Three Birds with One Stone: Design of Hydrophobic Bromine Capture Agents and Their Integration into Porous Carbon as an Optimal Solution for Flowless Zn-Br2 Batteries

Zinc-bromine batteries (ZBBs) hold great potential for large-scale energy storage due to their high energy density, sustainability, and cost-effectiveness. However, the practical application of flowless ZBBs is hindered by self-discharge (SD) from uncontrolled bromine diffusion and the overlap of the Br⁻/Br2 redox potential with the oxygen evolution reaction (OER). Additionally, the limited solubility of bromine complexing agents (BCAs) in aqueous media poses a significant challenge. Here, we introduce a targeted localized presence (TLP) strategy, encapsulating hydrophobic BCAs within porous activated carbon electrodes to address these limitations. By examining three BCA structures, we demonstrate that TLP effectively reduces SD and increases coulombic efficiency. We show that the formation of hydrophobic phases within the pores can be controlled by manipulating the BCA alkyl chain length. This tailored TLP approach minimizes OER susceptibility and extends the voltage window to 2.7V (0.1M ZnBr2). Nuclear magnetic resonance analysis highlights the aggregation behavior of BCAs, elucidating their role in stabilizing the system. Remarkably, insoluble BCAs with hexyl side chains achieved >98% CE at 200 mAh/g over 1000 cycles at 1 A/g. This work presents a robust pathway for advancing aqueous zinc-halide batteries towards scalable and durable energy storage solutions.