Evaluation of Carbon Foams and Membranes As the Cathode in Non-Flow Zinc Bromine Energy Storage Cells to Mitigating Liquid Bromine Convection

Zinc Bromine (Zn-Br₂) secondary batteries have been extensively studied as a low cost, fully rechargeable, high density energy storage system. However, large scale and widespread commercial implementation of this battery technology has been retarded due to two key limitations [1]: i) self discharging of the cell due to Br₂ (l) convection and diffusion through the liquid electrolyte towards the Zn anode. ii) shorting due to zinc dendrite formation after multiple cycles of the cell. Zn-Br₂ flow-cells alleviate these limitations by adding Br₂ (l) complexing agents, and flowing electrolyte to improve Zn plating, [2] respectively, at the cost of cell resistance, battery efficiency, size, and capital costs.

In this work, we address the Br₂ (l) convection issue by designing a simple, low cost non-flow Zn-Br₂ cell without the use of expensive complexing agents, via modification of the bromine cathodic electrode material composition and geometry. The Zn dendrite issue is addressed elsewhere [3]. We take advantage of the difference in density of the Br₂ (l) (~3.1g/cc) and a 2M ZnBr₂ electrolyte (~1.5 g/cc) to contain the liquid bromine below the cathode while cycling the cell. First, we demonstrate a thin membrane electrode composed of carbon black/graphite/PVDF having a density of 1.88 g/cc. The densities are matched such that the membrane electrode floats at the interface between the dense Br₂ (l) and the electrolyte. During charging bromine forms and drops below the semi-porous membrane electrode while the cathode rises, reducing the anode-cathode spacing and, hence, the cell resistance. During discharge the Br₂ (l) is prevented from convecting through the electrolyte by the physical membrane barrier, while allowing a small degree of diffusion. Second, we also demonstrate the use of highly conductive carbon foam bromine electrode, which selectively traps the non-polar Br₂ (l) in its highly porous structure and thus addresses the bromine convection issue. The electrochemical characteristics of the non-flow Zn-Br₂ cell based on both these bromine electrodes designs are discussed in terms of cell resistance, specific capacity, energy and coulombic efficiencies, energy storage capability and cycle life. The physical properties of the membrane and foam electrodes are analyzed using SEM, AFM and XRD.