Strategies for Improved Depth-of-Discharge of Zinc-Air Flow Batteries

As one of the proposed post lithium-ion technologies, zinc-air batteries have received revived interest in the recent years, which is driven by the increased demand of high-performance, efficient, safe and environmentally friendly energy storage solutions. These energy storage systems need to address variable power, capacity and profitability requests. Zinc-air batteries with specific energy density higher than lithium-ion and low-cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with high performance oxygen-reduction electrodes enables the development of high-power zinc-air batteries. This setup also enables the independent scaling of capacity and power. The cooperative project, ZnMobil, founded by the Federal Ministry of Economic Affairs and Energy in Germany combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; the fuel cell research center ZBT GmbH) to develop a high-performance and low-cost zinc-air flow battery. Here we present strategies to improve the depth-of-discharge of zinc-air flow batteries. The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with gas-diffusion layer (supplied by Covestro). The zinc-slurry contains zinc particles (supplied by Grillo) suspended in alkaline solution (30 wt-% KOH) and stabilized with polyacrylic acid. The battery performance and depth-of-discharge were measured for several additives added to the zinc-slurry. Furthermore, the influence of the discharge current density on the depth-of-discharge was evaluated for these setups. Depending on the current density and additive selection, it is possible to achieve depth-of-discharge up to 90 %.