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.

Electrochemical Degradation of a Dicationic Rhenium Complex via Hoffman-Type Elimination

“…We also note that the described electrolytic conversions is not expected to be reliant on the Re metal center. The metal-free ligand undergoes several irreversible reductions (Figure S12; E p = −1.88, −2.04, and −2.56 V vs Fc +/0 ) in contrast to metal-free bpy or other neutral derivatives . We speculate that the lack of reversibility observed in the CVs of the tmam ligand is due to a reductive deamination similar to that observed in complex 1 .…”

A Versatile Light‐Triggered Radical‐Releasing Surface Coating Technology

“…Furthermore, redox-active organic materials have attracted increasing attention as a result of their structural diversity and the ability to tailor the materials' redox properties and electrochemical performance. [7] Materials containing organic radical groups, such as nitroxides, constitute a widely studied group of functional materials as the nitroxide can reversibly switch between highly stable redox states under ambient conditions. [8] While nitroxides have been utilized extensively as catalysts to make new molecules and materials, [9][10][11] new materials are being of water insoluble PTEMPO copolymers, [37,38] plasma polymerization, [39][40][41] and biomimetic polydopamine chemistry.…”

Too Much of a Good Thing? Assessing Performance Tradeoffs of Two-Electron Compounds for Redox Flow Batteries

“…For example, many redox-active organic molecules that undergo multi-electron transfer in aqueous electrolytes (e.g., quinones, [13][14][15] phenothiazines, 16 and phenazines 17 ) typically exhibit multiple redox reactions occurring at similar potentials due to hydrogen bonding interactions present in these environments. 18 Conversely, similar molecules used in non-aqueous electrolytes (e.g., phenothiazines, 9,10 phenazines, 19 and viologens 20,21 ), some used in aqueous electrolytes (e.g., viologens [22][23][24] ), and metal-coordination complexes containing non-innocent ligands 11,25,26 often feature sequential electron transfer events with disparate and easily discernable redox potentials.…”