New Insights into Zinc-Bromine Flow Battery Design, Performance and Operation through an Improved Flow Battery Test-System

Abstract

The growing importance of renewable energy sources for our energy supply bring additional challenges for the electricity grid and require implementation of energy storage solutions. Zinc-bromine flow batteries (ZBFB) appear attractive for this purpose due to good scalability, high cycle efficiencies and comparatively low costs. While ZBFB systems are commercially available, many processes within the batteries are not sufficiently understood. Further improvements in terms of efficiency, durability and cost are possible, but require testing under realistic and reproducible test conditions. This PhD project set out to achieve such test conditions for ZBFB systems through the design, construction and application of a bench-scale and modular flow battery test system (FBTS). Primary focus was the implementation of comprehensive electrolyte temperature and flow control into the FBTS. The project involved design, manufacture and implementation of electrochemical cells, pumps, thermostats, sensors and system integration via a self-developed central process control program. The FBTS was validated with baseline cycle experiments. It was confirmed that electrolyte temperature and flow had strong influence on cycle performance and both must be thoroughly controlled to achieve reproducible results. Subsequently the FBTS was used to test the influence of flow profiles on battery performance. Three different flow configurations were tested at different flow velocities and significant differences in zinc deposition morphology and cycle performance were found. In another set of experiments, it was found that the type of BSA had significant influence on battery performance. BSA with weak bromine bonding insufficiently prevented self-discharge of the cell, while BSA with too strong bromine bonding caused cell polarization. Different BSA also produced significant variation in zinc morphologies ranging from smooth and homogenous to rough deposits with ramified growth in inlet areas.