Title

Investigation of transport processes and electrode kinetics in a zinc bromine battery

Date of Completion

January 1991

Keywords

Engineering, Chemical|Energy

Degree

Ph.D.

Abstract

Zinc bromine batteries are being developed as large rechargeable batteries for load leveling and automobile propulsion. Mathematical modeling is used to analyze the designs and predict the battery performance of the several designs being developed. A mathematical model for the flow-through porous electrode design of the flow cell was developed to analyze the flow cell design, identify design parameters and determine the interaction of the underlying physical phenomena. The flow cell model, based on material and charge balances, predicted the flow cell performance in terms of its energy efficiency, cell current and voltage. The model showed the concentration and potential distributions within the flow cell which were then used to identify the rate-limiting phenomena and evaluate the performance of individual components in the flow cell.^ In flow cell model was extended to analyze the entire zinc bromine battery system by taking into account the flow cell, the electrolyte storage tanks and the polybromide "red oil phase". The effect of the polybromide "red oil phase" on the battery performance was presented. It was shown that an optimum ratio of the volume of the "red oil phase" to the volume of the aqueous electrolyte exists and that it can be determined from the battery model.^ The flow cell model was compared with models for other flow cell designs. A qualitative comparison based on the bromine concentration was made. Changes in the present flow-through porous electrode design are suggested based this comparison.^ A current distribution model for the flow cell was used to assess the error introduced in the flow cell model by the linearization of the axial concentration gradients. It was shown that the flow cell model was accurate with an error of less than 5% and presented a substantial savings in the computation time.^ Rotating hemispherical electrode experiments into zinc electrodeposition and dissolution were undertaken to determine exchange current density and Tafel slopes. A laboratory scale flow battery was built to simulate the battery performance at the initial state of charge. The flow system experiments were used to observe ex situ the zinc deposition. A good agreement was found between the experimental data and the flow cell mode. The shielding effect of the plastic spacer was found to be important. ^