Simulation and experimental studies on a rechargeable hydrogen-bromine fuel cell

Rechargeable hydrogen-bromine fuel cells have high energy densities, and are excellent candidates for space power applications. The hydrogen electrode is composed of platinum particles that are bonded to a proton exchange membrane, typically Nafion. A flow-by porous bromine electrode is used in this system to enhance mass transfer rates. The research reported here has been directed at: (1) the development of a theoretical model capable of predicting overall cell performance for further system development and scale-up, (2) the development of a theoretical model capable of predicting the performance of the hydrogen membrane electrode assembly, and (3) experimental measurements to determine kinetic and transport properties of a Nafion coated electrode.; A mathematical model was developed to evaluate design and scale-up parameters on the hydrogen-bromine fuel cell. The model coupled porous electrode theory, applied to the bromine electrode, with membrane transport and planar treatment of the hydrogen electrode. Simulations performed on the model indicated that transport properties of the membrane were the most influential factors in controlling cell performance.; Therefore, a model was developed to examine the transport properties of the proton exchange membrane more closely. The distribution of the hydrogen electrode catalyst particles was also included in the model. The model predicted that the membrane electrode assembly performance was optimized when the volume ratio of polymer to catalyst was one in the catalyst zone. Lower ratios led to diffusion limitations and higher ratios led to kinetic limitations.; Electrochemical impedance spectroscopy measurements were performed on bare and Nafion coated platinum electrodes. Two parallel diffusion paths were observed in the Nafion film in hydrogen saturated phosphoric acid. Analysis of the transport information obtained from impedance scans in both hydrogen saturated phosphoric acid and hydrochloric acid solutions indicates that the concentrations of hydrogen in recast Nafion films is six to seven times greater than in the surrounding solutions. The oxygen concentration in the Nafion was also greater than in the surrounding oxygen saturated phosphoric acid solution.