Abstract
Despite the dramatically increasing research effort dedicated to redox-active electrolytes towards improved supercapacitor capacitance, the research concerning charging mechanisms remains sparse. This is particularly true for porous materials such as porous carbon. Whereas some charging mechanisms have been discussed for microporous materials, the mechanisms occurring within mesoporous materials are mostly unexplored. Herein, a new redox mediator - Thionine (Th) - in highly-concentrated LiTFSI, was used as a supercapacitor electrolyte to explore these mechanisms. The addition of Th modifies the viscosity and thermal properties of the electrolyte without affecting the ionic conductivity. Electrochemical measurements confirmed that the presence of Th endows an additional pseudocapacitance in mesoporous carbon but not in microporous carbon. Based on these contrasting phenomena, the charging mechanism and solvated structure were studied by Molecular Dynamics (MD) and Density Functional Theory (DFT) simulation. DFT simulation disclosed that it has an ion association effect between TFSI and Th due to their opposite charge. Two MD models were built with different porosities: 0.9 nm (micropores) and 3 nm (mesopores). It was found that Th molecules cannot enter micropores whereas their insertion into the mesopores was possible. Once inside the mesopores, Th can interact with the inner carbon wall via hydrogen bonding thus stabilizing the redox molecule within the mesopore which can then provide a redox contribution to the charging process. The present work helps to build a deeper understanding of the activity of redox-active molecules in mesoporous materials by elucidating the exact adsorption and charging mechanisms within a microporous system. This paves the way for the design of advanced redox-active electrolytes for supercapacitors.