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/cm 2 and a 91% voltaic efficiency at 0.4 W/cm 2 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. The environmental concerns and limited resources of fossil fuels have stimulated research for renewable energy sources such as wind and solar energy. Globally, there is 94 GW of electricity-generating wind power as of 2007, and it is estimated to reach 474 GW by 2020.
1The electricity from solar photovoltaics is growing at 40% per year worldwide, and the United States has targeted 100 GW of solar power by 2020.1 However, the electricity from these and other renewable resources is not constant and reliable due to their sensitive response to local weather conditions. To level out the variable generation of energy, large-scale electrical-energy storage (EES) is required. For the energy storage and load leveling, redox-flow batteries (RFB) have been considered as promising candidates due to their independently controllable power and energy, rapid response time, and high energy efficiency. Extensive research has been performed on RFB systems, including iron-chromium, all-vanadium, sodium-polysulfide, etc.
2-5However, due to the challenging issues such as low cell performance, power density, durability, and high electrolyte cost, their wide-spread adoption has not been realized. For example, the all-vanadium system, which is considered one of the closest to commercialization, utilizes a relatively expensive reactant and achieves power densities that are on the order of 0.2 to 0.7 W/cm 2 with relatively low efficiency. A hydrogen/bromine system is proposed as the reactants are earth-abundant and inexpensive and, as will be shown, high performance with high efficiency is obtainable.Yeo and Chin first investigated the hydrogen/bromine flow battery and reported excellent electric-to-electric efficiency, introducing it as a promising RFB system for energy-storage applications.6 The operating principle of the H 2 /Br 2 RFB can be described with a typical cell structure as in Figure 1. During discharge, a solution of Br 2 in HBr (aq) is fed into the cathode compartment where bromine reacts with protons supplied from the anode side and is reduced to bromide, generating the theoretical electric potential of 1.098 V at 25• C. The redu...