Dehydration of Ions in Voltage-Gated Carbon Nanopores Observed by in Situ NMR

Ion transport through nanochannels is of fundamental importance in voltage-gated protein ion channels and energy storage devices. Direct microscopic observations are critical for understanding the intricacy of ionic processes in nanoconfinement. Here we report an in situ nuclear magnetic resonance study of ion hydration in voltage-gated carbon nanopores. Nucleus-independent chemical shift was employed to monitor the ionic processes of NaF aqueous electrolyte in nanopores of carbon supercapacitors. The state of ion hydration was revealed by the chemical shift, which is sensitive to the hydration number. A large energy barrier was observed for ions to enter nanopores smaller than the hydrated ion size. Increasing the gating voltage above 0.4 V overcomes this barrier and brings F- into the nanopores without dehydration. Partial dehydration of F- occurs only at gating voltage above 0.7 V. No dehydration was observed for Na+ cations, in agreement with their strong ion hydration.