Electrochemical formation of graphite intercalation compounds in H2SO4-[1-C-14] CH,COOH solutions was studied by radiotracer analysis coupled with autoradiography. It was shown that a stage I ternary graphite intercalation compound TGIC can be synthesized in 60 and 80% H2SO4. The only formation of graphite bisulfate (GB) was observed in 20 and 40% H2SO4. The presence of ethanoic acid in the graphite matrix was confirmed by thermogravimetry coupled with mass-spectrometry and Fourier IR-spectroscopy. (c) 2006 Elsevier Ltd. All rights reserved.

Anodic oxidation of highly oriented pyrolytic graphite in an electrolyte containing concentrated sulfuric and anhydrous phosphoric acids is studied for the first time. The synthesis was carried out under galvanostatic conditions at a current I = 0.5 mA and an elevated temperature (t = 80°C). Intercalation compounds of graphite (ICG) are shown to form at all concentration ratios of H2SO4 and H3PO4 acids. The intercalation compound of step I forms in solutions containing more than 80 wt % H2SO4, a mixture of compounds of intercalation steps I and II forms in 60% H2SO4, intercalation step II is realized in the sulfuric acid concentration range from 10 to 40%, and a mixture of compounds of intercalation steps III and II is formed in 5% H2SO4 solutions. The threshold concentration of H2SO4 intercalation is ∼2%. With the decrease in active intercalate (H2SO4) concentration, the charging curves are gradually smoothed, the intercalation step number increases, and the potentials of ICG formation also increase. As the sulfuric acid concentration in the electrolyte changes from 96 to 40 wt %, the filled-layer thickness di in ICG monotonously increases from 0.803 to 0.820 nm, which apparently is associated with the greater size of phosphoric acid molecules. With further increase in H3PO4 concentration in solution, di remains unchanged. According to the results of chemical analysis, both acids are simultaneously incorporated into the graphite interplanar spacing and their ratio in ICG is determined by the electrolyte composition.

The electrochemical synthesis of graphite bisulfate in 94% H2SO4 is studied in galvanostatic (I = 10 μA) and potentiostatic modes. The formation potentials and detailed chemical formulas of stage I–V graphite bisulfate are determined. It is found that, within the stability range of a given stage, the molar ratio of intercalated H2SO4 to HSO–4 varies slightly. This variation is more significant at larger stage indices. In a potentiostatic mode, ordered stage I–V graphite bisulfate can be synthesized, with reproducible chemical and phase compositions. It is shown based on rough estimates of the diffusion coefficient D of sulfuric acid in the graphite host that the intercalate layer is indeed in a “quasi-liquid” state: the estimated D is substantially lower than typical diffusivities in solids. The thermal properties of stage I–V graphite bisulfate are studied systematically for the first time. It is found that, independent of the stage index, graphite bisulfate decomposition results in an unresolved endothermic doublet in the range 170–340°C. Heating to 170°C leads to partial decomposition of the graphite intercalation compounds, accompanied by an increase in stage index by unity. As a result, sulfuric acid is present in two states: free (deintercalated) and bound (in graphite bisulfate). The lower temperature endotherm in the unresolved doublet is due to the vaporization of free H2SO4 , and the higher temperature endotherm is due to the deintercalation and vaporization of bound sulfuric acid. The estimated heat of deintercalation from stage I–V graphite bisulfate (2.9–0.5 kJ/mol C) decreases steadily with increasing stage index.

The electrochemical intercalation of graphite in H2SO4–CH3COOH solutions is investigated by the radiotracer method in combination with autoradiography. The results attest to the formation of a stage I ternary intercalation compound in the solutions containing 60 and 80 wt % H2SO4 and stage II graphite bisulfate in the solutions containing 20 and 40 wt % H2SO4. The hydrolysis of the ternary compound leads to partial removal of acetic acid from the graphite host. Heat treatment of hydrolyzed samples results in complete deintercalation of the acids. The presence of both acetic and sulfuric acids in the synthesized compounds is confirmed by thermogravimetry combined with mass spectrometry and Fourier-transform IR spectroscopy.

The anodic oxidation of highly oriented pyrolytic graphite in H2SO4–CH3COOH electrolytes in a galvanostatic mode (I = 1.5 mA) is studied as a function of electrolyte composition. The concentration ranges for the formation of stage I–V graphite intercalation compounds (GICs) are determined. The concentration threshold for intercalation is ∼1 wt % H2SO4 . There are three distinct concentration ranges differing in the shape of the charging curve E(Q), which depends primarily on the content of H2SO4 (active intercalant). The potentials of formation of stage I–III GICs in H2SO4–CH3COOH electrolytes are found to be higher than those for graphite bisulfate, which points to an increase in the potential barrier for intercalation and is obviously associated with the intercalation of acetic acid into the graphite host. The specifics of the charging curves obtained in 60–80% H2SO4 , together with gravimetry and chemical analysis data, indicate the formation of ternary GICs with sulfuric and acetic acids. In this composition range, stage I* and II* cointercalation compounds are obtained, with an intercalate layer thickness di = 7.94 Å. The composition of the ternary GICs is shown to depend on the relative amounts of the acids in the electrolyte. A mechanism of the formation of graphite cointercalation compounds is proposed. In solutions containing less than 40 wt % H2SO4 , only graphite bisulfate is formed.

The anodic oxidation of highly oriented pyrolitic graphite (HOPG) in the mixture H2SO4–R (RCH3COOH, H3PO4) with concentration of H2SO4 in the range 0–100wt% was investigated in this research work. Galvanostatic oxidation of graphite in H2SO4–R solution allows one to obtain stages 1–5 graphite intercalation compounds (GICs). The presence of R in the electrolyte solutions significantly expands the concentration limit for intercalation down to 1wt% of H2SO4. The co-intercalated stages 1 and 2 H2SO4–CH3COOH–GICs with di=7.94Å were obtained in 60–80wt% of H2SO4 solution. The stages 1–3 co-intercalated H2SO4–H3PO4–GICs with di=8.08–8.20Å were synthesized for the first time. It was shown that the form of galvanostatic curve E(Q) is determinated by the content of strong intercalate H2SO4. It was established that the potential of intercalation Eint of the stages 1–3 co-intercalated GICs and the quantity of electricity (Q) are higher than Eint and Q for the formation of binary GICs (graphite hydrogensulfate). The enhancement of the potential barrier for intercalation gives the evidence of the co-intercalation of H2SO4 with CH3COOH or H3PO4 in graphite matrix. The mechanism of co-intercalation of graphite was proposed.