A high-capacity viologen-based anolyte for high energy density neutral pH aqueous redox-flow batteries

Redox-flow batteries (RFBs) are a promising energy storage technology with remarkable scalability and safety for storing vast amounts of renewable energy and mitigating output fluctuations of renewable power grids. We demonstrate a neutral pH aqueous RFB using a custom-designed 1',1',1'-(benzene1,3,5-triyltris(methylene))tris(1-(3-(trimethyl ammonio) propyl)-[4,4'''-bipyridine]-1,1'-diium) nonachloride (BTTMPB) as a 3 e- storage anolyte. The custom design with the high polarization in charge density has led to the excellent water solubility of 4.0 M in H2O (321.6 A h L-1) and 2.4 M in 2.0 M NaCl (192.9 A h L-1). The density functional theory (DFT) calculations and electrochemical experiments have shown 3 e- storage response of BTTMPB with a diffusion coefficient of 3.1 x 10-6 cm2 s-1 and rate constant of 1.6 x 10-2 cm s-1 for the first reduction process. The synthesized anolyte was paired with (Ferrocenylmethyl)trimethylammonium chloride (FcNCl) as catholyte enabling a 0.92 V aqueous RFB with 125.9 W h L-1 theoretical energy density. The aqueous RFB has an excellent cycling performance from 10-30 mA cm-2, energy efficiency up to 80%, capacity retention of 99.96% per cycle at 20 mA cm-2, and a high demonstrated energy density of 29.1 W h L-1. (c) 2022 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Automated summary

This article presents a neutral pH aqueous redox-flow battery (RFB) using a custom-designed BTTMPB as a 3 e- storage anolyte. The battery exhibits excellent water solubility and cycling performance, making it suitable for storing large amounts of renewable energy and mitigating output fluctuations in renewable power grids.