Ionic Liquid (BMIM+BF4–) Reactivity on Graphene Foam Electrodes: Infrared Fluorescence and Raman Monitoring of Reversible Cathodic BMIM+ Intercalation and Exfoliation
Highlights
- Small amounts of water can considerably alter reactivity at graphene foam electrode | ionic liquid interfaces
- Redox probes such as FeCp2+/0 or Fe3+/2+ in BMIM+BF4– are affected by humidity increasing diffusion rates. However, on graphene foam electrode surfaces with approx. 40 μm depth, both types of redox probes show voltammetric features associated with diffusional trapping within the porous graphene foam structure (i.e. peak currents scale with scan rate).
Abstract
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.
Introduction
Ionic liquids have attracted significant attention as electrolytes in electrochemical and energy systems due to their unique physicochemical properties, such as wide electrochemical potential window, low volatility, high ionic conductivity, and electrochemical stability.
Recently, ionic liquids have been considered in mineral processing and in rare earth recovery. The prototypical ionic liquid, 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIM+BF4–), has been studied for example for applications in electrochemistry and in energy storage and sensing devices. The reactivity of ionic liquids in redox processes is dependent on conditions (e.g. humidity) and electrode material.