suitable pore distribution GOLC-800
has． In addition，the surface functional groups，espe-
cially oxygen groups on the surface of GOLC-800 in-
duce pseudocapacitance，which could contribute to
the capacitance． From XＲD，EDX，TGA and FAAS
analysis，Ca and K could be detected． These original
mineral substances in green onion leaves act as pore-
generator during the carbonization． The porous car-
bons derived from green onion leaves are promising
electrode materials for supercapacitors，especially for
small devices，in which a high areal capacitance of
the electrode material is required．
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·484· 新 型 炭 材 料 第 31 卷