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Chemical charging and electrochemical discharging through graphite intercalation compounds with sulfuric acid
SOLID STATE IowIcs
Solid State Ionics 70/7 1 ( 1994) 425-428 North-Holland
Chemical charging and electrochemical discharging through graphite intercalation compounds with sulfuric acid M. Inagaki and N. Iwashita ’ Faculty OfEngineering, Hokkaido University, Kita-ku, Sapporo 060, Japan
The chemical charging and following electrochemical discharging through the formation and decomposition of graphite intercalation compounds with sulfuric acid were studied on three host materials with different textures, viz., natural graphite flakes, vapor-grown graphite fibers and mesophase-pitch-based graphite fibers. In the course of electrochemical discharging in sulfuric acid containing nitric acid as oxidant, a competition between the de-intercalation and re-intercalation of sulfuric acid into the graphite gallery was observed and, as a consequence, a strong effect of the texture of the host graphite on the discharging process. On the mesophase-pitch-based graphite fibers with radial arrangement of the graphite basal planes in the cross-section of the fibers, electrochemical discharging with rather steady state voltage in long period was possible.
1. Introduction The graphite intercalation compounds (GIG’s) with sulfuric acid have been studied by different authors and their formation has been known to occur by means of either electrochemical or chemical oxidation. Only recently it was shown, however, by following the potential of the host graphite that the formation process of H2S04-GIG’s by chemical oxidation is exactly the same as the one by electrochemical oxidation [ I,2 1. The process is characterized by different critical potentials; threshold potential showing the initiation of intercalation reaction, onset potentials corresponding to the establishment of each stage structure of the GIG’s, and saturated potential depending on the oxidant employed and its amount [ 21. By using these critical potential values, the criteria for the intercalation of sulfuric acid into various host carbon materials were discussed previously [ 3 1. The results of our work on the formation of H$O,-GIG’s [l-7] suggest that the intercalation reaction by chemical oxidation has to be a charging process and so the words “chemical charging” are employed here. In the present work, the processes of ’ Present address: Government Industrial Research Institute at Osaka, Ikeda-shi, Osaka 563, Japan 0167-2738/94/$
this chemical charging and the discharging through the intercalation and de-intercalation reactions of H,SO, were investigated using the different host materials. A remarkable effect of texture of the host on these processes was found.
2. Experimental Three host graphite materials were selected because they have different textures, i.e. different orientation of basal graphitic planes; natural graphite with an average flake size of 400 urn having an orientation of basal planes along the flake surface (called NG), a vapor-grown carbon fiber heat-treated at 3000°C having an annual ring texture along the fiber axis (VG), and a mesophase-pitch-based carbon fiber also heat-treated at 3000°C having radial arrangement of basal planes (MP). The host materials of 50 mg were held in between plastic plates as an electrode and dipped into sulfuric acid with a concentration of 18 mol/dm’, as shown in fig. 1. A platinum plate was used as a counter electrode. In the sulfuric acid, nitric acid with 13.8 mol/ dm3 was added as oxidant by controlling the mole ratio of HN03 to host carbon to be 3.3, which was an excess amount of oxidant to form the stage-l structure. Electrochemical discharging was carried
07.00 0 1994 Elsevier Science B.V. All rights reserved.
426
M. Inagaki, N. Iwashita /Graphite intercalation compounds
out by a constant current of 4 mA. The potential change was measured using a Hg/HgS04 reference electrode. The cell used was always kept at 25°C. The detailed experimental set-up and procedures are described in our previous papers [ 1,2]. The GIG’s formed were examined mostly by X-ray powder diffractometry and their decomposition was prevented by using a polyethylene bag.
Fig. 2. Changes in potential on the natural graphite (NG), the vapor-grown carbon fibers (VG) and the mesophase-pitch-based carbon fibers (MP): (a) chemical oxidation by HN02; (b) electrochemical reduction with a current of 4 mA.
3. Experimental results The changes in potential of the three host materials with time during chemical oxidation are compared in fig. 2a. On the flaky natural graphite (NG), a stepwise increase of potential is observed. This change in potential corresponds to the development of stage structures in the compound and the potential values for each plateau agree with the onset potentials for the stage-3, -2 and -1 structures which have been reported. On two other hosts (MP and VG), rapid increases in potential are found, a pacedown in potential increase above 0.9 V being related to the establishment of the stage-l structure in the resultant compounds. All the products obtained from these three hosts after 80 h reaction time were proved by their X-ray powder patterns to be the stage-l graphite intercalation compounds with H$O+ After the formation of the stage-l GIG’s, i.e., after the charging, the discharge process with a current of 4 mA was followed by measuring the potential. It has to be mentioned that this discharging was performed in electrolyte containing the oxidant HNOJ. The potential changes with discharged electric quantity, i.e., discharge capacity, on the GIG’s formed from three
Electric
quantity
(mAh/g-carbon)
Fig. 3. Discharge curves observed on the natural graphite (NG) and the mesophase-pitch-based carbon fiber (MP).
flake
hosts are shown in fig. 2b. On the sample NG, a stepwise decrease in potential is observed, of which the plateau corresponds to the stage change. The discharge capacity is about 150 mAh/g-carbon, which is a little larger than the theoretical value. On the sample VG, the step in potential was not clear, but the capacity reaches to about 600 mAh/g-carbon, much larger than that for the sample NG. Extremely large discharge capacity is obtained on the sample MP, of which the whole discharge curve is compared with the sample NG in fig. 3. After complete discharging the sample MP, the oxidant HNOs was added; the potential was recovered with the same rate and to the same level as the first charging and then we could discharge similar to the first discharge. Many repetitions of this cycle of
h4. Inagaki, N. Iwashita /Graphite intercalation compounds
chemical charging were possible.
and electrochemical
discharging
4. Discussion 4. I. Chemical charging The potential increase by chemical oxidation (chemical charging) was affected by the texture of the host. It is reasonable to suppose that the chemical oxidation proceeds at the edge of the graphite planes and that the intercalation of sulfuric acid starts also from the edge of the planes. In the case of flaky natural graphite (the sample NG) which has a limited area of the edge surface and well-developed basal planes, a well-defined stepwise increase in potential with time was observed, because the oxidation at the edge occurs quickly and the diffusion of the intercalated species in the interlayer space takes time to complete the stage structure. In the case of sample MP, the potential increases quickly, because it has a large edge surface area and its interlayer space can be occupied quickly. The characteristic annual ring texture of the sample VG was found to be partly broken during the intercalation, already before the formation of the stage- 1 structure, and the edge surface of the graphite crystallites was exposed to the electrolyte. Therefore, it showed a slow increase in potential above 0.9 V. The second cycle of chemical charging was found to give a stepwise increase in potential, very similar to the flaky natural graphite, 4.2. Electrochemical discharging A remarkable difference in the discharging behavior among three hosts is explained by the difference in texture. At the edge surface the intercalation into the vacant interlayer space formed by the de-intercalation by discharging can occur in the electrolyte containing enough amount of oxidant, and so these two reactions, de-intercalation and re-intercalation, can compete with each other. In the case of the sample MP, these two reactions seem to be balanced. This is why the discharging with the current of 4 mA can continue for a long time. In the case of sample NG, a large amount of re-intercalation cannot be expected and the main process must be the de-inter-
427
calation. As a consequence, the discharge capacity was only a little larger than the theoretical. The sample VG showed an intermediate behavior of discharging between these two hosts, because of the formation of edge surface due to the partial destruction of the annual ring texture by the first intercalation. Discharging current governs the de-intercalation reaction and the balance between de-intercalation and re-intercalation may be changed. On the sample MP, even the increase in potential was observed after the loading when the current of 0.4 mA was employed, and with 40 mA a rapid decrease in potential, like the case of the sample NG, was observed. On the sample NG in fig. 3, a prolonged discharging curve was obtained by using a current of 0.4 mA. 4.3. Repetition of the cycles The balance between the de-intercalation and reintercalation under loading was observed only when HN03 was used as the oxidant. This is because of the irreversible change of nitric ions, as expressed by the following reaction: NO,
+3H++2e-SHN02+H,0.
In the course of the oxidation of the host graphite (i.e. charging) the above reaction shifts to right-hand side, but the reaction products, which are supposed to be HNO, and H20, stay in the electrolyte. In the course of electrochemical reduction (i.e. discharging), the NO, ions are reproduced by a shift of the reaction to the left-hand side. However, a part of nitric ions seemed to be consumed by a release of some gaseous species, though bubbling was never observed. This is the main reason why the balance between de-intercalation and re-intercalation cannot last forever and also why the cell could be charged again by the addition of new nitric acid. If potassium permanganate KMn04 was used as oxidant, such a prolonged discharge performance was not obtained because the product formed from permanganate ions MnO; during the chemical oxidation of the host graphite was insoluble in sulfuric acid and some precipitates were formed. The present results revealed two important points: ( 1) the possibility of the coupling of chemical charging with electrochemical discharging and (2 ) a re-
428
M. Inagaki. N. Iwashita /Graphite intercalation compounds
markable effect of the texture of carbon materials on the electrochemical performance. If we could construct a sealed cell and select a host material with an appropriate texture, it might be possible to have a perfect balance between de-intercalation and re-intercalation, and so we might be able to have continuous discharge from the cell. It will be worthwhile trying.
Acknowledgement This work was partly supported by PICS-91 from CNRS, France, and by Monbusho International Scientific Research Program-Cooperation (No.
040440 15) from Japanese Culture and Science.
Ministry
of Education,
References [ 1 ] M. Inagaki, N. Iwashita and E. Kouno, Carbon 28 ( 1990) 49. [ 21 N. Iwashita and M. Inagaki, Synth. Met. 34 ( 1989) 139. [ 3 ] M. Inagaki, N. Iwashita and Y. Hishiyama, 7th Intern. Symp. Intercalation Compounds, May lo- 14, 1993, Louvain-laNeuve, Belgium. [4] N. Iwashita and M. Inagaki, TANS0 145 (1990) 228. [ 5 ] N. Iwashita and M. Inagaki, Mater. Sci. Forum 9 l-93 ( I992 ) 665. [ 61 N. Iwashita and M. Inagaki, Nihon Kagaku Kai Shi ( 1992) 1421. [ 71 N. Iwashita and M. Inagaki, TANS0 149 ( 199 1) 204.
Solid State Ionics 70/7 1 ( 1994) 425-428 North-Holland
Chemical charging and electrochemical discharging through graphite intercalation compounds with sulfuric acid M. Inagaki and N. Iwashita ’ Faculty OfEngineering, Hokkaido University, Kita-ku, Sapporo 060, Japan
The chemical charging and following electrochemical discharging through the formation and decomposition of graphite intercalation compounds with sulfuric acid were studied on three host materials with different textures, viz., natural graphite flakes, vapor-grown graphite fibers and mesophase-pitch-based graphite fibers. In the course of electrochemical discharging in sulfuric acid containing nitric acid as oxidant, a competition between the de-intercalation and re-intercalation of sulfuric acid into the graphite gallery was observed and, as a consequence, a strong effect of the texture of the host graphite on the discharging process. On the mesophase-pitch-based graphite fibers with radial arrangement of the graphite basal planes in the cross-section of the fibers, electrochemical discharging with rather steady state voltage in long period was possible.
1. Introduction The graphite intercalation compounds (GIG’s) with sulfuric acid have been studied by different authors and their formation has been known to occur by means of either electrochemical or chemical oxidation. Only recently it was shown, however, by following the potential of the host graphite that the formation process of H2S04-GIG’s by chemical oxidation is exactly the same as the one by electrochemical oxidation [ I,2 1. The process is characterized by different critical potentials; threshold potential showing the initiation of intercalation reaction, onset potentials corresponding to the establishment of each stage structure of the GIG’s, and saturated potential depending on the oxidant employed and its amount [ 21. By using these critical potential values, the criteria for the intercalation of sulfuric acid into various host carbon materials were discussed previously [ 3 1. The results of our work on the formation of H$O,-GIG’s [l-7] suggest that the intercalation reaction by chemical oxidation has to be a charging process and so the words “chemical charging” are employed here. In the present work, the processes of ’ Present address: Government Industrial Research Institute at Osaka, Ikeda-shi, Osaka 563, Japan 0167-2738/94/$
this chemical charging and the discharging through the intercalation and de-intercalation reactions of H,SO, were investigated using the different host materials. A remarkable effect of texture of the host on these processes was found.
2. Experimental Three host graphite materials were selected because they have different textures, i.e. different orientation of basal graphitic planes; natural graphite with an average flake size of 400 urn having an orientation of basal planes along the flake surface (called NG), a vapor-grown carbon fiber heat-treated at 3000°C having an annual ring texture along the fiber axis (VG), and a mesophase-pitch-based carbon fiber also heat-treated at 3000°C having radial arrangement of basal planes (MP). The host materials of 50 mg were held in between plastic plates as an electrode and dipped into sulfuric acid with a concentration of 18 mol/dm’, as shown in fig. 1. A platinum plate was used as a counter electrode. In the sulfuric acid, nitric acid with 13.8 mol/ dm3 was added as oxidant by controlling the mole ratio of HN03 to host carbon to be 3.3, which was an excess amount of oxidant to form the stage-l structure. Electrochemical discharging was carried
07.00 0 1994 Elsevier Science B.V. All rights reserved.
426
M. Inagaki, N. Iwashita /Graphite intercalation compounds
out by a constant current of 4 mA. The potential change was measured using a Hg/HgS04 reference electrode. The cell used was always kept at 25°C. The detailed experimental set-up and procedures are described in our previous papers [ 1,2]. The GIG’s formed were examined mostly by X-ray powder diffractometry and their decomposition was prevented by using a polyethylene bag.
Fig. 2. Changes in potential on the natural graphite (NG), the vapor-grown carbon fibers (VG) and the mesophase-pitch-based carbon fibers (MP): (a) chemical oxidation by HN02; (b) electrochemical reduction with a current of 4 mA.
3. Experimental results The changes in potential of the three host materials with time during chemical oxidation are compared in fig. 2a. On the flaky natural graphite (NG), a stepwise increase of potential is observed. This change in potential corresponds to the development of stage structures in the compound and the potential values for each plateau agree with the onset potentials for the stage-3, -2 and -1 structures which have been reported. On two other hosts (MP and VG), rapid increases in potential are found, a pacedown in potential increase above 0.9 V being related to the establishment of the stage-l structure in the resultant compounds. All the products obtained from these three hosts after 80 h reaction time were proved by their X-ray powder patterns to be the stage-l graphite intercalation compounds with H$O+ After the formation of the stage-l GIG’s, i.e., after the charging, the discharge process with a current of 4 mA was followed by measuring the potential. It has to be mentioned that this discharging was performed in electrolyte containing the oxidant HNOJ. The potential changes with discharged electric quantity, i.e., discharge capacity, on the GIG’s formed from three
Electric
quantity
(mAh/g-carbon)
Fig. 3. Discharge curves observed on the natural graphite (NG) and the mesophase-pitch-based carbon fiber (MP).
flake
hosts are shown in fig. 2b. On the sample NG, a stepwise decrease in potential is observed, of which the plateau corresponds to the stage change. The discharge capacity is about 150 mAh/g-carbon, which is a little larger than the theoretical value. On the sample VG, the step in potential was not clear, but the capacity reaches to about 600 mAh/g-carbon, much larger than that for the sample NG. Extremely large discharge capacity is obtained on the sample MP, of which the whole discharge curve is compared with the sample NG in fig. 3. After complete discharging the sample MP, the oxidant HNOs was added; the potential was recovered with the same rate and to the same level as the first charging and then we could discharge similar to the first discharge. Many repetitions of this cycle of
h4. Inagaki, N. Iwashita /Graphite intercalation compounds
chemical charging were possible.
and electrochemical
discharging
4. Discussion 4. I. Chemical charging The potential increase by chemical oxidation (chemical charging) was affected by the texture of the host. It is reasonable to suppose that the chemical oxidation proceeds at the edge of the graphite planes and that the intercalation of sulfuric acid starts also from the edge of the planes. In the case of flaky natural graphite (the sample NG) which has a limited area of the edge surface and well-developed basal planes, a well-defined stepwise increase in potential with time was observed, because the oxidation at the edge occurs quickly and the diffusion of the intercalated species in the interlayer space takes time to complete the stage structure. In the case of sample MP, the potential increases quickly, because it has a large edge surface area and its interlayer space can be occupied quickly. The characteristic annual ring texture of the sample VG was found to be partly broken during the intercalation, already before the formation of the stage- 1 structure, and the edge surface of the graphite crystallites was exposed to the electrolyte. Therefore, it showed a slow increase in potential above 0.9 V. The second cycle of chemical charging was found to give a stepwise increase in potential, very similar to the flaky natural graphite, 4.2. Electrochemical discharging A remarkable difference in the discharging behavior among three hosts is explained by the difference in texture. At the edge surface the intercalation into the vacant interlayer space formed by the de-intercalation by discharging can occur in the electrolyte containing enough amount of oxidant, and so these two reactions, de-intercalation and re-intercalation, can compete with each other. In the case of the sample MP, these two reactions seem to be balanced. This is why the discharging with the current of 4 mA can continue for a long time. In the case of sample NG, a large amount of re-intercalation cannot be expected and the main process must be the de-inter-
427
calation. As a consequence, the discharge capacity was only a little larger than the theoretical. The sample VG showed an intermediate behavior of discharging between these two hosts, because of the formation of edge surface due to the partial destruction of the annual ring texture by the first intercalation. Discharging current governs the de-intercalation reaction and the balance between de-intercalation and re-intercalation may be changed. On the sample MP, even the increase in potential was observed after the loading when the current of 0.4 mA was employed, and with 40 mA a rapid decrease in potential, like the case of the sample NG, was observed. On the sample NG in fig. 3, a prolonged discharging curve was obtained by using a current of 0.4 mA. 4.3. Repetition of the cycles The balance between the de-intercalation and reintercalation under loading was observed only when HN03 was used as the oxidant. This is because of the irreversible change of nitric ions, as expressed by the following reaction: NO,
+3H++2e-SHN02+H,0.
In the course of the oxidation of the host graphite (i.e. charging) the above reaction shifts to right-hand side, but the reaction products, which are supposed to be HNO, and H20, stay in the electrolyte. In the course of electrochemical reduction (i.e. discharging), the NO, ions are reproduced by a shift of the reaction to the left-hand side. However, a part of nitric ions seemed to be consumed by a release of some gaseous species, though bubbling was never observed. This is the main reason why the balance between de-intercalation and re-intercalation cannot last forever and also why the cell could be charged again by the addition of new nitric acid. If potassium permanganate KMn04 was used as oxidant, such a prolonged discharge performance was not obtained because the product formed from permanganate ions MnO; during the chemical oxidation of the host graphite was insoluble in sulfuric acid and some precipitates were formed. The present results revealed two important points: ( 1) the possibility of the coupling of chemical charging with electrochemical discharging and (2 ) a re-
428
M. Inagaki. N. Iwashita /Graphite intercalation compounds
markable effect of the texture of carbon materials on the electrochemical performance. If we could construct a sealed cell and select a host material with an appropriate texture, it might be possible to have a perfect balance between de-intercalation and re-intercalation, and so we might be able to have continuous discharge from the cell. It will be worthwhile trying.
Acknowledgement This work was partly supported by PICS-91 from CNRS, France, and by Monbusho International Scientific Research Program-Cooperation (No.
040440 15) from Japanese Culture and Science.
Ministry
of Education,
References [ 1 ] M. Inagaki, N. Iwashita and E. Kouno, Carbon 28 ( 1990) 49. [ 21 N. Iwashita and M. Inagaki, Synth. Met. 34 ( 1989) 139. [ 3 ] M. Inagaki, N. Iwashita and Y. Hishiyama, 7th Intern. Symp. Intercalation Compounds, May lo- 14, 1993, Louvain-laNeuve, Belgium. [4] N. Iwashita and M. Inagaki, TANS0 145 (1990) 228. [ 5 ] N. Iwashita and M. Inagaki, Mater. Sci. Forum 9 l-93 ( I992 ) 665. [ 61 N. Iwashita and M. Inagaki, Nihon Kagaku Kai Shi ( 1992) 1421. [ 71 N. Iwashita and M. Inagaki, TANS0 149 ( 199 1) 204.