Ionic Liquid (BMIM⁺BF₄^–) Reactivity on Graphene Foam Electrodes: Infrared Fluorescence and Raman Monitoring of Reversible Cathodic BMIM⁺ Intercalation and Exfoliation

Ionic liquids provide innovative technology solutions in electrochemical systems and in energy systems. On graphene foam electrodes, reactivity during interfacial electron transfer and during capacitive charging are affected by the type and purity of the ionic liquid, and in particular by the presence of humidity. Here, three redox systems are investigated in BMIM+BF4– ionic liquid on graphene foam surfaces: (i) dissolved ferrocene (FeCp2+/0), (ii) dissolved Fe3+/2+, and (iii) the cathodic intercalation/exfoliation reaction of BMIM+ into the graphene foam. In exploratory experiments, humidity (here typically 0.55 wt % H2O by Karl-Fischer titration) is shown to enhance diffusion of molecular redox probes (up to 1 mM concentration) as well as to affect speciation of Fe3+/2+ in solution close to the electrode surface. More importantly, humidity appears to catalyze BMIM+ intercalation and exfoliation. In operando spectroelectrochemical Raman and IR fluorescence data for intercalation/exfoliation are reported and a mechanism for water effects on intercalation is proposed based on charge density.