Abstract
Bromine complexing agents (BCA) are used to improve the safety of aqueous bromine electrolytes versus bromine outgassing in bromine electrolytes. In this work, cycling performance of hydrogen-bromine redox flow battery cells with 1-ethylpyridin-1-ium bromide ([C2Py]Br) as BCA in a bromine electrolyte with a theoretical capacity of 179.6 A h L−1 is investigated for the first time. The BCA leads to increased ohmic overvoltages. One cause of the ohmic drop can be attributed to [C2Py]+ cation interaction with the perfluorosulfonic acid (PFSA) membrane, which results in a drop of its conductivity. The BCA also interacts with bromine in the cell, by forming a non-aqueous fused salt second phase which exhibits a ten times lower conductivity compared to the aqueous electrolyte. A steep rise in cell voltage at the beginning of the charge curve followed by a regeneration of the cell voltage is attributed to this effect. Electrolyte crossover leads to an accumulation of [C2Py]+ in the electrolyte solution and intensifies both adverse processes. Under this condition only 30% of the theoretical electrolyte capacity of 179.6 A h L−1 is available under long term cycle conditions. However, electrolyte capacity is high enough to compete with other flow battery technologies.
Highlights
One opportunity for stationary energy storage are hydrogen-bromine redox flow batteries (H2/Br2-redox flow battery (RFB)) [1,2,3,4,5], which are currently on the cusp of commercialization [6,7]
In this study we investigate the influence of the bromine complexing agent [C2Py]Br on the cell performance of a H2/Br2-RFB single cell
At state of charge (SoC) 33% respectively a ratio of 1:1 between Br2 and Bromine complexing agents (BCA) is received, expecting all Br2 to be available as tribomide Br3− . [C2Py]Br is prepared from pyridine and bromoethane by alkylation [43,44] according to Dzyuba et al [45] and dried in a vacuum chamber. 1H NMR and 13C NMR are recorded for [C2Py]Br and confirmed by existing literature [44]. [C2Py]Br is solved in water and hydrobromic acid (HBr) 48 wt% and after Br2 is added
Summary
One opportunity for stationary energy storage are hydrogen-bromine redox flow batteries (H2/Br2-RFB) [1,2,3,4,5], which are currently on the cusp of commercialization [6,7]. H2/Br2-RFBs use a hydrogen gas diffusion electrode like in polymer electrolyte membrane fuel cells and a bromine/bromide half cell, which is operated with liquid electrolytes. Electrolytes are composed of hydrobromic acid (HBr), bromine (Br2) and water and are pumped through the positive H2/Br2-RFB half cell. Bromide (Br− ) is converted to Br2 by oxidation in the positive half cell. Discharging of the battery takes place in the same conversion unit, while the reactions proceed in opposite direction [3,4] according to equation Eq (1) [4,11]. The open cell voltage at standard conditions is 1.09 V [8,9,10]
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