摇
第 26 卷摇 第 3 期
2011 年 6 月
新摇 型摇 炭摇 材摇 料
NEW CARBON MATERIALS
Vol. 26摇 No. 3
摇
Jun. 2011
Article ID:摇 1007鄄8827(2011)03鄄0224鄄05
The effect of carbonyl, carboxyl and hydroxyl groups on
the capacitance of carbon nanotubes
LI Li鄄xiang1 ,摇 LI Feng2
(1. Institute of Materials Electrochemistry Research, University of Science and Technology Liaoning, Anshan 114051, China;
2. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China)
Abstract:摇 Concentrated H2 SO4 颐HNO3 mixed acids, air, nitric acid and potassium permanganate were used to oxidize
carbon nanotubes ( CNTs) to introduce surface functional groups ( SFGs) and the effects of the type and amount of SFGs
on the electrochemical properties of CNT supercapacitors were investigated. XPS analysis shows that the mixed acid oxi鄄
dation produces carbonyl ( C詤O ) and the carboxyl ( O C詤O ) groups, the air oxidation results in hydroxyl and
the smallest amount of carbonyl and carboxyl groups, and both the nitric acid and potassium permanganate treatments re鄄
sult in a moderate amount of carbonyl and carboxyl groups. It was found that the specific surface area and pore structures
of the four samples are similar and carbonyl and carboxyl groups contribute the most to pseudo鄄capacitance through a Far鄄
adic reaction. In particular, the carbonyl group has a proportional relationship to the capacitance of CNTs. However, the
hydroxyl group does not lead to an obvious increase of pseudo鄄capacitance, but can increase the electric double layer ca鄄
pacitance. The carbonyl and the carboxyl groups are advantageous for fast Faradic reactions to introduce pseudo鄄capaci鄄
tance, owing to their lower charge transfer resistance than that of the hydroxyl group.
Keywords:摇 Carbon nanotubes; Carbonyl; Carboxyl; Hydroxyl; Pseudo鄄capacitance
CLC number:摇 TB 383
Document code:摇 A
1摇 Introduction
Carbon nanotubes ( CNTs ) are typical one鄄di鄄
mensional carbon materials with a unique pore struc鄄
ture and have excellent electronic, mechanical and
chemical properties [1鄄5] , which along with their high
surface area and large accessible mesopores make
them suitable for transport of electrolyte ions. There鄄
fore, CNTs are good candidates for electrode materi鄄
als of electric double layer capacitors ( ECs ) [6鄄8] .
However, one impediment to the potential application
of CNTs as electrode materials of ECs is their poor
hydrophility, especially when aqueous electrolytes are
used. Therefore, the specific capacitance of CNTs is
low. In order to improve the hydrophility of CNTs for
further application, a variety of chemical treatments
or purification processes are used. Among them, the
chemical treatments using oxidation agents such as
acid, mixed acid and air are commonly used owing to
their simplicity and efficiency in removing impurities
( such as metallic catalysts) and producing oxygen鄄
containing surface functional groups ( SFGs) to en鄄
hance their hydrophility. Moreover, it is found that
these chemical treatments can result in an increase of
specific capacitance of CNT electrode because some
of SFGs can provide pseudo鄄capacitance by reacting
with electrolyte ions [9鄄11] , in addition to the electrical
double layer capacitance ( EDLC) formed by charge
separation on the interface between the electrode and
the electrolyte.
摇 摇 It has been known that the SFGs can obviously
affect the capacitance of CNTs. Generally, the higher
the amount of SFGs, the higher the capacitance.
However, SFGs from chemical treatment of CNTs of鄄
ten consist of the carbonyl, carboxyl and hydroxyl
groups [12] . The influence of these different kinds of
SFGs on the capacitance of CNTs is still not fully
studied.
摇 摇 In order to investigate the effects of different
kinds of SFGs on the capacitance of CNTs, four typi鄄
摇 Received date:2011鄄03鄄08;摇 Revised date:2011鄄06鄄02
摇 Foundation item: National Science Foundation of China (50328204, 50472084 and 50632040) , Natural Science Foundation of Liaoning Province
of China (20061078) and Foundation of Liaoning Education Committee of China ( L2010197) .
摇 Corresponding author: LI Feng, Professor. Fax: +86鄄24鄄23971471, E鄄mail: fli@ imr. ac. cn
摇 Author introduction: LI Li鄄xiang(1972-) , female, Dr. , working on synthesis and electrochemistry of advanced carbons.
E鄄mail: lxli2005@ 126. com
摇 English edition available online ScienceDirect ( http:蛐蛐www. sciencedirect. com蛐science蛐journal蛐18725805 ) .
摇 DOI: 10. 1016 / S1872鄄5805(11)60078鄄4
第3 期
LI Li鄄xiang et al: The effect of carbonyl, carboxyl and hydroxyl groups on the capacitance of …
cal treatment methods with different oxidation abilities
were used to control the amount and composition of
SFGs on the CNTs. They were the mixed acid oxida鄄
tion with nitric acid and vitriol, nitric acid oxidation,
potassium permanganate oxidation and air oxida鄄
tion [13鄄18] . Wet chemical treatments such as acid and
potassium permanganate oxidation can produce SFGs
containing more carboxyl and carbonyl groups. Espe鄄
cially, acid oxidation produces more acidic carboxylic
groups [19鄄20] and the gaseous oxidation generally re鄄
sults in more hydroxyl groups [21鄄22] . Furthermore, the
content of the SFGs on CNTs is dependent on the oxi鄄
dation ability of the treating agents. After the treat鄄
ment, the SFGs on CNTs were characterized and their
effects on the specific capacitance of CNT electrode
are compared. It was found that the SFGs of the hy鄄
droxyl ( C—O) , carbonyl ( C詤O ) , and carboxyl
( O C詤O ) groups have different contributions to
capacitance.
2摇 Experimental
2. 1摇 Chemical treatments
Multi鄄walled CNTs ( MCNTs) were synthesized
by catalytic decomposition of benzene at 1423 K using
iron compounds as catalysts [23] . The four types of
pretreatment are widely used, whether it is purifica鄄
tion or functionalization. Therefore, they were used
to control the SFGs of MCNTs in our study. The as鄄
prepared MCNT sample was heated in nitric acid
( mass fraction is 68% ) mixed with vitriol ( mass
fraction is 98% ) at 398 K in oil bath for 0. 5 h to ob鄄
tain the mixed acid oxidized sample [13] , which is de鄄
noted as MCNT鄄1. The as鄄prepared MCNT sample
was dispersed in 68% HNO3 and heated at 413 K for
4. 5 h in oil bath to obtain the nitric acid oxidized sam鄄
ple denoted as MCNT鄄2 [14] . The potassium perman鄄
ganate oxidized sample was denoted as MCNT鄄3 [15] .
The as鄄prepared MCNT sample was heated at 893 K
for 25 min in air, and then immersed in a 1 颐1 ( vol. /
vol. ) solution of deionized water and concentrated
hydrochloric acid to obtain the air oxidized sample de鄄
noted as MCNT鄄4 [16鄄18] .
2. 2摇 Characterization
The morphologies of the samples were character鄄
ized by a field鄄emission scanning electron spectro鄄
scope ( SEM, JEOL JSM鄄6301F) . The pore size dis鄄
tributions ( PSDs ) and the Brunauer鄄Emmett鄄Teller
( BET) specific surface area ( SSA) were measured
with
a
volumetric
adsorption
apparatus
( ASAP2010M) from nitrogen adsorption isotherms at
·225摇 ·
77 K. The chemical compositions and functional
group distributions of the CNTs were analyzed using
X鄄ray photoelectron spectroscopy ( XPS) . The XPS
was recorded with an ESCALAB250 spectrometer and
Mg K琢 radiation ( h淄 = 1253. 6 eV) . The deconvolu鄄
tion of the spectra was performed using a non鄄linear
least squares fitting program with a symmetric Gaussi鄄
an function.
2. 3摇 Electrochemical measurements
The samples were spread onto a 10鄄mm鄄diameter
nickel foam followed by pressing under 10 MPa as the
working electrode ( WE) . The mass of MCNTs on
each WE was about 10 mg. Two WEs covered with
the same amount of samples were separated by a ny鄄
lon paper and assembled in a sandwich type cell. The
electrolyte was a 6 mol·L -1 KOH aqueous solution.
The voltage window for the two electrode capacitor
was 0 to 1 V. The reported gravimetric capacitance
was normalized to the mass of MCNTs on a single
WE base, which was obtained on Arbin BT2000 at a
current of 1mA. Cyclic voltammograms ( CV) of the
four samples were measured in a three鄄electrode cell
by using a Solartron electrochemistry test system. A
saturated calomel electrode ( SCE) was used as a ref鄄
erence electrode and a Pt plate was used as a counter
electrode.
3摇 Results and discussion
3. 1摇 The morphologies and PSDs of MCNTs
The SEM images of four MCNT samples are
shown in Fig. 1, which show similar length / diameter
ratios and no obvious difference in the morphologies
of samples after four different treatments. The BET
SSAs and pore volumes of four samples are listed in
Table 1. It can be seen that four treatment methods do
not lead to significant differences in the SSA, which
2
are 33, 25, 23 and 21 m·
g -1 for MCNT鄄1, MCNT鄄2,
MCNT鄄3 and MCNT鄄4, respectively. The small me鄄
sopores (2鄄5 nm in pore diameter) are especially use鄄
ful to the EDLC using aqueous electrolyte, because
they are suitable for ion transport and storage, and
thus have a great contribution to EDLC [24] . From Ta鄄
ble1, it can be seen that MCNT鄄1, 2 and 4 have very
close values for mesoporous volumes, but MCNT鄄3
has a relatively low mesopore volume. On the basis
of the fact that the SSA and PSDs of CNTs treated by
the four methods are similar, the differences in the
specific capacitance of CNTs can be used to investi鄄
gate the influence of SFGs.
·摇 226·
第 26 卷
新摇 型摇 炭摇 材摇 料
Fig. 1摇 SEM images of four samples: ( a) MCNT鄄1, ( b) MCNT鄄2, ( c) MCNT鄄3 and ( d) MCNT鄄4
Table 1摇 BET surface areas and pore structure parameters
of the modified MCNTs
Characterization
2 -1
S BET / m·
g
3 -1
v meso a / ( cm·
g ) ´10 -2
MCNT鄄1 MCNT鄄2 MCNT鄄3 MCNT鄄4
33
2. 25
25
2. 03
23
1. 08
21
2. 33
Notation: Surface area ( S BET ) and mesopore volume ( v meso a ) were
calculated from the adsorption鄄desorption isotherm of N2 at 77 K by
multi鄄point BET and BJH equation, respectively.
3. 2摇 The contribution of SFGs to the capacitance
of MCNTs
Fig. 2 presents the ratios of surface functional
groups obtained by Gauss curve fitting of the peaks
from the original XPS spectra. MCNT鄄1 has the lar鄄
gest proportion of oxygen鄄containing SFGs of
27. 8% , the highest content of carbonyl and carboxyl
groups and the same amount of hydroxyl group as
MCNT鄄2 and MCNT鄄3, while the other samples have
almost the same quantity of oxygen鄄containing SFGs
from 18% to 18. 9% . MCNT鄄2 and MCNT鄄3 contain
more carbonyl and carboxyl groups, especially car鄄
bonyl, than MCNT鄄4. However, MCNT鄄4 has the
largest amount of hydroxyl group among the four
samples. The specific capacitances of four samples
are 35, 25, 20 and 8 F·g -1 for MCNT鄄1, MCNT鄄2,
MCNT鄄3 and MCNT鄄4, respectively. On the basis of
similar SSA and pore structure of samples, it can be
deduced that most of the capacitance of MCNT鄄1
should be contributed by the carbonyl and the carbox鄄
yl groups, because MCNT鄄1 has much more carbonyl
and carboxyl groups, especially carbonyl, than the
other samples. Furthermore, it is also worth noting
that MCNT鄄2, MCNT鄄3 and MCNT鄄4 have almost
the same amount of SFGs but the specific capacitance
of MCNT鄄4 is much lower than those of MCNT鄄2 and
MCNT鄄3. It can be found, by comparing the kinds of
SFGs of these three samples, that MCNT鄄4 contains
the largest amount of hydroxyl group but the lowest
carbonyl group, suggesting that carbonyl group is
more effective in increasing the capacitance of MC鄄
NTs. In order to analyze the relationship between the
capacitance and SFGs, the specific capacitances of the
four samples are plotted as a function of the content of
different SFGs. It can be observed that the capaci鄄
tance of MCNTs is proportional to the amount of car鄄
bonyl as shown in Fig. 3, but such relationship is not
found for the other two SFGs. It is also noted that the
capacitance of MCNT鄄3 is slightly lower than that of
MCNT鄄2 despite their similar composition of SFGs
and SSA, which can be attributed to the obviously
low mesopore volume of MCNT鄄3.
Fig. 2摇 The groups爷 ratio of samples. Sum stands
for the sum of functional groups
Fig. 3摇 Relationship between specific capacitance
and the proportion of carbonyl
Fig. 4 shows the cyclic voltammogram ( CV) of
the four samples in 6 mol·L -1 KOH electrolyte. It can
be noted that the strong electrochemical polarization
occurs in the potential range of 0. 4 to -0. 2 V for
第3 期
LI Li鄄xiang et al: The effect of carbonyl, carboxyl and hydroxyl groups on the capacitance of …
MCNT鄄1, MCNT鄄2 and MCNT鄄3 besides redox
peaks at about 0 V. This might be caused by a steric
hindrance for Faradic reaction from some groups such
as carboxyl [25] . However, the shape of CV for MC鄄
NT鄄4 is very close to a rectangular and only weak po鄄
larization can be observed, which suggest that the ca鄄
pacitance of MCNT鄄4 is mainly contributed by
EDLC. According to the characteristics of CV and the
composition of SFGs for the four samples, it can be
deduced that the contribution of carboxyl and carbonyl
groups to CNTs is pseudo鄄capacitance. In a basic
electrolyte, carbonyl and carboxyl groups can react
with the electrolyte ions via the following path鄄
way [12,26] :
> C詤O + OH - = —COOH,
—COOH + OH - = —COO - +H2 O.
Fig. 4摇 CV curves of four samples
In addition, hydroxyl groups may enhance
EDLC because of an improvement of hydrophility of
CNTs [25] . To further analyze the influence of differ鄄
ent SFGs on the electrochemical property of CNTs,
the electrochemical impedance spectrum of the four
samples was measured as shown in Fig. 5. All spectra
include semicircles in the high鄄frequency region and a
straight line in the low鄄frequency region, which repre鄄
sents the typical characteristics of a supercapacitor.
The semicircle in high frequency reflects the resistance
of charge transfer ( R ct ) for Faradic reaction and the
diameter of the semicircle is proportional to R ct .
Fig. 5 illustrates that R ct decreases with the increase on
the amount of carbonyl and carboxyl groups. It sug鄄
gests that these two SFGs enhance the charge transfer
for the fast faradic reaction, which is possibly attribu鄄
ted to their high electron density from the double
bonds between carbon and oxygen atoms. It is also
noted that SFGs for MCNT鄄2 and MCNT鄄3 have al鄄
most the same composition, but MCNT鄄3 has the lar鄄
ger R ct , which can be attributed to the lower ion
transport rate of MCNTs鄄3 due to its lower mesopore
volume than that of MCNT鄄2.
·227摇 ·
Fig. 5摇 Electrochemical impedance spectra of four samples
4摇 Conclusions
The influence of SFGs on the capacitance of
MCNTs is investigated by using different oxidation
treatments for CNTs to control the content and com鄄
position of SFGs. It is found that the carbonyl and
carboxyl groups can contribute to the capacitance of
CNTs electrode mostly via Faradic reactions and the
content of carbonyl is proportional to the capacitance.
The CNT treated by the mixed acid has the highest ca鄄
pacitance because of the largest amount of carbonyl
and carboxyl groups. The hydroxyl group does not
contribute obviously to pseudo鄄capacitance, but en鄄
hances electronic double layer capacitance by an in鄄
creased wettability. The MCNTs treated by air oxida鄄
tion have the lowest capacitance despite the largest
amount of hydroxyl group, as a result of the least
amount of carbonyl and carboxyl groups. Moreover,
the carbonyl and carboxyl groups have lower charge
transfer resistance and thus are advantageous to fast
faradic reactions to introduce pseudo鄄capacitance.
References
[1] 摇 Liu T, Sreekumar T V, Kumar S, et al. SWNT / PAN composite
film鄄based supercapacitors [ J] . Carbon, 2003, 41: 2427鄄2451.
[2] 摇 Park J H, Choi J H, Moon J S, et al. Simple approach for the
fabrication of carbon nanotube field emitter using conducting
paste[ J] . Carbon, 2005, 43: 698鄄703.
[3] 摇 Sayago I, Terrado E, Lafuente E, et al. Hydrogen sensors based
on carbon nanotubes thin films[ J] . Synth Met, 2005, 148: 15鄄
19.
[4] 摇 Zhao Q, Frogley M D, Wagner H D. Direction sensitive stress
measurements with carbon nanotube sensors [ J ] . Polym Adv
Technol, 2002, 13: 759鄄764.
[5] 摇 Somani P R, Somani S P, Flahaut E, et al. Improving the pho鄄
tovoltaic response of a poly( 3鄄octylthiophene) / n鄄Si heterojunc鄄
tion by incorporating double鄄walled carbon nanotubes[ J] . Nano鄄
technology, 2007, 18: 185708鄄1鄄185708鄄5.
[6] 摇 Frackowiak E, Jurewicz E, Delpeux S, et al. Nanotubular mate鄄
rials for supercapacitors [ J] . J Power Sources, 2001, 97鄄98:
822鄄825.
·摇 228·
第 26 卷
新摇 型摇 炭摇 材摇 料
[7] 摇 Ma R Z, Liang J, Wei B Q, et al. Study of electrochemical ca鄄
pacitors utilizing carbon nanotube electrodes [ J ] . J Power
Sources, 1999, 84: 126鄄131.
[8 ] 摇 Emmenegger C, Mauron P, Sudan P, et al. Investigation of
electrochemical double鄄layer capacitors electrodes based on car鄄
bon nanotubes and activated carbon materials [ J ] . J Power
Sources, 2003, 124: 321鄄329.
[9] 摇 Seo M K, Park S J. Electrochemical characteristics of activated
carbon nanofiber electrodes for supercapacitors [ J ] . Mater Sci
Eng B, 2009, 164: 106鄄111.
[10] 摇 Bleda鄄Martinez M J, Macia鄄Agullo J A, Lozano鄄Castello D,
et al. Role of surface chemistry on electric double layer capaci鄄
tance of carbon materials[ J] . Carbon, 2005, 43: 2677鄄2684.
[11] 摇 Ruiz V, Blanco C, Raymundo鄄Pinero E, et al. Effects of ther鄄
mal treatment of activated carbon on the electrochemical behav鄄
iour in supercapacitors[ J] . 2007, 52: 4969鄄4973.
[12] 摇 WANG Da鄄wei , LI Feng, LIU Min, et al. Improved capaci鄄
tance of SBA鄄15 templated mesoporous carbons after modifica鄄
method of opening and filling carbon nanotubes [ J] . Nature,
1994, 372: 159鄄162.
[18] 摇 Kosaka M, Ebbesen T W, Hiura H, et al. Electron鄄spin鄄reso鄄
nance of carbon nanotubes[ J] . Chem Phys Lett, 1994, 225:
161鄄164.
[19] 摇 H Ago, Kugler T, Cacialli F, et al. Work functions and surface
functional groups of multiwall carbon nanotubes [ J ] . J Phys
Chem B 1999, 103: 8116鄄21.
[20] 摇 Jung A, Graupner R, Ley L, et al. Quantitative determination
of oxidative defects on single walled carbon nanotubes [ J ] .
Phys Stat Sol ( b) 2006, 243: 3217鄄3220.
[21] 摇 Hsieh C, Teng H. Influence of oxygen treatment on electric
double鄄layer capacitance of activated carbon fabrics [ J] . Car鄄
bon, 2002, 40: 667鄄674.
[22] 摇 Hsieh C, Chen W, Cheng Y S. Influence of oxidation level on
capacitance of electrochemical capacitors fabricated with carbon
nanotube / carbon paper composites [ J ] . Electrochimica Acta,
2010, 55: 5294鄄5300.
tion with nitric acid oxidation [ J ] . New Carbon Materials,
[23] 摇 侯鹏翔. 多壁纳米碳管的大量制备、提纯及其储氢性能研究
( 王大伟,李摇 峰,刘摇 敏,等. 硝酸氧化改性 SBA鄄15 模板合
( Hou Peng鄄xiang. Large鄄scale synthesis, purification and hy鄄
2007, 24: 307鄄314.
成的中孔炭电容性能研究[ J] . 新型炭材料,2007, 22 (4 ) :
307鄄314. )
[13] 摇 Liu J, Rinzler A G, Dai H J, et al. Fullerene pipes[ J] . Sci鄄
ence, 1998, 280: 1253鄄1256.
[14] 摇 Dujardin E, Ebbesen T W, Krishnan A, et al. Purification of
single鄄shell nanotubes[ J] . Adv Mater, 1998, 10: 611鄄613.
[15] 摇 Hiura H, Ebbesen T W, Tanigaki K. Opening and purification
of carbon nanotubes in high yields[ J] . Adv Mater, 1995, 7:
275鄄276.
[16] 摇 Ajayan P M, Ebbesen T W. Opening carbon nanotubes with
oxygen and implications for filling [ J] . Nature, 1993, 362:
522鄄525.
[17] 摇 Tsang S C, Chen Y K, Harris P J F, et al. A simple chemical
[ D] . 沈阳: 中国科学院金属研究所, 2003.
drogen storage capacity investigations of multi鄄walled carbon
nanotubes[ D] . Shenyang: Institute of Metal Research, Chinese
Academy of Sciences, 2003. )
[24] 摇 Ciaz J, Paolicelli G, Ferrer S, et al. Separation of the sp3 and
sp2 components in the C1s photoemission spectra of amorphous
carbon films[ J] . Phys Rev B, 1996, 53: 8064鄄8069.
[25] 摇 Oda H, Yamashit A, Minour S, et al. Modification of the oxy鄄
gen鄄containing functional group on activated carbon fiber in e鄄
lectrodes of an electric double鄄layer capacitor [ J ] . J Power
Sources, 2006, 158: 1510鄄1516.
[26] 摇 Nian U R, Teng H. Influence of surface oxides on the imped鄄
ance behavior of carbon based electrochemical capacitors[ J] . J
Electroanalytical Chem, 2003, 540: 119鄄127.
羰基、羧基和羟基表面官能团
对碳纳米管电容量的影响
李莉香1 ,摇 李摇 峰2
(1. 辽宁科技大学 材料电化学研究所,辽宁 鞍山 114051;
2. 中国科学院金属研究所 沈阳材料科学国家( 联合) 实验室, 辽宁 沈阳 110016)
摘摇 要:摇 分别采用混酸、空气、硝酸和高锰酸钾对碳纳米管进行氧化处理,以在其表面引入官能团,进而研究了表
面官能团对碳纳米管电化学性能的影响。 X鄄射线光电子谱分析表明:混酸氧化处理引入的官能团主要为羰基和羧
基;空气氧化使碳纳米管表面链接较多的羟基,但羰基和羧基的含量最少;而硝酸处理和高锰酸钾处理引入了中等
数量的羰基和羧基。 经四种处理方法所得碳纳米管具有相近的比表面积和孔结构。 通过比较它们的比电容发现:
羰基和羧基贡献了最多的准电容,尤其羰基含量与碳纳米管的电容量呈正比关系;而羟基主要增强了双层电容,并
未引入明显的准法拉第容量。 由于羰基和羧基比羟基具有更低的电荷传递电阻,有利于快速的法拉第反应,从而
引入准电容。
关键词:摇 碳纳米管;羰基;羧基;羟基;准电容
基金项目:国家自然科学基金(50328204, 50472084, 50632040) ,辽宁省自然科学基金(20061078) 和辽宁省教育厅基金( L2010197) .
通讯作者:李摇 峰. E鄄mail: fli@ imr. ac. cn
作者介绍:李莉香(1972-) ,女,辽宁人,博士,研究方向:先进炭材料合成及电化学应用. E鄄mail: lxli2005@ 126. com