Reversible Electrochemical Ion Intercalation at an Electrified Liquid|liquid Interface Functionalised with Porphyrin Nanostructures
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Ion intercalation into solid matrices influences the performance of key components in most energy storage devices (Li-ion batteries, supercapacitors, fuel cells, etc.). Electrochemical methods provide key information on the thermodynamics and kinetics of these ion transfer processes but are restricted to matrices supported on electronically conductive substrates. In this article, the electrified liquid|liquid interface is introduced as an ideal platform to probe the thermodynamics and kinetics of reversible ion intercalation with non-electronically active matrices. Zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrins were self-assembled into floating films of ordered nanostructures at the water|a,a,a-trifluorotoluene interface. Electrochemically polarising the aqueous phase negatively with respect to the organic phase lead to organic ammonium cations intercalating into the zinc porphyrin nanostructures by binding to anionic carboxyl sites and displacing protons through ion exchange at neutral carboxyl sites. The cyclic voltammograms suggested a positive cooperativity mechanism for ion intercalation linked with structural rearrangements of the porphyrins within the nanostructures, and were modelled using a Frumkin isotherm. The model also provided a robust understanding of the dependence of the voltammetry on the pH and organic electrolyte concentration. Kinetic analysis was performed using potential step chronoamperometry, with the current transients composed of “adsorption” and nucleation components. The latter were associated with domains within the nanostructures where, due to structural rearrangments, ion binding and exchange took place faster. This work opens opportunities to study the thermodynamics and kinetics of purely ionic ion intercalation reactions (not induced by redox reactions) in floating solid matrices using any desired electrochemical method.