  In situ NMR spectroscopy is a powerful technique to investigate charge
storage mechanisms in carbon-based supercapacitors thanks to its ability to
distinguish ionic and molecular species adsorbed in the porous electrodes from
those in the bulk electrolyte. The NMR peak corresponding to the adsorbed
species shows a clear change of chemical shift as the applied potential
difference is varied. This variation in chemical shift is thought to originate
from a combination of ion reorganisation in the pores and changes in ring
current shifts due to the changes of electronic density in the carbon. While
previous Density Functional Theory calculations suggested that the electronic
density has a large effect, the relative contributions of these two effects is
challenging to untangle. Here, we use mesoscopic simulations to simulate NMR
spectra and investigate the relative importance of ion reorganisation and ring
currents on the resulting chemical shift. The model is able to predict chemical
shifts in good agreement with NMR experiments and indicates that the ring
currents are the dominant contribution. A thorough analysis of a specific
electrode/electrolyte combination for which detailed NMR experiments have been
reported allows us to confirm that local ion reorganisation has a very limited
effect but the relative quantities of ions in pores of different sizes, which
can change upon charging/discharging, can lead to a significant effect. Our
findings suggest that in situ NMR spectra of supercapacitors may provide
insights into the electronic structure of carbon materials in the future.
