# Boron‐Formazanate Complexes as Tunable Redox‐Active Materials for Non‐Aqueous Redox Flow Batteries

## TL;DR

Boron-formazanate complexes are explored as tunable and stable materials for non-aqueous redox flow batteries, offering a promising alternative to traditional metal-based systems.

## Contribution

The study introduces boron-formazanate complexes as a novel class of redox-active materials with tunable properties for energy storage.

## Key findings

- Boron-formazanate complexes showed stable electrochemical performance with minimal capacity fade in initial reduction processes.
- Degradation was observed when involving two-electron reduction, likely due to B─F bond cleavage and fluoride elimination.
- A diphenyl boron derivative was developed to mitigate degradation, achieving over 85% capacity retention after 15 days of cycling.

## Abstract

As a broad‐scale energy storage solution, redox flow batteries (RFBs) offer high efficiency and tunable design. However, conventional RFBs rely on transition‐metal ion couples, (e.g., vanadium or iron), whose implementation is limited by low energy densities, high cost, and environmental leaching. Main‐group compounds, comprising earth‐abundant, p‐block elements, represent highly promising, yet underexplored candidates for RFBs. Herein, we evaluate three boron‐formazanate complexes as negolyte and symmetric electrolytes in nonaqueous organic redox flow batteries (NAORFBs). Detailed electrochemical characterization of these complexes reveals two sequential reduction processes with the first being exceptionally stable (<3% capacity fade after charge/discharge cycling in a static H‐cell for 3 days). In contrast, cycling that includes the two‐electron reduced state results in rapid degradation (>59% capacity fade over 2.5 days in a static H‐cell), most likely due to fluoride elimination from the BF2 moiety. Guided by these insights, a B(Ph)2 unit was introduced to mitigate this degradation pathway. The elimination of labile B─F bonds as well as steric protection conferred by two phenyl groups led to improved cycling performance (>85% capacity retention after charge/discharge cycling in a flow battery for 15 days). These findings guide the rational design of inexpensive main‐group electrolytes for application in energy storage.

Boron‐formazanates were investigated as a chemically tunable and electrochemically robust class of redox‐active materials for nonaqueous flow batteries. Electrochemical characterization, coupled with postmortem analysis, uncovers B─F bond cleavage as a key degradation pathway contributing to capacity fade. Through rational design, a diphenyl boron derivative was developed that exhibited superior cycling stability in a symmetrical flow cell.