The influence of current density, rest time and electrolyte composition on the self-discharge of organic radical polymers

Abstract

In this work we report a systematic study about the influence of the charging current density and the rest time on the self-discharge of poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) based electrodes in two model electrolytes, namely 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI, IL) and 1 M Pyr14TFSI in propylene carbonate (PC). We show that for each electrolyte it is possible to identify a threshold of current density (and thus C-rate) which can be safely applied without increasing the electrode self-discharge. Furthermore, we show that the self-discharge of the PTMA-based electrodes is originated by a combination of activation controlled faradic processes, charge redistribution and diffusion limited processes. The use of current densities above the threshold is causing an improper charging process, which leads to faradic reactions on the surface of the eletrode followed by the redistribution of charge on the surface of the electrodes and causes a fast energy loss when a current is no longer applied. When current densities below the threshold are used, the diffusion of redox shuttles becomes the driving force of self-discharge taking place in PTMA based electrodes.