Designing Robust Two-Electron Storage Extended Bipyridinium Anolytes for pH-Neutral Aqueous Organic Redox Flow Batteries
Gonggen Tang, Yahua Liu, Yuanyuan Li, Kang Peng, Peipei Zuo, Zhengjin Yang, Tongwen Xu
Abstract
Bipyridinium derivatives represent the most extensively explored anolyte materials for pH-neutral aqueous organic redox flow batteries, and most derivatives feature two separate electron-transfer steps that cause a sharp decrease in cell voltage during discharge. Here, we propose a strategy to fulfill the concurrent two-electron electrochemical reaction by designing extended bipyridinium derivatives (exBPs) with a reduced energy difference between the lowest unoccupied molecular orbital of exBPs and the β-highest occupied molecular orbital of the singly reduced form. To demonstrate, a series of exBPs are synthesized and exhibit a single peak at redox potentials of −0.75 to −0.91 V (vs standard hydrogen electrode (SHE)), as opposed to the two peaks of most bipyridinium derivatives. Cyclic voltammetry along with diffusion-ordered spectroscopy and rotating disk electrode experiments confirm that this peak corresponds to a concurrent two-electron transfer. When examined in full-flowing cells, all exBPs demonstrate one charge/discharge plateau and two-electron storage. Continuous galvanostatic cell cycling reveals the side reactions leading to capacity fading, and we disclose the underlying mechanism by identifying the degradation products. By prohibiting the dimerization/β-elimination side reactions, we acquire a 0.5 M (1 M e–) exDMeBP/FcNCl cell with a high capacity of 22.35 Ah L–1 and a capacity retention rate of 99.95% per cycle.