Effect of Pulse Current Charging on the Zinc Morphology in Alkaline Zinc/Air Redox Flow Batteries
Authors: Zelger Christian , Laumen Jennifer , Gollas Bernhard 
Published: February 2020

The increasing fraction of intermittent and difficult-to-predict energy sources, like solar and wind power, necessitates easily scalable energy buffers with short response times on grid-level [1]. Due to the low cost, environment-friendliness and wide availability of the active materials, the zinc/air redox flow battery is a promising candidate for stationary energy storage [2]. The reversible air electrode in one half-cell reaction supersedes storage volume for the positive active material, leading to an increased energy density of the system. Besides the efficiency of the air electrode, also the electrodeposition/dissolution of zinc still is a challenge, because of dendritic growth. Although organic additives can be used to improve the morphology of the zinc deposits, their chemical stability is quite limited in hot highly alkaline electrolytes.
We present a study of the effect of pulsed current on zinc electrodeposition from additive-free potassium hydroxide electrolytes with 0.2 M and 0.5 M zincate concentrations as half-cell reaction for a zinc/air redox flow battery. Zinc deposition on brass cylinders has been studied in a Rota-Hull cell [3] at 60°C with different pulse patterns and rotation rates. The setup allows for a high throughput screening of the zinc morphology for a range of current densities under well-controlled mass transport conditions. The range of rotation rates corresponds to electrolyte flow velocities between 3.0 and 16.0 cm s-1. Low flow velocities and a wide swing of zincate concentration are crucial to achieve a high energy efficiency and a high energy density of the redox flow battery. Deposits were characterized by optical microscopy and scanning electron microscopy.
Depositions with direct current predominantly produced the filamentous mossy and sometimes dendritic zinc morphology [4] with low mechanical stability. In depositions with pulse interrupt current under the same experimental conditions and the same average current densities the filamentous mossy and dendritic morphologies could be avoided completely. The pulse current deposits showed heavy spongy and boulder morphologies [4] having much higher mechanical stability. With optimized pulse parameters it was possible to produce dendrite-free and mechanically stable zinc deposits up to average current densities of 50 mA/cm2 in additive-free electrolytes under nearly all experimental conditions.
The kinetic parameters for zincate reduction including diffusion coefficient, exchange current density, and Tafel slope have been measured by chronoamperometry and chronopotentiometry under the conditions used in the electrodepositions. The parameters were used to determine the practical current density distribution along the cylinder electrode of the Rota-Hull cell for a correlation with the observed zinc morphologies. This correlation should provide guidelines for defining operating parameter ranges of zinc/air redox flow batteries.
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