Abstract:The most critical disadvantages of the Zn-air flow battery system are corrosion of the zinc, which appears as a high self-discharge current density and a short cycle life due to the non-uniform, dendritic, zinc electrodeposition that can lead to internal short-circuit. In our efforts to find a dendrite-free Zn electrodeposition which can be utilized in the Zn-air flow battery, the surface morphology of the electrolytic Zn deposits on a polished polymer carbon composite anode in alkaline, additive-free solutions was studied. Experiments were carried out with 0.1 M, 0.2 M and 0.5 M zincate concentrations in 8 M KOH. The effects of different working conditions such as: elevated temperatures, different current densities and different flow velocities, on current efficiency and dendrite formation were investigated. Specially designed test flow-cell with a central transparent window was employed. The highest Coulombic efficiencies of 80%-93% were found for 0.5 M ZnO in 8 M KOH, at increased temperatures (50-70 °C), current densities of up to 100 mA·cm -2 and linear electrolyte flow velocities higher than 6.7 cm·s -1 .

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Rechargeable zinc-air flow batteries are investigated as possible technology for fast responding large-scale electrical energy storage due to the use of inexpensive, non-toxic and abundant materials, and compact system design. The operating ranges for several parameters such as flow rate (2-8 cm s −1 ), concentration of electrolyte (6 or 8 M KOH), charge/discharge current densities (up to 100 mA cm −2 mean), and active or passive air supply as well as their influence on the performance and stability of the electrodes are investigated and compared. Bi-catalyzed bifunctional air electrodes are tested by means of half-cell measurements achieving minimum 200 charge/discharge cycles at 50 mA cm −2 with the longest operation time being 800 h. At this current density, charge/discharge efficiencies are in the range of 50% for all tested air electrodes. End-of-life characterization by means of scanning electron microscopy reveals mechanical degradation of the electrode material. On the negative zinc electrode, zinc deposition morphology on different current collector materials (nickel, brass, and steel) is investigated using Rota-Hull cylinders showing nickel to be the most suitable material. The pulse interrupt current method is thereby successfully applied for compact zinc deposition in a broad current density range without any electrolyte additive. Subsequent scale-up of the rechargeable zinc-air flow battery and unit cell operation is finally performed for proof-of-concept. Graphical AbstractKeywords Zinc-air flow battery · Bifunctional air electrode · Pulse interrupt currents · Compact zinc morphology · Flow operation

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