A neutral pH all-polymer redox flow battery employing organic and organometallic redox active polymers

Abstract

Aqueous all-polymer redox flow batteries (APRFBs) working with size exclusion membranes are safe, low-cost, scalable solutions for energy storage applications. However, their development is still limited owing to challenges in optimizing the redox potential and solution viscosity of the polymers to deliver optimal energy density. Herein, we demonstrate an APRFB composed of ferrocene and viologen-based polymers with tunable redox potential. The theoretical calculations and electrochemical measurements reveal the contribution of HOMO energy change and polyelectrolyte dynamics in the charge transfer process with an increase in the molecular weight of the Catholyte-P. The electrochemical experiments yielded reasonable activity of the Catholyte-P24 and Anolyte-P12 with an oxidation rate constant of 1.26 × 10−4 cm s−1 and a reduction rate constant of 5.38 × 10−3 cm s−1, respectively. The application of synthesized polymers with size exclusion membrane yielded a 0.85 V APRFB with energy efficiency up to 72%, capacity retention of 99.93% per cycle at 10 mA cm−2, demonstrated energy density of 2.6 W h L−1, and capacity retention of >82% over continuous 250 cycles.