The bromine/nitrobenzene redox flow battery : mechanism of the bromide/bromine reaction in nitrobenzene and characterization of supporting electrolytes
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This work presents the first redox flow battery (RFB) to use the redox active liquids bromine (Br2) and nitrobenzene (NB) as both solvents and redox species. Because the supporting electrolyte (SE) is the only solute, instead of SE and an additional salt containing a redox species, the capacity of this RFB is limited only by the solubility of the SE and not by the solubility of the redox species coexisting in solution with SE. In addition to the increased capacity, this battery has a nominal cell voltage of 2.1 V, compared to 1.5 V or less for most aqueous systems. Therefore, this new all-liquid RFB system can reach energy densities more than twice that of traditional RFBs, over 100 Wh/L with 3 M SE and potentially close to 200 Wh/L with 5 M SE. In addition to the development of a prototype Br2/NB RFB, two specific aspects of this system were studied. First, the mechanism of the Br-/Br2 redox reaction in NB was characterized for the first time. As with the majority of halide/halogen reactions in nonaqueous solvents, this reaction proceeds in two distinct steps through a stable Br3- intermediate. An intermediate during Br- oxidation was identified using scanning electrochemical microscopy (SECM), and a mechanism for Br- oxidation was proposed to explain that observation. Then a full Br-/Br2 reaction mechanism, including the elementary reaction steps and relevant parameters, was developed using simulations of cyclic voltammograms in a variety of Br-, Br2, and Br3- solutions. Second, the solubility, conductivity, and viscosity of novel SEs containing the asymmetric cation dimethyldipropylammonium (DMDP+) in NB solutions were measured. This cation has the same molecular weight as the symmetric tetraethylammonium cation (TEA+), but its salts are roughly an order of magnitude more soluble in NB, sometimes exceeding 3 M. Compared to solutions with salts containing the larger tetrabutylammonium cation (TBA+), which are equally as soluble in NB, solutions with the smaller DMDP+ salts are up to seven times less viscous at 2 M concentrations. The conductivity of these solutions increases proportionally with the decrease in viscosity. Electrolysis of NB to NB-. was performed in solutions containing TBA+ and DMDP+ salts, and an increase in viscosity, accompanied by a decrease in conductivity, was observed as the electrolysis progressed. Finally, the performance of a Br2/NB RFB was evaluated using these same solutions.