Rota-Hull Cell Study on Pulse Current Zinc Electrodeposition from Alkaline Electrolytes

The electrodeposition of zinc has been studied in the rotating cylinder Hull (RCH) cell with continuous and pulse interrupt current as one of the half-cell reactions for a zinc/air redox flow battery. The morphology of all pulse current deposits was discussed on the basis of the practical current density distribution. The experiments were performed in additive-free 8 M potassium hydroxide electrolytes at 60 °C, electrolyte flow velocities of 3, 6 and 16 cm s−1, and zincate concentrations of 0.2 and 0.5 M. In contrast to continuous current deposition, compact zinc deposits have been obtained in Rota-Hull experiments with pulse interrupt current at all electrolyte flow velocities and zincate concentrations. The non-compact filamentous mossy morphology, predominant in continuous current deposition, was not observed. For the highest practical current densities, current efficiencies between 96 and 100% have been found in pulse current experiments. The local practical current densities of all pulse and one continuous current depositions could be determined from the thickness of the zinc deposits measured by X-ray fluorescence. Under the most critical conditions for the deposition of compact zinc, i.e. low zincate concentrations of 0.2 M and low electrolyte flow velocities of 3 cm s−1, pulse interrupt current produced compact zinc deposits up to the limiting current. The results of the pulse current electrodepositions are explained on the basis of the ratio of applied pulse current density to limiting pulse current density. The limiting currents in the Rota-Hull cell were calculated with zincate diffusion coefficients of 6.09 × 10−6 cm2 s−1 (0.2 M zincate) and 6.38 × 10−6 cm2 s−1 (0.5 M zincate) determined by rotating disc electrode experiments at 60 °C. The absence of the filamentous mossy zinc morphology in pulse interrupt current experiments is ascribed to an effect of the off-time.