NOVELTY - In an electrochemical flow cell which contains an electrolyte comprising aqueous hydrogen bromide, where the cell is hydrogen/bromine cell or vanadium bromine cell, at least one of 1-alkyl-2-alkyl pyridinium halide, 1-alkyl-3-alkyl pyridinium halide or 1-alkyl-3-alkyl imidazolium halide is used as bromine-complexing agent.    USE - As bromine-complexing agent in electrolyte suitable for use in an electrochemical flow cell; for operating an electrochemical flow cell selected from hydrogen/bromine or vanadium/bromine cells; in energy storage device (claimed); and for producing electric energy.    ADVANTAGE - The presence of compounds in the electrolyte allows the formation of polybromide complexes which do not solidify under the relevant working conditions.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an electrolyte suitable for use in an electrochemical flow cell comprising aqueous hydrogen bromide and a liquid complex composed of at least one of 1-alkyl-2-alkyl-pyridinium halide, 1-alkyl-3-alkyl-pyridinium halide or l-alkyl-3-alkyl imidazolium halide combined with at least one bromine molecule;    (2) a process for operating an electrochemical flow cell selected from hydrogen/bromine or vanadium/bromine cells, involving adding to HBr-containing electrolyte solution of the cell, an additive selected from 1-alkyl-2-alkyl pyridinium halide, l-alkyl-3-alkyl pyridinium halide, and/or 1-alkyl-3-alkyl imidazolium halide; and    (3) an energy storage device comprising: hydrogen/bromine cells arranged in a stack configuration, where each cell has in it spaced apart bromine and hydrogen electrodes which are in electrical contact with system for supplying electrical current to the cell and collecting electrical current generated by the cell; a separator positioned in the space between the electrodes dividing the cell into a first and second compartments; and an aqueous hydrogen bromide electrolyte in which 1-alkyl-2-alkyl pyridinium halide, and/or 1-alkyl-3-alkyl pyridinium halide is present; hydrogen storage tank and HBr/Br2 aqueous electrolyte storage tank connected by means of at least one conduit to the cell compartments, where at least one component of the device comprises high density polyethylene (HDPE).