Design and Optimization of Carbon Foam Electrode for Local Confinement of Bromine in Non-Flowing Single-Chamber Zinc Bromine Batteries

Zinc Bromine redox flow batteries are well studied, quite inexpensive, and have reasonably high power and energy densities. However, there has been limited commercial implementation of bromine bearing systems in general, and the Zn-Br₂ systems in particular, due to the following premises: i) Br₂(l), generated at the cathode during charging, can diffuse through the electrolyte and react with Zn deposited at the anode (crossover), leading to self-discharge; ii) Repeated electroplating and dissolution of zinc leads to dendrite formation, which can form a conductive bridge between the electrodes (shorting), and iii) Br₂(l) has low miscibility in aqueous solutions (~2.8 vol%) and tends to stratify, resulting in non-uniform concentration distributions. Standard Zn-Br₂ flow-cell designs alleviate these limitations with bromine-complexing agents to improve Br₂(l) solubility, separation membranes to prevent crossover and shorting, and flowing electrolyte to remove bromine and to minimize dendrite formation.

In the membrane-free, non-flowing single-chamber zinc-bromine (SC-Zn-Br₂) battery design zinc dendrites are allowed to form freely and Br₂(l) is allowed to stratify; expensive membranes, complexing agents, and pumps are eliminated; and yet overall energy efficiency is improved and cost is lowered. This is achieved by using a highly-porous carbon foam electrode (CFE) for local containment of Br₂ generated during cycling, thus preventing crossover. Zinc dendrites are allowed to grow, but react with the highly corrosive Br₂(l) on the surface of the CFE and dissolve back into the electrolyte as Zn²⁺ and Br^-ions. This prevents shorting.

Here, we discuss the design, composition, and fabrication of the CFE, and its effects on the performance of SC-Zn-Br₂ battery. In particular, we demonstrate the optimization of CFE porosity through processing parameters to maximize the coulombic and energy efficiencies of the cell. We also introduce a color tracking and feedback monitoring scheme to actively control the active species (deep red Br₂(l), yellow Br₂(aq), and colorless ZnBr₂ electrolyte) transport and prevent self-discharge. Finally, we demonstrate the effect of prolonged exposure to Br₂(l) after multiple cycles on various CFE compositions using XPS analysis, and the potential degradation mechanisms. We show coulombic and energy efficiencies of 95% and 75%, respectively, for over 1000 cycles for SC-Zn-Br₂ cells, with improvements possible with further optimization of the CFE.