Electrochemical Synthesis and Thermal Properties of Graphite Bisulfate

Abstract

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.