High energy density electrolytes for H₂/Br₂redox flow batteries, their polybromide composition and influence on battery cycling limits

Abstract

Polybromides formation in aqueous bromine electrolytes and influence on H2/Br2 redox flow battery performance is investigated the first time.

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

“…For all investigated BCAs, the storage of Br 2 in the aqueous electrolyte in the form of Br 5 − predominates, followed by storage in Br 3 − and finally storage in Br 7 − . This is in accordance with earlier studies on pure BCA-free HBr/Br 2 /H 2 O-electrolytes [ 65 ], leading to the result that [BCA] + has only a minor influence on the fractions of Br 2 in the different polybromide species in the aqueous solution. For SoC < 40%, the n -hexyl side chain BCAs [C6Py]Br and [C6MIm]Br have a slightly increased Br 2 content in Br 3 − , which means that less Br 2 is distributed to the Br 5 − and Br 7 − anions.…”

“…Presumably, a better solvation of the more stable Br 3 − compared to Br 5 − is present. This effect is obtained also for BCA-free HBr/Br 2 /Br − solutions [ 65 ]. Since the Br 2 concentration of c(Br 2 (aq)) < 0.35 M and the distribution of Br 2 to the polybromides is relatively constant in the whole range, no specific influence of this distribution on the performance or individual parameters is given and therefore cannot be discussed.…”

“…In general, for SoC ≥ 80% the Br 2 concentrations rise only slowly or form a plateau. In that range, the solubility of Br 2 in the aqueous solution is limited by a lack of bromide ions to form polybromides, known from BCA-free electrolytes [ 65 ].…”

“…Applying this theoretical assumption, the current densities of i = 723 mA cm −2 for c(Br 2 ) = 0.3 M, i = 482 mA cm −2 for c(Br 2 ) = 0.2 M and i = 241 mA cm −2 for c(Br 2 ) = 0.1 M would be possible in a cell with a geometrically active electrode/membrane area of 40 cm² and a volume flow rate of 30 mL min −1 . Data for this cell are described in [ 65 ]. For electrolytes with lower concentrations, the possible current densities would decrease further.…”

“…For a deeper understanding of the complexation behavior of the different BCAs, the Raman spectra of the aqueous phase have been analyzed regarding the distribution of bromine in the different polybromides. The method has been described in our earlier work [ 65 ]. The distribution of Br 2 on Br 3 − , Br 5 − and Br 7 − is shown in Figure 7 .…”