Electrochemical preparation of the ternary graphite intercalation compound with H2SO4 and H2SeO4

Carbon Vol. 27. No. 6. pp. 785-789.

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IW)OX-6223/X9 $3 00 + 00 0 1989 Pergamon Press plc

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ELECTROCHEMICAL PREPARATION OF THE TERNARY GRAPHITE INTERCALATION COMPOUND WITH H,SO, AND H,SeO, Government

H. SHIOYAMA and R. FUJII Industrial Research Institute, Osaka, Midorigaoka 1-8-31, Ikeda, Osaka 563, Japan

(Received 25 March 1988; accepted in revised form 22 December 1988) Abstract-Stage-n HSeO,-graphite intercalation compounds (GICs) (n = 2,3) prepared by galvanostatic oxidation of graphite in HSeO, were transferred into the electrolytic cell filled with H,SO,, and then continued the oxidation. From the observed electrode potential vs. electric quantity curves and the results of measured C/Se atomic ratio, it was suggested’that HSO, was intercalated-into stage-n H,SeO,-GICs and the nroducts were ternarv GICs of HSeO, and HSO,. When these ternarv GICs were reduced electrochemically. at first H2SO:was deintercalated, and then H,SeO, began to be released. These results may suggest that H,SO, and H,SeO, are intercalated in separate interlayer spaces of graphite, i.e. bi-intercalation. Intercalation of H$eO, into stage-2 H,SO,-GIC was also observed. 1. INTRODUCTION A large number of studies for graphite intercalation compounds (GICs) have been done as one of the

host-guest materials. Different from ordinary hostguest materials, GICs show a unique staging phenomena characterized by the periodic sequence of intercalate layers in the two-dimensional host matrix. The staging transition of GIC between graphite and stage-l GIC have been actively studied for alkaline metal-graphite[ 1,2], halogen_graphite[3,4], metal chloride-graphite[S,6] and acid-graphite systems[7-91. The experimental results suggest that a staging transition occurs (1) when a sample responds to a relative change of the chemical potential of an intercalate in the case of spontaneous intercalation or (2) when the quality or quantity of the applied energy to a sample is changed in case the reaction is not a spontaneous one. In case of a spontaneous intercalation of vapour phase of an intercalate, the variations of temperature of a host material and vapour pressure of an intercalate change the chemical potential, which causes staging transition[ l-61. Electrochemical preparation of GIC is an example of assisted intercalation (intercalation with absorption of energy): the composition and the stage structure are altered by change of applied potential to the sample or of electric quantity transported during the reaction[7-91. However, only a few discussions have been presented about the phenomena that arise during the reaction of stage-n GIC (n 2 2) with another kind of intercalate. In this article, the successive intercalation of sulfuric acid and selenic acid into host graphite is considered. Galvanostatic oxidation was adopted as a method of preparing GICs because the reaction rate can be readily controlled by setting the current density and the staging transition is easily detectable

from estimation of the electrode potential vs. electric quantity curve[7]. 2. EXPERIMENTAL

2.1 Electrochemical techniques The host graphite used was a monochromatorgrade HOPG of Union Carbide Corporation and the electrolytic solvents were concentrated sulfuric acid (18 mol/L) from Kishida Chemical Co., Ltd., G.R. Grade and selenic acid (16 mol/L) from Mitsuwa Pure Chemicals, G.R. Grade. A slab of HOPG (2 x 1 x 0.25 mms) was used for the working electrode with the electrical contact made by a platinum wire. A platinum plate (10 x 10 mm’) was used for the counter electrode. A Hg/Hg,SO, electrode with a Luggin capillary was adopted as a reference electrode in sulfuric acid electrolyte. A large piece of HOPG (5 x 2 x 0.5 mm’) served as a floating reference electrode when selenic acid was used as the electrolyte[lO]. These electrode potentials are converted to the values vs. SHE. The HOPG was oxidized galvanostatically in sulfuric and selenic acids with a programmable DC current generator (Takeda Riken, Model 6141). The relationship between potential of working electrode and electric quantity transferred in the cell was obtained by continuous recording of the potential with a high impedance X/t-recorder. The reductive deintercalation of GIC obtained with the above oxidation method was carried out in the same electrolytic cell by changing the current direction between the working and the counter electrodes. 2.2 Successive intercalation Galvanostatic oxidation in selenic acid was stopped at the time desired, and the working electrode was 785

H.

186

SHIOYAMA

taken out from the electrolytic cell. After washed with a large amount of sulfuric acid to remove adhered selenic acid, it was transferred into another cell filled with sulfuric acid, and then the galvanostatic oxidation was continued. During this treatment, exposure of the sample to air was avoided as much as possible to prevent the decomposition by humidity. The same procedure was also available in case the galvanostatic oxidation in selenic acid occurred after the oxidation in sulfuric acid. 2.3 Measurements The amount of selenium in the resulting GIC was determined by the following method: the compound was washed with concentrated sulfuric acid to remove adhered selenic acid and then decomposed by standing in distilled water for five days. The amount of selenium in the water was determined by an inductively coupled argon-plasma (ICP) emission spectrometry at a wavelength of 196.0 nm. The X-ray diffraction with CuKa radiation was measured with a powder diffractometer (Rigaku Denki Co., Ltd., Rad-B) to confirm the formation of GIC.

3. RESULTS AND DISCUSSION

3.1 Sulfuric acid GIG and selenic acid GZC Figure l(a) shows the electrode potential vs. electric quantity curve representing the galvanostatic oxidation process of graphite in sulfuric acid. It is noted Electric 0

ti 0

5

‘2.0

quantity 5

8..

1

. . I’

Ah-g-graphite” 15

*.

I

’

in the literature[7,8] that stage-3, -2, and -1 H,SO,GICs are formed when the oxidation attains to the marked points “A, ” “B,” and “c” on the curve, respectively. The electrode potential vs. electric quantity curve of the selenic acid-graphite system is shown in Fig. 2(a). To determine the assignment of stage structures corresponding to the slopes and plateaus of the curve, galvanostatic oxidation was stopped at several points indicated in Fig. 2(a) and X-ray powder pattern of the sample was observed. Table 1 shows the -E axis repeat distance (1,) determined from the pattern between 5” and 40” in 20 and its stage assignment. The points “a” and “b” correspond to the completion of the formation of stage-3 and -2 H,SeO,GICs, respectively. These assignments are quite similar to those of the sulfuric acid-graphite system: the points “A” and “B” in Fig. l(a) correspond to the complete formation of stage-3 and -2 H,SO,-GICs, respectively. However, the formation of pure stage1 H,SeO,-GIC was unsuccessful even though the oxidation was continued far beyond the point “c.” This is presumably due to the instability of stage-l H,SeO,GIC or the progress of the competitive side reactions at the graphite anode[lO]. The sample oxidized up to the point “c,” therefore, was a mixture of the stage-l and -2 structures. Table 1 also shows the C/Se atomic ratio of each sample determined by ICP emission spectrometry. The C/Se atomic ratio of the stage-2 H,SeO,-GIC was 13.4. On the other hand, the C/HSeOc ratio of the same GIC was 34.7, which was derived from the fact that an electric quantity of 0.064 Ah( = 1.44 x 102’ electrons) was necessary to oxidize 1 gram of graphite to stage-2 H,SeO,-GIC (Fig. 2(a)). This discrepancy is resolved if a certain structure similar to that of the stage-2 H,SO,-GIC (CL . HSO; * 2.5H,SO,) is adopted, i.e. every second interlayer space in graphite is filled with HSeO; ions

345678

2 5

0

/x10m2 10

and R. Furrr

1 .

Electric

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/x10m2

Ah.g-graphite-’

(b)

\

I

I

I

I

gOI %

zzn, al ; 1.5 5 i$ 1.0

B 0

Fig. 1. Galvanostatic oxidation in 18 mol/L H,S04 (a) of HOPG; (b) of the stage-3 H,SeO,-GIC; (c) of the stage-2 HISeOl-GIC; i = 20pA; weight of starting HOPG in each case is 1.0 mg.

5

0

10

(b)

d 12

3

4

5.

Tlme/hr.

Fig. 2. Galvanostatic oxidation in 16 mol/L H,SeO, (a) of HOPG; (b) of the stage-2 H,SO,-GIC; i = 20pA; weight of starting HOPG in each case is 1.0 mg.

Ternary GIC with H,SO, and H,SeO, Table 1, Resultsof x-rav diffractometrv

181

and auantitative analvses of HSeO,-GIC

Z axis repeat distance I,

Stage of GIC expected from

Amount of Se atoms

Samplet

(A)

diffraction pattern

(x 10e6 g-atom)

C/Se atomic ratio

in the sampIeS

a

14.74

Stage-3

4.2

19.8

b

11.37

Stage-2

6.3

13.4

C

11.36 8.04

Stage-2

8.9

9.4

Stage-l

tThe notation of samples corresponds to the letters in Fig. 2a. *The starting HOPG of the measured sample is l.Omg by weight

and H,SeO, molecules. The appropriate chemical formula for the stage-2 H,SeO,-GIC is C;, . HSeO, . l.SH,SeO,. In a similar manner, the C/Se atomic ratio of 19.8 and the electric quantity of 0.040 Ah/g-graphite suggest that the chemical formula of stage-3 H,SeO,-GIC is C,t * HSeO; . l.SH,SeO,. Judging from the above results, the general formula of stage-n H,SeO,-GIC could be written as C;,, . HSeO; . lSH,SeO,. 3.2 Intercalation of HJO, into H,SeO,-GIC The stage-3 H,SeO,-GIC prepared by the oxidation up to the point “a” in Fig. 2(a) was transferred into the electrolytic cell filled with sulfuric acid and then continued to be oxidized. The electrode potential vs. electric quantity curve is shown in Fig. l(b). The shape of the curve was resemble to that of the region A-D in Fig. l(a). This similarity in potential curve suggests that sulfuric acid intercalates into the stage-3 H,SeO,-GIC. The intercalation was confirmed from the analytical result of selenium content determined by ICP emission spectrometry. When oxidation of the stage3 H,SeO,-GIC was continued up to the point “E,” the C/Se ratio was 20.4 in agreement with that of the starting stage-3 GIC. In other words, H,SeO, in the interlayer of graphite was not released during the oxidation in H,SO,. The point “E” was selected because its electrode potential is similar to that of the point “C” where H,SO, is intercalated into all the interlayer of graphite in Fig. l(a). In fact, the sample obtained at the point “E” had a blue color, which suggested that all the interlayer space of graphite was filled. This presumption is based on the facts that graphite can be colored blue only when H,SO, or H,SeO, is intercalated up to the stage-l structure[7,10]. In a similar manner, the stage-2 H*SeO,-GIC obtained at the point “b” in Fig. 2(a) was transferred into sulfuric acid and then continued to be oxidized. The electrode potential vs. electric quantity curve is shown in Fig. l(c): the shape of the curve is similar to the curve B-D in Fig. l(a). The C/Se ratio of the compound oxidized up to the point “F” was found to be 14.1, which agreeded with that of the starting H,SeO,-GIC with stage-2 structure. Thus, sulfuric acid could be intercalated also into the stage-2 H*SeO,GIC to form a blue-colored sample at the point “F.”

The electrode potential change obtained in the subsequent intercalation is shown schematically in Figs. 3(a) and 3(b) for above two cases. The potential obtained at the beginning of oxidation in sulfuric acid is identical with that obtained at the end of oxidation in selenic acid. This comes from the fact that the analogous shape and the same plateau potential are observed in the electrode potential vs. electric quantity curves shown in Figs. l(a) and 2(a). Figure 4 shows the changes of electrode potential in the process of galvanostatic oxidation up to the point “E” and “F,” and subsequent reduction by starting from the stage-3 and stage-2 H*SeO,-GIC, respectively, in sulfuric acid electrolyte. The electrochemical treatment was stopped at points “E,” “G,” and “H” in Fig. 4(a), and also at “F,” “I,” and “J” in Fig. 4(b), and then C/Se ratio of each sample was measured with ICP emission spectrometry. The results are shown in Fig. 4. As mentioned above, the samples obtained at “E” and “F” contain equal amount of selenic acid to that of starting stage-3 and -2 H,SeO,-GICs, respectively. Even after the reductive treatments from “E” to “G” and from “F” to “I,” the ratio C/Se does not change. Therefore, only intercalated sulfuric acid

I

O3i33

1

p1 stage

Fig. 3. The electrode potential change obtained in the subsequent intercalation. The reciprocal of stage number is plotted as abscissa. HOPG was oxidized in H,SeO, (a) up to the stage-3 GIC; (b) up to the stage-2 GIC; and then it was transferred into H,SO,.

H. SHIOYAMAand R. FUJII

788

I,. As for the sample at “F,” the observed I, of 8.02A is close to a half of the expected 1,. These results may be explained from the supposition that the 001 X-ray diffraction line is detectable when l! is a multiple of 3 for the GBC at “E” and when f is even for the GBC at “F.” Nevertheless, these interpretations give insufficient evidence for the formation of GBC which has the Z, value identical to the sum of the 1, values of the associated binary GICs. The direct evidences is needed by using some techniques of structural analysis.

,,.

oxidation

i

reduction

L I

012’3L567 Time /hr.

Fig. 4. Galvanostatic oxidation and reduction in 18 mol/L H2SOI (a) of the stage-3 H$eO,-GIC; (b) of the stage-2 H,Se04-GE; i = 20kA; weight of starting HOPG is 1.0 mg. Time is plotted as abscissa because plotting of electric quantity cannot represent the change of current direction during the electrochemical treatment. molecules seem to be released. By further reduction beyond “I” in Fig. 4(b), however, selenic acid molecules are released, because no more sulfuric acid remains between the graphite layers of the sample at “I.” In the case of the sample shown in Fig. 4(a), the selenic acid content does not decrease, even by the reduction beyond “G,” until the compound recovers a stage-3 structure at point “H.” Results of measurements of X-ray powder pattern support the conclusions described above: the samples at “H” and “I” were identified as stage-3 and -2 H,SeO,-GICs, respectively. Such order of release of intercalates is explained simply and effectively under the assumption that the samples at “E” and “F” are graphite bi-intercalation compounds (GBCs)[ll]. GBCs obtained by the intercalation of H,SO, into the stage-n H,SeO,-GIC (n = 2,3) have separate layers of H,SeO, and H2S04. The GBCs can completely release one intercalate without loss of the other. Coexistence of these two kinds of intercalate in the same interlayer space of graphite seems not to be caused such a selective release. By assuming bi-intercalation of H,SO, and H,Se04, the Z axis repeat distance I, observed on X-ray powder pattern is expected to be a sum of 1, values for the binary GICs of H,SO, and H,SeO,, such as the case reported by H&old et al. on the metal chloride system[l2,13]. As the Zc of the sta e-l H,SO,-GIC obtained at “C” in Fig. l(a) is 7.92 1 and that of the stage-l H,SeO,-GIC is 8.04A, the Z, of the GBCs at “E” and “F” are expected to be 23.88 (=8.04 + 7.92 + 7.92)A and 15.96 (=8.04 + 7.92)& respectively. The observed Z, of the sample at “E” is 7.96& which is equal to one third of the expected

3.3 Intercalation of H,SeO, into H/90,-GIC Another kind of ternary GIC was prepared from the same combination of intercalates, i.e. H,SO, and H,SeO,, by changing the order of galvanostatic oxidations. First, HOPG was oxidized to stage-2 H,SO,GIC in sulfuric acid, and then the oxidation was continued in selenic acid. The change of electrode potential during the galvanostatic oxidation of the stage-2 H,SO,-GIC in H,SeO, is shown in Fig. 2(b). It is noteworthy that the potential at the starting point of the oxidation in H,SeO, is lower by =0.3V than that of the plateau for stage-2 H,SeO,-GIC, and some electric quantity is needed to raise the potential of the sample to the stage-2 structure. This is due to partial decomposition of the stage-2 H,SO,-GIC by transferring it into selenic acid. This explanation was proved by measuring the C/Se atomic ratio of the sample obtained at the point “e” in Fig. 2(b). The obtained C/Se ratio was 9.69, which was smaller than the expected ratio of the stage-2 H,SeO,-GIC (13.6), showing that the sample “e” contained much more H,SeO, than the stage-2 H,SeO,-GIC. This difference can be attributed to the intercalation of H,Se04 into the deficit of starting H,SO,-GIC caused by the partial decomposition. The partial decomposition is ascribed to the concentration difference between H,SO, and H,SeO,. The stage-2 GIC prepared in 18 mol/L H,SO, decomposes partially when it is transferred into diluted (16 mol/L) H,SeO,. The same kind of partial decomposition is observed when the stage-2 GIC prepared in 18 mol/L H,SO, is transferred into 16 mol/L H2S04, similar electrode potential vs. electric quantity curve being observed during the subsequent oxidation in 16 mol/L H,SO,. In this manner, a different kind of ternary GIC is obtained if the procedure of preparation is changed slightly. Therefore, the preparation method must be selected carefully to obtain a particular kind of ternary GIC. 4. CONCLUSIONS

We have reported the electrochemical intercalation of H,SO, into stage-n H,SeO,-GIC (n = 2,3). The intercalation of H2S04 was confirmed by the shape of the electrode potential vs. electric quantity curves during the galvanostatic oxidation. The products were determined to be the ternary GICs of

Ternary GIC with H,SO, and HSeO,

H,SO, and H,SeO,. When the ternary GICs were reduced electrochemically, the process of deintercalation was interesting: at first H,SO, that has intercalated later is released, and after its completion H,SeO, begins to be released. On the other hand, when H,SO, and H$eO, were intercalated in this order, H,SeO,-rich ternary GIC was obtained. Acknowledgements-The authors thank Prof. T. Tsuzuku for helpful advice to correct English expression in the manuscript.

3. T. Sass, Y. Takahasi and T. Mukaibo, 4. 5. 6. 7. 8.

9. IO. 11.

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