Supercapacitors (or electric double-layer capacitors) are high power energy storage devices that store charge by the non-faradaic adsorption of ions at the interface between porous carbon electrodes and an electrolyte solution. The development of new electrode materials and electrolytes with improved performances is an active area of research today, yet there are relatively few studies of the molecular mechanisms of the charge storage process. In this work, nuclear magnetic resonance (NMR) spectroscopy is developed for the study of the charge storage mechanisms of supercapacitors.

Importantly, NMR experiments show that electrolyte ions adsorbed inside the pores of the carbon electrodes can be resolved from those in bulk electrolyte for a range of supercapacitor electrode materials. Chemical shift calculations show that the adsorbed species are subject to ring current effects, whereby the delocalised electrons in the carbon shield the nearby nuclei. The calculated effects depend on the local carbon structure, helping to rationalise the variations observed when different porous carbons are studied experimentally, and allowing structural information to be extracted from the spectra.

NMR experiments performed on electrodes extracted from ionic liquid-based supercapacitors with different applied voltages allow the numbers of adsorbed ions to be measured upon charging. It is shown that supercapacitor charging involves the migration of both anions and cations in and out of the carbon pores in each electrode, with the anions dominating the charge storage process. When combined with lineshape measurements, which offer information about the diffusion of adsorbed ions, the power performances of supercapacitor devices with different electrolytes are rationalised.

In situ NMR methods are then developed to allow mechanistic studies of working supercapacitors as they are charged and discharged inside the NMR magnet. The experiments reveal that the charge storage mechanism depends on both the electrolyte and the electrode material studied. During charging, reversible chemical shift changes are also observed, arising from the introduction of paratropic ring currents. Finally, cross polarisation experiments allow the selective observation of the adsorbed electrolyte species, and show that their motion slows down during supercapacitor charging. Overall, the NMR approach offers unique insights into the molecular mechanisms of the supercapacitance phenomenon.