Planar carbenium ions for robust symmetrical all organic redox flow batteries

Abstract

Grid-scale energy storage can benefit from the potential of non-aqueous full organic redox flow batteries (NAORFBs). However, the majority of current NAORFBs rely on the utilization of distinct anolytes and catholytes, separated by a membrane or porous separator. Unfortunately, this setup can lead to crossover of redox active material from one side of the battery to the other, resulting in electrolyte mixing and irreversible fading in energy density and capacity. An attractive solution to tackle this crossover problem is the adoption of symmetric flow batteries, wherein a single bipolar molecule serves as both an anolyte and a catholyte. Herein, we report the use of of a diazatriangulenium ion, a heterocyclic fused carbenium ion, as a bipolar redox active material in such symmetric flow batteries. This redox active molecule exhibits promising characteristics, including a straightforward synthesis, high tunability, non-toxicity, and availability. Evaluation of this molecule through 3-electrode cell testing reveals excellent electrokinetic parameters suitable for NAORFB deployment. The performance is further demonstrated in a prototype of a fully organic symmetrical redox flow battery, exhibiting an large E_gap of 2.36 V, energy density exceeding 6 W h L⁻¹, and 99.93% capacity retention over 300 cycles, despite a moderate energetic efficiency.