Quantification of BF₄⁻ ions infiltrated into micropores of activated carbon fibers (ACFs) in 1 M (Et₄N)⁺(BF₄)⁻ in a propylene carbonate solvent using ex situ nuclear magnetic resonance (NMR) and a polytetrafluoroethylene internal standard, is presented. The measured concentrations of BF₄⁻ ions in the pores of the ACFs are correlated to their capacitances.