Understanding the Charge Storage Mechanism of Redox-Active Ionic Liquid Based Supercapacitors

Abstract

Redox-active ionic liquids are emerging as promising new electrolytes for supercapacitors, which provide higher capacitance and energy density than organic or ionic liquid electrolytes. The fundamental studies of charge storage mechanism in supercapacitors are of critical importance for the development and applications of devices. Solid-state NMR (SS-NMR) methodology that has the ability to give atomic information on local environments within electrodes has been recently developed to study the charge storage mechanism of supercapacitors at molecular level. The charge storage mechanisms in supercapacitors with organic or ionic liquid electrolytes have been studied by SS-NMR. However, there is until now no research on supercapacitors with redox-active electrolyte published. Therefore, the study of charge storage mechanism in supercapacitors with redox-active electrolyte are highly required. In this context, we employed SS-NMR techniques combined with electrochemical dilatometry measurements that are associated with charge induced strain of electrode to investigate in depth the charge storage during charging process in supercapacitors with redox-active ionic liquid electrolyte. It is revealed that the charging process of supercapacitors with redox-active ionic liquid electrolyte EMIM FcNTf/ACN is driven by different charge regimes for different voltages, that is, co-ion desorption at low voltage range and subsequently counter-ion adsorption at higher voltage range. The electrochemical dilatometry measurements show macroscopic change of the electrode during charging and further confirm the proposed mechanism obtained from SS-NMR. The results give a detailed picture of the charge storage mechanism of supercapacitors with redox ionic liquid electrolyte, providing new insights on the charge storage of supercapacitors.