Separating Reactants in Membraneless Redox Flow Batteries: Leveraging Fluid Mechanics

Redox flow batteries are considered a highly promising electrochemical technology for grid-scale renewable energy storage. However, their commercialization has been hindered by high capital cost and balance of plant complexity. Single-flow batteries are an emerging subclass employing a membraneless cell design and simplified flow systems. Recently, single-flow zinc-bromine batteries leveraging novel multiphase electrolyte emulsions have been demonstrated. Such electrolytes are a mixture of a bromine-poor aqueous phase and bromine-rich polybromide phase, where in the latter phase the bromine is largely electrochemically inactive. However, separating the zinc and bromine reactants in the absence of a membrane is a challenging problem requiring deep understanding of multiscale flow phenomena and its coupling to battery performance.<br/><br/>In this work, we will briefly present our novel 2D analytical flow model describing the multiscale flow phenomena within such flow batteries. The model captures the concentration field of the polybromide phase, including sedimentation as this phase is denser. The model was validated with numerical simulations and experimental data. We will focus on how the predicted fluid mechanics phenomena support effective separation between the reactants, and how the model explains several counter-intuitive battery observations, such as decreasing electrolyte conductivity for decreasing flowrate. The model also points towards future battery optimizations for systems employing multiphase flow electrolytes, and thus is a key aspect towards unlocking their potential for low-cost renewable energy storage.