A multiple-field 23Na NMR study of sodium species in porous carbons

The sodium environments in porous carbon materials prepared from NaOH activation of a char were investigated by means of multiple-field solid-state 23Na NMR measurements, carried out at magnetic fields of 4.7, 8.45 and 14.1 T, with single-pulse excitation and magic angle spinning (MAS). The recorded spectra showed a relatively featureless resonance with linewidth and peak shift strongly dependent on the magnetic field strength and on the hydration level of the samples. The existence of second-order quadrupolar effects was inferred, although the structural disorder and the mobile character associated with the Na environment precluded the direct observation of typical quadrupolar features in the MAS NMR spectra. The analysis of the spectra collected at multiple magnetic fields yielded the values of −2.8 ppm for the isotropic chemical shift and 1.8 MHz for the quadrupole coupling constant, which were interpreted as due to Na+ ions bonded to oxygenated groups at the edges of the graphene planes within the carbon pore network.