¹¹B NMR study of the BF₄^- anion in activated carbons at various stages of charge of EDLCs in organic electrolyte

The behavior of the electrolyte anion, tetrafluoroborate (BF₄^-), in an electric double-layer capacitor was studied by solid state ¹¹B NMR using two activated carbons of medium (M500) and large (M3000) surface area, which show capacitance of 35 F/ml and 20 F/ml by charging at 2.7 V, respectively. Magic angle spinning (MAS) ¹¹B NMR distinguishes two species (A and B) on the carbons at the impregnated, charged, and discharged stages. The A species corresponds to ions on the outer surface, showing free motion regardless of the activated carbon. The B species is located in the pores of the activated carbon, showing a downfield shift and line broadening. The B species in M500 was further separated into C and D species according to their chemical shifts in multiple quantum (MQ)MAS NMR. The very short relaxation time and large chemical shift of the C species suggest a very strong adsorption over the positive electrode of M500, whereas its spherical symmetry is maintained, as indicated by its small quadrupole constant. The amount of this species increases on the positive electrode of M500 by charging at 2.7 V, contributing to the larger capacitance (35 mF/ml) of this particular carbon at this voltage. By contrast, the B species was more moderately immobilized on the positive electrode of M3000, although the ion appeared to be deformed into a flattened shape on the carbon surface of large pores as indicated by its larger quadrupole constant. A rapid exchange between adsorbed and free B species within the same pores of larger size in M3000 may reduce the capacitance in spite of the more adsorption on this large surface area activated carbon.