The rapidly increasing deployment of renewable yet intermittent energy sources such as solar and wind power has raised an urgent demand of developing large-scale electrical energy storage systems to enhance the grid reliability and stability. Among emerging technologies, zinc-bromine flow battery (ZBFB) is widely regarded as one of the most promising candidates due to its nature of high energy density and low cost. Nevertheless, the widespread application of this type of flow battery is still hindered by several critical issues including low power density and zinc dendrite formation. The major goal of this thesis is to address these challenges and propel its commercialization pace.

This thesis begins with modifying the electrolyte for ZBFB with methanesulfonic acid (MSA), aimin...[ Read more ]

The rapidly increasing deployment of renewable yet intermittent energy sources such as solar and wind power has raised an urgent demand of developing large-scale electrical energy storage systems to enhance the grid reliability and stability. Among emerging technologies, zinc-bromine flow battery (ZBFB) is widely regarded as one of the most promising candidates due to its nature of high energy density and low cost. Nevertheless, the widespread application of this type of flow battery is still hindered by several critical issues including low power density and zinc dendrite formation. The major goal of this thesis is to address these challenges and propel its commercialization pace.

This thesis begins with modifying the electrolyte for ZBFB with methanesulfonic acid (MSA), aiming to improve the electrolyte conductivity meanwhile suppress the zinc dendrite formation. Results unravel that the addition of MSA improves the reversibility and kinetics of the Zn²⁺/Zn and Br₂/Br⁻ redox reactions. Along with the reduced internal resistance from 4.9 to 2.0 Ω cm², the ZBFB with MSA exhibits an improved energy efficiency of 75% at 40 mA cm⁻², much higher than that with a conventional electrolyte of only 64%. Moreover, dendritic zinc growth is significantly suppressed in the presence of MSA, thus enabling the ZBFB to be operated for 50 cycles without degradation, whereas its counterpart suffers from significant decay after only 40 cycles due to the severe dendrite growth. The addition of MSA, however, triggers hydrogen evolution reaction (HER), which will cause fast capacity decay and safety issues. Hence, near neutral chloride salts (i.e., KCl, NH₄Cl) are explored as alternative supporting electrolytes. It is revealed that the battery internal resistance is significantly reduced after adding these chloride salts and NH₄Cl shows a more positive effect in enhancing the electrolyte conductivity. With an optimized NH₄Cl concentration of 4 M, an energy efficiency of 74.3% is achieved at a current density of 40 mA cm⁻², outperforming the battery with a pristine electrolyte. Additionally, the presence of NH₄Cl is found to induce more zinc deposition within the porous graphite felt (GF) electrode, resulting in a more uniform zinc deposition and ameliorating zinc dendrite formation.

With regard to the large polarization in the positive electrode, thermal treatment of GF electrode is firstly investigated. The carbon fiber surface is found to be of abundant pores and oxygen-containing functional groups after thermally treated at 500 ⁰C, which not only increase the wettability and specific surface area of the GF electrode, but also benefit its catalytic activity toward Br₂/Br⁻ reaction. As a result, a ZBFB assembled with this thermally treated GF electrode exhibits an energy efficiency of as high as 81.8% at a current density of 40 mA cm⁻², far surpassing that employs a pristine GF electrode. Furthermore, carbonized tubular polypyrrole (CTPPy) with abundant nitrogen- and oxygen-containing functional groups is synthesized as a positive material for ZBFB to further improve its performance. Experimental results reveal that CTPPy exhibits a high activity toward the Br₂/Br⁻ redox reaction, and enables the ZBFB to be operated at current densities of 40 and 80 mA cm⁻² with high energy efficiencies of 83.9% and 76.0%, respectively. More importantly, ZBFBs equipped with these functionalized electrodes present excellent stability during repeated cycling test, indicating their great promise as high-performance electrodes for ZBFB application.

To further enhance the performance of ZBFB, the conventional thick GF electrode in the positive side is replaced with a thin carbon-paper (CP) electrode interfacing with a serpentine flow-field structure. With this improved cell structure and electrode, the ZBFB is capable of delivering an energy efficiency of more than 70% at a high current density of up to 100 mA cm⁻², demonstrating a much enhanced power capability of ZBFB.

Finally, we propose a novel high-energy-density positive electrolyte for RFBs, by incorporating multiple redox couples. Our calculated results reveal that the energy density can reach as high as 827 Wh L⁻¹ if Fe³⁺/Fe²⁺ and Br₂/Br⁻ are utilized at the same time. Detailed electrochemical characterizations are conducted to uncover the electrochemistry of this positive electrolyte and a proof-of-concept flow battery is also assembled and tested. Decent results obtained in this work suggest that this novel electrolyte offers great promise for application in high-energy-density RFBs.

Keywords: Zinc-bromine flow battery; energy storage; zinc dendrite; Br₂/Br⁻ redox reaction; high performance; multiple redox couples; high energy density.