Each carbon sample was made into a carbon film and dried for at least 24 h at 100 °C in vacuo before being transferred to a N2-filled glovebox. The film was then cut and weighed in the glovebox. The weighed film (around 5 mg) was put into a sealed vial overnight with 1 M NEt4BF4 (ACN) to fully saturate the sample (around 150 µL) before being packed into 2.5mm rotors. The rotors were quickly packed (typically within 3 minutes) to avoid evaporation of the solvent. For each sample, the rotor was weighed before and after being packed to calculate the mass of the electrolyte added to the system. Excess amount of electrolyte was carefully removed with tissue papers.