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 A. 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 A. 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.