Extending the Redox Potentials of Metal-Free Anolytes: Towards High Energy Density Redox Flow Batteries

Abstract

Non-aqueous organic redox flow batteries (NORFBs) have emerged as a promising technology for renewable energy storage and conversion. High capacity and power density can be achieved by virtue of high solubility and high operating voltage of the organic anolytes and catholytes in organic media. However, the lack of anolyte materials with high redox potentials and their poor electrochemical stability retard the wider application of NORFBs. Here, we investigated an evolutionary design of a set of bipyridines and their analogues as anolytes and examined their performance in full flow batteries. Using combined techniques of repeated voltammetry, lower scan rate cyclic voltammetry, proton nuclear magnetic resonance, and density functional theory calculations, we could rapidly evaluate the redox potential, stability, and reversibility of these redox candidates. The promising candidates, 4-pyridylpyridinium bis(trifluoromethanesulfonyl)imide (monoMebiPy) and 4,4′-bipyridine (4,4′-biPy), were subjected to battery cycling. Extended studies of the post-cycling electrolytes were conducted to analyze the pathway of capacity fading and revealed a reduction-promoted methyl group shift mechanism for monoMebiPy. A family of easily accessible anolyte molecules with high redox stability and redox potentials was discovered that can be applied in NORFBs.