Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Chemical Engineering

First Advisor

John W Weidner

Abstract

Recently, flow batteries are considered to be suitable for the large scale energy storage and different prototypes (vanadium, sodium polysulfide bromide, zinc-bromide, zinc-nickel, and lead acid etc) have been developed. These devices essentially store energy in the electrolyte externally and utilize redox electrode reactions for energy conversion. Therefore, highly reversible electrode reactions with good mass transfer are desired for improving its power density and efficiency. H2/H+ and Br2/Br-redox couples are known to own good reversibility, while discharge of Br2/Br- electrode are limited by mass transfer in liquid phase, a gas-phase Br2 H2 flow battery is investigated in this work. Equivalently important, in this flow battery, co-production of bromine and hydrogen, two important fundamental chemicals in the hydrogen economy and energy industry, is realized in the charge process.

This vapor-phase Br2 H2 flow battery is constructed and tested with respect to open circuit potential (OCV) and V-I polarization in both charge and discharge modes as function of bromine content. Operation of this flow battery is then analyzed via a mathematical model. A key feature of this model is water transport across the membrane, which determines membrane conductivity, reactant concentration and undesired condensation. The model predicts the operating conditions of the cell in both fuel-cell (i.e., charge) and electrolysis (i.e., discharge) mode as a function of inlet gas composition and pressure differential across the membrane. The analysis reveals that gas-phase Br2/HBr reactants significantly enhance mass transfer, which enables higher currents densities to be achieved compared to a liquid-fed system. The model is used to provide insight into cell operation, and predict conditions and battery performance where water condensation is avoided. To lower down the cost and improve the stability of the electrode materials, non Pt materials are developed. RuO2, carbon (Vulcan XC 72 R) and TiO2-Nb (10% wt.) are investigated for HBr electrochemical oxidation. IrO2/C and MoS2 are synthesized and characterized for hydrogen evolution reaction. They are prepared into membrane electrode assembly (MEAs) and evaluated in the HBr electrolyzer (charge mode).

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