Electrochemical Synthesis of Graphite-Tetrafluoroaluminate Intercalation Compounds

Abstract

Graphite tetrafluoroaluminate intercalation compounds (AlF₄-GICs) have been prepared by electrochemical oxidation of a natural graphite electrode in a 1.0 M nitromethane solution of tetraethylammmonium tetrafluoroaluminate ([TEA][AlF₄]). Galvanostatic electrolysis suggests that the intercalation reaction occurs above 0.8 V vs. Ag⁺/Ag. Powder X-ray diffraction measurements of the AlF₄-GIC obtained by potentiostatic electrolysis reveal that the most AlF₄-rich phase is the stage-3 GIC with a gallery height of 0.79 nm. This gallery height agrees with the theoretical value calculated from the size of AlF₄⁻ that locates its two-fold axis perpendicular to the graphite layers. Co-intercalation of the solvent is suggested by the composition of the stage-3 GIC (C₅₅AlF₄) and is confirmed by release of the solvent above 350 K during thermogravimetric analysis. Although the AlF₄-GIC shows the higher air stability than those of the GICs with typical inorganic complex anions, it slowly decomposes into GICs at higher stages after exposure to the air over 1000 h. Increase of gallery height was observed during this period, which possibly results from reorientation of AlF₄− between the layers. The thermodynamic stability of AlF₄-GIC is evaluated based on a Born-Harber cycle.