Improved coulombic efficiency of single-flow, multiphase flow batteries via the use of strong-binding complexing agents

Abstract

To support the energy transition, an inexpensive grid-scale energy storage device is needed to counteract the intermittency of renewable energy sources. Redox flow batteries (RFBs) offer the potential provide such storage, however, high capital costs have hampered market penetration. To reduce costs, single-flow configurations have been explored to eliminate expensive battery components and minimize balance of plant systems. Here, we report on a membraneless single-flow zinc–bromine battery leveraging a unique multiphase electrolyte. The use of such electrolyte emulsions, containing a bromine-poor aqueous phase and bromine-rich polybromide phase, have allowed for effective reactant separation in single-flow architectures, although at the cost of low cycling coulombic efficiency (CE). In this study, we show that significant improvements in CEs are possible when using strong-binding bromine complexing agents (BCAs) to form the polybromide phase. We compare battery performance when using widespread but relatively weak-binding BCA N-ethyl-N-methylpyrrolidinium bromide (MEP) or novel, stronger-binding 1-butyl-3-methylpyridinium bromide (3-MBPy). We characterize for the first time the ex situ viscosity, ionic conductivity and aqueous phase bromine concentration for such emulsive electrolytes, towards building a library of emulsive electrolyte properties. We show that the use of 3-MBPy significantly reduced zinc corrosion during cycling due to a reduced aqueous phase bromine concentration, enabling an up to 23% increase in CE when cycling at 30 mA cm⁻².