Raman scattering study of the staging kinetics in the c-face skin of pyrolytic graphite-H₂SO₄

Abstract

Raman scattering from the ∼ 1600 cm⁻¹ graphitic phonons is used to study the stage evolution of graphite-H₂SO₄ in the first ∼ 1000 Å of the bulk during electrochemical intercalation. The Raman results are compared to staging kinetics in the deep bulk studied previously by 00/ x-ray diffraction. For low cell currents, which establish quasiequilibrium conditions, the c-face surface of the highly oriented pyrolytic graphite (HOPG) in contact with the reactant rapidly changes stage index to n − 1 just as the bulk completes stage n. We conclude that the intercalant must cross the c-face plate boundary of the HOPG, probably entering at either grain boundaries, microcracks, or steps in the plate surface. During stage transitions, the Raman lines are observed to remain Lorentzian in shape, with constant width, indicating that an ordered stage n − 1 compound grows at the expense of an ordered stage n compound. In studies of partially submerged HOPG plates, the surface above the acid level is found to stage last, although the plate surface just below the acid level stages first. Lateral diffusion of the sulfate anions from regions below, to regions above the acid level, is apparently impeded for reasons that are not understood. During portions of the electrochemical reaction requiring only hydrogen rearrangement in the intercalate layers (“overcharging”), Raman spectra taken from above and below the acid level are observed to evolve in concert, indicating the protons are not similarly impeded.