A 1.51 V pH neutral redox flow battery towards scalable energy storage

Abstract

Aqueous redox flow batteries using low-cost organic and inorganic active materials have received growing interest for sustainable energy storage. In this study, a low-cost, high redox potential (1.08 V vs. NHE) and high capacity ammonium bromide (NH₄Br, 214.4 A h L⁻¹) catholyte was coupled with an organic viologen anolyte to demonstrate 1.51 V high voltage (SPr)₂V/Br⁻ aqueous redox flow batteries under pH neutral conditions for the first time. Benefitting from the high water solubility of both the NH₄Br catholyte and (SPr)₂V anolyte, the newly designed (SPr)₂V/Br⁻ organic flow battery was operated at up to 1.5 M and an energy density of up to 30.4 W h L⁻¹. Using multiwall carbon nanotubes as an electrochemical additive for the Br₃⁻/Br⁻ redox couple, the highly energy dense (SPr)₂V/Br⁻ flow battery manifested outstanding current performance, up to 78% energy efficiency at 40 mA cm⁻² current density and 227 mW cm⁻² power density, the highest power density known for pH neutral organic flow batteries.