The electrochemical behavior of a promising hydrogen/bromine redox flow battery is investigated for grid-scale energy-storage application with some of the best redox-flow-battery performance results to date, including a peak power of 1.4 W/cm2 and a 91% voltaic efficiency at 0.4 W/cm2 constant-power operation. The kinetics of bromine on various materials is discussed, with both rotating-disk-electrode and cell studies demonstrating that a carbon porous electrode for the bromine reaction can conduct platinumcomparable performance as long as sufficient surface area is realized. The effect of flow-cell designs and operating temperature is examined, and ohmic and mass-transfer losses are decreased by utilizing a flow-through electrode design and increasing cell temperature. Charge/discharge and discharge-rate tests also reveal that this system has highly reversible behavior and good rate capability. © 2012 The Electrochemical Society. [DOI: 10.1149/2.018211jes] All rights reserved.