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Electrochemical behavior of graphite in electrolyte of sulfuric and acetic acid
Carbon Vol. 35, No. 8, pp. 1167-l 173.1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 000%6223197 $17.00 + 0.00
Pergamon PII: SOOO8-6223(97)00097-3
ELECTROCHEMICAL BEHAVIOR OF GRAPHITE IN ELECTROLYTE OF SULFURIC AND ACETIC ACID F. KANG,+ T.-Y. Department
of Mechanical
Zhang,* and Y. LENG Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
(Received 11 January 1997; accepted in revised form 30 April 1997) Abstract-Electrochemical formation of graphite intercalation compounds (GICs) has been studied in the electrolyte with mixed sulfuric acid and acetic acid. The results show that, with addition of acetic acid, GICs can be synthesized in the electrolyte with sulfuric acid concentration as low as 3.6M, even though no GIC is formed in pure acetic acid. The stage structure of the synthesized GICs varies with the concentration of sulfuric acid, and stage 3 can be obtained in the electrolyte with 3.6M of sulfuric acid. The onset potential and current efficiency for intercalation changes linearly from 0.54 to 1.10 V (versus Hg/Hg,SO,) and from 0.90 to 0.68, respectively, with acetic acid addition. Two paths of graphite oxidation in an electrochemical reaction have been observed. Commonly the graphite oxide (GO) is formed via a stage 1 GIC, however, GO can be formed via a stage 2 GIC directly at elevated temperature when acetic acid is added in the electrolyte. The reduction of sulfur content in the GIC and efficiency of GIC synthesis might be balanced with an appropriate combination of sulfuric acid and acetic acid. 0 1997 Elsevier Science Ltd Key Words-A. Intercalation compounds properties, D. intercalation reactions.
(GICs),
1. INTRODUCTION
The sulfuric acid-GIC has been widely used to produce flexible graphite due to low cost and high efficiency of production. Many efforts have been concentrated on the reduction of sulfur content in GIC and exfoliated graphite since residual sulfur makes flexible graphite corrosive in service. For the electrochemical synthesis of H2S04-GIC, diluting sulfuric acid in electrolyte is a possible way to reduce sulfur content in the GIC. Most studies on the electrochemical behavior of graphite have been focused on high concentration sulfuric acid and its aqueous solution [l-7]. In the aqueous solutions, when the concentration of sulfuric acid is lower than lOM, severe water decomposition occurs and results in potential oscillation that interferes with the intercalation reaction during the electrochemical processing [ 3,4]. The influences of solvents other than water in the electrolyte and of reaction temperature on electrochemical formation of the H,SO,-GICs and GO have not yet been reported. Acetic acid has been used as a solvent of CrO, to chemically synthesize Cr03-CH,COOH-GIC [8,9], and of BF3 to synthesize BF,-CH,COOH-GIC [lo]. In this work, we investigate the possibility of using acetic acid to partially replace sulfuric acid in GIC formation in order to reduce the sulfur content in the GIC. We seek the understanding of the electrochemical beha*Corresponding author. +On leave from Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China. 1167
B. intercalation,
B. oxidation,
D. electrochemical
vior of natural graphite in the sulfuric acid electrolyte with acetic acid solvent.
2. EXPERIMENTAL
Natural graphite from China, with 99.9 wt% carbon content, was used as a host material. The graphite flakes had an average size of approximately 0.3 mm in diameter and 0.01 mm in thickness. Flakes of about 0.1 g were compressed, without any adhesive, to form a disk of 1.15 cm in diameter and approximately 0.6 mm in thickness. The graphite disk, held by a platinum gauze to prevent fragmentation during processing, served as the working electrode and a platinum plate was employed as the counter electrode. The saturated mercury sulfate electrode (Hg/Hg,SO,), 0.615 mV versus the standard hydrogen electrode (SHE), served as the reference electrode. A 100 ml electrolyte solution was mixed by sulfuric acid (purity: 95-97 wt%) and acetic acid (purity: 100 wt%) at different volume concentrations ranging from 0 to 100 ~01% of the concentration of sulfuric acid at an interval of 20 ~01%. All electrochemical reactions were performed using a computerized potentiostat (EG and G, Model 273). Two electrochemical techniques, cyclic voltammetry (CV) and chronopotentiometry (CE) were adopted to examine the influences of additive acetic acid on the intercalation of sulfuric acid. The scanning rate in CV was 0.1 mV s - ’ and the applied anodic current density in the CE varied from 0.1 to 20 mA cm-‘. A constant temperature was maintained at 24 or
80 & 1“C by an oil-bath surrounding the electrochemical cell. Stage structures of the GICs at different levels of synthesis processing were characterized by powder X-ray diffraction (XRD) with Cu KZ radiation, 40 kV anodic voltage and 50mA current. The samples for XRD characterization were taken directly from the electrochemical cell without any treatment. Under a given synthesis condition, the synthesized GIC reached its final structure and hence graphite oxidation occurred. The existence of graphite oxide (GO) formed during the synthesis was examined using the method proposed by Nakajima and Matsuo [ 111. i.e. the synthesized samples were immersed in methanol overnight. During this process the GICs were decomposed, while the GO remained unchanged, which was then characterized by XRD. The synthesized GICs were rinsed with acetone several times and then dried at 80°C overnight. After that, one part of the sample was exfoliated at 1000°C for 20 seconds, another part of the sample and their sulfur contents were also determined by a Sulfur-Carbon Analyzer (LECO model SC- 132) in which the process of high temperature burning and solid state infrared detection were adopted.
3. RESULTS
3.1 E#ect of concentration qf sulfuric ucid on the electrochemical behavior Figure 1 shows CE curves for concentrations varying from pure H,SO, to pure CH,COOH, where all the reactions were conducted at a constant charge current density, i= 10 mA cm-‘, and at a constant temperature, T= 24°C. The present experimental
,
results indicate that the concentration of the intercalate species in the electrolyte has a pronounced influence on the appearance of plateaus. The CE curve in pure sulfuric acid shows the potential plateau, corresponding to the stage transformation, which becomes more and more obscure and finally disappears when the concentration of sulfuric acid decreases. For example, no plateaus are recognizable when the concentration of sulfuric acid is lower than 50 ~01% and the potential increases abruptly up to the saturated potential value of 2.0 V for the concentration of 20 ~01%. Potential oscillation is not observed in the present solutions, while it occurs in aqueous sulfuric acid solutions lower than 10M [3,6] due to the decomposition of water. The sample was taken out for XRD examination under each concentration. The XRD patterns shown in Fig. 2 indicate that, at the same intercalation current and duration, a stage 1 GIC (F) was formed in 60 ~01% H,SO,, a stage 2 +( 1) GIC (G) was synthesized in 40 ~01% H,SO, and only stage 3 (D) was detected in 20 ~01% H,SO,, respectively, where the number inside the parentheses represents a minor stage structure. Clearly, the stage number increases with decreasing the concentration of sulfuric acid. No GIC was detected in the pure acetic acid even after a long time reaction. From the XRD patterns, the average thickness of the intercalate layer (d,) of the synthesized GICs in the mixed acids was evaluated to be 7.77 A. smaller than the value obtained in pure sulfuric acid, d, =7.89 A. Figure 3 shows the cyclic voltammetric curves in the electrolytes of CH,COOH and HISO at different concentrations and at the scanning rate of 0.1 mV ss’. There are several peaks in both the oxidation sweep (lower) and the reduction sweep
A: H,SO, B: 60% H,SO, C: 50% H,SO, D: 20% H,SO, E: CH,COOH
D ~--.-.-----_-._.____--
T = 24 ‘C
I/
I
i=lOmNcm’
/III// 0
2
4
6
8
10
12
t
E
P L= 8
F: Stage 1 ;: m 8
60% H,SO, I,=7.80A
14
Reaction Duration (ks)
0
10
20
30
40
50
60
28 / deg. (Cu Kcr) Fig. 1. Chronopotentiometric curves of graphite in different concentrations of electrolyte at i= 10mA crnm2 and T= 24’C.
Fig. 2. X-ray diffraction patterns different concentrations
of GICs synthesized of electrolyte.
in
Electrochemical synthesis of H,SO,-GICs
z g
-5
3
-10 G: 40% E:
I
H,SO,
T =
24 ‘C, SR = 0.1 mV/S
I
’
-20
I-
CH,COOH
0.0
2
.6
.4
.8
1.0
1.2
1.6
1.4
(V vs Hg/Hg2S04)
Potential
Fig. 3. Cyclic voltammetries of graphite in different concentrations of electrolyte at 0.1 mV SK’. to the stage trans(upper), which correspond formations from the high stages to the low stages and their inverse transformations. The current efficiency LXfor intercalation/deintercalation during the complete cycle is then calculated according to [ 11:
Qdeint
tl= ~
=0.76
(1)
Qinter
where Qcteintand Qinter denote,
respectively, the amounts of the intercalation and the deintercalation charges. The value of tl (OIC(I 1) gauges the reversibility of the electrochemical intercalation, a reversible process yields a= 1. According to the CV curves, the onset intercalation potential and the current efficiency are plotted in Fig. 4 as functions of the concentration of sulfuric acid. The onset potential increases from 0.54 to 1.lO V as the sulfuric acid concentration decreases
1169
from 18 to 3.6M. The value of the current efficiency is u = 0.90 in pure sulfuric acid and decreases to 0.84, 0.77 and 0.68, respectively, for 14.4, 10.8 and 7.2M of sulfuric acid. The intercalation in the mixed CH,COOH and H,SO, is less reversible than that in pure H,S04. Furthermore, no electrochemical reaction was observed in pure acetic acid, as shown by curve E in Fig. 3 within the potential range, where only a double layer current around 1 PA in both anodic and cathodic sweeps was detected. It is interesting to point out that GICs can be synthesized in dilute solutions of H,SO, and CH,COOH even when the concentration of sulfuric acid is 3.6M, while when the concentration of sulfuric acid is lower than IOM, it is impossible to synthesize GICs in aqueous dilute solutions due to the decomposition of water. As shown in Fig. 4, the onset intercalation potential behaves like a linear function of the concentration with a slope of -38 mV M -I. A similar behavior has been observed for the sulfuric acid aqueous solutions, but the slope is -47 mV M-’ [6]. If we fit the data of current efficiency c( also as a linear function of the concentration, we have GI= 0.60 +O.O165C (M). A relatively large deviation results from the linear fitting, but we will adopt the linear relationship for simplicity until a rational model is proposed.
3.2 Effects of reaction temperature on the intercalation process in the electrolyte of 50 ~01% CH,COOH and 50 ~01% H2S04 Figure 5 shows CE curves of graphite in a solution of 50 ~01% CHJOOH and 50 ~01% H,SO, at 24 or 80°C under applied current densities of 5 or 10 mA cm-‘, where the stage numbers identified by XRD are labeled. As can be seen in Fig. 5, GO forms directly from a stage 2 GIC at 8O”C, while from stage 1 at 24°C. The potential plateaus corresponding to
1 ‘.O 2+GO
GO /
a A: 10 mA!cm2. z B g
-t-
z
l
g
.2
2
4
i
Onset Intercalation Potential Current
8: 5 mNcm2,
Efficiency
/
I
1
I
I
6
8
10
12
14
16
/ _I,, 18
20
!I0
80 .X
D: 5 mA!cm2, 80 .C
(50 ~01% H,SO,
0.0
I
C: 10 m#cm’,
.5-
.4
24 & 24 .C
I
I
50
Reaction
+ 50 ~01% CH,COOH) I
,
,
,
100
Duration
150
200
(ks)
Concentration of H,SO, (M)
Fig. 4. Correlation between electrolyte concentration and onset of intercalation potential and current efficiency.
Fig. 5. Chronopotentiometric curves of graphite in 50 ~01% acetic acid + 50 ~01% sulfuric acid at different temperature and current density.
F. KANG rtul
1170
GIC transformation to GO occur obviously at 80°C while they are invisible at 24°C. The oxygen evolution is observed on the second plateau at 8O’C, while at the first (only) plateau at 24C. Figure 6(a) shows the XRD patterns of the samples directly taken from points a-e on the curves in Fig. 5 without any treatment. Pattern a illustrates the co-existence of graphite oxide and a stage 2 GIC. Both patterns b and c indicate graphite oxides but the diffraction peaks appear at different positions in the two patterns. This fact means the repeat distance (Z,) of GO varies in a large range depending on the extent of dehydration [ 121, i.e. on the co-intercalation of acetic acid in this case. Patterns d and e show the co-existence of GO and a stage 1 GIC. The samples
uu
0
10
GO
20
c: GO
30
40
20 I deg. (Cu Ka)
GO
used in Fig. 6(a) were immersed in methanol overnight and then analyzed again by XRD. The XRD patterns are shown in Fig. 6(b), where the GO peaks for a, b and c are sharper than those for d and e. This fact proves that GO formed via stage 1 has lower crystallinity than that formed via stage 2 [ 1 I]. Figure 7 shows the CE curves of graphite in a electrolyte of 50 ~01% CH,COOH and 50 ~01% H,SO, at 80°C under different current densities and the stage numbers are also labeled. When the charging rate is quite small, the potential plateaus are observed, for instance, the transformation of stage 4 to 3 or stage 3 to 2 is distinguishable in curve F, under current density i=O.l mA cm-‘. If i=O.5 mA cm-‘, the plateau for the transformation of stage 4 to 3 is obscure, but the plateau for the stage 3 to 2 transformation is still observed. When the density is increased to 5 mA cmm2, the plateaus corresponding to stage transformation for the GICs disappear, but the stage 2 GIC converts to GO at a constant potential of 1.06 V, and this plateau obscures when the density is increased to 20 mA cm -*. These results indicate that the charging rate is responsible for the appearance of potential plateaus corresponding to the transformations of stages and of the GIC to GO. Figure 8 shows the CE curves of graphite in 18M H,SO, with current density of 0.7 mA cm-’ at different temperatures. Potential plateaus corresponding to different stage transformations appear clearly at the charging rate. The onset potential for intercalation and transformation potentials for GICs from stage 3 to 2 and from 2 to 1 increases with increasing temperature. The potential for graphite oxide formation via a stage 1 GIC, however, decreases with the increase of temperature.
A:
I
20 mNcm2
t-
0
10
20
30
40
28 / deg. (Cu Ka)
I
T = 80 ‘C
(50WI% 50
H,SO,
+
50 ~1%
150
100
Reaction Duration Fig. 6. X-ray diffraction patterns of oxidized graphite by the electrochemical method in 50 ~01% acetic acid + 50 ~01% sulfuric acid at different temperatures. (a) After electrochemical oxidation, (b) after immersion in methanol overnight.
CH,COOH)
(ks)
Fig. 7. Chronopotentiometric curves of graphite in 50 ~01% acetic acid + 50 ~01% sulfuric acid at T= 80°C with different current densities.
Electrochemical
synthesis
of H,SO,-GICs
1171 4. DISCUSSION
1.8 /-L
20%
1.6
4.1 Appearance of plateaus corresponding to GICs stage transformation during the electrochemical intercalation Metrot
and
capacitance
2
1.0
B ’
.8 i = 0.7 mAkm2
.6
(Natural Graphite in 18 M H,SO,)
.4’,
’ 0
’ 10
’ 20
Reaction Fig. 8. Chronopotentiometric furic acid at i = 0.7 mA cm
j
’ 30
Duration
’ 40
11 50
(ks)
curves of graphite in 18M sul2 and different temperatures.
Tihli
model
[ 13,141
to explain
The sulfur contents of the GICs synthesized in a electrolyte of 50 ~01% CH,COOH and 50 ~01% H,S04 are examined before and after exfoliation and tabulated in Table 1. For comparison, Table 1 also lists the sulfur contents for the GICs synthesized in 18M H$O,. The sulfur content of the samples synthesized in diluted sulfuric acid is much lower than those synthesized in high concentration sulfuric acid under each stage number before exfoliation. This fact suggests that acetic acid may partially replace some sulfuric acid during the formation of bisulfate GICs, which may be expressed as Cz HSO; xH,SO, yCH,COOH, where y denotes the number of molecules of co-intercalation acetic acid with sulfuric acid. The co-intercalated acetic acid may have a smaller effective diameter than that of pure sulfuric acid, thus shortening the distance between the two graphite layers, as mentioned above. After exfoliation, the sulfur content in the product synthesized in the mixed acids is about one third of that synthesized in pure sulfuric acid. Clearly, synthesis of the GICs in the solution of acetic acid and sulfuric acid can produce exfoliation graphite with much lower sulfur content. Table 1. Sulfur content Stage no. of synthesized sample 1 2 3
Synthesized
of the synthesized
compounds
dQ
10.2 6.1 4.7
(wt%)
in different
electrolytes Synthesized
After exfoliationb
two-
CA
1
(2)
G
where n is the stage number of H,SO,-GICs, C, is defined as the internal interfacial capacitance and C, is the Fermi level lowering capacitance. C, relates to the band structure of graphite and seems effectively independent of the electrolyte. C, relates to the electrolyte nature and dominates the charging process. During the intercalation, the two processes may overlap to a certain degree. A high overall charging rate (dQ/dt) gives rise to a higher degree of overlapping [ 151. Thus, the shape of the curve is governed by the competition of reaction rates between the charging of carbon planes and the stage transformation in the gallery of graphite. The disappearance of the potential plateau by a rapid charging process was simulated using the two-capacitance model [ 151, which showed that, in 18M sulfuric acid, when the consumption ratio of electric quantity upon the further intercalation is 0.8, any plateaus are no longer recognizable. In the present work, the experimental observations have confirmed that the appearance of potential plateaus depends strongly on the charging rate not only at room temperature, but also at 80°C. Based on the charging process of stage 1 and 2 GICs in Fig. 8 and using eqn (2), the values of two capacitances of graphite flake in 18M H,S04
in 18M H,SO,
Before exfoliationa
a
quasi-equilibrium
potential versus electric quantity curve during electrochemical intercalation of sulfuric acid into graphite. According to this model, the overall intercalation reaction of sulfuric acid into graphite can be divided into two elementary processes. The charging process occurs at the interface between the graphite layer and intercalated sulfuric acid in pure stages, corresponding to a steep increase in potential. The stage transformation process caused by the further insertion in the graphite galleries shows a plateau on the potential change due to equilibrium between two successive stages. The slope of the charging curve, potential versus electric quantity, in each stage can be described as follows:
dE n -_=-+-
3.3 Co-intercalation of acetic acid with sulfuric acid in the mixing acids
proposed the
(ppm)
before and after exfoliation in 50 ~01%
CH,COOH + 50 ~01% H,SO, Before exfoliation (wt%)
2500 1500 1100
a The synthesized sample is rinsed with acetone and dried at 80°C overnight. b The rinsed and dried sample is exfoliated at 1000°C for 20 seconds.
6.2 4.5 3.6
After exfoliation 800 500 400
(ppm)
F. KANGc~iul.
1172
are evaluated to be CA=3.2 x IO3 F mol ml and at 20°C; C,=3.9 x lo3 F C,=l.6x 104Fmoll’ and C,=l.4x104Fmol-1 at 80°C. For mall’ HOPG in 18M H,SO,, the two capacitances are 103Fmol-’ CA = 3.0 x 1O3 F mol 1 and &=6.2x
[141. 4.2 Solvent efSects on the electrochemical jbrmation of GICs A solvent exerts a great influence on the dissolution of the solute and the intercalation process. The Gibbs’ free energy of solvation of an ion in a solvent with relative permittivity E, can be estimated from the Born equation [ 161: -N
A@=
-Ze2
A-
(3)
8nc,r
where the dominant quantities in this equation are the charge number 2, the ionic radius r and the relative permittivity E, of the solvent; NA is the Avogadro constant and c0 is the vacuum permittivity. The Born equation shows that the larger the relative permittivity, the more negative the value of AC?‘. Water has a relative permittivity e,=78.30 at 25°C and acetic acid has c,~6.15 at 20°C [ 171. Due to the smaller E,, the acidity of dilute H,SO, solution in CHaCOOH (pK, >O) is much weaker than that in water (pK,= -4), where pK, denotes the acidity constant. Thus, H,SO, is almost completely dissociated into HSO; and H30+ ions in aqueous solution. In the solution of H,SO, and CH,COOH, the following dissociation reaction may occur: CH,COOH
+ H,SO,oCH,COOH;
+ HSO,
reactions, however, may occur in the side H,SO,-CH,COOH solutions. For instance, graphite may be oxidized to graphite oxide or CO, in acetic acid at an overpotential [ 19,201. These side reactions also consume electric charge and facilitate a further change of potential in a positive direction. Thus, the current efficiency decreases and the onset intercalation potential increases with diluting sulfuric acid. 4.3 Formation mechanism of graphite oxides As described above, there are two types of formation processes of graphite oxides in electrochemical oxidation. One is via a stage 1 H,SO,-GIC and the other is via a stage 2 H,SO,-GIC. There may be three steps in the electrochemical formation of a graphite oxide: 1) formation of a stage 1 or 2 bisulfate GIG; 2) electro-oxidation of the formed GIC by the further charging; and 3) hydrolysis of the formed GIC and loss of sulfuric acid molecules. Thus, the formation of graphite oxide via a stage 1 or 2 bisulfate GIC may proceed according to: {C,+HSO,
;
.2SH,SO,}
{C,O,_,(OH),,}
+ f
(2+x)H,Oo
+3.5H,S04
[H+ +e-]
(O
(5)
where the bisulfate GIC is symbolically described as CZHSO, 2.5H2S0, with m =24 for stage 1 and m =48 for stage 2 and GO as C,O,_,(OH),,, (0
(4) The anion of HSO, usually reacts with H,SO, by hydrogen bonding to form a homoconjugate species of HSO, H,SO,, which is unstable in the solvent with low E,. Water is both an ionizing solvent with a donor number of DN= 18 and a strong dissociating solvent, whereas acetic acid has moderate ionizing property (DN= 11) and is a poor dissociating solvent. Therefore, the reaction of eqn (4) could be much lower. That is why the electrochemical behavior of graphite in H,S04-CH,COOH differs from that in H,S04-H20. The side reactions of the solvent itself must be taken into account in choosing an electrolyte solvent. Although the acidity and polarity in an HISO,-H,O solution are much stronger than in an H2S04-CH,COOH solution, water is decomposed into hydrogen and oxygen at the potential of 1.23 V (vs SHE) [ 181. The external oxidation of water occurs with oxygen evolution accompanying potential oscillation when the concentration of H,SO, in aqueous solution is smaller than 10M [3,6]. In this case, thus, it is difficult to get a complete cyclic voltammetry curve. In H2S04-CH,COOH solutions, such an oscillation phenomenon has not been observed. Other
i=
f
(2+x),
u=m/8, v=(m/2-
1)
(6)
where aGICHracO, aHzsO aH+ and a,- denote the activities of the GIC with4 a stage number n, graphite oxide, sulfuric acid and hydrogen ion, respectively. If the activities of the GIC and GO in the solid state are treated as unity, then eqn (6) reduces to RT E=E,+
W&J
(7)
For the case of natural graphite in 18M H2S04, as shown in Fig. 8, we define electrochemical potentials at points from a to e as GO formation potentials, i.e. the transition potentials from a stage 1 GIC to GO at each temperature. The variation of the GO formation potential with temperature is plotted in Fig. 9. The present experimental results show that the GO formation potential decreases linearly
Electrochemical synthesis of H,SO,-GICs
1.50
tration of sulfuric acid in the electrolyte. In dilute solutions, the potential plateaus in the CE curves gradually become obscure and finally are not recognizable, as the charging rate increases. Two types of formation processes of graphite oxide by electrochemical oxidation have been observed. In concentrated sulfuric acid, GO is formed via a stage 1 GIC, however, in the mixed acids of 50 ~01% H,SO, and 50~01% CH,COOH, GO formation depends strongly on temperature. For example, GO is formed completely via a stage 1 GIC at 24°C and may be formed via a stage 2 GIC at 80°C.
1.45
Acknowledgements-We
t
1.65 3 WI 5 1.60 2 2 1.55 s g
1173
1.40 260
I
/
I
280
300
320
,
I
340
are grateful for the financial support from the Hong Kong University of Science and Technology under RIG Grant, RI93/94,EG07.
,
360 REFERENCES
Temperature (K) Fig. 9. The formation potentials of graphite oxide via a stage 1 H,SO,-GIC in 96 wt% H,S04 as a function of temperature.
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with temperature at the rate of approximately -4mV K-’ with the exception of O”C, which is consistent with the data in ref. [4].
4. Jiang, J. and Beck, F., Carbon, 1992, 30(2), 223. 5. Inagaki, M., Iwashita, N. and Hishiyama, Y., Molecular
5. CONCLUDING REMARKS
I. Metrot, A. aid”Fuzellier, H., C&on; i984, 22(2), 131. 8. Metz, W. and Meyer-Spasche, H., Synthetic Metals, 1979/80, 1, 53163. 9. Kobayashi, T., Kurata, H. and Uyeda, N., J. Phys. Chem., 1986, 90( IO), 2231. IO. Schliigl, R., in Progress in Intercalation Research, ed. W. Miiller-Warmuth and R. Schiillhorn. Kluwer Academic Publishers, Dordrecht, 1994, pp. 104-106. II. Nakajima, T. and Matsuo, Y., Carbon, 1994, 32(3),
Science Forum, 1992, 91-93(2),
689.
Crystals and Liquid Crvstals, 1994, 244, 89.
6. King, F., Leng: Y. and Zhang, T.-Y ., Journal of Physical Chemistrv of Solids. 1996, 57(6-S),
The present work investigates the electrochemical behavior of natural graphite in the solutions of sulfuric acid and acetic acid. The results show that, with addition of acetic acid, GICs can be synthesized in the solution with sulfuric acid concentration as low as 3.6M, even though no GIC is formed in pure acetic acid. The stage structure of the synthesized GIC strongly changes with the concentration of sulfuric acid, stage 3 can be obtained in the electrolyte with 3.6M of sulfuric acid. The onset intercalation potential increases from 0.54 to 1.1 V (vs Hg/Hg,SO,) with addition of acetic acid and behaves like a linear function with the slope of -38 mV M -l. The current efficiency t( for intercalation/deintercalation can also be fitted by a linear function of sulfuric acid concentration C as cc=O.60+0.0165C (M). The reduction of sulfur content in the GIC and efficiency of GIC synthesis might be balanced with an appropriate combination of sulfuric acid and acetic acid. The appearance or disappearance of potential plateaus corresponding to the stage transformation depends strongly on the charging rate and the concen-
883.
469.
12. Nakajima, T., Mabuchi, A. and Hagiwara, R., Carbon, 1988, 26(3),
357.
13. Metrot, A., Synthetic Metals, 1983, 7, 177. 14. Metrot, A. and Tihli, M., Synthetic Metals, 1985, 12, 517.
15. Iwashita, N., Shioyama, H. and Inagaki, M., Synthetic Metals, 1995, 73(I),
33.
16. Shriver, D. F., Atkins, P. W. and Langford, C. H., Inorganic Chemistry, W.H. Freeman and Company, New York, 1990, p. 154. 17. Dean, J. A., Lange’s Handbook of Chemistry, 14th edn. McGraw-Hill, Inc., New York, 1992, p. 5.16, 5.18, 8.19. 18. Bard, A. J., Parsons, R. and Jordan, J., Standard Potential in Aqueous Solutions, Marcel Dekker, Inc., New York, 1985, p. 787. 19. Kinoshita, K., Carbon: Electrochemical and Physicochemical Properties, John Wiley and Sons, New York, 1988, p. 319. 20. Bagotzky, V. S., Fundamentals of Electrochemistry, Plenum Press, New York, 1993, p. 494.
Pergamon PII: SOOO8-6223(97)00097-3
ELECTROCHEMICAL BEHAVIOR OF GRAPHITE IN ELECTROLYTE OF SULFURIC AND ACETIC ACID F. KANG,+ T.-Y. Department
of Mechanical
Zhang,* and Y. LENG Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
(Received 11 January 1997; accepted in revised form 30 April 1997) Abstract-Electrochemical formation of graphite intercalation compounds (GICs) has been studied in the electrolyte with mixed sulfuric acid and acetic acid. The results show that, with addition of acetic acid, GICs can be synthesized in the electrolyte with sulfuric acid concentration as low as 3.6M, even though no GIC is formed in pure acetic acid. The stage structure of the synthesized GICs varies with the concentration of sulfuric acid, and stage 3 can be obtained in the electrolyte with 3.6M of sulfuric acid. The onset potential and current efficiency for intercalation changes linearly from 0.54 to 1.10 V (versus Hg/Hg,SO,) and from 0.90 to 0.68, respectively, with acetic acid addition. Two paths of graphite oxidation in an electrochemical reaction have been observed. Commonly the graphite oxide (GO) is formed via a stage 1 GIC, however, GO can be formed via a stage 2 GIC directly at elevated temperature when acetic acid is added in the electrolyte. The reduction of sulfur content in the GIC and efficiency of GIC synthesis might be balanced with an appropriate combination of sulfuric acid and acetic acid. 0 1997 Elsevier Science Ltd Key Words-A. Intercalation compounds properties, D. intercalation reactions.
(GICs),
1. INTRODUCTION
The sulfuric acid-GIC has been widely used to produce flexible graphite due to low cost and high efficiency of production. Many efforts have been concentrated on the reduction of sulfur content in GIC and exfoliated graphite since residual sulfur makes flexible graphite corrosive in service. For the electrochemical synthesis of H2S04-GIC, diluting sulfuric acid in electrolyte is a possible way to reduce sulfur content in the GIC. Most studies on the electrochemical behavior of graphite have been focused on high concentration sulfuric acid and its aqueous solution [l-7]. In the aqueous solutions, when the concentration of sulfuric acid is lower than lOM, severe water decomposition occurs and results in potential oscillation that interferes with the intercalation reaction during the electrochemical processing [ 3,4]. The influences of solvents other than water in the electrolyte and of reaction temperature on electrochemical formation of the H,SO,-GICs and GO have not yet been reported. Acetic acid has been used as a solvent of CrO, to chemically synthesize Cr03-CH,COOH-GIC [8,9], and of BF3 to synthesize BF,-CH,COOH-GIC [lo]. In this work, we investigate the possibility of using acetic acid to partially replace sulfuric acid in GIC formation in order to reduce the sulfur content in the GIC. We seek the understanding of the electrochemical beha*Corresponding author. +On leave from Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China. 1167
B. intercalation,
B. oxidation,
D. electrochemical
vior of natural graphite in the sulfuric acid electrolyte with acetic acid solvent.
2. EXPERIMENTAL
Natural graphite from China, with 99.9 wt% carbon content, was used as a host material. The graphite flakes had an average size of approximately 0.3 mm in diameter and 0.01 mm in thickness. Flakes of about 0.1 g were compressed, without any adhesive, to form a disk of 1.15 cm in diameter and approximately 0.6 mm in thickness. The graphite disk, held by a platinum gauze to prevent fragmentation during processing, served as the working electrode and a platinum plate was employed as the counter electrode. The saturated mercury sulfate electrode (Hg/Hg,SO,), 0.615 mV versus the standard hydrogen electrode (SHE), served as the reference electrode. A 100 ml electrolyte solution was mixed by sulfuric acid (purity: 95-97 wt%) and acetic acid (purity: 100 wt%) at different volume concentrations ranging from 0 to 100 ~01% of the concentration of sulfuric acid at an interval of 20 ~01%. All electrochemical reactions were performed using a computerized potentiostat (EG and G, Model 273). Two electrochemical techniques, cyclic voltammetry (CV) and chronopotentiometry (CE) were adopted to examine the influences of additive acetic acid on the intercalation of sulfuric acid. The scanning rate in CV was 0.1 mV s - ’ and the applied anodic current density in the CE varied from 0.1 to 20 mA cm-‘. A constant temperature was maintained at 24 or
80 & 1“C by an oil-bath surrounding the electrochemical cell. Stage structures of the GICs at different levels of synthesis processing were characterized by powder X-ray diffraction (XRD) with Cu KZ radiation, 40 kV anodic voltage and 50mA current. The samples for XRD characterization were taken directly from the electrochemical cell without any treatment. Under a given synthesis condition, the synthesized GIC reached its final structure and hence graphite oxidation occurred. The existence of graphite oxide (GO) formed during the synthesis was examined using the method proposed by Nakajima and Matsuo [ 111. i.e. the synthesized samples were immersed in methanol overnight. During this process the GICs were decomposed, while the GO remained unchanged, which was then characterized by XRD. The synthesized GICs were rinsed with acetone several times and then dried at 80°C overnight. After that, one part of the sample was exfoliated at 1000°C for 20 seconds, another part of the sample and their sulfur contents were also determined by a Sulfur-Carbon Analyzer (LECO model SC- 132) in which the process of high temperature burning and solid state infrared detection were adopted.
3. RESULTS
3.1 E#ect of concentration qf sulfuric ucid on the electrochemical behavior Figure 1 shows CE curves for concentrations varying from pure H,SO, to pure CH,COOH, where all the reactions were conducted at a constant charge current density, i= 10 mA cm-‘, and at a constant temperature, T= 24°C. The present experimental
,
results indicate that the concentration of the intercalate species in the electrolyte has a pronounced influence on the appearance of plateaus. The CE curve in pure sulfuric acid shows the potential plateau, corresponding to the stage transformation, which becomes more and more obscure and finally disappears when the concentration of sulfuric acid decreases. For example, no plateaus are recognizable when the concentration of sulfuric acid is lower than 50 ~01% and the potential increases abruptly up to the saturated potential value of 2.0 V for the concentration of 20 ~01%. Potential oscillation is not observed in the present solutions, while it occurs in aqueous sulfuric acid solutions lower than 10M [3,6] due to the decomposition of water. The sample was taken out for XRD examination under each concentration. The XRD patterns shown in Fig. 2 indicate that, at the same intercalation current and duration, a stage 1 GIC (F) was formed in 60 ~01% H,SO,, a stage 2 +( 1) GIC (G) was synthesized in 40 ~01% H,SO, and only stage 3 (D) was detected in 20 ~01% H,SO,, respectively, where the number inside the parentheses represents a minor stage structure. Clearly, the stage number increases with decreasing the concentration of sulfuric acid. No GIC was detected in the pure acetic acid even after a long time reaction. From the XRD patterns, the average thickness of the intercalate layer (d,) of the synthesized GICs in the mixed acids was evaluated to be 7.77 A. smaller than the value obtained in pure sulfuric acid, d, =7.89 A. Figure 3 shows the cyclic voltammetric curves in the electrolytes of CH,COOH and HISO at different concentrations and at the scanning rate of 0.1 mV ss’. There are several peaks in both the oxidation sweep (lower) and the reduction sweep
A: H,SO, B: 60% H,SO, C: 50% H,SO, D: 20% H,SO, E: CH,COOH
D ~--.-.-----_-._.____--
T = 24 ‘C
I/
I
i=lOmNcm’
/III// 0
2
4
6
8
10
12
t
E
P L= 8
F: Stage 1 ;: m 8
60% H,SO, I,=7.80A
14
Reaction Duration (ks)
0
10
20
30
40
50
60
28 / deg. (Cu Kcr) Fig. 1. Chronopotentiometric curves of graphite in different concentrations of electrolyte at i= 10mA crnm2 and T= 24’C.
Fig. 2. X-ray diffraction patterns different concentrations
of GICs synthesized of electrolyte.
in
Electrochemical synthesis of H,SO,-GICs
z g
-5
3
-10 G: 40% E:
I
H,SO,
T =
24 ‘C, SR = 0.1 mV/S
I
’
-20
I-
CH,COOH
0.0
2
.6
.4
.8
1.0
1.2
1.6
1.4
(V vs Hg/Hg2S04)
Potential
Fig. 3. Cyclic voltammetries of graphite in different concentrations of electrolyte at 0.1 mV SK’. to the stage trans(upper), which correspond formations from the high stages to the low stages and their inverse transformations. The current efficiency LXfor intercalation/deintercalation during the complete cycle is then calculated according to [ 11:
Qdeint
tl= ~
=0.76
(1)
Qinter
where Qcteintand Qinter denote,
respectively, the amounts of the intercalation and the deintercalation charges. The value of tl (OIC(I 1) gauges the reversibility of the electrochemical intercalation, a reversible process yields a= 1. According to the CV curves, the onset intercalation potential and the current efficiency are plotted in Fig. 4 as functions of the concentration of sulfuric acid. The onset potential increases from 0.54 to 1.lO V as the sulfuric acid concentration decreases
1169
from 18 to 3.6M. The value of the current efficiency is u = 0.90 in pure sulfuric acid and decreases to 0.84, 0.77 and 0.68, respectively, for 14.4, 10.8 and 7.2M of sulfuric acid. The intercalation in the mixed CH,COOH and H,SO, is less reversible than that in pure H,S04. Furthermore, no electrochemical reaction was observed in pure acetic acid, as shown by curve E in Fig. 3 within the potential range, where only a double layer current around 1 PA in both anodic and cathodic sweeps was detected. It is interesting to point out that GICs can be synthesized in dilute solutions of H,SO, and CH,COOH even when the concentration of sulfuric acid is 3.6M, while when the concentration of sulfuric acid is lower than IOM, it is impossible to synthesize GICs in aqueous dilute solutions due to the decomposition of water. As shown in Fig. 4, the onset intercalation potential behaves like a linear function of the concentration with a slope of -38 mV M -I. A similar behavior has been observed for the sulfuric acid aqueous solutions, but the slope is -47 mV M-’ [6]. If we fit the data of current efficiency c( also as a linear function of the concentration, we have GI= 0.60 +O.O165C (M). A relatively large deviation results from the linear fitting, but we will adopt the linear relationship for simplicity until a rational model is proposed.
3.2 Effects of reaction temperature on the intercalation process in the electrolyte of 50 ~01% CH,COOH and 50 ~01% H2S04 Figure 5 shows CE curves of graphite in a solution of 50 ~01% CHJOOH and 50 ~01% H,SO, at 24 or 80°C under applied current densities of 5 or 10 mA cm-‘, where the stage numbers identified by XRD are labeled. As can be seen in Fig. 5, GO forms directly from a stage 2 GIC at 8O”C, while from stage 1 at 24°C. The potential plateaus corresponding to
1 ‘.O 2+GO
GO /
a A: 10 mA!cm2. z B g
-t-
z
l
g
.2
2
4
i
Onset Intercalation Potential Current
8: 5 mNcm2,
Efficiency
/
I
1
I
I
6
8
10
12
14
16
/ _I,, 18
20
!I0
80 .X
D: 5 mA!cm2, 80 .C
(50 ~01% H,SO,
0.0
I
C: 10 m#cm’,
.5-
.4
24 & 24 .C
I
I
50
Reaction
+ 50 ~01% CH,COOH) I
,
,
,
100
Duration
150
200
(ks)
Concentration of H,SO, (M)
Fig. 4. Correlation between electrolyte concentration and onset of intercalation potential and current efficiency.
Fig. 5. Chronopotentiometric curves of graphite in 50 ~01% acetic acid + 50 ~01% sulfuric acid at different temperature and current density.
F. KANG rtul
1170
GIC transformation to GO occur obviously at 80°C while they are invisible at 24°C. The oxygen evolution is observed on the second plateau at 8O’C, while at the first (only) plateau at 24C. Figure 6(a) shows the XRD patterns of the samples directly taken from points a-e on the curves in Fig. 5 without any treatment. Pattern a illustrates the co-existence of graphite oxide and a stage 2 GIC. Both patterns b and c indicate graphite oxides but the diffraction peaks appear at different positions in the two patterns. This fact means the repeat distance (Z,) of GO varies in a large range depending on the extent of dehydration [ 121, i.e. on the co-intercalation of acetic acid in this case. Patterns d and e show the co-existence of GO and a stage 1 GIC. The samples
uu
0
10
GO
20
c: GO
30
40
20 I deg. (Cu Ka)
GO
used in Fig. 6(a) were immersed in methanol overnight and then analyzed again by XRD. The XRD patterns are shown in Fig. 6(b), where the GO peaks for a, b and c are sharper than those for d and e. This fact proves that GO formed via stage 1 has lower crystallinity than that formed via stage 2 [ 1 I]. Figure 7 shows the CE curves of graphite in a electrolyte of 50 ~01% CH,COOH and 50 ~01% H,SO, at 80°C under different current densities and the stage numbers are also labeled. When the charging rate is quite small, the potential plateaus are observed, for instance, the transformation of stage 4 to 3 or stage 3 to 2 is distinguishable in curve F, under current density i=O.l mA cm-‘. If i=O.5 mA cm-‘, the plateau for the transformation of stage 4 to 3 is obscure, but the plateau for the stage 3 to 2 transformation is still observed. When the density is increased to 5 mA cmm2, the plateaus corresponding to stage transformation for the GICs disappear, but the stage 2 GIC converts to GO at a constant potential of 1.06 V, and this plateau obscures when the density is increased to 20 mA cm -*. These results indicate that the charging rate is responsible for the appearance of potential plateaus corresponding to the transformations of stages and of the GIC to GO. Figure 8 shows the CE curves of graphite in 18M H,SO, with current density of 0.7 mA cm-’ at different temperatures. Potential plateaus corresponding to different stage transformations appear clearly at the charging rate. The onset potential for intercalation and transformation potentials for GICs from stage 3 to 2 and from 2 to 1 increases with increasing temperature. The potential for graphite oxide formation via a stage 1 GIC, however, decreases with the increase of temperature.
A:
I
20 mNcm2
t-
0
10
20
30
40
28 / deg. (Cu Ka)
I
T = 80 ‘C
(50WI% 50
H,SO,
+
50 ~1%
150
100
Reaction Duration Fig. 6. X-ray diffraction patterns of oxidized graphite by the electrochemical method in 50 ~01% acetic acid + 50 ~01% sulfuric acid at different temperatures. (a) After electrochemical oxidation, (b) after immersion in methanol overnight.
CH,COOH)
(ks)
Fig. 7. Chronopotentiometric curves of graphite in 50 ~01% acetic acid + 50 ~01% sulfuric acid at T= 80°C with different current densities.
Electrochemical
synthesis
of H,SO,-GICs
1171 4. DISCUSSION
1.8 /-L
20%
1.6
4.1 Appearance of plateaus corresponding to GICs stage transformation during the electrochemical intercalation Metrot
and
capacitance
2
1.0
B ’
.8 i = 0.7 mAkm2
.6
(Natural Graphite in 18 M H,SO,)
.4’,
’ 0
’ 10
’ 20
Reaction Fig. 8. Chronopotentiometric furic acid at i = 0.7 mA cm
j
’ 30
Duration
’ 40
11 50
(ks)
curves of graphite in 18M sul2 and different temperatures.
Tihli
model
[ 13,141
to explain
The sulfur contents of the GICs synthesized in a electrolyte of 50 ~01% CH,COOH and 50 ~01% H,S04 are examined before and after exfoliation and tabulated in Table 1. For comparison, Table 1 also lists the sulfur contents for the GICs synthesized in 18M H$O,. The sulfur content of the samples synthesized in diluted sulfuric acid is much lower than those synthesized in high concentration sulfuric acid under each stage number before exfoliation. This fact suggests that acetic acid may partially replace some sulfuric acid during the formation of bisulfate GICs, which may be expressed as Cz HSO; xH,SO, yCH,COOH, where y denotes the number of molecules of co-intercalation acetic acid with sulfuric acid. The co-intercalated acetic acid may have a smaller effective diameter than that of pure sulfuric acid, thus shortening the distance between the two graphite layers, as mentioned above. After exfoliation, the sulfur content in the product synthesized in the mixed acids is about one third of that synthesized in pure sulfuric acid. Clearly, synthesis of the GICs in the solution of acetic acid and sulfuric acid can produce exfoliation graphite with much lower sulfur content. Table 1. Sulfur content Stage no. of synthesized sample 1 2 3
Synthesized
of the synthesized
compounds
dQ
10.2 6.1 4.7
(wt%)
in different
electrolytes Synthesized
After exfoliationb
two-
CA
1
(2)
G
where n is the stage number of H,SO,-GICs, C, is defined as the internal interfacial capacitance and C, is the Fermi level lowering capacitance. C, relates to the band structure of graphite and seems effectively independent of the electrolyte. C, relates to the electrolyte nature and dominates the charging process. During the intercalation, the two processes may overlap to a certain degree. A high overall charging rate (dQ/dt) gives rise to a higher degree of overlapping [ 151. Thus, the shape of the curve is governed by the competition of reaction rates between the charging of carbon planes and the stage transformation in the gallery of graphite. The disappearance of the potential plateau by a rapid charging process was simulated using the two-capacitance model [ 151, which showed that, in 18M sulfuric acid, when the consumption ratio of electric quantity upon the further intercalation is 0.8, any plateaus are no longer recognizable. In the present work, the experimental observations have confirmed that the appearance of potential plateaus depends strongly on the charging rate not only at room temperature, but also at 80°C. Based on the charging process of stage 1 and 2 GICs in Fig. 8 and using eqn (2), the values of two capacitances of graphite flake in 18M H,S04
in 18M H,SO,
Before exfoliationa
a
quasi-equilibrium
potential versus electric quantity curve during electrochemical intercalation of sulfuric acid into graphite. According to this model, the overall intercalation reaction of sulfuric acid into graphite can be divided into two elementary processes. The charging process occurs at the interface between the graphite layer and intercalated sulfuric acid in pure stages, corresponding to a steep increase in potential. The stage transformation process caused by the further insertion in the graphite galleries shows a plateau on the potential change due to equilibrium between two successive stages. The slope of the charging curve, potential versus electric quantity, in each stage can be described as follows:
dE n -_=-+-
3.3 Co-intercalation of acetic acid with sulfuric acid in the mixing acids
proposed the
(ppm)
before and after exfoliation in 50 ~01%
CH,COOH + 50 ~01% H,SO, Before exfoliation (wt%)
2500 1500 1100
a The synthesized sample is rinsed with acetone and dried at 80°C overnight. b The rinsed and dried sample is exfoliated at 1000°C for 20 seconds.
6.2 4.5 3.6
After exfoliation 800 500 400
(ppm)
F. KANGc~iul.
1172
are evaluated to be CA=3.2 x IO3 F mol ml and at 20°C; C,=3.9 x lo3 F C,=l.6x 104Fmoll’ and C,=l.4x104Fmol-1 at 80°C. For mall’ HOPG in 18M H,SO,, the two capacitances are 103Fmol-’ CA = 3.0 x 1O3 F mol 1 and &=6.2x
[141. 4.2 Solvent efSects on the electrochemical jbrmation of GICs A solvent exerts a great influence on the dissolution of the solute and the intercalation process. The Gibbs’ free energy of solvation of an ion in a solvent with relative permittivity E, can be estimated from the Born equation [ 161: -N
A@=
-Ze2
A-
(3)
8nc,r
where the dominant quantities in this equation are the charge number 2, the ionic radius r and the relative permittivity E, of the solvent; NA is the Avogadro constant and c0 is the vacuum permittivity. The Born equation shows that the larger the relative permittivity, the more negative the value of AC?‘. Water has a relative permittivity e,=78.30 at 25°C and acetic acid has c,~6.15 at 20°C [ 171. Due to the smaller E,, the acidity of dilute H,SO, solution in CHaCOOH (pK, >O) is much weaker than that in water (pK,= -4), where pK, denotes the acidity constant. Thus, H,SO, is almost completely dissociated into HSO; and H30+ ions in aqueous solution. In the solution of H,SO, and CH,COOH, the following dissociation reaction may occur: CH,COOH
+ H,SO,oCH,COOH;
+ HSO,
reactions, however, may occur in the side H,SO,-CH,COOH solutions. For instance, graphite may be oxidized to graphite oxide or CO, in acetic acid at an overpotential [ 19,201. These side reactions also consume electric charge and facilitate a further change of potential in a positive direction. Thus, the current efficiency decreases and the onset intercalation potential increases with diluting sulfuric acid. 4.3 Formation mechanism of graphite oxides As described above, there are two types of formation processes of graphite oxides in electrochemical oxidation. One is via a stage 1 H,SO,-GIC and the other is via a stage 2 H,SO,-GIC. There may be three steps in the electrochemical formation of a graphite oxide: 1) formation of a stage 1 or 2 bisulfate GIG; 2) electro-oxidation of the formed GIC by the further charging; and 3) hydrolysis of the formed GIC and loss of sulfuric acid molecules. Thus, the formation of graphite oxide via a stage 1 or 2 bisulfate GIC may proceed according to: {C,+HSO,
;
.2SH,SO,}
{C,O,_,(OH),,}
+ f
(2+x)H,Oo
+3.5H,S04
[H+ +e-]
(O
(5)
where the bisulfate GIC is symbolically described as CZHSO, 2.5H2S0, with m =24 for stage 1 and m =48 for stage 2 and GO as C,O,_,(OH),,, (0
(4) The anion of HSO, usually reacts with H,SO, by hydrogen bonding to form a homoconjugate species of HSO, H,SO,, which is unstable in the solvent with low E,. Water is both an ionizing solvent with a donor number of DN= 18 and a strong dissociating solvent, whereas acetic acid has moderate ionizing property (DN= 11) and is a poor dissociating solvent. Therefore, the reaction of eqn (4) could be much lower. That is why the electrochemical behavior of graphite in H,S04-CH,COOH differs from that in H,S04-H20. The side reactions of the solvent itself must be taken into account in choosing an electrolyte solvent. Although the acidity and polarity in an HISO,-H,O solution are much stronger than in an H2S04-CH,COOH solution, water is decomposed into hydrogen and oxygen at the potential of 1.23 V (vs SHE) [ 181. The external oxidation of water occurs with oxygen evolution accompanying potential oscillation when the concentration of H,SO, in aqueous solution is smaller than 10M [3,6]. In this case, thus, it is difficult to get a complete cyclic voltammetry curve. In H2S04-CH,COOH solutions, such an oscillation phenomenon has not been observed. Other
i=
f
(2+x),
u=m/8, v=(m/2-
1)
(6)
where aGICHracO, aHzsO aH+ and a,- denote the activities of the GIC with4 a stage number n, graphite oxide, sulfuric acid and hydrogen ion, respectively. If the activities of the GIC and GO in the solid state are treated as unity, then eqn (6) reduces to RT E=E,+
W&J
(7)
For the case of natural graphite in 18M H2S04, as shown in Fig. 8, we define electrochemical potentials at points from a to e as GO formation potentials, i.e. the transition potentials from a stage 1 GIC to GO at each temperature. The variation of the GO formation potential with temperature is plotted in Fig. 9. The present experimental results show that the GO formation potential decreases linearly
Electrochemical synthesis of H,SO,-GICs
1.50
tration of sulfuric acid in the electrolyte. In dilute solutions, the potential plateaus in the CE curves gradually become obscure and finally are not recognizable, as the charging rate increases. Two types of formation processes of graphite oxide by electrochemical oxidation have been observed. In concentrated sulfuric acid, GO is formed via a stage 1 GIC, however, in the mixed acids of 50 ~01% H,SO, and 50~01% CH,COOH, GO formation depends strongly on temperature. For example, GO is formed completely via a stage 1 GIC at 24°C and may be formed via a stage 2 GIC at 80°C.
1.45
Acknowledgements-We
t
1.65 3 WI 5 1.60 2 2 1.55 s g
1173
1.40 260
I
/
I
280
300
320
,
I
340
are grateful for the financial support from the Hong Kong University of Science and Technology under RIG Grant, RI93/94,EG07.
,
360 REFERENCES
Temperature (K) Fig. 9. The formation potentials of graphite oxide via a stage 1 H,SO,-GIC in 96 wt% H,S04 as a function of temperature.
1. Beck, F. and Pru, A., Electrochim. Acta, 1983, 28( 12), 1847. 2. Beck, F. and Krohn, H., Synthetic Metals, 1986,14, 137. 3. Harrach, A., Douglade, J. and M&trot, A., Materials
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4. Jiang, J. and Beck, F., Carbon, 1992, 30(2), 223. 5. Inagaki, M., Iwashita, N. and Hishiyama, Y., Molecular
5. CONCLUDING REMARKS
I. Metrot, A. aid”Fuzellier, H., C&on; i984, 22(2), 131. 8. Metz, W. and Meyer-Spasche, H., Synthetic Metals, 1979/80, 1, 53163. 9. Kobayashi, T., Kurata, H. and Uyeda, N., J. Phys. Chem., 1986, 90( IO), 2231. IO. Schliigl, R., in Progress in Intercalation Research, ed. W. Miiller-Warmuth and R. Schiillhorn. Kluwer Academic Publishers, Dordrecht, 1994, pp. 104-106. II. Nakajima, T. and Matsuo, Y., Carbon, 1994, 32(3),
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689.
Crystals and Liquid Crvstals, 1994, 244, 89.
6. King, F., Leng: Y. and Zhang, T.-Y ., Journal of Physical Chemistrv of Solids. 1996, 57(6-S),
The present work investigates the electrochemical behavior of natural graphite in the solutions of sulfuric acid and acetic acid. The results show that, with addition of acetic acid, GICs can be synthesized in the solution with sulfuric acid concentration as low as 3.6M, even though no GIC is formed in pure acetic acid. The stage structure of the synthesized GIC strongly changes with the concentration of sulfuric acid, stage 3 can be obtained in the electrolyte with 3.6M of sulfuric acid. The onset intercalation potential increases from 0.54 to 1.1 V (vs Hg/Hg,SO,) with addition of acetic acid and behaves like a linear function with the slope of -38 mV M -l. The current efficiency t( for intercalation/deintercalation can also be fitted by a linear function of sulfuric acid concentration C as cc=O.60+0.0165C (M). The reduction of sulfur content in the GIC and efficiency of GIC synthesis might be balanced with an appropriate combination of sulfuric acid and acetic acid. The appearance or disappearance of potential plateaus corresponding to the stage transformation depends strongly on the charging rate and the concen-
883.
469.
12. Nakajima, T., Mabuchi, A. and Hagiwara, R., Carbon, 1988, 26(3),
357.
13. Metrot, A., Synthetic Metals, 1983, 7, 177. 14. Metrot, A. and Tihli, M., Synthetic Metals, 1985, 12, 517.
15. Iwashita, N., Shioyama, H. and Inagaki, M., Synthetic Metals, 1995, 73(I),
33.
16. Shriver, D. F., Atkins, P. W. and Langford, C. H., Inorganic Chemistry, W.H. Freeman and Company, New York, 1990, p. 154. 17. Dean, J. A., Lange’s Handbook of Chemistry, 14th edn. McGraw-Hill, Inc., New York, 1992, p. 5.16, 5.18, 8.19. 18. Bard, A. J., Parsons, R. and Jordan, J., Standard Potential in Aqueous Solutions, Marcel Dekker, Inc., New York, 1985, p. 787. 19. Kinoshita, K., Carbon: Electrochemical and Physicochemical Properties, John Wiley and Sons, New York, 1988, p. 319. 20. Bagotzky, V. S., Fundamentals of Electrochemistry, Plenum Press, New York, 1993, p. 494.