Novel Concepts for Hydrogen-Bromine Batteries

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A regenerable hydrogen-bromine battery is a high energy density system with potential applications in space projects, storage systems for solar electric power plants, and in transportation uses. The present hydrogen/bromine fuel cell is characterized by the presence of hydrogen on one side and HBr/Br2 on the other side of a solid Nafion and membrane. Nafion imposes certain constraints on the system which limit it's usability. Due to the water content required for Nafion (to maintain its ionic conductivity) as well as other engineering considerations, a 48 percent HBr solution is used as a fully charged cathode fluid. This requires a large amount of water which, although inert, adds considerable battery weight and volume. Second, the cycling of this aqueous liquid reactant concentration and therefore the Nernst potential varies continuously. Third, bromine and bromide ions migrate to the hydrogen side resulting in the poisoning of the electrode. The specific goal of this Small Business Innovation Research (SBIR) Phase I grant is to develop a high efficiency rechargeable hydrogen-bromine power source. A novel means to utilize bromine and hydrogen bromide separately on opposite sides of an anion exchange membrane as single phase reactants in the gaseous state will be explored in this project. It is expected that this will alleviate the changing cell voltage with the state-of-charge and reduce the water needed as well as offer the potential for other performance improvements. A central factor is finding a suitable anion exchange membrane which will support bromide ion transfer with high efficiency. The membrane needs to perform in a water free environment or with reduced water concentration and still maintain a low ohmic resistance. Based on preliminary work, ruthenium oxide catalyzed carbon as the positive electrode and platinum or palladium catalyzed carbon as the negative electrode will be tested. Successful completion of this project will yield a rechargeable hydrogen/bromine battery with relatively constant Nernst potential and a minimum water requirement. This will lead not only to higher efficiency and light weight, but also to improved system simplicity and reliability as well as presenting the potential for further performance improvements.