Tailored redox functionality of small organics for pseudocapacitive electrodes

Abstract

Recently, there has been an explosion of literature dedicated to organic electrodes for energy storage applications. While inorganic materials, especially oxides, have generally being explored for these applications, the guiding principles for successful electrical energy storage—maximizing capacity and energy density per unit mass and cost—have naturally led to the pursuit of organic materials. However, there has only been a modest focus on methods for systematic exploration, which could help establish rational design principles for their enhanced properties and performance. Here we focus on a specific class of pseudocapacitive cathodes based on conducting polymers with pendant redox sites. We have previously demonstrated that the addition of such a pendant charge storage component provides a significant increase in the capacity, in addition to well-defined voltage plateaus, all the while maintaining the superior rate capability of these materials. In this report, we present a computational screening study for downselection of pendant candidates and a systematic study of structure–electrochemical property relationships. From this study, a generalized approach for defining the formal potential of oxidation of “violene” organic pendants is presented. Surprisingly, many of the structural parameters with which the oxidations can be tuned are independently addressable. While the methods described here have only been applied to the violene system, it should be emphasized that similar formalisms can be applied to other systems where the ability to rationally tune redox active components is desirable. Notable examples include organic energy storage electrodes based on oxygen and sulfur redox couples, charge shuttles for Li-ion batteries, organic photovoltaics, synthetic metals and organic light-emitting diodes.