Systematic Study of Quaternary Ammonium Cations for Bromine Sequestering Application in High Energy Density Electrolytes for Hydrogen Bromine Redox Flow Batteries

Abstract

Bromine complexing agents (BCAs) are used to reduce the vapor pressure of bromine in the aqueous electrolytes of bromine flow batteries. BCAs bind hazardous, volatile bromine by forming a second, heavy liquid fused salt. The properties of BCAs in a strongly acidic bromine electrolyte are largely unexplored. A total of 38 different quaternary ammonium halides are investigated ex situ regarding their properties and applicability in bromine electrolytes as BCAs. The focus is on the development of safe and performant HBr/Br2/H2O electrolytes with a theoretical capacity of 180 Ah L−1 for hydrogen bromine redox flow batteries (H2/Br2-RFB). Stable liquid fused salts, moderate bromine complexation, large conductivities and large redox potentials in the aqueous phase of the electrolytes are investigated in order to determine the most applicable BCA for this kind of electrolyte. A detailed study on the properties of BCA cations in these parameters is provided for the first time, as well as for electrolyte mixtures at different states of charge of the electrolyte. 1-ethylpyridin-1-ium bromide [C2Py]Br is selected from 38 BCAs based on its properties as a BCA that should be focused on for application in electrolytes for H2/Br2-RFB in the future.

Properties of Bromine Fused Salts Based on Quaternary Ammonium Molecules and Their Relevance for Use in a Hydrogen Bromine Redox Flow Battery

“…Bromine‐complexing additives (BCA) are salts of organic cations utilised to bind volatile bromine (Br 2 ) in aqueous electrolytes of zinc/bromine redox flow batteries (Zn/Br 2 ‐RFB) and hydrogen/bromine redox flow batteries (H 2 /Br 2 ‐RFB). [ 1 , 2 , 3 , 4 , 5 , 6 ] In these batteries, the positive half cells are operated with bromide‐containing electrolytes, and the reactions are based on the redox couple bromine/bromide. [ 7 , 8 ] Electrolytes consist of zinc bromide, supporting electrolytes or HBr and Br 2 in aqueous solutions and the added BCAs.…”

“… [1] By the addition of [BCA] + cations, these polybromides are extracted into a fused salt phase which operates as a bromine and energy storage reservoir. [6] While bromide salts of many [BCA]Br are highly soluble in aqueous solutions, [2] there is poor solubility of the BCA‐polybromide form in the aqueous phase. [ 2 , 16 ] The solubility equilibrium is shown in Equation (1).…”

“… [6] While bromide salts of many [BCA]Br are highly soluble in aqueous solutions, [2] there is poor solubility of the BCA‐polybromide form in the aqueous phase. [ 2 , 16 ] The solubility equilibrium is shown in Equation (1). The fused salt precipitates to form an additional, heavy and liquid electrolyte phase.…”

“…The fused salt precipitates to form an additional, heavy and liquid electrolyte phase. [ 1 , 2 , 4 , 10 , 17 ] …”

“…Mainly investigated BCAs are from the groups of 1‐alkylpyridinium halides, 1‐alkyl‐3‐methylimidazolium halides, 1‐alkyl‐1‐methylpyrrolidinium halides, 1‐alkyl‐1‐methylmorpholinium halides or aliphatic ammonium compounds with ethyl, n ‐butyl or n ‐hexyl side chain groups. [ 1 , 2 , 4 , 5 , 10 , 11 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ] In the past BCAs have been mainly applied and investigated in Zn/Br 2 ‐RFB electrolytes. [ 4 , 12 , 18 , 20 , 21 , 23 , 25 , 26 ]…”