Graphite intercalation in the graphite-H₂SO₄-R (R = H₂O, C₂H₅OH, C₂H₅COOH) systems

Abstract

The electrochemical behavior of graphite in polar solvent-H₂SO₄ electrolytes is studied in a wide range of H₂SO₄ concentrations. The results demonstrate that, with decreasing H₂SO₄ concentration, the charging curves become smoother and shift to higher potentials, the stage index increases, and intercalation compounds are more difficult to obtain. At H₂SO₄ concentrations of 50% and lower, graphite polarization is accompanied by a significant overoxidation, as evidenced by the anomalously small intercalate layer thicknesses: 7.75–7.85 Å. Anodic polarization of graphite in electrolytes consisting of H₂SO₄ and a polar solvent (H₂O and C₂H₅OH) follows the same mechanism as in the case of the formation of graphite bisulfate. In going from water to C₂H₅OH, a less polar solvent, the intercalation threshold increases from 30 to 70% H₂SO₄. It is shown using a set of characterization techniques that, in the graphite-H₂SO₄-R (R = H₂O, C₂H₅OH) systems, the solvent is not intercalated into graphite. Stage I–III ternary graphite intercalation compounds (TGICs) are synthesized for the first time in the graphite-H₂SO₄-C₂H₅COOH system: stage I TGICs at H₂SO₄ concentrations above 70%, stage II in the range 30–70% H₂SO₄, and stage III at H₂SO₄ concentrations down to 10%. The intercalate layer thickness in the TGICs is 7.94 Å. The mechanism of TGIC formation in this system is shown to differ from those in mixtures of H₂SO₄ and other organic acids. Thermal analysis in combination with spectroscopic analysis of gaseous products provides clear evidence for intercalation of propionic acid into the TGIC and indicates that the thermal stability of this compound is lower than that of graphite bisulfate.