Nanoporous carbon powers were dried for at least 2 days in a vacuum oven at 100 ℃, and transferred to an N2-filled glovebox where enough material was loaded in a 5 mm tube to fill the NMR coil. The carbons were covered with an excess of dry 1 M TEABF4, TPABF4 or TBABF4 electrolyte in dry acetonitrile (≥ 99.0 %, Sigma-Aldrich). NMR tubes were sealed with PTFE tape, and the electrolyte was left to fill the sample porosity overnight at room temperature.