|M.Sc Student||Ragonis Danielle|
|Subject||Characterizing Zinc Deposition onto Flowable Electrodes|
of Zinc-Bromine Flow Batteries
|Department||Department of Chemical Engineering||Supervisor||Assistant Professor Matthew E. Suss|
|Full Thesis text|
In order to reduce our dependence on fossil fuels, there is currently significant growth in the use of renewable energy sources. Sources such as solar and wind energy are intermittent and thus plants based on such sources cannot supply constant power outputs, so that grid-scale energy storage systems are required to ensure level output power. Redox flow batteries (RFBs) are promising towards the application of grid-scale storage of renewable energy. In RFBs, the energy stored and power delivered can be decoupled spatially, allowing for highly scalable and potentially inexpensive, and geographically flexible systems. Charging RFBs converts electrical energy to chemical energy, and the chemical energy is stored in tanks until discharge of the battery, where the chemical energy is converted back to electricity with a round-trip energy efficiency which can exceed 80%. Zinc-bromine hybrid RFBs are attractive due to low-cost reactant materials, high specific energy density, and rapid reaction kinetics without precious metal catalysts. The main drawback of zinc-bromine RFBs is the growth of zinc dendrites during cell charging, which requires that the cell be deep discharged every few cycles to prevent dendritic short circuiting.
While dendrites are readily formed on static electrodes, one way to mitigate their growth is through use of flowable electrodes. Flowable electrodes are suspensions of solid conductive particles within liquid electrolytes, which form dynamic percolation networks for electric charge upon flowing. During flowable electrode charging, metal can be deposited on the surface of such particles, and due to the constant change in the orientation of the particles while flowing, dendrite formation is expected to be largely mitigated. While promising, metal deposition upon flowable electrodes of flow batteries is largely unexplored. Here, we present detailed characterizations of a custom-built zinc-bromine redox flow battery employing a flowable anode with multi-walled carbon nanotube particles. We present measurements of battery voltage during constant current charge-discharge cycling for various experimental conditions, as well as the results of a novel dual-discharge experimental technique which measures, in situ, the amount of active zinc deposited onto the current collector and that deposited onto the flowable electrode particles. Our dual discharge measurements demonstrated that we could not discharge zinc from the particles at all experimental conditions tested. We hypothesized this observation was due either to relatively low electronic conductivity of the suspensions percolating network for electric charge, or due to a fast corrosion of the zinc metal from the electrode particles. We present evidence which suggests that the main mechanism responsible for preventing zinc metal discharge from the particles is zinc corrosion. We conclude by suggesting possible improvements for next generation cells, in light of the insights gained from this work.