Improved voltage and solubility in hybrid non-aqueous redox flow batteries using a molecular 3,4-ethylenedioxythiophene (EDOT) derivative with a stable radical cation state

Abstract

Growing concern about climate change has spurred the rapid development of alternative energy sources; however, the lack of infrastructure required to store and distribute energy from such intermittent sources hinders their large-scale implementation. Redox flow batteries (RFBs) offer an excellent solution to this problem; however, the development of active materials with higher energy densities and stabilities is necessary. In this contribution, we explore the use of the 3,4-ethylenedioxythiophene (EDOT) moiety, an imminently useful p-type moiety in the context of organic semiconductors, as a radical cation carrying redox active molecule in RFBs. The 2,5-bis-t-butyl derivative of EDOT, t-Bu₂EDOT, was synthesized and characterized; analysis by quantum chemical calculations, cyclic voltammetry, and spectroscopic analysis indicates that it exhibits higher oxidation potential and potentially greater stability than the current state of the art benzene-based reference material, 1,4-di-tert-butyl-2,5-bis(2-methoxyethoxy)benzene (DBBB). Functioning hybrid RFBs were constructed and higher operating voltages and charge capacity were confirmed compared to those with DBBB. The molar solubility was measured to be 183% for DBBB, while the operating voltage was 150 mV higher, indicating potential for 90% greater energy density using t-Bu₂EDOT, compared to DBBB. The theoretical energy density of t-Bu₂EDOT (41 905 mA h L⁻¹) based on these values is much higher than that of DBBB (21 967 mA h L⁻¹).