Revealing Electrolytic Ion Sorption in Layered Graphene Galleries through Low-Temperature Solid-State NMR

Abstract

There is an ever-growing desire to use and store energy from sustainable resources. Pillared graphene materials offer high capacitive performances in supercapacitors, presumably through enhanced electrolytic ion sorption in their chemically engineered inter-layer graphene galleries. Herein, a judicious combination of the removal of excess electrolytes, isotopic enrichment of the pillar molecules, and the use of low temperatures (100 K) enables solid-state nuclear magnetic resonance spectroscopy to efficiently probe nuclear spin polarization exchange between the electrolyte and the electrode. This provides the direct detection of electrolyte ions in proximity to the gallery pillars, evidencing the adsorption of ions in such two-dimensional galleries. However, when the ions are larger than the gallery d-spacing, they are not observed to enter the galleries, and the total storage capacity is accordingly reduced. This methodology provides a means to locate electrolyte ions upon charging or discharging devices and thus will be invaluable in the quest for the design of materials with vastly improved power densities.