Characteristics of the Charge-Discharge Cycle of a Hydrogen-Bromine Battery with an IrO2-TiO2 Cathode on a Titanium Felt in the Full Capacity Utilization Mode

The charging/discharging cyclic process in a hydrogen-bromine battery is studied. Porous titanium felt with IrO2-TiO2-mixed-oxide coating in contact with aqueous HBr/Br2 solution is used as positive electrode (the cathode). A hydrogen gas-diffusion electrode with Pt-C catalytic layer served as negative electrode. The hydrogen ion is transferred between the electrodes through a GP-IEM 103 perfluorinated sulfocation-exchange membrane. The morphology, phase, and chemical composition of the cathode material are characterized using scanning electron microscopy with X-ray spectral microanalysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. The condition for switching between the charging and discharging stages within each cycle (the voltage upper limit) is so chosen as to minimize the amount of bromide and polybromide anions relative to the molecular bromine formed at the end of the charging stage (oxidation of Br-), instead of the traditionally used approach which includes only partial conversion of bromide to bromine, in order to increase the stability of the latter in the form of polybromide complexes. Charge-diacharge tests of the hydrogen-bromine battery are carried out in the galvanostatic mode at three current densities: 25, 50, and 75 mA/cm2. Comparison of the charge and average voltage values in the course of the electrical energy generation (the discharge stage) and storage (the charge stage) shows that the highest efficiency of the cycle is achieved at the current density of 50 mA/cm2. This value of the charging/discharging current density also corresponds to the maximal utilization of the electrolyte redox-capacity. The stability of the mixed-oxide cathode material used in contact with bromine compounds in acidic environment is found to exceed significantly that of the carbon paper. The principal reason of the decrease of the battery capacity from cycle to cycle is the molecular bromine absorption by elements of the system contacting the catholyte: components of the membrane-electrode assembly, pipelines, and elements of the pump that ensures the circulation.