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Potential change with intercalation of sulfuric acid into graphite by chemical oxidation
Carbon
Vol.
Printed
in Great Britain.
28, No.
I. pp
49-55.
1990
UUlW6223”YU $3.0(1+ 00 Copyright 0 1990 Pergamon Press plc
POTENTIAL CHANGE WITH INTERCALATION SULFURIC ACID INTO GRAPHITE BY CHEMICAL OXIDATION
OF
MICHIO INAGAKI, NORIO IWASHITA, and EIJI KOUNO Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 440, Japan (Received on February 1989) Abstract-Potential of graphite during the intercalation of sulfuric acid by chemical oxidation with either nitric acid or potassium permanganate in H,SO, of 18 molidm’ was found to increase to positive direction in stepwise. On potential change, a distinct plateau corresponding to the transformation from stage two to one compound and a faint one for the formation of stage three were observed, exactly the same as those observed during galvanostatic oxidation. The intercalation of sulfuric acid into graphite by chemical and electrochemical oxidations was shown to be fundamentally the same processes; potential of graphite increases with oxidation and the intercalation of sulfuric acid proceeds by showing potential plateaux. On the change of potential of graphite by chemical oxidation, there was found to be an upper limit of saturated potential, depending strongly on the oxidizer used and also on the concentration of sulfuric acid even if enough amounts of oxidizer was used. The relation between the threshold potential for stage transformation in H,SO,-GICs, increasing with dilution of sulfuric acid, and the upper limit of saturated potential by chemical oxidation, decreasing with dilution of the acid, governed the intercalation reaction by chemical oxidation in sulfuric acid with various concentrations. Kev Words-Graphite chemical oxidation.
intercalation
compounds,
1. INTRODUCTION
are two methods for synthesis of graphite intercalation compounds with sulfuric acid (H,SO,GICs); chemical and electrochemical oxidations. The latter has been quantitatively studied by many authors. For instance, Metrot[l,2] discussed the potential curve versus electric quantity measured on graphite electrode by using a two capacitor model. A remarkable effect of host graphite during electrochemical intercalation of sulfuric acid was shown by using different carbon fibers[3]. On the other hand, the former method has been used as the simplest one for the synthesis of H,SO,-GICs and industrially employed for the production of exfoliated graphite. The oxidizers used for the synthesis of H,SO,-GICs are HN03, CrO,, KMnO,, PbOz, etc.[4]. However, there has been no quantitative comparison between these two methods. In the present work, we followed potential change of graphite electrodes versus reference electrodes of Hg/HgSO, during chemical oxidation with HN03 and KMnO,, and compared with the potential change during electrochemical oxidation. The formation process of H,SO,-GICs and the effect of dilution of sulfuric acid were discussed by using the potential changes obtained.
There
2. EXPERIMENTAL
Flaky natural graphite powders with average size of 400 p,rn were used as host material. In order to measure potential, 50 mg of the graphite powders
sulfuric acid, potential, chemical oxidation, electro-
were held between two plastic plates (30mm x 30mm) with platinum wire as a supported electrode. The electrode of HglHgSO, with Luggin capillary was adopted as a reference potential. Sulfuric acid with different concentrations (10, 14, and 18 moli dm3) of 50 cm3 was used. Nitric acid (13.8 mol/dm3) and potassium permanganate (powders), KMnO,, were used as chemical oxidizer to synthesize the GICs. The test cell was covered with a plastic film to prevent absorption of moisture in air by sulfuric acid and also kept at constant temperature of 0°C by using a bath with ice-water mixture. The sulfuric acid solutions were bubbled by nitrogen gas and also magnetically stirred. The potential of the working electrode with time after the addition of certain amounts of an oxidizer was continuously recorded. Characterization of the synthesized H$O,-GICs was carried out with X-ray powder diffractometry by preventing the decomposition in a polyethylene bag. By immersing a graphite electrode into sulfuric acid, the potential gradually increased and saturated at a certain value after 20 hours, probably due either to the adsorption of H,SO, molecules on the surface or to the change in work function of the graphite electrode. Therefore, after making sure that the potential of graphite electrode became constant (usually after 20 hours), we added oxidizers and started in to measure the potential changes of the electrode. For comparison, electrochemical synthesis of H,SO,-GICs by galvanostatic oxidation was also carried out in exactly the same cell, except that a 49
50
M.
counter electrode of platinum mm) was inserted in the cell.
INAGAKI et
al.
plate (10mm x 10
3. RESULTS
3.1 Oxidation by nitric acid The curves of potential versus reaction time in H2S04 of 18 mol/dm3 are shown in Fig. 1, when nitric acid is used as a chemical oxidizer. The molar ratios of HNO, to carbon are 16.3, 3.3, 0.33, and 0.17, respectively. X-ray powder patterns of the sample electrodes oxidized up to the points a to e in Fig. 1 are shown in Fig. 2. The stepwise potential increase to positive direction and a plateau at a potential around 0.9 V are characteristic. Additionally, a faint plateau can be recognized at around 0.6 V. The rate of potential changes and the potential reached after long reaction time (hereafter, saturated potential) depends on the amount of nitric acid. In the solutions with HNOJ carbon = 16.3 and 3.3, the potential rapidly increases to 1.1 V and is saturated. In the former it is too quick to make clear the plateau at 0.9 V. The products at the potential of 1.1 V are always the stage one compound (Fig. 2a and c). All diffraction lines observed are indexed by using the identity period along the c-axis, d,, of 0.80 nm. The sample oxidized up to the point b (on the plateau at 0.9 V) is a mixture of the stage one and two structures (Fig. 2b), giving d, = 0.80 and 1.14 nm, respectively. In the solution with HNOJgraphite = 0.33; however, the value of saturated potential of the sample electrode is a little less than 0.9 V, and the final product at the point d is the stage two compound with d, = 1.14 nm (Fig. 2d). At the point e, the stage three compound with d, = 1.47 nm is obtained. In the blank test using a platinum electrode, which was unreactive to sulfuric acid, its potential increased instantly after adding nitric acid up to 1.1 V and became constant. Therefore, the stepwise increase in potential of the graphite electrode is due to the intercalation of sulfuric acid into graphite. The most pronounced I
I
’
350 Reaction
time
Fig. 1. Curves of potential of graphite in H,SO, of 18 mol/dmg by chemical acid. -: HNO,/carbon (in mole) _. -: 0.33, - .. -:
I h versus reaction time oxidation with nitric = 16.3, - - - -:3.3, 0.17.
3 .-
z
(I,
E
20
30
28,Cu
40
50
E
Koc I0
Fig. 2. X-ray powder patterns of HZSO,-GICs formed by chemical oxidation with nitric acid. Each pattern corresponds to the respective point on the potential curves in Fig. 1.
plateau around 0.9 V is produced by the transformation from stage two to one compounds. The value of the saturated potential of 1.1 V in the case of HNOJcarbon = 16.3 and 3.3, being added excess amount of oxidizer, seems to be an upper limit of saturated potential by the addition of nitric acid (with concentration of 13.8 mol/dm3) into H,SO, of 18 mol/dm3. 3.2 Oxidation by potassium permanganate The potential changes by addition of potassium permanganate in H,SO, of 18 mol/dm3 are shown in Fig. 3. The relative amounts of added potassium permanganate to carbon are 3.3, 1.5, and 0.33 in molar ratios, respectively. X-ray powder patterns at the points f to h in Fig. 3 are shown in Fig. 4. Characteristic plateaux in potential change around 0.9 and 0.6 V are also observed, and the compounds on the plateau at 0.9 V (at the point g) is a mixture of the stage one and two structures (Fig. 4g), exactly the same as the case of nitric acid. The plateau at 0.6 V appears more distinctly than in the case of nitric acid. The value of saturated potential by adding small amounts of oxidizer (KMnOJcarbon = 0.33) is below 0.5 V, so that the final product at the point h is a mixture of unreacted graphite and the compounds with undefined high stage numbers (Fig. 4h). At the point of plateau at 0.6 V, the stage three structure was detected. Therefore, the plateau at 0.6 V seems to correspond to stage transformation from high stage structure to stage three. Even if the oxidizer has more than KMnO,/ carbon = 1.5 added, the rate of potential increase does not change, suggesting that the potential increase depends mostly on the solubility of potassium permanganate into H,SO, of 18 mol/dm3. The saturated potential of 1.3 V in the case of KMnOJ
51
Potential change with intercalation
der pattern. From the weight change with heattreatment, the as-precipitates were supposed to have the composition of MnO,, though further study was necessary.
f!o,oi 0
Reaction
50 time
100 I h
Fig. 3. Curves of potential of graphite versus reaction time in H,SO, of 18 mol/dm3 by chemical oxidation with potassium permanganate. -: KNnOJcarbon (in mole) = 3.3, _ _ _ _: 1.5, -: 0.33. carbon = 1.5 and 3.3 seems to be the upper limit of saturated potential by the addition of potassium permanganate powders into H,SO, of 18 mol/dm3, and the value is higher than that by nitric acid. The identity period along the c-axis, d,, of the stage one compound finally obtained after attaining the upper limit of potential (at the pointf) was 0.79 nm, slightly smaller than the same stage compound obtained by nitric acid (the point a and c in Fig. 1). Precipitates with red-brown color were produced in the solution after formation of the stage one compound. The as-precipitates were amorphous but the ones heat-treated up to 1000°C in nitrogen atmosphere were identified as Mn,O, from its X-ray pow-
3.3 Electrolysis with galvanostatic oxidation The curve of potential versus electric quantity during galvanostatic oxidation with 200 FA (4 mA/gcarbon) is shown in Fig. 5. For comparison. the observed curves during chemical oxidation in H&GO, of 18 mol/dm3 are shown in the same figure. All potential changes in Fig. 5 shown characteristic plateaux, distinctly around 0.9 V and faintly around 0.6 V. The plateau at 0.9 V by the galvanostatic oxidation was also confirmed to correspond to the transformation from the stage two to one compounds by X-ray powder diffraction. The plateau at 0.6 V corresponded to the formation of the stage three compound. Although the potential changes in three cases in Fig. 5 were too fast to show the plateaux corresponding to all stage transformations between high stages, it is reasonable to suppose that the plateaux observed during chemical oxidation have the same potential values as those during galvanostatic oxidation. In other words, the chemical oxidation raises the potential of graphite in sulfuric acid and the intercalation of sulfuric acid occurs at the same potential values as the galvanostatic oxidation. 3.4 Formation of H,SO,-GICs sulfuric acid
in diluted
The potential changes of graphite by chemical and galvanostatic oxidations, and X-ray powder patterns for the compounds obtained after reaching the upper limit of saturated potential in H2S0, of 14 and 10 mol/dm3 are shown in Figs. 6 and 7, respectively. In Electric 1.5
O
quantity 20
/ mAh/g-carbon
40
60
80
100
U 54 I” T5
I
1.0
? > . 3 ._
0.5
E z a 0.c
20
30
28,Cu
40
50
60
K&I0
Fig. 4. X-ray powder patterns of H2S0,-GICs formed by chemical oxidation with potassium permanganate. Each pattern corresponds to the respective point on the potential curves in Fig. 3.
I 0
I 10
1 20
Reaction
30
time
40
50
60
70
/ h
Fig. 5. Potential changes during formation of H,SO,-GICs in H,SO, 18 molidm’. -: by galvanostatic oxidation. _ _ _ _: by oxidation with potassium permanganate (KMnOJcarbon = 1.5), - . -: by oxidation with nitric acid (HNOJcarbon = 3.3).
52
M. INAGAKIet al. Electric 1.51
quantity 0
201
0
V.”
20
I mAh/g-carbon 40I
40
60
Reaction
80
60I
80
100
100
time
120
140
20
/ h
30
40
20,Cu
Kd
50
60
lo
Fig. 6. Potential changes and X-ray powder patterns of H,SO,-GICs formed in H,SO, of 14 mol/dm’. a) Potential chances -: ealvanostatic. - - - -: potassium permanganate (KMnOJcarbon = 1.5), -: nitric acid”(HNOJcaibon = 3.3) oxidations, b) X-raypowde;patterns for the respective point on potential curves. I
-*
(the point i in Fig. 6). By diluting sulfuric acid to 14 mol/dm3, therefore, the threshold potential for the formation of stage one compound seems to shift up to 1.2 V. In addition, the sample finally obtained by galvanostatic oxidation in H2S04 of 10 mol/dm3 is a single phase of the stage two compound (in the point
the chemical oxidation process, enough amounts of the oxidizer is added to reach the upper limit of potential in each concentration of sulfuric acid. The final product obtained by galvanostatic oxidation at the potential of 1.2V in H,SO, of 14 mall dm3 is a mixture of the stage one and two structures
Electric 1.5%
quantity
I mAh/g-carbon
1 20I 1 60I 40
0
>
80
1
100
_._._._.-.-.1 /"_ n _ ,
0 a
0.0
I
I
I
I
1
0
20
40
60
80
Reaction
time (a>
I
I
100
120
I h
I
140
20
30
40
28,Cu
I
50
Kdl” (b)
Fig. 7. Potential changes and X-ray powder patterns of H$O,-GICs formed in H,SO, of 10 mol/dm3. a) Potential changes -: galvanostatic, - - - -..potassium permanganate (KMnO,/carbon = 1.5), - . -1 nitric acid (HNOJcarbon = 3.3) oxidations, b) X-ray powder patterns for the respective point on potential curves.
60
53
Potential change with intercalation 1 in Fig. 7), and the potential oscillates around 1.2 V before saturating, being related with oxygen evolution on the platinum current feeder[5]. The threshold potential for the stage one compound seems to increase above 1.4 V in H,SO, of 10 mall dm3. On the other hand, the upper limit of saturated potential by potassium permanganate oxidation decreases with dilution of sulfuric acid from 18 to 14 and 10 mol/dm3; from 1.3 to 1.1 and 1.05 V, respectively. The products at these potential values are stage one (the point fin Fig. 3), stage two (the point j in Fig. 6), and stage three (the point m in Fig. 7), respectively. This result is consistent with the increase of threshold potential for stage transformation in H,SO,-GICs with the dilution of acid. The upper limit of saturated potential by nitric acid oxidation also decreases with dilution of sulfuric acid and stage structure of the final product becomes a higher stage number. The product at upper limit of saturated potential of 0.9 V by nitric acid oxidation in H,SO, of 14 mol/dm3 is a mixture of the stage two and three structures (the point k in Fig. 6). In H,SO, of 10 mol/dm3, the potential slightly increases during chemical oxidation with nitric acid and, even at the upper limit of the potential of 0.6 V, no intercalation of sulfuric acid is detected (the point n in Fig. 7). The present results agree with the experimental fact that the formation of H,SO,-GICs by chemical oxidation gets the more difficult in the more dilute sulfuric acid[6]. We will discuss more quantitatively on the effect of dilution of sulfuric acid in the following section 4.3. 4. DISCUSSION
4.1 Formation process of H2S04-GICs For the formation of GICs, both donor and acceptor-type compounds, electron transfer between graphite layers and intercalates has to be accomplished. In the case of acceptor-type compounds, for example H,SO,-GICs, electrons have to be extracted from layers of host graphite either by electrochemical or by chemical oxidations. According to Metrot[l,2], as a consequence of extraction of electrons, i.e. oxidation, the Fermi level of energy bands of the host graphite is depressed, and the potential of host graphite is raised as proved experimentally in the present work. When the potential of the host reaches at a specific value, which is defined by threshold potential, the intercalation of bisulfate ions takes place, accompanying neutral sulfuric acid molecules. The potential is kept constant during intercalation until the completion of each stage structure, which is observed as the plateau in the curves of potential versus the electric quantity or reaction time, as shown in the previous figures. During galvanostatic oxidation, electrons extracted from host graphite flow to the counter electrode and are consumed by hydrogen evolution. During chemical oxidation, however, nitrate or per-
manganate ions pull out electrons from host graphite and reduce themselves. Permanganate ions, MnO,- , seemed to be reduced and then to precipitate as amorphous MnO,. On nitrate ions, reduction to HNO, seemed to be highly probable, which was soluble in sulfuric acid. So the following equations may be written; N03-
+ 3H+ + 2e- --+
MnO,-
+ 2H+ + e- -
HN02 + HzO,
(1)
MnO,(S)
(2)
+ HZO,
where e- in the left hand side means the extraction of electron from host graphite. In the synthesis of H,SO,-GICs by chemical and electrochemical oxidations, therefore, only the difference is how to consume the electrons extracted from host graphite. In the present results, the saturated potential reached by chemical oxidation depends on a relative amount of oxidizer to carbon. It is reasonable to assume that the value of saturated potential is related to the equilibrium potential given by Nernst’s equation. The equations for the two oxidizers used can be expressed as follows;
E = E,, + g
In
aivoj a3”+
(3) aHNO
E
=
E
+
0
g
ln
F
aH20
“MnOq ‘2H+ “MnO, aH20’
(4)
The equilibrium potential, E, depends strongly on the activities of each chemical species in reducing reactions (eqns (1) and (2), respectively). When a small amount of oxidizer is used for oxidation of host graphite in sulfuric acid, activity of reduced product, aHNO for example, is high in comparison with that of the remained oxidizer, aNoj, and so equilibrium potentials determined by the eqns (3) must be low. The value of saturated potential by a small amount of oxidizer was actually low, and high stage compounds were obtained, as experimentally shown in Figs. 1-4. When excess amount of oxidizer is used, activity aNOjis still high in comparison with aHNoZ,so that the equilibrium potential must be high. Actually the saturated potential reached to the value of upper limit for each oxidizer and consequently low stage compounds were obtained. At the saturated potential reached by chemical oxidation, the intercalation reaction of sulfuric acid into graphite is equilibrated. The present experiment suggests that there is an upper limit of saturated potential, even if an excess amount of the oxidizer is used. In other words, there is a limit of oxidation of host graphite by chemical oxidation depending on the oxidizer. This upper limit of saturated potential also depends on the concentration of sulfuric acid. It is probably explained also by eqns (3) and (4). In diluted sulfuric acid, the activity of water, aHZo,is so high that a value of upper limit of saturated potential is lowered.
M. INAGAKIet al.
54 4.2 Overcharging in the stage one H2S04-GIC
The upper limit of potential in H,SO, of 18 mol/ dm’ for the oxidizers used is higher than the plateau potential for the stage one structure of H,SO,-GIC, as shown in Figs. 1 and 3. Therefore, the oxidation of graphite layers is continued even after the completion of stage one structure and consequently the potential increases up to the upper limit of saturated potential for each oxidizer. This fact corresponds to so-called overcharging which has been pointed out in electrochemical oxidation[7] and it is reported to cause a slight decrease in identity period along the c-axis, d,, of the stage one compound with the increase of potential[8]. In the present work, the value of d, for the stage one compound obtained by permanganate oxidation was slightly smaller than that obtained by nitrate oxidation. These experimental results agree with the fact that potassium permanganate gives a higher upper limit of saturated potential than nitric acid, the former giving higher degree of overcharging to the stage one compound. The experimental result by Daioh and Mizutani[9] states that d, of stage one compound differs slightly depending on the kind of oxidizers that can be explained by taking account of the dependence of upper limit of saturated potential on oxidizer. 4.3 Effect of dilution of sulfuric acid for formation of H,SO,-GICs
by chemical oxidation
On the basis of the present results in different concentrations of sulfuric acid, the effect of dilution on the formation of H,SO,-GICs by chemical oxidation can be quantitatively discussed. The threshold potentials for stage transformation in H$O,-GICs by electrochemical methods, galvanostatic oxidation and cyclic voltammetry, increase with dilution of sulfuric acid, which has been pointed out by different authors[lO-121, and is also shown in the present work. During chemical oxidation, the potential of graphite is shown to increase, but to be saturated at a certain value, even if an excess amount of oxidizer is added. We call it the upper limit of saturated potential and is found to depend on the oxidizer used. The upper limit of potential is also shown experimentally to decrease with dilution of sulfuric acid. In Fig. 8, the threshold potentials for each stage, determined by galvanostatic oxidation, are plotted against the molarity of sulfuric acid by open marks, which are shown to be linear[l2]. In the same figure, the upper limit of saturated potential for nitric acid and potassium permanganate in three different concentrations of sulfuric acid are also plotted by closed marks. In order to synthesize the stage one compound, for example, we have to give higher potential to the host graphite than its threshold one (open circles in Fig. 8). By chemical oxidation, we can have only the potential lower than the upper limit of saturated potential (closed marks in Fig. 8). In H,SO, of 18 mol/dm3, therefore, the stage one compound can be
H2SO4 concentration
/ molldrn3
Fig. 8. Threshold potentials for the stage one, two, and three of H,SO,-GICs and upper limits of potential by chem-
ical oxidation as a function of molarity of H,SO,. Threshold potentials of the stage one (0), stage two (A) and stage three K3 of H,SO,-GICs. Uooer limits of ootential bv potassig permanganate ox&ion (0) anh by nit& acid (A).
synthesized by chemical oxidation with nitric acid and potassium permanganate because closed marks for two oxidizers are well above the open marks in Fig. 8. In 10 mol/dm3, however, only stage three compound can be obtained by potassium permanganate oxidation, but not by nitric acid, as is clear from Fig. 8. The X-ray diffraction studies on the GICs obtained agree with these predictions. Therefore, all of the experimental results on the formation of H2S04-GICs in different concentrations of sulfuric acid by using different chemical oxidizers can be understood from the relation between threshold potentials for each stage structure and upper limit of saturated potential for each oxidizer. 5. CONCLUSIVE
REMARKS
The present results lead to the conclusion that the intercalation process of sulfuric acid into graphite by chemical oxidation is fundamentally the same as that by electrochemical oxidation. The potential of sample graphite increases by the oxidation. When it reaches threshold potential for each stage structure, strongly depending on the concentration of sulfuric acid, intercalation reaction proceeds. The degree of oxidation of graphite by chemical oxidation, in other words, the potential of graphite, is limited by equilibrium potential which is given by Nernst’s equation of the oxidizer used and depends strongly on the activities of each chemical species in the solution. The evolution of potential of the sample graphite with chemical oxidation can explain various experimental results published on the intercalation of sulfuric acid. The present concept, potential increase with chemical oxidation and consequent intercalation, might be applied in order to understand the intercalation reactions in various solutions. We are now undertaking these experiments.
Potential change with intercalation REFERENCES
1. A. Metrot, Synth. Met. 7, 177 (1983). 2. A. Metrot and M. Tihli. Svnth. Met. 12. 517 (1985). 3. Y. Maeda, Y. Okemoto, and M. Inagaki, J. &&othem. Sot. 132, 2369 (1985). 4. H. Thiele, Z. anorg. allg. &em. 206, 407 (1932). 5. J. 0. Besenhard, M. Bindi. and H. Mohwald. Proc. of Carbon ‘88 Baden-Baden, pp. 414-416, (1986). 6. W. Rtidorff and U. Hofmann, Z. anorg. allg. Chem. 238, 1 (1938).
55
7. A. Metrot and J. E. Fischer, Synth. Met. 3, 201 (1981). 8. H. Shioyama and R. Fujii, Carbon 25, 771 (1987). 9. H. Daioh and Y. Mizutani. Tunso 1985 INo. 1231. >. 177 (1985). 10. E Beck, W. Kaiser, and H. Krohn, Angew. Chem. Suppl. 57, (1982). 11. R. Fuiii. Reoort of Gov. Ind. Res. Inst.. Osaka. No. 353, pp: 19-i4, (1978). 12. A. Pruss and F. Beck, J. Electroanal. Chem. 172.281 (1984).
Vol.
Printed
in Great Britain.
28, No.
I. pp
49-55.
1990
UUlW6223”YU $3.0(1+ 00 Copyright 0 1990 Pergamon Press plc
POTENTIAL CHANGE WITH INTERCALATION SULFURIC ACID INTO GRAPHITE BY CHEMICAL OXIDATION
OF
MICHIO INAGAKI, NORIO IWASHITA, and EIJI KOUNO Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 440, Japan (Received on February 1989) Abstract-Potential of graphite during the intercalation of sulfuric acid by chemical oxidation with either nitric acid or potassium permanganate in H,SO, of 18 molidm’ was found to increase to positive direction in stepwise. On potential change, a distinct plateau corresponding to the transformation from stage two to one compound and a faint one for the formation of stage three were observed, exactly the same as those observed during galvanostatic oxidation. The intercalation of sulfuric acid into graphite by chemical and electrochemical oxidations was shown to be fundamentally the same processes; potential of graphite increases with oxidation and the intercalation of sulfuric acid proceeds by showing potential plateaux. On the change of potential of graphite by chemical oxidation, there was found to be an upper limit of saturated potential, depending strongly on the oxidizer used and also on the concentration of sulfuric acid even if enough amounts of oxidizer was used. The relation between the threshold potential for stage transformation in H,SO,-GICs, increasing with dilution of sulfuric acid, and the upper limit of saturated potential by chemical oxidation, decreasing with dilution of the acid, governed the intercalation reaction by chemical oxidation in sulfuric acid with various concentrations. Kev Words-Graphite chemical oxidation.
intercalation
compounds,
1. INTRODUCTION
are two methods for synthesis of graphite intercalation compounds with sulfuric acid (H,SO,GICs); chemical and electrochemical oxidations. The latter has been quantitatively studied by many authors. For instance, Metrot[l,2] discussed the potential curve versus electric quantity measured on graphite electrode by using a two capacitor model. A remarkable effect of host graphite during electrochemical intercalation of sulfuric acid was shown by using different carbon fibers[3]. On the other hand, the former method has been used as the simplest one for the synthesis of H,SO,-GICs and industrially employed for the production of exfoliated graphite. The oxidizers used for the synthesis of H,SO,-GICs are HN03, CrO,, KMnO,, PbOz, etc.[4]. However, there has been no quantitative comparison between these two methods. In the present work, we followed potential change of graphite electrodes versus reference electrodes of Hg/HgSO, during chemical oxidation with HN03 and KMnO,, and compared with the potential change during electrochemical oxidation. The formation process of H,SO,-GICs and the effect of dilution of sulfuric acid were discussed by using the potential changes obtained.
There
2. EXPERIMENTAL
Flaky natural graphite powders with average size of 400 p,rn were used as host material. In order to measure potential, 50 mg of the graphite powders
sulfuric acid, potential, chemical oxidation, electro-
were held between two plastic plates (30mm x 30mm) with platinum wire as a supported electrode. The electrode of HglHgSO, with Luggin capillary was adopted as a reference potential. Sulfuric acid with different concentrations (10, 14, and 18 moli dm3) of 50 cm3 was used. Nitric acid (13.8 mol/dm3) and potassium permanganate (powders), KMnO,, were used as chemical oxidizer to synthesize the GICs. The test cell was covered with a plastic film to prevent absorption of moisture in air by sulfuric acid and also kept at constant temperature of 0°C by using a bath with ice-water mixture. The sulfuric acid solutions were bubbled by nitrogen gas and also magnetically stirred. The potential of the working electrode with time after the addition of certain amounts of an oxidizer was continuously recorded. Characterization of the synthesized H$O,-GICs was carried out with X-ray powder diffractometry by preventing the decomposition in a polyethylene bag. By immersing a graphite electrode into sulfuric acid, the potential gradually increased and saturated at a certain value after 20 hours, probably due either to the adsorption of H,SO, molecules on the surface or to the change in work function of the graphite electrode. Therefore, after making sure that the potential of graphite electrode became constant (usually after 20 hours), we added oxidizers and started in to measure the potential changes of the electrode. For comparison, electrochemical synthesis of H,SO,-GICs by galvanostatic oxidation was also carried out in exactly the same cell, except that a 49
50
M.
counter electrode of platinum mm) was inserted in the cell.
INAGAKI et
al.
plate (10mm x 10
3. RESULTS
3.1 Oxidation by nitric acid The curves of potential versus reaction time in H2S04 of 18 mol/dm3 are shown in Fig. 1, when nitric acid is used as a chemical oxidizer. The molar ratios of HNO, to carbon are 16.3, 3.3, 0.33, and 0.17, respectively. X-ray powder patterns of the sample electrodes oxidized up to the points a to e in Fig. 1 are shown in Fig. 2. The stepwise potential increase to positive direction and a plateau at a potential around 0.9 V are characteristic. Additionally, a faint plateau can be recognized at around 0.6 V. The rate of potential changes and the potential reached after long reaction time (hereafter, saturated potential) depends on the amount of nitric acid. In the solutions with HNOJ carbon = 16.3 and 3.3, the potential rapidly increases to 1.1 V and is saturated. In the former it is too quick to make clear the plateau at 0.9 V. The products at the potential of 1.1 V are always the stage one compound (Fig. 2a and c). All diffraction lines observed are indexed by using the identity period along the c-axis, d,, of 0.80 nm. The sample oxidized up to the point b (on the plateau at 0.9 V) is a mixture of the stage one and two structures (Fig. 2b), giving d, = 0.80 and 1.14 nm, respectively. In the solution with HNOJgraphite = 0.33; however, the value of saturated potential of the sample electrode is a little less than 0.9 V, and the final product at the point d is the stage two compound with d, = 1.14 nm (Fig. 2d). At the point e, the stage three compound with d, = 1.47 nm is obtained. In the blank test using a platinum electrode, which was unreactive to sulfuric acid, its potential increased instantly after adding nitric acid up to 1.1 V and became constant. Therefore, the stepwise increase in potential of the graphite electrode is due to the intercalation of sulfuric acid into graphite. The most pronounced I
I
’
350 Reaction
time
Fig. 1. Curves of potential of graphite in H,SO, of 18 mol/dmg by chemical acid. -: HNO,/carbon (in mole) _. -: 0.33, - .. -:
I h versus reaction time oxidation with nitric = 16.3, - - - -:3.3, 0.17.
3 .-
z
(I,
E
20
30
28,Cu
40
50
E
Koc I0
Fig. 2. X-ray powder patterns of HZSO,-GICs formed by chemical oxidation with nitric acid. Each pattern corresponds to the respective point on the potential curves in Fig. 1.
plateau around 0.9 V is produced by the transformation from stage two to one compounds. The value of the saturated potential of 1.1 V in the case of HNOJcarbon = 16.3 and 3.3, being added excess amount of oxidizer, seems to be an upper limit of saturated potential by the addition of nitric acid (with concentration of 13.8 mol/dm3) into H,SO, of 18 mol/dm3. 3.2 Oxidation by potassium permanganate The potential changes by addition of potassium permanganate in H,SO, of 18 mol/dm3 are shown in Fig. 3. The relative amounts of added potassium permanganate to carbon are 3.3, 1.5, and 0.33 in molar ratios, respectively. X-ray powder patterns at the points f to h in Fig. 3 are shown in Fig. 4. Characteristic plateaux in potential change around 0.9 and 0.6 V are also observed, and the compounds on the plateau at 0.9 V (at the point g) is a mixture of the stage one and two structures (Fig. 4g), exactly the same as the case of nitric acid. The plateau at 0.6 V appears more distinctly than in the case of nitric acid. The value of saturated potential by adding small amounts of oxidizer (KMnOJcarbon = 0.33) is below 0.5 V, so that the final product at the point h is a mixture of unreacted graphite and the compounds with undefined high stage numbers (Fig. 4h). At the point of plateau at 0.6 V, the stage three structure was detected. Therefore, the plateau at 0.6 V seems to correspond to stage transformation from high stage structure to stage three. Even if the oxidizer has more than KMnO,/ carbon = 1.5 added, the rate of potential increase does not change, suggesting that the potential increase depends mostly on the solubility of potassium permanganate into H,SO, of 18 mol/dm3. The saturated potential of 1.3 V in the case of KMnOJ
51
Potential change with intercalation
der pattern. From the weight change with heattreatment, the as-precipitates were supposed to have the composition of MnO,, though further study was necessary.
f!o,oi 0
Reaction
50 time
100 I h
Fig. 3. Curves of potential of graphite versus reaction time in H,SO, of 18 mol/dm3 by chemical oxidation with potassium permanganate. -: KNnOJcarbon (in mole) = 3.3, _ _ _ _: 1.5, -: 0.33. carbon = 1.5 and 3.3 seems to be the upper limit of saturated potential by the addition of potassium permanganate powders into H,SO, of 18 mol/dm3, and the value is higher than that by nitric acid. The identity period along the c-axis, d,, of the stage one compound finally obtained after attaining the upper limit of potential (at the pointf) was 0.79 nm, slightly smaller than the same stage compound obtained by nitric acid (the point a and c in Fig. 1). Precipitates with red-brown color were produced in the solution after formation of the stage one compound. The as-precipitates were amorphous but the ones heat-treated up to 1000°C in nitrogen atmosphere were identified as Mn,O, from its X-ray pow-
3.3 Electrolysis with galvanostatic oxidation The curve of potential versus electric quantity during galvanostatic oxidation with 200 FA (4 mA/gcarbon) is shown in Fig. 5. For comparison. the observed curves during chemical oxidation in H&GO, of 18 mol/dm3 are shown in the same figure. All potential changes in Fig. 5 shown characteristic plateaux, distinctly around 0.9 V and faintly around 0.6 V. The plateau at 0.9 V by the galvanostatic oxidation was also confirmed to correspond to the transformation from the stage two to one compounds by X-ray powder diffraction. The plateau at 0.6 V corresponded to the formation of the stage three compound. Although the potential changes in three cases in Fig. 5 were too fast to show the plateaux corresponding to all stage transformations between high stages, it is reasonable to suppose that the plateaux observed during chemical oxidation have the same potential values as those during galvanostatic oxidation. In other words, the chemical oxidation raises the potential of graphite in sulfuric acid and the intercalation of sulfuric acid occurs at the same potential values as the galvanostatic oxidation. 3.4 Formation of H,SO,-GICs sulfuric acid
in diluted
The potential changes of graphite by chemical and galvanostatic oxidations, and X-ray powder patterns for the compounds obtained after reaching the upper limit of saturated potential in H2S0, of 14 and 10 mol/dm3 are shown in Figs. 6 and 7, respectively. In Electric 1.5
O
quantity 20
/ mAh/g-carbon
40
60
80
100
U 54 I” T5
I
1.0
? > . 3 ._
0.5
E z a 0.c
20
30
28,Cu
40
50
60
K&I0
Fig. 4. X-ray powder patterns of H2S0,-GICs formed by chemical oxidation with potassium permanganate. Each pattern corresponds to the respective point on the potential curves in Fig. 3.
I 0
I 10
1 20
Reaction
30
time
40
50
60
70
/ h
Fig. 5. Potential changes during formation of H,SO,-GICs in H,SO, 18 molidm’. -: by galvanostatic oxidation. _ _ _ _: by oxidation with potassium permanganate (KMnOJcarbon = 1.5), - . -: by oxidation with nitric acid (HNOJcarbon = 3.3).
52
M. INAGAKIet al. Electric 1.51
quantity 0
201
0
V.”
20
I mAh/g-carbon 40I
40
60
Reaction
80
60I
80
100
100
time
120
140
20
/ h
30
40
20,Cu
Kd
50
60
lo
Fig. 6. Potential changes and X-ray powder patterns of H,SO,-GICs formed in H,SO, of 14 mol/dm’. a) Potential chances -: ealvanostatic. - - - -: potassium permanganate (KMnOJcarbon = 1.5), -: nitric acid”(HNOJcaibon = 3.3) oxidations, b) X-raypowde;patterns for the respective point on potential curves. I
-*
(the point i in Fig. 6). By diluting sulfuric acid to 14 mol/dm3, therefore, the threshold potential for the formation of stage one compound seems to shift up to 1.2 V. In addition, the sample finally obtained by galvanostatic oxidation in H2S04 of 10 mol/dm3 is a single phase of the stage two compound (in the point
the chemical oxidation process, enough amounts of the oxidizer is added to reach the upper limit of potential in each concentration of sulfuric acid. The final product obtained by galvanostatic oxidation at the potential of 1.2V in H,SO, of 14 mall dm3 is a mixture of the stage one and two structures
Electric 1.5%
quantity
I mAh/g-carbon
1 20I 1 60I 40
0
>
80
1
100
_._._._.-.-.1 /"_ n _ ,
0 a
0.0
I
I
I
I
1
0
20
40
60
80
Reaction
time (a>
I
I
100
120
I h
I
140
20
30
40
28,Cu
I
50
Kdl” (b)
Fig. 7. Potential changes and X-ray powder patterns of H$O,-GICs formed in H,SO, of 10 mol/dm3. a) Potential changes -: galvanostatic, - - - -..potassium permanganate (KMnO,/carbon = 1.5), - . -1 nitric acid (HNOJcarbon = 3.3) oxidations, b) X-ray powder patterns for the respective point on potential curves.
60
53
Potential change with intercalation 1 in Fig. 7), and the potential oscillates around 1.2 V before saturating, being related with oxygen evolution on the platinum current feeder[5]. The threshold potential for the stage one compound seems to increase above 1.4 V in H,SO, of 10 mall dm3. On the other hand, the upper limit of saturated potential by potassium permanganate oxidation decreases with dilution of sulfuric acid from 18 to 14 and 10 mol/dm3; from 1.3 to 1.1 and 1.05 V, respectively. The products at these potential values are stage one (the point fin Fig. 3), stage two (the point j in Fig. 6), and stage three (the point m in Fig. 7), respectively. This result is consistent with the increase of threshold potential for stage transformation in H,SO,-GICs with the dilution of acid. The upper limit of saturated potential by nitric acid oxidation also decreases with dilution of sulfuric acid and stage structure of the final product becomes a higher stage number. The product at upper limit of saturated potential of 0.9 V by nitric acid oxidation in H,SO, of 14 mol/dm3 is a mixture of the stage two and three structures (the point k in Fig. 6). In H,SO, of 10 mol/dm3, the potential slightly increases during chemical oxidation with nitric acid and, even at the upper limit of the potential of 0.6 V, no intercalation of sulfuric acid is detected (the point n in Fig. 7). The present results agree with the experimental fact that the formation of H,SO,-GICs by chemical oxidation gets the more difficult in the more dilute sulfuric acid[6]. We will discuss more quantitatively on the effect of dilution of sulfuric acid in the following section 4.3. 4. DISCUSSION
4.1 Formation process of H2S04-GICs For the formation of GICs, both donor and acceptor-type compounds, electron transfer between graphite layers and intercalates has to be accomplished. In the case of acceptor-type compounds, for example H,SO,-GICs, electrons have to be extracted from layers of host graphite either by electrochemical or by chemical oxidations. According to Metrot[l,2], as a consequence of extraction of electrons, i.e. oxidation, the Fermi level of energy bands of the host graphite is depressed, and the potential of host graphite is raised as proved experimentally in the present work. When the potential of the host reaches at a specific value, which is defined by threshold potential, the intercalation of bisulfate ions takes place, accompanying neutral sulfuric acid molecules. The potential is kept constant during intercalation until the completion of each stage structure, which is observed as the plateau in the curves of potential versus the electric quantity or reaction time, as shown in the previous figures. During galvanostatic oxidation, electrons extracted from host graphite flow to the counter electrode and are consumed by hydrogen evolution. During chemical oxidation, however, nitrate or per-
manganate ions pull out electrons from host graphite and reduce themselves. Permanganate ions, MnO,- , seemed to be reduced and then to precipitate as amorphous MnO,. On nitrate ions, reduction to HNO, seemed to be highly probable, which was soluble in sulfuric acid. So the following equations may be written; N03-
+ 3H+ + 2e- --+
MnO,-
+ 2H+ + e- -
HN02 + HzO,
(1)
MnO,(S)
(2)
+ HZO,
where e- in the left hand side means the extraction of electron from host graphite. In the synthesis of H,SO,-GICs by chemical and electrochemical oxidations, therefore, only the difference is how to consume the electrons extracted from host graphite. In the present results, the saturated potential reached by chemical oxidation depends on a relative amount of oxidizer to carbon. It is reasonable to assume that the value of saturated potential is related to the equilibrium potential given by Nernst’s equation. The equations for the two oxidizers used can be expressed as follows;
E = E,, + g
In
aivoj a3”+
(3) aHNO
E
=
E
+
0
g
ln
F
aH20
“MnOq ‘2H+ “MnO, aH20’
(4)
The equilibrium potential, E, depends strongly on the activities of each chemical species in reducing reactions (eqns (1) and (2), respectively). When a small amount of oxidizer is used for oxidation of host graphite in sulfuric acid, activity of reduced product, aHNO for example, is high in comparison with that of the remained oxidizer, aNoj, and so equilibrium potentials determined by the eqns (3) must be low. The value of saturated potential by a small amount of oxidizer was actually low, and high stage compounds were obtained, as experimentally shown in Figs. 1-4. When excess amount of oxidizer is used, activity aNOjis still high in comparison with aHNoZ,so that the equilibrium potential must be high. Actually the saturated potential reached to the value of upper limit for each oxidizer and consequently low stage compounds were obtained. At the saturated potential reached by chemical oxidation, the intercalation reaction of sulfuric acid into graphite is equilibrated. The present experiment suggests that there is an upper limit of saturated potential, even if an excess amount of the oxidizer is used. In other words, there is a limit of oxidation of host graphite by chemical oxidation depending on the oxidizer. This upper limit of saturated potential also depends on the concentration of sulfuric acid. It is probably explained also by eqns (3) and (4). In diluted sulfuric acid, the activity of water, aHZo,is so high that a value of upper limit of saturated potential is lowered.
M. INAGAKIet al.
54 4.2 Overcharging in the stage one H2S04-GIC
The upper limit of potential in H,SO, of 18 mol/ dm’ for the oxidizers used is higher than the plateau potential for the stage one structure of H,SO,-GIC, as shown in Figs. 1 and 3. Therefore, the oxidation of graphite layers is continued even after the completion of stage one structure and consequently the potential increases up to the upper limit of saturated potential for each oxidizer. This fact corresponds to so-called overcharging which has been pointed out in electrochemical oxidation[7] and it is reported to cause a slight decrease in identity period along the c-axis, d,, of the stage one compound with the increase of potential[8]. In the present work, the value of d, for the stage one compound obtained by permanganate oxidation was slightly smaller than that obtained by nitrate oxidation. These experimental results agree with the fact that potassium permanganate gives a higher upper limit of saturated potential than nitric acid, the former giving higher degree of overcharging to the stage one compound. The experimental result by Daioh and Mizutani[9] states that d, of stage one compound differs slightly depending on the kind of oxidizers that can be explained by taking account of the dependence of upper limit of saturated potential on oxidizer. 4.3 Effect of dilution of sulfuric acid for formation of H,SO,-GICs
by chemical oxidation
On the basis of the present results in different concentrations of sulfuric acid, the effect of dilution on the formation of H,SO,-GICs by chemical oxidation can be quantitatively discussed. The threshold potentials for stage transformation in H$O,-GICs by electrochemical methods, galvanostatic oxidation and cyclic voltammetry, increase with dilution of sulfuric acid, which has been pointed out by different authors[lO-121, and is also shown in the present work. During chemical oxidation, the potential of graphite is shown to increase, but to be saturated at a certain value, even if an excess amount of oxidizer is added. We call it the upper limit of saturated potential and is found to depend on the oxidizer used. The upper limit of potential is also shown experimentally to decrease with dilution of sulfuric acid. In Fig. 8, the threshold potentials for each stage, determined by galvanostatic oxidation, are plotted against the molarity of sulfuric acid by open marks, which are shown to be linear[l2]. In the same figure, the upper limit of saturated potential for nitric acid and potassium permanganate in three different concentrations of sulfuric acid are also plotted by closed marks. In order to synthesize the stage one compound, for example, we have to give higher potential to the host graphite than its threshold one (open circles in Fig. 8). By chemical oxidation, we can have only the potential lower than the upper limit of saturated potential (closed marks in Fig. 8). In H,SO, of 18 mol/dm3, therefore, the stage one compound can be
H2SO4 concentration
/ molldrn3
Fig. 8. Threshold potentials for the stage one, two, and three of H,SO,-GICs and upper limits of potential by chem-
ical oxidation as a function of molarity of H,SO,. Threshold potentials of the stage one (0), stage two (A) and stage three K3 of H,SO,-GICs. Uooer limits of ootential bv potassig permanganate ox&ion (0) anh by nit& acid (A).
synthesized by chemical oxidation with nitric acid and potassium permanganate because closed marks for two oxidizers are well above the open marks in Fig. 8. In 10 mol/dm3, however, only stage three compound can be obtained by potassium permanganate oxidation, but not by nitric acid, as is clear from Fig. 8. The X-ray diffraction studies on the GICs obtained agree with these predictions. Therefore, all of the experimental results on the formation of H2S04-GICs in different concentrations of sulfuric acid by using different chemical oxidizers can be understood from the relation between threshold potentials for each stage structure and upper limit of saturated potential for each oxidizer. 5. CONCLUSIVE
REMARKS
The present results lead to the conclusion that the intercalation process of sulfuric acid into graphite by chemical oxidation is fundamentally the same as that by electrochemical oxidation. The potential of sample graphite increases by the oxidation. When it reaches threshold potential for each stage structure, strongly depending on the concentration of sulfuric acid, intercalation reaction proceeds. The degree of oxidation of graphite by chemical oxidation, in other words, the potential of graphite, is limited by equilibrium potential which is given by Nernst’s equation of the oxidizer used and depends strongly on the activities of each chemical species in the solution. The evolution of potential of the sample graphite with chemical oxidation can explain various experimental results published on the intercalation of sulfuric acid. The present concept, potential increase with chemical oxidation and consequent intercalation, might be applied in order to understand the intercalation reactions in various solutions. We are now undertaking these experiments.
Potential change with intercalation REFERENCES
1. A. Metrot, Synth. Met. 7, 177 (1983). 2. A. Metrot and M. Tihli. Svnth. Met. 12. 517 (1985). 3. Y. Maeda, Y. Okemoto, and M. Inagaki, J. &&othem. Sot. 132, 2369 (1985). 4. H. Thiele, Z. anorg. allg. &em. 206, 407 (1932). 5. J. 0. Besenhard, M. Bindi. and H. Mohwald. Proc. of Carbon ‘88 Baden-Baden, pp. 414-416, (1986). 6. W. Rtidorff and U. Hofmann, Z. anorg. allg. Chem. 238, 1 (1938).
55
7. A. Metrot and J. E. Fischer, Synth. Met. 3, 201 (1981). 8. H. Shioyama and R. Fujii, Carbon 25, 771 (1987). 9. H. Daioh and Y. Mizutani. Tunso 1985 INo. 1231. >. 177 (1985). 10. E Beck, W. Kaiser, and H. Krohn, Angew. Chem. Suppl. 57, (1982). 11. R. Fuiii. Reoort of Gov. Ind. Res. Inst.. Osaka. No. 353, pp: 19-i4, (1978). 12. A. Pruss and F. Beck, J. Electroanal. Chem. 172.281 (1984).