Solid-state NMR and electrochemical dilatometry study of charge storage in supercapacitor with redox ionic liquid electrolyte

Abstract

Redox-active electrolyte supercapacitors have been proposed to increase the capacitance and energy density of double-layer electrochemical capacitors (EDLCs) by adding a faradaic contribution to the charging mechanism. While charge storage in EDLCs has been studied extensively, the implications of having a redox-active molecule participating in charge storage are still not entirely understood. Important questions about the ability of the redox center to penetrate in the carbon micropores, on the faradaic efficiency of these centers and on the self-discharge of redox-active electrolyte supercapacitors are still open. Solid-state NMR (SS-NMR) is a powerful technique to quantify ions inside the pores of activated carbon and has been used to study the charging mechanisms in EDLCs. In this work, we demonstrate for the first time its usefulness to investigate the charge storage in an activated carbon supercapacitor based on a redox-active electrolyte. The electroactive molecule is an ionic liquid in which a triflimide anion analogue is modified with ferrocene, allowing the use of 19F NMR for its quantification in the pore and chronocoulometry to determine the charge associated with the oxidation of ferrocene. We show that the charging process of such a system involves a voltage-dependant mechanism. Charge storage on the positive electrode occurs via co-ion desorption in the low voltage range and subsequently from a combination of the faradaic reaction and counter-ion adsorption at high voltage. At the negative electrode, charging occurs exclusively by counter-ion adsorption over the studied voltage range. In-situ electrochemical dilatometry measurements, done in complement to SS-NMR, further confirm the proposed mechanism. The approach described here provides a more detailed picture of the charging mechanism of redox-active electrolyte supercapacitors than electrochemical measurements alone.