High energy density electrolytes for H2/Br2 redox flow batteries, their polybromide composition and influence on battery cycling limits

Hydrogen-bromine redox flow batteries (H-2/Br-2-RFB) are a promising stationary energy storage solution, offering energy storage densities up to 200 W h L-1. In this study, high energy density electrolytes of concentrated hydrobromic acid of up to 7.7 M are investigated. Particular polybromide ion (Br-2n+1(-); n = 1-3) concentrations in the electrolyte at different states of charge, their effect on the electrolytic conductivity and cell operation limits are investigated for the first time. The concentrations of individual polybromides in the electrolytes are determined by Raman spectroscopy. Tribromide (Br-3(-)) and pentabromide (Br-5(-)) are predominantly present in equal concentrations over the entire concentration range. Besides Br-3(-) and Br-5(-), heptabromide (Br-7(-)) exists in the electrolyte solution at higher bromine concentrations. It is shown that polybromide equilibria and their constants of Br-3(-) and Br-5(-) from literature are not applicable for highly concentrated solutions. The conductivity of the electrolytes depends primarily on the high proton concentration. The presence of higher polybromides leads to lower conductivities. The solubility of bromine increases disproportionately with increasing bromide concentration, since higher polybromides such as Br-7(-) or Br-5(-) are preferably formed with increasing bromide concentration. Cycling experiments on electrolyte in a single cell are performed and combined with limitations due to electrolyte conductivity and bromine solubility. Based on these results concentrations of the electrolyte are defined for potential operation in a H-2/Br-2-RFB in the range 1.0 M < c(HBr) < 7.7 M and c(Br-2) < 3.35 M, leading to a theoretical energy density of 196 W h L-1.