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'''''-(benzene-1,3,5-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 × 10−6 cm2 s−1 and rate constant of 1.6 × 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.