Exfoliated graphite (obtained by rapid heating of sulfuric-acid intercalated and subsequently deacidified graphite flakes) is optionally subjected to residual acidity removal, which involves repeated washing with water, such that the pH of the wash water increases from 2 to 7. Compared to washed exfoliated graphite, the unwashed material exhibits lower specific surface area (24 vs. 45 m<sup>2</sup>/g), a higher value (360 vs. 38 at 50 Hz) of the relative dielectric constant (real part), a similar value of the conductivity (50 S/m), a higher value of the specific carbon-contact interfacial capacitance (1.17 vs. 0.04 μF/m<sup>2</sup>), and a lower value of the carbon-contact interfacial resistivity (0.08 vs. 0.27 Ω cm<sup>2</sup>). The greater concentration of residual intercalate (containing sulfur and oxygen) present without washing contributes to the polarizability without interfering the conduction. The carbon-contact interface is superior when the exfoliated graphite has not been washed. At 2.0 MHz, the relative dielectric constant (real part) remains high (280) and the carbon-contact interfacial specific capacitance remains high (1.13 μF/m<sup>2</sup>). The imaginary part of the relative dielectric constant and the dielectric loss angle are not affected by the washing. The relative dielectric constant of 360 is even higher than the value of 121 for potassium-hydroxide-activated graphite nanoplatelet.
Hong, X., & Chung, D. D. L. (2015). Exfoliated graphite with relative dielectric constant reaching 360, obtained by exfoliation of acid-intercalated graphite flakes without subsequent removal of the residual acidity. Carbon, 91, 1–10. https://doi.org/10.1016/j.carbon.2015.04.042