Electrolysis of HBr using molten, alkali-bromide electrolytes

Abstract

The electrolytic oxidation of HBr was investigated using molten alkali-bromide salts in a porous scaffold of yttria-stabilized zirconia (YSZ), with porous Pt electrodes. Despite a relatively thick (1.0 mm) and dense (35% porous) YSZ scaffold, a total cell impedance of less than 3 Ωcm 2 was achieved with NaBr at 1033 K. The open-circuit potentials also agreed with theoretical potentials for the reaction H 2 + Br 2 = 2HBr. Lower operating temperatures were made possible by using a eutectic mixture of alkali bromides, (Li 0.56K 0.19Cs 0.25)Br. The electrolyte losses for the (Li 0.56K 0.19Cs 0.25)Br electrolyte, determined from the ohmic component of the impedance spectra, were less than 1.5 Ωcm 2 at 673 and 773 K, similar to the value found for NaBr at 1033 K, indicating that the ionic conductivities of the alkali bromides are high so long as the salts are molten. However, the electrode losses were dependent on temperature, decreasing from 18.1 Ωcm 2 to 3.0 Ωcm 2 in going from 673 to 773 K. Implications of these results for recycling HBr in hydrocarbon bromination reactions are discussed.